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J. Phys. Chem. B 1997, 101, 11180-11184
Modulation of Cadmium Selenide Photoluminescence Intensity by Adsorption of Silapentanes and Chlorinated Silanes R. J. Brainard,† C. A. Paulson,† D. Saulys,† D. F. Gaines,† T. F. Kuech,‡ and Arthur B. Ellis*,† Department of Chemistry and Department of Chemical Engineering, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed: August 4, 1997; In Final Form: October 13, 1997X
The band gap photoluminescence (PL) intensity of n-CdSe single crystals is reversibly enhanced by adsorption of seven of the nine silapentane isomers, SiC4H12, from the gas phase onto the (0001) surface of this substrate; only tetramethylsilane and tert-butylsilane elicit no PL response. The adsorbate-induced increase in PL intensity is consistent with the species acting as Lewis bases toward the surface. Quantitatively, the PL changes can be fit by a dead-layer model, allowing estimation of the adduct-induced reduction in depletion width as ranging from ∼100 to 500 Å. Adduct formation constants, estimated from fits of the concentration-dependent PL changes to the Langmuir adsorption isotherm model, range from ∼10 to 103 atm-1 and are ∼102 atm-1 for most of the isomers. Two chlorinated silanes, H2SiCl2 and HSiCl3, yielded reversible PL quenching after an initial conditioning period, although the magnitude of the quenching was not in accord with the dead-layer model. Possible surface binding modes are discussed for these silanes, as are implications for their on-line detection as precursor molecules for growth of semiconductors by chemical vapor deposition (CVD).
Introduction We and others have observed that the photoluminescence (PL) intensity from semiconductors such as n-CdSe can be modulated by the adsorption and desorption of solution and gaseous analytes; typically, adsorption of Lewis acids and bases onto the surface of n-CdSe causes quenching and enhancement of PL intensity, respectively, relative to a reference ambient, permitting detection of a wide variety of gas- and solution-phase species.1 The PL effect appears to derive from adduct-induced modulation of the semiconductor’s surface depletion region. Dangling bonds at the semiconductor surface provide states within the band gap, which in an n-type semiconductor are filled to the Fermi level with electrons from the bulk. The resultant electric field separates photogenerated electron-hole pairs, preventing their radiative recombination in this region. Assuming the nearsurface depletion region to be nonemissive is the basis for the “dead-layer model” used to quantitatively interpret adductinduced PL changes: When a Lewis acid adsorbs, electron density is drawn to the surface, causing an increase in deadlayer thickness and a corresponding decrease in PL intensity; conversely, adsorption of a Lewis base shifts electron density back to the bulk semiconductor, causing a decrease in deadlayer thickness and increasing PL intensity. We have shown that transducing films of metal-containing complexes deposited onto the semiconductor can extend this methodology to embrace species that do not otherwise elicit a PL response from the CdSe substrate.1,2 When the PL response is reversible, on-line detection strategies are feasible and have been explored for precursor molecules used in the growth of materials by chemical vapor deposition (CVD). Past research has included the study of adsorption onto CdSe of CVD precursor gases such as boranes and group 15 hydrides.3 We report herein a study of a complete family of nine silapentane isomers, shown in Chart 1, that * Author to whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Chemical Engineering. X Abstract published in AdVance ACS Abstracts, November 15, 1997.
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CHART 1: Silapentane Isomers and Their 1 atm Boiling Points (Taken from Ref 4a)
permits a systematic investigation of steric and electronic effects on binding to CdSe. Two of the molecules, tert- and secbutylsilane, are known to decompose to form silane gas, a common CVD precursor gas used in the microelectronics industry, and tert-butylsilane has been used as a source of silicon dopants during the growth of GaAs films.4,5 Other commonly used silicon-delivering CVD precursor gases include H2SiCl2 and HSiCl3, both of which are included in the present investigation. We report in this paper that silapentane-induced enhancements in CdSe band-edge PL intensity are readily observable for seven of the nine isomers investigated and can be fit to a dead-layer model, permitting an estimation of adduct-induced contractions in the semiconductor’s depletion width. Equilibrium constants for the binding of this family of compounds to CdSe have been estimated using the Langmuir adsorption isotherm model, and a binding scheme reflecting variations in isomer properties is proposed. For comparison, H2SiCl2 and HSiCl3 have also been examined, and after causing an initial irreversible PL enhancement, these molecules are found to yield reversible PL quenching relative to a reference ambient, albeit not in accord with the dead-layer model. © 1997 American Chemical Society
Modulation of Cadmium Selenide PL Intensity
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11181
