eV Photoluminescence Research Articles

Time resolved photoluminescence study of Si modulation doped GaN/Al0.07Ga0.93N multiple quantum wells

AbstractThe effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation doped GaN/Al0.07Ga0.93N multiple‐quantum‐well (MQW) structures were studied by means of photoluminescence (PL) and time‐resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320–420 ps at 2 K for all doping levels. For the undoped and low‐doped samples (3 × 1018 cm−3), which have less free electrons in the QWs, a non‐exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm−3 to 1020 cm−3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. This localization effect appears even at high electron concentrations to cancel the expected lowering of the radiative lifetime with doping at 2 K; such a lowering is clearly observed at elevated temperatures for the highly doped samples, however. The internal polarization‐induced fields of the medium and highly‐doped samples are partly screened by the electrons originating from the doping in the barriers. Only the PL peak of the undoped and low‐doped samples shows a redshift with time delay, related to the photogenerated carriers. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum structures in Zn–Se–Te system containing submonolayer quantities of Te

AbstractIn this paper we show that a multilayer Zn‐Se‐Te system, grown by molecular beam epitaxy in such a way as to contain sub‐monolayer quantities of Te (so‐called delta‐doped ZnSe:Te), exhibits optical properties consistent with the presence of quantum size centers. We present results of excitation intensity and temperature dependent cw photoluminescence (PL) as well as time‐resolved PL measurements that explicitly show that the ensemble of type‐II quantum islands is formed in this system. Specifically, for the 2.46–2.51 eV PL we show that (1) its maximum shifts as a cube root of excitation intensity; (2) its full‐width‐at‐half‐maximum decreases with the temperature while its maximum undergoes a red shift in the temperature interval where the integrated PL intensity stays approximately constant; (3) the PL decay is non‐exponential at high excitation intensities and a single exponential decay at low excitation intensities; (4) the PL life‐time initially increases with temperature from of 86 ns at T = 15 K to 143 ns at T = 115 K before deceasing due to non‐radiative processes. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The interface mechanism of photoluminescence in carbonized porous silicon

The interface mechanism of photoluminescence (PL) in carbonized porous silicon is considered. The model involves processes in a heterojunction between p-Si and p-SiC nanodimensional grains and the interfacial SiOx and SixOyC1−x−y layers, where the radiative annihilation of charge carriers takes place. Irrespective of the size of nanograins, the bandgap width in the interfacial layers remains constant, which accounts for the fixed positions of red (1.8 eV) and blue-green (2.4 eV) PL bands observed in the experiment. The dimensions of nanograins were determined using X-ray diffraction and scanning tunneling microscopy.

Relationship of basal plane and prismatic stacking faults in GaN to low temperature photoluminescence peaks at ∼3.4 eV and ∼3.2 eV

ABSTRACTThe relationship between the optical properties and microstructure of GaN is of great interest due to the important optical and electronic applications of this material. Several different studies have been reported attempting to link the low temperature photoluminescence (PL) peak at ∼3.4 eV to the presence of various microstructural defects. However, no clear systematic studies have been reported establishing such a link for the PL peak observed at ∼3.2 eV. In this paper, we present evidence linking the ∼3.4 eV PL peak to the presence of a thin layer of cubic phase associated with basal plane stacking faults (BSF). This relationship is mainly established by studying a series of ammonothermally-grown GaN bulk crystals. The existence and strength of the ∼3.4 eV peak are found to be related to the I2 type BSF (RI2=1/3<1>) observed in these samples. To investigate the relationship between the ∼3.2 eV peak and structural defects, a series of GaN epilayers grown on either SiC or sapphire (of various off-cut angles) was investigated by TEM and PL spectroscopy. Samples grown on 3.5° off-cut SiC and 5° and 9° off-cut sapphire substrates exhibit PL peaks near ∼3.2 and ∼3.4 eV, which are absent in the on-axis SiC and sapphire cases. TEM shows that the former group of samples has defect configurations consisting of prismatic stacking faults (PSFs) folding back and forth between two different {1120} planes connected by stair rod dislocations, which in turn fold onto to I1 type BSFs again with stair rod dislocations at the fault intersections. The ∼3.2 eV PL peaks are proposed to possibly arise from transitions involving the PSFs and the stair rods associated with their mutual intersections and their intersections with the BSFs. The ∼3.4 eV peak is again attributed to the thin layer of cubic phase associated with the I1 type BSF (three bilayers as opposed to four bilayers for the I2 type BSF).

