Elsevier

Thin Solid Films

Volume 517, Issue 15, 1 June 2009, Pages 4286-4294
Thin Solid Films

Investigation of the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy

https://doi.org/10.1016/j.tsf.2008.11.134Get rights and content

Abstract

Thin films of In2O3 have been grown on Y-stabilised ZrO2(100) substrates by oxygen plasma assisted molecular beam epitaxy over a range of substrate temperatures between 650 °C and 900 °C. Growth at 650 °C leads to continuous but granular films and complete extinction of substrate core level structure in X-ray photoelectron spectroscopy. However with increasing substrate temperature the films break up into a series of discrete micrometer sized islands. Both the continuous and the island films have excellent epitaxial relationship with the substrate as gauged by X-ray diffraction and selected area electron diffraction and lattice imaging in high resolution transmission electron microscopy.

Introduction

Stoichiometric indium oxide (In2O3) is a transparent insulator. It is amenable to degenerate n-type doping with Sn cations to give so-called indium tin oxide (ITO). ITO is one of a very restricted range of materials which combines the property of optical transparency in the visible region with a high electrical conductivity. The optimal conductivity in ITO is higher than in related materials such as Sb-doped SnO2 and thus ITO is the transparent conducting oxide of choice in many technological areas [1], [2], [3], [4]. Despite the near ubiquitous application of ITO in liquid crystal displays, solar cells and electroluminescent display devices, little effort has been directed toward growth of high quality single crystal thin films of In2O3 or ITO. Not surprisingly then many aspects of the fundamental physics of In2O3 have remained controversial, including even the magnitude and nature of the bulk bandgap. Absorption measurements on single crystal In2O3 carried in 1967 showed a weak absorption onset at around 2.67 eV, attributed to indirect optical transitions [5], with a stronger absorption onset at 3.75 eV. Nonetheless the bandgap of In2O3 was for many years quoted to be 3.75 eV [6], [7], [8]. However the valence band onset in photoemission measurements is less than 3 eV below the Fermi energy [9]. This observation is inconsistent with a bandgap of 3.75 eV unless there is pronounced upward band bending at the surface [10], [11]. However it has recently been shown the bandgap is in fact direct, but transitions from states toward the top of valence band into the conduction band are either dipole forbidden or have minimal dipole intensity: this explains the ~ 1 eV shift between weak and strong optical absorption onsets [12].

To date most work on growth of high quality single crystal In2O3 films has concentrated on deposition of In2O3 on alumina [13] and yttria-stabilised zirconia single crystal substrates by carefully controlled pulsed laser deposition (PLD) [14], [15], [16] (i.e. “laser” molecular beam epitaxy), although there are some reports of single crystal growth metalloorganic chemical vapour deposition [17] and by molecular beam epitaxy (MBE) [18], [19], [20] using conventional indium Knudsen cells and oxygen atom plasma sources. These considerations have prompted us to initiate a programme concerned with growth of In2O3 thin films on cubic zirconia by oxygen plasma assisted MBE. ZrO2 itself has a low symmetry monoclinic structure at room temperature, but a cubic phase can be stabilized by replacement of some of the Zr(IV) with larger cations such as Ca(II) or Y(III), with concomitant introduction of compensating oxygen vacancies. The face centred cubic fluorite structure of Y-stabilised ZrO2 belongs to the space group Fm3 m. The lattice parameter of this phase increases with Y doping level. For the minimum Y concentration of around 17% required to stabilize the cubic phase the lattice parameter can be estimated as a = 5.1423 Å [21], [22], [23], whilst for 28% Y-doping a = 5.2100 Å [21]. The body centred cubic bixbyite structure of In2O3 belongs to the space group Ia3 [24] and has a tabulated cubic lattice parameter a = 10.1170 Å. It is derived from a 2 × 2 × 2 superstructure of the fluorite structure with 1/4 of the anion sites vacant. Thus at 17% Y-doping there is a mismatch of 1.6% between 2a for Y-ZrO2 and a for In2O3, increasing to 3.0% at 28% Y doping. Moreover the two structures involve basically similar cation arrays but with 1/4 of the anion sites of the fluorite structure vacant in In2O3 so that the cations are 6-coordinate rather than 8-coordinate as in fluorite. Thus Y-doped ZrO2 appears to be an ideal substrate for growth of well-ordered thin films of In2O3.

In the present communication we explore the mode of growth of In2O3 on Y-stabilised ZrO2(100) substrates. The main parameter that has been varied is the substrate temperature. It emerges that for films with a nominal thickness of about 120 nm, growth at 650 °C leads to near-continuous epitaxial films, but with a columnar or granular structure and a high density of macroscopic imperfections. However as the growth temperature is increased the films break up into islands and at the highest temperature studied of 900 °C there is spontaneous development of a growth mode which leads to striking arrays of highly oriented truncated square pyramidal micrometre sized epitaxial “dots” with a narrow size distribution. We have also studied the variation in growth morphology with coverage for films grown at 900 °C and the influence of high temperature annealing on films deposited at 650 °C.

Section snippets

Experimental details

Indium oxide layers were grown on 1 cm × 1 cm Y-stabilised ZrO2(100) substrates with a nominal Y doping level of 17% (as defined by the formula Zr1-xYxO2-x/2 with x = 0.17) in an ultrahigh vacuum oxide MBE system (SVT, USA) system with a base pressure of 5 × 10 8 Pa [25], [26]. This incorporated liquid nitrogen cooled cryopanels, a conventional indium Knudsen cell and a radio frequency plasma oxygen atom source operated at 200 mW RF power with an oxygen background pressure of 2 × 10 3 Pa. The nominal

AFM studies of growth morphology

AFM images over a 10 µm × 10 µm area of In2O3 deposited on 17% Y-doped ZrO2(100) in 3 × 103 s (50 min) deposition runs over a range of substrate temperatures between 650 °C and 900 °C are shown in Fig. 1. Deposition at 650 °C leads to continuous films, but with an obvious granular or columnar structure defined by square edges (Fig. 1a). Cross sectional transmission electron microscopy (TEM), discussed below, gave a film thickness of 120 nm. The root mean square (RMS) roughness of the film in 10 µm × 

Discussion

The combination of AFM, XPS and HRTEM establishes that In2O3 grows on 17% Y-doped ZrO2(100) as a continuous epitaxial film for a substrate temperature of 650 °C. There is however evidence for significant strain in the films and the surface is quite rough over length ranges of the order 1 µm. The obvious columnar structure suggests incipient break up of the film into the islands characteristic of higher temperature growth. Hall effect measurements discussed in detail elsewhere [29] show that the

Conclusions

We have found that the morphology of In2O3 films grown on Y-stabilised ZrO2(100) by oxygen plasma assisted molecular beam epitaxy shows a very strong dependence on the substrate temperature. Continuous and epitaxial but nonetheless rough films are obtained with a substrate temperature of 650 °C, but at 900 °C substrate temperature the films break up into an array of highly oriented square pyramidal islands with lateral dimensions of order 1 µm. The switchover from continuous to island growth is

Acknowledgements

This work was supported by EPSRC Grant GS/S94148. The NCESS facility at Daresbury Laboratory is funded by EPSRC Grant EP/025722/1.

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