Electron and hole conductivity in CuInS2

Single crystals of CuInS₂ have been grown from the melt and annealed in In or S to produce good n- or p-type conductivity, respectively. Two donor levels, one shallow and one deep (0.35 eV), and one acceptor level at 0.15 eV are identified. The hole-mobility data are best fitted with an effective mass $\text{m}{}_{\mathrm{<mtext>p</mtext>}}\text{1?1.3m}{}_{\mathrm{e}}$, which can be explained by simple, two band $\text{k}$. $\text{p}$ theory if the valence band has appreciable d character. Above 300°K, the hole mobility falls rapidly, evidently due to multiband conduction and/or interband scattering between the nondegenerate and degenerate valence bands. The conduction band mobility appears to be dominated, in many samples, by large concentrations ( >10¹⁸cm^?3) of native donors and acceptors, which are closely compensated.     

Study of the photoluminescence spectrum in high purity CdTe

In this work, a study of the photoluminescence produced by a high purity sample of n-type CdTe of $\text{?}\text{N}{}_{\mathrm{<mtext>D</mtext>}}\text{?}\text{N}{}_{\mathrm{<mtext>A</mtext>}}\text{? < 10}{}^{14}$ impurities per cm³ was done at several temperatures, varying from 10 to 35 K. Several sharp lines were observed in the spectral region between 1.5 to 1.6 eV, plus the well-known 1.4 eV band with several well-defined structures on it. The observed temperature behaviour of the line positions, linewidths and relative intensities allowed us to establish the presence of a new transition, located at 10 K 21.3 meV below in energy from the free exciton (FE) line, as well as its first phonon replica. Its nature seems to be transitions originating from the conduction band to an acceptor level, 32 meV above the valence band. These two lines appear at the same position where previous works had reported the first and second phonon replicas of the FE. A scheme of impurity level is proposed to explain the observed transitions in terms of previously established levels and this new acceptor level.     

Electronic structure of the GeGaAs (111) and (111) heterojunctions

The electronic density of states for GeGaAs (111) and ($\text{1}$$\text{1}$$\text{1}$) heterojunctions has been calculated. No interface states in the fundamental gap are found. A sizeable density of interface states below the top of the valence band is found for GeGa bonds-(111) junctions-interface states in the ionic gap are reported. The effect of varying the amount of the valence band discontinuity across the interface is discussed.     

Angular momentum decomposition of k = 0 Bloch functions in group IV and zincblende crystals

The angular momentum contents of $\text{k}\text{= 0}$ Bloch functions for a number of group IV and zincblende crystals are examined using a pseudopotential approach. The large amplitudes for d and f character functions found even for valence electron wave functions are related to the overlap of p and s symmetry states on different atoms. In zincblende crystals the strength of the d and f components increases around cations and decreases around anions for the valence bands. The f-like component is appreciable for the Γ₁ conduction band particularly around a cation.     

Electrical properties of narrow gap semiconductor PtSb2

Results of measurements of conductivity, Hall and Seebeck coefficients of tellurium doped n-type crystals of platinum antimonide are presented. The Hall coefficient and the Seebeck coefficient undergo sign inversion twice, below and above room temperature. The detailed analysis of the experimental results revealed that below 200 K PtSb₂ can be described by a simple conduction and valence band model. The energy gap E_g = (110?0.15 × T) (meV), the electron conductivity mass m_n^c/m₀ = 0.35, acoustic phonon limited electron mobility 〈μ_a〉_n = 3 × 10⁶ T^{?$\text{3}\text{2}$} ($\text{cm}{}^2\text{V · s}$) and mobility ratio 〈μ_a〉_n/〈μ_a〉_p = 0.4 are determined. However, at higher temperatures a more complicated valence band model is needed to account for the experimental results. The arguments for the existence of subsidiary valleys in the valence band are presented.     

