GaAs/AlGaAs异质结的界面束缚二维激子效应

TWO DIMENSIONAL EXCITON EFFECTS IN CARRIER CONFINEMENT AT GaAs/AlGaAs INTERFACE

本文讨论了GaAs/AlGaAs异质结界面的H线发光及其性能。用双晶X射线衍射及皮秒光致发光证明了H线与界面质量密切相关,并且有相似于激子跃迁的寿命行为。用限制于异质结界面势阱的二维电子(或空穴)与分布于GaAs边的三维空穴(或电子)组成的二维激子效应,解释了H线的实验结果。并讨论了不同外延生长的异质结与界面有关的发光行为。

The band offset occuring at an abrupt hetero-interface in heterojunction materials and the carrier confinement behaviour at this interface have been studied for many years. A new photoluminescence (PL) spectrum, H-line, has been observed from the undoped or lightly doped GaAs/AlGaAs hetero-junction grown by LPE.The luminescence of H-line was shown between ex-citon and acceptor related transitions at low temperature, with peak energy ranging from that of the bound exciton to that of the shallowest acceptor transitions, increasing with the excitation intensity. Also, it dependents on the experiment temperature and the composition of heterojunction materials. The H-line can be considered to originate from the GaAs/AlGaAs interface. A few researches about the H-line and related behaviour have been done on MBE growth GaAs/AlGaAs heterojunctions. The purpose of this paper is to describe the X-ray diffraction and pico-second(ps)PL results about the H-line transition. A model of two dimensional exciton effect has been used successfully to explain the H-line experimental results.LPE GaAs/AlGaAs samples with similar heterojunction structure Were used to study the relation between H-line and the crystal quality. Double crystal X-ray rocking curves of (400), (511) and (322) diffractive directions and reflective topography of hetero-structure crystal were taken by a Rigakn CN 1518B1 scanning X-ray double crystal aiffractor. In the sample whose H-line can be observed, the half width of rocking curve is sharp, and the topography shows uniform stress distribntion and less deffects.The H-line would be missing in that samples with more deffects and ununiform stress distribution. It can be confirmed that the H-line comes from good heterojunction qnality with perfect interface.Life-time of H-line and exciton transitions were measured with 7220A Ps laser pulse exciting PL at 2K. The life-time of H-line is about 1.3ns, while that of the free and bound excitons is in the order of hundred ps. The lifetime of H-line almost keeps constant as the excitation intensity increases, while that of the bound exciton becomes longer from a few hundred ps to 1ns. At the same time, the life time of acceptor related transitions is in the order of us or longer. From above behaviour of H-line, it can be concluded that the H-line has the same life-time result as the exciton related transitions and the H-line is very different from the acceptor related cases.The phenomena of H-line can be explained by the effect of two dimen-tional exciton at the GaAs/AlGaAs hetero-interface. This two dimensional exci-ton are formed from 2D electrons (or holes) which are confined at the interface notch, and 3D holes (or electrons) which are distributing at the flat part of GaAs energy band, where the 3D carriers are repelled by the interface built-in field and attracted by the different carriers confined at the interface notch. The wavefunction and the distribution of electrons and holes at the interface have been given from the theoretical research. The foldover between electrons and holes gives the recombination of H-line. In high excitation intensity, the foldover between electrons and holes distribution become large,so the intensity of H-line will be increased rapidly with nonlinear relation to the increase of excitation. In the same time, the eonfiaement of carriers at the interface notch will become weak due to the charge screen effect,so the peak energy of H-line will shift to high energy side and close to the bound exciton transition. The life-time of bound exciton increases roughly linearly following logarithm of excitation power as the excitation intensity increasing, which is resulted from the re-absorption and re-emission processes and the long distance exciton effects. However, the life-time of H-line dependents on the confinement of carriers at the interface. As the excitation intensity increasing, the charge screen effect will induce to a weak confinement which results in short life-time,as for the long distance excitons, re-absorption and re-emission effects will