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In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (Vf) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the Vf is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low Vf penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs. [ABSTRACT FROM AUTHOR]

Ultra-wide bandgap (UWBG) semiconductors are attracting increasing research interest for power device applications. While promising results have been reported for various materials, it remains unclear which material and technology will succeed. Many figure of merits (FOMs) were derived for power device applications to guide material choices, including the widely used Baliga's FOM (BFOM) to describe the resistive loss of power devices, and Baliga's high-frequency FOM (BHFFOM) to further consider the switching loss. However, key underlying assumptions for those widely cited FOMs, including the assumption of shallow hydrogenic dopants, tend to fail for UWBG semiconductors. In this work, we revisit several important FOMs in describing vertical power electronics to properly account for both incomplete ionization and background compensation effects. We suggest that it is necessary to include the dopant ionization term (for example n/Nd in an n-drift layer) for both BFOM and BHFFOM to fully capture the potential of the UWBG semiconductors for power device applications. Incomplete dopant ionization in materials like diamond and AlN substantially lowers their FOMs for power switching, leading to high conductive and switching losses. Due to the availability of shallow donors, low background impurity compensation, and bulk substrates, β-Ga2O3 promises the best performance among the investigated materials. The modified FOMs offer a valuable guidance in material choices for power device applications. [ABSTRACT FROM AUTHOR]

Tunnel junctions (TJs) offer alternative designs and promise in some cases improved performances for nitride-based light-emitting diode (LEDs) and laser diodes (LDs) and are widely used in academic studies. However, the voltage penalty of the LEDs and LDs, in comparison with standard contact technologies, has been a major concern especially for commercial applications. In this study, we investigated methods to achieve low excess voltage. Using ammonia molecular beam epitaxy (NH3 MBE), GaN TJs were grown on commercial metalorganic chemical vapor deposition (MOCVD) grown blue LED wafers. Atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) indicate 1 min buffered HF (BHF) clean of the regrowth interface reduced Mg and impurity incorporation into the n++ regrown TJ layers. The wafers were processed and measured in parallel to reference wafers using both university processes and industry processes. At 20 A cm−2, TJ LEDs grown with Si δ-doping at the junction interface processed in the university cleanroom had a forward voltage of 3.17 V in comparison to 2.86 V for LEDs processed with a standard indium tin oxide (ITO) contact. Unencapsulated TJ LEDs processed by industrial process without ITO or current blocking layer had about 0.3 V excess voltage compared to reference LEDs. The TJ LEDs also had more uniform light emission profile. The low excess voltage and consistent results acquired in both settings suggest that TJ can be scaled for industrial processes. [ABSTRACT FROM AUTHOR]

High performance GaN-based micro-light-emitting diodes (µLEDs) with epitaxial n-InGaN/n-GaN tunnel junctions (InGaN TJs) were grown by metalorganic chemical vapor deposition (MOCVD). The InGaN TJs µLEDs show a significant reduction of forward voltage (Vf) by ∼0.6 V compared to the common TJs µLEDs. The Vf at 20 A cm−2 is very low varied from 3.15 V to 3.19 V in small InGaN TJ µLEDs with a size less than 40 × 40 µm2, and then significantly increases in large LEDs. Selective area growth (SAG) of TJs can overcome such size limitation by vertical out diffusion of hydrogen through the apertures on top of p-GaN. The InGaN TJ µLEDs overgrown by SAG show a size-independent low Vf ranged from 3.08 V to 3.25 V. The external quantum efficiency (EQE) of the packaged TJ µLEDs was improved by 6% compared to the common µLEDs with indium tin oxide (ITO) contact. This work solves the key challenges of MOCVD-grown TJs. [ABSTRACT FROM AUTHOR]

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 × 1013 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 Ω/□. This structure had a threading dislocation density (TDD) of ∼5 × 108 cm−2. Through regrowth on a free-standing GaN template with low TDD (∼5 × 107 cm−2), an optimized metal-polar In0.17Al0.83N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 Ω/□ and mobility of 1822 cm2 V−1 s−1. High-electron-mobility transistors with output current densities above 1 A mm−1 were also demonstrated on the low-TDD structure. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy was performed on InGaN HEMTs and revealed a donor-like trap at the InGaN/GaN interface with net negative polarization. The trap was found to have an energy level of 0.071 eV. A photo-assisted capacitance-voltage (CV) measurement was performed on an InGaN diode. CV curves were measured before and after exposure to a broadband ultraviolet (UV) lamp. The UV lamp caused capacitance to increase as a result of traps being ionized. The trap density was found to be at least 4 × 1011 cm-2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper reviews the progress of N-polar (000 1) GaN high frequency electronics that aims at addressing the device scaling challenges faced by GaN high electron mobility transistors (HEMTs) for radio-frequency and mixed-signal applications. Device quality (Al, In, Ga)N materials for N-polar heterostructures are developed using molecular beam epitaxy and metalorganic chemical vapor deposition. The principles of polarization engineering for designing N-polar HEMT structures will be outlined. The performance, scaling behavior and challenges of microwave power devices as well as highly-scaled depletion- and enhancement-mode devices employing advanced technologies including self-aligned processes, n+ (In,Ga)N ohmic contact regrowth and high aspect ratio T-gates will be discussed. Recent research results on integrating N-polar GaN with Si for prospective novel applications will also be summarized. [ABSTRACT FROM AUTHOR]