Effect of thermal heat treatment on oxygen-deficiency-associated defect centers: Relation to 1.8 eV photoluminescence bands in silica glass

Our previous studies have reported the existence of a 1.8 eV photoluminescence (PL) band [full width at half-maximum (FWHM) of 0.2–0.4 eV, decay time τ of ∼200 ns, PL excitation peak of 2.1 eV] in silica glass exposed to visible light. Oxygen-deficiency-associated defect centers [Si clusters or SiOx (x<2) structure] were thought to be the origin of this PL band. In this article, we describe the thermal heat treatment characteristics of the 1.8 eV PL band in silica glass (excited by visible light). In the low-temperature region (∼600 °C), we observed a decrease in the intensity of this PL band. On the other hand, the 2.1 eV PL band (FWHM of ∼0.32 eV, τ unknown) was observed for the case of annealing at a temperature around 1050 °C. We propose that the high probability SiOx (x<2) structure rather than the Si clusters plays a predominant role in the generation of the 1.8 eV PL band, and that crystalline Si nanoparticles are probably the origin of the 2.1 eV PL band.

Temperature dependence of luminescence from silica glasses under in-reactor and 60Co gamma-ray irradiation

In order to investigate the temperature effects on the dynamic radiation damaging process, we have carried out in situ measurements of in-reactor luminescence (IRL) and gamma-ray induced luminescence (GIL) of a silica glass at temperatures ranging from 70 K to 370 K. Both luminescence spectra were found to consist of two broad emission centers at 3.1 eV and 4.1 eV with an additional temperature independent emission around 2.5 eV. The 2.5 eV emission different from the other two showed long tail to the lower energy side and was attributed to the Cherenkov radiation. The 3.1 eV band was attributed to a B 2 β oxygen deficient center on the basis of our photoluminescence measurement. The intensity of the 3.1 eV IRL increased with increasing temperature up to ca. 200 K and saturated above 200 K, which is clearly different from the reported temperature dependence of 3.1 eV photoluminescence, suggesting the existence of some different relaxation mechanism of excited electron under ionizing radiations.

Polarized micro‐photoluminescence spectroscopy of GaN nanocolumns

We propose inversion domains (IDs) to be the origin of the 3.42 eV photoluminescence (PL) band in GaN epilayers and nanocolumns. A shift of the band relatively to the near-edge PL band is induced presumably by different strain in the IDs. Micro-PL studies of nanocolumns enriched by IDs reveal anti-correlated intensity variation as well as a similarity between temperature and power dependences of both bands. A change of dominant polarization takes place across the spectra, being likely related to variation of exciton level ordering at tensile strain. Discrete narrow lines observed in the spectra are considered as manifestation of strain-induced one-dimensional carrier confinement in the IDs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect-related luminescence of GaN:Zn films thermally treated in a radio-frequency ammonia plasma

The effect of thermal annealing in nitrogen radicals obtained by the treatment of NH3 in a radio-frequency discharge on the luminescence properties of GaN:Zn films grown by MOCVD/hydride epitaxy on sapphire (0001) substrates is investigated. As the thermal treatment temperature was increased, a steady weakening of the violet (2.88 eV) and near-edge (3.48 eV) photoluminescence bands was observed. As a result of the thermal treatment in nitrogen radicals at 500–750°C, new bands that peaked at 3.27 and 3.42 eV were detected; the intensities of these bands increased with increasing treatment temperature. The mechanism of formation and the origin of all the bands are analyzed comprehensively. It is found that the luminescence bands at 2.88, 3.42, and 3.27 eV are characteristic of the GaN films obtained by practically each technology and are associated with the simple structural defects. The participation of O in the formation of the band at 3.42 eV is proved experimentally.

Critical layer thickness enhancement of InAs overgrowth on porous GaAs

In the present work we have investigated the initial stage of InAs layer grown on porous GaAs (π-GaAs) substrate by using reflection high-energy electron diffraction (RHEED) and low temperature photoluminescence (PL). RHEED measurements show that the 2D–3D growth mode transition appears after a deposition of 4.2 atomic monolayer (ML) of InAs, which is higher than that deposited on nominal GaAs (1.7 ML). PL investigations show two luminescence bands at 1.24 and 1.38 eV. The 1.24 eV PL peak emission is associated to the radiative transitions in InAs quantum dots (QDs), whereas the 1.38 eV PL peak emission is attributed to the InAs wetting layer (WL). The results show that π-GaAs is a promising candidate to obtain a reduced QDs size distribution, and to grow pseudomorphic epitaxial layer on GaAs substrate with higher indium concentration.

