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In this study, we covered Si nanopillar (NP) array with few-layer MoS2 films to convert their wettability characteristics from hydrophilic to hydrophobic for applications as a surface-enhanced Raman scattering (SERS) substrate. The Si NP array was fabricated using a semiconductor process. We then sulfurized and transferred MoO3 films coated onto the Si NP array to MoS2 films. The surface morphology and cross-sectional profile of the MoS2-coated Si NP array structure was examined using scanning electron microscopy and transmission electron microscopy. The SERS results indicate that the substrate exhibits a favorable enhancement factor of 1.76 × 103 and a detection limit of approximately 10−5M for Rhodamine 6G (R6G) utilized as the test molecule, attributed to the charge transfer (CT) mechanism at the interface between MoS2 and R6G. Contact angle measurements showed that the MoS2-coated Si NP array possesses a hydrophobic surface. Our results suggest that an MoS2-coated Si NP array with CT and hydrophobicity characteristics is extremely promising SERS substrates for SERS applications.

Searching for efficient surface-enhanced Raman scattering (SERS) substrates remains a challenge. In this study, we used metal–organic chemical vapor deposition to directly grow a pitted a-plane GaN thin film, subsequently covered by a thin Au layer (∼25 nm), for use as a SERS substrate, without the need for any additional etching or lithography process. The SERS substrate containing these micrometer-sized pits provided a low limit of detection (∼10−9 M) for Rhodamine 6G (R6G), with a high enhancement factor (4.27 × 108) relative to normal Raman spectroscopy. Furthermore, Raman spectral mapping indicated that most of the R6G molecules were concentrated in the pits, enhancing the localization of the probe molecules for further analysis. The same molecular localization phenomenon was also effective for polar methylene blue but not for nonpolar paraffin. The molecular aggregation became more ambiguous upon increasing the thickness of the Au layer, suggesting that the polarity of the Ga and N atoms in the pits was responsible for the efficient aggregation of the polar R6G molecules, which could be potentially beneficial for biomedical detection.

In the application of WS2 as a surface–enhanced Raman scattering (SERS) substrate, enhancing the charge transfer (CT) opportunity between WS2 and analyte is an important issue for SERS efficiency. In this study, we deposited few-layer WS2 (2–3 layers) on GaN and sapphire substrates with different bandgap characteristics to form heterojunctions using a chemical vapor deposition. Compared with sapphire, we found that using GaN as a substrate for WS2 can effectively enhance the SERS signal, with an enhancement factor of 6.45 × 104 and a limit of detection of 5 × 10−6 M for probe molecule Rhodamine 6G according to SERS measurement. Analysis of Raman, Raman mapping, atomic force microscopy, and SERS mechanism revealed that The SERS efficiency increased despite the lower quality of the WS2 films on GaN compared to those on sapphire, as a result of the increased number of transition pathways present in the interface between WS2 and GaN. These carrier transition pathways could increase the opportunity for CT, thus enhancing the SERS signal. The WS2/GaN heterostructure proposed in this study can serve as a reference for enhancing SERS efficiency.

By combining the excellent biocompatibility of molybdenum disulfide (MoS

Two-dimensional layered material Molybdenum disulfide (MoS2) exhibits a flat surface without dangling bonds and is expected to be a suitable surface-enhanced Raman scattering (SERS) substrate for the detection of organic molecules. However, further fabrication of nanostructures for enhancement of SERS is necessary because of the low detection efficiency of MoS2. In this paper, period-distribution Si/MoS2 core/shell nanopillar (NP) arrays were fabricated for SERS. The MoS2 thin films were formed on the surface of Si NPs by sulfurizing the MoO3 thin films coated on the Si NP arrays. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed to characterize Si/MoS2 core-shell nanostructure. In comparison with a bare Si substrate and MoS2 thin film, the use of Si/MoS2 core-shell NP arrays as SERS substrates enhances the intensity of each SERS signal peak for Rhodamine 6G (R6G) molecules, and especially exhibits about 75-fold and 7-fold enhancements in the 1361 cm−1 peak signal, respectively. We suggest that the Si/MoS2 core-shell NP arrays with larger area could absorb more R6G molecules and provide larger interfaces between MoS2 and R6G molecules, leading to higher opportunity of charge transfer process and exciton transitions. Therefore, the Si/MoS2 core/shell NP arrays could effectively enhance SERS signal and serve as excellent SERS substrates in biomedical detection.

