Photoemission Yield Spectroscopy Research Articles

Electronic properties of clean CdSe surfaces upon Fe adsorption

Clean surfaces of n-type CdSe single crystals have been prepared by cleavage along the c-axis under ultrahigh vacuum. They were exposed to increasing doses of Fe vapor and received from 10 −1 to 10 2 monolayer (ML) deposits. At each step they are studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results show that Fe dissociates the CdSe top layers. It forms an Fe Se compound on top of a Cd layer up to 2.5 ML deposit beyond which no more Se is extracted from the CdSe substrate. At that stage further Fe dilutes the Se atoms in the top alloyed layer to which the d electrons of Fe bring already a metallic character after 1 ML deposition.

Surface States for the GaAs(001) Surfaces Observed by Photoemission Yield Spectroscopy

The energy distributions of the occupied surface states of GaAs (001) surfaces are measured using photoemission yield spectroscopy. The surfaces are prepared by different kinds of techniques, including molecular beam epitaxy, As decapping, and chemical etching. The surface states are found to change in both distribution and density depending on the surface preparation techniques. The origins of the surface states are discussed in terms of surface atomic structures for the atomically clean surfaces. A great reduction by about one order of magnitude in the density of the surface states is revealed for the surface covered with native oxide compared with the other atomically clean surfaces: the surface state electron density is estimated to be about 3×1013 cm-2 for the latter, and ∼1012 cm-2 for the former.

Effect of NH3 on Si(100) vicinal surfaces

The structural and electronic properties of clean vicinal surfaces of Si(100) upon interaction with NH3 have been studied using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) in situ, under ultrahigh vacuum (UHV). Two eases of vicinal surfaces were compared to the nominal (100) face, namely one with a 6° angle towards the [011] direction, the other with a 5° angle towards the [010] direction. It is shown that the rougher the surface, the faster the common saturation coverage θs is reached, where θs corresponds to one NH3 molecule per two surface Si dimers. The dipole formed upon adsorption leads to a similar decrease of the work function, which can reach 0.9 eV at saturation. The best ordered surfaces (here 6 ° vicinal) show upon NH3 saturation a well pronounced electronic structure near the valence band edge, the origin of which is discussed.

Effect of vacuum annealings on the electronic properties of clean Si(111) surfaces.

Clean cleaved Si(111) samples of various degrees of doping have been vacuum annealed at increasing temperatures, up to 1150 \ifmmode^\circ\else\textdegree\fi{}C and studied in situ by photoemission-yield spectroscopy, with low-energy electron-diffraction and Auger-electron-spectroscopy controls. The photoyield-spectrum changes upon annealing of 7\ifmmode\times\else\texttimes\fi{}7 reconstructed surfaces are interpreted as a sharp increase of the density of acceptor levels in the surface layer probed by photoemission. The effect starts above 700 \ifmmode^\circ\else\textdegree\fi{}C and saturates beyond 1050 \ifmmode^\circ\else\textdegree\fi{}C at (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ acceptor states ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$; contamination decreases this p-type overdoping effect. It is attributed to the formation of the appropriate number of Si vacancies, which relaxes, and therefore measures, the residual surface stress of the perfect 7\ifmmode\times\else\texttimes\fi{}7 reconstruction.

Hydrogen action in the surface space charge region of highly doped silicon

The action of atomic hydrogen on clean cleaved (1 1 1) surfaces of highly doped silicon samples, both phosphorus ([n] = 2 × 1019 cm-3) and boron ([p] = 4 × 1019 cm-3) doped has been compared to the case of lightly doped samples ([n] = 1 × 1014 cm-3). Once cleaved under ultra high vacuum, the samples were exposed to increasing doses of atomic hydrogen up to saturation. Before and after each hydrogen exposure, the Si(1 1 1) 2 × 1 surface was studied by low energy electron diffraction (LEED) and photoemission yield spectroscopy (PYS). The compared PYS measurements show that H atoms adsorbed on the Si(1 1 1) surface at room temperature do totally compensate the shallow-acceptor impurities (boron) and only partially the shallow-donor impurities (phosphorus) in the space charge region. They also remove the surface dangling bond states. These effects are reversible upon heating under vacuum. Both surface stresses and space charge electric field play a role in this compensation effect.

