OPTICAL TRANSIENT CURRENT SPECTROSCOPY OF DEEP LEVELS IN SEMIINSULATING INDIUM-PHOSPHIDE

Deep levels in iron-compensated semi-insulating InP were investigated by a form of digital optical transient current spectroscopy (OTCS). The decay of the transient photocurrent following periods of illumination was analyzed by numerical method not previously applied to the problem. This could resolve a transient into a sum of exponential decays with time constants spanning a range of 10(4). Each time constant corresponds to the release of electrons or holes from a particular deep level. The temperature dependence gave information on the activation energy of the corresponding deep levels. By digitizing the entire transient, rather than recording two points, or their difference, as in the original OTCS method, only one sweep of temperature is required. Since the present algorithm determines the amplitudes and time constants of the exponential decays simultaneously, base-line induced artifacts and related problems encountered in previous work are avoided. The temperature range was limited by excessive dark current above about 340 K, and by "negative transients" and low-frequency oscillations below about 260 K. In the accessible range, nine deep levels were resolved. One level, previously unobserved in any type of InP, may be due to phosphorus vacancies. Of the remaining eight levels, one had previously been identified by OTCS in semi-insulating InP. The remaining seven had not previously been detected in semi-insulating InP but had activation energies close to levels previously found using capacitance deep level spectroscopy in doped (i.e., conducting) material. "Negative transients" are defined as transients in which the current initially undershoots the steady-state value. Low frequency oscillations were observed in the same range of temperature, depending upon the applied voltage. It is tentatively proposed that a dependence on voltage of both positive and negative transients and of the low-frequency oscillations arises from field-enhanced trapping. The oscillations are attributed to traveling high-field domains caused by hot-electron trapping following the original Ridley model.