Two comparative blue emitting InGaN/GaN multiple quantum well (MQW) structures, for lighting and laser diode
applications, with and without pre-strained layer, were grown by MOCVD. Temperature dependent photoluminescence
(TDPL) and time-resolved (TR) PL were used to study their optical and transient properties. PL signals from InGaN
MQWs were divided into two parts: one is the band to band transition of InGaN; the other is the broad defect band. It is
indicated that the InGaN/GaN MQW structure with prestrained layer has larger activation energy. TRPL measurements
were performed in 10-300 K and with the detection wavelength cross over the emission peak. It is found that the MQW
sample with prestrained layer has deeper localization depth. Temperature dependence of PL decay time shows an
interesting behavior of an increase from 10K to 30K and then a decrease till 300K.
Quaternary III-V compound InAlGaP, especially In0.5(AlxGa1-x)0.5P which is lattice matched with
GaAs, are important materials for visible red-green light emitting diode (LED) and laser diode (LD),
solar cell and other optoelectronic and electronic device applications. A set of In0.5(AlxGa1-x)0.5P thin films on GaAs substrates with a wide range of x up to ~80%, were grown by low pressure metalorganic chemical vapor deposition (MOCVD) and studied by a variety of nuclear science and optical analytical techniques, including Rutherford Backscattering Spectrometry (RBS), Raman scattering, photoluminescence (PL), Photoreflectance (PR) and FTIR. Temperature dependent PL-PR measurements over 10-300 K presented the band gap of these InAlGaP materials and variations with composition x and temperature (T). RBS was used to measure the microstructure of AlInGaP films, and through simulation, determine the film thickness and composition precisely. RBS measurement and simulation results indicate a quite fuzzy in the two interfaces, i.e. that there exists diffusion in
the majority samples, especially between the AlInGaP layer and substrate. For a certain number of incoming He+ ions,we have proposed a way to determine the error bar by RBS successful. For this series of samples, the error bar of content is around ±1.5%. The error bar of thickness is around ±5.0nm. Different InAlGaP films with different composition and thickness may present different error bars. The results illuminate that RBS is a precise tool to analysis the microstructure of quaternary semiconductor AlInGaP/GaAs samples.
Al-doped ZnO can replace tin-doped indium oxide (ITO) as a good transparent conductive oxide (TCO) in LEDs and
optoelectronic applications. We investigate on nanometer scale AlZnO thin film materials epitaxied on sapphire
substrates in 350-650°C from pulsed laser deposition (PLD). Synchrotron radiation X-ray absorption fine-structure
spectroscopy on O K-edge indicates that Al-doped ZnO can not form alloy at growth temperature 350°C without Al-O
bonding feature. The Al-O transition of AZO550 is stronger than AZO650. These are correlated to Raman scattering
measurements and analyses. Al-doped ZnO grown at 350°C possesses weak/broad Raman signals indicating a poor
crystalline film. The E2 (high) mode is strong and narrow in AZO550. All these experimental results indicate that PLD
grown AlZnO film on sapphire could get a better crystalline quality at 550°C than 350°C and 650°C.
We have performed a comparative structural and optical investigation of InGaN/GaN MQW LED wafers grown on (1122) facet GaN/sapphire templates by Metalorganic Chemical Vapor Deposition. The effect of the growth time of epitaxial
lateral overgrowth GaN with (11 2 2) facets on the structural and optical properties were investigated via
photoluminescence (PL), PL excitation, time-resolved PL, Raman and SEM measurement on two typical InGaN MQW
samples. From temperature dependence PL measurement, we can find that these two samples both exhibit two
distinguish peaks attributed to quantum wells with wide range wavelength. Raman E2 (high) signals revealed a partially
relaxation of compressive stress in the facet GaN template. Experimental observations have revealed: (a) a Stokes shift
between the emission peak energy and absorption edge and (b) a red shift behavior of emission with decay time
(equivalently, a rise in decay time with decreasing emission energy). The large Stokes shift can be attributed to the
quantum confined Stark effect (QCSE). The lower-energy side of the InGaN main emission peak is governed mainly by
carrier generation in the GaN barriers and subsequent carrier transfer to the InGaN wells. Different amounts of Stokes
shift indicate the interface imperfection from longer growth time of epitaxial lateral overgrowth GaN with (1122) facets.
