Doctor of Philosophy / Department of Physics / Hongxing Jiang / III-nitride photonic devices such as photodetectors (PDs), light emitting diode (LEDs), solar cells and optical waveguide amplifiers were designed, fabricated and characterized. High quality AlN epilayers were grown on sapphire and n-SiC substrates by metal organic chemical vapor deposition and utilized as active DUV photonic materials for the demonstration of metal-semiconductor-metal (MSM) detectors, Schottky barrier detectors, and avalanche photodetectors (APDs). AlN DUV PDs exhibited peak responsivity at 200 nm with a very sharp cutoff wavelength at 207 nm and extremely low dark current (<10 fA), very high breakdown voltages, high responsivity, and more than four orders of DUV to UV/visible rejection ratio. AlN Schottky PDs grown on n-SiC substrates exhibited high zero bias responsivity and a thermal energy limited detectivity of about 1.0 x 1015 cm Hz1/2 W-1. The linear mode operation of AlN APDs with the shortest cutoff wavelength (210 nm) and a photocurrent multiplication of 1200 was demonstrated. A linear relationship between device size and breakdown field was observed for AlN APDs. Photovoltaic operation of InGaN solar cells in wavelengths longer than that of previous attainments was demonstrated by utilizing InxGa1−xN/GaN MQWs as the active layer. InxGa1-xN/GaN MQWs solar cells with x =0.3 exhibited open circuit voltage of about 2 V, a fill factor of about 60% and external quantum efficiency of 40% at 420 nm and 10% at 450 nm. The performance of InxGa1-xN/GaN MQWs solar cell was found to be highly correlated with the crystalline quality of the InxGa1-xN active layer. The possible causes of poorer PV characteristics for higher In content in InGaN active layer were explained. Photoluminescence excitation studies of GaN:Er and In0.06Ga0.94N:Er epilayers showed that Er emission intensity at 1.54 µm increases significantly as the excitation energy is tuned from below to above the energy bandgap of these epilayers. Current-injected 1.54 µm LEDs based on heterogeneous integration of Er-doped III-nitride epilayers with III-nitride UV LEDs were demonstrated. Optical waveguide amplifiers based on AlGaN/GaN:Er/AlGaN heterostructures was designed, fabricated, and characterized. The measured optical loss of the devices was ~3.5 cm−1 at 1.54 µm. A relative signal enhancement of about 8 dB/cm under the excitation of a broadband 365 nm nitride LED was achieved. The advantages and possible applications of 1.54 µm emitters and optical amplifiers based on Er doped III-nitrides in optical communications have been discussed.

Doctor of Philosophy / Department of Physics / Jingyu Lin / III-nitride nanostructures and devices were synthesized by metal organic chemical vapor deposition (MOCVD) for their applications in various photonic, optoelectronic, and energy devices such as deep ultraviolet (DUV) photodetectors, solar cells, visible emitters, thermometric (TE) power generators, etc. Structural and optical properties in thicker AlN epilayers were found to be better than those in thinner AlN epilayers. Full-width at half maxima (FWHM) of x-ray diffraction (XRD) rocking curves as small as 63 and 437 arcsec were measured at (002) and (102) reflections, respectively in a thick AlN epilayer (4 m). The dark current of the fabricated AlN detectors decreases drastically as AlN epilayer thickness increases. DUV photoluminescence (PL) spectroscopy and x-ray diffraction (XRD) measurements were employed to study the effect of biaxial stress in AlN epilayers grown on different substrates. Stress-induced band gap shift of 45 meV/GPa was obtained in AlN epilayers. The potential of InGaN alloys as TE materials for thermopower generation has been investigated. It was found that as In content increases, thermal conductivity decreases and power factor increases, which leads to an increase in the TE figure of merit (ZT). The value of ZT was found to be 0.08 at 300 K and reached 0.23 at 450 K for In0.36Ga0.64N alloy, which is comparable to that of SiGe based alloys. Single phase InxGa1−xN alloys inside the theoretically predicted miscibility gap region (x = 0.4 to 0.7) were successfully synthesized. A single peak of XRD -2 scans of the (002) plane in InGaN alloys confirms that there is no phase separation. Electrical properties and surface morphologies were found to be reasonably good. It was found that growth rate should be high enough (>400 nm/hr) to achieve high quality and single phase InxGa1−xN alloys in this miscibility gap region. Mg-doped InxGa1-xN alloys were synthesized and characterized by Hall-effect and PL measurements for their application as visible emitters. P-type conductivity was measured up to x = 0.35 with accepter activation energy as low as 43 meV, which is about 4 times lower than that of Mg-doped p-type GaN. Resistivity as low as 0.4 -cm with a free hole concentration as high as 5x1018 cm-3 was measured in Mg-doped In0.22Ga0.78N. PL intensity decreased ~3 orders in magnitude when x increased from 0 to 0.22 in Mg-doped InxGa1-xN alloys.

