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Fermi level dependent native defect formation: Consequences for metal-semiconductor and semiconductor-semiconductor interfaces

Description: The amphoteric native defect model of the Schottky barrier formation is used to analyze the Fermi level pinning at metal/semiconductor interfaces for submonolayer metal coverages. It is assumed that the energy required for defect generation is released in the process of surface back-relaxation. Model calculations for metal/GaAs interfaces show a weak dependence of the Fermi level pinning on the thickness of metal deposited at room temperature. This weak dependence indicates a strong dependence of the defect formation energy on the Fermi level, a unique feature of amphoteric native defects. This result is in very good agreement with experimental data. It is shown that a very distinct asymmetry in the Fermi level pinning on p- and n-type GaAs observed at liquid nitrogen temperatures can be understood in terms of much different recombination rates for amphoteric native defects in those two types of materials. Also, it is demonstrated that the Fermi level stabilization energy, a central concept of the amphoteric defect system, plays a fundamental role in other phenomena in semiconductors such as semiconductor/semiconductor heterointerface intermixing and saturation of free carrier concentration. 33 refs., 6 figs.
Date: February 1, 1988
Creator: Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Activation of shallow dopants in II-VI compounds

Description: The amphoteric native defect model is applied to the understanding of the variations in the dopant activation efficiency in II-VI compounds. It is shown that the location of the common energy reference, the Fermi level stabilization energy, relative to the band edges can be used to determine the doping induced reduction of the formation energy and the enhancement of the concentration of compensating native defects. The model is applied to the most extensively studied compound semiconductors as well as to ternary and quaternary alloys. The effects of the compound ionicity on the dopant activation are briefly discussed.
Date: August 1, 1995
Creator: Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Electron scattering by native defects in III-V nitrides and their alloys

Description: We have calculated the electron mobilities in GaN and InN taking into consideration scattering by short range potentials, in addition to all standard scattering mechanisms. These potentials are produced by the native defects which are responsible for the high electron concentrations in nominally undoped nitrides. Comparison of the calculated mobilities with experimental data shows that scattering by short range potentials is the dominant mechanism limiting the electron mobilities in unintentionally doped nitrides with large electron concentrations. In the case of Al{sub x}Ga{sub 1-x}N alloys, the reduction in the electron concentration due to the upward shift of the conduction band relative to the native defect level can account for the experimentally measured mobilities. Resonant scattering is shown to be important when the defect and Fermi levels are close in energy.
Date: March 1, 1996
Creator: Hsu, L. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Surface recombination in semiconductors

Description: We propose two general criteria for a surface defect state to act as an efficient, nonradiative recombination center. The first is that the thermal ionization energy should not deviate from the mid-gap energy by more than the relaxation energy of the defect, In this case the activation energy for the recombination is given by the barrier for the capture of the first carrier, whereas the second carrier is captured athermally. The second citerion is related to the position of the average dangling bond energy relative to the band edges. If, as in the cases of InP or InAs, it is located close to a band edge, a low surface recombination velocity is expected. However a much faster recombination is predicated and experimentally observed in the materials with the average dangling bond energy located close to the mid-gap. The relevance of these criteria for the novel wide-gap optoelectronic materials is discussed.
Date: July 1, 1995
Creator: Langer, J.M. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Ultrahigh Efficiency Multiband Solar Cells Final Report forDirector's Innovation Initiative Project DII-2005-1221

Description: The unique properties of the semiconductor ZnTeO were explored and developed to make multiband solar cells. Like a multijunction cell, multiband solar cells use different energy gaps to convert the majority of the solar spectrum to electrical current while minimizing losses due to heating. Unlike a multijunction cell, this is accomplished within a single material in a multiband cell. ZnTe{sub 1-x}O{sub x} films with x up to 2% were synthesized and shown to have the requisite unique band structure (2 conduction bands) for multiband function. Prototype solar cells based on an n-type ZnTe{sub 1-x}O{sub x} multiband top layer and a p-type ZnTe substrate were fabricated. Contacts to the cell and the series resistance of the substrate were identified as challenges for good electrical performance. Both photovoltage and small photocurrents were demonstrated under AMO illumination. A second semiconductor system, GaN{sub x}As{sub 1-y-x}P{sub y}, was shown to have multiband function. This alloy system may have the greatest potential to realize the promise of high efficiency multiband solar cells because of the relatively advanced technology base that exists for the manufacturing of III-V-alloy-based IC and opto-electronic devices (including multijunction solar cells).
Date: March 29, 2006
Creator: Ager, Joel W., III; Walukiewicz, W. & Yu, Kin Man
Partner: UNT Libraries Government Documents Department