Temperature dependence of PL decay time τPL shows an interesting behavior of τPL with temperature.
A set of AlGaN epilayers were grown on sapphire (0001) substrate by MOCVD, with intermediate growths of low/high
temperature AlN nucleation layers. Variable flow rates of trimethylindium (TMIn), 0, 50 and 500 sccm were introduced
during growth. Three AlGaN samples were originally designed with similar Al composition of ~20%. Rutherford
backscattering (RBS), RT and 10-300K photoluminescence (PL) were used for analyzing the microstructure of thin
films. The Al content was calculated to decrease with increasing the In-flow rate. Main PL bands spread over 310-350
nm with peaks in 320-335 nm. PL (10-300K) exhibited anomalous temperature dependent emission behavior
(specifically an S-shaped shift, i.e. red-blue-red shifts) of the AlGaN related PL emission. Carriers transfer between
different luminescent centers. Abnormally high activation energy was obtained, which shows that the excitons are not in
the free states. Raman Scattering and spectral line shape analysis leaded to an optical determination of the electrical
property free carrier concentration of AlGaN. Our results on In-doped AlGaN provide useful information for designing
UV-LEDs.
A series of ZnO thin films with different thicknesses grown on sapphire substrates by metalorganic chemical vapor
deposition (MOCVD) have been studied by different characterization techniques. The optical properties are investigated
by photoluminescence (PL), optical transmission (OT) and 1st order derivatives, various angle scanning ellipsometry
(VASE). Rutherford Backscattering (RBS) shows the atomic Zn:O ratios with a few percentage aviation from 1:1, and
thicknesses in range of 10~230 nm, roughness layer with 10~30nm, which are corresponding to results from atomic force
microscopy (AFM), and scanning electron microscopy (SEM). The optical and structure characterization measurements
have confirmed the good quality of these epitaxial ZnO materials.
High brightness InGaN/GaN multiple quantum well structures have been grown on sapphire substrates by
metalorganic chemical vapor deposition, for wide range visible light emitting diode application. The compositions and
sizes within quantum wells were designed according to the requirements on the LED performance. Samples were
investigated by a variety of characterization techniques. Optimization of the growth parameters and process was realized
and evidenced by high resolution X-ray diffraction measurements. Optical spectroscopic properties were further studied
and quantum confined stokes shift was observed from room temperature and low temperature photoluminescence as well
as time resolved photoluminescence measurements.
AlGaN epilayers with higher Al-compositions were grown by Metalorganic Chemical Vapor Deposition (MOCVD)
on (0001) sapphire. Trimethylgallium (TMGa), trimethylaluminium (TMAl) and NH3 were used as the source precursors
for Ga, Al, and N, respectively. A 25 nm AlN nucleation layer was first grown at low-temperature of 590 °C at 300 Torr.
Followed, AlxGa1-xN layers were grown at 1080 °C on low-temperature AlN nucleation layers.
The heterostructures were characterized by a series of techniques, including x-ray diffraction (XRD), Rutherford
backscattering (RBS), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering. Precise Al
compositions were determined through XRD, RBS, and SEM combined measurements. Room Temperature Raman
Scattering spectra shows three major bands from AlGaN alloys, which are AlN-like, A1 longitudinal optical (LO) phonon
modes, and E2 transverse optical (TO) band, respectively, plus several peak comes from the substrate. Raman spectral
line shape analysis lead to an optical determination of the electrical property free carrier concentration of AlGaN. The
optical properties of AlGaN with high Al composition were presented here.
Rare earth (RE) elements are promising alternatives to transition metals (TMs) for use in developing a dilute
magnetic semiconductor (DMS) for spintronics applications. Instead of relying on the d-shell electrons of the TMs as the
magnetic element, the 4f electrons from the RE elements are used. The 4f RE elements can have larger magnetic
moments as compared to 3d TMs, because the 4f orbits are localized and the direct coupling between the 4f ions is weak.
There have been several reports of using RE elements for optoelectronic applications, as their various internal f-shell
electronic transitions vary in energy from infrared to visible.
