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Structure of oxygen-implanted (111) silicon before and after heat-pulse annealing

Description: The structure of oxygen-implanted silicon (dose -7.3 x 10/sup 16/cm/sup -2/) has been studied by transmission electron microscopy (TEM). The as-implanted material exhibited four structurally different layers: defect-free monocrystalline silicon, amorphous silicon, monocrystalline silicon with a high defect density, and the perfect crystalline substrate. After heat-pulse annealing for 20 s at 800/sup 0/C, 900/sup 0/C, or 1000/sup 0/C, the amorphous layer recrystallized resulting in polycrystalline silicon rich in oxygen. The uniform insulator buried layer was not formed under these specific implantation and annealing conditions.
Date: October 1, 1985
Creator: Liliental-Weber, Z.; Carpenter, R.W. & Kelly, J.C.
Partner: UNT Libraries Government Documents Department

Investigation on the lattice site location of the excess arsenic atoms in GaAs layers grown by low temperature molecular beam epitaxy

Description: We have measured the excess As atoms present in gaze layers grown by molecular beam epitaxy at low substrate temperatures using particle induced x-ray emission technique. The amount of excess As atoms in layers grown by MBE at 200{degrees}C were found to be {approximately} 4 {times} 10{sup 20} cm{sup {minus}2}. Subsequent annealing of the layers under As overpressure at 600{degrees}C did not result in any substantial As loss. However, transmission electron microscopy revealed that As precipitates (2-5nm in diameter) were present in the annealed layers. The lattice location of the excess As atoms in the as grown layers was investigated by ion channeling methods. Angular scans were performed in the <110> axis of the crystal. Our results strongly suggest that a large fraction of these excess As atoms are located in an interstitial position close to an As row. These As intersitials'' are located at a site slightly displaced from the tetrahedral site in a diamond cubic lattice. No interstitial As signal is observed in the annealed layers.
Date: November 1, 1991
Creator: Yu, Kin Man & Liliental-Weber, Z.
Partner: UNT Libraries Government Documents Department

Investigation on the lattice site location of the excess arsenic atoms in GaAs layers grown by low temperature molecular beam epitaxy

Description: We have measured the excess As atoms present in gaze layers grown by molecular beam epitaxy at low substrate temperatures using particle induced x-ray emission technique. The amount of excess As atoms in layers grown by MBE at 200{degrees}C were found to be {approximately} 4 {times} 10{sup 20} cm{sup {minus}2}. Subsequent annealing of the layers under As overpressure at 600{degrees}C did not result in any substantial As loss. However, transmission electron microscopy revealed that As precipitates (2-5nm in diameter) were present in the annealed layers. The lattice location of the excess As atoms in the as grown layers was investigated by ion channeling methods. Angular scans were performed in the <110> axis of the crystal. Our results strongly suggest that a large fraction of these excess As atoms are located in an interstitial position close to an As row. These As ``intersitials`` are located at a site slightly displaced from the tetrahedral site in a diamond cubic lattice. No interstitial As signal is observed in the annealed layers.
Date: November 1, 1991
Creator: Yu, Kin Man & Liliental-Weber, Z.
Partner: UNT Libraries Government Documents Department

TEM studies of laterally overgrown GaN layers grown on non-polarsubstrates

Description: Transmission electron microscopy (TEM) was used to study pendeo-epitaxial GaN layers grown on polar and non-polar 4H SiC substrates. The structural quality of the overgrown layers was evaluated using a number of TEM methods. Growth of pendeo-epitaxial layers on polar substrates leads to better structural quality of the overgrown areas, however edge-on dislocations are found at the meeting fronts of two wings. Some misorientation between the 'seed' area and wing area was detected by Convergent Beam Electron Diffraction. Growth of pendeo-epitaxial layers on non-polar substrates is more difficult. Two wings on the opposite site of the seed area grow in two different polar directions with different growth rates. Most dislocations in a wing grown with Ga polarity are 10 times wider than wings grown with N-polarity making coalescence of these layers difficult. Most dislocations in a wing grown with Ga polarity bend in a direction parallel to the substrate, but some of them also propagate to the sample surface. Stacking faults formed on the c-plane and prismatic plane occasionally were found. Some misorientation between the wings and seed was detected using Large Angle Convergent Beam Diffraction.
Date: January 5, 2006
Creator: Liliental-Weber, Z.; Ni, X. & Morkoc, H.
Partner: UNT Libraries Government Documents Department

