R. Hickman

ULTRAVIOLET-SENSITIVE, VISIBLE-BLIND GAN PHOTODIODES FABRICATED BY MOLECULAR BEAM EPITAXY

GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.

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.

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.

300°C GaN/AlGaN Heterojunction Bipolar Transistor

A GaN/AlGaN heterojunction bipolar transistor has been fabricated using Cl 2 /Ar dry etching for mesa formation. As the hole concentration increases due to more efficient ionization of the Mg acceptors at elevated temperatures (> 250°C), the device shows improved gain. Future efforts should focus on methods for reducing base resistance, which are briefly summarized.

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.

GaN PN junction issues and developments

Critical nitride-based p-n junction issues relating to wide bandgap bipolar device performance include minority carrier lifetime, defect related current characteristics and ohmic contact properties. Recent developments in p-GaN deposition processes resulted in GaN p-i-n UV photodetectors with improved deep UV responsivity, visible light rejection and shunt resistance characteristics. From the device data, the electron diffusion length in p-GaN doped at 1·1018 cm−3 was estimated to be 790 A, and the minority carrier lifetime in the p-GaN was estimated to be 24 ps to 0.24 ns. Improved junction electrical characteristics were achieved using MBE deposition on GaN buffers grown by MOCVD. NiAu ohmic contacts were also made to p-GaN with specific contact resistances less than 10−4 Ω·cm2.

Inductively coupled plasma damage in GaN Schottky diodes

The effects of H2 or N2 plasma exposure on the current–voltage characteristics of GaN Schottky diodes were examined as a function of source power and rf chuck power. Under all conditions there was a strong reduction in diode reverse breakdown voltage and an increase in forward and reverse currents. The results are consistent with creation of a thin (⩽600 A) n-type conducting surface region after ion bombardment of the GaN surface. Much of the degradation in diode quality can be recovered by annealing in N2 at 750 °C.

Molecular beam epitaxy grown InGaN multiple quantum well structures optimized using in situ cathodoluminescence

In this work, in situ cathodoluminescence (CL) is presented as a technique to optimize the molecular beam epitaxy (MBE) growth conditions for InGaN films and structures. InGaN was grown at 1 μm/h using a reactive nitrogen rf plasma source at substrate temperatures ranging from 550 to 650 °C. The quick determination of the emission wavelength and quality from the peak position and width allowed various growth conditions and structures to be tried without removal of the sample from the MBE system. CL scans are presented from samples grown under varying Ga/In flux ratios, III/nitrogen flux ratios, and substrate temperatures showing the usefulness of in situ monitoring for MBE InGaN growth.

Visible Blind uv GaN Photovoltaic Detector Arrays Grown by rf Atomic Nitrogen Plasma MBE

RF atomic nitrogen plasma molecular beam epitaxy (MBE) was used to deposit gallium nitride (GaN) p-i-n junction photovoltaic detectors on (0001) sapphire. The detectors consisted of a bottom contact layer n -type silicon doped to 5 × 10 18 cm −3 . The intrinsic layer was undoped and possessed an n -type background carrier concentration of 1 × 10 16 cm −3 . The top / p -GaN layer was doped with magnesium to give a Hall concentration of 5 × 10 17 cm −3 . The p -type GaN cathodoluminescence (CL) spectra showed a strong 372 nm emission level in contrast to the 430 nm level observed in MOCVD samples. These layers were fabricated into 1 × 10 element detector arrays using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. Peak responsivity of 0.11 AAV on detectors without anti-reflection coating were obtained at the GaN bandedge of 360 nm. The ultraviolet (UV) to visible rejection ratio was greater than 10 3 − 10 4 and was accredited to the reduction of the yellow defect levels in MBE material. Preliminary results on Al x Ga 1− x N detectors with responsivity peaks at 313 and 343 nm are presented as well.

Effect of thermal stability of GaN epi-layer on the Schottky diodes

Abstract The effect of annealing on the behavior of GaN Schottky diodes was investigated. Rapid thermal annealing experiments were performed in N 2 ambients for 30 s at temperatures of 500–900°C. After annealing, each sample was chemically treated using ozone/HCl to remove the thermally damaged layer prior to deposition of the metal. Samples were examined by current–voltage measurements and scanning electron microscopy or atomic force microscopy to monitor electrical and structural property changes. Only annealing at temperatures ⩾900°C significantly degraded the Schottky diode characteristics. This degradation is believed to be due to the preferential loss of surface nitrogen. Ozone/HCl surface chemical treatments were only partially successful in repairing the thermal damage.