Microstructure, Raman and optical studies on Cd0.6Zn0.4Te thin films

Materials Science and Engineering B107 (2004) 99–105

Microstructure, Raman and optical studies on Cd0.6Zn0.4 Te thin films K. Prabakar a , S. Venkatachalam a , Y.L. Jeyachandran a , Sa.K. Narayandass a,∗ , D. Mangalaraj b b

a Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, South Korea

Received 1 October 2003; accepted 16 October 2003

Abstract We report the structural, Raman and optical properties of Cd0.6 Zn0.4 Te polycrystalline thin films deposited onto well cleaned corning glass substrates by vacuum evaporation. X-ray diffraction pattern showed that the incorporation of zinc favours the growth of films preferentially oriented parallel to the (1 1 1) planes of cubic CdTe. The optical response of vacuum evaporated Cd0.6 Zn0.4 Te films in the 1.5–5.5 eV photon energy range at room temperature has been studied by spectroscopic ellipsometry. The measured dielectric-function spectra reveal distinct structures at energies of the E1 (3.53 eV), E1 + ∆1 (3.9 eV) and E2 (5 eV) critical points corresponding to the interband transitions. In order to check the film local atomic order, the samples were studied by Raman spectroscopy. The transverse and longitudinal optic modes regularly found in CdTe and ZnTe were also observed in Cd0.6 Zn0.4 Te thin films. From the optical transmittance and absorption coefficient, the band gaps of the films are found to be direct allowed. © 2003 Elsevier B.V. All rights reserved. Keywords: Cd0.6 Zn0.4 Te thin films; Raman; optical; Spectroscopic ellipsometry

1. Introduction Cd1−x Znx Te material with a band gap of 1.65–1.75 eV is especially attractive for use in highly efficient tandem solar cell structure [1]. The ternary compound semiconductors formed by CdTe and ZnTe exhibit a direct band gap ranging from 1.45 to 2.25 eV, respectively; this tunability has been exploited in recent years for designing optimized infrared detectors [2,3]. Cd0.6 Zn0.4 Te is a II–VI compound semiconductors, has extensively studied for application in CdS/Cd0.6 Zn0.4 Te heterojunction solar energy conversion devices [4,5]. These alloy thin films or mixed crystals offer a novel physical system whose properties are of considerable basic interest such as electronic band structure, lattice vibrations and localized electronic and vibrational levels present unique experimental and theoretical problems. The vibrational spectra of ternary thin films are of special interest in view of the many fundamental aspects of lattice vibrations they involve [6–9]. In order to get optoelectronic grade material with the desirable properties, it is necessary ∗

Corresponding author. Tel.: +91-422-2425458; fax: +91-422-2422387. E-mail addresses: k [email protected] (K. Prabakar), [email protected] (Sa.K. Narayandass). 0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.10.017

to conduct an intensive investigation of the influence of the growth parameters on the physical properties of the prepared materials. Knowledge of the optical constants of semiconductors is often of great interest in the design and analysis of semiconductors to be used in microelectronics. Spectroscopic ellipsometry (SE) is a non-invasive optical technique sensitive to fractions of atomic layer thickness, capable of determining surface changes, optical constants of bulk or layered materials, overlayer thickness and surface or interface roughness [10]. The evolution of optical constants of the interband transitions between important critical points can be deduced from the ⑀ spectra of alloys [11]. Besides this, optical measurements are widely used for characterization of composition and quality of the samples [12]. In this paper, the studies on the structural, Raman and optical properties studies were presented for the Cd0.6 Zn0.4 Te thin films deposited by vacuum evaporation.

