P2 Desorption from Phosphine Decomposition on Si(100) Surfaces

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Langmuir 1998, 14, 1428-1434

P2 Desorption from Phosphine Decomposition on Si(100) Surfaces Michael L. Jacobson,† Michael C. Chiu, and John E. Crowell* Department of Chemistry and Biochemistry, Materials Science Graduate Program, University of California, San Diego, La Jolla, California 92093-0314 Received July 16, 1997. In Final Form: November 25, 1997 Phosphorus desorption following phosphine (PH3) adsorption on the Si(100) surface was analyzed via temperature-programmed desorption (TPD) and Auger electron spectroscopy. Phosphine partially decomposes upon adsorption at room temperature. Hydrogen (H2) and phosphine are produced via recombinative desorption during TPD. At higher temperatures the remaining phosphorus desorbs in the form of P2. At low P coverages, a single R state is present in the desorption spectra which obeys secondorder kinetics with an activation energy of 72 kcal/mol. At coverages around 21% of a phosphorus monolayer, a second β state appears at lower temperatures which also obeys second-order kinetics with a desorption energy of 57 kcal/mol. At coverages greater than about 42% of a phosphorus monolayer, a third desorption state is observed.

Introduction As the minimum feature thickness and overall device size continue to shrink, traditional semiconductor manufacturing techniques will no longer be sufficient to provide adequate control over microstructure and film quality. Novel processes must be developed which will provide higher quality materials and lower the overall thermal budget. A current area of interest is the simultaneous growth and doping of silicon via chemical vapor deposition. In this scheme, the silane feed stream is mixed with a dopant gas such as phosphine and allowed to react with the surface to yield a mixture of silicon and phosphorus. Using low-temperature epitaxial silicon growth, one has control over the morphology at the atomic level by simply tailoring the phosphine-to-silane ratio. Deposition can occur at lower temperatures as the rate-determining step is the desorption of hydrogen, which occurs near 800 K. A variation of this technique has already been used to produce delta-doped layers of boron by chemical vapor deposition from a mixture of diborane, disilane, and hydrogen.1 Additionally, one can use this methodology to improve fabrication of nanoscale semiconductor devices. One can impose concentration gradients at the oxide interface to create retrograde wells. These provide a retarding field and reduce both the possibility of latch-up and the susceptibility to vertical punch through.2 One can also change the doping level of a region to produce lightly doped drains which serves to lower the carrier velocity at the gate oxide interface to minimize the effects of impact ionization and hot carrier injection.3 Despite the usefulness of this methodology, only a limited number of experiments have been performed on the PH3/SiH4/Si system. Crystalline silicon is a covalently bonded tetrahedral lattice; the bulk Si-Si bond length is reported to be 2.35 * To whom correspondence should be addressed. † Stacey Memorial Fellow. (1) Ozturk, M. C.; Wartman, J. J. In 1st International Rapid Thermal Processing Conference; Fair, R. B., Lojek, B., Eds.; Scottsdale, AZ, 1993; p 227. (2) Lewis, A. G.; Martin, R. A.; Huang, T. Y.; Chen, J. Y.; Koyanagi, M. IEEE Trans. Electron. Devices 1987, 34, 2156. (3) Ogura, S.; Tsang, P. J.; Walker, W. W.; Critchlow, D. L.; Shepard, J. F. IEEE Trans. Electron. Devices 1980, 27, 1359.

