Effects of γ-ray irradiation on photoluminescence spectra from Si-rich silicon oxide

Materials Research Bulletin, Vol. 32, No. 10, pp. 1427-1433, 1997 Copyright Q 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/97 $17.00 + .OO

Pergamon

PH SO0255408(97)00122-0

E:FFECTS OF y-RAY IRRADIATION ON PHOTOLUMINESCENCE SPECTRA FROM S&RICH SILICON OXIDE

S.Y. Ma, B.R. Zhang, and G.G. Qin* Department

of Physics, Peking University,

Beijing

100871, China

Z.C. Ma and W.H. Zong The 113thInstitute of Ministry of Electronic Industry, Shijiazhuang

050051, China

X.T. Meng Institute

of Nuclear Energy Technology,

(Received

Tsinghua

University,

Beijing

100084, China

(Communicated by Hailing Tu) March 26, 1997; accepted March 26, 1997)

ABSTRACT Si-rich silicon oxide films with thicknesses of about 1.2 pm were deposited on p-type Si substrates by the RF magnetron sputtering technique. After annealing at different temperatures in a N, atmosphere, the photoluminescence spectra of all samples show two main peaks located at about 710 and 800 nm. After y-ray irradiation, these two PL peaks increase 3-5 times in intensity. Moreover, a strong new 580 nm peak emerges from the PL spectra in all samples. The positions of all three PL peaks do not show any evident shift when the measurement temperature increases from 10 to 300 K. These experimental results can best be explained by the model that photoemission occurs through the luminescence centers, rather than the nanometer silicon particles, in the Si-rich silicon oxide films. Copyright Q 1997 Elsevier Science Ltd KEYWORDS: nescence

A. oxides, A. thin films, B. sputtering,

D. defects, D. lumi-

*Also with. the International Center for Materials Physics, Academia Sinica, Shenyang 110015, China. 1427

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INTRODUCTION Intensive research has been done on visible luminescence of porous silicon (PS) and Si-rich Si oxide (SSO) films in recent years. No consistent conclusion has been obtained on the mechanisms for their photoluminescence (PL). For the PL mechanisms of porous silicon, there are mainly three types of models: (1) Canham [l] suggested that both photoexcitation and radiative recombination of electron-hole pairs occur within nanometer silicon wires in PS, and their energy gaps become larger than that of the body Si due to the quantum confinement effect. This model is referred to as the quantum confinement model. Koch et al. [2] thought that electron-hole pairs are photoexcited in nanometer silicon particles and recombine via Si intrinsic surface states (from facetting, steps, ridges, and other structural irregularities on the nanometer silicon particle surfaces). Both these models consider luminescence to be an intrinsic effect of nanometer Si. (2) In this model type, origin of the luminescence of PS is attributed to certain luminescence materials, such as siloxene [3], SiH, complexes [4], polysilanes [5], or SiO, [6], rather than an intrinsic property of nanometer Si. (3) Qin and Jia [7] proposed the quantum confinement/luminescence center model for PS and SSO film luminescence. For oxidized PS and SSO films, it has been suggested that photoexcitation of electron-hole pairs carries out in nanometer silicon particles; however, photoemission occurs through the luminescence centers in SiO, or on Si/SiO, interfaces [7-111. More recently, our research results [12,13] indicate that there are two photoexcitation processes in PS: one occurring in the nanometer silicon particles and the other occurring in the Si oxide. In other words, we consider that for oxidized PS and SSO films, photoexcitation occurs in both nanometer silicon particles and SiO,, and photoemission occurs through the luminescence centers in SiO, or on Si/SiO, interfaces. As to the origin of the PL from SSO films, a debate exists. Takagi et al. [14] and Schuppler et al. [15] preferred the quantum confinement model; however, Morisaki et al. [16] and Min et al. [17] argued that luminescence originates from defects in dielectric medium films. We consider that the quantum confinement/luminescence center model is suitable for explaining the PL from SSO films. In this paper, we study the effects of y-ray irradiation on PL spectra from SSO films and discuss the PL mechanism for SSO films.

EXPERIMENTAL SSO films were deposited on p-type Si substrates, using an RF magnetron sputtering system MPS-3000FC with a Si and SiO, composite target. The area percentage ratio of the Si wafer in the target was 11%. The thicknesses of the SSO films deposited, as measured by ellipsometry, were about 1.2 mm. The samples were annealed in a N, atmosphere for 30 min at 300, 500, and 800°C. The samples were irradiated by a 60 Co radiative source at a dose rate of 330 rad/s. The total dosage of y-ray irradiation was 2.9 X 107 rad. The PL spectra of each sample were measured before and after the y-ray irradiation. A 4880 8, line from an Ar+ laser was used as a photoexcitation source for PL measurements. In order to exactly compare the PL intensities from a sample before and after y-ray irradiation and from different samples, the PL spectrum of an InGaP sample with a peak at 6560 A was measured first, to assure a constant sensitivity for the

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Wavelength ( nm ) The PL spectra were measured y-ray irradiation.

FIG. 1 for the SSO films annealed

at 300, 500, and 800°C before

measurement system. A SiO, wafer which was squeezed from analytically powder was used as a control sample in the study of SSO films.

