The authors review both the possibilities and problems of semiconductor quantum microstructures. Quantum microstructures such as quantum wires and quantum boxes where electrons are confined in low dimensions have potential for device applications. For example, they have high electron mobilities and long coherence lengths in transport devices, and low threshold current with weak temperature dependence in laser diodes. Among these possible applications, the authors propose a novel long-wavelength light-emitting device. It consists of asymmetric quantum microstructures and has large second-order susceptibility, leading to efficient down-conversion of two input lights. However, there still exist problems in realizing the expected performance of quantum microstructure devices. In fabrication technology, size fluctuation, roughness at the heterointerfaces, and residual impurities are the important subjects to be eliminated. Another problem is quantum mechanical granularity noise, which sets the fundamental limit for quantum interference devices.
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