Near-infrared photoluminescence in titania: Evidence for phonon-replica effect

The photoluminescence of rutile and anatase TiO2 has been investigated in the range of 360–890 nm at several temperatures. An unexpected intense near-infrared (1.53 eV) photoluminescence band was recorded for both phases. At low temperatures, the resulting bands form a substructure of equally spaced peaks irrespective of the phase. The spectra were interpreted in the framework of the single-configuration-coordinate model as the phonon-replica effect, originating from ionization of oxygen vacancies.

Optical properties of nanometre AgI–silica composite synthesized by a simplequenching method

Optical absorption and room temperature photoluminescence (PL)properties of nanometre AgI–silica composite synthesized by a simpleheating–quenching method were investigated. The absorption of quenchedAgI–silica was enhanced markedly. Three absorption bands at 440 (2.8eV), 260 (4.8 eV) and 220 nm (5.6 eV) were observed. The excitonicabsorption of AgI showed a red shift of ∼0.11 eV,in contrast to the usually observed blue shift in AgI nanocrystals.Besides two UV emissions from a non-bridging hole centre andE′centre defects formed in silica, the composite exhibited two PL emissions at 2.62and 2.40 eV. The 2.62 eV PL was ascribed to donor–acceptor recombination ofAgI, while the 2.40 eV one may correlate with energy levels induced by theinteraction between silica and AgI. The role of silica in improving the PL fornanometre AgI is discussed.

Contributions from gallium vacancies and carbon-related defects to the “yellow luminescence” in GaN

Carbon-doped GaN layers grown by molecular-beam epitaxy are studied with photoluminescence and positron annihilation spectroscopy. Semi-insulating layers doped with >1018 cm−3 carbon show a strong luminescence band centered at ∼2.2 eV (yellow luminescence). The absolute intensity of the 2.2 eV band is compared with the gallium vacancy concentration determined by positron annihilation spectroscopy. The results indicate that a high concentration of gallium vacancies is not necessary for yellow luminescence and that there is in fact a causal relationship between carbon and the 2.2 eV band. Markedly different deep-level ionization energies are found for the high-temperature quenching of the 2.2 eV photoluminescence in carbon-doped and reference samples. We propose that while the model of Neugebauer and Van de Walle [Appl. Phys. Lett. 69, 503 (1996)] applies for GaN of low carbon concentration, a different yellow luminescence mechanism is involved when the interstitial carbon concentration is comparable to or exceeds the gallium vacancy concentration.

Photoluminescence and its excitation mechanisms in Si wires and dots

Investigations of photoluminescence, its temperature dependence, Raman scattering and IR absorption spectra were done for the study of the photoluminescence mechanism in porous silicon and Si enriched silicon oxide films. Red (1.6-1.7 eV) and orange (1.9-2.2 eV) photoluminescence bands are observed in both objects. Comparative investigations indicate that an oxide defect related mechanism is involved in the emission of PL bands in Si wires and silicon oxide films. Photoluminescence excitation mechanisms are discussed as well.

Analysis of the 1.55 eV PL band of CdTe polycrystalline films

Photoluminescence (PL) spectra of CdTe at low temperatures usually show a narrow band at 1.55 eV, sometimes together with phonon replicas. It has been shown that this band is associated with a transition involving an acceptor level due to a cadmium vacancy; however, there is discrepancy about the high-energy level which causes this transition. It is assumed that this high-energy level comes from the bottom of the conduction band or arises from non-excited level due to a superficial donor. In order to clarify the origin of this level, we have carried out selective pair luminescence (SPL) studies at 10 K and also analyzed the temperature dependence of PL in CdTe polycrystalline samples. Our results allowed us to conclude that the 1.55 eV PL band arises from the overlap of two bands peaking at 1.550 and 1.556 eV, respectively. The first one is associated with a donor–acceptor pair transition and the second one (1.556 eV) is probably due to a transition involving excitons bound to a superficial acceptor of the type Cu i +–V Cd −.

Room temperature photoluminescence from Zr 4+-doped sol–gel silica

Intense room-temperature photoluminescence (PL) from the UV to the green region was observed from Zr 4+-doped silica synthesized by a sol–gel process using tetraethoxysilane as the precursor, followed by thermal treatment at 500 °C in air. The wide PL band can be resolved into three components centered at 3.70, 3.25, and 2.65 eV, respectively. The intensity of the 3.25 and 2.65 eV PL bands was greatly enhanced compared with pure sol–gel silica. The 3.70 eV emission was assigned to non-bridging oxygen hole centers, while the 2.65 eV one originated from neutral oxygen vacancies ( V O). The 3.25 eV PL band was most likely associated with E′ centers, as supported by electron spin resonance measurement. It was proposed that the Zr 4+-doping leads to oxygen deficiency in the silica, thus resulting in enhancement of the density of V O and E′ center defects.

