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• Azarov, Alexander; Radu, Cristian; Galeckas, Augustinas; Mercioniu, Ionel Florinel; Cernescu, Adrian & Venkatachalapathy, Vishnukanthan [Show all 11 contributors for this article] (2025). Self-Assembling of Multilayered Polymorphs with Ion Beams. Nano Letters. ISSN 1530-6984. 25(4), p. 1637–1643. doi: 10.1021/acs.nanolett.4c05727. Full text in Research Archive
• Abdullaev, Azat; Sekerbayev, Kairolla; Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Chauhan, Vinay S. & Utegulov, Zhandos [Show all 7 contributors for this article] (2024). Thermal conductivity of double polymorph Ga2O3 structures. APL Materials. 12(8). doi: 10.1063/5.0213985. Full text in Research Archive Show summary

  Recently discovered double gamma/beta (γ/β) polymorph Ga2O3 structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing the chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. In particular, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the γ-/β-Ga2O3 structures. In this way, the thermal conductivity of γ-Ga2O3 ranging from 1.84 to 2.11 W m−1 K−1 was found to be independent of the initial β-substrates orientations, in accordance with the cubic spinel structure of the γ-phase and consistently with the molecular dynamics simulation data. In turn, the thermal conductivity of monoclinic β-Ga2O3 showed a distinct anisotropy, with values ranging from 10 W m−1 K−1 for [−201] to 20 Wm−1 K−1 for [010] orientations. Thus, for double γ-/β-Ga2O3 polymorph structures formed on [010] β-substrates, there is an order of magnitude difference in thermal conductivity across the γ/β interface, which can potentially be exploited in thermal energy conversion applications.

• Azarov, Alexander; Galeckas, Augustinas; Cora, Ildikó; Fogarassy, Zsolt; Venkatachalapathy, Vishnukanthan & Monakhov, Eduard [Show all 7 contributors for this article] (2024). Optical Activity and Phase Transformations in γ/β Ga2O3 Bilayers Under Annealing. Advanced Optical Materials. doi: 10.1002/adom.202401325. Full text in Research Archive Show summary

  Gallium oxide (Ga2O3) can be crystallized in several polymorphs exhibiting different physical properties. In this work, polymorphic structures consisting of the cubic defective spinel (γ) film on the top of the monoclinic (β) substrate are fabricated by disorder-induced ordering, known to be a practical way to stack these polymorphs together. Such bilayer structures are annealed to investigate optical properties and phase transformations. Specifically, photoluminescence and diffuse reflectance spectroscopy are combined with transmission electron microscopy, Rutherford backscattering/channeling spectrometry and X-ray diffraction to monitor the evolutions. As a result, a two-stage annealing kinetics is observed in γ/β Ga2O3 bilayers associated with the epitaxial γ-to-β regrowth at the interface at temperatures below 700 °C and a non-planar γ-to-β phase transformation starting at higher temperatures. Thus, the present data enhance understanding of the polymorphism in Ga2O3, interconnecting the phase transformation kinetics with the evolution of the optical properties.

• Kaźmierczak-Bałata, Anna; Bodzenta, Jerzy; Szperlich, Piotr; Jesionek, Marcin; Michalewicz, Anna & Domanowska, Alina [Show all 9 contributors for this article] (2024). Impact of Annealing in Various Atmospheres on Characteristics of Tin-Doped Indium Oxide Layers towards Thermoelectric Applications. Materials. 17(18). doi: 10.3390/ma17184606. Full text in Research Archive Show summary

  The aim of this work was to investigate the possibility of modifying the physical properties of indium tin oxide (ITO) layers by annealing them in different atmospheres and temperatures. Samples were annealed in vacuum, air, oxygen, nitrogen, carbon dioxide and a mixture of nitrogen with hydrogen (NHM) at temperatures from 200 °C to 400 °C. Annealing impact on the crystal structure, optical, electrical, thermal and thermoelectric properties was examined. It has been found from XRD measurements that for samples annealed in air, nitrogen and NHM at 400 °C, the In2O3/In4Sn3O12 share ratio decreased, resulting in a significant increase of the In4Sn3O12 phase. The annealing at the highest temperature in air and nitrogen resulted in larger grains and the mean grain size increase, while vacuum, NHM and carbon dioxide atmospheres caused the decrease in the mean grain size. The post-processing in vacuum and oxidizing atmospheres effected in a drop in optical bandgap and poor electrical properties. The carbon dioxide seems to be an optimal atmosphere to obtain good TE generator parameters—high ZT. The general conclusion is that annealing in different atmospheres allows for controlled changes in the structure and physical properties of ITO layers.

• Devamanoharan, Arthi; Venkatachalapathy, Vishnukanthan; Veerapandy, Vasu & Ponniah, Vajeeston (2024). Investigating Stable Low-Energy Gallium Oxide (Ga2O3) Polytypes: Insights into Electronic and Optical Properties from First Principles. ACS Omega. 9(14), p. 16207–16220. doi: 10.1021/acsomega.3c10192. Full text in Research Archive Show summary

  This study provides a comprehensive analysis of the electronic and optical properties of low-energy gallium oxide (Ga2O3) polytypes not considered earlier. Among these polytypes, the monoclinic structure (β-Ga2O3) holds significant relevance for both research and practical applications due to its superior stability under typical conditions. The primary aim of this research is to identify new and stable Ga2O3 polytypes that may exist under zero-temperature and zero-pressure conditions. To achieve this objective, we employ the VASP code to investigate electrical and optical properties, as well as stability assessments. Additionally, we examine phonon and thermal properties, including heat capacity, for all polytypes. This study also encompasses the computation of full elastic tensors and elastic moduli for all polytypes at 0 K, with Poisson’s and Pugh’s ratios confirming their ductile nature. Furthermore, we present the first ever report on the Raman- and infrared (IR)-active modes of these stable Ga2O3 polytypes. Our findings reveal that these mechanically and dynamically stable Ga2O3 polytypes exhibit semiconductive properties, as evidenced by electronic band structure investigations. This research offers valuable insights into the optical characteristics of Ga2O3 polytypes with potential applications spanning various fields.

• Mahalakshmi, S.; Mayandi, Jeyanthinath; Sagadevan, Suresh; Ragavendran, V.; Manikandan, K. & Arumugam, S. [Show all 8 contributors for this article] (2023). Enriched second-harmonic generation in meta-phase barium titanate nanostructures synthesized by sol-gel hydrothermal method. Journal of Alloys and Compounds. ISSN 0925-8388. 936. doi: 10.1016/j.jallcom.2022.168171. Full text in Research Archive
• Polyakov, A.Y.; Kochkova, A.I.; Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Miakonkikh, A.V. & Vasilev, A.A. [Show all 11 contributors for this article] (2023). Tuning electrical properties in Ga2O3 polymorphs induced with ion beams. Journal of Applied Physics. ISSN 0021-8979. 133(9), p. 095701-1–095701-9. doi: 10.1063/5.0133181. Full text in Research Archive Show summary

  Ion beam fabrication of metastable polymorphs of Ga2O3, assisted by the controllable accumulation of the disorder in the lattice, is an interesting alternative to conventional deposition techniques. However, the adjustability of the electrical properties in such films is unexplored. In this work, we investigated two strategies for tuning the electron concentration in the ion beam created metastable κ-polymorph: adding silicon donors by ion implantation and adding hydrogen via plasma treatments. Importantly, all heat treatments were limited to ≤600 °C, set by the thermal stability of the ion beam fabricated polymorph. Under these conditions, silicon doping did not change the high resistive state caused by the iron acceptors in the initial wafer and residual defects accumulated upon the implants. Conversely, treating samples in a hydrogen plasma converted the ion beam fabricated κ-polymorph to n-type, with a net donor density in the low 1012 cm−3 range and dominating deep traps near 0.6 eV below the conduction band. The mechanism explaining this n-type conductivity change may be due to hydrogen forming shallow donor complexes with gallium vacancies and/or possibly passivating a fraction of the iron acceptors responsible for the high resistivity in the initial wafers.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Lee, In-Hwan & Kuznetsov, Andrej (2023). Thermal versus radiation-assisted defect annealing in β-Ga2O3. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 41(2). doi: 10.1116/6.0002388. Full text in Research Archive Show summary

  Gallium oxide (Ga2O3) exhibits complex behavior under ion irradiation since ion-induced disorder affects not only the functional properties but can provoke polymorphic transformations in Ga2O3. A conventional way used to minimize the lattice disorder is by doing postirradiation anneals. An alternative approach is to prevent the disorder accumulation from the beginning, by doing implants at elevated temperatures, so that a significant fraction of the disorder dynamically anneals out in radiation-assisted processes. Here, we use these two approaches for the minimization of radiation disorder in monoclinic β-Ga2O3 implanted to a dose below the threshold required for the polymorphic transformations. The results obtained by a combination of channeling and x-ray diffraction techniques revealed that implants at 300 °C effectively suppress the defect formation in β-Ga2O3. On the other hand, in order to reach similar crystalline quality in the samples implanted at room temperature, postirradiation anneals in excess of 900 °C are necessary.

