A. Azizur Rahman

Gate Leakage Mechanisms in AlGaN/GaN and AlInN/GaN HEMTs: Comparison and Modeling

The gate leakage mechanisms in AlInN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are compared using temperature-dependent gate current-voltage (IG-VG) characteristics. The reverse bias gate current of AlInN/GaN HEMTs is decomposed into three distinct components, which are thermionic emission (TE), Poole-Frenkel (PF) emission, and Fowler-Nordheim (FN) tunneling. The electric field across the barrier in AlGaN/GaN HEMTs is not sufficient to support FN tunneling. Hence, only TE and PF emission is observed in AlGaN/GaN HEMTs. In both sets of devices, however, an additional trap-assisted tunneling component of current is observed at low reverse bias. A model to describe the experimental IG-VG characteristics is proposed and the procedure to extract the associated parameters is described. The model follows the experimental gate leakage current closely over a wide range of bias and temperature for both AlGaN/GaN and AlInN/GaN HEMTs.

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.

Layered transition metal dichalcogenides: promising near-lattice-matched substrates for GaN growth

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area growth of GaN on CVD MoS2. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth.

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.

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...

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