Hole mobility in p-type HgTe

Experimental data concerning the electrical conduction and Hall coefficient in HgTe samples with acceptor states have been collected and analysed. In the analysis three ranges of acceptor concentration have been distinguished: a low concentration range up to about 5 × 10¹⁵ cm^?3 (pure samples), a high concentration range from 10¹⁶ to 10¹⁸ cm^?3 (p-like samples), and an extremely high concentration range above 10¹⁸ cm^?3 (p-type samples). In pure HgTe samples the holes are in the valence band, in p-like samples the “holes” are in the impurity band, and in p-type HgTe samples the holes are in a strong mixing impurity-valence band. The mobility of holes in the valence band is of the order of $\text{10}{}^5\text{cm}{}^2\text{Vs}$. The mobility of “holes” in the impurity band decreases with increasing impurity concentration from about $\text{5 × 10}{}^3\text{cm}{}^2\text{Vs}$ to $\text{125}\text{cm}{}^2\text{Vs}$. The mobility of holes in p-type HgTe samples is independent of the acceptor concentration and is equal to $\text{125}\text{cm}{}^2\text{Vs}$.     

Separation of the partial s- and p-densities of valence states of ZnO from UPS-measurements

UPS-spectra of the cleaved (0001) Zn and (000$\text{1}$) O surfaces of ZnO are taken at hv = 16.8, 21.2, 26.9, and 40.8 eV. Two maxima in the spectra at constant final energy are ascribed to high densities of conduction band states. Using the hv-dependence of the valence band emission, the partial s- and p-densities of states are separated. They yield similar excitation probabilities for Zn-4s-, 3d-, and O-2p-electrons.     

Optical absorption due to transitions between bands and local levels in CdCr2Se4

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr₂Se₄ is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)^{($\text{1}\text{3}$)}. The second type of absorption has a dependence on photon energy ?ω of the form (?ω ? E₃)², where E₃ is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr₂Se₄ at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.     

Energy loss mechanisms in low energy electron scattering from W(100) and their use as a surface sensitive spectroscopy

The energy loss spectrum of low energy (0 < E_p < 200 eV) electrons scattered from W(100) has been experimentally investigated, and mechanisms giving rise to the fine structure analyzed using a dielectric response formalism. The dielectric medium is characterized by available optical data and energy band calculations for tungsten. All of the structure for loss energies, w, less than 18 eV is attributed to intra- and interband transitions involving the bulk valence and conduction bands. The surface and bulk plasmon excitations are observed at w = 21 eV and w = 25.5 eV respectively which is in reasonable agreement with the optical data. A very narrow peak in the density of conduction d-band states apparently functions strongly in well defined excitations involving the $\text{5p}\text{3}\text{2}$ and 4f tungsten orbitais and the 2s and 2p orbitais of adsorbed oxygen. These conduction band states form a “window” with which to measure the electronic orbital structure of both the substrate and adsorbate during adsorption and reaction. We demonstrate this for the room temperature adsorption of oxygen on W(100) in which we observe the sequential filling of two electronically inequivalent binding states. The stability of the “d-band window” during thermally activated reaction, and the likelihood of its existence in other transition metals makes this an attractive surface sensitive spectroscopy.     

Addendum

Accurate valence band dispersions E$\text{k}$ of LaB₆ have been determined along three main symmetry lines ΓM, ΓX, and XM using one single crystal (001) surface by a simple new photoemission technique of general utility. Using direct interband transitions, E$\text{k}$dispersions are mapped out by suitably scanning polar emission angle and the photon energy. Detailed results agree well with recent Xα-APW energy band calculations.     

Subject index

Field-emission energy distributions from the (100) facet of Ge exhibit a double peak. Comparison of the measured distributions with theory shows that the lower energy peak arises from valence band emission while the higher energy peak represents emission from a band of surface states overlapping the valence band. The field-emission energy distribution from the surface states is a maximum at 0.18 eV above the valence band edge. The surface of the emitter is found to be 4kT degenerate n-type with an applied field of $\text{3 × 10}{}^7\text{V}\text{cm}$. This implies 6.3 × 10¹² surface states/cm² at the center of the clean, annealed (100) facet. The effect of the applied field is to broaden the surface state distribution. The degree of broadening can be accounted for by the Stark effect. Adsorption of contamination from the vacuum system ambient or geometric alteration of the surface from the annealed end form reduces the number of surface states.     

Splitting of the conduction bands of GaAs for k along [110]

The splitting of the spin degeneracy of the lowest conduction band of GaAs for $\text{k}$ along [110] is calculated by $\text{k}$·$\text{p}$ perturbation theory as well as by means of the self-consistent LMTO method. The magnitude and the sign of the calculated splitting agree with recent measurements of the spin precession in spin- polarized photoemission from GaAs [110] surfaces.     

