Molecular Beam Epitaxy Growth of Nanowires in the Hg1-xCdxTe Material System

The size of electronic components keeps decreasing as more computing power is packed into the volume of a personal computer. There are also clear advantages to shrinking sensors and other electronic devices; they will be lighter, smaller and require less power. More functionality can then be added to portable instruments, whether they are cellular phones or uniforms for soldiers. In the quest for miniaturization, nanotechnology is an obvious field of study. Nanostructures can have properties that differ from those of the bulk material, for example size-tunable effective band gap or high sensitivity to surface preparation due to the large surface-to-volume ratio. This can lead to miniaturized components with completely new properties. Nanowires are today grown in numerous material systems such as GaAs, Si, GaP, InP, ZnS, CdSe, ZnTe or GaAsSb (Olsson et al., 2001; Duan & Lieber, 2000; Shan et al., 2005; Janik et al., 2006; Dheeraj et al., 2008). Most of these are grown with vapor phase epitaxy techniques using the vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) mechanism, in which the component fluxes go through or around a gold catalyst particle and the nanowire grows underneath this particle (Wagner & Ellis, 1964; Persson et al., 2004). Many groups have also successfully grown segmented or heterostructured nanowires (Bjork et al., 2002; Wu et al., 2002; Gudiksen et al., 2002). Various nanowire devices have been demonstrated, for example a pn junction, field-effect transistor, photodetector, polarized light emitting diode (LED), laser, single electron transistor, optical switch, and detectors for biological and chemical molecules. Hg1-xCdxTe is an alloy between the semimetal HgTe and the semiconductor CdTe, and it has a direct band gap that is tunable from -0.26 eV (HgTe) to 1.61 eV (CdTe) at 77 K, covering the entire infrared (IR) region. The small effective electron mass of Hg1-xCdxTe (minimum of about 0.02 m0 for Hg0.66Cd0.34Te) leads to a quantum upshift for larger structures than in other materials and enables size-tunable electrical and optical properties. In HgTe particles there should, for example, be quantum effects for diameters smaller than 80 nm and a positive band gap below 18 nm (Green et al., 2003). HgCdTe is mostly used for high performance IR detectors, but the small lattice mismatch in this material system (maximum 0.3% between CdTe and HgTe), facilitates growing heterostructures, including quantum wells, with good crystallinity (Tonheim et al., 2008). Hg(Cd)Te also has a number of other