气相外延ZnSe单晶膜的自由激子发光

FREE EXCITON EMISSION IN VPE ZnSe EPILAYERS

本文在77K和N_2激光器3371谱线高密度激发的VPE ZnSe单晶膜上,首次得到了起因于自由激子与自由激子(Ex-Ex)散射的发光谱带(P带),理论拟合了该谱带的形状并讨论了它的发光特性。文中把在选择的VPE ZnSe外延单晶膜中得到P带的起因归结为这些ZnSe外延单晶膜的质量较高。

In recent years, the research on II-VI compound semiconductors such as ZnSe and CdS has special attention. The most interesting work is to study the free exciton emission in the II-VI crystals under high excitation density. Saito et al. observed E and P bands in ZnSe in photoluminescence (PL) within 4.2-65K,which they attributed to exciton-electron(Ex -e)and exciton-exciton (Ex-Ex) interaction, respectively. Colak et al. reported the stimulated emission in ZnSe crystal excited by electron beam. L. Ma et al. observed the P band excited by N2 Laser in VPE ZnSe epilayer at 64K. Up to now, there has been no report on Ex-Ex interaction in VPE ZnSe epilayer under high excitation density at 77K.In this work,a new band PA Was found at the low energy side of EA band under high excitation density. The origin of the new PA band was investigated.The ZnSe epilayer used in this work was grown on (100) GaAs substrate with VPE method. The ZnSe epilayer was excited by N2 Laser with the pulse width of 10ns, repetition frequency of 10Hz and the maximal pulse density of 2MW/cm2. PL spectra were measured using a grating spectrograph of model 44W with C31034 cooled photomultiplier.Fig.1 and Fig.2 show the PL spectra of the selected (specimen A) and ordinary (specimen B) ZnSe single crystal epilayers under different excitation . densities at 77K. In Fig.1, when excition density is lower than 0.4Io there is only a band labeled EA (4451 A) in PL. According to the peak position of EA band, we consider that EA band is attributed to Ex-e interaction. When excitation density is increased to 0.4I0, a shoulder appears at the low energy side of EA band. When excitation density is further increased to I0 continually, a new band labeled PA (4485A) appears instead of the shoulder and the intensity of the new PA band is higher than that of EA band (4458A).In order to find the origin of PA band,we calculated the theoretical curves of exciton emission according to the Ex-Ex interaction model and Ex -c interaction model (Fig.3,. 4). It is obvious .that the theoretical curves are in agreement with the experimental curves and the separation energy between EA and PA bands corresponds to exciton binding energy for ZnSe .These results further identify that the new PA band is just the P band which is attributed to the inelastic collision of two free excitons. The effective temperature of free excitons used in calculation of the theoretical curves was obtained with the method introduced by R. Baltramiejunas et al.Fig.5 shows the dependence of the intensity of Es and P bands on the excitation density. We find that with increasing the excitation density, the intensity of Es band becomes saturated. In this case, the intensity of P band increases rapidly. This is due to the increase of the intensity of P band which is proportional to the square of the number of free excitons in ZnSe . So that the Ex-e interaction is suppressed by Ex-Ex interaction.Fig.6 shows the Ex -e band separated from Fig.l under different excitation densities. Comparing Fig.2 with Fig.6, it is observed that EB band is the overlapping by Ex-e and Ex^Ex bands. That is why EB band becomes broader than E.x - e band does and the peak position of EB band shifts more to the low energy side than the peak position of E,-e band does, with increasing the excitation density.By analyses of the spectra in Fig.2 we consider that whether the P band can appear or not is determined by the quality of VPE ZnSe epilayer. This viewpoint is in agreement with the experimental result obtained in ZnSe bulk crystals by Zhang Jiying et al.In conclusion, the P band was first observed in VPE ZnSe epilayer on(100) GaAs substrate with high excitation density at 77K. In view of thisresult, we predict that it is possible to obtain the exciting stimulated emissionin the blue in MIS structure made of VPE ZnSe epilayer on low resistivityGaAs substrate under the excitation of electric field.