Current Applied Physics 10 (2010) S499–S501

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Current Applied Physics

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Vacuum evaporation of CdTe thin films on Ni, Mo, Ti and TiN-depositedsapphire single-crystal substrates

Yuichi Sato *, Hirotoshi Hatori, Suguru Igarashi, Manabu Arai, Kazuki Ito, Syota KikuchiDept. of Electrical and Electronic Engineering, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata-gakuen, Akita 010-8502, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 November 2009Received in revised form 11 January 2010Accepted 12 February 2010Available online 19 February 2010

Keywords:CdTeThin filmSapphireTiTiN

1567-1739/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cap.2010.02.040

* Corresponding author.E-mail address: yusato@ipc.akita-u.ac.jp (Y. Sato).

Cadmium telluride (CdTe) thin films were grown on sapphire c-face single-crystal substrates by vacuumevaporation. Prior to growing the CdTe thin films, conducting thin films of Ni, Mo, Ti and titanium nitride(TiN) were grown on the sapphire substrates. The crystallinities and photoluminescence (PL) propertiesof the CdTe thin films grown on the various conducting thin films were investigated and they were com-pared with those of CdTe thin films grown directly on sapphire substrates without a conducting thin film.CdTe thin films with relatively high crystallinities were obtained when Ti and TiN were used as the con-ducting thin film. Therefore, the PL properties of the CdTe thin films on such conducting thin films werenot inferior to those of the CdTe thin film grown directly on the sapphire substrate. The effect of the Cdand Te supply ratio on the properties of the CdTe thin films was also investigated. The CdTe thin films hadmuch higher crystallinities when they were grown in Te-rich conditions than in Cd-rich conditions.

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1. Introduction

Cadmium telluride (CdTe) has a direct band gap of about 1.5 eV,making it a highly suitable material for the absorber layer in solarcells. CdTe solar cells consist mainly of a CdTe polycrystalline absor-ber layer formed on a polycrystalline or amorphous underlayer, andthey exhibit high performances [1]. Sapphire single crystals havebeen widely used as substrates for epitaxial growth of semiconduc-tor thin films such as nitride-based III�V materials [2]. There is alsoan epitaxial relationship between sapphire and CdTe crystals. How-ever, since sapphire is an electrical insulator, conducting materialsneed to be deposited on its surface for solar cell applications. More-over, the conducting materials should be grown epitaxially on thesapphire substrate to form CdTe thin films with the high crystallin-ities necessary for producing high-performance solar cells.

In this study, thin films of conducting materials of Ni, Mo, Ti,and titanium nitride (TiN, which is often used as a barrier metalin LSI systems) were grown on sapphire c-face single-crystal sub-strates. CdTe thin films were then grown on the substrates andthe properties of the obtained CdTe thin films were compared withthose of CdTe thin films grown directly on sapphire substrateswithout a conducting thin film. In addition to the dependence ofthe properties on the type of conducting thin film, the effects ofthe Cd and Te supply ratio on the properties were also investigated.

ll rights reserved.

2. Experiment

CdTe thin films were prepared using a vacuum evaporationapparatus with two Knudsen cells (K cells) of Cd shots (6 N) andTe shots (6 N) and a turbo-molecular pumping system. In theprevious study, films were grown simultaneously on quartz andsapphire substrates using the same vacuum evaporation apparatuswith a K cell containing CdTe chunks, and properties of the filmswere semiquantitatively compared [3]. In this study, two K cellscontaining Cd and Te were used to investigate the effect of theCd/Te supply ratio. Sapphire c-face single-crystal substrates wereused for thin film growth, and thin films of various conductingmaterials were deposited on the sapphire substrate prior to grow-ing the CdTe thin films. Ni thin films were prepared by vacuumevaporation, Mo and Ti thin films were prepared by radio-fre-quency (RF) magnetron sputtering, and TiN thin films wereprepared by reactive RF magnetron sputtering using N2 sputteringgas. CdTe thin films were grown on the substrates at various Cd/Tesupply ratios by adjusting the temperatures of the two K cells. Thecrystallinities of the obtained thin films were evaluated by X-raydiffraction (XRD) and their photoluminescence (PL) propertieswere evaluated by using He�Cd laser irradiation at 12 K.

3. Results and discussion

First, CdTe thin films were grown directly on sapphire sub-strates with no conducting thin film at various Cd/Te supply ratiosand at a substrate temperature of 370 �C. The Cd/Te supply ratiowas adjusted by varying the temperatures of the two K cells.

