Amphoteric Doping Research Articles

Electron transport in molecular systems

Large-scale quantum electronic structure calculations coupled with nonequilibrium Green function theory are employed for determining quantum conductance on practical length scales. The combination of state-of-the-art quantum mechanical methods, efficient numerical algorithms, and high performance computing allows for realistic evaluation of properties at length scales that are routinely reached experimentally. Two illustrations of the method are presented. First, quantum chemical calculations using up to 104 basis functions are used to investigate the amphoteric doping of carbon nanotubes by encapsulation of organic molecules. As a second example, we investigate the electron transport properties of a Si/organic molecule/Si junction using a numerically optimized basis.

Production and recovery of defects in phosphorus-implanted ZnO

Phosphorus ions were implanted in ZnO single crystals with energies of 50–380keV having total doses of 4.2×1013–4.2×1015cm−2. Positron annihilation measurements reveal the introduction of vacancy clusters after implantation. These vacancy clusters grow to a larger size after annealing at a temperature of 600°C. Upon further annealing up to a temperature of 1100°C, the vacancy clusters gradually disappear. Raman-scattering measurements reveal the enhancement of the phonon mode at approximately 575cm−1 after P+ implantation, which is induced by the production of oxygen vacancies (VO). These oxygen vacancies are annealed out up to a temperature of 700°C accompanying the agglomeration of vacancy clusters. The light emissions of ZnO are suppressed after implantation. This is due to the competing nonradiative recombination centers introduced by implantation. The recovery of the light emission occurs at temperatures above 600°C. The vacancy-type defects detected by positrons might be part of the nonradiative recombination centers. The Hall measurement indicates an n-type conductivity for the P+-implanted ZnO layer, suggesting that phosphorus is an amphoteric dopant.

Amphoteric and Controllable Doping of Carbon Nanotubes by Encapsulation of Organic and Organometallic Molecules

By using first principles calculations, we show that fine tuning of both p- and n-type doping can be realized on single-wall carbon nanotubes (SWNTs) by tuning the electron affinity or ionization potential of the organic and organometallic molecules encapsulated inside SWNTs. This novel type of SWNT-based material offers great promise for molecular electronics because of its air stability, synthetic simplicity and the abundance of organic and organometallic molecules.

Si doped p- and n-type Al xGa 1− xAs epilayers for high density lateral-junction LED arrays on (3 1 1)A patterned substrate

The controlled incorporation of amphoteric Si dopant for p- and n- type conductivity in Al x Ga 1− x As layers simultaneously grown on GaAs (3 1 1)A and (1 0 0) substrates by molecular beam epitaxy (MBE) was investigated for different Al compositions (10–50%). The optimized growth conditions have been used to grow lateral junction (LJ) light emitting diodes (LEDs) structures on GaAs (3 1 1)A patterned substrate. High density (2400 device per inch) LJ-LED arrays were successfully fabricated. The electroluminescence spectrum shows a single peak around 813 nm at room temperature.

Boron Site Distribution in Doped GaAs

The lattice site distribution of boron in doped, melt-grown GaAs was studied by means of photoluminescence and local vibrational mode spectroscopy. The investigated samples were cut from nominally Si or Ge doped crystals grown under stoichiometric or Ga rich conditions. In GaAs:Si boron behaves like an amphoteric dopant. The formation of the antisite, B As , is favoured by the growth from a Ga rich melt. In Ge doped GaAs, on the other hand, only the isovalent boron defect, B Ga , was observed. Apart from infrared absorption spectra measured by the established Fourier transform technique the B As -related radiative recombination and the local Raman modes of the boron species are presented. These results demonstrate, that photoluminescence and near infrared Raman spectroscopy are viable tools for the detection of substitutionally incorporated boron in GaAs.

