DC transformer: Dream or reality?

We propose to use the exciton coupling between electrons and holes in different quantum wells to reach a strong Coulomb drag effect. The drag has to be really strong below the Mott transition when the most of the carriers are bound in excitons. We suggest to use the exciton drag for fabrication of DC transformer. Preliminary estimates for Si/SiO₂/Si structure give the Mott transition temperature of the order of 100 K.     

Electron–hole liquid formation by exciton and biexciton resonant excitation in CuCl

The dynamics of a cold photoexcited electron–hole system created by resonant excitation with picosecond optical pulses are studied by time-resolved emission and pump–probe measurements. A rapid build-up of strong metallic reflection in the mid-infrared region was observed, which indicates the creation of electron–hole plasma via the exciton or biexciton Mott transition. Transient reflection spectra clearly show the transformation of high-density electron–hole plasma into a metallic colloid-like state within 10 ps. In the meantime, broad plasma emission turns to a narrow emission band, in which the spectral profile does not change within several tens of picoseconds. These features in the emission spectra disappear when switched to the band-to-band excitation. An analysis of the reflection and emission spectra reveals that the density of metastable electron–hole liquid is as high as 10²⁰ cm⁻³. This indicates that the plasma formation via the Mott transition is crucial to eliminate excess heating for reaching the low-temperature states of an electron–hole ensemble.     

Direct observation of the mott transition in an optically excited semiconductor quantum well

We have studied density-dependent time-resolved photoluminescence from a 80 A InGaAs/GaAs single quantum well excited by picosecond pulses. We succeed in giving evidence for the transition from an exciton-dominated population to an unbound electron-hole pair population as the pair density increases. For pair densities below this excitonic Mott transition we observe a spectrally separate emission from free electron-hole pairs in addition to excitonic luminescence, thereby proving the coexistence of both species. Exciton binding energy and band gap remain unchanged even near the upper bound of this coexistence region. Above the Mott density we observe a purely exponential high energy tail of the photoluminescence and a redshift of the band gap with pair density. The transition occurs gradually between 1 x 10(10) and 1 x 10(11) cm(-2) at the carrier temperatures of our experiment.     

Subpicosecond time-resolved Mott transition in CuCl

The Mott transition in CuCl from a metallic phase of free electrons and holes towards an insulating phase of bound particles (excitons or biexcitons) has been studied by time-resolved luminescence with a resolution slightly better than 1 ps. The phase change takes place in a very short but finite time (about 3 ps) at a carrier density N 10¹⁹ cm^-3.     

A high resolution investigation of the recombination radiation from Si containing the acceptors B,Al, Ga,In and Tl

A high resolution investigation has been made of the luminescence from Si doped with the acceptors B, Al, Ga, In and Tl in the spectral energy range 0.96–1.16 eV using 50 keV electron excitation. Luminescence from Tl-doped material has not previously been reported. Particular attention is directed to the splitting of the no-phonon and phonon-assisted bound exciton lines which has been detected for the first time in luminescence studies. Thermalizing doublet structure has been observed for Al, Ga and In-doped Si and a thermalizing triplet structure for tl-doped material, and three possible models for the fine structure are considered. Satellite lines on the low energy side of the bound exciton lines, which have previously only been observed in B, P and Li-doped Si and attributed to multiple bound exciton complexes, are reported for Al and Ga-doped material. Recent models for these lines are discussed together with their relationship with electron-hole droplet condensation. Structure associated with free electron to bound hole transitions are reported and discussed.     

Phase transitions in nonequilibrium electron-hole systems of Si/SiGe/Si nanoheterostructures

The exciton condensation in a Si_1?x Ge _x solid solution layer of Si/Si_1?x Ge _x /Si heterostructures with the formation of electron-hole liquid has been investigated by low-temperature photoluminescence spectroscopy. In the temperature range above the critical temperature of the transition from an exciton gas to electron-hole liquid, a Mott transition from an exciton gas to electron-hole plasma has been found and investigated.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

Measurement of transport mean-free path of light in thin systems

We extensively investigate in-plane light diffusion in systems with thicknesses larger than but comparable with the transport mean free path. By exploiting amplified spontaneous emission from dye molecules placed in the same holder of the sample, we obtain a directional probe beam precisely aligned to the sample plane. By comparing spatial intensity distribution of laterally leaking photons with predictions from random walk simulations, we extract accurate values of transport mean free path, opening the way to the investigation of a previously inaccessible kind of sample.     