20 30 40 50 60 70

102

103

104

2 ( ), (CuK )

X-r

ay I

nten

sity

(co

unts

)

CdT

e (3

11)

CdT

e (1

11)

CdT

e (4

00)

CdT

e (4

22)

Te (

201)

sapp

hire

(b) Cd:Te=1:1.5

5 10 15 200

1

2

3]

FWHM=2.1 CdTe (111) XRC

20 30 40 50 60 70

102

103

104

2 ( ), (CuK )

X-r

ay I

nten

sity

(co

unts

)

CdT

e (1

11)

CdT

e (2

20)

CdT

e (4

22)

CdT

e (4

00)

CdT

e (3

11)

sapp

hire

(a) Cd:Te=1:0.8

5 10 15 200

2

4

6

8] FWHM=3.8

CdTe (111) XRC

Fig. 1. XRD patterns of CdTe thin films grown directly on a sapphire substrate at370 �C and at various Cd/Te supply ratios. The insets show XRC profiles of theCdTe(1 1 1) peaks.

20 30 40 50 60 70

102

103

104

105

2 ( ), (CuK )

X-r

ay I

nten

sity

(co

unts

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CdT

e (1

11)

Te (

201)

Ti (

0002

)

CdT

e (3

31)

(b) on Ti sa

pphi

re

5 10 15 200

2

4

6

]FWHM=1.7 CdTe (111) XRC

102

103

104

105

X-r

ay I

nten

sity

(co

unts

)

CdT

e (1

11)

Te (

201)

TiN

(11

1)sa

pphi

re

5 10 15 200

2

4

6

8]FWHM=1.4 CdTe (111) XRC

20 30 40 50 60 70

102

103

104

105

2 ( ), (CuK )

X-r

ay I

nten

sity

(co

unts

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CdT

e (3

11)

CdT

e (1

11)

CdT

e (3

31)

CdT

e (4

22)

Mo

(110

)

sapp

hire

CdT

e (4

00)

(a) on Mo

5 10 15 20

1

2

3]FWHM=7.2 CdTe (111) XRC

S500 Y. Sato et al. / Current Applied Physics 10 (2010) S499–S501

Fig. 1 shows the XRD patterns of the obtained CdTe thin films. Thefilms were approximately 1 lm thick. In these patterns, XRD peakscorresponding to CdTe(1 1 1) are mainly observed in addition tothe peak of the sapphire substrate. The intensities of these peaksincrease as the temperature of the K cell containing Te wasincreased. In addition, XRD peaks corresponding to Te wereobserved when the K cell temperature of Te was extremely high.X-ray rocking curves (XRCs) of the CdTe(1 1 1) peaks are also indi-cated in the figures. The full-widths at half-maximum (FWHM) ofthe peaks decreased with increasing Te K cell temperature. Thus,the crystallinity of the CdTe thin films increased with increasingTe supply temperature. An additional Cd and CdTe source and

1.3 1.4 1.5 1.6 1.7

0

200

400

600

800

PL In

tens

ity [a

rb. u

nits

]

Photon energy [eV]

Cd:Te=1:1.5

Cd:Te=1:0.8

Fig. 2. PL spectra of CdTe thin films grown directly on a sapphire substrate at 370 �Cand at various Cd/Te supply ratios. The PL spectra were measured at 12 K.

are often used in molecular beam epitaxy growth of CdTe thin filmsand Cd-rich conditions are generally maintained during growth [4].However, in this study, the crystallinity of the obtained CdTe thinfilms was found to increase when they were grown in Te-rich con-ditions. Fig. 2 shows the PL properties of CdTe thin films grown at

20 30 40 50 60 70

2 ( ), (CuK )

(c) on TiN

20 30 40 50 60 70

102

103

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2 ( ), (CuK )

X-r

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nten

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CdT

e (3

11)N

i (11

1)

Ni (

200)

CdT

e (1

11)

(d) on Ni

sapp

hire

5 10 15 20

0.4

0.6

0.8

1

1.2]FWHM=6.5 CdTe (111) XRC

Fig. 3. XRD patterns of CdTe thin films grown on sapphire substrates withconducting thin films. The insets show XRC profiles of the CdTe(1 1 1) peaks. TheCdTe thin films were grown at 370 �C, except for the film on the Ni thin film, whichwas grown at 300 �C.

