E. D. Marshall

Nonalloyed ohmic contacts to n‐GaAs by solid‐phase epitaxy of Ge

A low resistance nonalloyed ohmic contact to n‐GaAs is formed which utilizes the solid‐phase epitaxy of Ge through PdGe. Discussion focuses on the conditions necessary to attain low specific contact resistivity (∼10−6 Ω cm2 on 1018 cm−3 n‐GaAs) and on the interfacial morphology between the contact metallization and the GaAs substrate. MeV Rutherford backscattering spectrometry and channeling show the predominant reaction to be that of Pd with amorphous Ge to form PdGe followed by the solid‐phase transport and epitaxial growth of Ge on 〈100〉 GaAs. Cross‐sectional transmission electron microscopy and lattice imaging show a very limited initial Pd‐GaAs reaction and a final interface which is planar and structurally abrupt to within atomic dimensions. The presence of excess Ge over that necessary for PdGe formation and the placement of Pd initially in contact with GaAs are required to result in the lowest contact resistivity. The experimental data suggest a replacement mechanism in which an n+‐GaAs surface re...

Non‐alloyed ohmic contact to n‐GaAs by solid phase epitaxy

A non‐alloyed ohmic contact to n‐type GaAs has been demonstrated. The technique of solid phase epitaxy through a transport medium has been used to obtain a metal/Ge(n+, epi)/GaAs(n, 〈100〉) heterostructure. The resulting contact displays a smooth surface and low contact resistivity (∼10−6–10−5 Ω cm2) when compared with standard Au‐Ge contacts on n‐GaAs with similar doping concentrations (∼1018/cm3).

Ge redistribution in solid‐phase Ge/Pd/GaAs ohmic contact formation

A backside secondary ion mass spectrometry technique is employed to examine elemental redistribution in the Ge/Pd/GaAs ohmic contact as a function of annealing conditions. Dilute Al containing marker layers (Ga1−x Alx As) in the GaAs permit precise calibration and alignment of the elemental depth profiles. Double etch‐stop thinning yields high depth resolution. The onset of ohmic behavior is found to occur when Ge is detected at the GaAs surface. Good ohmic behavior is observed when an interfacial layer of reacted Pd4GaAs is dispersed and complete coverage of Ge occurs. The Ge/GaAs interface is abrupt with the Ge concentration dropping by over three orders of magnitude within 100 A. About 40 A of GaAs is found to be consumed during the ohmic contact formation. Degradation of the ohmic contacts, as a result of further heat treatment, was found to correlate with Ge in‐diffusion into the GaAs. The results place strict limitations on doping and heterointerface models of ohmic behavior for this contact.

An investigation of a nonspiking Ohmic contact to n-GaAs using the Si/Pd system

A low-resistance nonspiking Ohmic contact to n-GaAs is formed via solid-state reactions utilizing the Si/Pd/GaAs system. Samples with Si to Pd atomic ratios greater than 0.65 result in specific contact resistivity of the order of 10 −6 Ω cm 2 , whereas samples with atomic ratios less than 0.65 yield higher specific contact resistivities or rectifying contacts. Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, and electron diffraction patterns show that a Pd, Si layer is in contact with GaAs with excess Si on the surface after the Ohmic formation annealing. This observation contrasts with that on a previously studied Ge/Pd/GaAs contact where Ohmic behavior is detected after transport of Ge through PdGe to the interface with GaAs. Comparing the Ge/Pd/GaAs system with the present Si/Pd/GaAs system suggests that a low barrier heterojunction between Ge and GaAs is not the primary reason for Ohmic contact behavior. Low-temperature measurements suggest that Ohmic behavior results from tunneling current transport mechanisms. A regrowth mechanism involving the formation of an n + GaAs surface layer is proposed to explain the Ohmic contact formation.

The temperature dependence of contact resistivity of the Ge/Pd and the Si/Pd nonalloyed contact scheme on n‐GaAs

The temperature dependence of the contact resistance of the Ge/Pd and Si/Pd metalization scheme on n‐GaAs was investigated. These two contact systems are based on solid‐phase reactions, thus leading to nonspiking ohmic contacts to n‐GaAs. The experimental results show that the ohmic behavior is likely due to both a highly doped surface n+ region and/or a small barrier at the interface. The origin of this small barrier and nonlinear current‐voltage characteristics for certain samples are also discussed.The temperature dependence of the contact resistance of the Ge/Pd and Si/Pd metalization scheme on n‐GaAs was investigated. These two contact systems are based on solid‐phase reactions, thus leading to nonspiking ohmic contacts to n‐GaAs. The experimental results show that the ohmic behavior is likely due to both a highly doped surface n+ region and/or a small barrier at the interface. The origin of this small barrier and nonlinear current‐voltage characteristics for certain samples are also discussed.

