A. Sampath

High reflectivity and crack-free AlGaN/AlN ultraviolet distributed Bragg reflectors

High reflectivity and crack-free ultraviolet distributed Bragg reflectors (DBRs), based on AlGaN/AlN quarter-wave layers, have been designed and grown on (0001) sapphire by plasma-assisted molecular beam epitaxy. To minimize the tensile stress and thus prevent nucleation and propagation of cracks in the DBRs, the substrate was coated first with an AlN film, of thickness approximately equal to the total thickness of all the AlN layers in the epitaxially grown DBR structure. In such a configuration the AlN layers are stress-free, while the AlGaN layers are under compressive stress. Peak reflectivity higher than 99% was obtained in all samples. The design calculations and simulations of measured reflectivity data were performed using the transmission matrix method. The measured reflectivity spectra have a bandwidth of around 20% smaller than the simulated spectra, a result attributed to a combination of changes of the thickness and variations in the Al content in the quarter-wave layers.

Surface electronic structure of p-type GaN(0001̄)

The surface electronic structure of p-type wurtzite GaN(0001) 1×1 grown by molecular beam epitaxy has been investigated using synchrotron radiation excited angle resolved photoemission spectroscopy. Four discrete surface states were clearly identified and characterized. All the states were removed by exposure of the surface to atomic hydrogen. Three of the states were of pz orbital character, while the fourth is derived from s orbitals. Three of the surface states lie below the bulk valence band maximum throughout the surface Brillouin zone, and one surface state was observed to disperse into the bulk optical band gap. Comparison with theory suggests that this surface state is Ga-derived, consistent with a model of Ga-terminated, but N polar, GaN. The other three surface states compare well with states observed earlier from n-type GaN.

Growth and doping of GaN directly on 6H-SiC by MBE

The authors report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2 x 2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin, and the majority of recombination occurs through D-A transitions at 3.26 eV.

Metal Contacts to n- Al X Ga 1-x N

In this paper we report on the formation of ohmic contacts to n- Al x Ga 1-x N alloys. The films were produced by plasma-assisted MBE and doped n- type with silicon at doping levels between 10 18 to 10 19 cm -3 . Contacts were formed by sequential deposition of 200 A of Ti and 2000 A of Al and the contact resistivities were determined from TLM measurements. For low Al- content (x -4 to 10 -5 cm 2 . The contacts become progressively non-ohmic at Al concentrations greater than 10%. There results are consistent with the Schottky limit being applicable to these alloys and thus the Ti/Al contact forms Schottky barriers with higher barrier height as the conduction band of the alloy moves towards the vacuum level.

GaN photodiodes grown by MBE on HVPE and ELO-HVPE GaN\sapphire substrates

In this paper, we report on the growth by molecular beam epitaxy (MBE), the fabrication and the characterization of GaN diodes on HVPE n+-GaN/sapphire and ELO-HVPE n+-GaN/sapphire substrates. Specifically, such diodes were fabricated in the form of vertical schottky diodes or p-n junctions. In both cases we have seen a dramatic decrease in the leakage current in the reverse direction which is consistent with the reduction of threading dislocations in the active area of the device. The lowest leakage current measured at -5 V bias was approximately 10-8 A/cm2 for p-n junctions grown on ELO-HVPE n+-GaN/sapphire substrates. The spectral response of the vertical schottky diodes were evaluated and compared to similar devices grown wholely by MBE on sapphire substrates. The device grown on HVPE n+-GaN/sapphire substrate shows nearly ideal responsivity below 355 nm but also poorer visible light rejection than the fully grown MBE device. The observed exponential tail in the spectral response of the vertical schottky grown on the HVPE n+-GaN/sapphire substrate is attributed to the absorption and collection in the thick n+ GaN substrate.

A Comparative Study Of GaN Diodes Grown by MBE on Sapphire and HVPE-GaN /Sapphire Substrates

In this paper we report on the fabrication and characterization of GaN diodes (Schottky and p-n junctions) grown by plasma assisted MBE. We observed that Schottky diodes improve both in reverse as well as forward bias when deposited on 5 μm thick HVPE n + -GaN/sapphire instead of bare sapphire substrates. These improvements are attributed to the reduction of disloctions in the MBE homoepitaxially grown GaN. Similar benefits are observed in the reverse bias of the p-n junctions which according to EBIC measurements are attributed to the reduction of etch pits in the MBE grown p-GaN.

GaN Schottky diode ultraviolet detectors, grown by molecular beam epitaxy

Vertical geometry Schottky barrier photodiodes have been fabricated on n-GaN films grown by molecular beam epitaxy (MBE). Vertical mesas were fabricated by RIE and Schottky barriers were achieved by depositing Ni/Pt/Au metal contacts. I-V measurements show near ideal diode behavior, with reverse saturation current density of 1 X 109 A/cm2. Doping concentration and barrier height were determined to be 9 X 1016 cm-3 and 1.0V respectively, using C-V measurements. The diodes were then evaluated as UV photodiodes. The responsivity was measured to be 0.18A/W, corresponding to a quantum efficiency of 70 percent. Spectral response showed a sharp transition at 365 nm, and more than five orders of magnitude visible light rejection. Low frequency noise measurements indicate that 1/f noise is the dominant source of noise. The detectivity was determined to be 1.3 X 10-9 W/Hz1/2.

Generation Recombination Noise in GaN Photoconducting Detectors

Low frequency noise measurements are a powerful tool for detecting deep traps in semiconductor devices and investigating trapping-recombination mechanisms. We have performed low frequency noise measurements on a number of photoconducting detectors fabricated on autodoped n-GaN films grown by ECR-MBE. At room temperature, the noise spectrum is dominated by 1/f noise and thermal noise for low resistivity material and by generation-recombination (G-R) noise for high resistivity material. Noise characteristics were measured as a function of temperature in the 80K to 300K range. At temperatures below 150K, 1/f noise is dominant and at temperatures above 150K, G-R noise is dominant. Optical excitation revealed the presence of traps not observed in the dark, at room temperature.