Hall effect in GaTe single crystals

Hall effect measurements have been carried out over the temperature range 77–500K on p-type GaTe single crystals, grown from the melt. The results indicate that scattering by homopolar optical phonons polarized normally to the layers is largely dominant in GaTe. From the analysis of Hall mobility data, carried out according to the Schmid's model, a phonon energy $\text{h}\text{?}\text{ω = 9.8 meV}$ and the product between the coupling constant and the hole conduction mass along the layers g²m_{h⊥ = v.205me} has been found Finally, an acceptor center, lying at 138 meV from the valence band, displays an hydrogen-like behaviour, with its energy dependent from concentration according to the theory.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

The interaction of oxygen with gadolinium: UPS and XPS studies

The interaction of oxygen with evaporated Gd films at 300 K has been studied for the first time using AlK α XPS and Hel and Hell UPS. The characteristic changes in the Gd(6s5d) and O(2p) valence bands, Gd(4f), Gd(5p) and Gd(4d) core levels, and O(2s) and O(1s) core levels were studied. Evidence is presented for the initial formation of an intermediate oxidation state at low exposure (characterized by a new Gd valence band with a maximum in the DOS at ～ 2.5 ev below E_F and an ～ 0.6 eV shift in Gd(4f)) prior to formation of Gd₂O₃ where the Gd(6s5d) valence band disappears completely, as expected for Gd³⁺. In the higher exposure range there is little further increase in the oxide thickness, which is estimated as $\text{? 20}\text{A}\text{?}$, but there is a slow replacement of O by OH, as characterized by a second O(1s) feature at 532.3 eV and OH 1π and 3σ orbitals in UPS at ～ 6.7 and 11.5 eV. The interpretations are supported by parallel studies on bulk Gd₂O₃ and by Ar⁺ sputtering experiments. Comparisons are made to other rare earth oxidation studies.     

Angle resolved photoemission studies of the band structures of semiconductors: PbI2

Angle resolved photoemission data is presented for single crystals of the layered semiconductor PbI₂. The geometric form of the valence electron energy bands has been mapped using a simple approximation and the possible electronic origins of these bands are discussed. Such band dispersion effects in angle resolved measurements are contrasted with HeII excited data for the localised $\text{Pb}\text{5}\text{d}\text{5}\text{2}$ core level component, where “flat” band behaviour and only weak residual final state scattering effects may be discerned. Finally we comment on the role of the incoming photon in such experiments.     

Photoemission from single crystal xenon

Normal exit photoemission spectra from the (111) surface of a xenon single crystal indicate that the valence level emission is considerably broader than expected on the basis of band structure calculations. Similar observations from polycrystalline samples have lead Parrinello $\text{et al}$ to propose an excitonic screening mechanism which would cause an apparent increase in the spin-orbit splitting at the Γ point. The details of the spectra reported here show, however, that the effect could simply be due to a dispersion of the bands stronger than that predicted theoretically. Anomalous intensity effects at $\text{h}\text{?}\text{ω = 21.2}\text{eV}$ are explained in terms of electron-electron scattering.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Field-induced magnetic form factor of lutetium

The conduction electron field-induced magnetic form factor of Lu, a rare earth metal with a closed 4f shell, has been measured using the polarized neutron technique. The measurements demonstrate that the conduction electron density is more spatially extended than in the free atom. The experimental results are in good agreement with band calculations of the $\text{spin}$ magnetic form factor.     

The anisotropy of the hot-hole drift velocity in Ge

This paper presents a microscopical interpretation of the anisotropy of the hot-hole drift velocity in Ge at 77°K. The theory makes use of the Monte Carlo technique in solving the Boltzmann equation and is based on a single valence band model (the heavy mass one) warped and parabolic. The theoretical calculations carried out in a general form show that: (i) the anisotropy of the hot-hole drift velocity originates from the warping of the valence band structure; (ii) the strong interaction of holes with optical phonons via emission processes plays a fundamental role in the anisotropy supporting the reliability of the streaming motion model first suggested by Pinson and Bray.The comparison between theory and experiment at 77°K in Ge is satisfactory for the whole range of fields examined $\text{(50 ? E ? 10}{}^4\text{V}\text{cm}\text{)}$, confirming the basic validity of the present approach. The importance of optical and acoustic scattering mechanisms have been found comparable, the coupling strength ratio $\text{w}{}_{\mathrm{op}}\text{w}{}_{\mathrm{ac}}$ assuming the value 1.36.