InAs quantum dots on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$: STM and photoluminescence studies

The $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ surface was prepared by molecular beam epitaxy and analyzed in situ by scanning tunneling microscopy (STM) and low-energy electron diffraction. Atomically resolved STM images of $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ revealed a $1\times 1$ reconstruction, terminated by Ga dimers. The deposition of 1.5 ML of InAs onto $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ resulted in the two- to three-dimensional transition with appearance of small InAs quantum dots (QD’s) with a very narrow size distribution and a high number density. Low-index $\left(0\overline{1}\overline{1}\right)$, $\left(\overline{1}0\overline{1}\right)$, and $\left(\overline{1}\overline{1}\overline{1}\right)B$ facets, a rounded vicinal $\left(00\overline{1}\right)$ region for the main part, and a high-index $\left(\overline{1}\overline{3}\overline{5}\right)B$ surface for a flat base determine a shape of the QD’s that is totally unsymmetrical. Ex situ–performed photoluminescence measurements revealed a peak of the InAs QD’s on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ with a similar intensity to the peak from the InAs QD’s on the reference GaAs(001) surface, but with a higher emission energy and a smaller linewidth, indicating an ensemble of QD’s, smaller and more uniform in size. A small redshift (from $1.33\mathrm{eV}\text{to}1.20\mathrm{eV}$) of the emission energy was achieved by optimizing the preparation parameters.

Figure 1

Stereographic triangle for the GaAs $B$ surfaces. The position of the $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ is marked with a cross.

Figure 2

(a) LEED image of the $\mathrm{Ga}\mathrm{As}\left(2511\right)A$ surface $(E=59\mathrm{eV})$ with a schematic sketch of the surface net in reciprocal space (left-hand side) and in real space (right-hand side); (b) LEED image of the $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ surface $(E=56\mathrm{eV})$ with a schematic sketch of the surface net in real space (left-hand side) and in reciprocal space (right-hand side). The vertical line between figures (a) and (b) indicates a plane running perpendicular to the $\mathrm{Ga}\mathrm{As}\left(2511\right)A$ or $B$, around which the mirror operation has been carried out.

Figure 3

High-resolution STM image of the $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ surface. The solid markers at the image border indicate the trenches between the dimer stripes. The unit meshes $A$ and $B$ are depicted as black parallelograms. $(120\times 120){\mathrm{\AA }}^2$, $U=-2.55\mathrm{V}$, and $I=0.1\mathrm{nA}$.

Figure 4

Top view of a ball-and-stick model for the $1\times 1$ reconstructed $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ surface. The size of the circles representing the atoms was chosen according to their vertical distance from the uppermost atom. To arrive at the reconstructed surface, the lighter shaded Ga atoms have been added to the bulk-truncated surface. The numbers near the As and Ga atoms show which atomic layer they belong to, counting from the top. Within unit mesh $A$ DB’s are included, the As atoms are depicted with black (solid) DB’s, the Ga atoms with white (open) DB’s.

Figure 5

Overview STM images from two different locations on the nominal $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ surface: (a) $(1\times 1)\mu {\mathrm{m}}^2$, $U=-3.0\mathrm{V}$, $I=0.136\mathrm{nA}$, the off orientation is derived to 0.25° and (b) $5000\times 5000{\mathrm{\AA }}^2$, $U=-2.5\mathrm{V}$, $I=0.11\mathrm{nA}$, the off orientation is derived to 0.25°.

Figure 6

(a) Overview STM image with InAs QD’s grown on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$. The white arrow indicates a large incoherent island[$(5000\times 5000){\mathrm{\AA }}^2$, $U=-3.5\mathrm{V}$, $I=0.4\mathrm{nA}$, the sample temperature $T=430°\mathrm{C}$, InAs thickness is $4.5\pm 0.4\mathrm{\AA }\left(1.5\mathrm{ML}\right)$]. (b) Size distribution derived from 1560 islands in percent (the % values are given above the respective columns).

Figure 7

(a) STM image with InAs QD’s grown on a $\left(\overline{2}\overline{5}\overline{11}\right)B$ wetting layer ($U=-3.0\mathrm{V}$, $I=0.1\mathrm{nA}$, $T=430°\mathrm{C}$, 1.5 ML). (b) Atomically resolved STM image (error $\text{signal}\equiv \text{constant}$ height mode) of InAs QD’s on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ ($U=-3.0\mathrm{V}$, $I=0.3\mathrm{nA}$, $T=450°\mathrm{C}$).

Figure 8

Atomically resolved STM images (error $\text{signal}\equiv \text{constant}$ height mode, $U=-3.0\mathrm{V}$, $I=0.1\mathrm{nA}$) of (a) the $\left(0\overline{1}\overline{1}\right)$ facet [the size is $(47\times 47){\mathrm{\AA }}^2$], (b) the $\left(\overline{1}0\overline{1}\right)$ facet [the size is $(34\times 34){\mathrm{\AA }}^2$], and (c) the $\left(\overline{1}\overline{1}\overline{1}\right)B\text{−}(2\times 2)$ facet [the size is $(78\times 78){\mathrm{\AA }}^2$], and (d) the $\left(\overline{1}\overline{3}\overline{5}\right)B\text{−}c(2\times 2)$ facet [the size is $(48\times 48){\mathrm{\AA }}^2$]. Letters $m$ and $n$ mark unit cell vectors. (d) Schematic model of InAs QD’s on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$. Darker grey scales correspond to steeper facets.

Figure 9

Normalized linear scale PL spectra of InAs QD’s grown on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ ($T=430°\mathrm{C}$, 1.5 ML) acquired with different excitation densities. Excitation density was 5 and $5000\mathrm{W}∕{\mathrm{cm}}^2$ for solid and dotted lines, respectively. Peaks related to the QD’s, the WL, and the GaAs near-band-edge emission are marked.

Figure 10

Decimal logarithmic scale PL spectra of single InAs QD layer (solid line) and $5\times \mathrm{QD}$ stacking (dotted line) grown on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ ($T=430°\mathrm{C}$, 1.5 ML) acquired with low $\left(50\mathrm{W}{\mathrm{cm}}^{-2}\right)$ (a) and high $\left(5000\mathrm{W}{\mathrm{cm}}^{-2}\right)$ (b) excitation density.

Figure 11

Linear scale PL spectra of the InAs QDs grown on different GaAs substrates. The QD’s and GaAs bulk-related peaks are labeled. Excitation density was $5\mathrm{W}{\mathrm{cm}}^{-2}$.

Figure 12

Linear scale PL spectra of the InAs QD’s grown on $\mathrm{Ga}\mathrm{As}\left(\overline{2}\overline{5}\overline{11}\right)B$ with modified growth conditions. The solid, dotted, and dash-dotted lines correspond to the InAs thickness of 1.5 ML $(T=430°\mathrm{C})$, 2.0 ML $(T=500°\mathrm{C})$, and 3.0 ML $(500°\mathrm{C})$, respectively. The QD’s and GaAs bulk-related peaks are labeled. Excitation density was $50\mathrm{W}{\mathrm{cm}}^{-2}$.