Chemical structure of microcrystalline CdTe films grown by RF sputtering

196

Journal of C r’s sEal (ross tO ~6 I 95Sf

1 196 401) North—Holland, ~msterdam

CHEMICAL STRUCTURE OF MICROCRYSTALLINE CdTe FILMS GROWN BY RF SPUTTERING 1. HERNANDEZ-CALDERON, S. JIMENEZ-SANDOVAL, J.L. PENA and V. SAILER Physics Department, Centro de Inse~tigaion i’ sit’ Ettudiot 4t’anzados del IPV. .4pdo Postal 14 740, 07000 ~1ésico. Dl

We have applied X ray photoemission and Auger spectroscopy techniques to the stud’s of the stoichiometric properties of C’dfe thin films grown by RF sputtering. The microcrystalline films were deposited on glass substrates held at temperatures between 50 and 2000 C’. They contain a mixture of the cubic (zinc blende) and hexagonal (~urtzIte)phases which are nearly stoichiometric. Bs using hulk and suiface sejisitive photocmisssoii geonietnes it ,s ‘,hsswn that a tellurium uside u’s ci layer is always fyi med dftei esposwe Es, air. A simple calculation shows that this overlayer is at most 10 A thick. Cadmium seems to he insensitive to the presence of ox’sgen. as demonstrated by the absence of shifted Cd peaks in the X ray spectra. It is shown that the low kinetic energy features in the Auger spectra ( <100 eV) are very sensitive to the oxide overlaver and contamination

I. k~fr~dUCfiOfl

2. Experimental details

The study of the physical properties and methods of preparation of thin-film CdTe is important due to its potential application in the manufacture of optoelectronic devices such as all-thin-film solar cells. photodetectors and CdTe related heterostructures [1,2]. It has been shown by theoretical considerations that a nearly ideal solar cell can be constructed by using CdTe [3]. However, the production of economic and competitive devices has not been achieved, mainly due to problems involved in the preparation of films with the optical and electrical properties required for practical applications [4]. In contrast with most of the evaporation methods, the RF sputtering deposition process allows wide range control of n- and p-dopant concentrations, therefore appearing as a promising technique for the production of highquality CdTe films. In this paper we report the results obtained from the application of X-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES) to microcrystalline CdTe thin films grown by RF sputtering. Due to its influence in electrical contacts [sI. special attention has been devoted to the characterization of the Te oxide overlayer that forms after cxposure of the samples to air.

2.1. Sample preparation Cdle thin films were produced in a standard RE diode sputtering system. The sputtering chamber had a base pressure lower than 2 x 10 “ Torr. Corning glass (7059) plates situated 5 cm from the five-inch target of sintered high-purity CdTe were used as substrates. Ar pressures varied between I and 6 mTorr and RF power was in the 100 250 W range. Substrate temperature was varied between 50 and 2000 C: it was measured directly on the growing surface using a chromel aluniel thermoLouple. Depending on growing Londitions, giowth rates varied between 0.5 and 2.5 A ~. Films with thicknesses of 2000 to 14,000 A were produced. Films were allowed to cool down to room temperature in vacuum. Before deposition, 15 mm of pre-sputtering was performed. X-ray diffraction patterns indicated the presence of cubic (zincblende) and hexagonal (wurtzite) phases with preferential growing in the (111) and (002) directions, respectively [6]. Due to overlapping of the main diffraction peaks of both structures. it was not possible to obtain a quantitative relation of phase composttion from the diffraction patterns.

0022-0248/88/$03.50 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

I. Herndndez Calderbn et al

/

Microcrystalline CdTe films grown by RE sputtering

2.2. XPS Auger measurements

XPS

Analyses were performed in a PH1560/ESCASAM system which had a base pressure of approximately 4 x 10 ~‘° Torr. The double pass cylindrical mirror analyzer was operated at 25 eV pass energy in the XPS measurements; Al Kct radiation was used as excitation. For AES measurements, a 3 keV electron beam of typically 0.5 ~.tA current incident at 600 to the surface normal was employed. In the case of XPS measurements, surface and bulk sensitivities were obtained by collecting electrons in directions normal to or at glancing angles with the surface respectively A 900 collection aperture was employed.

