Structural and optical studies of CdS nanocrystals embedded in silicon dioxide films

Thin Solid Films 318 Ž1998. 108–112

Structural and optical studies of CdS nanocrystals embedded in silicon dioxide films A.G. Rolo

a,)

, O. Conde

b,1

, M.J.M. Gomes

a

a b

Departamento de Fısica, UniÕersidade do Minho, Largo do Pac¸o, 4709 Braga Codex, Portugal ´ Departamento de Fısica, UniÕersidade de Lisboa, Ed. C1 , Campo Grande, 1700 Lisboa, Portugal ´

Abstract

˚ have been grown using Silicon dioxide films doped with nanocrystallites of CdS with a mean radius value in the range of 20 to 40 A the magnetron rf-sputtering technique. We have studied the combined effects of rf-power deposition, the number of semiconductor chips on the SiO 2 target, and post-annealing temperature on the CdS crystal size. The optical transmission spectra display a marked blue shift of the absorption band edge of up to 400 meV which was attributed to quantum size confinement effects. The luminescence spectra show a narrow emission peak tentatively explained as being due to the recombination of electrons on shallow donor levels with confined valence band holes. q 1998 Elsevier Science S.A. Keywords: CdS-doped glass films; II–VI semiconductor; Rf-sputtering; Quantum dots; Optical absorption; Photoluminescence

1. Introduction Small semiconductor crystals embedded in a dielectric matrix, which have the properties of zero-dimensional quantum dots, have been extensively studied for several years w1,2x. Optical glasses doped with semiconductor nanocrystallites are one example of these systems. Optical and electronic properties of these semiconductor quantum dots have attracted much attention because, due to their low dimensions, these systems exhibit quantum confinement effects, which can give rise to interesting linear and nonlinear optical properties and have the potential of being developed into novel photonic devices w3x. 2. Experimental procedure CdS-doped glass films were prepared by a conventional rf-magnetron co-sputtering method using Alcatel SCM650 apparatus. A SiO 2 Ž99.99%. plate of 50 mm diameter, partially covered by polycrystalline chips of cadmium sulphide Žc-CdS., was used as target. Prior to sputtering, the chamber was always evacuated to 2 = 10y6 mbar. In situ Ar pre-sputtering of the target and substrates was ) Corresponding author. Tel.: q351-53604331; fax: q351-53678981; e-mail: [email protected]. 1 E-mail: [email protected].

0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 1 1 7 5 - 9

performed in order to clean and remove any impurities. The deposition parameters are presented in Table 1. The film deposition was carried out in highly pure Ar Ž99.996%. pressure of 5 = 10y3 mbar on glass slides, with the substrates kept at room temperature, resulting in typical ˚ sy1 . The target–substrate deposition rates of 0.1 to 0.9 A distance was kept at 60 mm. Relatively low rf-powers have been used in order to reduce the target heating that could cause a decomposition of the CdS chips and result in the loss of sulphur. In order to investigate the growth of semiconductor nanocrystals in the deposited films, post-annealing was carried out in a Kantal furnace in ambient atmosphere. Annealing temperature was varied between 1008C and 5508C for annealing times from 300 s to a few hours. X-ray diffraction ŽXRD. spectra of the films were recorded with a Siemens D5000 diffractometer, employing CuK a radiation in glazing incidence geometry ŽGIXRD.. Since the fraction of CdS crystals embedded in the glass films is rather small, a counting time longer than 15 s per 0.048 step and incidence angles of 1 and 28 were required. The XRD data were obtained between 2 u s 78 and 708. Optical absorption spectra were measured at room temperature using a double-beam Shimadzu model UV-3101 PC in the range from 200 to 3200 nm. Photoluminescence spectra in the range from 300 to 800 nm were obtained at room temperature by exciting the samples in the range 380 to 425 nm using a continuum Xenon lamp using a 0.22-m

A.G. Rolo et al.r Thin Solid Films 318 (1998) 108–112 Table 1 Deposition parameters of CdS-doped SiO 2 glass films Film

Rf power ŽW.

Deposition time Žs.

EDS CdrSi Ž%.