Experimental Section Materials and Sample Preparation. Single crystals of vapor-grown, c-axis-oriented n-CdSe with resistivities of 2 Ω cm were obtained from Atramet Crystals (Farmingdale, NY) and Cleveland Crystals (Cleveland, OH). Prior to use, the crystals were polished and etched in a 1:15 Br2/MeOH (v/v) solution until the shiny Cd-rich side of the crystal was revealed. They were then rinsed in methanol, sonicated, and dried under a flowing stream of nitrogen gas. No significant differences in PL characteristics were apparent for crystals obtained from the two sources. Dimethylethylsilane (98%), trichlorosilane (99%), n-pentane (99%), 2-methylbutane (99%), and tetramethylsilane (99.9%) were purchased from Aldrich Chemical; diethylsilane (97%) was obtained from Fluka. Both diethylsilane and dimethylethylsilane were twice distilled before use. Detailed syntheses and purification procedures of the other silapentanes have been previously described.4a All liquid reagents were handled with Schlenk techniques or in a N2-filled glovebag to exclude oxygen and water. Dichlorosilane gas (97% pure) was purchased from Aldrich and used without further purification. Apparatus. Gas handling for the silapentane molecules was performed on both stainless steel and glass vacuum lines, which have been described previously.3a The chlorinated silanes were handled only on the stainless steel line. Trichlorosilane, all silapentanes, n-pentane, and 2-methylbutane are liquids at room temperature and were held in glass bulbs; H2SiCl2 is a gas at room temperature and was contained in a high-pressure gas cylinder. CdSe crystals were positioned between two Teflon spacers within a glass cell that permitted sample excitation and PL collection. During PL experiments, the sample cell and analyte containers were connected to a vacuum line with flexible stainless steel tubing. The CdSe crystals could be exposed to vapors of these silanes at up to the room-temperature vapor pressure, which was at least 200 Torr for all of the compounds investigated.6 In addition to handling H2SiCl2 on the stainless steel vacuum line, a gas flow apparatus was constructed that permitted exposure of the crystal to flowing mixtures of dry N2 and H2SiCl2 gases while the sample was being illuminated.2a Fluoroelastomer tubing (VWR Scientific) was used to contain the flowing gases; gas partial pressures were controlled by adjusting the flow rates of the component gases. Total flow rates ranged from 100 to 180 mL/min, and total gas pressure was 1 atm. Optical Measurements. Semiconductor excitation was provided by Coherent I-90 Ar+ (457.9 and 514.5 nm) and Spectra-Physics He-Ne (632.8 nm) lasers. Incident laser intensities ranged from 1 to 20 mW/cm2. Band-edge PL (λmax ) 720 nm) was directed into an Oriel Instraspec II Si photodiode array spectrophotometer with an optical fiber and passed through a red cutoff filter to exclude background light. Output from the array was computer processed so that both the PL spectrum and the intensity at a particular wavelength (typically the PL band maximum) could be monitored simultaneously. Under the low spectral resolution (0.5 nm) conditions of these experiments, the PL spectral distribution was found to be unaffected by surface adduct formation. All experiments were performed at 295 K. NMR. Silicon-29 NMR spectra were recorded on a Bruker AM-500 spectrometer at 99.363 MHz and referenced to an external sample of tetramethylsilane in deuterated methylene chloride. Negative chemical shifts are at a lower frequency than (i.e., shielded from) the reference. The butylsilanes were diluted to concentrations between 0.80 and 0.90 M in CD2Cl2 and flame sealed under vacuum in 5 mm tubes. Surface Analysis. X-ray photoelectron spectra (XPS) were
Figure 1. Changes in PL intensity, relative to a vacuum reference ambient, of an etched n-CdSe crystal due to exposure to dimethylethylsilane at the indicated pressures. The crystal was excited with 457.9 nm light, and PL was monitored at 720 nm.
obtained via a Perkin Elmer PHI 5400 instrument with a Mg X-ray source. Dichlorosilane-treated CdSe samples were prepared in the following manner: Crystals were etched with Br2/MeOH as described above, then placed in a glass sample cell, and exposed to 400 Torr of H2SiCl2 gas for 10 min. The sample cell was then evacuated for ∼15 min before the sample was removed and placed in an airtight container for transport to the XPS instrument. The treated CdSe sample was then loaded into the XPS instrument for analysis. Results and Discussion Etched single crystals of n-CdSe emit red band-edge PL (Eg ≈ 1.7 eV; λmax ≈ 720 nm) when excited by ultra-band-gap light at room temperature. The PL intensity is affected by most of the silapentane isomers and by both chlorinated silanes. In sections below we characterize the PL enhancements caused by some silapentanes, estimate their equilibrium binding constants for adduct formation, discuss possible steric and electronic contributions to binding, and present corresponding data for the chlorinated silanes. The final section explores implications for on-line sensing of these species. PL Enhancements from Silapentanes. When a CdSe crystal is exposed to seven of the nine silapentanes shown in Chart 1, the PL intensity is reversibly enhanced relative to a vacuum baseline; only the tetramethylsilane and tert-butylsilane isomers do not significantly affect the PL intensity. Figure 1 illustrates a typical PL intensity modulation, relative to a vacuum ambient (∼10-3 Torr), when an etched n-CdSe crystal is exposed to dimethylethylsilane. The seven isomers that induced responses yielded reversible PL enhancements ranging from ∼10% to 100%, with rapid recovery of the baseline PL intensity within 5 min of evacuation of the sample cell. The PL enhancements observed upon gas adsorption onto CdSe are consistent with the silapentanes acting as Lewis bases toward the surface, presumably through hydridic Si-H bonds, as described below. The magnitude of the PL changes can be treated quantitatively using the aforementioned dead-layer model, which assumes that a region on the order of the depletion width is nonemissive. The quantitative form of the dead-layer model is given by the following equation:7
PLref/PLx ) exp(-R′∆D)
(1)
where PLref is the PL intensity in a reference ambient, PLx is the PL intensity in the presence of the silapentane, R′ ) R + β is the sum of the semiconductor’s absorptivities for exciting
11182 J. Phys. Chem. B, Vol. 101, No. 51, 1997
Brainard et al.