Structural Defects in Laterally Overgrown GaN Layers Grown onNon-polar Substrates

Description: Transmission electron microscopy was used to study defects in lateral epitaxial layers of GaN which were overgrown on a template of a-plane (11{und 2}0) GaN grown on (1{und 1}02) r-plane Al2O3. A high density of basal stacking faults is formed in these layers because the c-planes of wurtzite structure are arranged along the growth direction. Density of these faults is decreasing at least by two orders of magnitude lower in the wings compared to the seed areas. Prismatic stacking faults and threading dislocations are also observed, but their densities drastically decrease in the wings. The wings grow with opposite polarities and the Ga-wing width is at least 6 times larger than N-wing and coalescence is rather difficult. Some tilt and twist was detected using Large Angle Convergent Beam Electron Diffraction.
Date: February 14, 2007
Creator: Liliental-Weber, Z.; Ni, X. & Morkoc, H.
Partner: UNT Libraries Government Documents Department

Strain distribution in heterolayers with low misfit as revealed by convergent beam illumination methods

Description: Convergent beam electron diffraction (CBED) was applied to the local measurement of lattice parameter across a strained interface with small mismatch. GaAs layers grown at low temperature with excess As (with 0.15% misfit) on a GaAs substrate were chosen for these studies. Tetragonal distortion was detected in the layer up to 0.5 {mu}m from the interface. With an increase of the layer thickness lowering of the symmetry of these CBED patterns was observed. This lowering of symmetry is most probably due to saturation of As solubility and the strain build into these layers.
Date: November 1, 1992
Creator: Liliental-Weber, Z.; Kaneyama, T.; Terauchi, M. & Tanaka, M.
Partner: UNT Libraries Government Documents Department

Orthodox etching of HVPE-grown GaN

Description: Orthodox etching of HVPE-grown GaN in molten eutectic of KOH + NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.
Date: August 10, 2006
Creator: Weyher, J.L.; Lazar, S.; Macht, L.; Liliental-Weber, Z.; Molnar,R.J.; Muller, S. et al.
Partner: UNT Libraries Government Documents Department

Examination of the local structure in composite and lowdimensional semiconductor by X-ray Absorption Spectroscopy

Description: X-ray absorption methods have been successfully used to obtain quantitative information about local atomic composition of two different materials. X-ray Absorption Near Edge Structure analysis and X-Ray Photoelectron Spectroscopy allowed us to determine seven chemical compounds and their concentrations in c-BN composite. Use of Extended X-ray Absorption Fine Structure in combination with Transmission Electron Microscopy enabled us to determine the composition and size of buried Ge quantum dots. It was found that the quantum dots consisted out of pure Ge core covered by 1-2 monolayers of a layer rich in Si.
Date: September 25, 2006
Creator: Lawniczak-Jablonska, K.; Demchenko, I.N.; Piskorska, E.; Wolska,A.; Talik, E.; Zakharov, D.N. et al.
Partner: UNT Libraries Government Documents Department

