Increasing Excitation Intensity Research Articles

Optical properties of InAs epilayers grown on GaAs by molecular beam epitaxy

InAs epilayers with thicknesses of 400, 500, 750, and 1500 nm were grown on GaAs by molecular beam epitaxy and their properties were investigated by reflection high-energy electron diffraction and photoluminescence (PL). For all InAs epilayers, the PL peak position measured at 10 K is blue-shifted from that of bulk InAs, which could be largely due to the residual strain in the epilayer. InAs epilayers have PL peaks red-shifted to lower energy as the thickness of InAs layer is increased. This is the first observation on PL of sub-μm thick InAs epilayers grown on GaAs substrate. While the PL peak position of 400 nm thick InAs layer is linearly blue-shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of thicker InAs epilayers are gradually blue-shifted and then, saturated above a power of 75 mW.

Low-temperature visiblephotoluminescence spectra of Tl2GaInSe4 layered crystals

The photoluminescence (PL) spectra ofTl2GaInSe4 layered single crystals has beeninvestigated in the 10-95 K temperature,0.04-3.98 W cm-2 excitation laser intensity and 1.75-2.07 eVenergy ranges. We have observed an emission bandlocated at 1.912 eV (648 nm) at 10 K for an excitationintensity of 1.40 W cm-2. The band had a half-width of 0.105 eV and an asymmetric Gaussian line shape. Ithas been found to red-shift with increasingtemperature or decreasing excitation intensity. To explain theobserved PL behaviour, we propose that the emission is due tothe radiative recombination of a donor-acceptor pair (DAP) withan electron occupying a donor level located at 0.355 eV belowthe conduction band and a hole occupying an acceptor levellocated at 0.036 eV above the valance band. The blue-shift ofthe peak energy and the sub-linear increase of the bandintensity for the observed emission band with increasingexcitation intensity is explained using the in-homogeneously spaced DAP recombination model.

Luminescence studies of defects and piezoelectric fields in InGaN/GaN single quantum wells

Transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL) and photoluminescence (PL) studies were performed on a 30 nm GaN/2 nm In 0.28Ga 0.72N/2 μm GaN/(0 0 0 1) sapphire single quantum well (SQW) sample. SEM-CL was performed at low temperatures ≈8 K, and at an optimum accelerating voltage, around 4–6 kV to maximise the quantum well (QW) luminescence. The CL in the vicinity of characteristic “V-shaped” pits was investigated. The near band edge (BE) luminescence maps from the GaN showed bright rings inside the boundaries of the pits while the QW luminescence maps showed pits to be regions of low intensity. These observations are consistent with TEM observations showing the absence of QW material in the pits. Variations in both the BE and QW maps in the regions between the pits are ascribed to threading edge dislocations. The CL and PL QW luminescence was observed to blue-shift and broaden with increasing excitation intensity. This was accompanied by decreasing spatial resolution in the CL QW maps implying an increasing carrier diffusion length in the InGaN layer. The reasons for this behaviour are discussed. It is argued that screening of the piezoelectric field in the material may account for these observations.

Photoluminescence and photoreflectance of GaInNAs single quantum wells

The temperature dependence of photoluminescence (PL) and photoreflectance (PR) was studied on a 10 nm GaInNAs/GaAs single quantum well prepared by molecular-beam epitaxy using a solid As source. PL was dominated by the near-band edge PL peak with a full width at half maximum (FWHM) value of 16–28 meV for 50–280 K. This indicates that the GaInNAs epilayer was of good quality. The temperature dependence of the band gap energy was studied by the PR measurements, and the dependence was comparable to that of the GaInAs. The near-band edge PL at low temperature exhibited a blueshift with an increase in excitation intensity and temperature (8–50 K). It had a large FWHM value of 24–26 meV at 8 K. These results are discussed in terms of carrier localization.

