摘要:

We have carried out a study of the chemical reaction of silver, indium, and aluminium layers with cleaved GaP(110) surfaces using photoemission with synchrotron radiation. Core level photoelectron spectra show that silver and indium overlayers do not cause an interface reaction with GaP(110). The deposition of Al, on the other hand, leads to an extensive exchange reaction which also proceeds at low temperature, although influenced by changes in overlayer growth morphology. Surface band bending induced by the metallic overlayers was investigated as a function of deposition forn‐ andp‐type material. In contrast to earlier findings, almost identical Schottky barrier heights for In and Ag deposition are obtained, despite the large difference in work function between these two metals. Results for Al also suggest that a small range of pinning positions is responsible for the Schottky barrier heights for junctions of these metals with GaP(110). We find that large peak shifts due to a surface photovoltage induced by the photoemission light source affect the determination of the Schottky barrier heights. This and other possible reasons for the discrepancy with earlier work are discussed.

ISSN:0734-211X    

DOI:10.1116/1.584949

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The room temperature formation of GaP(110) interfaces with the transition metals (TM) vanadium and yttrium were studied by photoemission spectroscopy. Both the evolution of band bending and the interface chemical reactivity were deduced from the Ga 3dand P 2pcore level emission spectra for a metal coverage range of ∼0.003 to 50 Å. Free Ga is generated through a TM–Ga exchange reaction; it disperses in both metals with increasing coverages and with a noticeable tendency toward surface segregation. Two phosphorous reaction components were observed for both metals, one of which was attributed to free P, the other to TM–P bonding. Band gap emission attributed to an impurity state derived from the V 3dlevel was observed for coverages as low as 0.01 Å. A metallic Fermi edge developed at 1 Å of V, but required a thickness of ∼30 Å for Y. However, metallic character was observed in the replaced Ga for Y coverages ≳3 Å. The band bending for both metals onn‐GaP is characterized by a near‐constant value of the Fermi levelEFclose to midgap for a broad coverage range to ∼1 Å, followed by a rapid drop ofEFwith onset of metallicity in the V. For Y, this decrease extended to coverages beyond 20 Å, consistent with the delayed onset of metallicity for this system. Schottky barrier heights of 1.33 eV (V) and 1.24 eV (Y) were measured. For coverages below the onset of metallicityEFappears to be determined by the observed TM‐induced impurity states in the gap. The previously described delocalization model successfully accounts for both the observed drop inEFwith onset of metallicity and Schottky barrier heights for a number of metals, including V and Y.

ISSN:0734-211X    

DOI:10.1116/1.584950

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

We have used ultraviolet photoemission spectroscopy to study the morphology of the nonreactive materials In, Ag, Sb, and Bi deposited on the GaAs(100) surface. The attenuation of the substrate core levels and the electronic structure of the overlayers yield information on the growth modes and others properties of these interfaces. We find that in all cases the behavior is significantly different from the growth on the more heavily studied (110) surface. In the case of In, we observe isolated In atoms in between large clusters, while on the (110) surface, only strong clustering is observed. Ag forms a larger number of small clusters compared to the (110) surface. This indicates that the mobility of small Ag clusters is lower, and/or the number of nucleation sites is greater on our (100) surfaces. Both Sb and Bi grow in more laminar modes, as opposed to the Stranski–Krastanov mode observed on (110). The Sb film is found to be closer to perfectly laminar. After annealing at 250 °C for 10 min, the Bi film is found to cluster heavily except for a strongly bound single layer. In addition, the resulting Sb thin film is semiconducting up to a thickness of about 100 Å, while the Bi film appears to be semimetallic even at relatively low coverages. We find that the clean surface Fermi level is roughly 0.5 eV from the valence band maximum. However, we will concentrate on the growth mode of the overlayer. Band bending issues will be discussed elsewhere.

ISSN:0734-211X    

DOI:10.1116/1.584951

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The electronic structure of Na atoms adsorbed on the GaAs(110) surface is calculated in the tight‐binding approximation and the zero charge limit. For a monolayer and half a monolayer coverage, two different adsorption sites are considered. The surface atom and sodium core level shifts are calculated and compared to the experiment. The adatom binding energy is found to be maximum for adsorption on the surface Gallium dangling bonds. Finally the density of states and the interface state dispersion energy are compared with photoemission results.

