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.     

Micro-photoluminescence of GaN quantum dots embedded in 100 nm wide cylindrical AlN pillars

Individual pillars were etched from a sample embedding a single plane of GaN/AlN quantum dots, deposited by molecular beam epitaxy on a sapphire substrate. Pillars with diameters ranging from 0.1 to 5 μm were fabricated by electron-beam lithography and SiCl₄ reactive ion etching. The PL from a single pillar could be measured by using a confocal microscope, with a spatial resolution of 600 nm. We report an intense PL signal from pillar diameters as small as 0.1 μm at room temperature. By increasing the power of the excitation laser from 0.05 to 200 μW, we induced a blue-shift of the PL energy peak from 2.38 to 2.86 eV, accompanied by a substantial broadening of the PL line. This is explained by the photo-induced screening of the internal electric field, which is close to 10 MV/cm in GaN/AlN heterostructures. Finally we report and tentatively explain a photodarkening effect, i.e., the progressive decrease of the PL intensity over two orders of magnitude, after one hour of continuous laser excitation. However, this effect does not seem to be correlated to the etching process.     

Carrier dynamics in strain-induced InGaAsP/InP quantum dots

Carrier dynamics of strain-induced InGaAsP/InP quantum dots (QDs) is investigated. In this structure, self-assembled InAs islands on the surface act as stressors and create a lateral confinement potential in the near surface InGaAsP/InP quantum well. Photoluminescence (PL) measurements reveal that decreasing the distance from the QD to the surface significantly diminishes the QD–PL intensity, presumably due to surface states of the InAs islands. Moreover, time-resolved measurements show a faster decay of the QD–PL with decreasing distance. To analyze the carrier dynamics, rate equation model is applied and surface state-related transitions are taken into account. The model is found to agree with measurements, and thus provides a possible explanation for the observed temporal behavior of the carriers.     

Time- and frequency-domain measurements of carrier lifetimes in GaN epilayers

Results on time-resolved study of GaN photoluminescence (PL) in a power density range from 0.5 mW/cm² under CW excitation by ultraviolet light emitting diode (UV LED) to 1 GW/cm² under pulsed excitation by YAG:Nd laser in the temperature range from 8 to 300 K are presented. Measurements of PL response in the frequency domain by using amplitude-modulated emission of a UV LED as well as time-resolved PL measurements using a streak camera and light-induced transient grating technique have been used in the study. Yellow luminescence (YL) intensity increases with increasing temperature up to 120 K and faster components in YL decay switch to slower components with increasing temperature under UV LED excitation. At low carrier densities, the trapping decreases the carrier lifetime below 250 ps, while the carrier lifetime in the same GaN sample under excitation ensuring saturation of the traps equals 2 ns.     

Photoluminescence and excitation spectroscopy of the 1.5 μm Er-related band in MBE-grown GaN layers

The infrared photoluminescence at 1.5 m due to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions has been investigated for GaN:Er³⁺ layers grown by MBE. Low temperature high resolution measurements performed under continuous illumination at the wavelength  nm, resonant to one of the intra-4f-shell transitions, revealed that the 1.5 μm band consists of up to eight individual spectral components. In excitation spectroscopy, a temperature dependence splitting of resonant bands was observed. On the basis of these experimental results, a possible multiplicity of optically active centers formed by Er doping in GaN layers is discussed.     

Intra-dot relaxation and dephasing rates from time-resolved photoluminescence from InAs quantum dot ensembles

An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 10¹¹dots/cm² has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.     

Exciton localization behaviour in different well width undoped GaN/Al0.07Ga0.93N nanostructures

We report results from optical spectroscopy such as photoluminescence (PL) and time resolved photo-luminescence (TRPL) techniques from different well width MOCVD grown GaN/Al_0.07Ga_0.93N MQW samples. There is evidence of localization at low temperature in all samples. The decay time of all samples becomes non-exponential when the detection energy is increased with respect to the peak of the emission. Localization of carriers (excitons) is demonstrated by the “S-shape” dependences of the PL peak energies on the temperature. The time-resolved PL spectra of the 3-nm well multi quantum wells reveal that the spectral peak position shifts toward lower energies as the decay time increases and becomes red-shifted at longer decay times. There is a gradient in the PL decay time across the emission peak profile, so that the PL process at low temperatures is a free electron-localized hole transition.     

