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The deep levels in GaN associated with yellow luminescence transitions have been investigated using photoluminescence, Hall measurements, and deep level transient spectroscopy (DLTS). Hall measurements on Si-doped GaN show the presence of donor levels at ∼18, ∼35, and ∼70 meV, which are respectively associated with the Si shallow donors, O impurities, and the nitrogen vacancies (VN). DLTS measurements, on the other hand, reveal trap levels at Ec−0.1 eV, Ec−(0.2–0.24) eV, and Ev+0.87 eV. The trap level at Ec−0.1 eV obtained from DLTS can be correlated to the 70 meV deep donor (VN) obtained from Hall measurements. The deep donor band at Ec−(0.2–0.24) eV is attributed to the ON related defect complex decorated along dislocation sites while the hole level at Ev+0.87 eV is attributed to the Ga vacancy (VGa). Thermal annealing at 750 °C in nitrogen ambient results in reduction of yellow luminescence, which could be due to decrease in the concentration of VN and ON-related defect complexes. From these observatio...

A GaInP/AlGaInP multiple quantum well laser structure was grown by low-pressure metalorganic chemical vapor deposition with tertiarybutylarsine and tertiarybutylphosphine. Laser diodes fabricated from this structure lased at room temperature. Photoluminescence (PL) measurements were performed from 10 to 230 K. The PL energy increased with temperature from 10 to 70 K and decreased above 70 K. The former was attributed to thermal activation of trapped carriers due to localization in the quantum wells, while the latter was attributed to temperature-induced band-gap shrinkage. The PL intensity as a function of temperature was fitted by employing two nonradiative recombination mechanisms with good agreement, resulting in two activation energies that correspond to losses of photogenerated carriers to nonradiative centers.

In this article, a simple physical model describing the current transport mechanism in shallow Si p-n junctions containing silicide spikes is proposed and discussed. It includes analytical calculations of maximum electrical field around a silicide spike tip and tunneling probability of electrons at the silicide/silicon interface. The athermal behavior of the reverse current in shallow junctions containing silicide spikes is explained in terms of electric field enhancement at the sharp spikes and the resulting dominance of electron field-emission across the silicide/silicon Schottky barrier. The model also allows extraction of important parameters of such shallow junctions, such as the average radius of the silicide spikes.

We report the technique of controlled group V quantum well intermixing (QWI) in a compressively strained In0.76Ga0.24As0.85P0.15/In0.76Ga0.24As0.52P0.48 multiquantum well laser structure and its application to the fabrication of two-section tunable lasers. The blueshift of the band-gap energy was enhanced by capping the samples with films of SiO2 or low-temperature grown InP, while suppressed by a SixNy film with a refractive index of about 2.1. Spatially selective band-gap tuning was achieved by patterning the dielectric film into dot and strip arrays with different surface coverage. Time-of-flight secondary ion mass spectra showed that the enhanced blueshift was caused by the interdiffusion of group V atoms between the quantum wells and barriers. A group V interstitial interdiffusion mechanism is proposed for the sample capped with SiO2 and this is supported by the even more efficient intermixing induced by low-temperature InP, which contains a high concentration of excess phosphorus. A two-section tuna...

Plasma-induced damage to n-type GaN has been studied by atomic force microscopy and Raman spectroscopy. It was found that the extent of surface roughening is largely dependent on the surface defect density, since preferential sputtering at these sites takes place. Several Raman defect modes have emerged from the plasma-damaged samples, and have been compared to the defect modes observed from ion-implanted GaN. The defect peaks centered at 300 and 360 cm−1 have been assigned to disorder-activated Raman scattering modes, while the 453 and 639 cm−1 peaks have been attributed to vacancy scattering. It has also been demonstrated that structural damage can be annealed out at 900 °C for 60 s in flowing N2.