Doctor of Philosophy / Department of Physics / Talat S. Rahman / This dissertation is an extensive study of several issues related to post deposition morphological evolution of fcc metal surfaces. These studies were carried out by probing the energetics and the dynamics of underlying atomistic mechanisms responsible for surface diffusion. An important aspect is the determination of relative probability of competing atomistic mechanisms and their contribution to controlling shapes and step edge patterns of nano structures on surfaces. In this scenario, the descent of adatoms from Ag islands on Ag(111) surface is examined. It shows an exchange mechanism to dominate over hopping and the process to favor the formation of (100)-microfacetted steps (A-type) over the (111)-microfacetted ones (B-type). Molecular dynamics simulations support these results at low temperature while at high temperature B-type step formation dominates. This change in the trend could happen if these processes leading to the formation of the A and B type steps have different values of their diffusion prefactors. This difference is confirmed on the basis of our calculations of the diffusion coefficients. Further, to understand the macroscopic properties of a system on the basis of its atomic scale information, spatial and temporal fluctuations of step edges on vicinal Cu(1 1 13) and Cu(1 1 19) surfaces is studied using kinetic Monte Carlo (KMC) simulations. These results show excellent agreement with experimental data, highlighting the role of mass transport along step edges, and also showing the validity of tools like KMC which aims at bridging the gap in length and time scales at which a range of interesting phenomena take place. To facilitate unbiased modeling of material properties, a novel way of performing KMC simulations is presented. In this approach the lists of diffusion processes are automatically collected during the simulation using a saddle-point search method in the potential energy landscape. The speed of the simulations is thus enhanced along with a substantial gain in reliability. Using this method the diffusion and coalescence of two-dimensional Cu and Ag adatom-island on Cu(111) and Ag(111) is studied. Together with input from molecular dynamics simulations, new processes involving the concerted motion of smaller islands are revealed. A significant difference in the scaling of the effective diffusion barriers with island size is observed for the sets of smaller (less than 10 atoms) and larger islands. In particular, the presence of concerted island motion leads to an almost linear increase in the effective diffusion barrier with size, while its absence accounts for strong size-dependent oscillations and anomalous behavior for trimers and heptamers. A crossover from diffusion due to the collective motion of the smaller island to a regime in which the island diffuses through the periphery dominated mass transport (large islands, 19 to 100 atoms) is predicted. For islands containing 19 to 100 atoms the scaling exponent is found to be in good agreement with that found in previous studies.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study optical properties of AlGaN alloys, undoped and doped AlN epilayers and nanostructure AlN photonics crystals (PCs). Using a deep UV laser system with an excitation wave length at 197 nm, continuous wave PL, temperature dependent, and time-resolved PL have been carried out on these AlGaN and AlN epilayers and nanostructures. We have measured the compositional and temperature dependence of the energy bandgap of AlxGa1-xN alloys covering the entire alloy range of x, 0 ≤ x ≤ 1 and fitted with the Varshni equation. Varshni coefficients, alpha and beta in AlGaN alloys have a parabolic dependence with alloy concentration x. Based on the experimental data, an empirical relation was thus obtained for the energy gap of AlGaN alloys for the entire alloy concentration and at any temperature below 800 K. The exciton localization energy in AlxGa1-xN alloys the entire composition range (0 ≤ x ≤ 1) has been measured by fitting the band edge emission peak energy with the Varshni equation. Deviations of the excitonic emission peak energy from the Varshni equation at low temperatures provide directly the exciton localization energies, ELoc in AlGaN alloys. It was found that ELoc increases with x for x ≤ 0.7, and decreases with x for x ≥ 0.8. The relations between the