Peak Energy Shift Research Articles

Photoluminescence of Si-based nanotips fabricated by anodic aluminum oxide template

Large-area, high-density silicon-based nanotips were fabricated using electrochemical lithography. The morphology and optical properties of the samples were characterized by atomic force microscopy and photoluminescence. The distribution and size of the silicon-based nanotips were uniform. Two photoluminescence peaks were observed at 585 and 620 nm. The peak centered at 585 nm exhibited a narrow full-width at half maximum. No evident peak energy shift was observed when the measurement temperature was increased from 10 K to room temperature, which suggested that the photoluminescence should be attributed to the interface states and/or defects in the silicon-based nanotips.

Enhanced internal quantum efficiency of an InGaN/GaN quantum well as a function of silver thickness due to surface plasmon coupling

AbstractExcitation‐power‐dependent photoluminescence (PL) measurements have been carried out on a single InGaN quantum well structure with a different thickness of silver on its surface. This allows for investigation of surface plasmon (SP) coupling enhanced internal quantum efficiency as a function of the silver thickness. A maximal enhancement with a factor of up to 25 times in PL intensity has been observed on the sample with a silver film of 100 nm, compared with the as‐grown sample. A blue shift in peak energy has been observed as a function of silver thickness. Atomic force microscopy has been employed to examine the change in morphology of the silver films with different thickness. Finally an estimation of the internal quantum efficiency is made, showing ∼10.8 times enhancement on the sample with the 100 nm silver, compared to the as‐grown sample. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of samples produced by electroless wet chemical etching: Between silicon nanowires and porous structures

AbstractSamples containing silicon nanowires (Si‐NWs) and highly porous structures (P‐Si) were prepared by electroless wet chemical etching (EWCE) of crystalline silicon wafers using various etching parameters. Photoluminescence (PL) measurements were performed with excitation at 488 nm and a photon energy flux of 337 mW cm−2. According to the diameters of the Si‐NWs (>10 nm), from quantum confinement (QC) theory no shift in PL peak energy compared to the bandgap of crystalline silicon is expected. However, PL measurements show peak emission energies ranging between 1.4 and 1.6 eV. After further treatment of the samples with HF, substantial PL emission was still detectable with the measured PL peak pinned at 1.4 eV irrespective of etching time. We explain the observations by the hypothesis that the persistent part of PL emission is generated by nanocrystals located at the rough sidewalls of the Si‐NWs or residing within the porous sample structure. The part of the PL, which was present before HF treatment, but vanished after the treatment, is attributed to the presence of silicon suboxide surrounding the Si‐NWs or covering other Si surfaces. This hypothesis is explored by means of three sample series, prepared with different preparation parameters. In the first series the time used during the initial metallization step in order to prepare an Ag nanoparticle layer on the top surface was varied, in the second series the etching time was the changed parameter and in the third series the HF to H2O2 concentration ratio was varied.magnified image Strong visible orange colored PL of a sample produced by EWCE starting from a heavily doped wafer (n‐type c‐Si (111), As as dopant) and excited at 337 nm (the sample was mounted on a glass substrate. Blue luminescence is due to the substrate).

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton—longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements. With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang—Rhys factor, increases remarkably due to an enhancement of the internal electric field. With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang—Rhys factor decreases. These results reveal that there is a large built-in electric field in the well layer and the exciton—LO-phonon coupling is strongly affected by the thickness of the cap layer.

Estimation of the location of embedded InGaAs/GaAs quantum dots by measuring strain-induced blueshift of photoluminescence

A method was proposed to estimate the location of embedded InGaAs/GaAs quantum dots (QDs) as the distance from the center of a flat cylindrical nanoprobe of radius 600 nm, by measuring the blueshift of photoluminescence (PL) under elastic indentation of the nanoprobe onto the sample surface. The energy of the enhanced fine PL peaks originating from single QD was observed to blueshift linearly with indentation force. The PL peak energy shift per unit force, i.e., the blueshift rate, was measured in the range 22.7–87.8 meV/mN. Analyzing the nanoprobe-induced strain and energy shift of the ground state of QD, based on a numerical simulation, provided a quantitative relationship between the blueshift rate and the QD’s distance from the probe center. With this relationship, we succeeded in estimating the location of the embedded QDs from the experimentally obtained blueshift rate. The observed QDs were determined to be located around the nanoprobe edge, which coincides with the observation that only the PL emitted from the QDs located under the probe edge is enhanced and observable under nanoprobe indentation. The method presented in this paper provides a reliable and simple way to estimate the location of the embedded QDs with respect to the probe center by nanoprobe indentation.

