We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.

We report quantum dots fabricated on very shallow 2-dimensional electron gases, only 30 nm below the surface, in undoped GaAs/AlGaAs heterostuctures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of more than one order of magnitude in mobility (at 2×10 11 cm −2 ) with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited state energies (up to 0.5 meV).Electrostatically-defined quantum dots fabricated on high-mobility GaAs/AlGaAs heterostructure have been − and continue to be − invaluable in many fundamental investigations, e.g. Kondo physics and spin-based solid-state qubits. Unfortunately, the characteristics of these devices are extremely sensitive to seemingly random charge fluctuations in their local electrostatic potential, commonly known as Random Telegraph Signal (RTS) noise, or charge noise. Although one can perform a biased cooling 1,2 or a thermal cure 3 to attempt to drastically reduce the levels of charge noise on a given device, results from both techniques vary from device to device.Quantum dots fabricated in shallow two-dimensional electron gases (2DEGs) have two advantages over their deeper cousins. First, finer features can be transferred from the surface metallic gates to the 2DEG. Second, the energy scales of the dot levels tends to be larger, which enable operation at higher temperatures. However, shallow 2DEG depths (as little as 20 nm below the surface) come at the expense of mobility.4-14 Furthermore, the dopant layer may partially screen surface gates (through hopping conduction) and/or facilitate gate leakage, rendering many such wafers ungateable by surface metallic gates. The ungateability of some doped wafers is not only restricted to shallow 2DEGs, but also can occur in high-mobility doped wafers. 15-17The limitations described above can be circumvented or mitigated by using undoped heterostructures in different field-effect transistor (FET) geometries such as the SISFET 18-22 (semiconductor-insulator-semiconductor), the MISFET 23-25 (metal-insulator-semiconductor), or the HIGFET 26 (heterostructure-insulator-gate). Since there are no intentional dopants, the 2DEG can be brought much closer to the surface without sacrificing mobility. Furthermore, undoped quantum dots would not suffer from one possible source of charge noise: electrons hopping between dopant sites in AlGaAs. In doped wafers, intentional dopants typically outnumber unintentional dopants 10,000 to 1 (depending on mobility). Finally, undoped quantum dots may also interact with fewer undesirable impurities in the vicinity and are far more reproducible upon thermal cycling than their doped counterparts.27 In this Letter, we compare ultra-shallow undoped and doped GaAs/AlGaAs 2DEGs, and demonstrate gated quantum dots on ultra-shallow undoped heter...

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...

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both n-type and p-type Ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed.

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