Intensity Of Photoluminescence Spectra Research Articles

Polaron Interaction in InAs/GaAs Quantum Dots and Resonant Photoluminescence

In an adiabatic approach, the efficiency of the electron-phonon interaction (EPI) can be determined by measuring the ratio between the intensities of two of the phonon replicas that EPI induces in photoluminescence (PL) spectra. In low-dimensional structures such as InAs/GaAs quantum dots (QDs), this ratio depends on the excitation energy E exc . Moreover, the evolution of the PL spectra intensity and lineshape with E exc is quite elaborate. We reproduce well this evolution for E exc ranging from far below to above the GaAs bandgap by introducing an effective Huang-Rhys factor S, a commonly used measure of EPI. Nevertheless, the value of S remains much higher than predicted in an adiabatic model, which can hardly account for the reported dependence of S on QD size and shape. The likely source of this lack of consistency is briefly discussed.

Spatial Inhomogeneity of Photoluminescence in an InGaN-Based Light-Emitting Diode Structure Probed by Near-Field optical Microscopy Under Illumination-Collection Mode

Spatial distribution of photoluminescence (PL) spectra has been assessed in an InGaN single quantum well (SQW)-based light-emitting diode structure by near-field optical microscopy under the illumination-collection mode. The obtained PL mapping image revealed a variation in both peak and intensity of PL spectra according to the probing location with a scale less than about 200 nm. The variation in PL intensity is from 0.8 to 1.8 in arbitrary units indicating that the internal quantum efficiency fluctuates from 10% to 50% within the active layer.

Humidity-voltaic characteristics of Ag-PS-Si structures

Electrical characteristics of Ag-porous silicon (PS)-Si and PS-Si structures at the different ambient humidity and room temperature were investigated. Ag-PSSi structures have been fabricated by evaporation of Ag film onto the PS surface of PS-Si structures using electron-beam evaporator technique. It is shown that current-voltage characteristics of Ag-PS-Si structures strongly depend on ambient relative humidity (RH), whereas humidity-sensitiveness of electrical characteristics of PS-Si structures is weakly expressed. The humidity-voltaic effect, i.e. the origin of open-circuit voltage in Ag-PS-Si structures in humid atmosphere is discovered. The humidity-stimulated voltage approximately linearly increases with rise of the relative humidity. For the best Ag-PS-Si structure value of open-circuit voltage reached up to 350 mV at 90% RH. The possible mechanism responsible for humidity-stimulated generation of voltage in Ag-PS-Si structures is considered. The efficient photoluminescence from porous silicon (PS) observed by Canham [1] has stimulated extensive researches on formation mechanism of PS and physical mechanisms responsible for such luminescence [2]. Crystalline structure of PS is network of silicon nanometer-sized regions surrounded by void space with very large surface to volume ratios. PS surfaces are covered by silicon hydrides and silicon oxides. Surface bonds, in particular Si-H bonds play an important role in regulating of luminescence and other properties due to the large internal surface area of PS [3]. It is worth noting that environment influences on photoluminescence characteristics of PS [4]. The presence of water-vapor environment causes Si-Si bond breaking with formation Si-H bonds [5]. Influence of water on intensity of photoluminescence spectrum of PS is displayed in [6]. The porosity of the crystalline PS structure allows the fabrication of various different kinds of sensor device. Taliercio et al. [7, 8] suggested to use PS films as efficient membranes of oxygen sensors. Hydrogen detection with palladium modified porous silicon studied by Polishchuk et al. [9]. In Au-PS surface type structure Yamana et al. [10] discovered the dependence of the current on the relative humidity (RH) under applied voltage. A dependence of the reverse current of metalPS structure on the relative humidity was investigated by Dimitrov [11]. In this work the effect of ambient humidity on the current-voltage characteristics of Ag-PS diode structure is investigated. A rise of open-circuit voltage up to

Excitonic quantum confinement effects and exciton electroabsorption in semiconductor thin quantum boxes

