We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects that are responsible for carrier compensation.

Gold Schottky-barrier diodes ͑SBDs͒ were fabricated on vapor-phase-grown single-crystal ZnO. Deep-level transient spectroscopy, using these SBDs, revealed the presence of four electron traps, the major two having levels at 0.12 eV and 0.57 below the conduction band. Comparison with temperature-dependent Hall measurements suggests that the 0.12 eV level has a temperature activated capture cross section with a capture barrier of about 0.06 eV and that it may significantly contribute to the free-carrier density. Based on the concentrations of defects other than this shallow donor, we conclude that the quality of the vapor-phase-grown ZnO studied here supercedes that of other single-crystal ZnO reported up to now. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1452781͔During the past few decades, ZnO has been used in many, diverse products. Its applications, among others, include phosphors, piezoelectric transducers, varistors, alcohol synthesis catalysis, and gas sensing. In a recent review, however, where the properties of ZnO are summarized, 1 it was pointed out that ZnO can be used for several other, more advanced, electro-optical applications. Based on the fact that ZnO has a direct bandgap of 3.4 eV, it is expected to fulfill a similar role as GaN in optoelectronics, i.e., play an important role in realizing blue and ultraviolet light-emitting diodes and lasers, as well as daylight-blind UV detectors. Similar to that of GaN, its large band gap renders ZnO suitable for the fabrication of high-temperature, high-power devices with application, among others, in space where typical operating temperatures exceed 200°C. High-quality single-crystal ZnO can be successfully grown in bulk. 2 A very important consequence of this is that owing to the relatively close match in lattice constants, single-crystal ZnO may be used as a substrate to grow epitaxial GaN that is well oriented with respect to the substrate and that has a reduced defect density. 3 Further practical advantages of ZnO include amenability to conventional wet chemistry etching, which is compatible with Si technology 4 ͑unlike the case for GaN͒.An important issue in ZnO technology is establishing a technology for the fabrication of high-quality Schottkybarrier diodes ͑SBDs͒. SBDs are of key importance to probe defects in semiconductors by junction spectroscopic characterization techniques, such as deep-level transient spectroscopy ͑DLTS͒ ͑Ref. 5͒ and admittance spectroscopy ͑AS͒. 6 They are also important structures for metal-semiconductor field-effect transistors ͑MESFETs͒ and detectors. A limited amount of research regarding Schottky contacts to ZnO was carried out in the 1960s, where among other things the barrier height of some metal contacts to vacuum-cleaved ZnO was reported. 7 However, up to now, no systematic studies of surface characterization as a function of cleaning method have been performed; i.e., the influence of different cleaning methods on the surface quality of ZnO and the quality of Schottky contacts to ZnO have not been compared...

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Epitaxial n-GaN was irradiated with 5.4-MeV He ions. Capacitance–voltage (C–V) measurements showed that 5.4-MeV He ions remove free carriers at a rate of 6200±300 cm−1 in the first micron below the surface. Deep level transient spectroscopy (DLTS) revealed that, in addition to the radiation-induced defects previously detected by DLTS at 0.18–0.20 eV below the conduction band, He-ion irradiation introduced two additional prominent defects, ER4 (EC-0.78 eV) and ER5 (EC-0.95 eV) at rates of 1510±300 and 3030±500 cm−1, respectively. Capture cross-section measurements revealed that electron capture kinetics of ER5 is similar to that of a line defect.

We have investigated by deep level transient spectroscopy the hole and electron trap defects introduced in n-type Ge during electron beam deposition (EBD) of Pd Schottky contacts. We have also compared the properties of these defects with those introduced in the same material during high-energy electron irradiation. Our results show that EBD introduces several electron and hole traps at and near the surface of Ge. The main defect introduced during EBD has electronic properties similar to those of the V–Sb complex, or E center, introduced during high-energy particle irradiation of Ge. This defect has two levels E0.38 and H0.30 that correspond to its (−−,−) and (−,0) charge states.

We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction ͑UED͒ experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations.

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We report on the electrical and defect characterization of Au Schottky diodes formed on single-crystal ZnO, before and after irradiating with high-energy (1.8 MeV) protons. Prior to bombardment we observed that several electron traps (E1-E4), with energies between 0.10 and 0.57 eV below the conduction band, are present in the ZnO. High-energy proton bombardment introduces two electron traps (Ep1 and Ep2), with extremely low introduction rates (η) of 2.4 and 1.9 cm −1 , respectively. Schottky barrier properties such as the reverse leakage current deteriorated from 1 × 10 −9 A for an unirradiated diode to 1 × 10 −6 A after bombarding it with a dose of 4.2 × 10 14 cm −2 protons. Compared to GaN we found that ZnO is remarkably resistant to high-energy proton bombardment.