2. Experimental details The samples were grown onto well-cleaned corning glass substrates at room temperature by vacuum evaporation. The Cd, Zn and Te contents in the films were calculated using the X-ray photoelectron spectroscopy (XPS) and Energy

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Dispersive Analysis of X-ray (EDAX). The crystalline structure of the films was confirmed by X-ray diffractometer with an instrumental line width of 0.05◦ . Optical transmittance spectra of the films were obtained using a double beam spectrophotometer. The Raman measurements were taken in a laser of wavelength 514 nm, as the excitation source, having a power density of about 100 W/cm2 and with a charge coupled device thermoelectrically cooled detector. The Spectroscopic ellipsometry (SE) instrument used was of a rotating analyzer type. A 150 W xenon lamp was used as the light source. The SE data were measured over the photon energy range 1.5–5.5 eV at room temperature. The angle of incidence and the polarization azimuth were set at 70 and 45◦ , respectively. The surface microstructure of the vacuum evaporated Cd0.6 Zn0.4 Te film was investigated by ex-situ atomic force microscopy (AFM). The AFM images were acquired using the digital Nanoscope III in the tapping mode and in the repulsive force regime with a force constant of the order of 1 nN between the AFM tip and the sample surface.

3. Results and discussion 3.1. Structural, XPS and Raman studies

20

40

(422) (511)

(331)

0

60

80

Angle 2θ

20

(111)

Intensity (arb. units)

(a)

(b)

(220)

(311)

(111)

Intensity (arb. units)

The crystallinity of the deposited films was determined by the X-ray diffraction technique. Fig. 1a shows the X-ray diffraction pattern of the as prepared powder alloys and for the vacuum evaporated Cd0.6 Zn0.4 Te thin films of different

750 nm 450 nm 310 nm

30

40

50

2θ

60

70

80

Fig. 1. X-ray diffraction patterns of the (a) as prepared alloy (b) spectra of Cd0.6 Zn0.4 Te thin films of different thicknesses.

thicknesses deposited at room temperature on glass substrate is shown in Fig. 1b. The as prepared powder shows the reflection peaks associated with CdTe and ZnTe and compounds of the individual elements. Polycrystalline CdTe powder of random orientation known to show three diffraction peaks associated with (1 1 1), (2 2 0) and (3 1 1) reflections with d values of 3.742, 2.290 and 1.945 Å respectively and ZnTe powder shows the (1 1 1), (2 2 0) and (3 3 01) reflections with d values of 3.523, 2.159 and 1.840 Å respectively [5]. CdTe and ZnTe crystallize in the cubic structure with lattice parameter of 6.481 and 6.101 respectively [13–16]. From the X-ray diffraction patterns of as deposited Cd0.6 Zn0.4 Te films, it has been observed that the crystallites in the films are preferentially oriented along the (1 1 1) face. The results show that the films deposited on the glass substrates kept at room temperature has cubic structure. These results are in agreement with films prepared by other techniques [14]. The absence of diffraction peak associated with CdTe and ZnTe indicated that the Cd0.6 Zn0.4 Te films prepared in the present study were of single phase. Hence it is concluded that the crystalline Cd1−x Znx Te films can be grown highly oriented along the (1 1 1) direction by single source vacuum evaporation technique. The lattice parameters for the samples were calculated from the diffraction spectra using the Bragg law and the angular position of the (1 1 1) diffraction peak are shown in Table 1. The full width half maximum value decreases with increase in film thickness hence the crystallite size value increases from 13.46 to 21.75 nm as the thickness increases from 310 to 750 nm. It is due to decrease in RMS strain value. Such an increase of particle size with increasing thickness and substrate temperature has also been reported [17]. The increase in particle size with films thickness may be due to the coalescence of small crystals. The dislocation density decreases with the increase of film thickness. Since dislocation density and strain are the manifestation of dislocation network in the films, the decrease in dislocation density indicates the formation of high quality films at higher film thicknesses. Because of the room temperature deposition, thermal stress due to the difference in thermal expansion coefficients is negligible; therefore, the compressive stress in these films is probably due to intrinsic stress generated during the film growth [11]. Fig. 2 shows the XPS spectrum of the room temperature deposited Cd0.6 Zn0.4 Te thin film of thickness 750 nm. Based on the previous reported XPS studies of Te–Cd, Te–Zn bonding in CdTe and ZnTe and Te–O bonding in TeO2 , TeO3 and Cd–O, Zn–O, we have analyzed our results [18–21]. The spectral shift along with the abscissa due to the sample charging is eliminated by moving the measured C 1s peak to the binding energy value of 284.6 eV [20]. The XPS analysis clearly showed the films were oxidized in the surface region with thickness of the order of photoelectron escape depth, since both the signals of the oxide and the telluride were present in the spectra. The 3d peaks of both Cd and Te are doublets reflecting the 3d5/2 and 3d3/2 spin orbit splitting (6.38 ± 0.14 and 10.51 ± 0.18 eV, respectively). It has