Discovering a Defect that Imposes a Limit to Mg Doping in p-TypeGaN

Description: Gallium nitride (GaN) is the III-V semiconductor used to produce blue light-emitting diodes (LEDs) and blue and ultraviolet solid-state lasers. To be useful in electronic devices, GaN must be doped with elements that function either as electron donors or as acceptors to turn it into either an n-type semiconductor or a p-type semiconductor. It has been found that GaN can easily be grown with n-conductivity, even up to large concentrations of donors--in the few 10{sup 19}cm{sup -3} range. However, p-doping, the doping of the structure with atoms that provide electron sinks or holes, is not well understood and remains extremely difficult. The only efficient p-type dopant is Mg, but it is found that the free hole concentration is limited to 2 x 10{sup 18}cm{sup -3}, even when Mg concentrations are pushed into the low 10{sup 19}cm{sup -3} range. This saturation effect could place a limit on further development of GaN based devices. Further increase of the Mg concentration, up to 1 x 10{sup 20}cm{sup -3} leads to a decrease of the free hole concentration and an increase in defects. While low- to medium-brightness GaN light-emitting diodes (LEDs) are remarkably tolerant of crystal defects, blue and UV GaN lasers are much less so. We used electron microscopy to investigate Mg doping in GaN. Our transmission electron microscopy (TEM) studies revealed the formation of different types of Mg-rich defects [1,2]. In particular, high-resolution TEM allowed us to characterize a completely new type of defect in Mg-rich GaN. We found that the type of defect depended strongly on crystal growth polarity. For crystals grown with N-polarity, planar defects are distributed at equal distances (20 unit cells of GaN); these defects can be described as inversion domains [1]. For growth with Ga-polarity, we found a different type of defect [2]. These defects turn out ...
Date: July 20, 2006
Creator: Liliental-Weber, Z.; Tomaszewicz, T.; Zakharov, D. & O'Keefe, M.A.
Partner: UNT Libraries Government Documents Department

Defects in p-GaN and their atomic structure

Description: In this paper defects formed in p-doped GaN:Mg grown with Ga polarity will be discussed. The atomic structure of these characteristic defects (Mg-rich hexagonal pyramids and truncated pyramids) in bulk and thin GaN:Mg films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects. The inside walls of the cavities were covered by GaN which grew with reverse polarity compared to the matrix. It was proposed that lateral overgrowth of the cavities restores matrix polarity on the defect base. Exchange of Ga and N sublattices within the defect compared to the matrix lead to a 0.6 {+-} 0.2 {angstrom} displacement between the Ga sublattices of these two areas. A [1{und 1}00]/3 shift with change from AB stacking in the matrix to BC within the entire pyramid is observed
Date: October 8, 2004
Creator: Liliental-Weber, Z.; Tomaszewicz, T.; Zakharov, D.; Jasinski, J. & and O'Keefe, M.
Partner: UNT Libraries Government Documents Department

Effect of stoichiometry on defect distribution in cubic GaN grown on GaAs by plasma-assisted MBE

Description: High resolution electron microscopy was used to study the structure of {beta}-GaN epilayers grown on (001) GaAs substrates by plasma- assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N{sub 2} flow for fixed rf power). The best quality of GaN layers was achieved by ``stoichiometric`` growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the ``major flat`` of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN- GaAs interfaces.
Date: December 31, 1996
Creator: Ruvimov, S.; Liliental-Weber, Z.; Washburn, J.; Drummond, T.J.; Hafish, M. & Lee, S.R.
Partner: UNT Libraries Government Documents Department

Effects of structural defects on the activation of sulfur donors in GaN/x/As/1-x/ formed by N implantation

Description: The effects of structural defects on the electrical activity of S doped GaN{sub x}As{sub 1-x} layers formed by S and N coimplantation in GaAs are reported. S and N ions were implanted to the depth of about 0.4 {micro}m. Electrochemical capacitance voltage measurements on samples annealed at 945 C for 10s show that in a thin (&lt;0.1 {micro}m) surface layer the concentration of active shallow donors is almost an order of magnitude larger in S and N co-implanted samples than in samples implanted with S alone. The activation efficiency of S donors also shows a broad minimum at a depth of about 0.2 {micro}m below the surface. The results of these electrical measurements are correlated with the distribution of structural defects revealed by transmission electron microscopy (TEM). The TEM micrographs show that in addition to a band of dislocation loops commonly found in ion implanted GaAs, an additional band of small voids is observed in samples co-implanted with S and N. The location of this band correlates well with the region of reduced electrical activation of S donors, suggesting that formation of the voids through N accumulation results in a lower concentration of active, substitutional N atoms.
Date: July 16, 2001
Creator: Jasinski, J.; Yu, K.M.; Walukiewicz, W.; Liliental-Weber, Z. & Washburn, J.
Partner: UNT Libraries Government Documents Department

