Alastair McLean | Queen's University at Kingston (original) (raw)
Papers by Alastair McLean
Nature Chemistry, 2014
Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these struct... more Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.
Physical Review B, 2012
ABSTRACT The Si(111)-7×7 surface reconstruction can be used to template an array of mesitylene (1... more ABSTRACT The Si(111)-7×7 surface reconstruction can be used to template an array of mesitylene (1,3,5-trimethyl-benzene) molecules in which each molecule bridges a corner Si adatom and a neighboring Si rest atom. The molecules adsorb in corner sites to minimize a weak steric interaction associated with nearest-neighbor bridging sites. We demonstrate that the formation of the molecular array can be blocked by adding submonolayer amounts of Ge to the surface. The Ge atoms block the formation of the array because they substitute preferentially for Si adatoms located in corner sites, and the Ge–C bond strength is ≈0.8 eV weaker than the Si–C bond strength.
Applied Surface Science, 2005
A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab ini... more A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab initio theoretical technique and scanning tunnelling microscopy (STM). Our calculations demonstrate decisively that the recently proposed Haiku geometry is a lower energy configuration than any of the previously proposed line geometries. Furthermore, we have made comparisons between STM constant-current topographs of the lines and Tersoff-Haman STM simulations. Although the Haiku and the Miki geometries both reproduce the main features of the constant-current topographs, the simulated STM images of the Miki geometry have a dark stripe between the dimer rows that does not correspond well with experiment. #
Physical Review B, 2009
ABSTRACT The adsorption of thiophene on Si(111)-(7×7) has been studied using density functional t... more ABSTRACT The adsorption of thiophene on Si(111)-(7×7) has been studied using density functional theory within the generalized gradient approximation. For all adsorption geometries that have been experimentally identified, equilibrium binding energies have been calculated using a full 7×7 supercell to a depth of six Si monolayers. Despite the structural heterogeneity of the surface, all adsorption geometries are calculated to have the same binding energy, irrespective of their location within the 7×7 unit cell. A parallel survey of chemisorption sites, performed with scanning tunneling microscopy, demonstrated that thiophene prefers the faulted half of the unit cell over the unfaulted half, and edge over corner sites, in agreement with previous experimental studies. The theoretical and experimental results suggest that the activation energy barriers for chemisorption are site dependent. The physical factors leading to site dependent activation energies are briefly discussed.
Nanotechnology, 2005
The equilibrium geometry, electronic structure and energetic stability of Bi nanolines on clean a... more The equilibrium geometry, electronic structure and energetic stability of Bi nanolines on clean and hydrogenated Si(001) surfaces have been examined by means of ab initio total energy calculations and scanning tunnelling microscopy. For the Bi nanolines on a clean Si surface the two most plausible structural models, the Miki or M model (Miki et al 1999 Phys. Rev. B 59 14868) and the Haiku or H model (Owen et al 2002 Phys. Rev. Lett. 88 226104), have been examined in detail. The results of the total energy calculations support the stability of the H model over the M model, in agreement with previous theoretical results. For Bi nanolines on the hydrogenated Si(001) surface, we find that an atomic configuration derived from the H model is also more stable than an atomic configuration derived from the M model. However, the energetically less stable (M) model exhibits better agreement with experimental measurements for equilibrium geometry. The electronic structures of the H and M models are very similar. Both models exhibit a semiconducting character, with the highest occupied Bi-derived bands lying at approximately 0.5 eV below the valence band maximum. Simulated and experimental STM images confirm that at a low negative bias the Bi lines exhibit an 'antiwire' property for both structural models.