Å.4 The symmetry is broken at the surface, giving rise to high-energy unpaired electrons or dangling bonds. This excess energy drives a reconstruction in which the surface atoms form dimers and reduce the number of dangling bonds by a factor of 2. These dangling bonds give the Si(100) surface its characteristic reactivity. These dimers are aligned in the 〈110〉 direction. This gives rise to a low-energy electron diffraction (LEED) pattern reflective of 2 × 1 symmetry. Bond lengths for the Si-Si surface bond have been reported to be 2.37 Å (SLAB-MINDO calculations),5,6 2.40 Å (total-energy calculations),7 2.44 Å (cluster calculations)8 and 2.45 Å (cluster calculations),9 and 2.51 Å (cluster calculations).10 The surface energy is further minimized as the remaining dangling bonds on the dimer interact and form a weak π bond. This causes the dimer bond to shorten. Reported values for the π-bonded dimer are 2.13 Å (SLAB-MINDO calculations),6 2.30 Å (grazing incident X-ray diffraction analysis),11 and 2.32 Å (cluster calculations).9 Yu and Meyerson12,13 and Yu and co-workers14,15 performed studies of the interaction of PH3 with the Si(100) surface. They determined that PH3 molecularly adsorbs at room temperature on Si(100) on dangling bond sites with a high sticking probability, forming two σ bonds to yield a Si-P-Si bridge structure with a pentacoordinated phosphorus. A small amount of the molecular PH3 decomposed, and the resulting phosphorus was subsequently incorporated into the surface via substitution into surface dimers such that the surface retained its 2 × 1 (4) Properties of Silicon; Nielson, O. H., Ed.; Inspect: London, 1988. (5) Zheng, X. M.; Smith, P. V. Surf. Sci. 1992, 279, 127. (6) Craig, B. I.; Smith, P. V. Surf. Sci. 1990, 226, L55. (7) Northrup, J. E. Phys. Rev. B 1991, 44, 1419. (8) Wu, C. J.; Carter, E. A. Chem. Phys. Lett. 1991, 185, 172-178. (9) Nachtigall, P.; Jordan, K. D.; Janda, K. C. J. Chem. Phys. 1991, 95, 8652. (10) Chabal, Y. J.; Raghavachari, K. Phys. Rev. Lett. 1984, 53, 282. (11) Jedrecy, N.; Sauvage-Simkin, M.; Pinchaux, R.; Massies, J.; Greiser, N.; Etgens, V. H. Surf. Sci. 1990, 230, 197. (12) Yu, M. L.; Meyerson, B. S. J. Vacuum Sci. Technol. A 1984, 2, 446. (13) Meyerson, B. S.; Yu, M. L. J. Electrochem. Soc. 1984, 131, 23662368. (14) Yu, M. L.; Vitkavage, D. J.; Meyerson, B. S. J. Appl. Phys. 1986, 59, 4032-4037. (15) Yu, M. L.; Vitkavage, D. J.; Meyerson, B. S. J. Vacuum Sci. Technol. A 1985, 3, 861-862.

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P2 Desorption from PH3 Decomposition on Si(100)

symmetry. They also showed that the saturation phosphorus coverage obtainable from phosphine adsorption was temperature dependent, with the maximum phosphorus coverage upon PH3 adsorption at temperatures up to 673 K being 25%, increasing to a full monolayer for adsorption at 823 K. Phosphorus desorption in the form of P2 occurred at higher temperatures (g900 K) which resulted in a decrease in the phosphorus concentration. Van Brommel and Crobeen16 speculated that this full monolayer coverage could also be reached on the Si(111) surface. Taylor,17 Colaianni,18 Wallace,19 Chen,20 and co-workers used temperature-programmed desorption (TPD) and vibrational spectroscopy using high-resolution electron energy loss spectroscopy (HREELS) to study the PH3/Si system on Si(100) and Si(111). They showed that the extent of reaction for phosphine on silicon was much greater than that proposed by Yu et al.14 They observed significant PH3 decomposition even at temperatures as low as 100 K. Additionally, it was shown that phosphorus was stable on the Si(111) surface until approximately 1000 K at which point it desorbed in the form of P2 via a secondorder process. Hydrogen has been shown to desorb from silicon surfaces at a temperature of about 800 K. This difference in desorption temperature for P2 vs H2 indicates that the surface can be dosed with phosphine at low temperatures and heated to remove the hydrogen without removing the phosphorus. It could then be cooled and reexposed to PH3, which could adsorb on or react with the dangling bonds that were formerly occupied by hydrogen. Thus the phosphorus concentration could be “built up” by repeated adsorption/anneal cycles. The PH3/Si(100) adsorption system was studied using scanning tunneling microscopy (STM) by Kipp et al.,21 Hamers et al.,22 and Wang et al.23,24 Their surface STM images were qualitatively similar, but their interpretations of these images were slightly different. Both groups saw significant surface roughening and island formation on Si(100) following PH3 adsorption and its subsequent decomposition to deposit P atoms. Kipp et al. labeled these as phosphorus islands consisting of phosphorus-phosphorus dimers.21 Hamers et al. and Wang et al. concluded that at low coverages, these features were islands of displaced silicon resulting from the incorporation of phosphorus and the simultaneous ejection of silicon from the dimers.22,23 Wang et al.24 used P-P and P-Si bond lengths obtained from gas-phase molecules 25-30 to interpret their STM (16) van Brommel, A. J.; Crobeen, J. E. Surf. Sci. 1973, 36, 773. (17) Taylor, P. A.; Wallace, R. M.; Choyke, W. J.; Yates, J. T. Surf. Sci. 1990, 238, 1-12. (18) Colaianni, M. L.; Chen, P. J.; Yates, J. T. J. Vacuum Sci. Technol. A 1994, 12, 2995-2998. (19) Wallace, R. M.; Taylor, P. A.; Choyke, W. J.; Yates, J. T. J. Appl. Phys. 1990, 68, 3669-3678. (20) Chen, P. J.; Colaianni, M. L.; Wallace, R. M.; Yates, J. T. Surf. Sci. 1991, 244, 177-184. (21) Kipp, L.; Bringans, R. D.; Biegelsen, D. K.; Northrup, J. E.; Garcia, A.; Swartz, L. E. Phys. Rev. B 1995, 52, 5843-5850. (22) Hamers, R. J.; Wang, Y. J.; Shan, J. Appl. Surf. Sci. 1996, 107, 25-34. (23) Wang, Y. J.; Bronikowski, M. J.; Hamers, R. J. J. Phys. Chem. 1994, 98, 5966-5973. (24) Wang, Y. J.; Chen, X. X.; Hamers, R. J. Phys. Rev. B 1994, 50, 4534-4547. (25) Baudler, M.; Schmidt, L. Z. Anorg. Allg. Chem. 1957, 289, 219. (26) Kuczkowski, R. L.; Schiller, H. W.; Rudolph, R. W. Inorg. Chem. 1971, 10, 2505. (27) Leung, Y. C.; Waser, J. J. Phys. Chem. 1956, 60, 539. (28) McAdam, A.; Beagley, B.; Hewitt, T. G. Trans. Faraday Soc. 1970, 66, 2842. (29) Maxwell, L. R.; Hendricks, S. B.; Mosley, V. M. J. Chem. Phys. 1935, 3, 699.

