Fermi Level Movement Research Articles

Impact of surface processing on Al/GaAs(100) interface states

Discrete deep levels form at Al/GaAs(100) interfaces whose energies and state densities change with surface preparation conditions. Modifications in surface chemical composition and reconstruction with annealing temperature produce systematic changes in a set of interface states spanning the energy range ≈0.8 eV to 1.2 eV, as well as alter the interface barrier height. These results demonstrate close correlation between the interface states observed directly via low-energy cathodoluminescence spectroscopy and the Fermi level movement at Al/GaAs(100) interfaces and emphasize the central role of surface preparation in achieving controlled Schottky barrier behavior.

X-ray photoelectron spectroscopic study of the interaction of low energy carbon ions with GaAs and InP

To simulate the low energy ion bombardment that occurs in reactive ion etching (RIE) using alkanes, GaAs and InP were exposed to 20, 100, and 500 eV carbon ions, using a mass-separated carbon ion beam in an ultrahigh vacuum chamber. The InP sample structure consisted of a 40 Å ultrathin, epitaxial InP layer on InGaAs. The changes induced by ion bombardment and the effects of subsequent damage-removal treatments were determined by in situ polar-angle dependent x-ray photoelectron spectroscopy. Initially, carbon ion irradiation caused minor sputtering of the semiconductors and preferential removal of the group V constituents, with concurrent formation of carbon-semiconductor phases. The depth of these phases and the extent of the damage increased with increasing bombardment energy. An amorphous carbon residue formed after further bombardment. Several damage removal techniques were applied to the carbon bombarded surfaces. It was found that heating was ineffective in annealing the damage, although surface Fermi level movements were observed. Ozone oxidation did not remove the residual carbon efficiently, yet the underlying semiconductor components were readily oxidized. Hydrogen ion bombardment was effective in removing the carbon bombardment induced damage. The implications of these results for alkane based RIE processes are discussed.

Investigation of Schottky barrier formation for transition metal overlayers on InP and GaP(110) surfaces

Platinum overlayers on InP(110) and GaP(110) have been studied by photoelectron spectroscopy and measurement of current-voltage (I–V) and capacitance-voltage (C–V) characteristics. The reaction processes at these interfaces were monitored in parallel with the Fermi level movement in the gap up to the formation of the interfacial potential barrier, φb. Synchrotron radiation photoemission studies reveal the reaction between Pt and phosphorus, and also the alloying of indium and gallium with the Pt layer. For both surfaces, the band bending measurement is affected by a surface photovoltage caused by the photoemission light source. I–V (and C–V) measurements were carried out in situ, at different substrate temperatures in order to evaluate the bulk barrier height, and to obtain an experimental value for the Richardson constant. The values of φb(0.84 V for p-GaP(110) and 0.53 V for n-InP(110)) are in agreement with the values deduced from the photoemission measurements. These results are compared with data for other metals and are discussed in the context of current theories of barrier formation.

Sulphide passivation of GaAs: study of surface band bending

Abstract Optical and scanning tunnelling microscopy (STM) methods are used to characterize the electronic properties of sulphide-passivated GaAs surfaces. The optical technique makes it possible to follow the Fermi level movement on the surface in situ , the sample being immersed in sulphide solution. STM provides data on the thickness and electronic properties of the passivating coverage.

Effects of growth temperature on atom distributions, Fermi-level positions, and valence-band offsets for Ge/n-type InP(110) heterojunctions.

With synchrotron radiation photoemission, we have investigated Ge/InP(110) heterojunction formation at 300 and 60 K to correlate the movement of the surface Fermi level ${\mathit{E}}_{\mathit{F}}$ with the distributions of In and P in the growing Ge layer. Ge adatom deposition induces substrate disruption at both temperatures. For growth at 300 K, we observe In segregation to the surface. For growth at 60 K, In segregation is inhibited and the atoms remain at the interface, even when the system is warmed to 300 K. In contrast, P atoms, released by surface disruption, are retained at the buried interface at 60 and 300 K. The Fermi-level movement at 300 K as a function of coverage is unique in that it moves into the gap by \ensuremath{\sim}1-\AA{} deposition but then moves back toward the conduction-band minimum, demonstrating that the surface is not pinned. The surface photovoltage precludes measurement of the equilibrium ${\mathit{E}}_{\mathit{F}}$ position for formation at 60 K but annealing a 5-\AA{}-Ge/InP(110) interface to 300 K after formation at 60 K shows that the surface Fermi level was 440 meV from the conduction-band minimum. The fully developed valence-band discontinuity was 1.03\ifmmode\pm\else\textpm\fi{}0.08 eV, regardless of temperature.

