摘要:

Diamond exhibits very high, but widely varying, secondary-electron yields. In this study, we identified some of the factors that govern the secondary-electron yield from diamond by performing comparative studies on polycrystalline films with different dopants (boron or nitrogen), doping concentrations, and surface terminations. The total electron yield as a function of incident-electron energy and the energy distribution of the emitted secondary electrons showed that both bulk properties and surface chemistry are important in the secondary-electron-emission process. The dopant type and doping concentration affect the transport of secondary electrons through the sample bulk, as well as the electrical conductivity needed to replenish the emitted electrons. Surface adsorbates affect the electron transmission at the surface-vacuum interface because they change the vacuum barrier height. The presence of hydrogen termination at the diamond surface, the extent of the hydrogen coverage, and the coadsorption of hydrocarbon-containing species all correlated with significant yield changes. Extraordinarily high secondary-electron yields (as high as 84) were observed on B-doped diamond samples saturated with surface hydrogen. The secondary electrons were predominantly low-energy quasithermalized electrons residing in the bottom of the diamond conduction band. Two key reasons for the unusually high yields are (1) the wide band gap which allows the low-energy secondary electrons to have long mean-free paths, and (2) the very low or even negative electron affinity at the surface which permits the low-energy quasithermalized electrons that reach the surface to escape into vacuum.

ISSN:0021-8979    

DOI:10.1063/1.365990

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Filling of submicron width trenches with Cu from a highly ionized plasma is sensitive to both the directionality of the depositing flux and reflection and resputtering at the film surface. The spatial distributions of Cu atoms sputtered by He+, Ar+, and Xe+ions incident at 30°, 45°and 60°and over an ion energy range of∼55–600 eV have been investigated. In all cases, the distribution is forward directed and cannot be described by a cosine distribution about the surface normal. Decreasing energy, increasing angles of incidence, and increasing ion mass yield more forward directed distributions. For∼55 eV Ar+incident at 60°,∼85&percent; of the resputtered flux is in the forward direction. From these distributions, in a 50&percent; ionized physical vapor deposition system the depositing flux due to forward directed resputtering is estimated to be of the same order of magnitude as the neutral flux. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365991

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The process of ion bombardment is investigated for the fabrication of Mo/Si multilayer x-ray mirrors usinge-beam evaporation. The ion treatment is applied immediately after deposition of each of the Si layers to smoothen the layers by removing an additional thickness of the Si layer. In this study the parameters of Kr+ion bombardment have been optimized within the energy range 300 eV–2 keV and an angular range between 20°and 50°. The optical performance of the Mo/Si multilayers is determined by absolute measurements of the near-normal-incidence reflectivity at 14.4 nm wavelength. The multilayer structures are analyzed further with small-angle reflectivity measurements using both specular reflectivity and diffuse x-ray scattering. The optimal smoothening parameters are obtained by determining the effect of ion bombardment on the interface roughness of the Si layer. The optimal conditions are found to be 2 keV at 50°angle of incidence with respect to the surface. These settings result in 47&percent; reflectivity at 85°(&lgr;=14.4 nm) for a 16-period Mo/Si multilayer mirror, corresponding to an interface roughness of 0.21 nm rms. Analysis shows that the interface roughness is determined by ion induced viscous flow, an effect which increases with ion energy as well as angle of incidence. In order to determine the effect of intermixing of the Si and Mo atoms, the penetration depth of the Kr+ions is calculated as a function of ion energy and angle of incidence. Furthermore, the angular dependence of the etch yield, obtained from thein situreflectivity measurements, is investigated in order o determine the optimal ion beam parameters for the production of multilayer mirrors on curved substrates. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365992

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Plasma characteristics at high pressureYBa2Cu3O7−&dgr;reactive magnetron sputtering were investigated with a probe technique andin situfilm resistance measurements. The experimental features of probe measurements in oxygen plasma are discussed. Electron energy distribution is the sum of two Maxwell distributions withkTe≈1.5 eVandkTe≈0.3 eV.N2Oaddition to a gas mixture results in the generation of negative ions with a density virtually equal to the density of positive ions. The energies of ions, impinging the film surface under film biasing, are discussed in collisionless and drift approximations. Low energy ion bombardment of the film surface at temperatures≈400 °Cresults in a reduction of film oxygen content. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365993

