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Item Single crystalline Ge thin film growth on c-plane sapphire substrates by molecular beam epitaxy (MBE)(RSC, 2022-04) Kumar, RahulSingle crystalline Ge has been grown on c-plane sapphire substrates by molecular beam epitaxy. Direct growth of Ge on sapphire results in three-dimensional (3D) Ge islands, two growth directions, more than one primary domain, and twinned crystals. The introduction of a thin AlAs nucleation layer significantly improved the surface and material quality, which is evident from a smoother surface, single epitaxial orientation, sharper rocking curve, and a single domain. The AlAs nucleation layer thickness was also investigated, and a 10 nm AlAs layer resulted in the lowest surface roughness of 3.9 nm. We have been able to achieve a single primary domain and reduced twinning relative to previous works. A high-quality Ge buffer on sapphire has the potential as an effective platform for the subsequent growth of GeSn and SiGeSn for microwave photonics.Item Epitaxial growth of Ge1-xSnx on c – Plane sapphire substrate by molecular beam epitaxy(Elsevier, 2023-09) Kumar, RahulIn this paper, we have demonstrated the first epitaxial growth of Ge1-xSnx on c – plane Sapphire. This has been achieved by growing Ge1-xSnx on Ge(1 1 1)/AlAs/Al2O3(0 0 0 1) engineered substrates by low-temperature molecular beam epitaxy. The growth starts with three-dimensional islands growth, characteristics of the Volmer – Weber growth mode. The presence of type A and type B oriented domains of Ge1-xSnx epilayers was shown by reflection higher energy electron diffraction analysis. High-resolution X-rays diffraction measurement shows that the grown crystals are tetragonally strained. Further, from the recorded pole figure measurement of the two grown samples, the additional presence of reflection micro-twins was also revealed. The presence of twining has been significantly increased for the case of the sample grown with a higher Sn percentage of 5.2%, as compared to that of the one grown with 3.6% of Sn. However, the presence of defects in the epilayers may be suppressed by growing significantly a thicker layer.Item Improving the Material Quality of GaAs Grown on the c-Plane Sapphire by Molecular Beam Epitaxy to Achieve Room-Temperature Photoluminescence(ACS, 2023-09) Kumar, RahulHigh-quality GaAs on the c-plane sapphire has been achieved by employing a two-step growth technique, multiple annealing, and an AlAs nucleation layer using molecular beam epitaxy (MBE). The effect of growth parameters, namely, growth temperature, As2 flux, and low-temperature layer growth temperature (LTLGT) in two-step growth have been investigated. In all of the grown samples, the epitaxial orientation of the film is GaAs (111)A. Unlike the homoepitaxial GaAs (111)A MBE growth, where increasing the As2 flux improves the film quality, here the lowest As2 flux resulted in the best film quality. Very low LTLGT resulted in highly twinned material and film surface with many pits. Growth temperature also plays an important role, as seen by the exceptional structural and optical properties of samples grown at 650 °C, but at the cost of the rough film surface. We have observed low-temperature photoluminescence (PL) for all of the samples. However, for the first time, to the best of our knowledge, room-temperature PL (RT-PL) has been demonstrated from a heteroepitaxial GaAs (111)A film. This result is important because RT-PL from the epitaxial GaAs/c-plane sapphire will lead to the fabrication of GaAs laser on sapphire, which is an important functionality to realize photonic circuits on the sapphire platform.Item Theoretical Investigation of Thickness Variation on Ga2O3 MESFET: Depletion to Enhancement Mode Transition(IEEE, 2023) Kumar, RahulWe have investigated the DC characteristics of Ga 2 O 3 -based metal semiconductor field effect transistor (MESFET). Different channel thicknesses namely 1000 nm, 500 nm, 250 nm and 100 nm have been employed for simulation. By reducing the channel thickness, depletion mode (D-mode) devices turned into enhancement mode (E-mode) devices. Peculiar exponential output characteristics have been observed in the subthreshold region.Item Temperature dependent behavior of sub-monolayer quantum dot based solar cell(Elsevier, 2023-08) Kumar, RahulThis work investigates the temperature dependence of the performance of In(Ga)As-based solar cells made from sub-monolayer (SML) quantum dots (QDs), quantum wells (QWs), and Stranski-Krastanov QDs (SK-QDs). Well-defined sub-bandgap peaks are observed in external quantum efficiency (EQE) spectra for all samples. Apparently, changes in minority carrier lifetime affect both the sub-bandgap and the above bandgap EQE. Lateral confinement of carriers in the SMLs and QDs also affects the temperature dependence of the EQE. Finally, a “U” shaped curve was observed from the temperature dependence of the short-circuit current (ISC) from these nanostructure-based solar cells. This is due to the competition between the decreasing bandgap and the increasing carrier loss to recombination within the nanostructures as the temperature increases. In contrast, the solar cell's