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The surface structure of the hydrogen-saturated Si(110) surfaces after wet cleaning is studied on an atomic scale by means of scanning tunneling microscopy. When a surface oxide layer is stripped using a HF-containing solution, the surface consists of nanometer-scaled terraces and atomic steps along various directions. Coupled monohydride lines are formed inside a small terrace, as predicted by infrared spectra. The Si(110) surface after subsequent rinsing for a short period is occupied by a long terrace along the [110] direction in which the ideal 1×1 structure is formed. Atomic arrangements around step edges are determined in detail based on atomic images and first-principles calculations. A ridge-shaped structure is observed after excess rinsing with water, and infrared spectra indicate that the slope is composed of (111) microfacets. From these results, we present the mechanism underlying the formation of the hydrogen-terminated Si(110) surfaces during wet cleaning processes. [ABSTRACT FROM AUTHOR]

Atomic arrangements of the hydrogen-terminated Si(001) surface after wet cleaning are investigated by scanning tunneling microscopy and by first-principles calculations. The hydrogen-terminated Si(001) surface after dilute HF cleaning is atomically rough with a structure of terraces and steps. In addition, it is confirmed that 1 × 1 dihydride structures are formed inside the terraces. First-principles calculations reveal that the maxima of the local density of states string parallel to the Si-H direction on the 1 × 1 dihydride structure, which makes it possible to determine the atomic arrangement of the observed atomic images. When the surface after HF cleaning is subsequently rinsed with ultrapure water, rows emerge along the 〈110〉 direction. Atomic images show that the rows are mainly formed by a 2 × 1 unit cell. To elucidate the atomic structure of the 2 × 1 unit cell, it is proposed that every other row of the ideal 1 × 1 dihydride surface is preferentially etched with ultrapure water. Total energy calculations are performed based on the first principles of quantum mechanics, which justify the proposed etching mechanism of the Si(001) surface with ultrapure water. [ABSTRACT FROM AUTHOR]

Abstract: We studied processes of cleaning GaN(0001) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition (MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 C for 12h+400 C for 1h+500 C for 5min) without sputtering, after which the surface contamination of oxide and carbide was reduced to % of that before annealing. Clear GaN(0001) patterns were obtained by LEED and RHEED. STM images showed flat terraces of nm size and steps of nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar+ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear structure. Upon annealing at 500–600 C, the surface hydrogen was removed and a reconstruction structure partially appeared, although a structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure. [Copyright &y& Elsevier]

Atomically resolved scanning tunneling microscopy observations are performed on hydrogen-terminated Si(110) surfaces after wet cleaning. When a Si(110) wafer is dipped into dilute HF-containing solution, the surface is constructed by piling small terraces and steps. When the sample is consequently rinsed with ultrapure water, some characteristic features such as a zig–zag chain inside a terrace, a single row at step edges and an isolated zig–zag chain on a terrace are clearly observed, and their atomic arrangements are determined. Excessive rinsing, however, creates the ridge-shaped structure of nanometer height, which is explained by anisotropic etching by OH- ions in water. [ABSTRACT FROM AUTHOR]

Atomic structures of hydrogen-terminated Si(001) surfaces after HF cleaning are investigated by scanning tunneling microscopy. It is revealed that the surface is macroscopically rough but is composed of terraces and steps. Inside a terrace, 1x1 structures are formed. This corresponds to the ideal 1x1 dihydride structure. The step edges run along the <110> direction. On the other hand, the 1x1 dihydride structure disappears when the surface is subsequently rinsed with ultrapure water, because every other dihydride row of the ideal 1x1 structure is preferentially etched in ultrapure water. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2×2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ∼550°C but was not achieved by etching in HCl, NaOH, and HNO3. [Copyright &y& Elsevier]

Abstract: A surface preparation method with fine SiO2 particles in water is developed to flatten Si(001) surfaces on the nanometer scale. The flattening performance of Si(001) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100×100nm2) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈110〉 direction on terraces. [Copyright &y& Elsevier]

Continuous visible light irradiation is used to observe intrinsic hydrogenated amorphous silicon (a-Si:H) surfaces by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A deposited a-Si:H surface is wet-cleaned in order to terminate the a-Si:H surface with H atoms. A current-sample bias (I–V) curve shows a rectifying behavior in the dark, and the tunneling current increases dramatically only under the reverse-bias condition with light irradiation. This is attributed to the photoexcited minority carriers that lead to the appearance of a higher voltage across the vacuum than that in the dark. Experiments with both p-type and n-type substrates justify the suggested model. High tunneling current under irradiation enables us to obtain highly resolved STM images. Current imaging tunneling spectroscopy observations show that the detected tunneling current in the reverse direction is increased over the entire surface with irradiation, and the increment of the tunneling current is different at each surface site. [Copyright &y& Elsevier]