Silicon dioxide thin film luminescence in comparison with bulk silica

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Journal of Non-Crystalline Solids 223 (1998) 114-122

Silicon dioxide thin film luminescence in comparison with bulk silica A.N. Trukhin b, *, M. Goldberg a, j. Jansons b, H.-J. Fitting a, I.A. Tale b a Physics Department, Rostock University, Universitiitsplatz 3, D-18051 Rostock, Germany b Institute of Solid State Physics, University of Latvia, Kengaraga St. 8, LV-I063 Riga, Latvia Received 28 May 1997; revised 2 September 1997

Abstract The luminescence of the self-trapped exciton (STE) in SiOz films was measured at low temperatures on the background of defect luminescence under cathodoexcitation and compared with bulk silica luminescence. The defect luminescence is mainly caused by non-bridging oxygen centers (a red luminescence band at 1.8 eV) and twofold coordinated silicon centers (blue and ultraviolet luminescence with 2.7 and 4.4 eV bands, respectively). The STE luminescence with a band at 2.3 eV is uniformly distributed within SiO2 film volume. Contrary to defect luminescence, whose intensity increases with irradiation time, the STE luminescence decreases almost to zero in a few seconds of irradiation time. The defect luminescence increase is attributed to transformation of precursors whereas STE luminescence is produced in the continuous network. The decrease of STE luminescence is attributed to radiation damage in the continuous network. © 1998 Published by Elsevier Science B.V.

1. Introduction Thin films ( < 1000 nm) of silicon dioxide are an essential part of contemporary microelectronics and cathodoluminescence is one of several methods for thin film investigation. The role of the self-trapped exciton (STE) luminescence has not been discussed for the case of silicon dioxide films, with regard to the fact that STE luminescence is only observed under short time pulsed photoexcitation [1]. The STE luminescence band is found in the visible range of the spectrum [1,2], where other luminescence bands are also situated. The method of investigation of this

* Corresponding author. Tel.: +371 720 686; fax: +371 711 2583.

problem is a comparison with known silica luminescent bands. However, the situation is still complicated by incomplete knowledge of the excitation processes for luminescence even in silica. For example, there are various models for silica's visible blue band luminescence (2.7 eV), which can complicate resolution of the STE luminescence. On one hand the nature of blue luminescence is well explained by twofold coordinated silicon centers, due to oxygen deficit and irradiation effects [3]. On the other hand the intensity of the luminescence increases after implantation of oxygen into silica samples and the blue luminescence was attributed to some form of oxygen interstitiais [4]. Therefore, the aim of this work is the measurement of the self-trapped exciton luminescence in

0022-3093/98/$19.00 © 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 4 3 7 - 7

A.N. Trukhin et al. / Journal of Non-Crystalline Solids 223 (1998) 114-122

silicon dioxide films on the background of defect luminescence under cathodoexcitation. The data are compared with bulk silica luminescence. Thick sampies with different oxygen deficits were measured.

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[6]. The largest deficit was about 10 -2 wt% of excess silicon, see Ref. [6].

3. Results 2. Experimental The main experiment was done in a digital scanning electron microscope (Zeiss DSM 960). The electron irradiation (primary electron energy 0.5-30 kV and current densities between 10 -5 and 10 -3 A cm-2) of a sample was performed through a hole in an ellipsoidal mirror covering the sample and collecting the CL (cathodoluminescence) light. Luminescence spectra were analysed by a spectrograph (Spex-270M) and registered with a CCD camera (Princenton Instruments, EEV 1024 × 256). CL spectra ranging from 1.5 to 5.5 eV were accumulated in single shot mode within a short time of 2 s and with a spectral resolution of 4 nm. A cooling and heating temperature stage (Oxford Instruments) provided sample temperatures between 77 and 650 K. Details of this experimental setup can be taken from Ref. [5]. Additional time resolved experiments were done by means of a pulsed electron beam equipment with electron energies 6 kV, 1 p,s pulse and 250 kV, 20 ns pulse duration. From these data we deduced the luminescence kinetics. The cryostat allowed a change of sample temperature from 77 K to 360 K. Luminescence was analyzed by a grating monochromator and detected by a photomultiplier (FEU-106). The decay kinetics were recorded with a 256 channel analyzer made in the Institute of Solid State Physics, University of Latvia. The SiO 2 film samples were thermally grown films on Si-substrates. Furthermore, CVD TEOS (chemical vapour deposition from tetraethoxysilane) films of SiO 2 on silicon were investigated too. Samples of bulk silica of KS-4V type with different levels of excess silicon were measured. The method of preparation was by electrofusion of cristobalite synthetic silicon dioxide, made in the Institute of Silicate Chemistry of the Russian Academy of Science, St. Petersburg. The oxygen deficit was provided by reaction of cristobalite with silicon vapor

