Nitride semiconductors are ubiquitous in optoelectronic devices such as LEDs and Blu-Ray optical disks. A major limitation for further adoption of GaN in power electronics is its low hole mobility. In order to address this challenge, here we investigate the phonon-limited mobility of wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes, spin-orbit coupling, and many-body quasiparticle band structures. We demonstrate that the mobility is dominated by acoustic deformation-potential scattering, and we predict that the hole mobility can significantly be increased by lifting the split-off hole states above the light and heavy holes. This can be achieved by reversing the sign of the crystal-field splitting via strain or via coherent excitation the A$_1$ optical phonon through ultrafast infrared optical pulses.
The strain induced by lattice mismatch at the interface is responsible for the different value of the band discontinuities observed recently for the AlN/GaN (AlN on GaN) and the GaN/AlN (GaN on AlN) polar (0001) interface. We present a first-principles calculation of valence band offsets, interface dipoles, strain-induced piezoelectric fields, relaxed geometric structure, and formation energies. Our results confirm the existence of a large forward-backward asymmetry for this interface.
We report the first realization of molecular beam epitaxy grown strained GaN quantum well field-effect transistors on single-crystal bulk AlN substrates. The fabricated double heterostructure FETs exhibit a two- dimensional electron gas (2DEG) density in excess of 2x10^13/cm2. Ohmic contacts to the 2DEG channel were formed by n+ GaN MBE regrowth process, with a contact resistance of 0.13 Ohm-mm. Raman spectroscopy using the quantum well as an optical marker reveals the strain in the quantum well, and strain relaxation in the regrown GaN contacts. A 65-nm-long rectangular-gate device showed a r…
This work shows that the combination of ultrathin highly strained GaN quantum wells embedded in an AlN matrix, with controlled isotopic concentrations of Nitrogen enables a dual marker method for Raman spectroscopy. By combining these techniques, we demonstrate the effectiveness in studying strain in the vertical direction. This technique will enable the precise probing of properties of buried active layers in heterostructures, and can be extended in the future to vertical devices such as those used for optical emitters, and for power electronics.
The degenerate four-wave mixing spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.
Diluted magnetic semiconductors (DMSs) based on group-IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn-doped SiGe thin films were first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples became ferromagnetic (FM) semiconductors, in which the Curie temperature increased with increasing Mn doping concentration and reached 280 K with 5% Mn concentration. The data suggest that the ferromagnetism came from the hole-mediated proce…
The key obstacle toward realizing integrated gallium nitride (GaN) electronics is its low hole mobility. Here, we explore the possibility of improving the hole mobility of GaN via epitaxial matching to II-IV nitride materials that have recently become available, namely ZnGeN$_2$ and MgSiN$_2$. We perform state-of-the-art calculations of the hole mobility of GaN using the ab initio Boltzmann transport equation. We show that effective uniaxial compressive strain of GaN along the $[1\bar{1}00]$ by lattice matching to ZnGeN$_2$ and MgSiN$_2$ results in the inversion of the heavy hole band and spli…
We fabricate an undoped Ge quantum well under 30 nm Ge0.8Si0.2 shallow barrier with reverse grading technology. The under barrier is deposited by Ge0.8Si0.2 followed by Ge0.9Si0.1 so that the variation of Ge content forms a sharp interface which can suppress the threading dislocation density penetrating into undoped Ge quantum well. And the Ge0.8Si0.2 barrier introduces enough in-plane parallel strain -0.41% in the Ge quantum well. The heterostructure field-effect transistors with a shallow buried channel get a high two-dimensional hole gas (2DHG) mobility over 2E6 cm2/Vs at a low percolation …
First-principles carrier mobility and optical absorption of strained ZnO with self-consistent Hubbard interactions
Korea Institute for Advanced Study（韩国高等研究院）；Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin（德克萨斯大学奥斯汀分校奥登计算工程与科学学院）；Department of Physics, The University of Texas at Austin（德克萨斯大学奥斯汀分校物理系）
We report on the temperature dependence of the mobility, $μ$, of the two-dimensional electron gas in a variable density AlGaN/GaN field effect transistor, with carrier densities ranging from 0.4$\times10^{12}$ cm$^{-2}$ to 3.0$\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between 20 K and 50 K we observe a linear dependence $μ_{ac}^{-1} = α$T indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with $α$ being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scatte…
We have calculated band-edge energies for most combinations of zincblende AlN, GaN, InN, GaP, GaAs, InP, InAs, GaSb and InSb in which one material is strained to the other. Calculations were done for three different geometries, quantum wells, wires, and dots, and mean effective masses were computed in order to estimate confinement energies. For quantum wells, we have also calculated band-edges for ternary alloys. Energy gaps, including confinement, may be easily and accurately estimated using band energies and a simple effective mass approximation, yielding excellent agreement with experimenta…
In this letter we not only show improvement in the performance but also in the reliability of 30nm thick biaxially strained SiGe (20%Ge) channel on Si p-MOSFETs. Compared to Si channel, strained SiGe channel allows larger hole mobility (μh) in the transport direction and alleviates charge flow towards the gate oxide. μh enhancement by 40% in SiGe and 100% in Si-cap SiGe is observed compared to the Si hole universal mobility. A ~40% reduction in NBTI degradation, gate leakage and flicker noise (1/f) is observed which is attributed to a 4% increase in the hole-oxide barrier height (ϕ) in SiGe. S…
