Alloyed ohmic contacts (i.e. Au-Ge-Ni) to n-GaAs lead to non-planar interfaces which are unsuitable for devices with shallow junctions and small dimensions. In this study, the fabrication of non-alloyed ohmic contacts (via solid state reactions) is investigated. A layered structure involving the solid phase epitaxy of Ge using a transport medium (PdGe) is shown to produce low (1 — 5 × 10∼6Ω cm2) and reproducible values of contact resistivity. The resultant interface is shown to be abrupt by cross-sectional transmission electron microscopy.

The structure and electrical properties of Au contacts to GaAs have been studied by analytical and high-resolution transmission electron microscopy (TEM) combined with electrical characterization and photoemission spec-troscopy (PES). The contacts were prepared by in-situ deposition of Au onto ultra-high vacuum cleaved n-GaAs (110) surfaces. The effects of annealing on the electrical, morphological and chemical properties of the Au:GaAs contacts were studied. It was shown that the formation of Schottky contacts is correlated with the change in stoichiometry of the GaAs substrate near the Au/GaAs interface. The change to ohmic behavior for the samples annealed above Au-Ga eutectic was explained by leakage currents at the periphery of the devices. TEM micrographs of the structures revealed the existence of elongated Au crystallites on the GaAs surface at the periphery of the contacts. These leakage currents were eliminated by removing the current path at the periphery using a mesa-etch.

Interdiffusion and reaction in the GaAs/AuGe/Ni and GaAs/Ni/AuGe/Ni thin film systems have been investigated, using Auger Electron Spectroscopy depth profiling and Rutherford Backscattering Spectrometry. The roles of the individual metal layers were determined by studying the GaAs/AuGe, GaAs/Ni and GaAs/Ni/AuGe subsystems. It was found that the interfacial layer in the GaAs/Ni/AuGe/Ni contact scheme prevents Ge from diffusing away from the interface. In this way more efficient doping of the GaAe is possible. Current-Voltage measurements indicate that this layer structure forms a good ohmic contact to GaAs.

Ohmic contacts employing Au/Metal/Au/Ge/Ni (Metal=Ni or. TiB2) to n-type GaAs have been investigated. Alloying of the contacts, performed by both optical (rapid thermal) and strip heater methods, resulted in very low specific contact resistivities for the samples employing the TiB2 diffusion barrier. Such contacts are shown to be stable on aging at 350C for 180 hours.

We have performed a systematic study of the electrical properties of a large number of metal/n-GaAs and metal/n-InP diodes. Diodes were fabricated on clean cleaved InP and GaAs (110) surfaces in ultra-high vacuum with in-situ metal deposition of Cr, Mn, Sn, Ni, Al, Pd, Cu, Ag and Au. Using current-voltage (I-V) and capacitance-voltage (C-V) measuring techniques, we were able to obtain very reliable and consistent determinations of the barrier height, θ/b, and ideality factor, n. All of the metal-semiconductor systems formed on lightly doped (< 5×1016/cm3) substrates were characterized by near-unity (1.05) ideality factors.

The effects of doping on the electrical characteristics of the n-GaAs diodes were investigated. A decrease in the effective I-V barrier height, an increase in the ideality factor in forward bias and a strong voltage dependence on the thermionic emission currents in reverse bias were found for diodes formed on the more heavily doped samples. These changes are essentially metal-independent, but depend strongly on the doping of the substrate. The characterization (and elimination in some cases) of peripheral leakage currents from the thermionic emission current for the n-GaAs systems was found to be essential in obtaining consistent results in our work and in reinterpreting some of the prior work in the literature. The dominant leakage current in the GaAs diodes flows through a small area, low barrier at the periphery of the device and can be eliminated by mesa etching.

The structure of 500 Å Au/500 A Ge/500 Å Au/GaAs (100) was studied by transmission electron microscopy after annealing at 350 – 500°C. Annealing at 350 – 450°C caused the formation of AuGeAs with a (110) texture, but this phase disappeared after annealing at 500°C. The hexagonal a-AuGa (or AuGa) was formed after annealing at 400°C, such that (111)Au // (0001)a, and [110]AU // [1120]a and there was perfect lattice match between Au (i.e., Au-rich solid solution) and a-AuGa. After annealing at 450°C or above, a phase tentatively identified as the hexagonal Au3Ga was formed and Ge (i.e., Ge-rich solid solution) became epitaxial to (100) GaAs. Annealing at 400°C caused Au to change from no texture to a (110) texture.

