Decomposition of silicon hydrides following disilane adsorption on porous silicon surfaces

Surface Science Letters 295 (1993) L998-LI004 North-Holland

surface s c i e n c e letters

Surface Science Letters

Decomposition of silicon hydrides following disilane adsorption on porous silicon surfaces A.C. Dillon, M.B. Robinson and S.M. George Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Received 4 June 1993; accepted for publication 7 July 1993

Disilane (Si2H 6) is used for silicon chemical vapor deposition (CVD) and is a potential precursor for atomic layer epitaxy (ALE) on silicon surfaces. In this study, Fourier transform infrared (FTIR) transmission spectroscopy was employed to examine the adsorption and decomposition of disilane on high surface area porous silicon surfaces. The FTIR spectra revealed that disilane dissociatively adsorbs on porous silicon surfaces at 200 K to form a large fraction of Sill 3 surface species and some Sill 2 and Sill surface species. The Si-H x stretching modes between 2125-2156 c m - l , the Sill 3 degenerate deformation mode at 862 cm-1 and the Si-H 2 scissor mode at 907 cm - I were then employed to monitor the decomposition of the surface hydride species during thermal annealing. Between 200-600 K, the Sill 3 species decreased and were depleted from the silicon surface. Concurrently, the Sill 2 surface species were observed to increase between 200-440 K and subsequently to decrease between 440-620 K. Only monohydride species remained on the porous silicon surface at 620 K. Above 640 K, the Sill surface species decreased concurrently with H 2 desorption. Adsorption studies were also conducted at various isothermal temperatures. These disilane adsorption and decomposition experiments provide important insights to the surface chemistry during silicon CVD and ALE processing.

The decomposition of disilane ( S i 2 H 6) o n silicon surfaces is very important to semiconductor device technology. Presently, silane (Sill 4) is more frequently employed in silicon chemical vapor deposition (CVD) processes [1,2]. However, recent gas phase pyrolysis [3] and CVD film growth rate measurements [4,5] indicate that S i 2 H 6 plays a significant role during silicon CVD with Sill 4 at higher pressures. Disilane can also be used directly for silicon CVD and is preferable to silane because of its larger sticking coefficient [6]. As semiconductor technology moves towards smaller devices, the development of controlled deposition techniques has become a primary focus. For surface temperatures below the H 2 desorption temperature, disilane adsorption is self-regulating and limited by the number of available dangling bonds [7]. Consequently, disilane has recently been successfully employed for silicon atomic layer epitaxy (ALE) [8]. A complete understanding of the sur-

face chemistry of disilane is important for the further development of silicon ALE techniques. Many experiments have explored the interaction of disilane with silicon surfaces. The adsorption kinetics of Si2H 6 have been previously determined in temperature programmed desorption (TPD) studies on Si(lll)7 x 7 [6]. The dissociative chemisorption of Si2H 6 on Si(100)2 x 1 was also recently examined using direct recoil time of flight spectroscopy and static secondary ion mass spectrometry (SSIMS) [9]. Modulated molecular beam scattering (MMBS) [7,10,11] and scanning tunneling microscopy (STM) [12] investigations have revealed that disilane exposures at temperatures 670 < T < 770 K result in hydrogenated Si film growth on Si(lll)7 × 7. Recent ultraviolet photoemission spectroscopy (UPS) measurements have reported that disilane preferentially adsorbs on Si(lll)7 × 7 rest atoms [13]. These results are in conflict with a previous UPS study which re-

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A.C. Dillon et al. / Sill x decomposition after Si2H 6 adsorption on porous silicon

ported that the dangling bonds of the Si(lll)7 × 7 adatoms are more reactive to Si2H 6 [14]. Recent multiple internal reflection infrared spectroscopy studies have investigated the stable surface species and their decomposition mechanisms following disilane adsorption on Si(111)7 X 7 [15,16]. These studies revealed that SiEH 6 exposures at or below 200 K resulted in a surface dominated by chemisorbed Sill 3 species as a result of dissociative adsorption by Si-Si bond scission. At higher adsorption temperatures > 400 K, the surface was reported to be dominated by surface monohydrides [15,16]. Unfortunately, a conclusive identification of all the surface species present on the Si(lll)7 × 7 surface was not possible because of the limited resolution in the Si-H x stretching region. In addition, the infrared region below 1200 cm-1 is obscured in multiple-internal reflection experiments because of silicon lattice absorption [17,18]. The Si-H x vibrational modes in this region would have facilitated the differentiation of the Sill, Sill 2 and Sill 3 surface species. Electron energy loss spectroscopy (EELS) studies have also examined the surface species following Si2H 6 adsorption on Si(lll)7 x 7 [14]. These EELS studies were in good agreement with the multiple internal reflection infrared analysis. The EELS studies observed the formation of Sill 3 surface species following disilane adsorption at 170 K. By 520 K, the surface was reported to be dominated by surface monohydride species [14]. The vibrational features below 1200 cm -~ could have distinguished the Sill, Sill 2 and Sill 3 surface species. Unfortunately, these lower frequency features could not be resolved in this earlier EELS study. In the current study, the decomposition of Si2H 6 on porous silicon surfaces was examined using transmission Fourier transform infrared (FTIR) vibrational spectroscopy. Transmission FTIR spectroscopy studies are not limited by silicon lattice adsorption and can probe the infrared region below 1200 cm-~ with a resolution of 3-4 cm -~. With this resolution, the low frequency Si-Hx vibrational modes were easily resolved and observed following SiEH 6 exposure. This study identified all the surface hydride species at 300 K and subsequently determined