exciton localization energy, the activation energy, and the emission linewidth have been established. It thus provides three different and independent methods to determine the exciton localization energies in AlGaN alloys. Impurity transitions in AlGaN alloys have also been investigated. Continuous wave (CW) PL spectra of Si and undoped AlGaN alloys reveals groups of impurity transitions that have been assigned to the recombination between shallow donors and an isolated triply charged cation-vacancy (VIII)3-, a doubly charged cation-vacancy-complex (VIII-complex)2-, and a singly charged cation-vacancy-complex (VIII-complex)-1. The energy levels of these deep acceptors in AlxGa1-xN (0 ≤ x ≤ 1) alloys are pinned to a common energy level in the vacuum. AlGaN alloys predominantly exhibiting the bandedge and (VIII-complex)1- transitions possess improved conductivities over those emitting predominantly (VIII)3- and (VIII-complex)2- related transitions. These results thus answer the very basic question of high resistivity in Al-rich AlGaN alloys. Acceptor doped AlGaN alloys have been studied by deep UV PL. A PL emission line at 6.02 eV has been observed at 10 K in Mg-doped AlN. It is due to the recombination of an exciton bound to the neutral Mg acceptor (I1) with a binding energy, Ebx of 40 meV, which indicates large activation energy of the Mg acceptor. The observed large binding energy of the acceptor-bound exciton is consistent with relatively large binding energy of the Mg acceptor in AlN. With the energy level of 0.51 eV for Mg dopants in AlN, it is interesting and important to study other suitable acceptor dopants for AlN. Growth and optical studies of Zn-doped AlN epilayers has been carried out. The PL spectra of Zn-doped AlN epilayers exhibited two impurity emission lines at 5.40 and 4.50 eV, which were absent in undoped epilayers. They are assigned respectively, to the transitions of free electrons and electrons bound to triply positively charged nitrogen vacancies (0.90 eV deep) to the Zn0 acceptors. It was deduced that the Zn energy level is about 0.74 eV above the valence band edge, which is about 0.23 eV deeper than the Mg energy level in AlN. Nitrogen vacancies are the compensating defects in acceptor doped AlGaN alloys. A nitrogen vacancy (VN) related emission line was also observed in ion-implanted AlN at 5.87 eV and the energy level of singly charged VN1+ is found at 260 meV below the conduction band. As a consequence of large binding energy of VN1+ as well as high formation energy, VN1+ in AlN cannot contribute significant n-type conductivity, which is consistent with experimental observation. The temperature dependent PL study of the bandedge emissions in GaN and AlN epilayers up to 800 K has been carried out, which reveals two distinctive activation processes. The first process occurring below Tt = 325 K (Tt = 500 K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above Tt with an activation energy of 0.29 eV (0.3 eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (3) at about 0.3 eV above the conduction band minimum (1). These higher lying bands could affect device performance of GaN and AlN at elevated temperatures. Two-dimensional nanostructured AlN photonic crystals (PCs) with a varying periodicity/diameter down to 150 nm/75 nm have also been studied by deep UV PL. With PCs formation, a 20-fold enhancement in the band edge emission intensity at 208 nm over unpatterned AlN epilayer has been observed. The emission intensity increases with the decrease in the lattice constant of the AlN PCs. AlN PCs represent photonic crystals with highest (shortest) bandgap (wavelength) semiconductors, which open up new opportunities for exploring novel physical phenomena in the artificially structured photonic band gap material systems and their applications, particularly in the area of deep UV as well as nano-photonics.