CdTe

397

Cd core levels

3d5/2 404.8 3~312

0

I—

b)~

Cl)

Z LiJ

4051

I— Z

4118

1.-i

3. Results and discussion

C~

Results of the above measurements are described here, and conclusions based on these resuits are discussed briefly. I

3.1. xps measurements

413

411

I

409

407 4C6

403

BINDING ENERGY (eV) Figs. 1 and 2 show typical XPS spectra of Cd and Te 3d core levels of CdTe films before and after surface cleaning. Table I summarizes the important parameters from these spectra. Figs. Ia and 2a refer to bulk sensitivity, while figs. lb and 2b refer to surface sensitivity spectra of the films before surface cleaning. It is clear that the Cd 3d doublet shows only a change in intensity for both geometries, while the peak positions remain the same. Since the full width at half maximum (FWHM) corresponds to that of the clean CdTe surface (fig. ic), the conclusion is that Cd is tetrahedrally bonded to Te atoms and no other kind of bonding exists. On the other hand, for Te one observes two doublets and their ratios change from the bulk to the surface sensitive geometry, indicating the surface character of the doublet, with the Te 3d5 2 at 575.8 eV binding energy. Since this doublet can he easily eliminated by Ar~ erosion, these peaks are attributed to Te02 [7]. After short Ar~ bombardment the spectra of figs. ic and 2c are observed. Peaks are shifted 0.2 eV toward higher binding energies, and these peak

Fig. XPS data showing Cd(3d)sensitivity; core levels a CdTe film (a) 1. bulk sensitivity; (b) surface (c)of clean surface.

positions correspond to those of a bulk CdTe polycrystal. These results indicate the presence of a thin overlayer of Te oxide in films exposed to air and also that Cd does not oxidize. This conclusion is in agreement with investigations on single-crystal surfaces [5,8,9]. This is also consistent with the fact that Te02 has a higher heat of formation than does CdO: 78.1 and 62.3 g-cal/mol, respectively [10]. No evidence of Te or Cd clustering was observed, in agreement with Auger experiments. In order to make an estimate of the thickness of the Te oxide layer it can be assumed, as suggested by the results, that a Te02/CdTe interface exists as shown in fig. 3. We define for the case of normal emission I [Te(3d5 2)] —

r

oxsde

I FTe(3d5

2)]CdT~

195

/ Herndndez Calderon ci al.

XPS

CdTe

Te

Microcristalline CdTejilrnv grossn hi RI sputtering cc

core levels

Te 3d

312

k

ens I ye

surface sensitive

3d512 572

3

6~~,

-

Te02

__-

CdTe dTe

a 5)

4

Te2

I

Fig. 3 leO2 (‘dTe interface model emplosed for calculation of oxide oserlayer thicknesu

Cl)

z ILl I— z

5725

the fromI (‘dl e. i/sing the exponential contribution attenuation law I~ exp[ L]. where L represents the escape depth of the photoelectrons. .‘s

Lan

C)

and Lassuming illumination the sampled si ln(lvolume, +r).a homogeneous one obtains from eq. (2) in (1)

surface

Since thc maximum value of r was always less 582

586

578

574

570

BINDING ENERGY (eV) Fig. 2. XPS data showing Te(Jd) core levels of a C’dTe film (a) hulk sensitisit’s: (b) surface sensitivit’s. (c) clean surface

With the geometry of fig. 3. the same equation can be written as

f

than I for films several weeks old atid the “uni‘sersal curve” for the escape depth of electrons with 900 eV kinetic energy gives a value of ap proximately 15 A for L. one obtains a maximum thickness of 10 A for the oxide o’serlayer. This value is consistent with the fact that we were always able to detect Cd and Te signals from CdTe in the surface-sensitive geometry from the

(2)

contaminated samples. It is important to mention that the formation of the Te oxide in the films is a

where the first integral gives the contribution to the Te(3d3 peak in the oxide and the second

of exposure of clean surfaces to air did not show any detectable oxide formation.

1(d)

dI

1)0)

r

(

0

dI,

slow process. After surface cleaning, a few minutes 2)

Table I XPS 3d core levels Element

Compound

3d5

binding

FWHM

energy (eV)

(FV)

Te

CdTe leO

572.5 + 0.1 575.S-r-0.I

1 62 + 006 I 70-cOOS

Cd

CdTe

405.1+01

1 40+1) O~

~°

Spin orbit splitting. This column refers to ratio of intensities.

SC) “~

l( 3d5

10 4 + 0.1 10.4 +002

1.47 + 0.05 1 32-.-)) 05

675+0.07

I.4-+0 U’s

I) 3d,

1. Herndndez-Calderbn et al.

Microcrystalline CdTe films grown by RE sputtering

3.2. AES measurements

AES

399

CdTe

XPS and AES surveys of films exposed to air

N’~~’”~’

indicated taminantsC,(table 0, and2).S as In the general, main surface short conAr~