• Kaźmierczak-Bałata, Anna; Bodzenta, Jerzy; Dehbashi, Mohsen; Mayandi, Jeyanthinath & Venkatachalapathy, Vishnukanthan (2023). Influence of Post Processing on Thermal Conductivity of ITO Thin Films. Materials. 16(1). doi: 10.3390/ma16010362. Full text in Research Archive Show summary

  This work presents the influence of post processing on morphology, thermal and electrical properties of indium tin oxide (ITO) thin films annealed at 400 °C in different atmospheres. The commercially available 170 nm thick ITO layers deposited on glass were used as a starting material. The X-ray diffraction measurements revealed polycrystalline structure with dominant signal from (222) plane for all samples. The annealing reduces the intensity of this peak and causes increase of (221) and (440) peaks. Atomic force microscopy images showed that the surface morphology is typical for polycrystalline layers with roughness not exceeding few nm. Annealing in the oxygen and the nitrogen-hydrogen mixture (NHM) changes shapes of grains. The electrical conductivity decreases after annealing except the one of layer annealed in NHM. Thermal conductivities of annealed ITO thin films were in range from 6.4 to 10.6 W·m−1·K−1, and they were higher than the one for starting material—5.1 W·m−1·K−1. Present work showed that annealing can be used to modify properties of ITO layers to make them useful for specific applications e.g., in ITO based solar cells.

• Venkatesan, Ragavendran; Maideen, Sheik Moideen Thaha Sheik Kadar; Chandhiran, Saravanan; Kushvaha, Sunil Singh; Sagadevan, Suresh & Venkatachalapathy, Vishnukanthan [Show all 7 contributors for this article] (2022). Fabrication and Characterization of Si/PEDOT: PSS-Based Heterojunction Solar Cells. Electronics (Basel). 11(24). doi: 10.3390/electronics11244145. Full text in Research Archive Show summary

  In this study, we fabricated a planar Si/PEDOT: PSS heterojunction solar cell using three different solvents—ethylene glycol, acetonitrile, and dimethyl sulfoxide—to find the best one. The fabricated samples were characterized by diffuse reflectance spectroscopy, scanning electron microscopy, X-ray diffraction, and current–voltage. Diffused reflectance spectrum analysis showed reduced reflectance compared to the bare silicon wafers. The absorbance spectrum shows the change in absorption of the Si-coated PEDOT: PSS which was more than a 50% increase in the UV region, and for the EG sample, there was a 20% increase in the entire visible spectrum. This indicates that the solvent plays a major role in the bandgap between the Si and Si/PEDOT: PSS. Scanning electron microscope (SEM) was used to examine the surface morphology of Si/PEDOT: PSS as agglomerated, island-formed surfaces and carbon-layered Si-PEDOT: PSS. Cross-sectional images show the thickness of the PEDOT: PSS layer on the silicon wafer surface. The X-ray diffraction (XRD) pattern shows the characteristic peaks for silicon (69.5°), and Si/PEDOT: PSS shows a forbidden Si (200) peak at 32°. Current–voltage measurements have shown the characteristic diode curve for all fabricated cells. This characteristic diode curve indicated the presence of a heterojunction. Ethylene glycol-containing cells showed current output as 0.2 μA with Voc of 0.2 V.

• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Venkatachalapathy, Vishnukanthan; Mikšová, R. & MacKová, A. [Show all 9 contributors for this article] (2022). Radiation-induced defect accumulation and annealing in Si-implanted gallium oxide. Journal of Applied Physics. ISSN 0021-8979. 131(12). doi: 10.1063/5.0083858. Full text in Research Archive Show summary

  Defect accumulation and annealing phenomena in Si-implanted monoclinic gallium oxide (β-Ga 2 O 3 ) wafers, having [Formula: see text], (010), and (001) orientations, were studied by Rutherford backscattering spectrometry in channeling mode (RBS/c), x-ray diffraction (XRD), and (scanning) transmission electron microscopy [(S)TEM]. Initially, the samples with different surface orientations were implanted with 300 keV 28 Si + -ions, applying fluences in the range of 1 × 10 14 –2 × 10 16  Si/cm 2 , unveiling interesting disorder accumulation kinetics. In particular, the RBS/c, XRD, and (S)TEM combined data suggested that the radiation disorder buildup in Si-implanted β-Ga 2 O 3 is accompanied by significant strain accumulation, assisting crystalline-to-crystalline phase transitions instead of amorphization. Selected samples having [Formula: see text] orientation were subjected to isochronal (30 min) anneals in the range of 300–1300 °C in air. Systematic RBS/c and XRD characterization of these samples suggested complex structural transformations, which occurred as a function of the fluence and the temperature. Moreover, a detailed (S)TEM analysis of the sample implanted with 2 × 10 16  Si/cm 2 and annealed at 1100 °C was enhanced by applying dispersive x-ray and electron energy-loss spectroscopies. The analysis revealed silicon agglomerations in the form of silicon dioxide particles. Signal from silicon was also detected outside of the agglomerates, likely occurring as substitutional Si on Ga sites.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Karaseov, Platon; Titov, Andrei; Karabeshkin, Konstantin & Struchkov, Andrei [Show all 7 contributors for this article] (2022). Interplay of the disorder and strain in gallium oxide. Scientific Reports. 12(1). doi: 10.1038/s41598-022-19191-8. Full text in Research Archive Show summary

  Abstract Ion irradiation is a powerful tool to tune properties of semiconductors and, in particular, of gallium oxide (Ga 2 O 3 ) which is a promising ultra-wide bandgap semiconductor exhibiting phase instability for high enough strain/disorder levels. In the present paper we observed an interesting interplay between the disorder and strain in monoclinic β-Ga 2 O 3 single crystals by comparing atomic and cluster ion irradiations as well as atomic ions co-implants. The results obtained by a combination of the channeling technique, X-ray diffraction and theoretical calculations show that the disorder accumulation in β-Ga 2 O 3 exhibits superlinear behavior as a function of the collision cascade density. Moreover, the level of strain in the implanted region can be engineered by changing the disorder conditions in the near surface layer. The results can be used for better understanding of the radiation effects in β-Ga 2 O 3 and imply that disorder/strain interplay provides an additional degree of freedom to maintain desirable strain in Ga 2 O 3 , potentially applicable to modify the rate of the polymorphic transitions in this material.

• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Ponniah, Vajeeston; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-Induced Ordering in Gallium Oxide Polymorphs. Physical Review Letters. ISSN 0031-9007. 128(1). doi: 10.1103/PhysRevLett.128.015704. Full text in Research Archive Show summary

  olymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in Ga2O3 and, potentially, for similar polymorphic families in other materials.

