W. H. Price

Thermally stable ohmic contacts to n-type GaAs. III: GeInW and NiInW contact metals

Improvement in thermally stable, low‐resistance ohmic contacts to n‐type GaAs is reported for GeInW and NiInW contact metals. Coevaporation of In with Ge or In with Ni reduced the contact resistances by a factor of about 2 compared with those of the layered structures. The reduction is believed to be due to a uniform In distribution in the contact metals in the as‐deposited state which resulted in an increased area of InxGa1−xAs phases in direct contact with the GaAs substrate. Annealing the coevaporated GeInW contacts for a short time at temperatures between 900 and 980 °C resulted in a mean contact resistance of 0.5 Ω mm. Similar annealing of the coevaporated NiInW contacts at temperatures between 800 and 1000 °C resulted in a contact resistance of 0.3 Ω mm. Additionally, the thermal stability of these ohmic contacts at 400 °C after contact formation, which is required by subsequent integrated circuit process steps, was studied. Although a slight increase in the contact resistances was observed after an...

Thermally stable, low-resistance NiInW ohmic contacts to n-type GaAs

A new thermally stable, low‐resistance NiInW contact metal to n‐type GaAs has been developed by depositing a thin In layer with Ni and W layers and annealing at elevated temperatures for a short time. Low resistances of ∼0.3 Ω mm were obtained at annealing temperatures in the range of 800 to 1000 °C. The contact resistances were stable during subsequent annealings at 400 °C for 100 h and 500 °C for 10 h. The thermal stability of the contact resistance and the surface morphology of this contact are superior to those of the conventionally used AuNiGe contacts and this new contact is suitable for various device applications. Further reduction of the contact resistance can be achieved simply by reducing the sheet resistance of the contact metals.

Thermally stable ohmic contacts to n‐type GaAs. II. MoGeInW contact metal

It was previously found that a small amount of In impurity was able to convert MoGeW contacts from Schottky to ohmic behavior yielding thermally stable, low‐resistance ohmic contacts n‐type GaAs. In the present experiment transport measurements and materials studies were carried out for MoGeInW contacts in which a thin layer of In was directly added to the MoGeW contacts during deposition. The transition from Schottky to ohmic behavior was observed by adding an In layer as thin as ∼1 nm to the MoGeW. Contact resistances were found to be very sensitive to the deposition sequence, the annealing method, the annealing temperature, and the In layer thickness. Low resistances of ∼0.5 Ω mm were obtained in the MoGeInW contacts with 2‐nm‐thick In layers, annealed by the heat‐pulse method at temperatures in the range of 880–960 °C for 2 s. Contact resistances were stable during subsequent annealing at 400 °C for 100 h. Evidence of formation of the parallel diode areas with various barrier heights was obtained for ...

Thermally stable ohmic contacts to n‐type GaAs. IV. Role of Ni on NiInW contacts

Recently, thermally stable, low resistance NiInW ohmic contacts to n‐type GaAs have been developed using a conventional evaporation and lift‐off technique and annealing the contacts by a rapid thermal annealing method. This contact material has great potential for use in GaAs integrated circuits. In the present paper, the microstructure of the NiInW contact material has been studied extensively by cross‐sectional transmission electron microscopy. Special attention was paid to understanding the role of Ni in the NiInW contacts by analyzing samples prepared by different deposition sequences. In order to prepare the contacts with a large fractional coverage of InxGa1−xAs phases at the metal/GaAs interface, which is essential to produce low resistance contacts, Ni must prevent In from spreading vertically into the GaAs substrate during the heating process. The formation of a uniform Ni2GaAs layer at the GaAs surface and suppression of In diffusion toward the GaAs by intermixing In with Ni at the initial stage...

Thermally stable ohmic contacts to n‐type GaAs. I. MoGeW contact metal

Thermally stable, low‐resistance ohmic contacts on n‐type GaAs are required to fabricate high‐speed GaAs integrated circuits. MoGeW contacts prepared by annealing at high temperature around 800 °C in an InAs overpressure are attractive, because the contact is expected to be thermally stable during subsequent annealing at 400 °C, which is required by several process steps following ohmic contact formation. In the present experiment, the contact resistance measurements and microstructural analysis of MoGeW contacts were carried out to establish a fabrication process which forms ohmic contacts with low contact resistance. The contact metals were prepared by sequentiallydepositing Ge, Mo, Ge, and W, with various Mo/Ge layer thickness ratios, onto (100)‐oriented GaAs wafers. The conducting channels were formed by doping GaAs with about 1×1018 cm−3 Si. Contact resistances were determined by the transmission line method, and microstructural analysis was carried out by x‐ray diffraction, Auger electron spectrosco...

