A very effective, two-step chemical etching method to produce clean InP(100) surfaces when combined with thermal annealing has been developed. The hydrogen peroxide/sulfuric acid based solutions, which are successfully used to clean GaAs(100) surfaces, leave a significant amount of residual oxide on the InP surface which can not be removed by thermal annealing. Therefore, a second chemical etching step is needed to remove the oxide. We found that strong acid solutions with HCl or H 2 SO 4 are able to remove the surface oxide and leave the InP surface passivated with elemental P which is, in turn, terminated with H. This yields a hydrophobic surface and allows for lower temperatures to be used during annealing. We also determined that the effectiveness of oxide removal is strongly dependent on the concentration of the acid. Surfaces cleaned by HF solutions were also studied and result in a hydrophilic surface with F terminated surface In atoms. The chemical reactions leading to the differences in behavior between InP and GaAs are analyzed and the optimum cleaning method for InP is discussed.
We describe p-type gallium nitride (GaN) as a candidate for high brightness photocathodes. Experiments utilizing photoemission spectroscopy and quantum yield measurements were performed on GaN films to characterize various cesium and oxygen activations. Quantum efficiencies of 0.1%–4% were obtained in reflection for the cesiated p-type 0.5 μm thick GaN films and 25%–50% on the 0.1 μm thick GaN films. The corresponding emission currents are 142–300 nA for 0.5 μm thick films and 0.7–1.3 μA for the 0.1 μm thick films. This results in an increase of several orders of magnitude in the emission current from the starting GaN films. Furthermore, an initial desorption measurement was performed in order to evaluate the Cs binding strength to GaN relative to GaAs. We observe Cs was bound to the GaN surface (0001_) at 700 °C and completely desorbed at 450 °C for a (100) GaAs surface. Finally, an alternate barium activation on GaN is included for preliminary comparison with the various cesium activations.
Achieving clean surfaces is a major and challenging requirement for the study of surfaces and surface reactions. Nondestructive cleaning is a crucial step in semiconductor manufacturing, growth of materials, and processing. We use photoemission spectroscopy (PES) to systematically study the core and valence band electronic structure of various chemical treatments of InP(100), GaAs(100), and GaN(0001). These surfaces undergo wet chemical cleaning of H2SO2/H2O2/H2O followed by thermal heating. In order to achieve the necessary surface sensitivity and spectral resolution, synchrotron radiation in the energy range of 60–1000 eV is used for PES. In tuning the sulfuric acid based chemistry, we achieve oxygen free GaAs and InP surfaces, as shown in our valence band and core level PES analysis. Furthermore, core level PES shows oxygen coverage of the GaN surface is reduced to less than 0.1 monolayer (ML). The carbon coverage is also reduced dramatically for the III–V surfaces, <10% ML for InP and GaAs and approximately 1% ML for GaN. The chemical reactions and species at different cleaning stages are determined and cleaning mechanisms are proposed. Our study shows that material similarities do not imply exact correlation to the chemical cleaning properties among III–V materials.
Wet chemical cleaning of InP surfaces investigated by in situ and ex situ infrared spectroscopyDependence of indium-tin-oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopiesThe chemical cleaning of indium phosphide ͑InP͒,͑100͒ surfaces is studied systematically by using photoemission electron spectroscopy. In order to achieve the necessary surface sensitivity and spectral resolution, synchrotron radiation with photon energies ranging from 60 to 600 eV are used to study the indium 4d, phosphorus 2p, carbon 1s, and oxygen 1s core levels, and the valence band. Typical H 2 SO 4 :H 2 O 2 :H 2 O solutions used to etch GaAs͑100͒ surfaces are applied to InP͑100͒ surfaces. It is found that the resulting surface species are significantly different from those found on GaAs͑100͒ surfaces and that a second chemical cleaning step using a strong acid is required to remove residual surface oxide. This two-step cleaning process leaves the surface oxide free and with approximately 0.4 ML of elemental phosphorus, which is removed by vacuum annealing. The carbon coverage is also reduced dramatically from approximately 1 to about 0.05 ML. The chemical reactions are investigated, the resulting InP surface species at different cleaning stages are determined, and the optimum cleaning procedure is presented.
Atomic hydrogen-cleaned GaAs(100) negative electron affinity photocathode: Surface studies with reflection high-energy electron diffraction and quantum efficiency Source contamination, photovoltage effects, and stimulated electron desorption of cesium are factors that determine the initial high performance and longevity of negative electron affinity electron sources. As part of the study of these factors, we investigate the effect of oxygen contamination during the activation process and operation of a traditional GaAs ͑100͒ and a novel GaN ͑0001͒ emitter. We use synchrotron radiation photoemission spectroscopy, a focused mercury arc discharge lamp, and a helium neon ͑HeNe͒ laser to obtain simultaneously elemental analyses of the emitting surface and the corresponding total quantum yield at various stages, respectively, for GaN and GaAs. Our results indicate: ͑1͒ carbon uptake does not occur in our carbon free system over time, ͑2͒ oxygen uptake is observed for both GaN ͑0001͒ and GaAs ͑100͒ activated surfaces, a property common to the chemistry of the Cs/O adlayer, ͑3͒ the oxygen species appears to change over time and the initial species is assigned to an ion of nondissociated oxygen in the Cs/O activation layer, ͑4͒ the chemical changes of the Cs/O adlayer are not accompanied by a significant loss of cesium from the surface, and ͑5͒ the onset of decay of the quantum yield begins at a later time for the GaN ͑0001͒ emitter in comparison to the GaAs ͑100͒ electron source. A chemical model for the activation layer and its transformation over time is developed, consistent with points ͑1͒ through ͑4͒ in a separate report ͓F. Machuca et al. ͑unpublished͔͒. The complete account of the decay of the quantum yield of both Cs/O activated III-V emitters is compared and discussed in this article.
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