H. R. Philipp

Bio: H. R. Philipp is an academic researcher from General Electric. The author has contributed to research in topics: Absorption (logic) & Electron. The author has an hindex of 14, co-authored 16 publications receiving 4791 citations.

Optical Properties of Semiconductors

Abstract: Reflectance data are presented for Si, Ge, GaP, GaAs, InAs, and InSb in the range of photon energies between 1.5 and 25 eV. The real and imaginary parts of the dielectric constant and the function describing the energy loss of fast electrons traversing the materials are deduced from the Kramers-Kronig relations. The results can be described in terms of interband transitions and plasma oscillations. A theory based on the frequency-dependent dielectric constant in the random phase approximation is presented and used to analyze these data above 12 eV, where the oscillator strengths coupling the valence and conduction bands are practically exhausted. The theory predicts and the experiments confirm essentially free electron-like behavior before the onset of $d$-band excitations and a plasma frequency modified from that of free electrons due to oscillator strength coupling between valence and $d$ bands and $d$-band screening effects. These complications are absent in Si. The energy loss functions obtained from optical and characteristic energy loss experiments are also found to be in good agreement. Arguments for interpreting structure in the reflectance curves above 16 eV in terms of $d$-band excitations are given.

Optical Properties of Ag and Cu

Abstract: Experimental data for the optical constants of Ag and Cu extending to 25 eV are discussed in terms of three fundamental physical processes: (1) free-electron effects, (2) interband transitions, and (3) collective oscillations. Dispersion theory is used to obtain an accurate estimate of the average optical mass characterizing the free-electron behavior over the entire energy range below the onset of interband transitions. The values are ${m}_{a}=1.03\ifmmode\pm\else\textpm\fi{}0.06$ for Ag and 1.42\ifmmode\pm\else\textpm\fi{}0.05 for Cu. The interband transitions to 11 eV are identified tentatively using Segall's band calculations. Plasma resonances involving both the conduction band and $d$ electrons are identified and described physically.

Optical Properties of Graphite

Abstract: The complex dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})={\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2}$ and associated functions are derived by application of the Kramers-Kronig relation to reflectance data for graphite obtained in the energy range to 26 eV. It is possible to divide the optical properties into two spectral regions. In the range 0 to 9 eV, intra- and interband transitions involve mainly the $\ensuremath{\pi}$ bands. At higher energies, a broad absorption peak near 15 eV is associated with interband transitions involving the 3 $\ensuremath{\sigma}$ electrons per atom. This viewpoint is strongly supported by evaluation of the sum rules for ${n}_{\mathrm{eff}}$. Plasma resonances which produce peaks in the energy-loss function $\ensuremath{-}\mathrm{Im}{\ensuremath{\epsilon}}^{\ensuremath{-}1}$ at 7 and 25 eV are identified and described physically. At low energies, structure in the reflectance curve near 0.8 eV is attributed to the onset of transitions between the ${E}_{2}$ and ${E}_{3}$ bands at the point $K$. This yields a value for ${\ensuremath{\gamma}}_{1}$ of \ensuremath{\approx}0.4 eV.

Optical Constants of Silicon in the Region 1 to 10 ev

Abstract: The reflectance, ${|r(\ensuremath{\lambda})|}^{2}$, of single crystal silicon was measured in the range 1 to 11.3 ev. The phase, $\ensuremath{\theta}(\ensuremath{\lambda})$, was computed from these data using the Kramers-Kronig relation between the real and imaginary parts of the complex function $\mathrm{ln}r=\mathrm{ln}|r|+i\ensuremath{\theta}$. The optical constants, $n$ and $k$, were then determined from the Fresnel reflectivity equation. The real part of the refractive index, $n$, shows a sharp maximum of magnitude 6.9 at 3.3 ev. The extinction coefficient, $k$, shows maxima of magnitude 3.1 at 3.5 ev and 5.1 at 4.3 ev; optical absorption above 3 ev is associated with the onset of strong direct transitions. The results indicate that much useful information, applicable to band structure calculations for both silicon and germanium, could be obtained from limited reflectance studies (2 to 5 ev) on Ge-Si alloys.

