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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. [ABSTRACT FROM AUTHOR]

High room temperature n-type mobility, exceeding 300 cm2/Vs, was demonstrated in Si-doped AlN. Dislocations and CN−1 were identified as the main compensators for AlN grown on sapphire and AlN single crystalline substrates, respectively, limiting the lower doping limit and mobility. Once the dislocation density was reduced by the growth on AlN wafers, C-related compensation could be reduced by controlling the process supersaturation and Fermi level during growth. While the growth on sapphire substrates supported only high doping ([Si] > 5 × 1018 cm−3) and low mobility (∼20 cm2/Vs), growth on AlN with proper compensation management enabled controlled doping at two orders of magnitude lower dopant concentrations. This work is of crucial technological importance because it enables the growth of drift layers for AlN-based power devices. [ABSTRACT FROM AUTHOR]

We demonstrate a pathway employing crystal polarity controlled asymmetric impurity incorporation in the wide bandgap nitride material system to enable 3D doping control during the crystal growth process. The pathway involves polarity specific supersaturation modulated growth of lateral polar structures of alternating Ga- and N-polar GaN domains. A STEM technique of integrated differential phase contrast is used to image the atomic structure of the different polar domains and their single atomic plane boundaries. As a demonstration, 1 μm wide alternating Ga- and N-polar GaN domains exhibiting charge balanced and periodic domains for superjunction technology were grown. The challenges in characterizing the resulting 3D doping profile were addressed with atom probe tomography with atomic scale compositional resolution corroborating capacitance measurements and secondary-ion mass spectroscopy analysis. [ABSTRACT FROM AUTHOR]

Ge doping in AlGaN was studied over a wide dopant concentration range. For high Ge concentrations, the formation of VIII–nGeIII was determined to be the main point defect limiting the conductivity. It was shown that the complex formation could be suppressed by controlling chemical potentials during growth, leading to a higher maximum achievable carrier concentration and selective stabilization of a certain complex type. Chemical potential of the growth species was varied by changing the V/III ratio and growth temperature. Free carrier concentrations as high as 4 × 1019 cm−3 were achieved in Al0.4Ga0.6N:Ge grown on sapphire substrates under "metal-rich" conditions. The ability to control the onset of self-compensation and to stabilize a certain charge state of the compensating defect is of great technological importance for application of AlGaN in various devices. [ABSTRACT FROM AUTHOR]

Contrary to the arsenides where donors undergo stable DX transition, we find that Ge in AlGaN does not suffer from the DX transition; instead, it undergoes a shallow donor (30 meV) to deep donor (150 meV) transition at ∼50% Al content in the alloy. This finding is of profound technological importance as it removes fundamental doping limitations in AlGaN and AlN imposed by the presumed DX−1 acceptor state. The charge state of Ge below and above the transition was determined by co-doping with Si, which remains a shallow donor in AlGaN for up to 80% Al. It was found that Ge occupied a donor state with a (0/+) thermodynamic transition for AlGaN alloys below and above the transition. Ge as a shallow donor was completely ionized at room temperature; however, the ionization of the deep donor required elevated temperatures, commensurate with its higher ionization energy. This behavior is not unique to Ge; preliminary findings show that Si and O in AlGaN may behave similarly. [ABSTRACT FROM AUTHOR]

Reduction in compensation in Si-doped Al-rich AlGaN is demonstrated via chemical potential control (CPC). The chemical potentials and the resulting formation energies of carbon on the nitrogen site (CN) and cation vacancy complex with Si (VIII + nSiIII) were related to growth variables through a thermodynamic supersaturation model, which quantitatively predicted the incorporation of CN and the generation of the VIII + nSiIII complex. The compensation "knee" behavior, i.e., decreasing conductivity with increasing Si incorporation beyond a certain concentration, was successfully controlled. The maximum free carrier concentration was improved by impeding the formation of VIII + nSiIII complexes under III-richer conditions, while the impurity compensation by CN was reduced by making the growth environment N-richer. The results of Hall effect measurement and photoluminescence agreed well with quantitative theoretical predictions of the CPC model. Based on the developed model, the highest conductivity of 160 Ω−1 cm−1 with free carrier concentration of 3 × 1019 cm−3 in Al0.7Ga0.3N ever reported was achieved on single crystal AlN substrates. The demonstrated predictive power of the CPC model should greatly reduce the empirical analysis or iterative experimentation that would otherwise be necessary. [ABSTRACT FROM AUTHOR]

