Low-frequency oscillations in semi-insulating GaAs and InP: deep level domain spectroscopy

Abstract

Low-frequency-current oscillations in semi-insulating GaAs and InP, believed to be caused by the transit of high-field domains produced by the field-enhanced trapping of electrons, were investigated with the aim of finding the best method of extracting information on deep levels from the temperature dependence of the power spectra of the current, a procedure known as "deep-level domain spectroscopy." With low voltages applied to our specimens of undoped GaAs the conduction was "ohmic", the oscillations were fairly coherent over a range in temperature of about 300–350 K, and since the peaks in the spectra all had the same activation energy within experimental scatter, they were concluded to be harmonics determined by the shape of the high-field domains, with the activation energy corresponding to the trap causing the high-field domains. For higher voltages (above a "knee" in the l–V curve), the oscillations were much larger in amplitude and complex. As in our previous work with InP, the spectra after baseline removal contained peaks with differing activation energies. These peaks are believed to be due to deep levels other than those involved in the field-enhanced trapping process (EL2 in GaAs and an Fe-related level in InP:Fe). Rather than conclude that several traps independently cause high-field domains, it is suggested that the occupancy of such traps will be modulated by the passage of a high-field domain and this will produce the observed peaks. A simplified model is treated analytically. In practice, peaks were observed with differing activation energies only when the oscillations were incoherent and therefore had a broad spectral content. The oscillations occur at sufficiently low average fields to be a potentially serious problem for integrated circuits made on semi-insulating substrates. The maximum entropy (or "all poles") method was compared with windowed fast Fourier transform methods for use in deep-level domain spectroscopy. Results are presented for specimens of undoped and Cr-doped GaAs, and for Fe-doped InP.