Effect of annealing Sb/InP(110) interfaces and Schottky barrier formation of Ag on annealed Sb/InP(110) surfaces

The effect of annealing one monolayer of Sb on p-InP on the surface Fermi level position and the band bending due to Ag deposition on these well-ordered surfaces have been studied using photoemission spectroscopy. The adsorption of one monolayer of Sb on p-InP gives a Fermi level position 0.85 eV above the valence band maximum (VBM). However, with increasing annealing temperature, the band bending decreases and recovers to nearly the flatband condition above 200 °C. The Fermi level movement of annealed InP shows a correlation with the surface stoichiometry of phosphorus and indium. Ag deposition on these annealed Sb/p-InP interfaces gives an anomalously low band bending of 0.5 eV above the VBM.

Au interfaces with epitaxially grown CdTe(111): Chemistry and barrier heights

We have used high-resolution soft x-ray photoemission spectroscopy (SXPS) to study chemistry, atomic distributions, and Fermi level (EF) movement caused by Au deposition on molecular beam epitaxy (MBE) -grown CdTe(111)-B surfaces. Using etch-and-anneal cycles, we produce clean surfaces with characteristic valence-band and core-level photoemission spectra. Au atoms disrupt substrate bonds releasing both Cd and Te atoms into the overlayer. Anions segregate preferentially to the free surface, while cations are more interspersed in the Au matrix. These first SXPS studies of CdTe(111) surfaces indicate that the atom-induced disruption is comparable or less than that for cleaved (110) substrates. After correcting for photovoltaic effects, we obtain an equilibrium Fermi-level position 0.55 eV above the valence-band maximum (Ev), intermediate between stabilization positions for Au on (i) cleaved, bulk-grown CdTe(110) (Ev+0.8 to 1.1 eV), and (ii) cleaved, bulk-grown CdTe(110) with a Yb interlayer (Ev+0.45 eV). Despite clear evidence for interfacial diffusion, EF for MBE-grown CdTe(111) lies closer to the position expected on the basis of work function difference than does EF for bulk-grown CdTe(110). These results indicate that the structural and electronic quality of the CdTe crystal plays a major role in the Schottky barrier formation.

Surface photovoltage effects in photoemission from metal/GaP(110) interfaces: Temperature-dependent Fermi level movement

In recent experiments of metal deposition onto cleaved GaP(110) surfaces we have shown that light sources used in photoelectron spectroscopy may induce a surface photovoltage (SPV), which causes a substantial deviation from the ground state potential distribution, and may induce errors in the determination of band bending by photoemission. Here we analyze the temperature-dependent movement of the surface Fermi level in n- and p-type GaP(110) surfaces as a function of indium and silver deposition, taking into account the presence of the SPV. It is found that changes in the substrate temperature not only modify the adlayer morphology and metallicity, but also the surface electron-hole recombination rate. We observe that the temperature-dependent shift of the semiconductor core levels is always accompanied by a similar shift of the metal core level and Fermi edge, suggesting that the reversible temperature-dependent band bending recently reported for metal/III–V semiconductor interfaces is related to the SPV, and does not represent a ground state property of the interfacial electronic structure. Implications of these results on current models concerning Schottky barrier formation are discussed.

Erratum: Confirmation of the temperature-dependent photovoltaic effect on Fermi-level measurements by photoemission spectroscopy

Soft-x-ray photoemission spectroscopy (SXPS) measurements of Fermi-level positions within the band gap of clean, GaAs(100) surfaces demonstrate that photovoltaic charging produces a major shift in the apparent band bending at low (50--100 K) temperatures. These measurements confirm recent predictions of Hecht [M. H. Hecht, Phys. Rev. B 41, 7918 (1990)], based on restoring currents limited by carrier transport through the depletion region, highlight the effects of photovoltaic charging for clean (versus metallized) semiconductor surfaces, and justify Hecht's claims for a reassessment of many previous low-temperature photoemission studies of Fermi-level movement.