A fundamental obstacle toward the realization of GaN p-channel transistors is its low hole mobility. Here we investigate the intrinsic phonon-limited mobility of electrons and holes in wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes and many-body quasiparticle band structures. We predict that the hole mobility can be increased by reversing the sign of the crystal-field splitting, in such a way as to lift the split-off hole states above the light and heavy holes. We find that a 2% biaxial tensile strain can increase the hole mob…
Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first principles
Low temperature magnetotransport measurements and far infrared transmission spectroscopy are reported in molecular beam epitaxial grown two-dimensional hole systems confined in strained InAs quantum wells with magnetic impurities in the channel. The interactions of the free holes spin with the magnetic moment of 5/2 provided by manganese features intriguing localization phenomena and anomalies in the Hall and the quantum Hall resistance. In magnetic field dependent far infrared spectroscopy measurements well pronounced cyclotron resonance and an additional resonance are found that indicates an…
The quasiparticle (QP) band structures of both strainless and strained monolayer MoS$_{2}$ are investigated using more accurate many body perturbation \emph{GW} theory and maximally localized Wannier functions (MLWFs) approach. By solving the Bethe-Salpeter equation (BSE) including excitonic effects on top of the partially self-consistent \emph{GW$_{0}$} (sc\emph{GW$_{0}$}) calculation, the predicted optical gap magnitude is in a good agreement with available experimental data. With increasing strain, the exciton binding energy is nearly unchanged, while optical gap is reduced significantly. T…
The decoupling of epitaxial factors influencing on the dynamic instabilities of AlGaN/GaN metal-insulator semiconductor high electron mobility transistors is investigated. Three different sets of samples have been analyzed by means of dynamic instabilities in the threshold voltage (V$_{\mathrm{th}}$ shift). Secondary ion mass spectroscopy, steady-state photoluminescence (PL) measurements have been performed in conjunction with electrical characterization. The device dynamic performance is found to be significantly dependent on both the C concentration close to the channel as well as on the dis…
In this study the MOCVD growth and electrical properties of N-polar modulation doped p-AlGaN/GaN superlattices (SLs) were investigated. Hole sheet charge density and mobility were studied as a function of the concentration of the p-type dopant Mg in the SL and the number of SL periods. Room temperature Hall measurements were carried out to determine the hole mobility and the sheet charge density. While the hole density increased with increasing number of SL periods, the hole mobility was largely unaffected.Hole mobilities as high as 18cm2/Vs at a simultaneous high hole density of 6.5e13 cm-2 w…
Localized residual stress and elastic strain concentrations in microelectronic devices often affect the electronic performance, resistance to thermomechanical damage, and, likely, radiation tolerance. A primary challenge for characterization of these concentrations is that they exist over sub-$μ$m length-scales, precluding their characterization by more traditional residual stress measurement techniques. Here we demonstrate the use of synchrotron X-ray -based differential aperture X-ray microscopy (DAXM) as a viable, non-destructive means to characterize these stress and strain concentrations …
By systematically comparing experimental and theoretical transport properties, we identify the polar optical phonon scattering as the dominant mechanism limiting electron mobility in beta-Ga2O3 to lower than 200 cm2/Vs at 300 K for donor doping densities lower than 1018 cm-3. In spite of similar electron effective mass of beta-Ga2O3 to GaN, the electron mobility is 10x lower because of a massive Frohlich interaction, due to the low phonon energies stemming from the crystal structure and strong bond ionicity. Based on the theoretical and experimental analysis, we provide an empirical expression…
Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here, we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor (HEMT) heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic…
We report epitaxial growth and characterization of GaN thin films on sapphire (0001) substrates using low temperature GaN intermediate layer by plasma assisted molecular beam epitaxy (PA-MBE) technique. As grown and annealed GaN thin films were studied by high- resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Hall Effect and photoluminescence (PL). It has been found that there is a significant improvement in the quality of the GaN films after annealing at 725 \degree C in terms of electron mobility, full width at half maximum (FWHM) of omega (ω) scan around (0002) XRD peak …
Electron transport studies for AlGaN/GaN two-dimensional electron gases is presented. Novel defects in the III-V nitrides are treated theoretically for two-dimensional transport. Theory of electron scattering by charged dislocation lines is presented for realistic two-dimensional electron gases. The theory lets us draw new conclusions about the nature of charges residing in the dislocation in the III-V nitrides. The theory leads to the recognition of the fact that current state of the art highest mobility values are limited intrinsically and not due to removable defects. Ways of bypassing the …
The integration of advanced optoelectronic properties in nanoscale devices of group III nitride can be realized by understanding the coupling of charge carriers with optical excitations in these nanostructures. The native defect induced electron-phonon coupling in GaN nanowires are reported using various spectroscopic studies. The GaN nanowires having different native defects are grown in atmospheric pressure chemical vapor deposition technique. X-ray photoelectron spectroscopic analysis revealed the variation of Ga/N ratios in nanowires having possible native defects, with respect to their gr…
We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifeti…
We present a magneto-optical study of the three-dimensional topological insulator, strained HgTe using a technique which capitalizes on advantages of time-domain spectroscopy to amplify the signal from the surface states. This measurement delivers valuable and precise information regarding the surface state dispersion within <1 meV of the Fermi level. The technique is highly suitable for the pursuit of the topological magnetoelectric effect and axion electrodynamics.
