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J. Phys. Chem. B 2002, 106, 352-355
Island Formation at the Initial Stages of Epitaxial ZnS Films Grown by Single Source Chemical Vapor Deposition Nguyen H. Tran and Robert N. Lamb* Surface Science and Technology, School of Chemistry, UniVersity Of New South Wales, Sydney NSW 2052, Australia ReceiVed: August 1, 2001; In Final Form: October 12, 2001
Chemical vapor deposition of single-source precursor zinc diethyldithiocarbamate Zn[S2CN(C2H5)2]2 and subsequent decomposition on heated Si(111) has been shown to produce epitaxial ZnS films. During the initial growth studies, X-ray photoemission spectroscopy indicated a relatively high concentration of carbon at the interface, which decreased with increasing film thickness. The interfacial carbon is attributed to chemisorption of byproducts during precursor decomposition. The higher than expected binding energy of Zn 2p3/2 for the ultrathin films (∼5 Å) approached the bulk ZnS value as film thickness increased (∼2000 Å). This was ascribed to changes in crystallite size, which resulted in different core-hole screenings. The combined results are related to a kinetic process in which various carbon-terminated sites on Si(111) surface inhibited the two-dimensional coalescence of ZnS clusters. The films were initially grown via formation of epitaxial three-dimensional islands. Our results suggest that while the precursor chemistry and associated byproduct concentration can significantly influence the growth mechanism, the epitaxial driving force is sufficiently strong to overcome such chemical defects at the interface.
Introduction Single source chemical vapor deposition (SS CVD) is a simple technique for growing a range of high quality thin film ceramics. During SS CVD growth, an organo-metallic precursor is decomposed at a substrate surface leaving behind the film constituents while the volatile carbonaceous byproducts are readily removed from the system. Recently, SS CVD has been exploited for the production of epitaxial ZnS semiconductor films on lattice matched Si(111) with the aim of integrating semiconductor thin film properties with well-established silicon technology.1 In this paper, we are concerned about the precursor decomposition and impurity status at the initial stages given the epitaxial nature of the growth. In particular, we used X-ray photoemission spectroscopy (XPS) to analyze in situ thin and ultrathin epitaxial ZnS films grown on Si(111) using SS CVD of zinc diethyldithiocarbamate precursor. Experimental Procedure Details of the single source CVD growth procedure have been described previously.1,2 Film growth was carried out utilizing optimized growth conditions with the substrate and source temperature set to ∼400 and 130 °C, respectively. Zinc diethyldithiocarbamate (Zn[S2CN(C2H5)2]2, AR grade) (Figure 1) was sublimed from a resistively heated Knudsen cell at a partial pressure ∼2 × 10-6 Torr. Deposition chamber was attached to a sample preparation chamber (pressure ∼3 × 10-8 Torr) of the electron spectrometer. The in situ X-ray photoelectron spectroscopy (XPS) experiments were performed using a Kratos XSAM-800 spectrometer with an Al KR X-ray source (hν ) 1486.6 eV) and electron energy analyzer pass energy of 20 eV. The base pressure of the main analysis chamber was ∼3 × 10-9 Torr. * Corresponding author. Email:
[email protected].
Figure 1. Molecular structure of zinc diethyldithiocarbamate Zn[S2CN(C2H5)2]2 single-source precursor.
The preparation of Si(111) surfaces involved three cycles of H-passivation and reoxidization using buffered NH4F and standard H2O2 solution respectively.3,4 The final H-passivation was carried out by immersing Si wafer into 5% HF solution. The wafer was then rinsed thoroughly using absolute ethanol,5 immediately inserted into the XPS chamber and subsequently heated to 350 °C for 10 min. This final thermal treatment is aimed at the removal of adventitious surface contaminants. Results and Discussion Figure 2 shows the XPS region scans recorded at the polar angle θ ) 70° for a ∼5 Å thick film. (In this experiment, θ was determined by the electron takeoff axis and the sample surface normal.) The Zn 3p3/2 and Zn 3p1/2 features shown in Figure 2a have binding energy of EB ) 89.9 eV and EB ) 92.3 eV, respectively. The S 2p peak shown in Figure 2b has a binding energy EB of 161.7 eV. The Si 2p peak from the underlying Si(111) substrate (Figure 2a) was undetectable when film thickness reached ∼90 Å. The Zn 3p and S 2p XPS results are in agreement with those of ZnS references.6 Figure 2c shows the C 1s peak of the residual carbon in the films (EB ) 284.5 eV), which is related to the aliphatic hydrocarbons, and which predominantly resulted from the precursor decomposition byproducts.2 In addition, the results of depth-profile composition analysis for a 2000 Å thick film grown under identical conditions indicated that the total atomic concentration of carbon in the bulk of the films was approximately 2 at % (zinc diethyldithiocarbamate single precursor contains ∼40 at. % carbon).
