D. N. Lobanov

Condensation of excitons and the spectrum of multiparticle states in SiGe/Si quantum wells: The role of the barrier in the conduction band

It has been demonstrated that the barrier in the conduction band represented by the SiGe layer in SiGe/Si quantum wells affects the work function and equilibrium density of the quasi-two-dimensional condensed phase formed in these structures. The existence of a new recombination channel with unconventional characteristics is uncovered in the structures with barrier heights close to the critical value for the formation of the electron-hole liquid.

Raman Spectroscopy and Electroreflectance Studies of Self-Assembled SiGe Nanoislands Grown at Various Temperatures

SiGe nanoislands grown in a silicon matrix at temperatures of 300 to 600°C are studied using Raman spectroscopy and electroreflectance. For islands grown at relatively low temperatures (300–500°C), phonon bands are observed to have a doublet structure. It is shown that changes in the percentage composition, size, and shape of nanoislands and, hence, in the elastic stresses (depending on the growth temperature of the structures) have a significant effect on the energies of optical electronic interband transitions in the islands. As a consequence, the resonance conditions for Raman scattering also change. It is found that interdiffusion from the silicon substrate and the cover layer (determining the mixed composition of SiGe islands) is of importance even at low growth temperatures of nanostructures (300–400°C).

Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si1 − xGex/Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p0 ≈ 8.5 × 1011 cm−2 and n0 ≈ 4.8 × 1018 cm−3, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − xGex/Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si1 − xGex/Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si1 − xGex layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T+ and biexcitons, in the exciton gas.

Low-energy photoluminescence of structures with GeSi/Si(001) self-assembled nanoislands

The photoluminescence spectra of structures with self-assembled GeSi/Si(001) islands are investigated as functions of the growth temperature. It is shown that the shift of the peak of photoluminescence from islands toward lower energies on decreasing the growth temperature is due to the suppression of Si diffusion into islands and an increase in the fraction of Ge in islands. A photoluminescence signal from the GeSi islands is found in the region of energies down to 0.6 eV, which is considerably smaller than the band-gap width in bulk Ge. The position of the peak of photoluminescence from islands is described well by the model of a real-space indirect optical transition with account of the real composition and elastic strains of the islands. Mono-and multilayer structures are obtained with self-assembled GeSi/Si(001) nanoislands exhibiting a photoluminescence signal in the region 1.3–2 μm at room temperature.

The elastic strain and composition of self-assembled GeSi islands on Si(001)

The paper presents the results of investigation of self-assembled GeSi islands growth on Si(001) at 700°C and the evolution of Ge islands parameters during annealing. The islands with the narrow (standerd deviation ∼6%) lateral size and height distributions were grown. Dissolution of Si in islands was revealed from the Raman scattering and the X-ray diffraction measurements. Both the alloy composition and the elastic strain in the islands were determined. It was found that the content of Si in islands increased during annealing. This increase was shown to result in changes of the shape and sizes of islands.

TOF-SIMS 5 instrument sensitivity to matrix elements in GeSi Layers: Analysis based on recording of complex secondary ions

GexSi1 − x layers are investigated by means of secondary ion mass spectrometry (SIMS). Experimental results obtained with the use of a TOF-SIMS 5 instrument are presented. To surmount the so-called matrix effect, SIMS analysis is performed by using complex secondary ions: Ge2−, CsGe+, and Cs2Ge+.

Secondary cluster ions Ge2− and Ge3−for improving depth resolution of SIMS depth profiling of GeSi/Si heterostructures

New possibilities of improving depth resolution of SIMS depth profiling of GeSi/Si heterostructures using a TOF.SIMS-5 spectrometer are discussed. Contributions of ion sputtering artifacts and instrumental effects to depth resolution were analyzed in detail using a Talysurf CCI-2000 optical profilometer to control the shape and roughnesses of the sputtering crater bottom. It was found that the use of Cs+ ions for sputtering makes it possible to minimize roughness development during depth profiling of GeSi/Si structures to depths of 1–1.5 μm. It was shown that the use of secondary cluster ions Ge2− and Ge3− instead of Ge1− and Ge+ allows narrowing the transition regions in measured profiles.

Effect of parameters of Ge(Si)/Si(001) self-assembled islands on their electroluminescence at room temperature

The electroluminescence (EL) of multilayered p-i-n structures with the self-assembled Ge(Si)/Si(001) islands are investigated. It is found that the structures with islands grown at 600°C have the highest intensity of the electroluminescence signal at room temperature in the wavelength range of 1.3–1.55 μm. The annealing of structures with the Ge(Si) islands leads to an increase in the EL-signal intensity at low temperatures and hampers the temperature stability of this signal, which is related to the additional Si diffusion into islands during annealing. The found considerable increase in the electroluminescence-signal intensity with the thickness of the separating Si layer is associated with a decrease in the elastic stresses in the structure with an increase in this layer’s thickness. The highest EL quantum efficiency in the wavelength range of 1.3–1.55 μm obtained in investigated structures amounted to 0.01% at room temperature.

Comparative analysis of photoluminescence and electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) island

Comparative studies of the photoluminescence and electroluminescence of multilayer structures with self-assembled Ge(Si)/Si(001) islands are carried out. The luminescence signal from the islands is observable up to room temperature. Annealing of the structures induces a shift of the luminescence peak to shorter wavelengths. The shift is temperature dependent, making possible controllable variations in the spectral position of the luminescence peak of the Ge(Si) islands in the range from 1.3 to 1.55 μm. The enhancement of the temperature quenching of photoluminescence of the islands with increasing annealing temperature is attributed to the decrease in the Ge content in the islands during annealing and, as a result, to a decrease in the depth of the potential well for holes in the islands. The well-pronounced suppression of the temperature quenching of electroluminescence of the Ge(Si) islands in the unannealed structure with increasing pumping current is demonstrated.