ZnCdSe量子点的激子行为研究

Excitonic Properties of ZnCdSe Quantum Dots

用金属有机化学气相沉积(MOCVD)的方法在晶格失配较小的情况下制备了ZnCdSe量子点,并用原子力显微镜(AFM)和极低温度下的发光光谱确认了量子点的形成。原子力显微镜观测的形貌变化发现,随着生长后时间的增加,量子点的尺寸逐渐增大,而密度减小,这是由于熟化过程作用的结果。随着量子点生长完毕与加盖层之间间隔时间的增加,量子点的发光峰位明显红移,且由变温光谱得到的激子束缚能逐渐变小。这可以解释为随着间隔时间的增加,量子点的熟化过程导致量子点的尺寸增大,量子限域效应减弱所致。

Strain induced quantum dots (QDs) have attracted more and more attention in recent years due to its potential applications in optoelectronic devices such as lightemitting devices and lasing diodes. Practical devices based on QDs are expected to exhibit high differential gain, low threshold current, and high characteristic temperature comparing with those based on thin films, twodimensional quantum well and onedimensional quantum wire structures. To realize these priorities, the study on growth and optical properties of QDs is necessary. Up to now, the study on QDs mainly focuses on ⅢⅤ systems, In(Ga)As/GaAs system, for example. As for ⅡⅥ systems, CdSe/ZnSe attracts most attention. However, The CdSe/ZnSe system, which has relative narrow band gap, can only cover the spectrum range from red to green region. As is known, ZnxCd1-xSe can extend its spectrum to the whole visible range by varying x value. Therefore, the study on the optical properties of ZnCdSe QDs is not only important but also necessary. Additionally, the realization of longlifetime lightemitting devices based on ZnCdSe quantum well structures makes the study more attractive. In the present paper, ZnCdSe QDs have been fabricated under StranskiKrastanow (SK) mode on GaAs substrate. Atomic force microscopy investigation confirmed the formation of the dots. The excitonic properties of the QDs were investigated by photoluminescence (PL) measurements. The appearance of a kind point in the temperaturedependent PL intensity at about 20K verifies the observation of AFM. With increasing the interruption time between the dot formation and capping, the PL spectra of the QD structures show obvious broadening in full width at half maximum and redshift of peak energy, and the ionization energy, which is induced from the temperature dependence of integrated intensity, decreases significantly with increasing the interruption time. These facts can be attributed to the variations in dot size induced by the ripening process occurring in the growth interruption, in which, the larger dots become bigger at the cost of the smaller ones. Therefore, the PL spectrum broadens because of the increased nonuniformity of the size distribution of the dots. Meanwhile, the peak energy shifts to low energy side, and the ionization energy decreases, resulted from the increase of the dot size. 