POSITRON DEEP-LEVEL TRANSIENT SPECTROSCOPY - A NEW APPLICATION OF POSITRON-ANNIHILATION TO SEMICONDUCTOR PHYSICS

Recent positron mobility and lifetime measurements made on ac-biased m etal on semi-insulating GaAs junctions, which have identified the nati ve EL2 defect through a determination of the characteristic ionization energy of the donor level, are reviewed. It is shown that these measu rements point towards a new spectroscopy, tentatively named positron-D LTS (deep level transient spectroscopy), that is the direct complement to conventional DLTS in that it monitors transients in the electric f ield of the depletion region rather than the inversely related depleti on width, as deep levels undergo ionization. In this new spectroscopy, which may be applied to doped material by use of a suitable positron beam, electric field transients are monitored through the Doppler shif t of the annihilation radiation resulting from the drift velocity of t he positron in the depletion region. Two useful extensions of the new spectroscopy beyond conventional capacitance-DLTS are suggested. The f irst is that in some instances information on the microstructure of th e defect causing the deep level may be inferred from the sensitivity o f the positron to vacancy defects of negative and neutral charge state s. The second is that the positron annihilation technique is intrinsic ally much faster than conventional DLTS with the capability of observi ng transients some 10(6) times faster, thus allowing deep levels (and even shallow levels) to be investigated without problems associated wi th carrier freeze-out.