Self Assembled PbSe Quantum Dots with Almost Equal Sizes Grown by MBE on PbTe/Si(111)
- PDF / 675,565 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 18 Downloads / 166 Views
E5.5.1
Self Assembled PbSe Quantum Dots with Almost Equal Sizes Grown by MBE on PbTe/Si(111) Karim Alchalabi, Dmitri Zimin, Hans Zogg Thin Film Physics Group, Swiss Federal Institute of Technology, CH-8005 Zurich, Switzerland [email protected] http://www.tfp.ethz.ch phone +41 1 445 14 80, fax +41 1 445 1499 ABSTRACT We report on PbSe QDs grown on a few µm thick PbTe(111) layers on Si(111). The QDs form as pyramids with three (100) side faces and (111) base. Their sizes are almost equal, the standard deviation of their heights can be as low as 2-3%. This seems to be the narrowest distribution ever observed for self assembled QDs. Similar QDs of PbSe have first been described by Pinczolits et al. (Appl. Phys. Lett. 73, 250, 1998). These authors used cleaved BaF as a substrate, they observed two types of dots with somewhat wider size distributions. The results are compared and explained on the basis of nucleation and growth theories. 2
INTRODUCTION Self assembled quantum dots (QD) like SiGe on Si or InAs on GaAs normally exhibit a quite broad distribution of sizes (20-50%) and may undergo shape transitions during growth. For many applications, self assembled dots having all nearly the same shape and size would be advantageous. For the narrow gap lead chalcogenide system, PbSe QD on PbTe(111) quasi-substrates exhibit a pyramidal shape with (100) type side faces and a (111) base. The dots grow in the Stranski-Krastanov mode. Such dots, where the PbTe quasi substrate has been grown on (cleaved) bulk BaF (111), have first been described by Pinczolits et al. [1]. The authors observed a “remarkably narrow” size-distribution with a width of 7%-17%. In the following, we will analyse similar structures, but where the PbTe quasi-substrate is grown on a Si(111) wafer. We found still remarkably lower spread in the sizes, as low as 23% [2], to our best knowledge lower than any size distribution observed for any self assembled QDs. In the following, we will briefly review the experiments, and give an explanation for the findings. 2
EXPERIMENTAL The layers are grown by MBE on Si(111) substrates. First, a very thin CaF layer is grown for compatibility, followed by about 5 µm PbTe to form the substrate. PbSe dots are obtained on these substrates at a slightly higher growth temperature by depositing about 3 ML from a single source containing PbSe (Fig. 1). The layers are highly lattice mismatched. In addition, the thermal expansions with respect to Si are much higher (see Tab. 1). The PbTe quasi substrate layer is fully relaxed [3], while the PbSe-dots are believed to be completely strained, i.e. not to contain dislocations [1]. A general review on MBE growth and properties of lead-chalcogenides may be found in ref. [4]. 2
E5.5.2
PbSe QDs
PbTe 5 µm CaF 20 nm 2
Si(111) Fig. 1. Schematic view of PbSe quantum dots on PbTe quasi-substrates on CaF2 covered Si(111).
PbSe PbTe CaF Si
2
-6
a: 6.12 Å 6.46 Å
α: 19.4·10 /K 19.8·10 /K
5.46 Å 5.43 Å
19.1·10 /K 2.6·10 /K
-6
-6 -6
Table 1. Lattice constants a and linear thermal expan
Data Loading...
