The chemical stability of interfaces between metals and GaAs was discussed in terms of reaction sequence and diffusion path concepts. The factors which determine interface morphology were also given. These general ideas can be applied to any interfacial reactions between two dissimilar materials.

Epitaxial thin films of three different Pt-Ga intermetallic compounds have been grown on GaAs by molecular beam epitaxy (MBE). The resultant films have been annealed at various temperatures and then examined using X-ray two-theta diffraction. Both PtGa2 and PtGa thin films are chemically stable on GaAs under 1 atmosphere of N2 up to 450°C and 600°C, respectively. Thin films of Pt2Ga react with GaAs at temperatures as low as 200°C to form phases with higher Ga concentration.

In-situ annealing TEM experiments were performed on the Ti/GaAs system in order to study the dynamic behavior of interfacial reactions. Both plan-view and cross-sectional samples were investigated in either diffraction and imaging (both conventional and high resolution) modes. During experiments, we observed the following: (a) At the initial stage of reaction, the TiAs phase formed at the original Ti/GaAs interface with a distinct orientation with respect to the substrate; (b) as the reaction proceeded, the TiAs phase formed in a random manner; (c) finally, the liberated Ga species from the GaAs diffused out to the metal film and formed TiGa2 phase in the plan-view sample similar to the furnace-annealed case. For the cross-sectional sample, however, we did not observe any Ti:Ga phase formation. Instead, we observed the formation of voids both in the Ti film and in the GaAs substrate. The formation of different microstructure between in-situ and furnace annealed cases is explained by the sample geometry during annealing.

A fundamental and comprehensive approach has been taken to study Co//GaAs interfacial reactions, using phase diagram determination, bulk and thin-film diffusion couple studies, and electrical characterization. Phase formation sequences and interfacial morphologies are found to be similar in bulk and thin-film couples. Thermodynamic and kinetic analyses are used to rationalize the contact formations. The electrical properties of the contacts are correlated to the phase formation sequences and phase diagram information.

Bulk diffusion couples of Co/Si were annealed at 800, 900, 1000, 1050 and 1100°C for periods ranging from 24 hours to one month. Growth rates of the intermediate phases, Co2Si, CoSi and CoSi2, as well as the composition profiles across the couples were determined by optical microscopy and electron probe microanalysis (EPMA). Using the solution to the multiphase binary diffusion equations and the experimental data, the interdiffusion coefficients for Co2Si, CoSi and CoSi2 are obtained as a function of temperature. The activation energies obtained are 140, 160 and 190 KJ/mole for Co2Si, CoSi and CoSi2, respectively. The generally small interdiffusion coefficient of CoSi2 and its high activation energy cause the growth rate of CoSi2 to be extremely small at low temperatures.

The interdiffusion coefficients for Co2Si, CoSi and CoSi2 at 545°C are obtained by extrapolation of the high-temperatures data. Using these data and solving numerically the diffusion equations with the appropriate boundary conditions, the growth of Co2Si, CoSi and CoSi2 is calculated as a function of time. The calculated results are in good agreement with the experimental data reported in the literature. This study demonstrates clearly that the initial absence of the CoSi2 phase is due to diffusion-controlled rather than nucleation-controlled kinetics. This phenomenon may be quite common in many thin-fiflm metal/Si couples.

WNx,/GaAs Schottky contacts formed by reactive sputtering were found to be thermally stable up to an annealing temprature of ∼9400°C. The interface morphology and structure of this contact under high temperature annealing conditions ( > 700°C ) have been investigated by transmission electron microscopy (TEM) and x-ray diffractometry techniques. For the as-deposited samples, the thin film had an amorphous structure. After annealing at high temperatures, the amorphous phase transformed to α-W and W2N phases. However, the contact interface remained thermally stable up to 850°C. Cross-sectional TEM micrographs revealed that annealing at temperatures above 850°C resulted in the formation of ‘pockets’ beneath the interface. This phenomenon has been correlated with the electrical properties of the contacts, e. g., an enhancement of the barrier height of the contact. Comparisons between the interface morphology of this system and other refractory metal nitride contacts (e. g., TiN/GaAs) are also presented.

InP substrates with 40nm metal films of Pt were encapsulated in SiO2, and isochronally annealed up to 600°C in flowing forming gas. The composition and morphology of the phases that formed were studied using x-ray diffraction, Rutherford Backscattering, and transmission electron microscopy.

