Photoluminescence Wavelength Research Articles

Effects of surface adsorption on the photoluminescence wavelength of silicon nanocrystal

Surface-passivated silicon nanocrystallites were prepared by pulsed laser ablation in hydrogen gas, and the photoluminescence (PL) properties were measured by varying the ambient gas to reveal the effect of surface adsorption on recombination processes. The PL peak wavelength of the as-deposited sample was about 800 nm in vacuum. This peak shifted to about 660 nm when the sample was exposed to the atmosphere for about 5 min. This peak shift was partially reversible; the 660 nm peak red-shifted to 730 nm when the chamber was evacuated again. This reversible peak shift was not observed on exposure to pure oxygen gas, but the similar shift was observed in water vapor. These results indicate that this peak shift was caused by physical adsorption of water molecules on the surface of the Si nanocrystallites. Prolonged exposure results in a peak shift to about 580 nm, and this shift was not reversible. This result indicates that the defect created by oxidation create PL centers at 580 nm which is insensitive to the adsorption of molecules. A calculation with the semi-empirical molecular orbital method suggests that the adsorption of water molecules affects the lattice relaxation of the excited state.

Spatially resolved photoluminescence and Raman spectroscopy of bandgap gratings fabricated in GaAs/AlAs superlattice waveguide using quantum well intermixing

Photoluminescence (PL) and Raman spectroscopy were devised to study bandgap gratings fabricated in a waveguide with a core consisting of GaAs/AlAs short superlattice (SSL) structure using quantum well intermixing (QWI). The spatially resolved PL is carried out on the superlattice layer from the sample surface, as well as from a cleaved facet to the side of the epitaxial layer. The PL from the sample surface showed a modulation in the PL wavelength with shoulders being observed. The root cause of this observation was resolved via PL scans of the cleaved side of the grating. The results were further supplemented via Raman spectroscopy measurements, where Raman spectra were collected at different depths of the SSL through probing the SSL layer cross-section. SSL-related phonon states were clearly observed for the lower part of the SSL layer where intermixing did not take place, while the Raman spectra for the intermixed regions showed regular bulk phonon modes, indicating the SSL layer intermixing.

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Hydrogenated amorphous silicon (a-Si:H) nanoball films which include Si nanocrystals, are fabricated by the double-tubed coaxial-line-type microwave plasma chemical vapor deposition (DTCL-MPCVD) system. Photoluminescence (PL) is observed at room temperature after the a-Si:H nanoball films are oxidized at high temperature in air and/or pure oxygen atmosphere. In this study, the HF treatment is performed to a-Si:H nanoball. By the HF treatment, the photoluminescence wavelength has shifted from about 740 nm to 590 nm. We have measured the diameter of Si nanocrystal using the X-Ray Diffraction (XRD) and the Transmission Electron Microscope (TEM). It is found that the diameter of the Si nanocrystal decreases from about 5.0 nm to 3.0 nm, by the HF treatment. After changing the conditions of oxidation, PL peak wavelengths are shifted from about 740 nm to 500 nm. We have determined the diameter of Si nanocrystal contained in the a-Si:H nanoball using XRD and TEM. Consequently, it is found that the diameter of Si nanocrystal becomes small from about 5.1 nm to 3.4 nm.

Anomalous temperature dependence of photoluminescence peak energy in InAs/InAlAs/InP quantum dots

We have observed an unusual temperature sensitivity of the photoluminescence (PL) peak energy for InAs quantum dots grown on InAs quantum wires (QDOWs) on InP substrate. The net temperature shift of PL wavelength of the QDOWs ranges from 0.8 to −4 Å/°C depending upon the Si doping concentration in the samples. This unusual temperature behavior can be mainly ascribed to the stress amplification in the QDOWs when the thermal strain is transferred from the surrounding InAs wires. This offers an opportunity for realizing quantum dot laser devices with a temperature insensitive lasing wavelength.

