5713
J. Phys. Chem. 1993,97, 5713-5716
Photoluminescence as a Probe of the Adsorption of Gaseous Boranes onto the Surface of Cadmium Selenide Crystals Donald R. Neu, Joel A. Olson,and Arthur B. Ellis' Department of Chemistry, University of Wisconsin-Madison, Madison, WTsconsin 53706 Received: January 20, 1993
Gas-phase adsorption of several boranes (BF3, MeBBr2, MelBBr, and Et3B) onto the surface of etched, singlecrystal n-CdSe quenches the band gap photoluminescence (PL) relative to its intensity in vacuum (pressure Torr). The PL quenching is consistent with the Lewis acidic nature of the boranes and is dependent on substituents: PL quenching follows the order BF3 > MeBBr2 > MezBBr > Et3B. The magnitude of the PL quenching can be fit to a dead-layer model, permitting an estimate of the adduct-induced expansion of the depletion width in the semiconductor resulting from exposure to these Lewis acids; dead-layer expansions as large as -600 A have been measured. The borane-induced PL changes are pressure-dependent and, for all but BF3, can be fit by a simple Langmuir adsorption isotherm model, yielding equilibrium constants for adduct formation, K p , that range from on the order of 10 atm-' for Me2BBr and Et3B to lo3 atm-I for MeBBr2. Detection limits can reach values of as low as -0.001 Torr for BF3. The use of these effects as the basis for on-line sensors in chemical vapor deposition (CVD) processes is discussed.
-
I
In recent studies, we have shown that the photoluminescence (PL) intensity of single-crystal 11-VI semiconductors such as n-CdS and n-CdSe can be reversibly perturbed by adducts formed between gaseous molecules and the surface atoms of the solids.'*2 Adducts examined thus far, in both gas-phase and solution studies, define a Uluminescentlitmus test," with Lewis bases like amines enhancing PL intensity relative to a reference ambient and Lewis acids such as SO2 quenching the PL intensity relative to the reference ambient.'-8 In characterizing these systems, we have modeled the adductsas weak charge-transfer complexes that shift thedistributionof electronsbetween the bulkof thesemiconductor and its surface electronic states, thereby causing changes in PL intensity. The electronic structural changes accompanying adduct formation are illustrated in Figure 1 for adsorption of a Lewis acid. On the left of the figure is the hypothetical preadsorption state for an n-type semiconductor: danglingorbitals at the surfacecan give rise to intra-band gap surface electronic states that are filled up to the Fermi level with electrons transferred from the bulk of the solid. This creates an electric field in the solid, a depletion region, that is symbolized by the bent band edges. Interaction of the surfacestates with the lowest unoccupied molecularorbital (LUMO) of the adsorbingLewis acid stabilizesthesurfacestates, moving them closer to the valence band edge, and enabling them to accommodate additional electrons from the bulk of the solid, extending the depletion region further into the solid. Assuming the region that supports the electric field to be nonemissive (electron-hole pairs are efficiently separated in this region), PL intensity will decline with adsorption, reflecting the expanded depletion region. This effect has been successfully modeled quantitatively,using a dead-layer model (vide infra), for a variety of adsorbing molecules.'-8 We have begun to extend the library of semiconductor-molecule interactions characterized by PL methods to embrace gaseous species that are of potential use in chemical vapor deposition (CVD) processes. In principle, the relatively rapid response of PL changes to ambient concentration lends itself to the construction of on-line chemical sensors. In this paper we extend our methodology to a representative family of gaseous borane compounds. The particular molecules Address correspondence to this author.
0022-3654/93/2097-57 13$04.00/0
_-I.--.
