Chemically cleaned GaP͑001͒ surfaces in 25% NH 4 OH solution have been studied using spectroscopic ellipsometry ͑SE͒, ex situ atomic force microscopy ͑AFM͒, x-ray photoelectron spectroscopy ͑XPS͒, and wettability measurement techniques. The SE data clearly indicate that the solution causes removal of the native oxide film immediately upon immersing the sample. The SE data also indicate that when the native oxide film is completely etch removed, the resulting surface is still roughened. The estimated roughness thickness is ϳ1.2 nm, in excellent agreement with the AFM rms value ͑ϳ1.2 nm͒. The XPS spectra confirm the removal of the native oxide from the GaP surface. The XPS data also suggest a thin oxide overlayer, ϳ0.3 nm thick, on the etch-cleaned GaP surface. The wettability measurements indicate that the as-degreased surface is hydrophobic, while the NH 4 OH-cleaned surface is hydrophilic. This result is in direct contrast to those obtained from acid cleaned surfaces, which are usually hydrophobic. The origin of hydrophilicity may be singular and associated hydroxyl groups bonded on the GaP surface.

Chemically cleaned GaP͑001͒ surfaces in aqueous HF solutions have been studied using spectroscopic ellipsometry ͑SE͒, ex situ atomic force microscopy ͑AFM͒, x-ray photoelectron spectroscopy ͑XPS͒, wettability, and photoluminescence ͑PL͒ measurements. The SE data clearly indicate that the solutions cause removal of the native oxide film immediately upon immersing the sample ͑Յ1 min͒. The SE data, however, suggest that the native oxide film cannot be completely etch-removed. This is due to the fact that as soon as the etched sample is exposed to air, the oxide starts to regrow. The SE estimated roughness is ϳ1 nm, while the AFM roughness value is ϳ0.3 nm. The XPS spectra confirm the removal of the native oxide and also the presence of regrown oxide on the HF-etched GaP surface. The wettability measurements indicate that the HF-cleaned surface is hydrophobic, which is in direct contrast to those obtained from alkaline-cleaned surfaces ͑hydrophilic͒. A slight increase in the PL intensity is also observed after etching in aqueous HF solutions.

Structural and optical properties of GaAs(001) surfaces thermally annealed in dry N 2 atmosphere

Thermal degradation of GaP(0 0 1) surfaces has been studied using spectroellipsometry (SE), optical microscopy, scanning electron microscopy (SEM), ex situ atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) techniques. The SE data suggest that thermal annealing causes removal of the native oxide at temperatures T < 600 °C and surface roughening at T ≥ 600 °C. The XPS spectra confirm the native oxide removal from the GaP surface. The AFM images provide rms roughnesses of ∼0.6 nm at T < 600 °C and ∼2 nm (∼15 nm) at T = 600 °C (800 °C), suggesting a rapid increase in the rms roughness with an increase in T above 600 °C. The reduction in PL intensity is observed for T ≥ 500 °C. The Ga droplets, observed by optical microscopy and SEM, appear at T > 600 °C, i.e. after the native oxide is removed from the GaP surface. These facts allow us to draw the conclusion that the surface native oxide acts as a good passivation film against annealing-induced PL degradation and thermal decomposition.

Thermal annealing behaviors of GaAs͑001͒ surfaces in dry N 2 atmosphere have been studied by using spectroscopic ellipsometry ͑SE͒, optical microscopy, scanning electron microscopy, ex situ atomic force microscopy ͑AFM͒, x-ray photoelectron spectroscopy ͑XPS͒, and photoluminescence ͑PL͒ techniques. The SE data indicate that thermal annealing at temperatures T below 500°C causes no or a little influence on the surface morphology. Microscopic roughening starts to occur at T Ͼ 600°C and exhibits a saturated roughness value at ϳ800°C. AFM observation confirms the presence of a microscopically roughened surface overlayer after annealing at T ϳ 600-750°C and a macroscopically roughened overlayer at above 700°C. The XPS data indicate the removal of As 2 O 3 oxide after annealing at above 500°C; however, the Ga 2 O 3 oxide is always observed regardless of annealing temperature, although its peak intensity becomes remarkably large at T ϳ 700°C and small at T Ն 800°C. Annealing at T = 1000°C leads to deliquesced sample surface. The native GaAs oxide is also found to act as a passivation film against annealing-induced PL degradation for T below ϳ400°C.