Localization of biexcitons and mechanism of optical gain formation in semiconductor low-dimensional structures

我们研究了ZnS基量子阱结构中双激子的局域化和光增益的形成机制。采用<chi><I-chi>低压金属有机化学气相沉积技术制备了Cd_ Zn_ S-ZnS多量子阱样品，通过光致发光激发光谱，我们清晰地观察到了双激子的双光子吸收过程，即双激子从基态直接产生的过程。根据单光子激子共振和双光子激子共振的能量差，计算了不同Cd组分比和阱层厚度的一系列样品的双激子结合能.迄今为止获得的双激子结合能的最大值估计为34 meV。这个值大约是ZnS中双激子结合能的四倍，并且超过了室温下的26 meV的热能。双光子双激子共振与探测光子能量的关系清楚地表明了双激子的局域化.通过光泵浦实验研究了受激发射过程与温度的关系.在双激子发光的能量位置观察到紫外受激发射，发现阈值激发-受激发射的功率密度直到约240 K几乎是恒定的。这一结果强烈地表明，由于双激子的辐射复合过程而形成的光学增益直到约240 K才实现。我们还发现，双激子局域化的程度不是有效降低受激发射的阈值激发功率密度。为了在室温下实现双激子光学增益的形成，我们认为进一步提高双激子的结合能是必要的。

We have studied the localization of biexcitons and the mechanism of optical gain formation in ZnS-based quantum- well structures. The Cd_<chi> Zn_<I-chi> S-ZnS multiple-quantum-well samples used in this work were prepared by a low-pressure metalorganic chemical vapor deposition technique.By means of photoluminescence excitation spectroscopy, we clearly observed the two-photon absorption process of biexcitons, which was the direct creation of biexcitons from the ground state. On the basis of the energy difference between the one-photon exciton resonance and the two-photon biexciton resonance, the binding energy of biexcitons was obtained for a series of samples with various Cd composition ratios and well-layer thicknesses. The maximum value of the biexciton binding energy obtained to date was estimated to be 34 meV.This value is approximately four times as large as the binding energy of the biexciton in ZnS, and exceeds the thermal energy of 26 meV at room temperature. it was also noted that the detected-photon-energy dependence of the two-photon biexciton resonance clearly indicated the localization of biexcitons.Temperature dependence of stimulated emission processes was also investigated by means of optical pumping experiments. We observed the ultraviolet stimulated emission just at the energy position of the biexcilon lnminescence and found that the threshold excitation-power density for the stimulated emission was almost constant up to about 240 K.This result strongly indicates that the optical gain formation due to the radiative recombination process of biexcitons is achieved up to about 240 K.We also found that the degree of biexciton localization was not effective to lower the threshold excitation-power density for stiumlated emission. In order to realize the biexcitonic optical gain formation at room temperature, we consider that further increase in the biexciton binding energy is essential.