C. C. Ling

Current transport studies of ZnO∕p-Si heterostructures grown by plasma immersion ion implantation and deposition

Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.

Influence of gaseous annealing environment on the properties of indium-tin-oxide thin films

The influence of postannealing in different gaseous environments on the optical properties of indiu-tin-oxide (ITO) thin films deposited on glass substrates using e-beam evaporation has been systematically investigated. It is found that the annealing conditions affect the optical and electrical properties of the films. Atomic force microscopy, x-ray diffraction, and x-ray photoemission spectroscopy (XPS) were employed to obtain information on the chemical state and crystallization of the films. These data suggest that the chemical states and surface morphology of the ITO film are strongly influenced by the gaseous environment during the annealing process. The XPS data indicate that the observed variations in the optical transmittance can be explained by oxygen incorporation into the film, decomposition of the indium oxide phases, as well as the removal of metallic In.

Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction

Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient spectroscopy was used to investigate deep level defects induced by N+ implantation and the effect of air annealing. The N+ implantation enhanced the electron trap at EC−(0.31±0.01)eV (E3) and introduced another one at EC−(0.95±0.02)eV (D1), which were removed after annealing at 900 and 750°C, respectively. Another trap D2 (Ea=0.17±0.01eV) was formed after the 750°C annealing and persisted at 1200°C.

Arsenic doped p-type zinc oxide films grown by radio frequency magnetron sputtering

As-doped ZnO films were grown by the radio frequency magnetron sputtering method. As the substrate temperature during growth was raised above ∼400 °C, the films changed from n type to p type. Hole concentration and mobility of ∼6×1017 cm−3 and ∼6 cm2 V−1 s−1 were achieved. The ZnO films were studied by secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS), low temperature photoluminescence (PL), and positron annihilation spectroscopy (PAS). The results were consistent with the AsZn–2VZn shallow acceptor model proposed by Limpijumnong et al. [Phys. Rev. Lett. 92, 155504 (2004)]. The results of the XPS, PL, PAS, and thermal studies lead us to suggest a comprehensive picture of the As-related shallow acceptor formation.

Deep level transient spectroscopic study of neutron-irradiated n-type 6H–SiC

Deep level transient spectroscopy has been employed to study the deep level defects introduced in n-type 6H-SiC after neutron irradiation. Deep levels situated at E-C-0.23, E-C-0.36/0.44, E-C-0.50, and E-C-0.62/0.68 eV have been detected in the temperature range of 100-450 K, which have been identified with the previously reported deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2), respectively. Thermal annealing studies of these deep levels reveal that ED1 and E-i anneal at a temperature below 350degreesC, the Z(1)/Z(2) levels anneal out at 900degreesC, while the intensity of the E-1/E-2 peaks is increased with annealing temperature, reaching a maximum at about 500-750degreesC, and finally annealing out at 1400degreesC. The possible nature of the deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2) are discussed in the context of their annealing behavior. Upon further annealing at 1600degreesC, four deep levels labeled NE1 at E-C-0.44 eV, NE2 E-C-0.53 eV, NE3 E-C-0.64 eV, and NE4 E-C-0.68 eV are produced. Evidence is given that these levels are different in their origin to E-1/E-2 and Z(1)/Z(2)

Current transport and its effect on the determination of the Schottky-barrier height in a typical system: Gold on silicon

Abstract The measurements of photovoltage, internal photoemission, current-voltage (I-V) and capacitance-voltage (C-V) have been performed for Au on n-Si (111) within a temperature range of 7 to 300 K. The values of Schottky-barrier height at room temperature obtained from these four measurements are in very good agreement, but conflicting results have been obtained at low temperatures. The measurements of both the internal photoemission and the C-V show a negative temperature dependence of the barrier height which is almost identical to that of the indirect energy band gap in silicon, but the barrier height obtained from the photovoltage measurements and the I-V measurements strongly decreases as temperature decreases. In this paper, the influence of the current transport on the determination of the Schottky-barrier height is discussed, and it is demonstrated that the conflicting results may be explained well in terms of the recombination current involved in the photovoltage measurements and the I-V measurements.

Influence of annealing temperature and environment on the properties of indium tin oxide thin films

Indium tin oxide (ITO) thin films were deposited on glass substrates using the e-beam evaporating technique. The influence of deposition rate and post-deposition annealing on the optical properties of the films was investigated in detail. It is found that the deposition rate and annealing conditions strongly affect the optical properties of the films. The transmittance of films greatly increases with increasing annealing temperature below 300°C but drastically drops at 400°C when they are annealed in forming gas (mixed N2 and H2 gas). An interesting phenomenon observed is that the transmittance of the darkened film can recover under further 400°C annealing in air. Atomic force microscopy, x-ray diffraction and x-ray photoemission spectroscopy were employed to obtain information on the chemical state and crystallization of the films. Analysis of these data suggests that the loss and re-incorporating of oxygen are responsible for the reversible behaviour of the ITO thin films.

Nature of the acceptor responsible for p-type conduction in liquid encapsulated Czochralski-grown undoped gallium antimonide

Acceptors in liquid encapsulated Czochralski-grown undoped gallium antimonide (GaSb) were studied by temperature dependent Hall measurement and positron lifetime spectroscopy (PLS). Because of its high concentration and low ionization energy, a level at EV+34meV is found to be the important acceptor responsible for the p-type conduction of the samples. Two different kinds of VGa-related defects (lifetimes of 280ps and 315ps, respectively) having different microstructures were characterized by PLS. By comparing their annealing behaviors and charge state occupancies, the EV+34meV level could not be related to the two VGa-related defects.

STUDY OF DX CENTER IN CD0.8ZN0.2TE:CL BY POSITRON ANNIHILATION

Variable energy positron beam and positron annihilation lifetime experiments have been carried out to study the DX center in Cd0.8Zn0.2Te:Cl at 50 K. A short positron effective diffusion length of 275±25 A and a large intensity of 79.0%±0.3% for the long lifetime component indicate a strong trapping effect at DX centers. A trapping rate of κ=1.53±0.05×109 s−1 and a positron lifetime of 335±2 ps at the DX center were obtained. The concentration of DX centers is found to be 5.9±0.7×1016 cm−3, which is in good agreement with the results obtained using Hall effect and thermo-electric effect measurements.

Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator

Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (≈400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (≈10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling.