Transport Properties Of GaN Research Articles

Effects of Temperature and Bias Voltage on Electron Transport Properties in GaN High-Electron-Mobility Transistors

In this study, we investigated the effects of temperature and bias voltage on the electron transport properties of AlGaN/GaN high-electron-mobility transistors in the saturation region using a turn-on delay time. A sine wave signal was used to accurately acquire the delay time between the gate-source voltage and drain-source current signal of the device at temperatures ranging from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30\mathbf { \mathtt {^\circ }}\text{C}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$180\mathbf {\mathtt {^\circ }}\text{C}$ </tex-math></inline-formula> under operating conditions. With this method, we illustrated a non-monotonic relationship between the temperature and the delay time, which was closely related to the effect of temperature on saturation drift velocity and charge trapping. In addition, it suggested that the delay time increased with increasing drain-source voltage, which was ascribed to the effect of charge trapping. The validity of the proposed results has been thoroughly verified with similar phenomena on two samples. The results may be useful for further study on the electron transport of GaN HEMTs under high temperature and voltages.

Phonon thermal transport properties of GaN with symmetry-breaking and lattice deformation induced by the electric field

Electric fields commonly exist in semiconductor structures of electronics, bringing to bear on phonon thermal transport. Also, it is a popular method to tune thermal transport in solids. In this work, phonon and thermal transport properties of GaN with wurtzite and zincblende structures in the finite electric field are investigated using first-principles calculations from the perspectives of symmetry-breaking and lattice deformation. Effects of electric field on phonon transport properties including phonon dispersion and thermal conductivity from the response of electron density distribution only and response from lattice changes are studied in zincblende GaN. It is found that the former has a small but qualitative impact on phonon dispersion relations, i.e., splitting of phonon branches, since it breaks the symmetry of zincblende lattice. While the latter affects both lattice symmetry and size, causing significant changes in phonon properties and an increase in thermal conductivity. In wurtzite GaN, space-group-conserved lattice changes in the finite electric field are studied with lattice deformation only, where thermal conductivity decreases at electric fields significantly with the increase of anisotropy, much different from the changes in zincblende GaN. This work provides a comprehensive understanding of phonon thermal transport properties in GaN in the finite electric field, which promises to benefit phonon transport tuning and provide a reference for thermal management in GaN-based information and power electronics.

(Invited) Epitaxial Lift-Off of GaN and Related Materials for Device Applications

The unique material and transport properties of GaN and related III-N materials have led to their importance in both optoelectronic and RF/microwave power amplification applications, as well as to their promise for emerging applications including power control and conversion, harsh-environment and radiation-resistant sensing and signal processing, and biomedical applications. The use of epitaxial lift-off with III-N materials provides an approach that can improve the electrical and thermal performance of electronic and optoelectronic devices, while at the same time reducing cost and dramatically reducing device size and weight. However, in contrast to other III-V material systems, epitaxial lift-off of III-N materials is challenging due to the lack of high-selectivity, high-etch-rate wet etches in this material system. As an alternative, we have explored the use of band gap selective photoelectrochemical (PEC) wet etching to perform epitaxial lift-off of GaN-based materials and devices. By combining the use of a thin pseudomorphic InGaN release layer and KOH electrolyte with high-intensity filtered ultraviolet illumination tuned to generate electron-hole pairs only in the InGaN release layer (and not in the surrounding GaN material), sufficient selectivity and etch rate have been achieved to allow epitaxial lift-off of large-area films [1]. This process has been demonstrated to improve the electrical and thermal performance of Schottky diodes on non-native substrates [2, 3] and high-voltage, high-power pn junctions grown on low-dislocation-density bulk GaN substrates [4-6] without compromising material quality, while also resulting in ultra-thin devices with total thicknesses below 15 µm. The devices show no evidence of additional defects or recombination centers as a result of either the inclusion of the pseudomorphic InGaN release layer in the epitaxial layer structure or the epitaxial lift-off fabrication processing steps. In addition to device performance, device cost can be improved both through potential re-use of the substrate after lift-off [7] and reduction in die size due to improved thermal performance [8]. While demonstrated for power electronics applications, the ability to form high-quality, high-performance III-N based devices in an ultra-thin, flexible form factor may be an enabling technology for additional applications in flexible electronics, displays, wearable electronics, RF/microwave communication systems, and other fields.

Effect of Carbon Doping on Charging/Discharging Dynamics and Leakage Behavior of Carbon-Doped GaN

Capacitance transient spectroscopy and dc current–voltage ( I–V ) characterization are employed to analyze charging/discharging effects and transport in 200-nm-thin carbon-doped GaN layers (GaN:C) with a low ( ${N}_{\textsf {C}} = \textsf {10}^{\textsf {18}}$ cm $^{-\textsf {3}}$ ) and high ( ${N}_{\textsf {C}} = \textsf {10}^{\textsf {19}}$ or ${N}_{\textsf {C}}= \textsf {7}\times \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ ) carbon concentration ${N}_{\textsf {C}}$ . The discharging and charging events are found to be governed by transport properties of GaN:C for whole ${N}_{\textsf {C}}$ range. However, distinct temperature behavior has been found as a function of ${N}_{\textsf {C}}$ . In the samples with low ${N}_{\textsf {C}}$ , an Arrhenius-like behavior with an activation energy of 0.8 eV indicates that the hole transport in valence band (VB) is the limiting process and the carrier exchange occurs between VB and the defect level. In contrast, in samples with high ${N}_{\textsf {C}}$ , a non-Arrhenius charging/discharging behavior with exp( aT )-dependence ( ${a}$ being a constant) in a wide temperature range (150–560 K) indicates that the transport is governed by defect bands (DBs) where the carrier exchange occurs between DBs and the carbon defect level. For samples with ${N}_{\textsf {C}} \ge \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ , the large charge accumulation in GaN:C produces a large energy barrier to prevent electron injection from n-GaN to GaN:C, thus making GaN:C blocking. This is in contrast to samples with ${N}_{\textsf {C}} = \textsf {10}^{\textsf {18}}$ cm $^{-\textsf {3}}$ where the barrier is low, rendering thus the bilayer leaky. Detailed physical models for carrier capture and emission processes and carrier transport are provided for both cases. The developed knowledge is important for understanding the role of growth parameters such as ${N}_{\textsf {C}}$ and dislocation densities, as well as for reliability testing and defect analysis.