Langmuir, Vol. 14, No. 6, 1998 1429

images. They determined that both the P-P (2.25 Å) and P-Si (2.25 Å) bond lengths are shorter than the Si-Si (2.30 Å) bond length, and thus the surface strain energy increases as the P concentration increases. Hamers et al.22 and Wang et al.24 found that this surface was essentially a random P/Si alloy. However, the P-Si heterodimer is slightly preferred (by a couple of percent) over the P-P dimer, presumably because it can tilt and act as a stress relief mechanism. Kipp et al.21 also detected the presence of P-Si heterodimers but only at high P coverages. Both groups showed that line defects were created, particularly at high coverages. Kipp et al.21 reasoned that these were Si(111) microfacets, which lowered the surface free energy by reducing the residual stress. Hamers et al.22 and Wang et al.24 reported two different types of line defects at different coverage regimes. At low P coverages the defects present are the same antiphase boundaries present on the Si(100) surface during epitaxial growth of silicon.31 The defect lines that are present at higher P coverages act as stress relief mechanisms and are different in nature than the antiphase boundaries. It is important to understand the kinetics of P2 desorption from the silicon surface in order to make predictive models for P/Si co-deposition. To date there have been no systematic studies of the desorption behavior of P2 from the P/Si(100) surface. This paper addresses that topic. Prior to investigating the desorption behavior of P2 as a function of coverage, the actual amount of surface P must be quantified. This was done using Auger electron spectroscopy (AES), while the desorption kinetics were studied using temperature-programmed desorption (TPD). Kinetic parameters were extracted using both the Chan, Aris, and Weinberg32 and the Redhead33 methods. Methods and Materials Experiments were performed in an ultrahigh vacuum chamber. The stainless steel chamber is pumped by a liquid N2 cooled Ti sublimation pump and a 360 L/s. turbomolecular pump; the latter is backed by both a 2 in. diffusion pump and a two-stage mechanical pump. The chamber is equipped with a single pass cylindrical mirror analyzer (CMA) and electron gun for AES. The chamber is also equipped with an ion gun and power supply for ion sputtering, and a shielded quadrupole mass spectrometer for both TPD experiments and residual gas analysis. A 1 × 2.5 cm2 polished Si(100) sample is fixed to a liquid nitrogen cooled manipulator that has 3 degrees of translational freedom and 360° of rotational freedom and can be moved to within millimeters of the analytical probes within the chamber. Ultrahigh vacuum conditions were achieved by baking the chamber at a temperature of 473 K until the background pressure at this temperature was less than 1 × 10-8 Torr. Upon cooling to room temperature, typical pressures achieved after sample preparation were less than 2 × 10-10 Torr. Prior to conducting experiments, the silicon single-crystal sample was cleaned in a vacuum. This was accomplished by sputtering the sample using argon ions with energies of 1.5 keV at a sample temperature of 573 K. The beam current was kept at 3 µA until the carbon (KLL) signal was less than 1% of the silicon (LMM) signal and no oxygen could be detected by AES. The sample was then annealed at 1123 K for 7 min. No other special handling procedures for the crystal were employed. The sample temperature was monitored via a type K thermocouple embedded in a zirconia-based ceramic adhesive and attached to the back of the crystal. TPD experiments were (30) Horn, H. G. Chem.-Z. 1986, 110, 131. (31) Bronikowski, M. J.; Wang, Y. J.; Hamers, R. J. Phys. Rev., B 1993, 48, 12361-12364. (32) Chan, C. M.; Aris, R.; Weinberg, W. H. Appl. Surf. Sci. 1978, 1, 360. (33) Redhead, P. A. Vacuum 1962, 12, 203.