We investigate the excitonic optical properties in thin quantum boxes in the intermediate regime between the two-dimensional (2D) and zero-dimensional (0D) with a theoretical analysis that rigorously treats excitonic confinement effects. It is found that the exciton binding energy is substantially enhanced and that the oscillator strength concentrates to the lowest excitonic transition, even in a thin box whose lateral width is considerably (about five times) larger than the Bohr radius. Novel optical properties experimentally observed in semiconductor quantum disks, which are the intense photoluminescence spectrum and ultranarrow photoluminescence excitation spectrum, are explained well by the theoretical results. We also calculate exciton absorption in a thin box in which an electric field is applied in the lateral direction. The present theory can simulate how the electroabsorption evolves from the quantum confined Stark effect in the 0D to the quantum confined Franz–Keldysh effect in the 2D with an increase in the lateral size of the box. In the intermediate regime between 2D and 0D, a strong excitonic electric-field effect, distinct from the well-known electroabsorption effects at 0D and 2D, is found. These theoretical results demonstrate that even though the lateral confinement is weak, it considerably enhances the electron–hole Coulomb interaction and alters excitonic optical features markedly in the thin quantum box.

MBE growth of high-quality InP for heterostructures using incongruent evaporation of GaP

The incongruent evaporation of GaP source material using a conventional effusion cell equipped with a PBN scavenger at the orifice is demonstrated as a simple and cost-effective way to generate a pure P 2 molecular beam, suitable for the growth of InP layers in a solid-source MBE system. Low residual carrier concentrations in combination with high mobilities as well as narrow and intense photoluminescence spectra were achieved. While some arsenic incorporation from the growth environment was observed, gallium incorporation is limited to below 0.1% of the group-III lattice sites. n- and p-doping using Si and Be has been investigated. Successful growth of InP/GaInAs heterostructures indicates that the use of the GaP source technique is a viable way for growing AlGaInAs/InP device structures by solid source MBE.

High-temperature surface cleaning of AlGaAs without As flux for MBE regrowth

Abstract AlGaAs surfaces were cleaned by high-temperature heat treatment without As flux in a preparation chamber for regrowth of active layers by molecular beam epitaxy (MBE). Reflection high-energy electron diffraction from the cleaned AlGaAs surface showed a spotty pattern, but surface roughness observed by an atomic force microscope was smaller than the surface of cleaned GaAs. Auger electron spectroscopy showed that a cleaned surface was covered by Al, As and O 2 . Surface sublimation measured by quadrupole mass spectroscopy during the heat treatment decreased drastically with increasing alloy composition x , suggesting that the cleaned surface consisted of AlAs or its oxide. Quantum well (QW) structures were grown on the cleaned surfaces and photoluminescence (PL) spectra were measured. Well-defined PL peaks corresponding to stacked single QWs (SQWs) with 500 nm thick buffer layer were obtained at 300 K, which were in contrast to the SQWs grown on the surface cleaned by the conventional method. The PL intensity was influenced strongly by the thickness of the buffer layer when it was thinner than 300 nm. GaAs passivation of the AlGaAs surface was effective for obtaining intense PL spectrum from the regrown SQWs.

Influence of C + -implantation dose on blue emission and microstructures of Si-based porous β-SiC

Crystal Si were implanted with different doses of C+ from 1011 to 1017 cm-2 at an energy of 50 keV. β-SiC precipitates were formed by thermal annealing at 1050 °C for 1 h and porous structures were prepared by electrochemical anodization. Under the excitation of ultraviolet, the samples, with C+ dose ≥ 1015 cm-2 have intense blue emission which is stronger than the photoluminescence (PL) intensity of reference porous silicon (PS), and increases as C+ dose increases; the samples with C+ dose ≤ 1014 cm-2 show similar PL spectra to those of PS. The blue peak intensity in PL spectra is correlated with the TO phonon absorption strength of β-SiC in infrared absorption spectra. The transmission electron microscopy study shows that the blue peak is also correlated with the microstructures. Because porous β-SiC is nanometer in size, it is suggested that the quantum confinement effect be responsible for the blue light emission.

Conditions of luminescence degradation or enhancement in porous silicon

We report that the exposure of porous silicon(PS) to continuous laser light in air results in quenching the intensity of PL(photoluminescence) spectrum peaked at 800 nm, but enhancing that of PL spectrum peaked at 740 nm, which is not recovered by annealing the samples at 160 °C in vacuum for 1 hour. When the light exposure is done in vacuum, the intensity of the PL spectra is reduced over the entire range of wavelength and easily recovered at room temperature. From the FTIR (Fourier transform infrared spectroscopy) measurements, it has been shown that the light-induced change of PL in air occurs as a result of optically induced oxidation on the anodized surfaces of PS. These results suggest that whether the light soaking in air induces an enhancement or a degradation of PL depends on the degree of oxidation or hydride formation on the surface of as-prepared PS.

