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Applied Surface Science 264 (2013) 522– 526

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

ffects of CdZnTe buffer layer thickness on properties of HgCdTe thin film growny pulsed laser deposition

. Liua,∗, D. Bia, B.Y. Mana, D.M. Konga, X.Y. Xub

College of Physics and Electronics, Shandong Normal University, Jinan 250014, PR ChinaInformation Research Institute, Shandong Academy of Sciences, Jinan 250014, PR China

r t i c l e i n f o

rticle history:eceived 30 August 2012eceived in revised form 10 October 2012ccepted 10 October 2012

a b s t r a c t

HgCdTe thin films have been deposited on CdZnTe/Si(1 1 1) substrates by pulsed laser deposition (PLD). ANd:YAG pulsed laser with a wavelength of 1064 nm was used as laser source. The effects of CdZnTe bufferlayer thickness which varied with the deposition time in the range from 3 to 15 min on the crystalline,morphology and other properties of HgCdTe thin films were analysed. The results show that the crystalline

vailable online 17 October 2012

eywords:LDgCdTe thin filmsdZnTe buffer layer

quality and the composition of the HgCdTe epitaxial layer change with the increase of the depositiontime of the buffer layer. The CdZnTe buffer layer with a proper deposition time can improve the qualityof HgCdTe films, and the HgCdTe films deposited on the CdZnTe buffer layer with the deposition time of5 min exhibit the best crystalline quality and smooth surface in our experiment.

© 2012 Elsevier B.V. All rights reserved.

eposition time of buffer layer

. Introduction

Mercury cadmium telluride (MCT, Hg1−xCdxTe) is an importantnfrared detector material. But the difficulties and limitations inhe growth of MCT crystal make it unable to meet the needs of thenfrared focal plane array (FPA) development. While the MCT thinlm has many features, such as simple deposition, low dislocationensity and defects, equal composition, and can be widely used forigh performance detectors. Many techniques have been used torow MCT thin films, such as molecular beam epitaxy (MBE) [1],etal organic vapor phase epitaxy (MOVPE) [2], hot wall epitaxy

HWE) [3], liquid phase epitaxy (LPE) [4] and so on. Compared withhese methods, pulsed laser deposition (PLD) results many highinetic energy ablation which enable the preparation of thin filmsn low-temperature substrates due to high surface mobility [5].his method has been used for deposition of different thin films,nd it is particularly suited for the deposition of high quality com-ound semiconductor films. Many research groups have obtainedCT thin films by this method [6–8], and have investigated the

ffects on the different technological aspects and properties of PLDCT thin films, such as different Si substrates [9], substrate tem-

erature [10], electrical properties [9] and crystalline properties

10,11]. Some hetero-substrates, such as Si, GaAs, and Al2O3, weresed to be the substrates of MCT epitaxial growth [12,13]. But theyave a large mismatch with MCT, and may induce impurities in the

∗ Corresponding author. Tel.: +86 531 86182531; fax: +86 531 86182521.E-mail address: liumei@sdnu.edu.cn (M. Liu).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.10.057

film. Thus, we use cadmium zinc telluride (CZT, Cd1−yZnyTe) as thebuffer layer between MCT thin film and Si substrate to reduce thelattice mismatch and improve the quality of MCT thin film.

In this study, the influences of CZT buffer layer thickness whichwas determined by the deposition time on the properties of MCTepitaxial layer on Si(1 1 1) substrates were studied. It is an impor-tant aspect of the properties of MCT thin films using PLD. X-raydiffraction (XRD), scanning electron microscope (SEM) and X-rayphotoelectron spectroscopy (XPS) are used to evaluate the effectsof CZT buffer layer thickness on the properties of MCT thin films.

2. Experiments

MCT thin films and CZT buffer layer were deposited on undopedSi(1 1 1) substrates using a Nd:YAG laser with a wavelength of1064 nm, a pulse duration of 10 ns and a repetition rate of 10 Hz.The schematic representation of the experiment has been shown inour previous work [14]. The incident laser beam was focused onto arotating target at a 45◦ angle through a 60 cm focal length lens. Crys-talline Hg1−xCdxTe (x = 0.2) and Cd1−yZnyTe (y = 0.04) were used astargets. The target–substrate separation distance was 40 mm. TheAr partial pressure was kept at 5 × 10−3 Pa by adjusting the flowrate of the Ar gas. The substrate temperature was 200 ◦C duringthe deposition, and the laser incident energy was kept at 200 mJ.The thickness of the CZT buffer layer and the MCT epitaxial layer

were determined by the deposition time. The deposition time of CZTbuffer layers (TCZT) was 3, 5, 10 and 15 min, respectively, and thedeposition time of MCT epitaxial layer (TMCT) was 60 min. Samplenumbers are listed in Table 1.