physica status solidi (a), 2012
ABSTRACT We show that the Si(111)-7 x 7 surface reconstruction can be used to template an ordered... more ABSTRACT We show that the Si(111)-7 x 7 surface reconstruction can be used to template an ordered array of 1,3,5-trimethyl benzene (mesitylene) molecules. The disorder that normally derives from the multiplicity of admissible adsorption geometries, for small aromatic molecules on 7 x 7, is suppressed, and the molecules are found to occupy both halves of the 7 x 7 unit cell with equal probability. It is argued that a steric interaction, associated with the methyl groups, hinders nearest neighbor adsorption and this leads to the formation of an array that has the molecules at the corners of the 7 x 7 half unit cells. To understand the ordering kinetics we used: scanning tunneling microscopy to study site occupancy as a function of coverage, ab initio total energy calculation to study the stability of the attachment sites and kinetic Monte Carlo modeling to investigate the emergence of translational order in the overlayer. [GRAPHICS] When two mesitylene molecules adsorb in nearest neighbor bridging geometries on the 7 x 7 reconstruction, the hydrogen atoms located on the methyl groups interact. To reduce the interaction energy, the molecules form an ordered array that places each molecule at the corner of the half cell. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
physica status solidi (c), 2010
ABSTRACT The adsorption of thiophene on Si(111)–7×7 has been studied with scanning tunneling micr... more ABSTRACT The adsorption of thiophene on Si(111)–7×7 has been studied with scanning tunneling microscopy (STM) and kinetic Monte Carlo (kMC) modelling. Previous experimental studies of this system clearly demonstrated that thiophene prefers to chemisorb on the faulted half of the 7×7 unit cell. The STM studies reported here concur with this and provide further information about thiophene site preference as a function of coverage. Additionally, an ab initio theoretical investigation of this system demonstrated that the occupancy of available adsorption sites could not be explained using equilibrium binding energies, as these were calculated to be the same for all experimentally identified adsorption sites (≈1.0 eV). To investigate the possibility that site selection is kinetically controlled, a kMC model was developed. This model places the molecule in a mobile precursor state, allowing the molecule to traverse the surface before chemisorbing. The kMC model was found to reproduce the STM data, providing compelling evidence that site occupancy in this system is indeed kinetically controlled at room temperature. Activation energy differences, for each the four unique chemisorption geometries, could be extracted from a fit of the kMC model predictions to the experimental data (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physical Review B, 2011
ABSTRACT We demonstrate that the Si(111)-7×7 surface reconstruction can be used to template an or... more ABSTRACT We demonstrate that the Si(111)-7×7 surface reconstruction can be used to template an ordered array of 1,3,5-methyl benzene molecules that are uniformly distributed over both the faulted and the unfaulted halves of the 7×7 unit cell by covalent attachment in vacuo. An intermolecular steric interaction, which hinders nearest-neighbor adsorption, is shown to play an important role in the formation of the ordered array. The stable equilibrium structure is shown to be one where the molecules are located at the corner of the half unit cells maximizing the intermolecular separation. In addition to the intermolecular steric interaction, there is an interaction between the molecule and the surface that plays a important role in reducing disorder in the array. Moreover, as the coverage is increased, there is a switch in site preference, from edge to corner, that mitigates the effect of the intermolecular interaction. To investigate this system we used scanning tunneling microscopy to study site occupancy as a function of coverage, ab initio total energy calculation to study the stability of the attachment sites, and Monte Carlo modeling to examine the emergence of translational order in the overlayer. The switch in site preference from edge to corner is faithfully reproduced by the kinetic Monte Carlo model when an interaction term is included.
We present a theoretical investigation of the influence of domain boundaries on the Ge/Si(111)-5 ... more We present a theoretical investigation of the influence of domain boundaries on the Ge/Si(111)-5 × 5 phase using both large-scale DFT simulations and an analytical model. It is shown that different boundary types modify the atomic and electronic structure of the adjoining 5 × 5 domains in very different ways. A simple theoretical model, that describes the energy interaction J between the boundaries and the 5 × 5 phase, is presented and the interaction energy decay J(x) ≈ x −n for different domain boundaries is estimated. Additionally, the influence of the boundaries on the atomic and electronic structure of adatoms in the parental 5 × 5 phase is analyzed and it is argued that the presence of domain boundaries may strongly affect not only the physical but also the chemical properties of the Ge/Si(111)-5 × 5 phase.
