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We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimentaldata,aswellastheoreticalcalculations,tounderstandtheroleoftheMndopantintheemergenceof ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

Intermediate-band semiconductors have attracted much interest in the photovoltaic community, as they are expected to allow higher solar-cell efficiencies than traditional absorbers. The authors directly measure minority-carrier lifetimes in Ga(P,As,N), only to find that these times are too short for efficient solar cells. The observed lifetimes seem not to be fundamental material properties, though, but rather the result of recombination at defects. This identifies material quality as possibly the most important issue preventing the development of efficient solar cells based on these semiconductors.

Copper impurities affect optical and electronic properties in a diverse range of materials. Using IR spectroscopy, we provide evidence for a shallow Cu acceptor in Si. The observation of ${P}_{1/2}$ excitation lines indicates a Cu acceptor with a hole ionization energy of only 27 meV, which is shallower than any of the group-III acceptors. The observed center has an IR spectrum that is characteristic of a hydrogenic acceptor. Variable-temperature Hall-effect measurements confirm this result. The observation of shallow, hydrogenic behavior is consistent with a universal level model, in which the Cu acceptor level is fixed relative to vacuum, irrespective of the host.

Semiconductors known as highly mismatched alloys (HMAs) have unique properties that make them useful in the fabrication of intermediate band solar cells (IBSCs). Now a theoretical and experimental study of GaNPAs with ~40% phosphorus and up to 2% nitrogen finds that this HMA has an electronic band structure well suited for IBSC applications.

We have measured the spectrum of stimulated far-infrared emission from Be- and Zn-doped germanium single crystals in crossed electric and magnetic fields. The spectrum consists of broad radiation bands in the frequency range of $40--130{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ that are due to intervalence band transitions and new narrow emission bands with their center frequencies rising linearly with magnetic field. We demonstrate that the latter emission is due to cyclotron resonance hole transitions between Landau levels of the light-hole subband accompanied by simultaneous spin flip, ``combined resonances.''

We have developed a theoretical model describing carrier scattering by divalent impurities in semiconductors. The mobility predicted by the model based on the scattering of electrons by helium atoms shows excellent agreement with the low-temperature mobilities measured for three Ge samples doped with different double acceptors; Be, Zn, and Hg. We show that the scattering cross sections of these double acceptors are the same despite the large difference in ionization energies. This supports our assumption that the contribution of the central-cell potential to neutral impurity scattering is negligible. @S0163-1829~97!06328-5#

The effect of uniaxial stress on the hole lifetime in gallium-doped germanium single crystals has been studied by photo-Hall-effect measurements. We demonstrate that the stress dependence of the hole lifetime offers a critical test for competing models of the capture of holes by acceptors. We find that below 5 K the hole lifetime increases by over one order of magnitude upon the application of 4 kbar in a [l angle]100[r angle] direction. Our results show the hole lifetime to be proportional to the acoustic-phonon-scattering component of the hole mobility, in very good agreement with the cascade capture theory for Coulomb-attractive centers and in clear contradiction to the recently proposed model in which holes are directly captured into the acceptor ground state [Phys. Rev. Lett. [bold 69], 2839 (1992)].

We use scanning tunneling microscopy to study the structure of graphene islands on Au(111) grown by deposition of elemental carbon at 950 \ifmmode^\circ\else\textdegree\fi{}C. Consistent with low-energy electron microscopic observations, we find that the graphene islands have dendritic shapes. The islands tend to cover depressed regions of the Au surface, suggesting that Au is displaced as the graphene grows. If small tunneling currents are used, it is possible to image simultaneously the graphene/Au moir\'e and the Au herringbone reconstruction, which forms underneath the graphene on cooling from the growth temperature. The delicate herringbone structure and its periodicity remain unchanged from the bare Au surface. Using a Frenkel--Kontorova model, we deduce that this striking observation is consistent with an attraction between graphene and Au of less than 13 meV per C atom. Raman spectroscopy supports this weak interaction. However, at the tunneling currents necessary for atomic-resolution imaging of graphene, the Au reconstruction is altered, implying influential tip--sample interactions and a mobile Au surface beneath the graphene.