Using micro-Raman spectroscopy, we have studied the vibrational properties of GaN and Al0.5Ga0.5N/GaN long period superlattices (SLs) grown on Si(111). Crack-free areas of GaN layers grown on Si(111) exhibit residual tensile stress, which is evidenced by the red shift of the frequency of E2(TO) phonon. We have derived the strain cartography in GaN and Al0.5Ga0.5N/GaN long period SLs, which shows that cracking leads to strain relaxation. In addition, the AlGaN layers on GaN introduce an additional component of compressive strain into the GaN layers in these SLs. The amount of strain is quantified using micro-Raman analyses and by taking into account the elastic properties of GaN and AlGaN. By introducing a thin, low temperature InGaN interlayer, we could significantly reduce the crack density of the GaN layer.

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.

We have performed outgoing resonant Raman scattering and photoluminescence measurements on as-grown, Be- and C-implanted GaN in the temperature range of 77–330 K. In implanted GaN after postimplantation annealing at 1100 °C, the A1(LO) multiphonons up to the seventh order were observed with the very strong four longitudinal optical (LO) and five LO modes at ∼2955 and ∼3690 cm−1, respectively, showing extraordinary resonance behavior. With the sample temperature, these two modes significantly decreased and increased in intensity, respectively. The phenomenon is attributed to the variation of resonant conditions due to the shift of the band gap energy. Meanwhile, the combination of E2(high) and quasi-LO phonons was strongly enhanced by quasi-LO phonon involvement and thus the corresponding overtones can be clearly observed even up to the sixth order (m=6). The mechanisms that such strong outgoing multiphonon resonance Raman scattering occurred to implanted GaN instead of high-quality as-grown GaN samples ca...

We report a systematic study of the effects of wet chemical treatment, inductively coupled plasma etching, and thermal annealing on the surface and optical properties of Mg-doped p-type GaN. The chemical bonding and surface stoichiometry of the GaN surface subjected to different processing steps are analyzed based on the results of x-ray photoelectron spectroscopy. Atomic force microscopy has been employed to characterize the surface morphology. Photoluminescence (PL) and micro-Raman techniques have been used to investigate the electronic and vibrational properties of plasma etched surface. We have correlated the surface changes induced by dry etching of p-type GaN to the corresponding changes in the defect and impurity related states, through their manifestation in the PL spectra. We have observed several local vibrational modes (LVMs) in p-type GaN subjected to various processing steps. A broad structure in the low-temperature Raman spectra around 865 cm−1 is attributed to the electronic Raman scatterin...

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.

We report that the shift in the band gap of Al0.3Ga0.7As/GaAs quantum well structures can be precisely controlled by an Al layer buried between a spin-on silica film and a wet-oxidized GaAs surface. The blueshift in wavelength of the Al0.3Ga0.7As/GaAs quantum well photoluminescence (PL) depends linearly on the thickness of the buried Al layer. By changing the Al layer thickness, the PL peak wavelength can be tuned from 7870 A for the as-grown sample to 7300 and 7050 A after 20 and 45 s rapid thermal annealing at 850 °C, respectively. Applying this technology, Al layers with different thickness, i.e., no Al, 200 and 300 A thick, were applied to the oxidized GaAs surface in three adjacent regions with 200 μm spacing on a quantum well laser structure sample. Three wavelength lasers were successfully fabricated in a single chip by a one step rapid thermal annealing. All the lasers have similar threshold current and slope efficiency.

The structural properties of as-grown and rapid thermal oxidized Si1−x−yGexCy epitaxial layers have been examined using a combination of infrared, x-ray photoelectron, x-ray diffraction, secondary ion mass spectroscopy, and Raman spectroscopy techniques. Carbon incorporation into the Si1−x−yGexCy system can lead to compressive or tensile strain in the film. The structural properties of the oxidized Si1−x−yGexCy film depend on the type of strain (i.e., carbon concentration) of the as-prepared film. For compressive or fully compensated films, the oxidation process drastically reduces the carbon content so that the oxidized films closely resemble to Si1−xGex films. For tensile films, two broad regions, one with carbon content higher and the other lower than that required for full strain compensation, coexist in the oxidized films.

