The height and current profiles of the marked small QDs of sample A and sample B are plotted in (c) and (f), respectively.
The height and current profiles of the marked small QDs of sample C and sample D are plotted in (c) and (f), respectively.
By comparing the results of the large and small QDs of sample A with those of sample B, it can be found that the average current values of BL QDs are larger than those of their corresponding SL QDs at all biases, for both large and small QDs (Figures 4(a) and 4(b) respectively).
The possible reason may be due to the differences of the QDs' size, where the sizes of both the large and small QDs of higher-density samples C/D are smaller than those of the lower-density samples A/B, respectively.
The statistical results of diameters, heights and densities of both large and small QDs of the four samples are listed in Figure1.
For example, the average current of large and small QDs of the BL sample is about 17 and 21 times of their corresponding SL ones at −2 V, respectively.
(a) Single layer/low dot-density sample A, (b) bilayer/low dot-density sample B, (c) single layer/high dot-density sample C, and (d) bilayer/high dot- density sample D. All samples have two types of QDs, and the average sizes of both large and small QDs of the low dot-density samples (A and B) are larger than those of the high dot-density samples (C and D), respectively.
By comparing the averaging currents of large QDs (sample A in Figure4(a) and sample C in Figure4(c)), as well as those of small QDs (sample A in Figure4(b) and sample C in Figure4(d)), the current increased ratios of high-density QDs (sample C) to low-density QDs (sample A) also increased with the bias voltage, which are about 6 and 4 times at −2 V for large and small QDs respectively.
A better comparison can be achieved in the height and current profiles of single typical small QDs for SL and BL samples as shown in Figures 3(c) and 3(f) respectively.
Figure 3 Topography and current images of samples C/D with height and current profiles of small QDs.
