Carrier transport and bandgap shift in n-type degenerate ZnO thin films: The effect of band edge nonparabolicity
Introduction
Transparent conducting oxide (TCO) thin films are an essential part of most optoelectronic devices such as flat panel displays, light-emitting diodes and photovoltaic solar cells [1], [2], [3]. TCOs are very significant materials due to their relatively high conductivity and also high transparency in the visible region of spectrum [4]. These materials possess a relatively high concentration of free electrons (∼1020 cm−3 or higher) in their conduction band. The high carrier concentration, in turn, causes absorption of electromagnetic radiation in both visible and IR portions of spectrum. The conduction electrons in these films are supplied from donor sites associated with oxygen vacancies or excess metal ions [5]. For technological purposes, TCOs must simultaneously conduct electricity and transmit visible light, an unusual combination. A limited class of metal oxides, mostly n-type conductors, satisfies this criterion. Among all TCOs, transparent conducting ZnO thin films doped with impurities like Al and Ga have been attracting much attention due to facile accessibility and non-toxicity. ZnO is stable in hydrogen containing ambient, e.g., in a silan (SiH4) plasma discharge, which is used for the manufacturing of amorphous silicon thin film solar cells by plasma-enhanced chemical vapor deposition technique [6]. There are numerous reports on the preparation, characterization, electrical and optical properties of ZnO thin films and a large amount of experimental data have been given. However, few of them have reported the resistivity less than 1×10−4 Ω cm [7], [8]. Reducing the resistivity involves either increase in the carrier concentration or in their mobility. According to Coutts et al. [9], increasing the mobility would be the best direction to follow because it has no deleterious effect on the optical properties. The mobility, in turn, depends on different charge carrier scattering mechanisms in the film such as lattice scattering (LS), ionized impurity scattering (IIS), neutral impurity scattering (NIS), grain boundary scattering (GBS), etc. In this study we concentrate on the polycrystalline ZnO thin films. We shall look briefly at some main scattering mechanisms that affect the electrical properties of the material.
Despite numerous reports of transport phenomena in ZnO thin films, band edge nonparabolicity and carrier concentration dependence of the effective mass have been slightly considered. Hence, to get better understanding of physical constraints which are involved in the electrical and optical properties of the prepared thin films, introducing nonparabolicity into calculations seems essential. Meanwhile, to show the consequences of introducing nonparabolicity, the results then will be compared to the experimental data taken from the literature and also the case of constant effective mass with parabolic band edge.
On the other hand, most electro-optical properties of semiconductors are related to the bandgap which can be modified by doping[10]. ZnO is a wide bandgap semiconductor (3.37 eV) with high exciton binding energy (60 meV) [4]. The electrical and optical properties of ZnO may be changed by doping with a group III impurity, such as Al, In or Ga. The magnitude of bandgap shift due to moderate or heavy doping level is determined by two competing mechanisms; bandgap narrowing (BGN) which is a consequence of many body effects on the conduction and valence bands and the bandgap widening (BGW) which is referred to the well-known Burstein–Moss effect. In this paper the carrier concentration dependence of bandgap shift is also studied. The total bandgap shift is evaluated both with and without nonparabolicity modification and the results are compared to the data taken from the literature.
The approach taken in this study is not based on the experimental details of thin films preparation, but instead would be based on the theoretical concepts which may help determine the optimum properties desired in the thin films being produced.
Section snippets
Band edge nonparabolicity and effective mass
One crucial factor in the expressions for electrical and optical properties of materials is the effective mass of charge carriers. Fundamental studies of TCOs require knowledge of the effective mass. Regarding close relationship between effective mass and band edge shape, an initial study of band edge seems necessary. As well known for semiconductors, when optical transition or carrier transport occurs in a band positioned far from the conduction band bottom or valence band top, nonparabolicity
Carrier transport in zinc oxide thin films
As the carriers traveled through a semiconductor, they encounter various scattering centers that govern the carrier mobility in the electronic system. The dominant scattering mechanisms that generally govern the electron transport in TCOs may arise from impurity atoms (ionized or neutral), thermal vibrations (acoustical and optical) of the lattice atoms, structural defects (dislocations, vacancies) and other obstacles. For highly degenerate TCOs, carrier mobility can be defined by the classical
Bandgap shift in moderately and heavily doped ZnO films
Theoretically, the increase of carrier concentration in degenerate semiconductors causes two opposite effects, namely the bandgap widening (BGW) and bandgap narrowing (BGN) [42], [43]. The BGW is usually explained by Burstein–Moss (BM) effect whereby the conduction band becomes significantly filled at high doping concentration and the lowest energy states in the conduction band are blocked.
According to BM effect, the broadening of the optical band gap is [44]where is
Conclusion
In this study, we discussed the carrier transport and bandgap shifting of degenerate n-type ZnO thin films as a function of carrier concentration. To conduct this, the results of theoretical calculated mobility due to main scattering mechanisms in ZnO thin films and also bandgap shifting of the films were compared with experimental values taken from literatures, regardless of preparation details. It was found that the nonparabolicity correction led to satisfactory agreement between theoretical
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