• Krishnan, Saravanan Vanal; Ambalam, Muthumanickam Muthukaruppan; Venkatesan, Ragavendran; Mayandi, Jeyanthinath & Venkatachalapathy, Vishnukanthan (2021). Technical review: Improvement of mechanical properties and suitability towards armor applications – Alumina composites. Ceramics International. ISSN 0272-8842. 47(17), p. 23693–23701. doi: 10.1016/j.ceramint.2021.05.146. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Vines, Lasse; Monakhov, Eduard; Lee, In-Hwan & Kuznetsov, Andrej (2021). Activation energy of silicon diffusion in gallium oxide: Roles of the mediating defects charge states and phase modification. Applied Physics Letters. ISSN 0003-6951. 119(18). doi: 10.1063/5.0070045. Full text in Research Archive Show summary

  Silicon (Si) is an efficient n-type dopant in gallium oxide (Ga2O3)—an ultra-wide bandgap semiconductor promising in a number of applications. However, in spite of the technological importance for device fabrication, the activation energy for Si diffusion in Ga2O3 is missing in the literature. In the present work, we do such measurements in ion implanted monoclinic β-Ga2O3 samples employing anneals in air ambient, also admitting the influence of potential ion beam induced phase modifications on diffusion. Importantly, we show that Si diffusion in β-Ga2O3 fits with the concentration dependent diffusion model, involving neutral and single negatively charged point defects to mediate the process; so that we assumed gallium vacancies in the corresponding charge states to assist Si diffusion in β-Ga2O3 with activation energies of 3.2 ± 0.3 and 5.4 ± 0.4 eV, respectively. Moreover, we also found that a preexisting phase modified surface layer efficiently suppressed Si diffusion in β-Ga2O3 for temperatures up to 1000 °C.

• Kaźmierczak-Bałata, Anna; Grzadziel, Lucyna; Guziewicz, Marek; Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej & Krzywiecki, Maciej (2021). Correlations of thermal properties with grain structure, morphology, and defect balance in nanoscale polycrystalline ZnO films. Applied Surface Science. ISSN 0169-4332. doi: 10.1016/j.apsusc.2021.149095. Full text in Research Archive Show summary

  ZnO thin films grown by atomic layer deposition method were investigated to analyse the correlation between morphology, chemical defects and thermal properties. This analysis became possible by measuring series of similarly thick wurtzite ZnO films synthesized using identical number of deposition cycles at two temperatures – 100 °C and 200 °C. By comparing these sample series, exhibiting the thickness variations from 12 to 118 nm, several prominent correlations for the thermal properties was observed - measured by high-resolution scanning thermal microscopy at room temperature. Specifically, comparing thinnest samples in the series, a lower thermal conductivity was revealed in samples grown at 100 °C and these variations were interpreted in terms of changing populations of native donors, affecting the free carrier concentration and, as such, the thermal conductivity. Further, the increase in thickness for the samples grown at 200 °C was accompanied with a continuous shift to Zn-rich conditions and the corresponding increase in the thermal conductivity, specifically from 0.28 Wm−1 K−1 to 2.81 Wm−1 K−1. As such, we have decoupled the thickness effect on the thermal conductivity from the rest of the phenomena, explaining the thermal transport in terms of the bulk thermal contributions within the grains combined with additional thermal resistance introduced by the grain boundaries.

• Mayandi, Jeyanthinath; Madathil, Reshma Krishnan; Abinaya, C; Bethke, Kevin; Venkatachalapathy, Vishnukanthan & Rademann, Klaus [Show all 8 contributors for this article] (2021). Al-doped ZnO prepared by co-precipitation method and its thermoelectric characteristics. Materials Letters. ISSN 0167-577X. 288. doi: 10.1016/j.matlet.2021.129352. Full text in Research Archive Show summary

  We report on structural and electrical characterization of zinc oxide material containing aluminum (Al:ZnO) prepared by a co-precipitation technique using different zinc and aluminum precursors. The as-prepared powders were calcinated and pre-annealed at 600 °C and 1200 °C. These were cold-pressed to pellets and sintered at 1000 °C. We also prepared ZnO material with no Al for comparisons. The electrical and thermal conductivity, as well as the Seebeck coefficient, were measured. These properties were correlated with structural analysis by X-ray diffraction characterization and scanning electron microscopy. The process yields a nanostructured material with much reduced thermal conductivity.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dominating migration barrier for intrinsic defects in gallium oxide: Dose-rate effect measurements. Applied Physics Letters. ISSN 0003-6951. 118(23). doi: 10.1063/5.0051047. Full text in Research Archive Show summary

  Ion bombardment provides an opportunity to study basic properties of intrinsic defects in materials since the radiation-induced disorder accumulation depends on the balance between defect generation and migration rates. In particular, variation of such parameters as irradiation temperature and ion flux, known in the literature as dose-rate effect, interconnects the macroscopically measured lattice disorder with the migration barrier of the dominating defects. In this work, we measured the dose-rate effect in monoclinic gallium oxide (β-Ga2O3) and extracted its activation energy of 0.8 ± 0.1 eV in the range of 25–250 °C. Taking into account that the measurements were performed in the Ga-sublattice and considering 0.8 ± 0.1 eV in the context of theoretical data, we interpreted it as the migration barrier for Ga vacancies in β-Ga2O3, limiting the process. Additionally, we observed and took into account an interesting form of the lattice relaxation due to radiation-induced disorder buildup, interpreted in terms of the compressive strain accumulation, potentially trigging phase transitions in Ga2O3 lattice.

• Ellingsen, Josef Gert Åsheim; Venkatachalapathy, Vishnukanthan; Azarov, Alexander; Nilsen, Ola & Kuznetsov, Andrej (2020). Al incorporation during metal organic chemical vapour deposition of aluminium zinc oxide. Thin Solid Films. ISSN 0040-6090. 709. doi: 10.1016/j.tsf.2020.138245. Full text in Research Archive Show summary

  Aluminium zinc oxide (AZO) is a promising transparent conductive oxide this is difficult to fabricate by metal-organic chemical vapour deposition (MOCVD) because of prereactions between the precursors for Al and oxygen. One way of reducing prereactions is to use separate injectors. In this work, we investigated the effect of separate injectors and various flow configurations on the MOCVD fabrication of AZO using diethyl zinc (DEZn), tert-butanol (tBuOH), and trimethyl aluminium (TMAl) precursors. X-ray diffraction revealed that the resulting films maintained a purely wurtzite ZnO structure, and that the Al incorporation changed the c lattice parameters from 5.207 to 5.181 Å in agreement with theoretical models of mixing on the Zn sub-lattice. This indicates that Al was mainly incorporated onto the Zn sub-lattice. The maximum Al concentration was 6 × 1021 cm . The Al concentration was directly influenced by the TMAl flow rates, but the incorporation efficiency of Al also depended on the O/Zn balance as determined by the tBuOH supply and the total flow rate through the TMAl+DEZn injector. The tBuOH supply decreased the incorporation efficiency, while it increased for higher total flow in the TMAl+DEZn injector although both changes nominally increases the O/Zn balance. This dual, but contradicting, dependency on the O/Zn suggested that the incorporation efficiency depended not only on precursor supply, but also on the chemical stability of the Al species present at the growth interface. The concept of a reaction depth of TMAl was introduced to discuss the observed trends, and a qualitative model was suggested to explain how the Al-species present at the growth interface are determined by the relative depths of the reaction depth and the boundary layer. These findings illustrate the limits of the O/Zn parameter when using separate injectors because the properties of the boundary layer can affect Al incorporation during MOCVD growth. This should be relevant for MOCVD growth of other compound semiconductor systems that are grown using separate injectors.

