Phys. Status Solidi A 209, No. 5, 949–952 (2012) / DOI 10.1002/pssa.201127549 p s sa

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applications and materials science

Raman scattering characterizationof ZnSe/Zn0.6Cd0.4Se multilayers

prepared by thermal vacuum evaporation

Diana Nesheva*,1, Maja Scepanovic2, Zdravka Aneva1, Zelma Levi1, Zoran V. Popovic2, and Ilko Miloushev1

1 Institute of Solid State Physics, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria2Center for Solid State Physics and New Materials, Institute of Physics, University of Belgrade, Pregrevica 118,

11080 Belgrade, Serbia

Received 16 September 2011, revised 9 January 2012, accepted 18 January 2012

Published online 9 February 2012

Keywords multiple quantum wells, photoconductivity, Raman scattering, thermal evaporation, zink-cadmium-selenide

*Corresponding author: e-mail nesheva@issp.bas.bg, Phone: þ359 29795686, Fax: þ359 29753632

Nanocrystalline multilayers from ZnSe/Zn0.6Cd0.4Se with

three different layer thicknesses (3.5, 5.0, 10.0 nm) and, for

comparison, 400 nm thick Zn0.6Cd0.4Se nanocrystalline single

layers have been prepared by thermal evaporation of ZnSe and

CdSe in vacuum. Raman scattering spectra have beenmeasured

in the range 100–1000 cm�1 under excitation with the 488

and 514.5 nm lines of an Arþ laser. Series of four bands have

been seen in the spectra of all samples. They have been related

to replicas of the longitudinal optical LO-phonon in the

ZnxCd1�xSe layers and interpreted as an indication for

crystallinity of both kinds of samples. A blue shift of the 1 LO

maximum has been observed, which has been connected with

an increase of the compressive stress in the ternary layers when

the layer thickness decreases. It has also been obtained that

the ratio of the integrated intensity of the 1 LO band in the

spectra taken with the 488 and 514.5 nm lines decreases with

decreasing Zn0.6Cd0.4Se layer thickness. The result has been

assigned to size-induced increase of the optical band gap of

the layers. This conclusion has been confirmed by spectral

photocurrent and optical transmission measurements.

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1 Introduction Ternary alloys of II–VI semiconduc-tors included in zinc-selenide based quantum structures havedemonstrated considerable promise as short-wavelengthlight sources, fast switching devices, etc. [1–3]. The interestin inclusion of such alloys (i.e. ZnCdSe, ZnSSe, etc.) inlow-dimensional structures is high [4–8] since their latticeconstant and optical properties can be changed by varyingthe composition which gives them advantages over binarycompounds (ZnSe, CdSe, CdS).

For preparation of multilayers (MLs) with good stabilityand periodicity, molecular beam epitaxy or metal-organicchemical vapour deposition (MOCVD) are most frequentlyapplied. However, both techniques are quite expensiveand, in addition, MOCVD normally uses dangerous gases.Besides, high quality crystalline ZnSe or GaAs substratesare necessary for the epitaxial growth. Therefore aconsiderable research activity has been addressed topreparation of low-dimensional structures by nonepitaxialfilm growth and application of less expensive vapourdeposition ([9] and references therein) or electrodeposition[10] techniques.

In several previous papers [9, 11, 12] we have reportedon preparation of amorphous and amorphous/nanocrystal-line multilayers with good artificial periodicity and smoothinterfaces in which carrier confinement has been observed.In this study, preparation of ZnSe/ZnxCd1�xSe periodicMLsby thermal evaporation of ZnSe and CdSe in high vacuumis reported. Raman scattering, spectral photocurrent, andoptical transmissionmeasurements are carried out to explorethe film microstructure and size induced changes in theelectronic structure of the Zn0.6Cd0.4Se layers in MLs.

