Epitaxial growth of thick, relaxed, homogeneous, and crack-free AlGaN films was demonstrated on single-crystalline GaN substrates. A maskless heteroepitaxial facet-controlled epitaxial lateral overgrowth (FACELO) scheme was developed to relax the lattice mismatch-induced tensile stress via the generation and glide of misfit dislocations at the pyramidal heterointerfaces. Up to 10 mu m-thick AlGaN films were demonstrated, which was more than three orders of magnitude beyond the critical thickness for cracking. The expected relaxation mechanism was confirmed by the observation of misfit dislocations at the heterointerfaces via transmission electron microscopy. The developed growth scheme is amenable to the growth of thick, homogeneous, relaxed AlGaN films of any composition on native substrates.

We demonstrate that facet-controlled epitaxial lateral overgrowth enables the formation of regular, strain-relieving misfit dislocation (MD) arrays at {11-22} interfaces and results in relaxed growth of AlGaN on native GaN substrates (threading dislocation density <10(4 )cm(-2)). Complementary plan-view and cross-sectional transmission electron microscopy studies confirmed the presence of uniform MD arrays with spacing matching the value predicted for an ideal, strain-relieving array. The MDs exhibited a pure edge-type Burgers vector (b = 1/3 < 11 - 20 >) and were aligned parallel to [1-100]. Based on these results, we outline a pathway to achieve low threading dislocation densities in thick, relaxed, crack-free AlGaN epilayers grown on native GaN.

Wing tilt and associated defects at the coalescence boundary in GaN were completely eliminated in a maskless homoepitaxial overgrowth process, whereas SiO2-masked homoepitaxial overgrowth showed the same effects as samples grown on sapphire or SiC. X-ray diffraction and transmission electron microscopy were used to investigate the structures and defect behavior in the epitaxial layers. Crystallographic tilt and the generation of new dislocations with a-type Burgers vectors were observed in the SiO2-masked sample, consistent with the wing tilt phenomenon, while high crystal quality and dislocation-free layers were grown in the maskless process. This shows that the mask is the primary cause for tilt and dislocations during overgrowth, while lattice and thermal mismatch have a contributing but smaller role. The wing tilt phenomenon is completely eliminated via a homoepitaxial, mask-free scheme.

The transport-limiting scattering mechanisms in N-polar p-GaN were identified over a wide range of temperatures (220-770 K), Mg doping concentrations (3 & times; 10(18)-4 & times; 10(19) cm(-3)), and V/III ratios (100-8000). Temperature-dependent Hall measurements, combined with a charge balance model, were employed to extract the acceptor (N-A) and donor (N-D) concentrations. These values enabled the determination of the temperature-dependent Hall factor (r(H)), allowing the conversion of the measured Hall mobility (mu(Hall)) to drift mobility (mu(drift)). The experimental drift mobility was then fitted using a mobility model, which incorporates six scattering mechanisms, including coulomb scattering by charged dislocation lines. The model identified charged dislocation scattering as the transport-limiting scattering mechanism at both room and low temperatures. The origin of coulomb scattering-donor-like traps along threading edge dislocations-was attributed to charged nitrogen vacancy-related defects [(V-N-nMg(Ga))(3-n), where n = 0, 1, or 2], with their concentration controllable through [Mg] and the nitrogen chemical potential (i.e., V/III ratio).

Recent advancements in spintronics have opened a new avenue in terahertz (THz) radiation sources that may outperform the traditional contact-based metallic counterparts. Inspired by the generation of broadband spintronic THz signals at the interface of a ferromagnet and ultrawide bandgap semiconductors, here we investigated the generation of THz radiation from micro-structured heterostructures of a metallic ferromagnet (Ni80Fe20) and an ultrawide bandgap semiconductor (AlGaN/GaN) that contains a layer of 2D electron gas. By precisely tailoring the dimension of the subwavelength pillars of a THz device, the micro-structured spintronic THz emitter can achieve up to more than three times higher emission intensity compared to that of the un-patterned counterpart. Our study advances the development of the next generation of spintronic THz sources that allow a tailored emission frequency and intensity control and, further, are compatible with existing integrated wide-bandgap semiconductor circuits.

The growth of $\text{GaN} / \text{InN} / \text{GaN}$ quantum wells via MOCVD without flow-modulation or plasma-assistance is presented. A growth supersaturation model is employed which enables unification of all reaction conditions (temperature, pressure, flow rates, etc.) into a single parameter. Ultimately, state-of-the-art quantum wells with abrupt interfaces are produced.

Drive-in diffusion of Mg implanted into GaN during ultra-high pressure annealing leads to low surface acceptor concentrations. This favors p-type Schottky contact formation, which severely increases the on-state resistance of Mg-implanted GaN pn diodes (PNDs). This work aims to reduce the resistance of contacts to Mg-implanted p-GaN by incorporating Mg deposition and annealing into the contact stack, achieving a rectification ratio (RR) over 10 ^12 , a current density above 1 kA cm ^−2 and a record-low differential specific on-resistance ( R _ON ) of 0.65 mΩ.cm ^2 in Mg-implanted PNDs, offering a potential solution for improving the performance and manufacturability of vertical GaN devices that require contacts to Mg-implanted p-GaN.

Electron and hole impact ionization coefficients are obtained from measurements of high‐Al content Al x Ga 1‐ x N p–n diodes grown on AlN substrates. The photomultiplication method using a 193 nm pulsed laser is applied to measure the multiplication factor. The impact ionization coefficients are modeled using Chynoweth's formulation, based on electric field profiles determined from the solution of Poisson's equation. A least‐squares fit of the theoretical multiplication factor to the measured multiplication factor yields the impact ionization coefficients and the extracted electron impact ionization coefficients are consistent with previous numerical predictions. Furthermore, the theoretical breakdown voltage and critical electric field are computed based on the extracted impact ionization coefficients. These results provide much‐needed data to further optimize the design of optoelectronic, power switching, and high‐power RF devices based on ultrawide bandgap AlGaN.

Roles of Al-vacancy (VAl) complexes on the cathodoluminescence (CL) spectra of Si-doped AlN grown by halide vapor phase epitaxy (HVPE) on a physical-vapor-transported (0001) AlN substrate are described, making a connection with the results of positron annihilation measurements. A combination of HVPE and AlN substrate enabled decreasing deleterious carbon concentration and dislocation density, respectively, thus accentuating the influences of VAl-complexes on the luminescence processes. A low-temperature CL spectrum of unintentionally doped AlN exhibited predominant excitonic emissions at around 6 eV and a marginal deep-state emission band at around 3.7 eV that originates from residual carbon (<1016 cm−3) on nitrogen sites (CN). However, the sample was revealed to contain a considerable amount (∼1017 cm−3) of vacancy clusters, most likely comprising a VAl and nitrogen-vacancies (VN), namely, VAlVN1−2, which act as nonradiative recombination centers that decrease overall CL intensity at elevated temperatures. With increasing Si-doping concentration ([Si]), major vacancy species progressively changed from VAlVN1−2 to VAlON1−2, where ON is oxygen on N sites, which exhibit other deep-state emission bands ranging from 3.2 to 3.5 eV. Further increase in [Si] gave rise to the formation of donor-compensating defects comprising VAl and Si on the second-nearest-neighbor Al sites (SiAl), abbreviated by VAl−SiAln, which exhibit emission shoulders at around 2.9–3.0 eV. When [Si] exceeded 5 × 1018 cm−3, an emission band at around 4.5 eV emerged, which had been ascribed to originate from the nearest-neighbor SiAlCN complexes. Because VAl-complexes, including those containing impurities, are thermally stable, incorporation of vacancies should be blocked at the growth stage.

The neutral (VN-3MgGa)0 complex was found to be the primary compensator in Mg-doped N-polar GaN. The experimental data showed a sharp drop in hole concentration once [Mg] exceeded ∼1019 cm−3. Temperature-dependent Hall measurements, in conjunction with a charge balance model, revealed that the carrier loss was due to a drastic reduction in acceptor concentration (NA), suggesting that a significant fraction of Mg atoms was incorporated in an electrically neutral configuration. A quantitative semi-empirical model based on the grand canonical formalism pointed to the formation of (VN-3MgGa)0 complexes as the primary cause for the observed carrier loss.

We report on vertical GaN junction barrier Schottky (JBS) diodes formed by Mg ion implantation and ultrahigh -pressure annealing (UHPA). The static ON-state characteristics of the diodes show an ideality factor of 1.05, a turn-on voltage of $\sim$ 0.7 V, a current rectification ratio of $\sim$ 10 $^{11}$ , and a low differential specific ON-resistance that scales with Schottky stripe width in fair agreement with the analytical model. The reverse leakage dependence on Schottky stripe width also agrees well with the analytical model. Implanted p-n junction diodes fabricated on the same wafer exhibit avalanche breakdown in reverse bias with a positive temperature coefficient, but the forward current is limited by a series barrier. Temperature-dependent current–voltage measurements of th p-n diodes verify the presence of the implanted p-n junction and reveal an additional 0.43-eV barrier, which we hypothesize arises from a p-Schottky contact and forms a second diode back-to-back with the p-n junction. This interpretation is supported by analysis of the capacitance–voltage characteristics of the implanted p-n diodes, epitaxial p-n diodes fabricated with intentional p-Schottky contacts, and comparison to TCAD simulations. Ultimately, the presence of the p-Schottky contact does not hinder JBS diode operation. The use of diffusion-aware designs and/or diffusion reduction represents future directions for Mg implantation technology in GaN power devices.