1.3 1.4 1.5 1.6 1.70

500

1000

Photon energy [eV]

PL I

nten

sity

[ar

b. u

nits

]

on TiN

on Ti

on Mo

Fig. 4. PL spectra of CdTe thin films grown on sapphire substrates with conductingthin films. The CdTe thin films were grown at 370 �C. The spectra were measured at12 K.

Y. Sato et al. / Current Applied Physics 10 (2010) S499–S501 S501

various Cd/Te supply ratios. The PL peak intensities almost followthe improvement in the crystallinity of the CdTe thin films. Inthe PL spectra, two peaks at about 1.36 eV and 1.47 eV are mainlyobserved. Both peaks originate in donor–acceptor recombinationand the former peak is related to a deeper donor [5–9]. The inten-sity of the 1.47 eV peak increased when the CdTe thin film wasgrown in Te-rich conditions.

Next, CdTe thin films were grown on sapphire substrates thathad various conducting thin films on their surfaces. Despite 300-nm-thick Ni thin films being deposited on the sapphire substrateby vacuum evaporation with no substrate heating, highly (1 1 1)oriented Ni (cubic) thin films were obtained. Other conducting thinfilms were prepared by RF magnetron sputtering at a substratetemperature of 500 �C. 400-nm-thick Ti (hexagonal) and 70-nm-thick TiN (cubic) thin films were epitaxially grown on the sapphiresubstrate. However, 300-nm-thick Mo (cubic) thin films could notbe epitaxially grown. CdTe thin films were then grown on the sap-phire substrates. For the case of growth on a Ni thin film, the crys-tallinity of the CdTe thin films deteriorated when the growthtemperature exceeded 300 �C, probably due to diffusion of Ni intothe films. Consequently, CdTe thin films were grown below 300 �Con the Ni thin films. On the other hand, when conducting thin filmsof Mo, Ti, and TiN were used, the CdTe thin films could be grown athigher substrate temperatures, so they were grown at 370 �C.

Fig. 3 shows XRD patterns of the CdTe thin films grown on thesapphire substrates with various conducting thin films. XRD peaksof the conducting thin films are visible in these XRD patterns. Inaddition, XRC profiles of the CdTe(1 1 1) peaks are indicated inthese figures. In these cases, the K cell temperatures of Cd and Tewere respectively 200 �C and 365 �C, and the supply ratio of Teto Cd was about 2. The CdTe thin films were about 1.5 lm thick,except for the case of growth on Ni, where the CdTe was 400 nmthick. The Mo thin films were (1 1 0) oriented; that is, they didnot grow epitaxially to the sapphire c-face substrate. Therefore,the CdTe thin film on the Mo thin film had a lower crystallinitythan those grown on the other substrates. Since the CdTe thin filmscould not be grown on Ni conducting thin films at substrate tem-peratures over 300 �C, the crystallinity of the CdTe thin film wasrelatively poor, although a highly (1 1 1) oriented CdTe thin filmwas obtained. On the other hand, the Ti and TiN conducting thinfilms were epitaxially grown on the sapphire substrate and theircrystallinities were quite high. Therefore, there was no deteriora-tion in the crystallinities of CdTe thin films grown on Ti and TiNconducting thin films, and they had higher crystallinities thanthe CdTe thin film directly grown on the sapphire substrate.

The PL spectra of the CdTe thin films grown on various conduct-ing thin films are shown in Fig. 4. Since the CdTe thin film on the Nifilm was grown at lower substrate temperatures, PL peaks were notobserved from the CdTe. In contrast, PL peaks related to donor–acceptor pairs were observed from the CdTe thin films grown onTi, TiN, and Mo thin films. The PL intensity of the CdTe thin filmgrown on the Ti thin film was especially high, and it was compara-ble to that of the CdTe thin films grown directly on sapphire

substrates. Thus, CdTe thin films that have comparable propertiesto those of the CdTe thin film grown directly on a sapphire sub-strate could be obtained by selecting suitable conducting thinfilms.

4. Conclusion

CdTe thin films were grown on sapphire c-face single-crystalsubstrates after depositing various conducting thin films. The ef-fect of Cd/Te supply ratio on the properties of these CdTe thin filmswas investigated. CdTe thin films grown on conducting thin filmsof Ti and TiN exhibited no deterioration in their crystallinitiesand their PL properties were comparable to those of CdTe thinfilms grown directly on a sapphire substrate. Using Te-rich condi-tions was also effective in producing high-quality CdTe thin films.

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