Metamorphosis of a quantum wire into quantum dots
Metamorphosis of a quantum wire into quantum dots
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量子线转变为量子点
作者：
J. Hasen;L. Pfeiffer;A. Pinczuk;Song He;K. West;B. Dennis
半导体中电子-空穴对（激子）的束缚态具有理想的特性，例如增强了辐射复合的振荡器强度，这为下一代光学器件带来了希望。然而，在典型的设备操作条件下（室温和中等电荷密度），激子解离形成电子空穴等离子体。将激子限制在较低的维度上可以防止解离，因为在较低的维度上，它们的结合能有望显著增加。但这种约束反过来又可能影响激子的动力学性质。在这里，我们报告空间分辨光致发光图像的激子局限在一个孤立的砷化镓量子线。随着结构温度的降低，我们观察到从广泛和相当连续的光致发光到一组强烈的发射峰的显著转变，这些峰在能量上是尖锐的，在空间上是局域的。这种行为表明，在足够低的温度下，量子线就像一组稀疏的量子点。此外，在孤立量子点的位置，我们观察到激子弛豫速率的异常下降，这样它们在弛豫到基态之前从高能态辐射（通过重组）。我们认为这是激子弛豫“瓶颈”的表现，它的存在可能会给基于量子点的光学器件的发展带来问题。
Bound states of electron–hole pairs (excitons) in semiconductors possess desirable properties — such as an enhanced oscillator strength for radiative recombination — that hold promise for the next generation of optical devices. However, at typical device operating conditions (room temperature and moderate charge densities), excitons dissociate to form an electron–hole plasma. Dissociation may be prevented by confining excitons to lower dimensions, where their binding energy is expected to increase significantly. But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire. As the temperature of the structure is lowered, we observe a striking transition from broad and fairly continuous photoluminescence to an intense set of emission peaks which are both energetically sharp and spatially localized. Such behaviour indicates that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated quantum dot, we observe an unusual decrease in the relaxation rate of excitons, such that they radiate (via recombination) from higher energy states before relaxing to their ground state. We argue that this is the manifestation of an exciton relaxation ‘bottleneck’, the existence of which could pose problems for the development of optical devices based on quantum dots.