Ohmic Contact Formation Mechanism Research Articles

Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n -type GaN

We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN.

Microstructure and chemistry of Cu-Ge ohmic contact layers to GaAs

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10-7 Ωcm2 for n∼l x 1017cm-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0–40 at.%. Layers with Ge deficiency to form ζ-phase (average composition Cu5Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu3As phase which grows epitaxially on Ga111 planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing ζ-phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic e1 Cu3Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the e1Cu3Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.

NiGe-based ohmic contacts to n-type GaAs

Refractory NiGe ohmic contacts which have excellent thermal stability and smooth surface have been developed. To apply these contacts to the future very large scale integration GaAs devices, reduction of the contact resistance (Rc) of the NiGe contacts is mandatory. In the present paper, in order to obtain a guideline for the Rc reduction, the formation mechanism of the NiGe contacts was investigated. The NiGe contacts were found to have two different ohmic contact formation mechanisms. These mechanisms suggested that facilitation of heavy doping at the GaAs surface and/or in the Ge layer was very effective to reduce the Rc values of the NiGe contacts. Experimentally, the Rc reduction was demonstrated by adding a small amount of third elements. Direct doping elements (Sn, Sb, and Te) and indirect doping elements (Pd, Pt, Au, Ag, and Cu) were chosen as the third elements. In additon, the effect of addition of In, which forms alow barrier layer between metal and GaAs, was investigated. The contact resistance of these NiGe-based contacts were close to 0.3 Ω mm, and they provided smooth surface and shallow reaction depth. Finally, the NiGe-based contacts were compared with the conventional AuGeNi contact.

On the low resistance Au/Ge/Pd ohmic contact to n-GaAs

We report that a Au/Ge/Pd layered structure can result in low contact resistivities (∼10−6 Ω cm2) to n-GaAs processed in three temperature ranges (175–200, 340–350, and 425–450 °C). The contacts processed below the Au–Ge eutectic temperature (361 °C) show good surface and interface morphology, thermal stability, Au wire bondability, and reproducibility. The ohmic contact formation mechanisms are also presented.

Ohmic contact formation mechanism of the Au/Ge/Pd/n-GaAs system formed below 200 °C

We report the ohmic contact formation mechanism of the low resistance (∼1×10−6 Ω cm2) Au/Ge/Pd contact to n-GaAs annealed at 175 °C. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry were utilized in this study. It is found that the solid phase regrowth process, interdiffusion between Au and Ge, and the enhancement of the conductivity of the excess Ge layer are responsible for the observed low contact resistivity.

Low resistance Zn3P2/InP heterostructure Ohmic contact to p-InP

A low resistance Zn/Pd Ohmic contact scheme to p-InP based on solid phase reactions has been investigated. Contact resistivity in the mid-10−5 Ω cm−2 could be obtained for contacts with an atomic ratio of Zn to Pd of ∼1.5. In addition to the solid phase regrowth process, a Zn3P2/InP heterojunction is formed at the contact/semiconductor interface. The Ohmic contact formation mechanism is rationalized with the formation of Zn3P2/InP heterojunction.

Investigation of Aluminum and Titanium/Aluminum Contacts to n-Type Gallium Nitride

ABSTRACTWe report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7 x l017cm-3) annealed in forming gas at 600°C reached a minimum contact resistivity of 8x10~6Qcm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35nm/l 15nm) contacts on n-GaN (5xl0I7cm~3) had resistivities of 7 x l0"6Qcm2 and 5xl0"6Qcm2 after annealing in Ar at 400°C for 5min and 600°C for 15sec, respectively. Depth profiles of Ti/Al contacts annealed at 400°C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for ohmic contact formation in Ti/Al contacts annealed in the 400-600°C range includes reduction of the native oxide on GaN by Ti and formation of an Al-Ti intermetallic phase in intimate contact with the GaN. Contacts with different Ti/Al layer thicknesses were investigated and those with 50nm/100nm layers had the same low resistance and better stability than 25nm/125nm contacts.

Si/Pd ohmic contact to n-GaP based on the solid phase regrowth principle

A Si/Pd ohmic scheme (contact resistivity ~2 × 10-4 Ω-cm2 ) annealed between 400 and 650°C has been developed for n-GaP (n~5 × 1017 cm3). The ohmic contact formation mechanism can be rationalised in terms of the solid phase regrowth (SPR) principle and the solid phase epitaxy of Si on n-GaP.

Au/In/Pd/Te/Pd ohmic contact to n-GaSb

An Au/In/Pd/Te/Pd ohmic contact scheme has been developed to n-GaSb (n/spl sime/10/sup 18/ cm/sup -3/). Annealed at 250 and 400/spl deg/C, this contact scheme exhibits excellent electrical properties (contact resistivity <10/sup -6/ /spl Omega/-cm/sup 2/) and good thermal stability. The surface and interface morphologies were investigated using a scanning electron microscope (SEM). The ohmic contact formation mechanism is also proposed.

