Al-doped ZnO

The photovoltaic devices were then completed with a 50-nm layer of chemically deposited CdS, 50 nm of radio frequency (RF) sputtered intrinsic ZnO, and 350 nm of Al-doped ZnO and bilayer Ni/Al top contacts deposited by e-beam. Finally, a 100-nm layer of MgF2 is deposited by e-beam to minimize... 

Al-doped ZnO films were also deposited by adding AICI3 to the deposition solution. The amount of A1 in the films (given as at.% with respect to the Zn concentration) was somewhat smaller than that in the deposition solution but was proportional to the concentration in solution (up to the maximum measured concentration in the films of 5.5%). 

Fig. 1.15. Electron concentration (dashed line) of Sn-doped indium oxide and Al-doped ZnO in dependence on oxygen partial pressure for a dopant concentration of 1 % [117]. With increasing oxygen partial pressure the donors become compensated by oxygen interstitials (In20s) or by zinc vacancies (ZnO). Reprinted with permission from [117]. Copyright (2007) by the American Physical Society...

For polycrystalline films this limited understanding is nicely illustrated by our own mobility data for Al-doped ZnO films (see Fig. 2.15) deposited both on glass and sapphire substrates as a function of the carrier concentration... 

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]. 

Fig. 3.17. Experimental (dotted lines) and best-model (solid lines) IRSE spectra of an highly Al-doped ZnO thin film (d 1400 nm) grown by PLD on sapphire [43].

Electronic surface properties including Fermi level positions, work functions, and ionization potentials of sputter-deposited ZnO and Al-doped ZnO films in dependence on deposition parameters. The results provide insight into aspects of doping, surface chemistry, and terminations. 

Fig. 4.6. X-ray photoelectron spectra of undoped ZnO (a—c) and of Al-doped ZnO (d—f) prepared by magnetron sputtering. The spectra are excited with monochromatic A1 Ka radiation (hv = 1486.6 eV). ZnO Al films are prepared from a target containing 2 wt % Al. The films are prepared with 100 % Argon as sputter gas either at room temperature (a and d) or at a substrate temperature of 400° C (b and e). Spectra (c) and (f) are recorded from films deposited onto samples held at room temperature in a sputter gas mixture of 50 % argon and 50 % oxygen...

To study the influence of the preparation conditions on the interface properties, a number of different interfaces have been prepared. Details of the preparation and the determined valence band offsets are listed in Table 4.2. The experiments include not only both deposition sequences, but also interfaces of Al-doped ZnO films, which have been conducted to elucidate the role of the undoped ZnO film as part of the Cu(In,Ga)Se2 solar cell. Details of the experimental procedures and a full set of spectra for all experiments are given in [70]. Table 4.2 includes a number of interfaces between substrates of undoped ZnO films and evaporated CdS layers (ZOCS A-D). In a recent publication [90] different values were given for the valence band offsets, as the dependence of BEvb(CL) on the deposition conditions was not taken into account in this publication. 

X-ray photoelectron spectra recorded during interface formation of magnetron sputtered Al-doped ZnO with an evaporated In2S3 substrate are shown in Fig. 4.35. The In2S3 substrate has been deposited at 250°C substrate temperature and the ZnO Al was deposited at room temperature in pure Ar, resulting in a degenerately doped film. The valence band maximum after the last deposition step (not shown) is at Ep — Eyb = 3.9 0.1 eV. 

Fig. 4.35. Core levels and cation Auger levels of an In2S3 substrate during sputter deposition of Al-doped ZnO (experiment ISZA-B). The deposition times are indicated in seconds. In2S3 was deposited at 250°C and ZnO Al at room temperature in pure Ar. Reproduced with permission from [136]...

Interface formation between In2S3 and ZnO has also been studied for the reverse deposition sequence with Al-doped ZnO films used as substrates [70]. In this case, only degenerately doped substrates were used. Photoemission spectra indicate no chemical reactivity at the surface. 

A1 Al-doped ZnO thin him as ohmic back contact of ZnO hlms with Pd Schottky contact on top, improved frequency response enables capacitance spectroscopy DLTS [57,59]... 

Figure 18.3 Density of state for Al-doped ZnO. Al 3s donor level appears in condution bands. Al 3p is a condution level.