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Figure 1. Temperature-programmed desorption spectrum following a saturation exposure of PH3 on Si(100) at 298 K. The desorption products are amu 2 (H2), amu 34 (PH3), and amu 62 (P2). performed by resistively heating the crystal (P-doped, F ) 0.020 Ω cm) with a linear temperature ramp of 3 K/s unless otherwise noted.

Results and Discussion Desorption Behavior. Figure 1 shows the TPD spectrum of a Si(100) surface following a saturation (30 langmuir) exposure to PH3 at room temperature. The peaks shown represent PH3 (amu 34), H2 (amu 2), and P2 (amu 62). Desorption of these species from the PH3/Si surface has been characterized previously by others.12,14,18 The hydrogen desorption behavior is similar to that following H atom exposure to a clean Si surface in that there are no new desorption states apparent. The primary desorption feature centered near 800 K corresponds to the β1 (monohydride) desorption.34-38 The shoulder on the leading edge of the peak was also seen by Colaianni et al. on Si(100).18 This is nearly the same temperature as one would expect for β2 (dihydride) desorption to occur. These experiments are designed to examine the phosphorus desorption behavior as a function of phosphorus coverage. The inherent difficulty of this experiment is that the relevant coverage is the amount of surface phosphorus at the time of P2 desorption. However, as evident from Figure 1, phosphine desorbs at lower temperatures than P2. As this occurs, phosphorus will be removed from the surface in the form of phosphine. Thus, the phosphorus concentration will be lower at the time of P2 desorption than when it was measured by AES at low temperature. It is difficult to quantify the amount of PH3 desorption from TPD as the features are very broad compared to that of H2 (cf. amu 2 vs amu 34 in Figure 1), and the signal-to-noise ratio is significantly worse (as seen in Figure 1). One cannot simply desorb the molecular PH3 and then measure the P coverage by AES because of (34) Sinniah, K.; Sherman, M. G.; Lewis, L. B.; Weinberg, W. H.; Yates, J. T.; Janda, K. C. J. Chem. Phys. 1990, 92, 5700-5711. (35) Greenlief, C. M.; Gates, S. M.; Holbert, P. A. J. Vacuum Sci. Technol. A 1989, 7, 1845. (36) Greenlief, C. M.; Gates, S. M.; Holbert, P. A. Chem. Phys. Lett. 1989, 159, 202-206. (37) Ning, B. M. H.; Crowell, J. E. Surf. Sci. 1993, 295, 79-98. (38) Ning, B. M. H.; Crowell, J. E. Appl. Phys. Lett. 1992, 60, 29142916.

Jacobson et al.