The MoS2/ZnO and MoS2/Si heterojunction structures were fabricated by thermal evaporation and sol-gel methods. The crystal structures properties of MoS2/ZnO and MoS2/Si were characterized by X-ray diffraction (XRD) pattern, Raman spectroscopy, and transmission electron microscope (TEM). The XRD and Raman spectroscopy results indicate that the n-MoS2 film was successfully grown on p-doped ZnO or Si. The TEM images of MoS2/ZnO and MoS2/Si heterojunction structures shows the MoS2 stacking layer-by-layer covalented by van der Waals (vdW) force. The current–voltage (I–V) measurement shows the rectifying behavior of the heterojunction structures. The photoconductivity and photoresponsivity properties explore its carrier kinetic decay process. The results shows the potential applicability of MoS2/ZnO and MoS2/Si heterojunction structures as optoelectronic devices.

The investigation of optoelectronic devices based on two-dimensional materials and their heterostructures is a very active area of investigation with both fundamental and applied aspects involved. We present a description of a home-built scanning photocurrent microscope that we have designed and developed to perform electronic transport and optical measurements of two-dimensional materials based devices. The complete system is rather inexpensive (, 9 pages, 3 figures, 1 table, supporting information

The In 10 Ge x Sb 52-x Sn 23 Te 15 films (x = 2, 5, 7, and 9) were grown on silicon wafers, glass, and polycarbonate substrates at room temperature by dc magnetron sputtering. The thickness of In 10 Ge x Sb 52-x Sn 23 Te 15 films is fixed at 20 nm. The ZnS-SiO 2 films deposited by RF magnetron sputtering were employed as the protective layers. We have studied the crystallization behavior and optical property of the InGeSbSnTe phase change films. In addition, the recording characteristics for blue laser recording media were also investigated. It is found that the crystallization temperature of In 10 Ge x Sb 52-x Sn 23 Te 15 films was increased with increasing the Ge content. The optical contrasts of In 10 Ge x Sb 52-x Sn 23 Te 15 films with x = 2-9 are all higher than 30% at a wavelength of 405 nm. After ten direct overwriting cycles, the optimum jitter value of InGeSbSnTe optical disk with 4× recording speed is lower than 7%, indicating that the films have high potential in blue laser rewritable media.

In this study, SnS2(1-x)Se2x alloys layered crystals were grown by the chemical vapor transport method. The crystal structure characterization has been substantiated by X-ray diffraction (XRD) and transmission electron microscope technologies. From each sample's XRD pattern, the peaks corresponds to (001), (002), (003), (004), and (005) which were found and used to determine the lattice constants. The band gap energies modulated from 2.28 eV (SnS2) to 1.23 eV (SnSe2) with the increasing content of Se, which were verified by piezoreflectance (PzR) spectra and absorption spectra. By PzR spectra and absorption spectra, the empirical relation between band gap energies and composition ratio, according to the extended Vegard's law, has been predicted; accorded with a bowing parameter b of 0.56 and 0.54 eV, respectively.

In this paper, undoped and Ni-doped MoS 2 crystal layers were fabricated by chemical vapor transport method. Two direct band edge transitions of excitons at 1.9 and 2.1 eV were observed for both samples by optical reflectance measurement. Hall effect measurements were carried out to analyze the transport behavior of carriers in undoped and Ni-doped MoS 2 , which indicate that the Ni-doped MoS 2 sample is n-type and has a higher resistance and lower mobility than the undoped MoS 2 sample has. Photoconductivity was performed which shows Ni-doped MoS 2 has higher photocurrent than undoped MoS 2 . Meanwhile, an additional Ni doping level at 1.2 eV was observed in Ni-doped MoS 2 . According to a series of experimental results, this result indicates that the Ni dopants could improve their applications in light responsibility for solar cells and photo detectors.

In order to obtain a large area MoS 2 thin film, a thin Mo film were deposited on Si substrates using E-gun. The samples were placed on a quartz tube within a horizontal furnace to produce MoS 2 in a rich sulfur and argon environment from 700 to 1000°C. Both Scanning electron microscope (SEM) and X-ray diffraction (XRD) results reveal that MoS 2 films grown at the higher temperature possess better crystallization. Raman spectroscopy is employed to determine the behavior of atomic vibration which also provides information about layer structure in MoS 2 films. In addition, reflection measurement shows two obvious peaks located at 1.86 and 2.01 eV when the samples were grown at 900°C which are attributed to excitons A and B.