Effective metal screening and Schottky-barrier formation in metal-GaAs structures

The Fermi-level movement in a Schottky barrier is investigated using a bimetal thin-film structure. The functional dependence of the barrier height on the inner metal thickness is formulated in terms of the metal effective screening length and the interface trap states. This model is used to describe experimental results on Pt-Ti-GaAs and Ti-Pt-GaAs diodes. It is found that the effective screening lengths for Pt and Ti are 6.5 and 7.0 AA, respectively, significantly greater than the ideal values of the theory of N.F. Mott and H. Jones (1958). This indicates that the potential drop inside the metal electrode can evolve over several monolayers in a Schottky contact.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Confirmation of the temperature-dependent photovoltaic effect on Fermi-level measurements by photoemission spectroscopy.

Soft-x-ray photoemission spectroscopy (SXPS) measurements of Fermi-level positions within the band gap of clean, GaAs(100) surfaces demonstrate that photovoltaic charging produces a major shift in the apparent band bending at low (50--100 K) temperatures. These measurements confirm recent predictions of Hecht [M. H. Hecht, Phys. Rev. B 41, 7918 (1990)], based on restoring currents limited by carrier transport through the depletion region, highlight the effects of photovoltaic charging for clean (versus metallized) semiconductor surfaces, and justify Hecht's claims for a reassessment of many previous low-temperature photoemission studies of Fermi-level movement.

‘‘Pinning’’ and Fermi level movement at GaAs surfaces and interfaces

Band bending at GaAs interfaces with either metals or insulators is of considerable importance. Recently a literature has developed, which shows for both metals and insulators, that this band bending can be modified, i.e., the Fermi level position Efi at the interface changes. Of the five metals studied in detail, Ag, Au, Al, Ti, and Cr, three (Au, Al, and Ti) have been shown to produce Fermi level changes on the order of 0.1 eV (i.e., change in the Schottky barrier height φ) on thermal annealing. For n-type GaAs φ decreases for Au, and increases for Al and Ti. These changes of Efi can be explained in terms of the generation of excess As or Ga at the interface due to chemical reactions between the metal and the GaAs. The Fermi levels move toward the conduction-band minimum (CBM) when the As/Ga ratio is increased and toward the valence-band maximum (VBM) when it decreases. The changes in As/Ga ratio have been confirmed independently. No movement is seen for Ag, which is unique in having little reaction with GaAs, or for Cr, which bonds to both As and Ga. The Fermi level position can also be changed at the GaAs insulator interface. Na2S⋅H2O and (NH4)2S treatments of the GaAs surface reduce surface recombination. Recent studies show that such treatments increase rather than decrease the band bending on n-GaAs. All of these results agree with a model in which AsGa and GaAs antisites dominate the interface and Fermi level changes are explained in terms of changes in the relative numbers of these defects.

Use of x-ray photoelectron spectroscopy to investigate the deposition of metal overlayers onto the clean cleaved CdS surface

X-ray photoemission spectroscopy (XPS) is used to study the deposition of a range of metals onto both the clean cleaved and air exposed surfaces of n-type CdS. Al, Co and Pd are clearly observed to react with the CdS surface to form a metal sulphide together with metallic Cd. There is no evidence for metal sulphide formation after the evaporation of Au, Ag or Sb. The Sn 3d core level emission for Sn on CdS shows two components, indicative of SnSn and SnS bonding. However, in this case, there is no sign of metallic Cd being formed with increasing Sn coverage. Our XPS spectra also allow us to measure the initial Fermi level movement (band bending) which occurs as the metal/CdS interface is being formed. Schottky barrier heights obtained from XPS measurements show the same trend as barrier heights determined from current-voltage characteristics. Finally, by considering the contribution of defects to the electrical properties of metal/CdTe and metal/CdS diodes, we assess the relevance of these studies to the nature of electrical barriers at metal/CdS interfaces.

