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Development and Analysis of Wafer-Bonded Four-Junction Solar Cells Based on Antimonides With 42% Efficiency Under Concentration
摘要： The highest solar cell efficiencies today are reached with four-junction devices under concentrated illumination. The optimal bandgap combination for realistic four-junction cells is modelled to be 1.89/1.42/1.05/0.68 eV and indeed promises for efficiencies >50%. We present the development and analysis of a wafer-bonded four-junction solar cell based on GaInP/GaAs/GaInAs//GaInAsSb. This concept allows for the implementation of these ideal bandgaps and exhibits at present an efficiency of 42.0±2.5% at a concentration of 599x AM1.5d. The present loss mechanisms in this device are analyzed, which are dominated by current losses due to mismatched subcell currents and absorbing passive layers. Under the assumption of proper current matching, this device would achieve an efficiency above 44%.
关键词： metal-organic vapour phase epitaxy (MOVPE),multijunction solar cells,wafer bonding,Antimonides,four-junction solar cells
更新于2025-09-16 10:30:52
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MOVPE-Grown Quantum Cascade Laser Structures Studied by Kelvin Probe Force Microscopy
摘要： A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed pro?le of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication ?aws of QCLs.
关键词： MOVPE,quantum cascade laser,Kelvin probe force microscopy
更新于2025-09-16 10:30:52
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Mask-less MOVPE of arrayed n-GaN nanowires on site- and polarity-controlled AlN/Si templates
摘要： We present a novel approach to attain Ga-polar n-GaN nanowires on n-Si(111)/AlN templates, by site- and polarity-controlled metal organic vapor phase epitaxy. A three-stage process is developed to (i) form equally-sized Ga-polar GaN islands, (ii) change the growth direction towards the vertical direction and finally, to (iii) obtain continuous nanowire epitaxy. Homogeneous islands are achieved by minimizing parasitic nucleation and adjusting the adatom diffusion length to the used nanoimprint pattern. The influence of the carrier gas composition on the polarity is studied, achieving pure Ga-polarity by mostly using nitrogen carrier gas. Enhancing the Si/Ga-ratio leads to an amplification of the vertical growth, but also to a reduced number of NWs. 100% growth is attained by a height dependent V/III-ratio adjustment. The results are supported by a qualitative model, explaining how suppression of multi-pod, parasitic and inhomogeneous crystallization can be realized by trading off in situ SiNx passivation and localized GaN growth.
关键词： GaN nanowires,MOVPE,site-controlled,Si templates,polarity-controlled
更新于2025-09-12 10:27:22
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Terahertz intersubband absorption of GaN/AlGaN step quantum wells grown by MOVPE on Si(111) and Si(110) substrates
摘要： We demonstrate terahertz intersubband absorptions in nitride step quantum wells (SQWs) grown by metal organic vapor phase epitaxy simultaneously on two different substrate orientations [Si(111) and Si(110)]. The structure of the SQWs consists of a 3 nm thick Al0.1Ga0.9N barrier, a 3 nm thick GaN well, and an Al0.05Ga0.95N step barrier with various thicknesses. This structure design has been optimized to approach a ?atband potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moment. Structural characterizations prove the high quality of the samples. Intersubband absorptions at frequencies of 5.6 THz (k (cid:2) 54 lm), 7 THz (43 lm), and 8.9 THz (34 lm) are observed at 77 K on both substrate orientations. The observed absorption frequencies are in excellent agreement with calculations accounting for the depolarization shift induced by the electron concentration in the wells.
关键词： Si(111),GaN/AlGaN,step quantum wells,intersubband absorption,Si(110),MOVPE,terahertz
更新于2025-09-12 10:27:22
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Efficient fluorescence quenching of CdSe quantum dots on epitaxial GaAs nanostructures
摘要： Interaction of CdSe quantum dots (QDs) with epitaxially grown GaAs nanostructures has been studied using photoluminescence (PL) technique. Highly fluorescent CdSe QDs of size 3.9 nm were synthesized by colloidal method and coated over GaAs nanostructures grown on GaAs (111)B substrate by metal organic vapor phase epitaxy (MOVPE) using self-assembled Ga droplets as catalyst. Effect of conditions like catalyst growth time and temperature on the growth of GaAs nanostructures has also been studied. Highly uniform tapered hexagonal nanostructures of height around 500 nm were obtained in nearly 100% yield at 420 °C using Ga droplets grown for 10 s. The fluorescence emission of the CdSe QDs on sample bearing the GaAs nanostructures was measured by steady state and time-resolved photoluminescence (TRPL) techniques and compared with the one obtained on bare substrate. Enhanced quenching of the fluorescence of QDs has been witnessed on the sample in which GaAs nanostructures were present. It has been attributed to more efficient defect-related non-radiative relaxation of excited QDs on the surface of six {110} side facets of the GaAs nanostructures that were not present in the bare substrate. The detailed analysis of the TRPL characteristics of the samples has suggested F?rster-like resonance energy transfer (FRET)-based relaxation of CdSe QDs on GaAs nanostructures through shallow traps with significantly reduced average lifetime and the high quenching efficiency.
