Bernard R. Goldstein

High-voltage photovoltaic effect

Vacuum evaporated films of cadium telluride have been prepared that show photovoltages as high as 100 v/cm of film length. An oblique angle of deposition of the vapor onto the substrate is required. The photovoltage saturates at high light intensities and low temperatures. At all other light intensities and temperatures it has the same functional dependence on light intensity as that of ordinary p‐n junctions. The short‐circuit current varies linearly with light intensity at all temperatures and is only weakly temperature‐dependent. Analysis of the data suggests a series of p‐n junctions (or other photovoltaic elements) arrayed in an additive manner. Estimates of the linear density of photovoltaic elements based on the measurements described above vary from ∼200/cm at room temperature to ∼7000/cm at −170°C. A possible mechanism to explain the effect is proposed, based on an anisotropic growth of crystallites due to the angle of deposition, and on the presence of residual gases in the vacuum chamber.

LEED, Auger and plasmon studies of negative electron affinity on Si produced by the adsorption of Cs and O

Abstract We have made LEED, Auger, and Plasmon measurements to study how Cs and O adsorb onto the (100) surface of p-type degenerate Si to produce negative electron affinity (NEA). A key factor to producing NEA was found to be a highly ordered Si surface as reflected by very high quality 2×2 LEED patterns. When NEA is produced, both the adsorbed Cs and O give the same LEED pattern as the original Si surface, but with a general enhancement of the half-order spot intensity. The adsorption of both Cs and O is strongly self-limiting, apparently controlled by the number of available appropriate sites on the surface. If Cs and O adsorb amorphously, NEA is not achieved. The thermal desorption of Cs occurs over a fairly broad temperature range centered at about 550°C. After Cs desorbs, the remaining O reverts spontaneously from an ordered layer to an amorphous layer, and then desorbs at about 800°C with an activation energy of 3.3 eV. Measurements of backscattered electron energy losses due to plasmons have shown that the Si surface plasmon is reduced in energy from 12.5 V to 7.0 V by the Cs-O layer. From this, an effective dielectric constant e = 5.3 for this layer can be deduced which, in turn, enables us to characterize completely the Cs-O dipole layer. The geometrical model described by Levine for the NEA surface is consistent with our experimental results.

Diffusion length of holes in a‐Si:H by the surface photovoltage method

The diffusion length L for holes in undoped a‐Si:H films has been measured by using a variation of the surface photovoltage method. Values of L in the range 0.33–0.45 μ were found for samples prepared at substrate temperatures Ts = 240 °C and Ts = 330 °C. After prolonged illumination, a reduction to L<0.2 μ was observed; the original value of L was restored after annealing at 200 °C.

LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

Abstract A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 μA lumen . The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at −170°C has a broad maximum at about −50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at −170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 A thick on the GaAs.

Langmuir evaporation from the (100), (111A), and (111B) faces of GaAs

Abstract We have measured the Langmuir evaporation of Ga and As from the (100), (111A), and (111B) faces of GaAs above and below the congruent evaporation temperature T c . We have found that T c is lowest for the (111B) face and highest for the (111A) face. These differences can be understood in terms of the different lifetimes of surface Ga on these faces. Furthermore, we have deduced that the evaporation processes are the rate limiting steps in the decomposition of GaAs. Below T c , decomposition is controlled by the evaporation of Ga; above T c it is controlled by the evaporation of As.

Growth of MgO films with high secondary electron emission on Al-Mg alloys

We have identified the processes which take place during the formation of MgO layers on the surface of Al-1% Mg alloys by measuring surface composition, electron multiplication, and Mg evaporation. An initial layer of Al2O3 on the alloy plays a vital role in this process: it acts as a sink for Mg, and prevents the inward diffusion of O2 during the heating, thereby localizing the MgO formation on the outer surface. The Mg concentration in the Al2O3 layer increases about 20-fold over its original value, and the Al2O3 is partially reduced. For a given temperature and pressure of oxygen, the MgO growth rate depends on the thickness and crystallinity of the Al2O3. Electron multiplication factors 5 of 10 to 15 are obtained; part of the secondary electrons originate in the underlying Al2O3 layer.

Characterization of clean and oxidized (100)LaB6

Abstract We have studied clean and oxidized (100)LaB 6 grown from aluminum melts by making Auger, LEED, evaporation and work function measurements on well-defined surfaces. The clean surface shows no La enrichment when initially heated as high as 1700°C. Its LEED pattern is 1 × 1, indicating no surface reconstruction. Langmuir evaporation studies up to temperatures of 1700°C show only La and B evaporating non-congruently, and LaO. The activation energy for B evaporation from LaB 6 (and from CeB 6 and EuB 6 also) is abot 5.5 eV, very close to that from elemental B. The rare-earth activation energies, however, vary, being highest for the rare-earth whose pure metal vapor pressure is lowest. Oxidation was carried out at room temperature using O 2 pressures up to 10 −7 Torr and at 1000°C using O 2 pressures up to 10 −4 Torr. At room temperature oxygen adsorption proceeeds to a saturated value indicated from LEED behavior to be about one monolayer. It produces a monotonic work function increase, which also saturates (at 1.40 V), varying linearly with the oxygen uptake. Oxidation at 1000°C is much more extensive than at room temperature, involving at least several monolayers, and results in a work function increase of 2.42 V. Results are discussed in terms of a terminal plane composed of La atoms, and adsorbed oxygen which, when given sufficient mobility, prefers bonding to La atoms at sites atop the B octahedra.

Different bonding states of Cs and O on highly photoemissive GaAs by flash−desorption experiments

The thermal desorption of Cs and O from GaAs/Cs/O activated to negative electron affinity occurs as atomic Cs133 and (mostly) Ga2O. Flash−desorption experiments show that several bonding states exist for the adsorbed Cs which correlate with changes in work function. For example, (i) the Cs bonding state for GaAs/Cs changes when the Cs coverage exceeds that required for maximum photoemission, and (ii) the distribution of bonding states for Cs shifts markedly to higher energies as the GaAs surface passes from unactivated (GaAs/Cs) to activated (GaAs/Cs/O).

The Alfonsine tables of Toledo

List of Figures. Preface. 1: Introduction. 2: Text. 2.1. Need for a new edition. 2.2. The manuscript. 2.3. The text. 2.4. Transcription criteria. 2.5. A transcription of the Libro de las tables alfonsies. 3: Glossary of technical terms. 4: Astronomical commentary. 5: Context. 6: The legacy of Alfonsine astronomy. 6.1. Introduction. 6.2. The characteristics of Alfonsine astronomy in Paris. 6.3. The astronomers in the Alfonsine tradition in Paris. 6.4. Beyond Paris. Bibliography. Notation. Manuscripts cited. List of parameters. Index.

Surface photovoltage, band-bending and surface states on a-Si : H

Abstract We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between −168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at −168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (∼min), saturation at photon fluxes of about 10 11 /cm 2 · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster ( 11 / cm 2 , a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.