Martin Hundhausen

Origin of the D peak in the Raman spectrum of microcrystalline graphite

Abstract We have studied the D peak in the Raman spectrum of polycrystalline graphite as a function of excitation energy in the range from 1.16 to 4.3 eV. The D peak disperses with excitation energy (≈50 cm−1/eV) and its intensity decreases with increasing laser energy. The dispersion of the D peak is explained in terms of a novel k → -selective resonant Raman scattering process. The scattering cross-section is resonantly enhanced for phonons on one branch in the phonon dispersion curves which have the same k → -vector as the electronic transition responsible for the resonance.

Raman spectra of epitaxial graphene on SiC(0001)

We present Raman spectra of epitaxial graphene layers grown on 63×63 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature.

The quasi-free-standing nature of graphene on H-saturated SiC(0001)

We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy, we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p = 5.0 − 6.5 × 1012 cm−2) with a carrier mobility of 3100 cm2/Vs at room temperature. Compared to graphene on the buffer layer, a strongly reduced temperature dependence of the mobility is observed for graphene on H-terminated SiC(0001) which justifies the term “quasi-free-standing.”

Nanometer‐scale field‐induced oxidation of Si(111):H by a conducting‐probe scanning force microscope: Doping dependence and kinetics

Hydrogen‐terminated Si(111) was patterned on the nanometer scale by field‐induced oxidation using a biased conducting‐probe scanning force microscope. The kinetics of oxide growth as well as its dependence on doping are investigated. Field‐induced oxidation is observed for voltages exceeding a doping dependent threshold above which oxidation kinetics follows a power law.

Quantitative evaluation of biaxial strain in epitaxial 3C-SiC layers on Si(100) substrates by Raman spectroscopy

We present experimentally determined biaxial strain coefficients for the longitudinal optical (LO) and transversal optical (TO) Raman lines in 3C-SiC. Suspended 3C-SiC membranes with a (100) texture are deflected by a variable pressure load on one side and the strain-induced shifts of the LO and TO Raman lines are measured while the strain is simultaneously calculated from the membrane deflection vs pressure. Using these results we measure the residual strain of 3C-SiC films grown on Si as a function of preparation conditions. The largest residual strain is found in thin samples which relaxes as film thickness increases to a value imposed by the different thermal expansion coefficients of 3C-SiC and Si. As the residual strain decreases and the film thickness increases, the Raman lines narrow indicating an improved crystalline quality. We also find a reduction of the residual strain with increasing growth rate.

Growth of diamond on silicon during the bias pretreatment in chemical vapor deposition of polycrystalline diamond films

The processes that occur on the silicon substrate during the bias pretreatment in a microwave plasma chemical vapor deposition system for the growth of diamond are investigated using a variety of techniques. The direct current during the bias pretreatment is correlated with changes in the shape of the plasma and with the amount and chemical and crystalline nature of the deposit on the substrate as well as with the nucleation density for subsequent diamond growth. The spatio‐temporal nature of the nucleation process explains the evolution of the bias current. It is shown that the bias pretreatment alone already yields a closed layer of nanocrystalline diamond and a strong etching of the silicon substrate. An analysis of the nucleation conditions necessary for the formation of oriented and textured diamond crystallites leads to a choice of parameters during bias pretreatment that yield homogeneous layers of highly oriented and textured diamond crystals over an area of 100 mm2.

Influence of substrate bias on the properties of a-C:H films prepared by plasma CVD

Abstract We have measured photoluminescence (PL) and Raman spectra for a series of amorphous carbon (a-C:H) films prepared in a RF-plasma CVD system. The negative bias voltage was varied between 0V and 950V. The samples seem to consist of two components. One polymeric component has a high band gap and gives strong luminesence above 2.1 eV with 0.6 eV band width. The second component shows the Raman spectrum that is typical for the sp 2 -clusters of hard carbon. We find that the hard carbon component seems to be absent for zero bias voltage. The PL for this sample, however, is strongest. We find a fatiguing of the luminescence during laser excitation. This fatiguing is weaker at 120 K, resulting in a higher PL-efficiency for this temperature.

Contribution of the buffer layer to the Raman spectrum of epitaxial graphene on SiC(0001)

We report a Raman study of the so-called buffer layer with (6 p 3◊ 6 p 3)R30 periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a non-vanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer layer.

Quasi-freestanding Graphene on SiC(0001)

We report on a comprehensive study of the properties of quasi-freestanding monolayer and bilayer graphene produced by conversion of the (6√3×6√3)R30° reconstruction into graphene via intercalation of hydrogen. The conversion is confirmed by photoelectron spectroscopy and Raman spectroscopy. By using infrared absorption spectroscopy we show that the underlying SiC(0001) surface is terminated by hydrogen in the form of Si-H bonds. Using Hall effect measurements we have determined the carrier concentration and type as well as the mobility which lies well above 1000 cm2/Vs despite a significant amount of short range scatterers detected by Raman spectroscopy.

Deriving the kinetic parameters for Pt-silicide formation from temperature ramped in situ ellipsometric measurements ☆

Abstract Single-wavelength ellipsometry is employed to monitor in situ the reaction of 23 nm platinum layers with Si(100) to form platinum silicides during heating at constant rates up to 100 K/min. A previous study showed that by the use of ‘Kissinger’ plots the activation energy of the intermediate silicide phase Pt2Si and that of the final PtSi can be determined with an accuracy of 50 meV. It is the purpose of the present paper to extend the study one step further and obtain also the pre-exponential growth factor by directly modeling the evolution of the ellipsometric data as they were obtained during the temperature ramp. To do so, the chemical composition of the reaction products and the thickness of the formed layers were identified at crucial stages of the reaction via RBS. Additionally, the optical constants of the constituent phases Pt, Pt2Si and PtSi were calculated from spectroscopic ellipsometry measurements in the range from 1.5 to 4.5 eV. With this input information the evolution of the ellipsometric angles during the reaction of Pt and Si were modeled quantitatively. The analysis yields good fits to the data for different ramp rates with activation energies of 1.50 and 1.70 eV and reaction coefficients of 37 and 27 cm2/s for the Pt/Si to Pt2Si and Pt2Si to PtSi formation, respectively. A significant improvement of the fit is obtained when the activation energies are allowed to be distributed about their mean value with a width of 47 meV. This distribution takes inhomogeneities in the reaction process into account and leads to a broadening of the reaction fronts.