Shanthi Iyer

Effect of thermal processing on silver thin films of varying thickness deposited on zinc oxide and indium tin oxide

Silver films of varying thicknesses (25, 45, and 60 nm) were deposited on indium tin oxide (ITO) on silicon and zinc oxide (ZnO) on silicon. The films were annealed in vacuum for 1 h at different temperatures (300–650 °C). Four-point-probe measurements were used to determine the resistivity of the films. All films showed an abrupt change in resistivity beyond an onset temperature that varied with thickness. Rutherford backscattering spectrometry measurements revealed agglomeration of the Ag films upon annealing as being responsible for the resistivity change. X-ray pole figure analysis determined that the annealed films took on a preferential ⟨111⟩ texturing; however, the degree of texturing was significantly higher in Ag/ZnO/Si than in Ag/ITO/Si samples. This observation was accounted for by interface energy minimization. Atomic force microscopy (AFM) measurements revealed an increasing surface roughness of the annealed films with temperature. The resistivity behavior was explained by the counterbalancin...

Photoluminescence study of liquid phase electroepitaxially grown GaInAsSb on (100)GaSb

Low‐temperature (4.5 K) photoluminescence (PL) spectra of liquid phase electroepitaxially grown GaSb and GaInAsSb have been examined. The excitonic transitions observed in GaSb and GaInAsSb layers of compositions close to the GaSb corner of the phase diagram indicate an excellent quality of the grown layers. A systematic trend in the low‐temperature PL spectra is observed with the change in the alloy composition. The overall PL emission efficiency decreases and the number of excitonic transitions are fewer with the shift in the composition towards the lower band gap. Shift in the PL peak energy corresponding to the band to band transition with temperature was determined. The linear part of the shift above 100 K exhibits a slope of −0.3 meV/K.

Diffusion coefficient and differential mobility of As in In for LPE and current controlled LPE

InAs has been grown on undoped InAs substrates by liquid phase epitaxy (LPE) and current controlled LPE (CCLPE) in the temperature range of 570 to 670°C. From the LPE growth rate versus temperature results, the diffusion coefficient of As in In was calculated to be D = 525exp[-(1.4 × 104)T] cm2 with an activation energy for diffusion of 1.2 eV (27.8 kcal/mol). CCLPE growth results indicated a linear relationship between the growth rate of InAs and the current density. The differential mobility of As in In was computed as μ = 0.9 × 10-2 cm2/V·s at 620°C.

Low‐temperature photoluminescence of Te‐doped GaSb grown by liquid phase electroepitaxy

The results of a low‐temperature (4.5 K) photoluminescence study of Te‐doped GaSb layers grown by liquid phase electroepitaxy are reported. A doubly ionizable native residual acceptor (A/A−) with shallow and deep levels is observed at 34 and 97 meV, respectively, another native acceptor level for GaSb (B) is seen at 54 meV, and two Te‐related acceptor levels (C and D) are found at 68 and 83 meV, respectively. In addition, a few Te‐related deep levels are also seen between 114–129 meV at higher Te concentrations. The relative dominance of each of these transitions depends on the degree of Te compensation and the incident excitation intensity. At low excitation intensities, the spectra are dominated by deep impurity levels and with increasing intensity the transitions associated with the shallow acceptors become more prominent. The limited data on the PL integrated intensity dependance on excitation intensity further confirms the nature of these transitions. And finally, we also present preliminary results ...

Nitrogen incorporation and optical studies of GaAsSbN∕GaAs single quantum well heterostructures

In this work, the effects of N incorporation on the optical properties of GaAsSbN∕GaAs single quantum wells (SQWs) have been investigated using temperature, excitation, and magnetic dependencies of photoluminescence (PL) characteristics. These layers were grown in an elemental solid source molecular beam epitaxy system with a rf plasma N source. The N concentrations in the range of 0.5%–2.5% were investigated in this study. The SQW with N∼0.5% exhibits a behavior similar to that in an intermediate regime where the contributions from the localized states in the band gap are dominant. The temperature and excitation dependencies of the PL characteristics indicate that for the N concentration of 0.9% and above, the alloy behavior is analogous to that of a regular alloy and the changes in optical properties are only marginal. The conduction band effective mass (meff) values computed from the magnetophotoluminescence spectra using a variational formalism and the band anticrossing model are in good agreement and...

Annealing effects on the temperature dependence of photoluminescence characteristics of GaAsSbN single-quantum wells

In this work we investigate the effects of ex situ annealing in N ambient and in situ annealing in As ambient on the temperature dependence of photoluminescence (PL) spectral characteristics of GaAsSbN∕GaAs single-quantum-well heterostructures. The focus of this work is on three representative nitride samples grown by molecular-beam epitaxy. The widths of the quantum wells (QWs) varied from 8 to 9 nm and the concentrations of nitrogen and antimony as determined from high-resolution x-ray diffraction and secondary-ion-mass spectroscopy were in the range of 0.8%–1.4% and 26%–33%, respectively. One sample was ex situ annealed in N ambient at 700 °C for 10 min. Two other samples were in situ annealed in As ambient at 650 and 700 °C, respectively, also for 10 min. Excitonic transitions in the QWs exhibit the well-known “S-curve” behavior in the temperature dependence of the PL peak energy. In addition, the variation of the full width at half maximum with temperature exhibits an “inverted S-curve” behavior. The...