,/1
n
Semiconductor
Adduct
Valence
*-.-
Acid
Figure 1. Perturbation of the electronic structure of a semiconductor by adsorption of a Lewisacid. The initial, preadsorption electronic structure of the semiconductor is represented by the drawing on the left side of the figure. The electronic states at the surface are populated with electrons up to the Fermi level (Ef) in the material (shaded region). Interaction of the surface states with the LUMO of a Lewis acid destabilizes the LUMOand stabilizesthesurfacestates,movingthem closer to thevalence band edge, allowing more electrons from thesemiconductor bulk tooccupy surface states and increasing the depletion region (center part of figure).
examined-BF3, MeBBrl, Me2BBr, and Et3B--are sketched below and illustrate the effects of halide and alkyl substitution on adsorption properties. Moreover, Et3B has been used as a equilibriumconstantsfor adduct formation.
'F
I
KCH2
plasma-assisted CVD precursor for introducingboron as a dopant into amorphous hydrogenated silicon.9 The experimentsdescribed herein demonstrate the viabilityof PL-based sensing: borane-induced PL changes are readily detectable using CdSe band-edge PL and can be quantitatively fit to a dead-layermodel, permitting estimatesof adduct-induced expansionsin depletionwidth. The concentrationdependence of these PL changes yields estimates of detection limits and equilibrium constants for adduct formation.
0 1993 American Chemical Society
Neu et al.
5714 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
vacuum
I
To traps and diffusion Pump
I I
Gas source
1.0~10.2atm
I
4.3x10.2 atm
c . )
10 min 650
Time Figure 3. Changes in PL intensity at 713 nm resulting from alternating
collecting emitted light
light
Figure 2. Schematicof the cell and apparatusused in the PL experiments. A more detailed description is given in the Experimental Section.
Experimental Section Materials and Metbods. Single-crystal,vapor-grown, c-plates of n-CdSe, having a resistivity of -2 ohmem, were obtained from Cleveland Crystals, Inc. Thesecrystalswere etched in Br2/ MeOH (1:15 v/v), revealing the shiny, Cd-rich (OOO1) face that was illuminatedin these PL experiments. The boranesemployed, BF3, MeBBr2, MezBBr, and Et3B, were purchased from Alfa Products with at least 99% purity and used without further purification after IIB NMR spectra of neat MeBBr2, MezBBr, and Et3B (using a Bruker AM 360 NMR instrument with a IIB resonant frequency at 1 1 SSMHz, unlocked and referenced to a BF3-OEt2externalstandard) revealed no impuritiesin the liquids other than that MeBBr2 and MezBBr were present as trace impuritiesin each other. Thevapor pressuresat 0 "C of MeBBr2, MezBBr, and Et3B agreed with literature values.IO All of the boranes save BF3 were loaded into a vacuum flask in an Ar-filled drybox for use on the Schlenk line described below; BF3 was introduced from a lecture bottle. Apparatus. A sketch of the apparatus employed is shown in Figure 2: The vacuum line had an ultimate pressure of MeBBr2 > MezBBr > Et3B; this ordering was obtained regardless of the exposure sequence. In all cases, since PL quenching increases with concentration to a saturation value (vide infra), we used concentrations of the boranes that displayed their maximum quenching ability. Qualitatively, the observed quenching is consistent with the Lewisacidic, electrondeficientcharacterof boranes. Presumably, the LUMO of these molecules-a vacant p orbital (or vacant hybrid orbital derived from s and p orbitals, as adduct formation distorts the borane from planarity around the boron atom)-can accept electron density from donor semiconductorsurface atoms like selenium. This process, as noted in the Introduction and illustrated in Figure 1, can in principle stabilizethe surface state distributionof the solid,expand the nonemissive depletionregion, and thereby quench the PL intensity. The particular sequence of quenching activity found for the borane derivativescould reflect both steric and electronicfactors. Specifically,while electron-donating alkyl substituentswould be expected to reduce the Lewis acidity of boranes, as inferred from the weaker PL quenching observed, they are also more sterically demanding. Since it is not known whether all the boranes are occupying the same sites on the CdSe surface or even whether
The Journal of Physical Chemistry, Vol. 97, No. 21,1993 5715
Photoluminescence as a Probe of Adsorption
b
a 1.o
BF3
; . 1I m
-
1.0
-
'
W
0.8
Excitation h 633 nm
MeBBr,
Me,BBr
i;
0.6
0
0 514
0.4
458
0.2
Et3B
0I
100
200
300
400
m
-: .
5
0.2
0
5e3
le4
otm-1
MeBBrt, atm
500
AD, A Figure 4. Maximum values of the dead-layer expansion, AD,induced by adsorption of BF3, MeBBr2, MelBBr, and Et3Bonto the (O001) face of a single etched n-CdSe sample. Values of AD were calculated using eq 1 in the text. Typical pressures needed to produce the maximum PL changescorresponding to these ADvaluesaregiven in the text. Excitation wavelengths used in the PL measurements are indicated in the figure.
the absolute coverages are the same, the trend in PL quenching should be regarded strictly as empirical. Dead-Layer Model. The PL quenching can be treated quantitatively using a dead-layer model in which it is assumed that a distanceon the order of the depletionregion is nonemissive. The quantitative form of the dead-layer model is given by eq 1:ll-l4
PLref/PL,= exp (-dAD)
(1) where PLrer is the reference vacuum PL intensity; PL, is the PL intensity in the presence of the gaseous borane; d = ( a + 6)is the sum of the semiconductor absorptivities for exciting and emitted light; and AD = (Drer- D,) is the increase in dead-layer thickness resulting from switching from vacuum to the borane ambient. In using the dead-layer model, we assume that the surface recombination velocity S either is unaffected by adduct formation or very large (S >> L/T and S >> aL2/7, where L and T are the minority carrier (hole) diffusion length and lifetime, respectively) both in the absence and presence of the adductforming species.13 A quantitative test of the dead-layer model is that AD values derived from PL quenching experimentsshould be independent of the excitation wavelength. Figure 4 demonstratesthe good fit to this model for boranes, using 458-, 514-, and 633-nm laser lines spanning a factor of 3 in absorptivity.Is Note that for this CdSe sample, AD ranges from 100 %i for Et3B to -5500 A for BF3; the largest value measured was -600 %i for BF3. We should note that while the trends observed in Figure 4 were found with all the CdSe samples examined, AD values resulting from adsorption of a given boraneontothe differentCdSesamples employed varied by as much as a factor of 2. Concentration Effects. The quenching of CdSe PL intensity increases with borane concentration up to a saturation level. The concentrationdependence can be used to estimatethe equilibrium constant for adduct formation through the use of the Langmuir adsorption isotherm model. The quantitative form of this model is given by eq 2:'
-
+
8 = KP/(1+ KP) or l/8 = 1 l / ( K P ) (2) where 8 is the fractional surface coverage; K is the equilibrium constant; and P is the gas pressure of the uncomplexed Lewis acid. The saturated PL intensity, P L , , is taken to correspond to the maximum surface coverage, 8 = 1, and the PL intensity in vacuum is taken to be 8 = 0. At intermediate coverages, 8 is estimated using eq 3.3
(3)
m
BF3, atm
Figure 5. Fractional surface coverage, 8, obtained from adduct-induced PL changes by use of eq 3 in the text, vs the pressure of (a) MeBBr2 and (b) BF3. The insets display double-reciprocal plots of the same data; the linearity of the MeBBr2 plot represents a good fit to the Langmuir adsorption isotherm model and yields an equilibrium binding constant K p of 1.1 X lo3 atm-I. The double-reciprocal plot of BF3, however, is not linear and does not represent a good fit to the model. The excitation wavelength and intensity employed in these experiments is 5 14.5 nm and -2 mW/cm2, respectively.
