Stephen W. Kaun

Role of oxygen in the OFF-state degradation of AlGaN/GaN high electron mobility transistors

The physical degradation of AlGaN/GaN high electron mobility transistors during OFF-state stress experiments has been systematically studied. Oxide particles and stringers were found to form along the gate edge of stressed devices. When the gate electrode is removed, pits are seen to have formed underneath each particle. The observed room-temperature oxidation process is strongly dependent on the duration of the electrical stressing and the electric field. Moreover, the oxidation can be significantly reduced in vacuum (3 × 10−5 Torr), with a corresponding 30% reduction of current collapse. Finally, a degradation process with electric-field-driven oxidation of the AlGaN surface has been proposed.

Influence of threading dislocation density on early degradation in AlGaN/GaN high electron mobility transistors

Early stage degradation of AlGaN/GaN high electron mobility transistors (HEMTs) with different threading dislocation densities (TDDs) submitted to off-state voltage bias stress was studied. It was found that, for the stress conditions used, HEMTs with TDD ∼1010 cm−2 show pronounced degradation in terms of maximum drain current, gate-lag, and trap generation measured by drain current trapping characteristics, a slight degradation in gate leakage was observed also for HEMTs with TDD of ∼108 cm−2, and no significant degradation for devices with TDD in the ∼107 cm−2 range. The results illustrate the importance of TDD for degradation and reliability of AlGaN/GaN HEMTs.

Spatially-resolved spectroscopic measurements of Ec − 0.57 eV traps in AlGaN/GaN high electron mobility transistors

Simultaneous temperature-dependent measurements of resistance transients (RTs) and spatially resolved surface potential transients were made after bias switching on AlGaN/GaN high electron mobility transistors (HEMTs). We find an Ec − 0.57 eV trap, previously correlated with HEMT degradation, located in the GaN buffer and not in the AlGaN barrier or at the AlGaN surface. The amplitude of the Ec − 0.57 eV trap in RTs depends strongly on the Fe-concentration in the GaN buffer. Filling of this trap occurs only under bias conditions where electric fields penetrate into the GaN buffer.

Molecular beam epitaxy for high-performance Ga-face GaN electron devices

Molecular beam epitaxy (MBE) has emerged as a powerful technique for growing GaN-based high electron mobility transistor (HEMT) epistructures. Over the past decade, HEMT performance steadily improved, mainly through the optimization of device fabrication processes. Soon, HEMT performance will be limited by the crystalline quality of the epistructure. MBE offers heterostructure growth with highly abrupt interfaces, low point defect concentrations, and very low carbon and hydrogen impurity concentrations. Minimizing parasitic leakage pathways and resistances is essential in the growth of HEMTs for high-frequency and high-power applications. Through growth on native substrates with very low threading dislocation density, low-leakage HEMTs with very low on-resistance can be realized. Ga-rich plasma-assisted MBE (PAMBE) has been studied extensively, and it is clear that this technique has inherent limitations, including a high density of leakage pathways and a very small growth parameter space. Relatively new MBE growth techniques—high-temperature N-rich PAMBE and ammonia-based MBE—are being developed to circumvent the shortcomings of Ga-rich PAMBE.

Effects of Threading Dislocation Density on the Gate Leakage of AlGaN/GaN Heterostructures for High Electron Mobility Transistors

AlGaN/GaN heterostructures were regrown on three semi-insulating GaN templates with threading dislocation densities of ~2×1010, ~5×108, and ~5×107 cm-2. Regrowths were carried out under Ga-rich conditions by plasma-assisted molecular beam epitaxy to determine the effects of threading dislocation density on leakage through Schottky contacts on the AlGaN/GaN heterostructures. A similar AlGaN/GaN heterostructure was directly grown on 4H-SiC under Ga-rich conditions for comparison with the regrown heterostructures. High electron mobility transistors were fabricated. Decreasing the threading dislocation density from ~2×1010 to ~5×107 cm-2 yielded up to a 45-fold decrease in the average reverse Schottky diode current at -10 V bias.

Atom probe analysis of AlN interlayers in AlGaN/AlN/GaN heterostructures

Atom probe tomography was used to characterize AlN interlayers in AlGaN/AlN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy (PAMBE), NH3-based molecular beam epitaxy (NH3-MBE), and metal-organic chemical vapor deposition (MOCVD). The PAMBE-grown AlN interlayer had the highest purity, with nearly 100% of group-III sites occupied by Al. The group-III site concentrations of Al for interlayers grown by NH3-MBE and MOCVD were ∼85% and ∼47%, respectively. Hall measurements were performed to determine the two-dimensional electron gas mobility and sheet concentration. Sheet concentrations were ∼25%–45% higher with molecular beam epitaxy than with MOCVD, and these results matched well with atom probe data.

