Shift Of Photoluminescence Peak Energy Research Articles

Pressure induced transition of direct to indirect excitons in double quantum well: Effect of Γ-X crossover

Influence of Γ-X crossover due to applied hydrostatic pressure on exciton dynamics has been studied in symmetric GaAs/Al 0.3 Ga 0.7 As double quantum wells using variation technique and effective mass approximation. The effect of Γ-X crossover is significant in narrow wells irrespective of the coupling, type of excitons and nature of confinement. The stability of both direct and indirect excitons has been analysed from spatial extension and diamagnetic susceptibility. At P > P 2 Kbar, change of confinement of electron from Γ to X band state would form a spatially indirect exciton with reduced spatial extension. This study confirms that the complete transition of direct to indirect exciton can be achieved through the application of pressure. The change in inter-band transition energy due to Γ-X crossover may lead to a shift in Photoluminescence peak energy. • Effect of Γ-X crossover is significant in narrow wells irrespective of the coupling. • The change is also dependant on the type of exciton. • At P > P 2 Kbar, change of confinement of electron from Γ to X band form a spatially indirect exciton. • Transition of direct to indirect exciton can be achieved through the application of pressure. • Change in inter-band transition energy may lead to a shift in Photoluminescence peak.

Effect of Power on Crystallinity and Opto-Electronic Properties of Silicon Thin Films Grown Using VHF PECVD Process

The silicon thin films have been deposited using VHF (60 MHz) plasma enhanced chemical vapor deposition (PECVD) process. The influence of the power (10–50 W), used during the deposition, on the silicon thin films properties was studied. The structural transition (different crystalline fraction) along with their variable grain distribution within amorphous silicon matrix has been discussed with the help of optical, electrical and structural measurements. Raman characterization along with photoluminescence (PL), UV-VIS spectroscopy, micro-ellipsometry spectroscopy and Field emission scanning electron microscopy (FESEM) have been performed to recognize the transitions in deposited silicon films including the microstructure mainly related with crystallite fraction and optical constants. FESEM was used to investigate the characteristics and variation in the microstructure. From Raman results, the structural transition from amorphous to micro/nanocrystalline growth occur as indicated by narrow band at 515 cm−1 observed at 10 W applied power deposited film. From these results, it was found that with increasing power, the crystallinity increases and the films are highly crystalline (Xc > 50%) in nature above 20 W applied power. The contribution of nano-sized crystallites increases the crystalline volume fraction from 32 to 84%. The peak observed nearly at 2.0 eV in photoluminescence emission spectra (PL) confirms the presence of nanocrystallites in the deposited films which is well correlated with the band gap values observed from UV-VIS spectroscopy. However, red shift in PL peak energy and decrease in PL intensity observed and are explained in terms of grain boundaries. The shift in refractive index (n) and extinction coefficient (k) were also observed at different power variation. The investigation of spectroscopic ellipsometry result explains the microstructure of deposited film with changes in crystallite sizes and their volume fractions.

Engineering of carrier localization in BGaAs SQW for novel intermediate band solar cells: Thermal annealing effect

In this study, the effects of thermal annealing on the morphological, structural and optical properties of BGaAs/GaAs Quantum Wells (QWs) are investigated for the first time. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) show that boron islands nucleate on the surface and their rugosity increases after annealing due to boron segregation phenomenon. The segregation is further validated through high-resolution X-ray diffraction (HR-XRD) measurements, which also confirms a good crystalline quality of the system. After annealing at 710 °C for 10 min a significant compositional increment of boron is also observed. Nevertheless, a negligible enhancement of photoluminescence (PL) intensity and minor shift in PL peak energy are found, which is attributed to the defect density and the spatial reduction of localization effects. Time-resolved photoluminescence (TRPL) results exhibit an initial decay time in the microsecond regime from localization emission, followed by an even slower decay from deep levels states. Our results can be the first step towards the engineering of carriers localization in BGaAs/GaAs SQWs by thermal annealing and can orient the material system to be an active layer in intermediate band solar cells.

