Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Surface core level shifts of A II–VI compound: CdTe

The surface core level shifts (SCLS) of the Cd and Te 4d levels in the compound semiconductor CdTe have been measured. Values of 0.24 ± 0.05 eV for Cd and −0.26 ± 0.05 eV for Te were obtained. These results cannot be explained solely by the reduction in the Madelung potential at the surface; other factors are at least as important. We also show that SCLS can be related to heats of segregation of n-type impurities via a Born-Haber cycle.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Surface micromorphology of CdTe(310) layers grown by molecular beam epitaxy

Surface morphology of CdTe(310) buffer layers grown by molecular beam epitaxy has been investigated by the method of reflection of high-energy electron diffraction. It was established that a clean CdTe(310) surface is atomically flat. Its reconstruction can be described by a unit cell coinciding with the unit cell of the unreconstructed (310) surface. It is determined that Te₂ adsorption in amounts of less than 0.2 monolayers results in the surface reconstruction with the formation of terraces parallel to the (100) plane and are 3/2a long. A system of (100) + (210) facets develops on the CdTe(310) surface with the increase in the Te adsorption layer’s thickness up to 0.3 monolayers and more.     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

Studies of the surface structures formed by the alternated electrodeposition of Cd and Te on the low-index planes of Au: II. STM studies

Scanning tunneling microscopy (STM) studies of the alternated deposition of Te and Cd atomic layers on the low-index planes of Au are presented here. These investigations were designed to gain an understanding of the surface chemistry involved in the deposition of CdTe by electrochemical atomic layer epitaxy (ECALE). ECALE is the electrochemical analog of atomic layer epitaxy (ALE). In the present study, the structures formed by the alternated electrodeposition of Te and Cd atomic layers, at underpotential, were examined by STM on the low-index planes of Au.

Deposition of atomic layers of each element were achieved by underpotential deposition (UPD). Studies of the UPD of Cd by STM were not performed due to Cd's instability in aqueous solutions after withdrawal from solution and loss of potential control. Studies of Te UPD were performed, however, since Te is stable in aqueous media without potential control. Ordered Te structures were observed using STM on all three Au low-index planes. Atomic resolution imaging of Cd UPD on Te atomic layers proved more difficult than imaging simple Te deposits, however, ordered CdTe structures were observed on each Au plane. Higher bias voltages were generally required to image the CdTe deposits. On Au(100) the CdTe deposit appears to adopt a structure equivalent to a (100) slice of bulk CdTe (zinc blende), but contracted 10% due to the lattice missmatch on the Au surfaces. Similar structures were observed on the other two low index planes, as well; although Cd-Cd distances were closer to those found in bulk CdTe.     

Interface formation in CdTe solar cells: Nucleation of CdTe on CdS(0001) and (10$ \bar 1 $0)

The growth of CdTe on (0001) and (10 0) surfaces of CdS was studied using low energy electron diffraction and photoelectron spectroscopy. The results indicate oriented growth of the CdTe films with a faster nucleation of CdTe on CdS(0001), explaining the preferred (111) orientation of CdTe films on polycrystalline substrates. The faster nucleation on the (0001) surface is attributed to the formation of a stable CdS(0001):Te surface termination, which is identified from surface sensitive Te 4d spectra and a 2√3 × 2√3 surface reconstruction. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Type III - Type I transition and strain-effect in Hg1−xCdxTeCdTe and Hg1−xZnxTeCdTe superlattices

In this paper, the results of Hg_1−xZn_xTeCdTe strained layer superlattices grown by MBE are reported, and compared to Hg_1−xCd_xTeCdTe superlattices. Both Type III and Type I Hg_1−xZn_xTeCdTe superlattices with different strain have been grown on CdTe(111)B/GaAs(100) and CdTe(100)/GaAs(100) substrates and characterized by electron, X-ray diffraction, infrared transmission and Hall measurements. The values of hole mobility between 5×10³ up to 2×10⁴cm²v⁻¹s⁻¹ at T = 23K along (111)B growth orientation and up to 4.9×10⁴cm²v⁻¹s⁻¹ at T = 5K along (100) growth orientation are obtained for Type III superlattices whereas in Type I superlattices, the hole mobility is between 200–300cm²v⁻¹s⁻¹. This drastic change in the hole mobility between Type III and Type I superlattices along with the role of the strain are discussed in this paper.     