关键词： Nanostructures,MOVPE,GaAs,CdSe,Time-resolved photoluminescence,Quantum dots
更新于2025-09-11 14:15:04
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First-Principle Study of Ammonia Decomposition and Nitrogen Incorporation on the GaN Surface in Metal Organic Vapor Phase Epitaxy
摘要： Based on the density functional theory (DFT), we theoretically investigate the growth mechanism of GaN (0001) using the Metal-Organic Vapor-Phase Epitaxy (MOVPE). We first identify the structure of the growing Ga rich GaN (0001) surface, then study the adsorption of NHx (x = 0-3) on such surface. We find that NH2 and NH units spontaneously intervene in the Ga-Ga weak bonds on the Ga-rich GaN (0001) surface. A reaction pathway of decomposition of NH3 on Ga rich surface is revealed. During reaction, N is found to incorporate in the weak Ga-Ga bond and form - Ga - (NH) - Ga - structure. The activation barrier of NH3 is surprisingly small, just 0.63eV. We also explore the decomposition of NH2 to N to form Ga-N network and find the plausible reaction pathway with the energy barrier of 2 eV. Taking into account the chemical potential of an H2 molecule in the gas phase at the growth temperature, we find that this reaction can be overcome. This NH3 decomposition mechanism and the N incorporation on GaN is a new growth mechanism catalyzed by the growing surface.
关键词： NH3 decomposition,DFT,N incorporation,MOVPE,GaN
更新于2025-09-10 09:29:36
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The origin of carbon-related carrier compensation in p-type GaN layers grown by MOVPE
摘要： The role of carbon impurities in p-type GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) was investigated. The lightly Mg-doped (～1017 cm?3) p-type GaN samples with different carbon concentration [C] were prepared by controlling growth temperature and pressure. Temperature-dependent Hall-effect analyses exhibited an increase in donor concentration with increasing [C]. The low-temperature mobility also decreased with increasing [C], as a result of mobility limitation due to ionized impurity scattering. These results show that carbon atoms in MOVPE-grown p-GaN layers act as ionized donors and cause carrier compensation. Deep-level transient spectroscopy (DLTS) using bias pulses detected the existence of Hd traps (EV +0.88 eV) arising from the 0/?1 charge state of carbon on nitrogen sites (CN). The concentrations of Hd traps closely corresponded to [C] values in p-type GaN layers. Employing low-frequency capacitance DLTS to avoid carrier freeze-out at low temperatures, we newly discovered the Ha trap (EV +0.29 eV) whose concentration was directly proportional to the [C] value. These ?ndings suggest that the Ha trap originates from CN identical to the Hd trap. Based on prior theoretical calculations of energy levels, the Ha trap can reasonably be assigned to a +1/0 donor state of CN. These results strongly suggest that a CN having two different charged states can compensate an electron and a hole in n-type and p-type GaN layers, respectively.
关键词： DLTS,p-type GaN,carrier compensation,MOVPE,carbon impurities
更新于2025-09-09 09:28:46
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Gallium Oxide || Progress in MOVPE growth of Ga2O3
摘要： Semiconducting sesquioxides, especially Ga2O3, are known since decades [1–3]; however, it is only in the past few years that they are massively investigated. This is essentially due to the development of suitable technologies for growth of large single crystals [4–9] and homo- and heteroepitaxial layers [10–15]. The possibility of growing single crystals and films with relatively low defect density, opened the way to new application areas, in addition to the well-known transparent conducting oxides (TCO) electrodes, namely, (i) substrates for GaN epilayers and (ii) high-power transistors, and (iii) UV detectors.
关键词： homoepitaxial,β-Ga2O3,MOVPE,ε-Ga2O3,Ga2O3,heteroepitaxial
更新于2025-09-09 09:28:46
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Investigation of the Anisotropy of the Structural Properties of GaN(0001) Layers Grown by MOVPE on a-Plane (11$$\bar {2}$$0) Sapphire
摘要： The structural properties of GaN(0001) heteroepitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE) on a-plane (11 0) sapphire substrates are investigated by X-ray diffractometry. Anisotropy of the rocking-curve width for the symmetric (0004) and asymmetric {11 4} and {10 5} reflections of gallium nitride upon rotation of the sample is observed. A comparison of the anisotropy of the rocking-curve width for GaN(0001)/Al2O3(11 0) layers with two different variants of in-plane orientation relationships suggests that the anisotropy of the structural properties is independent of the thermoelastic stress arising upon cooling the heterostructure.
关键词： structural properties,MOVPE,anisotropy,sapphire,GaN,X-ray diffractometry
更新于2025-09-09 09:28:46
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Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface
摘要： The aim of this work is to elucidate how di?erent growth mode and composition of barriers can in?uence the QW properties and their PL and to ?nd optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL e?ciency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when di?erent temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In)GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.
关键词： A1. Interfaces,A3. Quantum wells,B1. Nitrides,A3. MOVPE,B2. Scintillators
更新于2025-09-04 15:30:14