Showing papers by "Le Si Dang published in 2006"

Bose-Einstein condensation of exciton polaritons

Abstract: Phase transitions to quantum condensed phases—such as Bose–Einstein condensation (BEC), superfluidity, and superconductivity—have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 109 times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase. Bose–Einstein condensation (BEC), a form of matter first postulated in 1924, has famously been demonstrated in dilute atomic gases at ultra-low temperatures. Much effort is now being devoted to exploring solid-state systems in which BEC can occur. In theory semiconductor microcavities, where photons are confined and coupled to electronic excitations leading to the creation of polaritons, could allow BEC at standard cryogenic temperatures. Kasprzak et al. now present experiments in which polaritons are excited in such a microcavity. Above a critical polariton density, spontaneous onset of a macroscopic quantum phase occurs, indicating a solid-state BEC. BEC should also be possible at higher temperatures if coupling of light with solid excitations is sufficiently strong. Demokritov et al. have achieved just that, BEC at room temperature in a gas of magnons, which are a type of magnetic excitation. This paper presents a comprehensive set of experiments in which polaritons are excited in a semiconductor microcavity. Above a critical density of polaritons, massive occupation of the ground state at 19 K is observed and various pieces of experimental evidence point to a spontaneous onset of a macroscopic quantum phase.

Si-doped GaN /AlN quantum dot superlattices for optoelectronics at telecommunication wavelengths

Abstract: We report on the controlled growth by molecular beam epitaxy of 20-period Si-doped GaN∕AlN quantum dot (QD) superlattices, in order to tailor their intraband absorption within the 1.3–1.55μm telecommunication spectral range. The QD size can be tuned by modifying the amount of GaN in the QDs, the growth temperature, or the growth interruption time (Ostwald ripening). By adjusting the growth conditions, QDs with height (diameter) within the range of 1–1.5nm (10–40nm), and density between 1011 and 1012cm−2 can be synthesized, fully strained on the AlN pseudosubstrate. To populate the first electronic level, silicon can be incorporated into the QDs without significant perturbation of the QD morphology. All the samples exhibit strong p-polarized intraband absorption at room temperature. The broadening of the absorption peak remains below 150meV and can be as small as ∼80meV. This absorption line is attributed to transition from the s ground level of the QD to the first excited level along the growth axis, pz. ...

Depth-resolved optical studies of excitonic and phonon-assisted transitions in ZnO epilayers

Abstract: The authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T>150K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO.

Si‐doped GaN/AlN quantum dot superlattices for optoelectronics at telecommunication wavelengths

Abstract: We report on the controlled growth of Si doped GaN/AIN quantum dot (QD) superlattices, in order to tailor their intersubband absorption within the 1.3-1.5 μm telecommunication wavelengths. The QD size is tuned by modifying the amount of GaN in the QDs and the growth temperature. Silicon can be incorporated in the QDs to populate the first electronic level, without significant perturbation of the QD morphology. As a proof of the capability of these structures for infrared detection, a quantum-dot intersubband photodetector at 1.38 μm with lateral carrier transport is demonstrated.

Observation of hot luminescence and slow inter-sub-band relaxation in Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multi-quantum-well structures

Abstract: We report on the growth, structural, electronic, and optical properties of Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multiple-quantum-well structures grown on SiC by plasma-assisted molecular-beam epitaxy. We have demonstrated that the use of In as a surfactant during growth improves the structural and optical properties of these layers. Photoluminescence studies have made possible the identification of the fundamental and excited electronic levels by comparison with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behavior of the photoluminescence intensity, which is the quenching of the e1−hh1 line, while the e2−hh1 and e3−hh1 transitions become dominant at room temperature in the samples with 11% and 25% Al in the barrier, respectively. This behavior can be explained by the population of the e2 and e3 electronic states by thermally excited carriers, and by the higher oscillator strength of e2−hh1 and e3−hh1 transitions compared with e1−hh1, due to the intense elec...

Optical properties of single non-polar GaN quantum dots

Abstract: We_present a microphotoluminescence study of non polar GaN/AIN quantum dots (QDs) grown along the [1120] axis. Despite the high QD density, single exciton lines could be isolated on the high energy side of the spectral distribution of the QD array emission. Linewidths down to 0.5 meV are reported, which is one order of magnitude lower than previously reported linewidths for polar GaN/AIN QDs. This difference is attributed to the drastic reduction of the internal field in non-polar quantum dots. Temperature dependent measurements were performed up to 180 K.

