Chemical functionalization and electronic passivation of gallium arsenide surfaces

摘要

Chemically controlled, low defect-density surfaces are essential for the incorporation of gallium arsenide into solar conversion and optoelectronic devices. Detailed X-ray photoelectron spectroscopic (XPS) studies have been conducted on chemically functionalized GaAs(111)A surfaces. Quantitative analysis of this surface after HCl(aq) etching reveals that it is completely free of observable oxide and As(0) contaminants, and is terminated with nearly a full monolayer of Cl. These surface Ga-Cl bonds have been reacted with the phosphine reagents PCl3 and PEt3, both of which introduce P atoms onto the surface. Direct reaction of PCl3 with the oxide-terminated surface leads to surfaces that are nearly oxide free but contain measurable amounts of As(0). Steady-state photoluminescence (PL) intensity measurements were used to evaluate the effectiveness of these techniques at passivating surface carrier recombination. Consistent with the chemical observations, etched and functionalized surfaces showed enhanced PL, while surfaces functionalized directly with PCl3 did not.ududThe effects of surface functionalization were explored on GaAs nanocrystals chemically synthsized with an oxide capping layer. Transmission electron microscopy and powder X-ray diffraction demonstrated that the particles were anisotropically etched by treatment with HCl(aq). XPS measurements showed that the Cl-terminated particles were almost entirely free of oxide but contained significant As(0) contamination. Further functionalization of the particles with N2H4 or NaSH replaced surface Cl atoms with N or S moieties but did not remove this As(0). The corresponding band gap PL of these particles was quite weak. Annealing the functionalized particles lead to the disappearance of the As(0) and strong enhancement of the PL intensity. These results imply that surface As(0) is a dominant carrier trap on nanoscale GaAs surfaces and should be broadly applicable for improving the performance of GaAs nanocrystals and nanowires.ududFinally, Fermi’s golden rule has been used to develop relationships between rate constants for electron transfer in donor-bridge-acceptor and electrode-bridge-acceptor systems and resistances across metal-bridge-electrode and metal-bridge-tip junctions. This formulation was used to predict resistances for alkanethiolate, oligophenylene, and DNA bridges from reported donor-acceptor electron-transfer measurements in these systems. These predicted values were compared to reported resistances measured for these molecules.