S. Rai

A New Selective Area Lateral Epitaxy Approach for Depositing a-Plane GaN over r-Plane Sapphire

We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.

AlGaN multiple-quantum-well-based, deep ultraviolet light-emitting diodes with significantly reduced long-wave emission

We report on a deep UV light-emitting diode over sapphire substrate with AlGaN multiple-quantum-well active region. Pulsed atomic-layer epitaxy deposited low-defect AlN/AlGaN buffers and an optimized active layer design yielded a sharp quantum-well emission peak at 287 nm and very little long-wave secondary emission. For a 100 μm×100 μm unpackaged device, a power of 27 μW at 20 mA dc and a peak external quantum efficiency of 0.1% at 100 mA pulse pumping were measured. Flip-chip packaging should increase these numbers nearly by a factor of 3.

Air-bridged lateral growth of crack-free Al0.24Ga0.76N on highly relaxed porous GaN

We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters.

Monolithically integrated high-power broad-band RF switch based on III-N insulated gate transistors

We report on the high-performance monolithically integrated RF switch based on metal-oxide-semiconductor III-N heterostructure field-effect transistors (MOSHFETs). The radio frequency (RF) switch microwave monolithic integrated circuit (MMIC) consists of three submicron-gate MOSHFETs connected into /spl pi/-type configuration. In the 0-10 GHz frequency range, the insertion loss is less than 1dB and the isolation is better than 20 dB. The switching powers well exceed 20 W per 1mm of the active element width. The high performance parameters of the switch are achieved due to unique properties of III-nitride MOSHFET, which combines a low channel resistance and high breakdown voltage features of AlGaN/GaN HFETs and extremely low gate leakage currents, large gate voltage swing and low gate capacitance specific to insulated gate design. The combination of these parameters makes MOSHFETs excellent candidates for high-power switching. The experimental data obtained from the RF switch are in close agreement with the results of simulations.

Low Threshold-14 W/mm ZrO2/AlGaN/GaN Metal–Oxide–Semiconductor Heterostructure Field Effect Transistors

We report for the first time on the RF performance of a low-threshold AlGaN/GaN metal–oxide–semiconductor heterostructure field transistor (MOSHFET) with zirconium dioxide as the gate dielectric. Low gate leakage current of 5×10-7 A/mm2 and a threshold voltage which was only 1 V higher than that of an HFET were achieved. The RF power of these devices at 2 GHz was 14.32 W/mm at 50 V drain bias.

Digital oxide deposition of SiO2 layers for III-nitride metal-oxide-semiconductor heterostructure field-effect transistors

We present a digital-oxide-deposition (DOD) technique to deposit high quality SiO2 dielectric layers by plasma-enhanced chemical vapor deposition using alternate pulses of silicon and oxygen precursors. The DOD procedure allows for a precise thickness control and results in extremely smooth insulating SiO2 layers. An insulating gate AlGaN∕GaN heterostructure field-effect transistor (HFET) with 8nm thick DOD SiO2 dielectric layer had a threshold voltage of −6V (only 1V higher than that of regular HFET), very low threshold voltage dispersion, and output continuous wave rf power of 15W∕mm at 55V drain bias.

AlGaN/GaN MOSHFET integrated circuit power converter

This work introduces the first step in a version of a wide-bandwidth, frequency-agile power interface that can sit between a simulation environment and real electrical hardware. Silicon (Si) technology is incapable of meeting the extreme switching demands of such a power interface, while gallium-nitride (GaN) technology is best suited for this application. A GaN power cell, in the form of an integrated H-bridge power block, is used as the core element in this new interface to take advantage of the III-V semiconductor material properties, resulting in enhanced operating characteristics. The GaN integrated H-bridge transistors are constructed out of AIGaN/GaN, MOS-Hetero-junction FETs (MOSHFETs). The H-bridge is mounted to an aluminum nitride (AIN) substrate for heat removal via a thermally conducting, electrically insulating, epoxy. This wide bandgap power converter utilizes a high- and low-side driver to modulate the gate-source voltage of each device between +5 V and -12 V. Control for the power converter is provided via a dual output pulse generator. The pulse generator operates open-loop with two outputs to experimentally test the H-bridge in a half-bridge converter topology under different loading conditions.

