Thin ZnO Films

Thin films of zinc oxide can be prepared by a variety of deposition methods  

In the following we will focus on the first three deposition methods, since these deliver the best films with respect to low resistivity and high transparency. Especially, magnetron sputtering is a technique, which is already 

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One can conclude that thin ZnO films may serve as absolute sensors for H- and O-atoms as well as for other atoms and radicals affecting their conductivity (CH3-, OH, N, and others) provided that values 03 and i9 are determined by above technique. 

Thin ZnO films can be used either as a transparent and conductive window layer, or as a buffer layer, within CuInS2 (CIS) and Cu(In,Ga)Se2 (CIGS) thin film solar cell devices (see Chaps. 4 and 9). In both cases, the ZnO layers... 

Research and development was mainly focnsed on the Ga-doped powders and thin ZnO film systems (Bourret-Courchesne et al. 2009, Choi et al. 2008, Derenzo et al. 2002, Ehrentraut et al. 2006, Makino et al. 2005, Neal et al. 2006, Yen et al. 2010) due to the aforementioned Ga-induced scintillation efficiency increase in the UV region and diminished reabsorption problem. An important improvement in the scintillation efficiency of the Ga-doped ZnO was reported recently (Bourret-Courchesne et al. 2007, 2009). Namely, when the annealing in an Ar-H2 atmosphere at a temperature of about 800°C is applied as the last step of the postpreparation treatment of the Zn-vacancy containing ZnO powder, the intensity of UV emission increases dramatically (Figure 4.5). The effect is interpreted as the UV radiative transition caused by the recombination of the Ga + donor-band electrons and the holes trapped at shallow acceptors. The latter are ascribed to H+ ions localized in Zn vacancies. 

ZnO is a widely used functional material due to its rniique optical and piezoelectric properties. It has wide band gap ( 3.37eV), good electrical conductivity ( 5 x lO fl cm ), and piezoelectric property. Incorporating the ZnO film onto IPMC electrodes was studied in a view of developing IPMC for optical applications [Kim et al. (2009)]. Namely, piezoelectric properties of ZnO are potentially able to convert mechanical energy into electrical energy, and in conjunction with the optical properties of the material, the IPMCs coated with thin ZnO film can be of interest for opto-electrical applications. 

Figure 1. SEM image of the as-prepared thin ZnO film prepared by electrochemical deposition. Inset shape of water droplet on the surface of ZnO thin film. Reproduced with permission from [23]. Copyright 2003 American Chemical Society.

Fig. 2.4. Microphotographs of sintered ZnO films with different structures a - structure consists of microcrystals connecting each other by thin crystal bridges b - lace structure is characterized by variety of branch thickness. Magnification 2 1(H.

Fig 2.9. The field dependence of electric conductivity of a thin sintered film (/) and a pressed ZnO sample (2) [37]... 

Cheng, H.-C. Chen, C.-F. Lee, C.-C. 2006. Thin-film transistors with active layers of zinc oxide (ZnO) fabricated by low-temperature chemical bath method. Thin Solid Films 498 142-145. 

Ito, K. Nakamura, K. 1996. Preparation of ZnO thin films using the flowing liquid film method. Thin Solid Films 286 35-36. 

Robles, M. Tagueena-Martinez, J. del Rio, J. A. 1997. Effective conductivity of chemically deposited ZnO thin films. Thin Solid Films 293 320-326. 

Ristov, M. Sinadinovski, G. Grozdanov, I. Mitreski, M. 1987. Chemical deposition of ZnO films. Thin Solid Films 149 65-71. 

Olson, D. C. Piris, J. Collins, R. T. Shaheen, S. E. Ginley, D. S. 2006. Hybrid photovoltaic devices of polymer and ZnO nanofiber composites. Thin Solid Films 496 26-29. 

Changes in the electrical conductivity of a thin-semiconductive film when an eluate is adsorbed on the surface was used by Seiyama and co-workers (45). The response on a ZnO film (20-1000 A) depended upon the nature of the interaction. For electron acceptors, such as 02, a decrease in the conductivity was observed, while for electron donors such as ethyl alcohol and C02, an increase in the conductivity was measured. Temperatures of 200 °C or greater were necessary to avoid slow desorption rates and concomittant loss in resolution. Sensitivities were poor, but the phenomena are worth further scrutiny in hopes of obtaining materials exhibiting stable characteristics as well as... 

ZnO thin films can be prepared by a variety of techniques such as magnetron sputtering, chemical vapor deposition, pulsed-laser deposition, molecular beam epitaxy, spray-pyrolysis, and (electro-)chemical deposition [24,74]. In this book, sputtering (Chap. 5), chemical vapor deposition (Chap. 6), and pulsed-laser deposition (Chap. 7) are described in detail, since these methods lead to the best ZnO films concerning high conductivity and transparency. The first two methods allow also large area depositions making them the industrially most advanced deposition techniques for ZnO. ZnO films easily crystallize, which is different for instance compared with ITO films that can... 

Raman scattering was often applied for studying the phonon modes of ZnO bulk samples [31-38], It has become a fast and reliable tool to study ZnO thin films [29,38-43], and ZnO nano- and/or microstructures [44-46]. Raman scattering studies were also reported for ZnO samples doped with Li [43, 47, 48], N [43, 49-51], A1 [48, 52-54], P [43, 55], Mn [43, 56-58], Fe [43, 48], Co [43], Ni [43], Cu [43], Ga [48, 51], As [59], Ce [60], or Sb [48, 61], and (Mg,Cd)xZni a 0 [43, 62] samples. IR reflection [63-65] and transmission measurements [66-68] were reported mainly for ZnO bulk materials. IR optical studies of doped ZnO and ZnO-based thin films are, in general, restricted to transmission and reflection measurements in the near-IR (NIR) spectral region, and to highly conductive Al-doped ZnO thin films. Some experiments were performed in the mid-IR (MIR) spectral region [52,69,70], where the optical phonon modes can be studied. Recently, IRSE was applied to study undoped and doped ZnO films, and ZnO-based alloy films [30,38,43,62,71-74]. 

ZnO films can provide substantial information on chemical and electronic properties of ZnO surfaces and interfaces, which occur in real thin film solar cell structures. In addition, general information on the interface formation of oxide materials can be extracted. In the following we describe ... 

The CdS/ZnO interface is of particular importance in Cu(In,Ga)Se2 thin film solar cells because it is used in the standard cell configuration (Fig. 4.2). A first experimental determination of the band alignment at the ZnO/CdS interface has been performed by Ruckh et al. [102]. The authors have used ex-situ sputter-deposited ZnO films as substrates. The interface formation was investigated by stepwise evaporation of the CdS compound from an effusion cell. Photoelectron spectroscopy revealed a valence band offset of A Vb = 1.2eV. An identical value of 1.18eV has been derived using first-principles calculations [103]. With the bulk band gaps of CdS and ZnO of 2.4 and... 

The results presented in this section further illustrate that there is a considerable dependence of the band alignment at the CdS/ZnO interface on the details of its preparation. An important factor is the local structure of the ZnO film. There is considerable local disorder when the films are deposited at room temperature in pure Ar, deposition conditions that are often used in thin film solar cells. It is recalled that the disorder is only on a local scale and does not affect the long range order of the films, as obvious from clear X-ray diffraction patterns recorded from such films (see discussion in Sect. 4.2.3.3). Growth of sputter deposited ZnO on CdS always results in an amorphous nucleation layer at the interface. The amorphous nucleation layer affects the valence band offset. 

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