The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology has been employed to
obtained Zn K-edge absorption spectra for Cd11-xZnxTe alloy with x = 0.03, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.00. Their
Fourier transform spectra were analyzed, which have shown a bimodal distribution of bond lengths, suggesting distortion
of the Te sub-lattice, so that the linear interpolation is true only in an approximate sense. Synthetic CdZnTe crystals can
be used for the room temperature-based detection of gamma radiation. The radiation detection properties of CZT crystals
vary widely. A common defect found in most high-quality CZT crystals is Te secondary phases, often located along
grain boundaries. The secondary phases can be both large inclusions (>50 μm) and smaller precipitates (<50 μm). The
Te secondary phases distributed throughout the crystal can cause changes to the detector leakage current, resulting in
decreased radiation spectrometer performance. This set of Cd1-xZnxTe crystals were also measured by Raman scattering
at room temperature. The two observed peaks at about 125 and 145 cm-1 which can be assigned to Te A1 and E phonon
mode, respectively. The induced damage on the crystal surfaces by Raman laser has been discussed. It is suggested that
in the case of highly Zn doping CdZnTe crystals, the ZnTe bond were broken by laser exposing and then free Te atoms
are migrating to these heated areas which cause Te precipitate. Further, the results of the soft X-ray measurements have
been also presented and this part of the experimental data needs to do more penetrating analysis in the future.
Far-infrared (FIR) reflectance spectroscopy has been employed to study the optical properties for a
series of bulk CdZnxTe1-x and CdSexTe1-x wafers. The zone-centre optical phonons for the ternary
alloys show a variety of behavior patterns: they exhibit a "one-mode", "two-mode" or
"intermediate-mode" behavior depending on the vibration characteristics of the end binary members.
The CdSeTe called CST were found to be single-crystal with the zincblende structure. These four
samples labeled with CST5, CST15, CST25, and CST35, which correspond with the composition of Se,
5%, 15%, 25%, 35%, respectively. The intensity of CdTe-like TO band decays with x increasing, and
the peak position increases from 140 cm-1 to 145 cm-1. In the other hand, the intensity of CdSe-like TO
band grows with x increasing, and the peak position of CdSe-like TO band increases from 174 cm-1 to
181 cm-1. We use the model of dielectric function and using Least-Square fit to find the optical and
transport parameters. By the infrared spectra analysis, we found the conductivity of CdZnxTe1-x
increase with increasing of x value and the conductivity of CdSexTe1-x decrease with increasing of x
value.
The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology was employed on Si Kedge
absorption spectra for bulk 6H-SiC with different doping concentration. Their Fourier transform spectra were
analyzed, which have shown a parabolic linear distribution of bond lengths. Through combined Raman and XAFS
studies, the coincidental results could be obtained. In the Raman spectroscopy, the LO mode intensity becomes weaker
and broader as the doping concentration increases. This indicates that the crystallinity is damaged by the heavy doping
concentration. The Raman curves have been fitted by theoretical formulas and the accurate information of the intensity,
peak position, and FWHM in each TO and LO modes have been obtained. By the EXAFS and the fitting program, the
bond length of Si-C in 6H-SiC decreases as the doping concentration increases. It is believed to be caused by the
nitrogen atoms substituting for carbon atoms in the SiC lattice. But further research work is needed to identify this. The
little change in Si-Si bond length indicates the influence of doping is still under local structure, near the absorbed atoms.