Quantum biexcitonic liquid in monoclinic ZnP2 crystals

A phonon–roton dispersion relation is proposed for the elementary excitations of a quantum biexcitonic liquid in semiconductors. The proposed dispersion relation is used as a starting point for a calculation of the photoluminescence spectrum of the liquid and an analysis of its behavior under variation of temperature and density of the biexcitonic liquid. The parameters of the dispersion curve of elementary excitations of the quantum biexcitonic liquid are evaluated by fitting the calculated photoluminescence spectrum to the experimental spectrum of the biexcitonic liquid of semiconducting β-ZnP2 crystals. Experimental studies of how the photoluminescence spectrum of a biexcitonic liquid in β-ZnP2 depends on the temperature and the intensity of the laser excitation confirm the initial theoretical model. The dependence of the temperature of the crystals on the excitation intensity is measured, and for some of the samples an anomalous dependence is found: the temperature of the crystal decreases as the excitation intensity increases. This effect is probably a consequence of the giant thermal conductivity of the superfluid biexcitonic liquid in β-ZnP2 crystals.

Diffusion photovoltage in poly(p -phenylenevinylene)

Photovoltage phenomena in poly(p-phenylenevinylene) (PPV) are investigated under pulsed laser illumination. The photovoltage transients are strongly retarded in time depending on sample thickness, laser intensity, and bias illumination. It is shown that the photovoltage in PPV originates from separation of excess electrons and holes due to their concentration gradient and different diffusion coefficients (diffusion photovoltage). The diffusion coefficient of excess holes is found to be on the order of 1×10−6 cm2/s and it increases with increasing excitation intensity and intensity of bias illumination. The diffusion coefficient of excess electrons is about 1–2 orders of magnitude smaller than for excess holes.

Tuneability of the ASE in thin organic films

We show that the amplified spontaneous emission (ASE) in poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) can be tuned by controlling the film thickness and the excitation intensity. For sufficiently thin films the peak position of the ASE is determined by the cut-off wavelength for the waveguide and is shifted to shorter wavelengths with decreasing film thickness. Increasing excitation intensity causes a blue-shift of the emission spectra. We also show that excitation wavelength has no effect on the position of the ASE.

Blue photoluminescence activated by surface states in GaN grown by molecular beam epitaxy

We have studied the broad blue band, which emerges in the photoluminescence (PL) spectrum of c-plane GaN layers after etching in hot H3PO4 and subsequent exposure to air. This band exhibited a 100 meV blueshift with increasing excitation intensity and a thermal quenching with activation energies of 12 and 100 meV. These observations led us to suggest that surface states may be formed on etched surfaces and cause bandbending, which leads to a shift in transition energy with excitation. The blue PL is related to transitions from the shallow donors filled with nonequilibrium electrons to the surface states, which capture the photogenerated holes. The observed irreversible bleaching of the blue luminescence may be attributed to the metastable nature of the surface states or to the oxygen desorption.

Optical and structural prorerties of InAs epilayer on graded InGaAs

AbstractWe have studied infrared photoluminescence (PL) and x-ray diffraction (XRD) of 400 nm and 1500 nm thick InAs epilayers on GaAs, and 4 nm thick InAs on graded InGaAs layer with total thickness of 300 nm grown by molecular beam epitaxy. The PL peak positions of 400 nm, 1500 nm and 4 nm InAs epilayer measured at 10 K are blue-shifted from that of InAs bulk by 6.5, 4.5, and 6 meV, respectively, which can be largely explained by the residual strain in the epilayer. The residual strain caused by the lattice mismatch between InAs and GaAs or graded InGaAs/GaAs was observed from XRD measurements. While the PL peak position of 400 nm thick InAs layer is linearly shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of 4 nm InAs epilayer on InGaAs and 1500 nm InAs layer on GaAs is gradually blue-shifted and then, saturated above a power of 75 mW. These results suggest that adopting a graded InGaAs layer between InAs and GaAs can efficiently reduce the strain due to lattice mismatch in the structure of InAs/GaAs.

InGaN/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy with mass transport

We report on studies of In0.12Ga0.88N/GaN heterostructures with three 35-Å-thick quantum wells (QWs) grown on sapphire substrates by metalorganic vapor phase epitaxy with employment of mass transport. The structure is demonstrated to show good structural and optical properties. The threading dislocation density is less than 107 cm−2 for the mass-transport regions. The photoluminescence (PL) spectrum is dominated by the rather narrow near-band gap emission at 2.97 eV with a linewidth of 40 meV. This emission has a typical PL decay time about 5 ns at 2 K within the PL contour. With increasing excitation intensity, an additional transition with longer decay time (about 200 ns) is enhanced at energy about 2.85 eV. The position of this line depends strongly on the excitation power. We explain the data in terms of a model, where the PL is a result of contribution from at least two nonequivalent QWs, which could be realized due to a potential gradient across the layers.

