Pehr E. Pehrsson

Infrared detection of gaseous species during the filament‐assisted growth of diamond

Infrared diode laser absorption spectroscopy is employed as an in situ method to examine gas phase species present during filament‐assisted deposition of diamond films. From a reactant mixture of 0.5% methane in hydrogen, methyl radical (CH3 ), acetylene (C2H2), and ethylene (C2H4 ) are detected above the growing surface, while ethane (C2H6 ), various C3 hydrocarbons, and methylene (CH2) radicals are below our sensitivity levels. The growth of polycrystalline diamond films on Si wafers and polycrystalline Ni is confirmed with x‐ray and Raman scattering, scanning electron microscopy, and Auger electron spectroscopy.

Effects of surface pretreatments on nucleation and growth of diamond films on a variety of substrates

The effects of surface pretreatments on nucleation and growth of chemical vapor deposited diamond films were studied. The pretreatments included scratching with diamond grit, coating with a low vapor pressure, high thermal stability hydrocarbon oil, and coating with a 100–200‐A‐thick layer of evaporated carbon. The effects of the treatments on carbide and noncarbide forming materials are described.

Secondary electron emission from diamond surfaces

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 hy...

A vibrational study of the adsorption and desorption of hydrogen on polycrystalline diamond

The adsorption and desorption of hydrogen from diamond films were studied in ultrahigh vacuum using high resolution electron energy loss spectroscopy as a probe of surface vibrations. Auger electron and energy loss spectroscopies were also used to characterize the diamond surface. The samples studied were boron‐doped polycrystalline diamond films with chiefly (111) oriented facets. We attribute the observed spectral features to a monohydride species and local sp3 bonding on the diamond surface exposed to atomic hydrogen. A significant fraction of the hydrogen desorbs from the surface between 950 and 1000 °C, although some hydrogen persists even after heating to 1050 °C.

Surface depletion effects in semiconducting nanowires

The impact of surface depletion on the electronic properties of semiconductor nanowires (NWs) is explored both theoretically and experimentally. The impact of dopant concentration, surface barrier height, and NW radius on surface depletion and extracted material properties are determined by solving Poisson’s equation for the cylindrical system. The theoretical results reveal a size-dependent systematic error in carrier concentration extraction, which is verified through experiment. Interrogation of GaN NWs with radii from 15 to 70 nm exposed an error that reaches over an order of magnitude for the samples studied. These data compared favorably to an analytical treatment assuming physically reasonable material properties. While this manuscript focuses on GaN, the systematic error discussed will be present for any semiconducting NW, which exhibits surface band bending and therefore influences the behavior and characterization of a wide range of semiconducting nanoelements.

Adsorption and abstraction of hydrogen on polycrystalline diamond

The processes of atomic hydrogen adsorption and abstraction on a diamond surface determine the fraction of sites available for reaction with carbon containing species during growth. The relative efficiencies of hydrogen atom adsorption and abstraction on a polycrystalline diamond surface were determined at surface temperatures of 80 and 600 °C using high resolution electron energy loss spectroscopy. Abstraction is seen to occur 0.05 times as efficiently as adsorption on a diamond surface at 80 °C. This is interpreted to indicate that the activation barrier to abstraction is higher than in analogous gas phase reactions. No change in either the adsorption or abstraction rate is seen for a diamond surface at 600 °C indicating that hydrogen atoms do not accommodate the surface during the reaction. Thus, abstraction proceeds via a generalized Eley–Rideal mechanism.

Low energy ion implantation and electrochemical separation of diamond films

We combine low energy and low dose ion implantation with an electrochemical etch to fabricate thin diamond layers suitable for seeding homoepitaxial or polycrystalline chemical vapor deposited (CVD) diamond growth. Implantation of a carbon ion dose of 1×1016 cm−2 at 175 keV creates subsurface damage in a bulk crystal which is selectively removed by the electrochemical etch. Implanted substrates were subjected to CVD diamond deposition prior to etching of the damage layer. We discuss the effect of implantation and subsequent annealing conditions on the morphology and Raman spectra of the CVD films. All results indicate that the seed layer nucleated high quality CVD diamond film growth.

