B. V. Kamenev

Photoluminescence and Raman scattering in three-dimensional Si/Si1−xGex nanostructures

We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.

Polarized Raman scattering and localized embedded strain in self-organized Si/Ge nanostructures

Using polarized Raman spectroscopy, we examine different vibrational modes (i.e., Si–Si, Si–Ge, and Ge–Ge) in Si/Ge self-organized nanostructures. Here, we present unambiguous proof that multilayers of Ge nanometer-size, “dome-shaped” islands grown on a 〈100〉 Si substrate are nearly fully relaxed and that the built-in strain field is substantially localized in the surrounding Si matrix. In contrast, multilayers with “pyramid-shaped” islands do not show observable relaxation. The large strain in the Si layers of the multilayer dome samples correlates with the greater self-organization in these structures compared to the multilayer pyramid samples.

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.

Laser-induced structural modifications in nanocrystalline silicon/amorphous silicon dioxide superlattices

We calculate and experimentally detect the laser melting threshold in nanocrystalline Si/amorphous SiO2 superlattices. Using laser energy density slightly above the melting threshold, we observe two types of laser-induced structural modifications: (i) disappearance of nanocrystalline Si phase in the samples with thin (∼2nm) SiO2 layers and (ii) amorphization of Si nanocrystals in the samples with thicker (⩾5nm) SiO2 layers. The observed Si nanocrystal amorphization increases optical absorption and intensity of visible photoluminescence in nanocrystalline Si/amorphous SiO2 superlattices.

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.

Strain-induced lateral self-organization in Si/SiO2 nanostructures

We show that strain, arising from the mismatch between Si and SiO2 thermal expansion coefficients, directs the thermal crystallization of amorphous Si along Si/SiO2 interfaces, and produces continuous, fully crystallized nanometer thick Si layers with a lateral-to-vertical aspect ratio close to 100:1. These Si nanolayers exhibit a low density of structural defects and are found to be elastically strained with respect to the crystal Si substrate.

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.

Optical Properties of Composition-Controlled Three-Dimensional Si/Si

We report the Raman, continuous-wave (CW), and time-resolved photoluminescence (PL) measurements in a series of multilayer Si/Si1-xGex samples with an island-like morphology and precise control over the chemical composition in the range of 0.096 les x les 0.61. In the samples with x continuously increasing from 0.096 to 0.55, an increase in the intensity of the Raman signal related to the 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 1-10-ms PL components. The highest observed PL quantum efficiency (better than 1% at low temperature) is found in the samples with x~0.5, where the carrier recombination presumably occurs at sharp Si/SiGe interfaces that exhibit type-II band alignment, with a small (to the order of several milli-electron volts) barrier for electrons and deep potential wells for the holes localized within the Ge-rich Si1-xGex islands. In the samples with Ge concentration close to 0.61, we observe a strong, step-like increase in the strain, and a significant evidence of strain-induced SiGe interdiffusion that results in the decrease in the PL quantum efficiency

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Reliable fabrication of high-speed, delta-doped transistors and better understanding of two-dimensional metal-insulator transitions can be achieved using silicon molecular beam epitaxy (MBE). However, this fabrication technique should be performed with care, avoiding dopant segregation on epitaxial Si surfaces and improving the doping efficiency. Here we report comprehensive structural and optical investigations of MBE-grown Si/delta-doped Si:B multilayer structures. Measurements of Auger electron spectroscopy, Raman scattering, optical reflection and photoluminescence are performed. Our results indicate nearly metallic conductivity at room temperature with metal-insulator phase transition near T ∼100 K. In contrast to recently reported data, no enhancement of photoluminescence at room temperature is found. Occasionally, a few samples in specific areas exhibit strong photoluminescence at 1.4-1.6 micron attributed to structural defects, most likely due to B segregation.