Redshift and blueshift of GaNAs/GaAs multiple quantum wells induced by rapid thermal annealing

The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs/GaAs multiple quantum wells (MQWs) grown by chemical beam epitaxy (CBE) are studied by photoluminescence (PL) at 77 K. The results show that the optical quality of the MQWs improves significantly after RTA. With increasing RTA temperature, PL peak energy of the MQWs redshifts below 1023 K, while it blueshifts above 1023 K. Two competitive processes which occur simultaneously during RTA result in redshift at low temperature and blueshift at high temperature. It is also found that PL peak energy shift can be explained neither by nitrogen diffusion out of quantum wells nor by nitrogen reorganization inside quantum wells. PL peak energy shift can be quantitatively explained by a modified recombination coupling model in which redshift nonradiative recombination and blueshift nonradiative recombination coexist. The results obtained have significant implication on the growth and RTA of GaNAs material for high performance optoelectronic device application.

Localization effect in green light emitting InGaN/GaN multiple quantum wells with varying well thickness

Green light emitting InGaN/GaN multiple-quantum-well (MQW) structures with varying well thickness are grown via metal-organic chemical vapor deposition (MOCVD). The localization effect in these samples is studied by means of temperature-dependent photoluminescence (PL) measurements. The S-shape shift of PL peak energy with increasing temperature is observed, from which the extent of localization effect is determined quantitatively by using a band-tail model. It is found that the composition-related deep localization states dominate the light emission in thin-well MQWs, while in thick-well MQWs the shallow localization states induced by the fluctuations of InGaN well thickness dominate the luminescence efficiency. It is considered that in the thinner wells the improved emitting efficiency may partially originate from the stronger localization effect.

Carrier redistribution between different potential sites in semipolar (202¯1) InGaN quantum wells studied by near-field photoluminescence

Scanning near-field photoluminescence (PL) spectroscopy at different excitation powers was applied to study nanoscale properties of carrier localization and recombination in semipolar (202¯1) InGaN quantum wells (QWs) emitting in violet, blue, and green-yellow spectral regions. With increased excitation power, an untypical PL peak energy shift to lower energies was observed. The shift was attributed to carrier density dependent carrier redistribution between nm-scale sites of different potentials. Near-field PL scans showed that in (202¯1) QWs the in-plane carrier diffusion is modest, and the recombination properties are uniform, which is advantageous for photonic applications.

Thermal annealing effects on the optical and structural properties of (1 0 0) GaAs1−xBix layers grown by Molecular Beam Epitaxy

The effects of long time thermal annealing at 200°C on the optical and structural properties of GaAs1−xBix alloys were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). FESEM images show that bismuth islands nucleate on the surface and their diameter increases after annealing. It was observed a PL intensity enhancement and a small blue shift in PL peak energy after thermal annealing at 200°C for 3h of GaAs1−xBix alloys which was associated to the reduction of the density of defects. However these defects are not completed removed by thermal annealing although an important PL intensity improvement is observed.

Photoluminescence of samples produced by electroless wet chemical etching: Between silicon nanowires and porous structures

AbstractSamples containing silicon nanowires (Si‐NWs) and highly porous structures (P‐Si) were prepared by electroless wet chemical etching (EWCE) of crystalline silicon wafers using various etching parameters. Photoluminescence (PL) measurements were performed with excitation at 488 nm and a photon energy flux of 337 mW cm−2. According to the diameters of the Si‐NWs (>10 nm), from quantum confinement (QC) theory no shift in PL peak energy compared to the bandgap of crystalline silicon is expected. However, PL measurements show peak emission energies ranging between 1.4 and 1.6 eV. After further treatment of the samples with HF, substantial PL emission was still detectable with the measured PL peak pinned at 1.4 eV irrespective of etching time. We explain the observations by the hypothesis that the persistent part of PL emission is generated by nanocrystals located at the rough sidewalls of the Si‐NWs or residing within the porous sample structure. The part of the PL, which was present before HF treatment, but vanished after the treatment, is attributed to the presence of silicon suboxide surrounding the Si‐NWs or covering other Si surfaces. This hypothesis is explored by means of three sample series, prepared with different preparation parameters. In the first series the time used during the initial metallization step in order to prepare an Ag nanoparticle layer on the top surface was varied, in the second series the etching time was the changed parameter and in the third series the HF to H2O2 concentration ratio was varied.magnified image Strong visible orange colored PL of a sample produced by EWCE starting from a heavily doped wafer (n‐type c‐Si (111), As as dopant) and excited at 337 nm (the sample was mounted on a glass substrate. Blue luminescence is due to the substrate).