We report the measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection technique. By fitting with the experimental data we obtain all the parameters of the lattice vibration oscillators and of the plasmon. From the plasmon frequency and the damping constant we have derived the carrier concentration and the electron mobility. The concentration agrees with the Hall data very well while the mobility values are smaller than that of the Hall measurement by a factor of about 0.5. We attribute such mobility lowering to the increase of scattering for the electrons coupling with the incident photons.

It is a well-established fact that the luminescence peak arising from spatially indirect transitions (effective band gap Egeff) of a n-i-p-i structure shifts to a higher energy when the intensity of the excitation is increased. In this article, the quantitative dependence of the effective band gap on the excitation light intensity IW is formulated and verified experimentally. Using AlGaAs based n-i-p-i samples, the plot of ln(IW) versus Egeff gives a slope of value 4×10−21 eV−1 which agrees well with the theoretical value of 5.2×10−21 eV−1. It is found that this blueshift is typically 50 meV for a doubling in the magnitude of the optical excitation intensity from 50 to 100 mW cm−2. Further, it is found that the carrier lifetime decreases exponentially with an increase in the excitation intensity from zero to 500 mW cm−2 by four orders of magnitude which also agrees well with our theoretical model.

Audible acoustic wave generation during excimer laser interaction with materials has been investigated. It is found that the amplitudes of acoustic waves depend on laser fluence, pulse number, and substrate material characteristics and can be used to determine the nature of laser–material interactions. When laser fluence is below the ablation threshold of the materials, the amplitudes are reduced to zero at large pulse number due to the cleaning of contaminants on the substrate surface. As laser fluence becomes higher than the ablation threshold, the amplitudes of acoustic waves also reduce with increasing pulse number but to a constant level instead of zero due to laser ablation of substrate materials. Since the surface contamination can be completely removed by a few pulses at high laser fluence, the constant level is attributed to the material ablation. It is also found that the constant level increases with laser fluence. By establishing a relationship between the amplitudes and laser parameters, real‐time monitoring of laser–solid interaction can be achieved. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves with a peak around 10.9 kHz as the dominant one, which is related to laser material ablation. The monitoring of the acoustic wave emission can, therefore, be used to find the nature of laser–substrate interaction (i.e., surface cleaning or ablation), and to find the ablation threshold.

Strained AlGaInP/GaInP multiple-quantum-well laser structures have been grown by metalorganic chemical vapor deposition using teriarybutylphosphine as the phosphorus precursor and ridge waveguide lasers of 4 μm wide have been fabricated. Room temperature continuous-wave lasing has been obtained with an emission wavelength of about 670 nm. A single-facet output power of more than 18 mW has been achieved for an as-cleaved laser chip. It can be concluded that it is feasible to fabricate AlGaInP red lasers using less toxic metalorganic source tertiarybutylphosphine in parallel with conventionally used highly toxic PH3.

GaN-based metal–insulator–semiconductor (MIS) structures were fabricated by depositing a SiO2 film on a metalorganic chemical vapor deposition-grown GaN/Al0.4Ga0.6N/GaN double heterojunction. Various-frequency capacitance–voltage (C−V) measurements were carried out on the MIS structures. The measured C−V curves show a notable flatband shift of up to about 12.5 V with a typical polarization hysteresis window (9.4 V in width). Moreover, the capacitance of the heterojunction MIS structure reaches a minimum value under 4.1 V bias (forward scan) or −6.5 V bias (reverse scan). Due to the strong polarization and piezoelectric effects existing in AlGaN/GaN heterostructures, these results are deemed to be due to the influence of the piezoelectricity and polarization effects in the structure. Conductance–voltage (G−V) measurements were also performed. Various-frequency C−V and the G−V behaviors indicate that interface states have little influence on the sample.