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Table 1 Microstructural and optical parameters of Cd0.6 Zn0.4 Te thin films Thickness (nm)

d (Å)

a (Å)

Grain size (nm)

Dislocation density 1016 line m−2

rms strain 10−3 line−2 m−4

Band gap (eV)

310 450 750

3.6627 3.6538 3.6579

6.3440 6.3286 6.3338

13.46 18.85 21.75

7.361 5.285 4.262

2.3116 2.3254 2.0175

1.74 1.72 1.69

been shown that the splitting seen in the Te 3d peak is due to the presence of excess tellurium (TeO in CdTe [19] and CdZnTe [21]). These two peaks appear to be asymmetric and they tail toward higher energy. Subbands of Te 3d5/2 and 3d3/2 with peaks at higher energy were also observed for bromine etched CdTe and CdZnTe [21,22] and are attributed to TeO2 . We assign the high energy doublet component at 587.5 and 583.9 eV to 3d5/2 TeO and TeO2 bonds, the low energy doublet components at 576.9 and 573.4 eV to TeO3 and Te [22], respectively. It is seen that the Cd peaks splits into two peaks as Cd 3d5/2 and 3d3/2 spectra and exhibit no subbands. The doublet may be because of Cd oxides and CdTeO3 . It is proposed that CdTeO3 is the most stable phase among several other oxide phases predicted based on the equilibrium phase diagram of the Cd–Te–O [12]. The Zn 2p spectral peak positions showed no shift within the experimental error and the Full Width at Half Maximum

(FWHM) are 1.7, 1.6 for 2p3/2 (1021 eV), 2p1/2 (1045 eV), respectively. The Cd + Zn/Te ratio shows the deficiency of Cd and Zn on the surface and confirms the stoichiometric nature of the prepared films after etching 10 nm and the results are in close agreement with that calculated from the EDAX analysis (Cd = 27.8%, Zn = 19.7%, Te = 52.5%) We used ex situ AFM measurement to independently access surface quality of the vacuum-evaporated Cd0.6 Zn0.4 Te thin film. Fig. 3a and b shows the large-scale (2 × 2 ␮m) three-dimensional and two-dimensional AFM image for the vacuum evaporated film. The root mean square (rms) roughness obtained for this image is 54 Å. The room temperature Raman spectrum of Cd0.6 Zn0.4 Te thin films is shown in Fig. 4. for a typical film of thickness 750 nm. The first-order Raman scattering of a cubic type crystal usually consists of two features corresponding

Fig. 2. XPS spectra of Cd0.6 Zn0.4 Te thin film deposited at room temperature.

Fig. 3. Cd0.6 Zn0.4 Te thin film surface at ambient as imaged by atomic force microscopy.

