C. Poblenz

High-power AlGaN/GaN HEMTs for Ka-band applications

We report on the fabrication and high-frequency characterization of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). In devices with a gate length of 160 nm, a record power density of 10.5 W/mm with 34% power added efficiency (PAE) has been measured at 40 GHz in MOCVD-grown HEMTs biased at V/sub DS/=30 V. Under similar bias conditions, more than 8.6 W/mm, with 32% PAE, were obtained on the MBE-grown sample. The dependence of output power, gain, and PAE on gate and drain voltages, and frequency have also been analyzed.

Optimization of the surface morphologies and electron mobilities in GaN grown by plasma-assisted molecular beam epitaxy

The morphology and electrical properties of homoepitaxial GaN layers grown by molecular beam epitaxy at 720 °C were investigated as a function of Ga/N ratio. GaN films grown with low Ga/N ratios (N-stable regime) are semi-insulating and have heavily pitted morphologies. GaN films grown with higher Ga/N ratios (intermediate regime) have fewer pits with areas of atomically flat surface. The room-temperature electron mobilities in samples grown in the intermediate regime are greater than 800 cm2/V s and increase with Ga/N ratio. At the highest Ga/N ratios (Ga-droplet regime), Ga droplets formed on the surface during growth. Although the surface morphology is free of pits and atomically flat for films grown within the Ga-droplet regime, the mobility decreases significantly compared to films grown in the intermediate regime. Room-temperature electron mobilities as high as 1191 cm2/V s were measured in a GaN film grown with the highest Ga/N ratio within the intermediate regime.

Effect of carbon doping on buffer leakage in AlGaN/GaN high electron mobility transistors

Carbon doping via CBr4 in AlGaN/GaN high electron mobility transistors grown by rf-plasma-assisted molecular beam epitaxy on 4H–SiC (0001) was investigated as a means to reduce buffer leakage. For carbon doping in the first 400 nm of the structure, a significant decrease in buffer leakage was observed with increasing overall carbon concentration. A carbon doping scheme in which the level of doping is tapered from 6×1017 cm−3 down to 2×1017 cm−3 was found to result in sufficiently low drain-source leakage currents. The effect of thickness of the GaN:C layer was explored as well as the effect of thickness of the subsequent unintentionally doped GaN layer. For structures with reduced leakage, rf I–V and power measurements revealed better performance in structures in which the two-dimensional electron gas was spaced at a large distance from the GaN:C layer. Possible sources and locations of unintentional free carriers contributing to leakage in these structures are discussed in light of the results.

Measurement of polarization charge and conduction-band offset at InxGa1−xN/GaN heterojunction interfaces

The spontaneous and piezoelectric polarization fields in group-III nitride semiconductors lead to the presence of large electrostatic sheet charge densities at nitride semiconductor heterojunction interfaces. Precise quantitative knowledge of these polarization-induced charge densities and of the band-edge discontinuities at nitride heterojunction interfaces is therefore essential in nitride semiconductor device design and analysis. We have used capacitance–voltage profiling to measure the conduction-band offset and polarization charge density at InxGa1−xN/GaN heterojunction interfaces with x=0.054 and x=0.09. We obtain conduction-band offsets ΔEC=0.09±0.07 eV for x=0.054 and ΔEC=0.22±0.05 eV for x=0.09, corresponding to an averaged conduction-to-valence-band offset ratio ΔEC:ΔEV of 58:42. Our measurements yield polarization charge densities of (1.80±0.32)×1012 e/cm2 for x=0.054 and (4.38±0.36)×1012 e/cm2 for x=0.09. These values are smaller than those predicted by recent theoretical calculations, but in ...

