https://digital.library.unt.edu/ark:/67531/metadc684550/m2/1/high_res_d/463682.pdf

A survey of ohmic contacts to III-V compound semiconductors

https://iopscience.iop.org/article/10.1016/S1468-6996(01)00150-4/pdf

Development of refractory ohmic contact materials for gallium arsenide compound semiconductors

XPS studies on SiOx thin films

In order to examine the possibility of preparing a nonreacted (nonalloyed) Ohmic contact to p-GaN, the effects of GaN surface treatments and work functions of the contact metals on the electrical properties between the metal contacts and p-GaN were investigated. A contamination layer consisting of GaOx and adsorbed carbons was found on the GaN substrate grown by metalorganic chemical vapor deposition. The contamination layer was not completely removed by sputtering the GaN surface with Ar and N ions where the ion densities were ∼10−2 μA/cm2. Although the contamination layer was partially removed by immersing in a buffered HF solution, little improvement of the electrical properties of the GaN/metal interfaces was obtained. Most of the contamination layer was removed by annealing the Ni and Ta contacts at temperatures close to 500 °C. These annealed contacts exhibited slightly enhanced current injection from the contact metal to the GaN. The present surface treatment study indicated that removal of the contamination layer did not significantly reduce the contact resistance. On the other hand, the resistance decreased exponentially with increasing the metal work functions, where Pt, Ni, Pd, Au, Cu, Ti, Al, Ta, and Ni/Au were deposited on the GaN. This result suggests that the Schottky barrier height at the p-GaN/metal interface might not be pinned at the GaN surface. The present study concluded that a contact metal with a large work function is desirable for nonreacted Ohmic contacts to p-GaN. However, these contacts did not provide the low contact resistance required for blue laser diodes.

Effects of surface treatments and metal work functions on electrical properties at p-GaN/metal interfaces

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.

Effects of interfacial microstructure on uniformity and thermal stability of AuNiGe ohmic contact to n-type GaAs

Microstructure analysis and contact resistance measurements of alloyed AuNiGe contacts to GaAs were performed to assist in the development of low resistance Ohmic contacts for metal–semiconductor field‐effect transistor (MESFET) devices. The contact metals were prepared by sequential deposition of 100 nm of Au–27 at. % Ge, 35 nm Ni, and 50 nm Au onto sputter‐cleaned GaAs wafers in which conducting channels were formed by Si doping to a level of about 1×1018 cm−3. The contact resistances were determined by the transmission line method. Analysis of the substrate and the film microstructure was carried out by x‐ray diffraction, Auger electron spectroscopy (AES), and x‐ray photoelectron spectroscopy (XPS). A strong correlation between the contact resistance and the film microstructure was observed. Low resistances were observed when NiAs compounds containing Ge were in contact with GaAs and the β‐AuGa phase was concentrated near the top of the contact. High resistances were measured when free Au, the α‐AuGa p...

Microstructure studies of AuNiGe Ohmic contacts to n‐type GaAs

Several types of thin dielectric films which can be deposited on substrates held near room temperature have been tested for use as insulators in integrated circuit structures. The types of insulation studied include single and double layer SiO films, Parylene polymer films, and SiO/Parylene composites. Test structures, which were fabricated with processes used in Josephson integrated circuits, allowed measurement of the number of electrical defects per unit area in the insulation between two Pb–In–Au films as well as the number of defects along edges of the lower metal film. The SiO/Parylene composite films had the lowest defect densities, as low as 0.2 cm−2 over planar metal layers and 0.001 cm−1 at insulated metal edges. Defect densities below those for single SiO films were also obtained by depositing the SiO in two layers through separate but indentical lift‐off stencils. The defect densities measured for these insulating films both before and after repeated thermal cycling to 4.2 K are believed to be...

John M. Baker

Papers

Microstructure studies of AuNiGe Ohmic contacts to n‐type GaAs

Low defect density insulating films deposited on room temperature substrates