The cathodoluminescence of film samples presented in Fig. la; Fig. lb shows, at 80 K, several bands with maxima at 1.8, 2.3, 2.7, and 4.4 eV. The last two bands appear after long duration (up to 20 min) of excitation and the band at 4.4 eV has a larger amplitude at higher temperatures [5]. The band at 2.3 eV practically disappears in a time of few minutes under continuous excitation. Therefore, careful conditions with a minimum of interference due to other bands is needed to measure the parameters of this luminescence band. We determined that a recovery of the 2.3 eV band can take place by heating after irradiation at low temperatures. The data show a decrease with irradiation time at 80 K and then a decrease after intermediate heating to 380 K for half an hour, presented in Fig. 2. There is a recovery of intensity for the 2.3 eV luminescence band, however, not a complete one. Therefore, the luminescence decay kinetics were measured on virgin samples within a few minutes after irradiation. Subsequent cycles of heating and cooling reproduce the intensity of the second curve in Fig. 2. Samples were heated to a recovery temperature and held for 5 min to determine the temperature dependence of the recovery process, Fig. 3. Obviously the recovery occurs over a range of temperatures with an approximate completion at temperatures > 400 K. The temperature dependence of the recovery part of the luminescence is presented in Fig. 4, together with the temperature dependence of X-ray excited luminescence for the 2.3 eV band in silica. In Fig. 5 a CL depth analysis is performed by decreasing the electron beam energy from 16 keV to 1 keV. This has been performed under constant power condition, i.e., E o I o = const., to keep the overall CL excitation dose constant (Eo; I0: primary electron energy and current, respectively). In the latter case E 0 = 1 keV the CL excitation is limited to a 30 nm distance beneath the surface. It is seen that the intensity of the STE luminescence band at 2.3 eV

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A.N. Trukhin et al./Journal of Non-Crystalline Solids 223 (1998) 114-122

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A.N. Trukhin et al. / Journal of Non-Crystalline Solids 223 (1998) 114-122 I

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2.3 eV band, T = 80 K, 850 nm TEOS SiO 2 on Si

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1 - 1st measurement, virgin sample 2 - 2rid measurement after intermediate heating to 360 K

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IRRADIATION TIME (s) Fig. 2. Kinetics of the 2.3 eV luminescence band intensity under continuous electron beam excitation of a SiO 2 thin film at 80 K, (1) First excitation of a virgin sample. (2) Second excitation after intermediate heating to 360 K.

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2.3 eV peak measured at T = 100 K, after intermediate heating to the recreation temperature 10 t"

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RECREATION TEMPERATURE (K) Fig, 3. Recreation of the 2.3 eV STE band intensity after intermediate heating of the thin film S i O 2 to the given temperature with 5 min at each temperature; each point then measured again at low temperature 100 K. The line is drawn as a guide for the eye. The error in the CL are + 2 units.

Fig. 1. Cathodoluminescence spectra of thennaUy grown 500 mn thin films of silicon dioxide on Si; R: red peak at 1.8 eV; STE band at 2.3 eV; B: blue band at 2.7 eV; UV-band at 4.4 eV; (a) at different excitation times 2 s to 30 rain; E 0 = 4 keV; J0" t = 2 × l0 -5 A s c m - 2 ; (b) at different excitation doses jot; E 0 = 8 kV.

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A.N. Trukhin et al. / Journal of Non-Crystalline Solids 223 (1998) 114-122 400

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T E M P E R A T U R E (K) Fig. 4. Temperatmre dependence of the 2.3 eV luminescence in silicon dioxide; circles: thin film CL; before each measurement the film was intermediately heated to 360 K; full line: X-ray excited luminescence (XL) in bulk silica. The line connecting the data symbols is drawn as a guide for the eye. The random errors in this data are _+5 units of intensity.

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Fig. 5. Luminescence spectra of different SiO 2 samples under different excitation conditions. Thin lines: CL spectra normalised to the 2.3 eV band intensity, excited with electron energies from 16 keV down to 1 keV in thin 500 nm SiO 2 layers; thick line: X-ray excited luminescence (XL) of pure bulk silica.