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / The correlation between polarity and material quality of un-doped Al[0.81subscript]Ga[0.19subscript]N was studied. The overall material quality is significantly influenced by the growth polarity. The epilayers with aluminum-polarity have a much higher crystalline quality and better surface morphology than those of nitrogen-polarity. Nitrogen-polar growth more readily incorporates unintentional impurities. A-plane AlN epilayers have been grown on r-plane sapphire substrates. The orientation and high crystalline quality were confirmed by x-ray diffraction (XRD) [Theta]-2[Theta] scan exhibiting a reflection peak at 2[Theta] = 59.4[0superscript] and rocking curve of the (110) reflection having a line width of 940 arcsec. Room temperature photoluminescence (PL) spectroscopy showed that the surface emission intensity of a-plane AlN epilayers is comparable to that of c-plane AlN. PL spectra of Mg-doped a- and c-plane AlN revealed that the Mg level in both a- and c-plane AlN is identical and is about ~ 0.5 eV. Identically designed a-plane and c-plane AlN/A1[0.65subscript]Ga[0.35subscript]N QWs have been grown on a-and c-plane AlN/Al[2subscript]O[3subscript] templates respectively, and their PL emission properties were studied. Low temperature PL characteristics of a-plane QWs are primarily governed by the quantum size effect, whereas those of c-plane QWs are significantly affected by the polarization fields. The growth of AlN epilayers on SiC substrates was investigated. A smooth, crack free AlN epilayer with high optical and crystalline quality was achieved. Because of its high quality, AlN was used as active layer in a hybrid Schottky photodetector. Highly conductive Si-doped Al[subscript0.75]Ga[0.25subscript]N alloys were grown on AlN/SiC templates. The effects of using Indium as a surfactant during the growth of Si-doped Al[0.75subscript]Ga[0.25subscript]N epilayers at relatively high temperature 1050 [degrees]C were studied. Indium significantly increases the doping efficiency as shown by RT Hall measurements. RT PL measurements show a clear correlation between emission intensity of the defect related transition and indium flow rate. P-type conductivity has been obtained in beryllium doped GaN by MOCVD. The activation energy of the beryllium acceptor was estimated to be 118 [plus or minus] 4 meV, which is about 40 meV less than the activation energy of the Mg acceptor in GaN.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / InGaN based, blue and green light emitting diodes (LEDs) have been successfully produced over the past decade. But the progress of these LEDs is often limited by the fundamental problems of InGaN such as differences in lattice constants, thermal expansion coefficients and physical properties between InN and GaN. This difficulty could be addressed by studying pure InN and In[subscript]xGa[subscript]1-xN alloys. In this context Ga-rich In[subscript]xGa[subscript]1-xN (x≤ 0.4) epilayers were grown by metal organic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) measurements showed In[subscript]xGa[subscript]1-xN films with x= 0.37 had single phase. Phase separation occurred for x ~ 0.4. To understand the issue of phase separation in Ga-rich In[subscript]xGa[subscript]1-xN, studies on growth of pure InN and In-rich In[subscript]xGa[subscript]1-xN alloys were carried out. InN and In-rich In[subscript]xGa[subscript]1-xN (x~0.97- 0.40) epilayers were grown on AlN/Al[subscript]2O[subscript]3 templates. A Hall mobility of 1400 cm[superscript]2/Vs with a carrier concentration of 7x1018cm[superscript]-3 was observed for InN epilayers grown on AlN templates. Photoluminescence (PL)emission spectra revealed a band to band emission peak at ~0.75 eV for InN. This peak shifted to 1.15 eV when In content was varied from 1.0 to 0.63 in In-rich In[subscript]xGa[subscript]1-xN epilayers. After growth parameter optimization of In- rich In[subscript]xGa[subscript]1-xN alloys with (x= 0.97-0.40) were successfully grown without phase separation. Effects of Mg doping on the PL properties of InN epilayers grown on GaN/Al[subscript]2O[subscript]3 templates were investigated. An emission line at ~ 0.76 eV, which was absent in undoped InN epilayers and was about 60 meV below the band edge emission peak at ~ 0.82 eV, was observed to be the dominant emission in Mg-doped InN epilayers. PL peak position and the temperature dependent emission intensity corroborated each other and suggested that Mg acceptor level in InN is about 60 meV above the valance band maximum. Strain effects on the emission properties of InGaN/GaN multiple quantum wells (MQWs) were studied using a single blue LED wafer possessing a continuous variation in compressive strain. EL emission peak position of LEDs varies linearly with the biaxial strain; a coefficient of 19 meV/GPa, characterizes the relationship between the band gap energy and biaxial stress of In[subscript]0.2Ga[subscript]0.8N/GaN MQWs.