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton—longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements. With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang—Rhys factor, increases remarkably due to an enhancement of the internal electric field. With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang—Rhys factor decreases. These results reveal that there is a large built-in electric field in the well layer and the exciton—LO-phonon coupling is strongly affected by the thickness of the cap layer.

Estimation of the location of embedded InGaAs/GaAs quantum dots by measuring strain-induced blueshift of photoluminescence

A method was proposed to estimate the location of embedded InGaAs/GaAs quantum dots (QDs) as the distance from the center of a flat cylindrical nanoprobe of radius 600 nm, by measuring the blueshift of photoluminescence (PL) under elastic indentation of the nanoprobe onto the sample surface. The energy of the enhanced fine PL peaks originating from single QD was observed to blueshift linearly with indentation force. The PL peak energy shift per unit force, i.e., the blueshift rate, was measured in the range 22.7–87.8 meV/mN. Analyzing the nanoprobe-induced strain and energy shift of the ground state of QD, based on a numerical simulation, provided a quantitative relationship between the blueshift rate and the QD’s distance from the probe center. With this relationship, we succeeded in estimating the location of the embedded QDs from the experimentally obtained blueshift rate. The observed QDs were determined to be located around the nanoprobe edge, which coincides with the observation that only the PL emitted from the QDs located under the probe edge is enhanced and observable under nanoprobe indentation. The method presented in this paper provides a reliable and simple way to estimate the location of the embedded QDs with respect to the probe center by nanoprobe indentation.

Carrier Dynamics of Deep-Level States in InGaN/GaN Multiquantum Wells

InGaN/GaN (3 nm/10 nm) multiquantum wells (MQWs) were grown on (0001) sapphire substrates by metal–organic chemical vapor deposition. To characterize the effect of internal electric fields on deep-level states in InGaN MQWs, deep-level transient spectroscopy (DLTS) and bias-dependent photoluminescence (PL) measurements were performed. The thermal activation energy of MQWs obtained from DLTS spectra decreased to 0.15 eV with an increase in reverse measure bias from -2 to -10 V, which indicates partially cancelled internal fields. The blue shift of PL peak energy was about 0.05 eV when the reverse measure bias was changed from 0 to -5 V. This is explained well by carrier dynamics in the conduction band of InGaN/GaN MQWs, which affects carrier recombination energies corresponding to the band edge to edge transitions with changing reverse measure bias.

Photoluminescence and excitation spectrum of Mg‐doped p‐type AlXGa1–XN

AbstractWe have been measured the near band edge photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg doped AlGaN. PL and PLE peak energies were almost corresponding to those of undoped AlGaN. From the temperature dependent measurement of the PL spectra, the PL peak energy shift as a function of temperature showed S ‐shaped behaviour. These results suggest that the mechanisms of the near band edge PL of AlGaN:Mg are basically similar to those of undoped AlGaN. Using the known spectral feature of undoped AlGaN which consists of main and LO phonon replica emission bands at even interval energy and same line width, the spectral decomposition of the PL spectrum reveals an additional band which is expected as the transition to the acceptor level arising from Mg doping. The temperature dependences of its peak energy and intensity suggest relative temperature shift of the acceptor level and also may imply the existence of another relaxation pathway. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the photoluminescence of self-assembled InAs/GaAs quantum dots studied in high magnetic fields

We have investigated the photoluminescence (PL) of self-assembled InAs/GaAs quantum dots (QDs) in high magnetic fields of up to 50 T and as a function of temperature. Our data clearly indicate that two different mechanisms are at work. At low temperatures (T < 80 K), the zero-field PL is increasingly dominated by lower energy dots. High-field measurements however demonstrate that these dots are larger in size only in the growth direction. At temperatures above 100 K, a strong decrease of the PL peak energy shift with field is observed, while the zero-field PL is characterized by a redshift according to the changes in the bandgap. We discuss these contradictory observations in terms of a phenomenon that we call field-assisted enhancement of the QD barrier potential. Since this effect is much stronger for small high-energy QDs, the latter progressively dominate the PL emission when temperature and magnetic field are increased.