The influence of the polarization field on the emission properties of the AlGaN-based quantum structures grown on AlN substrates was investigated as a function of well width, barrier width, and barrier height. A thin AlGaN well and a thin AlN barrier design reduced the polarization field to ∼0.5 MV/cm, resulting in an ultralow laser threshold of 3 kW/cm2 in an optically pumped configuration. These experimental results were used to validate the simulation. In the next step, a structure with Al0.7Ga0.3N barriers was designed to support carrier injection with a minimal loss in optical performance. This structure showed a threshold of 7 kW/cm2 under optical pumping and an estimated threshold current of 8 kA/cm2 for the electric injection. [ABSTRACT FROM AUTHOR]

A comprehensive energy map as a function of AlGaN composition over the whole alloy range is presented for commonly observed point defects in nominally intrinsic, n‐, and p‐doped material. The map covers intentional and unintentional impurities (CN, MgIII), vacancies (VIII, VN), passivating complexes (H), and self‐compensating complexes. The tracking of these defects is crucial to understand their impact on optical and electrical properties as well as for their mitigation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Recent successful integration of semiconductors into spintronic THz emitters has demonstrated a new pathway of control over terahertz (THz) radiation through ultrafast demagnetization dynamics. Here, the spintronic THz emission from different ultrawide bandgap (UWBG) semiconductors interfaced with ferromagnets is studied. The authors show that the Schottky barrier in the UWBG semiconductor AlN acts as a spin filter that increases the polarization of the spin current injected from the ferromagnet. Furthermore, the authors show that the two‐dimensional electron gas at the interface between Al0.25Ga0.75N and GaN enhances the magnitude of the emitted radiation due to the high spin‐to‐charge conversion efficiency induced by the Rashba effect that results in a hallmark asymmetry in emission amplitude. The results provide a framework for future engineering of semiconducting/ferromagnet heterostructures for ultrafast communications technologies beyond 5G. [ABSTRACT FROM AUTHOR]

A thermodynamic supersaturation model for growth of AlGaN by metalorganic chemical vapor deposition was developed for experimentally accessible growth parameters. The derived non-linear relationships enabled us to estimate Ga and Al supersaturation during AlGaN growth for given growth conditions. Calculations revealed that the GaN phase was close to chemical equilibrium, while the Al supersaturation was as high as 1010 for typical growth conditions. Such a disparity in the supersaturation of reaction species plays a significant role in the stability of the growth of the resulting ternary alloy. The agreement between experiment and simulation suggests that the parasitic gas phase reactions between trimethylaluminum and NH3 were not significant at low NH3 flow rates/partial pressures, indicating that, under these conditions, the AlGaN growth was thermodynamically limited. [ABSTRACT FROM AUTHOR]

The thermal conductivity of GaN crystals grown by different techniques is analyzed using the 3ω method in the temperature range of 30 K to 295 K. GaN wafers grown by the ammonothermal method show a significant variation in thermal conductivity at room temperature with values ranging between 164 W m−1 K−1 and 196 W m−1 K−1. GaN crystals produced with the sodium flux and hydride vapor phase epitaxy methods show results of 211 W m−1 K−1 and 224 W m−1 K−1, respectively, at room temperature. Analysis using secondary ion mass spectrometry indicates varying amounts of impurities between the respective crystals and explains the behavior of thermal conductivity trends in the samples. The observed difference between thermal conductivity curves suggests that scattering of phonons at point defects dominates the thermal conductivity of GaN within the investigated temperature range. Deviations of model curves from thermal conductivity measurements and disparities between modelled characteristic lengths and actual sample thicknesses indicate that phonon resonances are active in GaN. [ABSTRACT FROM AUTHOR]

The average bulk thermal conductivity of free-standing physical vapor transport and hydride vapor phase epitaxy single crystal AlN samples with different impurity concentrations is analyzed using the 3ω method in the temperature range of 30–325 K. AlN wafers grown by physical vapor transport show significant variation in thermal conductivity at room temperature with values ranging between 268 W/m K and 339 W/m K. AlN crystals grown by hydride vapor phase epitaxy yield values between 298 W/m K and 341 W/m K at room temperature, suggesting that the same fundamental mechanisms limit the thermal conductivity of AlN grown by both techniques. All samples in this work show phonon resonance behavior resulting from incorporated point defects. Samples shown by optical analysis to contain carbon-silicon complexes exhibit higher thermal conductivity above 100 K. Phonon scattering by point defects is determined to be the main limiting factor for thermal conductivity of AlN within the investigated temperature range. [ABSTRACT FROM AUTHOR]

We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics. [ABSTRACT FROM AUTHOR]

A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations. [ABSTRACT FROM AUTHOR]