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Journal of Crystal Growth 311 (2009) 2367–2371

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

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Cell: +9

E-m

(N. Abb

journal homepage: www.elsevier.com/locate/jcrysgro

Comparative study of HgxCd1�xTe films grown on CdTe thin films previouslydeposited from two different techniques

A. Ali a, N. Abbas Shah b,�, A. Maqsood c

a Optics Laboratories, Nilore, Islamabad 45320, Pakistanb Department of Physics, COMSATS Institute of Information Technology, Islamabad 45320, Pakistanc School of Chemical and Materials Engineering, National University of Sciences & Technology, Islamabad 45320, Pakistan

a r t i c l e i n f o

Article history:

Received 10 November 2008

Received in revised form

16 January 2009

Accepted 22 January 2009

Communicated by M. SchieberCdTe showed the preferential (111) orientation with smoother and larger grain size than those of TES-

Available online 23 February 2009

PACS:

07.30.Cy

81.05.Dz

81.15.Ef

81.40.Tv

82.56.Lz

Keywords:

A1. Crystal structure

A1. Optical microscopy

A1. Thin films

B2. Semiconductor

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.01.144

esponding author. Tel.: +92519235036; fax: +

23215105363.

ail addresses: nabbasqureshi@yahoo.com, abid

as Shah).

a b s t r a c t

High quality cadmium telluride (CdTe) thin films were grown on glass substrates with two different

techniques, two evaporation source (TES) and closed space sublimation (CSS). Further to the above

mercury telluride (HgTe) was then deposited by using single source on both CdTe thin films for

obtaining HgxCd1�xTe samples. The crystalline structure of the HgxCd1�xTe sample grown from CSS-

CdTe. The optical transmission for TES-CdTe sample was above 90% in the 1000–1500 nm range whereas

it was significantly below 80% for CSS-CdTe sample. The optical transmission for TES-HgxCd1�xTe and

CSS-HgxCd1�xTe was �60%. The resistivity at room temperature of TES-CdTe and CSS-CdTe was

�3.33�109O cm and �2.20�108O cm, respectively, while the resistivity of TES-HgxCd1�xTe and CSS-

HgxCd1�xTe samples was �1.73O cm and �5.34�105O cm, respectively. The comparative study of

ternary compound prepared with the above techniques has been carried out for the first time.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

It has been established that the properties of materials in theform of thin films differ significantly from that of in the bulk form.Similarly, the properties of polycrystalline thin films differmarkedly from that of amorphous films. Moreover the metastablenature of the amorphous state implies a strong dependence of thephysical properties, the electrical properties in particular in thepreparation technique [1]. In addition, the history and structuralmodifications result in marked variation in physical properties [2].

Mercury telluride (HgTe) is known to be a semimetalcompound in the polycrystalline form; it exhibits an invertedband structure [3]. Thus the forbidden gap in HgTe is zero. Thestructural, optical and electrical properties of such gaplessmaterials are important for applications, as well as for thefundamental solid-state physics. HgTe, as AII BVI compound, ischaracterized by having high-current carrier mobility in the order

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of 104 cm2/Vs at room temperature for epitaxial layers [4]. HgTe inthin film form behaves as a semiconductor with energy gap�0.02–0.3 eV [2,5].

Cadmium telluride (CdTe) is a wide band gap II–VI compoundwith promising applications as X-ray and g-ray detectors [6,7] andfor high efficiency solar cells [8,9]. It is also being employed as asubstrate for HgTe-based epitaxial layers to be used in infraredtechnology [10,11]. An additional interest has been stimulated bythe discovery of infrared photo refractivity. Infrared applicationsrequire high resistivity of the order of 108O cm at roomtemperature and good photoconductivity in infrared radiation[12,13].

CdTe and HgTe belong to important group of alloys which isparticularly useful for infrared applications. The band gap ofHgxCd1�xTe alloy ranges from 0 to 1.6 eV as ‘x’ is varied from zeroto unity. The alternating layers of CdTe, HgTe and CdTe/HgTe superlattices could also be interesting materials for applications ininfrared optoelectronic devices. CdTe/HgTe super lattice was firstproposed in 1979 by Schulman et al. [14,15].