Results show that the Pt/InP system begins interacting at 300°C. TEM analysis of the 350°C anneal shows unreacted Pt and and additional polycrystalline phases, with no observed orientation relationship with the substrate. The Pt layer has been completely consumed by 400°C, with a uniform reacted layer indicated by RBS. At high temperatures (between 500°C and 600°C), the reaction products are PtIn2 and PtP2. The two phases show a tendency for phase separation, with a higher concentration of PtP2 at the InP/reacted layer interface. The phosphide phase also shows a preferred orientation relationship with the substrate.

Results of the interaction of 40-nm Pd films on chemically cleaned InP substrates at temperatures from 175 to 650°C are reported. Comparisons are made with previous results from studies of Pd thin-films on GaAs. For both systems, the reaction began upon deposition of the metal. Ternary phases were found after annealing at temperatures up to 250°C. At 450°C and higher, the PdIn or PdGa phase was dominant because of loss of volatile P or As. Possibilities for making ohmic contacts based on these systems are discussed.

We have used Auger spectroscopy to monitor chemical reactions during solid phase epitaxy by contact reaction in the Ni/Si(ll1) ultrathin film system. We show that coexisting phases may be separated by numerically fitting the composite Si LVV lineshape using a linear combination of single phase “fingerprint” spectra. Sytematic measurements of coverage and temperature conditions are compiled into a kinetic phase diagram. Comparison with conventional (1000Å) thin film data suggest that the reactions forming Ni2Si and NiSi at > 20 Å thickness are bulk diffusion limited, while surface diffusion dominates at lower coverage. On the other hand, the formation of NiSi2 appears to be nucleation limited at all coverages, with dramatic variations in reaction rate with film thickness. This is discussed in terms of a competition between surface and bulk free energies.

During high temperature circuit fabrication, metallization layers can come in contact with both solids and gases. Their stability can be addressed with the aid of phase equilibria. Using the Gibbs phase rule as a basis, a method for generating phase diagrams for multicomponent systems can be established. This procedure is described and illustrated by reference to the quaternary phase diagram of Ti-Si-N-O. This phase diagram can then be used to predict stability and/or reactions in metallization layers and thin films.

Cross-sectional transmission and scanning transmission electron microscopy and thermodynamic and kinetic analysis have been used to characterize amorphous and crystalline nickel silicide formation in nickel/amorphous-silicon multilayer thin films. An amorphous-nickelsilicide layer was formed between the nickel and amorphous-silicon layers during deposition. Heating caused crystalline Ni2Si to form at the nickel/amorphous-nickel-silicide interface. The composition of the amorphous-siicide was determined to be approximately 1 Ni atom to 1 Si atom. Thermodynamic analysis indicates that amorphous-nickel-silicide could be in equilibrium with nickel and amorphous-silicon if there were kinetic barriers to the formation of the crystalline silicides. Kinetic analysis indicates that the “nucleation surface energies” of the crystalline silicides, other than Ni3Si, must be 1.6 to 3.0 times larger than that of amorphous-nickel-silicide.

We have derived a dynamical model for interdiffusion at aluminum-silicon interfaces which can explain the great variety of rates of interdiffusion observed for this system. The variables addressed by the model include the degree of covalent bonding which occurs at the chemical interface and the local atomic structure of that interface. Local solid solubility data and maximum rates of dissolution are derived analytically, and good agreement is observed between the model and both thermodynamic data for the bulk aluminum-silicon system and kinetic data for reaction rates at differently prepared interfaces. The influence of dopants and structural defects on the predictions of the model is in accord with experimental observations. The model points to some critical conditions for the preparation of thermally stable aluminum-silicon interfaces, which are potentially applicable to other metal-semiconductor systems.

We report electron-radiation-induced epitaxial growth of CoSi2 on Si(111) from an amorphous Co/Si 1:2 mixture with epitaxial CoSi2 nuclei. Under the electron beam of a Philips EM430 electron microscope, the epitaxial nuclei grow parallel to the surface with a growth rate orders of magnitude higher than that for thermally activated growth. The substrate temperature during irradiation and the electron energy dependence were studied. Electron-radiation-induced growth shows very weak temperature dependence in the temperatures between 100°K and 300°K and the activation energy is 0.03 eV. The growth rate increases significantly as the electron energy increased to 200 KeV which is about the threshold energy for displacing Si atoms.

