M. W. Allen

Influence of oxygen vacancies on Schottky contacts to ZnO

Ni, Ir, Pd, Pt, and silver oxide Schottky contacts were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO. A relationship was found between the barrier height of the contact and the free energy of formation of its “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately 0.7eV below the conduction band minimum. Therefore, a key goal in the fabrication of high quality Schottky contacts should be the minimization of oxygen vacancies near the metal-ZnO interface.

Metal Schottky diodes on Zn-polar and O-polar bulk ZnO

Planar Pd, Pt, Au, and Ag Schottky diodes with low ideality factors were fabricated on the Zn-polar (0001) and O-polar (0001¯) faces of bulk, single crystal ZnO wafers. The diodes were characterized by current-voltage and capacitance-voltage measurements. A polarity effect was observed for Pt and Pd diodes with higher quality barriers achieved on the O-polar face. No significant polarity effect was observed for Au or Ag diodes. The highest barriers were achieved with Ag as the Schottky metal with barrier heights varying between 0.77 and 1.02eV. This is possibly due to varying degrees of oxidation of the Ag contacts.

Influence of spontaneous polarization on the electrical and optical properties of bulk, single crystal ZnO

Hall effect, photoluminescence, and Schottky diode measurements were made on the Zn-polar and O-polar faces of undoped, bulk, single crystal, c-axis ZnO wafers. Significant polarity related differences were observed in the PL and Schottky diode characteristics of low carrier concentration, hydrothermally grown wafers. Increased emission from free exciton recombinations and from recombinations between 3.3725 and 3.3750eV was observed on the Zn-polar face. Conversely, emission between 3.3640 and 3.3680eV was more intense on the O-polar face. The barrier heights of silver oxide Schottky diodes were approximately 130meV larger on the Zn-polar face compared to the O-polar face.

Oxidized noble metal Schottky contacts to n-type ZnO

Deliberately oxidized iridium, platinum, and palladium Schottky contacts were fabricated on the Zn-polar and O-polar faces of hydrothermal bulk ZnO by eclipse pulsed laser deposition in an oxygen ambient. The barrier heights of these oxidized contacts were significantly higher than their plain metal counterparts, with ideality factors approaching the image-force-controlled limit for laterally homogeneous interfaces. The key aspects of this technique are a low deposition energy and the use of an oxidizing environment which reduces interfacial defects, particularly oxygen vacancies. In each case, the barriers on the Zn-polar face were 210–260 meV higher than those on the O-polar face.

Polarity effects in the x-ray photoemission of ZnO and other wurtzite semiconductors

Significant polarity-related effects were observed in the near-surface atomic composition and valence band electronic structure of ZnO single crystals, investigated by x-ray photoemission spectroscopy using both Al Kα (1486.6 eV) and synchrotron radiation (150 to 1486 eV). In particular, photoemission from the lowest binding energy valence band states was found to be significantly more intense on the Zn-polar face compared to the O-polar face. This is a consistent effect that can be used as a simple, nondestructive indicator of crystallographic polarity in ZnO and other wurtzite semiconductors.

Ion currents to cylindrical Langmuir probes in RF plasmas

The positive ion current collected by cylindrical Langmuir probes has been measured in an RF argon plasma. The current-voltage characteristics were measured using an RF compensation technique whereby the probe was forced to follow the RF fluctuation in the local plasma potential. The I-V curves from probes of different radii and material show an ion current which is always greater than that predicted by the orbital motion theory and that agrees well with the radial motion theory. This is because the ions travel to the probe from a distance which is limited by the vessel geometry or by the collisional mean free path. The role of the possible collisions, ion-ion and ion-neutral, in the ion trajectories is analysed. It is suggested that the criterion for the application of the orbital motion theory should be tested whenever this theory is used for the determination of the charged particle density.