The influence of vicinal sapphire substrate on GaN epilayers and LED structures grown by metalorganic chemical vapor deposition

The effects of the GaN epilayers and LED structures grown by metalorganic chemical vapor deposition on 1° off c-plane sapphire substrate were studied. Although the 1° off-axis GaN sample has slightly rough surface morphology, it has narrower full-width at half-maximum (FWHM) of X-ray asymmetrical reflection (10–12), higher mobility, lower compensating acceptor concentration and lower etching pits densities (EPD) than the on-axis GaN sample. And it also has lower etching pits densities (EPD). To reduce the threading dislocations in GaN films grown on vicinal substrates is attributed. A very weak yellow luminescence is observed in photoluminescence (PL) spectra of 1° off-axis GaN sample. Finally, comparing to LED structures grown on on-axis c-plane sapphire substrates, the LED structures grown on 1° off substrate show a blue shift of PL wavelength and 2.6 times stronger in PL intensity. The electroluminescence (EL) intensity of 1° LED sample rises faster in small amount of driving current region.

Correlation between Natural Oxidation Process and Photoluminescence Properties of Hydrogenated Si Nanocrystallites Prepared by Pulsed Laser Ablation

Natural oxidation processes of hydrogenated Si nanocrystallites were investigated to clarify effects of surface oxidation on photoluminescence wavelength. Hydrogenated Si nanocrystallites were prepared by pulsed laser ablation in hydrogen gas ambient. The Si–H bonds on the surface of the nanocrystallites enable us to estimate the local configuration of Si–O bonds using infrared frequency shifts. The natural oxidation process was investigated by measuring the density and local configuration of Si–O bonds. The oxidation process can be classified into first and second stages. The first stage is due to the diffusion of oxygen molecules in the nanocrystallites through voids in the porous structure, and the second stage is due to the oxidation of each nanocrystallite from the top surface to the sub-surface. The configurations of Si–O bonds in the first and second stages are silicon-rich and oxygen-rich compositions, respectively. The photoluminescence wavelength was blue-shifted with increasing Si–O bond density. This PL peak shift was not continuous, but three PL peak regions at 800, 600–700, and 400–500 nm were observed. This result indicates that the origin of this PL peak shift is not due to quantum confinement because of decreased diameter of Si nanocrystallites, but is due to the existence of surface oxide. A photoluminescence peak at 800 nm was observed in fresh specimens, and those at 600–700 and 400–500 nm were observed from the first and second stages of oxidation, respectively.

Photoluminescence of CdGeP2 and (Cd,Mn)GeP2

Photoluminescence of CdGeP 2 (112) single crystal and CdGeP 2 epitaxial film grown on GaAs (001) substrate have been studied and their spectral similarity found. Spectral bands associated with donor/acceptor transitions peak at close energies for both substances and all are lower than the energy gap of the chalcopyrite crystal. On the other hand, the growth of (Cd,Mn)GeP 2 ferromagnetic layer on CdGeP 2 (112) single crystal was performed to make it possible observation of PL from both the ferromagnetic layer and substrate. The green laser excitation (514, 532 nm) produces a proper photoluminescence similar to that in the undoped CdGeP 2 crystal and film. An extra emission from the ferromagnetic–nonmagnetic heterojunction occurs to extend up to photon energies exceeding E g of the host semiconductor. The short wavelength photoluminescence is to be due to (Cd,Mn)GeP 2 dilute magnetic semiconductor (DMS). This fact states that Mn-doped II–IV–V 2 chalcopyrites are closer to II–VI DMS than to another group III–V DMS, where the heavy Mn-doping suppresses photoluminescence at all. Features of the observed short wavelength emission are discussing based on the temperature and spectral analyses.