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

GaN films grown on (0001) 6H–SiC and (0001)Al2O3substrates using low-pressure chemical vapor deposition withGaCl3andNH3as precursors are comparatively explored by optical, scanning tunneling, and transmission electron microscopy. Independent of the substrate material used, the surface of the GaN layers is covered by hexagonally shaped islands. For GaN on 6H–SiC, the islands are larger in diameter (≈50 &mgr;m) and rather uniformly distributed. An atomically flat interface is observed for GaN onAl2O3in contrast to GaN grown on 6H–SiC, where the interface is characterized by large steps. For both substrates, faceted holes (named as pinholes) are observed in near-surface regions of the GaN layers occurring with a density of about7×108 cm−2. No unequivocal correlation between the density of pinholes and the density of threading dislocations (≈1.6×1010 cm−2forGaN/Al2O3and≈4×109 cm−2for GaN/6H–SiC) can be found. Rather, different types of defects are identified to be correlated with the pinholes, implying a dislocation-independent mechanism for the pinhole formation. Despite the small lattice mismatch between GaN and 6H–SiC, the pronounced original surface roughness of this substrate material is believed to account for both the marked interfacial roughness and the still existing high density of threading dislocations. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365994

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A novel etching effect of hydrogen ions on the growth of diamond films was observed. TheH+ion bombardment was performed by applying a negative substrate bias during a microwave plasma chemical vapor deposition process, using only hydrogen as a reactant gas. The effect of this bombardment was investigated by means of scanning electron microscopy. It was found that the etching efficiency ofH+ions on non-[001]-oriented grains is more significant than that on grains with their (001) faces parallel to the substrate. A lateral growth of the (001) faces can occur during the bombardment process. As a result, the size of (001) faces increases afterH+etching while grains with other directions are etched off. This effect provides a way to improve the orientation degree of [001] oriented diamond films and might be helpful for obtaining [001] oriented diamond films with small thickness. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365995

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The column densities of several molecular radicals are measured in an oxyacetylene torch flame during the growth of diamond films. The column densities of CH,C2,CN, and OH radicals are determined by highly sensitive absorption spectroscopy. The radical densities are measured as a function of position in the flame for different fuel/oxidant ratios. The growth of diamond is confirmed by scanning electron microscopy and Raman spectroscopy. Of the four radicals measured, theC2and CH densities are the most correlated with diamond growth. The measured densities are compared with numerical flame models. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365996

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Hydrogenated amorphous boron (a-B:H) thin films were prepared by radio-frequency plasma deposition usingB2H6(10&percent;) inH2as precursor gas. The influence of the substrate temperature and self-bias on the a-B:H film structure was investigated. The boron and hydrogen atom densities were determined by ion-beam analysis. The film structure, especially the bonding of hydrogen to boron, was investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR data were quantified by using a new formalism which allows a proper calculation of the extinction coefficient from the FTIR spectra. The intensities of the different boron-hydrogen absorption bands were compared with the ion-beam analyzed hydrogen atom densities to determine the absorption strength of the B–H terminal and B–H–B bridge bonds. A non-negligible fraction of hydrogen is shown to be bonded to boron in a B–H–B bridge bond. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365997

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Intrinsic hydrogenated amorphous silicon (a-Si:H) has been deposited using a hot tungsten filament in pure silane to drive the deposition chemistry—the “hot-wire” deposition method. The electronic and infrared properties of the film have been measured as a function of deposition parameters, leading to three principal conclusions. First, to obtain a high quality material, the Si atoms evaporated from the filament (distanceLfrom the substrate) must react with silane (densityns) before reaching the substrate; this requiresnsLgreater than a critical value. Second, radical-radical reactions cause deterioration of film properties at high values ofG(nsL),3whereGis the film growth rate; this requiresG(nsL)3less than a critical value. Finally, the film quality is a function ofG,and asGis increased the substrate temperature must be correspondingly increased to obtain high film quality. By optimizing these parameters, we have produced films with excellent electronic properties (e.g., ambipolar diffusion length >200 nm) at >5 nm/s deposition rate. Based on these insights, formulas are also given for optimizing film properties in multiple-filament geometries and in diluted silane. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365998

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Cubic (In,Ga)N layers with In contents up to 11&percent; are grown on GaAs(001) by dc plasma-assisted molecular beam epitaxy. Comparatively high substrate temperatures are used to desorb In accumulated on the growth front by the massive In segregation in this material system. We report the observation of band-edge photoluminescence and the dependence of the emission energy on In composition for the (In,Ga)N layers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365999

出版商:AIP    

年代:1997

数据来源: AIP