2 Experimental details Two kinds of samples: (i)multilayers of ZnSe/ZnxCd1�xSe and (ii) single layers ofZnxCd1�xSe, for comparison, were prepared by thermalevaporation of ZnSe and CdSe powder (Merck, Suprapure)loaded in two separate crucibles that can be independentlyheated. The crucibles were provided with cylindrical screenswhich were not intentionally heated. Thus each materialwas evaporated in a quasi-closed volume. When depositingZnxCd1�xSe layers the substrates, nominally at roomtemperature, were rotated and during each turn they passed

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

950 D. Nesheva et al.: Raman scattering characterization of ZnSe/Zn0.6Cd0.4Se multilayersp

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Figure 1 (online colour at: www.pss-a.com) Raman spectra of aZn0.6Cd0.4Se single layer and ZnSe/Zn0.6Cd0.4Se MLs with layerthicknesses of 10, 5.0 and 3.5 nm taken under excitation with the514.5 nm line. All spectra, recorded at identical experimental con-ditions, correspond to the same intensity scale with the top three ofthem vertically displaced for clarity.

over the two simultaneously heated crucibles spending 1/12of the turn time above each crucible and 5/12 of the turn timebetween the two crucibles. The substrate rotation was startedafter achieving a deposition rate of 1.5 nm/s for both CdSeand ZnSe. The deposition rates and nominal layer thick-nesses were controlled by two preliminary calibrated quartzmicrobalance systems MIKI FFV. Thus ZnxCd1�xSe layerswere prepared by alloying of CdSe and ZnSe sublayers withnominal thickness of �0.37 nm, deposited at each turn. TheZnSe layers of MLs were deposited at the same way but onlythe ZnSe cruciblewas heated. Both kind of layers inMLs hadequal thickness of 3.5, 5.0 or 10 nm and the total thickness ofall MLs was 200 nm; the thickness of the single layers was400 nm.

The ZnxCd1�xSe composition was determined on singlelayers by Energy Dispersive Spectroscopy measurementsusing a JEOL Scanning Electron Microscope (JSM-6390)operating at an accelerating voltage of 25.00 kV. Thesamples were uncoated; the element used for optimizationwas Iron. A value of x¼ 0.6 has been obtained.

Raman scattering measurements were performed inpseudo-backscattering geometry using the 488 nm (2.54 eV)and 514.5 nm (2.41 eV) lines of an Arþ laser with a powerdensity of�0.3W/cm2 at the sample surface). A Jobin-YvonU1000 double monochromator and a photomultiplier asdetector were used. All experiments were carried out at roomtemperature in air on layers deposited on crystalline p-Sisubstrate.

Spectral photocurrent (PC) measurements were carriedout at room temperature. The samples for these measure-ments were deposited on Corning 7059 glass substrates andprovided with planar contacts of melted indium (�10mmlong and�1mm spaced). They showed Ohmic-like current–voltage characteristics at the applied fields of 102–103V/cm.The samples were illuminated by a chopped (2Hz)monochromatic light from a diffraction grating monochro-mator MDR 2 at a spectral resolution of 4 nm. Themeasurements were carried out in the 0.4–1.0mm spectralrange. The samples on Corning 7059 glass substrates werealso used in optical transmissionmeasurements performed inthe 400–2500 nm spectral range at room temperature in airby means of a Perkin Elmer spectrophotometer, modelLamda 1050.

3 Results and discussion Raman scattering spectraof a Zn0.6Cd0.4Se single layer and three ZnSe/Zn0.6Cd0.4SeMLs with different layer thicknesses are shown in Fig. 1.

A series of four longitudinal optical (LO) Raman peaks(at �240, �480, �720, �960 cm�1) is observed in thespectra of all samples. This series originates from so-calledLO-phonon replicas, well known in polar crystallinesemiconductors. The observed LO-phonon replicas in thespectra of Zn0.6Cd0.4Se layers indicate that in both kinds ofsamples the Zn0.6Cd0.4Se layers are nanocrystalline ratherthan amorphous. Previous high resolution electron micro-scopy measurements on SiOx/CdSe MLs deposited in asimilar way have indicated that the CdSe layers are

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

nanocrystalline with nanocrystal size in direction perpen-dicular to the layer plane being equal to the layer thickness[9, 12]. Atomic force microscopy measurements performedusing a Multimode V (Veeco, Santa Barbara, CA) micro-scope operating in tapping mode have revealed [13] that theZn0.6Cd0.4Se single layers are nanocrystalline with a grainsize around or less than 20 nm. Based on these observationswe assume that the layers in ZnSe/Zn0.6Cd0.4Se MLs arenanocrystalline with a grain size which is close to the layerthickness.

