Fig. 2: Devices made using optical (OL) or electron-beam (EBL) lithography, displaying Coulomb-staircase behaviour (‘step curves’, StC).
The sweep direction is reversed when each device reaches a voltage where its staircase becomes unresolvable, so the voltage range never exceeds ± 1 V. The step width ΔV and height ΔI vary between and within devices. All sweeps show random telegraph noise (RTN) and resistances ~ MΩ. The selected sweeps are representative of the dataset. Devices show a high degree of stability over up to 100 sweeps. a I–V measurement showing a StC in an OL device with a ~5.4 μm2 area. The differential conductance at 0 V (G0) is 1.1 × 10−9 S/μm2 and the device shows distinct current plateaux in the range ∣V∣ < 1 V. This behaviour is stable: in > 20 sweeps only minor lateral shifts are observed. b, f A 4.6 μm2 EBL device with StC and a variety of ΔV across the trace and increasing ΔI with increasing V. The device has G0 = 4.1 × 10−10 S/μm2. f This junction’s smallest steps around the origin. The device is stable over >100 sweeps, except for some changes in the RTN height around − 0.6 V. c, g, k A 38100 nm2 EBL device showing a wide range of ΔI. The largest step height is 45 times bigger than the smallest. The device is stable for >50 sweeps and has G0 = 7.8 × 10−10 S/μm2. c Smaller step structures around the origin. k RTN at low voltage. d, h, l A 1.2 μm2 EBL device with G0 = 8.4 × 10−10 S/μm2. d Shows the finer step structures close to the origin that span one order of magnitude. Low-frequency RTN in (l) is likely a result of trapped charges in the QDs' surface states or electron excitations inside the QDs. e, i A 2800 nm2 OL device with ten steep steps in the ∣V∣ < 1 V range. The device has G0 = 4.7 × 10−10 S/μm2 and repeatable behaviour for >50 sweeps. i The first negative-bias step has a larger ΔI than the second negative step that follows as the voltage is made more negative. j A 12500 nm2 EBL device with a high G0 ~ 2.8 × 10−8 S/μm2. This device’s higher conductance combined with its smoother current steps suggests multiple QDs conducting in parallel, all contributing some current to each step.