Optical Properties of Aluminum

Abstract: The frequency-dependent complex dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})={\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2}$ and associated functions are derived in the range 0 to 22 eV by application of the Kramers-Kronig relations to existing reflectance data for clean Al surfaces. The results are quantitatively interpreted in terms of intra- and interband transitions as well as plasma oscillations. The decomposition of $\ensuremath{\epsilon}(\ensuremath{\omega})$ into intra- and interband parts given here is seen to be valid in the presence of electron-electron interactions. Due to these interactions the optical effective mass ${m}_{a}=1.5$, deduced from experiment in the free-carrier region, is appreciably larger than that obtained using Segall's band calculations (${m}_{\mathrm{ac}}\ensuremath{\cong}1.15$). The band calculations are extended to higher energies in order to examine the effect of interband transitions for the range of interest. It is found that the only interband transitions which lead to significant structure in ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ are those that occur around $W$ and $\ensuremath{\Sigma}$ in the vicinity of $K$ in the Brillouin zone and that these produce a peak near 1.4 eV. These conclusions are in accord with the experimentally determined ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ which exhibits a peak at 1.5 eV and has no further structure at higher energies. The result of a quantitative calculation of the structure in ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ using a fine mesh of points in k space and an approximate variation of the momentum matrix element with k is in good agreement with the experimental results with respect to shape but has a magnitude which is somewhat too low. From the known influence of many-electron effects on the intraband contribution to $\ensuremath{\epsilon}(\ensuremath{\omega})$ and a general sum rule, the corresponding effect on interband transitions may be estimated and shown roughly to account for the difference. The derived $\ensuremath{\epsilon}(\ensuremath{\omega})$ indicates the presence of a sharp plasma resonance at $\ensuremath{\hbar}{\ensuremath{\omega}}_{p}=15.2$ eV, in excellent agreement with the results of characteristic energy loss experiments. It is shown that this resonance may be interpreted either in terms of electrons characterized by the low-frequency optical mass and screened by the interband dielectric constant at ${\ensuremath{\omega}}_{p}$ or, since the $f$ sum has been essentially exhausted, in terms of the exact asymptotic formula for $\ensuremath{\epsilon}$ in which all carriers are unscreened and have the free-electron mass.

Optical properties of metallic films for vertical-cavity optoelectronic devices.

Abstract: We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV

Abstract: We report values of pseudodielectric functions $〈\ensuremath{\epsilon}〉=〈{\ensuremath{\epsilon}}_{1}〉+i〈{\ensuremath{\epsilon}}_{2}〉$ measured by spectroscopic ellipsometry and refractive indices $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=n+ik$, reflectivities $R$, and absorption coefficients $\ensuremath{\alpha}$ calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, we have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Our results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among our data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ in optical measurements.

Light Absorption by Carbonaceous Particles: An Investigative Review

Abstract: The optical properties of the light-absorbing, carbonaceous substance often called “soot,” “black carbon,” or “carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp 2-bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m2/g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for s...

Advanced carbon electrode materials for molecular electrochemistry.

Abstract: 3.6.1. Polishing and Cleaning 2663 3.6.2. Vacuum and Heat Treatments 2664 3.6.3. Carbon Electrode Activation 2665 3.7. Summary and Generalizations 2666 4. Advanced Carbon Electrode Materials 2666 4.1. Microfabricated Carbon Thin Films 2666 4.2. Boron-Doped Diamond for Electrochemistry 2668 4.3. Fibers and Nanotubes 2669 4.4. Carbon Composite Electrodes 2674 5. Carbon Surface Modification 2675 5.1. Diazonium Ion Reduction 2675 5.2. Thermal and Photochemical Modifications 2679 5.3. Amine and Carboxylate Oxidation 2680 5.4. Modification by “Click” Chemistry 2681 6. Synopsis and Outlook 2681 7. Acknowledgments 2682 8. References 2682