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Fig. 5. Spectral dependence of ε1 and ε2 of Cd0.6 Zn0.4 Te thin films. Fig. 4. Raman scattering spectrum of Cd0.6 Zn0.4 Te thin film of thickness 750 nm.

to the transverse optic (TO) and longitudinal optic (LO) zone-centre phonon modes. The second-order Raman scattering of CdTe and ZnTe, by combinations of phonons with total wave vector q = 0, provides additional information on phonon mode frequencies at critical points near the edge of the first Brillouin zone (BZ). In this geometry the phonons with A1 and E symmetry has been observed. The first-order Raman spectrum of pure CdTe and ZnTe consists [23,24] of a pair of LO and TO lines of F2 symmetry along with the A1 symmetry of Te precipitates. We note from Fig. 4 that the alloy thin films of Cd0.6 Zn0.4 Te exhibit a rather intense and quasi-continuous specrum below 190 cm−1 . This clearly showed that the Cd0.6 Zn0.4 Te thin film has a richer and more complex spectrum as compared to that of CdTe and ZnTe. Besides the Raman peaks corresponding to the TO and LO modes at the  point, several two phonons features corresponding to different wavevectors in the BZ are clearly observed. The 137.7 and 163 cm−1 are the transverse optic (TO) and longitudinal (LO) phonons in Cd0.6 Zn0.4 Te [6–9] in addition to the new peak at 261.8 cm−1 . The peak at 119.7 cm−1 is the phonon with A1 symmetry of Te precipitates in Cd0.6 Zn0.4 Te [6]. Due to the strong resonance in the Raman effect when Cd0.6 Zn0.4 Te films are excited with laser radiation of about 2.4 eV, very small amounts of Te aggregates embedded in any type of matrix in the films can be detected. The Raman measurements in our Cd0.6 Zn0.4 Te samples indicate that in those with an excess of Te, the Te segregates, at least part of it, forming particles with a crystalline structure. According to the data described above, one can conclude that samples with an excess of Te contain small amounts of crystalline Te embedded in the Cd0.6 Zn0.4 Te structure on the surface as is evidenced the XPS measurement. The features located at 122.5 cm−1 correspond to the E1 symmetry of the phonon vibrations in the TeO structure and XPS also confirms the presence of this TeO on the surface. Another feature at 261.8 cm−1 in the spectrum has been identified as the combination of the LO phonon with transverse acoustic phonons at X point of the Brillouin zone [19]. In the low-frequency region, we can find the structures related to TA(X) (92.4 cm−1 ) scattering. Above the LO region, a double structure, related to the LO (X) + TA (X) contributions is also observed. All the major peaks are iden-

tified and are in good agreement with those observed for single crystals and thin films [6,24]. 3.2. Optical constants from spectroscopic ellipsometry The complex dielectric function, ε (E) = ε1 (E) + iε2 (E), can describe the optical response of any homogeneous medium at all photon energies E = hυ. Fig. 5 shows the pseudodielectric function spectra of vacuum evaporated Cd0.6 Zn0.4 Te film measured by Spectroscopic Ellipsometry (SE). The three major features seen in the figure are the low energy peak at E1 (3.53 eV) has been assigned to Λ3 –Λ1 , the middle peak at E1 + ∆1 (3.9 eV) arises from spin-orbit splitting of the valence band in the Λ-direction. The high energy peak at E2 (5 eV) is attributed to the intense X5 –X1 transition at the X-point of the Brillouin zone [25]. The E1 and E1 + ∆1 critical points may be of 3D M1 type [26]. Since the M1 critical point longitudinal effective mass is much larger than its transverse counterparts, one can treat these 3D M1 critical points as a two dimensional minimum M0 . The more pronounced structure found in the optical spectra of Cd0.6 Zn0.4 Te thin films in the region higher than E1 + ∆1 in II–VI semiconductors could be labeled as E2 [27]. The origin of these distinct structures is explained as being due to interband transitions [26–30]. From this it can be seen that the optical behaviour data of Cd0.6 Zn0.4 Te thin films for x = 0.4 obtained in the present investigation by spectroscopic ellipsometry yielded results which are in fair agreement with those reported in Cd1−x Znx Te crystals [25]. Optical spectra such as the complex refractive index is connected to ε(E) by n∗ (E) = n(E) + ik (E), where n is refractive index and k is the extinction coefficient, also called the attenuation index. The absorption coefficient α(E) and normal incidence reflectivity R(E), can also be easily calculated from the SE ε(E) data. The real refractive index n (E) and extinction coefficient k (E) can now be written as
1/2 [ε1 (E)2 + ε2 (E)2 ]1/2 + ε1 (E) n(E) = (1) 2
k(E) =

