Siddharth Rajan

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.

N-polar GaN/AlGaN/GaN high electron mobility transistors

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN∕GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides.

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.

p-type doping of MoS2 thin films using Nb

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 3.1 × 1020 cm−3, Hall mobility of 8.5 cm2 V−1 s−1 was determined, which matches well with the theoretically expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered crystalline nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately ½ electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution, and the influence of different electrostatic boundary conditions are obtained.

Large area single crystal (0001) oriented MoS2

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, the potential for exciting physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area single crystal Molybdenum Disulfide (MoS2) thin films. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) oriented sapphire yielded highly oriented single crystal MoS2 films. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

Influence of the dynamic access resistance in the g/sub m/ and f/sub T/ linearity of AlGaN/GaN HEMTs

The decrease of transconductance g/sub m/ and current gain cutoff frequency f/sub T/ at high drain current levels in AlGaN/GaN high-electron mobility transistors (HEMTs) severely limits the linearity and power performance of these devices at high frequencies. In this paper, the increase of the differential source access resistance r/sub s/, with drain current is shown to play an important role in the fall of g/sub m/ and f/sub T/. The increase of r/sub s/ occurs due to the quasi-saturation of the electron velocity in the source region of the channel at electric fields higher than 10 kV/cm. This has been confirmed by both experimental measurements and two-dimensional drift-diffusion simulations. Through simulations, we have identified HEMT structures with source implanted regions (or n/sup ++/ cap layers) as good candidates in order to increase the linearity of the g/sub m/ and f/sub T/ versus current profile.

Electrical properties of atomic layer deposited aluminum oxide on gallium nitride

We report on our investigation of the electrical properties of metal/Al2O3/GaN metal-insulator-semiconductor capacitors. We determined the conduction band offset and interface charge density of the alumina/GaN interface by analyzing the capacitance-voltage characteristics of atomic layer deposited Al2O3 films on GaN substrates. The conduction band offset at the Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band conditions in the GaN was inferred, which we attribute to a fixed net positive charge density of magnitude 4.60 × 1012 cm−2 at the Al2O3/GaN interface. We provide hypotheses to explain the origin of this charge by analyzing the energy band line-up.

Suppression of electron overflow and efficiency droop in N-polar GaN green light emitting diodes

In this letter, we experimentally demonstrate direct correlation between efficiency droop and carrier overflow in InGaN/GaN green light emitting diodes (LEDs). Further, we demonstrate flat external quantum efficiency curve up to 400 A/cm2 in a plasma assisted molecular beam epitaxy grown N-polar double quantum well LED without electron blocking layers. This is achieved by exploring the superior properties of reverse polarization field of N-face polarity, such as effective carrier injection and higher potential barriers against carrier overflow mechanism. The LEDs were found to operate with a low (∼2.3 V) turn-on voltage.

Polarization-engineered GaN/InGaN/GaN tunnel diodes

We report on the design and demonstration of polarization-engineered GaN/InGaN/GaN tunnel junction diodes with high current density and low tunneling turn-on voltage. Wentzel–Kramers–Brillouin calculations were used to model and design tunnel junctions with narrow band gap InGaN-based barrier layers. N-polar p-GaN/In0.33Ga0.67N/n-GaN heterostructure tunnel diodes were grown using molecular beam epitaxy. Efficient interband tunneling was achieved close to zero bias with a high current density of 118 A/cm2 at a reverse bias of 1 V, reaching a maximum current density up to 9.2 kA/cm2. These results represent the highest current density reported in III-nitride tunnel junctions and demonstrate the potential of III-nitride tunnel devices for a broad range of optoelectronic and electronic applications.