M. R. Gokhale

Polarized photoluminescence and absorption in A-plane InN films

The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [112¯0] oriented A-plane wurtzite InN films grown on R-plane (11¯02) sapphire substrates using molecular beam epitaxy. For A-plane films the c axis lies in the film plane. The PL signal collected along [112¯0] with electric vector E⊥c is more than three times larger than for E‖c. Both PL signals peak around 0.67eV at 10K. The absorption edge for E‖c is shifted to higher energy by 20meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7eV at 10K.

Distorted wurtzite unit cells: Determination of lattice parameters of nonpolar a-plane AlGaN and estimation of solid phase Al content

Unlike c-plane nitrides, “nonpolar” nitrides, e.g., those grown in the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (xsolid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of nonpolar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for xsolid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.8 Unlike c-plane nitrides, “non-polar” nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (xsolid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for xsolid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.

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.

Long-wavelength photoluminescence from InGaP/GaAs heterointerfaces grown by metal organic vapour-phase epitaxy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to investigate the problems related to GaAs/InGaP/GaAs heterointerfaces grown by metal organic vapour-phase epitaxy (MOVPE). Normal PL features corresponding to the band gaps of GaAs and InGaP are seen for InGaP layer grown on GaAs. However, we observe an intense long-wavelength PL feature if we grow GaAs on InGaP while the features of InGaP and GaAs are suppressed. This PL feature varies significantly in wavelength with changes in the switching sequence and it could not be suppressed by the growth of a thin layer of GaP between InGaP and GaAs. Similar PL features are also observed for interrupted growth of InGaP on GaAs if we introduce AsH3 during the growth interruption. In this case, PLE measurements show that the corresponding PL features arise from an interfacial layer which is created during the interruption and lies sandwiched between the two halves of the InGaP layer. Thus, we show that a deleterious effect arises with the exposure of InGaP surface to AsH3 during the growth process with or without the Ga source and is attributed to the formation of an interfacial InGaAsP layer.

Facile fabrication of lateral nanowire wrap-gate devices with improved performance

We present a simple fabrication technique for lateral nanowire wrap-gate devices with high capacitive coupling and field-effect mobility. Our process uses e-beam lithography with a single resist-spinning step and does not require chemical etching. We measure, in the temperature range 1.5–250 K, a subthreshold slope of 5–54 mV/decade and mobility of 2800–2500 cm2/Vs—significantly larger than previously reported lateral wrap-gate devices. At depletion, the barrier height due to the gated region is proportional to applied wrap-gate voltage.

Anisotropic structural and optical properties of a-plane (112¯0) AlInN nearly-lattice-matched to GaN

We report epitaxial growth of a-plane (112¯0) 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 band gap of ∼140 meV between (electric field) E∥c[0001]-axis and E⊥c conditions with room-temperature photoluminescence peaked at 3.38eV strongly polarized with E∥c, in good agreement with strain-dependent band-structure calculations.

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.

Non-intrinsic superconductivity in InN epilayers : Role of Indium Oxide

In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from high-resolution x-ray diffraction measurements, we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.

Growth kinetics effects on self-assembled InAs∕InP quantum dots

A systematic manipulation of the morphology and the optical emission properties of metalorganic vapor phase epitaxy grown ensembles of InAs∕InP quantum dots is demonstrated by changing the growth kinetics parameters. Under nonequilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dots’ density, their average size and hence the peak emission wavelength can be tuned by changing efficiency of the surface diffusion (determined by the growth temperature) relative to the growth flux. We further observe that the distribution of quantum dot heights, for samples grown under varying conditions, if normalized to the mean height, can be nearly collapsed onto a single Gaussian curve.

Characterization of InGaP/GaAs heterointerfaces grown by metal organic vapour phase epitaxy

In GaAs/InGaP/GaAs structures grown by metal organic vapor-phase epitaxy (MOVPE), the two heterointerfaces are not identical. Normal photoluminescence (PL) features corresponding to the band gaps of GaAs and InGaP are seen for InGaP layer grown on GaAs. However, an intense long-wavelength feature is observed if we grow GaAs on InGaP (inverted structure) while the features of InGaP and GaAs are suppressed. The nature of interfacial regions is investigated by using different gas switching sequences, which can influence the interfacial region composition. Significantly, we find anomalous PL features similar to those observed in the case of inverted structure if we briefly interrupt the growth of InGaP on GaAs and introduce AsH 3 during the growth interruption. Secondary-ion mass spectrometry (SIMS) measurements and preliminary results of the compositional analysis of the interfacial layers based on high resolution X-ray diffraction (HRXRD) and PL measurements suggest that a deleterious effect arises with the exposure of InGaP surface to AsH 3 and is attributed to the formation of an interfacial InGaAsP layer.