A bit study of ZnO (part 1)

by Iwan

ZnO has wide band gap energy (3.37 eV at room temperature) (Yu et al. 1997; Makino et al. 2000, 2001, 2002; Gruber et al. 2004; Park and Ahn 2005; Zhang et al. 2005; Ahn et al. 2006) and large exciton (related to electron-hole pairs) binding energy (60 meV) (Makino et al. 2000). Based on these properties it has practical and potential applications for short wavelength optoelectronic devices and transparent conductive oxide (TCO) such as in ultra violet (UV) light emitting diodes (LED) and solar cell electrodes. From the economical side, ZnO has lower cost than GaN that well established in optoelectronics devices. Therefore, commercially ZnO is futuristic material for substitute in GaN. Doping with Manganese (Mn) or other 3d transition metals produces spintronic opportunities (V. Avrutin et all, Proc. Of SPIE vol 6122, 2006). In addition, many other features of ZnO make it very good replacement for GaN such as availability of wet chemical etching, high resistance to damage from radiation, high thermal conductivity, semi insulating capability and high quality production in bulk form for homoepitaxial substrate (M.Pan et all, Proc. Of SPIE vol 6122, 61220M (2006)).

Most of the II-VI compound crystallizes in either cubic zinc blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corner of tetrahedron. This tetrahedral coordination is typical of sp3 covalent bonding, but these materials also have a substantial ionic character. Schematically, ZnO structure is shown in figure 1 (O Uzgur et all, J appl physics, 2005).




















In contrast to other IIb–VI semiconductors, which exist both in the cubic zinc blende and the hexagonal wurtzite-type structures (like ZnS, which gave the name to both structures), ZnO crystallizes with great preference in the wurtzite-type structure. The cubic zinc blende type structure can be stabilized by epitaxial growth of ZnO on suitable cubic substrates, while the rock salt structure is stable only under pressure (S. Desgreniers, Phys. Rev. B 1998, 58, 14102).

In order to build ZnO film, some growth method has been studied such as ZnO single crystal films prepared by RF magnetron sputtering and other growth techniques allow a well control over the deposition procedure, such as molecular-beam epitaxy (MBE) pulsed-laser deposition (PLD), metal organic chemical-vapor deposition (MOCVD) and hydride or halide vapor-phase epitaxy (HVPE)(O Uzgur et all, J appl physics, 2005).

However, the main obstacle is to realize p type of ZnO based on optoelectronics device. Despite all the progress that has been made and the reports of p-type conductivity in ZnO films using various growth methods and various group-V dopant elements (N, P, As, and Sb), a reliable and reproducible high quality p-type conductivity has not yet been achieved for ZnO. Therefore, it remains to be the most essential topic in ZnO research today. Generally, most of the research efforts are directed just to solving this problem. In order to overcome this difficulty and to control the material’s properties, a clear understanding of physical processes in ZnO is necessary in addition to obtaining low n-type background. In spite of many decades of investigations, some of the basic properties of ZnO still remain unclear. For example, the nature of the residual n-type conductivity in undoped ZnO films, whether being due to impurities of some native defect or defects, is still under some amount of debate. Some authors ascribe the residual background to intrinsic defects (oxygen vacancies (VO) and interstitial zinc atoms (Zni)), and others to non controllable hydrogen impurities introduced during growth. The well known green band in ZnO luminescence spectra (manifesting itself as a broad peak around 500–530 nm), observed nearly in all samples regardless of growth conditions, is related to individually ionized oxygen vacancies by some and to residual copper impurities by others (O Uzgur et all, J appl physics, 2005). While p type ZnO is difficult to attain, the advantages of ZnO is being explored and exploited by another method. One of the methods to fabricated ZnO quantum well is for optoelectronics applications, such as for realize light amplification by stimulated emission of radiation (LASER) and ultra violet (UV) light using heterostructure based on quantum well.

Why ZnO Quantum Well?

The wide band-gap wurtzite semiconductors have attracted considerable attention due to their potential applications for optoelectronic devices in blue and ultraviolet (UV) regions (Nakamura and Fasol 1997). Wurtzite (WZ) GaN-based quantum-wells (QWs) with the (0001) crystal orientation require higher carrier densities to generate optical gain, in comparison with zinc-blende (ZB) GaAs- or InP-based QWs. This is ascribed to the heavy effective masses in the valence and conduction bands (Fang and Chuang 1995). In addition, the GaN-based QW structures have large internal field due to piezoelectric (PZ) and spontaneous (SP) polarizations (Martin et al.1996; Bernardini et al. 1997). The internal field in the QW structures causes the intrinsic quantum confined Stark effect, resulting in a red shift of the transition energy and the decrease of the transition probability. Thus, the control of the internal field and the reduction of the effective mass are very important for the realization of high performance GaN-based devices. As an additional element of band structure engineering, the crystal orientation effect on electronic and optical properties in WZ GaN-based QW lasers has been studied in order to decrease their threshold carrier density. It was reported in GaN-based QW structures that the average hole effective mass and the internal field are significantly reduced with increasing crystal angle (Domen et al. 1997; Niwa et al. 1997; Ohtoshi et al. 1998; Yeo et al. 1998; Park and Chuang 1999; Mireles and Ulloa 2000; Takeuchi et al. 2000).
Recently, ZnO and related oxides have been proposed as the new wide band-gap semiconductors for short wavelength optoelectronic applications (Yu et al. 1997; Makino et al. 2000, 2001, 2002; Gruber et al. 2004; Park and Ahn 2005; Zhang et al. 2005; Ahn et al. 2006) ZnO-based quantum well (QW) structures have several advantages compared to GaN-based QW structures. For example, the growth temperature of ZnO is usually around 500oC, which is much lower than typical growth temperature, 1000oC, of GaN (Zhang et al. 2005). In addition, ZnO system has a very large exciton binding energy (60 meV), which permits excitonic recombination even at room temperature (Makino et al. 2000).

According to these reason, in the next part I will review some experiment result of ZnO quantum well.