K. S. Zhuravlev

Mechanism of photoluminescence of Si nanocrystals fabricated in a SiO2 matrix

The luminescence properties of silicon nanocrystals fabricated by Si ion implantation into a SiO2 matrix and subsequent thermal annealing have been studied. To identify the mechanism of photoluminescence of Si nanocrystals, the dependencies of the steady-state photoluminescence on temperature and excitation power density, and the time-resolved photoluminescence have been investigated. The experimental results point to the mechanism of recombination via the levels of centers which are presumably localized at the silicon nanocrystal–silicon dioxide boundary.

Coexistence of direct and indirect band structures in arrays of InAs∕AlAs quantum dots

We report studies of energy structure of InAs self-assembled quantum dots (QDs) embedded in AlAs matrix by stationary and transient photoluminescence and calculations. Calculation reveals that the QDs have band alignment structure of type I. Photoluminescence demonstrates low-energy and high-energy bands with drastically different decay time related to carries recombination in QDs of different sizes. The experimental results evidence a direct-indirect transition of the QD conduction band structure with decreasing their size.

Photoluminescence from cadmium sulfide nanoclusters formed in the matrix of a Langmuir-Blodgett film

Photoluminescence (PL) from CdS nanoclusters formed in the matrix of a Langmuir-Blodgett film and from the same clusters with the matrix removed has been studied. The PL spectrum of clusters in the matrix has the form of a broad band (full width at half-maximum (FWHM) ∼ 0.6 eV) peaked at 2.4 eV. After removing the matrix with hexane, the PL spectrum consists of a high-energy band at 2.9 eV (FWHM ∼ 0.2 eV) and two low-energy bands at 2.4 and 2.0 eV (FWHM ∼ 0.5 eV). The high-energy band is attributed to exciton recombination in the nanoclusters, and the bands at 2.4 and 2.0 eV, to recombination via levels related to defects in the bulk of the matrix and at the nanocluster-matrix interface, respectively.

The influence of irradiation and subsequent annealing on Si nanocrystals formed in SiO2 layers

Luminescent Si nanocrystals formed in SiO2 layers were irradiated with electrons and He+ ions with energies of 400 and 25–130 keV, respectively. The effects of irradiation and subsequent annealing at 600–1000°C were studied by the methods of photoluminescence and electron microscopy. After irradiation with low doses (∼1 displacement per nanocrystal), it was found that photoluminescence of nanocrystals was quenched but the number of them increased simultaneously. After irradiation with high doses (∼103 displacements per nanocrystal), amorphization was observed, which is not characteristic of bulk Si. The observed phenomena are explained in terms of the generation of point defects and their trapping by Si-SiO2 interfaces. Photoluminescence of nanocrystals is recovered at annealing temperatures below 800°C; however, an annealing temperature of about 1000°C is required to crystallize the precipitates. An enhancement of photoluminescence observed after annealing is explained by the fact that the intensities of photoluminescence originated from initial nanocrystals and from nanocrystals formed as a result irradiation are summed.

Changes in optical properties of CdS nanoclusters in langmuir-blodgett films on passivation in ammonia

The optical properties of CdS nanoclusters are studied for samples in which the nanoclusters are embedded in the Langmuir-Blodgett film matrix or the matrix is removed by annealing in vacuum or ammonia atmosphere. After annealing the samples in vacuum or ammonia, the bands of emission from the nanoclusters and from the surface states appear in the photoluminescence spectrum, with the peaks at 2.9 or 2.7 eV and at 1.9 or 2.1 eV, respectively. It is found that, after treating the samples with ammonia, the photoluminescence of the nanoclusters becomes more intense, whereas the photoluminescence corresponding to recombination via the levels of the surface states becomes less intense. It is established that the increase in the photoluminescence intensity for the nanoclusters and the difference between the temperature dependence of the photoluminescence peak position and the temperature dependence of the band gap of bulk CdS are due to passivation of surface states in the nanoclusters. The experimental data are interpreted in the model of recombination of nonequilibrium charge carriers in the CdS nanoclusters, taking into account the exchange of charge carriers between the nanocrystals and trapping centers at their surface and the recombination of charge carriers via the levels of the surface states.

Millisecond photoluminescence kinetics in a system of direct-bandgap InAs quantum dots in an AlAs matrix

Anomalously long millisecond kinetics of photoluminescence (PL) is observed at low temperatures (4.2–50 K) in direct-bandgap InAs quantum dots formed in an AlAs matrix. An increase in temperature leads to a decrease in the duration of PL decay down to several nanoseconds at 300 K, whereas the integral PL intensity remains constant up to 210 K. In order to explain the experimental results, a model is proposed that takes into account the singlet-triplet splitting of exciton levels in small quantum dots.

Photoluminescence of high-quality AlGaAs layers grown by molecular-beam epitaxy

We report a photoluminescence study of high-purity AlxGa1−xAs layers grown by molecular-beam epitaxy over the 0⩽x⩽0.295 composition range. The intense excitonic line dominates in the photoluminescence spectra of the layers. The full width at half maximum of the excitonic line is in excellent agreement with values calculated by Lee and Bajaj [J. Appl. Phys. 73, 1788 (1993)] for perfectly random alloys, and in the spectra of the layers with AlAs fractions of x=0.15 and x=0.209 it equals to 1.24 and 1.48 meV, respectively. A linear dependence of the exciton line intensity on excitation power evidences negligible concentration of nonradiative recombination centers in the layers.

Prospects for the development of high-power field-effect transistors based on heterostructures with donor-acceptor doping

We report the first results on the development of high-power field-effect transistors on gallium-arsenide heterosrtuctures with a quantum well and additional potential barriers, formed from layers with different doping types, optimized to reduce transverse spatial electron transport and enhance quantum confinement. The transistors yield a doubled output power at a trapezoidal gate length of 0.4–0.5 μm and a total gate width of 0.8 mm at a frequency of 10 GHz in the continuous mode of operation. The gain exceeds 9.5 dB at a specific output power above 1.6 W/mm and a power-added efficiency of up to 50%. Prospects for the development of such devices are presented.

Recombination of self-trapped excitons in silicon nanocrystals grown in silicon oxide

The kinetics of photoluminescence (PL) and steady-state PL from silicon nanocrystals formed in the SiO2 matrix by silicon ion implantation were studied experimentally for the first time in the temperature range from liquid-helium to room temperature. A dramatic increase in the photoluminescence decay time, accompanied by PL intensity quenching, is observed below 70 K. The results obtained indicate that the silicon nanocrystal PL arises from radiative recombination of excitons self-trapped at the silicon nanocrystal-SiO2 interface.

Temperature dependence of photoluminescence of CdS nanoclusters formed in the Langmuir-Blodgett film matrix

The photoluminescence of the CdS nanoclusters formed in the matrix of a Langmuir-Blodgett film is studied in the temperature range 5–300 K. At the temperature 5 K, the photoluminescence spectrum of the nanocrystals consists of two bands, with peaks at 2.95 and 2.30 eV. The temperature dependence of the position of the high-energy photoluminescence band differs from the temperature dependence of the band gap of the CdS bulk crystal. The integrated photoluminescence intensity of this band decreases as temperature increases in the range from 5 to 75 K, increases in the range from 150 to 230 K, and decreases above 230 K. The experimental data are interpreted in the context of the model of recombination of nonequilibrium charge carriers in CdS nanoclusters with regard to the charge-carrier transport in locally coupled clusters different in size. In the model, the energy depth of the traps for electrons is estimated at 120 meV; the estimations of the activation energies of nonradiative recombination yield 5 and 100 meV.