R. Nishitani

Esca studies of Ga, As, GaAs, Ga2O3, As2O3 and As2O5

Abstract The binding energies of Ga 3 d , As 3 d , Ga L 3 M 4 , 5 M 4 , 5 and O 1 s in Ga, As, GaAs, Ga 2 O 3 , As 2 O 3 and As 2 O 5 are reevaluated by means of ESCA. The calibration lines of the C 1 s and the Au 4 f 7 2 gave different binding energies for the compound materials. In order to determine the absolute binding energies, the chemical shifts in Auger and photoelectron lines from a layered structure composed of thin layer oxide and substrate of a defined material were used. An energy calibration curve, E (Ga 3 d ) vs. Δ E (GA LMM - Ga 3 d ), was found to be useful for determination of binding energies in the material which contains gallium. In the case of the GaAs sample, both the chemical etching and the ion bombardment effects on the chemical structure of the GaAs surface are also discussed.

Surface structures and work functions of the LaB6 (100), (110) and (111) clean surfaces

The structures of the LaB6 (110) and (111) clean surfaces have been studied by angleresolved XPS, ISS and LEED. The LaB6 (110) clean surface has the relaxed and reconstructed c(2 × 2) structure where the surface lanthanum atoms are displaced toward the surface and a c(2 × 2) overlayer structure is caused by a displacement of the surface lanthanum atoms. The LaB6 (111) clean surface has the relaxed (1 × 1) structure where the surface lanthanum atoms are displaced toward the surface. The work functions of the (100), (110) and (111) surfaces have been measured from the width of UPS spectra and are ~2.3, ~2.5 and ~3.3 eV, respectively. The anisotropy of the work functions has been correlated with the surface structures. The origin of the unusually low work functions of the LaB6 (100) and (110) clean surfaces in refractory materials is interpreted by electric dipole moments produced by positive charges of surface lanthanum ions.

X-ray photoemission study of the initial oxidation of the cleaved (110) surfaces of GaAs, GaP and InSb

Abstract The room temperature oxidation of GaAs, GaP and InSb upon exposure to atmospheric dry O 2 has been studied by analyzing XP spectra quantitatively. The rate of oxidation decreases in the order InSb > GaP > GaAs. For GaAs and GaP, evidences of the formation of metastable surface complex are shown: the ratio of A III ue5f8O bonds to B V ue5f8O bonds are about 2 and 1, respectively. For InSb, it is shown that both elements are oxidized to comparable extent, giving rise to In 2 O 3 and Sb 2 O 3 probably, by using Auger spectra as well as photoemission spectra.

Surface states on the LaB6(100), (110) and (111) clean surfaces studied by angle-resolved ups

The surface states on the LaB6(100), (110) and (111) clean surfaces have been studied by means of angle-resolved UV photoelectron spectroscopy with unpolarized light. Surface states on LaB6(110) and (111) have been observed for the first time, and the energy-band structures have been determined. The surface states on LaB6(110) are located at ∼1.8 and ∼3.0 eV below EF. They are associated with the occurrence of the c(2 × 2) surface structure on LaB6(110). The surface states on LaB6(111) are located at ∼1.5 and ∼2 eV below EF. The dispersion of the lower band is relatively large (∼0.9 eV) compared with those on LaB6(100) and (110). The surface state at ∼2 eV below EF on LaB6(111) has a maximum energy at Γ point (k∥ = 0), while that on LaB6(100) has a minimum energy at Γpoint. The surface state at ∼2 eV below EFon LaB6(100) possesses Δ1 symmetry at k∥ = 0, and has even parity with respect to the (010) and (011) planes. The surface state at ∼1.8 eV below EF on LaB6(110) has even parity with respect to the (110) plane, and odd parity with respect to the (001) plane. The surface state at ∼3.0 eV below EF has even parity with respect to both the (110) and (001) planes. The low-lying surface state on LaB6(111) has Δ1 symmetry at k∥ = 0. All these surface states on the LaB6 clean surfaces are interpreted in terms of dangling bonds of the surface boron frameworks which are mainly B 2p in character.

