A. Babiński

Rapid thermal annealing of InAs/GaAs quantum dots under a GaAs proximity cap

The effect of postgrowth rapid thermal annealing (RTA) on GaAs proximity-capped structures with self-assembled InAs/GaAs quantum dots (QDs) is investigated using transmission electron microscopy (TEM) and photoluminescence (PL). As can be seen from the TEM images, QDs increase their lateral sizes with increasing annealing temperature (up to 700 °C). QDs cannot be distinguished after RTA at temperature 800 °C or higher, and substantial thickening of the wetting layer can be seen instead. The main PL peak blueshifts as a result of RTA. We propose that in the as-grown sample as well, as in samples annealed at temperatures up to 700 °C, the peak is due to the QDs. After RTA at 800 °C and higher the PL peak is due to a modified wetting layer. Relatively fast dissolution of QDs is explained in terms of strain-induced lateral Ga/In interdiffusion. It is proposed that such a process may be of importance in proximity-capped RTA, when no group-III vacancy formation takes place at the sample/capping interface.

Multiphonon resonant Raman scattering in MoS2

Optical emission spectrum of a resonantly (λ = 632.8 nm) excited molybdenum disulfide (MoS2) is studied at liquid helium temperature. More than 20 peaks in the energy range spanning up to 1400 cm−1 from the laser line, which are related to multiphonon resonant Raman scattering processes, are observed. The attribution of the observed lines involving basic lattice vibrational modes of MoS2 and both the longitudinal (LA(M)) and the transverse (TA(M) and/or ZA(M)) acoustic phonons from the vicinity of the high-symmetry M point of the MoS2 Brillouin zone is proposed.

Natural quantum dots in the InAs∕GaAs wetting layer

We report on microphotoluminescence study of excitons localized by potential fluctuations in a wetting layer (WL), which accompanies InAs∕GaAs quantum dots (QDs). Linear polarization of spectral lines due to localized excitons enable us to identify a neutral excitonic and biexcitonic emission. A charged exciton has also been identified. High resolution transmission electron microscopy measurements of the investigated structure reveal lateral fluctuations of In content in the WL, as well as its broadening. Both effects give rise to potential fluctuations, which can confine excitons observed in our measurements. The potential fluctuations can be regarded as “natural” QDs in the WL.

Post-growth thermal treatment of self-assembled InAs/GaAs quantum dots

Abstract Results of a post-growth rapid thermal annealing (RTA) on GaAs proximity-capped structures with high density (∼10 11 cm −2 ) of self-assembled InAs/GaAs quantum dots (QDs) are presented. Features due to the QDs, bi-dimensional platelets (2DP) and InAs wetting layer (WL) were identified in photoluminescence (PL) spectrum of the as-grown sample. It is shown, using transmission electron microscopy, that RTA at temperature up to 700 °C (for 30 s) results in an increase of QDs lateral sizes. After RTA at 800 °C or higher temperatures, no QDs can be distinguished and substantial thickening of the WL can be seen. The main PL peak blueshifts as a result of RTA in all investigated temperature ranges, which is accompanied by a quenching of the 2DP and WL PL. It is proposed that the main PL peak, which is due to the QDs in the as-grown sample, results from optical recombination in the modified WL in the samples, after RTA at 800 °C and higher temperatures. Laterally-enhanced Ga/In interdiffusion induced by strain is proposed to explain a relatively fast dissolution of QDs.

Raman scattering of few-layers MoTe2

We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe2) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with a recent study reported by Froehlicher et al (2015 Nano Lett. 15 6481) we observe a complex structure of the out-of-plane vibrational modes , which can be explained in terms of interlayer interactions between single atomic planes of MoTe2. In the case of low-energy shear and breathing modes of rigid interlayer vibrations, it is shown that their energy evolution with the number of layers can be well reproduced within a linear chain model with only the nearest neighbor interaction taken into account. Based on this model the corresponding in-plane and out-of-plane force constants are determined. We also show that the Raman scattering in MoTe2 measured using 514.5 nm (2.41 eV) laser light excitation results in much simpler spectra. We argue that the rich structure of the out-of-plane vibrational modes observed in Raman scattering spectra excited with the use of 632.8 nm laser light results from its resonance with the electronic transition at the M point of the MoTe2 first Brillouin zone.

