Mira Misra

PHOTOCONDUCTIVE DETECTORS BASED ON PARTIALLY ORDERED ALXGA1-XN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

Photoconductive detectors based on partially ordered AlxGa1−xN alloys with AlN mole fractions up to 45% were fabricated and evaluated. The degree of ordering in these alloys was found to increase with the AlN mole fraction and it has a maximum value at about 50%. The resistivity of the AlxGa1−xN films was found to increase from 10 to 108 Ω cm by increasing the Al content in the films. Correspondingly, the mobility-lifetime (μτ) product, which was determined by measuring the photoconductive gain, was found to decrease from 10−2 to 10−5 cm2/V. These high values of the μτ product at the high AlN mole fraction are attributed to spatial separation and indirect recombination of the photogenerated electron hole pairs, due to band-gap misalignment of the ordered and disordered domains in these films.

Photoconducting ultraviolet detectors based on GaN films grown by electron cyclotron resonance molecular beam epitaxy

We report for the first time, fabrication of photoconducting UV detectors made from GaN films grown by molecular beam epitaxy. Semi-insulating GaN films were grown by the method of electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy. Photoconductive devices with interdigitated electrodes were fabricated and their photoconducting properties were investigated. In this paper we report on the performance of the detectors in terms of UV responsivity, gain-quantum efficiency product, spectral response, and response time. We have measured responsivity of 125A/W and gain-quantum efficiency product of 600 at 254nm and 25V. The response time was measured to be on the order of 20ns for our detectors, corresponding to a bandwidth of 25Mhz. The spectral response showed a sharp long-wavelength cutoff at 265nm, and remained constant in the 200nm to 365nm range. The response of the detectors to low-energy x-rays was measured and found to be linear for x- rays with energies ranging from 60kVp to 90kVp.

Origin of the high photoconductive gain in AlGaN films

In this paper we report the properties of photoconductive detectors fabricated on GaN and AlGaN films produced by plasma assisted MBE. The spectral dependence of such devices shows a sharp increase over many orders of magnitude at the gap of the semiconductor but it remains constant at shorter wavelengths consistent with absence of surface recombination. The mobility-lifetime product, which is the intrinsic figure of merit of the photoconductive gain, decreases monotonically with the resistivity of GaN films. This result is attributed to the existence of exponential tails due to potential fluctuations arising from stacking faults, point defects and impurities. In the case of AlGaN alloys similar dependence of the mobility-lifetime product on film resistivity has been observed. However, the mobility-lifetime product for films with AlN mole fraction close to 50% is about two orders of magnitude higher than that of GaN films with comparable resistivity. This result was accounted for by the longer lifetime of the photogenerated carriers due to the partial atomic ordering in these alloys. The band structure of the ordered and random domains form a type-II heterostructure and thus photogenerated electrons and holes in these detectors are physically separated, leading to an increase in recombination lifetime.

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.