Study of band-gap narrowing effect and nonradiative recombination centers for heavily C-doped GaAs by photoluminescence spectroscopy

The band-gap narrowing effect of GaAs as a function of carbon doping concentration has been measured using photoluminescence (PL) spectroscopy on samples grown by metalorganic chemical-vapor deposition. The range of carbon concentration varies from 3.4×1018 to 1.1×1020 cm−3. The experimental results obtained from PL spectra taking tailing effects into account are in good agreement with recent theoretical calculations. The intensity of PL spectra decreases rapidly when the free-carrier concentration is higher than about 4×1019 cm−3. This phenomenon cannot be explained with only the varying tendency of the minority-electron lifetime of the radiative recombination process, indicating the introduction of additional nonradiative recombination centers in heavily C-doped GaAs.

The growth of resonant tunneling hot electron transistors using chemical beam epitaxy

A systematic growth study of InGaAs/AlAs/InGaAsP resonant tunneling hot electron transistors (RHETs) was performed using chemical beam epitaxy (CBE). The resonant tunneling hot electron transistors studied consist of a highly strained AlAs/In 0.75Ga 0.25As/AlAs double barrier structure and an undoped InP collector barrier with 1.1 and 1.2 μm InGaAsP graded layers. These quaternaries were lattice matched to InP within 2.6×10 −4 and showed an averaged full width at half-maximum (FWHM) of 6 meV from low temperature photoluminescence (PL) measurement. The effects of growth interrupt were studied using PL, X-ray diffraction and secondary ion mass spectrometry (SIMS) measurements. It was found that excessive growth interrupt induced high oxygen accumulation (8×10 18 cm −3) at the heterojunction and reduced the intensity of PL spectra. Moreover, for the growth of tunneling heterostructures, low substrate temperature, appropriate growth interrupts and use of hydride drying filters and high purity hydrides were helpful to improve device performance. The highest peak-to-valley current ratio (PVR) observed was 12.7, and maximum base transport ratio was 0.98 at 80 K. Furthermore, some digital functions such as flip-flop gate and exclusive NOR were demonstrated using a single RHET.

Electronic structure of an InAs monomolecular plane in GaAs.

The electronic structure of an InAs monomolecular plane in the host GaAs is calculated by a self-consistent pseudopotential method. In this monomolecular-plane--host-crystal system, both electrons and holes are found to be strongly localized near the inserted InAs plane. The inserted InAs plane behaves somewhat like a quantum well. This result successfully explains the observed intense photoluminescence spectra. We also calculate the band structure for a system of two InAs monomolecular planes separated by N monolayers of GaAs. The calculated band gap as a function of GaAs thickness between the two InAs planes is in good agreement with experimental data.

Improvements of electrical and optical properties of GaAs by substrate bias application during electron-cyclotron-resonance plasma-excited molecular beam epitaxy

Electrical and optical properties of GaAs layers are improved by applying substrate bias during electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE). The highest mobility obtained without substrate bias is 4500 cm2 V−1 s−1. By applying positive bias, the mobility of the grown layer is increased to as high as 6800 cm2 V−1 s−1. Intense photoluminescence spectra comparable to that of a high quality MBE-grown layer are also observed. The suppression of high-energy ion impingement on the growing surface plays an important role in obtaining high quality crystal.

Anomalous excitation-intensity dependence of photoluminescence properties of an asymmetric coupled quantum well structure

We report anomalous emission peak shifts as a function of excitation intensity in photoluminescence spectra from a p-i-n structure where asymmetric coupled quantum wells are embedded in the intrinsic region. Emission behavior can be explained in terms of an anticrossing of quantized levels: this occurs when the energy-band structure automatically becomes flat by means of reduction of the internal electric field by photogenerated carriers. This result may be of significance in creating novel all-optical devices where transition energies are able to be optically modulated.