Thin GaAs-oxide layers have been formed on GaAs (100) surfaces using deep UV light to enhance the oxidation reactions. Two different oxidation reactions were compared by using UV light of wavelengths below and above the O2 dissociation limit, viz, 248 nm and 193 nm respectively. For the light above the O2 dissociation limit which causes ozone formation, oxides formed by direct surface illumination were compared to oxides formed by illumination parallel to the GaAs surface. Metal contacts were deposited in situ to study the effect of the various thin oxides on the Schottky barrier height of contacts to GaAs.

Room-temperature, optically-induced oxidation of the gallium arsenide surface has been studied with laser radiation of different wavelengths. It was found that deep-ultraviolet light is much more effective in enhancing oxidation than near-ultraviolet or visible light. The growth rate of the oxide was also found to be drastically increased by the presence of chemisorbed water molecules on the surface.

Electrical contact resistivities of both Al and Cu single layer contacts as well as Cu/Au and Al/Au bilayer metal contacts to YBa2Cu3O7-x (YBCO) thin film have been studied. It was found that aluminum and copper make poor electrical contacts to YBCO due to interface reactions. These contacts have large contact resistivity (10(-1)-1 OMEGA cm2), orders of magnitude higher than that of a gold/YBCO contact (10(-6) OMEGA cm2). When an ultrathin Au interlayer (10-30 angstrom) was inserted between an Al or Cu overlayer and a YBCO film, interface reaction was greatly reduced. The contact resistivities of Al/Au and Cu/Au bilayer-metal contacts dropped by five orders of magnitude when Au interlayer thickness was increased from 0 to 10 angstrom. With the gold interlayer thickness of 15 angstrom, the Al/Au and Cu/Au bilayer-metal contacts reached a minimum contact resistivity, approaching that of a thick single-layer Au metal contact.

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We report on using a thin (∼200 Å) layer of photodeposited Cd to form high Schottky barrier height contacts to InP and In0.53Ga0.47As. Current-voltage measurements of the Schottky diodes yield barrier heights of 0.70 and 0.55 eV to InP and In0.53Ga0.47As, respectively. The photodeposition process has been integrated with conventional clean room processing to fabricate Au/Cd/In0.53Ga0.47As transistors with high transconductances (∼200 mS/mm) and operating frequencies ( fmax∼30 GHz). X-ray photoelectron spectroscopy of thin Cd photodeposits on InP shows that the process produces an interfacial (∼10 Å thick) Cd-InP reaction zone covered by metallic Cd.

The surface chemistry of GaAs-oxide removal with an electron cyclotron resonance (ECR) hydrogen plasma has been investigated with x-ray photoelectron spectroscopy. It is found that As oxide is efficiently removed at room temperature, and heating expedites the removal of Ga oxide. Band bending changes during ECR hydrogen-plasma oxide reduction are also discussed.

A variation of classical marker techniques has been combined with low-energy ion scattering spectroscopy (ISS) to study the diffusion of species during the initial formation of several reacted monolayers, allowing the study of thinner layers than standard marker techniques allow. Results are presented for the reaction of oxygen with GaAs under 248 nm laser illumination and at 400-degrees-C, as well as for the well-studied case of Ni oxidation at room temperature. We discuss the criteria for choosing an appropriate marker element, and the limitations on the information that can be obtained with this method. The results show that inward oxygen diffusion can be distinguished from outward metal diffusion as the reaction proceeds. This information is crucial for developing an understanding of reaction mechanisms in thin films.

Low resistivity normal metal contacts to YBa2Cu3O7-x (YBCO) films have been investigated. It has previously shown that the contact resistivity of Au contact exhibits a strong temperature dependence, decreasing 2-3 orders of magnitude at a transition temperature near T(c) of YBCO film. Other metal contacts, including Pd and Nb, do not show this effect. The contact resistivity of metal contacts has been correlated with interfacial reactions and disruption studied by x-ray photoelectron spectroscopy (XPS). In this work we demonstrate that a thin interlayer, specifically 10 angstrom of Au, between the YBCO and a metal contact such as Nb can allow the formation of a low resistance contact. XPS of the effect of the interlayer is presented, and the implications for carrier coupling are discussed. These results indicate a methodology for low contact resistance bilayer-metal structures for superconducting device applications.