Thermally stable ohmic contacts to n-type GaAs. V. Metal-semiconductor field-effect transistors with NiInW ohmic contacts

Thermal stability of self‐aligned refractory metal‐semiconductor field‐effect transistors (MESFETs) with the universally used AuNiGe ohmic contact metallurgy or the recently developed NiInW ohmic contact metallurgy has been studied. In these devices WSi0.1 films with length down to 1 μm were used as the gate material, and AlCu was used as the overlayer material on the ohmic contacts, where a very thin Ti layer was deposited prior to the AlCu deposition. The performance of the as‐fabricated devices with the NiInW ohmic contacts was as good as those with the AuNiGe ohmic contacts. During subsequent annealing at 400 °C, deterioration of the device performance (defined by the decrease in the FET square‐law coefficient) was observed after annealing at 400 °C for 2 h in the devices with the AuNiGe ohmic contacts. However, excellent stability was observed in the devices with the NiInW ohmic contacts; no deterioration was observed at 400 °C for 180 h, 450 °C for 18 h, and 500 °C for 2 h. The device deterioration ...

Thermally stable ohmic contacts to n‐type GaAs. VII. Addition of Ge or Si to NiInW ohmic contacts

The effects of Si or Ge addition to NiInW ohmic contacts on their electrical behavior were studied, where the samples were prepared by evaporating Ni(Si) or Ni(Ge) pellets with In and W and annealed by a rapid thermal annealing method. An addition of Si affected the contact resistances of NiInW contacts: the resistances decreased with increasing the Si concentrations in the Ni(Si) pellets and the lowest value of ∼0.1 Ω mm was obtained in the contact prepared with the Ni‐5 at. % Si pellets after annealing at temperatures around 800 °C. The contact resistances did not deteriorate during isothermal annealing at 400 °C for more than 100 h, far exceeding process requirements for self‐aligned GaAs metal‐semiconductor field‐effect‐transistor devices. In addition, the contacts were compatible with TiAlCu interconnects which have been widely used in the current Si process. Furthermore, the addition of Si to the NiInW contacts eliminated an annealing step for activation of implanted dopants and low resistance (∼0.2 Ω mm) contacts were fabricated for the first time by a ‘‘one‐step’’ anneal. In contrast, an addition of Ge to the NiInW contacts did not significantly reduce the contact resistances.

Thermally stable ohmic contacts to n‐type GaAs. VI. InW contact metal

The electrical properties and thermal stability of In/W Ohmic contacts in n‐type GaAs were studied by analyzing interfacial microstructure using cross‐sectional transmission electron microscopy and measuring the contact resistances by transmission line method. Indium layers with various thicknesses were deposited directly on GaAs substrates, which were kept at room or liquid‐nitrogen temperature. The lower contact resistances (Rc) were obtained when the contacts were prepared at liquid‐nitrogen temperature. These low Rc values were due to formation of large‐areal InxGa1−xAs phases on the GaAs substrate after annealing at temperatures higher than 600 °C. The In layer thicknesses of the In/W contacts prepared at liquid‐nitrogen temperature strongly affected the contact resistances as well as the thermal stability after contact formation. The optimum In layer thickness which provided the best electrical properties and thermal stability was determined to be 3 nm. The In(3 nm)/W contacts yielded Rc values less...

Thermally stable ohmic contacts to p‐type GaAs. IX. NiInW and NiIn(Mn)W contact metals

Thermally stable, low‐resistance p‐type ohmic contacts have been developed by depositing NiInW metals on GaAs substrates in which Be and F were coimplanted. The contacts provided resistances of about 1.4 Ω mm after annealing at temperatures in the range of 300–800 °C for short times. The electrical properties did not deteriorate after annealing at 400 °C for more than 100 h, which far exceeds the requirements for current GaAs device fabrication. The present study demonstrated for the first time that thermally stable, low‐resistance ohmic contacts to both n‐ and p‐type GaAs can be fabricated using the same metallurgy. In addition, NiInW ohmic contacts were prepared by simultaneous (one‐step) annealing for ion‐implant activation and contact formation, which simplifies significantly the device fabrication process. A factor‐of‐2 reduction of the contact resistances was achieved by slight etching of the GaAs surface prior to the contact metal deposition so that the metal/GaAs interface contacted the peak posit...

Thermally stable, low‐resistance NiInWNx ohmic contacts to n‐type GaAs prepared by sputter deposition

A new thermally stable, low‐resistance In‐based ohmic contact to n‐type GaAs has been developed. The contacts consist of ion‐beam sputtered Ni (5 nm)/In (5 nm)/Ni (5 nm) layers with a magnetron sputtered WNx overlayer. A low‐contact resistance of ∼0.3 Ω mm was obtained by rapid thermal annealing at 750 °C for ∼5 s. The contact resistance and the excellent contact morphology remained unchanged after annealing at 400 °C for more than 100 h. The present deposition technique provides several advantages over previously reported electron‐beam evaporated In‐based contacts. In particular, the ability to deposit a thick WNx overlayer simplifies GaAs integrated circuit (IC) fabrication by (a) eliminating the need for separate diffusion barrier deposition and patterning steps, and (b) providing for low‐sheet resistance (∼2 Ω/⧠) IC interconnect capabilities. In addition, sputter deposition allows for the controlled incorporation of n‐type dopants into the metallization if further reduction of the contact resistance i...