Band gap and cathode- and photoluminescences from LiInO2 films

LiInO 2 is one of the candidate materials as a solid-state scintillator for solar neutrinos due to an inverse β−-decay of In115 to Sn115. LiInO2 films (space group I41/amd) having a tetragonal phase are grown by a sol–gel spin-coating method using Li and In acetates as raw materials. Six Raman active modes related to the space group are observed at 156, 250, 278, 382, 520, and 576 cm−1. The band gap of LiInO2 is found to be direct with a forbidden gap of 4.2 eV from an optical absorption spectrum. As a virtual experiment for testing a scintillation due to β−-ray self-emitted from In115 in LiInO2 by solar neutrinos, a cathode luminescence (CL) study is performed. A CL peak is observed in the wavelength range of 533–556 nm (2.3–2.2 eV) for various electron energies (5–20 kV) at room temperature. Possible origins of the yellow emission (∼2.2 eV) are oxygen vacancies introduced during the sol–gel synthesis and/or residual impurities included in the coating solution. A 2.3 eV photoluminescence is also observed at 20 K.

Laser-induced treatment of silicon in air and formation of Si/SiO x photoluminescent nanostructured layers

Two different mechanisms of laser-induced treatment of silicon in atmospheric air are compared. In one, silicon target was ablated by pulsed UV radiation, which led to a formation of microscale spikes under the irradiation spot and a deposition of material around it. The treated material contained silicon nanocrystals and exhibited weak photoluminescence (PL) signals, whose peak position was not uniform over the treated surface and varied from 2.0 to 2.3 eV in different points. In another mechanism, silicon target served to provide first electrons in order to initiate a breakdown in surrounding air by pulsed IR radiation. As a result, a highly porous nanostructured layer was formed under the contact of the target with the breakdown plasma. All points of the layer exhibited only 1.95 eV PL signals, whose intensity was stronger than in the case of the UV ablation by at least an order of magnitude. Possible mechanisms of nanostructure formation and PL origin are discussed.

On the origin of the 2.2–2.3 eV photoluminescence from chemically etched germanium

The photoluminescence (PL) at ∼2.2–2.3 eV from Ge-based nanocrystalline materials is described in the literature as nanocrystal size-independent. We have observed visible luminescence from two different types of stain-etched Ge samples, one prepared after Sendova-Vassileva et al. (Thin Solid Films 255 (1995) 282) in a solution of H 2O 2:HF at 50:1 volume ratio, and the other in a solution of HF:H 3PO 4:H 2O 2 at 34:17:1 volume ratio. Energy dispersive X-ray analysis (EDX), Raman and FTIR spectroscopy, and the near edge X-ray absorption structure (XANES), indicate that the chemically etched Ge layers of the former type of samples are composed of non-stoichometric Ge oxides, i.e. GeO x (0< x<2), and free from any Ge nanoconstructions. It is also suggested from XANES that the latter type of chemically etched Ge samples comprise 8–9 nm nanocrystals of Ge, surface-covered with mainly oxygen. Photoluminescence occurred at ∼2.3 eV for all samples. The PL behavior of the latter type of chemically etched Ge on annealing in different chemical environments (air or H) allowed us to conclude that the PL from these materials, as well as that from those Ge-based nanocrystalline materials reported in the literature, is from GeO x s.

Oxygen-related red photoluminescence bands in silica glasses

Previous studies have reported the existence of a photoluminescence (PL) band in silica glass that originates from oxygen molecules (PL peak: ∼0.97 eV, full width at half maximum (FWHM): ∼10 meV, decay time ( τ): ⩾100 ms) and ozone (PL peak: ∼1.9 eV, FWHM: ∼0.12 eV, τ: uncertain) following irradiation by F 2 laser (7.9 eV), γ-rays, and neutrons. In this paper, we report the PL properties produced by high-temperature (1200–1500 °C) oxygen processing carried out on silica glass in air. The red PL band (peak: ∼1.85 eV, FWHM: ∼0.33 eV, τ: ∼5 ms) was observed for the case of high-temperature oxygen processing. Our studies indicate that the oxygen processing is closely related to the generation of the 1.85 eV PL.

Spectral Characteristics of Native Defects in BN

Spectral characteristics of native defects from the crystalline lattice of c-BN were studied. It is found that the photoluminescence (PL) spectrum under exposure to ultraviolet-blue light has a complex structure containing a predominant wide 2.5 eV luminescence band at room temperature. The photostimulated luminescence (PSL) consists of a band, which is coincident with the 2.5 eV PL. The results obtained allow to conclude that the same defects are responsible for both the PL and PSL formation.