A. P. Shah

The role of hydrostatic stress in determining the bandgap of InN epilayers

This letter establishes a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy, the authors find that the films are under primarily hydrostatic strain. The corresponding stress results in a shift in the band edge to higher energy. The effect is significant and may be responsible for some of the variations in InN bandgap reported in the literature.

Determination of InN–GaN heterostructure band offsets from internal photoemission measurements

Band discontinuities at the InN–GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n+ InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624eV and the other at 2.527eV. From these, we obtain the band offsets ΔEv=0.85eV and ΔEc=1.82eV.

Nanostructured MoS 2 /BiVO 4 Composites for Energy Storage Applications

We report the optimized synthesis and electrochemical characterization of a composite of few-layered nanostructured MoS2 along with an electroactive metal oxide BiVO4. In comparison to pristine BiVO4, and a composite of graphene/BiVO4, the MoS2/BiVO4 nanocomposite provides impressive values of charge storage with longer discharge times and improved cycling stability. Specific capacitance values of 610 Fg−1 (170 mAhg−1) at 1 Ag−1 and 166 Fg−1 (46 mAhg−1) at 10 Ag−1 were obtained for just 2.5 wt% MoS2 loaded BiVO4. The results suggest that the explicitly synthesized small lateral-dimensioned MoS2 particles provide a notable capacitive component that helps augment the specific capacitance. We discuss the optimized synthesis of monoclinic BiVO4, and few-layered nanostructured MoS2. We report the discharge capacities and cycling performance of the MoS2/BiVO4 nanocomposite using an aqueous electrolyte. The data obtained shows the MoS2/BiVO4 nanocomposite to be a promising candidate for supercapacitor energy storage applications.

Temperature dependence of surface photovoltage of bulk semiconductors and the effect of surface passivation

Surface photovoltage (SPV) of n-GaAs decreases both above and below a certain transition temperature. We explain this phenomenon in terms of a Schottky contact based model and relative dominance of thermal and nonthermal parts of the dark current. This also explains the observed increase of SPV of p-GaAs below room temperature. Our analysis is further confirmed from the temperature dependence of the SPV measurements on p-InP and n-InP samples. Surface passivation is seen to lower the transition temperature of n-GaAs.

Growth of high mobility InSb crystals

Abstract Experiments on the growth of high-mobility Te-doped n-type InSb and undoped p-type InSb crystals by vertical directional solidification are reported. Hall mobility of the Te-doped crystals is 26 000 cm2/V·s and of the undoped crystals 42 000 cm2/V·s at 300 K. The Hall coefficient of Te-doped samples is negative over the entire temperature range, but the Hall coefficient of the undoped samples is positive at low temperature, and changes sign at higher temperatures.

ICP-RIE etching of polar, semi-polar and non-polar AlN: comparison of Cl2/Ar and Cl2/BCl3/Ar plasma chemistry and surface pretreatment

We report a comprehensive investigation of inductively-coupled plasma reactive ion etching (ICP-RIE) of polar (0001) c-plane, semi-polar (11–22) and non-polar (11–20) a-plane AlN epilayers and show that under optimized conditions a combination of BCl3-based surface oxide removal pretreatment and Cl2/Ar ICP etching allows fast etch rates (750 nm min−1) with a smooth surface morphology. We compare samples of different orientation etched in Cl2/Ar and Cl2/BCl3/Ar plasmas, with and without BCl3/Ar ICP pretreatment, and show that the effective removal of surface oxide is a crucial step for reliable ICP-RIE etching of AlN layers. For such pretreated samples, optimization of etch parameters such as RF power, ICP power, and chamber pressure then permit very high etch rates to be obtained with a smooth surface morphology. We also study the effect of varying the BCl3 fraction in BCl3/Cl2/Ar plasmas on the etch rate and surface morphology and find that increasing the BCl3 fraction reduces the etch rate for AlN. However, above 20% BCl3 content, samples with and without pre-treatment show similar etch rates.

Inductively coupled plasma–reactive ion etching of c- and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry

Inductively coupled plasma (ICP)–reactive ion etching (RIE) patterning is a standard processing step for UV and optical photonic devices based on III-nitride materials. There is little research on ICP-RIE of high Al-content AlGaN alloys and for nonpolar nitride orientations. The authors present a comprehensive study of the ICP-RIE of c- and a-plane AlGaN in Cl2/Ar plasma over the entire Al composition range. The authors find that the etch rate decreases in general with increasing Al content, with different behavior for c- and a-plane AlGaN. They also study the effect of BCl3 deoxidizing plasma pretreatment. An ICP deoxidizing BCl3 plasma with the addition of argon is more efficient in removal of surface oxides from AlxGa1−xN than RIE alone. These experiments show that AlxGa1−xN etching is affected by the higher binding energy of AlN and the higher affinity of oxygen to aluminum compared to gallium, with oxides on a-plane AlGaN more difficult to etch as compared to oxides on c-plane AlGaN, specifically for...

