Ohmic Contacts to GaAs Transferred Electron Devices

Abstract

The specific contact resistance of A u / G e / N i alloy contacts to G a A s for transferred electron devices has been measured. It is found that a shallow sulfur diffusion under the contact is effective in reducing the specific contact resistance by up to two orders of magnitude. This procedure is expected to improve the uniformity of threshold and bias voltages in integrated circuit configurations by making the contact resistance only 1-10% of the total device resistance.Because of the low doping found in GaAs transferred electron devices (TED's) for microwave applications, the contact resistance to the device is the major part of the total device resistance. A survey of the literature indicates that for a given doping level, the contact resistance can vary uP to two orders of magnitude as the processing parameters are varied (1-5). Significant variation can also occur across a wafer in a given run. This situation is allowable for discrete device configurations where postsorting is possible. In integrated circuit configurations, however, local variations are intolerable because threshold and bias voltages must be uniform from one device to the next in the same circuit. The purpose of this paper is to show that a shallow sulfur diffusion u n d e r the metallization pads can reduce the contact resistance to 1-10% of the total device resistance. The specific contact resistance is lowered by approximately two orders of magnitude over that measured on nondiffused samples.The metallurgical and electrical properties of alloyed A u -G e / N i films on n -t y p e GaAs are relatively well understood (1). Edwards, Hartman, and Torrens (2) have summarized specific contact resistance data of this and other contact alloys to GaAs through mid-1971. In general, their data show two order of magnitude variation in the specific contact resistance which ranges from a m e a n of 3 • 10 -3 ~-c m at a doping level of mid-1015/cm 8 to 10 -4 ~-c m at 1017/cm ~. Robinson (3) and Yu (4) have independently confirmed that by process optimization, the mean specific contact resistance can be lowered by up to one order of magnitude. For GaAs TED's doped from mid-1015/cm ~ to mid-1016/cm 3, the specific contact resistance values reported as well as those measured in our laboratories give a device contact resistance far greater than the intrinsic device resistance (typically 3000~). To lower the contact resistance to 1-10% of the total device resistance, a specific contact resistance of ~10 -5 ~-c m is required. The exact value depends on the specific device configuration. This value is two orders of magnitude lower than that reported, and hence can only be achieved by the formation of a shallow n-t-layer below the contact to give a doping level of mid-1017/cm 3 to mid-101S/cm ~.The samples used in this experiment were semiinsulating GaAs substrates on which 25 #m vapor phase epitaxy layers were grown. The typical doping range was mid-1015/cm 8 to mid-1016/cm 3 n -t y p e (Si). Ohmic contacts were formed by evaporation of a gold/ 12 weight perc...

Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

“…The electrical performance of ohmic contacts to semiconductors is commonly described with the areaindependent parameter of specific contact resistance (or contact resistivity, c), which explicitly characterizes the resistance of the interface between the metal contact and semiconductor. In order to determine c, the current transport between lateral contacts of different geometries made to a semiconductor layer is analyzed using a transmission line model, which is a commonly accepted method since the 70s [20]. This approach can be realized by forming a sequence of rectangular pads with varying distance between them.…”

Defect reduction and dopant activation of in situ phosphorus-doped silicon on a (111) silicon substrate using nanosecond laser annealing

“…[1][2][3][4][5][6] Here, the phosphorus atoms are incorporated into the silicon lattice, leading to a significant reduction in the lattice parameter of ISPD silicon 2,3) at the source/drain and consequent increase of the electron mobility by imposing the strain to the channels. 1,[6][7][8][9][10][11][12] Moreover, recent requirement of ultra-low contact resistivity pushes the employment of the high doping processes such as ISPD silicon epitaxial process by the phosphorous concentration at the source/drain up to 4 × 10 21 cm −3 . [13][14][15] This reduces the metal-semiconductor Schottky barrier width, which in turn increases the carrier tunneling and reduces the contact resistivity.…”