Linear non-hysteretic gating of a very high density 2DEG in an undoped metal–semiconductor–metal sandwich structure

Abstract

Modulation doped GaAs-AlGaAs quantum well based structures are usually used to achieve very high mobility 2-dimensional electron (or hole) gases. Usually high mobilities (> 10 7 cm 2 V −1 s −1 ) are achieved at high densities. A loss of linear gateability is often associated with the highest mobilites, on account of a some residual hopping or parallel conduction in the doped regions. We have developed a method of using fully undoped GaAs-AlGaAs quantum wells, where densities ≈ 6 × 10 11 cm −2 can be achieved while maintaining fully linear and non-hysteretic gateability. We use these devices to understand the possible mobility limiting mechanisms at very high densities.Keywords: undoped quantum well, double-side processing, etch-stop layer, backgateThe ability to tune the carrier density of a 2dimensional electronic system (2DES) over large ranges with a linear and non-hysteretic gate is one of the most desirable and generic aspects in experiments that involve a 2DES. Fundamental aspects of a 2DES, like the ratio of Coulomb and kinetic energy, screening, relative importance of various scattering mechanisms are all functions of the carrier density. The low density end (∼ 10 9 cm −2 and lower) is of great interest because the very dilute 2DES is a strongly interacting system[1], where the Coulomb interaction energy outweighs the kinetic energy. One the other hand the very high density end (10 11 − 10 12 cm −2 ) is of interest because the highest electron mobilities [2]can be achieved at these densities. Qualitatively, this happens because the effect of ionized impurity scattering diminishes as k F (the Fermi wavevector) becomes larger compared to the Fourier components of the impurity potential (∼e −qd /q, where q is the scattering wavevector and d is the distance of the ionized impurity from the plane of the 2DES ). Study of several other phenomena like non-parabolic effects and anti-crossing of hole bands [3] , mobility limiting effect of interface roughness [4], study of novel Fractional Quantum Hall (FQHE) states [5,6]also require single-subband, parallel-conduction free, linearly gateable, non-hysteretic 2DES in the density range (10 11 − 10 12 cm −2 ).The advent of the quantum well structure with modulation doping [7] and an undoped spacer allowed higher densities and mobilities to be reached compared to what was possible with a heterostructure. Such structures have been the workhorse for 2DES based devices for last 30 years. But a limitation of this scheme becomes apparent at high densities ( Fig. 1 (a)&(b)). As the (as grown) carrier density is increased by increasing the doping concentration the slope of the conduction band (CB) just outside the well must also increase. This is necessary to satisfy electrostatics, because the flux of the electric field * Electronic address: kd241@cam.ac.uk, kdasgupta@phy.iitb.ac.in (slope of the conduction band) over a box that encloses the quantum well must equal the charge contained within. But the sharper slope of the conduction band forces the impurity band (∼30 meV bel...

Semiconductor membranes for electrostatic exciton trapping in optically addressable quantum transport devices

“…They can be dissolved by a 30 s dip in a hydroxide potassium solution (25 g/100 ml dionized water). Our experience showed that a full chemical etching of the substrate instead of a combination mechanical polishing/chemical etching [37,38] reduces the risk of cracks on the thin semiconductor heterostructure. Another cycle of (H 2 O 2 +citric acid) + HF etches the 300 nm GaAs buffer layer and the 30 nm Al 0.75 Ga 0.25 As second etch stop.…”

“…The electron transport in the two quantum wells being coupled, resonant tunneling phenomena were studied with this method. Rather high mobilities of about 3.3 × 10 5 cm 2 V −1 s −1 (for an electron density between 1 and 4 × 10 11 cm −2 ) in a doped [37] and nearly 10 7 cm 2 V −1 s −1 (for an electron density of 5.8 × 10 11 cm −2 ) in an undoped heterostructure were demonstrated [38]. However, the thickness of the heterostructure and its gate stack was a few microns, which is much too large to achieve sufficiently tight exciton confinement and to realize single-electron quantum dots.…”

Beyond modulation doping: Engineering a semiconductor to be ambipolar, or making an ON-OFF-ON transistor

“…The electrochemical potential itself goes down as one moves out from the quantum well, the positive voltage bias on the gates attract electrons (positive bias lowers the electrochemical potential, in the usual convention followed). The combination of these two also ensures that parallel conduction in dopant layers can be eliminated [17]. The bias on the two gates need to be adjusted to tune the shape and tilt of the wavefunction in the quantum well.…”

“…A lithographic backgate also has distinct advantages over an in-situ n+ backgate, such as the ability to only act upon a defined region. However the two-sided metalsemiconductor-metal sandwich structure shown in fig 2 requires a flip-chip process originally introduced by Weckwerth et al [18] and further developed by us [17]. Unlike modulation doped GaAs/AlGaAs structures, there are no intentional dopants in the system.…”

N-type ohmic contacts to undoped GaAs/AlGaAs quantum wells using only front-sided processing: application to ambipolar FETs

“…Field-effect-induced two-dimensional electron gases (2DEGs) and two-dimensional hole gases (2DHGs) in undoped GaAs/ AlGaAs heterostructures offer a number of advantages over their modulation-doped counterparts. The absence of doping results in higher carrier mobilities at low carrier densities [1] and more reliable control of the carrier density by an applied gate voltage [2,3]. 2DEGs and quantum dots can be formed as shallow as 30 nm below the surface [4,5], and it becomes possible to define finer features using surface gates, as a spacer layer (between the 2DEG and the dopants) is no longer needed.…”