In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensitive photodetectors.In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensi...

The crystal quality of a -plane GaN films was improved by using epitaxial lateral overgrowth on nano-rod GaN template. The scanning electron microscope images showed the fully coalesced regrowth process completed within only 2 μm thickness. The a -plane GaN films grown on nano-rod template showed superior surface quality and less surface pits. The on-axis and off-axis FWHMs of X-ray rocking curves decreased with the increase in the etching depth of the nano-rod template. TEM analysis revealed that the dislocation density was reduced to around 1.2×10 9 cm −2 by the nano-rod epitaxial lateral overgrowth. In addition, the photoluminescence intensity of the a -plane GaN was enhanced significantly. These results demonstrated the opportunity of achieving a -plane GaN films with high crystal quality via nano-rod epitaxial lateral overgrowth.

The non-polar a -plane Al x Ga 1− x N alloys on GaN epitaxial layer with different Al compositions (0≤ x ≤0.2) were grown on r -plane (1 1¯ 0 2) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (MOCVD) and the Al composition x were estimated from the X-ray diffraction measurements. According to the result of asymmetric X-ray reciprocal space mapping, AlGaN layer was coherently strained to the underlying GaN template. The a -plane AlGaN alloy with relatively lower Al composition showed a flat surface with reduction of pits. The best mean roughness of the surface morphology was 1.18 nm. The photoluminescence (PL) result revealed that the PL peak position shifted from 3.42 to 3.87 eV with 0≤ x ≤0.2. Apart from the shifted peak position with increasing Al content, the PL emission intensity and surface morphology of the a -plane AlGaN alloy with relatively low Al content show slightly better characteristics than that of the a -plane GaN and AlGaN with higher Al composition.

The physical mechanisms leading to the effi- ciency droop of InGaN/GaN light-emitting diodes (LEDs) are theoretically investigated. We first discuss the effect of Auger recombination loss on efficiency droop by taking dif- ferent Auger coefficients into account. It is found that the Auger recombination process plays a significant nonradia- tive part for carriers at typical LED operation currents when the Auger coefficient is on the order of 10 −30 cm 6 s −1 . Fur- thermore, the InGaN/GaN multiple-quantum-well (MQW) LEDs with varied indium compositions in InGaN quan- tum wells are studied to analyze the wavelength-dependent efficiency droop. The simulation results show that the wavelength-dependent efficiency droop is caused by sev- eral different effects including non-uniform carrier distrib- ution, electron overflow, built-in electrostatic field induced by spontaneous and piezoelectric polarization, and Auger recombination loss. These internal physical mechanisms are the critical factors resulting in the wavelength-dependent ef- ficiency droop in InGaN/GaN MQW LEDs.

In this work, we report the growth of smooth, high-quality N-face GaN on c-plane sapphire by metalorganic chemical vapor deposition. It is found that the nitridation temperature of sapphire has a critical effect on the surface morphology of N-face GaN. Sapphire after a severe nitridation gives rise to a high density of hexagonal hillocks during N-face GaN growth. Smooth N-face GaN has been grown on appropriately nitridized sapphire. The N-polarity of the GaN film has been confirmed with no inversion domain by convergent beam electron diffraction. Controlled growth interruption is carried out to study the nucleation evolution during N-face GaN growth, which is found distinctly different from the two-step growth of Ga-face GaN. Atomically smooth N-face GaN has been achieved with comparable structural quality to Ga-face GaN.

The optical properties of InGaN multi-quantum-well laser diodes with different polarization-matched AlInGaN barrier layers have been investigated numerically by employing an advanced device simulation program. The use of quaternary polarization-matched AlInGaN barrier layers enhances the electron–hole wave function overlap due to the compensation of polarization charges between InGaN quantum well and AlInGaN barrier layer. According to the simulation results, it is found that, among the polarization-matched quantum-well structures under study, lower threshold current and higher slope efficiency can be achieved simultaneously when the aluminum composition in AlInGaN barrier layers is about 10–15%. The optimal polarization-matched InGaN/AlInGaN laser diode shows lower threshold current and higher slope efficiency compared to conventional InGaN/InGaN laser diodes.