The challenge of growing GaN and its alloys, In1-xGaxN and Al1-xGaxN, is still formidable because of the lack of close
lattice match, stacking order match, and similar thermal expansion coefficient substrates, the same as GaN-based
optoelectronic materials. ZnO is the most promising optoelectronic materials in the next generation, with wide band gap
of 3.3eV and exciton binding energy of 60meV. In addition, ZnO also has been considered as a substrate for epitaxial
growth of III-Nitrides due to its close lattice and stacking order match. Our works cover the growth of n-type InGaN and
GaN epitaxial layers on lattice-matched ZnO substrates by metal-organic chemical vapor deposition (MOCVD). Since
MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need to more fully explore
this technique for ZnO substrates. However, the thermal stability of the ZnO substrate, out-diffusion of Zn from the ZnO
into the GaN, and H2 back etching into the substrate can cause growth of poor quality GaN. We use a GaN buffer layer
of about 40nm to avoid Zn/O diffusion. We can investigate the Zn/O diffusion in the InGaN epilayers by means of
second ion mass spectroscopy (SIMS) depth profiles, and analyze the surface bonding of different elements by x-ray
photoelectron spectroscopy (XPS), and investigate optical and structural characterization of InGaN epilayers on ZnO
substrates by various angles spectroscopic ellipsometry (VASE). Finally, from the Raman scattering, Photoluminescence
(PL) and Photoluminescence excitation (PLE) spectra, we can determine the qualities easily and prove that we have
grown the InGaN on ZnO with a GaN buffer layer successfully.
ZnO is a promising substrate for GaN growth due to a lattice match with In.18Ga.82N, similar thermal expansion
coefficient, and its ability to be easily chemically etched, which results in improved light extraction. A transition layer of
Al2O3 was also grown by ALD prior to MOCVD growth to prevent Zn diffusion, protect the ZnO substrate from H2 back
etching, and promote high quality nitride growth. Thick InGaN layers (~200-300nm) were grown in this study on bare
ZnO substrates and ALD/ZnO substrates. Various buffer layers were attempted, such as SLs of AlGaN/GaN, MQWs of
InGaN/GaN, and LT-GaN. These results are significant as previous studies showed decomposition of the layer at InGaN
thicknesses of 100nm or less. These layers allowed for the first LEDs to be grown on bare ZnO substrates. This study
demonstrated that InGaN LEDs showed emission in optical measurements as well as a high IQE of ~60%. The data
shows promise for LED structures on ZnO using InGaN as n- and p-type LED layers. Etching of the ZnO substrate also
showed that removal of the substrate can be performed easily.
Three blue emission multi-quantum well (MQW) light emitting diode (LED) samples with different indium
composition and different amount of quantum wells were studied. From x-ray diffraction and transmission electron
microscopy experimental measurements, the period thickness and indium composition of the sample have been
obtained. The temperature dependent photoluminescence (PL) shows that the signal from InGaN/GaN samples was
influenced by two kinds of factors. One is the band to band transition of InGaN; another is the localization effect
caused by the non-uniformity of the In composition in In-rich samples. While the temperature increases, full width
half maximum becomes larger, and the signal tends to shift to the red side. Through theoretical fitting on the
temperature dependent PL data, the activation energies (Ea) of the InGaN multi-quantum well samples were
obtained. Time-resolved photoluminescence (TRPL) results show that as the indium composition increases and the
QW number increases, a longer decay time will get. From the results of photoluminescence excitation (PLE)
experiment, a large Stokes shift (SS) was observed. The large Stokes shift can be attributed to the variation of
indium composition or the quantum confined Stark effect (QCSE). Also, the Photoluminescence spectra exhibit
weak blue peaks and the optical intensity is improved by increasing the number of wells.
A series of self-organized InAs/GaAs quantum dots with spacer layer under different thermal-treat
(annealing) temperature and environments were prepared by molecular beam epitaxy. They were investigated
by atomic force microscope and temperature-dependent photoluminescence (PL). Results showed that the
sample annealed at lower temperature has lager size quantum dots and smaller density of quantum dots. The
size of quantum dots is getting smaller and the density of quantum dots is getting larger as the annealing
temperature increase. Two broad PL peaks are attributed to the combined size distribution of the bimodal
quantum dots.
Carbon nanotubes (CNTs) arrays were prepared by microwave plasma-enhanced chemical vapor deposition
(MPCVD) method. Nickel layer of 7 nm in thickness on 100-nm thickness titanium nitride film was
transformed into discrete islands after hydrogen plasma pretreatment. CNTs were then grown up on Ni-coated
areas by MPCVD. Their field emission properties were studied and evaluated. From formulism analysis,
superior CNT films with very low emission onset electric field, about 0.425 V/micron (at J =10 micro-A/cm2), are attained without post-deposition treatment. The large field amplification factor arising from small
curvature radius of nanotube tips is responsible for good emission characteristics.
