Observations of deep levels in 4H-SiC using optoelectronic modulation spectroscopy

Journal of Electronic Materials, Oct 2001 by Chiu, Chi-Hsin, Parmiter, P J M, Hilton, K, Uren, M J, Swanson, J G

Optoelectronic modulation spectroscopy (OEMS) has been used to reveal defect states in 4H-SiC. Pairs of magnitude and phase spectra have been used to infer whether they were electron or hole traps. Eleven discrete trap responses have been observed, eight were assigned as electron traps and three as hole traps. Five of these had been observed previously using optical admittance spectroscopy (OAS). An electron trap at 1.20 eV gave the most prominent response with a distinctive signature indicating that these traps were spatially delocalized with an extent of at least 1.35 run, possibly associated with an extended defect structure. An unresolved continuum of hole traps was seen between 2.3 eV and 2.9 eV. A response at 0.72 eV appeared to be the superimposed response of an electron and hole trap at closely similar energies. Consistency has been demonstrated with previous work using DLOS and OAS.

Key words: OEMS, 4H-SiC, defects, semiconductor, optical, hole trap, electron trap, delocalized trap, MESFET

INTRODUCTION

SiC has outstanding device potential for high power, high voltage, high temperature and high frequency applications. This arises because of key material advantages compared to conventional silicon or gallium arsenide semiconductors.1 In particular, the wide bandgap results in a breakdown field that exceeds silicon and gallium arsenide by an order of magnitude, the thermal conductivity is more than twice that of silicon, and the saturated velocity is around twice that of silicon. Rapid progress in devices is being made with initial commercial availability of SiC microwave MESFETs from Cree Inc. and SiC power devices demonstrated with performance exceeding those of silicon devices.2,3 However, although the SiC material quality is rapidly improving, it still has a large density of extended and point defects which can adversely affect device performance.4,5 The aim of this work has been to study 4H-SiC using an essentially optical method to observe the responses from deep level states. The method used is optoelectronic modulation spectroscopy6,7 which can reveal the optical transition energy, whether the state is a hole or an electron trap and provide an indication of its extent of localization. A very useful basis for comparison is provided by previous work on 6H-SiCB8 and 4H-SiC9,10 using optical admittance spectroscopy11 and with DLOS.12

OPTOELECTRONIC MODULATION SPECTROSCOPY

OEMS senses the effect of light of definite photon energy on an electrical parameter associated with a semiconductor device structure. In this modulation spectroscopy the photon energy is periodically modulated and it is the modulation of the electrical parameter that is measured. The magnitude is then displayed as a spectrum as the mean photon energy is scanned. If the response is not in phase with the variation of photon energy the phase as well as magnitude can be plotted to form a pair of related spectra. The method has been used in various modes to explore the optical responses of FET's, pn and Schottky diodes as well as simple resistors.6,7

In this study OEMS is applied to a 411-SiC MESFET in which channel conduction is determined by the thickness of the gate and backplane depletion regions, and by the carrier concentration and their average carrier mobility. If any of these parameters were affected by the incident light a change in channel current would be seen. In this work sub band-gap photons were used to permit penetration into the semiconductor in order to excite charges in deep defect states.

MODULATION MECHANISMS

The effect of possible channel current modulation mechanisms arising from energy modulated light has already been modeled and discussed in detail. 13 Reference should be made to the energy diagram in Fig. la. A model is assumed in which a single trap level is assumed to exist throughout the thickness of the active layer extending into the two depletion regions bounding the channel. Since sub band-gap illumination is to be used electron hole pairs will not be of concern.

In order to assess the significance of the channel current responses to the modulated photon energy it is necessary to consider the effect of photon energy modulation on the population of traps.

The population of the electron trap level would not be affected if the photon energy were smaller than the trap energy depth. For the sake of initial simplicity it will be assumed that the optical cross-section of the traps increases abruptly when the photon energy is equal to the trap depth and that it remains constant for higher photon energies. The result would be that the amount of trapped negative charge, Q^sub t^, would reduce to a new constant level when the photon energy exceeds the threshold value. When a small modulation of photon energy is superimposed and the mean photon energy is swept through the threshold energy there would be a maximum in the modulation of the trap population at the threshold energy. Above and below this value the modulation would fall to zero. Figure lb depicts the variation in modulation of the population as a negative spectral peak, corresponding to a reduction in the number of trapped electrons for positive increases in photon energy.