Analysis of carrier transport properties in GaN/Al 0.3Ga 0.7N multiple quantum well nanostructures

Analysis of carrier transport properties in GaN based multiple quantum well nanostructure has been carried out with an applied bias. Effect of an applied bias and aluminium mole composition in the barrier on the scattering rate, capture time and escape rate has been investigated. The scattering rate was found to be decreased with an increase of applied bias voltage and aluminium mole composition. Capture time shows oscillatory nature with variations in mole composition of aluminium under biasing conditions. The escape rate was found to be increasing from 0.01 ps −1 to 0.69 ps −1 with applied bias voltage.

High-field properties of carrier transport in bulk wurtzite GaN: A Monte Carlo perspective

The transport properties of both electron and hole in bulk wurtzite phase GaN in the high electric field domain are presented by using an ensemble Monte Carlo (EMC) method. In our EMC simulation, the impact ionization process, which is seldom studied due to the lack of experimental data, is included. The impact ionization is treated as an additional scattering mechanism, and the impact ionization rate is described by the Keldysh formula, with the parameters in the formula determined by fitting the simulation results to the numerical calculation results. Such a treatment makes it convenient to simulate the impact ionization initiated by either an electron or hole with the EMC method compared to the previous study of carrier transport properties in GaN up to the high field. Steady-state properties of carriers under an applied electric field up to 1 MV/cm are presented and analyzed. Particularly, the impact ionization process here is further studied and detailed discussions are also given. It is found that the impact ionization coefficients of both the electron and hole upon applied electric field can be described by two simple experiential equations. Moreover, for the first time, to the best of our knowledge, we obtain the ratio of the electron impact ionization coefficient to the hole impact ionization coefficient in wurtzite GaN and find out that it can be smaller than that in InP, which means wurtzite GaN may have good gain noise behaviors according to the present noise theories.

GaN and InN nanowires grown by MBE: A comparison

Morphological, optical and transport properties of GaN and InN nanowires grown by molecular beam epitaxy (MBE) have been studied. The differences between the two materials in respect to growth parameters and optimization procedure was stressed. The nanowires crystalline quality has been investigated by means of their optical properties. A comparison of the transport characteristics was given. For each material a band schema was shown, which takes into account transport and optical features and is based on Fermi level pinning at the surface.

Influence of crystalline defects on transport properties of GaN grown by ammonia-molecular beam epitaxy and magnetron sputter epitaxy

GaN layers have been grown using an MBE/MSE (molecular beam epitaxy/magnetron sputter epitaxy) dual-mode system. The layers grown by the two techniques exhibited a large difference in crystalline quality and presented a broad spectrum of structural, optical, and transport properties that are useful for an analysis of the role of crystalline defects in GaN epilayers. The model of electron scattering by charged threading dislocations was applied in a theoretical fit of the mobility data. The theoretical fit in combination with x-ray diffraction and photoluminescence studies reveal the correlation between dislocation density, electron mobility, doping characteristics and yellow luminescence.

Plasma etch-induced conduction changes in gallium nitride

The effect of plasma-etching damage on carrier transport properties in GaN has been studied under various plasma conditions by monitoring the changes in sheet resistivity (ρ s) and mobility (μ s) or the resistivity (R). All the etching experiments were performed in an electron cyclotron resonance microwave plasma reactive ion etching (ECR-RIE) system. Consistent changes in the transport properties have been observed with increasing dc bias (ion energy) in all plasmas except in those containing chlorine. With noble gas plasmas, the largest change in conductance was created when Ar, the heaviest ion, was accelerated to its highest voltage. In these Ar sputtering cases, substantial surface micro-roughening has been observed. These surfaces also display considerable nitrogen deficiency as measured by Auger electron spectroscopy. These observations suggest that preferential sputtering of nitrogen from the surface of GaN is one form of ion damage. The other is displacement damage. Both of these forms of ion damage are considered to be the direct cause of the observed changes in the electrical properties.

Correlation between the surface defect distribution and minority carrier transport properties in GaN

We have studied linear dislocations and surface defects in p- and n-type metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy grown GaN films on sapphire with atomic force microscopy. The surface pits due to threading dislocations were found not to be distributed randomly but on the boundaries of growth columns. The dislocations are thought to be electrically active since the average distance between them (average column size) is comparable to minority carrier diffusion lengths as measured by electron beam induced current experiments on Schottky diodes fabricated with the same material. Diffusion lengths found for holes and electrons are on the order of Lp=0.28 μm and Le=0.16 μm which corresponded to the sizes of regions free from surface dislocations in both cases and can be described by a simple model of recombination on grain boundaries.

Influence of potential fluctuation on optical and electrical properties in GaN

We observed strong correlation between optical properties and transport properties in GaN. Both the intensity and the energy of near-band edge photoluminescence (PL) peak in GaN:Si vary with its mobility. Such behavior has been explained by the potential fluctuation associated with the inhomogeneous impurities or local defects, leading to the space-charge scattering of carriers and the redshift of the PL line. We also discuss the strain relaxation in GaN:Si.