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Our in situ experimental observations of the influence of electrical stress voltage on organic light-emitting device growth in dark spot areas are presented. We demonstrate the use of microsized silica particles to create uniformly sized defects on the protective layer. This is an efficient way to control the location and the number of dark spots. The growth in dark spot area was studied at different driving voltages from 0 up to 11 V. Dark field microscopy was used to monitor the dark spot size below the turn-on voltage. The bright field was used at or above the turn-on voltage. Our observations indicate that dark spot growth was strongly affected by the electrical stress voltage. A linear growth rate with respect to the voltage was observed with a fitting parameter better than 99.7% when the device is driven above the turn-on voltage. We interpret the dark spot growth in terms of the diffusion of moisture and oxygen accompanied by cathode layer chemical and physical changes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deals with a study which examined the high-field electron-transport properties of bulk silicon. Description of the distribution function of electron motion with the introduction on anisotropy by a high electric field; Velocity-field characteristics of electrons for samples of various mobilities; Significance of the findings.

Abstract: Epitaxial lateral overgrowth of gallium nitride with facets was realized by metal organic chemical vapor deposition on GaN/sapphire (0001) substrates with SiO2 stripe mask. Then AlGaN layers were grown on ELO GaN and on c-plane GaN templates simultaneously. The variation of the aluminum compositions was studied by photoluminescence spectroscopy, secondary ion mass spectrometry (SIMS) and XRD. Different aluminum content of 0.07 and 0.11 were obtained on facet AlGaN layers and on (0001) facet AlGaN layers, respectively. Compare to the c-plane AlGaN layer, it is found that polycrystalline AlGaN is grown on SiO2 layer and has higher Al content. These results indicate that aluminum incorporation depends significantly on the surface orientation. [Copyright &y& Elsevier]

Abstract: In this paper, we report experimental results regarding highly strained InP and InAs/InP quantum nanostructures grown on vicinal GaAs (001) substrate with 7° miscut towards (110) orientation by MOCVD using TBA and TBP as group V sources. Formation of well-aligned wire nanostructures has been observed, which may be attributed to the effects of multi-atomic steps formed during GaAs buffer layer growth. We also observed strong photoluminescence from these nanostructures even for the type II band gap alignment of the InP/GaAs quantum wires. Our results show a possible way for fabricating low-dimensional quantum nanostructures with band gap engineering. [Copyright &y& Elsevier]

Abstract: In this study, we investigated the effect of post-annealing treatment on the UV photoluminescence of ZnO nanorods deposited on (0001) GaN by hydrothermal synthesis. The temperature dependence of the photoluminescence spectra of ZnO nanorods annealed in nitrogen and oxygen was also studied. The as-grown ZnO nanorods were annealed in nitrogen and oxygen at 800°C in 30min. Results showed that UV light emission was greatly enhanced by annealing the as-grown ZnO nanorods in nitrogen ambient. Meanwhile, strong visible light emission was observed from the ZnO nanorods annealed in oxygen ambient. The effect of the annealing ambient and the photoluminescence temperature dependence were discussed with the relations to the structural defects. The time-resolved photoluminescence measurement was also proceeded to characterize the optical quality of the annealed ZnO nanorods. The decay times were 74 and 84ps for oxygen and nitrogen annealed ZnO nanorods, respectively. Once again, these results confirmed that the nitrogen annealed ZnO nanorods had better optical quality with less nonradiative related defects. [Copyright &y& Elsevier]

Abstract: High quality InAs films are grown on GaAs (001) substrates by low-pressure metalorganic vapor phase epitaxy. It is found that the combination of InAs buffer grown under low temperature and InAs epilayer grown under high temperature is essential to obtain high quality InAs films. The role of buffer layer is discussed. AFM observation on thick InAs film surface shows that step-flow growth mode takes place. The crystalline quality of InAs is characterized by transmission electron microscopy and X-ray diffraction. The crystalline quality improves with increase in film thickness. Residual strain due to thermal mismatch between InAs layers and GaAs substrates is investigated with X-ray diffraction. [Copyright &y& Elsevier]