10.1021/jp012955p CCC: $22.00 © 2002 American Chemical Society Published on Web 12/14/2001
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Figure 2. XPS region scans for (a) Zn 3p, (b) S 2p, and (c) C 1s photoelectrons recorded from a ZnS ultrathin film grown on Si(111).
Figure 3. Variation of Zn 3p, S 2p, and C 1s XPS intensity as a function of film thickness.
Figure 4. Variation of film average thickness with growth time.
At the early growth, however, the carbon concentration in films was relatively high, as shown in Figure 3. Initially, the C 1s intensity decreased exponentially and above 20 Å, the carbon concentration appeared to be less dependent on film thickness. Variation of carbon contents, however, did not alter the Zn:S intensity ratios, which remained almost unchanged (see Figure 3). We have previously shown that high carbon contents were associated with the incomplete decomposition of the precursor, which occurred only when the substrate temperatures were kept lower than 250 °C.2 The results in Figure 3 suggest that high carbon contents at the early stages, especially within the monolayer regime, are due to high sticking probabilities of these carbon on Si(111) surface and chemical interactions between them, as will be discussed below. Direct fragmentation of zinc diethyldithiocarbamate in the gas phase at 400-500 °C has been described previously:7
In addition, the sharp decrease of C 1s intensity between 7 and 12 Å would indicate that in this thickness region, a saturated coverage of chemisorbed carbon was almost achieved. Above this region, it is expected that the smaller amount of residual carbon would be incorporated into the ZnS lattice or would segregate to the grain boundary of the ZnS crystallites. As described above, the S:Zn atomic ratio was not affected by the relatively large carbon concentration at the interface. Residual carbon may influence other aspects of film growth. For example, as shown in Figure 4, the variation of film thickness with the growth time suggests that the growth rate was relatively slow at the early stages. This is consistent with chemisorbed carbon reducing the sticking probabilities of the subsequently adsorbed ZnS molecules at the surface. The core-level electron binding energies reflect the chemistry but is especially for ultrathin films also influenced by various crystallographic properties such as crystallographic strain, cluster sizes, and electrical properties.12-15 To understand the initial film growth stages in more details, we analyzed the XPS core electron binding energy as a function of film thickness. The results indicated that the Zn 2p3/2 photoelectron peaks were shifted toward lower binding energies with increasing film thickness (up to ∼2000 Å, see Figure 5). Similar energy shifts were also observed for the S 2p and Zn LMM Auger features. The Zn 2p3/2 binding energy decreased from 1021.65 to 1021.34 eV for film thickness increasing from ∼5 to 2000 Å (Figure 6). In general, the binding energy shifts in core-level electron spectroscopy are attributed to a variety of mechanisms related to initial-state and final-state effects.12-15 The observed binding energy shifts of the Zn 2p3/2 signals are interpreted as related to final-state effects (core-hole screening or cluster-charge). In larger film clusters, core holes can be screened more effectively than in smaller ones,14,15 which influences the Coulomb interaction between the emerging photoelectron and the residual charge left on the cluster. (The magnitude of the binding energy shift depends on, for example, the density and mobility of free electrons.) This could indicate
Zn[S2CN(C2H5)2]2 f ZnS + C2H5NCS + (C2H5)3NCS2 (1) Secondary fragmentation of the byproducts is likely to result in:
(C2H5)3NCS2 f (C2H5)2NH + CS2 + C2H4
(2)
Simultaneously with the adsorption of ZnS molecules at the initial growth stages, these fragments would be chemically adsorbed (chemisorption) on Si(111) surface. Secondary-ion mass spectrometry (SIMS) for the ultrathin films has indicated the presence of C2H5N+ (m/z ) 43) and SiC2H5+ (m/z ) 57) ions in which the latter was probably as a result of the residual ethylene C2H4 chemisorption. In agreement with these results, extensive characterization studies of the C2H4 chemisorption on Si surface have shown the formation of the -SiCxHy bond species.8-11 The C2H4 molecules have relatively high sticking probabilities on Si and chemical reaction between them involved the redistribution of π-electrons within the CdC double bonds.
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Figure 5. Charge-corrected Zn 2p3/2 binding energy signals at film thickness of (a) ∼5, (b) 8, (c) 12, (d) 23, (e) 81, and (f) 2000 Å.
Figure 7. Comparison between the calculation and XPS results of the film-to-substrate intensity ratios for ZnS on Si at different film thickness.