Band bending at the Ni/Si(100)-2×1 submonolayer interface

Band bending at the Ni/Si(100)-2×1 interface has been monitored by using Si 2p core level photoemission spectra. Two nickel-induced Si 2p components appear in the initial interaction between Ni and Si(100)-2×1, which is confined at the top surface and the first subsurface layers. At Ni coverage less than 0.0375 ML, Ni atoms prefer the adamantane interstitial sites on the first subsurface, but switch to the pedestal sites on Si dimer rows at higher Ni coverage. The change in the preferred occupation sites of Ni atoms on the Si(100)-2×1 surface strongly affects the amount of band bending shift. The shift towards higher binding energy, when Ni atoms occupy the adamantane interstitial sites, is attributed to metal-induced-gap states. While Ni atoms occupy the pedestal sites, the band bending shift is reduced which is attributed to the passivation of surface states.     

XPS investigation of electron beam effects on a trimethylsilane dosed Si(100) surface

A trimethylsilane covered Si(100) surface at temperature −120°C was bombarded by 1.3 keV electrons for various time intervals. The core level Si2p and C1s electrons were studied after each electron bombardment by use of X-ray photoelectron spectroscopy. The spontaneous dissociation of TMSiH on the Si(100) surface was observed judging from the formation of C---C bonds. The C---C and C---Si bonds increased initially and then saturated after 20 min of electron exposure. The binding energy of C1s in C---C and C---Si bonds and that of Si2p in Si---C bonds showed an opposite behavior under electron irradiation. The former increased and the latter decreased at the beginning of the irradiation and then both increase rate and decrease rate reduced. From the previous results of electron stimulated desorption and temperature programmed desorption, and the variation of electron density distribution around C and Si, it is concluded that the de-hydrogenation in C---H_n and Si---C---H_m bonds was induced and new Si---C and C---Si bonds were formed by electron irradiation on TMSiH covered Si(100) surface.     

Ag deposited onto the (100) surface in silicon studied by Density Functional Theory and Classical Molecular Dynamics

Numerous theoretical studies have been dedicated to Ag deposited on Si(111) and the properties of this system are now well assessed. On the contrary, the interface Ag/Si(100) is more elusive and several problems of this system have not been satisfactorily solved yet. In this work the Density Functional theory with GGA corrections and Molecular Dynamics simulations with classical interatomic potentials have been applied to the evaluation of adsorption of Ag on Si(100). Small clusters representing a fragment of the exposed surface are used in GGA calculations to study the chemistry of Ag/Si bonds in dependence of the shape, bulk-terminated or dimerized, of the Si(100) surface. Isothermal molecular dynamics simulations describe the motions of the diffusing adatoms in a temperature range from T = 10 to 1000 K. The results indicate that the Ag atoms are adsorbed in a cave site among surface atoms, a feature consistent with experimental observations. Furthermore it is shown that, though there are no contradiction between the two methods, the difference between the stationary GGA and the dynamical MD evaluation is noticeable. This result has both practical and conceptual relevance.Received: 21 July 2003, Published online: 24 October 2003PACS: 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations - 81.10.Aj Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation     

Spontaneous composition modulation during Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te(301) molecular beam epitaxy

The spontaneous composition modulation of a solid solution in Cd _x Hg_1?x Te during molecular beam epitaxy at CdTe/ZnTe/GaAs(301) substrates has been discovered. The study of the micromorphology of the film surfaces in an atomic-force microscope, together with the study of the film microstructure in a transmission electron microscope, has revealed that a periodic system of macrosteps oriented along the [010] direction and separated by (100) terraces is formed during epitaxial growth at temperatures higher than the optimal value on a Cd _x Hg_1?x Te(301) surface. The growth of the film of homogeneous composition is only possible at the fronts of macrosteps and it is not possible at (100) terraces, where a layered structure grows with composition modulated along the [100] direction having period of several tens of angström. The observed phenomenon is explained by decreased adsorption of mercury atoms in the (100) plane.     

