RF plasma treatment of porous silicon

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Journal of Non-Crystalline Solids 220 (1997) 261-266

RF plasma treatment of porous silicon L. Bedikjan a,,, p. Danesh b a Institute of Applied Physics, Bulgarian Academy of Sciences, 59 St. Petersburg bh,d., 4000 PIocdit, Bulgaria b Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee bIFd., 1784 Sofia, Bulgaria Received 6 February 1997; revised 23 June 1997

Abstract The effect of oxygen and hydrogen rf plasmas on the photoluminescence (PL) spectra of porous silicon (PS) was studied. PS samples were prepared from p-type, (100) oriented silicon wafers with a resistivity of 0.03 1~ cm by electrochemical anodization in an ethanol-containing solution. The plasma treatments were carried out in a planar reactor for 150 min, whereas the samples were placed on the ground electrode and heated to 250°C. The PL was excited by the 548 nm line of Hg lamp and measured using a monochromator and lock-in amplifier. Both hydrogen and oxygen plasma treatments led to a decrease in PL intensity. The decrease was with a factor of 5 after hydrogen and 2.5 after oxygen plasma treatments. It was established by means of Auger electron spectroscopy (AES) that both plasma treatments led to an increase in the oxygen content and to a decrease in the carbon content on the PS surface. However, no direct correlation between the AES data and the PL intensity could be established. We suggest that the appearance of plasma-induced electronic defects, rather than the change in the surface chemistry, accounts for the PL quenching. © 1997 Elsevier Science B.V. PACS: 78.55; 79.20.F; 79.60

1. I n t r o d u c t i o n While blue photoluminescence (PL) is consistent with quantum confinement effects in nanometer-sized silicon crystallites [1], the red PL observed in porous silicon (PS) may be determined by surface-related mechanisms [2-4]. The low dimensionality is a key factor, which provokes the light emission, but the luminescence mechanisms depend on the compositional and structural properties of the silicon-based system [5,6]. In the case of PS the effect of surface must be significant because of the large surface-tovolume ratio. There are models for the PL mechanism, which suggest that the generation of the elec-

* Corresponding author. E-mail: [email protected].

t r o n - h o l e pairs proceeds in the inside of the silicon crystallites, while the radiative recombination of carriers occurs on the surface or in interface regions [3,7]. Possible candidates for luminescent centers in the case of as-grown PS are SiH~ [8] or polysilanes [9]. In the case of oxidized PS there is a direct evidence that oxygen-related centers at the S i / S i O ~ interface or in SiO 2 are involved in the light emission mechanism [4]. For light emitting Si-based materials the passivation of the surface of silicon crystallites is a necessary condition for an efficient room-temperature (RT) luminescence [2,3,10]. It has been established that the replacement of S i - H bonds by a SiO 2 layer improves the stability of the light emission [11-14]. Experimentally, the effect of surface on the PL

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spectra was studied by immersion of PS in different chemical solutions [15], by using thermal annealing [16-18] and rapid thermal oxidation (RTO) in dry [4,10,19] and wet [11] oxygen, by boiling in water [20] and in hydrogen peroxide [21]. In comparison with wet chemistry, the RTO is a processing technique, more compatible with silicon device manufacture. The other technique widely used in integrated circuit technology is radio frequency (rf) plasma processing. The aim of the present work is to study the effect of hydrogen and oxygen rf plasmas on the PL of PS. The change of the surface of PS caused by these treatments has been studied by Auger emission spectroscopy (AES).

2. Experimental The PS samples were prepared by electrochemical anodization without illumination. The silicon wafers were p-type, B-doped with (100) orientation and resistivity of 0.03 f~ cm. The anodization was carried out under a constant current of 50 mA for 20 min. The electrolyte consisted of 1 part 49% HF and 1 part ethanol. After preparation PS samples were cut in pieces for further processing in plasma. The plasma treatment was carried out in a capacitively coupled stainless steel planar reactor with radial gas flow [22]. The applied radio frequency (rf) (13.56 MHz) power was 18 W. The gas (oxygen or hydrogen) pressure was 0.5 Torr. The samples were placed on the ground electrode and heated to 250°C. The heating effect was investigated by annealing some PS samples at the same temperature but in vacuum of 10 3 Tort. The annealing time was 150 min. Some samples were boiled in water for 8 min. The PL spectra were measured at room temperature, using as excitation source the 548 nm line of an Hg lamp. The emitted light from the PS samples was dispersed by a monochromator (SPM2), detected by a photomultiplier tube and analyzed by a standard lock-in amplifier. All the measurements were carried out at least 24 h after the sample preparation and treatments. During this time the samples were stored in air, so that an unavoidable oxidation of the sample surface occurred. No changes in the PL during the measurement were noticed and the obtained PL spectra were reproducible.