Doping mechanism in single-wall carbon nanotubes studied by optical absorption

We have separately probed the doping behavior of semiconducting and metallic single wall carbon nanotube (SWNT) films, by optical absorption and dc resistance measurements. Either electron acceptors (Br 2, I 2) or donors (K, Cs) were used as dopants with controlled stoichiometry. Disappearance of the absorption bands at 0.7, 1.2 (assigned to semiconducting SWNT) and 1.8 eV (assigned to metallic SWNT) and the concomitant decrease of resistance due to doping were attributed to electron depletion or filling in specific bands of semiconducting or metallic SWNT. This demonstrates the amphoteric doping behavior of SWNT. Semiconducting and metallic SWNT undergo charge transfer in a specific sequence: initially the transition at 0.7 eV, subsequently around 1.2 eV and finally the feature at 1.8 eV are affected depending on the concentration of the dopant. Changes in the electronic properties are discussed in terms of charge transfer mechanisms in the framework of the rigid-band model.

Fabrication of GaAs quantum wires by natural selective doping and its characterization by electrostatic force microscope

We report an in situ fabrication method of self-organized quantum wire structures on patterned GaAs substrates by molecular beam epitaxy. The present method is based upon a selective doping mechanism of amphoteric silicon dopant both on patterned (311)A and on (100) GaAs substrates. The surface electrostatic potential distribution on these patterns was investigated by electrostatic force microscope (EFM) and quantum wire structures were verified. A minimum wire width of ≈80 nm was obtained for samples based on both substrate orientations. Good agreement of the wire width estimation by the EFM estimation and the magnetoresistance measurement on modulation-doped heterojunction single quantum wire transistor confirms the validity of the present approach to fabricate self-assembled quantum wires.

A kinetic model for metalorganic chemical vapor deposition from trimethylgallium and arsine

A kinetic model based on the collision theory of chemical reactions is proposed for gallium arsenide (GaAs) metalorganic chemical vapor deposition from trimethylgallium and arsine. A simplified reaction mechanism is incorporated into the model, which includes four heterogeneous deposition reactions: Ga-containing and As-containing species with Ga and As sites, as well as carbon incorporation reactions. An equation for the overall growth rate of the four deposition reactions is derived, which is simplified under the Ga-rich or As-rich growth condition. A discussion about antisite defects leads to the conclusion that As-rich growth produces stoichiometric GaAs. The relation between temperature and arsine/trimethylgallium ratio for As-rich growth is defined. The concept of competitive adsorption is introduced to understand doping and ternary deposition. Carrier concentration for n-type, p-type, and amphoteric doping as a function of deposition conditions is derived. Conversion from p type to n type with arsine/trimethylgallium ratio due to residual carbon is, for the first time, quantitatively explained within the framework of doping. Film composition as a function of deposition conditions in ternary deposition of aluminum–gallium arsenide (Al1−xGaxAs) and gallium arsenide–phosphide (GaAs1−yPy) is also derived. The distributions of Al and P between gas phase and solid film follow the same physics rule but differ in the nature of impinging species. The quantitative agreement between the model and a wide range of experiments demonstrates the value of this model.

Proton degradation of light-emitting diodes

Proton degradation is investigated for several types of light-emitting diodes with wavelengths in the near infrared region. Several basic light-emitting diode (LED) technologies are compared, including homojunction and double-heterojunction devices. Homojunction LEDs fabricated with amphoteric dopants are far more sensitive to displacement damage than double-heterojunction LEDs, and are strongly affected by injection-enhanced annealing. Unit-to-unit variability remains an important issue for all LED technologies. For technologies, degradation of the forward voltage characteristics appears to be more significant than degradation of light output.

