K. Baskar

MOCVD growth of high efficiency current-matched AlGaAsSi tandem solar cell

Abstract The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs Si tandem solar cell have been studied by varying the growth conditions and solar cell structure. The crystal quality of the AlGaAs layer was evaluated by time resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2 and 21.4% (AM 0 and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configurations, respectively, by perfect photocurrent matching between the top cell and the bottom cell.

Heteroepitaxial technologies on Si for high-efficiency solar cells

Abstract The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs/Si tandem solar cell have been studied with varying the growth conditions. The crystal quality of the AlGaAs layer was evaluated by photoluminescence, deep level transient spectroscopy, time-resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs/Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2% and 21.4% (AMO and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configuration, respectively, by the perfect photocurrent matching between the top cell and the bottom cell.

Crystal growth of high quality hybrid GaAs heteroepitaxial layers on Si substrate by metalorganic chemical vapor deposition and liquid phase epitaxy

This paper reports the first successful liquid-phase epitaxial (LPE) growth of hybrid GaAs from a Bi solvent for the starting growth temperature (SGT) of 650°C on GaAs coated Si grown by metalorganic chemical vapor deposition (MOCVD) without oxide strip windows. The hybrid-grown GaAs on Si has shown better crystallinity than the MOCVD-grown one. The quality of the grown layers has been characterized by photoluminescence (PL), double-crystal X-ray diffraction (XRD) and etch pit density (EPD). The PL intensity increased nearly twice and the full-width at half-maximum (FWHM) of the double-crystal XRD is reduced to 125 arcsec after the growth of GaAs layers by LPE. The dislocation density was about 7 times lower than the dislocation density observed in the MOCVD GaAs on Si layer which served as a substrate for the LPE growth. The cross-sectional view was observed by scanning electron microscope (SEM) and transmission electron micrograph (TEM). The TEM results indicate that the high dislocation density has drastically reduced in the LPE layer.

Investigations on the concentration profiles of arsenic atoms during liquid phase epitaxial growth of GaAs from Ga-As-Bi solution

Abstract A one-dimensional diffusive transport model has been used to study the liquid-phase-epitaxial growth kinetics of GaAs binary system from gallium-bismuth (90% Ga + 10% Bi) mixed solution. The concentration profile for arsenic atoms has been constructed for different cooling rates at a constant equilibrium temperature of 1023 K using appropriate boundary conditions. The thickness of the grown epilayer has been calculated at different growth temperatures corresponding to 10 K, 1 K and 0.32 K cooling rates. The simulated layer thickness data agree well with the experimental results obtained by the authors.

High Quality GaAs Epitaxial Layers Grown from Ga–As–Bi Solutions by Liquid Phase Epitaxy

The liquidus isotherms for the Ga–As–Bi system were determined at 973 and 923 K in the Ga–As–GaAs region. The liquid phase epitaxial growth of GaAs from Ga–As and Ga–As–Bi solutions were investigated. The addition of ten atomic percent of Bi to the Ga–As solution increase the growth rate of the grown epilayers nearly 4.4 times than in the case of GaAs layers from Bi solution. Above 9 at.% of Bi in Ga solution the problems associated with the edge growth were almost eliminated. Optical measurements at 4.2 K revealed that Bi does not alter the band gap energy value of the GaAs epitaxial layers. The photoluminescence (PL) intensity and full width at half maximum (FWHM) revealed the good quality of undoped GaAs epilayers from Bi solvents compared to that of Ga solvents even though the purity of the Bi was relatively less.

