Shin Mou

Midinfrared type-II InAs/GaSb superlattice photodiodes toward room temperature operation

We study midinfrared type-II InAs∕GaSb superlattice p-i-n photodiodes for high temperature operation. Representative samples exhibit a 3.9μm cutoff wavelength at 250K and detectivity of 4.9×1013, 1.0×1010, and 2.4×109cmHz1∕2∕W at 78, 240, and 300K, respectively. Longer-wavelength devices exhibit a 5.2μm cutoff wavelength at 240K, and detectivity of 1.3×1013 and 1.5×109cmHz1∕2∕W at 78 and 240K, respectively. The electron beam induced current technique is used to investigate the spatially varying carrier collection efficiency contribution to the quantum efficiency at different biases and temperatures. The residual doping in the i region is determined to be 6.0×1013cm−3 (n type) at 78K. The prospect of operating focal plane arrays based on the sample studied around 240K is quite promising.

Heteroepitaxy of N-type β-Ga2O3 thin films on sapphire substrate by low pressure chemical vapor deposition

This paper presents the heteroepitaxial growth of ultrawide bandgap β-Ga2O3 thin films on c-plane sapphire substrates by low pressure chemical vapor deposition. N-type conductivity in silicon (Si)-doped β-Ga2O3 films grown on sapphire substrate is demonstrated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors. The morphology, crystal quality, and properties of the as-grown thin films were characterized and analyzed by field emission scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, photoluminescence and optical, photoluminescence excitation spectroscopy, and temperature dependent van der Pauw/Hall measurement. The optical bandgap is ∼4.76 eV, and room temperature electron mobility of 42.35 cm2/V s was measured for a Si-doped heteroepitaxial β-Ga2O3 film with a doping concentration of 1.32 × 1018 cm−3.

Metalorganic chemical vapor deposition growth of high-quality InAs∕GaSb type II superlattices on (001) GaAs substrates

InAs layers and InAs∕GaSb type II superlattices (SLs) were grown on (001) GaAs substrates by metalorganic chemical vapor deposition. A thin low-temperature GaSb nucleation layer and a thicker high-temperature metamorphic GaSb buffer layer were introduced before the growth of InAs or the SLs. By optimizing the growth temperature, the interface gas switching and the growth rate, morphology, and structural properties of the grown structures are significantly improved. In some cases, nanopipes are found in these structures. Morphology studies of the InAs heteroepitaxial layers show that (1) the diameter of these nanopipes decreases with the growth temperature; (2) nanopipes are nucleated from the interface of GaSb∕InAs; and (3) the density of nanopipes depends on the passivation of GaSb surface before the growth of the InAs. In growing InAs∕GaSb SLs, we show that the growth rate of GaSb has a strong effect on the morphology and that the unintentional interfacial InSb layer formed at the InAs∕GaSb interfaces i...

Improved surface and structural properties of InAs∕GaSb superlattices on (001) GaSb substrate by introducing an InAsSb layer at interfaces

InAs∕GaSb type-II superlattices (SLs) were grown on (001) GaSb substrates by metal organic chemical vapor deposition. Besides the expected tensile stress introduced by the InAs layers in the SLs, additional tensile stress is found in the InAs∕GaSb SLs from the simulation of x-ray diffraction (XRD) curves of the SLs. High-resolution transmission electron microscopy and XRD of the SLs grown with different interface gas switching procedures suggest that the additional tensile stress is mainly located at the GaSb→InAs interface. To compensate for the tensile stress in the SL structures, we show that introducing ∼2-ML-thick InAs0.8Sb0.2 layer at the interfaces of the SL improves the morphology and the structural properties of the SLs significantly.

Quantum Efficiency Analysis of InAs–GaSb Type-II Superlattice Photodiodes

We compare the experimentally measured and theoretically calculated quantum efficiency (QE), where an analytical drift-diffusion photocurrent model is used, of n+ -on-p InAs-GaSb superlattice (SL) photodiodes. With inputs of the transport parameters obtained by the electron-beam-induced current technique and absorption coefficient spectra calculated by the eight-band kldrp method for the p-SL, n+ -SL, and depletion region, taking into account the band filling effect, we show that the drift-diffusion photocurrent model is a good approximation for the InAs-GaSb type-II SL photodiodes, which implies that the SL depletion region in InAs-GaSb SL photodiodes is as effective as that in bulk semiconductor photodiodes in terms of collecting the photo-excited electron-hole pairs. Using this theoretical model, we also find that the high doping density in n-type SL degrades the QE by reducing the absorption coefficient. As a result, the n-type doping density is suggested to be below 1times1017cm-3 in order to optimize the QE for the studied InAs-GaSb SL photodiodes.

Surface channel current in InAs∕GaSb type-II superlattice photodiodes

We observed experimental evidences of surface channel current on the sidewall of InAs∕GaSb superlattice photodiodes. We investigated the surface channel current by measuring the current-voltage (I-V) characteristics of the diodes. The experimental data compare very well with our theoretical model before and after ammonium sulfide passivation. By using the passivation, we reduced the surface channel current by five times, which supports that the surface channel current is induced by surface carriers. We believe that the surface channel current results from the inversion layer of a p-type superlattice with surface Fermi levels pinned above the conduction-band minimum in InAs∕GaSb superlattices.

Midinfrared InAs∕GaSb type-II superlattice interband tunneling photodetectors

A photovoltaic InAs∕GaSb superlattice photodetector based on electron transfer using quantum energy levels and interband tunneling is presented: an interband tunneling detector. The quantum efficiency is about 7%, which is improved by ten times compared to the previous published interband cascade detectors. The R0A product is 0.03Ωcm2 at 200K and is comparable to that of state-of-the-art InAs∕GaSb superlattice photodiodes. Since the interband tunneling detector works without an applied bias, it is promising for small-pixel focal plane array applications.

P-type conduction in two-dimensional MoS2 via oxygen incorporation

The effects of oxygen incorporation on the electronic transport properties of two-dimensional (2D) MoS2 have been studied via temperature dependent and gate voltage dependent transport measurements of physical vapor deposited 2D MoS2. Gated micro-van der Pauw cross devices were fabricated from the MoS2 film for transport measurements. Field-effect measurements indicate that incorporated oxygen acts as a p-type dopant for MoS2. The combination of X-ray photoemission spectroscopy surface analysis and Raman measurements of the film indicates that acceptor states resulting from MoSxO3-x inclusions in the MoS2 film are the origin of the p-type doping. Temperature dependent van der Pauw conductivity measurements indicate an acceptor energy of 214 meV above the valence band edge for the acceptor state.

Donors and deep acceptors in β-Ga2O3

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in β-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed, Czochralski, molecular beam epitaxy, and low pressure chemical vapor deposition. Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in β-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. The high temperature Hall effect measurement of Fe doped β-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity.

Incomplete Ionization of a 110 meV Unintentional Donor in β-Ga2O3 and its Effect on Power Devices

Understanding the origin of unintentional doping in Ga2O3 is key to increasing breakdown voltages of Ga2O3 based power devices. Therefore, transport and capacitance spectroscopy studies have been performed to better understand the origin of unintentional doping in Ga2O3. Previously unobserved unintentional donors in commercially available