Thermal equilibrium concentrations of the amphoteric dopant Si and the associated carrier concentrations in GaAs

Expressions of the thermal equilibrium concentrations of Si in GaAs have been obtained in terms of fundamental constants of the involved materials. Silicon is an amphoteric dopant in GaAs, with four species: a neutral and an ionized shallow donor species occupying Ga sublattice sites, and a neutral and an ionized shallow acceptor species occupying As sublattice sites. The concentration of an ionized Si species is expressed by the concentration of the appropriate neutral species and the GaAs crystal Fermi level or the carrier concentration and the band gap energy level positions. The thermal equilibrium concentrations of the two neutral species are expressed by the relevant Gibbs free energies of formation and the As4 vapor phase pressure in the ambient. Using these equations, the long observed relations between the carrier and Si concentrations in different experiments involving both n- and p-type Si doping produced GaAs are quantitatively explained. A difference of ∼1.55 eV in the effective formation enthalpy between the neutral Si atoms occupying the As and Ga sublattice sites has been identified. Moreover, at high temperatures, the GaAs intrinsic Fermi level energy Ei appears to be higher than the midgap energy Eg/2 by ∼20–80 meV.

Amphoteric doping of single-wall carbon-nanotube thin films as probed by optical absorption spectroscopy

We have separately probed the doping behavior of semiconducting (S) and metallic (M) single-wall carbon-nanotube (SWNT) films, by optical absorption and dc resistance (R) measurements. Either electron acceptors $({\mathrm{Br}}_{2},$ ${\mathrm{I}}_{2})$ or donors (K, Cs) were used as dopants with controlled stoichiometry. Disappearance of absorption bands at 0.68, 1.2, and 1.8 eV, and concomitant decrease of R by doping have been attributed to electron depletion from or filling to specific bands in S- or M-SWNT's, demonstrating their amphoteric doping behavior. Changes in the electronic properties are discussed in terms of the charge-transfer mechanism in the framework of the rigid-band model.

Analysis of electronic levels in SiC: V, N, Al powders and crystals using thermally stimulated luminescence

The production of semi-insulating SiC depends on two issues: (i) the incorporation of a transition metal or further defects in the SiC lattice providing near midgap electronic levels; and (ii) the control of shallow and middle deep acceptors and donors. In this work a synthesis process for SiC powders doped with vanadium (V) as an amphoteric dopant is proposed. The incorporation of vanadium as electrically active V Si is monitored via photoluminescence (PL) measurements. Thermally stimulated luminescence (TSL) is used as a characterization technique for shallow levels in the crystals and powders. Recorded TSL glow curves are compared with numerical solutions of trap emptying processes revealing quantitative information on the activation energy of the ionization of shallow donors and acceptors. TSL emission and photoionization studies give information on existing deep levels.

Arsenic incorporation in MBE grown Hg1−xCdxTe

The p-type doping of Hg1−xCdxTe (MCT) has proven to be a significant challenge in present day MCT-based detector technology. One of the most promising acceptor candidates, arsenic, behaves as an amphoteric dopant which can be activated as an acceptor during Hg-rich, low temperature annealing of as-grown molecular beam epitaxy (MBE) samples. This study focuses on developing an understanding of the microscopic behavior of arsenic incorporation during MBE growth. In particular, the question of whether arsenic incorporates as individual As atoms, as As2 dimers, or as As4 tetramers is addressed for MBE growth with an As4 source. A quasithermodynamical model is employed to describe the MCT growth and As incorporation, with parameters fitted to an extensive database of samples grown at the Microphysics Laboratory. The best fits for growth temperatures between 175 and 185°C are obtained for arsenic incorporation as As4 or possibly as As4 clusters, with lower probabilities for As2 and individual As atoms. Based on these results, we investigate the relaxed atomic configurations of As4 and As2 in bulk HgTe by ab initio total energy calculations. The calculations are performed in the pseudopotential density-functional framework within the local density approximation, employing supercells with periodic boundary conditions. The lattice distortions due to As4 and As2 in bulk HgTe are predicted to be modest due to the small size of these arsenic clusters.

Self-assembled InAs dots and quantum wires fabricated on patterned (3 1 1)A GaAs substrates by molecular beam epitaxy

We have investigated one-dimensionally arranged self-assembled growth of InAs quantum dots and the formation of lateral n–i–p–i heterojunction quantum wire structures based on the facet-selective doping mechanism of an amphoteric silicon dopant. Both structures were successfully formed on the same facet stripes on patterned (3 1 1)A GaAs substrates by molecular beam epitaxy in a self-organized way, thus enabling quantum dots to be selectively formed along the quantum wires in a single growth sequence.