Unrelaxation of the semiconductor surface at low-coverage Ag/InP(110) interfaces as determined by photoemission extended x-ray-absorption fine structure.

The atomic geometries of Ag/InP(110) interfaces for metal coverages in the cluster regime have been determined by photoemission extended x-ray-absorption fine structure (PEXAFS). P 2p PEXAFS for InP(110)+0.5 \AA{} Ag and InP(110)+1 \AA{} Ag (at room temperature) were acquired. The data were analyzed by conventional Fourier-transform methods using the theoretical backscattering phase function of McKale et al. plus the absorber phase function of Teo and Lee. For both noble-metal coverages on the semiconductor surface, our measurements show that the relaxation (about 4% contraction) in the P-In bond length of the clean InP(110) surface is mostly removed. This is in contrast to our recent PEXAFS results, reported in the literature, for reactive-metal Al/InP(110) or Na/InP(110) interfaces, where low-coverage metal-induced reconstruction of the P-In bond length has been observed. The low-coverage noble-metal-induced unrelaxation of the P-In bond length might contribute to Fermi-level movements during Schottky-barrier formation.

Thermally reversible band bending for Bi/GaAs(110): Photoemission and inverse-photoemission investigations.

Results of synchrotron-radiation photoemission, inverse-photoemission, and low-energy electron diffraction studies of the Bi/GaAs(110) interface over the temperature range 20 to 300 K are presented. At 300 K, the first Bi monolayer grows in zigzag chains along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction to form an ordered (1\ifmmode\times\else\texttimes\fi{}1) overlayer. This overlayer is semiconducting with a gap of 0.7 eV. Continued deposition results in Bi island growth atop the initial monolayer (ML) and conversion from semiconducting to semimetallic character. Bi deposition at 50 K results in layer-by-layer growth with no long-range order. Temperature- and coverage-dependent studies of band bending show symmetric behavior for n- and p-type GaAs(110) with matched bulk dopant concentrations. For 300-K deposition, the Fermi level ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward final positions 0.74 and 0.98 eV below the conduction-band minimum for n- and p-type GaAs doped at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For 50-K deposition, the bands remain nearly flat to coverages of 1--2 ML where ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward midgap. Significantly, both techniques also show that Fermi-level movement is thermally reversible, such that the position of ${\mathit{E}}_{\mathit{F}}$ in the surface band gap depends on temperature. A small part of the ${\mathit{E}}_{\mathit{F}}$ movement is due to nonequilibrium processes involving the creation of electron-hole pairs by the incident photon or electron beam followed by the transport of the minority carriers to the surface region by the electric field in the depletion region. This surface voltage is most important at low temperature, and studies which vary the incident photon flux by over 3 orders of magnitude establish that the photovoltage has a small (\ensuremath{\le}100 meV), but non-negligible, effect on the band bending measured under ``normal'' experimental conditions. Hence, equilibrium processes control coupling of substrate and adsorbate-induced states in the GaAs band gap through the semiconductor depletion region and they determine the temperature-dependent interface band bending.

Fermi level movement for n- and p-GaAs interfaces: Effects of temperature and dopant concentration

Photoemission studies demonstrate that temperature and dopant concentration dependent movement of the surface Fermi level is controlled by coupling between adatom-induced and bulk states. At a low temperature for lightly doped n- or p-GaAs, initial band bending inhibits tunneling and EF remains near the band edges until the onset of metallicity. For heavy doping, greater band bending reflects a thinner depletion region. Thermal cycling for 20≤T≤300 K for low coverages demonstrates that band bending is reversible.

Temperature effects for Ti/GaAs(110) interface formation involving cluster and atom deposition.