Journal of Physics C Solid State Physics
The transport properties of over 400 Schottky diodes, fabricated on cleaved, atomically clean and... more The transport properties of over 400 Schottky diodes, fabricated on cleaved, atomically clean and on cleaved, air-exposed GaAs (110) surfaces, have been examined for a diverse range of metallisations using the conventional current-voltage and capacitance-voltage techniques. The resulting body of experimental data provides a reliable base for studying the correlation between the Schottky barrier height and fundamental quantities. In particular, the range of barrier heights obtained from the diodes fabricated on the clean surfaces are compared with the metal work function and the metal electronegativity. It is demonstrated that central transition metals, from groups Va-VIII of the periodic table, produce mid-gap Fermi-level pinning on GaAs. In contrast, the barrier heights extracted from simple- and noble-metal contacts, and from transition metals situated towards the beginning and end of a transition series, exhibit a near-linear dependence on electro-negativity.
Journal of Physics C Solid State Physics
The formation of the Al/GaAs(110) interface has been studied at room temperature using synchrotro... more The formation of the Al/GaAs(110) interface has been studied at room temperature using synchrotron-radiation-excited soft-X-ray photo-emission spectroscopy. It is demonstrated that the Al 2p core-level emission, at low Al coverages, provides evidence for at least three distinct Al phases. These are isolated Al adatoms, Al atoms in surface Ga sites and Al metallic clusters. The last two phases are observed at higher Al coverages and have been extensively studied by other groups. However, the nature of the first phase and the transition from the first to the second phase have been the subject of only a few studies. In this paper it is shown that the change from the first to the second phase, as determined from the Al2p emission, is similar to that observed when Al is deposited on InP(110). Additionally it is argued that the low-coverage Al2p core-level photo-emission spectra, which have been used previously to support the view that the Al adatoms are weakly interacting with the GaAs s...
Journal of Physics C: Solid State Physics, 1988
For pt.I see ibid., vol.21, p.783 (1988). Thermodynamic aspects of Schottky contact formation on ... more For pt.I see ibid., vol.21, p.783 (1988). Thermodynamic aspects of Schottky contact formation on cleaved GaAs (110) surfaces are examined. It is shown, for 18 different metals, that a simple binary-phase bulk thermodynamic model is a useful semi-quantitative guide to interfacial reactivity, provided both arsenide and gallium alloy formation are considered. Absence of reaction, reaction by arsenide formation alone, reaction by alloying alone and reaction by both arsenide and alloy formation are correctly predicted. In contrast, attempts to correlate any of the thermodynamic parameters, including the interface heat of reaction, with the Schottky barrier height were unsuccessful.
Solid State Communications, 1988
The transport properties of a diverse range of metal/GaAs(1 1 0) Schottky diodes have been examin... more The transport properties of a diverse range of metal/GaAs(1 1 0) Schottky diodes have been examined using the conventional current-voltage and capacitance-voltage techniques. The data are used to test for correlations between the Schottky barrier height and the interface heat of reaction, the metal work function and the metal electronegativity. Central transition metals, situated between groups Va-VIII of the periodic table, pin the Fermi level near mid-gap. The barrier heights obtained from most other metals exhibit a near-linear dependence on electronegativity.
Semiconductor Science and Technology, 1986
ABSTRACT By including the effects of electron-hole recombination and electron trapping in the ana... more ABSTRACT By including the effects of electron-hole recombination and electron trapping in the analysis of the transport properties of Schottky diodes it is possible to obtain a more accurate estimate of the Schottky-barrier height. The magnitude of the barrier height obtained from an analysis of the forward I-V characteristics of CdTe Schottky diodes is in better agreement with that obtained from C-V measurements than that obtained by using the conventional procedure of introducing an ideality factor into the 'ideal diode' equation.
Semiconductor Science and Technology, 1987
ABSTRACT The electrical properties of metal-CdTe contacts have been measured for a diverse group ... more ABSTRACT The electrical properties of metal-CdTe contacts have been measured for a diverse group of 18 metals by using current-voltage and capacitance-voltage techniques. The contacts were formed by metal evaporation onto chemically etched surfaces of n-CdTe single crystals. A large number of metals yield a barrier height of 0.72+or-0.03 V, independent of the work function of the metal. Mn, Cr and V were different, producing ohmic or very low barriers. The nature of the CdTe(110) surfaces produced by chemical etching using bromine in methanol was studied by an X-ray photo-emission technique. The authors have also carried out a detailed investigation into the microscopic interactions at selected interfaces by soft X-ray photo-emission using a synchrotron radiation source. Detailed comparisons of the microscopic interaction of Ag and Mn with the oxidised surfaces, using photo-emission are reported. In contrast to the behaviour of Ag, the Mn overlayer reduces completely the CdTe native oxide layer, leading to low-barrier contacts. It has been found that the bulk thermodynamic data provide an approximate description of the interface reactivity for most of the systems studied.
