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AlScN is attractive as a lattice-matched epitaxial barrier layer for incorporation in GaN high electron mobility transistors due to its large dielectric constant and polarization. The transport properties of polarization-induced two-dimensional (2D) electron gas of densities of [Formula: see text]/cm2 formed at the AlScN–GaN interface is studied by Hall-effect measurements down to cryogenic temperatures. The 2D electron gas densities exhibit mobilities limited to ∼300 cm2/V s down to 10 K at AlScN/GaN heterojunctions. The insertion of a ∼2 nm AlN interlayer boosts the room temperature mobility by more than five times from ∼300 cm2/V s to [Formula: see text] cm2/V s, and the 10 K mobility by more than 20 times to ∼6980 cm2/V s at 10 K. These measurements provide guidelines to the limits of electron conductivities of these highly polar heterostructures.

The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 [Formula: see text]/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 [Formula: see text] is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors.

Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure α-(Al xGa1− x)2O3 thin films (0 [Formula: see text] x [Formula: see text] 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular ([Formula: see text] = 1.31 eV) and parallel ([Formula: see text] = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between α-Ga2O3 and α-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in α-Ga2O3 whereas for α-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M1 type in α-Ga2O3 to M0 type van Hove singularity in α-Al2O3.

We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an $\alpha$-Ga$_2$O$_3$ thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on $m$-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73 eV to 8.75 eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with $M_0$-type van Hove singularities for polarization perpendicular to the $c$ axis, $E_{0,\perp}=5.46(6)$ eV and $E_{0,\perp}=6.04(1)$ eV, and one direct band-to-band transition with $M_1$-type van Hove singularity for polarization parallel with $E_{0,||}=5.44(2)$ eV. We further identify excitonic contributions with small binding energy of 7 meV associated with the lowest ordinary transition, and a hyperbolic exciton at the $M_1$-type critical point with large binding energy of 178 meV., Comment: 7 pages, 4 figures, in submission to Applied Physics Letters

N-polar GaN/AlN resonant tunneling diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current-voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8$\pm$ 0.8 kA/cm$^2$ at a forward bias of ~8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ~$-$4 V. These resonant and threshold voltages are well explained with the polarization field which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the RTDs. When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing$-$via resonant tunneling injection$-$a wide range of exotic materials with III-nitride semiconductors, providing a route to explore new device physics., 12 pages, 3 figures

We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(Al xGa1− x)2O3 alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al2O3 substrates with x = 0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga2O3 and α-Al2O3. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques.

Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultra-high temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, $>$1600$^\circ$C, and low boron fluxes, $\sim1\times10^{-8}$ Torr beam equivalent pressure. \emph{In-situ} reflection high energy electron diffraction (RHEED) revealed the growth of hBN layers with $60^\circ$ rotational symmetry and the $[11\bar20]$ axis of hBN parallel to the $[1\bar100]$ axis of the sapphire substrate. Unlike the rough, polycrystalline films previously reported, atomic force microscopy (AFM) and transmission electron microscopy (TEM) characterization of these films demonstrate smooth, layered, few-nanometer hBN films on a nitridated sapphire substrate. This demonstration of high-quality hBN growth by MBE is a step towards its integration into existing epitaxial growth platforms, applications, and technologies.

A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum α-(AlxGa1−x)2O3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular ( e∞,⊥) and parallel ( e∞,∥) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the high-frequency dielectric constants with b⊥=0.386 and b∥=0.307.

Here, we have explored the thermal stability of α-(Al,Ga)2O3 grown by the molecular-beam epitaxy on m-plane sapphire under high-temperature annealing conditions for various Al compositions (i.e., 0%, 46%, and 100%). Though uncapped α-Ga2O3 undergoes a structural phase transition to the thermodynamically stable β-phase at high temperatures, we find that an aluminum oxide cap grown by atomic layer deposition preserves the α-phase. Unlike uncapped α-Ga2O3, uncapped α-(Al,Ga)2O3 at 46% and 100% Al content remain stable at high temperatures. We quantify the evolution of the structural properties of α-Ga2O3, α-(Al,Ga)2O3, and α-Al2O3 and the energy bandgap of α-Ga2O3 up to 900 °C. Throughout the anneals, the α-Ga2O3 capped with aluminum oxide retains its high crystal quality, with no substantial roughening.

Using high magnetic fields up to 60 T, we report magneto-transport and photoluminescence (PL) studies of a two-dimensional electron gas (2DEG) in a GaN/AlGaN heterojunction grown by molecular-beam epitaxy. Transport measurements demonstrate that the quantum limit can be exceeded (Landau level filling factor $\nu < 1$), and show evidence for the $\nu =2/3$ fractional quantum Hall state. Simultaneous optical and transport measurements reveal synchronous quantum oscillations of both the PL intensity and longitudinal resistivity in the integer quantum Hall regime. PL spectra directly reveal the dispersion of occupied Landau levels in the 2DEG and therefore the electron mass. These results demonstrate the utility of high (pulsed) magnetic fields for detailed measurements of quantum phenomena in high-density 2DEGs., Comment: 5 pages, 4 figures

The evolution of surface morphology for single-crystal bulk Al-polar aluminum nitride substrates during ex situ cleaning, in situ cleaning, and subsequent homoepitaxy is investigated. Ex situ acid treatment is found to reveal atomic steps on the bulk AlN substrates. After in situ Al-assisted cleaning at high temperatures in a high vacuum environment monitored with reflection high-energy electron diffraction, cleaner atomic step edges are observed. Subsequent growth on the cleaned bulk AlN by molecular beam epitaxy is used to develop a phase-diagram for homoepitaxy on AlN single crystals. Secondary ion mass spectrometry profiles reveal high-purity epitaxial layers with undesired chemical impurity densities of Si, O, and C to be below detection limits. The grown homoepitaxial films are observed to oxidize in the ambient environment, but repeating the ex situ acid treatment again reveals atomic steps.