Effect of Y-Doping on the Dielectric Properties of BatiO3 Films Deposited in Reducing Atmospheres Using Pulsed Laser Deposition

AbstractBaTiO3-based multilayer capacitors with Ni electrodes are fired at low Po2 to prevent the oxidation of Ni. Amphoteric dopants such as Y, Ho and Dy are used to prevent electrical degradation due to oxygen vacancy migration. This paper describes use of this approach in preparing thin film BaTiO3 capacitors. Tolerance of reducing atmospheres is particularly important in films which are co-processed with resistors such as TaN for integrated resistor-capacitor networks. To assess this approach, films with and without Y-doping were grown in pure N2 atmospheres by pulsed laser deposition from undoped and 0.2wt% Y2O3 doped BaTiO3 targets, respectively. Films were deposited onto Pt/Ti/SiO2/Si substrates using a 5∼10Hz laser repetition rate, an energy density of 4.2J/cm2 and a working pressure of 100mTorr. It was found that polycrystalline perovskite films could be grown at 700°C and that Y-doped films had better crystallinity than the undoped ones. The dielectric constants of films without and with Y-doping are ∼1100 and ∼1300 at lkHz, respectively. The dielectric losses of Y-doped films were lower than for undoped films, possibly because the concentration of mobile charges including oxygen vacancies was decreased. Y-doped films also have a smaller coercive field (25kV/cm) and higher maximum polarization (∼20μC/cm2) than undoped films. The defect chemistry of the system is also discussed.

Growth and fabrication of strained-layer InGaAs/GaAs quantum well lasers grown on GaAs(311)A substrates using only a silicon dopant

The dependence of electrical properties as a function of substrate orientation and growth conditions for Si-doped GaAs grown by molecular beam epitaxy has been studied. A phase diagram concerning conduction type as a function of growth temperature Tg and V/III flux ratio has been examined for GaAs(311)A, GaAs(111)A, and GaAs(331)A. The conduction type of Si-doped GaAs(311)A abruptly changes from p to n type with varying Tg and V/III flux ratio. Furthermore, it was found that the dependence of electrical properties on growth conditions for Si-doped GaAs(311)A is more like GaAs(331)A rather than (111)A. In addition, InGaAs/GaAs strained-layer quantum well lasers have been fabricated on GaAs(311)A substrates using only Si dopant. The p-n junction was formed by changing the V/III flux ratio without growth interruption using the Si doping characteristic in GaAs(311)A as an amphoteric dopant. High crystal quality was maintained even when the V/III flux ratio was changed, and an abrupt p-n junction with a sharp boundary was achieved. Lasing operation at room temperature has been demonstrated for the first time, and a threshold current density of 260 A/cm2 and an external differential quantum efficiency of 67% were obtained for 720-μm-long broad area lasers. The dependence of threshold current density and external differential quantum efficiency on a laser cavity length was also examined. A laser structure with high crystal quality and abrupt p-n junction exhibits an internal quantum efficiency of 90%, an internal optical loss of 6 cm−1, and a transparency current density of 80 A/cm2.

Electron and γ irradiation effects in InP assessed by photoluminescence

Different InP samples were exposed to electron and γ radiation at 300 and 77 K, respectively. The effects of the exposure to high energy radiation were investigated by photoluminescence. A major reduction in the photoluminescence signal from all samples was observed due to the defects introduced. In the case of γ-irradiated samples, the photoluminescence characteristics were recovered after room temperature aging while for the electron irradiated ones a heat treatment either at 473 or 673 K was required for annealing out the radiation-induced defects. The different roles played by the amphoteric dopant Sn and the n dopant S in the changes in the photoluminescence near band-edge spectra are addressed.