The preparation and properties of CeB6, SmB6, and GdB6

Single crystals of CeB6, SmB6, and GdB6 have been prepared by a floating‐zone technique under a pressurized gas atmosphere of Ar. A zone leveling technique has successfully been applied to prepare single crystals of GdB6, a compound which peritectically decomposes. The crystals so obtained were almost stoichiometric and had a low‐impurity level. The measured work functions of (001) surfaces of CeB6 and GdB6 are 2.6±0.1 and 3.8±0.1 eV, respectively. The temperature dependence of the electrical resistivity of CeB6 resembles that of CeAl3. The temperature dependence of the resistivity of SmB6 single crystals is like that of a semiconductor and is similar to those reported for polycrystalline specimens. The resistivity has a very high saturation value of 70.7 Ωu2009cm at low temperatures. A new phase transition of GdB6 was observed at about 7 K, which is indicated by hysteretic behavior in the resistivitiy versus temperature curve.Single crystals of CeB/sub 6/, SmB/sub 6/, and GdB/sub 6/ have been prepared by a floating-zone technique under a pressurized gas atmosphere of Ar. A zone leveling technique has successfully been applied to prepare single crystals of GdB/sub 6/, a compound which peritectically decomposes. The crystals so obtained were almost stoichiometric and had a low-impurity level. The measured work functions of (001) surfaces of CeB/sub 6/ and GdB/sub 6/ are 2.6 +- 0.1 and 3.8 +- 0.1 eV, respectively. The temperature dependence of the electrical resistivity of CeB/sub 6/ resembles that of CeAl/sub 3/. The temperature dependence of the resistivity of SmB/sub 6/ single crystals is like that of a semiconductor and is similar to those reported for polycrystalline specimens. The resistivity has a very high saturation value of 70.7 ..cap omega.. cm at low temperatures. A new phase transition of GdB/sub 6/ was observed at about 7 K, which is indicated by hysteretic behavior in the resistivitiy versus temperature curve.

Oxygen adsorption on the LaB6(100) surface studied by UPS and LEED

Abstract The surface states (∼2 eV below E F ), which are originated mainly from the dangling bonds of boron atoms on the LaB 6 (100) clean surface, disappear at an oxygen exposure of ∼ 1.4 L. At the same exposure, an oxygen sticking-coefficient has a maximum value, ∼1.0. A change in the work function due to oxygen adsorption increases linearly with increasing surface oxygen and varies its slope at the above-mentioned exposure. At a low oxygen-exposure of ∼0.38 L, the first peak appears at ∼6.6 eV below e f in UPS spectra. The second overlapping oxygen peak at ∼6.0 eV below E F in UPS spectra, which grows around l L and overcomes the first peak, shifts to the low binding-energy side above ∼1.4 L. The (1 × 1) LEED pattern does not disappear until an oxygen exposure of several hundreds langmuir. It is suggested that the results support the presence of more than two adsorption states. The results are principally interpreted on the basis of two kinds of chemisorption sites; one is a boron site, and another a lanthanum site.

Direct observation of LaB6(001) surface at high temperatures by x‐ray and ultraviolet photoelectron spectroscopy, low‐energy electron diffraction, Auger electron spectroscopy, and work‐function measurements

The properties of the LaB6(001) surface which is in equilibrium with oxygen (ultrahigh vacuum—10−6 Torr) at high temperatures (room temperature ∼1500u2009°C) have been investigated by XPS, UPS, LEED, AES, work‐function measurements, and so on. In the case of the clean surface (experiments under ultrahigh vacuum), the chemical composition of the surface does not change up to ∼1500u2009°C, although lanthanum ions which form the outermost layer of the surface begin vigorous thermal motion at ∼1100u2009°C. A surface electronic state observed at room temperature undergoes no essential change even at ∼1400u2009°C. Under an oxygen atmosphere, the surface is covered by an oxide and has a high work function below a critical temperature Tc which depends on the oxygen pressure PO2. Above Tc, the surface oxide, which is rich in lanthanum content, evaporates (the activation energy of the evaporation is 7.8 eV), and the chemical composition and work function of the surface become identical to those of the clean surface under ultrahigh...

In-Depth Profiles of Oxide Films on GaAs Studied by XPS

Quantitative depth profiles of chemical composition of the thermal and the anodic oxide of GaAs have been studied by XPS in conjunction with argon ion sputtering. By analyzing the XPS spectra with taking account of the sputtering effects, both the amount and the chemical state of each elemental species in the oxides were obtained. In the thermal oxidation below 530°C in air, Ga2O3 was produced as a primary product and both the amounts of the evaporated-As and the accumulated-As increased with increasing oxide thickness. XPS spectra from the thermal oxides grown at 900°C in air or grown with As at ~530°C suggested that the products contained GaAsO4. The anodic oxidation resulted in films that had an As2O3/Ga2O3 ratio of about 1. The widths of the transition region between the ~500 A thick oxide and GaAs were about 70 A for the anodic oxides and 200 A for the thermal oxides.

Small changes in work function of the TiC(001) surface with chemisorption of O2 and H2O

Changes in work function of the TiC (001) surface have been measured against O2 and H2O exposure by using photoelectron spectroscopy. The sticking rates are about 1/100 of those for elemental Ti‐metal substrate. In the case of oxygen exposure, the work function increases monotonically from 3.8 to 4.2 eV, whereas for H2O a maximum value of work function occurs at an exposure of ∼300L, and its value finally reaches 4.5 eV.

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB6(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB6(100), (110) and (111) clean surfaces. The surface states on LaB6(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB6(110) is increased to ∼3.8 eV by an oxygen exposure of ∼2 L. The surface states on LaB6(111) disappear at an oxygen exposure of ∼2 L where the work function has a maximum value of ∼4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB6(111) until an exposure of ∼2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ∼4.4 to ∼3.8 eV until an oxygen exposure of ∼100L. The initial sticking coefficient on LaB6(110) has the highest value of ∼1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB6 surfaces.