Role of Intrinsic Defects in Nanowires

The properties of nanowires are distinctively different from those of bulk materials because they are affected by quantum confinement effect and large surface-to-volume ratios. A fundamental understanding of the intrinsic properties of nanowires, in terms of their structure, electronic and optical properties as well as a study of the nanowire surface is required to develop them for future device applications. The morphology and properties of the nanowires are strongly dependent on the synthesis process and growth parameters. Key to realizing the full potential of nanowires is the ability to synthesize uniform, defect free nanowires. Any deviation from stoichiometry leads to imperfections within the nanowire crystal lattice and affects its properties. Efforts to optimize growth conditions are spearheaded by investigations to identify and control native point defects inherent in semiconductors. Structural characterization using electron microscopy is usually the first step towards identifying the quality of the nanowire in terms of composition, morphology and defects like twinning and stacking faults. Optical characterization tools like Photoluminescence (PL) spectroscopy is useful for extracting information on band-to-band transition and other sub-band electronic transitions, thus providing information on any deviations from stoichiometry. However, it cannot provide a comprehensive view of every aspect of the optical properties of nanowires. Hence, complementary experiments like time-resolved PL, pump probe measurements, cathodoluminescence, absorption and reflection spectroscopy are often used. In this chapter, we will present a brief review of some of the techniques used in identifying and characterizing defects in semiconductor nanowires. Studies drawn primarily from the author’s work, addressing the role of intrinsic point defects in nanowires and discussion of strategies for determining the type, energy and concentration of defects in nanowires will be presented in this chapter. The presence of low energy luminescence bands in un-intentionally doped nanowires is evidence that intrinsic point defects (IPDs) play a significant role in the emission spectrum. IPDs in a compound semiconductor are vacancies, interstitials and antisites. In bulk semiconductors, IPDs are often studied by Optical Detection of Magnetic Resonance (ODMR) and Electron Paramagnetic Resonance (EPR) studies. However, the use of such studies on nanowires is only recent (Mal et al., 2010). It is important to understand the behavior of IPDs for the successful application of any semiconductor, since they control, directly or indirectly, doping, compensation, minority carrier lifetime, and luminescence efficiency. Semiconductor doping, in particular, can be adversely affected by IPDs, by causing self-compensation. For example, IPDs which act as donors may compensate the deliberately introduced acceptors. In group II-VI semiconductors like ZnO, specific native defects have long been believed to play 13