Dynamics of biexciton localization inAlxGa1−xNmixed crystals under exciton resonant excitation

We report the localization dynamics of biexcitons in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals under exciton resonant excitation at low temperatures. During a few tens of picoseconds just after intense laser excitation, the photoluminescence (PL) spectral shape obeys an inverse Maxwell--Boltzmann distribution and free biexcitons dominate the PL spectrum. With a further increase in the delay time, the biexciton PL peak energy and edge energy shift to lower energies. These redshift behaviors in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals are completely different from the behaviors of free biexcitons in GaN crystals. Our observations reveal the rapid transformation dynamics from free to localized biexcitons in band-tail states in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals.

Interwell coupling effect in Si/SiGe quantum wells grown by ultra high vacuum chemical vapor deposition

Si/Si0.66Ge0.34coupled quantum well (CQW) structures with different barrier thickness of 40, 4 and 2 nm were grown on Si substrates using an ultra high vacuum chemical vapor deposition (UHV-CVD) system. The samples were characterized using high resolution x-ray diffraction (HRXRD), cross-sectional transmission electron microscopy (XTEM) and photoluminescence (PL) spectroscopy. Blue shift in PL peak energy due to interwell coupling was observed in the CQWs following increase in the Si barrier thickness. The Si/SiGe heterostructure growth process and theoretical band structure model was validated by comparing the energy of the no-phonon peak calculated by the 6 + 2-bandk·pmethod with experimental PL data. Close agreement between theoretical calculations and experimental data was obtained.

Analysis of Time-Resolved Donor–Acceptor Photoluminescence of N-Doped ZnO

The wide-gap semiconductor ZnO has a band gap of ’3:3 eV at room temperature and is of current interest due to its potential optoelectronic applications. Since undoped ZnO is known to be grown under fairly high residual n-type, the trial of the p-type doping generally tends to result in an observation of the photoluminescence (PL) of donor–acceptor pair (DAP) bands. The optical properties of the DAP emissions have not been fully understood. In a previous report, we described the growth and the steady-state PL properties of ZnO:N epitaxial layers grown by repeated temperature modulation (RTM) epitaxy. Evidence of p-type conductivity for RTM-grown ZnO:N has been accumulated from a variety of experimental results, such as the Hall-effect and electroluminescence measurements in a p–n juction structure. For the analysis of PL peak energy and excitation-induced peak shift on p-type ZnO, we took the effect of donor wavefunction overlapping and the fact of impurity distribution inhomogeneity into account by adopting the fluctuation theory developed by Kuskovsky et al. Using this theory, we were able to consistently explain the excitation-intensity dependences of the PL properties of ZnO:N grown with three different techniques and conditions. In this short communication, we report on the analysis of the time-resolved PL (TRPL) spectra taken for the RTMgrown sample at an intermediate doping level to experimentally detemine the transition probability of the DAP (being W0) by using all the other material parameters that have been deduced in our previous work. It should be noted that the recombination dynamics of the DAPs has been studied for ZnO:N within the low-doping regime, where the approach of Thomas, Hopfield, and Augustyniak (THA) is still valid. The doped epilayer was grown by the RTM epitaxy on a lattice-matched ScAlMgO4 (SCAM) substrate. 7) The detailed methods of fabrication have been given elsewhere. Secondary-ion mass spectroscopy (SIMS) yielded a total nitrogen concentration [N] of 6 10 cm . Our ZnO:N sample dominantly showed the DAP band (D being a shallow donor) with zero-phonon energies of 3:235 eV, a donor activation energy of ED 1⁄4 60meV, and an acceptor activation energy of EA 1⁄4 171meV at 5K. These have been already determined from the energy of the bound exciton line (unchanged from that for undoped samples) and the excitation-intensity dependence of the PL. In addition, the energy of EA has turned out to be almost independent of the nitrogen concentration. For the time-resolved experiments, pulsed excitation was provided by the frequency-doubled beam of a picosecond mode-locked Ti:sapphire laser (360 nm, 2 ps). Here, we focus our attention to experimental total-lightdecay data (a spectrally integrated PL decay curve), because one can fit to the data with a smaller number of running parameters than the case of spectrally resolved decay curves, i.e., EA, ED, and ðNA NDÞ (acceptor and donor concentrations). Irrespective of the model adopted, in both cases of the theories of THA and of the fluctuation, the intensity IðtÞ of light emitted at time t is