The optical and structural properties of Charge Asymmetric Resonance Tunneling (CART) structure InGaN/GaN multiquantum
wells (MQWs) grown on sapphire by metalorganic chemical vapor deposition (MOCVD) have been
investigated by optical measurements of temperature-dependent photoluminescence (PL), photoluminescence excitation
(PLE) and time-resolved photoluminescence (TRPL), and
high-resolution transmission electron microscopy (HRTEM).
Two typical samples are studied, both consisting of six periods of CART InGaN wells with 3.3 nm thickness and with
8.5 nm thickness of GaN barrier, respectively, and two periods of InGaN wells with 2 nm thickness of 7 nm GaN barrier
with different well growth-temperature of 797°C and 782°C, respectively. According to the PL measurement results,
large values of activation energy are obtained. The decrease of well growth-temperature results higher In composition
and also in the increase of composition fluctuation in the InGaN MQW region, showing the stronger carrier localization
effect and large values of activation energy and Stokes' shift are obtained. The lifetime at the low-energy side of the
InGaN peaks is longer for higher indium composition.
Al2O3 films were deposited on the Zn face of ZnO (0001) substrates as a transition layer by atomic layer
deposition (ALD). The as-deposited 20 and 50nm Al2O3 films were transformed to polycrystalline α-Al2O3 phase
after optimal annealing at 1100°C after 10 and 20 minutes, respectively, as identified by high resolution x-ray
diffraction (HRXRD). Furthermore, GaN and InGaN layers were grown on annealed 20 and 50nm Al2O3 deposited
ZnO substrates by metalorganic chemical vapor deposition (MOCVD) using NH3 as a nitrogen source at high growth
temperature. Wurtzite GaN was only seen on the 20nm Al2O3/ZnO substrates. Room temperature
photoluminescence (RT-PL) shows the near band-edge emission of GaN red-shifted, which might be from oxygen
incorporation forming a shallow donor-related level in GaN. Raman scattering also indicated the presence of a wellcrystallized
GaN layer on the 20nm Al2O3/ZnO substrate. InGaN was grown on bare ZnO as well as Al2O3
deposited ZnO substrates. HRXRD measurements revealed that the thin Al2O3 layer after annealing was an effective
transition layer for the InGaN films grown epitaxially on ZnO substrates. Auger Electron Spectroscopy (AES)
atomic depth profile shows a decrease in Zn in the InGaN layer. Moreover, (0002) InGaN layers were successfully
grown on 20nm Al2O3/ZnO substrates after 10min annealing in a high temperature furnace.
Surface and material properties of dielectric Ga2O3 thin films deposited onto GaAs substrate with different
annealing temperature were studied via a variety of techniques, including X-ray diffraction (XRD), X-ray
Photoelectron Spectroscopy (XPS), scanning electron microscope (SEM) and energy dispersive X-ray
spectroscopy (EDX). The effects of annealing are investigated. The increase of ratio of oxygen to gallium
assuredly as the annealing temperature increased was found. The relationship between the interface quality
and annealing temperature is identified.
Optical and structural properties of InGaN/GaN multi-quantum well (MQW) structures with different well width, influenced by the nano-structural features in the MQWs, were investigated by optical measurements of photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL), as well as structural analysis methods, such as high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) measurements. Due to the quantum confined Stark effect (QCSE), larger Stokes shift is induced with larger well width. Thermally activated carrier screening model is established to well describe the so-called S-shaped spectral shift with temperature. Inhomogeneous line-width broadening induced by piezoelectric field is found to be dominant at low temperature, while homogeneous line-width broadening due to phonon scattering takes over at higher temperature. Additionally, two activation energies are extracted from the Arrhenius plot of PL intensity. One is assigned to be the exciton binding energy and the other one the confinement energy of electrons in the quantum well. TRPL study further indicated that the radiative lifetime was decreased with the decreased well width. All these are associated with the In-composition fluctuation and nano-structures in the MQWs.
