Wavelength resolved thermally stimulated luminescence of SiO2 films

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Journal of Non-Crystalline Solids 187 (1995) 124-128

Wavelength resolved thermally stimulated luminescence of SiO2 films M. Martini, F. Meinardi, E. Rosetta, G. Spinolo, A. Vedda* Dipartimento di Fisica, Universith di Milano, Via Celoria 16, 1-20133 Milan, ltaly

Abstract A wavelength resolved thermally stimulated luminescence (TSL) study has been carried out for the first time on X-irradiated chemical vapor deposition SiO 2 films deposited on a silicon substrate, from room temperature, to 400°C. Upon irradiation, the TSL glow curve features a prominent structure at a maximum temperature, Tmax, of approximately 62°C (heating rate = l°C/s); the analysis of the emission wavelength shows a peak at 457 nm (2.71 eV). The shape of the TSL peak is complex, and cannot be described in the frame of classical first- or second-order kinetics. Moreover, Tma x has a strong and monotonic shift to higher temperatures after partial pre-heating treatments while the spectral emission is not modified: a similar phenomenon has already been observed in bulk fused silica of various types, different from crystalline quartz which does not present any shift of Tma x as a function of pre-heating. These results suggest that the TSL peak is characterized by a distribution of trap parameters: specifically, a continuous distribution of trap energy or of the frequency factor can be taken into account. Considerations on the characteristics of the trap parameter distribution are made.

1. Introduction Although thermally stimulated luminescence (TSL) above r o o m temperature has been widely used in the study of defects in quartz and silica, very few papers have dealt with T S L in the other a m o r p h o u s form, i.e. thin films that are extensively used in the electronic industry I-1-3]. The principal drawbacks in studying the T S L of thin films have been the small a m o u n t of sample and the limited information gained due to the lack

*Corresponding author. Tel: +39-2 239 2352. Telefax: +39-2 239 2414. E-mail: [email protected].

of knowledge of the emitting wavelength. Both these disadvantages are o v e r c o m e by the measurement of T S L using newly designed high sensitivity spectrometers that give three-dimensional information, i.e. the intensity of T S L as a function of both temperature and emission wavelength I-4]. By means of such a spectrometer we have analyzed the T S L of C V D SiO2 films; the results obtained on the wavelength distribution of the TSL emission are discussed in c o m p a r i s o n with the spectral emission of bulk SiO2. Moreover, a t h o r o u g h analysis of the principal '62°C ' glow peak, using partial heating and the 'initial rise' method, leads us to consider the presence of a distribution of trap parameters.

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M. Martini et al. /Journal of Non-Crystalline Solids 187 (1995) 124-128

2. Experimental results This TSL study has been undertaken on 3 lam thick films of a m o r p h o u s SiO2 purchased from M E M C SpA, N o v a r a (Italy) : they were obtained by chemical v a p o r deposition (CVD) at 900°C on silicon wafers and cut for the measurements into a square shape of 1 cm 2 area. TSL measurements were performed from r o o m temperature to 400°C with a linear heating rate of 1°C/s by means of two different apparatuses: in the first the total emitted light was detected as a function of temperature by the technique of photon counting using an E M I 9635 QB photomultiplier tube with quantum efficiency peaking at 380 nm. The second consisted of a high sensitivity spectrometer measuring the TSL intensity as a function of both temperature and emission wavelength: the detector is a double stage Microchannel plate followed by a 512 diode array; the dispersive element is a 140 lines/mm holographic grating, the detection range being 200-800 nm. Irradiations have been done at room temperature using a Machlett O E G 50 X-ray tube operating at 40 kV. Fig. 1 (curve a) shows the TSL glow curve of a C V D sample irradiated with an X-ray dose of approximately 103 Gy: a strong peak at 62°C is evident. The shape of this peak appears to be complex. In fact, the TSL emission is asymmetrically extended to high temperatures contrary to the expectations both for the cases of first- and secondorder kinetics [5]. Moreover, the peak m a x i m u m temperature, T m , is shifted to higher temperature as a function of partial cleaning treatments: this is indicated by curves b and c of the same figure, which show measurements obtained after irradiation and partial heating, to 51 ° C and 94 ° C, respectively (Tstop), before the recording of the glow curve. The Tm shift is approximately linear as a function of pre-heating temperature, as evidenced by the inset of the figure. This phenomenon suggests that the trap responsible for this TSL peak is characterized by a distribution of trap parameters, i.e. of the thermal energy, E, or possibly of the frequency factor, v [6]. In order to evaluate whether indeed a distribution of trap depths is present, a simple and preliminary

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Fig. 1. T S L glow curves of a 3 g m C V D SiO2 s a m p l e after an X-ray dose of 103 Gy. The h e a t i n g rate was l°C/s. C u r v e a, as irradiated; curves b a n d c, after a p r e - h e a t i n g t r e a t m e n t of 51 ° C a n d 94 ° C, respectively. The inset s h o w s the shift of the p e a k m a x i m u m t e m p e r a t u r e as a function of p r e - h e a t i n g treatments.

analysis of the peak shape has been performed, based on the initial rise method. In Fig. 2 we show an enlargement of the 20-100°C range of the TSL glow curves obtained after different Tstop temperatures. Here, a semilog plot has been chosen to evidence the fact that the portions of the curves reported here follow an exponential behavior, which allows us to determine the trap energy using the initial rise method. The results of the exponential fitting are shown in Fig. 3, where the trap depths obtained are presented as a function of Tstop. The spread of the data has to be ascribed to different sources of errors such as the measurement of the sample temperature and background subtraction. However, the values appear to be independent of Tstop , and a mean energy has been calculated: E m = 0.81 eV ___0.06 eV). It is to be noted that for Tstop = 84°C, an appreciable lowering of the energy is in fact observed (E = 0.68 eV); at the present time no specific conclusions can be drawn about this. The significance and the limits of validity of this method will be discussed in the next section. The results of wavelength resolved TSL measurements are presented in Fig. 4 with a three-dimensional plot and in Fig. 5 as a contour plot. The '62°C ' peak is characterized by a single emission

M. Martini et al. /Journal of Non-Crystalline Solids 187 (1995) 124-128

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Fig. 4. Three-dimensional plot of a wavelength resolved TSL measurement performed on a CVD SiO2 sample irradiated with an X-ray dose of 103 Gy; Heating rate = l°C/s.