High‐field electron‐transport properties of bulk silicon are studied taking into account the multivalley band structure with ellipsoidal energy surface. A distribution function that takes into account the anisotropy introduced in electron motion by the high electric field is described. This puts some order in the otherwise completely random motion. The transition from linear to nonlinear behavior is obtained when the energy gained by an electron in traversing a mean free path is comparable to the thermal energy. The drift velocity is shown to be limited to the random thermal velocity for nondegenerate electrons and to the Fermi velocity for degenerate electrons. This indicates independence of the saturation velocity on momentum‐randomizing scattering events which control the mobility. The emission of an optical phonon is significant when inelastic scattering length is comparable to the momentum randomizing mean free path. This further lowers the saturation velocity. When an electric field is applied along the

A photonic bandgap structure was created on the 100 nm thick GaAs barrier layer with Au nanodisks deposited inside the holes. To mitigate the nonradiative surface recombination of GaAs, the Au nanodisks were formed on top of a 15 nm SiO2 deposited in the holes. A maximum 7.6-fold increase in photoluminescence intensity was obtained at the etch depth of 80 nm. In this configuration, the Au nanodisk is separated from the quantum well by 20 nm of GaAs and 15 nm of SiO2. The experimental result was verified by the simulation based on this structure. There was a good agreement between experiments with simulation results.

The first-principles pseudopotential method combined with virtual crystal approximation is used to calculate band offsets at the In0.53(AlzGa1−z)0.47As/In0.52Al0.48As (001) heterostructures lattice matched to an InP substrate. It is found that the valence-band offset (VBO) varies with respect to the aluminum composition as VBO=0.18–0.16z–0.02z2 eV, while the conduction-band offset (CBO) varies as CBO=0.51–0.33z–0.18z2 eV. Our results are in good agreement with the experimental data.

We report the influences of group-III source preflow, which were introduced prior to the growth of the low temperature GaN on the polarity, photoluminescence (PL), and crystallographic properties of GaN epilayers grown on nitridated c-plane sapphire substrates by metal-organic chemical vapor deposition. By studying the surface morphology evolutions under chemical etching in KOH, we found that with increasing the trimethyl-gallium (TMGa) preflow duration (t), the polarity of the GaN film can be changed from a complete N-polarity to a mixture of N- and Ga-polarity and further to a complete Ga-polarity. PL and high-resolution X-ray diffraction studies revealed that the impurity incorporation and the edge- and screw-type threading dislocations are strongly polarity dependent. A further study at the optimized t (i.e., 30 s for TMGa) shows that the polarity inversion of GaN can be realized not only by TMGa preflow but also by trimethyl-aluminium preflow and by trimethyl-indium preflow. A two-monolayer model was employed to explain the polarity inversion mechanism.

We demonstrated a novel and widely accessible method for determining the electron effective mass and scattering time of ZnO films with different carrier concentrations by combining terahertz time-domain spectroscopy with Hall measurement. The terahertz time domain spectroscopy (THz-TDS) transmission spectra (0.1–2THz) were well described by Drude model. It is found that electron effective mass varied from 0.23m0 to 0.26m0 as the electron concentration changes from 5.9 × 1017 cm−3 to 4.0 × 1019 cm−3. The carrier concentration dependent characteristic is ascribed to the non-parabolicity of conduction band. Free carrier localization mechanism explained the discrepancy in mobilities obtained from THz-TDS and Hall measurements.

High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the {111} facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si{111} growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (IYL/INBE) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E2(high) optical phonon mode at 565.224 ± 0.001 cm−1 with a narrow full width at half maximum of 1.526 ± 0.002 cm−1 was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si{111} surface etched on Si(100).