AlGaInP epilayers of different Al compositions have been grown on (1 0 0) GaAs substrates with a misorientation of 2° and 7° towards (1 1 1)A by metalorganic chemical vapor deposition using tertiarybutylphosphine. High-resolution X-ray diffraction measurements show that the In composition of the AlGaInP epilayer grown on 7° off substrate is lower than that of the AlGaInP epilayer grown on 2° off substrate. Also the difference in the In composition increases with increasing V/III ratio while decreasing with increasing Al composition and substrate temperature. Based on the effect of substrate orientation and temperature on the sticking coefficients and migration lengths of the adatoms, the dependence of In composition on the substrate orientation and growth temperature are discussed. [Copyright &y& Elsevier]

In this paper, the zero-bias resistance areas product R0A is calculated in the n+–n–p and p+–p–n Ga0.8In0.2As0.81Sb0.19 infrared detectors, on the base of the material parameters in the three layers. The calculated results show that parameters in the heavily doped layer in the different structures have different influences on R0A. Moreover, R0A in the n+–n–p structure is higher than that in the p+–p–n structure because the higher carrier concentration in the n+-region for the n+–n–p structure improves R0A whereas the one in the p+-region for the p+–p–n structure reduces R0A. [Copyright &y& Elsevier]

In this paper, the performance of GaN metal–semiconductor–metal (MSM) photodetectors is analyzed based on the active layer parameters. The calculated results show the dependence of dark current on carrier concentration for n- and p-type active layers in GaN MSM structures. It is found that the increase in the finger spacing and layer thickness in the MSM structures increase the steady-state photocurrent but do not show strong influence on the transient photocurrent response. [Copyright &y& Elsevier]

Electrical properties of Schottky contact of titanium on low boron doped SiGeC have been investigated as a function of annealing temperature. Annealing has been performed at a temperature range of 400–600 °C for 10 min. The Schottky barrier heights were deduced from current–voltage characteristics. Both rapid thermal annealing and bias voltage will lead to a decrease of the Schottky barrier height for Ti/p-SiGeC contact. The decrease of Schottky barrier heights caused by the increase of annealing temperatures is about 3.5–5.6 meV per 100 °C between 400 and 600 °C and by the increase of bias voltage is less than 1 meV/V. X-ray diffraction and secondary ion mass spectroscopy measurements were performed and showed that, after annealing at 600 °C for 10 min, the SiGeC peak has shifted a little to the right compared with the Si substrate peak and a slight strain relaxation was occurred in the SiGeC epilayer. [Copyright &y& Elsevier]

We investigate theoretically the influence of electron acceptors such as cyano groups and donors such as methoxy and amino groups on the geometric and electronic properties of the poly(9,9-dihexylfluorene-1,4-phenylene) unit cell. The trends derived from quantum chemistry calculations fully represent the properties observed at the scale of long polymer chains. It is shown that sizable effects can be obtained. In particular, the use of diamino or dicyano substituents on the phenylene ring allows the bandgaps to decrease significantly. This information is of prime importance in the design of new chemical structures aimed at a fine-tuning of the emitted color and at a significant improvement in quantum efficiency. [Copyright &y& Elsevier]

The effect of electrical stress on polymer degradation has been studied on the organic device structure ITO/HTL/PPV/Ca/Ag. The devices were electrically stressed for duration up to different hours in nitrogen environment. The electrically stressed polymer was characterised by X-ray diffraction (XRD), photoluminescence (PL) spectrum and photoluminescence quantum efficiency (PLQE). The measurements were conducted from the side of glass substrate. PL results show that there is a significant drop after the sample has been taken out from the nitrogen environment and the PL intensity decrease extent differently with different stressing time. PLQE decreases with prolonged stress time. XRD results showed that there was an increase in crystallinity peak with increasing driving electrical stress time. All the results compared with non-stressed polymer give us a better understanding of the polymer degradation mechanism. [Copyright &y& Elsevier]

The optical properties of two series of thiophene-based polymers are interpreted by torsion analysis. When approached by the calculated HOMO/LUMO energy gaps of model molecules, the wavelength shifts from solutions to films are reasonable in cyclohexyl-substituted series but not in hexyl-substituted series, indicating that solid packing force plays a more important role in the former than in the latter. The lengths of hexyl and cyclohexyl groups on bithiophene segment are the dominant factor that determines the distance of co-facial chain packing which affects the conformation of the polymer chain. In both series the photoluminescence (PL) efficiency increases as the equilibrium torsion angle decreases. [Copyright &y& Elsevier]

