Micro-photoluminescence Spectra Research Articles

Locally Strained Ge/SOI Structures with an Improved Heat Sink as an Active Medium for Silicon Optoelectronics

The results on the formation of locally strained Ge microstructures on silicon-on-insulator (SOI) substrates and investigation of their optical properties are presented. Suspended Ge structures are formed by optical lithography and plasmachemical and selective chemical etching using the “stress concentration” approach. To provide a heat sink from Ge microstructures, their formation scheme is modified so as to provide the mechanical contact of a part of the suspended microstructure with lower-lying layers. To implement this scheme, SOI substrates with a thin upper Si layer 100 nm in thickness are used. It is shown using the measurements of Raman spectra depending on the pumping power that local heating in such structures decreases. Measurements of the microphotoluminescence spectra show a considerable increase in the signal intensity from strained regions of Ge microstructures as well as the possibility of increasing the maximal optical pumping power (not leading to irreversible changes) for microstructures, in which the mechanical contact of the strained part with lower-lying layers is provided, when compared with suspended structures.

Epitaxial growth and characterization of Cd1−xMnxTe films on Si(1 1 1) substrates

Semiconductors structures with high quality and low cost have been developed and applied in industry on a large scale. The knowledge of basic properties of semiconductor materials growth is important for many technological applications. Cd1-xMnxTe films have been used successfully in optoelectronic, solar cells and high energy detectors applications. However, despite the high potential for application of these films, fundamental growth studies on Si(1 1 1) substrate are not extensively explored. In this study, two Cd1−xMnxTe (CMT) films (with different Mn concentrations) were successfully prepared on Si(1 1 1) substrate employing molecular beam epitaxy. The morphology and surface roughness of the films were investigated using atomic force microscopy (AFM). X-ray diffraction was used to investigate the structural quality and to determine the Mn concentration of the samples. It shows that the films have only (1 1 1) orientation despite a lattice mismatch of almost 19%. Room temperature micro-photoluminescence (µ-PL) spectra shows a high intensity band edge emission and very low intensity defect related emission. Raman spectroscopy of the samples was carried out in order to obtain the vibrational modes. First, second and third order CdTe-like and MnTe-like longitudinal-optical (LO) phonon modes were observed. These results demonstrate the potential of these structures for room temperature applications.

Fabrication and characterization of asymmetric metal/Ge/metal diodes with Ge-on-insulator substrate

We fabricated asymmetric metal/Ge/metal diodes with p-type Ge-on-insulator (GOI) and bulk Ge substrates, from which direct band gap electroluminescence (EL) was observed at room temperature. The integrated EL intensity for p-GOI diodes is approximately 16 times that for bulk p-Ge diodes, owing to the structure of GOI that confines carriers in the very thin Ge layer, and consequently enhances both the electron density in the conduction band and the efficiency of radiative recombination. Micro-photoluminescence spectra of p-GOI show defect-related peaks in the range of 0.70–0.75 eV, of which the intensity and energy depend on the measurement position, which implies fluctuation of defect type and density in p-GOI. Those defects contribute to the low-energy part of the broad EL spectra in p-GOI diodes.

Luminescence landscapes of nitrogen-vacancy centers in diamond: quasi-localized vibrational resonances and selective coupling

77 K micro-photoluminescence spectrum, room-temperature near-field photoluminescence image, and a local atomic arrangement of the nitrogen-vacancy (NV) center in diamond.

Study of femtosecond laser writing in the bulk of Nd3+, Y3+ co-doped CaF2 crystals

We reported on the femtosecond (fs) laser induced permanent refractive index changes, stress-induced birefringence, and micro-photoluminescence properties of Nd3+, Y3+ codoped CaF2 crystals. Permanent changes in the linear optical properties were studied for various pulse energies (0.01 - 5 µJ) at two different repetition rates (10 kHz and 500 kHz). Optimum quantitative phase changes are close to - 1.7π radians within an irradiated area, surrounded by positive phase changes due to the stress field that can reach + 0.7π. Provided that, we avoid heat-accumulation at 500 kHz, written laser lines offer higher linear birefringence on the order of λ/4 at 200 nm. From the spectroscopic point of view, we observed the Raman changes and micro-photoluminescence spectra of irradiated fs-laser Nd3+, Y3+ doped CaF2.