the significant overlap of PH3 and H2 desorption, as the H2 desorption is additionally used to quantify the P coverage (vide infra). The desorption peak maximum for PH3 desorption is lower than that of the β1 state but overlaps with that of the β2 state. Thus phosphine desorption occurs prior to and simultaneous with H2 desorption. For PH3 adsorption at temperatures where hydrogen is stable on the surface, PH3 desorption does not present a problem for quantifying the P coverage at the time of P2 desorption. One could compare the total H2 area due to PH3 decomposition to that of the β1 peak at saturation. The saturated β1 state corresponds to a full monolayer hydrogen coverage, where a monolayer is defined as one hydrogen per silicon surface site. Therefore, the amount of phosphorus in monolayers present at the time of phosphorus desorption can be determined by taking the total hydrogen area due to PH3 adsorption and decomposition and dividing by 3. Phosphorus that desorbs in the form of PH3 does not affect the 3:1 ratio and, thus, is of no consequence. However, the P/Si ratio measured at low temperature by AES does quantify the phosphorus present in both the PH3 and P2 desorption products. The saturation amount of phosphorus attainable on the silicon surface increases with adsorption temperature if the adsorption occurs in the regime where hydrogen desorption occurs. Yoo et al.39 performed careful studies of hydrogen desorption following PH3 exposure on Si(100). They performed their measurements by first saturating the surface with PH3 at various temperatures. The surface was then heated to desorb hydrogen and cooled to 673 K. The surface was then given an additional but specific dose of PH3, and TPD was performed monitoring only the hydrogen. An algebraic expression was developed which related the total phosphorus coverage to the hydrogen TPD area resulting from the PH3 decomposition. Wallace et al.,19 Chen et al.,20 Colaianni et al.18 and Shan et al.40 all showed that there are no PHx modes (where x ) 2 or 3) on the surface at this temperature using vibrational spectroscopies. This indicates that PH3 can only dissociatively adsorb at these temperatures resulting in little or no PH3 desorption. The results of Yoo et al.39 showed that the ratio of surface H to P dropped from the roomtemperature value of 3 to a value of 0.1 at 873 K. Shan et al.40 showed that the extent of PH3 reaction varied as a function of the dosing pressure. To make use of the method of Yoo et al.39 for determining the P concentration, the dosing pressure must be kept constant, or a new set of ratios must be developed for each dosing pressure. We elected to change both the dosing temperature and pressure to tailor the final concentrations. Thus we need to develop the means to calculate the P coverage for arbitrary dosing conditions. However, the amount of PH3 desorption (and thus the final P concentration at the time of P2 desorption) varies in a nonlinear manner in the course of our experiments. To avoid this difficulty, the following thermal pretreatment was performed. The Si(100) surface was exposed to PH3 at various temperatures. The surface was then heated to 873 K for 6 s and cooled to room temperature prior to TPD. A temperature of 873 K was chosen as it was above the highest dosing temperature thus ensuring that all surfaces had seen the same temperature. Quantification of the Surface Phosphorus Coverage. Figure 2A shows a typical AES spectrum of a P/Si (39) Yoo, D. S.; Suemitsu, M.; Miyamoto, N. J. Appl. Phys. 1995, 78, 4988-4993. (40) Shan, J.; Wang, Y. J.; Hamers, R. J. J. Phys. Chem. 1996, 100, 4961-4969.

P2 Desorption from PH3 Decomposition on Si(100)

Langmuir, Vol. 14, No. 6, 1998 1431

Figure 2. (A) Typical AES spectrum of a P/Si surface. (B) Series of AES spectra with phosphorus coverage increasing from left to right. The spectra correspond to P coverages of 0, 9.7, 20, 46, 60, and 77% P, respectively.

surface taken in the dN(E)/dE mode. Two prominent peaks are shown. The first occurs at 92 eV, which corresponds to the energy of a Si LMM process. The second peak occurs at 120 eV and is due to a P LMM process. The concentrations of both silicon and phosphorus can be calculated by calibrating the signals with a known standard.41 Figure 2B shows a series of spectra obtained by dosing the surface at various conditions. Each surface was then heated to a temperature of 873 K and cooled prior to the Auger analysis. The data shown were chosen to illustrate how the phosphorus signal increases with increasing phosphorus coverage and correspond to coverages of 0, 9.7, 20, 46, 60, and 77% P. The Si signal is seen to decrease as the phosphorus concentration increases. This is in good agreement with the results of Taylor et al.17 Equation 1 is the expression for quantitative AES

Y(t) ) Nx∆t σx e-(t cosθ/λ) I T dΩ/4π

(1)

analysis.42 YA is the number of Auger electrons produced from a thin layer of width ∆t at a depth t in the sample. Here Nx is the number of x atoms/unit volume, σx is the Auger cross section for element x, λ is the escape depth, θ is the analyzer angle, T is the transmission of the (41) Davis, L. E.; McDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division: Eden Prairie, MN, 1978. (42) Fundamentals of Surface and Thin Film Analysis; Feldman, L. C., Mayer, J. M., Eds.; Elsevier Science Publishing: Amsterdam, 1986.

analyzer, dΩ is the solid angle of the analyzer, and I is the electron flux. Equation 1 shows that the signal at the detector is a function of concentration as well as more subtle factors including the geometry of the experiment. The CMA has a specific acceptance angle. The electrons emitted from the surface have a cosine distribution. Slight changes in the distance from the CMA entrance to the crystal can result in greater or fewer electrons arriving at the detector. At least four AES measurements were made at distinct locations on the Si(100) crystal for every PH3 exposure. The purpose of recording multiple measurements at distinct locations was to get a better estimate of the surface coverage. Due to experimental constraints, only the angle of the crystal to the CMA could be kept constant. Attempts were made to keep the distance from the crystal to the CMA constant with each measurement; however, some uncertainty exists in this distance due to the orientation of the CMA with respect to the manipulator and the corresponding degrees of freedom in sample manipulation. The error in the measurement of the phosphorus concentration could be reduced by normalizing the absolute P signal (which is a function of the distance between the surface and the CMA) to the total signal from both the silicon and the phosphorus. Because this quantity is a ratio, the contributions from all the geometric factors are the same in the numerator and the denominator and thus cancel each other out. This quantity is then only a function of the P and Si concentrations. The Auger measurements were made using a 3 keV primary electron beam. These electrons have a penetration length of a few nanometers. The Auger electrons emitted by both the Si and P LMM transition (near 100 eV) have a mean-free path of approximately 5 Å.17 The signal arriving at the detector thus contains information from the first few monolayers, while only the surface concentration is relevant to TPD in this case. Analysis is further complicated by the fact that there is an exponential decay of the signal as a function of distance into the crystal, as shown in eq 1. All of the electrons generated on the surface within the acceptance angle of the CMA enter the analyzer. Due to screening effects from the topmost layers, only a fraction of the electrons generated in the bulk enter the CMA. The total current can be resolved into a contribution from the surface and from the bulk. The total Auger signal can be thought of as separate contributions from the surface and bulk