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their decomposition mechanisms during thermal annealing experiments. Transmission FFIR studies are limited to high-surface area materials. Porous silicon provides high surface area, crystalline silicon samples that greatly facilitate infrared studies of silicon surface chemistry [19-22]. The electrochemical techniques used to prepare porous silicon have been described previously [19]. Boron doped, ptype Si(100) wafers with a resistivity of p = 0.1-0.3 f~ cm were anodized to obtain a 2 /~m porous silicon layer [19]. These porous silicon films provided a surface area enhancement of × 400 compared with a single-crystal sample [23]. Although these porous silicon films were not photoluminescent [24], subsequent passive HF etching in 48% HF/Ethanol produces photoluminescent porous silicon in approximately 1-2 h [25]. A Nicolet 740 FTIR spectrometer with an MCT-B detector and an ultra-high vacuum (UHV) chamber designed for transmission FTIR investigations were employed in these studies [21]. For the thermal annealing studies, the porous silicon samples at 100 K were exposed to Si2H 6 at 1 × 10 -5 Torr for 3 min followed by a 1 x 10 -5 Torr Si2H 6 exposure at 200 K for 10 min. These exposures were sufficient to achieve a saturation coverage. The sample was then heated to the annealing temperature with a constant heating rate of 7 K/s. The porous silicon sample was held for 1 min at the annealing temperatures. FTIR spectra were subsequently recorded at 200 K. In the isothermal adsorption studies, porous silicon samples at 500 and 670 K were exposed to Si2H 6 at 1 × 10 -5 Torr. The FTIR spectra following these higher temperature adsorptions were then recorded at 500 K. Fig. 1 displays the FFIR spectrum of porous silicon observed following a saturation SiEH 6 exposure at 200 K. Infrared absorption features are observed at 2156, 2125, 923, 907, 862, 688, 644 and 626 cm -1. The absorption features in fig. 1 are tabulated with their assignments in table 1. The previous vibrational studies that provided the basis for these spectral assignments are also included in table 1. The highest frequency infrared absorption feature at 2156 cm -~ in fig. 1 is attributed to the Si-H 3 stretching vibrations of the

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surface trihydride species. This assignment is based on infrared studies of gas phase [26] and crystalline disilane [27] which have observed the S i - H 3 stretching vibrations between 2140-2158 cm -1. Multiple internal reflection infrared studies of disilane adsorption on S i ( l l l ) 7 x 7 have also reported the S i - H 3 stretching vibrations between 2130-2154 c m - I [15]. Reflectance infrared studies of hydrogen on Si(100) have assigned infrared features between 2080-2100 cm -~ to S i - H , stretching vibrations of surface Sill and Sill 2 species [28]. After elec-

trochemical anodization, porous silicon exhibits two absorption features at 2110 and 2087 cm ~ assigned to silicon monohydride and silicon dihydride stretching vibrations [19]. Consequently, the broad infrared feature centered at 2125 cm-~ in fig. 1 is attributed to the S i - H , stretching vibrations of Sill and Sill 2 surface species. The presence of both Sill and Sill 2 surface species following Si2H 6 exposure on porous silicon at 200 K is confirmed by the corresponding infrared absorption of the low frequency vibrational modes. The infrared feature at 907 c m - I in fig. 1 is assigned to the Sill 2 scissor mode. This assignment is consistent with the observation of the Sill 2 scissor mode on electrochemically anodized porous silicon at 910 cm -I [19]. The infrared features at 923 and 862 cm-J in fig. 1 are attributed to the S i - H 3 deformation modes. Infrared studies of disilane [26,27] have observed absorption peaks corresponding to S i - H 3 deformation modes at 939-940 cm - l and 835-943 c m - - 1.