Reduction of reverse-bias leakage current in Schottky diodes on GaN grown by molecular-beam epitaxy using surface modification with an atomic force microscope

The characteristics of dislocation-related leakage current paths in an AlGaN/GaN heterostructure grown by molecular-beam epitaxy and their mitigation by local surface modification have been investigated using conductive atomic force microscopy. When a voltage is applied between the tip in an atomic force microscope (AFM) and the sample, a thin insulating layer is formed in the vicinity of the leakage paths where current is observed. As the insulating layer reaches a thickness of 2–3 nm, the leakage current is blocked and subsequent growth of the layer is prevented. Although conductive screw or mixed dislocations are observed, dislocations with a screw component that do not conduct current are also apparent. The reverse-bias leakage current is reduced by a factor of two in a large-area diode fabricated on an area modified in this manner with an AFM compared to typical diodes fabricated on unmodified areas with comparable series resistances, confirming that dislocation-related leakage current paths are a ma...

Power performance of AlGaN-GaN HEMTs grown on SiC by plasma-assisted MBE

We report AlGaN-GaN high electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on SiC substrates with excellent microwave power and efficiency performance. The GaN buffers in these samples were doped with carbon to make them insulating. To reduce gate leakage, a thin silicon nitride film was deposited on the AlGaN surface by chemical vapor deposition. At 4 GHz, an output power density of 6.6 W/mm was obtained with 57% power-added efficiency (PAE) and a gain of 10 dB at a drain bias of 35 V. This is the highest PAE reported until now at 4 GHz in AlGaN-GaN HEMTs grown by MBE. At 10 GHz, we measured an output power density of 7.3 W/mm with a PAE of 36% and gain of 7.6 dB at 40-V drain bias.

High-Efficiency Blue and True-Green-Emitting Laser Diodes Based on Non-c-Plane Oriented GaN Substrates

Using non-c-plane bulk GaN substrates, we demonstrate continuous-wave single-mode blue-emitting laser diodes operating with over 23% wall plug efficiency and over 750 mW output power, which represent the highest values reported to date. Furthermore, we demonstrate continuous-wave 520 nm green-emitting laser diodes with over 60 mW output power and 1.9% wall plug efficiency. The rapid performance evolution of laser diodes fabricated on non-c-plane orientations is validation of the benefits resulting from increased electron–hole overlap, reduced effective hole mass, and increased design flexibility.

Deep level optical and thermal spectroscopy of traps in n-GaN grown by ammonia molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios sugge...

Nonalloyed ohmic contacts in AlGaN/GaN HEMTs by ion implantation with reduced activation annealing temperature

This letter reports AlGaN/GaN high-electron mobility transistors with capless activation annealing of implanted Si for nonalloyed ohmic contacts. Source and drain areas were implanted with an Si dose of 1/spl times/10/sup 16/ cm/sup -2/ and were activated at /spl sim/1260/spl deg/C in a metal-organic chemical vapor deposition system in ammonia and nitrogen at atmospheric pressure. Nonalloyed ohmic contacts to ion-implanted devices showed a contact resistance of 0.96 /spl Omega//spl middot/mm to the channel. An output power density of 5 W/mm was measured at 4 GHz, with 58% power-added efficiency and a gain of 11.7 dB at a drain bias of 30 V.

Structural Properties of GaN Buffer Layers on 4H-SiC(0001) Grown by Plasma-Assisted Molecular Beam Epitaxy for High Electron Mobility Transistors

The structural properties of GaN buffer layers grown on 4H-SiC(0001) by plasma-assisted molecular beam epitaxy were investigated. Films grown under intermediate and N-rich conditions exhibited lower dislocation densities and lower residual lattice mismatch strain than Ga-rich grown films. To overcome the rough surfaces associated with intermediate and N-rich growth, two-step GaN buffers were investigated where the first layer was grown in the intermediate growth regime and the second layer at the boundary to Ga droplet formation. It is demonstrated that for our growth conditions a buffer thickness of 100 nm grown in intermediate conditions was necessary and sufficient for dislocation reduction. High electron mobility transistor structures grown using this Ga-flux strategy exhibit mobilities, power densities and power added efficiencies in excess of 1400 cm2/Vs, 12 W/mm and 46% at 4 GHz, respectively.