Carrier localization in gallium nitride

Description: In wide bandgap GaN, a large number of interesting and important scientific questions remain to be answered. For example, the large free electron concentration reaching 10{sup 19} to 10{sup 20} cm{sup - 3} in nominally undoped material are ascribed to intrinsic defects because no chemical impurity has been found at such high concentrations. According to theoretical models, a nitrogen vacancy acts as a donor but its formation energy is very large in n-type materials, making this suggestion controversial. We have investigated the nature of this yet unidentified donor at large hydrostatic pressure. Results from infrared reflection and Raman scattering indicate strong evidence for localization of free carriers by large pressures. The carrier density is drastically decreased by two orders of magnitude between 20 and 30 GPa. Several techniques provide independent evidence for results in earlier reports and present the first quantitative analysis. A possible interpretation of this effect in terms of the resonant donor level is presented.
Date: September 1, 1996
Creator: Wetzel, C.; Walukiewicz, W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Electron-phonon scattering in Si doped GaN

Description: Phonon-plasmon scattering in non-resonant Raman spectroscopy is used to determine the free electron concentration in Si doped GaN films. For various doping concentration and variable temperature the correlation with magneto-transport data is established. The freeze-out of the carrier concentration at low temperature is thus observed in a purely optical detection scheme. We observe a very long transient time of several hours for the carrier concentration as a reaction to temperature variation. This indicates an indirect capture and emission process with a very small cross section. The value of the Faust-Henry coefficient is determined.
Date: December 1, 1996
Creator: Wetzel, C.; Walukiewicz, W. & Ager, J.W. III
Partner: UNT Libraries Government Documents Department

Band anticrossing in highly mismatched semiconductor alloys

Description: The basic theoretical aspects of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys are reviewed. The many-impurity Anderson model treated in the coherent potential approximation is applied to the semiconductor alloys, in which metallic anion atoms are partially substituted by atoms of a highly electronegative element. Analytical solutions for the Green's function describe dispersion relations and state broadening effects for the restructured conduction band. The solutions are identical to those obtained from the physically intuitive and widely used two-level band anticrossing model. It is shown that the model explains key experimental observations including the unusual composition and pressure dependence of the interband optical transitions and the large enhancement of the electron effective mass.
Date: July 26, 2002
Creator: Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Fermi level stabilization energy in cadmium oxide

Description: We have studied the effects of high concentrations of native point defects on the electrical and optical properties of CdO. The defects were introduced by irradiation with high energy He+, Ne+, Ar+ and C+ ions. Increasing the irradiation damage with particles heavier than He+ increases the electron concentration until a saturation level of 5x1020 cm-3 is reached. In contrast, due to the ionic character and hence strong dynamic annealing of CdO, irradiation with much lighter He+ stabilizes the electron concentration at a much lower level of 1.7x1020 cm-3. A large shift of the optical absorption edge with increasing electron concentration in irradiated samples is explained by the Burstein-Moss shift corrected for electron-electron and electron-ion interactions. The saturation of the electron concentration and the optical absorption edge energy are consistent with a defect induced stabilization of the Fermi energy at 1 eV above the conduction band edge. The result is in a good agreement with previously determined Fermi level pinning energies on CdO surfaces. The results indicate that CdO shares many similarities with InN, as both materials exhibit extremely large electron affinities and an unprecedented propensity for n-type conductivity.
Date: April 8, 2010
Creator: Speaks, D. T.; Mayer, M. A.; Yu, K. M.; Mao, S. S.; Haller, E. E. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Engineering the Electronic Band Structure for Multiband Solar Cells

