To enable the use of GaAs-based devices as chemical sensors, their surfaces must be chemically modified. Reproducible adsorption of molecules in the liquid phase on the GaAs surfaces requires controlled etching procedures. Several analytical methods were applied, including Fourier transform infrared spectroscopy (FTIRS) in attenuated total reflection and multiple internal reflection mode (ATR/MIR), high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for the analysis of GaAs (100) samples treated with different wet-etching procedures. The assignment of the different features due to surface oxides present in the vibrational and XPS spectra was made by comparison with those of powdered oxides (Ga 2 O 3 , As 2 O 3 and As 2 O 5 ). The etching procedures here described, namely, those using low concentration HF solutions, substantially decrease the amount of arsenic oxides and aliphatic contaminants present in the GaAs (100) surfaces and completely remove gallium oxides. The mean thickness of the surface oxide layer drops from 1.6 nm in the raw sample to 0.1 nm after etching. However, in presence of light, water dissolution of arsenic oxides is enhanced, and oxidized species of gallium cover the surface.

GaAs-based electronic devices have interesting applications in spintronics and as sensors. In the past, methods were developed to stabilize the surface of GaAs, since it is known to be highly sensitive and unstable. It turns out, however, that these particular properties can be used for controlling the electronic characteristics of the devices, by adsorbing molecules that affect the surface properties. Here, we concentrate on the adsorption of molecules that can be bound to GaAs through their phosphate group. Phosphate functional groups can be found in many biological molecules; therefore, the binding of organic phosphate to a semiconductor surface can provide the first step toward a new line of hybrid bioorganic/inorganic electronic devices. We investigated the adsorption of tridecyl phosphate (TDP) and compared its adsorption to that of dodecanoic acid (lauric acid), which contains a carboxylic binding group. The alkyl phosphate monolayer is found to bind to the GaAs surface more strongly than any other functional group known to date. In addition, we show that the adsorption of a DNA nucleotide (5‘-AMP), as well as single-stranded DNA (ssDNA), on the GaAs surface occurs through the phosphate groups. Hence, DNA can be bound to these surfaces with no need for chemical modifications.

Characterizing the structure and dynamic properties of a single monolayer is a challenge due to the minute amount of material that is probed. Here, EPR spectroscopy is used for investigating the spatial and temporal organization of self-assembled monolayers of 5- and 16-doxyl stearic acid (5 DSA and 16 DSA, respectively) adsorbed on a GaAs substrate. The results are complemented with FTIR and ellipsometery measurements, which provide the evidence for the formation of monolayers. Moreover, a comparison with the FTIR spectrum of a monolayer of stearic acid shows that the monolayers of the spin labeled molecules are less packed due to the hindrance introduced by the labeling group. The EPR spectra provide a new insight on the ordering in the layer and more interestingly, it reveals the time dependence of the organization. For 5DSA, with the spin-label group situated close to the substrate, the EPR spectrum immediately after adsorption is poorly resolved and dominated by the spin-exchange interaction between neighboring molecules. As time increases (up to 1 week) the resolution of the 14N hyperfine coupling increases, revealing a better organized monolayer where the molecules are more homogenously spaced. Moreover, the spectrum of the layer, after reaching equilibrium, shows that there is no motional freedom near the GaAs surface. Orientation dependence measurements on the equilibrated sample show the presence of a preferred orientation of the molecules, although with a wide distribution. The spectrum of the 16DSA monolayer, where the nitroxide spin label is situated at the end of the chain, far from the surface, also showed a poorly resolved spectrum at short times, but unlike 5DSA, it did not exhibit any time dependence. Through EPR line-shape simulations and by comparison with FTIR results, the differences between 5DSA and 16DSA were attributed to difference in coverage caused by the bulky spin label near the surface in the case of 5DSA.

We report that adsorption of monolayers of organic molecules onto ferromagnetic semiconductor heterostructures can produce large changes in magnetic properties [1]. The digital-alloy heterostructures studied have 1/2 monolayer MnAs planes embedded in GaAs. We investigate effects on magnetic properties of self-assembly of various organic molecules onto the heterostructure surface. Depending on the molecular structure, the monolayers can either strengthen or suppress ferromagnetism. We attribute this chemical modulation of magnetic properties to electronic changes brought about by molecular binding to the semiconductor surface.

No abstract