ZnO has been considered as a substrate for epitaxial growth of III-Nitrides due to its close lattice and stacking order
match. This paper will cover growth of InxGa1-xN epitaxial layer on lattice-matched ZnO substrates by metal-organic
chemical vapor deposition (MOCVD). InGaN of various indium compositions from different growth temperatures were
well controlled in the InGaN films on ZnO substrates. High-resolution X-ray diffraction (HRXRD) confirmed the
epitaxial growth of InGaN film on ZnO. The optical and structural characterization of InGaN epilayer on ZnO substrates
was measured by room temperature photoluminescence, temperature-dependent photoluminescence, and field-emission
secondary electron microscope. In addition, a transition layer of Al2O3 on ZnO substrates have been employed for
InGaN growth to help prevent Zn and O diffusion into the epilayers as well as assist nitride epilayer growth. HRXRD
results show a single crystal InGaN film has been successfully grown on annealed Al2O3 coated ZnO substrates.
The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) grown on sapphire by metalorganic
chemical vapor deposition (MOCVD) have been investigated by optical measurements of temperature-dependent
photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL), and
structural analysis methods of high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron
microscopy (HRTEM). Two typical samples are studied, both consisting of five periods of InGaN wells with 1.6 nm
thickness with different indium compositions of 21 and 24%, respectively. The thickness of the GaN barrier was 7.9
nm. According to the PL and PLE measurement results, large values of activation energy and Stokes' shift are obtained.
This indicates that higher In composition results in the increase of composition fluctuation in the InGaN MQW region,
showing the stronger carrier localization effect. The lifetime at the low-energy side of the InGaN peaks is longer for
higher indium composition, as expected from the larger Stokes shift.
InGaN/GaN multiple quantum well (MQW) light emitting diode (LED) structures with blue and green light emissions have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated by high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) and photoluminescence excitation (PLE). HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. HR-TEM determined the MQW structures and parameters, indicating the high quality of layer interfaces of these LED samples. Excitation power-dependent PL predicates that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties. Temperature-dependent PL shows that the QW PL emission peak exhibits a monotonic red-shift and that the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the PLE results, a large energy difference, so-called quantum confined Stokes shift, between the band-edge absorption and emission was observed. Penetrating TEM revealed the V-shape defects, and quantum dot-like structures within the InGaN well region, which leads to intense light emissions from these MQW LEDs.
Copper nanoparticles and microfibers have been prepared by solid state reaction using carbon nanotube (CNT) as template. Their optical and structural properties were investigated by scanning and transmission electron microscopy, X-ray diffraction (XRD), Raman scattering and photoluminescence. Carbon nanotube template was found to be the main contributor to the Raman spectra, however, after the introducing of copper, we found G band shifted to high frequency by a few wavenumbers and the shift was also related to the ratio of copper to carbon nanotube. With copper form change from nanoparticles to microfiber, the ratio of scattering intensity between D band and G band of carbon nanotube increase, which reveals a change in microcrystallinity. Compared with pure carbon nanotube, a broad photoluminescence background was found to be superimposed on the Raman spectrum of these copper related carbon nanotube, and the intensity of this photoluminescence background also increase with respect to the ratio of copper to carbon nanotube. High resolution X-ray diffraction indicated that copper lattice constant of copper fibers decreased with respect to its bulk form.
InGaN/GaN Multiple Quantum Well (MQW) structures have been grown on sapphire substrates by low pressure metalorganic chemical vapor deposition (MOCVD), for wide range of blue, blue-green and green light emission device application. The compositions and sizes within QWs were designed according to the requirements on the LED performance. Samples were investigated by a variety of characterization techniques. Optimization of the growth parameters and process was realized and evidenced by high resolution X-ray diffraction (XRD) measurements. High quality of MQW wafers have been achieved with excellent characteristics, showing XRD multiple satellite peaks up to 10th order due to the quantum well superlattice confinements and fine fringe structures among satellite peaks. Transmission electron microscopy confirmed the sharp MQW structures and dimensional parameters, and revealed the V-shape defects. Optical properties were further studied and Quantum confined Stokes effect was observed from photoluminescence and photoluminescence excitation measurements.
