No abstract

Using the technique of high-resolution energy-distribution analysis of electrons photoemitted from a cleaved GaAs surface coated with a layer of Cs, we have been able to determine many of the transport properties of GaAs which are important in the operation of the GaAs-Ca-0 photocathode and other GaAs devices. A two-minima diffusion model is presented which explains the photon energy dependence of the photocathode yield near threshold. Electron diffusion lengths for the Y\ and Xi minima have been determined from the spectral shape of quantum yield as a function of temperature and carrier concentration for heavily doped p-type material. The hot-electron scattering length for equivalent intervalley scattering has been measured by comparison with a computer scattering model. The coupling constant for equivalent intervalley scattering has been calculated from the hot-electron scattering length. The coupling constant for scattering between the Ti and Xi minima is calculated from the Xi diffusion length. These results, along with other recent data, are used to calculate the temperature dependence of the mobility in the Xi valleys and the intervalley scattering time. The temperature dependence of the energy spacing of the Vi and Xi valleys has been measured. The escape probability for the photocathode and the shape of the energy distribution curves is explained by a model which includes optical phonon scattering in the band-bending region, reflection at the surface, trapping in surface states, and lifetime broadening.

A detailed picture of the behavior of cesium oxide as a low work-function coating on III-V semiconductors and on silver has been obtained. Measurement of required cesium and oxygen exposure for optimum photoyield shows that the compound normally formed is close to CS2O, with variations in required exposure for very thin and very thick layers. By making simultaneous Kelvin work-function, photoyield-threshold, and thickness measurements, it was possible to establish that the CS2O, an n-type semiconductor, forms a heterojunction or Schottky barrier with its substrate. This provides a band bending which produces a gradual lowering of the vacuum level with increasing thickness to an ultimate work function of 0.6 eV. The photoyield and dark current from the substrate are limited by the interfacial barrier at the heterojunction. This barrier is 1.00±0.05 eV for a silver substrate and 1.23±0.03 eV for GaSb. The band-bending distance in the CS2O is about 50 Å and the hot electron scattering distance is 9 Å. These data have been used in an improved calculation of the maximum Γ escape probability and requisite CS2O thickness for electron emission from III-V semiconductors of different bandgaps. Electron emission from CS2O induced by an oxygen overpressure was also measured. CSOH is compared with CS2O as a work-function lowering coating.

The reaction of the OH radical with isoprene, CH (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k = (9.90 ± 0.09) × 10 cm molecule s) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + CH ⇌ HOCH equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C and C to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol for the C adduct). Addition to the less stable C and C adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k = (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k( T, [ M]). An analytical representation of k( T, [ M]) and k( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k = (9.5 ± 0.2) × 10( T/298 K) + (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s, significantly extend the measured temperature range of R1.

The ternary-phase diagram of GaAsSb has been calculated using Darken's quadratic formalism for a ternary liquid and assuming a regular solid solution. Liquid epitaxial layers of GaAsxSb1−x have been grown in the range 0.75>x>1 on {100} and {111} GaAs substrates. Results are in excellent agreement with the calculated phase diagram. Variation of bandgap with composition of the layer has been determined by transmission, photoemission, and x-ray fluorescence experiments. The data were fitted to a curve of the form EG=A+Bx+Cx2, where A=0.725 eV, B=−0.32 eV, and C=1.02 eV. Graded bandgap layers have been obtained, with gradients of 700 eV/cm near the substrate interface and 25 eV/cm for thick layers. For use as high-efficiency photoemitters, the samples were doped p type by the addition of elemental Zn to the melt. Cesium and oxygen surface layers were used to lower the work function. Quantum yields of 0.1%–0.2% at 1.06 μ were obtained. Field assisted photoemission in a graded bandgap sample has been calculated and demonstrated experimentally.