Electrical characterization of process, annealing and irradiation induced defects in ZnO

A study of defects in semiconductors is vital as defects tend to influence device operation by modifying their electrical and optoelectronic properties. This influence can at times be desirable in the case of fast switching devices and sometimes undesirable as they may reduce the efficiency of optoelectronic devices. ZnO is a wide bandgap material with a potential for fabricating UV light emitting diodes, lasers and white lighting devices only after the realization of reproducible p-type material. The realization of p-type material is greatly affected by doping asymmetry. The self-compensation behaviour by its native defects has hindered the success in obtaining the p-type material. Hence there is need to understand the electronic properties, formation and annealing-out of these defects for controlled material doping. Space charge spectroscopic techniques are powerful tools for studying the electronic properties of electrically active defects in semiconductors since they can reveal information about the defect “signatures”. In this study, novel Schottky contacts with low leakage currents of the order of 10-11 A at 2.0 V, barrier heights of 0.60 – 0.80 eV and low series resistance, fabricated on hydrogen peroxide treated melt-grown single crystal ZnO samples, were demonstrated. Investigations on the dependence of the Schottky contact parameters on fabrication techniques and different metals were performed. Resistive evaporation proved to produce contacts with lower series resistance, higher barrier heights and low reverse currents compared to the electron-beam deposition technique. Deep level transient spectroscopy (DLTS) and Laplace-DLTS have been employed to study the electronic properties of electrically active deep level defects in ZnO. Results revealed the presence of three prominent deep level defects (E1, E2 and E3) in the as-received ZnO samples. Electron-beam deposited contacts indicated the presence of the E1, E2 and E3 and the introduction of new deep level defects. These induced deep levels have been attributed to stray electrons and ionized particles, present in the deposition system during contact fabrication. Exposure of ZnO to high temperatures induces deep level defects. Annealing samples in the 300°C – 600°C temperature range in Ar + O2 induces the E4 deep level with a very high capture cross-section. This deep level transforms at every annealing temperature. Its instability at room temperature has been demonstrated by a change in the peak temperature position with time. This deep level was broad, indicating that it consists of two or more closely spaced energy levels. Laplace-DLTS was successfully employed to resolve the closely spaced energy levels. Annealing samples at 700°C in Ar and O2 anneals-out E4 and induces the Ex deep level defect with an activation enthalpy of approximately 160 – 180 meV. Vacuum annealing performed in the 400°C – 700°C temperature range did not induce any deep level defects. Since the radiation hardness of ZnO is crucial in space applications, 1.6 MeV proton irradiation was performed. DLTS revealed the introduction of the E4 deep level with an activation enthalpy of approximately 530 meV, which proved to be stable at room temperature and atmospheric pressure since its properties didn’t change over a period of 12 months.Thesis (PhD)--University of Pretoria, 2013.Physicsunrestricte

Electrical characterization of ZnO and metal ZnO contacts

The electrical properties of ZnO and contacts to ZnO have been investigated using different techniques. Temperature dependent Hall (TDH) effect measurements have been used to characterize the as-received melt grown ZnO samples in the 20 – 330 K temperature range. The effect of argon annealing on hydrogen peroxide treated ZnO samples has been investigated in the 200 – 800oC temperature range by the TDH effect measurement technique. The experimental data has been analysed by fitting a theoretical model written in Matlab to the data. Donor concentrations and acceptor concentrations together with the associated energy levels have been extracted by fitting the models to the experimentally obtained carrier concentration data by assuming a multi-donor and single charged acceptor in solving the charge balance equation. TDH measurements have revealed the dominance of surface conduction in melt grown ZnO in the 20 – 40 K temperature range. Surface conduction effects have proved to increase with the increase in annealing temperature. Surface donor volume concentrations have been determined in the 200 – 800oC by use of theory developed by D. C. Look. Good rectifying Schottky contacts have been fabricated on ZnO after treating the samples with boiling hydrogen peroxide. Electrical properties of these Schottky contacts have been investigated using current-voltage (IV) and capacitance-voltage (CV) measurements in the 60 – 300 K temperature range. The Schottky contacts have revealed the dominance of predominantly thermionic emission at room temperature and the existence of other current transport mechanisms at temperatures below room temperature. Polarity effects on the Schottky contacts deposited on the O-polar and Zn-polar faces of ZnO have been demonstrated by the IV technique on the Pd and Au Schottky contacts at room temperature. Results obtained indicate a strong dependence of the Schottky contact quality on the polarity of the samples at room temperature. The quality of the Schottky contacts have also indicated their dependence on the type of metal used with the Pd producing contacts with the better quality as compared to the Au. Schottky barrier heights determined using temperature dependent IV measurements have been observed to increase with increasing temperature and this has been explained as an effect of barrier inhomogeneities, while the ones obtained from CV measurements have proved to follow the negative temperature coefficient of the II – VI semiconductor material, i.e. a decrease in barrier height with increasing temperature. However, the values have proved to be larger than the energy gap of ZnO, an effect that has been explained as caused by an inversion layer. CopyrightDissertation (MSc)--University of Pretoria, 2010.Physicsunrestricte

