Photoluminescence Excitation Power Research Articles

Atomic-like UV emission generated in hexagonal boron nitride single crystals by thermal annealing

A series of atomic-like photoluminescence (PL) emission peaks in UV region near 4.0 eV were created by thermal annealing hexagonal boron nitride (h-BN) single crystals in air. The pristine h-BN did not have these peaks, emitting strong phonon-assisted band edge PL with peaks at 5.78 and 5.89 eV. After annealing the h-BN crystals in ambient air, a new atomic-like sharp emission in UV region at 4.09 eV with a line width of 0.2 nm appeared along with its phonon replicas at 3.89 and 3.69 eV in the low temperature (8 K) PL measurement. Further testing demonstrated that annealing the h-BN samples in the temperature window of 700–950 °C for 60 min generated the atomic-like emission. The peak position of the emission line is stable with the temperature and PL excitation power. Our study also suggests that the defect responsible for the atomic-like emission resides in the surface region.

Influence of the quantum-confined Stark effect on the temperature-induced photoluminescence blueshift of InGaN/GaN quantum wells in laser diode structures

Measurements of the excitation power-dependence and temperature-dependence photoluminescence (PL) are performed to investigate the emission mechanisms of InGaN/GaN quantum wells (QWs) in laser diode structures. The PL spectral peak is blueshifted with increasing temperature over a certain temperature range. It is found that the blueshift range was larger when the PL excitation power is smaller. This particular behavior indicates that carriers are thermally activated from localized states and partially screen the piezoelectric field present in the QWs. The small blueshift range corresponds to a weak quantum-confined Stark effect (QCSE) and a relatively high internal quantum efficiency (IQE) of the QWs.

Coupling Behaviors of Surface Plasmon Polariton and Localized Surface Plasmon with an InGaN/GaN Quantum Well

Surface plasmon (SP) coupling behaviors of an InGaN/GaN quantum well (QW) with surface plasmon polariton (SPP) induced on a smooth Ag-film/GaN interface and localized surface plasmon (LSP) induced on randomly distributed surface Ag nanoparticles (NPs) are compared based on temperature-dependent, excitation power-dependent, and time-resolved photoluminescence (PL) measurements. By comparing the variations of PL excitation power-dependent emission at 10 and 300 K, the contribution of SP coupling to the increase of PL intensity can be differentiated from that of PL excitation-intensity increase at the QW due to the enhanced reflection at the Ag/GaN interface. The observed stronger SP coupling effect through SPP, when compared with that through LSP, is attributed to its larger available SP state number per unit area for coupling with the QW. Although the SP coupling strength through LSP on the surface Ag NPs is weakly dependent on PL excitation power, that through SPP at the smooth Ag-film/GaN interface increases significantly with PL excitation power. Such different behavior is due to the abundant SPP states available for coupling with the QW of a higher carrier density. It can also be attributed to the spectral blueshift of QW emission through the screening of the quantum-confined Stark effect and the band filling effect for QW coupling with SPP of a higher density of state.

Effect of deposition rate on the characteristics of Ge quantum dots on Si (001) substrates

We have studied the effect of the deposition rate on the structural and optical characteristics of five-layer-stacked Ge self-assembled quantum dots (QDs) on a Si (001) substrate grown by molecular beam epitaxy. Ge QDs were formed using a pulse growth technique, which consists of high-rate Ge deposition and an interruption in growth. In this work, the deposition rate was varied from 1.1Å/s to 2.8Å/s. Pyramid-shaped QDs with a monomodal size distribution were obtained at 2.8Å/s, while a bimodal size distribution including pyramid-shaped QDs and multifaceted dome-shaped QDs was observed between 1.1Å/s and 2.2Å/s. As a result, an improved size fluctuation of 11.1% and highest sheet density of 5.6×1010cm−2 were achieved for the QD sample grown at 2.8Å/s. As for the optical characteristics, a single photoluminescence (PL) emission line at 0.825eV with a width of 86.1meV was observed at 12K. Furthermore, we found that the PL emission from the Ge QDs shows type-II emission characteristics, i.e., the shift of the PL peak energy with respect to the PL excitation power (Pexc) was proportional to Pexc1/3.

Observation of band alignment transition in InAs/GaAsSb quantum dots by photoluminescence

The band alignment of InAs quantum dots (QDs) embedded in GaAsSb barriers with various Sb compositions is investigated by photoluminescence (PL) measurements. InAs/GaAsSb samples with 13% and 15% Sb compositions show distinct differences in emission spectra as the PL excitation power increases. Whilst no discernible shift is seen for the 13% sample, a blue-shift of PL spectra following a 1/3 exponent of the excitation power is observed for the 15% sample suggesting a transition from a type I to type II band alignment. Time-resolved PL data show a significant increase in carrier lifetime as the Sb composition increases between 13% and 15% implying that the transformation from a type I to type II band alignment occurs between 13% and 15% Sb compositions. These results taken together lead to the conclusion that a zero valence band offset (VBO) can be achieved for the InAs/GaAsSb system in the vicinity of 14% Sb composition.