showed that theperiods films were homogeneous and free bombardment contamination; Auger were profiles enough and toscanning remove of contaminants, and no clustering was observed. The main peak for Cd was located at 378 eV and for Te at 482 eV in the dN/dE spectrum. The stoichiometry of the films was determined mainly by AES using single-crystal CdTe as a reference. Measurements of the peak-to-peak amplitude in the dN/dE distribution indicated a 51 ±3 at% concentration of Te for films grown with substrate temperatures in the 50 200°Crange. Variations were found for films grown at the same substrate temperature in different runs. However, most of the films showed small deviations from stoichiometry; the trend was for a higher Te content for films grown at higher temperatures. The excess of Te may be attributed to Cd losses with increasing substrate temperature [11]. Examination of Cd features in the Auger spectra before and after surface cleaning showed only minor changes, confirming the stability found by XPS. However, Te related features exhibited considerable variations. These are attributed to the existence of a Te02 overlayer, as demonstrated by the XPS results. Figs. 4a, 4b and 4c show the Auger spectra of contaminated, slightly contaminated and clean CdTe surfaces, respectively, The left portion of each figure corresponds to the low kinetic energy region, and the right portion to the Cd and Te regions. The peak at 515 eV is due

Table 2

Auger peaks Element

Peak (eV)

Cd

378+1

Te

384+1 482+1

490+1 S C

154+1 271 +1

0

535+1

“contam surface

Cd

dN(E) dE

“

20

40

60

80

100

surface

300 350 400 450 500

KINETIC ENERGY (eV) Fig. 4. Low kinetic energy (left) and MNN transitions (right) in Auger spectra of (a) contaminated, (b) slightly taminated, and (c) clean CdTe surfaces.

con-

to oxygen, which also contributes to the peak at 490 eV. It should be noted that the ratio of the peak-to-peak amplitudes of the features at 482 and 490 eV is inverted from the contaminated to the clean surface. The same behavior has been observed in elemental Te [6]. Stronger modifications are observed in the low kinetic energy region of the Auger spectra that contain Cd and Te contributions. In this energy region the electrons have minimum values for the escape depth and the CdTe features are strongly attenuated by contamination and the oxide overlayer. This sensitivity makes this region very important for monitoring contamination of CdTe surfaces.

4. Summary Microcrystalline CdTe thin films grown by RF sputtering at substrate temperatures between 50

40(1

/ Herndnde: C a/derón eta).

Mrs msri ito/line C dTefr/rpi.s gross u/ri RI sputteruig

and 200°Care homogeneous and free of contami nants. They are nearly stoichiometric, with a tendency to have excess Te in films grown at the higher substrate temperatures. Auger and

authors (S.J.S. and J.L.P.) thank CONA(’yT of Mexico for partial financial support of this work.

XPS

results indicate the presence of a thin TeO, o~er layer that forms after film exposure to air. The thickness of the overlayer was determined to he less than 10 A. Absence of modifications for Cd peaks in contaminated and clean surfaces, both in

References [Ii E.G (ourreges. ~ L Fahrenhruch and RH ~uppl Ph’ss. ‘sI (1980) 2175 [2] T.H Myers. ‘y I o R N Bicknell and AppI. Ph 1s I etters 42 (1983) 247

I F

Babe,

I

Schet,’in,i.

XPS and Auger experiments, suggest that Cd does not oxidize. Due to the high sensitivity of the low

(~]SM

kinetic energy region of the AES spectra to contamination and TeO, layer formation, it is recommended that this region he used in monitoring for

[4] MB. Das. S.V. Krishnasssarn’s, R Peikie. P. Sssah and K S edam, Solid-State Electron 27 (1984) 329 [5] I (~ Werthen. J P Harin~and R H. Buhe I \ppl Phys

determination of surface cleanliness.

[6] I Hernandez-C alderhn. J.L Pena and S Romero Surface Science Lectures, Eds. G.R Castro and

Sic Physies of Semiconductor Des’ices (Vi tIe’s. Ne~

York, 1981)

54 ~

1159 in:

M

(ardona (Springer Berlin, 1987) p. 56. [7] Handbook of X-Ray Photoelectron Spectroscop’.. Ed. C F’.

Acknowledgements

Muilenherg (Perkin Elmer. Minnesota 1975) Ehina, K Asano. Y Soda and T. Takahashi. J ‘vacuum Sci. Technol 17 (1980) 1074

I~IA.

The authors thank S.S. Chao of Energy Conker sion Devices (Troy. MI). for preltminary measurements of XPS spectra. One author (I.H.C.) wishes to thank the R.J. Zevada Foundation of Mexico for partial financial support. and two of the

[9] A.J Ricco. H S. White and M.S Wrighton. Sci. Technol. A2 (1984) 910 [10] Handbook ol (‘hemistry and Physics. Ed (CRC. Boca Raton, 1-1 1979) [II]

R.FC

Farrose. CR

Jones

(1 M

I Vacuum R(

We,ist

Williams and I M

Young. AppI Ph’.s Letters 39 (1981) 954