Polychromatic Photoluminescence of Polymorph Boron Dipyrromethene Crystals and Heterostructures

Micrometer-sized boron dipyrromethene crystalline rods were grown from solution. Fluorescence microscopy images reveal that each rod displays characteristic visible light emission of a different color. In a particular case, optical heterostructures with discrete, differently colored sections are observed within a single microrod. Microphotoluminescence (μ-PL) spectra of green and red rods at room temperature show multiple contributions, indicating the presence of microdomains. Temperature-dependent μ-PL measurements further confirm this, as red emission decreases and green emission increases at lower temperatures. These observations are discussed as a result of crystalline polymorphism, leading to a local variation of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy difference. An Arrhenius plot quantifies the hopping barrier for the charge carriers to reach the low emission energy (red) regions. A line scan of a single rod further supports that microdomains of green- and ...

Multiwall MoS2 tubes as optical resonators

We study the optical properties of MoS2 nanotubes (NTs) with walls comprising dozens of monolayers. We reveal strong peaks in micro-photoluminescence (μ-PL) spectra when detecting the light polarized along the NT axis. We develop a model describing the optical properties of the nanotubes acting as optical resonators which support the quantization of whispering gallery modes inside the NT wall. The experimental observation of the resonances in μ-PL allows one to use them as a contactless method of the estimation of the wall width. Our findings open a way to use such NTs as polarization-sensitive components of nanophotonic devices.

Selective Area Growth of InGaAs/InP Quantum Well Nanowires on SOI Substrate

In this paper, we directly grew the InGaAs/InP quantum well nanowires on the v-grooves patterned SOI substrate by metal organic chemical vapor deposition using aspect ratio trapping technique. The surface morphology of the III-V nanowires, the thickness of the quantum wells and the photoluminescence spectra of the InGaAs/InP quantum well nanowires were characterized by scanning electron microscopy, transmission electron microscopy, and micro photoluminescence, respectively. It was found that the thickness inhomogeneity of the quantum well, the composition inhomogeneity of the quantum well and the pump power density irradiated onto the surface of nanowire are the three major factors affecting the micro-photoluminescence spectra.

Modal Analysis of β−Ga2O3:Cr Widely Tunable Luminescent Optical Microcavities

Optical microcavities are key elements in many photonic devices, and those based on distributed Bragg reflectors (DBRs) enhance dramatically the end reflectivity, allowing for higher quality factors and finesse values. Besides, they allow for wide wavelength tunability, needed for nano-and microscale light sources to be used as photonic building blocks in the micro- and nanoscale. Understanding the complete behavior of light within the cavity is essential to obtaining an optimized design of properties and optical tunability. In this work, focused ion-beam fabrication of high refractive-index contrast DBR-based optical cavities within Ga₂O₃:Cr microwires grown and doped by the vapor-solid mechanism is reported. Room-temperature microphotoluminescence spectra show strong modulations from about 650 nm up to beyond 800 nm due to the microcavity resonance modes. Selectivity of the peak wavelength is achieved for two different cavities, demonstrating the tunability of this kind of optical system. Analysis of the confined modes is carried out by an analytical approximation and by finite-difference-time-domain simulations. A good agreement is obtained between the reflectivity values of the DBRs calculated from the experimental resonance spectra, and those obtained by finite-difference-time-domain simulations. Experimental reflectivities up to 70% are observed in the studied wavelength range and cavities, and simulations demonstrate that reflectivities up to about 90% could be reached. Therefore, Ga₂O₃:Cr high-reflectivity optical microcavities are shown as good candidates for single-material-based, widely tunable light emitters for micro- and nanodevices.