Effects of N incorporation on the structural and photoluminescence characteristics of GaSbN/GaSb single quantum wells

The structural and optical properties of GaSbN single quantum wells grown on GaSb substrates by solid source molecular beam epitaxy have been investigated for N concentrations up to 1.5%. The presence of well-resolved and pronounced Pendellosung fringes, dynamical diffraction rods seen in the corresponding reciprocal space map, and triple-axis x-ray full width at half maximum of 10–11 arcsec of the substrate and epilayer peak indicates epilayers of excellent quality with smooth interfaces. Low-temperature photoluminescence (PL) exhibited sharp and discrete N-related PL line features below the GaSb band edge. Their dependence on N concentration as well as measurement temperature and excitation intensity of the PL strongly suggests that these lines correspond to highly localized N pair/cluster states. No significant effect of in situ annealing in Sb ambient on the PL features was observed, while ex situ annealing in N ambient led to the annihilation of these features.

Current controlled liquid phase epitaxial growth and characterization of InGaAsP

The current controlled liquid phase epitaxy (CCLPE) or electroepitaxy growth technique has been employed for the first time to grow lattice matched In1 - xGaxAsyP1 - y quaternary layers with composition in the range of 0 < x < 0.47 on (100) InP substrates at a constant furnace temperature of 650°C. The growth results showed a linear relationship between the growth rate of the epilayer and current density (up to 15 A/cm2). The growth rate varied from 0.62 μm/min for In0.53Ga0.47As to 0.03–0.05 μm/min for quaternary layers corresponding to a wavelength of ≌ 1.3 μm, at a current density of 10 A/cm2. X-ray diffraction measurements showed good lattice match between the epilayer and the substrate. The experimental data are compared with the derived numerical results based on the existing model.

Bandgap Tuning of GaAs/GaAsSb Core-Shell Nanowires Grown by Molecular Beam Epitaxy

Semiconductor nanowires have been identified as a viable technology for next-generation infrared (IR) photodetectors with improved detectivity and detection across a range of energies as well as for novel single-photon detection in quantum networking. The GaAsSb materials system is especially promising in the 1.3–1.55 μm spectral range. In this work we present band-gap tuning up to 1.3 μm in GaAs/GaAsSb core–shell nanowires, by varying the Sb content using Ga-assisted molecular beam epitaxy. An increase in Sb content leads to strain accumulation in shell manifesting in rough surface morphology, multifaceted growths, curved nanowires, and deterioration in the microstructural and optical quality of the nanowires. The presence of multiple PL peaks for Sb compositions ≥12 at.% and degradation in the nanowire quality as attested by broadening of Raman and x-ray diffraction peaks reveal compositional instability in the nanowires. Transmission electron microscope (TEM) images show the presence of stacking faults and twins. Based on photoluminescence (PL) peak energies and their excitation power dependence behavior, an energy-band diagram for GaAs/GaAsSb core–shell nanowires is proposed. Optical transitions are dominated by type II transitions at lower Sb compositions and a combination of type I and type II transitions for compositions ≥12 at.%. Type I optical transitions as low as 0.93 eV (1.3 μm) from the GaAsSb for Sb composition of 26 at.% have been observed. The PL spectrum of a single nanowire is replicated in the ensemble nanowires, demonstrating good compositional homogeneity of the latter. A double-shell configuration for passivation of deleterious surface states leads to significant enhancement in the PL intensity resulting in the observation of room temperature emission, which provides significant potential for further improvement with important implications for nanostructured optoelectronic devices operating in the near-infrared regime.

Molecular beam epitaxial growth and characterization of InSb1 − xNx on GaAs for long wavelength infrared applications

Recent research progress and findings in InSbN have attracted great attention due to its use in long wavelength infrared applications. A large bandgap reduction in InSb resulting from high N incorporation with minimal crystal defects is challenging due to relatively small atomic size of N. Hence optimization of growth conditions plays an important role in the growth of high-quality InSbN epilayers for device purposes. In this paper, we report on the correlation of structural, vibrational, electrical, and optical properties of molecular beam epitaxially grown InSbN epilayers grown on GaAs substrates, as a function of varying growth temperatures. Two dimensional growths of InSb and InSbN were confirmed from dynamic reflection high energy electron diffraction patterns and growth parameters were optimized. High crystalline quality of the epilayers is attested to by a low full width at half maximum of 200 arcsec from high resolution x-ray diffraction (HRXRD) scans and by the high intensity and well-resolved In...