Biexciton emission from high-quality ZnO films grown on epitaxial GaN by plasma-assisted molecular-beam epitaxy

We have investigated the optical and structural properties of high-quality ZnO films grown on epitaxial GaN (epi-GaN) by plasma-assisted molecular-beam epitaxy employing low-temperature buffer layers. High-resolution x-ray diffraction for both symmetric and asymmetric reflexes shows that crystalline defects in ZnO films have a similarity to epi-GaN used as a substrate. The quality of ZnO epilayers grown on epi-GaN is basically determined by epi-GaN. The photoluminescence (PL) spectrum at 10 K exhibits very sharp exciton emission with a linewidth of 1.5 meV, while deep-level emission is negligible, indicative of small residual strain. At 77 K, PL is dominated by a free-exciton emission line in the low-excitation regime, while it is overtaken by a new emission band due to biexcitons at its low-energy side as the excitation intensity increases. This biexciton emission band emerges even under the intermediate excitation regime of 100 W/cm2, which is 100 times smaller than the previously reported threshold for bulk ZnO. The biexciton binding energy is estimated to be 15 meV, in agreement with previous results. At the higher excitation regime, the emission line due to exciton–exciton scattering dominates the PL spectrum.

Optical properties of InAs self-organized quantum dots in n–i–p–i GaAs superlattices

The optical properties of InAs quantum dots in n–i–p–i GaAs superlattices are investigated by photoluminescence (PL) characterization. We have observed an anomalously large blueshift of the PL peak and increase of the PL linewidth with increasing excitation intensity, much smaller PL intensity decrease, and faster PL peak redshift with increasing temperature as compared to conventional InAs quantum dots embedded in intrinsic GaAs barriers. The observed phenomena can all be attributed to the filling effects of the spatially separated photogenerated carriers.

Near-field fluorescence imaging by localized field enhancement near a sharp probe tip

This work describes near-field investigations of luminescent nanosamples based on monitoring fluorescence due to the enhanced field around a laser-illuminated probe tip. These fluorescence effects are investigated as a function of probe-sample separation, which identify a strong, spatially localized (≈7 nm) enhancement of the incident laser field in the vicinity of the probe tip. From a model fit to the fluorescence data, the localized enhancement of the electric field is estimated to be >tenfold, which predicts a significant increase in localized excitation intensity (>100-fold) for fluorescence imaging of molecular size samples.

Effect of carrier confinement on photoluminescence from modulation-doped AlxGa1−xN/GaN heterostructures

Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al0.12Ga0.88N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased.

Donor-Acceptor Pair Recombination in the Ordered Vacancy Compound CuIn5Te8

The photoluminescence (PL) spectra of the ordered vacancy compound CuIn5Te8 was measured as a function of temperature and excitation power intensity. The PL spectra show a band centered around 1.03 eV at excitation intensity of 0.3 W that shifts to higher energies with increasing excitation intensity and with decreasing temperature. The variation of the PL intensity of this band with excitation power is sublinear. Both these facts indicate that this is due to a donor-acceptor pair recombination. From the shift of this band to higher energies with P and the thermal quenching of the PL band, donor and acceptor levels with activation energy of 0.02 and 0.08 eV, respectively, were estimated.

Synthesis and Ultrafast Study of Cysteine- and Glutathione-Capped Ag2S Semiconductor Colloidal Nanoparticles