The mechanical properties, specifically, the tensile strength and the cyclic bending of two organic light-emitting materials, namely, Alq3 and PPV, coated on barrier substrates have been examined. Static tensile test revealed brittle failure of the polymer-based barrier film. This investigation gives insight into the feasibility of polymer-based barrier film for the application to thin, lightweight and flexible organic light-emitting displays (OLED). [Copyright &y& Elsevier]

A thin 3-nm-thick parylene layer is deposited by chemical vapor deposition at room temperature on the indium tin oxide (ITO) coated glass substrate to form a bilayer anode of an organic light emitting diode. The parylene layer forms a conformal film to cover the spikes present in the ITO film. This parylene film presents a smoother surface to the subsequent organic layers. The parylene film not only reduces the occurrence of dark spots, acting as a barrier for oxygen diffusion from either the ITO or from the atmosphere and stabilizing the migration of the electrodes during electrical stress, but also improves the injection of holes from the anode. By inserting another parylene layer in between the organic and cathode layers, the probability of formation of nonemissive areas is further reduced. [ABSTRACT FROM AUTHOR]

The degradation and failure of organic light-emitting device are observed via optical microscopy. The "degraded area" has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current. [ABSTRACT FROM AUTHOR]

Based on the two-mode interference principle, the high carrier injection effect, and the free-carrier plasma dispersion effect, a directional coupling active optical switch in SiGe/Si alloy has been fabricated. Its insertion loss and crosstalk were measured to be 2.74 and -15.5 dB, respectively, at the wavelength of 1.3 μm and the total switching current of 110 mA. The fastest response time of the switch is up to 30 ns. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The degradation in electroluminescence of poly(p-phenylene vinylene)-based organic light-emitting devices is studied using optical microscopy, scanning electron microscopy, and secondary ion mass spectroscopy. “Bubbles” are formed at the polymer and indium tin oxide interface or in the polymer layer within the nonemissive area. This formation, which occurs during device electrical stress, is accompanied by a fluctuation of the device current. The bubbles are formed by the degraded polymer and/or the gas released from disintegration of the polymer. High local current density flowing near the dark spot center and the resultant heating, decomposes the polymer layer. The resultant carbonized area causes either local short circuit and/or open circuit leading to the final light-emitting device failure. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Self-organized InP quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix by metalorganic chemical vapor deposition. Strong photoluminescence centered at 986 nm is observed for the sample of InP grown at 490 °C, which can be attributed to radiative recombination of zero-dimensional (0D) electrons located in the InP dots and holes located in the surrounding regions. The indirect recombination of photogenerated carriers has been confirmed by the measurement of luminescence at different excitation densities and temperatures. If the InP is grown at 600 °C, experimental results show that a thicker and much smoother wetting layer is formed which results in much stronger and narrower luminescence located at 875 nm. In addition, state filling of the 0D electrons is also observed for the type-II quantum dots. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A InGaAsP/InP self-aligned, native oxidized buried heterostructure (BH) distributed feedback (DFB) laser is proposed. It is as easy to process as the ridge waveguide DFB laser and has superior performance. The current aperture can be easily controlled without selective regrowth. The laser exhibits a low threshold of 5.0 mA with 36 dB side mode suppression ratio at the emission wavelength of 1.562 μm. It emits in a single lobe with full width at half maximum angles of 33.6° and 42.6° for the lateral and vertical fields, respectively. Its beam is more circular than that of the as-grown BH laser because the lower refractive index of oxide compared to the as-grown layer and results in a larger lateral optical confinement. Its characteristic temperature (T[sub 0]) is 50 K at room temperature but increases in value at the higher temperature range. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An InAlAs native oxide is used to replace the p-n reverse-biased junction in a conventional buried heterostructure InP-based laser. This technique reduces the number of regrowth steps and eliminates leakage current under high-temperature operation. The InAlAs native oxide buried heterostructure (NOBH) laser with strain-compensated InGaAsP/InP multiple quantum well active layers has a threshold current of 5.6 mA, a slope efficiency of 0.23 mW/mA, and a linear power up to 22.5 mW with a HR-coated facet. It exhibits single transverse mode with lasing wavelength at 1.532 μm. A characteristic temperature (T[sub 0]) of 50 K is obtained from the NOBH laser with a nonoptimized oxide layer width. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