Optically Pumped Lasing at 353 nm Using Non-polar a-plane AlGaN Multiple Quantum Wells over r-plane Sapphire

We report an optically-pumped alternative nitride-based laser with room-temperature emission at 353 nm. The active region comprised of non-polar a-plane Al0.04Ga0.96N/Al0.08Ga0.92N multiple quantum wells whereas the lasing cavity consisted of Al0.15Ga0.85N clad and Al0.10Ga0.85N waveguide layers and naturally cleaved facet mirrors. The layers were grown over r-plane sapphire substrates by metalorganic chemical vapor deposition technique. A room temperature lasing threshold for N2-laser photoexcitation of 110 kW/cm2 and a modal optical gain of 215 cm-1 was measured at the peak emission wavelength.

1.5 kV Power AlGaN/GaN HFETs

AlGaN/GaN power heterostructure field-effect transistors (HFETs) on sapphire substrate with up to 1600-V breakdown voltage (VBR) for power electronic applications have been fabricated. The devices have a low on-resistance (RON) of 3.4 mΩ.cm 2 , making the best VBR-RON relationship yet reported. The power device figure of merit VBR 2 / RON = 7.5x10 8 V 2 /(Ω cm -2 ) is among the best reported values for any AlGaN/GaN HEMT in their class. As silicon power semiconductor devices fail to meet the stringent frequency and temperature requirements imposed by new concepts and applications in the field of power electronics to produce the systems with increased efficiency, the SiC and GaN technologies emerge as the solutions for the future energy-conversion systems. SiC devices, although having the advantages of mature technology, possibility of vertical field effect transistor (FET) design, high breakdown voltages of several kilovolts, are limited by relatively high on-resistance and low switching frequencies. These are the key performance parameters for power conversion devices. On the other hand, AlGaN/GaN-based technology, thanks to a high-density two-dimensional electron gas (2DEG) (above 1x 10 13 cm -2 ) and high electron mobility (above 1500 cm 2 /V.s) allows for low on-resistance high-speed HFETs. For GaN- based high voltage lateral HFET the minimal value of ON-resistance, RON=VBR 2 /(qµnEC,GaN 2 ) while for vertical SiC devices it is given as RON=4VBR 2 /(erµEC,SiC 3 ). Substituting the material parameters for SiC and GaN one can see that the ratio RON,GaN/ RON,SiC ≈ 4.5 x 10 -2 . In this paper, we present a detailed study of AlGaN/GaN HFET breakdown voltage on device geometry and compare the achieved results with the theoretical limits and the best reported values. The HFETs devices were fabricated over sapphire substrate. The wafer sheet resistance was around 350 Ω, the threshold voltage, VT =-4.5V. The ohmic contacts were formed by Ti(200A)/Al(1000A)/Ti(500A)/Au(1500A) metal stacks. These were annealed at 850 °C for 1 min. in a forming gas ambient. The source-gate spacing was 2 µm, gate - drain spacing was varying from 2 to 20 µm. The Au/Ni gates with the gate length varying from 2µm to 12µm were formed using optical lithography. No field-plates or passivation layers have been deposited on the HFETs for this study. The breakdown voltage of AlGaN/GaN HFETs was found to depend linearly on the gate-drain spacing. This behavior is possible if (i) the breakdown is limited by the surface breakdown or (ii) the width of the 2DEG depletion region expands linearly with the gate- drain spacing. Detailed experiments using surface potential profiling and breakdown voltage - residual channel current dependencies have been carried out to confirm the breakdown mechanism. From these experiments, the breakdown of the HFETs was found to be surface limited. For devices with 20µm gate-drain spacing, the breakdown voltage VBD = 1600 V was measured with the corresponding value of RON = 3.4 mΩ.cm 2 . To the best of our knowledge, these are the highest reported results for the GaN based HFETs. Since the surface effects are dominating, an importance of the passivation or the encapsulation material used to cover the device will be discussed.

Insulated gate III-N devices and ICs

The MOSHFET design which combines the advantage of the MOS structure, which suppresses the gate leakage current, and an AlGaN/GaN heterointerface that provides high density, high mobility two-dimensional electron gas channel. This article presents a comparative review of the I-V characteristics, cut-off frequencies, RF output powers, power gain, and nonlinear distortions of AlGaN/GaN MOSHFET, and HFET device. The MOSHFETs possess significant advantages for the monolitic IC design. They sustain very high input impedance at elevated temperatures, even above 300 /spl deg/C. The results show that the MOSHFET based ICs are extremely promising for a large variety of high-power high-temperature applications.