TiO2 chemical vapor deposition on Si(111) in ultrahigh vacuum: Transition from interfacial phase to crystalline phase in the reaction limited regime

The interaction between the metal organic precursor molecule titanium(IV) isopropoxide (TTIP) and three different surfaces has been studied: Si(111)-(7 × 7), SiO_x/Si(111) and TiO₂. These surfaces represent the different surface compositions encountered during TTIP mediated TiO₂ chemical vapor deposition on Si(111). The surface chemistry of the titanium(IV) isopropoxide precursor and the film growth have been explored by core level photoelectron spectroscopy and x-ray absorption spectroscopy using synchrotron radiation. The resulting film morphology has been imaged with scanning tunneling microscopy. The growth rate depends on both surface temperature and surface composition. The behavior can be rationalized in terms of the surface stability of isopropoxy and isopropyl groups, confirming that growth at 573 K is a reaction limited process.     

Studies of the structures formed by the alternated electrodeposition of atomic layers of Cd and Te on the low-index planes of Au: I. LEED and Auger studies

We present here a low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) study of the surface chemistry resulting from electrodeposition of two monolayers of CdTe on the low-index planes of Au by electrochemical atomic layer epitaxy (ECALE). ECALE is the electrochemical analog of atomic layer epitaxy (ALE). In the present study, well-ordered monolayers of CdTe were formed by the alternated electrodeposition of atomic layers of Cd and Te, at underpotential, on all three Au planes. Deposition of the first monolayer of CdTe was performed by depositing Te either oxidatively at underpotential or reductively at underpotential, followed by reductive Cd underpotential deposition (UPD). The structures formed were the same in both cases: Au(100)( , Au(110)(2 × 3)-CdTe, and

Full-size image

. Varying the initial coverage of Te did not change the resulting CdTe structure, although the higher coverages of Te showed sharper LEED patterns and no emersed oxygen. Deposition of Cd as the first atomic layer followed by oxidative Te UPD resulted in the same structures as those found when Te was used for the first atomic layer. Optimal CdTe monolayers were formed on surfaces where the first atomic layer deposited was homogeneously distributed and of the correct coverage. The correct initial coverage is determined by the coverage needed to complete the subsequently formed CdTe monolayer. On Au(100), an initial coverage of 0.5 appears to be optimal in order to form the Au(100)( . Deposition of a second CdTe monolayer onto the first resulted in structures similar to the first monolayer, but with increased diffuse intensity in the resulting LEED patterns under the conditions used.     

Endotaxial silicide nanowires

We demonstrate the growth of self-assembled nanowires of cobalt silicide on Si(111), (100), and (110) substrates during deposition of Co onto a heated Si substrate. Silicide islands form via an endotaxial mechanism, growing into the substrate along inclined Si{111} planes, which breaks the symmetry of the surface and leads to a long, thin nanowire shape. During growth, both the length and width of the islands increase with time in a fixed proportion that varies strongly with growth temperature, which shows that the nanowire shape is kinetically determined. It is expected that nanowires could form in many other overlayer/substrate systems via this mechanism.     

Following the oxidation of yttrium silicide epitaxially grown on Si(1 1 1) by core level photoemission spectroscopy

We have identified, by means of synchrotron radiation X-ray photoemission spectroscopy, several core-level shifted components in the Si-2p photoemission core level peak from a thin yttrium silicide layer epitaxially grown on a Si(1 1 1) surface. We have unequivocally assigned these components to different environments of the Si atoms in the silicide structure. This information has been used to monitor a surface oxidation process promoted by room temperature oxygen adsorption, identifying the final product of this reaction as a silicate-type ternary compound.     

STRUCTURE STUDY OF MODULATION-DOPED Cd1-xMnxTe: In/CdTe STRAINED LAYER MULTIPLE QUANTUM WELLS

The photomodulated reflectivity (PR) spectroscopy of modulation-doped diluted mag-netic semiconductor Cd_1-xMn_xTe: In/CdTe multiple quantum wells has been measured at 20-300K. Several spectral features associated with intersubband transitions have been found. The band structure of Cd_1-xMn_xTe: In/CdTe has been calculated by the Hartree self-consistent method. The results show that the theory is in agreement with experiments. In addition, an abnormal transition intensity ratio of 22H (the second heavy hole subband to the second electroa subband) to 11H (the first heavy hole subband to the first electron subband) caused by electron filled effect has been reported. At low temperature, a feature associated with Fermi level is observed, which has not been reported before.