High-reflectivity ultraviolet distributed Bragg reflectors (DBRs), based on AlN/AlGaN quarter-wave layers, have been designed and grown on 2 in (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition. The growth of 20-pair AlN/Al0.23Ga0.77N DBR shows no observable cracks in the structure and achieves peak reflectivity of 90% at 367 nm together with a stop-band width of 24 nm. Furthermore, the growth of 34-pair AlN/Al0.23Ga0.77N DBR shows partial cracks from optical microscopy images. According to the room-temperature photoluminescence measurement, the emission spectrum of 34-pair AlN/Al0.23Ga0.77N DBR is broader than that of 20-pair AlN/Al0.23Ga0.77N DBR, which could be due to strain inhomogeneity generated by cracks. Despite the crystal quality problem, the peak reflectivity of 34-pair AlN/Al0.23Ga0.77N DBR could still achieve 97% at 358 nm and the stop-band width is 16 nm.

Theoretical analysis for different active layer structures is performed to minimize the laser threshold current of the ultraviolet GaN/AlGaN multiple-quantum-well laser diodes by using advanced device simulation. The simulation results show that the lower threshold current can be obtained when the number of quantum wells is two or three and the aluminum composition in the barrier layer is about 10%-12%. This result is attributed to several different effects including electron leakage current, nonuniform carrier distribution, interface charge density induced by spontaneous and piezoelectric polarization, and optical confinement factor. These internal physical mechanisms are investigated by theoretical calculation to analyze the effects of quantum-well number and different aluminum compositions in barrier layer on laser threshold properties. Furthermore, the effect of quantum-well thickness is discussed as well. It is found that the optimal quantum-well thickness is about 3 nm due to the balance of the advantages of a large confinement factor against the disadvantages of significant quantum-confined Stark effect (QCSE).

InGaP/AlGaInP 660-nm resonant-cavity light-emitting diodes (RCLEDs) with stable temperature characteristics have been achieved by extending the resonant cavity length from one optical wavelength (1 lambda) to three optical wavelengths (3 lambda) and tripling the number of quantum wells. When the operation temperature increases from 25degC to 95degC, the degree of power variation at 20 mA is reduced from -2.1 dB to -0.6 dB for the conventional 1- lambda cavity RCLEDs and 3- lambda cavity RCLEDs, respectively. In order to interpret the temperature-dependent experimental results, advanced device simulation is applied to model the RCLEDs with different cavity designs. According to the numerical simulation results, we deduce that the stable temperature-dependent output performance should originate from the reduction of electron leakage current and thermally enhanced hole transport for the 3- lambda cavity AlGaInP RCLEDs.

This study reports on the reduction of dislocations in the GaN grown on the stripe-patterned r-plane sapphire substrates via metal-organic chemical vapor deposition (MOCVD). The stripes oriented along the sapphire [1 1 2¯ 0] direction with asymmetrical side faces were fabricated by lithographic and wet-etching processes. The two etching sides of sapphire-striped mesa are {0 0 0 1} and {1 1¯ 0 1} faces. GaN grown on both etching facets exhibits different epitaxial relationships with the sapphire substrate. The GaN grown from the {0 0 0 1} side face of the sapphire mesa contains a low-dislocation density in the order of 107 cm−2. The interfacial regions between the GaN and patterned sapphire substrate are also studied to clarify the behavior of GaN epitaxial growth on the inclined sapphire faces and defect-reduction mechanism.

We report the synthesis of the In 2 O 3 nanostructures grown at different growth temperatures by using the thermal evaporation method. The gold nanoparticles were used as the catalyst and were dispersed on the silicon wafer to facilitate the growth of In 2 O 3 nanostructures. The nanostructures of the In 2 O 3 were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The photoluminescence study reveals that In 2 O 3 nanostructures could emit different luminescence peaks in the range of 400–600 nm with broad bands by adjusting different growth temperatures. The coverage of the wavelength tuning in the emission peaks of the In 2 O 3 nanostructures could be beneficial for possible applications in white light illumination through manipulating the ratio of each wavelength component.

