Si

0 downloads 0 Views 181KB Size Report
gmal/LG the MODFETs presented here exceed the transconductance of convention MESFETs by a factor of 2 . /1/ T.P. Pearsall et al., proc. 1st Int. Symp. on SiĀ ...

FABRICATION AND PROPERTIES OF n-CHANNEL SiGe/Si MODULATION DOPED FIELDEFFECT TRANSISTORS GROWN BY MBE H. Daembkes, H.-J. Herzog, H. Jorke, H. Kibble and E. Kasper AEG Research Center Ulm, West Germany

Silicon MBE has become an interesting technique for growing improvedor even completely new layer structures. Especially the heteroepitaxyof Sil-xGex/Si offers fascinating prospects for novel high speed circuits and optoelectronic circuits. From these structures high performance FET-devices can be fabricated /1/ utilizing the enhancedmobility in thequantum well at theSiGe/Si heterointerface/2/. We report the first fabrication of n-channelmodulation doped SiGe/Si FETs. The layer sequence isgrown 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 of0.2 m thickness, 20 nm undoped Si, 10 nm Sio.5 Ge0.5 doped by an Sb spike in thecenter 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-layerwas done by themethode of doping by secondary implantation 131. Separate investigations by SDH-measurements /4/ have demonstrated that theelectrons 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 isan evaporated sandwich structure of Pt-Ti-Au. The selection of Pt and an undoped Si top layerensures a high quality Schottky contact. A slight plasma etching was done before the evaporation. The gate length, the gatewidth and thechannel length of the experimental devices are 1.5, 160, and 4.5pm, respectively. The dc investigations demonstrate characteristics without any loops andwith 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 leastcomparable to thevalues 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 thetransconductance of convention MESFETs by a factor of2 . T.P. Pearsall et al., proc. 1st Int. Symp. on Si MBE, Toronto 1985 /2/ H. Jorke, H.-J. Herzog, ibid. / 3 / H. Jorke, et al., Appl. Phys. Lett 47, 511, (1985) / 4 / G. Abstreiter et al., Phys. Rev. Lett. 54 No 22 (19851, 2441 /5/ G. Fernholz, H. Beneking, IEEE Ed-30 No 7, (19831, 837 /1/

28.7 768

- IEDM 85

Source

Gate

Drain

Au top layer Si, -x Ge, 10 n m

zzzzzzz

X = 0.5-0

Ge,,, 10 nm

Si,

graded layer Sbspike doping

I

20 nm

Si

undoped buffer layer

Si

-

substrate loL SI. c m

Fig. 1 : Schematical cross-sectlonal view of the SiGe/Si MESFET

F i g . 2: Photograph o f t h e experimental device

28.7 IEDM 85

- 769

Sb doping spike 2nm

i-Si

Si Ge 1-X

X

Si Ge 0.5 Q5

i -Si

buffer layer

graded layer Fig. 3: Schematical band diagram of the modulation doped heterostructure at thermal equilibrium

Fig. 4:

I-v characteristics of the SiGe/Si MODFET with L = 1.5 pm and W 160 pm at 300 K

28.7 77Q

- IEDM 85

si075Ge025