CdTe/HgTe super lattices are fabricated with two differenttechniques and both crystallize in zinc-blende structure. They arenearly lattice matched with bulk lattice constants differing about

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A. Ali et al. / Journal of Crystal Growth 311 (2009) 2367–23712368

0.3% (aHgTe=6.46 A and aCdTe=6.48 A). The present study aims toinvestigate and compare the structural, morphology, optical andelectrical properties of HgxCd1�xTe compound resulting fromCdTe/HgTe structures deposited on glass substrates by twodifferent vacuum-evaporation techniques; two evaporation sourceand closed space sublimation. Due to problem of instability ofweak Hg–Te bonding, the data on HgxCd1�xTe thin films is verylimited. To our knowledge this is a novel report on comparison ofHgxCd1�xTe thin films fabricated by two different techniques.

2. Experimental details

2.1. Two evaporation source technique

Cadmium granulated about 3–6 mm of BDH chemicals ofPoole, England and tellurium powder, �325 mesh, 99.998%(metals basis) of Alfa Aesar were used as two evaporation source(TES) materials inside Edwards E306A vacuum coating unit [16].The materials were loaded into two cylindrical graphite boatshaving a hole of about 2 mm diameter on the top to act as pointsource. The boat is tightly wrapped with 100 amperes tungstenwire. Boats were heated indirectly by passing current through theCd and Te heaters, respectively. The evaporation temperatures ofCd and Te were maintained within 71 1C error by temperaturecontrollers using K-type thermocouples. The thermocouples wereplaced inside the rare of the graphite boats. Scratch free corning7051 glass substrates with surface area of 5�5 cm2 were cleaned,in pure isopropyl alcohol (MERCK Company) by ultrasonic cleanerof Cole Parmer model 8890E-DTH for about 3 h. These substrateshave very high softening temperature i.e. 900 1C and above 90%transmittance. The substrate was placed at the stainless steelholder, at a distance of about 15 cm from the source materials;whereas an IR heater provided heat to the substrate. During theevaporation, the substrate temperature was controlled by a thirdK-type thermocouple placed on the substrate. The chamber wasthen closed and allowed to evacuate to around 10�4 Pa by rotaryand diffusion pumps attached with the liquid nitrogen trap. Thevacuum during the evaporation was about 1.5�10�3 Pa. Thethickness of the film and the deposition rate were measured withthe help of a quartz crystal film thickness monitor, model FTM7 ofEdwards. To prepare high quality TES-CdTe thin film, the substratetemperature was fixed at 300 1C for 1 h before the deposition ofthe film. The source materials were sintered for about 5 minbefore opening the shutter which is necessary to have stable andconstant evaporation rate. As a first step the Cd and Te sourceswere kept at pre-calibrated temperatures of 400 and 510 1C,respectively. After film-deposition, the shutter was closed and thefilm remained at substrate temperature for further 20 min. Thesubstrate heater was then switched off to cool down thetemperature to 100 1C for completing deposition process of TES-CdTe thin film. In the next step this thin film was used as asubstrate for HgTe deposition by using single source, howeverwith the same geometry, at 450 1C source and 300 1C substratetemperatures, respectively, to complete the deposition of TES-HgxCd1�xTe sample.

2.2. Closed space sublimation technique

The high quality thin film of CdTe was deposited onto ultra-cleaned corning glass substrate by closed space sublimation (CSS)technique [17,18]. As a single source, CdTe material was kept in thegraphite boat of size (20� 45) mm. Heat was provided indirectlyby resistive heating of Mo filament which is in contact with baseof graphite boat attached with a temperature controller with K-

type thermocouple. The substrate was fixed at 4 mm from thesource material and heated by an IR heater. Another thermocouplewas placed over the substrate to monitor the surface temperature.Source and substrate temperatures were fixed at 450 and 300 1C,respectively. The chamber was evacuated down to 10�1 Pa withthe help of rotary pump. The deposition time for each film was5 min. After that the source and substrate heaters were switchedoff for cooling down to 100 1C before opening the chamber to getCSS-CdTe thin film.