Role of the phase transition at GaN QDs formation on (0001)AlN surface by ammonia molecular beam epitaxy

We report an original method of GaN/AlN quantum dots (QDs) formation with low density by ammonia MBE on the (0001)AlN surface using a decomposition process of GaN thin layer. The QDs formation has been investigated in situ by reflection high-energy electron diffraction technique. Low density of quantum dots has been obtained in the range 107–109 cm−2. Single quantum dots photoluminescence lines corresponding to exciton and biexiton transitions were observed in micro-photoluminescence spectra. A lattice gas model was developed for correct description of the GaN QDs statistical ensemble on the surface. Effective interaction between QDs results in the discontinuous phase transitions (first-order phase transitions) from low density of QDs (gas branch) to condensed phase of QDs. The GaN QDs formation has been confirmed by high-resolution transmission electron microscopy and micro-photoluminescence of single quantum dots.

Waveguiding behavior of VLS-grown one-dimensional Ga-doped In2O3 nanostructures

Highly crystalline undoped and Ga-doped indium oxide nanorods with square-shaped faceted morphology were fabricated through the vapor-liquid-solid process at moderate temperature. Effects of Ga incorporation on the growth rate, morphology, and crystallinity of the nanostructures were evaluated by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Defect structure and waveguiding behavior of the 1-D In2O3 nanostructures have been studied using microRaman and micro photoluminescence spectroscopies. The appearance of several resonant modes superposed over the broad room temperature micro-photoluminescence spectra of the nanostructures demonstrates their waveguiding behaviors. While the pristine or undoped In2O3 nanostructures of 20–150 nm widths revealed Fabry-Pérot resonance modes, the Ga-incorporated nanostructures of 20–100 nm width revealed whispering gallery modes due to their smaller widths. The quality factor (Q) of the resonators was estimated to be about 20.86 and 188.79 for the pristine and Ga-incorporated nanostructures, respectively, indicating a huge enhancement due to Ga incorporation. The increment in the Q factor on Ga incorporation in In2O3 nanorods opens up the possibility of their utilization for the development of new optical transmitters and resonators, and fabrication of nanoscopic lasing devices.

Role of quantum-confined stark effect on bias dependent photoluminescence of N-polar GaN/InGaN multi-quantum disk amber light emitting diodes

We study the impact of quantum-confined stark effect (QCSE) on bias dependent micro-photoluminescence emission of the quantum disk (Q-disk) based nanowires light emitting diodes (NWs-LED) exhibiting the amber colored emission. The NWs are found to be nitrogen polar (N-polar) verified using KOH wet chemical etching and valence band spectrum analysis of high-resolution X-ray photoelectron spectroscopy. The crystal structure and quality of the NWs were investigated by high-angle annular dark field - scanning transmission electron microscopy. The LEDs were fabricated to acquire the bias dependent micro-photoluminescence spectra. We observe a redshift and a blueshift of the μPL peak in the forward and reverse bias conditions, respectively, with reference to zero bias, which is in contrast to the metal-polar InGaN well-based LEDs in the literature. Such opposite shifts of μPL peak emission observed for N-polar NWs-LEDs, in our study, are due to the change in the direction of the internal piezoelectric field. The quenching of PL intensity, under the reverse bias conditions, is ascribed to the reduction of electron-hole overlap. Furthermore, the blueshift of μPL emission with increasing excitation power reveals the suppression of QCSE resulting from the photo-generated carriers. Thereby, our study confirms the presence of QCSE for NWs-LEDs from both bias and power dependent μPL measurements. Thus, this study serves to understand the QCSE in N-polar InGaN Q-disk NWs-LEDs and other related wide-bandgap nitride nanowires, in general.

EMP Formation in the Co(II) Doped ZnTe Nanowires

Co(II) doped ZnTe nanowires are prepared by a thermal evaporation method. The power and temperature dependent micro-photoluminescence spectra of single nanowire demonstrate the double bands near its band edge, and the ferromagnetism behavior for these nanowires is identified. The occurrence of excitonic magnetic polaron (EMP) can account for the second emission band for its higher binding energy and ferromagnetic coupling. This EMP formation in a nanostructure will facilitate to realize magnetic modulation on confined excitons and will find new applications for spinpolarized nanophotonic devices.

