The formation process of COSi2 films grown from Co deposited on a Si single crystal was investigated as a function of very short annealing times by transmission electron microscopy and x-ray diffraction. Information on the interfacial reactions and atomic transport phenomena was obtained from a microstructural analysis of the CoxSiy layers formed. It was demonstrated that Co is the dominant diffusion species during COSi2 formation. Co atoms are generated at the COSi2/COSi2 interface via the reaction 2CoSi=CoSi2+Co and diffuse to the COSi2/Si interface, where they react with Si by Co+2Si=CoSi2. Direct microscopic evidence indicates that diffusivity of Co atoms along a COSi2 grain boundary greatly exceeds that through the COSi2 lattice. The grain boundary diffusion coefficient is estimated to be up to 100 times larger than the lattice diffusion coefficient. On this basis the influence of grain size on COSi2 film formation is discussed.

We have extended the recent work1,2 on grain boundary grooving, agglomeration and islanding during the reaction of thin Co films on single crystal Si<100> substrates (source/drain contacts) to the case of poly silicon substrates3 (gate contact and local interconnects). The stability of 27 to 50 nm CoSi2 films formed via rapid thermal annealing on polysilicon substrates with and without P+ or Ge+ implantation was examined using XTEM, RBS, XRD, and Four-probe measurements. For an undoped polysilicon, partial agglomeration of a 30 nm CoSi2 film has been observed, similar to that seen on a single crystal Si substrate.2For heavily Phosphorus doped polysilicon, a transition from low resistance, stoichiometric cobalt disilicide films (700 °C, 10 sec RTA) to high resistance films at 700 °C, 30 sec RTA (600–1000 Ω per square and discontinuous suicide grain growth) has been observed. Concurrent suicide formation on preamorphized polysilicon (Ge+ I/I and no anneal) has been found to suppress agglomeration of nanoscale cobalt suicide layers, since small grain size and low grain boundary energy are favorable for improving thermal stability4. These continuous layers also exhibited the lowest sheet resistance corresponding to a resistivity of ≤ 21 μΩ-cm.

The change of the doping concentration (including type) near the metal-semiconductor interface influences the Schottky barrier height (BH). In many cases this phenomenon is apparently spontaneous, or it is a side effect of the technology. The goal of this paper is to summarize the effect of the near-interface concentration change on the apparent and real Schottky BHs, and to demonstrate its importance with experimental results obtained in GaAs Schottky contacts. The question of the definition of the real BH for some of these structures is also treated.

The influence of metallization and processing on Schottky barrier formation provides the basis for one of several fruitful approaches for controlling junction electronic properties. Interface cathodo-and photoluminescence measurements reveal that electrically-active deep levels form on GaAs(100) surfaces and metal interfaces which depend on thermally-driven surface stoichiometry and reconstruction, chemical interaction, as well as surface misorientation and bulk crystal quality. These interface states are discrete and occur at multiple gap energies which can account for observed band bending. Characteristic trends in such deep level emission with interface processing provide guides for optimizing interface electronic behavior. Correspondingly, photoemission and internal photoemission spectroscopy measurements indicate self-consistent changes in barrier heights which may be heterogeneous and attributable to interface chemical reactions observed on a monolayer scale. These results highlight the multiple roles of atomic-scale structure in forming macroscopic electronic properties of compound semiconductor-metal junctions.

Low resistance, alloyed AuGeNi Ohmic contacts have been extensively used in the current manufacturing GaAs devices. However, extension of usage of these devices to Very Large Scale Integration levels requires the contacts with excellent thermal stability, shallow diffusion depth, and smooth contact surface in addition to low contact resistance. In the present paper recent studies for development of “non-gold” Ohmic contacts which improve the poor contact properties of the alloyed Ohmic contacts are rev i ewed.

HREM,TEM, and EDX were employed to study the cross sections of Au/Zn/Au and Au/Te/Au contacts to [001]-oriented GaAs substrates. The metal contacts fabricated by sequential vapor deposition were capped with an A12O3 thin film to form a closed system when subjected to a 420°C anneal for 3 min. For an as-deposited Au/Zn/Au/GaAs specimen, Zn was found to react with Au to form ordered phases of Au3Zn. These phases were found to be stable upon annealing. The interface between the annealed metallization and GaAs exhibited the same degree of uniformity and planarity as that of the as-deposited specimen. For a Au/Te/Au/GaAs specimen, annealing resulted in epitaxial growth of Ga2Te3 grains, forming a basically flat interface with GaAs, while Au single-crystal grains were found to protrude rather extensively into GaAs to form a facetted interface with the substrate.