A new synthetic method has been developed for preparing silver sulfide, Ag2S, nanoparticles capped with cysteine or glutathione. The average particle diameter has been determined to be around 9 nm using transmission electron microscopy. The ground-state electronic absorption spectra of the Ag2S nanoparticles show a continuous increase in absorption cross section toward shorter wavelengths starting from the red (600−800 nm). Ultrafast dynamics of photoinduced electrons in these nanoparticles have been measured using femtosecond transient absorption/bleach spectroscopy. In most cases studied, the early time transient profiles feature a pulse-width limited (<150 fs) rise followed by a fast decay (750 fs) and a slower rise (4.5 ps). The signal has contribution from both transient absorption and transient bleach. On longer time scales, three (Cys-1, Cys-2, and GSH-2) of the four samples studied show a recovery with 4.5 ps time constant that goes above the baseline and then decays gradually toward the baseline with a time constant of >1 ns. One sample (GSH-1) shows a bleach recovery that gradually approaches the baseline with a similar time constant (>1 ns) following the fast 4.5 ps rise. An interesting power dependence was observed for all the samples: the transient absorption contribution becomes more dominant over bleach with increasing excitation intensity. A simple four-state kinetic model developed to account for the main features of the dynamics suggests that initial photoexcitation populates the conduction band and depletes the valence band within the laser pulse (<150 fs). The conduction band electrons are first trapped in shallow trap states with a time constant of 500 fs and then further trapped into deep traps with a constant of 4 ps. The deep trapped electrons finally recombine with the hole with a time constant of >1 ns. This model suggests that the difference in dynamics observed between the different samples is due to different absorption cross sections of deep trap states. The observed excitation intensity dependence of the dynamics is attributed to shallow trap state saturation at high intensities.

The dynamic Stark effect under a not-too-strong femtosecond excitation regime in quantum well structures

Using straightforward time-dependent perturbation treatment, transient coherent radiation-semiconductor interaction mechanisms are investigated analytically in a quantum well structure (QWS) duly irradiated by a moderate power femtosecond laser with photon energy slightly less than the crystal bandgap energy. The transition energy is calculated by recognizing the finite potential well depth of the QWS. The effects of higher excitonic states on the Stark shift and self-energy correction are examined in GaAs/AlGaAs QWS. The study shows a red shift with increasing excitation intensity and the shift is also found to be less when all the discrete excitonic states below the band edge are considered together. An increased self-energy correction accompanies this red shift.

Quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells grown by gas-source molecular-beam epitaxy

A quantum dot-like behavior of GaInNAs in Ga0.7In0.3NxAs1−x/GaAs quantum wells (QWs) grown by gas-source molecular-beam epitaxy has been studied using high-resolution x-ray rocking curves (XRC), cross-sectional transmission electron microscopy (XTEM), and photoluminescence (PL) spectroscopy. XRC determines the average alloy composition and thickness of the QWs to be In 0.30 and N 0–0.030, and 6.2 nm, respectively. XTEM images show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated with lateral variations in strain, but the latter is much rougher. For Ga0.7In0.3As/GaAs QWs, rapid thermal annealing results in the splitting of a broad excitonic emission into two peaks presumably due to In composition fluctuation. The separation between these two peaks increases with increasing N concentration. Increasing excitation intensity results in a significant blueshift for the low-energy peak, while little blueshift for the high-energy peak. The earlier PL behavior can be interpreted by a simple model where the low-energy peak originates from In and N-rich regions in the wells acting as quantum dots. The high-energy peak is likely due to the excitons of a more two-dimensional QW.

Photoluminescence of modulation doped GaAs quantum wires on vicinal GaAs (110) substrates

Modulation-doped GaAs quantum wires are grown on the surfaces with giant steps which are naturally formed on vicinal GaAs (110) substrates by molecular beam epitaxy (MBE). It is found that photoluminescence (PL) peaks shift to lower energy with increasing doping level and to higher energy with increasing excitation intensity. These results indicate that the GaAs quantum wires are successfully modulation-doped.

Observation of quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells

We report a quantum dot-like behavior of GaInNAs due to composition nonuniformity of N and In in GaInNAs/GaAs quantum wells (QWs). Images of cross-sectional transmission electron microscopy show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated due to lateral variations in strain. This effect is more pronounced in the N-containing QWs due to nonuniform In and N concentrations. Rapid thermal annealing causes a blueshift of the photoluminescence (PL) peak, and results in a splitting of the as-grown broad PL emission into two peaks. The In and N composition fluctuation after annealing becomes predominantly bimodal. The low-energy PL peak is attributed to excitons localized at deep levels, which are originated from In- and N-rich regions in the wells acting as quantum dots (QD). The high-energy peak PH is likely due to the excitons of the 2D QWs. To reduce the local strain, N atoms are preferentially localized in the In-rich regions, so the separation between these two peaks increases with increasing N concentration. Increasing excitation intensity results in a significant blueshift for PL and little blueshift for PH, due to the easier band filling of the localized QD states than QW states.