Depth dependent properties of ITO thin films grown by pulsed DC sputtering

Abstract

A systematically prepared set of ITO layers for solar cell applications has been analyzed by spectroscopic variable angle ellipsometry in order to trace the dependence of free carriers' distribution along the film depth as a function of film thickness as well as its change upon annealing. Samples were deposited on silicon substrates with various thicknesses in steps of approximately 10-20 nm. This set was duplicated and these samples were annealed, so that for each thickness an as-deposited and an annealed sample is available. Conventionally measured electrical conductivity and morphological properties (AFM measurements) of the films have been compared with the optical constants' inhomogeneity, i.e. material properties along the film thickness modelled by variable-angle spectroscopic ellipsometry. The obtained results show that the optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. For the deposition conditions used a well-determined reproducible non-uniform distribution of free carriers within the film thickness was determined. In particular it has been found that the majority of free carriers in as-deposited ultra-thin ITO films is concentrated at sample half-depth, while their distribution becomes asymmetric for the thicker films, with a maximum located at approximately 40 nm depth. The distribution of free carriers in annealed samples is qualitatively different from that of as-deposited layers.

Limitations of ultra-thin transparent conducting oxides for integration into plasmonic-enhanced thin-film solar photovoltaic devices

“…This is a well-known phenomenon (Burstein-Mess shift) which is a result of ITO optical band gap shifting towards higher energies when annealed in FG or H 2 gas. This is attributed to increase in carrier concentration and is well documented [25]. In addition, it appears that among the annealed samples, 20 min gives the optimum transmittance for thicknesses below 50 nm, especially at large wavelengths.…”

“…The few exceptions include Sychkova et al [24], who reported both optical and electrical properties of 9-80 nm ITO films deposited by pulsed DC sputtering varied with thickness and showed a general increase in resistivity with decrease in film thickness [24]. Other notable studies on ultra-thin ITO films using various deposition techniques include the following: Chen et al who used filtered cathodic vacuum arc (FCVA) to deposit 30-50 nm on heated quartz and Si substrates [25]; Tseng and Lo, who used DC magnetron sputter for 34.71-71.64 nm ITO film on PET (polyethylene terephthalate) [26]; Kim et al who used RF magnetron sputter for films between 40 and 280 nm deposited on PMMA substrate heated at 70°C [27]; Alam and Cameron, who used sol-gel process for 50-250 nm film deposited on titanium dioxide film [20]; and Betz et al who used planar DC magnetron sputtering for 50, 100 and 300 nm films on glass substrates [28]. The results from these few thin TCO studies reveal a pattern in which resistivity increases rapidly as film thickness decreases from 50 to 10 nm.…”

Fabrication of ITO particles using a combination of a homogeneous precipitation method and a seeding technique and their electrical conductivity

“…Volume resistivities of ITO films are in the range of 10 −3 -10 −4 cm, which have been reported by several researchers [4,5,[11][12][13]. The volume resistivities obtained in the present work were larger than the reported volume resistivities.…”

“…Tin-doped indium oxide, or indium tin oxide (ITO), is the most representative among transparent electro-conductive materials used for opto-electronic devices such as displays, solar cells and sensors [1][2][3][4][5][6]. ITO films can be produced using various methods such as electron beam evaporation, PVD, sputtering, CVD and spray [5,[7][8][9][10][11][12][13][14][15]. Although these methods are conventionally used and fundamentally and industrially studied, they have some disadvantages as follows.…”

Transparent and conductive Al‐doped ZnO films for solar cells applications

“…It is worth noting, that the growth process of AZO film ensures an inhomogeneous profile of optical constants through the film depth. Similarly, to the case of ITO films 23, 24, angularly resolved measurements are necessary for a more precise characterization of the film optical constants, that is out of the scope of this work. Further investigations are in progress and will be published in a dedicated paper.…”