Ternary Zn1-x Cdx O films were grown on (0001) sapphire substrates by pulsed laser deposition. The energy band gap of Zn1-x Cdx O films decreases with increasing Cd content. An increase of Cd content also leads to the emission broadening, absorption edge broadening, and crystallinity degradation. The absorption edge and ultraviolet emission energy shift to lower energy from 3.357 eV to 3.295 eV and 3.338 eV to 3.157 eV, respectively, with increasing Cd content from 0.3% to 3% at 4 K. The Stokes' shift between the absorption and emission is observed and that indicates the increase of exciton localization with Cd content. © 2006 American Institute of Physics

Crooker, SA

Jia, QX

Kang, HS

Kim, JH

Kim, JW

Lee, JK

Lee, JS

Lee, SY

Li, Y

Nastasi, MA

Ternary Zn1-xCdxO films were grown on (0001) sapphire substrates by pulsed laser deposition. The energy band gap of Zn1-xCdxO films decreases with increasing Cd content. An increase of Cd content also leads to the emission broadening, absorption edge broadening, and crystallinity degradation. The absorption edge and ultraviolet emission energy shift to lower energy from 3.357 eV to 3.295 eV and 3.338 eV to 3.157 eV, respectively, with increasing Cd content from 0.3% to 3% at 4 K. The Stokes’ shift between the absorption and emission is observed and that indicates the increase of exciton localization with Cd content

Kang, Hong Seong

Kim, Jae Won

Kim, Jong Hoon

Lee, Sang Yeol

Li, Y.

Lee, Jang-Sik

Lee, Jung-Kun

Nastasi, M. A.

Crooker, S. A.

Jia, Q. X.

Name not available

Optical property and Stokes’ shift of Zn1-xCdxO thin films depending on Cd content\ud