open-circuit voltage and power efficiency decrease monotonically with temperature.Item Submonolayer InAs Quantum Dot Based Solar Cell: A New Approach Towards Intermediate Band Solar Cell(Bentham Science, 2023) Mourya, Satyendra Kumar; Kumar, RahulThis chapter summarizes the progress of InAs submonolayer (SML) quantum dot (QD) based intermediate band solar cell (IBSC). A brief background of intermediate band solar cells (IBSC) will be presented. Different IBSC prototypes will be discussed. The importance of quantum dots (QDs) for IBSC prototyping will be illustrated. An alternative of the most extensively used Stranski-Krastanow (SK)-QDs named SML QDs will be introduced. The fabrication of SML-QD-based IBSC will be discussed from the material point of view. We will also discuss the physics behind the improved performance of these SCs. Important research in this field will be reviewed. Finally, the future direction will be suggested to further improve the performance.Item AlN/β-Ga₂O₃ MOSHEMT as Biosensor(IEEE, 2024) Kumar, RahulGallium Oxide (Ga 2 O 3 ), as an ultra-wide bandgap semiconductor, has shown great promise for power electronics. Due to its chemical and thermal stability and biocompatibility, it can be an excellent material for biosensing applications. In this pioneering work, we have explored the possibility of Ga 2 O 3 metal oxide semiconductor-high electron mobility transistor (MOSHEMT) for biosensing applications. We have tested neutral biomolecules on cavity-based MOSHEMT biosensors. Initial simulation results are promising enough to test the experimental viability of the Ga 2 O 3 field effect transistor (FET) as a biosensor.Item Reduction of off-state drain current in AlN/β-Ga₂O₃ HEMT by trap state engineering(Elsevier, 2024-05) Mourya, Satyendra Kumar; Kumar, RahulIn this work, we report various strategies to reduce the off-state drain leakage current () in AlN/ high electron mobility transistor (HEMT) by 2D device simulation. We have investigated the effect of access region, channel doping concentration, barrier layer thickness, and trap state engineering on . The formation of a parallel channel deep into the substrate has been found to be responsible for large . All other strategies except trap state engineering have an incremental effect on . However, the device’s was reduced by around 12 orders of magnitude by trap-state engineering. Simultaneously, the on-state performance was unaffected, resulting in an elevated / current ratio of (). A steep subthreshold slope of 0.267 (V/dec) was also obtained. Further, we have investigated the impact of both donor- and acceptor-type traps on subthreshold characteristics. These promising results highlight the potential of AlN/ HEMT as a switch and for future high-power nanoelectronics applications.Item Temperature dependent optical properties of ultrathin InAs quantum well(Elsevier, 2024-10) Kumar, RahulTemperature-dependent photoluminescence (TDPL) and time-resolved photoluminescence (TRPL) of ultrathin InAs quantum wells (QWs) in GaAs matrix have been investigated to understand the optical properties of carriers. Samples containing different thicknesses of InAs (0.5, 0.75, 1, 1.2, 1.4 monolayers) have been used for this study. The PL peak position of InAs with temperature does not follow the Varshni model at low temperatures. The activation energy (EA) of these QWs has been calculated from TDPL. As expected, the thinnest QW sample (0.5 monolayer) results in the smallest EA of 23 meV, whereas the thickest QW sample (1.4 monolayer) results in the highest EA of 79 meV. Carrier lifetime has been calculated from TRPL measurement for varying temperatures. At 10 K, the carrier lifetime increased almost linearly from 250 to 800 ps with the InAs QW thickness. Thicker InAs QW results in a longer carrier lifetime, which has been explained by the carrier escape model. Higher temperatures resulted in a decrease in carrier lifetime, which suggests carrier escape is dominating the temporal decay behavior.Item AlN/β-Ga2O3 HEMT for Low-Noise Amplifier(Springer, 2024-10) Kumar, RahulThis study proposes an AlN/β-Ga2O3 high electron mobility transistor (HEMT) using a 0.10 µm gate length where AIN is used as a barrier and β-Ga2O3 as a channel material. DC and Noise characteristics of AlN/Ga2O3 with different AIN barrier thicknesses at 1–20 GHz microwave frequencies have been explored. The simulation has been performed using the Silvaco TCAD software. The device has exhibited a high unity current gain cut-off frequency (ft) of 100 GHz and extrinsic transconductance of 255 mS/mm. Also, the device showed current drive capability as high as 2000 mA/mm. The microwave noise characteristics of the device were investigated from 0 to 20 GHz with respect to different gate bias voltages and drain currents. At a gate bias of −2 V and drain bias of 15 V, the device has shown a minimum noise figure (Fmin) of 0.71 dB and max unilateral power gain (GUmax) of 24 dB at 10 GHz. Also, it shows a noise resistance of 47.94 KΏ at 10 GHz, which is very appropriate for low-noise applications in the X-band frequency range. These findings indicate the expertise of AlN/Ga2O3 in the field of low-noise and high-power amplifiers.