Figure 6. Zn 2p3/2 binding energy as a function of film thickness.
that, at the early stages, the average film crystallite sizes increased with increasing film thickness, which would be in agreement with three-dimensional growth. These results have shown that there is a correlation between the size of film crystallite and the amount of chemisorbed carbon on Si(111) surface. Residual carbon is trapped at various sites of the surface and therefore reduced the number of Si dangling bonds. This resulted in the formation of the relatively small ZnS crystallites on the surface at the early growth stages. The carbon-terminated sites would inhibit the entire two-dimensional coalescence at the interface, and alternatively, give rise to a three-dimensional growth where the crystallites grow larger and form islands. (The nucleation and growth are similar to those for the systems with a large lattice-mismatch, where islands are formed to relieve mismatch strain between film and substrate.) To obtain further information about the model of growth, we compared the variation of photoelectron intensities for various films deposited with those calculated for a uniform thin film with layer-by-layer growth. Figure 7 shows the S 2p/Si 2p intensity ratios as a function of polar angle θ for various film thicknesses. The dashed curves of Figure 7 show the theoretical plots of the intensity ratios for a completely covered ZnS film on Si. These ratios were calculated using the following relationship:16
()
( )
If,d If,o ) 1Is,d Is,o
( (
) )
-d λf sin θ -d e λs sin θ e
where If,d and Is,d are the S 2p and Si 2p intensities for a film with thickness d, If,o is the S 2p intensity from an infinitely thick film, and Is,o is the Si 2p intensity for a clean substrate, and λf and λs are the attenuation lengths for the S 2p and Si 2p photoelectrons, respectively. The ratio of the absolute intensities If,o/Is,o was approximately 0.94 measured using clean ZnS films and Si(111).
The measured intensity ratio curve for the 5 Å thick film exhibits significant deviations from the theoretical data for the corresponding thickness and indicated that the actual growth mode was not completely layer-by-layer. In fact, the relatively low intensity ratios recorded for the film indicated that the initial coverage of the Si surface plane is associated with a fraction of ZnS less than that originally required for a complete 5 Å ZnS coverage. These results are another indication for the continuous surface coverage where carbon and ZnS molecules were chemisorbed onto Si(111) surface atoms, which resulted in the formation of ZnS islands. Increasing the film thickness, however, leads to a change of the growth mode to two-dimensional nucleation. At film thickness greater than 12 Å, the layer-by-layer growth of ZnS films on Si(111) was almost achieved. This was indicated by the measured intensity ratios being nearly identical to those calculated, especially at low polar angles. Furthermore, the measured intensity ratios exhibited a significant decrease at high polar angles, which is analogous to the theoretical data. The carbon chemisorption initially altered the nucleation and growth rate of ZnS film crystallites on Si(111). It is interesting that, despite high carbon contents, films with crystallographic ordering were obtained at the early stages.1 Angle-dependent X-ray photoelectron diffraction (XPD) measurements have indicated that the films were of sphalerite (cubic) structure with [111] orientation. Figure 8 shows the Zn 2p3/2 and S 2p photoelectron intensity distribution for variously thick films, recorded as a function of polar angle. The diffraction patterns were consistent with cubic (111) stacking geometry, which resulted in enhanced Zn 2p3/2 intensity and substantially suppressed S 2p intensity at θ ) 0°. Thus, film growth and ordering occurred via a subsequent stacking of S and Zn latticeplanes perpendicular to the growth direction. (In the cases with a large mismatch, stacking faults occur in order to relieve mismatch strain and give rise to formation of disordered films.) The well-defined stacking sequence observed at various levels of film thickness can be attributed to the following: (i) below a certain level of thickness where island growth is dominant, stacking is initiated directly from Si surface, which produces epitaxial forces to accommodate the subsequent lattice planes, and (ii) with increasing thickness carbon chemisorption leads to formation of mixed C and ZnS islands in which subsequent
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J. Phys. Chem. B, Vol. 106, No. 2, 2002 355 the growth mechanism, it suggests that the driving force for film structuring is sufficiently strong to overcome such chemical defects at the interface. Acknowledgment. The authors thank Drs. A. Hartmann and A. Buckley for stimulating discussions. References and Notes
Figure 8. Intensity distribution of (1) Zn 2p3/2 and (2) S 2p photoelectrons for ZnS films with thickness at (a) ∼5, (b) 12, and (c) 2000 Å. Figure 1.1d,1.2d shows the intensity distribution of Zn 2p3/2 and S 2p for the 2000 Å film after Argon etching.
growth occurs preferentially on the ZnS island sites and therefore stacking faults are minimized. Conclusion Single source CVD of epitaxial ZnS films has produced hydrocarbon byproducts, which are randomly distributed on Si(111) substrate surface. The byproduct chemisorption led to formation of carbonaceous islands which kinetically limited the two-dimensional coalescence of ZnS clusters. Layer-by-layer growth was therefore inhibited despite the fact that single crystalline films resulted. While the precursor chemistry and associated byproduct concentration can significantly influence
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