Semiconductor Science and Technology, 1987
ABSTRACT The electrical properties of moderately doped Ga/nGaAs(110) Schottky diodes, fabricated ... more ABSTRACT The electrical properties of moderately doped Ga/nGaAs(110) Schottky diodes, fabricated on both the clean cleaved and on the cleaved and air exposed GaAs(110) surface, have been studied. The current-voltage characteristics of both interfaces have an extremely large electron-hole recombination current. It is argued that this is produced by a disordered GaAs interfacial layer. Additionally, the capacitance-voltage characteristics of Ga/nGaAs(110) diodes, fabricated on the clean cleaved surface, are non-ideal and they preclude conventional analysis. The aging behaviour of these interfaces has also been studied in detail. The electrical characteristics and the changes in the electrical characteristics with time are discussed in terms of the available microscopic theories of Schottky barrier formation.
AIP Conference Proceedings, 1989
ABSTRACT
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1986
ABSTRACT Detailed studies are reported of interfaces formed between cleaved surfaces of GaAs and ... more ABSTRACT Detailed studies are reported of interfaces formed between cleaved surfaces of GaAs and InP, and the metals, Ag, Al, and Mn. Microscopic interactions have been examined by soft x‐ray photoemission, with synchrotron radiation, and electrical transport at interfaces have been studied by I–V and C–V methods. Ag is unreactive with clean GaAs and InP, and with oxidized GaAs, but is reactive with oxidized InP. Al and Mn chemically react with air cleaved and vacuum cleaved surfaces. Metals on InP are generally more reactive than on GaAs and Schottky barriers formed are very variable and dependent on intermediate oxide layers. Variable Schottky barriers are also measured for Mn on GaAs. The data are discussed in terms of current models of barrier formation at metal–semiconductor interfaces.
Nature Chemistry, 2014
Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these struct... more Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.
Physical Review B, 2012
ABSTRACT The Si(111)-7×7 surface reconstruction can be used to template an array of mesitylene (1... more ABSTRACT The Si(111)-7×7 surface reconstruction can be used to template an array of mesitylene (1,3,5-trimethyl-benzene) molecules in which each molecule bridges a corner Si adatom and a neighboring Si rest atom. The molecules adsorb in corner sites to minimize a weak steric interaction associated with nearest-neighbor bridging sites. We demonstrate that the formation of the molecular array can be blocked by adding submonolayer amounts of Ge to the surface. The Ge atoms block the formation of the array because they substitute preferentially for Si adatoms located in corner sites, and the Ge–C bond strength is ≈0.8 eV weaker than the Si–C bond strength.
Applied Surface Science, 2005
A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab ini... more A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab initio theoretical technique and scanning tunnelling microscopy (STM). Our calculations demonstrate decisively that the recently proposed Haiku geometry is a lower energy configuration than any of the previously proposed line geometries. Furthermore, we have made comparisons between STM constant-current topographs of the lines and Tersoff-Haman STM simulations. Although the Haiku and the Miki geometries both reproduce the main features of the constant-current topographs, the simulated STM images of the Miki geometry have a dark stripe between the dimer rows that does not correspond well with experiment. #
Physical Review B, 2009
ABSTRACT The adsorption of thiophene on Si(111)-(7×7) has been studied using density functional t... more ABSTRACT The adsorption of thiophene on Si(111)-(7×7) has been studied using density functional theory within the generalized gradient approximation. For all adsorption geometries that have been experimentally identified, equilibrium binding energies have been calculated using a full 7×7 supercell to a depth of six Si monolayers. Despite the structural heterogeneity of the surface, all adsorption geometries are calculated to have the same binding energy, irrespective of their location within the 7×7 unit cell. A parallel survey of chemisorption sites, performed with scanning tunneling microscopy, demonstrated that thiophene prefers the faulted half of the unit cell over the unfaulted half, and edge over corner sites, in agreement with previous experimental studies. The theoretical and experimental results suggest that the activation energy barriers for chemisorption are site dependent. The physical factors leading to site dependent activation energies are briefly discussed.
