In this study, we investigate a new technique to fabricate DNA-CTMA films with tunable properties. MAPLE is, for the first time, explored to deposit DNA-CTMA dielectric films on top of epitaxially grown graphene on silicon carbide (SiC) substrate. Silicon dioxide (SiO2) is commonly used as a gate insulator in graphene based field effect transistors (GFETs) in a top gate configuration. The high temperature deposition of SiO2 on graphene is known to cause damage to the surface of the graphene leading to poor device operation. We propose an alternative gate insulator based on a bio-organic (DNA-CTMA) material processed and deposited at room temperature (RT) using MAPLE. Hall measurements run before and after DNA-CTMA deposition showed no change in the type of conductivity as well as charge carrier mobility.
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.
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.
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.
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.
Aligned carbon nanotubes (CNT’s) have been found to form on both the Si and C faces of silicon carbide (SiC) wafers at high temperature. The CNT’s form when the SiC wafer is exposed to temperatures in the range 1400-1700°C under moderate vacuum. The CNT’s are aligned roughly parallel to the surface. After a half hour at 1700°C under vacuum of 10-4torr, a near continuous CNT layer about 250nm thick is formed. The entire surface of the SiC is covered with CNT’s including both single and multiwalled tubes, and some graphitic carbon. SEM, TEM, AFM, XPS and Raman scattering measurements have been used to analyse the CNT/SiC structures. The metal catalyst free CNT’s on SiC exhibit low density of structural defects and are very straight. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. It is speculated that CNT's growth is catalysed by low concentrations of residual oxygen in the chamber during growth. The vacuum conditions can significantly affect CNT's growth. Single wall carbon nanotubes are evident in Raman spectra on the samples grown at 10-3 Torr, not on these grown at 10-5Torr.
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