Material quality improvements of ultra-broadband gain materials grown by selective-area-growth techniques

Bulk InGaAs, InGaAsP and multi-quantum-well (MQW) gain materials were grown with selective-area-growth (SAG) techniques using a metal organic chemical vapor deposition (MOCVD) system. A systematic study of wavelength shift, thickness enhancement, and photoluminescence (PL) intensity as a function of oxide and opening widths was performed. From the results, we found that the material degradation is independent of the width of the oxide opening and only depends on the ratio of oxide width and opening width. From the growth thickness enhancement and the wavelength shift studies, we have also found that the wavelength shift in the MQW materials is near 50% contributed from the quantum size effect and 50% from the material composition changes. With the composition change in bulk materials, the amount of compressive strain was calculated. By using a right amount of strain compensation we have improved the long wavelength material quality and achieved high PL gain materials with a very wide wavelength shift.

HigHighly strained InGaAs/GaAs quantum wells emitting beyond 1.2 µm

AbstractHighly strained InxGa1–xAs quantum wells (QWs) with GaAs barriers emitting around 1.2 µm are grown on GaAs substrates by metal organic vapour phase epitaxy (MOVPE) at low growth temperatures using conventional precursors. The effects of growth temperature, V/III ratio and growth rate on QW composition and luminescence properties are studied. The variation of indium incorporation with V/III ratio at a growth temperature of 510 °C is found to be opposite to the results reported for 700 °C. By an appropriate choice of the growth parameters, we could extend the room temperature photoluminescence (PL) wavelength of InGaAs/GaAs QWs up to about 1.24 µm which corresponds to an average indium content of 41% in the QW. The results of the growth study were applied to broad area laser diodes emitting at 1193 nm with low threshold current densities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A 10-GHz Bandwidth Electroabsorption Modulated Laser byUltra-Low-Pressure Selective Area Growth

A novel integration technique has been developed using band-gap energycontrol of InGaAsP/InGaAsP multi-quantum-well (MQW) structures duringsimultaneous ultra-low-pressure (22 mbar) selective-area-growth (SAG)process in metal-organic chemical vapour deposition. A fundamentalstudy of the controllability of band gap energy by the SAG method isperformed. A large band-gap photoluminescence wavelength shift of83 nm is obtained with a small mask width variation (0–30 μm).The method is then applied to fabricate an MQW distributed-feedbacklaser monolithically integrated with an electroabsorption modulator.The experimental results exhibit superior device characteristics withlow threshold of 19 mA, over 24 dB extinction ratio when coupled into asingle mode fibre. More than 10 GHz modulation bandwidth is alsoachieved, which demonstrates that the ultra-low-pressure SAG techniqueis a promising approach for high-speed transmission photonic integratedcircuits.

Luminescence 3D-Ordered Porous Materials Composed of CdSe and CdTe Nanocrystals

The luminescence porous materials of CdTe or CdSe nanocrystals (NCs) were prepared by filling the corresponding NCs into the voids of colloidal crystal by co-deposition of polymer beads and NCs. After removing the beads with tetrahydrofuran (THF), the 3D-ordered porous materials of CdTe (or CdSe) NCs were obtained. The wavelength of maximum photoluminescence of the NCs porous material shows obvious red shift compared with their aqueous dispersion. Under the excitation of high-energy electron the porous materials of CdTe and CdSe NCs will emit photons that can be collected to form a cathode luminescence (CL) image.

Stain etching of silicon pillars and macropores

AbstractWe used a XeCl excimer laser (308 nm, ∼3 J cm–2) and chemically enhanced laser ablation in the presence of SF6 to create arrays of silicon pillars. Etching of these pillars with KOH leads to arrays of macropores whose morphology depends on surface crystallography. Several new stain etchants – containing some combination of HF, NH4HF2, HCl, HNO3, Fe(III), Mn(VII) and water – have been used to cover these structures with a layer of microporous silicon. Both pillars and macropores are black, i.e., exhibit low reflectivity across the visible spectrum. When porosified, pillars exhibit visible photoluminescence whose angular distribution differs from that of planar porous silicon with significantly more intensity at high angles. The peak photoluminescence wavelength depends on the stain etchant composition and exposure to air. The use of metal ions in place of HNO3 in the stain etchant ameliorates the production of gas bubbles and leads to more uniform films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray investigation of quantum well intermixing after postgrowth rapid thermal processing