Ultraviolet nonpolar InGaN/GaN light-emitting diodes grown on trench epitaxial lateral overgrowth (TELOG) a -plane GaN template by metalorganic chemical vapor deposition were fabricated. Two emission peaks at 373 and 443 nm are observed from each fabricated device. The double emission peaks feature is identified by cathodoluminescence images, which show that the ultraviolet peak is emitted from the low-defect density wings on the TELOG and the blue peak is emitted from the TELOG-coalesced seed areas due to different incorporation of indium. The L–I–V diagram revealed that there are leakage current pathways due to the many threading dislocations in seed regions, and that the output power reached 0.2 mW at 140 mA. Two electroluminescence (EL) peaks are observed simultaneously when the driving current is below 50 mA. However, the EL peak at 373 nm dominates when current is above 50 mA. In addition, the degree of polarization of the ultraviolet peak was measured and found to be 28.7%.

In this study, we proposed a method to prepare GeO 2 by treating porous Ge thin film with thermal annealing in O 2 ambient. After annealing, the morphological transformation from porous thin film to an island structure was observed. The crystallization and composition of the porous Ge thin film prepared using different annealing time in O 2 ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO 2 with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO 2 , it was found that the visible photoluminescence originated from the germanium oxide. Photoluminescence measurements obtained at different temperatures exhibited a maximum integrated PL intensity at around 200 K. A possible explanation for this behavior might be the competition between radiative recombination and nonradiative hopping process.

We report the synthesis of the In2O3 nanoloquat grown at different oxygen flow rates by using the thermal evaporation method. The gold nanoparticles were used as the catalyst and were dispersed on the silicon wafer to facilitate the growth of In2O3 nanoloquats. The nanostructures of the In2O3 nanoloquats were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The photoluminescence (PL) study reveals that In2O3 nanoloquats could emit different luminescence peaks in the range of 500–600 nm with broad bands by adjusting different oxygen flow rates. The coverage of the wavelength tuning in the emission peaks of the In2O3 nanoloquats could be beneficial for possible applications in white light illumination through manipulating the ratio of each wavelength component. © 2007 Published by Elsevier B.V.

Effects of built-in polarization and carrier overflow on InGaN quantum-well lasers with a ternary AlGaN or a quaternary AlInGaN electronic blocking layer (EBL) have been numerically investigated by employing an advanced device-simulation program. The simulation results indicate that the characteristics of InGaN quantum-well lasers can be improved by using the quaternary AlInGaN EBL. When the aluminum and indium compositions in the AlInGaN EBL are appropriately designed, the built-in charge density at the interface between the InGaN barrier and the AlInGaN EBL can be reduced. Under this circumstance, the electron leakage current and the laser threshold current can obviously be decreased as compared with the laser structure with a conventional AlGaN EBL when the built-in polarization is taken into account in the calculation. Furthermore, the AlInGaN EBL also gives a higher refractive index than the AlGaN EBL, which is a benefit for a higher quantum-well optical confinement factor in laser operations.

The authors reported the synthesis of indium oxide nanocrystal chains on silicon substrates using gold nanoparticles as catalyst on native silicon dioxide. Scanning electron microscopy investigations showed that In2O3 nanocrystal chains were formed. The nanocrystal chains comprised many well shaped octahedron nanoparticles with their size ranging from 20to100nm connected with each other to form a network. Both high resolution transmission electron microscopy and diffraction pattern revealed that the In2O3 nanocrystal chains were single crystalline grown along the [111] direction. Photoluminescence study of In2O3 nanocrystal chains exhibited main photoemission at 544nm due to the recombination between electrons in localized state induced by oxygen vacancies and holes in valence band. After the annealing treatment, the main photoluminescence peak not only slightly redshifted to 558nm but also became narrower, which was attributed to the total localized state away from the conduction band caused by less defect distribution.