Surface properties of Si(111)7 × 7 upon NH 3 adsorption and vacuum annealing

The adsorption of NH 3 on a clean Si(111)7 × 7 reconstructed surface and the effect of vacuum annealings of the NH 3-saturated surface have been studied by low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The analysis of the surface order, of the nitrogen coverage and of the workfunction, ionization energy and filled surface state band changes upon NH 3 exposure or annealing temperatures gives the following results: the adsorption is dissociative into NH 2 + H and occurs in a two-step process, a fast one with an initial sticking coefficient equal to one up to half saturation, then a much slower one. At saturation, 10 ± 1 NH 3 molecules per 7 × 7 unit cell are adsorbed, the NH 2 radicals occupying the 7 rest atom dangling bonds plus three sites on the average which are attributed to the presence of steps or missing adatoms. Annealings lead to a two-step process, first a hydrogen desorption from 450 °C to 700 °C then a surface nitridation beyond 800 °C.

Ionization-energy dependence on GaAs(001) surface superstructure measured by photoemission-yield spectroscopy.

The ionization energies for GaAs(001) are determined by photoemission-yield spectroscopy as a function of surface superstructure. The ionization energy changes by as much as 0.5 eV in accordance with the surface superstructure.

Interface formation on n- and p-CdTe(110) surfaces with Cu and Au

The first stages of Cu and Au ultra-high vacuum deposition onto clean, in-situ cleaved (110) surfaces of n- and p-type CdTe single crystals at room temperature have been studied using low energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy. The changes of the measured spectra as a function of metal coverage are well explained for both Cu and Au by the formation of metallic islands which are essentially two-dimensional up to a fraction of a monolayer before becoming more (for Au) or less (for Cu) three-dimensional at higher coverages. The corresponding band bending changes are tentatively evaluated, in spite of the inhomogeneous character of the surface. The presence of a Au induced shallow donor surface state is also recognized below monolayer coverage.

Local roughness and surface states on Si(001): Analysis of vacuum annealings and oxygen adsorption-desorption processes

The clean (001) face of silicon has been studied as a function of surface preparation both prior to and after ultra-high vacuum immersion, using photoemission yield spectroscopy assisted by LEED and AES characterization. It was found that (i) the density of surface defects differs strongly as a function of preparation, (ii) this density can be significantly changed by proper UHV treatments, and (iii) the density of surface states at the Fermi level EF as well as the surface position of EF in the gap depend strongly on the degree of roughness.

The influence of oxygen adsorption on the electronic properties of the polar GaAs(111)As surface

Photoemission yield spectroscopy (PYS) associated with Auger electron spectroscopy (AES) control have been used in the investigation of the influence of the first steps of oxygen adsorption on the electronic properties of the clean, polar GaAs(111)As surface obtained by ion bombardent cleaning with subsequent annealing at 770 K. Depending on the oxygen coverage the variation of the work function and absolute band bending are analyzed together with the changes of the effective density of the filled electronic states localized in the band gap below the Fermi level E F and in the upper part of the valence band within 1 eV below the top. Their possible origins are briefly discussed.

Photoemission yield study of semimagnetic semiconductor Hg 1−xFe xSe crystals

Photoemission yield spectroscopy (PYS) was used to investigate HgSe and Hg 0.88Fe 0.12Se crystal surface freshly cleaved in a UHV chamber at a pressure of about 10 −9 Torr. The spectral photoemission yield curves were measured in the vacuum-ultraviolet range (4 < hv < 11 eV). The experimental results show that the contribution of the Fe3d 6 electrons to the electronic structure of the valence band leads to a shift of the work function from 4.30 to 4.15 eV. This contribution appears as an additional shoulder on the edge of the spectral photoemission yield curve. The interaction of the ambient atmosphere with the surfaces of freshly cleaved Hg 1− x Fe x Se ( x = 0.0, 0.12) crystals causes a decrease of the ionization energy E i and the work function W.

