Low-resistance ohmic contacts to p-type GaAs using Zn/Pd/Au metallization

We have fabricated the low resistance ohmic contacts to p-type GaAs. Specific contact resistances as low as 7 × 10^-7Ω.cm²have been obtained for contacts prepared by heat treating Zn/Pd/Au metallizations deposited on p-type epitaxial GaAs layers with an acceptor concentration of 1.5 × 10¹⁹cm^-3. These contacts are reproducible, simple to fabricate, exhibit excellent adhesion, and have a uniformly smooth surface morphology.     

Passivation of ohmic contacts to GaAs

A film of silicon dioxide was used to passivate Au-Ge-Ni metallization during the alloying cycle. Ohmic contacts prepared in this way had smooth surfaces, better edge definition, and more uniform electrical characteristics. Also the time-temperature alloying cycle for fabricating suitable contacts was significantly broadened. These improvements are most likely due to the prevention of arsenic loss during the alloying procedure.     

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 (R_c) of the NiGe contacts is mandatory. In the present paper, in order to obtain a guideline for the R_c 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 R_c values of the NiGe contacts. Experimentally, the R_c 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.     

Characteristics of AuGeNi ohmic contacts to GaAs

We have studied AuGeNi ohmic contacts to n-type MBE grown GaAs epitaxial-layer with doping in the (10¹⁶?10¹⁹) cm^?3 range, and found several new effects: (a) Contact resistivity exhibit a weak dependence on carrier concentration (much weaker than 1/N_D depencence); (b) We find evidence for a high resistivity layer under the contact at least several thousands angstroms deep, which dominate the contact resistance in most cases; (c) We find a peripheral zone around the contact, about 1 μm wide which differs chemically from the GaAs epi-layer; (d) SIMS analysis reveals a deep diffusion into the GaAs of Ni and Ge; (e) Correlation between density of GeNi clusters in the contact and the contact resistivity are found; (f) Temperature measurements justify that tunneling is responsible for the ohmic contact. We discuss also the validity of the transmission line method and the commonly accepted model of the contact.     

Hot-plate alloying for ohmic contacts to GaAs

The fabrication and optimization of ohmic contacts to GaAs prepared by heating the wafers on a hot plate is described. The method offers high throughput and is production adaptable. The apparatus consists of a hot plate constructed of a heat pipe with a high surface temperature uniformity (±2°C over 4-in diameter) located inside a glove box with an ambient controlled to 5 percent H2/95 percent N2forming gas. Specific resistance and morphology of AuGe/Ni/Au contacts were characterized as a function of hot-plate surface temperature. At the optimum alloy temperature, specific resistances of less than 10^-6Ω-cm²were obtained repeatably for VPE GaAs with active layers of n ∼ 10¹⁷cm^-3and thicknesses of ∼0.4 µm as well as for VPE GaAs with an n⁺surface layer greater than 10¹⁸cm^-3. The contact surface morphologies were smoothest for those alloyed at temperatures below the optimum, and, for those alloyed at or above the optimum, they were increasingly less smooth for increasingly higher alloy temperatures. A general discussion of this method's potential application to high-volume GaAs processing is given.     

Characterization of reacted ohmic contacts to GaAs

In the calculation of the true value of specific resistivity of reacted ohmic contacts, a modification in the conductivity of the semiconductor that may occur below the contact must be considered. Contacts to GaAs are discussed. A substantial decrease in the sheet resistance of p-type GaAs is measured for Pt-reacted contacts in which a single thin layer of Mg is interposed. It is pointed out that this lowering, which is attributed to the doping action of Mg, if not taken into account, can lead to serious errors in the estimation of specific contact resistivity.     

Development of InxGa1−xAs-based ohmic contacts for p-type GaAs by radio-frequency sputtering

In_xGa_1−xAs-based ohmic contacts which showed excellent contact properties for n-GaAs were demonstrated to be applicable to p-GaAs ohmic contacts. These contacts, prepared by radio-frequency sputtering, provided low contact resistance (0.2 Ω-mm), excellent thermal stability, smooth surface, and good reproducibility. The contact resistances had a weak dependence on the annealing temperatures, which was desirable in a manufacturing view point. This weak temperature dependence was explained to be due to a unique Schottky barrier height at the metal/p-In_xGa_1−xAs interface which does not depend on the In concentration in the In_xGa_1−xAs layer. The present experiment showed the possibility of simultaneous preparation of ohmic contacts for both n and p-GaAs using the same contact materials.     

Gallium-vacancy-dependent diffusion model of ohmic contacts to GaAs

A new model based on gallium-vacancy-dependent diffusion of germanium into gallium arsenide is proposed to explain the ohmic behaviour of an alloyed AuGeNi?n GaAs system. For constructing the model, the following dominating states are assumed to form during alloying; germanium on gallium sites (Ge_Ga^?), germanium on arsenic sites (Ge_As^?), the vacancy complex (Ge_GaV_Ga^?) and neutral pairs (Ge_GaGe_As). This suggests that the formation of donor states (Ge_Ga⁺) is a gallium-vacancy phenomenon and attains a maximum level at an optimum temperature at which acceptor formation rate begins to exceed donor generation. The established theory of grain-boundary diffusion and thermodynamical results are used to compute concentrations of gallium vacancies and donors, respectively. The results indicate that the maximum achievable doping, in practice, is of the order of 5 × 10¹⁸ cm^?3, when an AuGeNi-n GaAs system is alloyed at the optimum temperature. This explains the limitations in obtaining minimum values of contact resistance by alloying techniques.     

