Giacinta Parish

Gallium nitride based high power heterojunction field effect transistors: process development and present status at UCSB

The development of GaN based devices for microwave power electronics at the University of California, Santa Barbara (UCSB), is reviewed. From 1995 to 2000, the power performance of AlGaN/GaN-on-sapphire heterojunction field effect transistors improved from 1.1 W/mm to 6.6 W/mm, respectively. Compensating the disadvantages of the low thermal conductivity of the sapphire substrate through heat management via flip chip bonding onto AlN substrates, large periphery devices with an output power of 7.6 W were demonstrated. UCSB also fabricated the first GaN based amplifier integrated circuits. Critical issues involved in the growth of high quality AlGaN/GaN heterostructures by metal-organic chemical vapor deposition and the device fabrication are discussed.

Gallium nitride based transistors

An overview is presented of progress in GaN electronic devices along with recent results from work at UCSB. From 1995 to 2001, the power performance of AlGaN/GaN high electron mobility transistors (HEMT) improved from 1.1 to 11 W mm-1, respectively. The disadvantage of the low thermal conductivity of the sapphire substrate was mitigated by flip-chip bonding onto AlN substrates, yielding large periphery devices with an output power of 7.6 W. A variety of HEMT amplifier circuits have been demonstrated. The first AlGaN/GaN heterojunction bipolar transistor (HBT) was demonstrated in 1998, with a current gain of about 3. By developing the technique of emitter regrowth, a current gain of 10 was achieved in both GaN BJTs and AlGaN/GaN HBTs. A common emitter current gain cutoff frequency of 2 GHz was measured. Critical issues involved in the growth of high quality AlGaN/(AlN)/GaN heterostructures and GaN:Mg by metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) and the device fabrication are discussed.

/sup 60/Co gamma irradiation effects on n-GaN Schottky diodes

The effect of /spl gamma/-ray exposure on the electrical characteristics of nickel/n-GaN Schottky barrier diodes has been investigated using current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The results indicate that /spl gamma/-irradiation induces an increase in the effective Schottky barrier height extracted from C-V measurements. Increasing radiation dose was found to degrade the reverse leakage current, whereas its effect on the forward I-V characteristics was negligible. Low temperature (/spl les/50) post-irradiation annealing after a cumulative irradiation dose of 21 Mrad(Si) was found to restore the reverse I-V characteristics to pre-irradiation levels without significantly affecting the radiation-induced changes in C-V and forward I-V characteristics. Three shallow radiation-induced defect centers with thermal activation energies of 88 104 and 144 meV were detected by DLTS with a combined production rate of 2.12 /spl times/ 10/sup -3/ cm/sup -1/. These centers are likely to be related to nitrogen-vacancies. The effect of high-energy radiation exposure on device characteristics is discussed taking into account possible contact inhomogeneities arising from dislocations and interfacial defects. The DLTS results indicate that GaN has an intrinsically low susceptibility to radiation-induced material degradation, yet the effects observed in the Schottky diode I-V and C-V characteristics indicate that the total-dose radiation hardness of GaN devices may be limited by susceptibility of the metal-GaN interface to radiation-induced damage.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Characterisation of Multiple Carrier Transport in Indium Nitride Grown by Molecular Beam Epitaxy

Transport properties of two distinct electron species in indium nitride grown by molecular beam epitaxy (MBE) have been measured. Variable field Hall and resisitivity voltages were used in a quantitative mobility spectrum analysis (QMSA) to extract the concentrations and mobilities of the two electron species, attributed to the bulk electrons and a surface accumulation layer. Single magnetic field data corresponds to neither electron species. The bulk electron distribution has an extracted average mobility of 3570 cm2/(V s) at 300 K, which rises to over 5100 cm2/(V s) at 150 K. Bulk electron concentration in the sample is 1.5 ×1017 cm-3 . The surface electrons have a higher sheet charge density and an order of magnitude lower average mobility than those in the bulk.

60Co gamma-irradiation-induced defects in n-GaN

Transient capacitance measurements of Schottky diodes fabricated on nominally undoped n-type GaN exposed to 60Co gamma irradiation indicate the introduction of two defect levels with thermal activation energies of 89±6 and 132±11 meV. While the emission characteristics of these defects manifest significant broadening, their parameters are consistent with reported electron-irradiation-induced nitrogen-vacancy related centers. Three deep-level defects present before irradiation exposure with activation energies of 265, 355, and 581 meV were found to remain unaffected for cumulative gamma-ray doses up to 21 Mrad(Si).

Development of an Alkaline-Compatible Porous-Silicon Photolithographic Process

A technique for masking porous-silicon (PS) films by optical photolithography without significant film degradation is demonstrated for the first time. The chemical resistance of the PS films is achieved by low-temperature passivation via nitrogen annealing. The effect of the various photolithographic process steps is investigated by determining the changes in the optical properties of the films. The passivated PS films are shown to be resistant to strong alkaline-based solutions (dilute AZ400K developer) for up to 100 s with minimal reduction in optical thickness. Fourier transform infrared and spectral reflectance measurements show that the passivated PS films are relatively unaffected by a complete photolithography process, apart from the seepage of photoresist into the pores, which can be prevented by applying a thin protective polymer layer. Metal was deposited and patterned via the standard liftoff process on PS surface to demonstrate the feasibility of this technique.

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

A crystallographic alignment method in silicon for deep, long microchannel fabrication

The aim of this work was to develop an alignment technique to be used in the production of long, deep, large area microchannel devices. The microchannel design specifications used for the investigation were 800 µm deep channels of 100 µm width, with a 200 µm pitch, over an area of 40 mm × 40 mm. The device was fabricated with (1 1 0) orientated silicon, to take advantage of the large wet etch ratio between the {110} and {111} planes. Silicon nitride was used as the channel etchant mask, and was patterned by reactive ion etching. The channels were wet etched in a KOH 40 wt% solution at 80 °C to minimize undercut of the silicon nitride mask, while maintaining a reasonable etch rate of 2 µm min−1. The {111} crystal plane normal to the {110} wafer surface needed to be determined with high accuracy for the fabrication of microchannels of such a large size. Investigations of several established alignment techniques revealed only one suitable technique: the use of a wet etched alignment feature that is self-aligned to the {111} crystal planes. This resulted in an silicon nitride mask undercut of 10 µm for channels 800 µm deep and 45 mm in length.

Multilayer porous silicon diffraction gratings operating in the infrared

Transmission diffraction gratings operating at 1,565 nm based on multilayer porous silicon films are modeled, fabricated, and tested. Features down to 2 μm have been patterned into submicron-thick mesoporous films using standard photolithographic and dry etching techniques. After patterning of the top porous film, a second anodization can be performed, allowing an under-layer of highly uniform porosity and thickness to be achieved. High transmission greater than 40% is measured, and modeling results suggest that a change in diffraction efficiency of 1 dB for a 1% change in normalized refractive index can be achieved. Preliminary measurement of solvent vapor shows a large signal change from the grating sensor in agreement with models.