Nirupam Hatui

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.

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 lattice parameters, strain state and composition in semipolar III-nitrides using high resolution X-ray diffraction

In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2ϑ-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and co...

Free-standing semipolar III-nitride quantum well structures grown on chemical vapor deposited graphene layers

We report the synthesis and optical characterization of semipolar-oriented III-nitride quantum well (QW) structures obtained by growth on chemical vapor deposited graphene layers using metalorganic vapor phase epitaxy. Various multi-quantum well stacks of GaN(QW)/AlGaN(barrier) and InGaN (QW)/GaN (barrier) were grown. Growth on graphene not only helps achieve a semipolar orientation but also allows facile transfer of the QW multilayer stack to other cheap, flexible substrates. We demonstrate room-temperature photoluminescence from layers transferred to flexible Kapton films.

Fabrication and characterization of GaN nanowire doubly clamped resonators

Gallium nitride (GaN) nanowires (NWs) have been intensely researched as building blocks for nanoscale electronic and photonic device applications; however, the mechanical properties of GaN nanostructures have not been explored in detail. The rigidity, thermal stability, and piezoelectric properties of GaN make it an interesting candidate for nano-electromechanical systems. We have fabricated doubly clamped GaN NW electromechanical resonators on sapphire using electron beam lithography and estimated the Youngs modulus of GaN from resonance frequency measurements. For wires of triangular cross section with side ∼90 nm, we obtained values for the Youngs modulus to be about 218 and 691 GPa, which are of the same order of magnitude as the values reported for bulk GaN. We also discuss the role of residual strain in the nanowire on the resonant frequency and the orientation dependence of the Youngs modulus in wurtzite crystals.

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.

Comparison of GaN nanowires grown on c-, r- and m-plane sapphire substrates

Gallium nitride nanowires were grown on c-plane, r-plane and m-plane sapphire substrates in a showerhead metalorganic chemical vapor deposition system using nickel catalyst with trimethylgallium and ammonia as precursors. We studied the inuence of carrier gas, growth temperature, reactor pressure, reactant ow rates and substrate orientation in order to obtain thin nanowires. The nanowires grew along the and axes depending on the substrate orientation. These nanowires were further characterized using x-ray diraction, electron microscopy, photoluminescence and Raman

Large exciton g-factors in anisotropically strained A-plane GaN film measured using magneto-optical Kerr effect spectroscopy

Exciton Lande g-factors in wurtzite GaN epitaxial films with (0001) C-plane and (112¯0) A-plane orientations have been measured in magnetic fields B up to 1.8 T, using polar magneto-optical Kerr effect (MOKE) spectroscopy. A procedure is developed for extracting the Zeeman splitting and thereby the g-factor, from Kerr ellipticity and rotation spectra of A-plane films, which have in-plane polarization anisotropy. In the C-plane film the measured g-factors for the A, B, and C exciton transitions were gA=0.09±0.02, gB=0.74±0.05, and gC=3.9±0.2, respectively, with B∥c-axis and comparable to earlier reports. The MOKE spectra of the A-plane film have one dominant exciton feature each for analyzer axis ⊥ and ∥ to the c-axis of GaN, and they arise at different energies. The measured g-factors for these were much larger, with values g⊥c=4.7±1 and g||c=7.1±1.2 with B⊥c-axis. Comparison with a k·p perturbation theory based calculation, which included the influence of strain, indicates that the features in the A-plan...

Optoelectronic devices based on III-N quantum wells grown on CVD graphene

We report the synthesis and optical characterization of GaN/AlGaN and InGaN/GaN quantum well structures grown on CVD graphene layers. We demonstrate a novel process allowing facile transfer of QW multilayers to other (cheap, flexible) substrates.