Photoluminescence from ultrathin Ge-rich multiple quantum wells observed up to room temperature: Experiments and modeling

Employing a low-temperature growth mode, we fabricated ultrathin $\mathrm{S}{\mathrm{i}}_{1-x}\mathrm{G}{\mathrm{e}}_x$/Si multiple quantum well structures with a well thickness of less than 1.5 nm and a Ge concentration above 60% directly on a Si substrate. We identified an unusual temperature-dependent blueshift of the photoluminescence (PL) and exceptionally low thermal quenching. We find that this behavior is related to the relative intensities of the no-phonon (NP) peak and a phonon-assisted replica that are the main contributors to the total PL signal. To investigate these aspects in more detail, we developed a strategy to calculate the PL spectrum employing a self-consistent multivalley effective mass model, in combination with second-order perturbation theory. Through our investigation, we find that while the phonon-assisted feature decreases with temperature, the NP feature shows a strong increase in the recombination rate. Besides leading to the observed robustness against thermal quenching, this causes the observed blueshift of the total PL signal.

Figure 1

Schematic of SiGe/Si multiple-QW sample stack sequence.

Figure 2

(a) BF-TEM and (b) HR-TEM cross-section images of the Ge multiple-QW structure. (c) Overlay of c-Si (red) and Ge multiple-QW sample (black) Raman signal. (d) XRD ω-2θ scan along the (004) direction of the Ge multiple-QW sample (black) and fit with a multiple square well model (red).

Figure 3

(a) Experimentally observed PL spectra at $T_L=80\mathrm{K}$ for different excitation densities. (b) Measured energy separation between phonon-assisted and NP peaks as a function of excitation density; in the inset, the deconvolution of the PL signal taken at 80 K and $1.60\mathrm{MWc}{\mathrm{m}}^{-2}$ with two Gaussian peaks is shown. (c) Experimentally measured integrated PL intensity for the phonon-assisted peak as a function of the excitation power density.

Figure 4

(a) PL peak energy of the phonon-assisted and NP line according to theory and experiment as a function of the excitation density. (b) Comparison of experimental and simulated phonon-assisted PL at $T_L=80\mathrm{K}$ for different pump powers.

Figure 5

Near-gap subband states calculated for $T_L=80\mathrm{K}$ at low (a) and high (b) optical excitation densities. Confinement energies in the valence band are unchanged, while at high excitation density, the conduction subband states are found at higher energies due to larger band bending.

Figure 6

(a) Experimental PL spectra at a $0.10\mathrm{Wc}{\mathrm{m}}^{-2}$ excitation density for different lattice temperatures. (b) Comparison between phonon-assisted peaks extracted from experiment and theory for different lattice temperatures.

Figure 7

(a) PL peak energies as a function of $T_L$ for theory and experiment. (b) Experimental integrated PL signal as a function of $T_L$. (c) Measured energy separation of phonon-assisted and NP peaks as a function of $T_L$; in the inset, the deconvolution of the PL signal taken at 100 K and $0.1\mathrm{MW}\mathrm{c}{\mathrm{m}}^{-2}$ with two Gaussian peaks is shown.