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Applied Surface Science 256 (2010) 4879–4882

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

rowth and characterization of Cd1−xZnxTe thin films prepared from elementalultilayer deposition

ajiv Gangulya, Sumana Hajraa, Tamosha Mandala, Pushan Banerjeeb,∗, Biswajit Ghoshb

Institute of Engg. & Management, Saltlake Electronic Complex, Kolkata 700091, IndiaSchool of Energy Studies, Jadavpur University, Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 18 December 2009eceived in revised form 23 February 2010ccepted 23 February 2010vailable online 3 March 2010

ACS:8.67.Pt1.15.Dj8.70.Ck

a b s t r a c t

Cd1−xZnxTe is a key material for fabrication of high-energy radiation detectors and optical devices. Con-ventionally it is fabricated using single crystal growth techniques. The method adopted here is thedeposition of elemental multilayer followed by thermal annealing in vacuum. The multilayer structurewas annealed at different temperatures using one to five repetitions of Cd–Zn–Te sequence. X-ray diffrac-tion pattern for the multilayer with five repetitions revealed that annealing at 475 ◦C yielded single-phasematerial compared to other annealing conditions. EDX spectroscopy was carried out to study the corre-sponding compositions. Photoluminescence properties and change of resistance of the multilayer underillumination were also studied. The resistivity of the best sample was found to be a few hundreds of � cm.

8.55.−m

eywords:ZTultilayer

apor deposition

© 2010 Elsevier B.V. All rights reserved.

-ray diffractionuminescence

. Introduction

Cd1−xZnxTe (CZT) is the basic element in the fabrication ofolid state detectors for X-ray, gamma ray, cosmic ray and infraredlectro-magnetic radiation detection observed in medicine, astron-my, and high-energy physics [1]. The physical properties of CZTuch as constituent atoms with high atomic number, a sufficientlyarge tunable energy band gap (from the visible to the infraredortion of the electro-magnetic spectrum) to minimize leakageurrents at room temperature, high detective quantum efficiency,ood charge transport, high resistivity and high intrinsic mobility-ifetime (��) products for electrons and holes give it an attractiveandidature as a good detector material [2]. However, despitehe tremendous promise of this material, problems clearly exist.ZT crystals are difficult to grow in large sizes and with ultra

igh purity. There is a need to further lower the leakage currents

n detector grade material and also to increase the efficiency ofharge collection. Uniform CZT single crystals can be obtained bysing conventional or modified Bridgman method [3,4], the trav-

∗ Corresponding author. Tel.: +91 33 24146823; fax: +91 33 24146853.E-mail address: b pushan@rediffmail.com (P. Banerjee).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.02.076

eling heater technique [5,6] and the temperature gradient solutiongrowth [7–9]. Several methods have been used to prepare CdZnTefilms, such that molecular beam epitaxy [10], liquid phase epi-taxy [11], electro-deposition [12], close space vapor transport [13],laser ablation [14], thermal evaporation [15,16], sputtering [17],and metal-organic chemical vapor deposition (MOCVD) [18]. Multi-layer method of deposition was also tried by diffusion of elementalZn into CdTe [19]. Here we have tried to fabricate CZT thin film usingall elemental precursors—Cd, Zn and Te. As an alternative to singlecrystals, this method of multilayer elemental deposition in vacuumcan be utilized for the production of large-area polycrystalline CZTthin films.

2. Experimental methods

The substrate used in the present study was chromium coatedsodalime glass slides, because cadmium and zinc were found tostick poorly over glass. Chromium was found earlier to act as a good

inter-adhesive layer between glass and thin films [20]. So, prior todeposition of Cd, Zn and Te, a thin (∼10 nm) layer of chromiumwas deposited over glass slides at a substrate temperature of 190 ◦C,using thermal evaporation of chromium in high vacuum. Then pureCd, Zn and Te raw materials were taken in separate quartz crucibles

4880 R. Ganguly et al. / Applied Surface Sc

Table 1Calculated individual thickness required for deposition of elemental layers.

