The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

A comprehensive review of zno materials and devices

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

/pdf/fundamentals-of-zinc-oxide-as-a-semiconductor-1s4l8bh009.pdf

Fundamentals of zinc oxide as a semiconductor

Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

Stability of polymer solar cells.

Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

Defects in ZnO

A method for forming a conductive material on a substrate includes laser annealing a selected portion of a blanket coated material to form a conductive region.

System and method for forming conductive material on a substrate

E-beam deposited Ti/Al/Pt/Au contacts on undoped (n∼1017 cm−3) bulk ZnO showed minimum specific contact resistance, ρc, of ∼6×10−4 Ω cm2 after annealing at 250 °C. This value was essentially independent of the surface cleaning procedure employed, including sequential solvent cleaning or H2 plasma exposure. Higher annealing temperatures degraded the ρc, and Auger electron spectroscopy depth profiling revealed increasing intermixing of the metal layers. The Al outdiffuses to the surface at temperatures as low as 350 °C, and the contact metallization is almost completely intermixed by 600 °C.

Annealing temperature dependence of contact resistance and stablity for Ti/Al/Pt/Au ohmic contacts to bulk n-ZnO

GaN Schottky rectifiers employing guard-ring and SiO2 edge termination show almost ideal forward current characteristics, with ideality factor 1.08 and specific on-state resistance as low as 2.6×10−3 Ω  cm2. A maximum forward current of 1.72 A at 6.28 V was achieved under pulsed (10% duty cycle) conditions. The reverse breakdown voltage was inversely dependent on rectifier area. The presence of defects in the GaN still dominates the reverse leakage, with both field emission and thermionic field emission contributions present. The parallel-plane breakdown voltage is never reached, even with the use of multiple edge termination methods, but the results show the promise of GaN rectifiers for power conditioning and electric utility applications.

High current bulk GaN Schottky rectifiers

Electrical and magnetic properties, room temperature optical absorption bands, and 300 and 90K microcathodo luminescence (MCL) bands were studied in heavily Mn and Co doped (1–5at.%) bulk ZnO crystals. Optical absorption bands near 1.9eV (Co) and 2eV (Mn) and MCL bands near 1.84eV (Co) and 1.89eV (Mn) are found to be associated with transition metal (TM) ions. These bands are assigned to internal transitions between the levels of the substitutional TM ions. The temperature dependence of the resistivity of the ZnO showed activation energies of ∼35meV in all cases and the electron mobilities were decreased relative to the undoped material.

/pdf/properties-of-mn-and-co-doped-bulk-zno-crystals-3l2klzlcg5.pdf

Properties of Mn- and Co-doped bulk ZnO crystals

The carrier concentration dependence of Ti/Al/Pt/Au ohmic contact resistance on P-doped n-type ZnO thin films is reported. Ti (200 A)/Al (800 A)/Pt (400 A)/Au (800 A) was deposited by electron-beam evaporation on ZnO thin films grown by pulsed laser deposition on (0001) sapphire substrates using a ZnO:P0.02 source. Postgrowth annealing from 30 to 600 °C resulted in carrier concentrations of 7.5×1015 cm−3–1.5×1020 cm−3 in the ZnO. After metal deposition, the specific contact resistances were measured at temperatures in the range 30–100 °C prior to alloying annealing at 200 °C and at 30–200 °C after this anneal. The lowest specific contact resistance of 8.7×10−7 Ω cm2 for nonalloyed ohmic contacts was achieved in the sample with carrier concentration of 1.5×1020 cm−3 when measured at 30 °C. In the annealed samples, minimum specific contact resistances of 3.9×10−7 Ω cm2 and 2.2×10−8 Ω cm2 were obtained in samples with carrier concentrations of 6.0×1019 cm−3 measured at 30 °C and 2.4×1018 cm−3 measured at 200...

Specific contact resistance of Ti/Al/Pt/Au ohmic contacts to phosphorus-doped ZnO thin films

4H-SiC Schottky rectifiers were exposed to inductively coupled Ar plasmas as a function of source power (150–750 W), rf chuck power (75–350 W), and process pressure (5–30 mTorr) The reverse breakdown voltage (VB) was increased by increases in both incident ion energy and ion flux, with the former having the strongest influence As an example, Ar plasma exposure at ion energies of ∼335 eV led to an increase in VB from approximately −500 to −950 V These results suggest strategies for minimizing plasma-induced damage during both deposition and etching steps used in the fabrication of SiC rectifiers

Effects of Ar inductively coupled plasma exposure on 4H-SiC Schottky rectifiers

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