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1.
Strain Effect on Photoluminescences from InGaN MQWs with Different Barriers Grown by MOCVD
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InGaN/GaN MQWs, InGaN/AlGaN MQWs and InGaN/AlInGaN MQWs are grown on (0001) sapphire substrates by MOCVD. Membrane samples are fabricated by laser lift-off technology. The photoluminescence spec-ra of membranes show a blue shift of peak positions in InGaN/GaN MQWs, a red shift of peak positions in InGaN/AlGaN MQWs and no shift of peak positions in InGaN/AIlnGaN MQWs from those of samples with substrates. Different changes in Raman scattering spectra and HR-XRD (0002) profile of InGaN/AlInGaN MQWs, from those of InGaN/GaN MQWs and InGaN/AlGaN MQWs, are observed. The fact that the strain changes differently among InGaN MQWs with different barriers is confirmed. The AIlnGaN barrier could adjust the residual stress for the least strain-induced electric field in InGaN/AIlnGaN quantum wells. 相似文献
2.
Within the framework of the effective-mass approximation and variational approach, we present calculations of the bound exciton binding energy, due to an ionized donor, in wurtzite InxGa1−xN/GaN strained quantum dots (QDs), considering three-dimensional confinement of the electron and hole in the QDs and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Our results show that the position of the ionized donor, the strong built-in electric field, and the structural parameters of the QDs have a strong influence on the donor binding energy. The variation of this energy versus position of the donor ion is in double figures of milli-electron volt. Realistic cases, including the donor in the QD and in the surrounding barriers, are considered. 相似文献
3.
L.V. Butov 《Solid State Communications》2003,127(2):89-98
Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly. 相似文献
4.
We report on dynamics of excitons in CdxZn1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (rHB=2700 ns−1) is two orders of magnitude more efficient than in the low QD-density samples (rHB=33 ns−1). Radiative recombination rates are similar in both types of samples, rR=1-1.3 ns−1. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs. 相似文献
5.
We theoretically investigate a new type of lateral trap that can be used to confine quantum well excitons. By sending an ultrahigh frequency bulk acoustic wave from the substrate of III-V semiconductors such as GaAs or GaN, an oscillating piezoelectric field is generated. The effective potential induced by the oscillating piezoelectric field implements a type I lateral trap. Such a controllable quantum confinement is essential in many semiconductor nano-electronics and nano-photonics applications. 相似文献
6.
The suitability of bound exciton system in semiconductors is studied for use in nonlinear optical schemes based on EIT, such as “slow” or “stored” photons. We match the desired properties of such a system exhibiting EIT with the known physical realities of a semiconductor system, and suggest, in particular, two suitable schemes using donor impurities in GaAs. In addition to generic properties, we also focus on the influence of many neighboring levels and continuum levels, and on the effect of strong hole-mixing. 相似文献
7.
Optical spectra of highly excited quantum wires at low temperatures have been studied within the dynamical screening approximation. We found a strong Fermi-edge singularity (FES) in the photoluminescence spectra. The spectral shape and FES intensity strongly depend on temperature in agreement with recent experimental results. 相似文献
8.
Microphotoluminescence mapping measurements were performed in a magnetic field on a (Cd,Mn)Te quantum well, modulation n-doped with iodine at about 1010 cm−2. Photoluminescence spectra contain neutral (X0) and negatively charged (X−) exciton lines. The Zeeman effect shows a significant role of heating of the Mn system even under lowest excitation densities. The effective temperature of the magnetic system exhibits strong fluctuations anticorrelated with the total intensity of PL signal. An interpretation of these fluctuations in terms of the influence of non-radiative recombination centers is proposed (in two alternative versions). Maps of the local X0/X− intensity ratio indicate a minor role of electrostatic potential fluctuations. 相似文献
9.
A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor. 相似文献
10.
Applying the method of spectral interferometry we investigate the phase of reflected light at a ZnSe-ZnSxSe1−x heterostructure. We find a series of polariton modes propagating through the ZnSe layer. They can be related to the different polariton branches split of at the heavy- and light-hole excitons. The phase shows pronounced changes around these modes. The strongest changes by 2π appear at the modes of lowest order located weakly above the exciton resonances, while they are smaller for higher modes. Our experimental findings can be explained considering spatial dispersion, Pekar's additional boundary conditions and a weak extension of the excitonic polarization into the ZnSxSe1−x cladding layers. 相似文献
11.