[ε1 (E)2 + ε2 (E)2 ]1/2 − ε1 (E) 2

1/2 (2)

Fig. 6 shows the numerically calculated spectral dependence of n(E) and k(E) for the vacuum evaporated Cd0.6 Zn0.4 Te

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half integer for minimum and t the thickness of the film. The refractive index can be computed from the relation [32] n2 =

n2a + n2g

+

Fig. 6. Numerically calculated spectral dependence of refractive index and extinction coefficient of Cd0.6 Zn0.4 Te thin films.

thin films of thickness 750 nm. Like the real and imaginary dielectric constant it shows the E1 , E1 + ∆1 and E2 critical point structures. This type of interband transitions has already been observed by the Prasada Rao et al. [31] for the two-source vacuum evaporated Cd1−x Znx Te thin films deposited at higher substrate temperatures. 3.3. Optical constants from transmittance spectra Fig. 7 shows the optical transmittance spectrum of the Cd0.6 Zn0.4 Te thin films in the wavelength range 500–2500 nm. The spectrum shows interference pattern with a sharp fall of transmittance at the band edge, which is an indication of good crystallinity. The transmittance T varied with wavelength λ according to the relation [32] T =

n2 exp(−αt)

R21

16na ng 2 + R2 exp(−αt) + 2R1 R2 exp(−αt) cos(4πnt/λ) (3)

+ 2na ng T  2  2 (na + n2g + 4na ng T  )2 4

1/2 − n2a n2g

(5)

where T  = (Tmax − Tmin )/(Tmax Tmin ). Tmax Tmin represent the envelopes of the maximum and minimum positions of the T–λ curve. Exponential variation of T with absorption coefficient α is most probable near the absorption edge, therefore α may be determined from the relation T = A exp(−αd)

(6)

where A=

16na ng (n2 + k2 ) [(na + n)2 + k2 ][(ng + n)2 + k2 ]

where k is the extinction coefficient. A is found to be nearly equal to unity at the absorption edge. The relation between ␣ and incident photon energy hυ can be written as αhυ = C1 (hυ − Egd )1/2

(7)

αhυ = C2 (hυ − Egi )2

(8)

for direct allowed and indirect allowed transitions, respectively, where C1 and C2 are two constants, Egd , Egi are the direct and indirect band gaps, respectively. The (αhυ)2 versus hυ plot for Cd0.6 Zn0.4 Te films deposited at room temperature on glass substrates is shown in Fig. 8. The graphs

where α is the absorption coefficient R1 = (n + na )(ng + n), R2 = (n − na )(ng − n) and n, na and ng are the refractive indices of the film, air and substrate, respectively. The maxima and minima in the T–λ plot occurs when 4πnt = Mπ λ

(4)

where M represents the order number of each interference extreme which takes the value of integer for maximum and

Transmittance (%)

100 80

220 nm 310 nm 450 nm 750 nm

60 40 20 0 500

1000 1500 2000 2500

Wavelength (nm) Fig. 7. Transmittance spectra against wavelength for Cd0.6 Zn0.4 Te thin films.

Fig. 8. Plots of (αhυ)2 against hυ for Cd0.6 Zn0.4 Te thin films of different thicknesses.

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3.2

n2 (¯hω) = 1 +

Refractive index

3.1 3.0 2.9 2.8 2.7 2.6 2.5

1000

1500

2000

Wavelength (nm)

(a)

0.18 0.17

E02

Ed E0 − (¯hω)2

(9)

where h ¯ ω is the photon energy, E0 is the single oscillator energy and Ed is the dispersion energy. The parameter Ed , which is a measure of the intensity of the interband optical transitions, does not depend significantly on the band gap or the density of the valence electrons. The oscillator strength is derived from intercept and the slope of the straight line portion of the 1/(n2 − 1) versus (hυ)2 . In Fig. 9b, a typical 1/(n2 − 1) versus (hυ)2 plot for the Cd0.6 Zn0.4 Te films of thickness 750 nm deposited at room temperature is shown. The average strength of the oscillator E0 is 1.95 and Ed is 11.74 obtained from the slope and intercept of the straight line fit, respectively. This type of behavior is similar to that observed in CdSe films by Pal et al. [37].