Fukuoka O, Matsunami N, Tazawa M, Shimura T, Sataka M, Sugai H, Okayasu S, Irradiation effects with 100 MeV Xe ions on optical properties of Al-doped ZnO films, Nucl. Instrum. Methods Phys. Res. B, 250, 295-299, 2006. 

Matsunami N, Fukuokaa O, Tazawa M, Sataka M, Electronic structure modification of ZnO and Al-doped ZnO films by ions. Surf. Coat. Tech., 196, 50-55, 2005. 

Motivated by this prediction we developed the sample preparation process that uses photolithography and self-aligned pattern transfer for defining lateral interfaces in thin-film structures and that will be detailed in the following. In fig. 1, a lateral structure with interfaces between undoped ZnO and Al-doped ZnO is shown as the typical result of this process. A... 

Elm, M. T, Henning, T., Klar, P. J. Szyszka, B. (2008). Effects of artificially structured micrometer holes on the transport behavior of Al-doped ZnO layers. Applied Physics... 

Tsubota, T., Ohtaki, M., Eguchi, K., and Arai, K. (1997) Thermoelectric properties of Al-doped ZnO as a promising oxide material for high temperature thermoelectric conversion. /. Mater. Chem., 7 (1), 85. 

Wiff, J.P., Kinemuchi, Y., Kaga, H., Ito, C., and Watari, K. (2009) Correlations between thermoelectric properties and effective mass caused by lattice distortion in Al-doped ZnO ceramics. /. Eur. 

Cai K.F., Muller E., Drasar C., Mrotzek A. Preparation and thermoelectric properties of Al-doped ZnO ceramics. Mater. Sci. Eng. B 2003 104 45 8 Candy J.P., Fouilloux P., Keddam M., Takenouti H. The characterization of porous-electrodes by impedance measurements. Electrochimica Acta 1981 26 1029-1034 Choi Y.M., Pyun S.I., Moon S.I., Hyung Y.E. A study of the electrochemical lithium intercalation behaviour of porous LiNi02 electrodes prepared by solid-state reaction and sol-gel methods. J. Power Sources 1998 72 83-90... 

The most famous study of doped ZnO for TE purpose was reported by Tsubota et who showed that in the Al -doped ZnO series, a ZT 0.3 at 1273 K is reached for air-prepared ceramics Zno.98Alo.02O. As shown in Figure 4.35, the Al " doping, expected to create donor levels (electrons) to the conduction band, strongly reduces the electrical resistivity p) as compared with ZnO. At RT, p decreases by more than 3 orders of magnitude, the semiconducting behaviour of ZnO being replaced by almost T independent p values. 

Klemm, S. Pust, S. Hassel, A. Hiipkes, J. Mayihofer, K. J. 2012. Electrochemical texturing of Al-doped ZnO thin films for photovoltaic apphcations. J. Solid State Electrochem., 16, 283—290. 

Yun SN, Lim S (2011) Improved conversion efficiency in dye-sensitized solar cells based on electrospun Al-doped ZnO nanofiber electrodes prepared by seed layer treatment. J Solid State Chem 184(2) 273-279... 

Transparent conductive oxides such as indium-tin oxide (ITO), fluorine-doped tin oxide (FTO) and Al-doped ZnO (AZO) are the most commonly used transparent electrodes for optoelectronic devices. Among them, ITO has been extensively applied for both opaque cells and ST-OPV cells owing to its low sheet resistance (10-15 Q sq ) and high optical transmittance (-90%). ITO can be coated on either glass substrate or plastic substrate and can be used as the bottom transparent electrode in conjunction with other types of transparent electrode as the top electrode. Indeed, in one of the earliest demonstrations of the concept of the ST-OPV device, ITO was used as both the bottom and top electrodes, facilitated by a lamination method to stick together two individually... 

Matsubara et al. [126] used oxygen radical-assisted PLD to grow highly transparent and low-resistivity Al-doped ZnO films at room temperature. A KrF excimer laser (X = 248 nm, 30 ns pulse width, 10 Hz repetition rate) was used for ablation. The oxygen partial pressure during deposition was 0.7-1.4 x 10 Torr, and the applied RF power was 150W. The distance between the target and the substrate was approximately 6 cm. The minimum resistivity of the obtained transparent films was... 

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