Nanotechnology, 2005
The equilibrium geometry, electronic structure and energetic stability of Bi nanolines on clean a... more The equilibrium geometry, electronic structure and energetic stability of Bi nanolines on clean and hydrogenated Si(001) surfaces have been examined by means of ab initio total energy calculations and scanning tunnelling microscopy. For the Bi nanolines on a clean Si surface the two most plausible structural models, the Miki or M model (Miki et al 1999 Phys. Rev. B 59 14868) and the Haiku or H model (Owen et al 2002 Phys. Rev. Lett. 88 226104), have been examined in detail. The results of the total energy calculations support the stability of the H model over the M model, in agreement with previous theoretical results. For Bi nanolines on the hydrogenated Si(001) surface, we find that an atomic configuration derived from the H model is also more stable than an atomic configuration derived from the M model. However, the energetically less stable (M) model exhibits better agreement with experimental measurements for equilibrium geometry. The electronic structures of the H and M models are very similar. Both models exhibit a semiconducting character, with the highest occupied Bi-derived bands lying at approximately 0.5 eV below the valence band maximum. Simulated and experimental STM images confirm that at a low negative bias the Bi lines exhibit an 'antiwire' property for both structural models.
physica status solidi (a), 2012
ABSTRACT We show that the Si(111)-7 x 7 surface reconstruction can be used to template an ordered... more ABSTRACT We show that the Si(111)-7 x 7 surface reconstruction can be used to template an ordered array of 1,3,5-trimethyl benzene (mesitylene) molecules. The disorder that normally derives from the multiplicity of admissible adsorption geometries, for small aromatic molecules on 7 x 7, is suppressed, and the molecules are found to occupy both halves of the 7 x 7 unit cell with equal probability. It is argued that a steric interaction, associated with the methyl groups, hinders nearest neighbor adsorption and this leads to the formation of an array that has the molecules at the corners of the 7 x 7 half unit cells. To understand the ordering kinetics we used: scanning tunneling microscopy to study site occupancy as a function of coverage, ab initio total energy calculation to study the stability of the attachment sites and kinetic Monte Carlo modeling to investigate the emergence of translational order in the overlayer. [GRAPHICS] When two mesitylene molecules adsorb in nearest neighbor bridging geometries on the 7 x 7 reconstruction, the hydrogen atoms located on the methyl groups interact. To reduce the interaction energy, the molecules form an ordered array that places each molecule at the corner of the half cell. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
physica status solidi (c), 2010
ABSTRACT The adsorption of thiophene on Si(111)–7×7 has been studied with scanning tunneling micr... more ABSTRACT The adsorption of thiophene on Si(111)–7×7 has been studied with scanning tunneling microscopy (STM) and kinetic Monte Carlo (kMC) modelling. Previous experimental studies of this system clearly demonstrated that thiophene prefers to chemisorb on the faulted half of the 7×7 unit cell. The STM studies reported here concur with this and provide further information about thiophene site preference as a function of coverage. Additionally, an ab initio theoretical investigation of this system demonstrated that the occupancy of available adsorption sites could not be explained using equilibrium binding energies, as these were calculated to be the same for all experimentally identified adsorption sites (≈1.0 eV). To investigate the possibility that site selection is kinetically controlled, a kMC model was developed. This model places the molecule in a mobile precursor state, allowing the molecule to traverse the surface before chemisorbing. The kMC model was found to reproduce the STM data, providing compelling evidence that site occupancy in this system is indeed kinetically controlled at room temperature. Activation energy differences, for each the four unique chemisorption geometries, could be extracted from a fit of the kMC model predictions to the experimental data (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physical Review B, 2011
ABSTRACT We demonstrate that the Si(111)-7×7 surface reconstruction can be used to template an or... more ABSTRACT We demonstrate that the Si(111)-7×7 surface reconstruction can be used to template an ordered array of 1,3,5-methyl benzene molecules that are uniformly distributed over both the faulted and the unfaulted halves of the 7×7 unit cell by covalent attachment in vacuo. An intermolecular steric interaction, which hinders nearest-neighbor adsorption, is shown to play an important role in the formation of the ordered array. The stable equilibrium structure is shown to be one where the molecules are located at the corner of the half unit cells maximizing the intermolecular separation. In addition to the intermolecular steric interaction, there is an interaction between the molecule and the surface that plays a important role in reducing disorder in the array. Moreover, as the coverage is increased, there is a switch in site preference, from edge to corner, that mitigates the effect of the intermolecular interaction. To investigate this system we used scanning tunneling microscopy to study site occupancy as a function of coverage, ab initio total energy calculation to study the stability of the attachment sites, and Monte Carlo modeling to examine the emergence of translational order in the overlayer. The switch in site preference from edge to corner is faithfully reproduced by the kinetic Monte Carlo model when an interaction term is included.