Target semiconductor area Ž%.

aA3 aA4 aA5 aA6 aA7 aA8 aA10 aA11 aA12

30 30 15 30 50 40 30 15 50

25 200 23 180 11 102 7200 14 400 18 000 14 400 21 600 14 460

y 30 20 y 4 y y y 6

1.3 6.4 6.4 2.6 2.6 2.6 3.8 3.8 3.8

double-beam spectrometer Spex 1680. Control of nanocrystals size can be achieved through the combined effect of deposition parameters, the number of CdS chips on the SiO 2 target, and post-annealing temperature. Energy dispersive spectroscopy ŽEDS. measurements confirmed that the constituent dopants Cd and S were deposited in about an 1:1 ratio.

3. Results and discussion Fig. 1 shows the spectral optical transmission measured at room temperature for the as-deposited CdS doped-glass films. A large change in the spectra of the films below 500 nm is observed. Since the absorption edge in the bulk CdS at room temperature is situated at approximately 496 nm

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ŽEg s 2.50 eV. w4x, there is a blue shift of bandgap energy in our samples. With increasing rf-power, the optical bandgap energy Eg moves to lower energies, as can be seen on Fig. 1, by comparing the spectra changes between the films on the films groups ŽaA6, aA8, aA7., and ŽaA5, aA4. and ŽaA11, aA10, aA12.. By increasing the concentration of CdS in the SiO 2 film Žby increasing the ratio of CdSrSiO 2 on the target., Eg decreases and moves closest to that of the CdS bulk, as can be observed in Fig. 1 Žby comparing the films aA3, aA6, aA10 and aA4.. The absorption edge becomes abrupt with a higher number of CdS chips, as observed for film aA4. The films subjected to a post-deposition annealing display a red shift of the absorption band edge, comparatively to as-deposited one, as shown on Fig. 2. As previously reported w5,6x, the asymmetric lineshape of the first-order Raman scattering LO peak observed experimentally in the post-annealed films can be fitted fairly well, considering the model of phonons confined in a finite volume of semiconductor nanocrystallites. Thus, the silicon dioxide glass films doped with CdS crystallites have size-confinement effects. The shifts observed in the absorption bandgap are caused by changing the crystal size. In fact, due to the size dependence of the nanocrystal energy gap, the glass colour changes from a transparent yellow of the as-deposited films Žsuggesting that some crystals of CdS were already formed during deposition., to a darker yellow after the heat treatment. The blueshift in the optical absorption spectra arises from the confinement of the charge carriers in the nanocrystallites, and should result in sharp optical absorption peaks w7x as already observed in similar systems.

Fig. 1. Room temperature optical transmission spectra of the as-deposited CdS-doped glass films for different deposition conditions Žsee Table 1. and that of the glass substrate for comparison.

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A.G. Rolo et al.r Thin Solid Films 318 (1998) 108–112

Fig. 2. Room temperature optical transmission of several films as-deposited and after annealing ŽR.: Ža. aA7-2 and RaA7-2 Ž3008C, 6000 s.; Žb. aA5-13 and RaA5-13 Ž4008C, 3600 s.; and Žc. SiO 2 film Žthicknessf 2530 nm., aA4-19 and RaA4-19 Ž5508C, 10800 s..

However, our spectra do not show sharp peaks, which is probably because of an inhomogeneous broadening due to crystal size distribution in our samples. The quantum size effects were examined using the model developed by Efros and Efros w8x including the Coulombic interaction term proposed by Brus w9x. From the minimum of the second derivative of the absorption curves we estimated the mean radius of the CdS nanocrystals. The absorption edge varies from 2.6 to 2.9 eV corresponding to a mean radius value ˚ ranging from 20 to 40 A. Fig. 3 shows the GIXRD spectra of the CdS doped-glass films after annealing at different temperatures, and that of a SiO 2 glass film for comparison. The XRD spectrum of the SiO 2 glass film shows a broad peak centred at about 268 indicating its amorphous character, and those of the semiconductor-doped glasses consist of several broad and low-intensity reflection peaks due to CdS. For the as-deposited films, as the numberrsize of semiconductor crystals was very low, the scattering intensity was too weak. Thus, the diffraction peaks were indistinctive in the diffractograms, only clearly appearing after heat treatment. A narrowing of the peaks with an increase of the annealing temperature is observed. Similar results have been reported in the literature for this kind of systems w10–12x. CdS is a II–VI semiconductor, which can manifest the wurtzite Žhexagonal. and zincblende Žcubic. structures 2 w13x. Due to the overlapping of the main diffraction peaks of both structures, it is rather difficult to obtain a quantitative relation of phase composition from the diffraction patterns. However, the broad CdS peaks are rather well described 2

Table JCPDS-ICDD, PDF-2 Sets 1-42 database Ž1992..