Temperature-dependent current-voltage characteristics of Pd/ZnO Schottky barrier diodes and the determination of the Richardson constant

We report on a systematic investigation of temperature dependent current-voltage (I-V) characteristics of Pd/ZnO Schottky barrier diodes in the 30-300 K temperature range. The ideality factor was observed to decrease with increase in temperature, whilst the barrier height increases with increase in temperature. The observed trend has been attributed to barrier inhomogeneities, which results in a distribution of barrier heights at the interface. Using the dependence of saturation current values on temperature, we have calculated the Richardson constant (A*) which was investigated in the two distinct temperature regions: 140–200 K and 210–300 K and values 3 x10-12 of and 3x10-9 A cm-2 K-2 were obtained, respectively. A mean barrier height of 0.97 eV was obtained in the 140-300 K temperature range. Applying the barrier height inhomogeneities correction, the value of A* was obtained from the modified Richardson plots as 39.43 and 39.03 A cm-2 K-2 in the 140-200 K and 210-300 K temperature range. The modified Richardson constant (A**) has proved to be strongly affected by barrier inhomogeneities and dependent on contact quality.South African National Research Foundation (NRF) and the University of Pretoria.http://www.elsevier.com/locate/mssp2016-06-30hb201

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

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 theory developed by D. C. Look [1]. Results indicate an increase in surface volume concentration with increasing annealing temperature from 6.0´1017 cm-3 at 200°C to 4.37´1018 cm-3 at 800°C.South African National Research Foundationhttp://www.elsevier.com/locate/physbhb2013ai201

Determination of the laterally homogeneous barrier height of palladium Schottky barrier diodes on n-Ge (1 1 1)

We have studied the experimental linear relationship between barrier heights and ideality factors for palladium (Pd) on bulk-grown (1 1 1) Sb-doped n-type germanium (Ge) metal-semiconductor structures with a doping density of about 2.5×1015 cm−3. The Pd Schottky contacts were fabricated by vacuum resistive evaporation. The electrical analysis of the contacts was investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at a temperature of 296 K. The effective barrier heights from I–V characteristics varied from 0.492 to 0.550 eV, the ideality factor n varied from 1.140 to 1.950, and from reverse bias capacitance–voltage (C−2–V) characteristics the barrier height varied from 0.427 to 0.509 eV. The lateral homogenous barrier height value of 0.558 eV for the contacts was obtained from the linear relationship between experimental barrier heights and ideality factors. Furthermore the experimental barrier height distribution obtained from I–V and (C−2−V) characteristics were fitted by Gaussian distribution function, and their mean values were found to be 0.529 and 0.463 eV, respectively

Electrical characterisation of ruthenium Schottky contacts on n-Ge (1 0 0)

Please read abstract in the article.The South African National Research Foundationhttp://www.elsevier.com/locate/physbnf201