Influence of low temperature-grown GaAs on lateral thermal oxidation of Al{sub 0.98}Ga{sub 0.02}As

Description: The lateral thermal oxidation process of Al{sub 0.98}Ga{sub 0.02}As layers has been studied by transmission electron microscopy. Growing a low-temperature GaAs layer below the Al{sub 0.98}Ga{sub 0.02}As has been shown to result in better quality of the oxide/GaAs interfaces compared to reference samples. While the later have As precipitation above and below the oxide layer and roughness and voids at the oxide/GaAs interface, the structures with low-temperature have less As precipitation and develop interfaces without voids. These results are explained in terms of the diffusion of the As toward the low temperature layer. The effect of the addition of a SiO{sub 2} cap layer is also discussed.
Date: June 29, 2000
Creator: Ferrer, J.C.; Liliental-Weber, Z.; Reese, H.; Chiu, Y.J. & Hu, E.
Partner: UNT Libraries Government Documents Department

V-shaped inversion domains in InN grown on c-plane sapphire

Description: Inversion domains with a V-shape were found to nucleate inside a Mg-doped InN heteroepitaxial layer. They resemble Al-polarity domains, observed recently, in N-polarity AlN films. However, the angle between the side-walls of the V-shaped domain and the c-axis differs in these two cases. In InN, this angle is almost two times bigger than that reported for AlN. The origin of V-shaped inversion domains in InN film is not yet clear.
Date: April 27, 2004
Creator: Jasinski, J.; Liliental-Weber, Z.; Lu, H. & Schaff, W.J.
Partner: UNT Libraries Government Documents Department

Defects in GaAs bulk crystals and multi-layers caused by In diffusion

Description: The objective was to study by transmission electron microscopy the lattice defects in GaAs bulk crystals and heterostructures formed by In diffusion. In such samples hints for the existence of superconductivity have been found. Indium was found to move more than 100 {mu}m into bulk GaAs during lh annealing at 550C (such conditions are typical for molecular beam epitaxy growth on GaAs wafers). This rapid diffusion is accompanied by the creation of dislocation networks and metallic In droplets that show evidence for lattice strain. To study the interaction of In with the GaAs lattice, In/GaAs multi-layers were grown by MBE at about 450C on a GaAs buffer layer. The interfaces of these structures showed misfit dislocations at islands of InAs besides the presence of lattice strain. Both types of samples showed microwave absorption signals typical for superconductivity. The most likely superconductive phases are small metastable inclusions, probably consisting amorphous Ga or In.
Date: April 1, 1993
Creator: Werner, P.; Liliental-Weber, Z.; Swider, W.; Sohn, H.; Yau, W.; Weber, E. R. et al.
Partner: UNT Libraries Government Documents Department

Reorientation of misfit dislocations during annealing in InGaAs/GaAs(001) interfaces