Description: Using the unique features of the electronic band structure of GaNxAs1-x alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the Band Anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.
Date: July 12, 2010
Creator: Lopez, N.; Reichertz, L. A.; Yu, K. M.; Campman, K. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

Local vibrational modes of Se-H complexes in AlSb

Description: Using infrared spectroscopy the authors have observed local vibrational modes (LVMs) arising from Se-H complexes in AlSb. At liquid-helium temperatures, hydrogenated AlSb:Se samples have three stretch mode peaks at 1,606.3, 1,608.6, and 1,615.7 cm{sup {minus}1}, whereas deuterated samples have only one peak at 1,173.4 cm{sup {minus}1}. The anomalous splitting of the se-H stretch mode may be explained by a resonance between the stretch mode and two multi-phonon modes. As the temperature or pressure is increased, the stretch mode and multi-phonon modes show anti-crossing behavior.
Date: September 1, 1996
Creator: McCluskey, M.D.; Hsu, L.; Haller, E.E.; Walukiewicz, W. & Becla, P.
Partner: UNT Libraries Government Documents Department

Si in GaN -- On the nature of the background donor

Description: A characterization of the Si impurity in GaN is performed by Raman spectroscopy. Applying hydrostatic pressure up to 25 GPa the authors study the behavior of the LO phonon-plasmon mode in a series of high mobility Si doped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observed in Si doped samples. The authors find that Si is a shallow hydrogenic donor throughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.
Date: August 1, 1996
Creator: Wetzel, C.; Chen, A.L.; Suski, T.; Ager, J.W. III & Walukiewicz, W.
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 (<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

Large persistent photochromic effect due to DX centers in AlSb doped with selenium

Description: A large photochromic effect has been observed in bulk AlSb crystals doped with Se. Illumination with light of energy higher than 1 eV leads to an increase of the absorption coefficient in the spectral range 0.1 to 1.6 eV. The enhanced absorption is persistent at temperatures below about K. The effect is a manifestation of a DX-like bistability of Se donors. The illumination transfers the from the DX center to a metastable hydrogenic level. The increased absorption with peaks around 0.2 eV and 0.5 is due to photoionization from the donor level to X{sub l} and X{sub 3} minima of the conduction band.
Date: April 1, 1995
Creator: Becla, P.; Witt, A.G.; Lagowski, J. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

The effect of co-implantation on the electrical activity of implanted carbon in GaAs

Description: We have undertaken a systematic study of the effect of co- implantation on the electrical properties of C implanted in GaAs. Two effects have been studied, the additional damage caused by co- implantation and the stoichiometry in the implanted layer. A series of co-implant ions were used: group III (B, Al, Ga), group V (N, P, As) and noble gases (Ar, Kr). Co-implantation of ions which create an amorphous layer was found to increase the electrical activity of C. Once damage was created, maintaining stoichiometric balance by co-implantation of a group III further increased the fraction of electrically active carbon impurities. Co-implantation of Ga and rapid thermal annealing at 950{degree}C for 10s resulted in carbon activation as high as 68%, the highest value ever reported.
Date: November 1, 1991
Creator: Moll, A.J.; Walukiewicz, W.; Yu, K.M.; Hansen, W.L. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Electrical and optical properties of p-type InN

Description: We have performed comprehensive studies of optical, thermoelectric and electrical properties of Mg doped InN with varying Mg doping levels and sample thicknesses. Room temperature photoluminescence spectra show a Mg acceptor related emission and the thermopower provides clear evidence for the presence of mobile holes. Although the effects of the hole transport are clearly observed in the temperature dependent electrical properties, the sign of the apparent Hall coefficient remains negative in all samples. We show that the standard model of two electrically well connected layers (n-type surface electron accumulation and p-type bulk) does not properly describe Hall effect in p-type InN.
Date: November 20, 2011
Creator: Mayer, M. A.; Choi, S.; Bierwagen, O.; Smith, III, H. M.; Haller, E. E.; Speck, J. S. et al.
Partner: UNT Libraries Government Documents Department