We present a theoretical investigation of the influence of domain boundaries on the Ge/Si(111)-5 ... more We present a theoretical investigation of the influence of domain boundaries on the Ge/Si(111)-5 × 5 phase using both large-scale DFT simulations and an analytical model. It is shown that different boundary types modify the atomic and electronic structure of the adjoining 5 × 5 domains in very different ways. A simple theoretical model, that describes the energy interaction J between the boundaries and the 5 × 5 phase, is presented and the interaction energy decay J(x) ≈ x −n for different domain boundaries is estimated. Additionally, the influence of the boundaries on the atomic and electronic structure of adatoms in the parental 5 × 5 phase is analyzed and it is argued that the presence of domain boundaries may strongly affect not only the physical but also the chemical properties of the Ge/Si(111)-5 × 5 phase.
Journal of Physics C Solid State Physics
The transport properties of over 400 Schottky diodes, fabricated on cleaved, atomically clean and... more The transport properties of over 400 Schottky diodes, fabricated on cleaved, atomically clean and on cleaved, air-exposed GaAs (110) surfaces, have been examined for a diverse range of metallisations using the conventional current-voltage and capacitance-voltage techniques. The resulting body of experimental data provides a reliable base for studying the correlation between the Schottky barrier height and fundamental quantities. In particular, the range of barrier heights obtained from the diodes fabricated on the clean surfaces are compared with the metal work function and the metal electronegativity. It is demonstrated that central transition metals, from groups Va-VIII of the periodic table, produce mid-gap Fermi-level pinning on GaAs. In contrast, the barrier heights extracted from simple- and noble-metal contacts, and from transition metals situated towards the beginning and end of a transition series, exhibit a near-linear dependence on electro-negativity.
Journal of Physics C Solid State Physics
The formation of the Al/GaAs(110) interface has been studied at room temperature using synchrotro... more The formation of the Al/GaAs(110) interface has been studied at room temperature using synchrotron-radiation-excited soft-X-ray photo-emission spectroscopy. It is demonstrated that the Al 2p core-level emission, at low Al coverages, provides evidence for at least three distinct Al phases. These are isolated Al adatoms, Al atoms in surface Ga sites and Al metallic clusters. The last two phases are observed at higher Al coverages and have been extensively studied by other groups. However, the nature of the first phase and the transition from the first to the second phase have been the subject of only a few studies. In this paper it is shown that the change from the first to the second phase, as determined from the Al2p emission, is similar to that observed when Al is deposited on InP(110). Additionally it is argued that the low-coverage Al2p core-level photo-emission spectra, which have been used previously to support the view that the Al adatoms are weakly interacting with the GaAs s...
Journal of Physics C: Solid State Physics, 1988
For pt.I see ibid., vol.21, p.783 (1988). Thermodynamic aspects of Schottky contact formation on ... more For pt.I see ibid., vol.21, p.783 (1988). Thermodynamic aspects of Schottky contact formation on cleaved GaAs (110) surfaces are examined. It is shown, for 18 different metals, that a simple binary-phase bulk thermodynamic model is a useful semi-quantitative guide to interfacial reactivity, provided both arsenide and gallium alloy formation are considered. Absence of reaction, reaction by arsenide formation alone, reaction by alloying alone and reaction by both arsenide and alloy formation are correctly predicted. In contrast, attempts to correlate any of the thermodynamic parameters, including the interface heat of reaction, with the Schottky barrier height were unsuccessful.