ZnO nanorod arrays (ZNAs) were hydrothermally synthesized on the surface of patterned indium-doped tin oxide p-type contact (PIPC) of GaN-based light-emitting diodes (GaN-LEDs) for enhancing the light extraction efficiency (LEE). It was found that the alignment of the ZnO nanorod arrays in the grooves of the PIPC was poorer than these grown on the ridges of the PIPC. By comparing the light output of the GaN-LEDs with and without ZNAs grown in the grooves of PIPC, the influence of the alignment of ZNAs on the LEE of GaN-LEDs was revealed and investigated. Numerical analysis based on the finite difference of time domain (FDTD) method suggested that the poorer alignment of ZNAs grown on GaN-LEDs resulted in more energy reflected back into GaN-LEDs and lower light extraction efficiency. [ABSTRACT FROM AUTHOR]

In this paper, we study the multiple scattering by electrically small (the radius of the cylinder is much smaller than the wavelength) plasmonic nanocylinders near surface plasmon resonance. The cylinders are assumed to be identical in dimension and composition. The incident plane wave is assumed to be TE polarized so that the plasmon resonance of two-dimensional cylindrical structures (for both individual and group of cylinders) can be excited. It is found that multiple plasmonic cylinders enhance the near-field magnetic field intensity due to mutual coupling. When the electrical dimension q of the cylinders (q=k0R, where k0 is the wave number of the free space and R is the radius of the cylinder) is fixed, the magnitude of the field distribution primarily depends on the positions of the cylinders at normal incidence. [ABSTRACT FROM AUTHOR]

The formation and growth of bubbles within dark spots in organic light-emitting diodes comprised of an electroluminescent polymer and a Ca–Ag cathode have been observed and studied. Our studies indicate that electric stress and pinholes are two important parameters affecting the formation of bubbles. More detailed analysis reveals the dependence of bubble area on the pinhole diameter. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this report, we designed a light emitting diode (LED) structure in which an N-polar p-GaN layer is grown on top of Ga-polar In 0.1 Ga 0.9 N/GaN quantum wells (QWs) on an n-GaN layer. Numerical simulation reveals that the large polarization field at the polarity inversion interface induces a potential barrier in the conduction band, which can block electron overflow out of the QWs. Compared with a conventional LED structure with an Al 0.2 Ga 0.8 N electron blocking layer (EBL), the proposed LED structure shows much lower electron current leakage, higher hole injection, and a significant improvement in the internal quantum efficiency (IQE). These results suggest that the polarization induced barrier (PIB) is more effective than the AlGaN EBL in suppressing electron overflow and improving hole transport in GaN-based LEDs. [ABSTRACT FROM AUTHOR]

Mg-doped p-GaN films have been grown on sapphire substrate with a multilayered buffer (MLB) by metalorganic chemical vapor deposition and characterized by photoluminescence spectroscopy. The MLB consists of GaN/Al[sub x]Ga[sub 1-x]N (0

Si-doped n-GaN films grown by metal–organic chemical vapor deposition were studied by photoreflectance (PR) spectroscopy. Based on the intense optical transitions observed in the room-temperature PR spectra, the energy positions of the near-band-edge transition for n-GaN samples with different Si-doping levels were determined through theoretical curve fitting under the weak-field approximation. Furthermore, based on the observed dependence of the redshift in the near-band-edge transition energy with the carrier concentration and the approach using the many-body theory, the band-gap renormalization coefficient for GaN was derived to be (2.4±0.5)×10[sup -8] eV cm. This value was found to be nearly 35% larger than that for GaAs.© 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Strong luminescence from the nitrogen–nitrogen (N–N) interstitials had been observed in GaAsN epilayers grown by metal organic chemical vapour deposition (MOCVD) using nitrogen as the carrier gas. These N–N interstitials in the GaAsN films were thoroughly studied through photoluminescence (PL) and secondary ion mass spectroscopy (SIMS) measurement combined with X-ray diffraction (XRD) measurement. The N–N interstitial emission peak resided at deep level of 0.2–0.23 eV below the inter-band gap transition energy of the GaAsN epilayer. The N–N interstitial PL intensity decreased after the GaAsN samples were thermal annealed. It had also been found that the N–N interstitials contribute to the substitutional nitrogen composition of the GaAsN epilayer after rapid thermal annealing were carried out on the GaAsN epilayer. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we report experimental results of photoluminescence (PL) of nanopatterned organic light-emitting diode (OLED). Nanosphere lithography is used to create the nanopatterns. Comparing the emission of the OLED from regions with and without the nanopatterns, the PL intensity resulting from the regions with the nanopatterns is increased by 43%. In addition, we also observed that the PL intensity is increased ∼6 times if the organic active layer is deposited on the nanopatterns and covered by a thin metal film of Al. This strong increase of PL intensity may be attributed to surface plasmon effects. [Copyright &y& Elsevier]