Figure 5 shows typical data for adsorption of MeBBrz and BF3 ontothe (0001) faceof n-CdSe, as plots of 8 vs the boranepressure. All of the boranes except BF3 gave good fits to the Langmuir model. From thedoublereciprocalplotsof 8-tvs reciprocal borane pressure, like that shown for MeBBr2 in the inset of Figure 5a, weestimatethatKpison theorderofabout 1O3atm-Ifor MeBBr2, 10 atm-1for Me2BBr,and about 30 atm-1for Et3B. The relative role of stericand electronic factors influencingthese equilibrium constants is intriguing but not directly addressable by our data. With BF3, detection through PL quenching is easily accomplished at pressures of as low as 0.005 Torr, but the data do not fit the Langmuir model; this is particularly evident in the nonlinear double-reciprocal plot shown in the inset of Figure 5b. We are uncertain as to the origin of this nonadherence to the Langmuir model, although,as noted above, the PLchangesare stillreversible. ChemicalSensing. The abilitytodetect the presence of boranes at low concentrations may lend itself to the construction of optically-coupled,on-line chemical sensors for these derivatives using optical fibers.I6 In the optimum case, 0.001 Torr of BF3 can be measured up to a saturationconcentrationof 10 Torr. For the other boranes, concentrationsover ranges of 0.05-30 Torr for MeBBr2; 20-400 Torr for Me2BBr; and 5-1 00 Torr for Et3B are resolvable. Reversibility at r c " temperature is relatively slow but could be rapidly accelerated by slightly higher temperatures based on previous studies." Further studies of CVD precursor molecules are underway in our laboratories. Acknowledgment. We thank Professors Donald Gaines and Thomas Kuech for helpful discussions and Mr. Robert McGaff for experimental assistance. We are grateful to the National Science Foundation for support of this research.
References and Notes (1) Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J . Am. Chem. Soc. 1988, 110,4914. (2) Meyer, G. J.; Leung, L. K.; Yu, J. C.; Lisensky, G. C.; Ellis, A. B. J . Am. Chem.Soc. 1989,111,5146. (3) Lisensky,G. C.; Penn, R. L.; Murphy, C. J.; Ellis, A. B. Science 1990, 248, 840. (4) Murphy, C. J.; Ellis, A. B. J . Phys. Chem. 1990, 94, 3082. (5) Murphy, C. J.; Lisensky, G. C.; Leung, L. K.; Kowach, G. R.; Ellis, A. B. J . Am. Chem. Soc. 1990,112,8344. (6) Murphy, C. J.; Ellis, A. B. Polyhedron 1990, 9, 1913. (7) Leung, L. K.; Komplin, N. J.; Ellis, A. B.; Tabatabaie, N. J . Phys. Chem. 1991, 95,5918. (8) Zhang, J. Z.; Ellis, A. B. J . Phys. Chem. 1992,96,2700. (9) Suchaneck, G.; Albert, M.; Beyer, W.; St6tzel, H.; Schade, K. J . Noncryst. Solids 1991, 137&138, 701. (10) Beilsrein, Series 11, Vol. 4, p 1022 (Et,B); Series 111, Vol. 4, p 1959 (MeZBBr), p 1962 (MeBBrz).
5716 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 (1 1) Hollingsworth,R. E.; Sites, J. R.J . Appl. Phys. 1982,53, 5357 and references therein. (12) Ellis, A. B. In Chemistry and Structure at Interfaces; New Luser and Oprical Techniques; Hall, R. B., Ellis, A. B., Eds.; VCH: Deerfield Beach, FL, 1986; Chapter 6. (13) Burk, A. A., Jr.; Johnson, P. B.; Hobson, W. S.; Ellis, A. B. J . Appl. Phys. 1986,59, 1621. (14) Mettler, K. J . Appl. Phys. 1977, 12, 75.
Neu et al. (15) Parsons, R.B.; Wardzynski, W.;Yoffe, A. D. Proc. R. Soc. London, A 1961, 262, 120. (16) Luebker, E. R.M.; hung, L. K.; Murphy,C. J.; Liscnsky, G. C.; Ellis, A. B. Biolechnology: Bridging Resdarch And Applications; Kamely, D., Chakrabarty, A. M., Kornguth, S.E.,Us.Kluwer ; Academic Publishers: Dordrccht, The Netherlands, 1991; pp 317-331. (17) Lisensky, G. C.; Meyer, G. J.; Ellis, A. 8. Anal. Chem. 1988, 60, 2531.