GaN/AlN quantum dot photodetectors at 1.3–1.5 μm

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c-sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 10¹¹–10¹² cm⁻². Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–p_z transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–p_z intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.     

Growth and characterisation of Eu doped GaN thin films

We have studied the optical properties of Eu doped GaN thin films. We have grown high quality Eu doped GaN thin films by using Gas Source Molecular Beam Epitaxy (GSMBE), with 1.4% Eu concentration. The Full Width at Half Maximum (FWHM) of the X-ray diffraction in an omega scan was found to be 288 arcsecs. Low Eu concentration (0.08%) doped GaN thin films were grown, where Eu-related photoluminescence at 622 and 613 nm was detected using above band-gap excitation at 2 K. For high Eu concentration of 30% GaN:Eu crystal photoluminescence (PL) and cathodoluminescence (CL) spectra show strong and intense transitions at 622 and 664 nm, but also at 593 nm for CL spectra, with a similar transition observed from the low Eu concentration sample.     

Time-resolved spectroscopy of freestanding GaN layers grown by halide vapour phase epitaxy

Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness; however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 μm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms; namely, for layers with thickness less than 400 μm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies.     

Sb/As intermixing in self-assembled GaSb/GaAs type II quantum dot systems and control of their photoluminescence spectra

The intermixing of Sb and As atoms induced by rapid thermal annealing (RTA) was investigated for type II GaSb/GaAs self-assembled quantum dots (QD) formed by molecular beam epitaxy growth. Just as in InAs/GaAs QD systems, the intermixing induces a remarkable blueshift of the photoluminescence (PL) peak of QDs and reduces the inhomogeneous broadening of PL peaks for both QD ensemble and wetting layer (WL) as consequences of the weakening of quantum confinement. Contrary to InAs/GaAs QDs systems, however, the intermixing has led to a pronounced exponential increase in PL intensity for GaSb QDs with annealing temperature up to 875 °C. By analyzing the temperature dependence of PL for QDs annealed at 700, 750 and 800 °C, activation energies of PL quenching from QDs at high temperatures are 176.4, 146 and 73.9 meV. The decrease of QD activation energy with annealing temperatures indicates the reduction of hole localization energy in type II QDs due to the Sb/As intermixing. The activation energy for the WL PL was found to drastically decrease when annealed at 800 °C where the QD PL intensity surpassed WL.     

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).     

Third-order nonlinear optical response in double-walled carbon nanotubes

Resonant behavior and magnitudes of third-order nonlinear optical susceptibilities in double-walled carbon nanotubes (DWNTs) have been investigated by means of femtosecond pump-probe spectroscopy with different pump-photon energies. With the selective excitation of the E₂₂ exciton transition of the inner tubes labeled by the chiral vector indices (7,5) and (7,6), the imaginary part of nonlinear susceptibility Imχ⁽³⁾ has shown the resonant enhancement compared with the case of the nonresonant excitation of the specific tube. The nonlinear response signal at the E₂₂ transition energy of the (8,7) tube has been also enhanced for the excitation of the G-band phonon sideband of its E₂₂ transition. This result is consistent with the phonon-mediated nonlinear optical process observed for the E₂₂ transitions in single-walled carbon nanotubes (SWNTs). It has been also found that the values of the figure of merit Im χ⁽³⁾/α (α: absorption coefficient) of the inner tubes in DWNTs are smaller than those of the corresponding SWNTs, which is interpreted in terms of decay time shortening due to the energy relaxation between the inner and outer tubes.     

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range 1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of 3.35–3.68 eV with decreasing thickness in agreement with the calculated values.     