Oxidation of heated diamond C(100):H surfaces

This paper extends a previous study (Pehrsson and Mercer, submitted to Surf. Sci.) on unheated, hydrogenated, natural diamond (100) surfaces oxidized with thermally activated oxygen (O 2 . ). In this paper, the oxidation is performed at substrate temperatures from T sub =24 to 670 C. The diamond surface composition and structure were then investigated with high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and low energy electron diffraction (LEED). The oxygen coverage (0) increased in two stages, as it did during oxidation at T<80°C. However, there are fundamental differences between the oxidation of nominally unheated and heated diamond surfaces. This difference is attributed to simultaneous adsorption and rapid desorption of oxygen species at higher temperatures; the desorption step is much slower without heating. The initial oxidation rates were similar regardless of the substrate temperatures, but the peak coverage (0) was lower at higher temperatures. For example, 0 plateaued at 0.4±0.1 ML at 600°C. The lower saturation coverage is again attributed to oxygen desorption during oxidation. Consistent results were obtained on fully oxidized surfaces, which when heated in vacuum to T sub =600°C, lost ∼60% of their adsorbed oxygen. ELS revealed few C=C dimers on the oxidized surfaces, and more graphitization than on unheated surfaces. Oxidation at elevated temperatures also increased the carbonyl to ether ratio, reflecting etching-induced changes in the types of surface sites. The carbonyl and C-H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. The oxygen types did not interconvert when the oxidized surfaces were heated in vacuum. Oxygen desorption generated a much more reactive surface than heating-induced dehydrogenation of the smooth, hydrogenated surface.

Periodically porous top electrodes on vertical nanowire arrays for highly sensitive gas detection

Nanowires of various materials and configurations have been shown to be highly effective in the detection of chemical and biological species. In this paper, we report a novel, nanosphere-enabled approach to fabricating highly sensitive gas sensors based on ordered arrays of vertically aligned silicon nanowires topped with a periodically porous top electrode. The vertical array configuration helps to greatly increase the sensitivity of the sensor while the pores in the top electrode layer significantly improve sensing response times by allowing analyte gases to pass through freely. Herein, we show highly sensitive detection to both nitrogen dioxide (NO(2)) and ammonia (NH(3)) in humidified air. NO(2) detection down to 10 parts per billion (ppb) is demonstrated and an order-of-magnitude improvement in sensor response time is shown in the detection of NH(3).

Surface oxidation chemistry of β-SiC

Well-ordered, Si- and C-terminated β-SiC (100) surfaces were oxidized at low oxygen pressure and high temperature to elucidate the mechanism of oxidation. The samples were studied by high resolution electron energy loss spectroscopy (HREELS), Auger electron and electron loss spectroscopies, and low energy electron diffraction. Si-terminated surfaces exposed at Tsub=1050 °C to either O2 or hot filament-activated oxygen (O+O2*) and then cooled without oxygen were C-terminated and exhibited similar oxidation rates, surface composition, and HREELS spectra. The change from Si- to C-termination slightly shifted the surface optical phonon. Activated oxygen played no discernible role in the removal of the Si overlayer at this temperature. Samples oxidized under similar conditions but cooled under oxygen were Si-terminated, indicating that the surface lost carbon as CO and CO2 during cooldown. The conversion from Si- to C-termination during oxidation did not occur because of self-limiting etching of a Si monolayer, but rather because the oxidation products of silicon volatilized more rapidly than those of carbon. Oxidation at room temperature resulted in formation of a Si oxide layer.Well-ordered, Si- and C-terminated β-SiC (100) surfaces were oxidized at low oxygen pressure and high temperature to elucidate the mechanism of oxidation. The samples were studied by high resolution electron energy loss spectroscopy (HREELS), Auger electron and electron loss spectroscopies, and low energy electron diffraction. Si-terminated surfaces exposed at Tsub=1050 °C to either O2 or hot filament-activated oxygen (O+O2*) and then cooled without oxygen were C-terminated and exhibited similar oxidation rates, surface composition, and HREELS spectra. The change from Si- to C-termination slightly shifted the surface optical phonon. Activated oxygen played no discernible role in the removal of the Si overlayer at this temperature. Samples oxidized under similar conditions but cooled under oxygen were Si-terminated, indicating that the surface lost carbon as CO and CO2 during cooldown. The conversion from Si- to C-termination during oxidation did not occur because of self-limiting etching of a Si monolayer...