Photogenerated electron transfer can play a large role in surface chemical reactions. These electrons have been shown to play a role in adsorption, desorption, and dissociation of semiconductor surfaces.1 Deep UV photons can photoemit carriers into energy bands and states not accessible by photogenerated carriers created by visible light. As an example, we describe the sharp increase in the photoenhanced oxidation rate on GaAs in the presence of deep UV radiation (4.1 eV < hv < 5.1 eV). In contrast to previous experiments using visible radiation,2 the deep UV reaction enhancement occurs at oxide coverages of more than one monolayer (ML), and the oxide formation under deep UV irradiation can exceed several monolayers.

A new technique is developed to study the atomic movement in ultraviolet laser enhanced and low temperature (⩽ 400 ° C) thermal oxidation of GaAs. The new method is a combination of the classical marker technique and low energy ion scattering spectroscopy (ISS). In the formation of thin GaAs oxide layers (∼ 10 Å), the marker is found to remain on the oxide surface, indicating that oxidation is occurring at the intrface of GaAs/oxide by the diffusion of an oxygen species. This is in contrast to the oxidation of metals such as Ni and Cu where the same technique supports earlier observations that oxidation occurs at the oxide/ambient interface. The diffusion of a metal species results in the marker being buried during the oxidation of the metal surfaces.

X-ray photoelectron spectroscopy (XPS) and electrical characterization have been used to investigate metal contacts to thin films of superconducting YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} (YBCO). The metals studied are Au, Pt, Pd, Sn, and Ti, which cover a wide range of physical properties including reactivity with oxygen and bulk resistivity. XPS was also used to investigate the sputter cleaning and subsequent heating of the YBCO films prior to contact formation. Contacts for electrical study were defined by a shadow mask, while contacts studied by XPS were formed by sequential deposition of several A of metal. XPS of the O 1s, Cu 2p, and the various metal core levels confirm that the more reactive metals cause more disruption at the metal/YBCO interface. These observations of interface chemistry are correlated with contact resistivity measured at room temperature and at 77 K. We find that although Au, Pt and Pd have similar contact resistivity at room temperature, only the Au contacts show a large decrease in contact resistance at 77 K. Ti and Sn have much higher contact resistivities at room temperature than the noble metals. These results are discussed in terms of interface chemistry and material properties.

The recent observation of the optical rectification of femtosecond electromagnetic waves provides an alternative optical technique to characterize the depletion fields at semiconductor surfaces and interfaces 1 The basic physical mechanism of the optical rectification is the electromagnetic radiation from the transient photocurrent within the depletion width. The outward rectified field E1 radiated from a bare semiconductor surface can be expressed as follows.

Fermi level movements at Pt/GaAs and Ti/GaAs interfaces have been investigated using a direct measurement of Schottky barrier heights in a bimetal Schottky structure. Using thin interfacial layers, the Schottky barrier was smoothly varied from the characteristic value of the thick metal to that of the interfacial metal. The variation of barrier height versus the inner metal thickness was found to exhibit an exponential behavior extending over a few monolayers coverage. This experiment indicates a new approach to the fundamental study of metal-semiconductor interfaces and could be useful in device applications.

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.

Rapid thermal annealing has been used for fabrication of YBaCuO thin films from Cu/BaO/Y2O3 layered structures. The films were deposited on Si substrates by electron-beam evaporation. The interdiffusion at the film/substrate interface has been investigated using Auger depth profiling. With a metal barrier layer, the film showed the superconducting transition between 74–85 K. At anneal temperature above 980°C, Si was found to diffuse throughout the film and degrade the superconductivity of the films.

Light-enhanced reaction of oxygen with gallium arsenide surfaces by irradiation with deep-UV, near-UV, and visible light was studied using Auger electron spectroscopy and x-ray photoelectron spectroscopy for submonolayer and above monolayer regimes, respectively. The onset of a strong wavelength dependence of the enhanced oxidation was observed after the oxygen coverage reached more than one-half a monolayer. An abrupt threshold for this wavelength dependence was also observed at ∼4.1-eV photon energy. A possible mechanism is presented to explain this strong wavelength dependence.

Ultraviolet illumination has been used to form a few monolayers of GaAs oxide in dry oxygen on both p-type and n-type GaAs(100) surfaces. Some of the oxides have been annealed to form Ga-rich oxide surfaces. Six metals with a wide range of work functions and different reactivity with the oxide layer have been used to form Schottky contacts on clean GaAs and on the thin-oxide layers. Large variations from the Schottky barrier height on clean GaAs have been observed for both doping types due to the photo-oxidation of the GaAs surfaces. The trend in Schottky barrier change is toward the ideal Schottky limit where the metal work function determines the barrier height. The GaAs Fermi level is shown to move in a much wider region of the band gap for the oxidized surfaces than for clean surfaces, however, it is still confined to a midgap region.