A.N. Trukhin et al. / Journal of Non-Crystalline Solids 223 (1998) 114-122

does not deminish as the red band at 1.8 eV does. Thus, we conclude that the red band due to defect luminescence, intensity decreases with decreasing

600

500

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119

distance to the sample surface [9] whereas the STE band intensity remains constant with decreasing penetration depth of the electrons under constant power

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Fig. 6. X-ray excited luminescencespectra of pure bulk silica samples with differentoxygendeficiencyare compared with thermostimulated luminescence(TSL) after X-ray irradiation,as well as with virgin sample photoluminescence.The upper part presents intensitiesof different bands in dependenceon oxygen deficit; the lower part illustratesthe spectral contents of normalisedintensities.

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A.N. Trukh in et al. // Journal of Non-Crystalline Solids 223 (1998) 114-122

with photoluminescence spectra as well as with afterglow and thermostimulated luminescence (TSL) following X-ray irradiation. It can be seen that the intensity of the 2.7 eV band increases with increasing excess silicon. In Fig. 7 time resolved luminescence of the luminescence bands under cathodoexcitation are presented. The decay curves are non-exponential in the

condition. Obviously STE luminescence occurs at depths < 30 nm. On the other hand the X-ray excited spectrum is broader and extends over the main blue band at 2.7 eV and the STE part at 2.3 eV. In Fig. 6 the X-ray excited luminescence spectra of silica samples with different oxygen deficiency, measured as quantity of excess silicon, are shown

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CL decay 500 nm SiO2B - 2.7 eV, T = 290 K = 6.5 ms (fitted line) STE - 2.3 eV band, T = 77 K

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DECAY TIME (l~s)

Fig. 7. CL decay kinetics for different luminescencebands at 1.8, 2.3, and 2.7 eV under cathodoexcitation.

A.N. Trukhin et al. / Journal of Non-Crystalline Solids 223 (1998) 114-122

case of cathodoluminescence. The actual decay time of the UV luminescence band at 4.4 eV could not be determined in our apparatus since it is less than the time resolution (10 ns) of the apparatus.

4. Discussion Although the cathodoluminescence decay curves are non-exponential, the decay takes place in the same range of time as the corresponding photoluminescence bands. The decay time of red luminescence at 1.8 eV ranges from 5 to 40 p~s. However, its photoluminescence decay is approximated by an exponential law with a time constant, 14 ~s [7]. Obviously, the red band at 1.8 eV belongs to non-bridging oxygen [7]. This band has not been observed in photoluminescence spectra of SiO2 thin films [8] and therefore we assume these centers are created by electron beam excitation. Also this red luminescence does not appear under X-irradiation in pure samples even in samples prepared with oxygen deficiency. Differences in luminescence decay times are attributed to differences in the atoms surrounding a center, e.g., in bulk and thin films, as well as to differences in intracenter excitation and recombination processes. The decay time of the blue luminescence at 2.7 eV is 6.5 ms at 300 K. Therefore it can be compared with the photoluminescence mean decay time of 10 ms [3] of the triplet-singlet luminescence of a two-fold coordinated silicon center. Such interpretation of this band correlates well with results obtained here, Fig. 6. The blue band at 2.7 eV is the main band in oxygen deficient samples. Indeed the oxygen deficient samples contains two-fold coordinated silicon centers because the UV photoluminescence band at 4.4 eV, which is due to singlet-singlet transitions in such a center [3] has a larger amplitude in such samples. The ratio of intensities of the blue band to the UV band in photoluminescence is << 1 because of low conversion efficiency from the singlet excited state to the triplet state [3]. The band at 4.4 eV has a smaller amplitude whom excited in recombination process and, in this case, it is a broader band than in the case of photoluminescence and in X-ray excited TSL and afterglow spectra the main band is at 2.7 eV. In case of cathodoexcitation the UV band is better resolved as in the case of X-ray