The degradation of organic light-emitting devices (OLEDs) often correlates to the formation of dark spots in active area of the devices. An optical image analysis technique is developed to investigate the evolution of the dark spots. The circular features observed in the optical images of non-operated devices correlated directly to the dark spots seen in the same locations in the electroluminescent images. It is found that the growth of dark spots is dependent on the contact between the anode and the first organic layer. An ultra-thin interlayer of tris-(8-hydroxyquinoline) aluminum modified anode favors the efficient operation of the OLEDs. [Copyright &y& Elsevier]

GaxIn1-xP/(Al0.4Ga0.6)0.52In0.48P multiple quantum well structures (MQW) have been grown by metalorganic chemical vapor deposition and characterized by photoluminescence (PL) and time-resolved PL. It is found that the low-temperature PL intensities are almost the same for GaInP/AlGaInP MQW structures with different strains. However, the room temperature (RT) PL intensity of the 0.5% compressively strained QW structure is about three times stronger than that of the unstrained one. The mechanism of RT PL intensity enhancement in the strained sample is discussed, and it is believed that the deeper potential well in the strained QW structure is the main reason for the higher PL intensity at RT. The time-resolved PL measurements indicate that the 0.5% strained sample shows the shortest PL decay time, indicating the strongest carrier confinement in the well. It is also concluded that RT PL is more suitable for evaluating the optical quality of the GaInP/AlGaInP MQW structures. [Copyright &y& Elsevier]

AlGaInP epilayers of different Al compositions and AlGaInP/GaInP multiple quantum well (MQW) structures have been grown by metalorganic chemical vapor deposition using tertiarybutylphosphine (TBP) as phosphorus precursor. Low-temperature photoluminescence (PL) measurements have been performed to evaluate the quality of the materials. The line widths of the PL spectra of AlGaInP epilayers grown with TBP are comparable to those grown using PH3, suggesting the high quality of AlGaInP grown by TBP. Finally, AlGaInP MQW red laser structures have been grown, and for the first time the electrically pumped AlGaInP red lasers grown by TBP have been reported with the central emission wavelength of 645 nm, indicating that TBP can be used to grow high quality AlGaInP epilayers and AlGaInP-based red lasers in parallel with the widely used highly toxic gas source PH3. [Copyright &y& Elsevier]

Using tertiarybutylphosphine (TBP) as phosphorus precursor, high-quality AlGaInP epilayers and AlGaInP/GaInP multiple-quantum-well (MQW) structures have been grown by metalorganic chemical vapor deposition. The photoluminescence results indicate that the AlGaInP materials are as good as those grown using PH&sub3; in terms of optical quality. Finally, AlGaInP MQW red laser structures have been grown, and the electrically pumped AlGaInP red lasers grown by TBP have been demonstrated with the emission wavelength of 647 nm, indicating that TBP can be used to grow high-quality AlGaInP epilayers and AlGaInP-based red lasers, which presently is dominated by the highly toxic gas source PH&sub3;. [ABSTRACT FROM AUTHOR]

Polymer degradation under electrical stress was studied using the device structure indium tin oxide/poly(p-phenylene vinylene)/Ca/Ag. The devices that were used in this study do not show any significant decrease in electroluminescent intensity for up to several thousand hours of operation. However for devices that were electrically stressed and then exposed to normal ambient, there was a significant decrease in the photoluminescence. Those that were not electrically stressed but exposed to normal ambient or were electrically stressed but not exposed to normal ambient, there was no significant degradation in the photoluminescence. These results showed that both conditions of electrical stress and photo-oxidation are necessary for a significant decrease of the photoluminescence intensity. Degradation is accompanied by an increase in the x-ray crystallization peak. We interpret that the photoluminescence intensity degradation is due to the change in polymer crystallinity caused by electrical stress followed by photo-oxidation. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Indium tin oxide (ITO) has a wide range of applications at its epsilon-near-zero (ENZ) wavelength due to its unique optical properties. Post-annealing is a simple way to tune the ENZ wavelength. We show that the ENZ wavelength of ITO films could be red-shifted over a wide range from 1200 nm to 1550 nm by thermal annealing in air for durations up to 130 min at 330 °. Optical transmittance and reflectance spectra were measured for these ITO samples, along with electron densities, to extract the Drude model parameters of plasma frequency, damping factor, electron mobility and effective mass. The results show that the changes in electron density and effective mass collectively cause the red-shift in the plasma frequency and ENZ wavelength. The oxygen uptake and crystallite size increase during the annealing in the air are the main reasons for the change in electronic properties. This versatile method of tuning the ITO's ENZ wavelength can expand the wavelength range for applications and adapt it to working at wavelengths of plasmonic devices in the telecommunication wavebands. [ABSTRACT FROM AUTHOR]

Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene) with different channel lengths were fabricated under different substrate pretreatment process conditions. These OFET devices were characterized using low frequency noise (LFN) spectroscopy, and the device performance parameters were correlated with the level of LFN. It is observed that the devices with higher noise levels showed poorer device properties when compared with the devices operated at same Ids of the same channel length. It is also observed that the noise level increased with the increase in channel length for devices with the same pretreatment conditions, which is due to increased trapping and detrapping in the channel material interface domain. The OFET device operating around the threshold voltage Vth will have a 1/f noise slope that is flatter, having a gradient that is smaller in magnitude. The threshold voltage of a device can thus be observed to be at the gate voltage in which 1/f noise intensity is the highest at low frequencies. [ABSTRACT FROM AUTHOR]

Degradation induced changes in the structural and optical properties of polyfluorene-based light-emitting diodes are examined by using electroluminescence and low frequency noise (LFN) spectroscopic techniques. The materials studied are poly[2,7-(9,9′-dihexylfluorene)-alt-bithiophene] (P1) and poly[2,7-(9,9′-dihexylfluorene)-alt-thieno[3,2-b]thiophene] (P2). Improved emission spectra for a light-emitting device based on polymer P2 in terms of current efficiency, spectra stability, and lifetime are observed. A polymer P2-based device also presents long lifetime predicted by the smaller slope in the initial LFN spectra. Correlation of device LFN spectra with polymer structure change and lifetime is established. The increase in noise level predicts the undergoing degradation in bulk material and the increase in the noise slope predicts the fluctuation of carrier number and change in polymer structure. The redshift in emission spectrum for P2 after long-time driving is also picked up by the LFN spectrum. [ABSTRACT FROM AUTHOR]

A GaAs defect-free epitaxial layer has been grown on Si via a Ge concentration graded SiGe on insulator (SGOI) for application in high channel-mobility metal-oxide-semiconductor field effect transistor. The SGOI layer, 42 nm thick, serves as the compliant and intermediate buffer to reduce the lattice and thermal expansion mismatches between Si and GaAs. A modified two-step Ge condensation technique achieves the surface Ge concentration in SGOI as high as 71%. It is also found that low-temperature migration enhanced epitaxy during the initial GaAs nucleation on the SGOI surface is critical to obtain a device quality GaAs layer by epitaxial growth. [ABSTRACT FROM AUTHOR]

Abstract High quality GaN was grown on 200 mm Si (111) substrates by using AlN and 3 step-graded Al x Ga 1-x N as the buffer layer in a metalorganic chemical vapor deposition system. We have investigated the influence of NH 3 pre-flow time on the threading dislocation density (TDD) of AlN, AlGaN buffer layers and GaN layers. It was observed that the compressive stress introduced into the buffer layer and GaN is dependent on the nitridation time. The lowest TDD for GaN obtained in our samples was ~1 × 109 cm−2 for screw type and 3.2 × 109 cm−2 for edge type dislocations, as obtained from atomic force microscopy and further confirmed by high resolution X-ray diffraction analysis. The threading dislocations generated in the first buffer layer (AlN) during its nucleation are found to influence the TDD in the subsequent layers. Samples without an intentional nitridation step exhibit higher TDD compared to the samples with optimal nitridation time. Longer nitridation time also leads to poor crystalline quality likely because of amorphous SiN x formation at the interface. Highlights • Wafer scale MOCVD growth of high-quality GaN on 200 mm Si (111) substrates. • Substrate nitridation (NH 3 pre-flow) controls crystallinity in each nitride layer. • Compressive stress in AlGaN depends on AlN quality which determines final wafer bow. • (Screw + mixed) type TDD of ~1 × 109 cm−2 & edge type TDD of 3.2 × 109 cm−2 are achieved. [ABSTRACT FROM AUTHOR]

