Growth-temperature-dependent band alignment in $\mathrm{Si}∕\mathrm{Ge}$ quantum dots from photoluminescence spectroscopy

The present work is a photoluminescence study of Si-embedded Stranski-Krastanov Ge quantum dots. The value of the conduction band offset is a result of the magnitude of the tensile strain in the Si surrounding the compressive strained Ge dot. Due to the increased $\mathrm{Si}∕\mathrm{Ge}$ intermixing and reduced strain in the Si barrier, a reduction of the conduction band offset is observed at increased growth temperatures. The optical properties as derived from photoluminescence spectroscopy are correlated with structural properties obtained as a function of the growth temperature. High growth temperatures result in large Ge dots with low density due to the pronounced surface diffusion and $\mathrm{Si}∕\mathrm{Ge}$ intermixing. As confirmed by photoluminescence, the band gap of the Ge dots increases with increased growth temperature due to the higher degree of $\mathrm{Si}∕\mathrm{Ge}$ intermixing. The band alignment is of type II in these structures, but the occurrence of both spatially indirect and spatially direct transitions are confirmed in temperature-dependent photoluminescence measurements with varied excitation power conditions. An increasing temperature results in a gradual transition from the spatially indirect to the spatially direct recombination in the type-II band lineup, due to higher oscillator strength for the spatially direct transition combined with a higher population factor at higher temperatures.

Figure 1

Schematic picture of the type-II energy band lineup in the $\mathrm{Si}∕\mathrm{Ge}$ QD system along the growth direction. (a) Two different recombination processes are illustrated, one spatially indirect $\left(E_A\right)$ across the $\mathrm{Si}∕\mathrm{Ge}$ interface and one spatially direct $\left(E_B\right)$ inside the Ge dot. (b) The influence on the energy bands at two different carrier concentrations is demonstrated. A high carrier concentration will bend the energy bands which results in a blueshift of the spatially indirect transition, i.e., $E_{\mathrm{LD}}<E_{\mathrm{HD}}$. The energy of the spatially direct transition is practically unaffected by the increased carrier density.

Figure 2

AFM images of Ge dots on Si for samples grown at (a) $480°\mathrm{C}$, (b) $580°\mathrm{C}$, and (c) $730°\mathrm{C}$. Note the differences in scale for (a), (b), and (c)

Figure 3

PL spectra of Ge QD samples grown at different temperatures. The no-phonon (NP) and TO-phonon related emission of the WL is indicated in the figure. Si related peaks are observed at approximately $1.14\mathrm{eV}$ (TA phonon), $1.10\mathrm{eV}$ (TO phonon), and $1.04\mathrm{eV}$ ($\mathrm{TO}+\Gamma$ phonon). All spectra are measured at $10\mathrm{K}$ with an excitation power of $20\mathrm{mW}$. The spectra are shifted for clarity reasons, and the growth temperatures are indicated in the figure.

Figure 4

Temperature dependence of the Ge QD PL spectra of samples grown at two different temperatures; (a) $480°\mathrm{C}$ and (b) $630°\mathrm{C}$, respectively. The sample temperature for each spectrum is indicated in the figures. The excitation power was set to $20\mathrm{mW}$.

Figure 5

Excitation power dependent PL of Ge dots grown at $700°\mathrm{C}$, at a sample temperature of (a) $2\mathrm{K}$ and (b) $30\mathrm{K}$. The excitation power was varied between 3 and $100\mathrm{mW}$ as indicated in the figure.

Figure 6

Temperature dependent PL of Ge dots grown at $700°\mathrm{C}$. The sample temperatures are specified in the figure and the excitation power was kept constant at $10\mathrm{mW}$.

Figure 7

(a) Peak energy positions for the spatially indirect and direct transitions as functions of the growth temperature. (b) The energy separation between the spatially indirect and direct transitions is shown as a function of the growth temperature.