Figure 2. Maximum values of the dead-layer contraction, ∆D, induced by adsorption of the indicated silapentanes onto an etched n-CdSe crystal and calculated from PL enhancements using eq 1. The entries for diethylsilane and isobutylsilane show the dead-layer contraction caused by 225 and 270 Torr of these gases (their vapor pressures at 295 K), respectively; the PL enhancements were not yet saturated at these pressures. The same CdSe sample was used for all of these experiments; however, the crystal was etched after exposure to each gas, so comparisons of ∆D values between gases cannot be made. The excitation wavelengths used to produce the observed dead-layer changes are shown in the legend, and horizontal lines at the right-hand side of each bar indicate the corresponding error associated with the estimate of ∆D.
and emitted radiation, and ∆D ) Dref - Dx is the change in dead-layer thickness upon adsorption. An assumption of the model is that the surface recombination velocity S either is very large before and after adduct formation or is unaffected by adsorption (S . L/τ and S . RL2/τ, where L and τ are the minority (hole) diffusion length and lifetime, respectively). The dead-layer model predicts that ∆D values calculated using eq 1 should be independent of incident wavelength. A test of the model is to compare ∆D values measured using different laser excitation lines. Values of ∆D were calculated for 458, 514, and 633 nm illumination, with absorptivities for CdSe at these wavelengths spanning a factor of 3.8 Except for isobutylsilane, good fits to the model were obtained for the silapentanes that produce a response, as demonstrated in Figure 2. The data in Figure 2 were not all obtained on the same surface, and there can be considerable variability between surface preparations (compare data in Figures 2 and 4, for example). This presumably reflects differences in morphology and/or the presence of surface impurities, caused by etching and handling procedures, as discussed below. Calculated values of ∆D typically ranged from ∼100 to 500 Å. Concentration Dependence. As exemplified by Figure 1, the PL intensity enhancements due to silapentane vapor increase with pressure until a saturation intensity is achieved. Typically, the onset of a PL response is seen at about 1-10 Torr, and saturation occurs near 50-200 Torr. Adduct formation constants can be estimated from such data using the Langmuir adsorption isotherm model.9 The model has the following quantitative form:
θ ) KP/(1 + KP) or 1/θ ) 1 + 1/(KP)
(2)
where θ is the fractional surface coverage, P is analyte pressure, and K is the equilibrium binding constant. PL data can be used to estimate θ between its limits of zero in the reference ambient (PLref) and one at analyte concentrations corresponding to the saturation of the PL enhancement (PLsat). Assuming an
Figure 3. Plot of fractional surface coverage θ, calculated using eq 3, vs pressure of dimethylethylsilane (DMES), using the data from Figure 1. The inset shows the double reciprocal plot of the data; the linearity demonstrates a good fit to the Langmuir adsorption isotherm model, yielding an equilibrium binding constant of 900 ( 100 atm-1. The incident wavelength used was 458 nm.
TABLE 1: Summary of PL-Derived Equilibrium Binding Constants
a Equilibrium binding constants for adsorption of the indicated compound onto CdSe, obtained from fits to the Langmuir adsorption isotherm model, eq 2. Samples of CdSe were etched before exposure to each of these isomers, permitting only rough comparisons of binding constants. The excitation wavelength was 458 nm and the error was (20%. b PL enhancements had not saturated at the vapor pressures of these silanes at 295 K, so values are estimated upper limits.
unpinned interface, θ is taken to be the fractional change in ∆D at intermediate concentrations, eq. 3:
θ ) ln(PLref/PLx)/ln(PLref/PLsat)
(3)
From eq 2, a double-reciprocal plot of 1/θ vs 1/P yields a straight line with an intercept of unity and a slope of the reciprocal binding constant. Such a plot is shown as an inset in Figure 3 for dimethylethylsilane; the extracted equilibrium binding constant is ∼900 atm-1. Table 1 summarizes the binding constants obtained in this manner for the seven silapentane isomers producing PL enhancements. Because the CdSe samples were freshly etched before exposure to each of these isomers, the Table 1 data permit only rough comparisons of binding constants. Most values are on the order of 100 atm-1. The exceptions are isobutylsilane and diethylsilane, having values of