Blueshift of bandgap energy and reduction of non-radiative defect density due to precise control of InAs-on-GaSb interface in type-II InAs/GaSb superlattice

We have investigated the influence of As-soak parameters at InAs-on-GaSb interfaces on the structural and optical parameters of InAs/GaSb superlattices (SLs). The lattice-matched SLs were obtained for two sets of technological parameters. For a long As-soak time of 12.0 s and high V/III ratio of 10, the photoluminescence (PL) analysis indicates the presence of a non-radiative recombination channel. For a shorter time of 2.8 s and a reduced V/III ratio of 5.6, the PL excitation power dependence is close to that relevant for a clear excitonic recombination. The spectral blueshift of the bandgap energy of 23 meV was observed for SLs with GaAs-like interface thicknesses of 0.4 ML and 1.0 ML and other fixed parameters.

Controllability of the subband occupation of InAs quantum dots on a delta-doped GaAsSb barrier

Optical properties of InAs quantum dots (QDs) embedded in GaAsSb barriers with delta-doping levels equivalent to 0, 2, 4, and 6 electrons per dot (e/dot) are studied using time-integrated photoluminescence (PL). When the PL excitation power is increased the full width at half maximum (FWHM) of the 4 and 6 e/dot samples is found to increase at a much greater rate than the FWHMs for the 0 and 2 e/dot samples. PL spectra of the 4 e/dot sample show a high energy peak attributed to emission from the first excited states of the QDs, a result deduced to be due to preoccupation of states by electrons supplied by the delta-doping plane. When temperature dependent PL results are fitted using an Arrhenius function, the thermal activation energies for the 4 and 6 e/dot samples are similar and greater than the thermal activation energies for the 0 and 2 e/dot samples (which are similar to each other). This increased thermal activation energy is attributed to the enhanced Coulombic interaction in the InAs QD area by the delta-doping plane for higher doping levels. It is concluded that delta-doping of the barrier in QD systems is a feasible method for controlling the level of carrier occupation in a QD mediated intermediate band.

Role of photovoltaic effects on characterizing emission properties of InGaN/GaN light emitting diodes

Strong photovoltaic effects on photoluminescence (PL) spectra in InGaN/GaN blue light emitting diodes were investigated. Due to severe carrier escape from quantum wells, significant photovoltaic effects occur in PL measurement in open-circuit condition, which strongly affect the PL peak position and intensity. We reveal that proper correlation between electroluminescence and PL peak positions cannot be obtained without proper consideration of the photovoltaic effects. By changing sample temperature and the PL excitation power, the generated photovoltage varies in the range of 2.0 to 2.6 V. We show that in the open-circuit condition, which is the usual case, the determination of radiative efficiency by measuring the PL intensity ratio of low-and high-temperature cannot be accurate, and the excitation intensity dependent PL cannot be solely intrinsic either. Both the absorption of incident laser and the carrier escape from the quantum well are bias-sensitive. By a simple and straightforward method, we determined that 51% of photogenerated carriers escaped in short-circuit condition.

Some aspects of exciton thermal exchange in InAs quantum dots coupled with InGaAs/GaAs quantum wells

Photoluminescence (PL), its temperature and excitation power dependences, and PL excitation spectra have been investigated in InAs quantum dots (QDs) embedded in In0.15Ga0.85As/GaAs quantum wells (QWs) as a function of QD density. The QD density varied from 1.1×1011 down to 1.3×1010 cm−2 with the increase in QD growth temperature at the molecular beam epitaxy processing. A set of rate equations for exciton dynamics (relaxation into QWs and QDs, and thermal escape) has been solved to analyze the mechanism of PL thermal quenching in studied structures. Three stages have been revealed in thermal decay of the PL intensity of InAs QDs. Presented mathematical analysis provides the explanations of the mechanism of PL thermal decay as well as the peculiarities of PL excitation power dependences and PL excitation spectra. A variety of activation energies of PL thermal decay and the localization of nonradiative defects in InGaAs/GaAs QW structures with different InAs QD density are discussed as well.