Comparison of carrier dynamics in GaN quantum dots and GaN quantum wells embedded in low-Al-content AlGaN waveguides

Abstract: Comparative analysis of the carrier dynamics of GaN quantum dot (QD) and GaN quantum well (QW) separated confinement heterostructures (SCHs) with low-Al-content AlGaN waveguide layers is reported. A redshift (blueshift) of QD (wetting layer) emission is found with respect to QW emission, as expected from the thickness hierarchy of these objects. The influence of nonradiative processes on QD emission in QD SCH is dramatically reduced compared to the case of QW SCH. It is concluded that GaN QDs in low-Al-content AlGaN matrix are robust localization centers and that the carrier dynamics is seriously affected by the built-in internal field effect.

Coexistence of type-I and type-II band lineups in Cd(Te,Se)/ZnSe quantum-dot structures

Abstract: The authors report on transmission electron microscopy, cathodoluminescence, and time resolved photoluminescence studies of thin Cd(Te,Se) layers in a ZnSe matrix, grown by molecular beam epitaxy. All observations confirm strain-induced self-assembly of quantum dots (QD’s), induced primarily by the 14% lattice mismatch between CdTe and ZnSe. The emission spectrum of the structure is the superposition of a relatively narrow luminescence line originating from CdSe-enriched type-I QD’s and a broad band attributed to the emission of an ultrathin ZnTeSe∕ZnSe layer with type-II band lineup, formed in between the QD’s.

III-Nitride Nanostructures for Infrared Optoelectronics

Abstract: 8 eV for the GaN/AlNsystem) and subpicosecond intersubband scattering rates, III-nitride het-erostructures in the form of quantum wells or quantum dots are excellentcandidates for high-speed unipolar devices operating at optical-flber telecom-munication wavelengths, and relying on the quantum conflnement of elec-trons. In this work, we present the plasma-assisted molecular-beam epitaxialgrowth of quantum well infrared photodetector structures. The growth ofSi-doped GaN/AlN multiple quantum well structures is optimized by control-ling substrate temperature, metal excess and growth interruptions. Struc-tural characterization conflrms a reduction of the interface roughness to themonolayer scale.

The dynamics of amplified spontaneous emission in CdSe/ZnSe quantum dots

Abstract: We have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32cm−1 for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300ps and carrier capture times of 15 and 40ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into the active region from the uppermost ZnMgSSe cladding layer, yielding a longer rise time for the pump-probe signals for this sample.

Influence of stacking on optical characteristics of GaN/AlN self-organized quantum dots

Abstract: The effect of stacking on the optical properties of self-organized GaN/AlN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy was investigated by photoluminescence (PL), PL excitation (PLE), and cathodoluminescence (CL) spectroscopic techniques. With an increase in the number of the GaN QDs stacking layer, we observed a red shift in the main QD PL peaks. A Stokes-like shift was observed between PL emission and PLE absorption edge for all the samples. From the depth-resolved CL of a 200-period GaN QD sample, we observed that the main QD CL emission peak energy position does not change much, while the higher-energy side CL peak around ∼3.7 eV shows a variation of its peak position with the accelerating voltage probably due to the change in either distribution of QD size/shape or wetting layer properties during the growth of multiple stacking of GaN QDs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous phase condensation of CdTe exciton-polaritons

Abstract: We have studied CdTe/CdMgTe microcavity samples containing 4 quantum wells and characterized by a vacuum field Rabi-splitting of 13.2 meV. Electron-hole pairs were excited at 7.3K by linearly-polarized light at 1.74 eV provided by a Ti:sapphire laser operating in the pulsed mode (at 80 MHz rate). Spectroscopic imaging in reciprocal space as well as near-field spectroscopy show that, above a critical stimulation threshold, polaritons in our microcavity undergo a spontaneous transition to a condensed phase characterized by the massive occupation of the polariton ground state. The coherence of this quantum state is evidenced by the spectral and angular narrowing of its emission. We point out the conditions which should be fulfilled to achieve BEC of microcavity polaritons. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN/AlGaN superlattices for optoelectronics in the mid‐infrared

Abstract: In this work we discuss the optical properties of a GaN/Al 0.11 Ga 0.89 N multiple-quantum-well structure grown on 4H-SiC by plasma-assisted molecular-beam epitaxy. Photoluminescence lines related to the fundamental and excited electronic levels are identified, in good agreement with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behaviour with quenching of the e 1 -hh 1 while the e 2 -hh 1 line becomes dominant at room temperature. This behaviour could be explained by the increasing thermal population of the e 2 state and the higher radiative efficiency of the e 2 -hh 1 transition compared to the e 1 -hh 1 transition due to the intense electric field in the well.