Materials Letters 62 (2008) 3373–3375

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Materials Letters

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Structural, optical and electrical properties of In doped CdO thin films foroptoelectronic applications

R.K. Gupta a,⁎, K. Ghosh a, R. Patel b, S.R. Mishra c, P.K. Kahol a,⁎a Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri-65897, USAb Roy Blunt Jordan Valley Innovation Center, Missouri State University, Springfield, Missouri-65806, USAc Department of Physics, The University of Memphis, Memphis, TN 38152, USA

⁎ Corresponding authors. Tel.: +1 417 836 4467; fax:E-mail addresses: ramguptamsu@gmail.com (R.K. Gu

PawanKahol@missouristate.edu (P.K. Kahol).

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.03.015

A B S T R A C T

A R T I C L E I N F O

Article history:

Thin films of indium doped Received 17 December 2007Accepted 6 March 2008Available online 14 March 2008

Keywords:SemiconductorElectrical propertiesThin filmsCadmium oxideOptical materials and properties

cadmium oxide were deposited on quartz substrate using pulsed laser depositiontechnique. The effect of growth temperature and partial oxygen pressure on structural, optical and electricalproperties was studied. We find that the optical transparency of the films largely depends on the growthtemperature, while partial oxygen pressure has virtually no effect on the transparency of the films. Electricalproperties are found to be sensitive to both the growth temperature and oxygen pressure. It is observed thatconductivity and carrier concentration decreases with temperature. The film grown at 200 °C under anoxygen pressure of 5.0×10−4 mbar shows high mobility (155 cm2/V s), high carrier concentration(1.41×1021 cm3), and low resistivity (2.86×10−5 Ω cm).

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxides (TCOs), such as doped zinc oxide,indium oxide, cadmium oxide, etc have been widely used fortransparent electrodes [1–3]. These materials have attracted research-ers due to their high electrical conductivity and high optical trans-mittance. Due to their unique optical and electrical properties, TCOsare used for photovoltaic solar cells, phototransistors, liquid crystaldisplay, optical heaters, gas sensors, transparent electrodes and otheroptoelectronic devices [4–6]. Among these TCOs, cadmium oxide(CdO) has attracted considerable attention for various applicationssuch as solar cells, photodiodes etc, due to its low electrical resistivity,high carrier concentration and high optical transmittance in thevisible region of the solar spectrum [7]. CdO is an n-type semicon-ductor with band gap of approximately 2.5 eV [8].

Thin films of CdO and doped CdO have been prepared by variousdeposition techniques such as spray pyrolysis [9], sol–gel [10], ionbeam sputtering [11], magnetic sputtering [7,12], chemical vapordeposition, etc [13]. Martin et al have deposited multilayer of TCOsusing Sn doped CdO and Sn doped CdIn2O4 using pulsed laserdeposition technique [14]. To the best of our knowledge no detailsstudy based on growth parameters such as substrate temperature andoxygen pressure have been done. In this communication we are

+1 417 836 6226.pta),

l rights reserved.

reporting the effect of growth temperature and oxygen pressure onstructural, optical and electrical properties of indium doped cadmiumoxide (CdInO) thin films grown by pulsed laser deposition (PLD).

2. Experimental details

An indium doped CdO target for the pulsed laser deposition wasprepared by standard solid-state reaction method using high purity

Fig. 1. Transmittance spectra of CdInO films grown at different temperatures (in vacuum).

Fig. 2. XRD patterns of CdInO film grown at different temperatures on quartz substrate.

Fig. 3. AFM image of CdInO film grown at 300 °C.

3374 R.K. Gupta et al. / Materials Letters 62 (2008) 3373–3375

In2O3 (99.999%) and CdO (99.999%). Required amounts of In2O3 andCdO were taken by molecular weight and mixed thoroughly to obtaina target for PLD. The well-ground mixture was heated in air at 800 °Cfor 8 h. The powder mixture was cold pressed using hydraulic press at6×106 N/m2 load and sintered at 900 °C for 10 h.

The thin films were deposited on quartz substrate using KrF excimerlaser (Lambda Physik COMPex, λ=248 nm and pulsed duration of 20 ns)under different growth conditions such as at different substrate

Fig. 4. (a) Effect of substrate temperature and (b) oxygen pressure on res

temperature and at different oxygen pressure in the PLD chamber. Thelaserwasoperatedat apulse rateof 10Hz,withanenergyof 300mJ/pulse.The laser beam was focused onto a rotating target at a 45° angle ofincidence. Thinfilmsweredeposited at room temperature, 200 °C, 400 °C,and 600 °C (under vacuum of base pressure 1.0×10−6 mbar) and underoxygen pressures of 2.5×10−4 mbar, 5.0×10−4 mbar, 7.5×10−4 mbar,1.0×10−3 mbar, 5.0×10−3 mbar, 1.0×10−2 mbar, 5.0×10−2 mbar, and1.0×10−1 mbar (at substrate temperature of 200 °C). The depositionchamber was initially evacuated to 1.0×10−6 mbar and during depositionoxygen gas was introduced into the chamber to obtain the pressuresmentioned above. The growth rate for the films was ∼3 nm/min.