The LO phonons demonstrate a single phonon beha-viour, expected for ZnxCd1�xSe solid solutions [14] with the1 LO maximum at �240 cm�1. In the spectra of all samplesit is disposed between the frequencies of the 1 LO modes instoichiometric ZnSe (252 cm�1) and CdSe (210 cm�1).The LO band is asymmetric and has quite large FWHM(�17 cm�1), which has been related [15] to lattice distortiondue to the interfaces between nanocrystals and somevariations in the film composition on the nanoscale. TheRaman spectra of Zn0.6Cd0.4Se layers were fitted [15]to a sum of three Lorentzian bands: a band at 190 cm�1

assigned to transverse optical (1 TO) phonons, a second band(at 225 cm�1) related to 1 LO phonons in Cd-rich nanosizedregions and a third band (at�239 cm�1) related to scatteringfrom 1LO phonons in most of the film volume. Theformation of Cd-enriched regions in ZnxCd1�xSe filmsgrown by MBE has been confirmed by several researchgroups [16–18]. No appreciable scattering from the ZnSelayers of MLs (with 1 LO band expected at �252 cm�1)is seen. This could be explained with the low total thicknessof ZnSe (100 nm) and the larger Eo

g of ZnSe single layers(2.6 eV) than the energy of the incident laser lines.

As known the size reduction relaxes the Raman selectionrules and therefore LO and TO Raman lines ‘red’ shift andasymmetrically broaden to the low energy side. Relativelysimple formulas can be found in the literature [19, 20], giving

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Phys. Status Solidi A 209, No. 5 (2012) 951

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Figure 2 (online colour at: www.pss-a.com) Raman shift of the1 LOmaximumwith decreasingZn0.6Cd0.4Se layer thickness. Exci-tation by the 514.5 nm (a) and 488 nm (b) laser line was applied. Allspectra in each figure correspond to the same intensity scalewith thetop three of them vertically displaced for clarity.

Figure 3 Ratioof the integrated intensity Iof the1 LORamanbandin the spectra measured under excitation by the 488 and 514.5 nmlaser lines.

Figure 4 (online colour at: www.pss-a.com) Photocurrent spectraof a Zn0.6Cd0.4Se single layer and a 10 nm ML.

a relationship between the Raman shift/full width at halfmaximum of the Raman band and the crystal size.Experimental Raman spectra have been used to evaluatethe average size of crystals in nanocrystalline Si andGe filmsas well as of CdS, CdSe nanocrystals embedded in variousmatrices. However, in the case of ZnxCd1�xSe the existenceof the second band at 225 cm�1 strongly affects the shape ofthe main band at 239 cm�1 and this makes impossiblenanocrystal size estimation in our case.

Figure 2(a) and (b) depicts the 1 LO maximum in thespectra of a single layer and three MLs with different layerthickness taken under excitation with the 514.5 and 488 nmlaser lines, respectively.A gradual blue shift is observedwithdecreasing Zn0.6Cd0.4Se layer thickness. Since no appreci-able compositional changes are expected, one can relatethe blue shift to an increase of the internal strain in theZn0.6Cd0.4Se layers with decreasing layer thickness.