• Muthuvel, Ramuvel; Arunachalam, Manimaran; Karanthamalai, Vinayagar; Venkatesan, Ragavendran; Venkatachalapathy, Vishnukanthan & Mayandi, Jeyanthinath (2020). Mechanical, Structural and Optical Properties of the Silicon Nanowire Arrays. Zeitschrift für Physikalische Chemie. ISSN 0942-9352. doi: 10.1515/zpch-2019-1588. Full text in Research Archive
• Azarov, Alexander; Galeckas, Augustinas; Venkatachalapathy, Vishnukanthan; Mei, Zengxia; Du, Xiaolong & Monakhov, Eduard [Show all 7 contributors for this article] (2020). Acceptor complex signatures in oxygen-rich ZnO thin films implanted with chlorine ions. Journal of Applied Physics. ISSN 0021-8979. 128. doi: 10.1063/5.0021089. Full text in Research Archive Show summary

  Spectroscopic identification of defects and impurities is crucial for understanding doping asymmetry issues in ZnO and, therefore, realization of true ZnO-based bipolar devices. Chlorine (Cl) is an amphoteric impurity in ZnO exhibiting acceptor behavior in the interstitial configuration and donor action once on substitutional oxygen sites (ClO). In its turn, the incorporation of Cl atoms depends on the material growth conditions and a ClO fraction should be suppressed in O-rich materials. In the present work, Cl ions were implanted into ZnO thin films synthesized under O-rich conditions. In contrast to a negligible effect of Cl incorporation to electrical conductivity, photoluminescence measurements revealed dramatic developments of optical properties with a strong acceptor-like spectral signature emerging after 900 °C anneals. We discuss the origins of a new excitonic I* line (3.355 eV) induced by Cl-implantation and propose two alternative defect models based on shallow acceptor and shallow donor complexes.

• Venkatesan, Ragavendran; Mayandi, Jeyanthinath; Søndenå, Rune; Finstad, Terje & Venkatachalapathy, Vishnukanthan (2020). Investigating antireflection properties of hybrid silicon nanostructures comprising rod-like nanopores and nano-textured surface. Materials Letters. ISSN 0167-577X. 275. doi: 10.1016/j.matlet.2020.128087. Full text in Research Archive Show summary

  In the present work, we have fabricated hybrid silicon (Si) nanostructures comprising vertical rod-like nanopores and nano-textured surface by metal assisted chemical etching (MACE) method at room temperature. The as-received p-type Upgraded Metallurgical grade (UMG) Si wafers were chemical polished, prior to investigating the etching effects at the metal nanoparticle semiconductor interface. The influence of metal silver nanoparticle (AgNPs) concentration on the formation of hybrid nanostructures were studied systematically. Depending on the surface morphology, the hybrid structures exhibited constant 10% average reflectance in the UV–Visible spectral region or average 7.5% reflectance in range of 200–400 nm

• Mayandi, Jeyanthinath; Finstad, Terje; Venkatesan, Ragavendran; Vajeeston, Ponniah; Karazhanov, Smagul & Venkatachalapathy, Vishnukanthan (2020). Carbon-dioxide as annealing atmosphere to retain the electrical properties of indium-tin oxide. Materials Letters. ISSN 0167-577X. 276. doi: 10.1016/j.matlet.2020.128195. Full text in Research Archive Show summary

  In practical applications of indium-tin-oxide (ITO) annealing at temperatures ~400 °C without degrading its electrical and optical properties is an important challenge. In the present work, commercial Indium-tin oxide (ITO) coated on glass was subjected to post-annealing treatment in the range of 200-400 °C at different annealing atmospheres; oxygen, nitrogen and carbon-dioxide. The annealed samples were characterized by X-ray diffraction, UV-visible spectroscopy and Hall measurements to evaluate the structural, optical and electrical properties. Both oxygen and nitrogen treated samples degrades the structural, optical and conducting properties of ITO, while carbon-dioxide atmosphere inhibits the degradation of ITO at 400 °C. The obtained results suggest that carbon-dioxide can be well utilized as annealing ambient to retain opto-electronic, structural and electrical properties of ITO and thereby improve the efficiency of ITO based solar cells.

• Venkatesan, Ragavendran; Mayandi, Jeyanthinath; Pearce, Joshua M. & Venkatachalapathy, Vishnukanthan (2019). Influence of metal assisted chemical etching time period on mesoporous structure in as-cut upgraded metallurgical grade silicon for solar cell application. Journal of materials science. Materials in electronics. ISSN 0957-4522. 30(9), p. 8676–8685. doi: 10.1007/s10854-019-01191-6. Full text in Research Archive Show summary

  In this work, upgraded metallurgical grade silicon (UMG-Si) wafer was used to fabricate mesoporous nanostructures, as an effective antireflection layer for solar photovoltaic cells. The length of the vertical Si nanostructure (SiNS) arrays was altered by varying the etching time period during metal assisted chemical etching process, using a silver catalyst. The optical, structural, morphological changes and the antireflection properties of Si nanostructures formed on UMG-Siwafer were analysed. SEM and photoluminescence studies indicate that Si nanocrystals are formed on the surface and along the vertical nanowires. The pore size depends on the Ag nanoparticle size distribution. All the samples demonstrated a luminescence band centred around 2.2 eV. From the optical results, samples etched for 45 min show strong absorption in the visible spectrum. The minimum and maximum surface reflectance in the visible region was observed for 15 min and 60 min etched SiNS. Based on the observed results, 15 min etched Si with a uniform porous structure has minimum reflectance across the entire silicon UV–Vis absorption spectrum, making it worth further investigation as a candidate for use as an antireflection layer in silicon based solar cells.

• Selvaraj, Mahalakshmi; Venkatesan, Ragavendran; Mayandi, Jeyanthinath & Venkatachalapathy, Vishnukanthan (2019). Misidentification of hexagonal phase as barium carbonate during chemical synthesis of barium titanate nanopowders. Materials Today: Proceedings. 23, p. 81–84. doi: 10.1016/j.matpr.2019.06.713. Full text in Research Archive
• Krishnan, Saravanan Vanal; Ambalam, Muthu Manickam Muthukaruppan; Venkatesan, Ragavendran; Arivalagan, Muthukumar; Venkatachalapathy, Vishnukanthan & Mayandi, Jeyanthinath (2019). Reinforcement of alumina with carbon nano cones and characterization. Materials Today: Proceedings. doi: 10.1016/j.matpr.2019.05.442. Full text in Research Archive
• Selvaraj, Mahalakshmi; Venkatesan, Ragavendran; J, Mayandi & Venkatachalapathy, Vishnukanthan (2019). Influence of tin (IV) doping on structural and optical properties of rhombohedral barium titanate (BaTiO3). Materials Today: Proceedings. doi: 10.1016/j.matpr.2019.05.302. Full text in Research Archive
• Zhan, Wei; Venkatachalapathy, Vishnukanthan; Aarholt, Thomas; Kuznetsov, Andrej & Prytz, Øystein (2018). Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale. Scientific Reports. 8(848), p. 1–7. doi: 10.1038/s41598-017-18949-9. Full text in Research Archive Show summary

  Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1−xCd x O/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.

• Murugesan, Marikkan; A., Dinesh; J, Mayandi; Venkatachalapathy, Vishnukanthan & Pearce, Joshua M. (2018). Properties of Al-Doped Zinc Oxide and In-Doped Zinc Oxide Bilayer Transparent Conducting Oxides for Solar Cell Applications. Materials Letters. ISSN 0167-577X. 222, p. 50–53. doi: 10.1016/j.matlet.2018.03.097. Full text in Research Archive Show summary

  Novel aluminum and indium doped zinc oxide (ZnO) bilayer transparent conducting oxide thin films was fabricated by simple sol-gel spin coating method and post-annealed at 500 °C for an hour under nitrogen ambient for solar cell applications. The structural, electrical and optical properties of both the as-deposited and annealed bilayer thin films were characterized. X-ray diffraction studies show a hexagonal wurtzite-type structure of ZnO with (0 0 2) orientation, which enhanced with annealing. In atomic force microscopy studies, minimum surface roughness was attained for Al-doped ZnO (AZO)/In-doped ZnO (IZO) bilayer TCO film compared to IZO/AZO bilayer film. The AZO/IZO film sheet resistance improved to 0.057 M ohm/square after post-annealing, while the single layer AZO film sheet resistance degraded upon annealing in nitrogen atmosphere. All the films had an average transmittance in the visible region over 96%.