In the second step, the prepared thin film was used assubstrate for deposition of HgTe as single source, however insimilar setup to get CSS-HgxCd1�xTe sample. The sample wasannealed at 200 1C for 30 min under the same vacuum.

2.3. Characterization

The structure of the films was studied by X-ray diffraction(XRD) by using Cu–Ka (1.5418 A) radiation with operatingconditions voltage/current 40 kV/30 mA and scan speed 81/min.The microstructure and composition analysis of the samples wereperformed using scanning electron microscope (SEM) with energydispersive X-ray (EDX) model JEOL 5910 at a magnification of5000 and an accelerating voltage of 20 kV. The optical transmis-sion was recorded between 200 and 2500 nm by Lambda 900 UV/vis/NIR spectrophotometer. The dc electrical resistivity andmobility of cut samples with gold-coated contacts were deter-mined according to van der Pauw geometry by Hall effectmeasurements. The electrical resistivities as a function oftemperature (40–200 1C) was measured under vacuum �10�1 Paby two probe method. Silver paste was used above the goldcoating for ohmic contacts. The current voltage linearity of ohmiccontacts was checked and ensured in the working temperaturerange.

3. Results and discussion

The deposition parameters, optical and electrical characteris-tics for the studied samples are listed in Table 1. We put in aneffort to compare the physical properties of the samples preparedunder different situations.

3.1. Structural properties

The structures of the TES-CdTe, CSS-CdTe substrates and TES-HgxCd1�xTe, CSS-HgxCd1�xTe samples were examined by XRD areshown in Fig. 1. It can be seen that TES-CdTe and CSS-CdTesamples were polycrystalline. The sharp diffraction peaks at2yffi23.7501, 39.2861 and 46.4331 correspond to (111), (2 2 0)and (311) planes of the cubic CdTe structure. It is clear from XRDpeaks of TES-HgxCd1�xTe and CSS-HgxCd1�xTe samples in Fig. 1that HgxCd1�xTe structure is deposited along (111) plane. SEMimages of all samples are shown in Fig. 2. TES-CdTe samples wereuniform and grains were small whereas TES-HgxCd1�xTemorphology was relatively rough and nonuniform. Well facetedgrains in the SEM images of the CSS-CdTe and CSS-HgxCd1�xTesamples can be seen in Fig. 2.

The composition of the samples was determined by EDXanalysis. Hg was detected in HgxCd1�xTe structures deposited byTES and CSS as shown in Fig. 3. Hg concentration in CSS-HgxCd1�xTe is less than half as compared to the same in TES-HgxCd1�xTe sample; this is may be due to re-evaporation causedby the high vapor pressure of Hg in closed source and substratesystem. In TES system this may be due to sufficient time for theatoms and molecules to make bonds with other species in the

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Table 1The composition analysis, optical parameter and electrical characteristic of the samples (at% atomic percent, d film thickness, Eg energy gap, r dc electrical resistivity and

mobility at room temperature).

Sample no. Composition d (nm) 75 Eg(eV) 70.01 r (O cm) Mobility (cm2/Vs)

Cd (at%) Te (at%) Hg (at%) x

TES-CdTe 44 56 0 0 730 �1.51 3.33�109 –

TES-HgxCd1�xTe 39 56 5 0.11 977 – �1.73 7.79�102

CSS-CdTe 44 56 0 0 732 1.50 2.20�108 –

CSS-HgxCd1�xTe 46 52 2 0.04 784 1.47 5.34�106 36.2

Fig. 1. XRD patterns for TES-CdTe, CSS-CdTe, TES-HgxCd1�xTe and CSS-HgxCd1�xTe

samples.