Formula omitted] reconstruction along the (111) face of highly boron-doped Si upon vacuum annealing

Low energy electron diffraction, Auger electron spectroscopy (AES), and photoemission yield spectroscopy measurements were performed on vacuum cleaved Si(111) surfaces. The samples were p-type boron-doped at 4 × 10 19 B cm −3; they were treated with successive annealings in the ultrahigh vacuum system up to 1150°C. After annealing, the usual reconstruction change from 2 × 1 to 7 × 7 in the 300° C range is followed by the appearance of a blurred 1 × 1 pattern in the 900–1000° C range when traces of surface boron become detectable by AES. Then, in the 1050–1150° C range, the surface displays a sharp 3 × 3 R30° diagram with a saturated surface concentration evaluated at about 0.1 monolayer (less than 8 × 10 13 B cm −2). The results are discussed in terms of the relaxation of residual strains and lead to a new model for the boron-induced 3 surface structure.

The effect of oxygen adsorption on the electronic properties of the polar GaAs(111) surface after thermal cleaning in ultrahigh vacuum

The influence of oxygen adsorption on the electronic properties of clean, polar n-type GaAs(111)As surfaces prepared by prolonged ultrahigh vacuum heating at 770 K have been investigated by means of photoemission yield spectroscopy combined with Auger electron spectroscopy control. For the clean surface the evaluation of arsenic content strongly depends on the theoretical models used whereas the work function φ and absolute band bending eVs were 4.05±0.03 eV and −0.21±0.07 eV respectively. Moreover, two stable filled electronic surface state bands localized in the band gap tailing up to Fermi level E F and above the top of the valence band at the surface were observed. After exposure of this surface to 10 3 L of oxygen the work function φ and absolute band bending eVs increased by 0.13 eV and the filled surface state band close to the Fermi level E F drastically decreased in amplitude. For oxygen exposures of 10 4 L and 10 5 L the above parameters increased only by 0.09 eV and 0.04 eV respectively whereas the filled surface state band above the top of the valence band at the surface was stable.

Room temperature interaction of ionised nitrogen with cleaved GaAs

The room temperature interaction of ionised nitrogen with GaAs (110) prepared by cleavage in ultrahigh vacuum has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The ions, having a kinetic energy of about 75 eV, were produced by an excitation cell connected to the working chamber by a hole 0.5 mm in diameter. They could be deflected onto the sample surface and slowed down to the 15 eV kinetic energy range. A great disorder is observed in LEED through the rapid decrease of the diffraction spot intensities at low exposures whatever the kinetic energy of the ions. AES reveals that the N Auger peak saturates at large exposures but that oxygen is always present. Comparison with the neutral species effects in PYS shows that the active species are indeed the ions which, at low exposures, drastically change the surface electronic properties of GaAs.

Hydrogen-induced contamination of III-V compound surfaces

Specific examples are reported from the study of H/GaAs(110), H2+/GaAs(110), and H/InP(110) systems in which the initially clean surfaces, prepared by cleavage in ultrahigh vacuum, became contaminated through enhanced or displacive reactions involving excited hydrogen. They show that elements forming volatile hydrides can be displaced by atomic or ionized hydrogen onto the sample surface from those parts of the ultrahigh vacuum chamber (walls, equipments) on which they are present. The surface electronic properties were followed by photoemission yield spectroscopy. Comparison with the clean systems shows that the main effects of contamination on these properties concern (i) an irreproducible variation of the ionization energy during the adsorption stage at the lower exposures, and (ii) the nonobservation of the yield quenching phenomenon which appears during the dissociation stage upon heavy hydrogenation in the clean cases.