Ti/GaAs(110) interfaces formed at 60 and 300 K by atom-by-atom deposition and by the deposition of preformed metallic clusters exhibit very different Schottky-barrier behavior and morphologies. For atom deposition, temperature-dependent Fermi-level pinning is similar to that observed for nonreactive metals, despite the fact that the deposition process leads to the disruption of \ensuremath{\sim}3 monolayers of GaAs at all temperatures. Although disruption is apparent even at 0.02 A\r{}, atom deposition at 60 K on n-type GaAs shows almost no Fermi-level movement until \ensuremath{\sim}2 A\r{} when ${E}_{F}$ moves rapidly toward midgap. Our results show that the adatoms do not act as nonreactive donors at low temperature, that thermal changes do not quench surface chemistry, and that surface reaction alone does not lead to midgap pinning. In contrast, for interfaces formed by preformed Ti cluster deposition, unique pinning positions are observed far from the point expected for known defects or metal-induced gap states. The instability of the Ti(cluster)/GaAs system is demonstrated by warming to 300 K. We postulate that kinetically restricted reactions occur beneath the metallic clusters and the midgap pinning requires disruption in the presence of a metallic overlayer.

Low-temperature alkali metal/III–V interfaces: A study of metallization and Fermi level movement

The interfaces of alkali metals (Cs and Rb) on GaAs and InP(110) surfaces prepared at 110 K low temperature have been studied using photoemission. At low temperature, multilayers of alkali metals with laminar growth can be obtained. The overlayer metallization is investigated by following the density of states near the Fermi cutoff, the free electron plasma loss, and the Fermi level movement at the semiconductor surfaces. Several criteria of metallicity are proposed. It is found that one monolayer of Cs or Rb is not metallic, and full metallicity is established at around two monolayers of coverage. The Fermi level movement relative to the semiconductor band edges as a function of alkali metal coverage has been closely followed. The semiconductor band bendings at low coverages are attributed to the surface-donor states originating from alkali metal atom chemisorption. The Fermi level stabilization at these interfaces occurs when the overlayers become metallic. This pinning behavior is explained in terms of the metal-induced gap states. The Fermi level pinning at the room temperature interfaces is also discussed and compared with the low-temperature behavior.

Annealing Ag on GaAs: Interplay between cluster formation and Fermi level unpinning

It has been shown that upon annealing a 2-monolayer coverage of Ag on UHV-cleaved GaAs at 500 °C, the Ag clusters into islands and the surface Fermi level moves back to within 0.2 eV of the bulk position. In this study, this Fermi level unpinning behavior has been investigated further by using various substrate doping levels and varying the anneal temperature. The Ag clustering process was observed using both ultraviolet photoelectron spectroscopy (UPS) and scanning electron microscopy. Through UPS, the surface Fermi level (Efs) movement was monitored simultaneously with the clustering process. No distinct temperature for the onset of clustering is observed. Rather, the clustering process occurs continuously over the range of temperatures studied (room temperature to 500 °C), with the Ag clusters growing and increasing in separation as anneals are performed at successively higher temperatures. A 10 min annealing time was sufficient to achieve a stable, equilibrium configuration at each annealing temperature. Movement of the Efs back to near the bulk position occurred between 375–450 °C for high doped n-GaAs (6×1018/cm3); whereas no movement of the Efs was observed for low doped n-GaAs (4×1016/cm3) up to 475 °C. Similar results were obtained when the experiment was repeated for high doped p-GaAs (1.4×1019/cm3) and low doped p-GaAs (5×1016/cm3). The absence of movement of Efs in the low doped GaAs is attributed to the longer substrate depletion length, implying that only the areas beneath and within the depletion length of the Ag clusters are pinned.

Chemically modified GaAs Schottky barrier variation

Chemical modification of GaAs surfaces with and without photoenhancement can produce large variations in the Schottky barrier for metals which are subsequently deposited. Deep ultraviolet light enhanced oxidation of the GaAs surface produces barrier variation toward the ideal Schottky limit, however, there is no evidence that the number of interface states is reduced. A calculation suggests that this can be explained by the interface state energy distribution being altered to shift the states toward the band gap edges, possibly by a photochemical reaction of oxygen. This allows a wider range of Fermi level movement. Different procedures to introduce oxygen at the metal/GaAs interface without photoenhancement result in different barrier variation. Therefore, altered interface chemistry, as well as the mere presence of a species at the interface, is shown to be important for Schottky barrier formation.