A.K. Green

The photodissociative ionization of amorphous ice

Within the energy range 17 ≦ hv < 35 eV, the ionic species desorbed and their excitation spectra are reported. The only positive ion desorbed is H+. A model for the surface is suggested which explains the absence of OH+ desorption. The desorption mechanisms are discussed in terms of an energy analysis.

Preparation, tunneling, resistivity, and critical current measurements on homogeneous highTc A15 Nb3Ge thin films

We have prepared homogeneous films of high Tc Nb3Ge as demonstrated by a total transition width of less than 1 K with a resistive Tc onset of 21.7 K, by paying particular attention to the constancy of substrate temperature. Xray diffraction analysis done both at Stanford and at Westinghouse shows no evidence of a second phase to the limits of the instruments (<1%). Tunneling as a function of thickness shows the material to be of good quality (high, sharp gaps; low excess conductance below the sum gap) at the surface for thicknesses of up to 1 μm and only slightly degraded at 5 μm. Further evidence of homogeneity is demonstrated by critical current measurements which give an extrapolated T*c of 20 K on material with resistive Tc onset of 21.9 K. Critical current densities at 14 K in a 0.5 T field are as high as 9×106 A/cm2, which are the highest ever reported for any material at this temperature. In an applied magnetic field of 7.5 tesla, Jc was 2.5×106 A/cm2 at 4.2 K. Tunneling as a function of thickness ...

Investigation of interfacial roughness of InxGa1−xAs epitaxial layers on GaAs and InP substrates by soft x‐ray reflectivity

The grazing incidence x‐ray reflectivity is a nondestructive and sensitive technique for probing the depth profile of electron density in layered structures. This method has been utilized in the soft x‐ray regime to determine the roughness of interfaces, and the epilayer thickness in InxGa1−xAs/InP and InxGa1−xAs/GaAs heterostructures, for x=0.57 and x=0.60, grown by molecular beam epitaxy. By fitting the experimental results to our model, assuming uncorrelated interfacial roughness, we conclude that the top surface roughness does not depend on the type of the substrate or presence of stress in the epilayer, and is always smaller than interfacial roughness. The main factors which control the interfacial roughness are the quality of substrate and/or growth conditions rather than strain or lattice mismatch.

Photon stimulated ion desorption of H+ ions from GaAs (110)/H2O

Abstract The photon stimulated ion desorption yield of H+ ions from a H2O dosed GaAs (110) surface has been measured in the range 18eV ⩽ hυ ⩽ 30eV. There is a direct correspondence between the PSID H+ yield, reflectance, and the secondary electron yield spectrum of GaAs (110). The data provides evidence that the initial stages of PSID involve core level (Ga(3d), O(2s)) → conduction band excitation followed by Auger decay.

Uv-Induced Photodeposition Of Fe Films From Iron Carbonyl

We present results on the 77K photodecomposition of Fe(C0)5 to produce Fe films using synchrotron and excimer laser radiation. Analysis of these films included in situ photon-stimulated ion desorption (PSID) and total electron yield (TEY) spectroscopies, and ex situ Auger electron spectroscopy (AES), scanning electron microscopy (SEM), X-ray fluorescence, and resistivity measurements. For the films grown using synchrotron radiation, the various ion products were identified and their intensities monitored during photolysis. For the films grown using excimer laser radiation, appropriate laser power densities and Fe(C0)5 pressures produced adherent metallic films which contained less than 13 atomic % oxygen and carbon contamination.

The chemical makeup of Nb and Nb3Sn films

Abstract We report the identification and distribution of impurities found in films of Nb and Nb 3 Sn which were grown by multiple-source electron-beam coevaporation. Both Auger-electron spectroscopy (AES) and photon-stimulated ion desorption (PSID) were used. The PSID results show large desorption yields of hydrogen from surface carbon, oxygen, silicon, and aluminum impurity sites on the Nb films. From the Nb 3 Sn films, the dominant PSID yield was again hydrogen, which desorbed primarily from surface oxygen sites. Only after removal of 1–6 nm of material by Ar-ion sputtering was 0 + observed to be desorbed from Nb. 0 + desorption from Sn was never observed. The AES results showed only carbon, oxygen, and a trace of aluminum impurities in either material. Using Ar-ion sputter etching, the depth profiles of these impurities within the films were measured.