RESULTS

pure SiO,

AND DISCUSSION

Figures 1 and 2 show the PL spectra of the SSO films annealed at 300, 500, and 800°C before and after y-ray irradiation, respectively. Note that although the intensity scale in Figure Z!is the same as that in Figure 1, intensities of the three PL curves in Figure 2 have been multiplied by a factor of l/4. Before y-ray irradiation, all these spectra have three-peak structures with two main peaks being located at about 7 10 nm and 800 nm and a small peak about 640 nm. After y-ray irradiation, intensities of the PL peaks with positions about 710 nm and 800 nm increase to about 3 to 5 times as large as those of the initial spectra. Additionally, a strong peak at about 580 nm emerges in all irradiation samples, and the weak PL peak at about 640 nm is covered by the strong 580 nm peak and cannot be seen again. Figure 3 shows three PL spectra measured at different temperatures for a SiO, wafer. The peak position of the PL spectrum is located at about 580 nm and independent of the measured temperature and has almost the same position as that of the PL peak emerging in the SSO

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FIG. 2 The PL spectra were measured for the SSO films annealed at 300,500, and 800°C after y-ray irradiation. The PL intensity scale in Figure 2 is the same as that in Figure 1 and intensities of the three PL spectra in Figure 2 have been multiplied by a factor of l/4. films being subject to y-ray irradiation. The 580 nm (2.1 eV) PL peak observed in the SiO, wafer has a position very close to those reported in refs 18 and 19. Temperature-variation PL measurements have been carried out for the three main PL peaks observed in the SSO films with positions of -710, -800, and -580 nm. All these peak positions do not show any evident shift when measurement temperature increases from 10 to 300 K, and the results together with those for the temperature-variation PL measurements of the SiO, wafer are shown in Figure 4. We try to explain the results of these experiments by using the quantum confinement model. According to this model, the PL peak position is determined by the sizes of nanometer silicon particles in the SSO films. Since two main peaks exist in the SSO films before y-ray irradiation, the sizes of the nanometer silicon particles should centralize in two different ranges, In order to explain the fact that a new PL peak with wavelength about 580 nm emerges and the intensities of the PL peaks at about 710 nm and 800 nm increase in the y-ray irradiation process, we have to suppose that y-ray irradiation has created nanometer silicon particles, some of which have a size distribution which is almost the same as that before y-ray

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Wavelength ( nm ) FIG. 3 The PL spectra were measured for the Si02 wafer at measurement temperatures of 10, 77, and 300 K and the intensity of the 300 K spectrum has been multiplied by a factor of 2. irradiation and the others concentrate in an obviously smaller size. However, it is very difficult to imagine that y-rays can have such an effect. For most semiconductors, the variation of energy gap Es with temperature can be described by the Vashni equation [20]: E, = E, - cxT*/(T + B) This means that the energy gap decreases with increasing temperature. By combining the quantum confinement model with the Vashni equation, the PL peak will shift red with increasing measurement temperature. This predication is contrary to the experimental results shown in Figure 4. y-ray irradiation has had to induce defects in the SSO films. Some of these defects are luminescence centers and the others are nonradiative recombination centers. Suppose that the y-ray irradiation has induced several types of luminescence centers in the Si oxide films and they are responsible for, respectively, the three PL peaks located at about 710, 800, and 580 nm. These experimental results then can be explained by the quantum confinementkminescence center model with a supplement that photoexcitation can also occur in Si oxide. If y-ray irradiation can induce luminescence centers which are responsible for the PL peaks being located at about 710 nm and 800 nm, it can be understood that intensities of the two PL peaks increase, rather than decrease, in the y-ray irradiation process. If y-ray irradiation can induce

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100150200250300350 Temperature ( K ) FIG.

4

Temperature-variation PL measurement for the three main PL peaks (diamonds for -800 nm peak, squares for -710 nm peak, and circles for -580 nm peak) of the SSO films and that of the SiO, wafer. Because the experimental points for the -580 nm PL peak (circles) in the SSO film and that in the SiO, wafer (triangles) coincide, each triangle has been put a little away in the abscissa from the circle. some new luminescence centers which do no exist in the SSO films before irradiation, the fact that a new PL peak emerges at about 580 nm after irradiation can be realized. The PL peak emerging at about 580 nm in the SSO films during y-ray irradiation is located at almost the same position as the PL peak in the SiO, wafer and both their positions are independent of the measurement temperature. Experimental results indicate that the new type of luminescence centers being created in the SSO films by y-ray irradiation and the type of luminescence centers being responsible for the 580 nm PL peak in SiO, wafer are possibly the same. Since the 580 nm PL peak of SiO, wafer originates from luminescence centers (impurities or self-trapped exciton or, most probably, defects) in SiO, matrix, and its peak position is independent of temperature, the fact that three main PL peaks of the SSO films are independent of temperature, rather than red shift with increasing temperature, indicates that they originate from several types of luminescence centers in the Si oxide, rather than from nanometer silicon particles.

ACKNOWLEDGMENT This work was supported by the National

Natural Science Foundation

of China.

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