Anomalous photoresponse of GaN x-ray Schottky detectors

Abstract

GaN based materials are believed to be very stable materials, in particular, under irradiation by high energy photons such as x rays. We have studied x-ray detectors based on GaN Schottky diodes. Vertical Schottky diodes were fabricated based on a 20μm thick undoped GaN layer grown on a conductive GaN substrate. Their photoresponse to near UV light and to x rays was measured. While the response to near UV light was fast and linear as expected, anomalous behaviors were observed under x-ray illumination. The photocurrent increases as the third power of the incident x-ray flux. The photocurrent transient when the x rays is turned on are long and nonexponential (S shape) and strongly differs from the off transient which is fast and exponential. Also, a very strong quenching of the x-ray photoresponse is observed when the detector is simultaneously illuminated with visible light. All of these anomalous behaviors are explained in the frame of a complete model involving traps and tunnel currents. A reasonable quantitative agreement between the model and the experimental data is obtained.

Review of using gallium nitride for ionizing radiation detection

“…, and the CCE of the 12-lm device under a fluence of 10 16 cm À2 decreased from 53% to 20%. They suggested that the degradation is mainly due to an increased number of recombination/trapping centers.…”

“…4 shows the transient characteristics after the X-ray source was switched on and off at two different power levels. 16 X-ray detection has also been performed with a p-i-n structure, 22 but despite the different structures employed, a fast transient increase or decrease (of less than 1 s) followed by a slow exponential increase or decay is seen in all the measured photocurrents. According to Duboz et al, 15 this photocurrent characteristic can be explained in terms of two currents: photovoltaic and photoconductive.…”

“…16,17 The MSM structure was fabricated on 10-lm-thick undoped GaN grown by MOCVD and employed a Pt/Au Schottky contact with the finger width varying from 2 to 20 lm and the spacing varying from 2 to 10 lm. When the X-ray source was switched off, the signal showed a fast decrease in less than 1 s followed by a long exponential transient with a time constant of 40 s. The Schottky diode detector was fabricated on 20-lm homogeneous-growth, undoped GaN with a Pt/Au Schottky contact area that varied between 1 and 2 mm 2 .…”

“…11 The article concluded that GaN is a very promising candidate for use in such experiments, despite it still being a relatively immature semiconductor material. Subsequent studies have further demonstrated that GaN-based sensors can detect a-particles [12][13][14] and X-rays, [15][16][17] indicating the growing potential use of GaN for ionizing radiation detection. Furthermore, a study investigating alternative materials for neutron detection, driven by the shortage of 3 He gas, has considered the use of GaN for neutron sensors in harsh environments.…”

“…For a-particle detectors, the lateral double-Schottky contact (DSC), [8][9][10] mesa, 6,12,13,19 sandwich, 14,20 and p-i-n structures 21 have been tested. For X-ray detectors, Schottky Metal-Semiconductor-Metal (MSM), 15 Schottky diode, 16,17 and p-i-n structures 22 have all been reported. For electron detection, the applications for making betavoltaic energy converters, 23,24 using pn, 25 p-i-n, 23,[26][27][28] and Schottky type 29 structures have now been tested.…”