X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films

Heavily doped zinc oxide films are used as transparent and conductive electrodes, especially in thin film solar cells. Despite decades of research on zinc oxide it is not yet clear what the lower limit of the resistivity of such films is. Therefore, the electrical parameters of zinc oxide films deposited by magnetron sputtering, metal organic chemical vapour deposition and pulsed laser ablation are reviewed and related to the deposition parameters. It is found that the lowest resistivities are in the range of 1.4 to 2×10-4 Ω cm, independently of the deposition method. The highest reported Hall mobilities are about 60 cm2 V-1 s-1. The thin film electrical data are compared with the corresponding values of single crystalline zinc oxide and with that of boron and phosphorous doped crystalline silicon. From this comparison it can be seen that the dependence of the Hall mobilities on the carrier concentration n are quite similar for silicon and zinc oxide. In the region n>5×1020 cm-3, which is most important for the application of zinc oxide as a transparent and conductive electrode, phosphorous doped silicon has a mobility only slightly higher than zinc oxide. The experimental data on the electron and hole mobilities in silicon as a function of the impurity concentration have been described by a fit function (Masetti et al 1983), which can also be applied with different fitting parameters to the available zinc oxide mobility data. A comparison of the experimental data with the well known ionized impurity scattering theories of Conwell-Weisskopf (1946) and Brooks-Herring-Dingle (1955) shows that these theories are not able to describe the data very well, even if the non-parabolic band structure is taken into account. As in the case of silicon, an additional reduction of the mobility also occurs for zinc oxide for concentrations n>5×1020 cm-3, which can be ascribed qualitatively to the clustering of charge carriers connected with increased scattering due to the Z-2 dependence of the scattering cross section on the charge Z of the scattering centre. The presented review of the charge carrier transport in zinc oxide indicates that a physical limit due to ionized impurity scattering is reached for homogeneously doped layers. Due to the universal nature of this limitation it is suggested that it also applies to the other important materials indium-tin (ITO) and tin oxide. Experiments are proposed to overcome this limit.

https://iopscience.iop.org/article/10.1088/0022-3727/34/21/301/pdf

Resistivity of polycrystalline zinc oxide films: current status and physical limit

Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes

Magnetron sputtering of transparent conductive oxides (zinc oxide, indium tin oxide, tin oxide) is a promising technique which allows the deposition of films at low temperatures with good optical and electronic properties. A special advantage is the scalability to large areas. The principles underlying magnetron sputtering are reviewed in this paper. The growth process during magnetron sputtering is characterized by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to sputtered atoms with energies in the eV range, ions from the plasma (mostly argon) and neutral atoms (also argon) reflected at the target hit the growing film. Depending on the energy of these species and on the ion-to-neutral ratio the properties of the films vary. High energies ( 100 eV), which occur mainly at low sputtering pressures lead to damage of the growing film, connected with mechanical stress, small crystallites and bad electrical parameters. Ion assisted growth with low ion energies (below about 50 eV) is advantageous as is a high ion-to-neutral ratio. A compilation of resistivities of magnetron sputtered zinc oxide films yields a limiting resistivity of 2 × 10-4 cm for polycrystalline films. Based on the correlation between plasma parameters and film properties new research fields are anticipated.

https://iopscience.iop.org/article/10.1088/0022-3727/33/4/201/pdf

Magnetron sputtering of transparent conductive zinc oxide: relation between the sputtering parameters and the electronic properties

(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0–50at.%. The structural, optical, and electrical properties of (Zn,Al)O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region (<10at.%), Al-doped ZnO (AZO) thin films could be obtained at 400°C in an Ar–O2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4at.%, with low resistivity ∼10−4Ωcm, high transmittance ∼90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52eV at 4at.% Al due to the Burstein-Moss shift [E. Burstein, Phys. Rev. 93, 632 (1954)] modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the r...

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Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions

A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio ji/jn on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about Ei≈17 eV, the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio ji/jn on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances (ρ≈6×10−4 Ω...

Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties

Transparent and conducting ZnO(:Al) films deposited by simultaneous RF- and DC-excitation of a magnetron

A general drawback of the RF-magnetron sputtering technique is the small deposition rate, compared to DC-sputtering. This is caused by the low DC-voltage (40–160 V for RF-powers of 10–100 W) that develops at the target, since the sputtering rate and therefore the deposition rate in the energy range below 1 keV depend linearly on the acceleration voltage in the cathode fall. In order to increase the target voltage, a simultaneous excitation of the plasma by RF (13.56 MHz) and DC has been used. The DC-excitation is about 1.5 times as effective concerning the deposition rate compared to an RF-excitation. The mass selected ion energy distributions measured on a floating substrate display the characteristic differences between the two discharge modes. While the RF-excitation shows broad energy distributions, caused by the oscillating movement of the plasma sheath, and high plasma potentials (45 V), the DC-ion energy distributions exhibit sharp peaks at much lower plasma potentials (3 V). By a combination of DC- and RF-excitation the plasma potential and therefore the ion energy at a floating substrate can be shifted continuously between low (DC) and high (RF) values. ZnO- and ZnO:Al-layers from ceramic targets were prepared by this new combination for magnetron sputtering. The influence of the different ion energies at a floating substrate on the film structure and the electrical parameters is shown.

Characterization of a magnetron sputtering discharge with simultaneous RF- and DC-excitation of the plasma for the deposition of transparent and conductive ZnO:Al-films

Doped and undoped zinc oxide fims are widely used as transparent window and contact layers in thin film solar cells. Up to now the role of film stress on the layer and solar cell properties was not investigated in detail. In this paper the influence of the energy input from the magnetron sputtering plasma on the structural and electrical layer properties was investigated. The energy input during the deposition could be varied by using a simultaneous RF- and DC-excitation at a fixed sputtering pressure. The energy and the density of the ions bombarding the substrate at floating potential have been measured by a plasma monitor and by a Langmuir probe. Plasma monitor measurements displayed the different plasma potentials and ion energies at a floating substrate. While the DC-discharge is characterized by low plasma potentials and low ion energies, the RF-mode shows up to 2.5 times higher ion energies of EAr+ ≈ 50 eV. The structural and electrical properties of doped and undoped ZnO-films, deposited with different RF/DC-power ratios can be explained by taking into account the different ion energies and densities during the deposition. Higher ion energies (RF-excitation) improve the structural quality and reduce the film stress, which decreases the specific resistance, both by a higher charge carrier mobility as well as higher carrier concentrations. The lowest resistivities and film stresses were ≈ 6.10-4 Ωcm and 0.5 GPa, respectively. Layers with different film stresses have been used to prepare CuInS2/CdS/ZnO-solar cells. Lower stress in the ZnO-films leads to higher cell efficiencies. For the first time, these results can explain why RF-sputtering is almost exclusively used for the window preparation of thin film solar cells.

Polycrystalline ZnO- and ZnO:Al-Layers: Dependence of film stress and Electrical Properties on the Energy Input During the Magnetron Sputtering Deposition

R. Cebulla

Papers

Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties

Transparent and conducting ZnO(:Al) films deposited by simultaneous RF- and DC-excitation of a magnetron

Characterization of a magnetron sputtering discharge with simultaneous RF- and DC-excitation of the plasma for the deposition of transparent and conductive ZnO:Al-films

Polycrystalline ZnO- and ZnO:Al-Layers: Dependence of film stress and Electrical Properties on the Energy Input During the Magnetron Sputtering Deposition