Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Indirect exciton absorption and Raman scattering in GaAs-AlGaAs superlattices

Longitudinal optic (LO) phonon assisted indirect exciton creation (X_LO), hot carrier relaxation ((e-h)_LO) and Raman scattering phenomena are reported in the optical spectra of GaAs-AlGaAs superlattices. Structures of the same dimensions both with and without double heterostructure confining barriers are studied. For the structures without confining barriers, continuum transitions are suppressed in photoluminescence excitation (PLE) spectra, and as a result the X_LO, (e-h)_LO and Raman peaks are observed. The X_LO absorption peaks are identified from the observation of a clear threshold in PLE at ℏω_LO (36.4 meV) above the heavy hole exciton peak. The intensity of X_LO is a maximum at 6 meV above the threshold, probably due to dissociation into free carriers at the exciton binding energy (6meV) above ℏω_LO. The influence of non-radiative processes on incoherent (PLE) and coherent (Raman) processes is compared.     

Photoluminescence and photoluminescence excitation of AlGaAs/GaAs quantum wells with growth-interrupted heterointerfaces grown by molecular beam epitaxy

We have measured the photoluminescence (PL) and PL excitation (PLE) of AlGaAs/GaAs single quantum wells with growth-interrupted heterointerfaces. PLE shows the small Stokes shifts of less than 1 meV indicating the extremely flat heterointerfaces without microroughness. Photoluminescence spectra show four peaks originating from different monolayer terraces. These peaks exhibit a doublet splitting. We assigned this doublet to free excitons and excitons bound to neutral donors from the strong well width dependence of doublet splitting.     

Coupled ultrathin InAs layers in GaAs as a tool for the determination of band offsets

We have determined the band offsets at the highly strained InAs/GaAs heterointerface by photoluminescence excitation (PLE) measurements of the symmetric and antisymmetric states in two coupled ultrathin InAs layers embedded in a GaAs matrix. The conduction band offset ΔE_ccould be separated from the valence band offsets, since in a 32 monolayer (ML) barrier sample, the splitting between the heavy-hole exciton transitions is solely determined by ΔE_c. Knowing ΔE_c, the heavy-hole (hh) and light-hole (lh) band offsets ΔE_hhand ΔE_lhcould subsequently be determined from the coupling-induced shift and splitting in samples with a 16, 8 and 4 ML barrier. We find a conduction band offset of 535 meV, a conduction band offset ratio ofQ_c= 0.58 and a strain induced splitting between the hh and lh subbands of 160 meV.     

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ～5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ～1.99 THz (8.2 meV) and ～2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ～3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ～17 nW per 1 W of the input power.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.     

Temperature dependent effects on luminescence polarization and recombination lifetime in serpentine superlattice quantum wire arrays

We have measured photoluminescence excitation (PLE) spectra and radiative lifetimes as functions of temperature for serpentine superlattice quantum-wire arrays. The (Al, Ga)As arrays have lateral periods near 10 nm, and lateral confining potentials of 120 meV in the conduction band. At low temperature the excitons are strongly localized within potential fluctuations along the wires. The radiative lifetime of these localized states is 340 ps at 2 K. The degree of exciton localization decreases with increasing temperature, from which we estimate the strongly-localizing potential fluctuations to be approximately 10 meV deep. Above 80 K the excitons have sufficient thermal energy for motion along the wires. The radiative lifetimes increase with temperature, to 20 ns for free carriers at 325 K. The lateral potential barriers inhibit diffusion to non-radiative recombination sites.     

PLE spectra analysis of the sub-structure in the absorption spectra of CdSeS quantum dots

The semiconductor CdSeS quantum dots (QDs) embedded in glass are analysed by means of absorption spectra, photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra. The peaks of absorption spectra shift to lower energies with the size of QD increasing, which obviously shows a quantum-size effect. Using the PLE spectra, the physical origin of the lowest absorption peak is analysed. In PLE spectra, the lowest absorption peak can be deconvoluted into two peaks that stem from the transitions of 1S_3/2--1S_e and 2S_3/2--1S_e respectively. The measured energy difference between the two peaks is found to decrease with the size of QD increasing, which agrees well with the theoretical calculation for the two transitions. The luminescence peak of defect states is also analysed by PLE spectra. Two transitions are present in the PLE, which indicates that the transitions of 1S_3/2--1S_e and 2S_3/2--1S_e are responsible for the defect states luminescence.     

Excitons associated with miniband dispersion in (InGa)As---GaAs strained layer superlattices

Photoluminescence excitation (PLE) spectroscopy has been used to characterise miniband formation in (InGa)-As---GaAs superlattices with nominally 50 Å wide wells and barriers between 200 Å and 50 Å. The nominal composition of the alloy layers was 0.06. The observed exciton features are consistent with theoreical predictions of both parity allowed and forbidden transitions, at the mini-Brillouin zone centre and edge, including transitions associated with M₁ critical points in the superlattice bandstructure. Furthermore, as the GaAs thickness is varied we monitor changes in shape of the PLE spectra in the region of the first free electron to heavy-hole subband continuum, brought about by the electron-hole Coulomb interaction within the miniband. We also report PLE measurements on a structure which has been designed specifically to maximise the possibility of revealing a Δn = 0 exciton resonance below the saddle point.     