Effects of stress imposed by individual nitride layers on structural properties of an AlGaN/GaN high-electron-mobility-transistor (HEMT) structure, which was grown on a 150 mm diameter Si (111) substrate by metal-organic chemical vapor deposition employing high-temperature step-graded AlxGa1−xN/AlN buffer layers, were studied using transmission electron microscopy, visible micro-Raman spectroscopy, and high-resolution x-ray diffraction. It is revealed that all the nitride layers are more or less tensile strained on the Si (111) substrate; however, strain relaxations occurred at all the heterointerfaces except for the AlGaN/(AlN/)GaN two-dimensional electron gas interface, which is desired for achieving high performance HEMT. The wafer curvature, an important parameter for large area epitaxy of GaN-on-Si, is modeled on the basis of stress distribution within individual layers of the multilayered AlGaN/GaN HEMT structure via the close-form expression developed by Olsen and Ettenberg [J. Appl. Phys. 48, 2543...

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.

Indium rich InGaN nanostructures grown by metalorganic chemical vapor deposition were incorporated in InGaN/GaN quantum wells for long wavelength generation. These results were achieved by optimizing the growth temperature of the nanostructures, InGaN quantum well, the AlN capping layer and the GaN barrier layers. Before the growth of nanostructures, a thin InGaN wetting layer was included to reduce the lattice mismatch as well as to enhance the deposition of indium-rich InGaN nanostructures These individual quantum wells were each subsequently capped with an AlN layer which better preserved the In-rich phase in the nanostructures and prevented the indium interdiffusion between the InGaN/GaN heterojunctions. The AlN capping layer also reduces the effect of piezeoelectric field in the active layers of the light emitting diodes as seen from the reduction in the blueshift in the electroluminescence peaks with higher injection currents. The energy band profile of such a structure is discussed.

Zinc oxide (ZnO) thin films annealed at different temperatures were studied with photoluminescence (PL), electrical resistivity, Hall mobility, and 1/f noise spectroscopy. Relatively high electrical conductivity and carrier concentration in sample annealed at 400 °C suggested the presence of ZnO interstitials. Rapid reduction in electrical conductivity and carrier concentration upon increasing the annealing temperature suggested that ZnO interstitials could be eliminated by high temperature annealing. Presence of G-R noise in sample annealed at 400 °C indicated high level of electron trapping activities. Density of Zn vacancies acting as electron traps was estimated by Lorentzian fitting on the G-R noise. PL spectra exhibiting dominant green emission in all samples suggested the presence of Zn vacancies in high concentration. Yellow-orange emission in PL in samples annealed at 600 °C and below indicated the presence of O interstitials, while the same emission in samples annealed at higher temperatures wer...

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 t...

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=k(0)R, where k(0) 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. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2975214]

InGaN/GaN microdisk light emitting diodes (LEDs) on (111)-oriented nanosilicon-on-insulator (nano-SOI) substrates are demonstrated. The (111)-oriented thin SOI substrates are prepared by separation by implantation of oxygen (SIMOX) method. The InGaN/GaN LED structures are grown on these SIMOX templates by metal-organic chemical vapor deposition. The circular mesa patterns are created by standard LED processing steps including photolithography, inductively coupled plasma etching, and contact metallization. Due to the reflectivity changes at the bottom Si/SiO2 interfaces beneath AlN buffer, an improved light extraction from these LEDs has been observed. The room-temperature electroluminescence (EL) spectrum of the microdisk LEDs on SOI shows multiple interference peaks due to the reflections at the Fabry-Perot boundaries and such effect resulted in an increased integrated EL intensity. For a comparative analysis of light extraction with respect to similar LED structures grown on bulk Si(111), a detailed ref...

In this work, a simple method to reduce the absorption recovery time of the InGaN/GaN quantum well saturable absorber was demonstrated. The recovery time of the saturable absorber grown by metal organic chemical vapor deposition was effectively controlled by controlling the crystal quality. Transmission electron microscopy results showed that, for saturable absorbers with reduced GaN buffer thicknesses, increased dislocations were introduced into the quantum well regions. The degraded crystal quality would therefore cause an increased density of nonradiative recombination centers, which were responsible for the fast recovery of the absorption. In addition, with the as-grown thin GaN buffers, the severe interference-induced reflectivity fluctuations were successfully suppressed.

