Transition between grain boundary and intragrain scattering transport mechanisms in boron-doped zinc oxide thin films

Abstract

A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3 ϫ 10 19 to 2 ϫ 10 20 cm −3Transparent conductive oxides ͑TCOs͒ are an essential part of thin-film silicon solar cells. To contact the cell and act as a transparent window, TCOs have to exhibit a high conductivity and a high optical transmittance. In addition, they have to scatter the light at the TCO-cell interface in order to increase the effective absorption of light within the active layers.1,2 Boron-doped zinc oxide ͑ZnO:B͒ layers deposited by low pressure chemical vapour deposition ͑LPCVD͒ have been intensively developed in our institute.3 This material is especially attractive for thin-film solar cell technology, because of its low cost, and of the wide availability of its constituent raw materials. Furthermore, the LPCVD technique is well suited for large-scale device fabrication. 4 These films are constituted of large grains with a pronounced 9-18 preferential crystallographic direction. The extremities of the grains appear at the growing surface as large pyramids, which yield an as-grown rough surface texture that efficiently diffuses the light that enters into the solar cell. 1,3The optical and electrical properties of TCO films have been extensively investigated and recently reviewed. Ellmer 5 and Minami 6 discussed the limit of the resistivity of such films by analyzing reported data for ZnO films deposited by different techniques. They found that the electron mobility in undoped films is mainly limited by grain boundary scattering, whereas for doped layers intragrain scattering mechanism is predominant. But the transition between these two mechanisms within a given ZnO film series could not be evidenced yet. In the present work, the scattering mechanism limiting the electron mobility is determined by the comparison, of the value of the optical mobility ͑as evaluated by using the classical Drude model͒ with the value of the Hall mobility. When both values are markedly different, the mechanism limiting the electron mobility is attributed to grain boundary scattering. When both values are similar, the limiting mechanism is attributed to intragrain scattering. This approach is commonly applied to other TCOs. [7][8][9][10][11] The wide range of carrier density easily achievable in boron-doped LPCVD ZnO by varying the gas flow ratio allows us to observe within one single film system the transition from one transport mechanism to the other. T...

Electron scattering mechanisms in GZO films grown on a-sapphire substrates by plasma-enhanced molecular beam epitaxy

“…4 The aforementioned is supported by Ellmer and Mientus, 5 Minami et al, 6 Steinhauser et al, 7 and Ruske et al 8 Minami et al 6 have concluded that grain boundary scattering is mainly dominant in AZO films with carrier concentrations in the range 10 19 -10 20 cm À3 while the ionized impurity scattering is dominant in layers with carrier concentrations in the range 10 20 -10 21 cm À3 . Steinhauser et al 7 concluded that the grain boundaries do not limit the conductivity for carrier concentrations exceeding 1 Â 10 20 cm À3 based on the comparison between Hall mobility and optical mobility for boron-doped ZnO. By comparing the Hall mobility and the optical mobility, Ruske et al 8 boundaries do not limit the conductivity only for ZnO:Al films when the electron concentration is above $5 Â 10 20 cm À3 .…”

Controlling the resistivity gradient in aluminum-doped zinc oxide grown by plasma-enhanced chemical vapor deposition

“…8 There are several deposition techniques applied to synthesize ZnO, such as sol-gel, 14 spray pyrolisis, 15 magnetron sputtering, [16][17][18] pulsed laser deposition, 19,20 atomic layer deposition, 21,22 and metalorganic chemical vapor deposition (MO-CVD). 23 Gas phase-based techniques, as MO-CVD, have also shown potential to grow high quality aluminumdoped ZnO (ZnO:Al) layers 24,25 at deposition rates as high as $14 nm/s 26 on a large surface area (>10 cm 2 ). 25,26 In the plasma CVD techniques, the overall heat load of the process is lowered, as the substrate temperature is reduced down to the range of 100-200 C. For example, in the past years, we have shown that the expanding thermal plasma (ETP) leads to good quality ZnO:Al layers deposited up to 1 nm/s with a resistivity of 8 Â 10 À4 X cm for 1100 nm film thickness, at a substrate temperature of 200 C, as reported by Volintiru et al 27 One of the drawbacks of the CVD processes, [27][28][29] unlike the sputtering approach, is the development of a gradient in resistivity as function of the film thickness, usually present over a large thickness range.…”

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

“…Instead of q opt , most authors compute the mobility l opt . [29][30][31][32][33][34][35][36][37][38][39][40] The determination of l opt by modeling of optical spectra requires the effective mass m*, whereas the resistivity q opt can be evaluated without knowing m*. As the effective mass is prone to considerable uncertainties, 30,35,36,40 we will focus on the resistivity q opt .…”