Refractory Metal Contacts to N-Type InP and InGaAs
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performance. Traditional Au-based contacts are generally formed by a two step process; film deposition, followed by alloying at elevated temperatures. Due to the alloying process, contacts formed in such a manner have inherent disadvantages such as poor surface morphology, bad edge definition and a non-abrupt interface which penetrates up to several hundred nanometers (typically 2-3 times the thickness of the initially deposited metal film).(2 ) When these devices are further processed at elevated temperatures, the interaction and diffusion between the metal and semiconductor become 3 even more severe. In the worst situation, Au may penetrate into the active layer of the device.( ) This effect is especially detrimental for some photoelectric devices whose performance depends mostly on the properties and reliability of the ohmic contact layer in the immediate vicinity of the active region. Tungsten has been used for years as a low resistivity and thermally stable contact and interconnects in silicon based devices, because of its low resistivity, refractory nature, high thermal stability, high resistance to electromigration and chemical inertness to the semiconductor substrate. In recent years, W and W-based alloys have been investigated as refractory metal contacts to III-V compound semiconductor devices (5-8). Another motivation for introducing refractory metal contacts is for self-aligned contacts where a stable ohmic contact is in place during further device processing steps. It is used to reduce the parasitic capacitances and resistances which have detrimental effects on the overall performance of ultra-high speed devices. This contact metal film, in addition to its role as an ohmic contact is also used as a selective mask for both mesa etching and ion implantation. Therefore the contact should have good edge definition and high thermal stability to withstand post implantation anneal temperatures. It is obvious that the current Au-based contact films cannot meet these requirements. 233 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
Refractory metals such as tungsten, molybdenum, titanium and their related alloys are therefore becoming the most attractive systems to be used as self-aligned contacts. The present work evaluates a non-alloyed refractory metal ohmic contact to InP-based materials. EXPERIMENTAL PROCEDURE The samples used in this study included n-type InP and InGaAs grown by gas source molecular beam epitaxy (GSMBE) with different doping levels, on semi-insulating InP substrates. In addition, the effect of shallow-range dopant ion implantation on the specific contact resistance was determined. These samples were implanted with 60 keVSe+ ions at doses of 0.1-1xl015 cm"2 . The carrier concentration was obtained from Hall effect measurements or by C-V profiling. The samples were cleaned using a standard degrease and oxide removal processes. Prior to transferring the samples to the deposition chamber, the tungsten and titanium metal sources were outgassed by pre-melting them for a
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