Masihhur R. Laskar

p-type doping of MoS2 thin films using Nb

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 3.1 × 1020 cm−3, Hall mobility of 8.5 cm2 V−1 s−1 was determined, which matches well with the theoretically expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.

Large area single crystal (0001) oriented MoS2

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, the potential for exciting physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area single crystal Molybdenum Disulfide (MoS2) thin films. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) oriented sapphire yielded highly oriented single crystal MoS2 films. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

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.

Molecular Beam Epitaxy of Graded-Composition InGaN Nanowires

We report the growth of graded InGaN nanowires by plasma-assisted molecular beam epitaxy. Wire composition is linearly graded from InN to GaN along the length of each wire. The large lattice mismatch between GaN and InN (11%) introduces tensile strain in the graded region, which results in cracking of the wires. Growing with reverse grading (i.e., GaN to InN) results in crack-free nanowires. The composition is measured by energy-dispersive x-ray spectroscopy of individual nanowires performed in a scanning transmission electron microscope, and strain is measured by high-resolution x-ray diffraction.

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.

Low frequency noise in chemical vapor deposited MoS 2

Inherent low frequency noise is a ubiquitous phenomenon, which limits operation and performance of electronic devices and circuits. This limiting factor is very important for nanoscale electronic devices, such as 2D semiconductor devices. In this work, low frequency noise in high mobility single crystal MoS2 grown by chemical vapor deposition (CVD) is investigated. The measured low frequency noise follows an empirical formulation of mobility fluctuations with Hooges parameter ranging between 1.44×10-3 and 3.51×10-2. Small variation of Hooges parameter suggests superior material uniformity and processing control of CVD grown MoS2 devices than reported single-layer MoS2 FET. The extracted Hooges parameter is one order of magnitude lower than CVD-grown graphene. The Hooges parameter shows an inverse relationship with the field mobility.

1/f hopping noise in molybdenum disulphide

Molybdenum disulphide (MoS2), a layered metal dichalcogenide material, has attracted significant attention recently for potential application in next-generation electronics, light detection and emission, and chemical sensing due to its unique electrical and optical properties. The intrinsic 2-dimensional nature of carriers in MoS2 offers superior vertical scaling for device structure, leading to potentially low-cost, flexible, and transparent 2D electronic devices. However, the nature of charge transport still remains elusive, esp., a much lower mobility than theoretical limit set by phonon scattering. In this study, we focus on the study of low frequency noise (i.e., 1/f noise) of MoS2 devices working in the hopping regime since 1/f noise limits the performance of devices. There has been scarce 1/f noise study on monolayer or few-layer MoS2 based semiconductor devices. To the best of our knowledge, this is the first report focusing on 1/f hopping noise in MoS2. In this work, the low frequency noise of high mobility single crystal MoS2 is investigated by using transmission line measurements (TLM). At room temperature, the Hooges parameter is ranged between 1.44×10-3 and 3.51×10-2, and it shows an inverse relationship with the field mobility. At low temperatures, the 1/f noise performance reveals the hopping is nearest neighbor hopping.