Bound magnetic polaron in Zn-rich cobalt-doped ZnSe nanowires

The micro-luminescence spectra of the diluted magnetic semiconductor (DMS) can reflect the spin–exciton interaction and related relaxation process. Here the micro-photoluminescence (micro-PL) spectra and PL lifetime measurements have been done on an individual ferromagnetic (FM)-coupled cobalt (Co) doped zinc selenide (ZnSe) nanowire. There occurs a double-peak profile in its near bandedge emission spectrum: the first peak is from free exciton (FX) and the second comes from magnetic polaron (MP). In their temperature dependent PL spectra, the MP emission peak demonstrates obviously temperature-independent behavior, in contrast to the behaviors of FX and reported exciton MP in nanobelt. It is found that in this Co(II) doped ZnSe nanowires, this MP’s temperature-independent emission is related to the coupling between exciton and a FM nanocluster (↑↑↓). The nanocluster is likely due to the interaction of Se vacancies of the wide bandgap semiconductors with the antiferromagnetic (AFM) arrangement transition metal (TM) ions in these Se-deficient Co doped ZnSe nanowires. These results reflect that the AFM coupling TM ions pair can give rise to FM behavior with the involvement of positive charge defect, also indicating that the micro-luminescence detection can be used to study the magnetic coupling in DMS.

Original method of GaN and InGaN quantum dots formation on (0001)AlN surface by ammonia molecular beam epitaxy

We report original method of formation Ga(In)N/AlN quantum dots with low density by ammonia MBE on the (0001)AlN surface by using a decomposition process of Ga(In)N thin layer. Low density of quantum dots have been obtained in the range 107-109 cm-2. Single quantum dots photoluminescence lines corresponding to exciton and biexciton transitions were observed in micro-photoluminescence spectra.

Bent Polytypic ZnSe and CdSe Nanowires Probed by Photoluminescence.

Nanowires (NWs) have witnessed tremendous development over the past two decades owing to their varying potential applications. Semiconductor NWs often contain stacking faults due to the presence of coexisting phases, which frequently hampers their use. Herein, it is investigated how stacking faults affect the optical properties of bent ZnSe and CdSe NWs, which are synthesized using the vapor transport method. Polytypic zinc blende-wurtzite structures are produced for both these NWs by altering the growth conditions. The NWs are bent by the mechanical buckling of poly(dimethylsilioxane), and micro-photoluminescence (PL) spectra were then collected for individual NWs with various bending strains (0-2%). The PL measurements show peak broadening and red shifts of the near-band-edge emission as the bending strain increases, indicating that the bandgap decreases with increasing the bending strain. Remarkably, the bandgap decrease is more significant for the polytypic NWs than for the single phase NWs. This work provides insights into flexible electronic devices of 1D nanostructures by engineering the polytypic structures.

Band Alignment at GaN/Single-Layer WSe2 Interface.

We study the band discontinuity at the GaN/single-layer (SL) WSe2 heterointerface. The GaN thin layer is epitaxially grown by molecular beam epitaxy on chemically vapor deposited SL-WSe2/c-sapphire. We confirm that the WSe2 was formed as an SL from structural and optical analyses using atomic force microscopy, scanning transmission electron microscopy, micro-Raman, absorbance, and microphotoluminescence spectra. The determination of band offset parameters at the GaN/SL-WSe2 heterojunction is obtained by high-resolution X-ray photoelectron spectroscopy, electron affinities, and the electronic bandgap values of SL-WSe2 and GaN. The valence band and conduction band offset values are determined to be 2.25 ± 0.15 and 0.80 ± 0.15 eV, respectively, with type II band alignment. The band alignment parameters determined here provide a route toward the integration of group III nitride semiconducting materials with transition metal dichalcogenides (TMDs) for designing and modeling of their heterojunction-based electronic and optoelectronic devices.