0.15

4. Conclusion

2

(n -1)

-1

0.16

0.14 0.13 0.12 0.11

(b)

0.4 0.6 0.8 1.0 1.2 1.4 1.6

(hν)

2

Fig. 9. (a) Variation of refractive index with wavelength (b) 1/(n2 − 1) vs. (hυ)2 plot for a typical Cd0.6 Zn0.4 Te thin film of thickness 750 nm.

of (αhυ)2 versus hυ are found to lead straight lines over any part of the optical absorption spectrum, thus supporting the interpretation of direct rather than the indirect band gap for all Cd0.6 Zn0.4 Te films. It is observed that the direct band gap decreases from 1.74 to 1.69 eV with the increase of film thicknesses. According to the band structure of ZnTe and CdTe nonlocal pseudopotential method [33,34] there is unlikely to be any indirect optical energy gap less than the direct gap. It is known that at lower photon energy transitions, transitions rules are relaxed in the presence of a high density of defects, charge impurities and disorders at the grain boundaries may cause the decrease in the direct band gap. There is another reason for the decrease of direct band gap, which is likely to be attributed to an increase in particle size and a decrease in rms strain as observed from the XRD, leads to decrease in band gap energy. The dependence of refractive index on photon energy is shown in Fig. 9a. The refractive index was found to vary with photon energy and such type of behaviour was also observed by Chattopadhyay et al. [14] The data on the dispersion of the refractive index were evaluated according to the single effective oscillator model proposed by Wemple and Di Domenico [35,36]. It is well known from the dispersion theory that in the region of low absorption the index of refraction n is given in the single oscillator model by the expression

We have presented the results of investigations carried out to determine the structural and electronic band gap properties of vacuum evaporated thin films of the novel semiconductor alloy Cd0.6 Zn0.4 Te. X-ray diffraction experiment showed that the growth of thin films oriented preferentially along the cubic (1 1 1) direction. The Raman scattering experiments showed that the films surface are aggregated with Te and are oxidized, which are in agreement with the XPS measurement. The optical transmittance experiments showed that the trend of the electronic band gap of the Cd0.6 Zn0.4 Te films was found to increase with respect to that of pure CdTe by increasing the amount of Zn. The dispersion of the refractive index follows a single oscillator model.

References [1] J.C.C. Fan, B.J. Palm, Solar cells 12 (1984) 401. [2] A. Ruzin, Y. Nemirovsky, J. Appl. Phys. 82 (1997) 4166. [3] Y. Nemirovsky, A. Ruzin, G. Asa, J. Gorelik, J. Electron. Mater. 25 (1996) 1221. [4] M.G. Peters, A.L. Fahrenbruch, R.H. Bube, J. Appl. Phys. 64 (1988) 3106. [5] T.L. Chu, S.S. Xhu, C. Ferekides, J. Britt, J. Appl. Phys. 71 (1992) 5635. [6] K. Prabakar, S.K. Narayandass, D. Mangalaraj, Physica B 328 (2003) 355. [7] M. Levy, N. Amir, E. Khanin, A. Muranevich, Y. Nemirovsky, R. Beserman, J. Cryst. Growth 187 (1998) 367. [8] S. Venugopalan, A. Petrou, P.R. Galazka, A.K. Ramdas, S. Rodriguez, Phys. Rev. B 25 (1982) 2681. [9] B. Li, J. Zhu, X. Zhang, J. Chu, J. Cryst. Growth 181 (1997) 204. [10] D.E. Aspnes, Handbook of Optical Constants of Solids, Academic Press, New York, 1985, p. 89. [11] K. Suzuki, S. Adachi, J. Appl. Phys. 83 (1998) 1018. [12] M. Ambrico, D. Smaldone, C. Spezzacatena, V. Stagno, G. Perna, V. Capozz, Semicond. Sci. Technol. 13 (1998) 1446. [13] U. Pal, S. Saha, B.K. Samantaray, H.D. Panerjee, A.K. Chaudhuri, Phys. Stat. Sol. (a) 111 (1989) 532.