In this study, we grew non-polar a-plane GaN thin films on r-plane sapphire using a series of growth conditions by metal-organic chemical vapor deposition. The results showed that high temperature and low-pressure conditions benefited two-dimension growth could lead to a fully coalesced a-plane GaN layer with a very smooth surface. The best surface morphology with an excellent mean roughness of 10.5 A was obtained. The different thickness AlN as a nucleation layer and the different δ/β ratio were also considered. The results revealed that the surface morphology would get worse when the thickness of nucleation layer and δ/β ratio were away from the values of optimal condition. The observation of transmission electronic microscopy shown the lowest density of threading dislocations was 1.85×1010/cm2. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Non-polar a -plane ( 1 1 2 ¯ 0 ) GaN thin films were grown on r -plane ( 1 1 ¯ 0 2 ) sapphire substrates by metal-organic chemical vapor deposition. In order to obtain a -plane GaN films with better crystal quality and surface morphology, detailed comparisons between different growth conditions were investigated. The results showed that high-temperature and low-pressure conditions facilitating two-dimensional growth could lead to a fully coalesced a -plane GaN layer with a very smooth surface. The best mean roughness of the surface morphology was 10.5 A. Various thickness values of AlN nucleation layers and the V/III ratios for growth of the a -plane GaN bulk were also studied to determine the best condition for obtaining a smooth surface morphology of the a -plane GaN layer.

In this study, the carrier blocking effect on 850 nm InAlGaAs/AlGaAs vertical-cavity surface-emitting layers (VCSELs) was theoretically and experimentally investigated. By means of inserting a high-bandgap electron blocking layer, which was either 10 nm thick Al0.75Ga0.25As or 13 nm thick Al0.9Ga0.1As, on the p-side of a quantum-well active region, the laser output performance was theoretically found to be improved. VCSELs with and without an electron blocking layer were also experimentally demonstrated. It was found that the threshold current was reduced from 1.47 to 1.33 mA and the slope efficiency was increased from 0.37 to 0.53 mW mA −1 by inserting a 10 nm thick Al0.75Ga0.25As electron blocking layer. Also, the device became less sensitive to the device temperature, where the amount of increase in the threshold current at an elevated temperature of 95 ◦ Cw as only 0.27 mA and the slope efficiency dropped by only 24.5%. A peak frequency response of nearly 9 GHz at 5 mA, measured from relative intensity noise (RIN), was obtained in these VCSEL devices.

Ultrafine ZnO nanopowders were successfully synthesized by the dc thermal plasma process with a high production rate of 1.2 kg/h. The combination of plasma was found to affect the nanoparticles morphology. Both X-ray diffraction and Raman spectrum analysis confirmed a high quality wurtzite structure of the ZnO nanopowders. Photoluminescence study exhibited strong ultraviolet emission corresponding to the near band edge emission in the ZnO nanopowders. Random laser action observed in the ZnO nanostructures by optical pumping demonstrated high optical and crystal quality of the ZnO nanopowders fabricated by the dc thermal plasma synthesis. © 2006 Elsevier B.V. All rights reserved.

Indium oxynitride nanoparticles were synthesized on a silicon substrate in nitrogen atmosphere using the method involving thermal evaporation of pure indium in a two-zone reactor. Nanoscale compositional analysis by energy dispersion spectrum showed the existence of indium oxynitride compound. X-ray diffraction analysis further confirmed high crystallization and nitrogen atom existence within the nanoparticles. Scanning electron microscopy investigations showed shape transformation from amorphous sphere to well-shaped octahedron with an average nanoparticle size ranging from 50nmto1μm when the growth temperature of the substrate was increased from 600to900°C. Photoluminescence study was performed on the indium oxynitride nanoparticle samples grown at different temperatures. It was found that with increasing growth temperatures there was not only the formation of high quality indium oxynitride nanoparticles but also an increase in the intensities of emissions. These nanoparticles grown at 900°C could emit ...

We report on the fabrication of germanium quantum dots on silicon oxide and their growth mechanism. Germanium quantum dots were deposited by inductively-coupled plasma CVD at 400°C. Gold nanoparticles, attached to silicon oxide through a self-assembled monolayer, were adopted as catalysts to allow access to a vapor-liquid-solid process. The density of polycrystalline germanium dots is 1.46 × 10 11 cm -2 , which is consistent with the density of the gold nanoparticles. The mechanism by which the undesirable gold catalysts are removed during the germanium dot growth process has been elucidated. This technique provides a low-temperature process for the fabrication of devices consisting of germanium quantum dots on an insulator surface.

