Evidence of Normal EL2 State Changes in the Temperature Range 120 to 150 K in Semi‐Insulating GaAs

Abstract

The main deep donor EL2 in si GaAs can be transformed to a metastable state below 120 K. The metastable EL2* was assumed to be electrically and optically nonactive and totally filled with electrons [l]. Thus the EL2 4 EL2* transition leads to a change of sample compensation [2, 31 and gives the possibility to observe electron and hole traps independently in thermally stimulated measurements [4 to 61.EL2 can be transformed to EL2* exclusively below the temperature region 120 to 150 K. Thus, the normal states of EL2 below 120 K and above 150 K should be different. In this note we give some evidence of the changes of the n-type state occurring in si GaAs in the dark in the typical temperature range of EL2 recovery.The experiments have been performed on the same semi-insulating (si) undoped GaAs sample which has been used in our previous work [4].In this note the results on persistent photocurrent (PPC) and persistent Hall mobility (PHM) effects are presented. A high input resistance electrometer ( R > 1OI6 R) has been used for Hall mobility measurements. The PPC effect was reported to be associated with free holes which dominate after EL2 has been quenched [7]. Persistent electrons were observed by infrared investigation in [8]. As we know, no PHM effect has been reported in literature. The PPC and PHM effects were observed in the normal and metastable states of EL2 as well, after switching off the short (= 1 s) 1.08 eV excitation pulse of low intensity in order to prevent the quenching of the normal state of EL2 in the first case. In the second case EL2 has been quenched before the measurement. The PPC and PHM transients were measured at a temperature of % 110 K in the time region from lo-' up to lo4 s. The transients of PPC can be approximated by theoretical curves close to a hyperbolic law. Such dependences are typical for semiconductors containing inhomogeneities, which produce drift and recombination barriers of different height for the nonequilibrium charge carriers [9]. It must be pointed out that the initial values of photoconductivity oo and the power indexes of hyperbolas y differ significantly in both cases. For the normal EL2 state o,, = 1 . 2~ R-' cm-' and y = 0.4, while for the metastable one oo = 7.5 x R-' cm-' and y z 1. The total change of photoconductivity reached about four orders of magnitude in the first case, and more than six orders in the second. The conclusion is that the quenching of EL2 leads not only to simple charge transfer between defects, but also to the rearrangement of the defect ensemble in si GaAs.The Hall mobility in contrast to photoconductivity remained almost constant in the whole time region after the excitation was off in both cases. For the normal EL2 state the ') Sauletekio 9, 2054 Vilnius, Lithuania.

Model of Charge Transfer Induced by EL2→EL2* Transformation in Semi‐Insulating GaAs

“…Nevertheless, it is obvious that the proposed model can be developed taking into account more details of real si GaAs crystals. For example, as was reported earlier [4], the change of photoconductivity during EL2 + EL2* transformation is not exactly exponential (or the sum of few exponents), but demonstrates a more complicated behaviour. Probably, this fact could be explained, firstly, including into the system of equations more donors and acceptors with appropriate parameters.…”

“…At lower temperatures more shallow traps take part in charge transfer and modify quenching transients. 4. We propose that the complex behaviour of EL2 optical recovery spectra in the range 0.7 to 1.0 eV and 1.4 to 1.5 eV (see e.g.…”