GaN thin film materials, un-doped, Si- and Mg-doped, have been grown on c-sapphire substrates by low pressure metalorganic chemical vapor deposition, and have been characterized by micro-Raman scattering ((mu) -RS) and micro- photoluminescence ((mu) -PL) spectroscopy. Basic Raman scattering modes, and in particular, their variations have been observed with the laser incident on the cross section of a few micron thick GaN film. Raman line shape analysis on the E2 mode is presented, based upon the spatial correlation theoretical model. Through the theoretical modeling of the LO-phonon-plasmon coupling, the free carrier concentration can be determined via Raman measurements and curve fitting. Using a newly designed and developed UV Raman-PL microscope system, room temperature PL and its variation with the SiH4 doping level for a series of n- type GaN epitaxial materials have been studied. Combined UV (mu) -RS-PL spectra from p-type GaN are also investigated.
In this work we present optical investigations of metalorganic chemical vapor deposition (MOCVD) grown InSb thin films on GaAs(100) substrates and MeV ion implanted InSb(111) bulk crystals. Far-infrared (FIR) reflectance spectroscopy has been used to study the lattice vibration behavior of all the samples. For the MOCVD films the effects of III-V source ratios on the films crystalline quality may have been reported. Two additional weak modes in the wavenumber regions of 210-240 cm-1 are observed and they appeared more prominent at low temperatures. Interference fringe effects modify the FIR reflectance band of the GaAs substrate. They are related to the uniformity of film thickness and crystalline perfection. The relationship between these interference features and film quality as well as thickness uniformity was obtained from the measured spectra. The carrier concentration, mobility, effective mass as well as the dielectric constant of these films have been determined. For the bulk InSb crystals, high energy C+ ions have been implanted and MeV energies and, the optical as well as transport properties of the implanted layers have been presented and discussed.
Room Temperature (RT) Infrared (IR) Reflectance spectra are studied both theoretically and experimentally on 3C-SiC films grown on silicon (100) substrate by low pressure chemical vapor deposition (LPCVD). By the use of transfer matrix method, the spectral features influenced by film thickness, phonon-damping constant and free carrier concentration are systematically studied. Comparisons of reflectance spectra are made between experimental spectra and those of ideal samples. A modified effective medium model with consideration of the presence of interfacial layer is introduced to interpret some unusual features. Although some of our results are qualitative, careful analysis of reflectance spectra does provide valuable information about film quality.
GaN thin films, undoped, Si- and Mg-doped, and InGaN-GaN multiple quantum well (MQW) structures have been grown on Si substrates with specially designed composite intermediate layers consisting of a ultra-thin amorphous Si layer and a GaN/AlGaN multilayered buffer by low pressure metalorganic chemical vapor deposition. The structural and optical properties of these new grown material were studied. X-ray diffraction, Raman scattering and Fourier transform IR reflectance measurement confirmed their wurtzite structure. Scanning electron microscopy exhibited the single crystalline grain size up to approximately 2 micrometers . Photoluminescence showed strong GaN near edge emission, with only very weak deep defect-related emissions, for GaN films, and strong MQW emissions. The film surface morphology and material properties are improved by adjusting the growth conditions and buffer layer structural design.
A comparative study on GaN/sapphire has been performed by transmission electron microscopy (TEM) and IR reflectance (IR). TEM observations reveal that both the undoped and Si doped GaN epilayers have large density of threading dislocations. Dislocations in the undoped GaN tend to from open core structure, while dislocation lines in the Si-doped GaN are very sharp and the strain contrast is much more confirmed. It is believed that Si-doping causes the increase in undoped GaN to much more confirmed dislocation lines. Frank dislocation loops are also found lined up at a depth of about 110 +/- 10 nm from the interface. High resolution TEM study also reveals that the GaN buffer layer grown at low temperature has transformed into its thermodynamically stable wurtzite structure during the high temperature post- buffer GaN epilayer growth process. The comparative IR reflectance hows the corresponding behavior. The interference fringes of the Si doped sample, compared with the undoped ones, shows a contrast damping and reflectance reduction behavior, suggesting the presence of a transition/defect layers near the interface.