total AES signal ) P + Si ) surface + bulk ) (P + Sisurface) + Sibulk (2) where P and Si refer to the measured Auger signals. Provided that there is no subsurface phosphorus (or at least present only at a concentration below the sensitivity of the AES) and that the surface remains relatively flat (i.e., no three-dimensional island growth), the total number of atoms sampled by the beam and detected by the CMA (P + Si) will remain a constant. Thus the quantity P/(P + Si) is proportional to the P surface concentration. Figure 3A is a plot of P/(P + Si) vs the P signal (i.e., the peak to peak AES intensity). For convenience, the quantity P/(P + Si) will be referred to as the Pratio. The x and y error bars are the standard deviations of the P signal and Pratio for the multiple measurements made, respectively. Using a linear regression, the best fit of the data is given by

Pratio ) 0.0202 + (0.0205)P;

R2 ) 0.978

(3)

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Jacobson et al.

Figure 5. P2 desorption spectra from P/Si(100) surfaces having 0, 4.8, 21, 36, 42, 62, and 77% P. The R state is present at all coverages, the β and γ states develop at successively higher coverages.

Figure 3. (A) Normalized phosphorus concentration (Pratio) vs the P signal (peak to peak AES intensity). The P concentration is linear with the P signal. (B) Plot of the scatter in the measurements of both the Pratio and the P signal. The scatter is less for Pratio compared to P signal and decreases at higher values for both.

Figure 4. Pratio vs PH3 dose at various adsorption temperatures. The diamonds are from PH3 exposures at 298 K, triangles are from exposures at 773 K, and squares are from PH3 exposures at 873 K. The maximum value of Pratio increases with the temperature of PH3 adsorption.

Figure 3B is a plot of the scatter of the data in Figure 3A. The open circles represent the relative error of the P signal measurements, and the closed circles represent the error of the Pratio calculations. The average error in the P signal was 9.7 ( 4.5% and the average error in the Pratio was 5.3 ( 2.8%. Thus Pratio is a better statistical quantity than the absolute P signal. Phosphorus Surface Coverage vs Adsorption Temperature. Figure 4 is a plot of Pratio vs PH3 exposure taken at various adsorption temperatures. The diamonds were taken at room temperature, the triangles were taken at 773 K, and the squares were taken at 873 K. The results

are in general agreement with Yu et al. 12 and Kipp et al.21 The saturation coverage increases with adsorption temperature. The saturation value of Pratio was 0.16 for roomtemperature PH3 adsorption and 0.36 for PH3 adsorption at 873 K. Wang et al.24 performed STM analysis on what they termed a “phosphorus terminated surface”. The surface was prepared by exposing Si(100) to 20 langmuirs of PH3 at a temperature of 823 K. They reported a phosphorus to silicon Auger peak ratio of 0.57. This would correspond to a Pratio of 0.36. This agrees extremely well with our value of 0.36 obtained using the same dosing conditions. These researchers were able to distinguish between surface Si and P by changing the bias of the STM tip with respect to the surface.23,24 They determined that the surface contained a mixture of 54% P-P dimers, 43% P-Si dimers, and 3% Si-Si dimers, which corresponds to a P concentration of 77%. We used these results to quantify our coverages, and the proportionality between the Pratio and the coverage in monolayer was determined to be 2.1. Hence, a Pratio of 0.36 corresponds to a P coverage of 0.77 ML. P2 Desorption from PH3-Dosed Si(100) Surfaces. Figure 5 shows a series of P2 TPD spectra from PH3-dosed Si(100) surfaces with phosphorus coverages ranging from 0 to 77% of a monolayer. The surface layers were prepared by dosing the clean Si(100) surface with PH3 at either 298, 773, or 873 K. The coverage was measured by AES at 298 K prior to performing the thermal desorption measurement. At the lowest coverage a single desorption state is seen which grows with increasing coverage. We will refer to this feature as the R state. Qualitative inspection of this feature shows it growing with increasing P coverage and eventually saturating. As the phosphorus coverage increases, a second desorption feature becomes prominent which will be referred to as the β state. The β state begins as a slight asymmetry in the R state at low coverage. As the coverage increases, it develops into a shoulder on the leading edge of the higher temperature R state and finally becomes the dominant desorption feature at the highest coverages. A third state, γ, develops at high coverages. The peak temperatures are lower for each successive new desorption state. The TPD spectra shown in Figure 5 were fit by either one (R), two (R, β), or three (R, β, γ) individual features using Lorentzian fits (as a matter of convenience). The

