Fabrication and properties of n-channel SiGe/Si modulation doped field-effect transistors grown by MBE

Abstract

/1/ utilizing the enhanced mobility in the quantum well at the SiGe/Si heterointerface/2/. We report the first fabrication of n-channel modulation doped SiGe/Si FETs. The layer sequence is grown by Si-MBE on (100) Si substrates. High resistivity substrates (104Qcm, p-) are used in order to reduce parasitic substrate influences. The active structure was prepared by successively growing a Sio.75 Ge0.25 undoped buffer layer of 0.2 m thickness, 20 nm undoped Si, 10 nm Sio.5 Ge0.5 doped by an Sb spike in the center of the layer, a 10 nm Sil-xGex graded layer with x - 0.5 + 0, and an undoped Si top layer. The doping of the Si0.5Ge0.5-layer was done by the methode of doping by secondary implantation 131. Separate investigations by SDH-measurements /4/ have demonstrated that the electrons are transfered from the Sb doped Sio.5Geo .5 layer into the adjacent undoped Si layer, there forming a two-dimensional electron gas in a single quantum well. The MESFETs are fabricated in conventional mesa and lift-off technique. Dry etching technology was used. AuSb was evaporated and ailoyed at about 320 C for 30 seconds to form the ohmic contacts. No additional contact or channel region for doping was used. The gate is an evaporated sandwich structure of Pt-Ti-Au. The selection of Pt and an undoped Si top layer ensures a high quality Schottky contact. A slight plasma etching was done before the evaporation. The gate length, the gate width and the channel length of the experimental devices are 1.5, 160, and 4.5pm, respectively. The dc investigations demonstrate characteristics without any loops and with a neat pinch-off behaviour. Despite of fairly high access-resistances the best devices show an extrinsic transconductance of 40 mS/mm, typical values are 36 mS/rmn. Though our devices are in no way optimized these values are at least comparable to the values of optimized Si-MESFETs with only half the gate length of our devices and an even high carrier concentration in the channel 151. Using the scaling gmal/LG the MODFETs presented here exceed the transconductance of convention MESFETs by a factor of 2.