Auger and XPS measurements were made on samples without ion etching. The Auger spectra were obtained using an electron beam with energy of 3 keV. The X-ray source was MgK~ with energy of 1256.3 eV.

3. Experimental results The as-prepared PS samples had a red PL at 750 nm (Fig. l(a)). The treatment of the samples in hydrogen plasma leads to a reduction in intensity of about 5 times and to a wavelength shift of about 20 nm (Fig. l(b)). The effect of oxygen plasma is similar, but the intensity of the PL is less reduced, with a factor of 2.5 (Fig. l(c)). In order to reveal the effect of the thermal treatment, the samples were annealed in vacuum at the same temperature. The PL spectrum after this treatment is shown in Fig. l(d) and is similar to that of the sample treated in hydrogen plasma. The XPS measurements established carbon and oxygen on the PS surface and their bonding with Si

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Wavelength (nm) Fig. 1. Photoluminescence spectra of porous silicon samples: (a) as-prepared; (b) after treatment in hydrogen plasma; (c) after treatment in oxygen plasma; (d) after thermal annealing in vacuum; (e) after boiling in water following the thermal annealing.

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variation in the peak positions. The Auger spectrum of PS shown in Fig. 3(b) is apparently shifted due to surface charge accumulation. This effect is in agreement with the observations of Shinar et al. [26] and seems to be typical for PS layers prepared in ethanol-containing solutions. If the peak positions are corrected with the shift of 20 eV of the C(KLL) line the spectrum, and namely the absence of Si(LVV) line at ~ 90 eV, suggests that the sample surface must be completely covered with a layer. Since the O ( K L L ) / C ( K L L ) ratio of ~ 3 indicate an increase in the oxygen level, we assume the presence of SiO x and SiO~H,, groups in the top layer. The small peak at 400 eV could be ascribed to the adsorbed nitrogen. The treatment in rf plasma removes the charge

Binding energy (eV) Fig. 2. XPS spectra of O ls (a), S2p (b) and C ls (c) core levels.

atoms. Fig. 2 represents the XPS spectra of Si2p, C Is and O ls core levels. The position of the emission peak of S i 2 p at 103.2 eV and the asymmetry at 101.2 eV can be related with the presence of silicates and hydride groups [23,24]. The C l s peak is centered at 285 eV. The asymmetries at 283 eV and 286.6 eV are probably due to bonding with silicon and OH groups, respectively. The spectrum of O l s is characteristic of SiO 2. The Auger emission measurements have been focused on Si(LVV) transition, which is very sensitive to the chemical ambient and therefore is strongly affected by the surface layers. Also C(KLL) and O(KLL) transitions are analyzed, which appear at 272 eV and 510 eV, respectively [25]. The obtained spectra are compared with Auger emission of the original Si wafer (Fig. 3(a)). The peak at 91 eV is associated with Si(LVV) line of elemental Si [25] and is characteristic of pure Si surface. The peaks at 510 eV and 271 eV are most probably due to the adsorbed oxygen and carbon. Note the intensity ratio O ( K L L ) / C ( K L L ) in this case, being less than unity. The peaks at 84 eV and 75 eV, associated respectively with SiO and s i o , [25], reflect the presence of native oxide on the Si surface. The electrochemical anodization of Si leads to a

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Electron energy (eV) Fig. 3. Auger spectra of crystalline silicon (a) and porous silicon samples: (b) as-prepared, (c) alter treatment in oxygen plasma; (d) after treatment in hydrogen plasma.

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accumulation (Fig. 3(c) and (d)). Such an effect has also been observed after reactive ion etching (RIE) in oxygen and hydrogen [26], which is essentially a similar plasma processing but with more intensive ion bombardment. The treatment in oxygen plasma further increases the O(KLL)/C(KLL) ratio to ~ 5, which is also in correspondence with the influence of oxygen RIE and might be related with the removal of carbon through chemical reactions producing CO 2 and CO. On the other hand, although RIE leads to the appearance of strong Si-O related peak at ~ 80 eV, the Si(LVV) line at ~ 90 eV is still retained and we suggest that this is consistent with the presence of weak Si-Si bonds and Sill X configurations near the surface [26]. In our case, the presence only of Si(LVV) at 78 eV suggests that most of Si atoms on the surface are terminated by oxygen. Hydrogen plasma completely removes the C(KLL) peak in the Auger spectrum (Fig. 3(d)). The Si(LVV) is at 79 eV. The O(KLL) peak and the absence of a peak at ~ 90 eV suggest that the effect of hydrogen plasma on the surface is similar to oxygen plasma. This observation is consistent with the effect of hydrogen RIE which reduced the Si(LVV) at ~ 90 eV and, like oxygen RIE, increased the oxygen level [26]. The peak at 400 eV is not observed after both hydrogen and oxygen plasma.

