K. Ip

Recent advances in processing of ZnO

A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

ZnO: growth, doping & processing

Abstract A review is given here of recent results in developing improved control of growth, doping, and fabrication processes for ZnO devices with possible applications to ultraviolet (UV) light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. ZnO can be grown on cheap substrates such as glass at relatively low temperatures and may have advantages over the GaN system in some of these applications.

Hydrogen incorporation and diffusivity in plasma-exposed bulk ZnO

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to 2H plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8×10−10 cm2/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case.

Effects of high-dose Mn implantation into ZnO grown on sapphire

ZnO films grown by pulsed-laser deposition on c-plane Al2O3 substrates were annealed at temperatures up to 600 °C to produce n-type carrier concentrations in the range 7.5×1015–1.5×1020 cm−3. After high-dose (3×1016 cm−2) Mn implantation and subsequent annealing at 600 °C, all the films show n-type carrier concentrations in the range 2–5×1020 cm−3 and room temperature hysteresis in magnetization loops. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn, and that factors such as crystalline quality and residual defects play a role.

Electrical characteristics of Au and Ag Schottky contacts on n-ZnO

Au and Ag Schottky contacts on the epiready (0001)Zn surface of bulk n-ZnO crystals show Schottky barrier heights of 0.65–0.70 eV from capacitance–voltage measurements, activation energies for reverse saturation currents of 0.3–0.4 eV and saturation current densities ranging from 10−5 A cm−2 on surfaces etched in HCl to 8×10−7 A cm−2 on solvent cleaned samples. The diode ideality factors were in the range 1.6–1.8 under all conditions. The properties of both the Au and the Ag Schottky diodes were degraded by heating in vacuum to temperatures even as low as 365 K. The degradation mechanisms during annealing were different in each case, with the Au showing reaction with the ZnO surface and the Ag contacts showing localized delamination. Mechanical polishing of the ZnO surface prior to contact deposition produced a high-resistivity damaged layer with prominent deep level defects present with activation energies of 0.55 and 0.65 eV.

Transport properties of phosphorus-doped ZnO thin films

The doping behavior of phosphorus in ZnO thin films grown by pulsed laser deposition is examined. The transport properties of epitaxial ZnO films doped with 1–5 at. % P were characterized via room temperature Hall measurements. As-deposited films doped with phosphorus are highly conductive and n type. The origin of the shallow donor level appears to be either substitution of P on the Zn site or formation of a donor complex. Annealing these phosphorus-doped films significantly reduces the carrier density, transforming the transport from highly conducting to semi-insulating. These results indicate that the phosphorus-related donor defect is relatively unstable, and suggests the formation of a deep level upon annealing. The latter is consistent with phosphorus substitution on the O site yielding a deep level in the gap.

Thermal stability of ion-implanted hydrogen in ZnO

The evolution of implanted 2H profiles in single-crystal ZnO was examined as a function of annealing temperature (500–700 °C) by secondary ion mass spectrometry. The as-implanted profiles show a peak concentration of ∼2.7×1019 cm−3 at a depth of ∼0.96 μm for a dose of 1015 cm−2. Subsequent annealing causes outdiffusion of 2H from the ZnO, with the remaining hydrogen decorating the residual implant damage. Only 0.2% of the original dose is retained after annealing at 600 °C. Rutherford backscattering/channeling of samples implanted with 1H at a dose of 1016 cm−2 showed no change in backscattering yield near the ZnO surface, but did result in an increase near the end-of-range from 6.5% of the random level before 1H implantation to ∼7.8% after implantation. Results of both cathodoluminescence and photoluminescence studies show that even for a 1H dose of 1015 cm−2, the intensity of the near gap emission from ZnO is reduced more than 2 orders of magnitude from the values in unimplanted samples. This is due to ...

Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition

The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.

Temperature-dependent characteristics of Pt Schottky contacts on n-type ZnO

The Schottky barrier height of Pt contacts on n-type (n∼1016 cm−3) thin film ZnO deposited by pulsed laser deposition was obtained from current–voltage measurements as a function of temperature. The resulting values ranged from 0.61±0.04 eV at 25 °C to 0.46±0.06 eV at 100 °C with saturation current densities of 1.5×10−4 A cm−2 (25 °C) to 6.0×10−2 A cm−2 (100 °C), respectively. The reverse current magnitude was larger than predicted by thermionic emission alone. The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material.

Carrier concentration dependence of Ti/Al/Pt/Au contact resistance on n-type ZnO

Ti/Al/Pt/Au ohmic contacts on n-type ZnO with a range of carrier concentrations (7.5×1015–1.5×1020 cm−3) show as-deposited specific contact resistances in the range from 3×10−4 to 8×10−7 Ω cm2. Temperature-dependent measurements showed that the dominant transport mechanisms were tunneling in the contacts in the most highly doped films and thermionic emission in the more lightly doped films. After annealing at 200 °C, the lowest specific contact resistance achieved was 2.2×10−8 Ω cm2. However, the contacts show evidence of reactions between the Ti and the ZnO film even for this low annealing temperature, suggesting that applications requiring good thermal stability will need metallurgy with better thermal stability.