Non-linear exciton spin-splitting in single InAs/GaAs self-assembled quantum structures in ultrahigh magnetic fields

A. Babinski, G. Ortner, S. Raymond, M. Potemski, M. Bayer, P. Hawrylak, A. Forchel, Z. Wasilewski, and S. Fafard 1 Grenoble High Magnetic Field Laboratory, MPI/FKF and CNRS, BP166, 38042, Grenoble Cedex 9, France 2 Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland 3 Experimentelle Physik II, Universität Dortmund, D-44221 Dortmund, Germany 4 Institute for Microstructural Sciences, National Research Council, Ottawa, K1A 0R6, Canada and 5 Technische Physik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany (Dated: November 12, 2018)

Electron transfer efficiency of Si δ-modulation-doped pseudomorphic GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum wells

In Si δ-modulation-doped GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum well structures (QWs), the electrons from the ionized Si donors are initially confined in the V-shaped potential well (V-PW) formed at the position of a Si δ-doped layer. The efficiency of electrons transferring from the V-PW to the QW was investigated as a function of Si δ-doping concentration in the symmetric GaAs/In0.2Ga0.8As/GaAs QW at 1.7 K. The electron density in the QW increases linearly with an increase of Si δ-doping concentration, while the electron transfer efficiency remains unchanged either in the dark or under the illumination. The asymmetric GaAs/In0.2Ga0.8As/Al0.2Ga0.8As QW has a relatively higher electron transfer efficiency. The effect of grading the Al mole fraction over the AlxGa1−xAs spacer layer on the electron transfer efficiency was also reported.

The persistent photoconductivity effect in modulation Si δ-doped pseudomorphic In0.2Ga0.8As/GaAs quantum well structure

Persistent conductivity effect in modulation Si δ-doped In0.2Ga0.8As/GaAs quantum well (QW) structure grown by metal organic vapor phase epitaxy was examined using Hall effect and magnetotransport measurements in magnetic fields up to 12 T at T=1.7 K. No measurable electron density was found in the QW after cooling down the sample in the dark and the electron density in the V-shaped δ-doped potential well (V-QW) of the modulation Si δ-doped layer was two times lower than the electron density of the same Si δ-doped layer in GaAs. The illumination resulted in the increase of electron density in the V-QW at the beginning and consequently in the population of the ground subband in the InGaAs QW. Due to parallel conduction, a nonmonotonic dependence of Hall density as a function of illumination time was observed. The total electron density in the modulation doped InGaAs/GaAs heterostructure after the illumination became approximately equal to the electron density in the Si δ-doped layer in GaAs.

Electroreflectance bias-wavelength mapping of the modulation Si δ-doped pseudomorphic GaAs/InGaAs/AlGaAs structure

The electroreflectance bias-wavelength mapping is proposed as a tool for characterization of low-dimensional structures. The results of room-temperature measurements on modulation Si δ-doped pseudomorphic GaAs/InGaAs/AlGaAs heterostructure with high mobility two-dimensional electron gas are presented. Franz–Keldysh oscillations (FKO) in GaAs layer are analyzed using fast Fourier transform (FFT) mapping in order to find an electric field in the GaAs layer. Two frequencies of FKO are identified in the FFT spectra, which are attributed to transitions involving heavy and light holes. Two transitions within the InGaAs quantum well are found at zero bias and an additional transition becomes apparent in reversely biased structure. Spectral features due to spin-orbit split holes in GaAs, back AlGaAs barrier, and AlGaAs/GaAs superlattice are also identified.The electroreflectance bias-wavelength mapping is proposed as a tool for characterization of low-dimensional structures. The results of room-temperature measurements on modulation Si δ-doped pseudomorphic GaAs/InGaAs/AlGaAs heterostructure with high mobility two-dimensional electron gas are presented. Franz–Keldysh oscillations (FKO) in GaAs layer are analyzed using fast Fourier transform (FFT) mapping in order to find an electric field in the GaAs layer. Two frequencies of FKO are identified in the FFT spectra, which are attributed to transitions involving heavy and light holes. Two transitions within the InGaAs quantum well are found at zero bias and an additional transition becomes apparent in reversely biased structure. Spectral features due to spin-orbit split holes in GaAs, back AlGaAs barrier, and AlGaAs/GaAs superlattice are also identified.

Transport and quantum electron mobility in the modulation Si δ-doped pseudomorphic GaAs/In0.2Ga0.8As/Al0.2Ga0.8As quantum well grown by metalorganic vapor phase epitaxy

A study of transport and quantum mobility of electrons in two-dimensional electron gas (2DEG) in the modulation Si δ-doped pseudomorphic GaAs/In0.2Ga0.8As/Al0.2Ga0.8As quantum well (QW) grown by metalorganic vapor phase epitaxy is presented. Well-resolved Shubnikov–de Haas oscillations of the magnetoresistivity observed at T=4.2 K suggest that the 2DEG with high electron mobility (μt≈46 000 cm2/V s) formed in the QW with no significant parallel conduction. A persistent photoconductivity effect resulted in an increase in electron sheet density. An increase of transport and quantum mobilities up to the onset of the second subband occupancy was observed. Further illumination resulted in a decrease of both mobilities. Strong dependence of the quantum mobility on the thermal history of the investigated sample was attributed to the effect of actual distribution of ionized centers in the sample.