In order to better understand interdiffusion in quantum well based optoelectronic AlxGa1−xAs/GaAs devices, we have performed rapid thermal processing on similar periodic superlattices (SLs) grown by solid source molecular beam epitaxy. The interdiffusion has been studied by x-ray diffraction measurements. The intermixing of the atoms at the barrier–well interfaces in the temperature range between 800°C and 950°C shifts the energy levels of the bound states and thus the photoluminescence wavelength of the structures. X-ray ω–2θ scans confirm Al–Ga interdiffusion at the barrier–well interfaces and are quantitatively investigated by fitting the envelope function of the SL satellites. A fit to the data yields diffusion lengths in the range 0.2–2.7 nm. Diffusion coefficients and activation energies can be derived from the temperature dependence of diffusion lengths. In the samples that exhibit rougher interfaces, the early stages of interdiffusion at up to 800°C for 1 min show the narrowing and increasing of satellite peak intensities followed by their broadening and decrease of peak maxima at higher temperatures above 850°C. The observation correlates with the initial smoothing of the interfaces

1.3 μ m InAs quantum dots grown with an As2 source using molecular-beam epitaxy

We demonstrate the effects of using an As2 source to fabricate self-organized InAs∕GaAs quantum dot (QD) structures. QDs grown with As2 and As4 sources have narrow photoluminescence (PL) linewidths (22 and 20meV, respectively) and their respective emissions at room temperature are 1.30 and 1.29μm. QDs grown with an As2 source have a longer wavelength emission than those grown with an As4 source under all growth conditions. The density of QDs grown with an As4 source is larger and the dot size smaller than those of QDs grown with an As2 source. These results indicate that QDs grown with As2 are larger, resulting in a longer PL wavelength.

Monolithic integration of an InGaAsP–InP strained DFB laser and an electroabsorption modulator by ultra-low-pressure selective-area-growth MOCVD

The design and basic characteristics of a strained InGaAsP–InP multiple-quantum-well (MQW) DFB laser monolithically integrated with an electroabsorption modulator (EAM) by ultra-low-pressure (22 mbar) selective-area-growth (SAG) MOCVD are presented. A fundamental study of the controllability and the applicability of band-gap energy by using the SAG method is performed. A large band-gap photoluminescence wavelength shift of 88 nm was obtained with a small mask width variation (0–30 µm). The technique is then applied to fabricate a high performance strained MQW EAM integrated with a DFB laser. The threshold current of 26 mA at CW operation of the device with DFB laser length of 300 µm and EAM length of 150 µm has been realized at a modulator bias of 0 V. The devices also exhibit 15 dB on/off ratio at an applied bias voltage of 5 V.

Dilute nitride absorbers in passive devices for mode locking of solid-state lasers

We report on the growth, material and nonlinear optical properties of GaInNAs quantum wells (QWs) used in semiconductor saturable absorber mirrors (SESAMs) for passive mode locking of solid-state lasers in the telecommunication wavelength range from 1.3 to 1.55 μm. The antiresonant SESAMs were grown by molecular beam epitaxy with a nitrogen concentration of 1.6% and 2.6%, respectively. They were subject to rapid thermal annealing to fine-tune the absorption wavelength by blueshifting the photoluminescence wavelength. The appearance of QW intermixing upon thermal annealing was studied by X-ray rocking curve measurements. The thermal annealing procedure was proved not to alter the GaInNAs SESAM by QW intermixing for temperatures up to 800 °C. Optical characterization was applied to investigate the nonlinear SESAM properties. Degenerate pump-probe experiments revealed similar recovery times for both SESAMs in the tens of picoseconds range. We demonstrated stable self-starting passive cw mode locking with sub-10 ps pulses at 1314 and 1534 nm.

Photoluminescent core-shell particles of organic dye in silica

Using a single silica precursor, Rhodamine 6G organic dye molecules have been entrapped in silica particles resulting into core-shell particles of ∼500 nm diameter. Energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and transmission electron microscopy analysis reveals that dye molecules are trapped inside the silica particles. Photoluminescence investigations show that highly luminescent and photostable core-shell particles are formed. Such core-shell particles can be easily suspended in water and would be useful for a variety of applications. However, there is a blue shift in the photoluminescence wavelength in case of core-shell particles compared to bare dye powder sample.