ISSN:0734-211X    

DOI:10.1116/1.584952

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

We have investigated the compositional, structural, and band bending effects of growing ultrathin AlAs interlayers between epitaxial NiAl andn‐GaAs(001) by molecular‐beam epitaxy (MBE). Such an interface is of interest because of the possibility of increasing the already large Schottky barrier height at the NiAl/GaAs interface by inserting a lattice‐matched interlayer of larger band gap. We have found that the Fermi energy in the GaAs band gap, as measured by x‐ray photoemission spectroscopy (XPS), is only 0.35–0.40 eV below the conduction‐band minimum when very high quality AlAs is grown onn‐GaAs(001) by MBE. This result is presumably due to the creation of an interface of very high crystallographic quality and the related low density of interface states. We have also measured the valence band offset by XPS to be 0.40±0.07 eV, independent of band bending. Subsequent MBE growth of NiAl at 250 °C invariably increases band bending at the AlAs/GaAs interface, leaving the Fermi energy ∼0.8 eV below the conduction‐band minimum on the GaAs side. This increase in band bending is accompanied by disruption of the AlAs/GaAs interface, which appears to be driven by diffusion of Ni atoms through the thin AlAs interlayer. Post‐growth annealing at 570 °C increases the extent of disruption at the AlAs/GaAs interface and drives the Fermi level to ∼1.0 eV below the conduction‐band minimum. Combining the valence‐band offset and band bending measurements, the barrier height of the NiAl/AlAs/GaAs(001) interfacial system is estimated to be 1.15–1.35 eV, which is nearly a factor of 2 larger than values exhibited by conventional metal/GaAs interfaces.

ISSN:0734-211X    

DOI:10.1116/1.584953

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

We calculate the true electrostatic potential that forms a Schottky barrier, arising from a random distribution of dopants in the depletion region. It is customary to model the electrostatic potential in a Schottky barrier by a one‐dimensional quadratic potential, as would obtain from a jellium of charge in the depletion region. The difference between the true potential and the ‘‘ideal’’ one acts as a perturbation to electrons traversing the depletion region. This perturbation is found to be small, and effectively one dimensional. Its effect on the transmission probability of an electron tunneling through the depletion region is shown to be negligible. It is shown, however, that because of the large quantum dipoles near the metal–semiconductor interface, the image‐force lowering correction does not in general vary asN1/4d, as is widely thought, at least for heavily doped materials. Local‐density calculations of the Schottky barrier height were made for several metal/GaAs interfaces, to obtain the magnitude and range of the interfacial dipoles. They are found to be large and persist sufficiently far from the interface that they dominate the slowly varying potential from the ionized dopants. Thus, the interplay between the image‐force potential and the interfacial dipoles determines the maximum in Schottky barrier height; moreover the quantum dipoles add an additional correction of the same magnitude as the image‐force correction in the heavily doped case.

ISSN:0734-211X    

DOI:10.1116/1.584954

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

The atomic geometries of Ag/InP(110) interfaces for 0.5 and 1 Å nominal metal coverages have been determined by photoemission extended x‐ray absorption fine structure (PEXAFS). The coverages correspond to 3×1014and 6×1014atoms/cm2, if we assume that the Ag atoms are metallic, although we know that the Ag atoms are clustered at these low coverages. P 2pPEXAFS for 0.5 and 1 Å Ag covered InP(110) surfaces were acquired. The data were analyzed by conventional Fourier analysis procedures using the theoretical backscattering phase function of McKaleetal. plus absorber phase function of Teo and Lee. For both silver coverages, the P–In bond length at the interface was determined as 2.52±0.04 Å, which is 0.08±0.02 Å greater than its value for the clean surface. For the clean InP(110) surface, the average (first) P–In bond length is 2.43±0.04 Å [Choudharyetal., Phys. Rev. B38, 1566 (1988)], whereas in bulk InP the P–In interatomic distance is 2.54 Å. Hence, the PEXAFS results for low‐coverage Ag/InP(110) interfaces indicate that the relaxation (contraction) of the clean InP(110) surface is mostly removed by the deposited noble metal atoms. The structural results show that the low‐coverage Ag/InP(110) interfaces are nonreactive, since the unrelaxation is not a significant reaction. The low‐coverage metal‐induced unrelaxation in the P–In bond length at Ag/InP(110) interfaces might contribute to Fermi‐level shifts during the Schottky barrier formation.