In this study, Ni-doped MoS2 was grown by chemical vapor transportation. Photoconductivity results reveal that Ni-doped MoS2 has an obvious doping level of 1.2 eV and an electrical conductivity of σ 9.92 × 10−7 S cm−1 at room temperature, which is lower than that of undoped MoS2 (σ 7.913 × 10−5 S cm−1) owing to the impurity state caused by Ni atoms. Hall effect measurement results indicate that Ni-doped MoS2 sample is of the n-type and has a higher resistance and a lower mobility than undoped MoS2. We further fabricated undoped and Ni-doped MoS2 photodetectors to understand the operation characteristics of MoS2-based photodetectors. Persistent photoconductivity shows that both rise and fall times decreased from 0.33/0.68 to 0.14/0.43 s as Ni atoms were doped in MoS2 PDs. This work shows that Ni atoms could cause small lattice imperfections to form trap states leading to high resistance, low mobility, small activation energy and short decay time. Therefore, doping Ni atoms in MoS2 is beneficial for the application of photodetectors.

The crystal quality improvement of a-plane GaN grown on r-plane sapphire was demonstrated by applying thermal annealing on as-grown samples in nitrogen ambient. The root mean square roughness of the 1000 °C-annealed a-plane GaN was only 0.4 nm measured by atomic force microscopy. Transmission electron microscopy results further indicate threading dislocations were decreased from 5×1010 cm−2 to 1.5×1010 cm−2 along [0001]GaN and stacking faults were decreased from 8.7× 105 cm−1 to 4.8× 105 cm−1 after the sample was annealed at 1000 °C. Room temperature photo luminescence measurements showed band edge emission intensity was enhanced up to 2.6 folds compared to the regular a-plane GaN film. Furthermore, corresponding cathode luminescence images reveal larger emission area for a-plane GaN with annealing than those without annealing, which was attributed to reduction of the nonradiative recombination centers. A series of experiments confirm this annealing process could be useful for further applications of GaN-based optoelectric devices.

Four p–i–n GaInP solar cells were grown by metal organic chemical vapor deposition (MOCVD) with different intrinsic layer thicknesses from 0.25 to 1 µm. A series of optical measurements, including electroreflectance (ER), photoluminescence (PL), electric luminescence (EL), and photocurrent (PC) measurements, have been performed to study the built-in electric field effect and to determine the suitable thickness of an intrinsic layer. The PL and EL spectra reflected the crystal quality of the GaInP layers. Furthermore, from the obtained ER spectrum, the built-in electric field in the intrinsic layer can be revealed. From the PC spectra under various bias voltages, the effect of built-in electric fields on the collection of photogenerated carriers has been studied.

In this work, we have successfully grown a-plane green light-emitting diodes (LEDs) on r-plane sapphire and investigated the device characteristics of a-plane green LEDs. The apparent emission polarization anisotropy was observed and the polarization degree was as high as 67.4%. In addition, the electroluminescence (EL) spectra first revealed a wavelength blue-shift with increasing drive current to 20 mA, which could be attributed to the band-filling effect, and then the EL peak become constant. The current-voltage curve showed the forward voltage of a-plane LED grown on r-plane sapphire substrate was 3.43 V and the differential series resistance was measured to be about 24 Omega as 20-mA injected current. Furthermore, the output power was 240 muW at 100-mA drive current.

We had investigated the potential of non-polar a-plane InGaN/GaN multiple quantum well (MQW) for high indium content blue/green emission. By comparisons of a-plane and c-plane MQW with different indium compositions, we concluded that without piezoelectric field, high indium content non-polar a-plane InGaN/GaN MQW is suitable for green light emitting diode (LED). Furthermore, similar to c-plane GaN InGaN/GaN MQW, the localized state might enhance the performance of a-plane MQW while the indium composition about 24% although the threading dislocation density (TDD) is quite high. On the other hand, the indium phase separation might also occur on a -plane InGaN/GaN MQW while the indium composition about 30% so that the photoluminescence intensity of the sample 810 °C low down and spectrum bandwidth broaden. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the observation of green emission centered at 500nm, accompanied with luminescence enhancement and spectral blue shift (∼31meV) in the bandedge emission from a -plane grown gallium nitride (GaN) passivated with gallium hydroxide (GaOOH). From study of the photo-current response and polarization-resolved low-temperature micro-photoluminescence (μ -PL) we resolved an evolution of gain competition process between the green and the UV emission bands. By dissolving the oxide we recover the emission characteristics of the as-grown a -GaN. These observations can be ascribed to a formation of compressively-strained a -GaN due to a coherently grown GaOOH atop layer. The latter is responsible for the spectral blue shift and the generation of acceptor like deep-levels causing the green-band transition in the emission and photo-current response. Enhancement of bandedge emission from the oxide covered a -GaN is due to the GaOOH surface passivation effect which was prepared by the photo-enhance wet oxidation method. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of light extraction of GaN-based flip-chip indium-tin-oxide light-emitting diodes (FC ITO LEDs) with an omnidirectional reflector (ODR) is presented. The ODR consisting of alternating layers of TiO2 and SiO2 is designed to possess a complete photonic bandgap within the blue region of interest, and it is fabricated by E-beam deposition. At a driving current of 300mA and a chip size of 1 mmtimes1 mm, the light output power of the FC ITO LEDs with the ODR reaches 156 mW. This is an enhancement of 31% when compared with the same device with an Al mirror instead. Furthermore, by examining the radiation patterns, the FC ITO LED with the ODR shows stronger enhancement around the vertical direction. Our work offers promising potential for enhancing output powers of commercial light-emitting devices