Solid State Communications, 1988
The transport properties of a diverse range of metal/GaAs(1 1 0) Schottky diodes have been examin... more The transport properties of a diverse range of metal/GaAs(1 1 0) Schottky diodes have been examined using the conventional current-voltage and capacitance-voltage techniques. The data are used to test for correlations between the Schottky barrier height and the interface heat of reaction, the metal work function and the metal electronegativity. Central transition metals, situated between groups Va-VIII of the periodic table, pin the Fermi level near mid-gap. The barrier heights obtained from most other metals exhibit a near-linear dependence on electronegativity.
Semiconductor Science and Technology, 1986
ABSTRACT By including the effects of electron-hole recombination and electron trapping in the ana... more ABSTRACT By including the effects of electron-hole recombination and electron trapping in the analysis of the transport properties of Schottky diodes it is possible to obtain a more accurate estimate of the Schottky-barrier height. The magnitude of the barrier height obtained from an analysis of the forward I-V characteristics of CdTe Schottky diodes is in better agreement with that obtained from C-V measurements than that obtained by using the conventional procedure of introducing an ideality factor into the 'ideal diode' equation.
Semiconductor Science and Technology, 1987
ABSTRACT The electrical properties of metal-CdTe contacts have been measured for a diverse group ... more ABSTRACT The electrical properties of metal-CdTe contacts have been measured for a diverse group of 18 metals by using current-voltage and capacitance-voltage techniques. The contacts were formed by metal evaporation onto chemically etched surfaces of n-CdTe single crystals. A large number of metals yield a barrier height of 0.72+or-0.03 V, independent of the work function of the metal. Mn, Cr and V were different, producing ohmic or very low barriers. The nature of the CdTe(110) surfaces produced by chemical etching using bromine in methanol was studied by an X-ray photo-emission technique. The authors have also carried out a detailed investigation into the microscopic interactions at selected interfaces by soft X-ray photo-emission using a synchrotron radiation source. Detailed comparisons of the microscopic interaction of Ag and Mn with the oxidised surfaces, using photo-emission are reported. In contrast to the behaviour of Ag, the Mn overlayer reduces completely the CdTe native oxide layer, leading to low-barrier contacts. It has been found that the bulk thermodynamic data provide an approximate description of the interface reactivity for most of the systems studied.
Semiconductor Science and Technology, 1987
ABSTRACT The electrical properties of moderately doped Ga/nGaAs(110) Schottky diodes, fabricated ... more ABSTRACT The electrical properties of moderately doped Ga/nGaAs(110) Schottky diodes, fabricated on both the clean cleaved and on the cleaved and air exposed GaAs(110) surface, have been studied. The current-voltage characteristics of both interfaces have an extremely large electron-hole recombination current. It is argued that this is produced by a disordered GaAs interfacial layer. Additionally, the capacitance-voltage characteristics of Ga/nGaAs(110) diodes, fabricated on the clean cleaved surface, are non-ideal and they preclude conventional analysis. The aging behaviour of these interfaces has also been studied in detail. The electrical characteristics and the changes in the electrical characteristics with time are discussed in terms of the available microscopic theories of Schottky barrier formation.
AIP Conference Proceedings, 1989
ABSTRACT
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1986
ABSTRACT Detailed studies are reported of interfaces formed between cleaved surfaces of GaAs and ... more ABSTRACT Detailed studies are reported of interfaces formed between cleaved surfaces of GaAs and InP, and the metals, Ag, Al, and Mn. Microscopic interactions have been examined by soft x‐ray photoemission, with synchrotron radiation, and electrical transport at interfaces have been studied by I–V and C–V methods. Ag is unreactive with clean GaAs and InP, and with oxidized GaAs, but is reactive with oxidized InP. Al and Mn chemically react with air cleaved and vacuum cleaved surfaces. Metals on InP are generally more reactive than on GaAs and Schottky barriers formed are very variable and dependent on intermediate oxide layers. Variable Schottky barriers are also measured for Mn on GaAs. The data are discussed in terms of current models of barrier formation at metal–semiconductor interfaces.