Journal of Applied Physics

The morphology and distribution of the stripes caused by Cu surface reconstruction were measured, and the effects of stripes on graphene stability were studied by oxidation and corrosion. The results reveal that the stripes are determined by the crystal orientation of both the Cu surface and graphene, which can both change the stripe distribution, and the stripes can also be influenced by the graphene thickness. The stripes would not induce cracks or destruction to the graphene. The oxidation resistance of graphene can be improved by Cu surface reconstruction. The local nonuniform distortion of the stripe area may induce a bigger strain in the graphene which, in turn, may induce structure instability and result in local stability degeneration in the stripe area.

Photoemission experiments on 3d transition metals are reviewed. The emphasis is on understanding the results of experiments, not on experimental details and methods. Extensive use is made of simple models. Much of the review pertains to resonances associated with the autoionization 3p53dn+1 →3p63dn−1 +e and their implications for electronic structure. Nonresonant ultraviolet and x-ray photoemission spectroscopies are also discussed. Photoemission and photoabsorption of transition-metal atoms are discussed first. Results for Mn are described at length to establish the validity of the autoionization mechanism. The results from atomic spectroscopy are used to interpret experiments on solids. The role of atomiclike excitations in solids is examined. Compounds of transition metals are analyzed in terms of ligand-field theory, which is shown to be inadequate. Newer theories involving configuration interaction are shown to agree better with experiment. Various mechanisms for the excitation of photoemission satellites are presented. In the metallic state, effects similar to those observed for the compounds occur. The existence of two-bound-hole final states is demonstrated. Their importance in Auger spectroscopy, valence- and core-emission satellites, and resonant photoemission is discussed. The effects of closely related electron correlations on the band structure are described.

This paper reviews the electrical, magnetic, and optical properties of diluted magnetic semiconductors (sometimes also referred to as ‘‘semimagnetic’’ semiconductors). These materials are ternary semiconductor alloys whose lattice is made up in part of substitutional magnetic ions. Cd1−xMnxTe and Hg1−xMnxTe are examples of such systems. As semiconductors, these alloys display interesting and important properties, such as the variation of the energy gap and of effective mass with composition. They also exhibit magnetic properties which are interesting in their own right, e.g., a low temperature spin glass transition and magnon excitations. Most importantly, however, the presence of substitutional magnetic ions in these alloys leads to spin–spin exchange interaction between the localized magnetic moments and the band electrons. This in turn has rather important consequences on band structure and on donor and acceptor states, leading to dramatic effects in quantum transport, impurity conduction, and magneto-optics. Specifically, the presence of exchange interaction results in extremely large and temperature dependent g-factors of electrons and holes; in gigantic values of Faraday rotation; in anomalously large negative magnetoresistance; and in the formation of the bound magnetic polaron.

Ferromagnetic resonance (FMR) has been used to study a wide variety of very thin single crystals of Fe grown on (110) GaAs substrates by molecular beam epitaxy. Data were taken at room and liquid nitrogen temperatures for films with thicknesses L in the range 18–200 Å. Due to surface anisotropy, the easy axis of the magnetization switches from [100] to [110] when L≤50 Å, independent of whether the the film surface is passivated by an Al-overcoat or has a thin Fe oxide surface layer. We suggest that this is an effective surface anisotropy arising in part from a depth dependent strain near the film-substrate interface. The changes in the parameters describing the angular dependence of the FMR spectra upon cooling to 77 K can be explained as due to magnetostriction arising from thermally induced strains plus the temperature dependence of the cubic volume anisotropy. The FMR linewidth is shown to be a linear function of frequency in the range 5–40 GHz.

Techniques are developed for deriving both qualitative and quantitative information on the kinetics of gas desorption from measurements at continuously changing temperature. First- and second-order processes can be distinguished immediately by the constancy of the end point of the former. Quantitative values for activation energy and frequency factors are deduced from the experimental time-temperature curve and the instantaneous slopes of the evolution curve, even for systems with concentration-dependent rate parameters. It is shown that for multiple desorption peaks, qualitative detection is simplified by slow heating, but may result in interconversion. The experimental basis of desorption measurements using the Bayard-Alpert gauge is also analyzed, together with artifacts arising from negative pressures, bistable gauge operation, formation of new species in the gauge, and the delay in sensing density pulses transmitted through tubes.

