Non-Contact Characterization of Recombination Processes in 4H-SiC
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Non-Contact Characterization of Recombination Processes in 4H-SiC K. Matocha, T.P. Chow, R.J. Gutmann Center for Integrated Electronics and Electronics Manufacturing Rensselaer Polytechnic Institute, Troy, NY USA
Abstract Carrier decay transients in 4H-SiC n-type and p-type epilayers have been characterized using a non-destructive, non-contact microwave photoconductivity technique. Decay transients show a two-stage exponential decay with rst decay constants as high as 400 ns in 10 m p-type epilayers. The second decay constant increases with temperature and is dominated by interface recombination.
1 Introduction Silicon Carbide (SiC) has interesting electronic properties, namely high breakdown eld strength, high thermal conductivity, low intrinsic carrier concentration, and high saturation velocity. Silicon carbide has an indirect band structure and can be formed in several polytypes, each with a di erent bandgap. 4H-SiC, with a bandgap of 3.26 eV and an critical eld of 2.0 MV/cm, is available in high-quality wafers and is being investigated for use in high-voltage, high-frequency, and high-temperature devices. Carrier lifetime is a critical parameter which a ects the performance of bipolar devices and provides a measure of material and processing quality. Carrier recombination processes in SiC polytypes have been studied using photoluminescence decay[1, 2], free-carrier absorption[3, 4, 5, 6], and diode switching[7, 8]. Microwave photoconductivity decay[9] has been used to study 3C-SiC carrier recombination. This study examines the carrier recombination mechanisms in 4H-SiC using a non-destructive, non-contact, microwave photoconductive decay technique.
2 Experimental setup The measurement apparatus consists of a Gunn diode oscillating at 35 GHz, a 3 dB hybrid coupler, a crystal detector, and a rectangular to ridge waveguide adapter con gured as shown in Figure 1. The microwave signal couples to the sample through an open-ended 1 T4.4.1
Gunn diode
Laser beam
Xtal det.
sample
sliding short
variable attenuators
Figure 1: Lifetime measurement system block diagram. ridge waveguide. The microwave re ection provides a signal representing the conductivity of the semiconductor sample. The conductivity of the epilayer is modulated by a 266 nm pulsed laser beam from the fourth harmonic generation of a Nd:YAG laser having a nominal pulse width of 15 ns and decay time of 4 ns. The absorption depth of 266 nm radiation in 4H-SiC has been estimated to be less than 1 m[10]. The attenuator/sliding-short leg of the bridge can be used to null the output of the detector, or the leg can be fully attenuated. When the bridge is fully attenuated, the decay time represents the actual decay of the microwave signal. However, when the bridge is nulled, the decay measured by the oscilloscope represents half the decay time of the microwave signal. The bridge null is typically used to prevent the power entering the detector from exceeding the square law regime of the detector. Measurements performed using both the nulled and attenuat
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