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D-Scholarship@Pitt

Optical property and Stokes’ shift of Zn 1 − x Cd x O thin films depending on CdcontentHong Seong Kang, Jae Won Kim, Jong Hoon Kim, Sang Yeol Lee, Y. Li, Jang-Sik Lee, J. K. Lee, M. A. Nastasi,S. A. Crooker, and Q. X. Jia  Citation: Journal of Applied Physics 99, 066113 (2006); doi: 10.1063/1.2186372 View online: http://dx.doi.org/10.1063/1.2186372 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/99/6?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Oxygen effects on radiation hardness of ZnO thin films J. Vac. Sci. Technol. B 27, 2232 (2009); 10.1116/1.3222865  Effect of annealing on the spectral and nonlinear optical characteristics of thin films of nano-ZnO J. Appl. Phys. 104, 033118 (2008); 10.1063/1.2949400  Composition dependence of electrical and optical properties in sol-gel Mg x Zn 1 − x O thin films J. Appl. 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JiaMaterials Science and Technology Division, Los Alamos National Laboratory, Los Alamos,New Mexico 87545Received 15 November 2004; accepted 19 February 2006; published online 29 March 2006Ternary Zn1−xCdxO films were grown on 0001 sapphire substrates by pulsed laser deposition. Theenergy band gap of Zn1−xCdxO films decreases with increasing Cd content. An increase of Cdcontent also leads to the emission broadening, absorption edge broadening, and crystallinitydegradation. The absorption edge and ultraviolet emission energy shift to lower energy from3.357 eV to 3.295 eV and 3.338 eV to 3.157 eV, respectively, with increasing Cd content from0.3% to 3% at 4 K. The Stokes’ shift between the absorption and emission is observed and thatindicates the increase of exciton localization with Cd content. © 2006 American Institute of Physics.DOI: 10.1063/1.2186372ZnO, a wide and direct band gap about 3.37 eV II-VIsemiconductor, has found many applications in differentfields. For example, it can be used as the material base forheterojunction laser diodes, ultraviolet UV lasers, UVsolid-state emitters and detectors, transparent conductivecontacts, thin-film gas sensors, varistors, solar cells, surfaceelectroacoustic wave devices, and others.1–3 Interest in shortwavelength display devices based on ZnO has also been get-ting more attention recently.1–3 An important step to designZnO-based devices with high emitting efficiency is the bandgap engineering in order to fabricate the quantum wells thatare necessary in devices such as light emitting diode LEDand laser diode LD.4 Fabrication and characterization ofalloys such as Mg,ZnO or Cd,ZnO are important from theviewpoint of band gap engineering as well as a p-n junction.5Many results are reported related to ZnMgO.5–7 However,ZnCdO has not been widely studied.2,5,8 Moreover, theStokes’ shift in ZnCdO is rarely reported. In this paper, wereport the experimental results of band gap modulation,structural characteristics, spectral broadening, and Stokes’shift in ZnCdO films as a function of Cd content.Thin Zn1−xCdxO films were grown on 0001 sapphiresubstrates by pulsed-laser deposition PLD, using a pulsedXeCl laser operating at a wavelength of 308 nm and a rep-etition rate of 5 Hz. The target was a ceramic pellet whichconsists of 80 wt % ZnO and 20 wt % CdO. The substratetemperatures during deposition were systematically variedfrom 200 °C to 800 °C while the oxygen pressure wasmaintained at 350 mTorr. Cd content in ZnO depending ondeposition temperature from 200 °C to 800 °C was changedfrom 3% to 0.3%. Experimental details to deposit Zn1−xCdxOfilms and mechanism can be found elsewhere.2 The structuralproperties of the Zn1−xCdxO films were investigated by x-raydiffraction XRD. Zn1−xCdxO films have the thickness of700 nm. The stoichiometry of thin films was estimated usingRutherford backscattering spectroscopy RBS. The opticalproperties of the films were characterized by photolumines-cence PL using a He–Cd laser as a light source having anexcitation wavelength of 325 nm and absorption measure-ments, both at 4 K.All of the films studied were preferentially oriented asindicated by the only 0002 diffraction. Figure 1 shows thefull width at half-maximum FWHM values of the x-rayrocking curve from 0002 diffraction peaks and emissionbroadening from PL spectra as a function of Cd content. TheFWHMs of the x-ray rocking curve from 0002 diffractionpeaks and emission broadening increase with the Cd contentx in Zn1−xCdxO films. This indicates the structural degrada-tion of film and optical properties’ degradation as the Cdcontent increases.A comparison of photoluminescence PL and absorptionmeasured at 4 K for Zn1−xCdxO films is shown in Fig. 2. Thebroadening of both emission energy and absorption edge in-aElectronic mail: sylee@yonsei.ac.krFIG. 1. The FWHM values of rocking curve from 0002 diffraction andemission broadening from PL spectra as a function of Cd content.JOURNAL OF APPLIED PHYSICS 99, 066113 20060021-8979/2006/996/066113/3/$23.00 © 2006 American Institute of Physics99, 066113-1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.49.166.8 On: Fri, 07 Nov 2014 17:23:06creases as the emission peak energy decreases. This increaseof broadening is mainly due to the increase of disorder in thesemiconductor, which leads to the appearance of localizedelectron and/or hole states.9–11 Two localization mechanismshave been suggested.11 One is that exciton is localized byCoulomb interaction in very deep potential wells. The otheris a case of wide potential well with dimensions larger thanthe excitonic Bohr radius. In this case, the exciton is local-ized as a whole in a wide potential well. R. W. Martin et al.suggested the additional mechanism.10 An additional localiz-ing mechanism may be found in the nature of the segregatedquantum dots confinement effect or alternatively from thelarge piezoelectric