Optical probe of InAs/GaAs self-assembled quantum dots grown using low growth rate and growth interruptions

We have investigated the optical properties of InAs/GaAs self-assembled quantum dots (QDs), grown at 500 °C using a low growth rate (0.014 ML/s), growth interruptions and a two-stage capping process. The samples exhibited large-size dots with densities in the range (3-4.5) × 10⁹ cm⁻². Macro-photoluminescence (macro-PL) measurements revealed the presence of five electronic sub-bands in the dots, with the ground state (GS) emission exhibiting a linewidth of ∼70 meV. Because of the dots large size and composition dispersions, associated with the growth method, it was possible to resolve single dots emissions using micro-PL (μ-PL) excitation in the barrier layers of the as-grown samples. The sharp PL lines were detected 60-140 meV above the GS peak energy. High-resolution resonant optical excitation of the dots PL evidenced that these fine lines originate from exciton complexes confined to the GS of individual dots. Non-resonant power dependence μ-PL spectroscopy results further confirmed the occurrence of both single exciton (X) and biexciton (XX) radiative recombinations. Finally, with increasing lattice temperature up to 95 K, PL emissions from most of these nanostructures suffered the usual thermal quenching, with activation energies (E_a) ranging between 12 and 41 meV. The relatively small values of E_a suggest that the growth technique implemented here favors the formation of defects centers in the vicinity of the QDs.     

Temperature dependent growth of InGaN/GaN single quantum well

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690 °C, respectively, whereas the samples grown at 670 and 650 °C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.     

Kinetics of Low-Temperature Microphotoluminescence of Exciton-Impurity Complexes in CdZnTe Single Crystals

Low-temperature (6.5 K) microphotoluminescence near the intrinsic absorption edge of CdZnTe alloy single crystals is studied under conditions of non-resonant and resonant excitation by picosecond pulses. Characteristic relaxation times for free excitons and exciton-impurity complexes of various types are determined. A significant (by a factor of 4–8) decrease in the photoluminescence signal decay time of exciton-impurity complexes on neutral donors during the transition to the resonant excitation mode is detected. The detected sharp decrease in the photoluminescence signal decay time can indicate the manifestation of collective effects in this system of emitters.     

Observation of many-body Coulomb interaction effects on the photoluminescence spectra of InAs/GaAs quantum dots

InAs quantum dots (QDs) on GaAs (0 0 1) substrates were grown by Molecular Beam Epitaxy (MBE) using two growth temperatures. Photoluminescence (PL) pump power dependence measurements at low temperature were carried out for sample grown at higher temperature (520 °C). With increasing excitation density, the ground-state transition energy is found to decrease by 8 meV, while the excited-state transition energies exhibit resonance behaviour. The redshift of the ground-state emission was related to the band-gap renomalization (BGR) effect whereas the blueshift of the excited-state emissions was assigned to the compensation between filling of fine structure states and BGR effects. Using a quasi-resonant PL measurement, we have shown that the renormalization of the band-gap had to occur in the QD barrier.     

Optical and structural properties of rare earth doped GaN quantum dots

We have studied the structural and optical properties of GaN quantum dots (QDs) doped with Tm and Eu. It has been found that the morphology of the dots was affected by the presence of the rare earth atoms. Differences in incorporation have also been pointed out, Eu being located inside the QDs while Tm is more likely to be found at the interface between GaN QDs and AlN matrix. Intense and sharp Eu- and Tm-related transition lines are observed in the photoluminescence and cathodoluminescence spectra of doped QDs, with no thermal quenching between 20 and 300 K. They exhibit a saturation effect with increasing excitation power, independently of the temperature. It is tentatively proposed that some transition lines in GaN:Tm QDs could be induced by the presence of the internal electric field.     

InAs/GaAs单量子点中电子/空穴自旋弛豫

利用分子束外延制备了三种类型量子点样品,它们分别是:未掺杂样品、n型Si调制掺杂样品和p型Be调制掺杂样品。在5 K温度下,采用共聚焦显微镜系统,测量了单量子点的光致发光谱和时间分辨光谱, 研究了单量子点中三种类型激子(本征激子、负电荷激子和正电荷激子)的电子/空穴自旋翻转时间。它们的自旋翻转时间常数分别为: 本征激子的自旋翻转时间约16 ns, 正电荷激子中电子的自旋翻转时间约2 ns, 负电荷激子中空穴的自旋翻转时间约50 ps。