• Sky, Thomas Neset; Johansen, Klaus Magnus H; Venkatachalapathy, Vishnukanthan; Svensson, Bengt Gunnar; Tuomisto, Filip & Vines, Lasse (2018). Influence of Fermi level position on vacancy-assisted diffusion of aluminum in zinc oxide. Physical review B (PRB). ISSN 2469-9950. 98(24), p. 245204-1–245204-8. doi: 10.1103/PhysRevB.98.245204. Full text in Research Archive Show summary

  The influence of Fermi level position and annealing ambient on the zinc vacancy VZn generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of VZn and Al is established, demonstrating the Fermi level dependence of the formation of the electrically compensating −2 charge state of VZn in conductive n-type ZnO crystals. In contrast, thermal treatment in the zinc-rich ambient is shown to efficiently reduce the VZn concentration and related complexes. Using a reaction-diffusion model, the diffusion characteristics of Al at different donor background concentrations are fully accounted for by mobile (AlZnVZn)− pairs. These pairs form via the migration and reaction of isolated V2−Zn with the essentially immobile Al+Zn. We obtain a migration barrier for the (AlZnVZn)− pair of 2.4±0.2 eV, in good agreement with theoretical predictions. In addition to strongly alter the shape of the Al diffusion profiles, increasing the donor background concentration also results in an enhanced effective Al diffusivity, attributed to a reduction in the V2−Zn formation energy as the Fermi level position increases.

• Venkatesan, Ragavendran; Arivalagan, Muthu Kumar; Venkatachalapathy, Vishnukanthan; Pearce, Joshua M. & J, Mayandi (2018). Effects of silver catalyst concentration in metal assisted chemical etching of silicon. Materials Letters. ISSN 0167-577X. 221, p. 206–210. doi: 10.1016/j.matlet.2018.03.053. Full text in Research Archive Show summary

  A systematic investigation is performed to determine the effects of the concentration of silver on metal assisted chemical etching (MaCE) for nanostructure formation mechanisms on silicon as well as their resultant optical properties. Silver nitrate concentrations of 8 mM, 4 mM, 3 mM and 2 mM with hydrogen fluoride were used for the preparation of p-type silicon nanostructures. Experimentally, it is observed that when the catalysis molarity concentration is decreased in the etching processes, it resulted in nanostructures ranging from 140 to 60 nm, respectively over the concentrations investigated. A detailed analysis of the optical properties and structure provided insight into the physics of their formation. In addition, the results show the silicon nanostructures formed black silicon where in the visible region of the spectrum the reflectance dropped by an order of magnitude. The results indicate MaCE is a promising approach to the manufacturing of antireflection coatings on black silicon-based solar photovoltaic cells. MaCE is a simple and scalable approach to enhance the optical absorption of silicon and improve the overall efficiency of the solar cell without adding significantly to the complexity, capital expenditure or cost of production.

• Ţălu, Ştefan; Kulesza, Slawomir; Bramowicz, Miroslaw; Pringle, Adam M.; Pearce, Joshua M. & Murugesan, Marikkan [Show all 8 contributors for this article] (2018). Micromorphology analysis of sputtered indium tin oxide fabricated with variable ambient combinations. Materials Letters. ISSN 0167-577X. 220, p. 169–171. doi: 10.1016/j.matlet.2018.03.005. Full text in Research Archive
• Jensen, Ingvild Julie Thue; Johansen, Klaus Magnus H; Zhan, Wei; Venkatachalapathy, Vishnukanthan; Brillson, Len & Kuznetsov, Andrej [Show all 7 contributors for this article] (2018). Bandgap and band edge positions in compositionally graded ZnCdO. Journal of Applied Physics. ISSN 0021-8979. 124(1), p. 015302-1–015302-8. doi: 10.1063/1.5036710. Full text in Research Archive Show summary

  Introducing Cd into ZnO allows for bandgap engineering, potentially with particularly interesting properties to observe in compositionally graded samples. In this work, compositionally graded Zn1–xCdxO samples with 0 ≤ x < 0.16 were made using metal organic vapour phase epitaxy. The chemical composition was studied using scanning transmission electron microscopy, while the band structure of the samples was investigated using a combination of cathodoluminescence spectroscopy and X-ray photoelectron spectroscopy (XPS). It is found that the reduction of the bandgap in our samples is caused by changes in the conduction band. The position of the Fermi level relative to the vacuum level, i.e., the workfunction, was also found to change upon addition of Cd, giving an apparent shift in the valence band when evaluated from the XPS valence spectra. © 2018 AIP Publishing

• Tang, Aihua; Mei, Zengxia; Hou, Yao-Nan; Liu, Lishu; Venkatachalapathy, Vishnukanthan & Azarov, Alexander [Show all 8 contributors for this article] (2018). GaZn-VZn acceptor complex defect in Ga-doped ZnO. Science China Physics, Mechanics & Astronomy. ISSN 1674-7348. 61(7), p. 1–6. doi: 10.1007/s11433-018-9195-7. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Azarov, Alexander; Vines, Lasse & Kuznetsov, Andrej (2018). Phase stability and strain accumulation in CdO as a function of Cd/O supply during MOVPE synthesis. Superlattices and Microstructures. ISSN 0749-6036. 120, p. 569–577. doi: 10.1016/j.spmi.2018.06.017. Full text in Research Archive
• Schifano, Ramon; Riise, Heine Nygard; Domagala, J.; Azarov, Alexander; Ratajczak, R. & Monakhov, Edouard [Show all 11 contributors for this article] (2017). Comparison of the structural properties of Zn-face and O-face single crystal homoepitaxial ZnO epilayers grown by RF-magnetron sputtering. Journal of Applied Physics. ISSN 0021-8979. 121(1). doi: 10.1063/1.4973342. Full text in Research Archive Show summary

  Homoepitaxial ZnO growth is demonstrated from conventional RF-sputtering at 400 °C on both Zn and O polar faces of hydrothermally grown ZnO substrates. A minimum yield for the Rutherford backscattering and channeling spectrum, χmin, equal to ∼3% and ∼12% and a full width at half maximum of the 00.2 diffraction peak rocking curve of (70 ± 10) arc sec and (1400 ± 100) arc sec have been found for samples grown on the Zn and O face, respectively. The structural characteristics of the film deposited on the Zn face are comparable with those of epilayers grown by more complex techniques like molecular beam epitaxy. In contrast, the film simultaneously deposited on the O-face exhibits an inferior crystalline structure ∼0.7% strained in the c-direction and a higher atomic number contrast compared with the substrate, as revealed by high angle annular dark field imaging measurements. These differences between the Zn- and O-face films are discussed in detail and associated with the different growth mechanisms prevailing on the two surfaces. © 2016 AIP Publishing

• Zhan, Wei; Granerød, Cecilie Skjold; Venkatachalapathy, Vishnukanthan; Johansen, Klaus Magnus H; Jensen, Ingvild Julie Thue & Kuznetsov, Andrej [Show all 7 contributors for this article] (2017). Nanoscale mapping of optical band gaps using monochromated Electron Energy Loss Spectroscopy. Nanotechnology. ISSN 0957-4484. 28(10). doi: 10.1088/1361-6528/aa5962. Full text in Research Archive Show summary

  Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

• Malik, Nishant; Venkatachalapathy, Vishnukanthan; Dall, Wilhelm; Schjølberg-Henriksen, Kari; Poppe, Erik Utne & Taklo, Maaike M. Visser [Show all 7 contributors for this article] (2017). Texture of Al films for wafer-level thermocompression bonding. Superlattices and Microstructures. ISSN 0749-6036. 106, p. 216–233. doi: 10.1016/j.spmi.2017.03.053. Full text in Research Archive Show summary