Fig. 2. Plan-view SEM images of TES-CdTe, CSS-CdTe, TES-HgxCd1�xTe and CSS-

HgxCd1�xTe samples.

Fig. 3. Compositional analysis of TES-HgxCd1�xTe and CSS-HgxCd1�xTe samples.

A. Ali et al. / Journal of Crystal Growth 311 (2009) 2367–2371 2369

compound. The composition of Cd and Te are also different in thesamples obtained from both techniques.

3.2. Optical properties

The recorded optical transmittance spectra between200–2500 nm for TES and CSS-samples are shown in Fig. 4. TES-CdTe sample which is prepared at the substrate temperature of300 1C is uniform and more transparent than the CSS-CdTesample. Maxima and minima occur from the interference effectand their number increases with increasing film thickness [19].The spectral behavior of the TES-CdTe sample shows neitherabsorption nor scattering of light above 800 nm wavelength. In

contrast, after HgTe deposition, the TES-HgxCd1�xTe structure hasa rough surface and there is no sharp band edge as can be seen inFig. 4; which implies introduction of more interband levels. In thecase of CSS-CdTe sample, the transmittance spectrum is typical ofnonuniform rough surface, indicating the presence of scatteringlosses. The optical properties such as refractive index, absorptioncoefficient, film thickness and optical band gap were calculatedfrom the transmission spectra. Fitting of these spectra was doneaccordingly [18,20].

Tequals;Ax

B�Cx cosðfÞ þ Dx2(1)

where T is the normal transmittance for the system consisting of athin film on a transparent substrate surrounded by air (refractiveindex=1) and taking into account all multiple reflections at theinterface for the case of k2

5n2, which is true for this kind ofsemiconductor thin films. The other parameters are defined as,A=16n2s, B=(n+1)3(n+s2), C=2(n2

�1)(n2�s2), D=(n�1)3(n�s2),

f=4pnd/l, x=exp(�ad) and k=al/4p. Here n and s are therefractive indices of the film and the substrate; d and a arethickness and absorption coefficient of the film, respectively, andl is the wavelength. Here n varies with the wavelength as,

nequals; aþb

l2(2)

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Fig. 4. Transmittance spectra for TES-CdTe, TES-HgxCd1�xTe, CSS-CdTe and CSS-HgxCd1�xTe samples.

Fig. 5. Curve fitted transmittance data of TES-HgxCd1�xTe sample.Fig. 6. Plot of (ahv)2 vs. photon energy (hv) for CSS-HgxCd1�xTe sample.

A. Ali et al. / Journal of Crystal Growth 311 (2009) 2367–23712370

and polynomial approximation for a as

aequals; c þf

lþ

g

l2(3)

where a, b, c, g and f are constants obtained from the fitting of thetransmission spectra. The fitting parameters c, f, g are good onlyfor transparent region where the values of a are very small andvarying slowly with wavelength. In the medium and highabsorption regions, exact solution for x of Eq. (1) gives the valuefor a at every wavelength.

xequals;ðC0 þ A=TÞ�½ðC0 þ A=TÞ2�4BD�1=2

2D(4)

Here C0=Ccosj and a=�ln(x)/d. The values of n and d weretaken from the transmittance. For the photon energy above theoptical band gap energy, the transmittance loss due to absorption

is much higher than those from light scattering which is due tofilm roughness, so that Eq. (4) can be used with a good precision.

Once the thickness and refractive indices of the films as afunction of wavelength are determined from the fitting data, exactsolution of Eq. (1) may provide the values of a for all wavelengths.A fitted transmission spectrum of TES-HgxCd1�xTe sample isshown in Fig. 5. The values of direct band gap were obtained fromthe plots of (ahv)2 versus photon energy hv as shown in Fig. 6.