Graded heterojunction InGaAs ohmic contacts to n-GaAs have been prepared which show improved electrical and morphological properties compared with other diffused contacts. The improvements result primarily from the use of a thin 400 A Pt layer between the 4000 Â In layer and the substrate to control the reaction of the In and the GaAs. A study of chemically etched samples using energy dispersive x-ray (EDX) analysis has revealed the formation of a smooth In Ga, As heterojunction interface. Evidence is also presented that the heterojunction regions are epitaxial. A smooth, uniform interface of this type is not formed in other diffused contact systems, such as In/GaAs and Ni/Au-Ge/GaAs.

GaAs (100) substrates were subjected to UV-ozone, wet chemical, and thermal treatments prior to deposition of Ni contact structures. The use of UV-ozone showed marked improvement in the electrical characteristics compared to the wet chemical and thermal treatments alone. The major effect of UV-ozone is the nondestructive removal of carbonous contaminants. Results of C-V and I-V barrier height measurements on Ni contact structures fabricated on the treated surfaces are presented.

The Schottky barrier heights of single crystal NiSi2 layers on Si(111) have been studied by current-voltage, capacitance-voltage and activation energy techniques. Near ideal behavior is found for Schottky barriers grown on substrates cleaned at ∼820°C in ultrahigh vacuum. The Fermi level positions at the interfaces of single crystal type A and type B NiSi2 are shown to differ by ∼0.14 eV. Transmission electron microscopy demonstrated the epitaxial perfection of these suicide layers. At a cleaning temperature of 1050° C, the near surface region of lightly doped n-type Si was converted to p-type. The presence of a p-n junction was directly revealed by spreading resistance measurements and resulted in a high apparent Schottky barrier height (≥0.75 eV) which no longer bears immediate relationship to the interface Fermi level position.

This paper reviews the recent studies on Schottky barrier and interface states at silicide-silicon interfaces, with emphasis placed on the results obtained from the epitaxial Ni suicides. A model based on interfacial defect states has been proposed to account for the overall chemical correlation between the barrier height and the metal electronegativity. Measurements on the barrier heights of type A, B and C epitaxial Ni suicides show that these three interfaces can be formed with high degrees of perfection to yield a barrier of 0.78 eV. Similar interfaces formed under less ideal conditions or with impurity incorporation decrease the barrier to 0.66 eV. The density and distribution of the interface states measured by a capacitance spectroscopy method correlate well with the structural perfection of the single and mixed-phase interfaces. A consistent picture seems to have emerged suggesting that the barrier height at silicide-Si interfaces is formed as a result of Fermi level pinning by interfacial defect states which are controlled primarily by the degree of perfection of the interface instead of the specific epitaxy.

Single crystal NiSi2 films of type A and type B orientations with thicknesses ranging from 70–600Å have been grown on (111), n-type Si substrates. TEM and channeling measurements indicate that these films are of excellent epitaxial quality with uniform orientations over the entire range of observation. HRTEM studies show regular and atomically abrupt interfaces for both NiSi2 orientations with occasional localized planar defects. I-V and photoresponse measurements of the Schottky barrier heights(SBH) of the type A films yield consistent values of 0.62±.01eV. However, for type B films I-V measurements give a SBH of 0.69±.01eV while the photoresponse results give 0.77±.05eV. This discrepancy can be explained quantitatively by a phenomenological model in which a small percentage of low barrier height regions is incorporated into the type B films.

The existence and nature of localized states at metal-semiconductor interfaces are long-standing scientific issues that have not been resolved. As device dimensions continue to shrink and packing density continues to increase, interfaces at silicide-silicon contacts will be increasingly influential in determining the circuit behavior. Characterization of these interfaces, whether as ohmic contacts or rectifying junctions, is a necessity for microelectronics. Therefore, the study of silicide-silicon interfaces are important both scientifically and technologically. In this talk, we shall review the theoretical models of the Schottky barrier and the importance of interface states. We shall then introduce available experimental tools for interface state studies. In particular, we shall discuss the measurement of silicide-silicon interface states by an accurate-phase capacitance spectroscopy (APCS). Some of the devices were fabricated with standard chemical cleaning before metal evaporation. Others were prepared in an UHV chamber with heat cleaning before metal deposition. For palladium suicide diodes, the behavior of both chemically-cleaned and UHV-prepared samples is similar. The peak position of the capacitance spectrum correlates with the barrier height, and the magnitude of the peak decreases by a factor of three after annealing. In the case of nickel suicide diodes, a comparison between epitaxial and non-epitaxial interfaces shows much lower interface state density for the former. Experimental data indicate that suicide formation and interfacial perfection are important in specifying the Fermi level.

A microscopic many-electron model is utilized to calculate the self-consistent one-electron potential in the vicinity of a metal-semiconductor or semiconductor-semiconductor interface. The boundary conditions ensuring thermal, mechanical and electron-transfer equilibrium are imposed explicitly. Numerical calculations predict the validity of Schottky's phenomenological boundary condition for metal-semiconductor contacts and the applicability of the electron affinity rule for semiconductor heterojunctions. The occurrence of interfacial atomic rearrangements and/or chemical reactions is incorporated into the model via the inclusion of charge centers near the interface.