Valence band anticrossing in GaBixAs1-x

Description: The optical properties of GaBixAs1-x (0.04< x< 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.
Date: July 11, 2007
Creator: Alberi, K.; Dubon, O. D.; Walukiewicz, W.; Yu, K. M.; Bertulis, K. & Krotkus, A.
Partner: UNT Libraries Government Documents Department

Metal-insulator transition by isovalent anion substitution in Ga1-xMnxAs: Implications to ferromagnetism

Description: We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4percent of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4 percent As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature TC from 113 K to 60 K (100 K to 65 K) upon the introduction of 3.1 percent P (1percent N) into the As sublattice.
Date: February 7, 2008
Creator: Stone, P. R.; Alberi, K.; Tardif, S. K. Z.; Beeman, J. W.; Yu, K. M.; Walukiewicz, W. et al.
Partner: UNT Libraries Government Documents Department

Fundamental Curie temperature limit in ferromagnetic Ga1-xMnxAs

Description: We provide unambiguous experimental evidence that the upper limit of {approx}110 K commonly observed for the Curie temperature TC of Ga{sub 1-x}Mn{sub x}As is caused by the Fermi-level-induced hole saturation. This conclusion is based on parallel studies of the location of Mn in the lattice, the effectiveness of acceptor center, and ferromagnetism on a series of Ga{sub 1-x-y}Mn{sub x}Be{sub y}As layers, in which the concentration of magnetic moments and of free holes can be independently controlled by the Mn and Be contents. Ion channeling and magnetization measurements show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T{sub C} with increasing Be concentration. At the same time the free hole concentration remains relatively constant at {approx}5 x 10{sup 20}cm{sup -3}. These results indicate that the concentrations of free holes as well as of ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors. Based on these results, we propose to use heavy n-type counter-doping of Ga{sub 1-x}Mn{sub x}As (by, e.g., Te) to suppress the formation of Mn interstitials at high x, and thus improve the T{sub C} of the alloy system.
Date: September 24, 2002
Creator: Yu, K. M.; Walukiewicz, W.; Wojtowicz, T.; Lim, W. L.; Liu, X.; Bindley, U. et al.
Partner: UNT Libraries Government Documents Department

Mutual passivation effects in Si-doped diluted In{sub y}Ga{sub 1-y}As{sub 1-x}N{sub x} alloys

Description: We report systematic investigations of the mutual passivation effects of Si hydrogenic donors and isovalent nitrogen in dilute InGaAs{sub 1-x}N{sub x} alloys. Upon thermal annealing at temperatures above {approx}650 C, the Si atoms diffuse assisted by the formation and migration of Ga vacancies. When they find nitrogen atoms, they form stable Si{sub Ga}-N{sub As} nearest-neighbor pairs. As a result of the pair formation, the electrical activity of Si{sub Ga} donors is passivated. At the same time, the effect of an equal number of N{sub As} atoms is also deactivated. The passivation of the shallow donors and the N{sub As} atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental band gap. Analytical calculations of the passivation process based on Ga vacancies mediated diffusion show good agreement with the experimental results. Monte Carlo simulations have also been performed for a comparison with these results. The effects of mutual passivation on the mobility of free electrons are quantitatively explained on the basis of the band anticrossing model. Optical properties of annealed Si-doped InGaAs{sub 1-x}N{sub x} samples are also discussed.
Date: July 21, 2003
Creator: Wu, J.; Yu, K.M.; Walukiewicz, W.; He, G.; Haller, E.E.; Mars, D.E. et al.
Partner: UNT Libraries Government Documents Department

Giant negative piezoresistance effect in copper-doped germanium

Description: We have observed a stress-induced decrease of over ten orders of magnitude in the low-temperature electrical resistivity of copper- doped germanium single crystals. The application of large uniaxial stresses in a <001> direction leas to a change in the copper ground- state wavefunction from the highly localized (1s){sup 3} to the much more extended (1s){sup 2}(2s){sup 1} configuration. We attribute the decrease in the resistivity to impurity band conduction by the 2s - holes of the high pressure configuration.
Date: September 1, 1996
Creator: Dubon, O.D.; Haller, E.E.; Walukiewicz, W. & Beeman, J.W.
Partner: UNT Libraries Government Documents Department