Current-voltage temperature characteristics of Au/n-Ge (1 0 0) Schottky diodes

The variation in electrical characteristics of Au/n-Ge (1 0 0) Schottky contacts have been systematically investigated as a function of temperature using current-voltage (I-V) measurements in the temperature range 140-300 K. The I-V characteristics of the diodes indicate very strong temperature dependence. While the ideality factor n decreases, the zero-bias Schottky barrier height (SBH) ( ɸʙ) increases with the increasing temperature. The I-V characteristics are analysed using the thermionic emission (TE) model and the assumption of a Gaussian distribution of the barrier heights ɸʙ vs. ½ kT plot has been used to show the evidence of a Gaussian distribution of barrier heights and values of ɸʙ = 0.615 eV and standard deviation σs0 = 0.00858 eV for the mean barrier height and zero-bias standard deviation have been obtained from this plot, respectively. The Richardson constant and the mean barrier height from the modified Richardson plot were obtained as 1.37 A cm-2 K-2 and 0.639 eV, respectively. This Richardson constant is much smaller than the reported of 50 A cm-2 K-2. This may be due to greater inhomogeneities at the interface.The South African National Research Foundationhttp://www.elsevier.com/locate/physbnf201

Effects of high temperature annealing on single crystal ZnO and ZnO devices

We have systematically investigated the effects of high-temperature annealing on ZnO and ZnO devices using current voltage, deep level transient spectroscopy (DLTS) and Laplace DLTS measurements. Current–voltage measurements reveal the decrease in the quality of devices fabricated on the annealed samples, with the high-temperature annealed samples yielding devices with low barrier heights and high reverse currents. DLTS results indicate the presence of three prominent defects in the as-received samples. Annealing the ZnO samples at 300 C, 500 C, and 600 C in Ar results in an increase in reverse leakage current of the Schottky contacts and an introduction of a new broad peak. After 700 C annealing, the broad peak is no longer present, but a new defect with an activation enthalpy of 0.18 eV is observed. Further annealing of the samples in oxygen after Ar annealing causes an increase in intensity of the broad peak. High-resolution Laplace DLTS has been successfully employed to resolve the closely spaced energy levels.The South African National Research Foundation (NRF), A. R. Peaker (Centre for Electronic Materials Devices and Nanostructures, University of Manchester) and L. Dobaczewski (Institute of Physics, Polish Academy of Sciences).http://jap.aip.org/nf201

Thermal annealing behaviour of Pd Schottky contacts on melt-grown single crystal ZnO studied by IV and CV measurements

Current–voltage (IV) and capacitance–voltage (CV) measurement techniques have successfully been employed to study the effects of annealing highly rectifying Pd/ZnO Schottky contacts. IV results reveal a decrease in the contact quality with increasing annealing temperature as confirmed by a decrease in the zero bias barrier height and an increase in the reverse current measured at −1.5 V. An average barrier height of (0.77 ± 0.02) eV has been calculated by assuming pure thermionic emission for the as-deposited material and as (0.56 ± 0.03) eV after annealing at 550 ◦C. The reverse current has been measured as (2.10 ± 0.01) × 10−10 A for the as-deposited and increases by 5 orders of magnitude after annealing at 550 ◦C to (1.56 ± 0.01) × 10−5 A. The depletion layer width measured at −2.0 V has shown a strong dependence on thermal annealing as it decreases from 1.09 m after annealing at 200 ◦C to 0.24 m after annealing at 500 ◦C, resulting in the modification of the dopant concentration within the depletion region and hence the current flowing through the interface from pure thermionic emission to thermionic field emission with the donor concentrations increasing from 6.90 × 1015 cm−3 at 200 ◦C to 6.06 × 1016 cm−3 after annealing at 550 ◦C. This increase in the volume concentration has been explained as an effect of a conductive channel that shifts closer to the surface after sample annealing. The series resistance has been observed to decrease with increase in annealing temperature. The Pd contacts have shown high stability up to an annealing temperature of 250 ◦C as revealed by the IV and CV characteristics after which the quality of the contacts deteriorates with increase in annealing temperature.http://www.elsevier.com/locate/msebhb201