M. Liu et al. / Applied Surface Science 264 (2013) 522– 526 523

Table 1Characteristics of the MCT films grown on different CZT buffer layers by PLD.

Sample ID S0 S1 S2 S3 S4

Deposition time of CZT buffer layer (min) 0 3 5 10 150.28 0.24 0.3 0.34

309 361 289 2550.44 0.45 0.61 0.63

mclMtomTot

3

d(dsiibeM3pfiTtfiwd2mt

0cwcttdtw5Stltaatlod

FWHM of HgCdTe(1 1 1) 0.32

Size of the grains (A) 271Parameter (1 − x) of Hg1−xCdxTe 0.59

The average thickness of the film was measured by the weighingethod using a Sartoius R200D analytical balance which has a pre-

ision of 0.001 mg. Rigaku D/max-rB XRD spectroscopy with a Cu K�ine radiation source was used to characterize the structure of the

CT thin film. A standard �–2� scanning method was employed inhe XRD for the crystallographic phase analysis and determinationf the preferential orientation of the deposition films. The surfaceorphologies were examined with a Hitachi S-570 SEM system.

he XPS spectra and the compositions of the MCT thin films werebtained using a PHI-ESCA 5300 spectrometer. The monochroma-ized Mg K� radiation (1253.6 eV) was used as the photon source.

. Results and discussion

The thicknesses of CZT buffer layers deposited with variouseposition time are 0.94 �m (3 min), 1.45 �m (5 min), 2.19 �m10 min) and 1.69 �m (15 min), respectively. The MCT thin filmseposited on different CZT buffer layers are 35 �m thick. The resulthows that the thickness of the buffer layer increases with thencrease of the deposition time. While, when the deposition times too long (over 10 min), the average thickness of the buffer layerecomes small. It implied that the buffer layer film may be partiallyxfoliated from the substrate. Fig. 1 shows the XRD patterns of theCT thin films with different deposition time of CZT buffer layers (0,

, 5, 10 and 15 min). The main peaks corresponding to the HgCdTelanes are shown in the figure. Because the thickness of MCT thinlm is large, the XRD data of CZT buffer layer could not be detected.he MCT thin films deposited at different conditions are polycrys-alline. The full width at half maximum (FWHM) of the XRD curveor HgCdTe(1 1 1) peak is presented in Table 1. The crystalline qual-ty of MCT epitaxial layer improves with the increase of the TCZT,

hereas above 10 min, an increase in deposition time of buffer layereteriorates the epitaxial growth. Furthermore, the weak peak at� = 38.24◦ is corresponding to the (2 1 0) plane of TeO2, and the for-ation of TeO2 is feasible only on extended exposure of the sample

o the atmosphere [15].The lattice constants of Si and Hg1−xCdxTe are 0.5431 nm and

.6462–0.6481 nm, respectively. The lattice-mismatched interfacean induce a large lattice distortion and internal strain in the filmhich will cause the peak broadening in XRD patterns. The lattice

onstant of Cd1−yZnyTe is 0.6095–0.6482 nm. The CZT is used ashe buffer layer material that can reduce the effects of lattice dis-ortion between the MCT epitaxial layer and Si substrate. When theeposition time of the buffer layer increases the strain in the epi-axial film is relaxed. So the crystalline quality of the MCT/CZT filmsith the deposition time of the buffer layer varying between 3 and

min is better than that of the MCT thin film deposited directly oni substrate, and it is improved by increasing the CZT buffer layerhickness. While when the deposition time of the buffer layer is tooong, many defects, such as steps, holes and dislocations, appear inhe buffer layer. The strain in the CZT buffer layers forms a largeccumulation in the film and could not be relaxed in time [16]. Thedhesion between CZT film and Si substrate becomes weak with

he growth of the crystalline grains, which causes the partial exfo-iation of the CZT film from the Si substrate. The crystalline qualityf MCT epitaxial layers deposited on CZT buffer layers with manyefects (S3, S4) will degrade.