Cu3Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10^-7 Ω cm² is obtained on n-type GaAs with doping concentrations of～ 1 x 10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ε₁-Cu₃Ge (average lattice parameters: a₀= 5.30Å, b₀= 4.20Å, c₀= 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of theξ and ε-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.     

Al—Ge ohmic contacts to n-type GaAs

Ohmic contacts have been fabricated to n-type GaAs with an alloy of AL-Ge which has an eutectic temperature of 424°C with 53- weight percent Germanium. The lowest contact resistance of 1.4 × 10^-6Ω.cm²was measured with a transfer length transmission line structure. The application to GaAs integrated-circuit design is demonstrated.     

Reproducible low-resistivity AuMn ohmic contact for p-type GaAs

A novel ohmic contact 96% Au-4% Mn has been established for p-type GaAs. A specific contact resistivity of 2 × 10?7 ?cm2 has been obtained on 2 × 1019 cm?3 epitaxial layers after alloying, and a resistivity of 2 × 10?6 ?cm2 has been obtained on 2 × 1020 cm?3 doped layers without alloying. The contact is stable and reproducible.     

Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN

We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 × 10¹⁷ cm⁻³). It is shown that annealing at 500°C for 2 min in a N₂ ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(±0.2) × 10⁻⁴ and 2.4(±0.2) × 10⁻⁵ Ωcm² for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltage-temperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.     

Transition resistances of ohmic contacts to p-type cdte and their time-dependent variation

Ohmic contacts to p-type CdTe are important for the development of solar cells based on this semiconductor, as for instance CdS/CdTe or ITO/ CdTe solar cells. Ohmic contacts to CdTe Bridgman crystals, doped with phosphorus, have been examined with respect to their resistivity dependence and their variation as a function of time. The ‘specific’ contact resistance r shows a linear dependence on the bulk resistivity; in addition, it is affected by the oxygen content of the CdTe. The lowest r obtained was 0.07Ω cm. With one exception, ali the contacts with nickel, gold and platinum deposited on different crystals show a more or less pronounced increase of r as a function of time.     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.     

Use of Sn-doped GaAs for non-alloyed ohmic contacts to HEMTs

Segregation of Sn to the surface of MBE grown n⁺-GaAs layers allows fabrication of non-alloyed Ti/Pt/Au, Al or Ti/W ohmic contacts with low specific contact resistivities (1.1×10^-6 Ω·cm^-2). These contacts were used to realise high performance HEMTs (g_m=230 mS/mm for 1.0 μm gate length) in which Si is used as the dopant in the donor AlGaAs layer and Sn is employed in the GaAs contact layer     

Nitride-based light-emitting diodes with Ni/ITO p-type ohmic contacts

The optical and electrical properties of Ni(5 nm)-Au(5 nm) and Ni(3.5 nm)-indium tin oxide (ITO) (60 nm) films were studied. It was found that the normalized transmittance of Ni/ITO film could reach 87% at 470 nm, which was much larger than that of the Ni-Au film. It was also found that the specific contact resistance was 5 /spl times/ 10/sup -4/ /spl Omega/ /spl middot/ cm/sup 2/ and 1 /spl times/ 10/sup -3/ /spl Omega/ /spl middot/ cm/sup 2/, respectively, for Ni-Au and Ni/ITO on p-GaN. Furthermore, it was found that the 20 mA output power of light-emitting diode (LED) with Ni-Au p-contact layer was 5.26 mW. In contrast, the output power could reach 6.59 mW for the LED with Ni/ITO p-contact layer.     

Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -3/) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.     

Evaluation of ohmic contacts to p-type 6H-SiC created by C and Alcoimplantation

A comparative study of specific contact resistance and sheet resistivity of p-type 6H-SiC created by Al implantation only and by C and Al coimplantation into n^--6H-SiC epilayer grown on n⁺-6H-SiC has been performed to address the challenging issue of ohmic contacts to the anode of SiC thyristors and other thyristor-based advanced devices. Direct experimental evidence has been obtained which shows the obvious advantage of C and Al coimplantation in terms of contact resistance and sheet resistivity. Under our experimental conditions, it is found that the specific contact resistance can be reduced by three orders of magnitude and the sheet resistivity can be improved by a factor of 6 when C and Al are coimplanted into 6H-SiC     

Electrical and structural properties of W-In based ohmic contacts to GaAs

A low resistance, nonalloyed, nongold ohmic contact to n-GaAs is fabricated by cosputtering W and In targets with one of the dopant elements like Si, Ge or Te. The as-deposited rectifying contacts become ohmic when annealed to 500°C, and show an average specific contact resistance of 5 × 10⁻⁶ cm². A contact resistance as low as 3 × 10⁻⁶ is achieved in W-In-Si/n-GaAs system. The Auger and Rutherford back scattering analysis of the interface reveals an InGaAs phase formation prior to ohmic behavior. The contacts are stable up to 500°C and surface morphology is much superior to presently used AuNiGe contacts. The contact pads can be patterned by dry plasma etching without affecting the GaAs substrate.