  Properties of aluminum thin films for thermocompression bonding have been studied in terms of surface roughness, grain size, and grain orientation by AFM, SEM, XRD and EBSD for thermocompression bonding. Al films were sputter deposited directly on Si and thermally oxidized Si wafers, respectively. The resulting Si/Al and Si/SiO2/Al sample types were compared after annealing (300–550 °C) in vacuum. The Si/SiO2/Al film samples showed higher surface roughness than the Si/Al samples. The as-deposited films had (111) preferred orientation, while (100) and (110) oriented Al grains were also present in Si/SiO2/Al samples. The Si/SiO2/Al samples and Si/Al sample annealed at 550 °C had a conical <111> texture. The observed evolution of the grain structure with annealing temperature is discussed in terms of native oxide, surface roughness, diffusivity and grain orientation dependent mechanical properties in order to shine light on previously observed differences in Alsingle bondAl thermocompression wafer-level bonding with Si/SiO2/Al and Si/Al wafers.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Mei, Zengxia; Liu, Lishu; Du, Xiaolong & Galeckas, Augustinas [Show all 9 contributors for this article] (2016). Self-diffusion measurements in isotopic heterostructures of undoped and in situ doped ZnO: Zinc vacancy energetics. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 94, p. 195208–195208. doi: 10.1103/PhysRevB.94.195208. Full text in Research Archive
• Asokan, Vijayshankar; Venkatachalapathy, Vishnukanthan; Rajavel, Krishnamoorthy & Madsen, Dorte Nørgaard (2016). Microwave irradiation on carbon black: Studies on the transformation of particles into nano-balls, nano-sticks and nano-onion like structures. Journal of Physics and Chemistry of Solids. ISSN 0022-3697. 99, p. 173–181. doi: 10.1016/j.jpcs.2016.09.002. Full text in Research Archive
• Prabhu, Muthusamy Ramesh; Mayandi, Jeyanthinath; Mariammal, Rajkumar N.; Venkatachalapathy, Vishnukanthan; Pearce, Joshua M. & Soundararajan, Neelam [Show all 7 contributors for this article] (2015). Peanut shaped ZnO microstructures: Controlled synthesis and nucleation growth toward low-cost dye sensitized solar cells. Materials Research Express. 2:066202 (6). doi: 10.1088/2053-1591/2/6/066202. Full text in Research Archive
• Gan, Jiantuo; Venkatachalapathy, Vishnukanthan; Svensson, Bengt Gunnar & Monakhov, Edouard (2015). Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering. Thin Solid Films. ISSN 0040-6090. 594, p. 250–255. doi: 10.1016/j.tsf.2015.05.029. Full text in Research Archive
• Liu, Lishu; Mei, Zengxia; Hou, Yaonan; Liang, Huili; Azarov, Alexander & Venkatachalapathy, Vishnukanthan [Show all 8 contributors for this article] (2015). Fluorine doping: A feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.52 Zn0.49O active components. Scientific Reports. 5. doi: 10.1038/srep15516. Full text in Research Archive
• Maharajan, Marikkannan; Venkatachalapathy, Vishnukanthan; Asokan, Vijayshankar; Jeyanthinath, Mayandi & Pearce, Joshua M. (2015). A novel synthesis of tin oxide thin films by the sol-gel process for optoelectronic applications. AIP Advances. 5:027122(2). doi: 10.1063/1.4909542. Full text in Research Archive Show summary

  A novel and simple chemical method based on sol-gel processing was proposed to deposit metastable orthorhombic tin oxide (SnOx) thin films on glass substrates at room temperature. The resultant samples are labeled according to the solvents used: ethanol (SnO-EtOH), isopropanol (SnO-IPA) and methanol (SnO-MeOH). The variations in the structural, morphological and optical properties of the thin films deposited using different solvents were characterized by X-ray diffraction, atomic force microscopy, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy and photoluminescence (PL) analysis. The XRD patterns confirm that all the films, irrespective of the solvents used for preparation, were polycrystalline in nature and contained a mixed phases of tin (II) oxide and tin (IV) oxide in a metastable orthorhombic crystal structure. FTIR spectra confirmed the presence of Sn=O and Sn-O in all of the samples. PL spectra showed a violet emission band centered at 380 nm (3.25 eV) for all of the solvents. The UV-vis spectra indicated a maximum absorption band shown at 332 nm and the highest average transmittance around 97% was observed for the SnO-IPA and SnO-MeOH thin film samples. The AFM results show variations in the grain size with solvent. The structural and optical properties of the SnO thin films indicate that this method of fabricating tin oxide is promising and that future work is warranted to analyze the electrical properties of the films in order to determine the viability of these films for various transparent conducting oxide applications.

• M, Marikannan; M, Subramanian; J, Mayandi; Tanemura, M; Venkatachalapathy, Vishnukanthan & Pearce, J.M. (2015). Effect of ambient combinations of argon, oxygen, and hydrogen on the properties of DC magnetron sputtered indium tin oxide films. AIP Advances. 5(1). doi: 10.1063/1.4906566. Full text in Research Archive
• Selvaraj, Mahalakshmi; Venkatachalapathy, Vishnukanthan; Mayandi, J.; Karazhanov, Smagul & Pearce, J.M. (2015). Preparation of meta-stable phases of barium titanate by Sol-hydrothermal method. AIP Advances. 5:117119(11), p. 1–11. doi: 10.1063/1.4935645. Full text in Research Archive
• Sivanantham, A; Abinaya, C; Venkatachalapathy, Vishnukanthan; Jayabal, P.; Boobalan, K. & Mohanraj, S. [Show all 7 contributors for this article] (2015). Zinc oxide formation in galvanized metallic wire by simple selective growth method. Superlattices and Microstructures. ISSN 0749-6036. 82, p. 327–335. doi: 10.1016/j.spmi.2015.01.040. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2014). Tunneling in ZnO/ZnCdO quantum wells towards next generation photovoltaic cells. Solar Energy. ISSN 0038-092X. 106, p. 82–87. doi: 10.1016/j.solener.2013.11.032. Full text in Research Archive
• M, Prabhu; A, Sivanantham; P, Karthick Kannan; Venkatachalapathy, Vishnukanthan & J, Mayandi (2013). Preparation of DC Reactive Magnetron Sputtered ZnO Thin Film Towards Photovoltaic Applications. AIP Conference Proceedings. ISSN 0094-243X. 1536, p. 531–532. doi: 10.1063/1.4810335. Full text in Research Archive
• A, Sivanantham; P, Jayabal; C, Vasanthi; Venkatachalapathy, Vishnukanthan & J, Mayandi (2013). Novel Method of Synthesis and Device Formation of ZnO Layered Dye Sensitized Solar Cell. Invertis Journal of Renewable Energy. ISSN 2231-3419. 3(2), p. 74–78. Full text in Research Archive
• Trunk, Mareike; Galeckas, Augustinas; Venkatachalapathy, Vishnukanthan; Azarov, Alexander & Kuznetsov, Andrej (2013). Carrier dynamics in linearly and step graded bandgap Zn1-xCdxO structures. Applied Physics Letters. ISSN 0003-6951. 102(19). doi: 10.1063/1.4807387. Full text in Research Archive
• Trunk, Mareike; Gorzkowska-Sobas, Agnieszka A; Venkatachalapathy, Vishnukanthan; Zhang, Tianchong; Galeckas, Augustinas & Kuznetsov, Andrej (2012). Testing ZnO based photoanodes for PEC applications. Energy Procedia. 22, p. 101–107. doi: 10.1016/j.egypro.2012.05.221. Full text in Research Archive
• Trunk, Mareike; Venkatachalapathy, Vishnukanthan; Zhang, Tianchong; Azarov, Alexander; Galeckas, Augustinas & Kuznetsov, Andrej (2012). Time-resolved spectroscopy of carrier dynamics in graded ZnCdxO multilayer structures. Physica Status Solidi. C, Current topics in solid state physics. ISSN 1610-1634. 9(8-9), p. 1805–1808. doi: 10.1002/pssc.201100612. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Lee, In-Hwan & Kuznetsov, Andrej (2012). Engineering of nearly strain-free ZnO films on Si(1 1 1) by tuning AlN buffer thickness. Physica. B, Condensed matter. ISSN 0921-4526. 407(10), p. 1476–1480. doi: 10.1016/j.physb.2011.09.065. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Trunk, Mareike; Zhang, Tianchong; Azarov, Alexander & Kuznetsov, Andrej (2011). Understanding phase separation in ZnCdO by a combination of structural and optical analysis. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 83(12). doi: 10.1103/PhysRevB.83.125315. Full text in Research Archive Show summary