This paper reviews our recent studies of electrical transport in thin suicide films. Resistivity as a function of temperature, Hall effect, and magnetoresistance have been characterized for CoSi2 and NiSi2 and in conjunction with band theory provide estimates of important electronic parameters, viz. carrier densities and carrier scattering lengths. Resistivity data for TiSi2 and TaSi2 are included. Also examined in resistivity are (i) effects produced by ion bombardment which show CoSi2 to have an unusual susceptibility to radiation damage and (ii) classical size effects in the very thin film regime which show the boundary scattering to be principally specular. As an application we describe a Si/CoSi2/Si heterostructure transistor recently developed.

Ohmic contacts to n-type Si have been formed using a thin n layer of Ge grown by selective-area molecular beam epitaxy. The n+Ge is used to lower the metal-semiconductor barrier energy and thus reduce the contact resistivity of the thin film structure. Al/n+Ge/n+Si test devices were fabricated in a manner compatible with conventional VLSI processing and consisted of a 3500 A thick layer of arsenic-doped Ge grown at 250°C on a selectively diffused, phosphorus-doped Si substrate. Using both four terminal Kelvin and transmission line devices, unsintered Al/Ge/Si structures exhibited contact resistivities a factor of five lower than Al/Si control devices, in agreement with theoretical calculations based on a simple tunneling model for the metal/Ge/Si heterojunction ohmic contact.

A series of PtSi on p-type Si diodes have been characterized in order to establish correlations among processing parameters, metallurgical features and electrical properties. Characterization techniques include analytical (TED, TEM), electrical (current-voltage characteristics), and optical (photoemission and absorption). The fabrication techniques involve e-beam evaporation of platinum layers at UHV levels onto VLSI grade (100) p-type silicon substrates. The silicide layers are formed via sub-eutectic solid state diffusion at 350°C. The main trends with thickness as well as possible interrelationships are described. An unexpected result is the presence of unreacted polycrystalline Pt and Pt2Si at the interface.

Amorphous Si:H and Si1−xGex:H films were prepared by mixing electron beam evaporated silicon with a molecular beam of germanium from a Knudsen cell and with a beam of ionized hydrogen produced by a 0–3 keV ion source. Aluminum Schottky barriers on two types of samples: (1) a-Si1−xGex:H with.15<×<.85 and (2) modulated structures of 50 × 100 Å layers of a-Si:H/a-Si.8Ge.2:H (10-5 Torr PH hydrogen) were investigated. Barrier height was found to depend on the Ge concentration and possible Fermi-level pinning due to the dangling bond deep level. The modulated structures showed a negative resistance region and a barrier height determined only by the composition of the first layer.

We discuss a new low temperature process for the deposition of electronic quality thin films of silicon oxide and nitride. In contrast to conventional plasma enhanced chemical vapor deposition [PECVD], this process involves the remote excitation of one of the gas reactants followed by the extraction of the active species out of the plasma region where they react to generate precursor molecules. The precursors undergo a CVD reaction at a heated substrate to form the desired thin film. The process is called remote PECVD [RPECVD]. Insulators produced in this way show significant reductions in the incorporation of impurity groups such as SiH and SiOH relative to films grown by the PECVD process at the same substrate temperatures.

The Ion and vapour deposition method (IVD method) has been used for the first time to form silicon nitride films. Silicon nitride formation using IVD method, which consists of a nitrogen ion implantation and a simultaneous electron beam evaporation of silicon, has been confirmed.

These silicon nitride films formed by IVD method have been characterized using FTIR, ellipsometry and AES analyses. It has been observed that these IVD silicon nitride films characteristics are sensitive to the nitrogen ion implantation conditions and the silicon evaporation rate.

Applications of these IVD films to selective oxidation have exhibited a good resistance to oxidation. It has also permitted the bird's beak to be reduced.

Ti-N films were deposited by reactive ion plating on various substrates. The N/Ti-ratio was systematically varied in order to produce films of varying structure and properties from pure titanium to overstoichiometric TiN. Broad diffraction peaks typical of PVD Ti-N films were found. Metastable ct-titanium was observed over a wide range up to about 50 at. % N. No marked differences in the phase composition or in the crystallographic texture could be found in the films deposited on the various substrates. The hardness of the films was observed to increase with increasing nitrogen content, reaching a maximum around 35 – 40 at. % N corresponding to a mixed ε-Ti2N and δ-TiN structure. Extended annealing at 773 K did not result in any major phase transformations. After annealing at 1173 K, α-Ti could no longer be found in nitrogen-rich films. Also ε-Ti2 N transformed to δ-TiN after annealing at 1173 K for 2 h.