J. Ristein

A comparative analysis of a-C:H by infrared spectroscopy and mass selected thermal effusion

A comparative analysis of quantitative infrared absorption spectra and mass selected thermal effusion transients measured for a large number of hydrogenated amorphous carbon (a-C:H) films is presented in order to establish reliable IR absorption cross sections for the different hydrogen bonding configurations in the material. The structural character of the material and the hydrogen concentration were varied over a wide range from soft and wide band gap polymerlike a-C:H, via so-called diamondlike carbon, to the tetrahedral form of a-C:H deposited from a plasma beam source. The results show that the IR absorption cross sections from molecular hydrocarbons can well be transferred to the solid state as long as the material is of polymeric character, but that this procedure fails as soon as the material converts into a three-dimensional cross-linked and mechanically hard structure. For the latter material an empirical average IR absorption cross section for the C–H stretch band can nevertheless be determined...

Surface transfer doping of diamond

The electronic properties of many materials can be controlled by introducing appropriate impurities into the bulk crystal lattice in a process known as doping. In this way, diamond (a well-known insulator) can be transformed into a semiconductor, and recent progress in thin-film diamond synthesis has sparked interest in the potential applications of semiconducting diamond. However, the high dopant activation energies (in excess of 0.36 eV) and the limitation of donor incorporation to (111) growth facets only have hampered the development of diamond-based devices. Here we report a doping mechanism for diamond, using a method that does not require the introduction of foreign atoms into the diamond lattice. Instead, C60 molecules are evaporated onto the hydrogen-terminated diamond surface, where they induce a subsurface hole accumulation and a significant rise in two-dimensional conductivity. Our observations bear a resemblance to the so-called surface conductivity of diamond seen when hydrogenated diamond surfaces are exposed to air, and support an electrochemical model in which the reduction of hydrated protons in an aqueous surface layer gives rise to a hole accumulation layer. We expect that transfer doping by C60 will open a broad vista of possible semiconductor applications for diamond.

Structure and electronic states of ultrathin SiO2 thermally grown on Si(100) and Si(111) surfaces

Abstract The chemical and electronic structures of ultrathin SiO2 thermally grown on Si(100) and Si(111) have been investigated by using Fourier-transform infrared attenuated total reflection (FT-IR-ATR) and X-ray or ultraviolet excited photoelectron spectroscopy (XPS/UPS), respectively. A red-shift of the p-polarized LO phonon peak observed for oxides thinner than 2 nm indicates that compressively strained SiOSi bonds exist near the SiO 2 Si interface. The extent of the structural strain induced in the interface region is found to be smaller for SiO2 grown at 1000°C on Si(100) than 1000°C SiO2 on Si(111) or 800°C SiO2 on Si(100). It is also found that, from the onset of the energy loss signal for O1s photoelectrons, the bandgap of the oxides thicker than ∼2.3 nm is 8.95 ± 0.05 eV irrespective of the oxide thickness. For oxides thinner than ∼2.3 nm, a remarkable increase in the 5–9 eV energy loss signal for O1s photoelectrons is observed. This could be attributed to not only the contribution of suboxides at the interface but also the built-in stress in the interface region which causes the band edge tailing.

Noncontact temperature measurements of diamond by Raman scattering spectroscopy

The possibility of determining the temperature of diamond by noncontact Raman spectroscopy is assessed critically. The intensity ratio of Stokes to anti-Stokes lines is shown to be ill suited for temperatures above ∼750 K. Employing the temperature coefficient of the Raman line position, on the other hand, turns out to be a straightforward and highly reliable means to measure diamond temperatures between 300 and 2000 K with an accuracy of ±10 K. A prerequisite for the application of this method is an empirically developed formula which describes the temperature coefficient of the Raman active phonon frequency with high accuracy. Examples of temperature measurements on single crystal diamond and diamond films grown by chemical vapor deposition are given. The application of this procedure to the temperature measurement of silicon and germanium is demonstrated.

Origin of Doping in Quasi-Free-Standing Graphene on Silicon Carbide

We explain the robust p-type doping observed for quasi-free-standing graphene on hexagonal silicon carbide by the spontaneous polarization of the substrate. This mechanism is based on a bulk property of SiC, unavoidable for any hexagonal polytype of the material and independent of any details of the interface formation. We show that sign and magnitude of the polarization are in perfect agreement with the doping level observed in the graphene layer. With this mechanism, models based on hypothetical acceptor-type defects as they are discussed so far are obsolete. The n-type doping of epitaxial graphene is explained conventionally by donorlike states associated with the buffer layer and its interface to the substrate that overcompensate the polarization doping.