Ａｒイオンソフトエッチングを用いた微細接続孔内の自然酸化膜除去効果

An anisotropic inductive coupled plasma (ICP) soft etching using high-density Ar plasma and low incident energy, generally performed before metallization of interconnection, and which can uniformly remove the native oxide at the bottom of fine contact holes, is introduced. As a result, excellent ohmic characteristics can be achieved using ICP soft etching. A cross-sectional TEM (Transmission Electron Microscopy) -EELS (Electron Energy Loss Spectroscopy) was performed in order to compare interface layer generation at the contact hole in the ICP system with that generated in standard RF (Radio Frequency) diode etching. In the sample treated by ICP soft etching, reacted Ti silicide was observed at the bottom of the contact holes. Furthermore, the formation mechanism of stable ohmic contacts was theoretically analyzed using a Monte-Carlo topological simulation. From the results, we found that it is possible to prevent rem-iner denosition on the bottom of holes with 0.1μm diameter.

Low-temperature-processed (150–175 °C) Ge/Pd-based Ohmic contacts (ρc∼1×10−6 Ω cm2) to n-GaAs

We have developed low resistance (ρc∼1×10−6 Ω cm2) Ge/Pd-based (the Au/Ge/Pd and the Ag/Ge/Pd contacts) Ohmic contact schemes processed at temperatures 150–175 °C to n-GaAs (n∼1×1018 cm−3). The Ohmic contact formation mechanism can be rationalized in terms of the solid phase regrowth (SPR) principle and the interdiffusion between Au (or Ag) and Ge.

Characteristics of PdIn ohmic contacts to n-GaAs with a thin Ge layer

Effect of a thin (2 nm) Ge layer in PdIn ohmic contacts with different deposition structure were studied. An appropriate deposition structure may extend the optimum annealing temperature window down to 400°C, whereas an inappropriate one lets the effect of a thin Ge layer be negligible. In the former case, specific contact resistances ( ϱ c) were reduced significantly in low temperature range because a regrown GaAs layer doped heavily with Ge formed. Special attention was paid to confirm the mechanism of ohmic contact formation and a possible reaction sequence was derived. After rapid thermal annealing at 600°C for 10 s, a ϱ c value of 2 × 10 −6 Ω- cm 2 was obtained. Also, a thin (4 nm) In x Ga 1− x As layer with an average x value of 0.7 was observed in the metal/GaAs interface. Increment in ϱ c values was observed after sequential aging at 400°C.

Low resistance (~10-6 Ωcm2) ohmic contact to n-GaAs processed at 175°C

A low resistance (10-6 Ωcm2) Au/Ge/Pd ohmic contact processed at 175°C has been developed to n-GaAs (n ≃ 1018 cm-3). The ohmic contact formation mechanism can be rationalised in terms of the solid phase regrowth (SPR) principle and the interdiffusion of Au and Ge.

A combined x-ray diffraction and Raman analysis of Ni/Au/Te-ohmic contacts to n-GaAs

The ohmic contact formation mechanism in furnace alloyed Ni/Au/Te/Au/GaAs structures is investigated by the combined application of x-ray diffraction and Raman spectroscopy. It was found that the Ni top layer promotes a more uniform dissociation of the GaAs substrate by the formation of a ternary Nix GaAs phase in the initial stages of contact formation (T=350 °C). At an alloy temperature T=550 °C, which induces low resistive electrical behavior, the formation of epitaxial Ga2Te3 was observed, indicating the formation of a Ga2Te3/GaAs heterojunction. After alloying at 600 °C, Ga2Te3 was still found to be present in the contact zone. In addition, evidence for a regrowth of GaAs crystallites could be derived from an increase of the intensity ratio ITO/ILO of the GaAs Raman signals. No indication for the presence of a high density of shallow donors (≳1019 cm−3) in the GaAs top layers could be adduced.

Correlation Between the Microstructures and the Electrical Properties of Ni/Au/Te/Au Contacts on n-GaAs

ABSTRACTThe structural evolution of Ni/Au/Te/Au contacts on n-GaAs (001) was examined, in correlation with their electrical properties as a function of rapid thermal annealing in the temperature range 350 - 600°C. It was found that heating at temperatures ≥ 550°C results in the formation of ohmic contacts, while contacts annealed at lower temperatures remain nonohmic. Transmission electron microscopy revealed that heating ≥ 450°C leads to extensive reactions between Ni/Au/Te/Au and GaAs and deep spike formation into the GaAs. The major reaction products were identified as NiAs and β-AuGa. Ga2Te3 grains, growing epitaxially on GaAs, were detected only in 550°C annealed samples. Heating to 600°C caused considerable Ga2Te3 loss. Implications of these results concerning the ohmic contact formation mechanism are discussed.