The effect of edge-type dislocation wall strain field on the Hall mobility in n-type epitaxial GaN was theoretically investigated through deformation potential within the relaxation time approach. It was found that this channel of scattering can play a considerable role in the low-temperature transport at the certain set of the model parameters. The low temperature experimental data were fitted by including this mechanism of scattering along with ionized impurities and charge dislocation ones.
A study of the mobility of n-doped wurtzite III-Nitrides is reported. We have determined the nonequilibrium thermodynamic state of the III-Nitrides systems, driven far away from equilibrium by a strong electric field, in the steady state, which follows after a very fast transient. The dependence of the mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the electric field strength is derived, which decreases with the strength of electric field. We analyzed the contributions to the mobility arising out of the different channels of electron scattering, namely, the…
Gallium nitride (GaN), a wide band-gap semiconductor, has been broadly used in power electronic devices due to its high electron mobility and high breakdown voltage. Its relatively high thermal conductivity makes GaN a favorable material for such applications, where heat dissipation is a major concern for device efficiency and long-term stability. However, in GaN-based transistors, where the active region can withstand extremely strong electric fields, the field effect on the thermal transport properties has drawn little attention so far. In this work, we apply first-principles methods to inve…
First-principles calculations of $e$-ph interactions are becoming a pillar of electronic structure theory. However, the current approach is incomplete. The piezoelectric (PE) $e$-ph interaction, a long-range scattering mechanism due to acoustic phonons in non-centrosymmetric polar materials, is not accurately described at present. Current calculations include short-range $e$-ph interactions (obtained by interpolation) and the dipole-like Fröhlich long-range coupling in polar materials, but lack important quadrupole effects for acoustic modes and PE materials. Here we derive and compute the lon…
A theory of scattering by charged dislocation lines in a two-dimensional electron gas (2DEG) is developed. The theory is directed towards understanding transport in AlGaN/GaN high-electron-mobility transistors (HEMT), which have a large number of line dislocations piercing through the 2DEG. The scattering time due to dislocations is derived for a 2DEG in closed form. This work identifies dislocation scattering as a mobility-limiting scattering mechanism in 2DEGs with high dislocation densities. The insensitivity of the 2DEG (as compared to bulk) to dislocation scattering is explained by the th…
We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nano selective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defective crystals in the heteroepitaxial growth of nitrides, and the high mobility graphene film can readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. The process consists in first growing a 5-8 graphene layers film on the C-face of 4H- SiC by confinement-controlled sublimation of silicon carbide. The graphene film is t…
Silicon-Germanium (Si$_{1-x}$Ge$_x$) layers are commonly used as stressors in the gate of MOSFET devices. They are expected to introduce a beneficial stress in the drift and channel regions to enhance the electron mobility. When reducing the gate lateral size, one of the major issues is the stress relaxation which results in a significant decrease in the electron mobility. We report a new morphological evolution of a strained epitaxial SiGe nanolayer on a silicon gate (mesa) driven by strain inhomogeneity due to finite-size effects. Unlike the self-induced instability of strained films, this e…
In this work, the threshold voltage instability of normally-off p-GaN high electron mobility transistors (HEMTs) has been investigated by monitoring the gate current density during device on-state. The origin of the gate current variations under stress has been ascribed to charge trapping occurring at the different interfaces in the metal/p-GaN/AlGaN/GaN system. In particular, depending on the stress bias level, electrons (VG < 6 V) or holes (VG > 6 V) are trapped, causing a positive or negative threshold voltage shift {DVTH, respectively. By monitoring the gate current variations at different…
We study the linear and nonlinear optical properties of wurtzite GaN under c-axial stress field, using density functional theory calculations. The fully structural optimization at each c-axial strain was performed. The calculated dielectric functions show that tensile c-axial strain effectively improves the linear optical efficiency, especially for the band-edge transitions, and significantly increase the mobility of electrons in the conduction band. Second-order nonlinear optical susceptibilities show that the tensile c-axial strain will enhance the zero- and low-frequency nonlinear responses…
Gate-all-around nanowire transistor, due to its extremely tight electrostatic control and vertical integration capability, is a highly promising candidate for sub-5 nm technology node. In particular, the junctionless nanowire transistors are highly scalable with reduced variability due to avoidance of steep source/drain junction formation by ion implantation. Here we demonstrate a dual-gated junctionless nanowire \emph{p}-type field effect transistor using tellurium nanowire as the channel. The dangling-bond-free surface due to the unique helical crystal structure of the nanowire, coupled with…