121

excitation. Essentially this resolution was observed, if the excitation was done with a short pulsed (20 ns) 200 kV electron beam. In previous work [9] an impact excitation mechanism was introduced. It is based on a correlation of cathodoluminescence and electron beam induced current (EBIC) [9]. By this mechanism a resonance interaction takes place between inner secondary electron creation and center excitation. When the primary electron energy is large, e.g., 200 keV, then the quantity of straggling and inner secondary electrons with energy corresponding to band-band and singlet-singlet transition is larger [9] and we observe mainly the UV singlet-singlet luminescence. For primary electrons of small energy 1 to 16 kV the relative number of secondary electrons inducing singlet-singlet transitions is less than the number of secondary electrons inducing tripletsinglet transitions by impact excitation mechanism and we observe the blue wiplet-singlet luminescence as the main band. The luminescence decay kinetics of the 2.3 eV band in thin films ranged from ns to ms and are non-exponential under various excitations. It may be approximated by a power law, t -n, with n --- 1. Both decay kinetics and temperature dependence of luminescence intensity correspond to that of self-trapped exciton luminescence in silica [1,10]. CL depth analysis in Fig. 5, i.e., spectra recorded for different electron energies, show STE luminescence uniformly distributed across the thin film depth whereas for other luminescence bands there is a layer near the surface of the sample in which they are not observed (dead layer; see Ref. [9]). Therefore, for low energy electron excitation we observe mainly STE luminescence but only at low temperatures and for a short time of irradiation. The STE luminescence could be observed only in the undamaged part of lattice; therefore the decrease of STE luminescence with excitation time can be related to radiation induced changes of the glassy network. However this damage is related neither to creation of non-bridging oxygen centers (NBO) nor to twofold coordinated silicon, because the inactive layer of the other luminescence bands does not change with irradiation time. This damage can be devided into two parts, the recovering and the nonrecovering part. The recovering part is attributed to charge carrier trapping a n d / o r slight straining or

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destruction of the glassy network. The non-recovering part is attributed to electron beam induced loss of oxygen. This loss could produce the oxygen vacancies and that, most probably, correlates with increase of silicon clusters, which has been observed many times by means of Auger spectroscopy [11-13]. After electron beam irradiation the Auger signal at 92 eV appears corresponding to clusters of silicon. The twofold coordinated silicon centers are produced from precursors. Therefore, they do not appear with the loss of oxygen from the surface. These precursors could be produced by oxygen deficiency in silica preparation time in thermal oxydation conditions or by irradiation with ions, therefore as silicon implantation and oxygen implantation provide such a precursors.

5. Conclusions Cathodoluminescence spectra of silicon dioxide thin films possess a self-trapped exciton (STE) luminescence band at 2.3 eV. This luminescence manifests itself only at low temperatures T < 200 K and decreases with electron beam irradiation time. The STE luminescence band at 2.3 eV can be resolved from the blue defect luminescence band at 2.7 eV. For larger electron doses the defect luminescence is dominating. The dose dependence of the kinetics of defect luminescence was modelled in previous work [5]. Here we conclude that the decrease of STE luminescence is not correlated with the defect luminescence growth. Whereas the defect luminescence is attributed to precursor subnetwork transformation,

STE luminescence should be related to the defect free network of glass. Bulk silica samples with different deficits of oxygen show that blue luminescence is a transition in the twofold coordinated silicon defect.

Acknowledgements This work was supported by the Grants 96.0665 of the Scientific Society of Latvia and the Project 436LETl13 of the Deutsche Forschungsgemeinschaft (DFG).

References [1] A.N. Trukhin, J. Non-Cryst. Solids 149 (1992) 32. [2] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 899. [3] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [4] R.A. Weeks, Proc. 12th Conf. on Glasses and Ceramics, Varna, Bulgaria, Sept. 1996. [5] M. Goldberg, A.N. Trukhin, H.-J. Fitting, Mater. Sci. Eng. B 42 (1996) 293. [6] A.N. Trukhin, A.G. Boganov, A.M. Praulinsh, Phys. Chem. Glass (Sov.) 6 (1979) 346. [7] L.N. Skuja, A.R. Silin, A.G. Boganov, J. Non-Cryst. Solids 63 (1984) 431. [8] A.M. Praulinsh, M. Goldberg, A.N. Trukhin, H.-J. Fitting, Phys. Status Solidi (a) 133 (1992) 385. [9] M. Goldberg, H.-J. Fitting, A.N. Trukhin, J. Non-Cryst. Solids 220 (1997) 69. [10] T. Tanaka, T. Eshita, K. Tanimura, N. Itoh, Cryst. Latt. Def. Amorph. Mater. 11 (1985) 221. [11] B. Carriere, B. Lang, Surf. Sci. 64 (1977) 209. [12] J. Cazaux, J. Appl. Phys. 59 (1986) 1418. [13] R.A.B. Devine, J. Amdt, Phys. Rev. B 39 (1989) 5132.