Proton-Induced Dehydrogenation of Defects in AlGaN/GaN HEMTs

The responses to 1.8 MeV proton irradiation of AlGaN/GaN HEMTs grown under Ga-rich and ammonia-rich conditions are investigated in this work. Changes in defect energy distributions of AlGaN/GaN HEMTs during proton irradiation are characterized via temperature-dependent low-frequency noise measurements. Density functional theory calculations show these changes are consistent with the reconfiguration and/or dehydrogenation of oxygen-related defects in Ga-rich devices.

GaN-based high-electron-mobility transistor structures with homogeneous lattice-matched InAlN barriers grown by plasma-assisted molecular beam epitaxy

Metal-polar In0.17Al0.83N barriers, lattice-matched to GaN, were grown under N-rich conditions by plasma-assisted molecular beam epitaxy. The compositional homogeneity of these barriers was confirmed by plan-view high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Metal-polar In0.17Al0.83N/(GaN)/(AlN)/GaN structures were grown with a range of AlN and GaN interlayer (IL) thicknesses to determine the optimal structure for achieving a low two-dimensional electron gas (2DEG) sheet resistance. It was determined that the presence of a GaN IL was necessary to yield a 2DEG sheet density above 2 × 10 13 cm −2 . By including AlN and GaN ILs with thicknesses of 3 nm and 2 nm, respectively, a metal-polar In0.17Al0.83N/GaN/AlN/GaN structure regrown on a GaN-on-sapphire template yielded a room temperature (RT) 2DEG sheet resistance of 163 � /. This structure had a threading dislocation density (TDD) of ∼5 × 10 8 cm −2 . Through regrowth on a free-standing GaN template with low TDD (∼5 × 10 7 cm −2 ), an optimized metal-polar In0.17Al0.83N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 � / and mobility of 1822 cm 2 V −1 s −1 . High-electron-mobility transistors with output current densities above 1 A mm −1 were also demonstrated on the low-TDD

β-(AlxGa1−x)2O3/Ga2O3 (010) heterostructures grown on β-Ga2O3 (010) substrates by plasma-assisted molecular beam epitaxy

By systematically changing growth parameters, the growth of β-(AlxGa1−x)2O3/Ga2O3 (010) heterostructures by plasma-assisted molecular beam epitaxy was optimized. Through variation of the Al flux under O-rich conditions at 600 °C, β-(AlxGa1−x)2O3 (010) layers spanning ∼10% to ∼18% Al2O3 were grown directly on β-Ga2O3 (010) substrates. Nominal β-(AlxGa1−x)2O3 (010) compositions were determined through Al:Ga flux ratios. With x = ∼0.18, the β-(AlxGa1−x)2O3 (020) layer peak in a high-resolution x-ray diffraction (HRXRD) ω-2θ scan was barely discernible, and Pendellosung fringes were not visible. This indicated that the phase stability limit of Al2O3 in β-Ga2O3 (010) at 600 °C was less than ∼18%. The substrate temperature was then varied for a series of β-(Al∼0.15Ga∼0.85)2O3 (010) layers, and the smoothest layer was grown at 650 °C. The phase stability limit of Al2O3 in β-Ga2O3 (010) appeared to increase with growth temperature, as the β-(AlxGa1−x)2O3 (020) layer peak with x = ∼0.18 was easily distinguishable ...

Effect of dislocations on electron mobility in AlGaN/GaN and AlGaN/AlN/GaN heterostructures

AlxGa1−xN/GaN (x = 0.06, 0.12, 0.24) and AlGaN/AlN/GaN heterostructures were grown on 6 H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ∼2 × 1010, ∼5 × 108, and ∼5 × 107 cm−2, respectively. All growths were performed under Ga-rich conditions by plasma-assisted molecular beam epitaxy. Dominant scattering mechanisms with variations in threading dislocation density and sheet concentration were indicated through temperature-dependent Hall measurements. The inclusion of an AlN interlayer was also considered. Dislocation scattering contributed to reduced mobility in these heterostructures, especially when sheet concentration was low or when an AlN interlayer was present.