Temperature dependence of exciton localization dynamics in InxGa1−xN epitaxial films

We have studied the temperature dependence of exciton localization dynamics in InxGa1−xN epitaxial films (x=0.09) by means of optical Kerr-gate time-resolved photoluminescence (PL) spectral measurements. During 30ps after 150fs laser excitation, the PL dynamics is sensitive to the measurement temperature. In the temperature range of 6–50K, the PL rise time decreases and the PL peak energy shifts to higher energy with an increase of temperature. At high temperatures above 80K, the thermal quenching of the PL at shallow localized states occurs. The energy relaxation processes of excitons in localized states of InxGa1−xN films are discussed.

Effect of strain variation on photoluminescence from InGaAs quantum dots in air‐bridge structures

AbstractWe have fabricated freestanding wire (air‐bridge) structures with a bowed shape by introducing a strain layer to vary the strain around quantum dots. The photoluminescence peak energy shift following the shape change of the bridge can be observed for individual InGaAs quantum dots. We find systematic dependence of the peak shift on the dot position along the growth direction. The dependence of the peak shift is explained by strain distribution in the bridge. The strain distribution in the bridge as well as in the dot is calculated using a finite element method. Using the strain data, the electronic structures of the dots embedded in the bridge structures are calculated within the effective mass approximation. The calculated energy shifts agree well with the experimental ones.

Fabrication of wavelength-shifted In0.2Ga0.8As/GaAs multiple quantum well laser diodes by impurity-free vacancy disordering at different thermal annealing temperatures

We fabricated ridge-waveguide laser diodes with In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) that have undergone impurity-free vacancy disordering (IFVD) using the SiO2 capping layer. The SiO2-capped MQW samples were annealed by rapid thermal annealing (RTA) at different temperatures. The magnitude of the blueshift estimated by the shift of photoluminescence peak energy increases with the increase of RTA temperature due to the enhanced interdiffusion of the MQWs. The electrical characteristics and lasing performance of the annealed laser diodes were compared with those of the as-grown laser diode. The operation wavelengths of 967, 946 and 927 nm with device performance comparable to that of the as-grown device were obtained in the laser diodes fabricated by the MQWs that have undergone IFVD at 850, 900 and 950 °C for 50 s, respectively. Thus, the RTA for the IFVD employed to fabricate wavelength-shifted laser diodes is believed not to seriously degrade the electrical and optical properties of the devices.

Controlled Passivation and Luminescence Blue Shifts of Isolated Silicon Nanocrystals

AbstractWe have performed a comparative study of oxide- and nonoxide-passivated silicon nanocrystals to probe the role of the silicon/oxygen interface in low coverage, non-interacting silicon nanocrystal systems. Ensembles of Si nanocrystals characterized by a narrow distribution and diameters of 2–5 nm were synthesized by ion implantation into SiO2 films followed by a high-temperature anneal in Ar. The nanocrystals were removed from the SiO2 film matrix and deposited on Si substrates using a chemical etch in HF, leaving a hydrogen-terminated surface. A natural oxide layer grows on these surfaces in air. We characterized the morphology of the samples with atomic force microscopy (AFM) and the spectroscopic properties with photoluminescence (PL) and X-Ray photoelectron spectroscopy. We found that the PL energy of Si nanocrystals can be shifted by particle size reduction and hydrogen or oxygen termination. Further, PL peak energy shifts upon etching and oxidation were consistent with the model of Wolkin et al. that proposes that for very small radii, a silicon-oxygen double bond will produce deep interface states which red shift the luminescence.