InGaN thin films were grown by low-pressure metalorganic chemical vapor deposition and characterized by photoluminescence with variable excitation intensity and temperature, room-temperature cathodoluminescence (CL), high resolution X-ray diffraction, scanning-electron-microscopy (SEM) and atomic force microscopy (AFM). For PL, all the sample show dominant peaks at around 2.9 eV and extra peaks or shoulders at 2.8 eV at 6K. We concluded that the low energy peak is due to the localized near-band edge transition from the phase-separated InGaN mesoscopic structure with high In-content. The strong luminescence of the low of nanostructure or quantum dots. AFM images showed that phase-separated InGaN samples have inverted hexagonal pits which are formed by the In segregation on the surfaces. Room temperatures cathodoluminescence and imags at wavelengths corresponding to the GaN band edge, the In-poor and In rich regions were studied. It was shown that phase separated In-rich regions formed at the periphery of the hexagonal pits.
AlGaN based interdigital metal-semiconductor-metal (MSM) photodetectors with 14 percent Al have been successfully grown and fabricate don sapphire substrates. The devices exhibit large gains up to 106 at high bias voltages, but with very high dark currents, > 1 mA and very long detector responses, > 60 seconds. A negative temperature coefficient for the breakdown voltage was observed indicating that tunneling is occurring. However, at high bias voltages, avalanche breakdown also appears to be present since a constant breakdown field of 105 V/cm was obtained independent of MSM geometry. Avalanche breakdown is nucleated at the non-uniform field distribution at the edge of the MSM finger.
High brightness visible light emitting didoes (LEDs) based on GaAlInP/GaAs are now in great demand. In order to meet the requirements for mass production with low unit cost, metalorganic chemical vapor deposition (MOCVD) is the primary deposition process for cost effective large area and multiple wafer growth of compound semiconductors. In this work low pressure MOCVD growth and non-destructive materials characterization on 100 mm wafer size epitaxial films of quaternary GaAlInP grown on GaAs substrates for LED applications is demonstrated. The ability to scale the deposition and fabrication process from the traditionally used 50 mm to 100 mm will be key in further reducing costs. MOCVD system design requires that growth to be laterally uniform, abruptly switchable, and robust against variations in process parameters that can be achieved so that production of high quality and high uniformity GaAlInP films are obtained. In parallel, to this effort there is the need to develop rapid while wafer and non-destructive mapping characterization techniques to investigate GaAlInP materials properties such as sheet resistivity, film thickness, photoluminescence (PL), Fourier transform IR and Raman scattering spectra for both material and for the on-going qualification of material during production. Typical uniformities of GaAlInP epitaxial film thickness, sheet resistivity, major PL band peak wavelength and width are 1-3 percent. For techniques without automatic mapping abilities, multiple point measurements and employed to obtain information over the entire wafer. Variations of these characteristic features, such as sheet resistivity, PL and Raman properties, with different Al compositions and doping are discussed in this work.
Interdigital metal-semiconductor-metal (MSM) and p-n UV photodetectors have been successfully grown and fabricated from GaN based materials. The MSM devices were produced using two types of GaN; high-resistive GaN and Mg doped GaN. For the high-resistive GaN detector, the lowest dark current is approximately 0.1 nA and the UV responsivity of the device was about 460 A/W at a DC bias of 30 V. The Mg doped GaN exhibited larger gains, 1150 A/W at 2.0 V, but at much higher dark currents, 400 nA. The high gain in this device is not well understood but has attributed to an 'avalanche' effect and is under further investigation. The feasibility of a photovoltage detector structure based on alloys of GaN has also been proven. A GaN/GaInN structure exhibited a cut- off at 2.9 eV with a responsivity of 0.28 A/W at zero bias for an active region of only 500 angstrom thick.
Recent advancements in the technique of Raman spectroscopy now make it possible to achieve rapid, minimally invasive and non-destructive characterization of tissues. In order to evaluate the efficacy of this technique for diagnosis, the Raman spectra of normal and neoplastic human tissues (e.g., breast, kidney, liver and colon) were obtained utilizing visible and near-IR excitation. Normal breast tissue and colon adenocarcinoma showed major Raman features due to the presence of carotenoids and lipids. In breast carcinoma, the features due to lipids were attenuated and as fibrosis (desmoplasia) increased, new spectral features attributable to collagen were observed. Samples of normal and neoplastic liver and kidney show unique spectral differences sufficient to permit tissue differentiation.
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