4. Discussion Comparing the results obtained from Auger analysis and PL measurements no direct correlation between the surface chemistry and light emission of PS can be inferred. The obtained data shows that AES spectroscopy is less sensitive to the changes in the material than the PL. Therefore, although without direct ESR measurements, the influence of the different treatments is discussed in relation with the generation and passivation of the defects, which could act as non-radiative recombination centers. Such defects could be present both on the crystallite surface and at their oxidized surface/core interface [7]. The samples exposed to rf plasma are subjected to ion and electron bombardment and UV and visible light exposure, which lead to the generation of electronic defects in the near-surface region [27-29].

Our experiments on plasma treatment of PS samples at room temperature have led to a total quenching of the PL, which is apparently related to the plasma-induced defects. This fact pointed out the necessity of the plasma processing at greater substrate temperatures. Moreover, since the transport of plasma species into the sample proceeds by their diffusion from the surface, the content and the distribution of the hydrogen and oxygen atoms bonded in the Si network is affected by the substrate temperature and exposure time [29,30]. Therefore, in our experiments the substrate temperature of 250°C and an exposure time of 150 rain have been chosen. However, the observed decrease in the PL of the samples treated in vacuum under these conditions (Fig. l(d)) suggests that there is an increase in the number of the non-radiative recombination centers, most probably related with the hydrogen evolution from the material [16-18]. The treatment in oxygen plasma and boiling in water lead to similar PL spectra (Fig. l(c) and (e)). The PL quenched by the preliminary thermal annealing recovers to some extent after boiling in water due to the oxidation of the crystallite surface [20]. Oxygen plasma exposure of the as-prepared PS samples also leads to the oxidation of the silicon surface [29]. Their PL spectrum suggests that this surface modification probably compensates for the deleterious effect of the thermal treatment, similarly to the former case. In addition, the oxygen plasma can produce the radiative non-bridging oxygen hole centers at the newly formed Si/SiO 2 interface [4], which could compensate to some extent the PL quenching due to the plasma-induced defects. Plasma hydrogenation has been shown to be effective at 250-300°C for post-hydrogenation of amorphous Si films and passivation of the defects in crystalline Si [27,30,31]. However, the observed decrease in the PL of PS samples after our hydrogenation experiments (Fig. l(d)) suggests that either a change in the crystallite surface composition occurs, a n d / o r the increase in the defect centers is more significant, as compared with the oxygen plasmatreated samples. The only difference in the AES spectra of oxygen and hydrogen plasma-treated samples is the absence of the carbon peak for the hydrogenated PS samples (Fig. 3). However, this difference could not be considered as a reason for the reduced PL of the hydrogenated samples, since there

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are data, w h i c h suggest that carbon does not contribute to the light e m i s s i o n [26,32]. The PL amplitude of the h y d r o g e n a t e d PS samples is nearly as large as o f the v a c u u m annealed ones (Fig. l(b) and (d)). This similarity could be an indication that the h y d r o g e n passivation o f the n e w l y created defects is less e f f e c t i v e than the crystallite oxidation. Actually, in addition to h y d r o g e n in-diffusion there are other p l a s m a - i n d u c e d processes, such as a h y d r o g e n abstraction reaction and out-diffusion [33]. The prevailing o f one o f these processes m i g h t have resulted in increase [34] or in decrease o f h y d r o g e n content in the silicon n e t w o r k [35]. The balance o f the processes might lead to a small change in the defect density [33]. Our data suggest that under the used conditions p l a s m a h y d r o g e n a t i o n o f PS is not an e f f e c t i v e treatment for the defect passivation.

5. Conclusion The obtained results point out that under the used conditions (substrate temperature, applied rf power) the p l a s m a treatment o f the PS samples leads to a decrease in the intensity of PL. The decrease in the light e m i s s i o n ability is related to the increase in the density o f the defects, w h i c h act as non-radiative r e c o m b i n a t i o n centers. The generation o f the additional defects might be caused by h y d r o g e n out-diffusion f r o m the samples due to the e n h a n c e d temperature a n d / o r by the direct e x p o s u r e o f the samples to the plasma irradiation. In the present w o r k we established that h y d r o g e n p l a s m a has a m o r e deleterious effect than o x y g e n plasma. The latter reduces the PL quenching, probably, due to an additional oxidation of the crystallites. The A E S data indicate that the PS e x p o s u r e to rf p l a s m a increases the o x y g e n content and decreases the carbon content on the sample surface. There is little difference b e t w e e n the effects o f h y d r o g e n and o x y g e n plasmas, e x c e p t for the carbon elimination, more evident in the case o f the h y d r o g e n plasma. On the basis o f the obtained results no correlation between the A E S data and the PL intensity c o u l d be inferred. Evidently, the degree of the b o n d i n g rearr a n g e m e n t on the crystallites' surface, w h i c h affects the PL, is b e y o n d the sensitivity o f the A E S .

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