No. of repetitions Thickness (nm) of

Cd Zn Te

1 185.5 132.4 582.12 92.8 66.2 291.0

(uctpfidsaCfttpnvmfta

1spw

3 61.8 44.1 194.04 46.4 33.1 145.55 37.1 26.5 116.3

acting as effusion sources) and placed individually (for individ-al element depositions) in the physical vapor deposition (PVD)hamber. At a high vacuum (10−5 mbar), the crucible was heatedo suitable temperature to evaporate the corresponding elementlaced inside. Stacked multilayers comprising of Cd, Zn and Te thinlms – Cd at the bottom and Te at the top – were consecutivelyeposited onto chromium coated unheated sodalime glass sub-trates of size 7.4 cm × 2.4 cm. Tellurium was kept at the top sos to ease the future electrical contact fabrication to the structure.d was preferred as the layer just above chromium because it was

ound that keeping Zn after chromium had resulted in peeling off ofhe entire multilayer after annealing. The required thicknesses ofhe individual layers were calculated so as to keep an equal molarroportion of cadmium and zinc, and are given in Table 1. The thick-ess of all the layers was controlled within ±5 nm of the requiredalues and measured during evaporation using a digital thicknessonitor (DTM-101, Hindhivac, India). The process was carried out

or one to five numbers of Cd–Zn–Te repetitions and the individualhickness was so adjusted as to keep the total thickness of the filmsround 900 nm.

The evaporated multilayers were annealed in a vacuum of

0−5 mbar for 1 h, at temperatures of 425, 450 and 475 ◦C (i.e. threeets), because for the formation of tellurides from elements, tem-erature greater than 400 ◦C is required. The annealed structuresere then tested structurally using Bruker D8 powder X-ray diffrac-

Fig. 1. XRD pattern f

ience 256 (2010) 4879–4882

tometer, Hitachi S3200 scanning electron microscope (Departmentof Materials Science, University of Surrey, UK); and optically usingPerkinElmer LS-55 luminescence spectrometer (UGC-DAE CSR,Kolkata) through an excitation at 490 nm at room temperature. Thechange of resistance of the multilayer with time under illumina-tion (photoresponse) was also recorded using vacuum evaporatedsilver contacts placed 1 cm apart over the film surface. Electronicparameters were determined using Van der Pauw Hall measure-ment technique with silver dots evaporated at the corners of thesquare shaped film of side 1 cm.

3. Results and discussion

3.1. Structural analysis using XRD, EDX and SEM

Fig. 1 shows the X-ray diffraction pattern for five of the multi-layers, from which it becomes evident that the multilayer structurewith five repetitions annealed at 475 ◦C (will be referred as sam-ple 5L hereafter) showed single-phase behaviour with no dominantphase other than CZT present. While on reducing the no. of repeti-tions or the annealing temperature, peaks for ZnTe and especiallyunreacted Te (JCPDS card no. 040555) appeared. For Cd1−xZnxTe,the d-value of XRD peak for a particular plane lies between that forpure CdTe and pure ZnTe. For this purpose JCPDS card no. 150770was taken for CdTe with lattice parameter aCdTe = 6.4810 Å and cardno. 150746 for ZnTe with lattice parameter aZnTe = 6.1026 Å [21].The average value of “x” for sample 5L was determined from theobserved d-values three XRD peaks using the following formula(Vegard’s law):

d(Cd1−xZnxTe) = (1 − x) · d(CdTe) + x · d(ZnTe)

Using the above formula, the value of “x” was found to be 0.40.As only this particular sample turned out to be single phase

among all, analysis of the subsequent properties was carried outonly for this sample.

or multilayers.

R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882 4881

Fig. 2. EDX spectrum for sample 5L.

Table 2Composition of sample 5L from EDX.

Element Weight% Atomic%

iaXea

3

plaTtpftbefasT

Zn K 13 22Cd L 29 28Te L 58 50

The composition of sample 5L, as determined from EDX (Fig. 2),s shown in Table 2. Using this table, the value of “x” was determineds 0.43, which is in close agreement with the value obtained fromRD. The SEM micrograph for sample 5L is shown in Fig. 3. It isvident that the particles are smoothly and uniformly distributedt the surface, the particle size being less than 100 nm.

.2. Luminescent property

The PL spectrum of sample 5L is shown in Fig. 4. In thehotoluminescence spectrum two distinct peaks are visible—a

ower intensity broader peak centered about 662 nm (1.87 eV) andnother higher intensity sharper one at around 740 nm (1.67 eV).he origin of the peaks can be explained from the property ofellurium clusters as an iso-electronic exciton trap in II–VI com-ounds, as reported earlier by several authors. Roessler et al. [22]ound in CdS:Te that for concentration of Te around 1018 cm−3,he PL spectra (with excitation of 435 nm) showed a higher energyand around 600 nm. As the Te doping was raised gradually, lowernergy band appeared and eventually dominated the spectrum

or Te concentrations above 1020 cm−3. The high-energy band wasttributed to an exciton trapped at an isolated Te atom on a sulfurite, whereas the lower energy band resulted from trapping by twoe atoms on nearest neighbour sulfur sites. Cuthbert and Thomas

Fig. 3. SEM for sample 5L.