We have studied uniaxially stressed Ge doped with Cu, a substitutional triple acceptor, by photoconductivity spectroscopy. We find that the ground state of this triple acceptor, normally accommodating three 1[ital s] holes, reconfigures into an atomic Li-like 1[ital s][sup 2] 2[ital s][sup 1] arrangement by the application of [100] stress. Above 4 kbar the first ionization potential reaches a constant value of 17.4 meV corresponding to the hole excitation from the now partially occupied 2[ital s] energy level. Excellent agreement was obtained between the experimental results and variational calculations.

We use low-energy electron microscopy to investigate how graphene grows on Cu(111). Graphene islands first nucleate at substrate defects such as step bunches and impurities. A considerable fraction of these islands can be rotationally misaligned with the substrate, generating grain boundaries upon interisland impingement. New rotational boundaries are also generated as graphene grows across substrate step bunches. Thus, rougher substrates lead to higher degrees of mosaicity than do flatter substrates. Increasing the growth temperature improves crystallographic alignment. We demonstrate that graphene growth on Cu(111) is surface diffusion limited by comparing simulations of the time evolution of island shapes with experiments. Islands are dendritic with distinct lobes, but unlike the polycrystalline, four-lobed islands observed on (100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial graphene on smooth, clean Cu(111) has fewer structural defects than it does on Cu(100)., Article revised following reviewer comments

We demonstrate a direct connection between the magnetic easy axis in Mn-doped GaP and epitaxial strain by a combined ferromagnetic resonance, x-ray diffraction and superconducting quantum interference device magnetometry study. The magnetic easy axis of ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{P}$ is gradually rotated from the in-plane $[0\overline{1}1]$ direction toward the film normal [100] through alloying with isovalent N which changes the strain state of the film from compressive to tensile. For a nearly lattice-matched film the strain-related component to the out-of-plane uniaxial anisotropy field is close to zero. Both in-plane and out-of-plane magnetization reversal processes are explored by a simple model that considers the combination of coherent spin rotation and noncoherent spin switching. We use our results to estimate domain-wall sizes and energetics, which have yet to be directly measured in this materials system. The band structure and electrical properties of ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{P}$ imply that holes localized within a Mn-derived impurity band are capable of mediating the same anisotropic exchange interactions as the itinerant carriers in the canonical ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ system.

We study the effects of variable hole concentration on the transport, thermoelectric, and magnetic properties of ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$. The hole concentration in samples with fixed Mn content has been varied using high energy particle irradiation, which introduces donorlike defects that compensate Mn acceptors without changing the concentration of localized Mn spins. As expected, a decrease of the hole concentration results in a reduction of the Curie temperature and an increase in electrical resistivity and thermoelectric power. The mobility and thermopower data are then analyzed in terms of models based on free holes in the valence band and holes localized in a Mn impurity band. The energetic structure of the impurity band is described by the valence-band anticrossing model. We show that the electronic structure provided by the impurity band model is consistent with the experimental results.

A systematic investigation of ferromagnetic resonance (FMR) was carried out on ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ layers synthesized by Mn ion implantation into GaAs followed by pulsed laser melting. Angular and temperature dependences of FMR were measured on layers prepared on GaAs (001), (110), and (311) surfaces. The observed angular dependence of FMR can be understood in terms of contributions from cubic anisotropy fields defined by the crystal symmetry of ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ and uniaxial anisotropy fields perpendicular or parallel to the film plane. For completeness, the angular dependence of the FMR linewidth was also investigated and was found to be dominated by broadening ascribed to local inhomogeneities in magnetic anisotropy. Our results show that both the magnetic anisotropy and the FMR linewidth in (Ga,Mn)As prepared by ion implantation are similar to those observed on ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ samples grown by low-temperature molecular beam epitaxy, indicating that the two very different growth methods lead to materials with fundamentally similar magnetic properties.

We employ Faraday and Kerr effect spectroscopy in the infrared range to investigate the electronic structure of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ near the Fermi energy. The band structure of this archetypical dilute-moment ferromagnetic semiconductor has been a matter of controversy, fueled partly by previous measurements of the unpolarized infrared absorption and their phenomenological impurity-band interpretation. Unlike the unpolarized absorption, the infrared magneto-optical effects we study are intimately related to ferromagnetism, and their interpretation is much more microscopically constrained in terms of the orbital character of the relevant band states. We show that the conventional theory of the disordered valence band with an antiferromatnetic exchange term accounts semiquantitatively for the overall characteristics of the measured infrared magneto-optical spectra.