Electron-hole plasma luminescence in GaP

We have studied the photoluminescence spectrum of the electron-hole gas phase in presence of electron-hole liquid in GaP. At low temperature, free excitons are clearly present. With increasing temperatures, i.e. density of the vapour phase, the exciton features evolve into a broad band shifted in energy, which we attribute to an electron-hole plasma. The results are discussed in view of an insulator-metal Mott transition.     

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si_1?x Ge_x/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.     

Ultrafast breathinglike oscillation in the exciton density of ZnSe quantum wells

The spatial density profile of a low-density exciton ensemble in ZnSe quantum wells shows a breathinglike oscillation on a 30-ps time scale. This breathing results from the emission of the first acoustic phonon at the end of the quasiballistic transport phase of the excitons which reverses their direction of propagation. Since the scattering destroys the phase of the excitonic wave function, one can deduce simultaneously the coherence length and the coherence time of excitonic transport by evaluation of the oscillation measured from a single experiment. The breathing, which can be modeled by Monte Carlo simulations, is quenched for rising lattice temperature, i.e., increasing phonon absorption, and in samples with significant disorder. These results were obtained by time-resolved nanophotoluminescence with 5 ps and 250 nm temporal and spatial resolution, respectively.     

Phase diagram of electron-hole systems: Interplay between exciton Mott transition and quantum pair condensation

Quasi-thermal-equilibrium states of electron-hole (e-h) systems in photoexcited insulators are studied from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type pair-condensed state, and the exciton Mott transition between an insulating exciton/biexciton gas phase and a metallic e-h plasma phase. We determine the quasi-equilibrium phase diagram of the e-h system at zero and finite temperatures with applying the dynamical mean-field theory (DMFT) to the e-h Hubbard model with both repulsive and attractive on-site interactions. Effects of inter-site interactions on the exciton Mott transition are also clarified with applying the extended DMFT to the extended e-h Hubbard model.     

微纳跨尺度ZnO结构的紫外发射机理研究

利用气相输运的方法在Si(100)衬底上生长了ZnO的微纳跨尺度结构.扫描电镜照片可以明显地看到样品表面为椎顶六角微米柱-纳米棒的复合结构.样品在室温下的光致发光谱出现了很强的紫外发射峰,没有观察到与杂质或缺陷相关的深能级发射,表明样品有很好的光学质量.通过详细的研究样品的紫外发射谱与温度(83—307K)的依赖关系,发现在室温下样品的近带边发射包含两个部分,分别与自由激子发射和自由载流子到施主(受主)的跃迁(FB跃迁)相关,这个施主(受主)束缚态的离化能为124.6meV. 关键词： ZnO微纳跨尺度结构 光致发光谱 自由载流子到施主(受主)的跃迁 自由激子发射     

Photoluminescence study of Si doped and undoped Chalcopyrite CuGaSe2 thin films

Photoluminescence (PL) characteristics have been studied on undoped and Si-doped CuGaSe₂ single crystal thin films grown on GaAs (001) substrate by migration-enhanced epitaxy. Room temperature PL spectrum of an undoped layer clearly shows free excitonic emission bands related to the minimum band-edge and to the split-off valence band, but no discernible emission has been observed in the low energy area. At 4.2 K, the excitonic emission due to the split-off valence band disappears. Instead, two additional emissions appear at 1.68 and 1.715 eV which are attributed to the bound exciton and band-to-acceptor transition. The Si doping to CuGaSe₂ produces two additional PL bands around 1.61 and 1.64 eV. These PL bands are attributed to the donor acceptor pair emissions due to the doped Si impurity which probably occupies Cu or Ga sites and intrinsic Cu vacancy.     