In this work, the effects of large V-pits on the morphological and optical properties of InGaN∕GaN multiple quantum wells (MQWs) were studied using scanning electron microscopy, transmission electron microscopy, and photoluminescence. InGaN∕GaN MQWs with high-density large V-pits were grown by metal organic chemical vapor deposition. In addition to the regular c-plane MQWs, the MQWs grown on the {101¯1} faceted sidewalls of the V-pits were also observed, which gave much higher emission energies than those of the c-plane MQWs. Furthermore, when the low-temperature GaN buffer was very thin, the {112¯m} (m⩾2) faceted sidewalls of the V-pits were observed. It was then found that MQWs grown on such sidewalls had emission energies between those of the c-plane MQWs and those of the {101¯1} faceted sidewall MQWs.

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al2O3(0002) (c-plane), and Al2O3(11¯02) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al2O3 substrate while a ZnO(112¯0) single crystal is formed on an r-plane Al2O3 substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO∕GaAs(001), ZnO∕GaAs(111), and ZnO∕Al2O3(0002), while the photoluminescence from ZnO∕Al2O3(11¯02) shows a reduced intensity together with an increased linewidth, which is, likely, due...

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.

The effects of thermal annealing on Hall-effect measurement and photoluminescence (PL) from undoped n-type ZnO/GaAs thin-film samples have been studied. The evolutions of carrier concentration, electrical resistivity, and PL spectrum at various annealing conditions reveal that the dominant mechanism that affects the electrical and PL properties is dependent on the amount of thermal energy and the ambient pressure applied during the annealing process. At low annealing temperatures, annihilation of native defects is dominant in reducing the carrier concentration and weakening the low-energy tail of the main PL peak, while the GaAs substrate plays only a minor role in carrier compensations. For the higher temperatures, diffusion of Ga atoms from the GaAs substrate into ZnO film leads to a more n-type conduction of the sample. As a result, the PL exhibits a high-energy tail due to the high-level doping.

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, 42nm 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.

The authors studied the growth and wet thermal oxidation (WTO) of ZnO thin films using a radio-frequency magnetron sputtering technique. X-ray diffraction reveals a preferred orientation of [101¯0]ZnO(0002)∕∕[112¯0]Al2O3(0002) coexisted with a small amount of ZnO (101¯1) and ZnO (101¯3) crystals on the Al2O3 (0001) substrate. The ZnO (101¯1) and ZnO (101¯3) crystals, as well as the in-plane preferred orientation, are absent from the growth of ZnO on the GaAs(001) substrate. WTO at 550 °C improves the crystalline and the photoluminescence more significantly than annealing in air, N2 and O2 ambient; it also tends to convert the crystal from ZnO (101¯1) and ZnO (101¯3) to ZnO (0002). The evolution of the photoluminescence upon WTO and annealing reveals that the green and orange emissions, centered at 520 and 650 nm, are likely originated from oxygen vacancies and oxygen interstitials, respectively; while the 420 nm emission, which is very sensitive to the postgrowth thermal processing regardless of the subst...

Evolution of photoluminescence (PL) and strain is investigated in GaIn(N)As/Ga(N)As multiple quantum wells (MQWs) over a wide range of annealing temperatures from 700 to 900 °C. We observe two optimal annealing temperatures (Topt) that result in reduced PL linewidth and increased PL intensity irrespective of the thermal-induced strain relaxation. The decrease of PL intensity accompanied by peak splitting after the first Topt is mainly associated with the deterioration of the GaAs cap layer and the optical quenching after the second Topt is due to the overannealing-induced degradation of the bottom QWs. The strain relaxation in GaInAs/GaAs MQWs at elevated temperatures, which gives rise to extrinsic defects at the bottom of the MQWs stack, only plays a minor role in PL evolution, while no strain relaxation is observed in GaInNAs/GaNAs MQWs.