Probing the strain effect on near band edge emission of a curved ZnO nanowire viaspatially resolved cathodoluminescence

Curved ZnO nanowires were deliberately prepared on a Si substrate and the strain effecton their near band edge (NBE) emission was investigated by spatially resolvedcathodoluminescence (CL). By moving the electron beam step-by-step across individualcurved nanowires and acquiring the CL spectra simultaneously, we found that the NBEemissions from the inner region of the curved nanowires with compressive strain showblueshift, while those from the outer region with tensile strain show redshift. Both thestrains have been estimated from the local curvature by a geometrical model andhave been further examined by high-resolution transmission electron microscopy.A nearly linear relation between the strain and the peak energy shift in NBEemission was obtained. The result indicates that the optical band gap of ZnOnanowire is quite sensitive to and can be readily modulated by the induced strainvia simply curving the nanowire, which has potential applications for designingnew optical-electromechanical (OEM) and flexible optoelectronic nanodevices.

Poly(vinyl chloride) plasticized with succinate esters: synthesis and characterization

Phthalates pose adverse health effects due to their propensity to leach and the most common, di(2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DOP), are petroleum-based. Conversely, di-esters, succinates are biobased (produced from fermentation of biomass), biodegradable, and therefore potential sustainable replacements for phthalates. A series of succinates, di-octyl succinate (DOS), di-hexyl succinate (DHS), di-butyl succinate (DBS), and di-ethyl succinate (DES), were mixed with poly(vinyl chloride) (PVC). The interaction of the plasticizer ester carbonyl with PVC shows an average −5 cm−1 shift of the carbonyl absorbance peak energy. The glass transition temperatures (T g), were monitored by differential scanning calorimetry and dynamic mechanical analyses. The T gs of DOS and DHS plasticized PVC were significantly lower than DOP plasticized PVC at a lower percent mass. On the other hand, PVC plasticized with either DBS or DES exhibited a similar trend in lowering the T g as that of DOP plasticized PVC.

Cathodoluminescence study on in composition inhomogeneity of thick InGaN layer

We have investigated the optical properties of thick InGaN film grown on GaN by cathodeluminescence (CL) spectroscopy. It is found that there is obvious In composition variation in both growth and lateral direction of InGaN film. The depth distribution of In composition is closely related to the strain relaxation process of InGaN film. Accompanied with the relaxation of compressive strain, the In composition of InGaN layer increases and the CL peak energy shifts towards red. Moreover, a rather apparent In composition fluctuation is found in the relaxed upper part of InGaN layer as confirmed by CL imaging.

Quantum-confined direct band transitions in tensile strained Ge/SiGe quantum wells onsilicon substrates

We directly demonstrate quantum-confined direct band transitions in the tensile strainedGe/SiGe multiple quantum wells grown on silicon substrates by room temperaturephotoluminescence. The tensile strained Ge/SiGe multiple quantum wells withvarious thicknesses of Ge well layers are grown on silicon substrates with a lowtemperature Ge buffer layer by ultrahigh vacuum chemical vapor deposition.The strain status, crystallographic, and surface morphology are systematicallycharacterized by high-resolution transmission electron microscopy, atomic forcemicroscopy, x-ray diffraction, and Raman spectroscopy. It is indicated that thephotoluminescence peak energy of the tensile strained Ge/SiGe quantum wellsshifts to higher energy with the reduction in thickness of Ge well layers. Thisblue shift of the luminescence peak energy can be quantitatively explained bythe direct band transitions due to the quantum confinement effect at theΓ point of the conduction band.

Customizing electron confinement in plasma-assembled Si/AlN nanodots for solar cell applications

Size-uniform Si nanodots (NDs) are synthesized on an AlN buffer layer at low Si(111) substrate temperatures using inductively coupled plasma-assisted magnetron sputtering deposition. High-resolution electron microscopy reveals that the sizes of the Si NDs range from 9 to 30 nm. Room-temperature photoluminescence (PL) spectra indicate that the energy peak shifts from 738 to 778 nm with increasing the ND size. In this system, the quantum confinement effect is fairly strong even for relatively large (up to 25 nm in diameter) NDs, which is promising for the development of the next-generation all-Si tandem solar cells capable of effectively capturing sunlight photons with the energies between 1.7 (infrared: large NDs) and 3.4 eV (ultraviolet: small NDs). The strength of the resulting electron confinement in the Si/AlN ND system is evaluated and justified by analyzing the measured PL spectra using the ionization energy theory approximation.

Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure

This paper reports on a comparative study of the spatial distributions of the electrical, optical, and structural properties in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN template layer is shown to decrease in the regions of the wafer where the heterostructure sheet resistance increases and the GaN photoluminescence band-edge energy peak shifts to a high wavelength. This phenomenon is found to be attributed to the local compressive strain surrounding edge dislocation, which will generate a local piezoelectric polarization field in the GaN layer in the opposite direction to the piezoelectric polarization field in the AlGaN layer and thus help to increase the two-dimensional electron gas concentration.

Photoluminescence and magneto-optical properties of multilayered type-II ZnTe/ZnSe quantum dots

Multilayered Zn–Se–Te structures grown by migration enhanced epitaxy are studied by temperature- and excitation-dependent photoluminescence (PL) as well as magneto-PL. The PL consists of two bands: a blue band, overlaid with band edge sharp lines, dominant at low temperatures and high excitation, and a green band, which appears at elevated temperature and low excitation. Upon varying excitation intensity by four orders of magnitude, the green band peak energy shifts by ∼60 meV, indicating recombination of excitons in type-II quantum dots (QDs); no significant shift is observed for the blue band. Therefore, the green emission is attributed to ZnTe/ZnSe type-II QDs, which co-exist with isoelectronic centers, responsible for the blue and band edge emissions. The existence of type-II ZnTe/ZnSe QDs is further confirmed by magneto-PL, for which the observed oscillations in the PL intensity as a function of magnetic field is explained in terms of the optical Aharonov–Bohm effect.

Carrier Dynamics of Deep-Level States in InGaN/GaN Multiquantum Wells

InGaN/GaN (3 nm/10 nm) multiquantum wells (MQWs) were grown on (0001) sapphire substrates by metal–organic chemical vapor deposition. To characterize the effect of internal electric fields on deep-level states in InGaN MQWs, deep-level transient spectroscopy (DLTS) and bias-dependent photoluminescence (PL) measurements were performed. The thermal activation energy of MQWs obtained from DLTS spectra decreased to 0.15 eV with an increase in reverse measure bias from -2 to -10 V, which indicates partially cancelled internal fields. The blue shift of PL peak energy was about 0.05 eV when the reverse measure bias was changed from 0 to -5 V. This is explained well by carrier dynamics in the conduction band of InGaN/GaN MQWs, which affects carrier recombination energies corresponding to the band edge to edge transitions with changing reverse measure bias.

Photoluminescence and excitation spectrum of Mg‐doped p‐type AlXGa1–XN

AbstractWe have been measured the near band edge photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg doped AlGaN. PL and PLE peak energies were almost corresponding to those of undoped AlGaN. From the temperature dependent measurement of the PL spectra, the PL peak energy shift as a function of temperature showed S ‐shaped behaviour. These results suggest that the mechanisms of the near band edge PL of AlGaN:Mg are basically similar to those of undoped AlGaN. Using the known spectral feature of undoped AlGaN which consists of main and LO phonon replica emission bands at even interval energy and same line width, the spectral decomposition of the PL spectrum reveals an additional band which is expected as the transition to the acceptor level arising from Mg doping. The temperature dependences of its peak energy and intensity suggest relative temperature shift of the acceptor level and also may imply the existence of another relaxation pathway. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The crystal field interaction at the rare earth site inErNiAl4

Inelastic neutron scattering spectra are reported for orthorhombicErNiAl4 at temperatures ranging from 2.1 to 280 K. The neutron transitions are interpreted in terms ofthree reliably identified excited crystal field (CF) levels and four tentative excited levels for theJ = 7/2 groundterm of Er3+ at the single Er site. A shift in transition peak energy between 2.1 and 8.6 K isattributed to Zeeman splitting induced by magnetic order of the Er sub-lattice belowTN = 5.8 K. With the aid ofa suite of possible 155Gd-Mössbauer spectroscopy derivations of the rankn = 2 CFparameters and a simple point charge model calculation of within-rank ratios for the higher rank(n = 4,6) parameters, estimates are made for all nine CF parameters required for the orthorhombicC2v (mm) Er-site symmetry.

