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Sc0.18Al0.82N/GaN with state-of-the-art x-ray diffraction figures of merit grown by metal modulated epitaxy under metal-rich conditions and a low substrate temperature of 400 °C is demonstrated to have improved crystalline order [250 arc sec for the (0002) reflection and 469 arc sec for the (10 1 ¯ 5)] compared to a previous state-of-the-art sample grown at a more conventional temperature of 650 °C. While both samples show a columnar structure, the higher substrate temperature sample has a good symmetric rocking curve (RC) of 229 arc sec, but unlike the lower temperature sample, the RC of the (10 1 ¯ 5) asymmetric reflection could not be measured, indicating a more columnar structure common among ScAlN films. Local lattice constant maps (LLCMs) from 4D-STEM depict abrupt strain relaxation within ∼2 nm from the ScAlN/GaN interface for the sample grown at Tsub = 400 °C. Since these LLCMs suggest a lattice mismatch in the a-lattice constant, and since the films show a sudden roughening, the composition for lattice match to GaN may be less than the accepted 18%–20% Sc, consistent with the average GaN lattice match from lattice constant values reported in the literature of 12%. Compared to traditional III-Nitrides, ScAlN films have substantially more screw and mixed-type threading dislocations, suggesting substantial shear forces that result in significant twist and distortion leading to orthorhombic diffraction patterns as viewed from plan-view TEM in the Tsub = 650 °C sample. These results offer the possibility of ScAlN integration into low-thermal-budget processes including CMOS but further indicate that structural understanding of ScAlN remains lacking. [ABSTRACT FROM AUTHOR]

While metal-rich ScAlN epitaxy has traditionally led to mixed phase films by controlling the surface chemistry with transient metal doses utilizing a pulsed method of molecular beam epitaxy, phase-pure, metal-rich epitaxy of ScAlN was demonstrated, showing improved structural and electrical characteristics. The effects of substrate temperature and III/V ratio were studied, and an x-ray diffraction figure of merit and surface roughness as low as 225 arcsec and 0.68 nm, respectively, were demonstrated. A significant catalytic effect is observed with the use of Sc in metal-rich conditions, resulting in varied growth rates with substrate temperature and Sc surface coverage. This catalytic effect results in complications when selecting synthesis conditions and for in situ monitoring and can be accounted for improved phase purity. The variation of growth rates with Sc surface coverage introduces non-linearities to the transient initiation stage of growth but also introduces a feedback stabilization of the surface chemistry. Accounting for these complexities, a Sc0.2Al0.8N high electron mobility transistor (HEMT) heterostructure is demonstrated with a sheet resistance of 152 Ω/□, a mobility of 700 cm2/Vs, and a sheet charge of 5.9 × 1013 cm−2. [ABSTRACT FROM AUTHOR]

AlGaN self-assembled superlattices (SASLs) and their customization were investigated via metal modulated epitaxy. Using the dynamics of metal accumulation, surface strain, and surface segregation, coherent SASL structures were demonstrated with high repeatability over 910 nm (63 periods) with no growth interrupts at rates as high as 2.8 μm/h. High customizability was shown with control over period thickness as well as the thickness of the individual layers. Factors effecting the thickness and composition of each layer are discussed and methods of customization are presented. Transmission electron microscopy shows abrupt interfaces between individual layers of the SASL and good coherency throughout the structure and strong wavelength tunable ultraviolet photoluminescence was observed. Such a method of SASL fabrication offers promise for the growth of multi-quantum well structures, distributed Bragg reflectors, strain relaxation buffers, and beyond without the need for growth interrupts, and thus unintentional contaminant incorporation, between layers. [ABSTRACT FROM AUTHOR]

The structural quality of indium-rich Al0.3In0.7N grown by metal modulated epitaxy (MME), previously demonstrating x-ray diffraction (XRD) figures of merit ∼11 times better than the previous literature is investigated to explain the origin of such a large quality improvement. Four-dimensional scanning transmission electron microscope was used to map the lattice parameter near the AlInN/GaN heterojunction and indicate a 5.4% lattice constant change, suggesting 75% relaxation within ∼2 nm from the interface. Cross-sectional TEM Moiré fringes are observed at the AlInN/GaN heterointerface, indicating that there are misfit dislocations between AlInN and GaN which, while rare, have been observed for other highly mismatched In-rich III-Nitrides. The TEM measurements show regions of contrast indicating larger scale variations in strain, but defect contrast associated with dislocations and/or intrinsic basal stacking faults was minimal, indicating a good quality AlInN film and confirming prior XRD results. Significant electron beam induced damage can occur and depended strongly on operational conditions. The damage threshold current density was estimated using time-dependent TEM to be ∼5.7 A/cm2, significantly lower than from prior studies of InGaN. Damage also strongly depends on the thickness of the TEM foil examined and occurred at thicknesses greater than found for InGaN. The present study suggests that the MME technique is an excellent candidate for growing high-quality indium-rich AlInN films as compared to the traditional molecular beam epitaxy or metal organic chemical vapor deposition techniques. [ABSTRACT FROM AUTHOR]

7Li and 71Ga NMR measurements have been performed to study the Li+ ionic motion and vacancy ordering in the lithium semimetal beta-LiGa. The temperature dependence of the spin-lattice relaxation rate, T1(-1) of the 7Li nuclei in the 50 atom% Li sample shows an asymmetric broad peak around 175 K and is interpreted in terms of fast Li ionic diffusion. The activation energy of hopping is estimated as 0.11 eV using a non-Debye type relaxation model. In the temperature dependence of T1(-1) of the 7Li nuclei in 44 and 47 atom% Li samples, steep peaks are observed at 225 and 195 K, respectively. The origin of these anomalous peaks is attributed to the order-disorder transformation of Li+ vacancies. The temperature dependence of T1(-1) of the 71Ga nuclei measured above 200 K is interpreted in terms of the relaxation originating from the fluctuation of the electric field gradient at the 71Ga nuclei due to mobile Li+ ions. The activation energy for the Li+ ionic diffusion estimated from T1(-1) of the 71Ga nuclei is comparable with that obtained from T1(-1) of the 7Li nuclei.