Electrical characterization of electron irradiated X-rays detectors based on 4H-SiC epitaxial layers

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Abstract

In this work, we report and discuss the results of minority carriers diffusion length measurements carried out in order to quantify the radiation damage induced by electron irradiation of X-rays detectors based on a metal–silicon carbide junction. The devices examined consisted of 4H-SiC n-type epitaxial layers on 4H-SiC n+-type substrate wafers, on which Schottky contacts were formed by gold deposition. The minority carriers diffusion length, extracted from the results of photocurrent measurements using a fitting procedure, was shown to exhibit a strong decrease with the radiation dose.

Introduction

The potential impact of silicon carbide (SiC) in high power and high temperature electronic devices and integrated circuits has been considered in recent times, with considerably successful applications [1], [2]. In the last few years, SiC has been also considered for the realization of neutron and charge particle detectors and spectrometers, showing good performances and the potentiality of operating in hard environments, due to its intrinsic properties, which makes it potentially superior to other semiconductor materials, such as silicon and gallium arsenide [3]. This is, in particular, related to its wide band gap, resulting in low leakage current, even at high operating temperature, and to high displacement energy, making SiC detectors less susceptible to bulk radiation damage effects. Moreover, recent works have demonstrated that a metal–SiC junction shows extremely low current densities [4], even at room temperature, and this feature is very favourable for the realization of low-noise X-ray semiconductor detectors [5]. This work is aimed at investigating the radiation damage effects on electron irradiated X-rays detectors based on a gold/silicon carbide junction by monitoring the minority carriers diffusion length (Ld) as function of the radiation dose. In fact, it is well known [6] that defects in semiconductor materials, such as those induced by radiation damage, create recombination centers which reduce the local generation/recombination lifetimes of charge carriers, thus affecting their diffusion lengths. Large defect densities, therefore, severely limit the performance of minority carrier devices such as p-n junctions and photodetectors and they also deteriorate the performance of majority carrier devices such as MESFETs and MOSFETs, by contributing to the sub-threshold current and deteriorating the carrier mobility.

There are many different methods to measure or extrapolate the carrier diffusion lengths [7]. Here, photocurrent measurements have been employed to determine the minority carriers diffusion length of all the electron irradiated samples.

Section snippets

Experimental details

The detectors have been realized using heavily N-doped (approx. 1018 cm−3) single-crystal 4H-SiC wafers, 35 mm in diameter and 363 μm in thickness (supplied by CREE Research Inc.), as substrates, on which n-type, 30-μm thick layers with a net nitrogen concentration of 2.5×1015 cm−3 have been deposited. The metal/SiC junction has been prepared at Alenia–Marconi System by high vacuum e-beam deposition of a circular Au Schottky contact (2 mm in diameter and 1000 Å in thickness) on the epitaxial

Theoretical basis for the determination of the minority carriers diffusion length (Ld)

The first step to obtain the minority carriers diffusion length from photocurrent measurements is the determination of the photogenerated minority carriers concentration, in the case schematically illustrated in Fig. 2 [7].

A chopped light beam, with variable wavelength, probes a collector junction, consisting of a semitransparent Schottky diode. The short-circuit photocurrent is collected while varying the depth of the region where the carrier generation occurs. In particular, referring to Fig.

Results and discussion

The results of a typical photocurrent measurement as a function of the penetration depth (where z>0.9 μm, which is the xd value calculated [10] for the present experimental conditions and confirmed by CV measurements [11]) are reported in Fig. 3 for the sample G7. As in the case of the other irradiated samples, the average value of the collected photocurrent is of the order of 10−11 A, while the corresponding value of the virgin sample is instead of approximately 10−9 A. The black squares in

Conclusions

An experimental method and a fitting procedure was developed to obtain the minority carriers Ld values from photocurrent measurements in the case of SiC. The experimental results show a good correlation between the decrease of the diffusion length and the increase of the irradiation dose and a non-negligible radiation damage recovery effect by low temperature annealing, which requires further investigations.

Acknowledgements

The authors wish to thank C. Lanzieri of Alenia–Marconi System for the device production, and A. Arcari for his technical support in the photocurrent measurement system assembly. This work has been carried out with the financial support of the project COFIN 2001.

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