P2 Desorption from PH3 Decomposition on Si(100)

Langmuir, Vol. 14, No. 6, 1998 1433 Table 1. Chan, Aris, and Weinberg Parameters Obtained from Analysis of the r-P2 Desorption State from Si(100) Surfaces Containing 4.8, 21, and 36% P

Figure 6. P2 desorption spectrum for 77% P. The circles are the data and the solid line is the best fit. The best fit is determined by summing the individual R, β, and γ features (dashed lines). The residuals (i.e., the difference between the data and the fit) are shown beneath the spectrum.

Figure 7. Results from the best fits of the spectra in Figure 5. (A) Peak areas for the R state (circles), β state (triangles), γ state (diamonds), and total (squares). (B) Peak desorption temperatures for the R state (circles), β state (triangles), and γ state (squares).

peak area, width, and peak maximum temperature for each feature were allowed to vary until the residuals were minimized. Figure 6 shows a sample fit for the 77% P surface. The sum of the three features (solid line) needed to fit the raw data (solid circles) is shown, along with the individual peak fits (dashed lines) and difference between this sum and the raw data (bottom line in Figure 6). Figure 7A shows the area of all of the spectra shown in Figure 5 as calculated by the Lorentzian fit as described above. The R state indeed saturates at a coverage of about 60% of a monolayer. The β state begins to populate at coverages above 20%, while the γ state populates at coverages greater than 42%.

θP (%)

Ea (kcal/mol)

ν

4.8 21 36

96 70 61

3.5 × 1019 1.4 × 1014 4.2 × 1011

Figure 7B shows the temperature shifts of each state as a function of P coverage. Both the R and β states shift to lower temperatures with increasing coverage, a behavior characteristic of second order processes. The observed shifts are -65 K and -55 K for the R and β states, respectively. The γ state peak temperature increases with coverage, behavior that is inconsistent with either firstor second-order dependence on the P coverage and implies a more complicated process. The total integrated peak area for all three desorption states (i.e., the sum of the R, β and γ states), shown in Figure 7A, is linear with the calculated P coverage. This suggests that most (if not all) of the phosphorus atoms remain on the surface. However, Taylor et al.17 reported that some phosphorus diffuses from the Si(111) surface into the near surface region upon heating to 875 K. The phosphorus could then resurface upon further heating and desorb as P2. If this occurred on the Si(100) surface as well, it could explain the linearity of Figure 7A as well as the scatter in Figure 3B. However, short of performing depth-sensitive measurements such as static secondary ion mass spectrometry or a combination of sputtering and AES, we cannot quantify the actual concentration gradients perpendicular to the surface. For the purpose of this paper, we will use the effective concentration as measured by AES as the surface concentration. Desorption Energetics. The temperature shifts can be used to calculate the desorption energy provided that the desorption order is known. The desorption peak temperatures for both the R and β states shift to lower temperatures with increasing P coverage, consistent with second-order desorption kinetics. Similar behavior was seen in the results presented by Taylor et al.17 on Si(111). Taylor et al. reported a value for the single P2 desorption state seen on the Si(111) surface of 87 kcal/mol with a prefactor of 102(1 cm-2 s-1 using the method of Chan, Aris, and Weinberg (CAW).32 When the surface area for the Si(111) surface (3 × 1014 dangling bonds/cm2 43 ) is taken into account, the preexponential factor is on the order of 1016(1 s-1. The CAW method makes use of the initial surface concentration, using the peak maximum temperature and peak width to extract information about the kinetics responsible for simple desorption spectra. We performed a Chan, Aris, and Weinberg analysis for P2 desorption, and the results are shown in Table 1. Both the activation energy and the preexponential factor appear to decrease for the R-P2 desorption state with increasing P coverage. Ning and Crowell37 saw a similar trend for hydrogen desorption from Ge/Si surfaces. They argued that the trend was due to the fact that the CAW method proves to be inadequate for describing systems in which there are overlapping desorption states. Because the analysis relies on the energy spread of the desorption state, the “true” width of the peaks must be known. In all but the lowest P coverage, there is overlap between the R and β states. The most reliable values for both the activation energy and the preexponential factor would then be the ones (43) Wolkow, R.; Avouris, P. Phys. Rev. Lett. 1988, 60, 1049.