  A phenomenon of wurtzite (w), zincblende (zb), and rock-salt (rs) phase separation was investigated in ZnCdO films having Cd contents in the range of 0%–60% settling a discussion on the phase stability issues in ZnCdO. First, low-Cd-content (⩽17%) ZnCdO was realized preferentially in a w matrix determining optimal Zn-lean conditions by tuning the precursor decomposition rates during synthesis. However, more detailed analysis of x-ray diffraction and photoluminescence (PL) data revealed that the w single-phase stability range is likely to be as narrow as 0%–2% Cd, while samples containing 7%–17% of Cd exhibit a mixture of w and zb phases. Second, high-Cd-content (32%–60%) ZnCdO samples were realized, supplying more of the Cd precursor utilizing Zn-lean growth conditions, however, resulting in a mixture of w, zb, and rs phases. Characteristic PL signatures at 2.54 and 2.31 eV were attributed to zb-ZnCdO and rs-CdO, respectively, while the band gap variation in w-Zn1−xCdxO is given by (3.36–0.063x) as determined at 10 K. The phase separation is interpreted in terms of corresponding changes in the charge distribution and reduced stacking fault energy. © 2011 American Physical Society

• Zhang, Tianchong; Venkatachalapathy, Vishnukanthan; Azarov, Alexander; Trunk, Mareike; Galeckas, Augustinas & Kuznetsov, Andrej (2011). MgZnO synthesis employing weak oxidants for accurate Mg incorporation control. Journal of Crystal Growth. ISSN 0022-0248. 333(1), p. 66–69. doi: 10.1016/j.jcrysgro.2011.08.008. Full text in Research Archive
• Neuvonen, Pekka Tapio; Vines, Lasse; Venkatachalapathy, Vishnukanthan; Zubiaga, Asier; Tuomisto, Filip & Hallén, Anders [Show all 8 contributors for this article] (2011). Defect evolution and impurity migration in Na-implanted ZnO. Physical Review B. Condensed Matter and Materials Physics. ISSN 1098-0121. 84(20). doi: 10.1103/PhysRevB.84.205202. Full text in Research Archive Show summary

  Secondary ion mass spectrometry (SIMS) and positron annihilation spectroscopy (PAS) have been applied to study impurity migration and open volume defect evolution in Na+ implanted hydrothermally grown ZnO samples. In contrast to most other elements, the presence of Na tends to decrease the concentration of open volume defects upon annealing and for temperatures above 600 ◦C, Na exhibits trap-limited diffusion correlating with the concentration of Li. A dominating trap for the migrating Na atoms is most likely Li residing on Zn site, but a systematic analysis of the data suggests that zinc vacancies also play an important role in the trapping process.

• Sellappan, Raja; Galeckas, Augustinas; Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej & Chakarov, Dinko (2011). On the mechanism of enhanced photocatalytic activity of composite TiO2/carbon nanofilms. Applied Catalysis B: Environmental. ISSN 0926-3373. doi: 10.1016/j.apcatb.2011.05.036. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Sellappan, Raja; Chakarov, Dinko & Kuznetsov, Andrej (2011). Tuning light absorption by band gap engineering in ZnCdO as a function of MOVPE-synthesis conditions and annealing. Journal of Crystal Growth. ISSN 0022-0248. 315(1), p. 301–304. doi: 10.1016/j.jcrysgro.2010.09.056. Full text in Research Archive
• Trunk, Mareike; Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2010). Deep level related photoluminescence in ZnMgO. Applied Physics Letters. ISSN 0003-6951. 97(21). doi: 10.1063/1.3518480. Full text in Research Archive

View all works in NVA

• Azarov, Alexander; Galeckas, Augustinas; Cora, Ildikó; Fogarassy, Zsolt; Venkatachalapathy, Vishnukanthan & Monakhov, Eduard [Show all 7 contributors for this article] (2024). Thermal stability of double-layer gamma/beta Ga2O3 structures fabricated by ion beams. Full text in Research Archive
• Karazhanov, Smagul; Mayandi, Jeyanthinath; Venkatachalapathy, Vishnukanthan; Balakumar, S. & Jose, Sujin (2022). Functional MAterials for EneRgy, Environment And BiOmedical ApplicatioNs (FARAON-2021). Materials Today: Proceedings. 64, p. A1–A2. doi: 10.1016/j.matpr.2022.08.016. Full text in Research Archive
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-induced ordering in Ga2O3 polymorphs. Full text in Research Archive
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Monakhov, Eduard & Kuznetsov, Andrej (2022). Ion-beam-induced phase transitions in gallium oxide: impact of the implantation temperature. Full text in Research Archive
• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Venkatachalapathy, Vishnukanthan; Mikšová, Romana & Macková, Anna [Show all 10 contributors for this article] (2022). Structural Transformation of β-Ga2O3 through Si-implantation. Full text in Research Archive
• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Fernandez, Javier Garcia; Venkatachalapathy, Vishnukanthan & Mikšová, Romana [Show all 10 contributors for this article] (2022). Structure and luminescence properties of Si-implanted β-Ga2O3. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dose-rate effect in β-Ga2O3. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dominating migration barrier for intrinsic defects in gallium oxide at and above room temperature. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Radiation defect dynamics in β-Ga2O3: ion flux vs irradiation temperature. Full text in Research Archive
• Karazhanov, Smagul; Venkatachalapathy, Vishnukanthan & Senthilnanthanan, Meena (2020). Proceedings of the international conference on advanced materials for clean energy and health applications (AMCEHA 2019). Materials Today: Proceedings. 23(1). doi: 10.1016/j.matpr.2019.07.698. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Vines, Lasse & Kuznetsov, Andrej (2019). Polarization-controlled interface charges for generation of open-circuit voltage higher than the band gap towards “Polarization Photovoltaics” solar cell. Full text in Research Archive
• Zhan, Wei; Granerød, Cecilie Skjold; Venkatachalapathy, Vishnukanthan; Johansen, Klaus Magnus H; Jensen, Ingvild Julie Thue & Kuznetsov, Andrej [Show all 7 contributors for this article] (2018). Reply to Comment on 'Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy'. Nanotechnology. ISSN 0957-4484. 29(31). doi: 10.1088/1361-6528/aac3ed. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Galeckas, Augustinas; Vines, Lasse & Kuznetsov, Andrej (2018). Direct and indirect bandgaps of Cadmium Oxide. Full text in Research Archive
• Johansen, Klaus Magnus H; Sky, Thomas Neset; Frodason, Ymir Kalmann; Venkatachalapathy, Vishnukanthan; Svensson, Bengt Gunnar & Vines, Lasse (2017). Tailoring donor diffusion in ZnO by defects and doping. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej & Vines, Lasse (2017). Polarization PhotoVoltaics – P2V New Concept for Solar cell. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Vines, Lasse; Monakhov, Edouard & Kuznetsov, Andrej (2017). Ion irradiation induced phase transformation in crystalline Si. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Ayedh, Hussein Mohammed Hussein; Vajeeston, Ponniah; Vines, Lasse; Monakhov, Edouard & Kuznetsov, Andrej (2017). Quest for Si based multi-junction solar cells covering the entire solar spectrum. Full text in Research Archive
• Johansen, Klaus Magnus H; Zhan, Wei; Jensen, Ingvild Julie Thue; Granerød, Cecilie Skjold; Venkatachalapathy, Vishnukanthan & Brillson, Leonard J. [Show all 8 contributors for this article] (2016). Mapping of Optical band gaps using Electron Energy Loss Spectroscopy in ZnCdO-alloys. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Vines, Lasse & Kuznetsov, Andrej (2016). Polarization-Control of the Potential Barrier at the ZnO/Si Hetero-junction. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Azarov, Alexander & Kuznetsov, Andrej (2016). Structural and optical properties of cubic Cd1-xZnxO. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Bakke, Marius Mordal; Galeckas, Augustinas; Vines, Lasse & Kuznetsov, Andrej (2016). Investigation of polar and non-polar Zn(Mg,Cd)O-based Multiple Quantum Wells. Full text in Research Archive
• Granerød, Cecilie Skjold; Zhan, Wei; Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej; Nilsen, Ola & Monakhov, Edouard [Show all 7 contributors for this article] (2015). Bandgap of Functional Oxides in STEM-EELS. Full text in Research Archive
• Zhan, Wei; Venkatachalapathy, Vishnukanthan; Jensen, Ingvild Julie Thue; Granerød, Cecilie Skjold; Flage-Larsen, Espen & Johansen, Klaus Magnus H [Show all 9 contributors for this article] (2015). STEM-EELS Band Gap Measurement in Zn1-XCdXO Thin Films. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Mei, Zengxia; Liu, Lishu; Du, Xiaolong & Galeckas, Augustinas [Show all 7 contributors for this article] (2015). Zinc vacancy energetics revealed by self-diffusion in isotopic ZnO heterostructures. Full text in Research Archive
• Malik, Nishant; Venkatachalapathy, Vishnukanthan; Poppe, Erik; Schjølberg-Henriksen, Kari; Taklo, Maaike Margrete Visser & Finstad, Terje (2015). Grain structure of Aluminium films for wafer-level thermocompression bonding. Full text in Research Archive
• Vereshchagina, Elizaveta; Poppe, Erik; Schjølberg-Henriksen, Kari; Venkatachalapathy, Vishnukanthan & Moe, Sigurd T. (2014). ANALYSIS OF RESIDUAL STRESS AND THERMAL HYSTERESIS IN SPUTTERED ALUMINIUM THIN FILMS. Full text in Research Archive Show summary