A photoemission study of coherently strained InxGa1−xAs/InP(100)

Abstract Coherently strained 8 nm thick films of In 0.30 Ga 0.70 As (tensional strain = 1.7%) and In 0.75 Ga 0.25 As (compressional strain = −1.5%) were grown by molecular beam epitaxy on InP(100), on which In 0.53 Ga 0.47 As is the lattice-matched composition. Samples were studied with synchrotron-radiation photoemission measurements of the d-core levels of each ion. The results show (i) shifts of the core-level energies of surface ions consistent with previous results on As-rich (100) III–V surfaces, and (ii) significant intrinsic broadening of the core levels, especially the In 4d. Comparison of the two compositions showed (i) an increase in the binding energies of the Ga 3d and In 4d peak by 0.17 eV at the higher In concentration, and (ii) a constant separation of the In 4d from the Ga 3d of (1.80 ± 0.03) eV. A rigid-ion model of the alloy-semiconductor surface successfully predicts the observed halfwidths. The model also predicts that surface rumpling occurs in the As-terminated cation layer: The In ions are pushed out beyond the Ga ions. This rumpling has the effect of increasing the shift of the 4d-core binding energy for the “surface” In atoms.

Infrared-Wavelength Modulation Spectra Of InGaAs Grown By MBE And LPE

We present the infrared wavelength-modulation spectra of molecular-beam-epitaxy- (MBE) and liquid-phase-epitaxy- (LPE) grown Ini_xGaxAs (0.45 < x < 0.50) epilayers on (100) InP substrates. The transmittance, reflectance, and their wavelength derivatives are measured in the neighborhood of the band gap at room temperature. The dependence of band gap on strain is presented and analyzed. It is shown that epilayers partially relax the interfacial strain according to their thickness, giving different dependences of band gap on alloy composition. The results are analyzed by introducing a normalized fractional-strain parameter and applying Hookes-law elasticity and deformation potential theories.

Experimental And Theoretical X-Ray Absorption Near-Edge Study Of III-V Compound Semiconductor Materials

We report experimental and theoretical near-edge x-ray absorption fine structure (NEXAFS) spectra of clean and arsenic-capped gallium arsenide and the pseudo-binary alloy indium-gallium arsenide. Experimental data were obtained using synchrotron-radiation total-photoelectron-yield spectroscopy from the Ga(M2,3), As(M2,3), In(N2,3), and In(M4,5) edges. In addition, both C(K) and 0(K) NEXAFS spectra, and photon-stimulated ion-desorption mass spectra were obtained to assess and monitor the sample cleanliness. The samples studied were grown by molecular-beam epitaxy at China Lake and capped with arsenic for protection during transit to the Stanford Synchrotron-Radiation Laboratory. We have found by monitoring the As edges that heating the samples to 300 or 350°C completely removes the arsenic cap. Also, we find that after evaporation of the As cap, the NEXAFS spectra are identical for capped and uncapped samples. Theoretical calculations of the arsenic NEXAFS spectra were performed using a full multiple-scattering theory. The inputs to the calculations were ab initio phase shifts, calculated using pseudopotentials and a model geometry. Theoretical calculations of the arsenic NEXAFS spectra of In0.53Ga0.47As crystals are reported also. Here, the calculations are performed by creating many model crystals that have the appropriate stoichiometry and averaging the resulting spectra.

The phase stability of Nb3Ge by interstitial oxygen

Abstract A15 Nb3Ge with a high Tc is a metastable structure. One of the way that it is formed is by doping the codeposition of Nb and Ge with oxygen. Measurements show that there is no oxygen in the deposit when examined later. However, we have determined that oxygen was present (at a concentration of 1–2 at%) during the deposition at 900°C, and that it has diffused out during the cool down to 500°C. We propose that this interstitial oxygen has stabilized the A15 phase near to stoichiometry at 900°C by a combination of entropy change and electronic effects. These ideas will be discussed along with recent tests to substantiate them.