1434 Langmuir, Vol. 14, No. 6, 1998

Jacobson et al.

excluded from the analysis. When a linear regression is performed on the remaining points for the R state, the following is obtained

ln

Figure 8. Second-order Redhead analysis of the R and β states. The activation energies are proportional to the slopes for 1/(σ0Tm2) vs 1/Tm.

derived from analysis of the 4.8% P surface. The activation energy calculated from this desorption spectrum is 96 kcal/ mol. The Redhead method for calculating kinetic parameters from TPD data is less sensitive to the line shape than the CAW method. For second-order kinetics, the quantitative equation developed by Redhead33 is

Ed RTm2

)

(

)

σ0ν Ed exp β RTm

(4)

Here Ed is the desorption energy, R is the gas constant, Tm is the desorption peak maximum temperature, σ0 is the initial concentration, ν is the preexponential factor, and β is the scan rate. This equation can be linearized to yield the following relationship

ln

( ) [( )( )] ( ) 1 R ν ) ln 2 E σ0Tm d β

-

Ed 1 R Tm

(5)

Figure 8 is a plot of ln(1/σ0Tm2) vs 1/Tm using data from Figure 7B. The desorption energy can be calculated from the slope of the best-fit line, and the intercept is related to the prefactor, ν. Calculation of the best-fit line is straightforward for the β state. Regression analysis yields

ln

( )

( )

1 1 ) (17) - (28000) 2 T σ0Tm m

(6)

with a correlation coefficient R2 ) 0.985. Analysis for the R state is more complicated. The above relationship assumes that the peak maximum temperature continues to decrease as the concentration increases. This relationship is no longer valid if this is not the case. Figure 7B shows that at the highest coverage (77%) there is basically no change in the temperature of the peak maximum of the R state from that of 62% P. This represents a deviation from second-order kinetics. However, Figure 7A shows that there is no change in the area either. This indicates that the state has saturated and, thus, the data point which corresponds to 77% P must be

( )

( )

1 1 ) (22) - (36000) 2 T σ0Tm m

(7)

with a correlation coefficient R2 ) 0.965. This yields desorption energy values of 72 and 57 kcal/mol, and preexponentials of 1 × 1012 and 5 × 1014 for the R and β states, respectively. These values agree somewhat with the values for the activation energy and preexponential factor as calculated by the CAW method for a coverage of 21% P. The fact that multiple states appear for the Si(100) surface and only one appears for the Si(111) surface could be the result of different behavior on the two distinct surface reconstructions. If we assume that the peak shown by Taylor et al.17 for Si(111) was the R state, then our values for the Si(100) surface are slightly lower than those observed for Si(111). Conclusions Using a combination of TPD and AES, we have characterized the desorption of P2 from the Si(100) surface following PH3 adsorption. We have developed a new means to calibrate the surface phosphorus coverage that is insensitive to losses due to the desorption of PH3. This method has the advantage that additional deposition or etching precursors can be exposed to the P/Si(100) surface. Reactions on these surfaces can be compared to those on the clean Si(100) surface to determine the effects of the surface phosphorus on parameters such as desorption and growth rate. We have used P/Si surfaces similar to those described here to study the effect of phosphorus on hydrogen desorption44 and on disilane adsorption and decomposition.45 At the relevant industrial growth temperatures, the phosphorus would be incorporated into surface dimers. Thus the surfaces that are prepared in this manner would better approximate chemical vapor deposition processing conditions. P2 desorption from the Si(100) surface exhibits three distinct desorption features. An R feature appears instantaneously, which displays second-order kinetics. A β state appears at coverages of around 21% of a phosphorus monolayer. This state also displays second-order behavior. Finally, a γ state appears at coverages greater than 42% of a phosphorus monolayer. Analysis of the desorption spectra yields desorption energy values of 72 and 57 kcal/mol, respectively, for the R and β P2 desorption states. Acknowledgment. We gratefully acknowledge support of this research by the Office of Naval Research. M.L.J. also acknowledges support by the Stacey Memorial Fellowship Program. This work is dedicated to the memory of Professor Brian E. Bent. His scientific creativity and intellectual passion inspired us, while his genuine warmth, kindness, and generosity of spirit defined the best in human decency. LA970795W (44) Jacobson, M. L.; Chiu, M. C.; Crowell, J. E. In preparation. (45) Jacobson, M. L.; Chiu, M. C.; Crowell, J. E. In preparation.