Description: Transmission electron microscopy is applied to investigate the effect of postannealing on misfit dislocations in an In{sup 0.2}Ga{sup 0.8}As/GaAs(001) heterostructure. An orthogonal array of 60{degree} dislocations along [110] and [110] directions was observed in the interfaces of the samples grown by MBE at 520C. When the as-grown samples were annealed at temperatures ranging from 600 to 800C, the 60{degree} dislocations were gradually reoriented by dislocation reactions occurring at the 90{degree} intersections followed by nonconservative motion driven by dislocation line tension and the residual elastic misfit strain. The final result of this process was a dislocation array lying along [100] and {center_dot} [010] directions. The reoriented u =<100> dislocation has a Burgers vector b = a/2 <101>, which is the same as that of 60{degree} dislocation, but the edge component of its Burgers vector in the (001) interfacial plane is larger than that of 60{degree} dislocation by a factor of {radical}2, resulting in a greater contribution to elastic strain relaxation. This nonconservative reorientation of 60{degree} dislocations to form the u=<100> dislocations represents a new strain relaxation mechanism in diamond or zinc blende semiconductor heterostructures.
Date: April 1, 1993
Creator: Chen, Y.; Liliental-Weber, Z.; Washburn, J.; Klem, J. F. & Tsao, J. Y.
Partner: UNT Libraries Government Documents Department

Transmission Electron Microscopy Study of InN Nanorods

Description: InN nanorods were grown on a, c-, and r-plane of sapphire and also on Si (111) and GaN (0001) by non-catalytic, template-free hydride metal-organic vapor phase epitaxy and studied by transmission electron microscopy, electron energy loss (EELS) and photoluminescence (PL) at room temperature. These nanocrystals have different shapes and different faceting depending on the substrate used and their crystallographic orientation. EELS measurements have confirmed the high purity of these crystals. The observed PL peak was in the range of 0.9-0.95 eV. The strongest PL intensity was observed for the nanocrystals with the larger diameters.
Date: July 13, 2006
Creator: Liliental-Weber, Z.; Li, X.; Kryliouk, Olga; Park, H.J.; Mangum,J. & Anderson, T.
Partner: UNT Libraries Government Documents Department

InN Nanorods and Epi-layers: Similarities and Differences

Description: Transmission electron microscopy was applied to study InN nanorods grown on the a-, c-and r-plane of Al{sub 2}O{sub 3}, and (111) Si substrates by non-catalytic, template-free hydride metal-organic vapor phase epitaxy (H-MOVPE). Single crystal nanorod growth was obtained on all substrates. However, the shape of the nanorods varied depending on the substrate used. For example, nanorods grown on r-plane sapphire and (111) Si have sharp tips. In contrast, growth on a- and c- planes of Al{sub 2}O{sub 3} results in flat tips with clear facets on their sides. The structural quality of these nanorods and their growth polarity are compared to crystalline quality, surface roughness, defects and growth polarity of InN layers grown by MBE on the same planes of Al{sub 2}O{sub 3}.
Date: March 30, 2007
Creator: Liliental-Weber, Z.; Kryliouk, O.; Park, H.J.; Mangum, J.; Anderson, T. & Schaff, W.
Partner: UNT Libraries Government Documents Department

Atomic Structure of Pyramidal Defects in GaN:Mg; Influence ofAnnealing

Description: The atomic structure of the characteristic defects (Mg-rich hexagonal pyramids) in p-doped bulk and MOCVD GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects, confirmed also with positron annihilation. The inside walls of the cavities were covered by GaN of reverse polarity compared to the matrix. Defects in bulk GaN:Mg were almost one order of magnitude larger than in thin films. An exchange of Ga and N sublattices within the defect compared to the matrix lead to a 0.6 {+-} 0.2 {angstrom} displacement between the Ga sublattices of these two areas. A [1100]/3 shift with change from AB stacking in the matrix to BC within the entire pyramid was observed. Annealing of the MOCVD layers lead to slight increase of the defect size and an increase of the photoluminescence intensity. Positron annihilation confirms presence of vacancies of different sizes triggered by the Mg doping in as-grown samples and decrease of their concentration upon annealing at 900 and 1000 C.
Date: October 3, 2005
Creator: Liliental-Weber, Z.; Tomaszewicz, T.; Zakharov, D.; O'Keefe, M.; Hautakangas, S.; Saarinen, K. et al.
Partner: UNT Libraries Government Documents Department