Transient distortion and nth order filtering in deep level transient spectroscopy (D nLTS)

Abstract

Problems associated with intrinsic sensitivity and time constant selectivity of capacitive DLTS systems are discussed as well as intrinsic limitations in predominantly deep level doped semiconductors. A previously published figure of merit for the optimized exponential correlator for capacitive DLTS studies is shown to be too high because of improper consideration of effects of level restoration: when correctly compared, a filter with a 12% lower figure of merit can be constructed based purely on gating and a weighted phase inversion before the integrator and a phase sensitive detector has a 20% lower figure of merit. The exponential DLTS correlator is also inadequate for analysis of continuous spectra because of its slow drop off in response (∝ T S for T S shorter than the peak response time constant and ∝ T S −1 for longer times). Blanking is necessary to achieve more selectivity relative to short time constants. When performed on-line, D 2LTS gives a response ∝ T S −2 for longer times. Still more selective filters of order n, or D nLTS, are considered based on weighted averages over time intervals in geometric progression that are suitable for DLTS and a system with ±1 weighting and suitably chosen time intervals for use with DDLTS. For these filters there is no penalty in figure of merit associated with choice of DDLTS which also appears to be easier to achieve than DLTS. On-line filters with long time constant responses ∝ T S −3 or higher order are shown to exact a large penalty in figure of merit. Equivalent filters can, however, be synthesized with much better figure of merit by a software compensation of multichannel data. The channels are then selected so that the responses of successive channels peak a factor of 2 away in T S and the number of pulses used is decreased by a factor of two. Relative to a multipoint averager, the software compensated analyzer requires a factor of 100 less in measurement time for comparable accuracy when a spectrum must cover a range of 1000:1 in relaxation times. There are also comparable improvements in the holding time specifications and the number of A/ D conversions if the system is to be coupled to a computer. When the deep level concentration is comparable to the shallow doping concentration, the peak responses of both DLTS and DDLTS are broadened, the sensitivity increases, the response peaks are shifted to larger relaxation times, and the system becomes essentially nonlinear in response to the individual deep level constituents. This distortion can, in principle, be corrected by going to a constant capacitance mode of operation both during the initial driving pulse and during the recovery transient.