3.3. Electrical properties

For electrical studies, squarely cut pieces of both samples weregold coated for making ohmic contacts. At room temperature dcelectrical resistivity and mobility of samples were measured byvan der Pauw method. A very high resistivity of TES-CdTe and CSS-CdTe thin films can be seen in Table 1. The molar ratio of

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Fig. 7. Plot of ln r vs. 1000/T for TES-HgxCd1�xTe sample.

A. Ali et al. / Journal of Crystal Growth 311 (2009) 2367–2371 2371

tellurium-to-cadmium in the reaction mixture is slightly above 1,the deposited film becomes p-type and at decreased ratio oftellurium-to-cadmium, CdTe changes to n-type (cadmium vacan-cies and tellurium vacancies are acceptors and donors, respec-tively). This transition from p-type to n-type CdTe occurs over avery narrow range of the tellurium-to-cadmium molar ratio in thereaction mixture. The resistivity at room temperature of CSS-CdTethin film was comparably small as in the TES-CdTe thin film (Table1). This could be due to slow growth of CdTe compound betweenCd and Te atoms.

In case of TES-HgxCd1�xTe structure the resistivity falls to�1.73O cm. This decrease in resistivity is due to the presence ofhigher concentration of Te (�56 at%) as well as Hg (�5 at%) as isevident from Table 1. The mobility and carrier concentration of thesame sample was measured to be 779 cm2/Vs and 4.61�1015 /cm3, respectively, by Hall effect measurements. Similar observa-tions were reported earlier [21].

The temperature dependence of dark resistivity of TES-HgxCd1�xTe sample is presented in Fig. 7. It can be seen that thedark resistivity decreases with increasing temperature. Theexistence of two different slopes indicates a competitionbetween different conduction mechanisms. Therefore, in thelow-temperature range p100 1C some sort of aggregation ispossible, which causes reduction in electrical resistivity. TheXRD analysis suggests the transformation of polycrystalline stateinto single crystalline; however it is quite possible that these filmshave more than one plane, but the smaller peaks were notobserved in XRD spectrum, which is shown in Fig. 1. In addition,the diffusion in the film strain causes reduced resistivity. Theconduction might occur through hopping between localized statessince the deviation from the linear dependence in the case ofamorphous semiconductors is usually attributed to hoppingjumps [14,22]. For the high-temperature region, transition to thepolycrystalline state is expected and the diffusion of localizedstates band conduction takes part in the conduction mechanism.

4. Conclusions

The results showed that TES-CdTe and CSS-CdTe thin filmswere cubic in nature whereas TES-HgxCd1�xTe and CSS-HgxCd1�xTe structures were formed along (111) plane. Grain sizeof CSS deposited samples was larger than TES samples. Mercurycontent in TES-HgxCd1�xTe sample is higher than in CSS-HgxCd1�xTe sample. Re-evaporation due to high vapor pressureof mercury in CSS system is responsible for lower Hg content inCSS-HgxCd1�xTe sample whereas in TES-HgxCd1�xTe sample,mercury atoms get enough bonding time with other species inthe compound. TES sample was comparably more uniform andtransparent than CSS deposited sample. Very high electricalresistivity was found for both TES-CdTe and CSS-CdTe thin films.Semiconducting trend was observed for CSS-HgxCd1�xTe, whereasthe resistivity for TES-HgxCd1�xTe dropped to about 1.73O cm. Theadjustable direct band gap from 0.7 to 25mm with high absorptioncoefficient in infrared region qualifies HgxCd1�xTe thin films ashighly useful for infrared detectors as find a noticeable differencein optical properties and quite significant difference in electricalresistivity.

Acknowledgements

The authors acknowledge the Higher Education Commission,Islamabad, Pakistan for the financial support through a Projectno.20-694. The grant through Project 16-16/CRGP/CIIT/IBD/08 ofCOMSATS Institute of Information Technology, Islamabad is highlyappreciated and acknowledged. Thanks to Dr. Javed Akhtar ofOptics laboratories Nilore. The authors would like to thank Dr. AdilSyed, M.Aslam Khan and A.S.Bhatti of CIIT, Islamabad, for valuablediscussion.

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