H. Cho

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.

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.

High density plasma via hole etching in SiC

Throughwafer vias up to 100 μm deep were formed in 4H-SiC substrates by inductively coupled plasma etching with SF6/O2 at a controlled rate of ∼0.6 μm min−1 and use of Al masks. Selectivities of >50 for SiC over Al were achieved. Electrical (capacitance–voltage: current–voltage) and chemical (Auger electron spectroscopy) analysis techniques showed that the etching produced only minor changes in reverse breakdown voltage, Schottky barrier height, and near surface stoichiometry of the SiC and had high selectivity over common frontside metallization. The SiC etch rate was a strong function of the incident ion energy during plasma exposure. This process is attractive for power SiC transistors intended for high current, high temperature applications and also for SiC micromachining.

Relative merits of Cl2 and CO/NH3 plasma chemistries for dry etching of magnetic random access memory device elements

A typical magnetic random access memory stack consists of NiFe/Cu/NiFeCo multilayers, sandwiched by contact and antioxidation layers. For patterning of submicron features without redeposition on the sidewalls, it is desirable to develop plasma etch processes with a significant chlorinated etch component in addition to simple physical sputtering. Under conventional reactive ion etch conditions with Cl2-based plasmas, the magnetic layers do not etch because of the relatively involatile nature of the chlorinated reaction products. However, in high ion density plasmas, such as inductively coupled plasma, etch rates for NiFe and NiFeCo up to ∼700 A min−1 are achievable. The main disadvantage of the process is residual chlorine on the feature sidewalls, which can lead to corrosion. We have explored several options for avoiding this problem, including use of in situ and ex situ cleaning processes after the Cl2-etching, or by use of a noncorrosive plasma chemistry, namely CO/NH3. In the former case, removal of th...

Via-hole etching for SiC

Four different F2-based plasma chemistries for high-rate etching of SiC under inductively coupled plasma (ICP) conditions were examined. Much higher rates (up to 8000 A min−1) were achieved with NF3 and SF6 compared with BF3 and PF5, in good correlation with their bond energies and their dissociation efficiency in the ICP source. Three different materials (Al, Ni, and indium–tin oxide) were compared as possible masks during deep SiC etching for through-wafer via holes. Al appears to produce the best etch resistance, particularly when O2 is added to the plasma chemistry. With the correct choice of plasma chemistry and mask material, ICP etching appears to be capable of producing via holes in SiC substrates.

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.

Effect of additive noble gases in chlorine-based inductively coupled plasma etching of GaN, InN, and AlN

The effects of additive noble gases, He, Ar and Xe to chlorine-based inductively coupled plasmas (ICPs) for etching of GaN, AlN and InN were studied in terms of etch rate and selectivity. The etch rates were greatly affected by the chlorine concentration, rf chuck power and ICP source power. The highest etch rates for InN were obtained with Cl2/Xe, while the highest rates for AlN and GaN were obtained with Cl2/He. It was confirmed that efficient breaking of the III-nitrogen bond is crucial for higher etch rates. The InN etching was dominated by physical sputtering; the GaN and AlN etch rates were limited by initial breaking of the III-nitrogen bond. Maximum selectivities of ∼80 for InN to GaN and InN to AlN were obtained with the Cl2-based discharges.

Surface and bulk leakage currents in high breakdown GaN rectifiers

Abstract GaN Schottky diode rectifiers with contact diameters 125–1100 μm were fabricated on thick (4 μm) epi layers. At low reverse bias voltages the leakage current was proportional to contact perimeter size while at voltages approximately half the breakdown value, the reverse current was proportional to contact area. These results suggest that surface leakage dominated at low biases, while at higher biases the main contribution was from bulk leakage. The reverse leakage currents were several orders of magnitude higher than the theoretical values, while the forward turn-on voltages were approximately a factor of two higher than the theoretical value.

GaN/AlGaN HBT fabrication

Abstract Discrete GaN/AlGaN heterojunction bipolar transistors (HBTs) were fabricated on material grown by both metal organic chemical vapor deposition and molecular beam epitaxy. For both types of material, DC current gains of ∼10 were achieved in 90 μm emitter diameter devices measured at 300°C. Some of the key processing steps, such as ohmic contact annealing temperature and mesa fabrication by low damage dry etching, are described, together with secondary ion mass spectrometry measurements of the dopant and background impurity profiles.