Previous measurements of the critical temperature (Tcrit) for the first-order antiferromagnetic-ferromagnetic transition in FeRh as a function of magnetic field have established that the total entropy change at the transition (ΔS) is much larger than the estimated change in lattice entropy (ΔS)lat. This study has now been extended to pseudobinary variants of FeRh where the Rh is partially replaced by Pd, Pt, or Ir, resulting in a large decrease or increase of Tcrit from its value (about 330°K) for FeRh. In each case, a value for ΔS was deduced from the measured field dependence of Tcrit. The difference between each ΔS and an estimated (ΔS)lat value, when plotted vs Tcrit, defines a smooth curve with a maximum at about 500°K, which is just below the Curie points of these alloys. It is therefore concluded that ΔS−(ΔS)lat represents an entropy change of magnetic origin. This anomalous change in magnetic entropy is attributed to thermal excitation of the Rh moments which in the ferromagnetic state are induced by the net exchange field from neighboring Fe moments; in the antiferromagnetic state this net exchange field vanishes, and the induced Rh moments and their contribution to the entropy go essentially to zero.

The temperature dependences of heat expansion, elastocaloric effect, magnetocaloric effect, and the shift in the critical temperature of the transition due to tensile stress have been measured using samples of the same concentration Fe49Rh51 in the antiferromagnetic-ferromagnetic (AF-F) transition range. Using data on specific heat and magnetocaloric effects, entropy-temperature diagrams for the alloy were made for various magnetic fields. The ratios ΔS(TcH)/TcH, where ΔS(TcH) is the entropy change during transition and TcH is the transition’s critical temperature, were found to be ∼3.98×102 erg/g K2 which is close to the value of the change in the electronic specific heat coefficient Δγ obtained by other researchers. It has been concluded that the change in the electronic part of entropy is the main mechanism of the transition. The phenomenological model is proposed, taking into account the electronic entropy change during the transition. Calculations using the model give values for the main thermodynamic parameters of the transition (free energy change ΔF=1.91×106 erg/g, the critical temperature shift ∂Tc/∂P=3.08×10−9 K cm2/Dyn and ∂Tc/∂H=0.788 K/kOe due to hydrostatic pressure and magnetic field respectively), which are in agreement with experimental data.

The chemical composition of the CuInSe2/CdS heterojunction interface is investigated by angle resolved x-ray photoelectron spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy in combination with selective etching of CdS. We demonstrate that ∼0.8 monolayer of Cd is incorporated into the first 1–3 atomic layers of the CuInSe2. This is accompanied by significant Cu depletion with respect to In in the same region. The results suggest that CdCu defects heavily dope CuInSe2 surface n type and cause the observed large band bending on the CuInSe2 side of the heterojunction.

Microstructural and chemical properties of the interfaces between Cu(In,Ga)Se2 (CIGS) and In2S3 layers in dependence on the In2S3 deposition temperature and Na concentration were investigated. The In2S3 layers were deposited by atomic layer deposition on CIGS layers at substrate temperatures ranging from 140°C to 240°C. Interfaces were investigated by means of scanning electron microscopy, bright-field and high-resolution transmission electron microscopy, electron diffraction, and energy-dispersive x-ray spectrometry. An orientation relationship between CIGS {112) and In2S3 {103) planes was found for the sample deposited at 210°C, whereas no orientation relationship was detected for the 240°C sample. Cu diffusion from CIGS into In2S3 was detected, as well as Cu depletion and In enrichment on the CIGS side of the interface. All three effects are enhanced with increasing deposition temperature. These results indicate the formation of a buried junction in the CIGS layer. In addition, a Na-free solar cell was investigated. The results show that In2S3 grain sizes are smaller than in solar cells containing Na. Also, enhanced Cu and Ga diffusion from the CIGS absorber into the In2S3 buffer as well as enhanced Cu depletion and In enrichment on the CIGS side of the interface were detected. This may indicate that both Na and Cu occupy vacancies and In sites in the In2S3 tetragonal spinel structure.

The surface Cu-depletion of chalcopyrite thin films and its influence on the interface properties of related solar cells have been subject of a controversial debate for many years. Although the nature of this Cu-depletion and its extension in depth are crucial for the device physics, there are only a few contradictory experimental results that address this topic. To clarify this issue, we performed depth-dependent compositional analysis by angle dependent soft x-ray emission spectroscopy (AXES) on Cu(In,Ga)Se2 thin films with different integral Cu-contents. By considering depth profiles from literature and by taking the accuracy of AXES into account, our numerical AXES simulations predict a pronounced angle dependence for our samples. However, our experimental data show only a minor angle dependence, which leads to the conclusion that the Cu-depleted surface layer must be restricted to a very thin surface layer, which is not accessible by AXES. This conclusion is consistent with the result from our previous investigation by hard x-ray photoelectron spectroscopy, where we found a Cu-depleted surface layer in the subnanometer regime. Consequently the present study gives further experimental evidence for the surface reconstruction model proposed by first-principles calculations. Supported by secondary neutral mass spectroscopy, we show that the minor angle dependence in our AXES data can be attributed to a Ga-gradient in the chalcopyrite material.