Contribution of the electron-electron interaction to the optical properties of dense arrays of Ge/Si quantum dots

We present the results of an investigation of the light absorption due to interband and interlevel transitions and the photoconductivity in dense arrays of Ge quantum dots (QDs) in Si formed using the effect of self-organization during molecular-beam heteroepitaxy. It was found that the formation of charged exciton complexes composed of two holes and one electron, as well as of the be-exciton complexes in QDs of type II, leads to an increase in the energy of indirect (in real space) exciton transition, which is explained by the spatial separation of electron and hole. Self-consistent calculations of the wavefunctions for electrons and holes in exciton and in the exciton complexes showed that an electron in a single exciton is localized in the region of maximum stress for Si in the vicinity of the Ge pyramid apex, while a hole is localized near the pyramid base. In a be-exciton complex, electrons exhibit repulsion leading to their spatial separation. As a result, the second electron is bound at the boundary between Si and a continuous Ge layer in which the pyramid bases reside. The experimental data show that an increase in the charge carrier concentration in the ground state of QDs leads to a shortwave shift of the interband resonance and to the narrowing and shape change of the light absorption band, which is explained by depolarization of the external electromagnetic wave due to interaction with the collective charge density oscillations in the lateral direction of the array of Ge nanoclusters. It is established that the hole injection into an excited state of QDs leads to a longwave shift of the photoconductivity peak as a result of decay of the collective excitations and suppression of the depolarization effect.     

Channels of radiative recombination and phase transitions in a system of nonequilibrium carriers in a Si0.93Ge0.07/Si thin quantum well

The “exciton gas-plasma” transition (the Mott transition) in a Si_0.93Ge_0.07/Si thin quantum well is investigated using low-temperature photoluminescence. It is demonstrated that this transition is smooth and occurs in the concentration range from approximately 6 × 10¹⁰ to 1.2 × 10¹² cm^?2. At a temperature of 23 K and excitation densities of higher than 10 W/cm², the shape and location of the luminescence line associated with the electron-hole plasma remain unchanged with an increase in the pump density. This can indicate the occurrence of an “electron-hole gas-liquid” transition. It is shown that, in the spectrum of the quantum well, the luminescence of boron-bound excitons dominates at liquid-helium temperatures and low excitation densities, whereas the free-exciton luminescence dominates at temperatures above 10 K. The influence of the homogeneous and inhomogeneous broadening on the electron-hole plasma and exciton luminescence is discussed.     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

Formation and decay of electron-hole drops in Si

Using a single photon counting technique we were able to measure with very high time resolution the rise and decay times of the luminescence of free excitons and electron hole drops in Si. It was found that the formation and decay of droplets can be described by kinetic equations. The experimental data concerning the time behavior of the free excitons cannot be described by analogous simple kinetic equations. Therefore it is concluded that droplets are formed directly from a dense plasma rather than from a free exciton gas.     

Time-resolved photoluminescence on the stimulated emission in polycrystalline ZnO nanoparticles

The detailed room temperature stimulated emission including its optical characteristics from ZnO nanoparticles, which were prepared by a homogenous precipitation method, has been investigated by the time-resolved spectroscopy. The light emission originates from a free exciton recombination at a lower excitation level; the amplified spontaneous emission appears at a moderate excitation level, in which the threshold excitation intensity is 0.65 GW cm⁻². The resonant stimulated emission was observed in ZnO nanoparticles at a higher excitation intensity. Also, the emission lifetime is drastically reduced. Compared to the fluorescence decay curves, the time-resolved spectrum of the stimulated emission suggests the Gaussian-like decay time with only a few of picoseconds. The dynamic processes of lasing behavior and the characteristics of lasing emission in ZnO nanoparticles could provide the information on the crystal quality, the exciton and the lasing action in the particles.     

Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition

The photoluminescence study of self-assembled ZnO nanorods grown on a pre-treated Si substrate by a simple chemical bath deposition method at a temperature of 80 °C is hereby reported. By annealing in O₂ environment the UV emission is enhanced with diminishing deep level emission suggesting that most of the deep level emission is due to oxygen vacancies. The photoluminescence was investigated from 10 K to room temperature. The low temperature photoluminescence spectrum is dominated by donor-bound exciton. The activation energy and binding energy of shallow donors giving rise to bound exciton emission were calculated to be around 13.2 meV, 46 meV, respectively. Depending on these energy values and nature of growth environment, hydrogen is suggested to be the possible contaminating element acting as a donor.