Browsing by Author "Robert Nemanich, Committee Member"

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Aluminum Nitride Bulk Crystal Growth in a Resistively Heated Reactor
(2005-08-23) Dalmau, Rafael Federico; Jon-Paul Maria, Committee Member; Raoul Schlesser, Committee Member; Robert Nemanich, Committee Member; Zlatko Sitar, Committee Chair
A resistively heated reactor capable of temperatures in excess of 2300°C was used to grow aluminum nitride (AlN) bulk single crystals from an AlN powder source by physical vapor transport (PVT) in nitrogen atmosphere. AlN crystals were grown at elevated temperatures by two different methods. Self-seeded crystals were obtained by spontaneous nucleation on the crucible walls, while seeded growth was performed on singular and vicinal (0001) surfaces of silicon carbide (SiC) seeds. During self-seeded growth experiments a variety of crucible materials, such as boron nitride, tungsten, tantalum, rhenium, tantalum nitride, and tantalum carbide, were evaluated. These studies showed that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, influencing nucleation, coalescence, and crystal morphology. In terms of high-temperature stability and compatibility with the growth process, the best results for AlN crystal growth were obtained in crucibles made of sintered tantalum carbide or tantalum nitride. In addition, contamination from the commercially purchased AlN powder source was reduced by pre-sintering the powder prior to growth, which resulted in a drastic reduction of nearly all impurities. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction, x-ray topography, glow discharge mass spectrometry, and secondary ion mass spectrometry. Average dislocation densities were on the order of 10³ cm⁻³, with extended areas virtually free of dislocations. High resolution rocking curves routinely showed peak widths as narrow as 7 arcsec, indicating a high degree of crystalline perfection. Low-temperature partially polarized optical reflectance measurements were used to calculate the crystal-field splitting parameter of AlN, Δ[subscript cr] = -230 meV, and from this, a low-temperature (1.7 K) band gap energy of 6.096 eV was obtained for unstrained wurtzite AlN. Seeded growth of AlN bulk crystals on on-axis and off-axis (0001), Si-face SiC seeds was investigated as a means to scale up maximum single crystal size and pre-define crystal orientation. A two-step deposition process was developed for the growth of thick layers. AlN layers 0.1—3 mm thick were deposited on inch-sized seeds. X-ray diffraction analysis evidenced that the AlN grew in the direction of the seed. A one-dimensional isotropic model was formulated to calculate the thermal stress distribution in AlN/SiC heterostructures. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increasing AlN thickness, in agreement with model calculations. Crack-free AlN crystals were obtained from grown layers by evaporating the SiC seed in situ during high-temperature PVT growth. Based on these results, a reproducible seeded growth process was developed for production of crack-free AlN crystals having pre-determined polarity and orientation.
Fixed Charge Reduction and Tunneling in Stacked Dielectrics
(2005-07-27) Hinkle, Christopher; Gerald Lucovsky, Committee Chair; Jon-Paul Maria, Committee Member; Robert Nemanich, Committee Member; Hans Hallen, Committee Member
Stacked gate dielectrics using high-k materials were deposited using a RPECVD method. Aluminum oxide, hafnium oxide, hafnium silicate, nitrided films of each of the above, and multi-layer stacks of the above as well as silicon dioxide were deposited. These films were analyzed using AES, XPS, NRA, RBS, SIMS, XAS, cathodoluminescence, spectroscopic ellipsometry, capacitance-voltage, and current-voltage techniques. Fixed charge was found to be present in all high-k films and was practically impossible to reduce in a significant way. Nitridation of the films was unsuccessful at reducing the charge, but was helpful in enhancing some electrical measurements. Sandwich stack structures showed enhanced tunneling which led to a novel approach of calculating the E[subscript b]-m[subscript eff] product in the transmission probability equation. This tunneling also gives some clues as to which types of gate stacks cannot be used in technology. Gate stacks containing an HfO₂ layer below an Al₂O₃ layer were studied and also showed enhanced tunneling. Analysis of this tunneling found two significant trapping sites in the HfO₂ layer, one located ~0.5 eV below the HfO₂ conduction band offset and the other located in the Si bandgap. Fixed charge reduction was again expected in these laminates, but again remained despite theoretical predictions.
