Picosecond dynamics of absorption bleaching in polycrystalline ZnCdTe films

Abstract

The spectral and temporal behavior of absorption bleaching and of self-diffraction in Zn$!d,-,Te polycrystalline films with a nominal composition x=0.87 have been investigated. It has been found that the optical transmission increases 15 times at a wavelength of 575 nm for a laser light intensity of 0.2 GW/cm*. In a pump and test experiment, the buildup and the relaxation times T, of the bleaching were measured. Their values, between 10 and 40 and 20 and 45 ps, respectively, depend on the wavelengths of the pump and the test beams. The saturation intensity 1, was also determined (between 1.5 and 6 GW/cm2). This absorption saturation effect can be related to the high density of crystallite grain surface states as well as to other imperfections in the polycrystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photogenerated carriers. Then, in self-diffraction experiments, the phase relaxation time T2= (lOA5) ps of the localized surface states has been determined. The long duration of T, and the short energy lifetime T1 indicate that elastic scattering processes are highly suppressed in polycrystalline material when compared to monocrystals, while their coherence time is increased. 5729

Méthodes expérimentales de l'optique non linéaire

“…Afin d'améliorer l'accord entre points expérimentaux et le résultat de notre intégration à des fortes intensités, nous introduisons un terme de saturation I sa , dans l'équation. (34). Dans ce cas, a 2 prend la forme…”

“…Le même type d'expérience de pompe sonde a été largement appliqué à d'autres corps semi-conducteurs. Nous pouvons citer comme exemple nos travaux sur des films de CdZnTe sur substrat amorphe ou GaAs [34,35], qui montrent une diminution de l'absorption à la place de l'absorption induite. Comme nous le verrons, ce comportement dû au caractère polycristallin de l'échantillon est très différent de celui du monocristal massif.…”

Structural and Optical Properties of Cd1—xZnxTe Epilayers Grown on GaAs (001) by Pulsed Laser Deposition

“…Thin polycrystalline films of Cd 1± ±x Zn x Te of variable composition (x = 0 to 1) have been deposited on glass substrates using either a hot wall evaporation technique (HWE) or a pulsed laser ablation technique [6,7]. In both cases, a preferential orientation of the grains in the [111] direction has been observed.…”