Moiré pattern images have been used to investigate the crystalline quality of thin films deposited on (100)Si substrates. Observations performed on TiSi2 show a three-dimensional growth process and two different epitaxial modes. In the case of GaAs epilayers, it is shown that the residual strains are not uniformly distributed in the layer. Residual strain and threading dislocations are related to imperfections of the misfit dislocation network.

Recent studies of interface states and band bending at metal / III-V compound semiconductor interfaces reveal that these junctions are much more controllable and predictable than commonly believed. Soft x-ray photoemission spectroscopy studies demonstrate a wide range of band bending for metals on many III-V compounds, including GaAs. Cathodoluminescence spectroscopy measurements show that discrete states form at the microscopic junction which can have a dominant effect on the band bending properties. Internal photoemission measurements confirm the bulk barrier heights inferred by photoemission methods. After separating out surface chemical and bulk crystal quality effects, one finds simple, predictive barrier height variations which follow classical Schottky behavior.

We have carried out a systematic study of the electrical properties of Schottky barriers formed on atomically-clean and contaminated n-type and p-type GaAs surfaces[1-11]. Diodes were fabricated by in-situ deposition on clean GaAs (110) surfaces prepared by cleavage in ultrahigh vacuum and on contaminated surfaces prepared by cleavage and exposure to the atmosphere[1-4]. The consistent and reproducible barrier height determinations from the electrical measurements of unannealed and annealed diodes, when combined with results of transmission electron microscopy (TEM)[5,6] and surface sensitive studies on identically prepared samples[7,8], are found to be a particularly critical test of models of Schottky barrier formation. A strong correlation between annealing-induced changes in the Schottky barrier height and the stoichiometry of the near interfacial GaAs is found.

Studies of Schottky barrier formation at metal-semiconductor interfaces have been complicated by the difficulty of producing an abrupt, “ideal” interface. The commonly used methods of producing metal-semiconductor interfaces result in complex interfacial morphology and chemistry including substrate disruption, atomic interdiffusion, alloy or compound formation, and substrate surface structural changes. The complicated nature of such interfaces makes it difficult from a fundamental point of view to identify the mechanisms dominating Schottky barrier formation for the various stages of development.

A new approach for extending the range of the Schottky barrier height ϕB of metal contacts to (100) GaAs is described. Very thin (∼ 10-30Å) heavily n-type and p-type Si or Ge interlayers are found to directly alter the GaAi interface Fermi energy EF. X-ray photoemission spectroscopy is used to determine EF during contact formation and the corresponding ϕB for thick contacts is measured by electrical methods. In an appropriate structure the ϕB range for contacts to n-type GaAs is ∼ 0.25 to 1.0 eV. For p-type GaAs ϕB has been increased to as much as 0.9 eV. This method of ϕBcontrol can be used for both Schottky barrier contact and nonalloyed ohmic contact applications. The results are interpreted in terms of a simple heterojunction model.

A recently proposed, new approach to the problem of native defect formation in compound semiconductors is presented. The approach is based on the concept of amphoteric native defects. It is shown that the defect formation energy as well as structure and properties of simple native defects depend on the location of the Fermi level with respect to an internal energy reference: the Fermi level stabilization energy. The known location of the stabilization energy determines the electronic part of the defect formation energy and allows for a quantitative description of a variety of phenomena including: the formation of defects at metal-semiconductor interfaces, doping induced superlattice intermixing and limitations of free carrier concentrations in semiconductors.

Development of low resistance ohmic contacts to n-type GaAs which withstand high temperature cycles without degrading their electrical properties is crucial for fabrication of high performance GaAs integrated circuits. Prior to our work, indium-based ohmic contact materials were not attractive for actual devices, because the contacts provided resistances higher than those of the widely used AuNiGe contacts, were thermally unstable after contact formation, and had rough surface morphology. Recently, based on analysis of the interfacial microstructure of these contacts, several thermally stable, low resistance In-based ohmic contacts to n-type GaAs have been developed in our laboratories using a standard evaporation and lift-off technique and annealing by a rapid thermal annealing method. The present paper points out the reasons for the poor electrical properties and thermal stability of the “traditional” In-based ohmic contacts, and reviews the recent progress.