Fig. 1. XRD patterns of HgCdTe thin films with various deposition time of CdZnTebuffer layer.

The Debye Scherrer formula can be used to calculate the sizeof the grains, and the results are shown in Table 1. The crystallitesize of MCT films increases and then decreases as the TCZT is 5 min.Fig. 2(a)–(e) shows the SEM images of different MCT/CZT thin films(TCZT = 0, 3, 5, 10 and 15 min). In Fig. 2(a), the film has many smallcrystalline grains, and the surface is rough. With the increase of thedeposition time of the buffer layer, the crystalline grains grow in atwo-dimensional mode. The crystallite size becomes large and thesurface of the film becomes smooth. When the TCZT is over 10 min,some large grains appear in the film, the grains are not uniform insize and the film surface becomes rough again.

The grains of the buffer layer can act as the nucleation centresfor the further growth of the epitaxial layer, which can increase theorderliness of the atoms in the epitaxial layer. The nucleation rate,density and nucleus size increase with the increase of the bufferlayer thickness. A large nucleation density causes the grains to growin a two-dimensional mode. Therefore, the grains become large andthe surfaces of the MCT/CZT thin films with proper TCZT (≤5 min)

are smoother than that of the MCT thin films without buffer layer.When TCZT exceeds 10 min, the lattice mismatch between the CZTand Si substrate causes a large strain in the buffer layer. It notonly affects the quality of the buffer layer but also extends into the

524 M. Liu et al. / Applied Surface Science 264 (2013) 522– 526

Fig. 2. SEM images of the different MCT thin films deposited at CZT buffer layer with various deposition time: (a) 0 min, (b) 3 min, (c) 5 min, (d) 10 min, and (e) 15 min.

390 39 5 40 0 40 5 41 0 41 5 42 0 42 5

CP

S

MCTMCT/CZT (3 min)MCT/CZT (5 min)MCT/CZT (10 min)MCT/CZT (15 min)

Cd 3d

560 56 5 57 0 57 5 58 0 58 5 59 0 59 5

CPS

MCT/C ZT (10 min)

Te ( 3d)

MCT/C ZT (15 min)

MCT/C ZT (5 min)MCT/C ZT (3 min)MCT

85 90 95 10 0 10 5 11 0 11 5

CP

S

Bin din g ene rgy (eV)

MCT/C ZT(3 m in) MCT/C ZT(3 m in)

MCT/C ZT(3 m in) MCT/C ZT(3 m in)

MCT Hg (4f)

94 96 98 100 102 104 106 108 110 112

Hg 4f

MCT/CZT (3 min)

CP

S

Bindin g energ y (eV)

3d an

et

ieOoeITsds

Bindin g ene rgy (eV)

Fig. 3. XPS spectra of Hg 4f, Cd

pitaxial layer, which causes the surface of MCT epitaxial layer filmo become rough again.

XPS is an important method of surface analysis that provides thenformation about the elemental composition, chemical state andlectronic state of the elements that exist within the films. The C 1s,

1s, Hg 4f, Cd 3d and Te 3d peaks were detected in the XPS spectraf all samples (not shown). The appearance of carbon and oxygenlements may arise from the surface contamination and oxidation.t is due to the exposure of the samples to atmospheric air [15].

he extent of contamination by the impurities is negligible. Fig. 3hows the Hg 4f, Cd 3d and Te 3d XPS spectra of the MCT thin filmseposited on Si substrates and CZT buffer layers with various depo-ition time of buffer layers. The corresponding core level binding

Binding energy ( eV)

d Te 3d for different samples.

energy values of the MCT thin films deposited on different condi-tions are given in Table 2. The variation of the parameter (1 − x) ofHg1−xCdxTe is shown in Table 1, and the compositions of the dif-ferent MCT/CZT thin films are shown in Fig. 4. The results showthat the MCT films deposited at different conditions are rich in tel-lurium, and the concentration of mercury decreases at first and thenincreases with the increase of the deposition time of buffer layer.The XPS spectra for Hg 4f in MCT thin films shown in Fig. 3 have sim-ilar signatures except for the MCT/CZT (3 min) thin film. The inset

shows the result of the peak separation of XPS 4f spectrum for theMCT/CZT (3 min) thin film. The Hg 4f7/2 and Hg 4f5/2 core level XPSpeaks with a shift compared with the elemental mercury bindingenergy correspond to the Hg Te bonding. The Cd 3d5/2 and Cd 3d3/2

M. Liu et al. / Applied Surface Science 264 (2013) 522– 526 525

Table 2Binding energies for MCT/CZT thin films with various deposition time of buffer layers.