Hydrothermal selective growth of low aspect ratio isolated ZnO nanorods

We have grown patterned, low-aspect-ratio, well-separated single ZnO nanorods using a hydrothermal method on two different substrates with dissimilar crystal orientations. ZnO nuclei have been used as a seed layer to compensate the crystal mismatch between the substrates and nanorods. Based on XRD results, in order to have highly oriented nanorods, the seed layer must be annealed over 300°C. Micro-Raman spectra show that our patterned nanorods have a wurtzite crystal structure, with most nanorods presenting vertical orientation relative to the substrate. Room-temperature micro-photoluminescence (PL) spectra from the nanorods show sharp band edge emission at 385nm and a common broadband defect emission in the visible range. Our method is a significant step towards an economical controlled synthesis of 1D ZnO for application in mass-production advanced devices.

3.5-μm radius race-track microlasers operating at room temperature with 1.3-μm quantum dot active region

We present detailed studies of optically pumped InAs/InGaAs quantum dot based racetrack microlasers with 3.5-μm bend radius operating at room temperature. Q factor over 8000 and room temperature threshold power in the mW-range were achieved in the racetrack microlasers with straight section length ranging from 0 to 4 μm. A systematic investigation of the influence of the racetrack straight section length on spatial distribution of optical modes is presented. The microcavity eigenmodes and electromagnetic field distribution calculated by means of three-dimensional numerical simulation demonstrate a good agreement with the experimental results obtained by micro-photoluminescence and scanning near-field optical microscopy. The racetracks demonstrate zigzagging behavior of the modes inside the cavity and the energy switching between the radial maxima in second-order modes. Higher-order modes are found to be suppressed in micro-photoluminescence spectra.

Asymmetric waveguide and the dual-wavelength stimulated emission for CdS/CdS0.48Se0.52 axial nanowire heterostructures

Semiconductor axial nanowire heterostructures are important for realizing the high-performance nano-photonics and opto-electronics devices. Although different IV and III-V semiconductor axial nanowire heterostructures have been successfully prepared in recent decade, few of them focused on the optical properties, such as the waveguide, due to their low light emission efficiencies. The II-VI semiconductor nanowires grown by chemical vapor deposition strategy, such as CdS, CdSe and their alloys, can act as nanoscale waveguide, nanolasers, etc., because of their high optical gains and atomically smooth surfaces. However, it is still a challenge to growing the high-quality II-VI semiconductor axial nanowire heterostructures, owning to the poor controllability of the vapor growth techniques. Here, the CdS/CdSSe axial nanowire heterostructures are prepared with well controlled CVD method under the catalysis of annealed Au nanoparticles. The scanning electron microscope characterization shows that the wires have smooth surfaces with Au particles at the tips, indicating the vapor-liquid-solid growth mechanism for the nanowire heterostructures. The microscope images of the dispersed wires illuminated with a 405 nm laser show that the red and the green segment align axially with a sharp interface, demonstrating the axial alignment of CdS and CdSSe segments. The position related micro-photoluminescence spectra exhibit near band edge emissions of CdS and CdSSe without obvious emission from defect states, which suggests that the wires have highly crystalline quality. The waveguide of the nanowire heterostructures is studied through respectively locally exciting the two ends of the wire with a focused 488 nm laser. The local illuminations at both the CdS end and the CdSSe end result in red emission at the corresponding remote ends of the wires, with the emission intensity of the former being one order lower than that of the later, which is caused by the reabsorption of the green light emission (from CdS segment) in the CdSSe segment. This indicates the asymmetric waveguide in these heterosturctures, which implies that the CdS/CdSSe nanowire heterostructures have the potential applications in the photodiode. Under the pumping of 470 nm femtosecond laser, dual-color (red and green) lasing is realized based on these wires, with the lasing threshold of red light lasing being lower than that of the green one, which results from the larger round-trip loss for the green light arising from the self-absorption in CdSSe segment. To prove that the light can be transfer between the two segments with different refractivities, the waveguide of the nanowire heterostructure is simulated by the COMSOL. The result shows that the light can effectively propagate between CdS and CdSSe segments, which ensures the light-matter interaction in the axial CdS/CdSSe nanowire heterostructures as discussed above. These high-quality CdS/CdSSe axial nanowire heterostructures can be found to have the potential applications in photodiodes, dual-color nanolasers and photodetectors.