In this study, the optical constants of gold nanoparticles are evaluated for surface plasmon-based sensor applications. Using an effective medium approximation (EMA) and ellipsometry, approaches to monitor the self-assembly of gold nanoparticles are also demonstrated. Spectroscopic ellipsometric parameters measured (tan Ψ, cos Δ) before and after adding gold nanoparticles to a substrate are used to calculate the optical constants of gold nanoparticles. The film thickness is measured by grazing incidence X-ray reflectivity (XRR). The optical constants (refractive index, extinction coefficient) of gold nanoparticles can be obtained from the measured ellipsometric parameters and thickness. We also show that particles density can be well predicted and detected nondestructively by this method.

In this paper, MoS2 and Ni-MoS2 crystal layers were fabricated by the chemical vapor transport method with iodine as the transport agent. Two direct band edge transitions of excitons at 1.9 and 2.1 eV were observed successfully for both MoS2 and Ni-MoS2 samples using temperature-dependent optical reflectance (R) measurement. Hall effect measurements were carried out to analyze the transport behavior of carriers in MoS2 and Ni-MoS2, which indicate that the Ni-MoS2 sample is n-type and has a higher resistance and lower mobility than the MoS2 sample has. A photoconductivity spectrum was performed which shows an additional Ni doping level existing at 1.2 eV and a higher photocurrent generating only for Ni-MoS2. The differences between MoS2 and Ni-MoS2 could be attributed to the effect of Ni atoms causing small lattice imperfections to form trap states around 1.2 eV. The temperature-dependent conductivity shows the presence of two shallow levels with activation energies (84 and 6.7 meV in MoS2; 57 and 6.5 meV in Ni-MoS2). Therefore, the Ni doping level leads to high resistance, low mobility and small activation energies. A series of experimental results could provide useful guidance for the fabrication of optoelectronic devices using MoS2 structures.

Co-doped and undoped layered MoS2 crystals were grown by the chemical vapor transport method using iodine as the transport agent. Both reflectance and piezoreflectance measurements reveal two exciton transitions of the direct band edge around 1.86 and 2.06 eV for undoped MoS2 and 1.84 and 2.03 eV for Co-doped MoS2. Hall effect measurements show that the Co-doped MoS2 sample has a lower carrier concentration and mobility than the undoped sample. These differences between undoped and Co-doped MoS2 were attributed to the effect of cobalt atoms causing a small lattice distortion, lattice imperfections and/or impurity states that form trap states between the conduction band and valence band. Furthermore, photoconductivity (PC) and persistent PC results show that Co-doped MoS2 has a longer time constant and better responsivity than undoped MoS2. This work discusses the advantages of Co-doped MoS2 for photodetector applications.

Received 18 March 2009; revised manuscript received 17 September 2009; published 6 November 2009 Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.

We describe synthesis of In2O3 nanoloquats grown by thermal evaporation under different oxygen flow rates and temperatures. Gold nanoparticles were used the catalysts and were dispersed on the silicon wafer to assist growth of In2O3 nanoloquats. The nanostructures of In2O3 nanoloquats were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The photoluminescence study reveals that In2O3 nanoloquats could emit different broad-band luminescence peaks in the range of 410∼620 nm by tuning different oxygen flow rates and temperatures. The wide tuning range in the emission peaks of In2O3 nanoloquats has potential in applications of white light illumination.