Sample ID S0 S1 S2 S3 S4

Hg 4f7/2 100.8 99.08 100.82 100.9 100.9Hg 4f5/2 104.65 102.06 104.36 104.7 104.84Cd 3d5/2 405.36 405.62 405.49 405.39 404.57Cd 3d3/2 412.3 412.57

Te 3d5/2 576.67 576.5

Te 3d3/2 587.14 586.43

0 2 4 6 8 10 12 14 1610

20

30

40

50

60

70

Con

cent

ratio

n (%

)

Deposition Time (min)

HgCdTe

Te

Hg

Cd

Fig. 4. Concentration variation of Hg, Cd and Te for the HgCdTe films with dif-fc

XsCo(aF(TabtCbIfT

atteabaeTatbltt

[

[

[

pulse laser deposition, Thin Solid Films 480–481 (2005) 318–321.

erent deposition time of CdZnTe buffer layers. Dashed line shows the elementalomposition of the target.

PS signals of all our samples imply that Cd is present as Cd2+. Thehift in binding energy values for the peaks corresponding to Hg andd confirms the complete compound formation and bonding withther atoms [17]. It is clear that with the variation of the parameter1 − x) of Hg1−xCdxTe in the samples, the Hg 4f7/2, Hg 4f5/2, Cd 3d5/2nd Cd 3d3/2 peaks exhibit a corresponding shift in binding energy.ig. 3 shows the presence of tellurium atom in two different levelsTe 3d5/2 and Te 3d3/2). The peaks at about 572 eV correspond toe2− state or element Te0. Combined with the analysis about Cd 3dnd Hg 4f XPS lines above, the presence of tellurium is deduced toe Te2−. The XPS peaks at binding energy of 576 eV are attributed tohe tellurium, bonded with oxygen [18]. Thus, except for Hg Te andd Te bonding, the residual tellurium presents tellurium oxygenond corresponding to the presence of TeO2 in MCT epitaxial layer.

t is consistent with the results of XRD above. There is no evidencerom the XPS spectrum, for the presence of elemental Cd, Hg ande in all samples.

The buffer layer can improve the adsorption of the incidenttoms on the surface. When the buffer layer is too thin, the large lat-ice distortion between the buffer layer and the Si substrate causeshe atoms in the buffer layer to have a high kinetic energy and toscape the bondage of the substrate easily. When MCT thin filmsre grown on a thin CZT buffer layer, the Cd and Zn atoms in theuffer layer can enter into the epitaxial layer, substitute Hg atomsnd combine with Te. The Hg elements are unstable. They can formlemental Hg and can be volatilized from the substrate directly.hus, the concentration of Hg in the epitaxial layer deposited on

thin CZT buffer layer is relatively small. When the buffer layerhickness is in proper, most of the atoms in the buffer layer areound. The probability of the Hg atoms substituted by the buffer

ayer atoms becomes small. The volatilization of Hg decreases andhe concentration of Hg in the epitaxial layer increases. If we con-inue to increase the deposition time of the buffer layer, a numbers

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412.32 412.23 411.91576.73 576.66 576.81587.12 587.16 587.26

of defects in the buffer layer cause a large binding force to theincident atoms. The Hg composition increases with increasing thedeposition time of buffer layer.

4. Conclusions

The effects of CZT buffer layer thickness which varied withthe deposition time in the range from 3 to 15 min on the crys-talline, morphology and other properties of MCT films depositedon Si(1 1 1) substrates by PLD were analysed. The results show thatproper deposition time of CZT buffer layers can improve the qualityof MCT epitaxial layer. The composition varies with the increase ofthe deposition time of the buffer layer. All of the samples are richin tellurium, and the redundant tellurium presents TeO2. The MCTfilms deposited on the CZT buffer layer with 5 min exhibit a goodcrystalline quality and smooth surface.

Acknowledgements

The authors are grateful for the financial support by the Projectof Shandong Province Higher Educational Science and TechnologyProgram (J12LA07) and the National Natural Science Foundation ofChina (11274204 and 11047161).

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