Injection current‐dependent quantum efficiency of InGaN‐based light‐emitting diodes on sapphire and GaN substrates

AbstractWe studied the nonthermal mechanism of quantum efficiency (QE) roll‐off in InGaN‐based multiple‐quantum‐well LEDs on sapphire and bulk GaN substrates with In content ranging from 1% to 28%. Both the efficiency evolution and peak energy shift appear to be strongly dependent on the In content, but nearly independent of the dislocation density in the active region. The QE of the green LED peaks at a current density as low as 1.4 A/cm2, and decreases dramatically as current is increased, whereas the UV LED has a nearly constant QE at currents up to 1 kA/cm2. In contrast to minimal current‐induced peak shift and spectral broadening in the near‐UV and UV LED, the green LED has a monotonic peak blueshift of a total amount of ∼110 meV and a spectral broadening by a factor of 2. These results lead to a conclusion that carrier delocalization followed by nonradiative recombination at misfit defects is the major nonthermal mechanism of the efficiency roll‐off in InGaN‐based visible LEDs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the photoluminescence of self-assembled InAs/GaAs quantum dots studied in high magnetic fields

We have investigated the photoluminescence (PL) of self-assembled InAs/GaAs quantum dots (QDs) in high magnetic fields of up to 50 T and as a function of temperature. Our data clearly indicate that two different mechanisms are at work. At low temperatures (T < 80 K), the zero-field PL is increasingly dominated by lower energy dots. High-field measurements however demonstrate that these dots are larger in size only in the growth direction. At temperatures above 100 K, a strong decrease of the PL peak energy shift with field is observed, while the zero-field PL is characterized by a redshift according to the changes in the bandgap. We discuss these contradictory observations in terms of a phenomenon that we call field-assisted enhancement of the QD barrier potential. Since this effect is much stronger for small high-energy QDs, the latter progressively dominate the PL emission when temperature and magnetic field are increased.

Dynamics of biexciton localization inAlxGa1−xNmixed crystals under exciton resonant excitation

We report the localization dynamics of biexcitons in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals under exciton resonant excitation at low temperatures. During a few tens of picoseconds just after intense laser excitation, the photoluminescence (PL) spectral shape obeys an inverse Maxwell--Boltzmann distribution and free biexcitons dominate the PL spectrum. With a further increase in the delay time, the biexciton PL peak energy and edge energy shift to lower energies. These redshift behaviors in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals are completely different from the behaviors of free biexcitons in GaN crystals. Our observations reveal the rapid transformation dynamics from free to localized biexcitons in band-tail states in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals.

Magneto-optical spectroscopy of asymmetric double quantum wells CdSe/CdMnSe/CdSe with semimagnetic coupling barriers

We have studied magneto-photoluminescence and magneto-reflection spectra of novel asymmetric double quantum well structures CdSe/CdMnSe/CdSe with semimagnetic coupling barriers of different thicknesses. Large energy shifts of the exciton peaks of CdSe quantum wells were observed at magnetic fields applied in Faraday configuration. The shifts exceeded considerably the values calculated in a standard theory for abrupt interfaces of the coupling barrier. A model of a graded barrier was utilized originating from inter-diffusion of Cd and Mn ions over a distance of 2–3 monolayers. Increased magnetization of the interface layers of the coupling barrier due to the decreased number of antiferromagnetic Mn2+ pairs was also taken into account. This model allowed us to fit experimental data adequately.

Influence of substrate misorientation on properties of InGaN layers grown on freestanding GaN

AbstractThe influence of GaN substrate misorientation on properties of Inx Ga1‐x N epilayers (0.05 &lt; x &lt; 0.1) has been examined. X‐ray Bragg reflection, photoluminescence (PL) peak energy and full width at half maximum were analyzed. Higher misorientation angles with respect to the c‐plane (up to 2.30°) give lower average In incorporation in InGaN layers, causing a blue shift of the PL peak energy. In contrast with expectations, higher misorientation (i.e., lower In content) leads to higher alloy disorder. This is reflected in the spectral width of PL lines and in a well‐pronounced S‐shape of PL vs. temperature for more misoriented samples. The obtained results clearly show that the misorientation of the substrate can be used as a parameter for the growth of InGaN layers, enabling the control of crucial light emitting device parameters such as emission energy and In fluctuation effects. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)