Photoluminescence as a probe of the adsorption of gaseous boranes onto the surface of cadmium selenide crystals

Abstract

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.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 Uluminescent litmus 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 adducts as weak charge-transfer complexes that shift thedistributionof electrons between 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: dangling orbitals at the surface can 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 molecular orbital (LUMO) of the adsorbing Lewis acid stabilizes thesurfacestates, 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...

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