Optical studies of carbon nanotubes and nanographites

Resonance Raman and photoluminescence excitation (PLE) spectroscopies are used to study the optical properties of different types of carbon nanostructures such as carbon nanotube, nanoribbons, nanographites and graphite edges. In the resonance Raman experiments of carbon nanotubes, the (n,m) assignment is obtained by comparing the experimental and theoretical diameter and chirality dependence of the optical transitions. The influence of the environment on the optical transitions of the nanotubes is also obtained in the Raman experiments. The PLE measurements in different samples of carbon nanotubes show both direct and phonon-assisted optical transitions, and the results give new evidences that the optical transitions in nanotubes have an excitonic character, which is very strong for the low energy transitions. We also analyze the Raman spectra of nanoribbons and nanographites, showing that this technique is an important tool for defect characterization in graphitic materials, and can be used to distinguish the atomic structure of the graphite edges.     

The effect of temperature on the photoluminescence ofp-CuGaSe2 single crystals

We studied the temperature dependences of spectral bands and of the outer quantum yield of the edge photoluminescence (PhL) of p-CuGaSe₂ single crystals obtained by the method of chemical transport reactions. We determined the activation energies of acceptor levels and showed that the temperature dependence of the intensity of PhL is determined by an acceptor level of 35 meV below the Debye temperature and of 150 meV above the Debye temperature. The spectra of the edge PhL at 300 and 80 K are determined mainly by band-band optical transitions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 590–593, July–August, 1998.     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

A donor-acceptor pair broad band emission in AgGaS2

We interpret in this paper a broad band emission of silver thiogallate (AgGaS₂) peaked at 1.642 eV with a width of 395 meV. We show that the recombination kinetics are governed by donor-acceptor processes, with the band shape dominated by vibronic interaction. A theoretical calculation of the time-resolved spectra built with the parameters of donor-acceptor transitions gives good agreement with the experimental spectra.     

Luminescence and lattice defects in Cu In S2

We have studied the cathodoluminescence of single crystals of Cu In S₂ obtained by iodine chemical transport. As grown crystals and crystals annealed in In, S, (In + S), or in vacuum, were used. Two types of spectrum were observed, one for n type crystals due to S vacancies, and the other for p type or compensated crystals, which is interpreted as donor-acceptor pair transitions (determined by Time Resolved Speetroscopy) between the S vacancy (donor ~90 meV) and the Cu vacancy (acceptor ~45 meV), and by the corresponding free-to-bound transitions.     

Optical studies on a coherent InGaN/GaN layer

Photoluminescence (PL), photoluminescence excitation (PLE) and selective excitation (SE-PL) studies were performed in an attempt to identify the origin of the emission bands in a pseudomorphic In_0.05Ga_0.95N/GaN film. Besides the InGaN near-band-edge PL emission centred at 3.25 eV an additional blue band centred at 2.74 eV was observed. The lower energy PL peak is characterized by an energy separation between absorption and emission–the Stokes’ shift–(500 meV) much larger than expected. A systematic PLE and selective excitation analysis has shown that the PL peak at 2.74 eV is related to an absorption band observed below the InGaN band gap. We propose the blue emission and its related absorption band are associated to defect levels, which can be formed inside either the InGaN or GaN band gap.     

Intraexciton and Intracenter Terahertz Radiation from Doped Silicon under Interband Photoexcitation

Terahertz photoluminescence of boron- and phosphorus-doped silicon at low temperatures under interband photoexcitation is investigated. The lines of radiative transitions between free-exciton levels and between the levels of shallow impurity centers are observed. The intensities of these lines exhibit different dependences on temperature and excitation intensity. At temperatures near the temperature of liquid helium (T ~ 5 K), the terahertz radiation spectrum features a broad band (about 18–20 meV wide) with a peak at an energy of about 20–22 meV. This band is apparently associated with radiative transitions of nonequilibrium charge carriers from the states of the continuum to the state of an electron–hole liquid.     

应力作用下吸附过渡金属原子的蓝磷单层中的拓扑转变

本文通过第一性原理计算方法研究了被第四B族过渡金属吸附原子(Cr,Mo,W)修饰的蓝磷单层的电子结构性质,发现Cr修饰的蓝磷单层为磁性半金属,而Mo或W修饰的蓝磷单层为半导体,其带隙均小于0.2 eV. 对Mo或W修饰的蓝磷单层施加双轴压应力使得带隙先闭合再打开,且在此过程中发生了能带反转的现象,说明Mo或W修饰的蓝磷单层发生了拓扑转变. Mo和W修饰的蓝磷单层的拓扑转变压应力分别为-5.75%和-4.25%,其拓扑绝缘带隙分别为94 meV和218 meV. 如此大的拓扑绝缘带隙意味着在较高温度条件下有可能在蓝磷单层中通过吸附过渡金属原子实现拓扑绝缘态.     

Photoluminescence studies in N,P, As implanted cadmium telluride

High purity p-type Cdte crystals have been implanted with N⁺, P⁺ and As⁺⁺ ions. After appropriate low temperature annealing, samples have been studied with high resolution photoluminescence technique, and with a tunable dye laser as the excitation source. A chemical doping effect by the acceptors N, P and As on tellurium site, has been evidenced. The bound exciton lines, the two-hole transitions, the donor acceptor pairs bands and the free electron-neutral acceptor transitions have been identified for the first time. The ground state of the acceptors N, P and As are respectively at 56.0 meV, 68.2 meV and 92.0 meV from the valence band.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.