32 research outputs found
A study of the T2 defect and the emission properties of the E3 deep level in annealed melt grown ZnO single crystals
published_or_final_versio
Control of Electrons' Spin Eliminates Hydrogen Peroxide Formation during Water Splitting
The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells
Comparative study of the electrical properties of Pd/ZnO Schottky contacts fabricated using electron beam deposition and resistive/thermal evaporation techniques
A systematic investigation to check the quality of Pd Schottky contacts deposited on ZnO has
been performed on electron beam (e-beam) deposited and resistively/thermally evaporated
samples using current-voltage, IV, and conventional deep level transient spectroscopy (DLTS)
measurements. Room temperature IV measurements reveal the dominance of pure thermionic
emission on the resistively evaporated contacts, while the e-beam deposited contacts show the
dominance of generation recombination at low voltages,<0.30 V, and the dominance of pure
thermionic emission at high voltages, greater than 0.30 V. The resistively evaporated contacts have
very low reverse currents of the order of 10 10 A at a reverse voltage of 1.0 V whereas the e-beam
deposited contacts have reverse currents of the order of 10 6 A at 1.0 V. Average ideality factors
have been determined as (1.4360.01) and (1.6660.02) for the resistively evaporated contacts and
e-beam deposited contacts, respectively. The IV barrier heights have been calculated as
(0.72160.002) eV and (0.62460.005) eV for the resistively evaporated and e-beam deposited
contacts, respectively. Conventional DLTS measurements reveal the presence of three prominent
defects in both the resistive and e-beam contacts. Two extra peaks with energy levels of 0.60 and
0.81 eV below the conduction band minimum have been observed in the e-beam deposited
contacts. These have been explained as contributing to the generation recombination current that
dominates at low voltages and high leakage currents. Based on the reverse current at 1.0 V, the
degree of rectification, the dominant current transport mechanism and the observed defects, we
conclude that the resistive evaporation technique yields better quality Schottky contacts for use in
solar cells and ultraviolet detectors compared to the e-beam deposition technique. The 0.60 eV has
been identified as possibly related to the unoccupied level for the doubly charged oxygen vacancy,
Vо2+.The National Research Foundation of South Africahttp://dx.doi.org/10.1063/1.3658027nf201
Annealing and surface conduction on Hydrogen peroxide treated bulk melt-grown, single crystal ZnO
We report on the studies carried out on hydrogen peroxide treated melt-grown, bulk single crystal ZnO samples. Results show the existence of two shallow donors in the as-received ZnO samples with energy levels (37.8±0.3) meV that has been suggested as Zni related and possibly H-complex related and (54.5±0.9) meV, which has been assigned to an Al-related donor. Annealing studies performed on the hydrogen peroxide treated samples reveal the existence of a conductive channel in the samples in which new energy levels have been observed, Zn vacancies, related to the Group I elements, XZn. The surface donor volume concentration of the conductive channel was calculated from a theory developed by Look (2007) [1]. Results indicate an increase in the surface volume concentration with increasing annealing temperature from 60×1017 cm−3 at 200 °C to 4.37×1018 cm-3 at 800 °C
Chiral “doped” MOFs: an electrochemical and theoretical integrated study
This work reports on the electrochemical behaviour of Fe and Zn based metal-organic framework (MOF) compounds, which are “doped” with chiral molecules, namely: cysteine and camphor sulfonic acid. Their electrochemical behaviour was thoroughly investigated via “solid-state” electrochemical measurements, exploiting an “ad hoc” tailored experimental set-up: a paste obtained by carefully mixing the MOF with graphite powder is deposited on a glassy carbon (GC) surface. The latter serves as the working electrode (WE) in cyclic voltammetry (CV) measurements. Infrared (IR), X-ray diffraction (XRD) and absorbance (UV-Vis) techniques are exploited for a further characterization of the MOFs’ structural and electronic properties. The experimental results are then compared with DFT based quantum mechanical calculations. The electronic and structural properties of the MOFs synthesized in this study depend mainly on the type of metal center, and to a minor extent on the chemical nature of the dopant
Effect of thermal treatment on the characteristics of iridium Schottky barrier diodes on n-Ge (1 0 0)