Effective correlation energies for defects in a-C:H from a comparison of photelectron yield and electron spin resonance experiments

Abstract Amorphous hydrogenated carbon films (a-C:H) were deposited by r.f. plasma CVD from methane, varying the self bias potential of the substrate electrode by means of the r.f. power coupled into the discharge. Films were characterized by IR and optical spectroscopy, confirming a transition from polymer-like to diamond-like (DLC) material with increasing self bias. One set of samples was investigated in situ by photoelectron and photoelectron yield spectroscopy, from which the density of gap states and their spectral distribution was derived. An identical set of samples was then examined by electron spin resonance to determine the density of paramagnetic defects. From a comparison of the results of both experiments, a lower limit for the effective correlation energy of the defect states was extracted which gave surprisingly large values for the correlation energy of the DLC material. In addition to the interpretation of the results within a spatially uniform model, the influence of a possible surface band bending on the evaluation of the correlation energies is also discussed.

Diamond surface conductivity experiments and photoelectron spectroscopy

A unique feature of diamond surfaces is a highly conductive p-type layer which is usually observed when the surfaces are hydrogen terminated. We present a combination of conductivity and photoelectron yield measurements on a variety of different diamond samples in order to elucidate the role of hydrogen and adsorbates for this phenomenon. The experiments show that hydrogen termination is a necessary but not a sufficient condition for the appearance of the surface conductivity. Additionally, adsorbates from the atmosphere are needed. On the basis of the experiments an electrochemical model is developed which can explain the effect of the hydrogen termination and also shows why hydrogen terminated diamond is the only semiconductor with p-type surface conductivity.

Fermi level on hydrogen terminated diamond surfaces

Atomic force microscopy and Kelvin probe experiments are applied to characterize hydrogen terminated patterns contacted with gold and aluminum on (100) diamond surfaces. On hydrogen terminated diamond the work function of 4.9 eV is detected, with an accuracy of about 0.1 eV. Taking into account the negative electron affinity of −1.3 eV and a band gap of 5.5 eV the Fermi energy is 0.7 eV deep in the valence band. Illumination of the sample results in a shift of the surface Fermi level by as much as 0.2 eV. This is attributed to a surface photovoltage effect.

Electronic and chemical passivation of hexagonal 6H–SiC surfaces by hydrogen termination

Hydrogenation of 6H–SiC (0001) and (0001) is achieved by high-temperature hydrogen treatment. Both surfaces show a low-energy electron diffraction pattern representative of unreconstructed surfaces of extremely high crystallographic order. On SiC(0001), hydrogenation is confirmed by the observation of sharp Si–H stretching modes. The absence of surface band bending for n- and p-type samples is indicative of electronically passivated surfaces with densities of charged surface states in the gap of below 7×1010 cm−2 for p-type and 1.7×1012 cm−2 for n- type samples, respectively. Even after two days in air, the surfaces show no sign of surface oxide in x-ray photoelectron spectroscopy.

Electronic properties of diamond surfaces — blessing or curse for devices?

Abstract For the potential electronic applications of diamond as a wide band gap semiconductor, the properties of its surfaces are of fundamental importance. These properties in general depend on the kind of passivation of the surface dangling bonds, either by chemisorbed adsorbates or by the formation of mutual chemical bonds as a consequence of surface reconstruction. The principal crystallographic surfaces, (100), (111) and (110), are essentially understood in their adsorbate free form in terms of surface states and reconstructions. The corresponding results are briefly summarized in Section 2 . The consequences of adsorbate passivation are then discussed for the case of hydrogen as the most important radical for surface termination with emphasis on different aspects. Hydrogen passivation leads to a negative electron affinity due to a dipole layer which is induced by the heteropolar carbon–hydrogen bonds of the surface atoms. This aspect is discussed quantitatively in Section 3 . Hydrogenation also reduces the amount of sp 2 bonded carbon at the surface, which turns out to be the crucial parameter determining the position of the surface Fermi level and thus the surface band bending in diamond. This aspect is covered in Section 4 . Finally, the combination of the electron affinity reduction by hydrogen and the supply of electrons at the Fermi level by sp 2 bonded carbon phases has important consequences for the low threshold electron emission into vacuum. This is dealt with in the framework of an inhomogeneous emission model in Section 5 . The model can be expected to be applicable for all system containing sp 2 bonded or graphitic phases together with an sp 3 bonded diamond matrix, especially for CVD diamond, and it should have important implications also for field emission from diamond films.