Optical properties of epitaxially grown submonolayer $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures

The optical properties of epitaxially grown $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures with nominal CdSe coverage between 0.15 monolayer (ML) and 0.58 ML are investigated by photoluminescence (PL) and PL excitation spectroscopy at low and high excitation density, time-resolved PL, temperature dependent linewidth analysis, and two-photon excitation spectroscopy. We discuss exciton-phonon complexes in a potential and analyze their dominant dimensionality. The exciton-polaron picture, which is mostly applied to the extreme case of very strong exciton-phonon coupling, is introduced here as a well-suited description of the optical properties of ultra-small semiconductor nanostructures with relaxed quantum confinement.

Figure 1

Normalized PL (photoluminescence) spectra of sub-ML $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures at $4\mathrm{K}$. Shadowed curves relate to the samples grown on (001)-oriented GaAs. The spectra of 4° and 10° misorientation of the substrate are shown for 0.58 and 0.15 ML CdSe coverages, respectively. Vertical arrows denote the spectral positions of the ZnSe bandgap $\left(E_g\right)$ and the ZnSe free exciton (FX). The length of the horizontal arrows corresponds to the energy $\left(E_{\mathrm{LO}}\right)$ of the ZnSe LO-phonon. The inset shows the structure of the samples.

Figure 2

Normalized photoluminescence (a) and (b) and PL excitation (c) and (d) spectra of 0.15 ML (a) and (c) and 0.58 ML [lower parts (b) and (d)] $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures grown on (001)-oriented GaAs substrates. The length of horizontal arrows (on the left) shows the energy of ZnSe LO-phonon $E_{\mathrm{LO}}$. HH and LH mark the heavy- and the light-hole components of the free ZnSe exciton FX. Vertical arrows (on the right) are set at photon energies which exceed the emission peak energy by an integer multiple of $E_{\mathrm{LO}}$.

Figure 3

Luminescence decay kinetics of sub-ML $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures. (a) Time scale covers a $10\mathrm{ns}$ time range. (b) Time scale for the first $500\mathrm{ps}$. Closed symbols: 0.15 ML; open symbols: 0.58 ML. The solid line in (b) shows the response function of the detection system with respect to the laser pulse. The data were obtained after excitation with $10\mathrm{ps}$-laser pulses of an attenuated second harmonic from a Ti:sapphire laser and with the detection by a Hamamatsu streak-camera (time resolution of $\sim 30\mathrm{ps}$). Taking into account the $80\mathrm{MHz}$ repetition rate of the excitation pulses, the accumulation effect due to longer life time components is obvious for the 0.58 ML samples, while the luminescence of the 0.15 ML structures is completely decayed before arrival of the next laser pulse.

Figure 4

Spectral broadening (a) and shift (b) of the photoluminescence line of the 0.15 ML $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructure grown on (001)-oriented GaAs substrate (solid squares) and of the 0.58 ML $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ sample with 4° misoriented substrate (open squares). Low-temperature offsets in (a) are $\mathrm{FWHM}=1.38\mathrm{meV}$ for 0.15 ML and $6.4\mathrm{meV}$ for 0.58 ML, with peak maxima at $2.7944\mathrm{eV}$ for 0.15 ML and at $2.7641\mathrm{eV}$ for 0.58 ML. The solid line in (a) shows the fit of the FWHM with Eq. (1) using the acoustic $\left(a\right)$ and optical $\left(b\right)$ phonon coupling coefficients known from bulk ZnSe: $a=4\mu \mathrm{eV}∕\mathrm{K}$, $b=65\mathrm{meV}$. The dotted line has been added as an example for a fit with a completely different set of fit parameters: $a=10\mu \mathrm{eV}∕\mathrm{K}$, $b=25\mathrm{meV}$ (for discussion see text).

Figure 5

PLE spectra measured at temperatures of 3.5, 20, 50, and $60\mathrm{K}$. The sample grown on (001)-oriented GaAs substrate contains CdSe inclusions of nominal thickness of 0.15 ML. The detection energy is adjusted to follow the temperature-dependent shift of the maximum of the luminescence peak. The spectra are shifted vertically for clarity and the corresponding zeroth levels are visualized with horizontal lines. The numbers in parentheses show the factors of magnification.

Figure 6

Two-photon PLE spectra of $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures grown on (001)-GaAs. Nominal CdSe coverage is 0.15 ML (solid squares) and 0.58 ML (open squares). The vertical scale is linear in (a) and logarithmic in (b). The position of the ZnSe band gap and the PL peak of the 0.15 ML sample are marked by arrows. The excitation source is a mode-locked Ti:sapphire laser with $3\mathrm{ps}$ pulse width, $80\mathrm{MHz}$ repetition rate, and $60\mathrm{mJ}∕\mathrm{s}$ average power.

Figure 7

Upper part: High-excitation PLE spectrum of the 0.15 ML $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructure [(001)-GaAs substrate] recorded by using a $10\mathrm{ns}$-pulsed Coumarin 440 dye laser with $10\mathrm{Hz}$ repetition rate and $3\mu \mathrm{J}∕\text{pulse}$ energy as the excitation source. Vertical arrows mark the energies expected for the first two LO-phonon satellites. Lower part: PL spectra of the same sample for different excitation levels from top to bottom: $30\mathrm{mW}$ $(0.36\mathrm{nJ}∕\text{pulse})$, $3\mathrm{mW}$, $30\mu \mathrm{W}$, and $3\mu \mathrm{W}$ of the second harmonic of a mode-locked Ti:sapphire laser (constant excitation spot of about $0.1{\mathrm{mm}}^2$). The pump pulse photon energy of $2.8735\mathrm{eV}$ is close to the energy of the $+2.5\mathrm{LO}$ minimum in the low-intensity PLE spectrum of Fig. 2c Raman peaks are denoted with “R.”

Figure 8

(a) Emission intensity divided by excitation intensity and plot versus excitation intensity for the luminescence of excitons in a shallow potential of 0.15 ML CdSe inclusions. Solid triangles correspond to the excitation energy $2.8564\mathrm{eV}$ close to the $+2\mathrm{LO}$ maximum and open triangles to $2.8735\mathrm{eV}$ in the vicinity of the $+2.5\mathrm{LO}$ minimum (as in Fig. 7) of PLE spectrum shown in Fig. 2c. (b) Ratio of the PL intensity measured with excitation energies fixed at either $+2\mathrm{LO}$ or $+2.5\mathrm{LO}$ calculated from the data sets shown in (a).

Figure 9

Spectral profiles of the principal (or zero-phonon line: dotted) and its first ($-1\mathrm{LO}$: thick line) and second ($-2\mathrm{LO}$: thin line) LO satellites in the luminescence of 0.15 ML (a) and 0.58 ML (b) $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures grown on (001)-oriented GaAs wafer. The phonon side bands are scaled up by 1000 (a) and by 100 [$-1\mathrm{LO}$ in (b)] and by 800 [$-2\mathrm{LO}$ in (b)] in order to optimize the visibility. The energy “zero” corresponds to the maximum of the main PL line. The satellites are moved toward the “zero” by a corresponding number of $E_{\mathrm{LO}}$ derived from Raman replicae of the excitation source, a He-Cd laser operated at $441.6\mathrm{nm}$.