It is also seen from Fig. 2 that the intensity of thescattered light increaseswith decreasing layer thickness. Theincrease is by a factor of�3 and�5 for the 514.5 and 488 nmlines, respectively, were used when excitation. The observedeffect is possibly a manifestation of resonance enhancementof the Raman scattering when the energy of incident lightapproaches the optical band gap energy,Eo

g. This implies that

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the energies of the incident light (2.41 and 2.54 eV) areclose to the optical band gap energy of the 5.0 nm ML andespecially to the Eo

g of the 3.5 nmML, but not to the Eog of the

Zn0.6Cd0.4Se single layer and the layers in the 10 nm ML.Indeed, previous optical transmission investigations haveshown [14] that the optical band gap of the Zn0.6Cd0.4Sesingle layer is Eo

g � 2.1 eV. From the results shown inFig. 2 one can assume that the Eo

g of the 3.5 nm MLs lies inthe 2.41–2.54 eV energy range while Eo

g of the 5.0 nm MLis close but less than 2.41 eV.

Figure 3 shows the ratio of the integrated intensity of the1 LO Raman band in the spectra excited by the 488 and514.5 nm lines. One can see that the intensity of the Ramanscattering from the 3.5 nm ML excited by the 488 nm line(I488) is higher than that excited by the 514.5 nm line (I514).The situation is opposite for the other three samples, wherethe intensity ratio I488/I514 is lower than 1 and decreases withincreasing layer thickness. This result supports the aboveassumption for the optical band gap energies of the samples.Based on the Raman data it can be concluded that a sizeinduced increase of Eo

g takes place in the MLs with thethinnest layers, which is in the range 0.3–0.4 eV. This is anindication of carrier confinement in Zn0.6Cd0.4Se quantumwells.

Experimental data related to the spectral photosensi-tivity of Zn0.6Cd0.4Se layers and 10 nmMLs are presented inFig. 4. The photocurrent spectra of the 3.5 and 5.0MLs are

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

952 D. Nesheva et al.: Raman scattering characterization of ZnSe/Zn0.6Cd0.4Se multilayersp

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Figure 5 (online colour at: www.pss-a.com) Optical transmissionspectra of a Zn0.6Cd0.4Se single layer and three ZnSe/Zn0.6Cd0.4SeMLs with different layer thickness denoted in the figure.

not shown, because of low signal-to-noise ratio, which is dueto a strong decrease of the photocurrent. Most likely, thisincrease is caused by the increase of the integrated interfacearea both between the layers in MLs and between the grainsof the layers when the layer thickness decreases. One can seefrom the photocurrent spectra shown in Fig. 4 that the PCmaximum of the ML is blue shifted by about 0.1 eVcompared to the PC maximum of the Zn0.6Cd0.4Se singlelayer. This result confirms the above assumption for sizeinduced increase of Eo

g of the Zn0.6Cd0.4Se layers of MLs,which is due to carrier confinement. In addition, it indicatesthat, indeed, Eo

g of the 10 nm ML is rather far even from theenergy of green laser line.

Optical transmission spectra of the samples here studiedare depicted in Fig. 5. They show a gradual blue shift withdecreasing layer thickness that gives an additional evidencefor the above assumed size induced increase of the opticalband gap of the Zn0.6Cd0.4Se layers in MLs with decreasingthickness.

4 Conclusions A new technique for preparation ofZnSe/Zn0.6Cd0.4Se multilayers by thermal vacuum evapor-ation of ZnSe and CdSe has been reported. A blue shift ofthe 1 LO band related to the Raman scattering from theZn0.6Cd0.4Se layers has been observed with decreasing layerthickness. The shift has been correlated with an increase ofthe compressive stress in these layers.

Size-induced increase of the optical band gap of theZn0.6Cd0.4Se layers has been assumed on the basis of Ramanscattering results and confirmed by spectral photocurrentand optical transmission measurements. This conclusionshows that multiple quantum well structures fromZnSe/Zn0.6Cd0.4Se have been successfully produced by thedeveloped technique.

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Acknowledgements Thisworkhasbeenpartially supportedby the Bulgarian Ministry of Education and Science under grantBM – 1, the Serbian Ministry of Science and TechnologicalDevelopment under the Project No. 141047 and Bilateral projectbetween BAS and SASA.

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