We compare the effectiveness of in situ thermal cleaning with that of Al-assisted cleaning of native surface oxides of bulk AlN for homoepitaxial growth by molecular beam epitaxy. Thermal deoxidation performed at 1450 ° C in vacuum results in voids in the AlN substrate. On the other hand, Al-assisted deoxidation at ≈ 900 ° C results in high-quality AlN homoepitaxy, evidenced by clean and wide atomic terraces on the surface and no extended defects at the growth interface. This study shows that Al-assisted in situ deoxidation is effective in removing native oxides on AlN, providing a clean surface to enable homoepitaxial growth of AlN and its heterostructures; furthermore, it is more attractive over thermal deoxidation, which needs to be conducted at much higher temperatures due to the strong bonding strength of native oxides on AlN.

Blue light-emitting diodes (LEDs) consisting of a buried n+-p+ GaN tunnel junction, (In,Ga)N multiple quantum wells (MQWs) and a n+-GaN top layer are grown on single-crystal Ga-polar n+-GaN bulk wafers by plasma-assisted molecular beam epitaxy. The (In,Ga)N MQW active regions overgrown on the p+-GaN show chemically abrupt and sharp interfaces in a wide range of compositions and are seen to have high structural and optical properties as verified by X-ray diffraction and spatially resolved cathodoluminescence measurements. The processed LEDs reveal clear rectifying behavior with a low contact and buried tunnel junction resistivity. By virtue of the top n+-GaN layer with a low resistance, excellent current spreading in the LEDs is observed at low currents in this device structure. A few of new device possibilities based on this unique design are discussed.

The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of new nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors., 16 pages, 3 figures

N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk wafers by plasma-assisted molecular beam epitaxy growth. The current-voltage characteristics show high-quality rectification and electroluminescence characteristics with a high on/off current ratio and interband photon emission. The measured electroluminescence spectrum is dominated by strong near-band edge emission, while deep level luminescence is greatly suppressed. A very low dislocation density leads to a high reverse breakdown electric field. The low leakage current N-polar diodes open up several potential applications in polarization-engineered photonic and electronic devices., Comment: 13 pages, 3 figures, to appear in Applied Physics Letters

A single phase InN epitaxial film is grown on a bulk In2O3(111) wafer by plasma-assisted molecular beam epitaxy. The InN/In2O3 orientation relationship is found to be (0001) ‖ (111) and [11¯00] ‖ [112¯]. High quality of the layer is confirmed by the small widths of the x-ray rocking curves, the sharp interfaces revealed by transmission electron microscopy, the narrow spectral width of the Raman E2h vibrational mode, and the position of the photoluminescence band close to the fundamental band gap of InN.

We compare the structural properties of InN and In0.95Ga0.05N films grown on O-face ZnO(000 (1) over bar) substrates at different temperatures. The small amount of Ga results in dramatic changes in the morphology and structural properties of InN. In particular, inversion domains start to appear at higher temperatures in the In0.95Ga0.05N film. This process is a consequence of the chemical reaction of ZnO with Ga which can be prevented by choosing the substrate temperature to be 450 degrees C or below. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4739941]

We study plasmon-related excitations in InN by micro-Raman spectroscopy. The surface sensitivity of Raman scattering is found to be strongly enhanced for backscattering from cleaved edges of thick films. This fact enables us to observe the electron accumulation layer at the surface of InN and to deduce the associated volume charge density.

We describe magneto-optical and magnetic properties of quaternary Cd1−x−yMnxCryTe crystals (x=0.37, 0≤y≤0.03) grown by the Bridgman method. The presence of Cr in this alloy is observed to induce ferromagnetic behavior and to enhance magneto-optical effects. Using magnetic circular dichroism and magnetization measurements, we describe the dependence of this behavior on the Cr concentration, and we discuss possible mechanisms for the observed effects.

We investigate the effect compensating Mn acceptors in Ga1−xMnxAs films by doping with Si donors. For Ga1−xMnxAs with low Mn content (e.g., x 0.10), Si doping is found to increase TC. We ascribe this to an increase in the hole mobility in high x samples due to changes in the relative occupancy of the hole impurity band associated with compensation by the Si donors.

The coupling between the two magnetic layers in a series of (Ga,Mn)As∕GaAs∕(Ga,Mn)As magnetic tunnel junctions with different nonmagnetic spacer thicknesses tNM were studied by magnetization and planar Hall effect (PHE) measurements. The PHE data indicate that the magnetization reversals of the two layers are strongly correlated when the tNM is less than 3nm and are independent when tNM is larger than 15nm. From the results, it is concluded that considerable redistribution of hole wave functions plays a major role for small tNM. The PHE results for the sample with 6nm spacer also suggest an antiferromagnetic interlayer exchange coupling.