  This work concerns optimization of properties of aluminium (Al) thin films with respect to residual stress and thermal hysteresis. Controlling residual stress and minimizing the effect of thermal hysteresis is critical in a wide range of MEMS components. Throughout the study we used wafer curvature, White Light Interferometry (WLI), High Resolution Scanning Electron Microscopy(HRSEM), X-Ray Diffraction (XRD) to assess mechanical and microstructural properties of Al thin films as a function of deposition conditions and annealing. The maximum thermal hysteresis in Al was measured to be below 500 MPa. Plastic yielding occurred from ca. 150 oC at a yield compressive stress of 120 MPa. The mean grain size increased from 250 to 850 nm when increasing sputter power from 2 to 9 kW for annealed samples. A significant degree of temperature induced voiding and hillocking was observed on Al surface after annealing at 350oC. Integration of thin films of sputtered Ti and TiW on top of the Al film suppressed the hillock formation and is suggested as a possible solution to minimize thermal hysteresis.

• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2014). Interplay of Structural and Optical properties in CdO. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Azarov, Alexander & Kuznetsov, Andrej (2014). Investigation of Ga-Doping Effects in ZnO. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Azarov, Alexander; Galeckas, Augustinas & Kuznetsov, Andrej (2014). Synthesis and Characterization of Highly Conductive and Optically Transparent Al-Doped Zn1-xMgxO Films. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2013). Band gap engineering in ZnO for Solar energy harvesting - Multiple Quantum Well Solar Cells. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2013). Strained ZnCdO/ZnO multiple quantum well structures. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2013). CdO-ZnO Multiple Quantum Wells as Components for Next Generation Solar Cells. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2012). Internal field and composition trade-offs in light emission from ZnCdO/ZnO multiple quantum wells. Full text in Research Archive
• Trunk, Mareike; Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2012). Charge carrier dynamics in linearly graded bandgap Zn1-xCdxO structures. Full text in Research Archive
• Trunk, Mareike; Gorzkowska-Sobas, Agnieszka A; Venkatachalapathy, Vishnukanthan; Zhang, Tianchong; Galeckas, Augustinas & Kuznetsov, Andrej (2011). Bandgap-graded ZnCdO thin film PEC electrodes enhancing light absorption and carrier separation. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Trunk, Mareike; Azarov, Alexander; Galeckas, Augustinas & Kuznetsov, Andrej (2010). ZnMgO synthesis employing weak oxidants for accurate determination of alloying effects. Full text in Research Archive
• Trunk, Mareike; Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas & Kuznetsov, Andrej (2010). Deep level luminescence in ZnMgO. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Galeckas, Augustinas; Zubiaga, Asier; Tuomisto, Filip & Kuznetsov, Andrej (2010). Precursor decomposition/supply limited evolution of intrinsic defects in ZnO studied by a combination of positron and photoluminescence spectroscopy. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan; Trunk, Mareike; Sellappan, Raja; Zhang, Tianchong; Azarov, Alexander & Galeckas, Augustinas [Show all 8 contributors for this article] (2010). Band gap engineering in epitaxial ZnO-based alloy films. Full text in Research Archive
• Venkatachalapathy, Vishnukanthan (2011). ZnO and ZnCdO metal organic vapor phase epitaxy: epitaxy, defects and band gap engineering. Universitetet i Oslo. Full text in Research Archive Show summary

  Zinc oxide (ZnO) and its ternary alloys have high potential to compete with III-V nitrides for optoelectronic applications. Furthermore, oxide semiconductors receive considerable attention due to their low cost of fabrication, chemical robustness and high thermal conductance. The goal of this work was two fold: (i) to explore manufacturing route of ZnO and ZnCdO films using metal organic vapor phase epitaxy (MOVPE) in vector flow epitaxy mode and (ii) to master structural/optical properties of these films for preparing such as components in electronics, optoelectronics and solar energy conversion. The starting point was to study the influence of basic synthesis parameters on the structural and luminescence properties of pure ZnO films on c-axis oriented sapphire substrates. The samples were synthesized using previously unexplored for ZnO vector flow epitaxy mode of MOVPE employing systematic variations of fundamental synthesis parameters such as temperature, pressure, II/VI molar ratio, total carrier gas flow ratio, susceptor rotation rate, etc. It was concluded that the growth temperature affects the precursor pyrolysis and in these terms pre-determines the actual II/VI molar ratio available at the reaction zone. Concurrently, direct II/VI molar ratio variations by supplying different amount of precursors influences the properties too, for example, changing intrinsic defect balance in the films. Variations of other parameters like chamber pressure, total gas flow rate and susceptor rotation rate resulted in minor deviations in the growth, uniformity and properties. Further, exploring lower-cost substrates, ZnO films have also been successfully fabricated on Si(111) substrates by using AlN buffer layers. The process resulted in ZnO/AlN/Si heterostructures, where ZnO films were grown epitaxially on AlN buffers of different thicknesses and on Si(111) by so called domain-matching epitaxy. An optimal thickness of the AlN buffer was determined, resulting in nearly in-plane strain free ZnO films. Such films exhibited excellent crystalline quality and extremely bright excitonic emissions. The control of point defects in the crystal is essential for realization of any device, and we have specifically investigated the changes in the defect balance as a function of synthesis parameters in our films. By manipulating the growth temperature, we could achieve either Zn-lean or O-lean conditions. Positron annihilation spectroscopy and photoluminescence were employed to study point defects in such films. A range of vacancy complexes was identified from signal variations going consistently with variations in the synthesis conditions. Specifically, a synthesis temperature window has been determined allowing to control the concentration balance of zinc vacancies (VZn). Finally, manufacturing routes of wurtzite ZnCdO alloys were explored utilizing the knowledge obtained in the process of mastering VZn-enriched material. The alloys exhibited mixed wurtzite, zincblende and rocksalt phases for Cd contents > 7 % also demonstrating general decrease in excitonic luminescence. The phase separation is interpreted in terms of corresponding changes in charge distribution and reduced stacking fault energy. A narrow Cd content region (< 2%) was attributed to the wurtzite single phase equilibrium. The band gap of ZnCdO thin was found to decrease with increasing Cd concentration consistently with literature. In present work, the band gap of ZnCdO was tuned from 3.4 eV to 2.3 eV by changing Cd content up to 60 %, providing an excellent opportunity for band gap engineering in novel optoelectronic applications.

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