Low ohmic contact resistance is essential for high performance microwave and millimeter wave transistors. Rapid thermal processing (RTP) has been used to optimize the ohmic contact resistance of gold - germanium / nickel / gold metallizations on gallium arsenide (GaAs) layers for high electron mobility transistor (HEMT) applications. A HEMT layer structure consisting of a 9000Å buffer layer grown on a semi-insulating substrate followed by a 20Å undoped AlGaAs spacer layer, a 700Å Al0.22Ga0.78 As layer doped at 1.0 × 1018cm-3and a 500Å GaAs cap layer doped at 1.5 × 1018 cm°C to 450°C. Time at temperature was varied from 10 seconds to 1 minute. Optimum conditions for our equipment and layer structure were found to be 365°C for 30 seconds. These conditions produced contact resistances of 0.08 ohm-mm (approximately 2.0 times better than the standard furnace alloy process).

Degradation of the Au/GaAs interface during heating is a problem which limits device reliability. The “flow chart” method was used to construct the ternary Au-Ga-As phase diagram and determine the equilibrium reactions which are responsible for interfacial degradation. Interfacial degradation was correlated with the removal of Asx from the interface, which lowered the liquidus temperature and enhanced the dissolution of GaAs. The dissolution proceeds by a series of class II reactions within the Au-AuGa-GaAs ternary system and results in the incorporation of Ga into the Au phase, and growth of several AuxGay phases. However, when the processing is accomplished in a manner which prevents the interface from being depleted in As, no interfacial degradation is expected.

Electron-beam (e-beam) evaporated low Au content NiGe(Au)W ohmic contacts with a contact resistance (Rc) as low as 0.15 Ω-mra have been reported. Due to the high melting point of W it is desirable to deposit this layer by means other than e-beam evaporation. However, the use of nearly oxygen-free sputtered W, yields contact resistances in excess of 0.7 Ω-mm. By replacing the sputtered W by a reactively sputtered metallic W oxide, containing ∼25 at. % oxygen, the low contact resistance (Rc < 0.15 Ω-mm) is restored. Contacts employing a reactively sputtered W nitride in place of W oxide also yield high Rc's (∼0.75 Ω-mm). Auger depth profiles of the reacted contacts show a significant outdiffusion of Ga from the GaAs substrate in the presence the oxygenated W but not in the low oxygen and the W nitride contacts. These results and the fact that our previously reported e-beam evaporated W contacts also contain ∼25 at. % oxygen, suggest an oxygen assisted ohmic contact formation mechanism.

A thermodynamic and kinetic rationale was utilized to select Pt3In7 as an ohmic contact to n-GaAs. This analysis predicted that the contact metallurgy would be very simple and extremely uniform upon annealing. Preliminary electrical results showed that annealing samples at temperatures of 650 °C or higher led to ohmic behavior. A contact resistance of 3.4 × 10-5 Ω-cm2 was found upon annealing an uncapped contact at 750 °C for 15 s.

X-ray diffraction studies and current-voltage measurements have been performed on a (100) oriented single crystal thin film of CoxGa1-x (x = 0.42) grown epitaxially on n-GaAs, from 300°C to 900°C. At this composition, CoxGa1-x, which has a broad range of homogeneity and a variable lattice parameter, is lattice matched to GaAs better than 0.5%. A Schottky barrier height of 0.68eV and an ideality factor of 1.07 have been measured up to 500°C, with significant barrier degradation at 600°C. At 700°C formation of the CoGa3 phase and a shift in CoxGa1-x stoichiometry to its bulk thermodynamically most stable composition of Co.45Ga.55 was observed with x-ray diffraction. At 800°C Co2AS formed, and at 900°C only CoGa3 and Co2As phases remained in contact with GaAs.

The interfacial stability, surface morphology and electrical characteristics of MoAlx contacts to n-GaAs have been investigated by using x-ray diffraction, scanning electron microscopy, sheet resistance and current-voltage measurements. The compositions of rf-cosputtered MoAlx films were x = 0.35, 2.7, and 7.0, respectively. The contacts were annealed by rapid thermal processing in the temperature range 500–1000 °C for 20 s. The interfaces of MoAl0.35/GaAs and MoAl2,7/GaAs were stable up to 900 °C, while the interfaces of MoAl7.0/GaAs were less stable and reactions occurred above 800 °C. The variations of sheet resistances and the barrier heights of the Schottky diodes as a function of annealing temperatures can be well correlated to the interfacial stability. The MoAl2.7/n-GaAs diodes exhibited the best stability and were characterized by the highest barrier height (0.98 V) and nearly unit ideality factor (1.11) after annealing at 900 °C. For all thermally stable MoAlx/n-GaAs Schottky diodes, the barrier heights increased with annealing temperature.

Phase equilibria in the M-In-P (M = transition metal) ternary systems is determined through a combination of thermodynamic calculation and experimentation. Palladium is identified as a particularly attractive metal for contacts to InP. One useful feature of this system for contact design is the occurance of PdχInP ternary phases. Additionally, Pdln is found to be in equilibrium with InP and is identified as a promising contact material to InP because of its high melting point, low resistivity, and favorable properties for processing. Progress in the development of Pdln-based contacts to InP is briefly discussed.