ISSN:0734-211X    

DOI:10.1116/1.584955

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Electron‐induced surface electrovoltaic effects are determined as a function of temperature and dopant concentration for epitaxial Bi/GaAs(110) by using inverse photoemission. Results obtained with an incident electron flux of 1014‐1015cm−2 s−1show opposite shifts of empty state features of the Bi overlayers onp‐ andn‐type GaAs(110). These shifts depend upon the bulk dopant concentration and overlayer thickness, and they increase as the temperature is cooled from 300 to 60 K. They reveal a nonequilibrium charge distribution associated with electron–hole creation in the GaAs depletion region. Reversible, symmetric steady‐state band bending movements forn‐ andp‐type GaAs(110) are determined as a function of temperature for a variety of coverages by using thermal cycling 300→60→300 K. The suppression of electron–hole recombination at low temperature results in significant band flattening similar to that observed in low temperature photoemission studies for metal–GaAs(110) interfaces.

ISSN:0734-211X    

DOI:10.1116/1.585020

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Synchrotron radiation photoemission results for metals on intentionally misoriented molecular‐beam epitaxy GaAs(100) surfaces reveal an orientation‐dependent interfacial chemistry and Schottky barrier heights at low temperature (90 K). Previous measurements for different metals on aligned GaAs(100) surfaces show a Fermi stabilization energy range of 0.95 eV. However, for Au and Al on 2° misoriented specimens, narrower ranges are observed: 0.65 eV for GaAs surfaces cut towards [110] and 0.45 eV for surfaces cut towards [111]with either Ga or As dangling bonds perpendicular to the step edges. The decrease in the range of the Fermi level stabilization energies is mainly due to the increase in Schottky barrier heights for Al contacts on misoriented specimens, where a more pronounced interface reaction is observed. Our results indicate that the observed degree of chemical reaction and diffusion increases in the order of GaAs(100) surfaces tilted toward [110], [111]A, and [111]B. Our bonding results emphasize the importance of interfacial chemistry and the perfection of the substrate GaAs in the metal/GaAs junction electronic properties.

ISSN:0734-211X    

DOI:10.1116/1.585021

出版商:American Vacuum Society    

年代:1990

数据来源: AIP

摘要:

Submonolayer to several monolayers of In were evaporated onto the GaP(1̄ 1̄ 1̄) (1×1) clean surface at room temperature to investigate the formation of In/GaP(1̄ 1̄ 1̄) interface. From the electron energy loss spectra, it has been shown that the surface electronic states related with the Ga dangling bonds are significantly affected by the deposited In atoms even at initial stage. The growth mode of In overlayers on GaP(1̄ 1̄ 1̄) is island like rather than layer‐by‐layer form. By increasing the amount of deposited In atoms, the In islands become larger and thicker, as evaluated from the relative peak intensities of the bulk and plasmon losses. The core‐level photoelectron specrtra show there is no indication of the occurrence of In–Ga interfacial exchange reaction. The valence band ultraviolet photoelecton spectroscopy reveals a new peak structure at the energy of 5.2 eV below Fermi level, which is believed to be attributed by metallic In. In the case of Ga on InP, it has already been verified that the Ga–In interchange reaction does exist and is quite remarkable. It does not seem sufficient to explain the irreversibility of surface interchange reaction between Ga/InP and In/GaP simply by means of thermodynamical consideration. The surface migration coefficient and the stability of metallic clusters may play a complementary role in determining the surface chemical reactivity.

ISSN:0734-211X    

DOI:10.1116/1.585022

出版商:American Vacuum Society    

年代:1990

数据来源: AIP