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

  • View

  • Download

Embed Size (px)


  • s of In doped CdO thin lms for

    . Kahol a,ngeld, Missouri-65897, USAri-65806, USA

    Thin lms


    (1.4110 cm ), and low resistivity (2.8610 cm).

    Os), suhaveterials

    display, optical heaters, gas sensors, transparent electrodes and other

    Materials Letters 62 (2008) 33733375

    Contents lists available at ScienceDirect

    Materials Letters

    e lsev ie locate /mat le toptoelectronic devices [46]. 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 lms of CdO and doped CdO have been prepared by variousdeposition techniques such as spray pyrolysis [9], solgel [10], ionbeam sputtering [11], magnetic sputtering [7,12], chemical vapordeposition, etc [13]. Martin et al have deposited multilayer of TCOs

    using Sn doped CdO and Sn doped CdIndeposition technique [14]. To the best of oustudy based on growth parameters such as suoxygen pressure have been done. In this

    Corresponding authors. Tel.: +1 417 836 4467; fax:E-mail addresses: (R.K. Gu (P.K. Kahol).

    0167-577X/$ see front matter 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.03.015ectrical properties, TCOsransistors, liquid crystal

    An indium doped CdO target for the pulsed laser deposition wasprepared by standard solid-state reaction method using high puritymittance. Due to their unique optical and elare used for photovoltaic solar cells, phototers due to their high electrical conductivity and high optical trans-Optical materials and properties

    1. Introduction

    Transparent conducting oxides (TCindium oxide, cadmium oxide, etctransparent electrodes [13]. These mach as doped zinc oxide,been widely used forhave attracted research-

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

    2. Experimental detailsCadmium oxide 2008 Elsevier B.V. All rights reserved.SemiconductorElectrical properties oxygen pressure of 5.01

    21 3 5Structural, optical and electrical propertieoptoelectronic applications

    R.K. Gupta a,, K. Ghosh a, R. Patel b, S.R. Mishra c, P.Ka Department of Physics, Astronomy, and Materials Science, Missouri State University, Sprib Roy Blunt Jordan Valley Innovation Center, Missouri State University, Springeld, Missouc Department of Physics, The University of Memphis, Memphis, TN 38152, USA

    A B S T R A C TA R T I C L E I N F O

    Article history:Received 17 December 2007Accepted 6 March 2008Available online 14 March 2008


    Thin lms of indium dopedtechnique. The effect of groproperties was studied. Wetemperature, while partial oproperties are found to be sconductivity and carrier co

    j ourna l homepage: www.2O4 using pulsed laserr knowledge no detailsbstrate temperature andcommunication we are

    +1 417 836 6226.pta),

    l rights reserved.mium oxide were deposited on quartz substrate using pulsed laser depositiontemperature and partial oxygen pressure on structural, optical and electricald that the optical transparency of the lms largely depends on the growthen pressure has virtually no effect on the transparency of the lms. Electricalitive to both the growth temperature and oxygen pressure. It is observed thatntration decreases with temperature. The lm grown at 200 C under anmbar shows high mobility (155 cm2/V s), high carrier concentrationFig. 1. Transmittance spectra of CdInO lms grown at different temperatures (in vacuum).

  • 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. Thinlmsweredeposited at room temperature, 200 C, 400 C,and 600 C (under vacuum of base pressure 1.0106 mbar) and underoxygen pressures of 2.5104 mbar, 5.0104 mbar, 7.5104 mbar,1.0103 mbar, 5.0103 mbar, 1.0102 mbar, 5.0102 mbar, and1.0101 mbar (at substrate temperature of 200 C). The depositionchamber was initially evacuated to 1.0106 mbar and during depositionoxygen gas was introduced into the chamber to obtain the pressuresmentioned above. The growth rate for the lms was 3 nm/min.

    The structural characterization was performed using X-RayDiffraction (XRD) and Atomic Force Microscopy (AFM). The XRDspectra of all the lms were recorded with Bruker AXS x-ray dif-fractometer using the 2 scan with CuK (= 1.5405 ) 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 UVvisible

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

    3374 R.K. Gupta et al. / Materials Letters 62 (2008) 33733375In2O3 (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 at6106 N/m2 load and sintered at 900 C for 10 h.

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

    Fig. 3. AFM image of CdInO lm grown at 300 C.

    Fig. 4. (a) Effect of substrate temperature and (b) oxygen pressure on resspectrophotometer (Ocean Optics HR4000).The resistivity and Hall coefcient measurements were carried out

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

    3. Results and discussions

    Fig. 1 shows the effect of substrate temperature on optical transmittance of CdInOthin lms. It has been observed that optical transparency of the lms rst increases withincreases in temperature and then decreases with increase in substrate temperature.The lms grown at 200 C show the highest optical transmittance. The averagepercentage transmittance in the range of 400700 nm is 67%, 85%, 81% and 77% for thelms 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 lms.

    Fig. 2 shows the representative XRD patterns for the lms 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: 0050640 data. No extra peaks due to the addition of indium in cadmiumoxide lms were observed which indicates the absence of an impurity phase in thelms.

    Fig. 3 shows the AFM image of a CdInO thin lm grown at 200 C under vacuum on ascale of 55 m. It can be seen that the surface of the lm 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 roughnessistivity (), carrier concentration (n), and mobility () of CdInO lms.

  • 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 lms are discussed next. The carrier concentration (n) is derivedfrom the relation n=1/e RH, where RH is the Hall coefcient and e is the absolute valueof the electron charge. The carrier mobility () is determined using the relation =1/ne, where is resistivity [16].

    The effect of growth temperature on electrical resistivity of CdInO thin lms isshown in Fig. 4(a). We nd 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 theselms. 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 lms. It is observed that the resistivity, carrier concentration, and mobility aresensitive to oxygen pressure. The electrical resistivity of the lms increases wi