Iridium (Ir) Schottky barrier diodes were deposited on bulk grown (1 0 0) Sb-doped n-type germanium by using the electron beam deposition system. Electrical characterization of these contacts using current–voltage (I–V) and capacitance–voltage (C–V) measurements was performed under various annealing conditions. The variation of the electrical properties of these Schottky diodes can be attributed to combined effects of interfacial reaction and phase transformation during the annealing process. Thermal stability of the Ir/n-Ge (1 0 0) was observed up to annealing temperature of 500 °C. Furthermore, structural characterization of these samples was performed by using a scanning electron microscopy (SEM) at different annealing temperatures. Results have also revealed that the onset temperature for agglomeration in a 20 nm Ir/n-Ge (1 0 0) system occurs between 600 and 700 °C
Comparison of metal Schottky contacts on n-Ge (100) at different annealing temperatures
SiOC films made by the inductively coupled plasma chemical vapor deposition were researched the relationship between the dielectric constant and the chemical shift. SiOC film had the main Si-O-C bond with the molecule vibration mode in the range of 930∼1230 cm-1 which consists of C-O and Si-O bonds related to the cross link formation according to the dissociation and recombination. The C-O bond originated from the elongation effect by the neighboring highly electronegative oxygen atoms at terminal C-H bond in Si-CH3 of 1270 cm-1. However, the Si-O bond was formed from the second ionic sites recombined after the dissociation of Si-CH3 of 1270 cm-1. Increment of Si-O bond in the Si-O-C main bond contributed to increase the hardness and decrease the roughness because of the decreasing of the carbon content witch is related to the space effect. The dielectric constant of SiOC film was decreased by lowering the polarization which consists of the ionic and electronic polarity. Increment of Si-O bond in a right shoulder at the main bond was interpreted by the red shift observed in FTIR spectra. Moreover, the increment of Si-O bond and decrement of carbon content induced to decrease the ionic polarity and then decreased the dielectric constant
The dependence of barrier height on temperature for Pd Schottky contacts on ZnO
Temperature dependent current–voltage (I–V) and capacitance–voltage (C–V) measurements have been performed on Pd/ZnO Schottky barrier diodes in the range 60–300 K. The room temperature values for the zero bias barrier height from the I–Vmeasurements (ΦI–V) was found to be 0.52 eV and from the C–V measurements (ΦC–V) as 3.83 eV. From the temperature dependence of forward bias I–V, the barrier height was observed to increase with temperature, a trend that disagrees with the negative temperature coefficient for semiconductor material. The C–V barrier height decreases with temperature, a trend that is in agreement with the negative temperature coefficient of semiconductor material. This has enabled us to fit two curves in two regions (60–120 K and 140–300 K). We have attributed this behaviour to a defect observed by DLTS with energy level 0.31 eV below the conduction band and defect concentration of between 4×1016 and 6×1016 cm−3 that traps carriers, influencing the determination of the barrier height
Lithium and electrical properties of ZnO
Hydrothermally grown n-type ZnO samples have been investigated by deep level transient spectroscopy
(DLTS), thermal admittance spectroscopy (TAS), temperature dependent Hall effect
(TDH) temperature, and secondary ion mass spectrometry (SIMS) after thermal treatments
up to 1500◦C, in order to study the electrical properties of samples with different lithium content.
The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Ni and Fe
with concentrations up to ∼ 5 × 1017cm−3. The Li concentration was reduced from ∼ 1017 cm−3
in as-grown samples to ∼ 1015 cm−3 for samples treated at 1500◦C, while the concentration of
all the other major impurities appeared stable. The results from DLTS and TAS displayed at
least five different levels having energy positions of Ec − 20 meV , Ec − 55 meV , Ec − 0.22 eV ,
Ec − 0.30 eV , and Ec − 0.57 eV (Ec denotes the conduction band edge), where the Ec − 55meV
level is the dominant freeze out level for conduction electrons in samples treated at temperatures
< 1300◦C, while higher annealing temperatures revealed the shallower (Ec − 20 meV ) level. The
TDH measurements showed a pronounced increase in the electron mobility for the heat treated
samples, where a peak mobility of 1180 cm2/V s was reached for a sample treated at 1300◦C. The
results provide strong evidence that Li in hydrothermal ZnO is almost exclusively in the substitutional
configuration (LiZn), supporting theoretical predictions that the formation of LiZn prevails
over Li on the interstitial site for Fermi level positions at and above the middle of the band gap