Gallium Nitride (GaN) with its large bandgap, high electron saturation velocity, and critical electric field has found many applications in illumination, high frequency and high-power devices. Recently, researchers have started exploring radiation sensing as a probable application for GaN devices on the virtue of the high Displacement Energy (ED) of GaN (20 eV) which is twice of any other popular semiconductors such as GaAs and Si. GaN-based radiation sensors can improve the functioning of the detector by improving the lifetime of the detector, operational temperature range and in detection of higher energies. These advantages open new avenues for applications in high energy particle research, imaging tools in medicine, nuclear reactors, arial radiation detection and surveillance. In short, the development of GaN radiation sensors could transform the whole solid-state radiation sensors industry.
Development of GaN Radiation Sensors
In view of the requirement of solid-state radiation sensors researcher have explored Si and GaAs as possible candidates which have demonstrated high energy detection at low voltages making them ideal for portable α-particle detectors. Although commercially available solid-state detectors are able to detect high energies, they have a low lifetime and their reliability at high temperatures is questionable. Recently developed GaN-based α-particle detectors have been explored to overcome both these disadvantages due to its higher ED. Up till now, different research groups have successfully fabricated GaN Schottky Barrier Diodes (SBD) and tested them in alpha particle detection. While SBDs with thin epitaxial layers (<12 μm) could only detect low energy alpha particles (<4.5 MeV) at -120 V, SBDs on bulk GaN substrate required very high voltage (-550V) to detect higher energies (5.48 MeV). However, both bulk GaN SBDs and SBDs with thin epitaxial layers show extremely poor performance at low voltages. The poor performance of GaN-based radiation detector is due to high Threading Dislocation Density (TDD) and limited Depletion Width (DW). While high TDD increases leakage current (IR) thus reducing the sensitivity of the detector, thin DW restricts the maximum energy which can be detected by a GaN detector.
Mg-compensated GaN SBD
In Nanyang Technological University in Singapore, to improve the portability of GaN-based solid state α-particle detectors, a research team lead by Prof. Ng Geok Ing in collaboration with Prof. Hiroshi Amano from Nagoya University in Japan and Dr. John Kennedy from GNSS in New Zealand have worked on reducing the voltage requirement by employing a first of its kind Mg-compensated GaN SBD as a detector. The use of this novel compensated structure has enabled the formation of a thick DW even at low voltages. The thick DW has helped improve the reverse bias electrical characterization of the diodes by reducing IR by 6 orders of magnitude and improving the reverse breakdown voltage to -2400 V  which is the highest reported breakdown voltage on a vertical GaN SBD. Apart from the improvements in electrical characterization, the devices have also helped detect α-particles of 5.48 MeV with 65% charge collection efficiency (CCE) (at -20 V) which is 30% higher than previously reported value . Compensated GaN-based detectors were also able to detect α-particle radiation with 100% CCE at -300 V which is 250 V lower than previous reports. A detailed explanation on the development and working of these novel GaN-based α-particle detectors can be obtained in a review paper . These promising results have demonstrated great potential of GaN-based α-particle detectors in radiation sensing applications.
- Sandupatla, ; Arulkumaran, S.; Ng, G.I.; Ranjan, K.; Deki, M.; Nitta, S.; Honda, Y.; Amano, H. GaN drift-layer thickness effects in vertical Schottky barrier diodes on free-standing HVPE GaN substrates. AIP Adv., vol. 9, p. 045007, 2019.
- Sandupatla, S. Arulkumaran, K. Ranjan, N. G. Ing, P. Murmu, J. Kennedy, S. Nitta, Y. Honda, M. Deki and a. H. Amano, “Low voltage high-energy α-particle detectors by GaN-on-GaN Schottky Diodes with record-high charge collection efficiency,” Sensors, vol. 19, no. 23, p. 5107, 2019.
- Sandupatla, A., Arulkumaran, S., Geok, N., Nitta, S., Kennedy, J., & Amano, H. (2020). Vertical GaN-on-GaN Schottky Diodes as α-Particle Radiation Sensors. Micromachines, vol. 11, no. 5, p. 519, 2020.
Figure 1: Applications of α-particle detectors
Figure 2: Benchmarking of extracted CCE of our detectors with epitaxial-grown GaN detectors (squares) and bulk GaN detectors (triangles) at low voltages
ABOUT THE AUTHOR
Prof. Ng Geok Ing
School of Electrical and Electronic Engineering,Nanyang Technological University
School of Electrical and Electronic Engineering, Nanyang Technological University
Dr. Subramaniam Arulkumaran
Temasek Laboratories@NTU, Nanyang Technological University