Electron-hole interactions in coupled InAs-GaSb quantum dots based on nanowire crystal phase templates

We report growth and characterization of a coupled quantum dot structure that utilizes nanowire templates for selective epitaxy of radial heterostructures. The starting point is a zinc blende InAs nanowire with thin segments of wurtzite structure. These segments have dual roles: they act as tunnel barriers for electron transport in the InAs core, and they also locally suppress growth of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated electrical probes. The parallel quantum dot structure hosts spatially separated electrons and holes that interact due to the type-II broken gap of InAs-GaSb heterojunctions. The Coulomb blockade in the electron and hole transport is studied, and periodic interactions of electrons and holes are observed and can be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced ground-state transitions, possibly a result of changes in the spatial distribution of holes in the thin GaSb shell.

Figure 1

(a) Schematic illustrations of the core-shell QD structure where the hexagonal cross section is indicated. (b) A simplified energy band diagram of the double QD, where dark red corresponds to the lowest hole energy states $E_h$ in the GaSb valence band, and black corresponds to the lowest electron energy states $E_e$ in the InAs conduction band. (c) A SEM image of a core-shell QD NW device, where the WZ segments and the QD are indicated.

Figure 2

(a) Bright-field TEM image of a nanowire containing a core-shell QD viewed along a $\langle 11\overline{2}\rangle$-type direction. (b) Dark-field TEM image recorded along a $\langle 21\overline{3}\rangle$-type direction and selecting the WZ $000\overline{1}$ double diffraction spot for imaging, where WZ appears as bright contrast and ZB as dark contrast. (c) EDX line scan recorded across the ZB QD perpendicular to the nanowire growth axis, averaged over a width of 20 lines and showing the Ga $K\alpha$ (red) and In $L\alpha$ (blue) signal. The inset is a magnification and highlights the signals from the GaSb and InAs shells. The STEM-HAADF intensity profile is also given (gray background), which reflects the morphology of the nanowire at that position.

Figure 3

SEM images together with intensity profiles of measured QDs; (a) reference bare InAs (“WZ1” = 29 nm, “QD” = 24 nm, “WZ2” = 24 nm) and (b) core-shell InAs-GaSb (“WZ1” = 29 nm, “QD” = 73 nm, “WZ2” = 28 nm). The diameters of the NWs at the QD segment are 84 and 76 nm, respectively. The accuracy of all given dimensions is $\pm 2$ nm. (c) Conductance as a function of gate voltage at a bias of 10 mV recorded for the reference QD (green) and core-shell QD (blue). Charge stability diagrams recorded for (d) the reference QD, and (e) and (f) the core-shell QD. The vertical red arrows indicate lifting of the hole blockade.

Figure 4

(a) SEM image of a second core-shell QD device including an intensity profile (“WZ1” = 36 nm, “QD” = 37 nm, “WZ2” = 36 nm). The diameter of the segment containing the QDs is 91 nm. The accuracy of all given dimensions is $\pm 2$ nm. (b) Conductance as a function of gate voltage. (c) Measured charge stability diagram, where the larger diamonds corresponding to hole blockade are marked and where the dashed rectangle indicates the area displayed in panel (d). (d) A closeup of a transition between two hole–Coulomb-blockade regions. The black and red notations correspond to a suggested electron and hole occupation, respectively. An electron-hole interaction strength ${\mathrm{\Delta }}_{e-h}$ of approximately 4.5 meV is extracted from the shift in the onset of hole conduction. (e) Equivalent circuit schematic of the system used in the modeling. (f) and (g) Simulated charge stability diagrams of a coupled QD system with two interacting islands connected to a source and drain electrode. The values for all capacitances are the same in the two simulations. (f) Both QDs modeled as metallic islands with no confinement (gate voltage span of 0.5 V). (g) Fit to panel (d), based on a system of one metallic island and a QD with a single level only.

Figure 5

(a) SEM image of a third core-shell QD device including an intensity profile (“WZ1” = 31 nm, “QD” = 33 nm, “WZ2” = 30 nm). The diameter of the segment containing the QDs is 83 nm. The accuracy of all given dimensions is $\pm 2$ nm. (b) and (c) Charge stability diagrams for two different $V_g$ intervals for the corresponding device. In (b), ${\mathrm{\Delta }}_{e-h}$ of approximately 4.2 meV is extracted from the shift in the onset of hole conduction (red dashed lines). In (c), dashed blue and red lines indicate the expected borders of unperturbed Coulomb diamonds.

Figure 6

Charge stability diagram recorded from the core-shell device discussed in Fig. 3.

Figure 7

Equivalent circuit of the parallel coupled QD system, with the values for capacitances and resistances that were used in the simulation displayed in Fig. 4.