Formation of Metal Silicide and Metal Germanosilicide Contacts to Si[subscript 1-x]subscript Ge[subscript x] Alloys
(2004-04-20) Burnette, James E. Jr.; Dale Sayers, Committee Member; Robert Nemanich, Committee Member; Gregory Parsons, Committee Member; David Aspnes, Committee Member
The goals of this research were to study the phase stability and formation of Ti-Si[subscript 1-x]Ge[subscript x] and Co-Si[subscript 1-x]Ge[subscript x] thin film reactions. The Ti-Si[subscript 1-x]Ge[subscript x] and Co-Si[subscript 1-x]Ge[subscript x] solid phase reactions result in the formation of precipitates within the grain boundaries of the films thus formed. The precipitates are either Ge or a Si-Ge compound, depending on the type of metal used in the reaction. The formation of Ti(Si[subscript 1-y]Ge[subscript y])₂ thin films on Si[subscript 1-x]Ge[subscript x] has been examined. It has been found that the generation of Ge-rich Si-Ge precipitates which form in the Ti-Si[subscript 1-x]Ge[subscript x] solid phase reaction could be reduced or eliminated by the insertion of an amorphous Si layer before the metallization step. A Gibbs free energy model, which was parameterized in terms of Ge concentration by atomic percentage was used to determine stability between the Ti(Si[subscript 1-y]Ge[subscript y])₂ layer and the Si[subscript 1-x] Ge[subscript x] substrate. The films in this study were characterized using x-ray diffraction (XRD) to investigate phase formation, stability, and the composition of the Ti(Si[subscript 1-y]Ge[subscript y])₂ layer. Scanning electron microscopy (SEM) was used to determine the surface morphology and phase stability. It was found that amorphous Si layers of a certain thickness could prevent precipitate formation, depending on the composition of the underlying Si[subscript 1-x] Ge[subscript x] layer. The formation of CoSi₂ on Si[subscript 1-x]Ge[subscript x] was also examined. The solid phase reaction of Co and Si[subscript 1-x]Ge[subscript x] results in the formation of a poly-crystalline CoSi₂ layer, and the occurrence of a Ge precipitate. The TIME (Titanium Interlayer Mediated Epitaxy) process has been used in the formation of epitaxial CoSi₂ on Si (100). A Ti layer of varying thicknesses, which serves as a barrier to retard the diffusion of Co atoms was deposited on a c-Si/Si[subscript 1-x]Ge[subscript x] substrate pseudomorphically strained to Si (100), before the final Co metallization step. The films in this study were characterized using x-ray absorption fine structure (XAFS) to determine the short-range crystalline order, XRD to determine phase formation and long-range crystalline order, Auger electron spectroscopy (AES) to determine surface chemistry, and SEM to determine the surface morphology. This work shows that the formation of epitaxial CoSi₂ on Si[subscript1-x]Ge[subscript x] can be achieved, depending on the thickness of the diffusion barrier. In addition, the optimal diffusion barrier thickness has been determined for the Co layer thickness used in these studies.
Growth and Characterization of GaN and AlGaN Thin Films and Heterostructures and the Associated Development and Evaluation of Ultraviolet Light Emitting Diodes
(2005-06-28) Park, Ji-Soo; John Muth, Committee Member; Mark Johnson, Committee Member; Robert F. Davis, Committee Chair; Robert Nemanich, Committee Member
AlGaN-based thin film heterostructures have been grown and fabricated into ultraviolet light emitting diodes with and without p-type and/or n-type AlGaN carrier-blocking layers at the top and the bottom of the quantum wells, respectively, and having the principal emission at 353 nm. The highest values of this peak intensity and light output power were measured in the devices containing p-type carrier-blocking layers. Growth of an n-type carrier-blocking layer had an adverse effect on these device characteristics. A broad peak centered at ~540nm exhibited yellow luminescence and was present in the spectra acquired from all the devices. This peak is attributed to absorption of the ultraviolet emission by and re-emission from the p-GaN and/or to the luminescence from the AlGaN within quantum wells by current injection. Individual AlxGa1-xN films (x<0<1) have been grown on Si- and C-terminated 6H-SiC{0001} substrates and characterized for electron emission applications. The large range in the values of x was achieved by changing the fraction of Al in the gas phase from 0 to 0.45. The ionized donor concentration in the n-type, Si-doped AlxGa1-xN films decreased as the mole fraction of Al was increased due to the reduction in the donor energy level and compensation. The use of the SiH4 flow rate, which yields a Si concentration of ~1E19 cm-3 in GaN, established the upper limit of the mole fraction of Al wherein n-type doping could be achieved at ~0.61. The electron affinity of the Si-doped Al0.61Ga0.39N films was as low as 0.1 eV. Increasing the Si doping concentration in AlN films to as high as 1E21cm-3 caused slight degradation in crystal perfection. No difference was found in the Al core level binding energies between undoped and Si-doped AlN films. The results of XPS and UPS experiments showed that the work function of N-polar AlN films was 0.6 eV lower than that of Al-polar films.