Fig. 4. PL spectra for sample 5L.

[23] also showed that in case of CdS1−xTex the spectrum with lowtellurium concentrations was due to radiative recombination of anexciton bound to a Te atom. Similarly, tellurium was found as iso-electronic exciton traps in several other II–VI compounds, such asCdSe [24], ZnSe1−xTex [25] as well as in ZnSe–ZnTe superlattice [26]and CdTe/CdS combination [27]. Thus, tellurium related photolumi-nescence peaks were observed not only for systems with Te dopantbut also where Te was one of the chief elements.

However, no earlier report on room temperature PL was noticedby us with Cd–Zn–Te multilayer system adopted here, with excita-tion at 490 nm. So, we cannot get a direct evidence regarding theorigin of the peaks at those particular positions reported by us. Butin view of the discussions made by the above authors, it can beinferred here that the higher energy band around 662 nm possiblyoriginated from a bound hole and electron recombining at a tel-lurium atom. The 740 nm band corresponds to the radiative decayof a hole and electron bound to two Te atoms at nearest neighboursites. It is to be noted here that earlier workers [22,23] also reportedthe generation of two PL peaks due to Te traps—the position of thepeaks being dependent on the excitation wavelength.

3.3. Change in resistance under illumination

Sample 5L were kept under a tungsten halogen lamp with100 mW/cm2 intensity and the changes in the resistance (measuredacross two silver contacts over them) were recorded as a functiontime of illumination up to 3 min. The dark resistances were takenas that at time “0”. The normalized values of the resistances wereplotted by taking the dark resistance as unity and have been shownin Fig. 5 against the time of illumination.

When the film was illuminated, the measured resistance fellwith time and after some time the change in resistance was verysmall. This was due to the fact that the rate of photogenerationof carriers decreased with time. Also the process of recombinationtook place with respect to time, thereby decreasing the value ofphotocurrent. Ultimately a situation was obtained where the pro-cess of generation of charge carrier and recombination reachedequilibrium under constant illumination. This resulted in a nearlyflat profile with time at the end. The resistance at the end of record-ing was about 25% to that at dark (time = 0). However, the response

under illumination is rather slow and this is because the chargecarriers had to cross a large number of grain boundaries to flowfrom one contact to the other, undergoing frequent scattering andrecombination.

4882 R. Ganguly et al. / Applied Surface Science 256 (2010) 4879–4882

Table 3Parameters from Hall measurement.

Sample Bulk resistivity (� cm) Carrier density (cm−3

5L 416 1.0 × 1019

3

aTp1

4

saCmsueotd

[[[[

[

[

[[

[[[[

[[[

Fig. 5. Photoresponse for sample 5L.

.4. Electronic properties using Hall effect

The result of the Van der Pauw Hall measurement (carried outt a magnetic field of 5 kG) of sample 5L is shown in Table 3.he resistivity was of the order of a few hundreds of � cm with-type conductivity, and the carrier density was in the range of019 cm−3.

. Conclusion

Cd1−xZnxTe thin films have been prepared from multilayertructures comprising of Cd, Zn and Te. The structures werennealed at various temperatures with different number ofd–Zn–Te repetitions. Results of X-ray diffraction proved that theultilayer annealed at 475 ◦C with five repetitions of Cd–Zn–Te

howed best behaviour in regard to single-phase nature. The val-

es of “x” determined from XRD and EDX matched closely. SEMxhibited smooth and uniform particle distribution at the surfacef the film. Further works in this area are on the way to improvehe performance of the film and its application in large-area sensingevices.

[

[[

) Hall mobility (cm2 V−1 S−1) Conductivity type

0.001 p

Acknowledgements

One of the authors (Pushan Banerjee) gratefully acknowl-edges the support provided by Council of Scientific and IndustrialResearch, Govt. of India for carrying out this research. The helpextended by Dr. Abhijit Saha of UGC-DAE CSR, Kolkata center,towards taking PL measurements is also thankfully acknowledged.

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