Comparison of the Stability of Functionalized GaN and GaP.

Surface functionalization via 1 H,1 H,2 H,2H-perfluoro octanephosphonic acid was done in the presence of phosphoric acid to provide a simplified surface passivation technique for gallium nitride (GaN) and gallium phosphide (GaP). In an effort to identify the leading causes of surface instabilities, hydrogen peroxide was utilized as an additional chemical modification to cap unsatisfied bonds. The stability of the surfaces was studied in an aqueous environment and subsequently characterized. A physical characterization was carried out to evaluate the surface roughness and water hydrophobicity pre and post stability testing via atomic force microscopy and water goniometry. Surface-chemistry changes and solution leaching were quantified by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry. The results indicate a sensitivity to hydroxyl terminated species for both GaN and GaP under aqueous environments, as the increase of the degree of leaching was more significant for hydrogen peroxide treated samples. The results support the notion that hydroxyl species act as precursors to gallium oxide formation and lead to subsequent instability in aqueous solutions.

Surface characterization of gallium nitride modified with peptides before and after exposure to ionizing radiation in solution.

An aqueous surface modification of gallium nitride was employed to attach biomolecules to the surface. The modification was a simple two-step process using a single linker molecule and mild temperatures. The presence of the peptide on the surface was confirmed with X-ray photoelectron spectroscopy. Subsequently, the samples were placed in water baths and exposed to ionizing radiation to examine the effects of the radiation on the material in an environment similar to the body. Surface analysis confirmed degradation of the surface of GaN after radiation exposure in water; however, the peptide molecules successfully remained on the surface following exposure to ionizing radiation. We hypothesize that during radiation exposure of the samples, the radiolysis of water produces peroxide and other reactive species on the sample surface. Peroxide exposure promotes the formation of a more stable layer of gallium oxyhydroxide which passivates the surface better than other oxide species.

In situ chemical functionalization of gallium nitride with phosphonic acid derivatives during etching.

In situ functionalization of polar (c plane) and nonpolar (a plane) gallium nitride (GaN) was performed by adding (3-bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate the emission properties and oxide formation of GaN, which was explored through surface characterization with atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), inductively coupled plasma-mass spectrometry, and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth-related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High-quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room-temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.

Modification of the Surface Properties of Al x Ga1-x N Substrates with Gradient Aluminum Composition Using Wet Chemical Treatments.

The surface properties of biomolecular gradients are widely known to be important for controlling cell dynamics, but there is a lack of platforms for studying them in vitro using inorganic materials. The changes in various surface properties of an Al x Ga1-x N film (0.173 ≤ x ≤ 0.220) with gradient aluminum content were quantified to demonstrate the ability to modify interfacial characteristics. Four wet chemical treatments were used to modify the surface of the film: (i) oxide passivation by hydrogen peroxide, (ii) two-step functionalization with a carboxylic acid following hydrogen peroxide pretreatment, (iii) phosphoric acid etch, and (iv) in situ functionalization with a phosphonic acid in phosphoric acid. The characterization confirmed changes in the topography, nanostructures, and hydrophobicity after chemical treatment. Additionally, X-ray photoelectron spectroscopy was used to confirm that the chemical composition of the surfaces, in particular, Ga2O3 and Al2O3 content, was dependent on both the chemical treatment and the Al content of the gradient. Spectroscopic evaluation showed red shifts in strain-sensitive Raman peaks as the Al content gradually increased, but the same peaks blue-shifted after chemical treatment. Kelvin probe force microscopy measurements demonstrated that one can modify the surface charge using the chemical treatments. There were no predictable or controllable surface charge trends because of the spontaneous oxide-based nanostructured formations of the bulk material that varied based on treatment and were defect-dependent. The reported methodology and characterization can be utilized in future interfacial studies that rely on water-based wet chemical functionalization of inorganic materials.

Biomolecular gradients via semiconductor gradients: characterization of amino acid adsorption to InxGa1-xN surfaces.

The band gap of indium gallium nitride can be tuned by varying the compositional ratio of indium to gallium, spanning the entire visible region and extending into the near-infrared and near-ultraviolet. This tunability allows for device optimization specific to different applications, including as a biosensor or platform for studying biological interactions. However, these rely on chemically dependent interactions between the device surface and the biostructures of interest. This study presents a material gradient of changing In:Ga composition and the subsequent evaluation of amino acid adsorption to this surface. Arginine is adsorbed to the surface in conditions both above and below the isoelectric point, providing insight to the role of electrostatic interactions in interface formation. These electrostatics are the driving force of the observed adsorption behaviors, with protonated amino acid demonstrating increased adsorption as a function of native surface oxide buildup. We thus present a gradient inorganic substrate featuring varying affinity for amino acid adhesion, which can be applied in generating gradient architectures for biosensors and studying cellular behaviors without application of specialized patterning processes.