Strong visible emission from porous GaP doped with Eu and Tb ions

We demonstrate strong room-temperature photoluminescence (PL) in the visible region of the spectrum from europium (Eu 3+)-doped porous GaP (por-GaP) layers. Eu 3+ ions were infiltrated into the host matrix by simple impregnation from chloride solution of europium, followed by high temperature annealing in air. From RBS analysis, a rather uniform incorporation of Eu 3+ ions over the porous layer has been evidenced. A systematic study of the PL spectrum versus annealing temperature was performed. It was found that the Eu 3+ ions in GaP crystallites are optically activated after annealing at 900 °C. The excitation mechanism of Eu 3+ in por-GaP is discussed taking into account the dependence on the excitation beam wavelength of PL and the evolution of both the PL intensity and the PL lifetime versus temperature. The PL decay shows a non-exponential behaviour due to multisite phenomena or other processes like resonant and non-resonant energy transfer. The insignificant dependence of the PL intensity and the PL life time on temperature suggests a weak back transfer process from Eu to the host. A schematic band diagram is proposed to illustrate the excitation mechanism at low and high temperature. The PL intensity of Eu 3+ ions was found to be strongly enhanced by co-doping por-GaP with Eu 3+ and Tb 3+. We show that excitation transfer can occur between Tb 3+ and Eu 3+.

Effects of N2O Fluence on the PECVD-grown Si-rich SiOx with Buried Si Nanocrystals

AbstractThe optimized N2O fluence is demonstrated for plasma enhanced chemical vapor deposition (PECVD) of Si-rich substoichiometric silicon oxide (SiOx) films with buried Si nanocrystals. Strong room-temperature photoluminescence (PL) at 550-870 nm has been observed in SiOx films grown by PECVD under a constant SiH4 fluence of 20 sccm with an N2O fluence varying from 105 sccm to 130 sccm. A 22-nm-redshift in the central PL wavelength has been detected after annealing from 15 min to 180 min. The maximum PL irradiance is observed from the SiOx film grown at the optimal N2O fluence of 120 sccm after annealing for 30 minutes. Larger N2O fluence or longer annealing time leads to a PL band that is blue-shifted by 65 nm and 20 nm, respectively. Such a blue shift is attributed to shrinkage in the size of the Si nanocrystals with the participation of oxygen atoms from N2O incorporated within the SiOx matrix. The (220)-oriented Si nanocrystals exhibit radii ranging from 4.4 nm to 5.0 nm as determined by transmission electron microscopy (TEM). The luminescent lifetime lengthens to 52 μs as the nc-Si size increase to > 4 nm. Optimal annealing times for SiOx films prepared at different N2O fluences are also reported. A longer annealing process results in a stronger oxidation effect in SiOx films prepared at higher N2O fluences, yielding a lower PL irradiance at shorter wavelengths. In contrast, larger Si nanocrystals can be precipitated when the N2O fluence becomes lower; however, such a SiOx film usually exhibits weaker PL at longer wavelength due to a lower nc-Si density. These results indicate that a N2O/SiH4 fluence ratio of 6:1 is the optimized PECVD growth condition for the Si-rich SiO2 wherein dense Si nanocrystals are obtained after annealing.

Control of photoluminescence wavelength from uniform InAs quantum dots by annealing

Post-growth annealing of uniform InAs quantum dots (QDs) grown on GaAs(0 0 1) substrates was investigated, and obtained results gave some useful information about control of QD energy level and an intermixing effect between In and Ga atoms. In particular, a wide control of photoluminescence (PL) peak energy (312 meV) and an extremely narrow PL linewidth (13 meV) were obtained from uniform QDs annealed at 700 °C. Photoluminescence properties of annealed QDs depending on anneal conditions were explained by modification of the QD structure and, an interdiffussion effect was discussed.