Optimization of a -plane (112¯0) InN grown via MOVPE on a-plane GaN buffer layers on r -plane (11¯02) sapphire

We report epitaxial growth of a-plane (112̄0) AlInN layers nearly-lattice-matched to GaN. Unlike for c-plane oriented epilayers, a-plane Al1−xInxN cannot be simultaneously lattice-matched to GaN in both in-plane directions. We study the influence of temperature on indium incorporation and obtain nearly-lattice-matched Al0.81In0.19N at a growth temperature of 760 C. We outline a procedure to check in-plane lattice mismatch using high resolution x-ray diffraction, and evaluate the strain and critical thickness. Polarization-resolved optical transmission measurements of the Al0.81In0.19N epilayer reveal a difference in bandgap of ∼140 meV between (electric field) E‖c [0001]-axis and E⊥c conditions with room-temperature photoluminescence peaked at 3.38 eV strongly polarized with E ‖ c, in good agreement with strain-dependent band-structure calculations.We have performed a comprehensive investigation of the growth parameter space for the MOVPE of a- plane (11 20) InN on a-plane GaN buffer layers deposited on r-plane (1 102) sapphire substrates. About 0:2 m thick a-plane InN epilayers were grown on 1 m thick a-plane GaN buffer layers in a close-coupled showerhead reactor. The growth parameters - substrate temperature, reactor pressure, V/III ratio - were systematically varied and their effect on structural, electrical, optical and morphological properties of a- plane InN films were studied. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity. The (11 20) !-fwhm varies depending on the scattering vector being parallel to the c-direction or the m- direction. The magnitude and nature of this anisotropy is strongly influenced by the growth parameters. In general compared to c-plane InN, we observed a higher growth rate and a slightly higher optimum growth temperature for the a-plane InN epilayers. The optimum growth conditions are found at T = 550 o C, P = 500 Torr, V/III = 11; 000, where the !-fwhm for symmetric (11 20) reflection are 0:83 degree and 1:04 degree along (0001) and (1 100) direction respectively and for the skew-symmetric (10 11) plane is 1:47 degree. The optimized a-plane InN has a photoluminescence peak emission at 1750 nm ( 0:71 eV) at low temperature (11 K) and a mobility of 234cm 2 =V:s, carrier concentration 1:4 10 19 cm 3 at room temperature.

Inductively coupled plasma reactive-ion etching of β-Ga2O3: Comprehensive investigation of plasma chemistry and temperature

The authors investigated the inductively coupled plasma reactive-ion etching (ICP-RIE) of β-Ga2O3 using different fluorine and chlorine-based plasmas. Sn-doped (-201) oriented β-Ga2O3 substrates were etched using SF6/Ar, CHF3/Ar, O2/Ar, BCl3/Ar, and Cl2/Ar based ICP-RIE. Appreciable etch rates were obtained only with chlorine and boron-trichloride based plasmas, and the authors performed a comprehensive study on the composition and temperature-dependence of ICP-RIE of β-Ga2O3 in BCl3/Cl2/Ar plasmas in a temperature range of 22 to 205 °C. In general, the etch rate decreased with increasing Cl2 content in BCl3/Cl2/Ar plasmas. A high etch rate of 144 nm/min with a smooth surface morphology was obtained in BCl3/Ar plasmas, compared to 19 nm/min in Cl2/Ar plasmas. The etching behavior of Ga2O3 shows more similarity to that of Al2O3 than to that of GaN.

Short interval open tube diffusion of Zn in GaAs at low temperatures

Short interval open tube Zn diffusion in GaAs at low temperatures is studied for application to heterostructure lasers. The electrochemical capacitance voltage (ECV) profiling technique is used to obtain the carrier concentration versus depth profiles for Zn diffused samples. Diffusion rate is found to be somewhat different from the values obtained for longer durations employing similar techniques. Results are applied to improve the ohmic contact quality for Al-free semiconductor lasers grown in our laboratory. No deterioration is observed in the light versus current (L-I) characteristics of these devices fabricated after Zn diffusion.