Chakrabarti Subhananda. - Optimisation of ZnO Thin Films: Implants, Properties, and Device Fabrication

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Most of the IIVI binary compound semiconductors crystallise as either zinc blende or hexagonal wurtzite structure where each anion is surrounded by four cations or vice versa. The crystal structure of ZnO is wurtzite (B4), zinc blende (B3) and rock salt (B1), as shown in Fig.. Of these, wurtzite structure is the thermodynamically stable phase in ambient conditions. The zinc blende structure is stabilised only by growth on cubic structures, while rock salt structure can be obtained at high pressures [1].

The wurtzite structure has a hexagonal unit cell with lattice parameters, a and c. Figure shows the wurtzite structure of ZnO [4]. The structure is composed of two hexagonal-close-packed sub-lattices, each of which consists of one type of atom displaced with respect to each along the c-axis [1].

As mentioned earlier, ZnO has many important properties that make it a deserving candidate for primacy in the semiconductor industry. Some of its properties are wide bandgap, large exciton-binding energy and high resistance to radiation damage, among others. ZnO has a melting point of 1975 C and is insoluble in water and alcohol, but can be dissolved in dilute acids. Some of the other properties of ZnO are given in Table [5].

ZnO is intrinsically an n-type semiconductor material due to the existence of native defects such as zinc interstitials (Zn i ) and oxygen vacancies (VO), as well as the presence of impurities in the form of hydrogen, which act as donor states in ZnO [6, 7]. A vacancy is an unoccupied lattice site, which results in an unsatisfied bond within lattice. A vacancy is created when species leave lattice sites and move to the surface. Interstitial defects are present if species do not occupy the lattice site but remain in crystal and disturb the ideal structure of the crystal. Figure shows the various defects present in a crystal [8]. The bulk electron mobility is in the range of 200400 cm2/Vs, which is limited by the acoustic and phonon scattering. However, in the case of thin films, the electron mobility is typically in the range of 1100 cm2/Vs due to the interface roughness and grain boundary scattering [3]. A major obstacle in fabricating ZnO-based bipolar devices is the difficulty of achieving high-quality, reliable and reproducible p-type ZnO. Thus, there are not much data on the hole transport in p-type ZnO due to the problems associated with obtaining p-type ZnO. This lack of data explains why the commercial usage of ZnO-based bipolar devices has not yet been realised in the research community, although it has so many advantages over other materials.