Doctor of Philosophy / Department of Chemistry / Kenneth J. Klabunde / This is an account of the synthesis of several drastically different forms of silver nanoparticles: Bare metal nanoparticles, dry nanoparticulate powders, aqueous soluble particles, and organic ligand coated monodisperse silver nanoparticles were all produced. The synthetic method was adapted from previous studies on gold nanoparticles and investigated to understand the optimal conditions for silver nanoparticle synthesis. Also the procedure for refinement of the nanoparticles was studied and applied to the formation of alloy nanoparticles. This extraordinary procedure produces beautifully colored colloids of spherical metal nanoparticles of the highest quality which under suitable conditions self-assemble into extensive three dimensional superlattice structures. The silver nanoparticle products were later tested against several biological pathogens to find dramatic increases in antimicrobial potency in comparison to commercially available silver preparations.

Doctor of Philosophy / Department of Physics / Talat S. Rahman / In this work we present a study of the geometric, electronic, vibrational and magnetic properties of several nanostructured systems for which experimental data call for a theoretical understanding. In order to investigate the effect of magnetic dipolar interactions on the magnetization of nanomagnets arranged in finite lattices, we utilize a phenomenological classical approach, which is based on the Landau-Lifshitz equation. Dipolar interactions lead to hysteretic behavior of the magnetization curves and established that the external field sweep rate, sample temperature, and shape anisotropy play a role in determining the specifics. Our results (derived from a classical approach) for magnets arranged in a square lattice suggest that stepped hysteresis curves do not have necessarily a quantum origin (quantum tunneling of the magnetization). We also find that in the square lattice small changes in the dipolar strength introduce sudden transitions in the magnetic hysteresis. For the examination of geometric vibrational and electronic structure of systems of interest, we turn to density functional theory (DFT), which is the leading technique for modeling nanoscale systems from first principles. We have applied DFT to either address some old queries of surface science, such as the dynamics of the CO-chemisorbed Cu(001) surface, or to contribute to the forefront of hydrogen-based economy through the comprehension of the growth and diffusion of Pt islets on Ru(0001), or to predict the geometric and electronic properties of materials to-be-created, as in the case of core-shell bimetallic nanoclusters. In the case of CO on Cu(001), although the bond has been considered to be weak enough so as to treat the adsorbate and substrate separately, our calculations are able to reproduce measurements and provide evidence that the dynamics of the molecule is influenced by the substrate and vice versa, as well as by intermolecular interactions. Taking into account the adsorbate-substrate interplay, has furthermore clarified issues that were pending for the clean surface and led to the correct interpretation of some features in the phonon dispersion of the chemisorbed surface. DFT has also directed us to the conclusion that the catalytic properties of few-atom Pt islets on Ru nanoclusters are preserved by the low probability of these islets to diffuse through the edges of the Ru nanoclusters. Moreover, the analysis of the Ag_{27}Cu_7 nanoalloy from ab initio methods has opened a wide panorama in terms of the geometry, coordination, energetics, and electronic structure of alloyed phases, in general,that may aid in the assembling on new materials.