Time‐resolved differential reflection measurements on GaMnAs grown by molecular beam epitaxy

Abstract

We investigated time-resolved differential reflection in GaMnAs by pump-probe method at various wavelengths. The signals in GaMnAs were found to be more influenced by the compensating nature of the samples, rather than by the Mn concentrations. The time-resolved reflectivity of highly compensated samples exhibited stronger negative ∆R component at photon energies higher than the bandgap energy. This behaviour was similar to that of As + -ion implanted GaAs. Our results suggest that arsenic antisites (As Ga ) act as deep donors as in low-temperature grown GaAs and play an important role in compensating holes in GaMnAs.

Rapid diffusion of electrons in GaMnAs

“…Though the τ 0 = 8 ps that we measure at 80 K may seem too short to be the free-electron lifetime, it is consistent with previous observations in low-temperaturegrown GaAs 32,33 and in (Ga,Mn)As, 24,[34][35][36][37] where a transient lasting 6 to 14 ps was attributed to non-radiative recombination of trapped carriers. 35,36 Our signal arises from free electrons, not trapped, but is sensitive to recombination because, when all traps are filled, free electrons are trapped only as quickly as recombination empties traps.…”

“…Of these three components of our signal, we will see below that the τ (q) term reveals the dynamics of free electrons diffusing at about 70 cm 2 /s. Previous ultrafast experiments on (Ga,Mn)As have observed signals with a very slow component corresponding to our C term, and attributed it to trapped carriers [23][24][25] or to residual heat from the excitation pulse. 26,27 Such experiments have also observed fast processes on the scale of τ fast , [23][24][25] which were variously attributed to intraband relaxation 24 or trapping 23 of electrons at As antisite defects.…”

“…Previous ultrafast experiments on (Ga,Mn)As have observed signals with a very slow component corresponding to our C term, and attributed it to trapped carriers [23][24][25] or to residual heat from the excitation pulse. 26,27 Such experiments have also observed fast processes on the scale of τ fast , [23][24][25] which were variously attributed to intraband relaxation 24 or trapping 23 of electrons at As antisite defects. Rapid trapping is consistent with our observation of electrons' diffusion at later times: the number of electrons photoexcited likely exceeds the number of trapping sites, leaving some electrons untrapped and free to diffuse.…”

Effect of point defect and Mn concentration in time-resolved differential reflection in GaMnAs

“…Figure 2 shows the differential reflectivity ⌬R of sample A ͑Mn 3%͒, B ͑Mn 1.5%͒, and C ͑Mn 5%͒ for pump-probe wavelengths of 830, 850, and 860 nm, at room temperature. The sign of ⌬R was positive in semi-insulating ͑SI͒ GaAs substrates for these wavelengths, 8 Fig. 2.…”

“…The thicknesses of the Ga 1−x Mn x As layers and the Al 0.2 Ga 0. 8 As layers were about 200 and 500 nm, respectively. The growth temperature of GaMnAs was 215°C.…”