The absorption feature at 688 cm-~ in fig. 1 is also attributed to a vibrational mode of Sill 3 surface species. This identification is based on the observation that the infrared absorption feature at 688 cm -1 decreases concurrently in the thermal annealing studies with the absorption features at 923 and 862 cm -1 assigned to the S i - H 3 deformation modes. In addition, the disilane S i - H 3 rocking mode is Raman active and has been previously observed at 625 cm-1 [26].

Table 1 Absorption features Si surface frequency ( c m - 1)

Vibrational mode

Corresponding frequency ( c m - 1)

Molecule or surface species

Reference

2156

Si-H 3 stretch

2158-2140

2125

Si-H, S i - H 2 stretch

2110-2080

923 907 862 688 644 626

S i - H 3 deformation S i - H 2 scissor S i - H 3 deformation S i - H 3 rock Si-H 2 wag S i - H wag

940-939 910 843-835 625 640 625

Si2H 6 Sill3, Si(lll)7 × 7 Sill, Si(100) Sill, porous Si Si2H 6 Sill 2 porous Si Si2H 6 SiEH 6 Sill 2 amorphous Si Sill porous Si

[26,27] [15] [28] [19] [26,27] [19] [26,27] [26] [29] [19]

A.C. Dillon et al. / Sill x decomposition after Si 2 H6 adsorption on porous silicon

The lowest frequency absorption features at 644 and 626 cm -1 in fig. 1 are attributed to S i - H x wagging vibrations of Sill 2 and Sill surface species, respectively. These assignments are based on vibrational studies on hydrogenated amorphous silicon which have observed the Si-H 2 wagging vibration for isolated Sill 2 groups at 640 cm -1 [29]. Previous infrared studies on porous silicon have also observed the S i - H wagging vibration at 625 cm -1 [19]. Previous TPD studies of the adsorption kinetics of Si2H 6 and SiED 6 on Si(lll)7 × 7 have revealed that these two molecules exhibit the same initial sticking coefficient [6]. The absence of a deuterium kinetic isotope effect suggested that disilane adsorption occurs as a result of Si-Si bond scission, i.e., Si2H6(g) + 2Si* - , 2SiSiH3 where * indicates a dangling bond. Further studies on Si(100)2 × 1 [9] suggested that an alternative Si-Si bond breakage adsorption mechanism may also occur, i.e., SiEH6(g)+2Si*---) SiH4(g) + Si2-SiH 2 [9]. The FTIR spectra in fig. 1 following disilane adsorption at 200 K is consistent with Si-Si bond scission to produce a large fraction of Sill 3 surface species. However, the spectral features also reveal the existence of some Sill 2 and Sill surface species species. The presence of both Sill 2 and Sill indicates the partial decomposition of the Sill 3 surface species, i.e., Sill 3 + Si* ~ Sill 2 + Sill after disilane adsorption on porous silicon at 200 K. Some of the Sill 2 surface species may also be attributed to the alternative Si-Si bond breakage adsorption mechanism [9]. Fig. 2 displays the changes in the S i - H x stretching region of the infrared absorption spectra versus annealing temperature following a saturation Si2H 6 exposure on porous silicon at 200 K. Fig. 3 shows the corresponding changes in the S i - H x scissor, deformation and bending regions of the infrared absorption spectra. The normalized integrated absorbances of the Si-H2 scissor mode (907 cm-l), the S i - H 3 deformation mode (862 cm -1) and the Si-H~ stretching vibrations (2125-2156 cm -1) are plotted versus annealing temperature in fig. 4. The absorption feature of the Sill 2 scissor mode at 907 cm-~ was resolved from the absorption peak of the S i - H 3 deforma-

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tion modes at 923 cm-1 using a Lorentzian curve fitting analysis. The temperature dependence of the integrated absorhances of the S i - H 3 deformation modes at 923 and 862 cm-1 were proportional from 200-360 K until the curve fitting of

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Temperature (K) Fig. 4. Integrated infrared absorbances versus annealing temperature for the S i - H z scissors mode at 907 c m - l , the S i - H x stretching vibrations at 2125-2156 cm -1 and the S i - H 3 deformation mode at 862 cm -~. The annealing experiments were performed after a saturation Si2H 6 exposure at 200 K.

the 923 cm-1 mode became difficult at low absorbance values. Fig. 3 displays the gradual decrease between 200-400 K of the infrared absorption peaks at 923, 862 and 688 cm-1 attributed to vibrational modes of Sill a surface species. A low frequency shoulder at 440 K on the S i - H 3 deformation mode at 862 cm-1 may indicate an inhomogeneity in the Sill 3 surface sites. A similar decrease in the S i - H 3 stretching vibration at 2156 cm -1 is observed in fig. 2. Concurrently, the absorption features of the Sill 2 and Sill surface species at 2125, 907, 644 and 626 cm -1 are observed to increase over this same temperature range. Fig. 4 reveals that the integrated absorbance of the S i - H 2 scissor vibration increases by more than a factor of 2 for changes in annealing temperature between 200-400 K. These results confirm the Sill 3 decomposition reaction, i.e., Sill 3 + S i * ~ Sill 2 + Sill. In addition, no increase in the background H 2 pressure was measured by the mass spectrometer for annealing temperatures from 200-400 K. These results argue for the transfer of hydrogen from Sill 3 species to silicon surface sites.

