High mobility GaAs heterostructure field effect transistor for nanofabricationin which dopant-induced disorder is eliminatedB. E. Kane,a)L. N. Pfeiffer, and K. W. WestAT&T Bell Laboratories, Murray Hill, New Jersey 07974~Received 14 February 1995; accepted for publication 6 July 1995!We have fabricated field effect transistors with undoped GaAs channels, undoped AlxGa12 xAsbarriers, and either n1GaAs or epitaxial Al gates. Low resistance ohmic contacts are madeseparately to the gate and channel in samples with 250 Å barriers and in which the depth of thechannel below the top surface is 900 Å. Because electrons in the channel are neutralized byconducting charge on the gate, they do not experience the dopant-induced disorder inevitable inmodulation doped structures. Electron mobility is above 106cm2/V s, even when their Fermiwavelength exceeds 1000 Å, making these devices ideally suited for nanofabrication. © 1995American Institute of Physics.An important and elusive goal in research on nanostruc-tures is to fabricate devices in which electrons can be con-fined in potentials abrupt on the scale of their Fermi wave-length lF. While epitaxial techniques can be used to achievethis goal in a single spatial dimension ~along the direction ofcrystal growth!, further processing is required to confineelectrons in the plane perpendicular to this axis.1Usually, amodulation doped two-dimensional electron gas ~2DEG! isplaced as close as possible to the top surface of the crystal.The electrons are then confined within the plane either byselective etching of the 2DEG or by selective depletion ofthe electrons with a top gate.A fundamental problem with this approach is that theelectrons experience the disorder potential caused by the ran-dom positions of the donors in addition to the potential im-posed by the gate or etching. While this disorder has notprevented 2DEGs in GaAs/AlxGa12 xAs heterostructuresfrom having mobilities ~m! in excess of 107cm2/V s, it nec-essarily is more important in nanostructures for several rea-sons: First, using standard processing techniques, the size ofa structure can be no smaller than the separation between theelectrons and the top surface. Reducing this distance neces-sarily reduces the separation between donors and electrons,and hence, increases the disorder potential in the 2DEGplane.2Consequently, there is an inherent conflict betweenreducing the size of a structure and reducing the disorderpotential which the electrons experience. Second, the abilityof the electrons to screen disorder decreases as they are con-fined in more dimensions. Nixon, Davies, and Baranger3modeled the effect of dopant disorder on point contacts andfound that size quantization effects can be nullified by dop-ant induced disorder, even though the device scales are overan order of magnitude smaller than the 2D mean-free path ofthe electrons. This result suggests that many interesting de-vices, including long wires and arrays of point contacts inparallel and in series will have quantum properties eitherobscured or eliminated by dopant induced disorder.This pessimistic prognosis can be avoided, however, ifthe donor layer adjacent to the electrons is itself a conductinglayer and can screen any disorder associated with it.4In sucha structure, an undoped barrier layer lies between a channelcontaining the electrons and a conducting gate, made eitherof a metal or a heavily doped semiconductor. Density of theelectrons (n) can be varied by adjusting the potential differ-ence (VG) between the electrons and the gate. Nanostruc-tures can be created by patterning the top gate, with mini-mum feature sizes determined by the spacing between thegate and the electrons.The structure described above is nothing more than anenhancement mode field effect transistor ~FET!, and it hasbeen extensively studied in device research in GaAs/AlxGa12 xAs heterostructures.5It has not been favored innanostructures research because of its inherent complexity.Electrons drawn into the channel by the gate must comefrom a contact that does not short to the gate ~Fig. 1!,arequirement that becomes more difficult as the gate-channelspacing is reduced.6We have previously reported an ap-proach in which self-aligned etching and subsequent evapo-ration of contact metals can be used to realize the structureshown in Fig. 1;7however, the large barrier layer used inthose devices ~6000 Å! limits their use in nanostructures re-search. Recently, by developing a technique to accuratelycontrol the diffusion depth of alloyed contacts, we have fab-ricated FETs with AlxGa12 xAs barriers only 250 Å thick.Contact resistance (RC) in these devices is comparable to thelowest reported for GaAs heterostructure FETs. Also, wehave applied the same technique to make FETs with epitaxi-ally grown Al gates that have extremely low sheet resistivityat low temperatures. While our primary motivation for mak-ing these devices is to make low disorder FETs for nano-a!Current address: School of Physics, University of New South Wales, Syd-ney 2052 Australia. Electronic mail: [email protected]. 1. Schematic representation of our FET structure. In order to be ofvalue for nanostructures, both the barrier thickness and the depth of the2DEG from the top surface should be as small as possible. The contactscannot, however, diffuse through the barrier and short to the gate.1262 Appl. Phys. Lett. 67 (9), 28 August 1995 0003-6951/95/67(9)/1262/3/$6.00 © 1995 American Institute of PhysicsDownloaded¬27¬Sep¬2005¬to¬129.2.95.183.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright,¬see¬http://apl.aip.org/apl/copyright.jspstructures, their low RCand low gate resistivity may alsoprove of value for device FETs.Samples were grown for these experiments using proce-dures similar to those used for highmmodulation dopedGaAs/AlxGa12 xAs heterostructures.8The GaAs channel isdeposited after buffer layers are grown on a ~100! GaAssubstrate. The Al0.3Ga0.7As barrier layer is then grown, fol-lowed by a 150 Å GaAs spacer layer and a 500 Å n1GaAsgate layer, typically doped at 4–631018/cm3. Growth of Algated structures proceeds similarly until after the depositionof the GaAs spacer layer, at which point all molecular beamepitaxy ~MBE! furnaces ~except Al! are turned off, and thesubstrate is cooled to room temperature.9After the MBEchamber has fallen to its base pressure ~typically, 1 h!, the Alis deposited at ;1 Å/s. An 1100 Å thick gate grown in thismanner has