Thermal and chemical stability of Schottky metallization on GaAs

The high‐temperature stability of Schottky barriers on GaAs has been correlated with the thermodynamic driving force for chemical reaction between the metallic contacts and the substrate. The chemical stability of a gate metallurgy can result in the stability of the electrical characteristics of the contact after high‐temperature anneal. Since single element metal contacts on GaAs are chemically unstable, thermally stable Schottky barriers are not expected from these systems. Alternatively, most of the common metal silicides are chemically stable on GaAs and hence are more likely to form Schottky barriers which are stable against high‐temperature annealing. The chemically stable silicides of Ni and Co, which exhibit low electrical resistivities, are suggested as improved gate metallurgies in self‐aligned metal‐semiconductor field‐effect transistor technologies.

Partial epitaxial growth of cobalt germanides on (111)Ge

Localized  epitaxial  Co5Ge7  and  CoGe2  have  been  grown  in  cobalt  thin  films  on (111)Ge  in  the solid phase epitaxy regime. The orientation relationships between epitaxial germanides and the substrates as well as the configuration of the interfacial dislocations were analyzed by transmission  electron  microscopy (TEM) in detail.  Surface morphology was examined by scanning  electron  microscopy.  The  results  obtained  from  Read  camera  glancing  angle  x‐ray diffraction and Rutherford backscattering channeling analysis were found to corroborate with those from TEM examinations.

Partial epitaxial growth of Ni2Ge and NiGe on Ge(111)

Abstract Transmission electron microscopy (TEM), scanning electron microscopy (SEM), Rutherford backscattering (RBS) channeling and Read camera glancing X-ray diffraction analysis have been applied to study the interfacial reactions of nickel thin films on Ge(111) with particular emphasis on the epitaxial growth of Ni2Ge and NiGe at this surface. Ni2Ge and NiGe were found to grow epitaxially at 160°C and 250°C respectively. Epitaxial Ni2Ge regions, 30 nm in average size, were observed to cover about 100% of the surface area. The orientation relationships were analyzed to be Ni2Ge[0001]Ge[111] and Ni2Ge( 10 10) Ge 220). The best NiGe epitaxy was obtained in samples annealed at 500°C for 1 h. The orientation relationships were determined to be NiGe[010]Ge[111] and NiGe(002) Ge 220). The average size of epitaxial regions was measured to be about 4.5 μm. The areal fraction of epitaxial regions for NiGe was found to be about 70%. NiGe was found to agglomerate to form islands after 600°C annealing. The results obtained from Read camera glancing-angle X-ray diffraction and RBS channeling analysis were found to agree with those from TEM and SEM examinations.

Backside secondary ion mass spectrometry investigation of ohmic and Schottky contacts on GaAs

A backside secondary ion mass spectrometry (SIMS) technique is employed to examine seven distinctive metallic contacts on GaAs at various stages of formation. The contacts are formed on multilayered GaAs/AlGaAs structures grown by molecular beam epitaxy (MBE), each containing an AlGaAs etch‐stop layer and AlGaAs marker layers for precise alignment of the SIMS depth profiles. After removal of the substrate, the contact structures are profiled from the backside to avoid depth resolution degradation, which results when sputtering through a nonuniform multilayered metallic contact. The seven contacts examined are TaSix, Au/Ge/Ni, Au/Ge/Pd, Ge/Pd, Si/Pd, In/Pd, and Si/Ni/Mg. The TaSix Schottky contact exhibits extensive interdiffusion. The Au/Ge/Ni contact reveals extensive GaAs consumption. Much less consumption occurs in the Au/Ge/Pd contact. The absence of Au in the Ge/Pd contact leads to an extremely abrupt contact interface, with the Ge concentration dropping by four orders of magnitude within ∼100 A of t...

Nonspiking ohmic contact to p‐GaAs by solid‐phase regrowth

A low‐resistance and nonspiking contact consisting of a layered structure of Si/Ni(Mg) on p‐GaAs is formed by solid‐phase regrowth. Backside secondary‐ion mass spectrometry and cross‐sectional transmission electron microscopy show an initial reaction between Ni and GaAs to form NixGaAs which is later decomposed to form NiSi by reacting with the Si overlayer. This reaction leads to the solid‐phase epitaxial regrowth of a p+ ‐GaAs layer doped with Mg. The total consumption of substrate is limited to a few hundred angstroms. The as‐formed ohmic contact structure is uniform and planar with an average specific contact resistivity of ∼7×10−7 Ω cm2 on substrates doped to 8×1018 cm−3. The thermal stability of this contact scheme is also reported.