Strong optical emissions from two-dimensional metal photonic crystals with semiconductor multiple quantum wells

We investigated two-dimensional (2D) metal photonic crystals (PhCs) with semiconductor multiple quantum wells as an active material. The 2D metal PhCs were fabricated using only a lift-off process. The lift-off process leads to a simpler fabrication process than the one for conventional PhCs and avoids any process damage induced by dry etching. The 2D metal PhCs exhibited clear photonic band effects: (1) photoluminescence (PL) peak positions changed depending on the lattice constant and (2) the PL intensity from 2D metal PhCs was higher than that without 2D metal PhCs. These effects are suitable for fabricating light-emitting diodes with a high external quantum efficiency. PL intensity depending on PL excitation power revealed slightly improved luminescence efficiency than the sample without 2D metal PhCs.

Carrier localization effect in polarized InGaN multiple quantum wells

Carrier localization effects in polarized InGaN/GaN multiple quantum wells (MQWs) were investigated as a function of well width, d, and In content, x. Using photoreflectance (PR), photoluminescence (PL), PL excitation (PLE), selective excitation of PL, PL excitation power, and time-resolved PL spectroscopy, the dominance of the localization effect against the built-in field effect on carrier recombination dynamics in InxGa1–xN MQWs of different well width (d = 2.0–4.0 nm, x ≈ 0.15) and In content (x ≈ 0.22–0.27, d = 2.5 nm) was revealed. Based on the modeling of the PL spectra by Monte Carlo simulation of exciton hopping and the spectroscopic reference provided by PR, increased In content and well width were found to increase the band potential fluctuations and carrier localization depth. The density of localized states deduced from the simulation was found to be in a fair agreement with the PLE data. The built-in field strength in InGaN QWs containing 15% of In was estimated to be of 0.5 MV/cm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy growth and photoluminescence study of room temperature 1.31 μm (In yGa 1−yAs/GaAs 1−x Sb x)/GaAs bilayer quantum wells

Molecular beam epitaxy (MBE) growth of (In yGa 1− y As/GaAs 1− x Sb x )/GaAs bilayer quantum well (BQW) structures has been investigated. It is evidenced by photoluminescence (PL) that a strong blue shift of the PL peak energy of 47 meV with increasing PL excitation power from 0.63 to 20 mW was observed, indicating type II band alignment of the BQW. The emission wavelength at room temperature from (In yGa 1− y As/GaAs 1− x Sb x )/GaAs BQW is longer (above 1.2 μm) than that from InGaAs/GaAs and GaAsSb/GaAs SQW structures (1.1 μm range), while the emission efficiency from the BQW structures is comparable to that of the SQW. Through optimizing growth conditions, we have obtained room temperature 1.31 μm wavelength emission from the (In yGa 1− y As/GaAs 1− x Sb x )/GaAs BQW. Our results have proved experimentally that the GaAs-based bilayer (In yGa 1− y As/GaAs 1− x Sb x )/GaAs quantum well is a useful structure for the fabrication of near-infrared wavelength optoelectronic devices.

Optical properties of silicon-doped (100) GaAs layers grown by molecular-beam epitaxy

The photoluminescence spectra of (100) GaAs layers, both undoped and doped with silicon, is investigated at T=77 K. It is found that along with the B-band, which corresponds to interband radiative recombination, the spectra of doped layers also exhibit a so-called Si-band located near hν⋍1.4 eV. In multilayer δ-doped structures, an additional band appears in the region hν⋍1.47–1.48 eV, which is called here the δ-band. The dependence of the energy positions, intensities, and shapes of these photoluminescence bands on the doping dose N Si, laser excitation power, and temperature are investigated. It is shown that the Si-band is caused by optical transitions between the conduction band and a deep acceptor level (∼100 meV) connected with Si atoms on As sites. It is also established that the dependences of the shape and intensity of the δ-band on temperature and photoluminescence excitation power are identical to the corresponding dependences for the B-band. The behavior of the δ-band in the photoluminescence spectra is viewed as evidence of quantum-well effects in the δ-doped structures.

Design and photoluminescence study on a multiquantum barrier

The structure of a multiquantum barrier (MQB) is discussed, and possible choices of well and barrier thickness and pair number for a visible AlGaInP laser in the 630 nm wavelength range are examined. Is it confirmed that an effective potential barrier can be two times as high as a classical potential barrier. The potential barrier height under biased conditions is shown to be larger than 50 meV, considering the variation of Fermi level changes in the MQB region. This may contribute to higher temperature CW operation when introduced into visible AlGaInP lasers. By photoluminescence (PL) measurement, the effect of electron-wave confinement of a GaAs/AlGaAs MQB is experimentally verified. A weak saturation in PL excitation power dependence and less sensitive temperature dependence were observed. These effects could be due to the increase in potential barrier height, and show a superior carrier confinement of MQB. >