Ultra-shallow quantum dots in an undoped GaAs/AlGaAs two-dimensional electron gas

Abstract

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...

Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates

“…In most accumulation-mode devices the top-gate spans the entire conduction region and slightly overlaps the Ohmic contacts in order to effectively contact, and induce, a continuous 2D conducting region. However the close proximity of the contacts to the top-gate can cause electrical shorts between the two [15][16][17] . The shorting of the top-gate to the contacts is particularly problematic in a) Electronic mail: Alex.Hamilton@unsw.edu.au shallow semiconductor-insulator-semiconductor field effect transistors (SISFETs) 12 in which the top-gate is an in situ, degenerately-doped GaAs cap.…”

“…Circumventing the top-gate leakage problem can be done with an alternative architecture whereby a metallic gate replaces the doped-cap. The metallic top-gate is isolated from the contacts by an amorphous dielectric layer in a metal-insulator-semiconductor field effect transistor (MISFET) configuration 15,19 . Furthermore MISFETs are advantageous because dual-gating is possible by using two metallic gates (with insulating layers in between) to either realize sub-micron features in accumulation-mode devices [20][21][22] , or to independently control the accumulation layers near the contacts and in the conducting channel.…”

Few-hole double quantum dot in an undoped GaAs/AlGaAs heterostructure

“…Although the effective hole mass will depend on the details of the dot confinement potential 24 , here we use the effective mass for heavy holes in 2D systems in GaAs/AlGaAs single-interface heterostructures, m * HH ∼ 0.5m e 25 . For future devices, it would be of interest to utilize heterostructures with shallow 2D hole layers (< 100 nm depth) 26 . This should help to decrease the size of the electrostatic confinement potential, which may be required in order to achieve similar orbital level spacings to those obtained in electron quantum dots, since the heavy hole effective mass is larger than the electron effective mass in GaAs (m * HH ∼ 0.2m e − 0.5m e > m * e ≈ 0.07m e ).…”

Field-effect-induced two-dimensional electron gas utilizing modulation-doped ohmic contacts

“…Field-effect-induced 2DEG transistors (FETs) in GaAs/AlGaAs heterostructures have been investigated extensively [14][15][16][17][18][19][20][21][22][23][24][25][26][27] and might find utility as a platform to investigate nanoscale phenomena in a low-noise environment if certain limitations can be overcome. The most widely studied device is the heterostructure-insulated-gate field-effect transistor (HIGFET), in which a highly-conducting n+ GaAs gate is grown on top of an insulating Al x Ga 1−x As barrier layer by molecular beam epitaxy (MBE) [14-16, 18, 22].…”

“…In this configuration the global top-gate must extend over the ohmic contact pad to ensure a continuous 2DEG to the ohmic contact. Progress has been made in creating a 2DEG on shallow undoped structures by this method [20,21] and the induced 2DEG can have high mobility [24,25] in deeper structures. Nevertheless, the requirement that the gate electrode overlaps the rough annealed ohmic metal enhances the possibility of undesired leakage paths [27,28].…”