We report the effect of dilute alloying of the anion sublattice with S on the in-plane uniaxial magnetic anisotropy and magnetization reversal process in ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{P}$ as measured by both ferromagnetic resonance (FMR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry. At $T=5\text{ }\text{K}$, raising the S concentration increases the uniaxial magnetic anisotropy between in-plane $⟨011⟩$ directions while decreasing the magnitude of the (negative) cubic anisotropy field. Simulation of the SQUID magnetometry indicates that the energy required for the nucleation and growth of domain walls decreases with increasing $y$. These combined effects have a marked influence on the shape of the field-dependent magnetization curves; while the $[0\overline{1}1]$ direction remains the easy axis in the plane of the film, the field dependence of the magnetization develops double hysteresis loops in the [011] direction as the S concentration increases, similar to those observed for perpendicular magnetization reversal in lightly doped ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$. The incidence of double hysteresis loops is explained with a simple model whereby magnetization reversal occurs by a combination of coherent spin rotation and noncoherent spin switching, which is consistent with both FMR and magnetometry experiments. The evolution of magnetic properties with S concentration is attributed to compensation of Mn acceptors by S donors, which results in a lowering of the concentration of holes that mediate ferromagnetism.

We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4percent of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4 percent As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature TC from 113 K to 60 K (100 K to 65 K) upon the introduction of 3.1 percent P (1percent N) into the As sublattice.

While the support for the existence of a Mn-derived impurity band in the diluted magnetic semiconductor ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ has recently increased, a detailed quantitative analysis of its formation and properties is still incomplete. Here, we show that such an impurity band arises as the result of an anticrossing interaction between the extended states of the GaAs valence band and the strongly localized Mn states according to the valence band anticrossing model. The anticrossing interpretation is substantiated by optical measurements that reveal a shift in the band gap of GaAs upon the addition of Mn and it also explains the remarkably low hole mobility in this alloy. Furthermore, the presence of a Mn-derived impurity band correctly accounts for the metal-to-insulator transition experimentally observed in ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}{\text{As}}_{1\ensuremath{-}y}{(\text{N},\text{P})}_{y}$ with $y\ensuremath{\le}0.02$.

We report the observation of ferromagnetic resonance (FMR) and the determination of the magnetocrystalline anisotropy in (100)-oriented single-crystalline thin film samples of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{P}$ with $x=0.042$. The contributions to the magnetic anisotropy were determined by measuring the angular and the temperature dependencies of the FMR resonance fields and by superconducting quantum interference device magnetometry. The largest contribution to the anisotropy is a uniaxial component perpendicular to the film plane; however, a negative contribution from cubic anisotropy is also found. Additional in-plane uniaxial components are observed at low temperatures, which lift the degeneracy between the in-plane [011] and $[01\overline{1}]$ directions as well as between the in-plane [010] and [001] directions. Near $T=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the easy magnetization axis is close to the in-plane $[01\overline{1}]$ direction. All anisotropy parameters decrease with increasing temperature and disappear above the Curie temperature ${T}_{C}$. A consistent picture of the magnetic anisotropy of ferromagnetic ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{P}$ emerges from the FMR and magnetometry data. The latter can be successfully modeled when both coherent magnetization rotation and magnetic domain nucleation are considered.

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.

We show that the band gap bowing trends observed in III-V alloys containing dilute concentrations of Sb or Bi can be explained within the framework of the valence-band anticrossing model. Hybridization of the extended $p$-like states comprising the valence band of the host semiconductor with the close-lying localized $p$-like states of Sb or Bi leads to a nonlinear shift of the valence-band edge and a reduction of the band gap. The two alloys $\mathrm{Ga}{\mathrm{Sb}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ and $\mathrm{Ga}{\mathrm{Bi}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ are explored in detail, and the results are extrapolated to additional systems.

We report an energy gap for hole photoexcitation in ferromagnetic Ga{sub 1-x}Mn{sub x}P that is tunable by Mn concentration (x {le} 0.06) and by compensation with Te donors. For x{approx}0.06, electrical transport is dominated by excitation across this gap above the Curie temperature (T{sub c}) of 60 K and by thermally-activated hopping below T{sub c}. Magnetization measurements reveal a moment of 3.9 {+-} 0.4 {micro}{sub B} per substitutional Mn while the large anomalous Hall signal unambiguously demonstrates that the ferromagnetism is carrier-mediated. In aggregate these data indicate that ferromagnetic exchange is mediated by holes localized in a Mn-derived band that is detached from the valence band.