E.-K. Lee

Excitation-dependent photoluminescence in Ge∕Si Stranski-Krastanov nanostructures

In Ge∕Si Stranski-Krastanov nanostructures grown by chemical vapor deposition, the authors find ∼30meV/decade photoluminescence (PL) spectral shift toward greater photon energies as excitation intensity increases from 0.1to104W∕cm2. The PL lifetime exhibits strong spectral dependence, and it decreases from ∼20μs at 0.77eVto200ns at 0.89eV. The authros attribute the observed PL spectral shift and shorter PL lifetime at higher photon energies to an increasing contribution from recombination between holes populating excited Ge cluster energy states and electrons in SiGe alloy cluster regions.

Structural and optical properties of three-dimensional Si1−xGex/Si nanostructures

Steady-state and time-resolved photoluminescence (PL) combined with x-ray and Raman measurements have been performed on a series of well-characterized Si1−xGex/Si superlattice samples with an island-like morphology and with precise control over the alloy chemical composition in the range 0.091 ≤ x ≤ 0.61. In the samples with x increasing from 0.091 to 0.53, an increase in the intensity of the Raman signal related to Ge–Ge vibrations correlates with a red shift in the PL peak position and an increase in the activation energy of the PL thermal quenching. Time-resolved PL measurements reveal two PL components with relaxation times of a microsecond and up to 10 ms, respectively. The highest PL quantum efficiency observed (better than 1% at low temperature) is found in the samples with x ≈ 0.5 where carrier recombination presumably occurs at sharp Si/Si1−xGex interfaces which exhibit type-II band alignment, with a small (of the order of several milli-electron volts) barrier for electrons and deep potential wells for holes localized within Ge-rich Si1−xGex islands. In the samples with Ge concentration close to 0.61, we observe a strong, step-like increase in strain and significant evidence of strain-induced Si/Ge interdiffusion resulting in a decrease of the PL quantum efficiency.

Photoluminescence thermal quenching in three-dimensional multilayer Si/SiGe nanostructures

We find that in Si∕SiGe three-dimensional multilayer nanostructures, photoluminescence intensity as a function of temperature depends on the excitation intensity. The experimental results are explained using a model where electron-hole separation and nonradiative recombination is controlled by a competition between hole tunneling and thermally activated hole diffusion over the valence band energy barriers at Si∕SiGe heterointerfaces.

Three-Dimensional Silicon-Germanium Nanostructures for CMOS Compatible Light Emitters and Optical Interconnects

Three-dimensional SiGe nanostructures grown on Si (SiGe/Si) using molecular beam epitaxy or low-pressure chemical vapor deposition exhibit photoluminescence and electroluminescence in the important spectral range of 1.3–1.6 𝜇m. At a high level of photoexcitation or carrier injection, thermal quenching of the luminescence intensity is suppressed and the previously confirmed type-II energy band alignment at Si/SiGe cluster heterointerfaces no longer controls radiative carrier recombination. Instead, a recently proposed dynamic type-I energy band alignment is found to be responsible for the strong decrease in carrier radiative lifetime and further increase in the luminescence quantum efficiency.

Carrier transport in Ge nanowire/Si substrate heterojunctions

Low impedance and negligible conductivity temperature dependence are found for micron-long Ge nanowires (NWs) grown on (p+)Si substrates. In contrast, Ge NW/(n+)Si substrate samples exhibit many orders of magnitude higher impedance, an exponential dependence of conductivity on temperature, current instabilities, and negative differential photoconductivity. Our experimental results are explained by a model that considers energy-band alignment and carrier transport in abrupt Ge NW/Si substrate heterojunctions.