Interference effects in the spectra of the III-nitride epilayers can produce ambiguities and misinterpretations. This problem has been addressed in the literature (e.g. [L. Siozade, J. Leymarie, P. Disseix, A. Vasson, M. Mihailovic, N. Grandjean, M. Leroux, J. Massies, Solid State Commun. 115 (2000) 575]) and a few approaches are given to produce an interference function with which the interference effects can be removed. However, the calculated interference functions may provide over or underestimated corrections because of scattering in the values of the optical parameters of the materials used in the calculation. This paper first describes a model that is developed to derive an interference function. Then, it presents the fit parameters required in the calculation of the interference function that are found experimentally. The experiments compare the corrected spectrum with an interference free spectrum, which can be obtained if the geometry of the light collection is adjusted to the Brewster angle. In this way, one avoids large estimations errors, since the validation data are derived experimentally from the same material, and under the same excitation conditions. The model has been applied to correct the low-temperature, low excitation density photoluminescence measurements of a set of five GaN/InGaN single quantum well samples with different indium concentrations. The effectiveness of this proposed technique is illustrated by the so found results. 相似文献
12.
M. Dybiec S. Ostapenko J.L. Casas Espinola K.J. Malloy 《Applied Surface Science》2006,252(15):5542-5545
The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of InxGa1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm2) as the excitation source. The structures with x = 0.15 In/Ga composition in the InxGa1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well. 相似文献
13.
O. Rubel S.D. Baranovskii K. Hantke B. Kunert W.W. Rühle P. Thomas K. Volz W. Stolz 《Journal of luminescence》2007,127(2):285-290
An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures. 相似文献
14.
The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5×10−2. The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values. 相似文献
15.
Chun-Nan Chen Wei-Long Su Sheng-Hsiung Chang Ikai Lo Hsiu-Fen Kao 《Physics letters. A》2008,372(37):5915-5917
The optical polarization of GaN/AlGaN wurtzite quantum wells in various orientations is studied using an arbitrarily-oriented [hkil] Hamiltonian potential matrix. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The results reveal the presence of giant in-plane optical anisotropy (polarized normal to [0001]) in the M-plane (i.e., the -oriented layer plane) GaN/Al0.2Ga0.8N quantum well, due to the positive crystal-field split energy effect (ΔCR>0). The present theoretical results are consistent with the photoluminescence measurements presented in the literature [B. Rau, et al., Appl. Phys. Lett. 77 (2000) 3343]. 相似文献
16.
This study investigates the optical anisotropy spectrum in the R-plane (i.e., the -oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k⋅p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in -oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x′-polarization to y′-polarization as the well width increases. 相似文献
17.
We review the results of an extensive study of the novel luminescence rings found in GaAs and InGaAs double quantum well structures. We propose that the rings define the edge of a metastable population of carriers between the laser excitation spot and the ring. This population carries wavelike excitations at supersonic speeds. Evidence for and against associating this effect with excitonic superfluidity is reviewed. The effect cannot be easily understood in terms of a Kosterlitz-Thouless transition to a superfluid state. We examine the effects of free carriers in the system and suggest a possible mechanism for the luminescence ring effect. 相似文献
18.
Ultrafast optical pulses and coherent techniques are used to create spin entangled states of non-interacting electrons bound to donors in a CdTe quantum well. Our method, relying on the exchange interaction between optically excited holes and the impurities, can in principle, be applied to entangle a large number of spins. Results are presented for resonant excitation of localized excitons below the gap, and above the gap where the signatures of entanglement are significantly enhanced. 相似文献
19.
The present work investigates the excitonic effects on the bilayer graphene with layers of different thickness under the influence of external electric field through a simple numerical approach. The band structure and energy gap have been calculated using a tight-binding model including parameters like the second-nearest-neighbor-hopping energies t′ (in-plane) and γ (intra-layer) and the on-site energy Δ, in details. The binding energy of exciton for bilayer graphene has been calculated by Wannier model and Hartree–Fock approximation through the Bethe–Salpeter equation. Finally the optical conductivity spectrum of bilayer graphene has been calculated by using the effective mass approximation in two band model. 相似文献
20.
Photoluminescence (PL) spectra and time-resolved PL are measured from around 10 to 300 K for the InGaN/GaN single quantum wells (SQWs) with well widths of 1.5, 2.5, 4 and 5 nm. For the SQWs with the well widths of 1.5 and 2.5 nm, the peak position of PL exhibits an S-shaped shift with increasing temperature. The radiative recombination time τRAD begins to increase at the temperature for the position to change from the red-shift to the blue-shift. The steep increase of τRAD is observed beyond the temperature from the blue-shift to the red-shift. For the SQWs with the well widths of 4 and 5 nm, the peak position of PL exhibits a monotonic red-shift. τRAD decreases at first and then increases with temperature. It is about 100-times longer in the low temperature region and about 10-times longer at room temperature as compared with those of the SQWs with narrower widths. 相似文献