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Notes: 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 annealing for 100 h at 400 °C for the GeInW contacts, no change in the contact resistances was observed for the NiInW contacts after annealing for 100 h at 400 °C and for 10 h at 500 °C. This excellent thermal stability of the NiInW contacts is believed to be due to the formation of Ni3In intermetallic compounds which have high melting points. The present study suggests that in order to prepare thermally stable, low-resistance contacts it is desirable to deposit a metal which forms high melting point intermetallic compounds with In and which promotes formation of uniform InxGa1−xAs phases at the metal/GaAs interfaces. Further reduction in the measured contact resistances was achieved by reducing the sheet resistance of the contact metals.

Notes: To investigate the effects of microstructure of the Schottky characteristics of WSix contacts to n-type GaAs, cross-sectional transmission electron microscopy, x-ray diffraction, and secondary-ion mass spectrometry have been used to study the interfacial and bulk film microstructures. The barrier heights and ideality factors of WSi0.1 and WSi0.6 contacts were obtained by forward current-voltage and capacitance-voltage measurements. These Schottky characteristics were found to be unrelated to the bulk film microstructure, but closely related to the interfacial microstructure at the WSix/GaAs interfaces. Both the WSi0.1/GaAs and WSi0.6/GaAs interface morphologies were observed to be stable and remain smooth during annealing at 800 °C for 10 min, while a rough interface with W protrusions and Ga and As out-diffusion was observed in two-layer W/WSi0.6 contacts. The stability of the WSix interfacial microstructure is suggested to depend on both the chemical stability of the WSix films with GaAs and the intervening oxides between WSix and GaAs. Nontrivial amounts of W and Si were observed to diffuse from the WSi0.1 film into the GaAs substrate during annealing at 800 °C for 10 min. Although these in-diffused impurities in the GaAs substrate do not seem to affect the Schottky characteristics after the 800 °C annealing, they could be a potential problem in long-term stability. Of the three WSix film compositions, the single-layer WSi0.6 films were found to have the least W and Si in-diffusion and thus the best thermal stability.

Notes: 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 these contacts after annealing at elevated temperatures. These low-barrier-height areas are believed to be the interfaces between the contact metals and InGaAs phases. The composition within the ternary phases was uniform, and no composition gradient was observed. The composition was determined by small-probe x-ray energy dispersive spectrum to be close to In0.2Ga0.8As. The distribution of these ternary phases, influenced by the contact fabrication process parameters, strongly affected the contact resistance.

Notes: 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 spectroscopy, secondary ion mass spectroscopy (SIMS), and transmission electron microscopy. Contact resistance (Rc) was found to be strongly influenced by the Mo/Ge layer thickness ratio and annealing temperature. Rc values lower than 0.5 Ω mm were obtained for samples with a Mo/Ge thickness ratio in the range 0.6–1.3 and annealed at around 800 °C. The lowest mean Rc value obtained in the present experiment was 0.3 Ω mm. The major compound formed in this contact was identified to be Mo5As4, which has a high melting point. No changes in the microstructure and the Rc values were observed after annealing the contacts at 400 °C for more than 100 h. Finally, an attempt to understand the carrier transport mechanism was carried out by correlating the electrical behavior with the film microstructure. For this purpose the samples were annealed in an InAs overpressure with or without a Si3N4 cap, by flash annealing, and in an arsine atmosphere. The ohmic behavior was observed only in the samples annealed in an InAs overpressure. The SIMS analysis indicated that a small amount of In, less than 1 at. %, was segregated at the metal/GaAs interfaces in this sample. The In could form compounds with GaAs and reduce the barrier height, resulting in reduction of the contact resistances.

Notes: 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 stages of annealing were found to be critical to prepare such contacts with large fractional coverage of the InxGa1−xAs phases. In addition, Ni2GaAs phases seem to remove native oxides at the GaAs substrate, which is also important in attaining good coverage by InxGa1−xAs phases at the GaAs surface. Also, excess In was found to form high melting point Ni3In compounds which improved thermal stability.

Notes: As part of the investigation of the use of AuNiGe as the ohmic contact to n-type GaAs at a high integration level, cross-sectional transmission electron microscopy was used to explore the uniformity at the metal/GaAs interface and the thermal stability of the AuNiGe contact after the ohmic contact formation. A close relation between spread of the contact resistance and nonuniformity of the interfacial microstructure of the contact was found. Deposition of 5-nm-thick Ni as the first layer of the AuNiGe ohmic contact significantly reduced the spread of the contact resistance and led to the formation of a uniform interface without large protrusions. The improvement in uniformity of compound distribution and the reduction of interface roughness are believed to be due to a change in the sequence of alloying reactions, compared to those in the contact without a Ni first layer. This suggests an ideal interface structure for a low resistance AuNiGe ohmic contact after alloying to be a uniform two layer structure: a high density of the NiAs(Ge) grains contacting the GaAs substrate, and a homogeneous β-AuGa phase close to the top surface. However, due to the existence of β-AuGa phases with a low melting point of around 375 °C, the thermal stability of the contact at 400 °C is of serious concern. Segregation of the NiAs(Ge) grains was observed after annealing at 400 °C for 10 h, which reduced the contact areas between the NiAs(Ge) grains and GaAs. During subsequent annealing at this temperature for up to 90 h, liquidlike flow of the β-AuGa phase was observed which deteriorated the interface uniformity, causing an increase in contact resistance. A typical contact edge slide distance after contact alloying at 440 °C for 2 min was measured to be 0.2 μm and the longest distance among specimens examined was 0.47 μm. This edge deterioration could limit the use of the AuNiGe contact in GaAs submicron devices.