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / The optical and structural properties of Er-doped GaN/InGaN materials and devices synthesized by metal organic chemical vapor deposition (MOCVD) were investigated. Er-doped GaN via MOCVD emits a strong photoluminescence (PL) emission at 1.54 um using both above and below-bandgap excitation. In contrast to other growth methods, MOCVD-grown Er-doped GaN epilayers exhibit virtually no visible emission lines. A small thermal quenching effect, with only a 20% decrease in the integrated intensity of the 1.54 um PL emission, occurred between 10 and 300 K. The dominant bandedge emission of Er-doped GaN at 3.23 eV was observed at room temperature, which is red-shifted by 0.19 eV from the bandedge emission of undoped GaN. An activation energy of 191 meV was obtained from the thermal quenching of the integrated intensity of the 1.54 um emission line. It was observed that surface morphology and 1.54 um PL emission intensity was strongly dependent upon the Er/NH3 flow rate ratio and the growth temperature. XRD measurements showed that the crystalline ordering of the (002) plane was relatively unperturbed for the changing growth environment. Least-squares fitting of 1.54 um PL measurements from Er-doped GaN of different growth temperatures was utilized to determine a formation energy of 1.82 ± 0.1 eV for the Er-emitting centers. The crystalline quality and surface morphology of Er-doped InGaN (5% In fraction) was nearly identical to that of Er-doped GaN, yet the PL intensity of the 1.54 um emission from Er-doped InGaN (5% In fraction) was 16 x smaller than that of Er-doped GaN. The drop in PL intensity is attributed to the much lower growth temperature in conjunction with the high formation energy of the Er- emitting centers. Er-doped InGaN grown at fixed growth temperature with different growth pressures, NH3 flow rates, and Ga flow rates was also investigated, and showed that increased In fractions also resulted in a smaller 1.54 um PL intensity. Er-doped InGaN p-i-n diodes were synthesized and tested. The electroluminescence (EL) spectra under forward bias shows strong Er based emission in the infrared and visible region. The different emission lines from EL spectra in contrast to PL spectra implies different excitation methods for the Er based emission in the p-i-n diode than in the PL excited epilayer.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / Patrick Richard / This work focuses on the ultrafast dynamics of electrons and phonons in graphitic materials. In particular, we experimentally investigated the factors which influence the transport properties of graphite and carbon nanotubes. In the first part of this dissertation, we used Time-resolved Two Photon photoemission (TR-TPP) spectroscopy to probe the dynamics of optically excited charge carriers above the Fermi energy of double-wall carbon nanotubes (DWNTs). In the second part of this study, time-resolved anti-Stokes Raman (ASR) spectroscopy is applied to investigating in real time the phonon-phonon interactions, and addressing the way the temperature affects the dynamics of single-wall carbon nanotubes (SWNTs) and graphite. With respect to the first part, we aim to deeply understand the dynamics of the charge carriers and electron-phonon interactions, in order to achieve an as complete as possible knowledge of DWNTs. We measured the energy transfer rate from the electronic system to the lattice, and we observed a strong non-linear increase with the temperature of the electrons. In addition, we determined the electron-phonon coupling parameter, and the mean-free path of the electrons. The TR-TPP technique enables us to measure the above quantities without any electrical contacts, with the advantage of reducing the errors introduced by the metallic electrodes. The second investigation uses time-resolved ASR spectroscopy to probe in real time the G-mode non-equilibrium phonon dynamics and the energy relaxation paths towards the lattice by variation of the temperature in SWNTs and graphite. The lifetime range of the optically excited phonons obtained is 1.23 ps to 0.70 ps in the lowest (cryogenic temperatures) and highest temperature limits, respectively. We have also observed an increase in the energy of the G-mode optical phonons in graphite with the transient temperature. The findings of this study are important since the non-equilibrium phonon population has been invoked to explain the negative differential conductance and current saturation in high biased transport phenomena.