Exciton relaxation in chalcogenide semiconductor investigated by time-resolved photoluminescence measurement

含有第VI族元素的非晶态硫化物半导体表现出各种光致结构变化，如光变暗、光漂白、光结晶等。这些变化是由晶格形变引起的，晶格形变是由于光激电子通过强烈的电子-声子相互作用而弛豫而产生的。同样重要的是，非晶态不处于热平衡状态。硫族原子是二重配位的，被认为起着重要的作用，但具体的作用机制尚不清楚。为了了解非晶态和晶态GeSe_2中激发态电子的驰豫过程，我们进行了时间分辨光致发光测量。两相在可见光区(2.2-2.7 eV)都有带隙。带隙光的激发在近红外区(-1 eV)产生了宽广的高斯型光致发光。大的斯托克斯位移是由于强的电子-声子相互作用造成的。利用声光调制器对连续波激光进行调制，作为时间分辨测量的激励源。测量的时间范围为激发光截止后100 ns-3ms。非晶态和晶态GeSe_2在光谱形状上有许多相似之处，这表明两种材料的发光初始态是相似的。在时间分布上，晶态GeSe_2表现为单指数衰减，而非晶态GeSe_2表现为扩展指数衰减。后者是由于玻璃内部结构的起伏引起发光中心的能量分布所致。

Amorphous chalcogenide semiconductors which contain the elements of group VI show various kinds of photo-induced structural changes such as photo-darkening, photo-bleaching, photo-crystallization, etc.. These changes are caused by lattice deformations which result from the relaxation of the photo-excited electrons through the strong electron-phonon interaction. It is also important that the amorphous states are not in the thermal equilibrium. The chalcogen atoms which are in two-fold coordinated are considered to play an important role, but the details of the mechanism is still not clear. For example, it is still unknown how the excited electron distorts the lattice.To know the relaxation process of the excited electron in the amorphous and crystalline GeSe_2 which is a typical chalcogenide semiconductor, the time resolved photo-luminescence measurement was carried out. Both phases have the band-gap in visible region (2.2-2.7eV). The excitation by band-gap light causes a broad Gaussian-shaped photo-luminescence in near-infra-red region (-1eV). The large Stokes-shift is due to the strong electron-phonon interaction. The continuous wave laser light is modulated by an acousto-optic modulator and then used for the excitation source of the time-resolved measurement. The measured time region was 100ns-3ms after the cut-off of the excitation light. There are many similarities in the spectral shape between the amorphous and crystalline GeSe_2. This result shows that the initial states of the luminescence are similar in both materials. As for the time-profile, the crystalline GeSe_2 shows the single exponential decay, while the amorphous GeSe_2 shows the extended exponential decay. The later is caused by the energy distribution of the luminescent centers by structural fluctuation in the glass.