Defects in hydrothermally grown bulk ZnO

Hydrothermally grown bulk ZnO (Tokyo Denpa) was investigated using junction-capacitance spectroscopy on silver oxide Schottky contacts (barrier height of 1.20eV, ideality factor of 1.04). Two main shallow defects, T1 and T2, with thermal activation energies of 13 and 52meV, respectively, were identified. Two closely lying, deep defect levels E3∕E3′ at approximately 320meV below the conduction band were found in higher concentrations (mid-1014cm−3) than the shallow donors. 4K photoluminescence showed dominant emission from excitons bound to three neutral donors, aluminum, hydrogen, and an unassigned impurity, with donor binding energies close to the thermal activation energy of T2.

A Critical Assessment of Two Types of Personal UV Dosimeters

Doses of erythemally weighted irradiances derived from polysulphone (PS) and electronic ultraviolet (EUV) dosimeters have been compared with measurements obtained using a reference spectroradiometer. PS dosimeters showed mean absolute deviations of 26% with a maximum deviation of 44%, the calibrated EUV dosimeters showed mean absolute deviations of 15% (maximum 33%) around noon during several test days in the northern hemisphere autumn. In the case of EUV dosimeters, measurements with various cut‐off filters showed that part of the deviation from the CIE erythema action spectrum was due to a small, but significant sensitivity to visible radiation that varies between devices and which may be avoided by careful preselection. Usually the method of calibrating UV sensors by direct comparison to a reference instrument leads to reliable results. However, in some circumstances the quality of measurements made with simple sensors may be over‐estimated. In the extreme case, a simple pyranometer can be used as a UV instrument, providing acceptable results for cloudless skies, but very poor results under cloudy conditions. It is concluded that while UV dosimeters are useful for their design purpose, namely to estimate personal UV exposures, they should not be regarded as an inexpensive replacement for meteorological grade instruments.

Validation of brief questionnaire measures of sun exposure and skin pigmentation against detailed and objective measures including vitamin D status

Self‐reported sun exposure is commonly used in research, but how well this represents actual sun exposure is poorly understood. From February to July 2011, a volunteer sample (n = 47) of older adults (≥45 years) in Canberra, Australia, answered brief questions on time outdoors (weekdays and weekends) and natural skin color. They subsequently maintained a sun diary and wore an ultraviolet radiation (UVR) digital dosimeter for 7 days. Melanin density was estimated using reflectance spectrophotometry; lifetime sun damage was assessed using silicone casts of the back of the hand; and serum 25‐hydroxyvitamin D (25(OH)D) concentration was assayed. Questionnaire‐reported time outdoors correlated significantly with diary‐recorded time outdoors (Spearman correlation rs = 0.66; 95% CI 0.46, 0.80; P < 0.001) and UVR dosimeter dose (rs = 0.46; 95% CI 0.18, 0.68; P = 0.003), but not 25(OH)D concentration (rs = 0.24; 95% CI −0.05, 0.50; P = 0.10). Questionnaire‐reported untanned skin color correlated significantly with measured melanin density at the inner upper arm (rs = 0.49; 95% CI 0.24, 0.68; P < 0.001). In a multiple linear regression model, statistically significant predictors of 25(OH)D concentration were self‐reported frequency of physical activity, skin color and recent osteoporosis treatment (R2 = 0.54). In this study, brief questionnaire items provided valid rankings of sun exposure and skin color, and enabled the development of a predictive model for 25(OH)D concentration.

Temperature-Dependent Properties of Nearly Ideal ZnO Schottky Diodes

The current-voltage characteristics of low-ideality factor (1.09 at 300 K) ZnO Schottky diodes were investigated over the temperature range of 40-423 K. Planar geometry devices were fabricated on the Zn-polar face of a low carrier concentration (n = 1 times 1014 cm-3) hydrothermally grown bulk ZnO single crystal wafer. The current transport was dominated by thermionic emission between 293 and 423 K, provided the ZnO surface was exposed to air. The nearly ideal characteristics of the diodes yielded an experimental Richardson constant of 10 plusmn 6 Amiddotcm-2middotK-2, close to the theoretical value of 32 Amiddotcm-2middotK-2, and two orders of magnitude larger than previously reported values.