Between 440-620 K, fig. 4 reveals that thc Sill 3 surface species continue to decrease and are absent from the porous silicon surface at 620 K. The Sill 2 surface species also begin to decrease at annealing temperatures above 420 K and are extinguished from the silicon surface at 620 K. Concurrently, the S i - H x stretching and wagging vibrations evolve into sharp infrared features at 2090 and 621 cm-~, respectively. These features have been attributed to silicon monohydride stretching and wagging vibrations [19]. These results suggest the reaction SiH~ + Si* Sill + Sill and indicate that Si2H 6 decomposition on porous silicon yields only Sill monohydride species at annealing temperatures above 600 K. Above 640 K, the S i - H stretching vibration is observed to decrease as H 2 desorbs from the surface [19,30-33]. Following the isothermal adsorption of SizH 6 on porous silicon at 500 K, only silicon monohydride surface species are observed in the infrared absorption spectra. In contrast, the thermal annealing studies summarized by fig. 4 indicate that both Sill 3 and Sill 2 surface species are present at 500 K. These differences occur because the porous silicon sample was held at the adsorption temperature for the entire adsorption experiment. In contrast, the normalized integrated infrared absorbances shown in fig. 4 were obtained after annealing the sample at each temperature for only 1 min. Similar behavior has been observed in previous porous silicon F T I R studies [20,21]. As expected from the thermal annealing studies, only monohydride surface species are observed following SizH 6 exposure on porous silicon at 670 K. The integrated absorbance of the silicon monohydride S i - H stretching vibration versus Si2H 6 exposure at 500 and 670 K are displayed in fig. 5. The dissociative adsorption of disilane on porous silicon displayed in fig. 5 is characterized by a rapid initial Si2H 6 adsorption, followed by decreasing adsorption rates that ultimately reach a saturation hydrogen coverage. The saturation hydrogen coverage observed following Si2H 6 adsorption at 500 K is greater than the steady state coverage observed at 670 K. U 2 desorption occurs for temperatures above T > 640 K [19,30-33].

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The Sill 2 surface species increase during Sill 3 decomposition between 200-440 K before decreasing at annealing temperatures above 440 K. By 620 K, only Sill species remain on the porous silicon surface. These monohydride species are lost as H 2 desorbs at temperatures T >_640 K. For temperatures below the H 2 desorption temperature, SiEH 6 adsorption is self-limiting and saturation coverages are obtained after long SiEH 6 exposure times. This behavior indicates that disilane is a promising molecular precursor for the controlled deposition of silicon on silicon surfaces.

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Time (eec) Fig. 5. Integrated infrared ahsorbance versus Si2H 6 exposure time for the Si-H monohydride stretching vibration at 2090 cm-~ on porous silicon surfaces at temperatures of 500 and 670 K.

Consequently, the steady-state hydrogen coverage at 670 K is lower than the saturation coverages observed at T < 640 K. For temperatures below the H 2 desorption temperature, Si2H 6 adsorption is self-limiting. This observation suggests the following recipe for silicon ALE employing Si2H 6 as a molecular precursor. First, adsorb Si2H 6 at silicon surface temperatures less than 640 K. Second, anneal the sample to 800 K to desorb the surface hydrogen as H 2. Lastly, cool the sample down to the initial adsorption temperature and repeat the cycle. Each adsorption cycle should deposit approximately O = 0.4 ML of silicon where 1 ML is defined as 1 M L = 1 monolayer= 6.8 x 1014 atoms/cm 2 [8]. In conclusion, Sill 3 is the main surface species observed following the adsorption of Si2H 6 on porous silicon at 200 K. These Sill 3 surface species are consistent with the dissociative adsorption of Si2H 6 by Si-Si bond scission: Si2H6(g) + 2Si* ---> 2Si-SiH 3. The additional presence of both Sill 2 and Sill surface species indicates that the surface decomposition reaction Sill 3 + Si* ~ Sill 2 + Sill is competitive at 200 K. The Sill 3 species decrease and disappear from the surface after annealing between 200-600 K.

This work was supported by the Office of Naval Research under Contract N00014-92-J1353. S.M.G. also acknowledges the National Science Foundation for a Presidential Young INvestigator Award (1988-1993).

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A.C. Dillon et al. / Sitt~ decomposition after Si 2 He, adsorption on porous silicon

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