III–V/III-nitride p–n junctions were realized via crystal heterogeneous integration, and the resulting diodes were characterized to analyze electrical behavior and junction quality. p-type In0.53Ga0.47As, which is a well-established base layer in InP heterojunction bipolar transistor (HBT) technology, was used in combination with a homoepitaxial n-type GaN. The latter offers low dislocation density, coupled with high critical electric field and saturation velocity, which are attractive for use in future HBT collector layers. Transmission electron microscopy confirms an abrupt interface in the fabricated heterogeneous diodes. Electrical characterization of the diodes reveals a near-unity ideality factor (n ∼ 1.07) up to 145 °C, a high rectification ratio of ∼108, and a low interface trap density of 3.7 × 1012 cm−2.

We investigate the electrical characteristics of Ni Schottky contacts on n-type GaN films that have undergone ultra-high-pressure annealing (UHPA), a key processing step for activating implanted Mg. Contacts deposited on these films exhibit low rectification and high leakage current compared to contacts on as-grown films. By employing an optimized surface treatment to restore the GaN surface following UHPA, we obtain Schottky contacts with a high rectification ratio of ∼10 9 , a near-unity ideality factor of 1.03, and a barrier height of ∼0.9 eV. These characteristics enable the development of GaN junction barrier Schottky diodes employing Mg implantation and UHPA.

Record-low p-type resistivities of 9.7 and 37 Ω cm were achieved in Al0.7Ga0.3N and Al0.8Ga0.2N films, respectively, grown on single-crystal AlN substrate by metalorganic chemical vapor deposition. A two-band conduction model was introduced to explain the anomalous thermal behavior of resistivity and the Hall coefficient. Relatively heavy Mg doping (5 × 1019 cm−3), in conjunction with compensation control, enabled the formation of an impurity band exhibiting a shallow activation energy of ∼30 meV for a wide temperature range. Valence band conduction associated with a large Mg ionization energy was dominant above 500 K. The apparently anomalous results deviating from the classical semiconductor physics were attributed to fundamentally different Hall scattering factors for impurity and valence band conduction. This work demonstrates the utility of impurity band conduction to achieve technologically relevant p-type conductivity in Al-rich AlGaN.

Heteroepitaxial crystallographic tilt has been investigated as a possible strain‐relief mechanism in Al‐rich (Al>50%) AlGaN heteroepitaxial layers grown on single‐crystal (0001) AlN substrates with varying miscuts from 0.05° to 4.30°. The magnitude of the elastic lattice deformation‐induced tilt increases monotonically with the miscut angle, tightly following the Nagai tilt model. Although tilt angles as high as 0.1° are recorded, reciprocal space mapping (RSM) broadening and wafer bow measurements do not show any significant changes as a function of the heteroepitaxial tilt angle. While crystallographic tilting has been shown to be effective in controlling strain in some other heteroepitaxial systems, it does not provide any appreciable strain relief of the compressive strain in AlGaN/AlN heteroepitaxy.

High p-conductivity (0.7 Ω−1 cm−1) was achieved in high-Al content AlGaN via Mg doping and compositional grading. A clear transition between the valence band and impurity band conduction mechanisms was observed. The transition temperature depended strongly on the compositional gradient and to some degree on the Mg doping level. A model is proposed to explain the role of the polarization field in enhancing the conductivity in Mg-doped graded AlGaN films and the transition between the two conduction types. This study offers a viable path to technologically useful p-conductivity in AlGaN.

Highly conductive Ge-doped AlN with conductivity of 0.3 (Ω cm)−1 and electron concentration of 2 × 1018 cm−3 was realized via a non-equilibrium process comprising ion implantation and annealing at a moderate thermal budget. Similar to a previously demonstrated shallow donor state in Si-implanted AlN, Ge implantation also showed a shallow donor behavior in AlN with an ionization energy ∼80 meV. Ge showed a 3× higher conductivity than its Si counterpart for a similar doping level. Photoluminescence spectroscopy indicated that higher conductivity for Ge-doped AlN was achieved primarily due to lower compensation. This is the highest n-type conductivity reported for AlN doped with Ge to date and demonstration of technologically useful conductivity in Ge-doped AlN.

Mg-doped, graded AlGaN films showed the formation of an impurity band and high, temperature-invariant p-conductivity even for doping levels well below the Mott transition. However, compensating point defects disrupted the impurity band, resulting in an Anderson transition from the impurity band to valence band conduction and a more than tenfold reduction in room-temperature conductivity. This is the first demonstration of Anderson-like localization in AlGaN films.