G. Dang

Electrical effects of plasma damage in p-GaN

The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 A based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.

Comparison of GaN p-i-n and Schottky rectifier performance

The performance of GaN p-i-n and Schottky rectifiers fabricated on the same wafer was investigated as a function of device size and operating temperature. There was a significant difference in reverse breakdown voltage (490 V for p-i-n diodes; 347 V for the Schottky diodes) and forward turn-on voltage (/spl sim/5 V for the p-i-n diodes; /spl sim/3.5 V for the Schottky diodes). Both types of device showed a negative temperature coefficient for reverse breakdown, with value -0.34/spl plusmn/0.05 V/spl middot/K/sup -1/.

Depth and thermal stability of dry etch damage in GaN Schottky diodes

GaN Schottky diodes were exposed to N2 or H2 inductively coupled plasmas prior to deposition of the rectifying contact. Subsequent annealing, wet photochemical etching, or (NH4)2S surface passivation treatments were examined for their effect on diode current–voltage (I–V) characteristics. We found that either annealing at 750 °C under N2, or removal of ∼500–600 A of the surface essentially restored the initial I–V characteristics. There was no measurable improvement in the plasma-exposed diode behavior with (NH4)2S treatments.

Growth and Fabrication of GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor structure with Mg doping in the base and Si Doping in the emitter and collector regions was grown by Metal Organic Chemical Vapor Deposition in c-axis Al(2)O(3). Secondary Ion Mass Spectrometry measurements showed no increase in the O concentration (2-3x10(18) cm(-3)) in the AlGaN emitter and fairly low levels of C (~4-5x10(17) cm (-3)) throughout the structure. Due to the non-ohmic behavior of the base contact at room temperature, the current gain of large area (~90 um diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the mg acceptors in the base, and current gains of ~10 were obtained at 300 degree C.

High voltage GaN Schottky rectifiers

Mesa and planar GaN Schottky diode rectifiers with reverse breakdown voltages (V/sub RB/) up to 550 and >2000 V, respectively, have been fabricated. The on-state resistance, R/sub ON/, was 6 m/spl Omega//spl middot/cm/sup 2/ and 0.8 /spl Omega/ cm/sup 2/, respectively, producing figure-of-merit values for (V/sub RB/)/sup 2//R/sub ON/ in the range 5-48 MW/spl middot/cm/sup -2/. At low biases the reverse leakage current was proportional to the size of the rectifying contact perimeter, while at high biases the current was proportional to the area of this contact. These results suggest that at low reverse biases, the leakage is dominated by the surface component, while at higher biases the bulk component dominates. On-state voltages were 3.5 V for the 550 V diodes and /spl ges/15 for the 2 kV diodes. Reverse recovery times were <0.2 /spl mu/s for devices switched from a forward current density of /spl sim/500 A/spl middot/cm/sup -2/ to a reverse bias of 100 V.

GaN electronics for high power, high temperature applications

A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

Creation of high resistivity GaN by implantation of Ti, O, Fe, or Cr

Implantation of n- and p-type GaN with Ti+, O+, Fe+, or Cr+ was found to produce defect levels which pinned the Fermi level in these materials at EC−(0.20–0.49) eV (n type) or EV+0.44 eV (p type). Maximum sheet resistances of ∼1012 Ω/□ (n type) and ∼1010 Ω/□ (p type) were obtained after implantation and annealing in the range of 300–600 °C. At higher annealing temperatures, the sheet resistance decreased to near the unimplanted values (3×104 Ω/□ in p type, 7×102 Ω/□ in n type). The evolution of the sheet resistance with annealing temperature is consistent with damage-related trap sites removing carriers from the conduction or valence bands.

Al composition dependence of breakdown voltage in AlxGa1−xN Schottky rectifiers

Planar geometry, lateral Schottky rectifiers were fabricated on high resistivity AlxGa1−xN (x=0–0.25) epitaxial layers grown on sapphire substrates. The reverse breakdown voltages of unpassivated devices increased with Al composition, varying from 2.3 kV for GaN to 4.3 kV for Al0.25Ga0.75N. The reverse current–voltage (I–V) characteristics showed classical Shockley–Read–Hall recombination as the dominant mechanism, with I∝V0.5. The reverse current density in all diodes was in the range 5–10×10−6 A cm−2 at 2 kV. The use of p+ guard rings was effective in preventing premature edge breakdown and with optimum ring width increased VB from 2.3 to 3.1 kV in GaN diodes.

GaN n- and p-type Schottky diodes: Effect of dry etch damage

The reverse breakdown voltage (V/sub B/) and forward turn-on voltage (V/sub F/) of n- and p-GaN Schottky diodes were used to examine the effects of Cl/sub 2//Ar and Ar plasma damage. Even short plasma exposures (4 secs) produced large changes in both V/sub B/ and V/sub F/, with ion mass being a critical factor in determining the magnitude of the changes. The damage depth was established to be 500-600 /spl Aring/ and the damaged material could be removed in boiling NaOH solutions, producing a full recovery of the diode properties. Annealing at 700 to 800/spl deg/C under N/sub 2/ produced only a partial recovery of V/sub B/ and V/sub F/.

Temperature dependence and current transport mechanisms in AlxGa1−xN Schottky rectifiers

GaN and Al0.25Ga0.75N lateral Schottky rectifiers were fabricated either with (GaN) or without (AlGaN) edge termination. The reverse breakdown voltage VB (3.1 kV for GaN; 4.3 kV for AlGaN) displayed a negative temperature coefficient of −6.0±0.4 V K−1 for both types of rectifiers. The reverse current originated from contact periphery leakage at moderate bias, while the forward turn-on voltage at a current density of 100 A cm−2 was ∼5 V for GaN and ∼7.5 V for AlGaN. The on-state resistances, RON, were 50 mΩ cm2 for GaN and 75 mΩ cm2 for AlGaN, producing figures-of-merit (VRB)2/RON of 192 and 246 MW cm−2, respectively. The activation energy of the reverse leakage was 0.13 eV at moderate bias.