Carrier Confinement to One and Zero Degrees of Freedom.

Abstract

Since the early 1970s when the basic ideas on quantum wells [11.1, 2] were introduced, much progress has been realized in this field. The main catalysts for this development have been the advent of molecular beam epitaxy (MBE) which allows for the fabrication of ultra thin and highly perfect semiconductor epitaxial layers and the introduction of new devices based on the novel physical properties associated with carrier confinement. Carrier confinement in such structures, in the simplest way, is achieved by sandwiching the semiconductor layer with two wider band gap epitaxial semiconductor layers. If the narrower band gap material is in the form of a thin epitaxial layer, the carriers have 2 degrees of freedom within this layer (Fig. 11.1). The quantum properties appear in such structure for layer thicknesses smaller than ≈ 500 Å. The structure presenting this type of confinement are the well-known quantum wells (QW). Progress in reducing the carriers degrees of freedom i.e. increasing the degrees of confinement have been hampered by the complexity of the processing procedures. Indeed there is great interest from a technological point of view in realizing such structures since new device properties are expected [11.3]. For a smaller band gap region in the form of a thin wire (width smaller than: ≈ 500 Å) surrounded by wider band gap material, carriers will have one degree of freedom for motion along the wire axis. If the region of smaller gap material is in the form of a box (again with dimensions ≤ 500 Å), the carrier motion is confined to zero degrees of freedom (Fig. 11.1). When the wire and box exhibit dimensions which are smaller than the carrier deBroglie wavelength, their energy levels are quantized and these structures will be defined as quantum well wires (QWW) and quantum well boxes (QWB) respectively.