High resolution SIMS (secondary ion mass spectrometry) depth profiles of Ge/Pd and Si/Pd/Ge/Pd non-alloyed ohmic contacts on InP are obtained by sputter-etching from the back (semiconductor) side. The samples contain an InGaAs etch stop layer, to allow chemical thinning, and InGaAsP marker layers, which allow alignment and calibration of the depth profiles. Detailed comparisons are made to corresponding contacts on GaAs, processed and analyzed under similar conditions. The importance of interfacial alloy formation, semiconductor regrowth, and dopant incorporation are highlighted.

Ohmic contact formation and thermal stability in a Au/Ge/Pd metallization to n-type InP, doped to a level of 1017 cm-3, have been investigated. Contact resistance was measured using a transmission line method, while microstructural changes were examined by means of TEM, EDX, CBED and SAD. Contacts became ohmic after annealing at temperatures ranging from 300°C to 375°C. A minimum contact resistance of 2.5×l0-6 Ω-cm2 was obtained after annealing at 350°C for 320s. The drop in resistance to ohmic behavior corresponded to the decomposition of an epitaxial quaternary phase (Au-Ge-Pd-P). Annealing at 400°C and above resulted in Au10In3 spiking into InP and a break down of contact lateral uniformity, which increased contact resistance.

We have investigated the electrical and metallurgical behavior of Ni, Au-Ni, and Au-Ge-Ni contacts on n-InP. We have found that very low values of contact resistivity (pc) in the E-7 Ω-cm2 range are obtained with Ni-only contacts. We show that the addition of Au to Ni contact metallization effects an additional order of magnitude reduction in pc. Ultra-low contact resistivities in the E-8 Ω-cm2 range are obtained with both the Au-Ni and the Au-Ge-Ni systems, effectively eliminating the need for the presence of Ge in the Au-Ge-Ni system. The formation of various nickel phosphides at the metal-InP interface is shown to be responsible for the observed pc values in the Ni and the Au-Ni systems. We show, finally, that the order in which the constituents of Au-Ni and Au-Ge-Ni contacts are deposited has a significant bearing on the composition of the reaction products formed at the metal-InP interface and therefore on the contact resistivity at that interface.

Ni and Pd layers have been elaborated from chemical process on (100) oriented and 1018 doped n-InP substrates. The electrical and structural characteristics of these contacts have been investigated. Good refractory ohmic contacts were obtained by the annealing of these heterostructures. The ohmic quality of chemical contacts as well as those obtained with evaporated contacts is attributed to both the presence of crystallized compounds (NiP; Ni2P; PdIn3 and Pdln) and to superficial degenerescence of InP due to a phosphorus excess laying in the interphase.

Ti/Pt metallization was used to form contacts on both n+-InAs emitter cap and p+ base layers of heterojunction bipolar transistors (HBTs). The as-deposited contacts were found to be ohmic for both the base and emitter cap layers. Rapid thermal processing of the contact metallizations was performed in the temperature range of 3 00–500 C for 30 seconds. Minimum contact resistivities of l×10-6 ohm-cm2 for the base and 3×l0-7 ohm-cm2 for the emitter layer were achieved. The influence of heat treatment on contact morphology was also examined.

Thin films of sputtered, amorphous Ta36Si14N50 (a metallic conductor) and Si3N4 (an insulator) were evaluated as encapsulants for (100)-oriented InP substrates. Thicknesses of both films were approximately 100 nm. During a 15 min annealing in Ar, liberated phosphorus was gettered by a <Si>ISiO2ITa(100 nm) collector placed face-to-face on encapsulated or non-encapsulated InP. The stability of the InP with the encapsulant was characterized by backscattering spectrometry, scanning electron microscopy, and x-ray diffraction. As measured by 4He++ backscattering spectrometry, detectable amounts of phosphorus do not arise in the Ta collectors for the Ta-Si-N and Si3N4 encapsulation schemes until 650 and 700°C, respectively. Failure of the Ta36Si14N50 film is catastrophic at 700°C whereas the Si3N4 film degrades locally commencing at 600°C.

The stability of rcactively sputtered TiN films on InP for application as a diffusion barrier has been examined using electrical measurements, Auger profiling and scanning electron microscopy (SEM). The samples were subjected to rapid-thermal-annealing (RTA) in a N2 atmosphere at temperatures between 400°C and 900°C. The SEM pictures of “as deposited” and RTA stoichiometric films show that the morphology is smooth, fine-grained and stable until 800°C. Auger depth profiling shows little interdiffusion between TiN and InP for RTA below 800°C. Annealing at temperatures of about 700°C reduces the sheet resistance of TiN relative to the “as-deposited” films by about 50%. Annealing at temperatures above 800°C results in a large sheet resistance. This may be associated with the deterioration of the TiN/InP morphology at high anneal temperatures as observed by SEM.