The structural characterization was performed using X-RayDiffraction (XRD) and Atomic Force Microscopy (AFM). The XRDspectra of all the films were recorded with Bruker AXS x-ray dif-fractometer using the 2θ–θ scan with CuKα (λ= 1.5405 Å) radiationoperated at 40 kV and 40 mA. AFM imaging was performed underambient conditions using a Digital Instruments (Veeco) Dimension-3100 unit with Nanoscope® III controller, operated in tapping mode.The optical transmittance measurements weremade using UV–visiblespectrophotometer (Ocean Optics HR4000).

The resistivity and Hall coefficient measurements were carried outby a standard four-probe technique. Gold contacts were used for allelectrical measurements. The thickness of the films were measuredusing AFM and is approximately 100 nm. The film resistivities havebeen determined by taking the product of resistance and filmthickness. The Hall effect was measured with the magnetic field ap-plied perpendicular to film surface in the Van der Pauw configuration[15]. Carrier concentration and carrier mobility were calculated atroom temperature using Hall coefficient and resistivity data [16].

3. Results and discussions

Fig. 1 shows the effect of substrate temperature on optical transmittance of CdInOthin films. It has been observed that optical transparency of the films first increases withincreases in temperature and then decreases with increase in substrate temperature.The films grown at 200 °C show the highest optical transmittance. The averagepercentage transmittance in the range of 400–700 nm is 67%, 85%, 81% and 77% for thefilms grown at 27 °C, 200 °C, 400 °C, and 600 °C respectively. It is observed that partialoxygen pressure has virtually no effect on the transparency of the films.

Fig. 2 shows the representative XRD patterns for the films grown at differenttemperature. The XRD patterns obtained for the samples produced on quartz substrateshave preferential growth along the (200) direction. The observed diffraction patternsindicate the polycrystalline nature of the CdO with cubic structure on the basis of PDFCard No: 005–0640 data. No extra peaks due to the addition of indium in cadmiumoxide films were observed which indicates the absence of an impurity phase in thefilms.

Fig. 3 shows the AFM image of a CdInO thin film grown at 200 °C under vacuum on ascale of 5×5 μm. It can be seen that the surface of the film is very smooth which isbelieved to be due to the formation of a solid solution with a crystal structure. The rootmean square (rms) roughness, the average roughness and the peak to valley roughness

istivity (ρ), carrier concentration (n), and mobility (μ) of CdInO films.

3375R.K. Gupta et al. / Materials Letters 62 (2008) 3373–3375

was found to be 1.1 nm, 0.83 nm and 9.9 nm respectively. The peak to valley roughnessis a very important parameter compared to the rms roughness for optoelectronicsdevices [17]. The leakage current of the device increases with increase in peak to valleyroughness. The peak to valley roughness of the devices based on tin doped indium oxideis 16.4 nm [17].

The effect of substrate temperature and oxygen pressure on the electricalproperties CdInO thin films are discussed next. The carrier concentration (n) is derivedfrom the relation n=1/e ·RH, where RH is the Hall coefficient and e is the absolute valueof the electron charge. The carrier mobility (μ) is determined using the relation μ=1/neρ, where ρ is resistivity [16].

The effect of growth temperature on electrical resistivity of CdInO thin films isshown in Fig. 4(a). We find that the resistivity increases continuously with increase inthe substrate temperature. Also the carrier concentration decreases with increase insubstrate temperature. The substrate temperature strongly affects the mobility of thesefilms. The electron mobility continuously increases with increase in growth tem-perature. The mobility increases form 56 to 96 cm2/V s as the growth temperatureincreases from room temperature to 600 °C. The decrease in carrier concentration maybe caused by the annealing out of point defects and interstitial impurities [18]. Theannealing out of point defects and interstitial impurities results in decrease in impurityscattering and an increase in the mobility. From the Hall mobility and carrier con-centration, the resistivity is expected to decrease with increasing substrate tempera-ture. Fig. 4(b) shows the dependence of oxygen pressure on the electrical properties ofCdInO films. It is observed that the resistivity, carrier concentration, and mobility aresensitive to oxygen pressure. The electrical resistivity of the films increases with oxygenpressure. The carrier concentration, however, continuously decreases with oxygenpressure, decreasing from 1.97×1021 cm−3 to 1.56×1020 cm−3 as oxygen pressure in-creases from 2.5×10−4 mbar to 1.0×10−1 mbar. The mobility, on the other hand, initiallyincreases with an increase in the oxygen pressure and then it decreases. Mobility ashigh as 155 cm2/V s has been obtained. The low mobility of the films grown under highoxygen pressure is believed to be due to collisional energy loss of the particles withoxygen during their arrival toward the substrate surface [19].

4. Conclusions

Thinfilms of indiumdoped cadmiumoxideweredeposited onquartzsubstrate to study the effect of growth temperature and oxygenpressureon structural, optical and electrical properties. These films show highmobility (155 cm2/V s), high carrier concentration (1.41×1021 cm3), and

lowresistivity (2.86×10−5Ω cm). IndiumdopedCdOthinfilms grownviapulsed laser deposition could be an excellent candidate for futureoptoelectronic devices.

Acknowledgement

This work is partially supported by Research Corporation (awardnumber CC6166) for KG. RKG thankfully acknowledges Missouri StateUniversity for the Term Research Assistant Professor position.

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