Carrier transport in polycrystalline ITO and ZnO:Al II: The influence of grain barriers and boundaries

doi:10.1016/j.tsf.2007.10.082

Thin Solid Films

Volume 516, Issue 17, 1 July 2008, Pages 5829-5835

https://doi.org/10.1016/j.tsf.2007.10.082 Get rights and content

Abstract

ITO and ZnO:Al films have been deposited by magnetron sputtering from ceramic and metallic targets at different substrate temperatures and with different plasma excitation modes: DC and RF (13.56 and 27.12 MHz). Temperature-dependent conductivity and Hall measurements (down to 50 K) were used to determine the carrier concentrations N_D and the Hall mobilities μ. From the μ(N_D) dependences, which were fitted by a carrier transport model taking into account ionized impurity and grain barrier scattering, the trap densities at the grain boundaries were estimated. ITO films show much lower trap densities down to N_t ≈ 1.5 · 10¹² cm^− 2, compared to N_t values up to 3.10¹³ cm^− 2 for ZnO:Al films. The temperature-dependent mobilities were fitted by a phenomenological model with a T-independent term and a metal-like contribution or a thermally-activated part due to grain barrier-limited transport.

Seebeck coefficient measurements as a function of the carrier concentration give hints to different transport mechanisms in ITO and ZnO.

Introduction

Though transparent conductive oxides (TCOs) which combine high transparency in the visible and near infrared spectral range with a high electrical conductivity are today of high technological importance for flat panel displays and thin film solar cells, the electrical transport mechanisms are not well understood. Mostly, in papers reporting on electrical properties of TCOs either no theoretical explanation at all [1] or only ionized impurity scattering as dominant mobility limitation are given [2], [3]. Only few thorough papers dealt with other scattering processes than ionized impurity scattering: Pisarkiewicz et al. measured the mobility of CdIn₂O₄ and SnO₂ thin films as a function of the carrier concentration N [4], [5]. In the theoretical description of the mobility dependence μ(N) they took into account grain barrier and ionized impurity scattering, but not the relatively low lattice mobility of these TCO materials. Also Minami et al. used these two scattering processes to describe his comprehensive μ(N) data for magnetron-sputtered ZnO and ZnO:Al films [6], which were referenced already in our earlier review papers [7], [8]. Minami et al. also did not include the lattice mobility of ZnO, which is about 200 V s/cm² (see our review [7] and [8]). Pisarkiewicz et al. estimated the trap density at grain boundaries for CdIn₂O₄, inducing the electrical grain barriers, in the order of 1.5 · 10¹³ cm^− 2. Minami et al. mentioned the grain barrier mobility limitation but did not give a trap density value.

Recent reviews of the electrical parameters (carrier concentration and Hall mobility) of TCO films show a significant scattering of the experimental data [7], [9], which point to the probable influence of other scattering processes not yet taken into consideration. Recently, we have presented a comparison of the carrier transport in ZnO and ITO [8]. There we have shown that for sufficiently high carrier concentrations the grain barrier scattering is not active due to the narrow width of the barriers between grains which can be tunneled by the electrons. It was found that a significant variation of the mobilities in the carrier concentration range N > 3 · 10²⁰ cm^− 3 occurs. For lower carrier concentrations, i.e. with increasing width of the grain barriers the mobilities decrease, especially for ZnO. Both, own data and data reported in literature were taken into account for this comparison. The striking difference between ZnO and ITO was that this decrease occurred at much lower carrier concentrations in ITO than in ZnO, which was explained by significantly different grain barrier trap densities N_t for both TCOs. While undoped and doped ZnO layers exhibit, depending on the deposition method, N_t values between 5 · 10¹² and 3 · 10¹³ cm^− 2, ITO films typically show a lower grain boundary trap density as low as 1.5 · 10¹² cm^− 2. Furthermore, up to now it is not clear, why the resistivity of ITO films is significantly lower (about a factor of 2 to 4) than that of ZnO, though the general material data of both TCO materials do not favor ITO with respect to carrier transport, see also our earlier review [7]. Therefore, in this paper we present experiments to achieve a better understanding of the general transport mechanisms in TCO films. It is well known, that in sputtering discharges in electronegative species, in our case oxygen, ions with high energies occur, which can significantly influence the film growth and its properties [10], [11], [12]. In our recent paper [8] on the comparison between carrier transport in ZnO and ITO we had presented arguments for an influence of the particle energies in the deposition process on the mobility.

In the present paper the influence of the energy of the species (ions, energetic neutrals, sputtered atoms) contributing to the film growth is investigated by varying the discharge voltages using different plasma excitation frequencies: DC, 13 and 27 MHz.

Section snippets

Transport processes in polycrystalline semiconductors

The transport in polycrystalline materials and especially semiconductors is much more complex compared to that in single crystals. The best investigated polycrystalline semiconductor is poly-Si, for which the model of the grain barrier-limited transport was demonstrated convincingly by Seto [13]. A review of polycrystalline silicon was given in [14]. In earlier papers we had already reviewed both ionized impurity scattering [7] as well as neutral impurity and dislocation scattering and the

Experimental details

Both, doped and undoped zinc oxide (ZnO) as well as tin-doped indium oxide (ITO) films were prepared by magnetron sputtering from 76 mm targets in two load-lock sputtering systems. The target-to-substrate distance was 6.5 cm. Reactive sputtering from metallic InSn10 wt.% and nonreactive sputtering from ceramic In₂O₃SnO₂10 wt.% were performed. The plasma excitation was done by DC and by radio frequency (13.56 and 27.12 MHz) with sputtering powers from 25 to 300 W. Typical sputtering pressures

Mobility versus carrier concentration

Fig. 1 compares the mobilities of ITO films with that of ZnO:Al films deposited at different substrate temperatures and plasma excitation modes. Part of these data was reported in an earlier publication [8]. In this work the plasma excitation frequency has been extended to 27 MHz for the ITO films. Depositions at room temperature and at 300 °C have been performed. For comparison semiempirical fit curves of the single crystal data both for ZnO and In₂O₃ have been included, which were taken from

Conclusions

The mobilities as a function of the carrier concentration μ(N_D) have been measured for ZnO:Al and ITO films, deposited by magnetron sputtering with different plasma excitation frequencies and at different substrate temperatures. These μ(N_D) curves can be analyzed by a combined transport model taking into account ionized impurity and grain barrier scattering, leading to different trap densities N_t at the grain boundaries for ZnO:Al (up to 3 · 10¹³ cm^− 3) and for ITO (down to 1.5 · 10¹² cm^− 3). While

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Carrier transport in polycrystalline ITO and ZnO:Al II: The influence of grain barriers and boundaries

Abstract

Introduction

Section snippets

Transport processes in polycrystalline semiconductors

Experimental details

Mobility versus carrier concentration

Conclusions

Thin Solid Films

Thin Solid Films

Thin Solid Films

J. Crystal Growth

Thin Solid Films

Microelectron. Eng.

Thin Solid Films

Thin Solid Films

Surf. Coat. Technol.

Thin Solid Films

Solid-State Electr.

J. Appl. Phys.

Phys. Status. Solidi. (A)

J. Phys. D: Appl. Phys.

MRS Bull.

Jap. J. Appl. Phys.

J. Appl. Phys.

J. Appl. Phys.

J. Appl. Phys.

Polycrystalline Silicon for Integrated Circuit Applications

The Electrical Properties of Metals and Alloys