Electronic properties of the polar GaAs(111)As surface

Photoemission yield spectroscopy (PYS) has been used to investigate the electronic properties of the polar GaAs(111)As surface cleaned by ion bombardment with subsequent annealing at 770 K under UHV conditions of about 10 −7 Pa. The elemental composition was recorded simultaneously by means of Auger electron spectroscopy (AES). After the above described cleaning procedure As has been completely removed from the first surface monolayer which corresponds to the (1×1) atomic surface structure whereas the values of the work function ø and absolute band bending were 4.00±0.02 and −0.16±0.06 eV respectively. Moreover two filled electronic surface state bands were observed which have been described as the Ga-derived surface state band and As-derived “back-bond” surface state band respectively.

Interaction of atomic hydrogen with cleaved InP. II. The decomposition stage

Auger electron spectroscopy, electron energy-loss spectroscopy, low-energy electron diffraction, and photoemission yield spectroscopy have been used to study the interaction of atomic hydrogen with cleaved InP(110) upon heavy hydrogenation. Beyond the initial adsorption stage, the substrate dissociates into In metal and possibly two hydrogenated compounds of P, among which most likely phosphine PH3, which remain adsorbed on the surface. This interaction stage is characterized by the existence of the ‘‘black hole’’ phenomenon in photoemission yield spectroscopy, i.e., the complete disappearance of the photoemission signal at 5.3 and ∼5 eV, photon energies specific of the adsorbed species. The hydrogenated compounds desorb upon heating at 525 °C leaving a surface close to its H-adsorbed state. From there on, the properties of the surface can by cycled by, alternately, introducing a new heavy dose of H and annealing.

Interaction of atomic hydrogen with cleaved InP. I. The adsorption stage

The interaction of atomic hydrogen with the (110) face of InP, prepared by cleavage under ultrahigh vacuum, has been studied by Auger electron spectroscopy, electron energy-loss spectroscopy, low-energy electron diffraction, and photoemission yield spectroscopy. The interaction occurs in two stages: first, an adsorption of H, then a decomposition of the substrate. The adsorption stage is characterized by the covalent bonding of, most likely, one H atom per surface unit cell which induces a change in the surface reconstruction of the substrate, the surface unit mesh remaining 1×1. A hydrogen-induced surface state band, about 0.26 eV wide, is observed around 0.06 eV below the valence-band edge. Upon H adsorption, the Fermi level gets pinned in the gap at about 0.4 eV below the conduction-band edge on both n- and p-type samples, while the ionization energy decreases, the decrease reaching 0.35 eV at saturation.

Interaction of atomic hydrogen with InP(100): Comparison with the cleaved surface interaction

Photoemission yield spectroscopy (PYS) in the photon energy range from 3.8 to 6.7 eV has been used to characterize clean InP(100) and study its interaction with atomic hydrogen. Some measurements in low energy electron diffraction, Auger electron spectroscopy and electron energy loss spectroscopy have also been performed. The surface was cleaned by cycles of ion sputtering and annealing in ultrahigh vacuum. It is shown to contain a slight excess of In metal; the ionization energy of InP is that of the clean cleaved surface, and an effective density of surface states which exists at the top of the valence band is minimized when the surface is clean. Upon interaction with H, an adsorption and a decomposition stage are evidenced as with cleaved InP. However, during the adsorption stage on the (100) face, the ionization energy remains equal to that of the clean surface, while it decreased on (110). The decomposition stage on the (100) face, as on (110), leads to the formation of In metal and a phosphine-like compound which remains adsorbed on the surface and is at the origin of a peculiar phenomenon observed in PYS. The system is, however, far less stable than on the cleaved surface since the P-hydrogenated species desorbs spontaneously at room temperature leaving an In covered surface which appears insensitive to H.

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10 3 L of oxygen, the electronic surface state band localized just below the Fermi level E F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10 4L and 10 5L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.