K. Prabakar et al. / Materials Science and Engineering B107 (2004) 99–105 [14] K.K. Chattopadhyay, A. Sarkar, S. Chaudhuri, A.K. Pal, Vacuum 42 (1991) 1113. [15] J. Touskova, D. Kindl, J. Tousek, Phys. Stat. Sol. (a) 98 (1986) K197. [16] B. Samanta, A.K. Chaudhuri, S.L. Sharma, U. Pal, J. Appl. Phys. 75 (1994) 2733. [17] U. Pal, D. Samanta, S. Ghoral, B.K. Samantaray, A.K. Chaudhuri, J. Phys. D: Appl. Phys. 25 (1992) 1488. [18] S.S. Chol, G. Lucovsky, J. Vac. Sci. Technol. B 6 (1988) 1198. [19] J.P. Haring, J.G. Werthen, R.H. Bube, L. Gulbrandsen, W. Jansen, P. Luscher, J. Vac. Sci. Technol. A 1 (1983) 1469. [20] K.T. Chen, D.T. Shi, H. Chen, B. Granderson, M.A. George, W.E. Collins, A. Burger, R.B. James, J. Vac. Sci. Technol. A 15 (1997) 850. [21] M.A. George, M. Azoulay, H.N. Jayatirtha, A. Burger, W.E. Collins, E. Silberman, Surf. Sci. 296 (1993) 231. [22] J. Ramiro, L. Galan, E. Garcia Camarero, I. Montero, Y. Laaziz, J. Mater. Res. 16 (2001) 1942. [23] S.L. Lopez, G.T. Delgado, S.J. Sandoval, O.J. Sandoval, R.C. Perez, M.M. Lira, J. Vac. Sci. Technol. A 17 (1999) 1958.

105

[24] J. Camacho, I. Loa, A. Cantarero, K. Syassen, J. Phys.: Condens. Matter 14 (2002) 739. [25] M. Cardona, D.L. Greenaway, Phys. Rev. 131 (1963) 98. [26] S. Adachi, T. Kimura, Jpn. J. Appl. Phys. 32 (1993) 3888. [27] S. Adachi, T. Kimura, Jpn. J. Appl. Phys. 32 (1993) 3496. [28] J.P. Walter, M.L. Cohen, Phys. Rev. B 1 (1970) 2661. [29] O. Castaing, R. Granger, J.T. Benhlal, R. Triboulet, J. Phys.: Condens. Matter 8 (1996) 5757. [30] O. Castaing, J.T. Benhlal, R. Granger, Eur. Phys. J. B 7 (1999) 563. [31] K. Prasada Rao, O.M. Hussain, B. Srinivasulu Naidu, P. Jayarama Reddy, Adv. Mater. Opt. Electron. 7 (1997) 109. [32] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E: Sci. Instrum. 9 (1976) 1002. [33] J.R. Chelikowsky, M.L. Cohen, Phys. Rev. B 14 (1976) 555. [34] J.E. Lewis, Phys. Stat. Sol. (b) 143 (1987) 307. [35] S.H. Wemble, Di Domenico, Phys. Rev. B 7 (1971) 1338. [36] S.H. Wemple, Phys. Rev. B 7 (1973) 3767. [37] U. Pal, D. Samanta, S. Ghorai, A.K. Chaudhuri, J. Appl. Phys. 74 (1993) 6368.