We report on the growth of In0.30Ga0.70As channel high electron mobility transistor (HEMT) epi-layers on a 200-mm Si substrate by metal-organic-chemical-vapor-deposition. The HEMT layers were grown on the Si substrate by using a ~3- \mu \textm thick epitaxial buffer composing of a Ge layer, a GaAs layer, and a compositionally graded and strain relaxed InAlAs layer. The optimized epitaxy has a threading dislocation density of less than 2 \times 10^7 cm−2 and a root mean square surface roughness of ~6.7 nm. The device active layers include a \delta -doped InAlAs bottom barrier, a ~15-nm thick InGaAs channel, a ~8-nm InGaP top barrier layer and a heavily doped InGaAs contact layer. MOSHEMTs with channel length down to 130 nm were fabricated. The devices achieve a peak transconductance of \sim 450 ~\mu \textS/ \mu \textm at V D of 0.5 V. The peak effective mobility ( \mu \text {eff} ) in a device with a channel length of 20 \mu \textm device channel was ~3700 cm2/ \textV \cdot \texts . [ABSTRACT FROM AUTHOR]

A series of diketopyrrolopyrrole (DPP) and propylenedioxythiophene (ProDOT)-containing random copolymers with different donor-to-acceptor ratios is synthesized through Stille coupling polymerizations. The low-bandgap polymers display dark tones with colors ranging from magenta to blue, and reveal reversible colored-to-transmissive electrochromism in absorption/transmission-type devices with high optical contrasts (up to 48 and 77 % in the visible and near-infrared regions, respectively), modest switching speeds (a few to tens of seconds) and coloration efficiencies (267-574 cm2 C−1), as well as good long-term ambient redox stabilities. The structure-performance relationship of the polymers, in particular, the role of donor-to-acceptor ratio, is investigated, and it is shown that an increase in the amount of acceptor in the polymers leads to slower oxidative but faster reductive switching, accompanied with enhancement of the redox stability. In addition, further study on the influence of film thickness and film morphology reveals that devices with higher optical contrasts are attainable from thicker polymer films at the expense of switching speeds; films with high uniformity and connectedness together with open, loose structures at submicron to micron scale are favorable for achieving good electrochromic performance. [ABSTRACT FROM AUTHOR]

InGaN/Si multijunction solar cells form a cost-effective approach for full-spectrum photovoltaics. However, heteroepitaxial InGaN films with high In content are highly defective. To overcome this, nanostructured InGaN growth is explored in this work. Ordered arrays of In x Ga 1− x N nanopyramids were grown by metalorganic chemical vapor deposition on AlN(0001)/Si(111) substrates in a selective area heteroepitaxy approach. The effect of growth temperature and pressure on the morphology, structure and photoluminescence of the nanopyramids, and their In content x were studied. Reduction in temperature increases x but temperatures <750 °C are correlated with lower structural quality, polycrystalline deposits and In droplet formation. To maintain the quality, a moderately high temperature (~775 °C) may be used. Together with lower pressure, x can be improved. This also enhances the growth rate and improves nanopyramid size uniformity. Nanopyramid arrays generally exhibit greater lattice tilt than their control films due to dislocation bending. While lattice twist is increased with higher x , it is offset by epitaxial lateral overgrowth. Compared to control films, nanopyramid samples consistently yielded higher x , lower average reflectance (<5.5%) and a multi-fold increase in photoluminescence intensity with tunable emission from 3.05 eV to 1.93 eV. [ABSTRACT FROM AUTHOR]

A series of 4,9-dihydro- s -indecano[1,2- b :5,6- b’ ]dithiophene containing conjugated polymers with thiophene, 2,2׳-bithiophene, and thieno[3,2-b]thiophene as co-monomers was synthesized through Stille coupling reaction with good yields as well as high molecular weights. The physical and electrochemical properties of the resulting polymers were fully characterized. The polymers displayed exceptional solubility in common organic solvents. The polymers were fabricated as the electrochromic layer in absorption/transmission type electrochromic devices. Upon oxidation, the polymer films switched from a dark red to a grayish hue, displaying optical contrasts of about 20% and 40% in the visible and near-infrared (NIR) regions respectively, as well as reasonable ambient stabilities. In particular, the polymers exhibited rapid sub-second coloration times with exceptionally high coloration efficiencies of up to 1066 cm 2 /C in the visible region. Contrary to the traditional electrochromic conjugated polymers that typically show prolonged optical memory, these polymers exhibited a tendency to quickly lose their optical memory. [ABSTRACT FROM AUTHOR]