On the Interactions of Point Defects, Dopants and Light Element Impurities in Silicon as Stimulated by 200 kV Electron Irradiation.
(2005-07-21) Stoddard, Nathan Gregory; Robert Nemanich, Committee Member; George Rozgonyi, Committee Co-Chair; Gerd Duscher, Committee Co-Chair; Nadia El-Masry, Committee Member; Phil Russell, Committee Member
The purpose of this research has been the investigation of atomic manipulation in silicon. It has been demonstrated that bulk vacancies and interstitials are created and spatially separated one Frenkel pair at a time during 200 kV electron irradiation of nitrogen-doped silicon. The mechanism by which the nitrogen pair allows Frenkel pair separation is shown to be a combination of the lowering of the energy barrier to a knock-on event combined with a more stable end-state. Anomalous nitrogen diffusion has been observed as a result of low energy ion milling, and the diffusion of nitrogen is studied theoretically, revealing a new, low energy model for N2 pair diffusion. For the first time, 200 kV irradiation has been demonstrated not only to create Frenkel pairs during broad-beam irradiation, but also to allow the formation of extended defects like voids, oxygen precipitates and interstitial complexes. Using electron energy loss spectroscopy combined with first principles simulations, dark and bright areas induced in Z contrast images by 200 kV irradiation are demonstrated to be due to vacancy and self-interstitial complexes, respectively, with N>2. Finally, the manipulation of dopants in silicon is induced by using the difference in energy transferable from a 200 kV electron to light versus heavy elements (e.g. B vs. Sb). Atomic Force Microscopy is used to demonstrate that n-type regions with a size corresponding to the beam diameter are created in p-type material by short periods of 200 kV e-beam exposure. In this way, a method can be developed to create p-n-p type devices of arbitrary size in codoped silicon using a room temperature process.
Properties of Zr Silicate and Zr-Si Oxynitride High-k Dielectric Alloys for Advanced Microelectronic Applications; Chemical and Electrical Characterizations
(2005-09-27) Ju, Byongsun; Gerald Lucovsky, Committee Chair; Jon-Paul Maria, Committee Member; Robert Nemanich, Committee Member; Carl Osburn, Committee Member
As the microelectronic devices are aggressively scaled down to the 1999 International Technology Roadmap, the advanced complementary metal oxide semiconductor (CMOS) is required to increase packing density of ultra-large scale integrated circuits (ULSI). However, SiO2 or Si oxynitride (SiOxNy) films which is a traditional gate oxide materials shows its limitations in direct tunneling current density at the below about 3nm thickness, and moreover, the priority of leakage current is ranked high in device performance and reliability as the portable device prevails. High-k alternative dielectrics can provide the required levels of EOT for device scaling at larger physical thickness, thereby providing a materials pathway for reducing the tunneling current. Zr silicates and its end members (SiO2 and ZrO2) and Zr-Si oxynitride films, (ZrO2)x(Si3N4)y(SiO2)z, have been deposited using a remote plasma-enhanced chemical vapor deposition (RPECVD) system. After deposition of Zr silicate, the films were exposed to He/N2 plasma to incorporate nitrogen atoms into the surface of films. The amount of incorporated nitrogen atoms was measured by on-line Auger electron spectrometry (AES) as a function of silicate composition and showed its local minimum around the 30% silicate. Characterization by AES and x-ray photoelectron spectroscopy (XPS) indicated that the nitrogen atoms were substituted for the oxygen atoms' position and made a bond with Si and Zr depending on the silicate composition. The effect of nitrogen atoms on capacitance-voltage (C-V) and leakage-voltage (J-V) were also investigated by fabricating metal-oxide-semiconductor (MOS) capacitors. Results suggested that incorporating nitrogen into silicate decreased the leakage current in SiO2-rich silicate, whereas the leakage increased in the middle range of silicate. The pseudo-ternary alloy composition was determined by Rutherford back scattering (RBS) that was calibrated by on-line Auger electron spectroscopy (AES) and showed the composition's thermodynamically stable boundary composition in ternary phase diagrams. Zr-Si oxynitride was a pseudo-ternary alloy and no phase separation was detected by x-ray photoelectron spectroscopy (XPS) analysis up to 1100°C annealing. The leakage current of Zr-Si oxynitride films showed two different temperature dependent activation energies, 0.02 eV for low temperature and 0.3 eV for high temperature. Poole-Frenkel emission was the dominant leakage mechanism. Zr silicate alloys with no Si3N4 phase were chemically separated into the SiO2 and ZrO2 phase as annealed above 900°C. While chemical phase separation in Zr silicate films with Si3N4 phase (Zr-Si oxynitride) were suppressed as increasing the amount of Si3N4 phase due to the narrow bonding network in Si3N4 phase. (3.4 bonds/atom for Si3N4 network, 2.67 bonds/atom for SiO2 network)