A hole facet wire formed by MBE regrowth over an ex-situ patterned GaAs substrate

The transport properties of a series of long (5–100 μm) channels formed by a novel technique has been investigated at low temperatures. The wire is formed by growing a quantum well structure over a patterned (100) substrate which is etched to expose the (311)A plane on the side-wall face. In our initial structures we have observed a narrow two dimensional hole gas (2DHG) with a carried concentration of 2.5 × 10 11 cm −2 and mobility 2.4 × 10 4 cm 2V −1s −1 in the dark assuming the 2DHG extends over the geometric width of the facet. The (100) GaAs substrate is patterned using standard photolithographic techniques and etched using a buffered hydrofluoric acid based etch to expose a 4.8 μm wide facet, the (311)A plane. The wafer is cleaned immediately prior to reloading into the molecular beam epitaxy (MBE) growth chamber. The wafer is then regrown with a 140 nm buffer layer, a 60 nm GaAs quantum well, 20 nm Al 0.3Ga 0.7As (undoped), 40 nm Al 0.3Ga 0.7As silicon doped (10 18cm −3) and a 10 nm GaAs cap layer. Owing to the amphoteric doping nature of silicon in GaAs, a 2DHG is formed on the narrow (311)A facet and a two dimensional electron gas (2DEG) is formed on the corresponding (100) planes surrounding the (311)A facet. Here we will report on our ongoing experiments enlarging on our initial two terminal characterization of the facet 2DHG. Applying a positive bias to the 2DEG on either side of the narrow facet results in squeezing of the 2DHG, in effect, the 2DEG acts as an ‘electron-gate’.

The roles played by Ag and Al dopants in controlling the electrical properties of ZnO varistors

Four sets of ZnO-based ceramic varistors (reference samples-ZNR; doped with aluminium-AL; doped with silver-AG; doped with aluminium and silver-AA) have been prepared by the conventional mixed oxide route. The current-voltage (I-V) characteristics, determined at current densities up to 1 mA/cm2, yielded nonlinear coefficients (α) of 38, 60, 22 and 56, respectively. Dc and ac degradation tests were performed at 115 °C. Ac impedance analysis was used to determine grain (rg) and grain boundary (Rb) resistances; capacitance-voltage analysis enabled the donor density (Nd) and the barrier height (φ) to be determined. Doping the varistors with Al increased Nd, reduced rg, and improved the I-V characteristics, but caused an increase in Zn interstitials which degraded the stability. In contrast, Ag acts as an amphoteric dopant in ZnO causing a decrease in Nd, an increase in rg and Rb, and a reduction in α ; Ag located in the interstitial sites is able to block the formation and migration of new Zn interstitials, thereby improving the stability. Doping with both Al and Ag at optimum levels gives benefits from both additives, i.e., Al doping (increases Nd and decreases rg) improves nonlinearity, while Ag doping (blocks the formation and migration of Zn interstitials) improves the stability of ZnO varistors.

SIMS and X‐ray diffraction characterization of carbon‐doped GaAs, AlxGa1 − xAs films grown by MBE

AbstractIn this article we report on analyses of GaAs and AlxGa1 − xAs carbon‐doped films, grown by solid source molecular beam epitaxy by using secondary ion mass spectrometry, x‐ray diffraction, and Hall effect measurement.Carbon is an amphoteric dopant, i.e. it behaves as an acceptor or a donor impurity when it occupies an As or a Ga site, respectively. Comparison between x‐ray diffraction and Hall effect measurements indicates that, for concentrations below 4 × 1019 cm−3, carbon is preferentially incorporated on As site, both in GaAs and AlxGa1 − xAs layers. We find that the presence of Al strongly enhances the substitutional carbon incorporation. SIMS analyses show that the total amount of carbon largely exceeds the substitutional carbon content recorded by HRDXD. The results can be explained considering that carbon is on substitutional as well as interstitial sites. Furthermore, we show that the interstitial carbon concentration can be reduced for both GaAs:C and AlxGa1 − xAs:C growing at an increased As4 flux and higher substrate temperature.