We report on the dual-character of an InGaN/GaN multiple quantum well (MQW) device structure epitaxially grown by MOCVD on a 150 mm Si-on-insulator (SOI) wafer. The fabricated mesa device exhibits photovoltaic (PV) response mainly for λ < 500 nm and strong electroluminescence (EL) for 470 nm < λ < 750 nm with a major amber component about 580 nm < λ < 620 nm. Multiple, high amplitude oscillations are observed in the luminescence spectra and verified by theoretical modelling as strong Fabry-Perot interference effects. HR-XRD ω - 2θ (0002) scan estimates a mean In composition of ∼22% in the MQWs, consistent with the PV absorption edge and weak photoluminescence (PL) emission when excited by a 518 nm laser at 20K. Under 405 nm laser excitation at low temperatures, PL emission for 600 nm < λ < 740 nm is greatly enhanced. Asymmetric broadening of the spectral peak to 570 nm < λ < 740 nm occurs when the excitation energy is increased with a 325 nm laser, and is attributed to radiative transitions associated with MQW interface defects. This is supported by the (105) reciprocal space map which shows slight relaxation of the MQWs. The strong but broad long-wavelength emission, is ascribed to the MQW interface defects and highly recombinative In-rich clusters. These hardly contribute to PV response and are responsible for the protracted Stokes shift that constitutes the apparent near-mutual exclusion of EL and PV absorption wavelengths. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Highlights: [•] First systematic studies of macro-morphology of large inkjet-printed thin films. [•] High performance organic solar cells by inkjet printing. [•] Significant advance over high/low boiling point solvent combination strategy. [•] No coffee-stain, contact-line depinning, dewetting and other fluid flow issues. [•] No halogen containing solvents, applicable to other coating methods. [ABSTRACT FROM AUTHOR]

We examine the structural and morphological qualities of In x Ga1−x N grown directly on AlN/Si(111) substrates by MOCVD as a function of growth pressure and temperature. The use of elevated pressures (up to 300Torr) resulted in the suppression of InGaN phase separation and indium droplet formation allowing single phase, textured epitaxial (0002)-oriented In x Ga1−x N to be grown on the highly mismatched substrates. Various indium compositions x, up to ~0.4, can subsequently be achieved by adjusting the growth temperature over the range of 655°C–795°C. Increase in growth temperature reduces the indium composition x but is accompanied by a decrease in the FWHM of the (002)-ω and asymmetric (105)-ω rocking curves indicating lower crystallographic tilt and improved crystal quality. The reduction in tilt saturates at ~705°C. This corroborates with room-temperature photoluminescence (PL) measurements where PL is not detectable below ~705°C but emerges above this temperature and narrows in FWHM with further temperature increase. SEM shows that films grown at low pressure are compositionally and morphologically non-uniform, while films grown at elevated pressure are homogeneous, single phase and composed of densely packed, interconnected epitaxial islands, with lower temperature favouring a smaller island size. We conclude that while lower temperatures favour increased indium incorporation, the ensuing smaller island size and greater extent of island boundaries, arising from larger lattice mismatch and lower surface mobility of species, degrades crystal quality appreciably. Above 705°C, improvement in crystallographic quality is limited by the AlN growth template and requires innovative MOCVD growth strategies. [ABSTRACT FROM AUTHOR]

Lineshape-fitting, using a model that takes into account of band tails, was performed on the photoluminescence spectra measured from InN samples at temperatures varying from 5 to 300 K. By analyzing how the fitted parameters varied with temperature, it was found that the previously observed blue and then red shift of emission peak position with increasing temperature is related to valence band tails. The initial blue-shift at low temperature is due to filling of valence band tails while the subsequent Varshni red-shift is due to lattice expansion. In a series of samples grown using increasing trimethylindium flow rates during metal organic chemical vapor deposition, the impurity potential that describes the width of the valence band tail increased from 18 to 26 meV, possibly due to more nitrogen vacancies being created in the material. [ABSTRACT FROM AUTHOR]