Anomalous room temperature magnetoresistance in brownmillerite Ca2Fe2O5

We demonstrate the hitherto unreported field induced magnetoresistance in wet chemical synthesized bulk polycrystalline brownmillerite Ca2Fe2O5. A magnetoresistance of 12% is observed at room temperature, well below its antiferromagnetic transition temperature of 775 K. The variation in resistivity with increasing and decreasing magnetic field shows a significant memory effect. When synthesized in a nanostructured form it exhibits weak ferromagnetism due to the appearance of an Fe oxide phase, besides exhibiting a shift to a higher antiferromagnetic Neel temperature of 820 K. Spectroscopic investigations (X-ray photoelectron and Raman spectroscopy) as well as magnetic measurements have been employed to investigate the above behavior of brownmillerite Ca2Fe2O5.

Growth of high quality Al0.22Ga0.78As layers on Si substrates by metalorganic chemical vapor deposition

High quality epitaxial layers of Al0.22Ga0.78As have been grown on Si substrates by adopting thermal cycle annealing. The quality of the Al0.22Ga0.78As has been assessed by photoluminescence, deep‐level transient spectroscopy, and double‐crystal x‐ray diffraction studies. The emergence of a new luminescence emission (1.718 eV), high concentrations of shallow levels, passivation of a deep level (0.43 eV), and dislocation reduction in the high‐temperature thermal cycle annealed samples have been explained by a silicon diffusion mechanism and the formation of complex point defects. Deep‐level emission at 0.64 eV has been attributed to disordering in the epitaxial layers.

Reactive ion etching of GaN in SF6 + Ar and SF6 + N2 plasma

Reactive ion etching (RIE) of GaN using SF6 + Ar and SF6 + N2 plasma has been carried out. An improvement in etch rate is observed with SF6 + Ar over SF6 + N2 plasma. Schottky diodes fabricated on the etched surface show the evidence of damage. However, damage is less for the samples etched with SF6 + Ar plasma. The reverse leakage current density and the ideality factor are found to depend on the perimeter to area ratio of the device. This suggests that the damage caused by etching is surface damage and the leakage might be because of the formation of a surface channel. Thermal annealing and surface passivation using ammonium sulfide help in recovering the damage. Optimization of annealing as well as passivation schedule is essential, since increased annealing or passivation time degrades the device characteristics.

A comparison on the electrical characteristics of SiO2, SiON and SiN as the gate insulators for the fabrication of AlGaN/GaN metal-oxide/insulator-semiconductor high-electron mobility-transistors

We report on the effect of SiO2, SiN and SiON insertion for the demonstration of AlGaN/GaN metal–oxide/insulator–semiconductor high-electron mobility-transistors (MOSHEMTs/MISHEMTs) on sapphire substrates. The compositions of the three dielectrics were confirmed using X-ray photoelectron spectroscopy (XPS). All the three dielectrics showed enhanced drain saturation current density compared to unpassivated high-electron mobility-transistors (HEMTs). The SiO2 MOSHEMTs though exhibited a very low leakage current, suffered from a relatively high degree of current collapse. The SiN MISHEMTs displayed a low drain current collapse, but, suffered from high degree of leakage current compared with other two dielectrics. SiON MOSHEMTs, taking the advantage of both SiO2 and SiN, resulted both in low drain current collapse as well as moderate leakage current. Thus based on the improved dc characteristics, SiON proves to be a potential dielectric candidate as it simultaneously mitigates the problem of current collapse and achieves moderate leakage current.

On the Bismuth Composition Dependent Concentration of Arsenic Atoms during LPE Growth of GaAs Layers from Ga–As–Bi Solution

The concentration of arsenic atoms from Ga + Bi mixed solvent has been calculated in the range of 0.4 to 100 at% of bismuth for different saturation temperatures corresponding to the cooling rates of 0.32, 1 and l0 K min -1 at a constant equilibrium temperature of 1023 K using appropriate boundary conditions. It has been observed that the thickness of the epitaxial layer for 0 to 20% of bismuth has been found to be very high due to the immiscible region. The change in arsenic concentration in the mixed Ga-Bi-As solution has been observed to be dependent on the bismuth composition and compared with our theoretical values. The simulated layer thickness value agrees well with the experimental results obtained by the authors.