GaN/AlN axial-heterostructure single-electron transistors
Mastering the electron tunneling transport opens the way towards single-electron transistors (SETs). These devices have been intensively studied in III-As and Si (Tarucha et al. 1996; Kouwenhoven 2001; Hanson et al. 2007). In contrast, there are very few reports on the potential of such devices in III-N materials (Chou et al. 2005, 2006; Nakaoka et al. 2007). It was firstly shown that InP/InAs NW heterostructure can be operated as single-electron transistors (Thelander et al. 2003), which offers a way to study few-electrons InP/InAs quantum dots (Bjork et al. 2004, 2005).
The potential of GaN/AlN NW heterostructures for single-electron tunneling devices was recently demonstrated (Songmuang et al. 2010b). Devices were fabricated with back-gate geometry using defect-free GaN/AlN double-barrier NW axial heterostructures grown by PAMBE. An essential requirement for single-electron transport is a sizeable conduction at small bias voltages, attained by lowering quantized energy levels at or below the Fermi level, EF (Tarucha et al. 1996). In III-N nanostructures grown along the polar direction, the polarization-induced triangular shape of the potential profile along the NW growth axis enables the lowest electron confined level E to be below EF simply by increasing the GaN insertion height (Fig. 9.15(a)). Following this principle,
Fig. 9.15. (a) Plot showing the 1D calculation of the three lowest quantized levels for a GaN quantum dot between two 2-nm thick AlN barriers as a function of the dot thickness. (b) Isd — Vsd characteristic of a GaN NW with two AlN tunnel barriers at different Vg (c) Differential conductance versus Vg, revealing Coulomb blockade peaks. The measurement was made by using the lock-in technique with an alternative frequency of 13.305 Hz and an excitation amplitude of 500 pV. (d) Color scale plot of dIsd/dVsd vs. Vg and Vsd. All measurements were taken at 4.2 K. (e) Zoom-in on (d) at the region delimited by a black square. Peaks in dIsd/dVsd denoting the onset of tunneling via ground and excited states have been highlighted by dotted and dashed lines, respectively. (Adapted with permission from Songmuang et al. (2010b), © 2010 by the American Chemical Society.)
the distance between AlN double-barrier were designed to be around 6 nm, larger than the one required for the fabrication of GaN/AlN NW-RTDs.
Figure 9.15(b) shows two Isd — Vsd characteristics at 4.2 K and different gate voltages. The gate-dependent suppression of Isd on a 10-mV range around zero bias is due to the Coulomb-blockade effect. This is confirmed by the conductance oscillations and by the characteristic diamond-shape features in the color plot of dIsd/dVsd(Vg, Vsd) (Fig. 9.15(c)-(d)). The asymmetric shape of the Coulomb diamonds denotes a different capacitive coupling of the GaN island to the source and the drain leads, ascribed to the polarization-induced asymmetry of the conduction-band profile. The characteristic size of the Coulomb diamonds gives a charging energy Ec « 10 meV and a gate capacitance Cg « 0.1 aF, corresponding to an island size of a few nm, which matches well with the height of the GaN insertion between the AlN barriers.
The magnified view in Fig. 9.15(e) shows additional structures appearing as lines (i.e. dIsd/dVsd peaks) parallel to the diamond edges, ascribed to the onset of single-electron tunneling through some excited states in the GaN island. Their presence constitutes direct evidence of a discrete energy spectrum: the motion of electrons within the dot is quantized not only along the NW axis but also in the transverse plane due to the finite NW diameter. From the separation between the observed excited-state lines and the corresponding diamond edges, the level separation is in the range of 1 — 10 meV. Because the NW diameter is an order of magnitude larger than the GaN quantum-dot height, this energy distance should be associated to the size quantization in the transverse plane.