Fig. 3. Glancing incidence XRD spectra for CdS-doped SiO 2 glass films at different annealing temperature: Ža. RaA4-3 Ž6008C, 4 h.; Žb. RaA4-19 Ž5508C, 3 h.; Žc. RaA4-4 Ž4008C, 10 h.; Žd. RaA4-2 Ž3008C, 10 h.; Že. as-deposited SiO 2 glass film, for comparison.

A.G. Rolo et al.r Thin Solid Films 318 (1998) 108–112

ŽFig. 4. by considering that they consist of the diffraction peaks of wurtzite structure that overlaps one another. Since the average size of the crystal can be related to the broadening of the diffraction peaks by the Scherrer equation w14x, the mean diameter of the crystals could be estimated. This analysis was carried out for all the peaks after background subtraction. In order to deconvolute the overlapping peaks, several Gaussian functions, one for each crystallographic diffraction orientation of hexagonal CdS structure, were used. It is apparent that the data profile for CdS diffraction peaks is fitted well, as shown on Fig. 4, confirming that hexagonal wurtzite CdS crystals have been formed. Thus, the mean radius of the crystal ˚ could be estimated in the range of 18 to 33 A. Room-temperature luminescence measurements have also been performed in our samples. Two photoluminescence bands are seen in the spectra for CdS nanocrystals as shown in Fig. 5. The blue emission peak centred around 440 nm is coincident with the low-energy wing of the absorption edge, suggesting that the origin of this emission could be the direct band edge recombination of the electron-hole pair w12,15x. However, the position of this peak changes only slightly from sample to sample, and does not clearly present a size dependence as observed for CdS– glass composites w16x. In addition, the relative intensity of the blue band decreases after annealing the films. There-

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Fig. 5. Room temperature luminescence spectra of several as-deposited films Žsee Table 1.. The insert shows the luminescence of aA10 film Ža. before and Žb. after annealing. Excitation wavelength of 380 nm.

fore, we think that the blue band observed in the as-deposited films is rather due to the recombination of electrons on a donor level and holes in the valence band. This level may be due to an interstitial sulphur Ž0.023 eV below the conduction band w17x.. Quantum confinement of holes is responsible for the transition energy being higher than bulk CdS bandgap energy. Another broad emission band situated in the transparent region of the doped glass and extending into the red is also observed. It is well known in CdS glass systems, usually being attributed to transitions involving a deep trap levels, presumably due to Cd vacancies w15,18x. 4. Conclusions

Fig. 4. Experimental and simulated XRD spectra of the annealed aA4-19 ˚ was obtained from the fitting, Ž5508C, 3 h.. A mean diameter of 51 A taking into account both CuK a 1 and the CuK a 2 radiation lines.

The magnetron rf-sputtering technique has been successfully applied to the fabrication of small crystal-size CdS-doped silica glass films presenting a significant quantum size effect. The growth of CdS nanocrystals in glass ˚ films, with a mean radius value ranging from 20 to 40 A, has been confirmed by using XRD and optical absorption techniques. From the XRD pattern, we estimated the mean radius of the wurtzite type nanocrystals structures that agree quite well with those deduced from optical absorption and Raman spectra. As the size of the CdS crystals decreases, the absorption edge shifts to the high-energy side, thus demonstrating the quantum size effect. These lead us to qualitatively establish the nanocrystals size

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dependence on the fabrication conditions: the size increases both with the increasing of rf-power and the increasing of CdS:SiO 2 target ratio, as well as are strongly dependent of post-deposition annealing temperature. The origin of the blue emission peak centred around 440 nm is not completely explained, and seems to be due to the recombination of electrons on a donor level and holes in the valence band. Quantum confinement of holes is responsible for the transition energy being higher than bulk CdS bandgap energy.

Acknowledgements This work has been supported by the Junta Nacional de Investigac¸ao e Tecnologia ŽJNICT. under the ˜ Cientıfica ´ project number PBICrCrCTMr1923r95. The authors would like to thank Prof. M.I. Vasilevskiy and Prof. M.S. Belsley for helpful discussions.

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