fields present in wurtzite semiconductorsCoulomb effect. Emission and absorption spectra arebroadened due to increase of these localized states. Gener-ally, absorption spectrum has three regions11 as shown in Fig.2. In region I, the absorption is weak and is caused by im-purities in the alloy or amorphous semiconductor, i.e., atomsof a different chemical nature. The region II is where theabsorption coefficient increases exponentially with photonenergy. This region is also known as the “Urbach tail.”11 Itsorigin in disordered semiconductor is, however, not the in-teraction with phonons, but simply the exponential tail of thedensity of states of localized excitons. Region III is related toan optical and mobility gap. The region II, which is related tothe density of localized states, increases with the Cd contentshown in Fig. 2. The increase of localized states in semicon-ductor leads to the increase of emission and absorptionbroadening. This, in turn, causes the Stokes’ shift.Figure 3 shows the measured and referred values of PLpeak position of Zn1−xCdxO depending on Cd content. Forthe comparison, the results reported by Th. Gruber et al.8 inFig. 3 were included. The solid line shown in Fig. 3 showsthe estimated curve that roughly follows a nonlinear behav-ior, Eq. 1.EPL = 3.37 − 12.4x + 92.28x2eV . 1The difference between the solid line peak and the dottedline edge in Fig. 2 exhibits the Stokes’ shift of Zn1−xCdxO.The Stokes’ shift, defined as the energy difference betweenenergy band gap and the peak of the emission spectrum,9 isplotted as a function of Cd content shown in Fig. 4. Theo-retical results reported by Th. Gruber et al. were introducedin Fig. 4 to suggest the more exact Stokes’ shift data ofZn1−xCdxO depending on Cd content. Th. Gruber et al. re-ported that they performed absorption measurements to de-termine the Stokes’ shift, but could not be analyzed becauseFIG. 2. Photoluminescence solid lines and absorption dotted lines spec-tra of Zn1−xCdxO thin films at 4 K as a function of Cd content x.FIG. 3. Photoluminescence peak energy of Zn1−xCdxO as a function of Cdcontent x. Solid line shows the estimated curve of results of Ref. 8 and thiswork, Eq. 1.FIG. 4. Stokes’ shift plotted as a function of Cd content for Zn1−xCdxO.Solid line shows the estimated curve of results of theoretical results inducedby Ref. 8 and this work, Eq. 2.066113-2 Kang et al. J. Appl. Phys. 99, 066113 2006 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.49.166.8 On: Fri, 07 Nov 2014 17:23:06of the absence of clear absorption edge from the Zn1−xCdxOlayers. However, Stokes’ shift was clearly observed in thiswork as shown in Fig. 4. The Stokes’ shift increased as Cdcontent increased. The solid line shown in Fig. 4 exhibitsStokes’ shift curve of Zn1−xCdxO that follows a nonlinearbehavior, Eq. 2. The curve of Stokes’ shift of Zn1−xCdxOwas represented like a linear curve as shown in Fig. 4 despitethe nonlinear equation, Eq. 2.Ess = 0.02 + 4.34x + 46.77x2eV . 2The mechanism of Stokes’ shift of ZnCdO in this workcould be explained as follows. Disorder in ZnCdO increaseswith the Cd concentration. The increase of disorder inZnCdO leads to degraded crystallinity and introduces morelocalized states. The increase of localized states in ZnCdOgives rise to the increase of Stokes’ shift as a function of Cdcontent as shown in Figs. 3 and 4. The results as shown inFigs. 3 and 4 exhibit a little difference between theoreticaland experimental results, results in this work and referredresults. This is caused by crystallinity degradation and crys-tallinity difference.Disorder and potential fluctuation generated in alloyedand heavily extrinsic dopant doped semiconductors cause theincrease of localized states. This increase of localized stateswith increasing content of alloyed atoms and dopants leadsto an increase of Stokes’ shift and broadening of the emissionand absorption edges.7,9–13 These effects are observed invarious structures such as quantum well structures,14–20quantum dot structures in alloyed semiconductors,7,9,10 andheavily doped semiconductors.12,13 In view of the structureand experimental results, the results in this investigation arecloser to quantum dot model in alloyed semiconductors.7,9,10However, these models are based on ideal structure materialshaving high crystallinity and homogenity. Therefore, the ex-act application and analysis using above mentioned modelare necessary for more investigation on the homogeneity ofZnCdO, the relationship between optical properties and thecrystallinity of ZnCdO, and the relationship between Stokes’shift and crystallinity degradation.In summary, II-VI semiconductor alloying Zn1−xCdxOthin films 0.003x0.03 were grown by PLD. A system-atic increase in emission broadening with decreasing emis-sion peak energy is observed. The investigation on emissionand absorption spectra of Zn1−xCdxO has enabled the dem-onstration of a linear dependence of the Stokes’ shift on theemission peak energy, in which the Stokes’ shift increaseswith Cd content caused mainly by the localized states inZnCdO films.This work was supported by Grant No. R01-2004-000-10195-0 2005 from the Basic Research Program of the Ko-rea Science and Engineering Foundation.1Hong Seong Kang, Jeong Seok Kang, Jae Won Kim, and Sang Yeol Lee,J. Appl. Phys. 95, 1246 2004.2Sang Yeol Lee, Y. Li, Jang-Sik Lee, J. K. Lee, M. Natasi, S. A. Crooker,Q. X. Jia, Hong Seong Kang, and Jeong Seok Kang, Appl. Phys. Lett. 85,218 2004.3D. C. Look, D. C. Reynolds, C. W. Litton, R. L. Jones, D. B. Eason, andG. Cantwell, Appl. Phys. Lett. 81, 1830 2002.4M. Fukuda, Optical Semiconductor Devices John Wiley & Sons, Inc,New York, 1998, pp. 93–210.5T. Makino, Y. Segawa, M. 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Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:130.49.166.8 On: Fri, 07 Nov 2014 17:23:06