Synthesis and Properties of Barium Titanate Solid Solution Thin Films on Copper Substrates
(2006-09-26) Ihlefeld, Jon F.; Jon-Paul Maria, Committee Chair; Angus Kingon, Committee Member; Robert Nemanich, Committee Member; Zlatko Sitar, Committee Member; William Borland, Committee Member
Barium titanate thin films were deposited via chemical solution deposition using a hybrid-chelate chemistry directly on copper foil substrates. A process was developed to crystallize and densify the ferroelectric films at 900C by using a reductive atmosphere containing nitrogen, hydrogen, water vapor, and oxygen impurities such that film constituents were oxidized to form barium titanate and the foil substrate remained metallic. The crystallized films are polycrystalline with equiaxed morphology and average grain diameters in excess of 100 nm. The dielectric properties exhibit permittivities in excess of 1800 at room temperature and zero bias with tunabilites of greater than 90% and high field loss tangents of less than 1%. A series of samples was prepared with varying grain and crystallite sizes by dividing and processing a single film over a range of temperature from 700 to 900C. This ensures that the chemical composition and film thickness is invariant for each sample. It is shown that the grain size increases with higher process temperatures and results in a concomitant increase in permittivity and tunability. These enhancements, combined with the constant paraelectric⁄ferroelectric phase transition temperature, indicated that a combination of film crystallinity and grain size is responsible for diminished performance. The phase transition temperature and temperature coefficient of capacitance modified by partially substituting zirconium, hafnium, and tin for titanium. The resulting films were single phase and the phase transition shifts were consistent with bulk materials. A reduction in permittivity was observed for increasing substituent level and was attributed to a reduction in grain size for both barium titanate zirconate and barium titanate hafnate. Processing conditions were chosen to stabilize Sn2+ during the firing process in an attempt to flux the system and increase grain size. The barium titanate stannate films had less reduction in grain size per substituent level than either zirconium or hafnium, however a similar reduction in permittivity was observed. The diminished dielectric response was explained by a defect reaction involving divalent tin and oxygen vacancies that quenched the extrinsic domain response to the dielectric constant. Defect equilibria were investigated with respect to processing atmosphere, stoichiometry, and dopant concentration. The solubility of excess barium and titanium was found to be greater in the films than is expected in the bulk, however it is unclear that equilibrium is achieved in the process. It was demonstrated that dopants could successfully eliminate the necessity of a reoxidation anneal to compensate for oxygen point defects resulting from the low pO2 atmospheres. The dopant levels necessary and insulation resistance of pure BaTiO3 were greater than expected from thermodynamic calculations. It was suggested that this is the result of a reduction in the enthalpy of reduction, stemming from an increase in grain boundary volume. Barium borate fluxes were used to improve densification and crystallinity. Barium borate additions between 0 and 3% uniformly increased grain size and density, while levels greater than 3% resulted in anomalous grain growth. Films with exaggerated grains show tetragonal peak splitting in the X-ray diffraction patterns, consistent with bulk barium titanate. In materials without exaggerated grain growth, dielectric measurements revealed permittivities in excess of 3000 at room temperature (for average grain sizes of approximately 160 nm). This value is equivalent to the finest-prepared bulk ceramics and substantially greater than any polycrystalline film ever reported. This has been attributed to in improvement in film crystallinity. These two accomplishments — tetragonal crystal symmetry and permittivities in excess of 3000 — represent dramatic breakthroughs in ferroelectric thin film technology.