Molecular Beam Epitaxy

4Nanostructure Formation Process of MBE

Koichi Yamaguchi

Department of Engineering Science, The University of Electro‐Communications, Tokyo, 182‐8585, Japan

4.1 Introduction

Recent progress in semiconductor epitaxial growth techniques, such as molecular beam epitaxy (MBE) and metal‐organic chemical vapor deposition (MOCVD), has enabled the fabrication of semiconductor nanostructures having low‐dimensional electronic characteristics. When the semiconductor crystal size is reduced to the de Broglie wavelength, electrons in the crystal have discretized density of states. In addition, since the degree of freedom of their movement is extremely limited, their kinetic energy increases. Therefore, as the crystal size decreases, the band‐gap energy increases. Owing to this quantum size effect, the absorption and emission wavelengths of light can be controlled by adjusting the semiconductor crystal size.

Figure 4.1 illustrates the density of states in the conduction bands of a bulk semiconductor, quantum well (QW), quantum well wire (QWR), and quantum dot (QD). The density of states for QW, QWR, and QD show, respectively, step, sawtooth, and delta functions. Semiconductor materials having low‐dimensional electronic characteristics are expected to enable the development of high‐performance optoelectronic devices. The thickness of the epitaxial layer can be controlled atomically by conventional MBE growth. Many QW devices have already been developed and put to practical use, such as semiconductor ...