3. D i s c u s s i o n

This study has demonstrated that the TSL features of CVD films present characteristic properties that can be ascribed to the amorphous nature of the material. As already suggested [6], some similarities with the glow peak of crystalline quartz are found, but the detrapping-recombination phenomena appear to be specific of the amorphous

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centered at 2.71 eV (457 nm), with a halfwidth of 0.45 eV. Measurements performed as a function of pre-heating at 50°C and 100°C, respectively, are presented in Fig. 5 (curves b and c) : they show that the emission energy is not influenced by this treatment.

In the following, two main aspects will be discussed: one concerns the emission spectrum; the second relates to the observed detrapping phenomena. Only one emission energy at 2.71 eV has been detected for the broad TSL glow peak under study. Interestingly a similar emission, denoted as the 7 band, has already been found in photoluminescence and phosphorescence studies of amorphous bulk SiO2 and neutron irradiated crystalline quartz [7 9]. The defect responsible for the emission appears to be of intrinsic origin. Specifically, two models have been proposed up to now, both related to oxygen deficiency: the neutral oxygen vacancy, a bond between two silicon atoms r7], and the twofold coordinated silicon [10]. In this context,

M. Martini et al. / Journal o f Non-Crystalline Solids 187 (1995) 124-128

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this conclusion is in agreement with the fact that up to now different experimental evidence has been obtained that oxygen deficient defects are commonly present in the structure of thin SiO2 films [11]. Let us consider now the complex detrapping phenomena observed. As illustrated in Fig. 1, the TSL peak detected at 62°C has a broad and asymmetric shape and manifests a strong shift of the m a x i m u m temperature upon pre-heating treatments. F r o m the analysis of the thermoluminescent processes, a simple relation can be derived between the m a x i m u m temperature of a peak and the trap parameters: flE/kT2m = s exp( - E/kTm),

(1)

where fl is the heating rate, k is the Boltzmann's constant, E is the trap depth and s is the frequency factor; s can be expressed as vz exp(S/k), where v is the thermal vibration frequency, Z is the transmission coefficient of the trap and exp(S/k) is an entropy factor. The variation of Tm upon heating treatments suggests that the TSL peak observed is characterized by a distribution of lifetimes q defined as zi = (1/si) exp(Ei/kT).

(2)

Such a lifetime distribution can in principle reflect the existence of both a distribution of energy trap depths or of frequency factors. Actually, in a study of the TSL properties of Ge-silica optical fibers doped with Nd the existence of a continuous distribution of trap depths has already been suggested [12-1. We have made an effort in order to evaluate the existence of a continuous distribution of trap depths by using the initial rise method on measurements performed with different pre-heating temperatures; the results obtained do not show any evident increase of E as a function of the Tstop temperature. The spread of the data does not allow us to exclude the existence of a narrow distribution, but in this case its width should probably be lower than 0.1 eV. Alternatively, a distribution of frequency factors can be proposed. In this respect, the operating mechanism could be tunnel recombination of electrons from traps to emitting centers specifically giving rise to a distribution of transmission coefficients Z of the trap, as already suggested in the case of the TSL of different materials [13, 14]. Further experimental work, and a more thorough numerical analysis of the TSL glow curve, is needed

128

M. Martini et al. / Journal o f Non-Crystalline Solids 187 (1995) 124 128

in o r d e r to verify w h e t h e r this p a r t i c u l a r r e c o m b i n a t i o n p r o c e s s is in fact r e s p o n s i b l e for the o b served phenomenology.

References 1-1] J.P. Mitchell, IEEE Trans. Nucl. Sci. 15 (1968) 154. [2] T.W. Hickmott, J. Appl. Phys. 43 (1972) 2339. [3] T.W. Hickmott, J. Appl. Phys. 45 (1974) 1060. [4] P.D. Townsend and Y. Kirsh, Contemp. Phys. 30 (1989) 337. [5] S.W.S. Mc Keever, Thermoluminescence of Solids, Cambridge Solid State Science Series (Cambridge University, Cambridge, 1985).

[6] M. Martini, G. Spinolo, A. Vedda and C. Arena, Solid State Commun 91 (1994) 751. [7] R. Tohmon, H. Mizuno, Y. Ohki, K. Sasagane, K. Nagasawa and Y. Hama, Phys. Rev. B39 (1989) 1337. I-8] J.H. Stathis and M.A. Kastner, Phys. Rev. B35 (1987) 2972. [9] M. Guzzi, G. Lucchini, M. Martini, F. Pio, A. Vedda and E. Grilli, Solid State Commun. 75 (1990) 75. [10] L.N. Skuja, A.N. Streletsky and A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [11] S.T. Pantelides, ed., The Physics of SiO2 and its Interfaces (Pergamon, New York, 1978). [12] Y. Kirsh, J.E. Townsend and P.D. Townsend, Phys. Status Solidi A114 (1989) 739. [-13] P. Avouris and T.N. Morgan, J. Chem. Phys. 74 (1981) 4347. [14] R. Visocekas, T. Ceva, C. Marti, F. Lefaucheux and M.C. Robert, Phys. Status Solidi A35 (1976) 315.