The ohmic contact formation mechanism in laser beam mixed Au/Te/Au/GaAs structures

The electronic and structural properties of the intermixed interfaces of Au/Te/Au/n-GaAs structures subjected to continuous wave and to pulsed laser irradiations were studied. The transition from Schottky-type to ohmic contact was examined to identify the ohmic contact formation mechanism for which three types of conflicting models exist. The electrical characteristics have been correlated to the structural properties investigated by the combined application of X-ray diffraction, laser Raman spectroscopy and Mössbauer spectroscopy. In summary of the results, first the ohmic contact model based on the formation of a highly Te-doped GaAs sueface layer could be questioned. For the low fluence pulsed laser irradiations, the formation of a Ga 2Te 3 interface layer is crucial for the ohmic character of the contact, corresponding to the model based on the formation of a crystalline heterojunction. Ohmic contact structures formed by CW and by high energy fluence laser irradiations were found to be correlated to the formation of a high density of defect complexes in the GaAs top layers, corresponding to the highly defective/amorphous heterojunction model.

Structural and electronic properties of mixed Au/Te/n-GaAs structures

The structural and electronic properties of the intermixed interfaces of layered Au/Te/n-GaAs structures were investigated by the combined application of129I Mossbauer spectroscopy (in the decay of129mTe), X-ray diffraction, Raman spectroscopy and electrical measurements. The transition from Schottky-type to ohmic contact by high fluence pulsed laser irradiation was examined and compared to rapid time furnace heat treatment. Distinctly different ohmic contact formation mechanisms have been observed. Whereas for furnace alloying the formation of a crystalline (graded As doped) Ga2Te3 interface layer is crucial, strong evidence is adduced that the ohmic character of high-fluence laser mixed structures is correlated to the formation of a high density of defect complexes in the GaAs top layer.

The Development of Solid Phase Regrowth on GaAs and its applications

AbstractA solid phase regrowth process on GaAs has been observed in Pd- and Ni- based bi-layer structures, e.g. the Si/Ni, the Ge/Pd, the In/Pd, and the Sb/Pd structures. Due to the regrowth, uniform epitaxial layers of Ge, GaAs, InxGa1-xAs, and GaSbl-xAsx on GaAs substrates by solid phase reactions can achieved. The model of this regrowth process will be presented. Based on this regrowth mechanism, a series of non-spiking planar ohmic contacts on n and p type GaAs have been developed. Low contact resistivity in the range of mid 10−7 Ω-cm2 was obtained. The ohmic contact formation mechanism of these contacts will also be discussed. All the studies suggest that the ohmic behavior is a result of the formation of an n+ or p+ surface layer via solid phase reactions. The regrowth process has also been utilized to achieve compositional disordering of GaAs/AlGaAs superlattices, and low loss AlGaAs/GaAs waveguide has been obtained.

Identification of the Ohmic-contact formation mechanism in the Au/Te/Au/GaAs system.

The Ohmic-contact formation mechanism in furnace alloyed Au/Te/Au/(n-type GaAs) structures is investigated by the combined application of M\"ossbauer spectroscopy, x-ray diffraction, Raman scattering, and Auger-electron-spectroscopy depth profiling. A dominant shallow-donor-dopant behavior of the Te atoms (substitutional on As sites) can be questioned. The observed low-resistance conductivity is related to the presence of a (${\mathit{n}}^{+\mathrm{\ensuremath{-}}}$type ${\mathrm{Ga}}_{2}$${\mathrm{Te}}_{3}$)/(n-type GaAs) heterojunction. Although the possibility of some residual n-type doping of the GaAs substrate remains, evidence is adduced that, in the Ohmic-contact formation mechanism, at least an equally important role is played by a degenerate As n-type doping of the ${\mathrm{Ga}}_{2}$${\mathrm{Te}}_{3}$. Similar arguments are developed for the (Au-Ge)/(n-type GaAs) and Ge/Pd/(n-type GaAs) Ohmic-contact systems.

Regrowth of a GaAs layer for n-GaAs ohmic contacts

Reaction of Au with GaAs in conventional NiAuGe/GaAs ohmic contacts results in rough interfaces, poor surface morphology, and deterioration at the edges of films. To improve these properties, ohmic contacts to GaAs were formed by heating initially discrete layers of volatile arsenic and nonvolatile gallium under a Ge–Au alloy thin film. The layer sequence beginning at the air interface was AuGe(1500 Å)/As(500 Å)/Ga(400 Å), all evaporated onto chemically cleaned (100) GaAs surfaces, followed by annealing at 400 to 650 °C in forming gas. A regrown layer of highly doped GaAs was formed between the AuGe layer and the original GaAs substrate. This layer was characterized by scanning Auger electron spectroscopy, scanning electron microscopy, and electrical measurements. The carrier concentration of the regrown GaAs layer was estimated from four-point probe data to be in the range of 1019 to 1020 cm−3. The specific contact resistivity for the AuGe/As/Ga/GaAs regrown at 550 °C for 10 min was ≊1.5×10−5 Ω cm2, which is similar to values measured for AuGe/GaAs annealed at 450 °C for 2 min. The mechanism of GaAs regrowth and ohmic contact formation are discussed.