Bulk ZnO Substrates Research Articles

ZnO/(Zn)MgO polar and nonpolar superlattices

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results.

Step-flow growth of homoepitaxial ZnO thin layers by halide vapor phase epitaxy using ZnCl 2 and H 2O source gases

The surface morphology of homoepitaxial (0 0 0 1)ZnO thin layers grown by halide vapor phase epitaxy (HVPE) using ZnCl 2 and H 2O source gases was investigated. Atomic force microscopy (AFM) observations showed that high temperature growth at 1000 °C with a low input H 2O/ZnCl 2 (VI/II) partial pressure ratio of 20 strongly promoted step-flow growth. X-ray diffraction (XRD) analyses revealed the crystalline quality of the homoepitaxial ZnO layer grown at 1000 °C with input VI/II ratio of 20 is comparable to that of bulk ZnO substrates. Under a constant input partial pressure of ZnCl 2, the growth rate of ZnO is constant within the temperature range 700–900 °C, and the growth rate decreases above 900 °C due to the shift of thermodynamic equilibrium of the growth reaction.

Nitrogen acceptors in bulk ZnO (0001¯) substrates and homoepitaxial ZnO films

Bulk single crystals of unintentionally doped ZnO having charge carrier concentration, ND−NA values of ∼1017 cm−3 were implanted with N+ ions at dosages of 1015 and 1016 cm−2 at 95 keV to a depth of 150 nm. The resulting p−n structure having acceptor concentrations ranging from 1017 to 1018 cm−3 was compared with nitrogen doped homoepitaxial films with ∼8×1017 cm−3 acceptors. Photoluminescence spectra acquired at 8 K showed an increase in the peak for the neutral donor-bound to acceptor-bound transition at 3.210 eV with increasing annealing temperature, thermal activation of a unique donor to acceptor transition due to nitrogen at 3.067 and 3.057 eV for implanted and epitaxial films, respectively; and an increase in the intensity of the defect-related green band at selected temperatures. Electroluminescence measurements at 300 K revealed an ultraviolet band, direct band-to-band recombination at 3.34 eV, donor-acceptor pair recombinations at 3.19 and 3.0 eV, and recombination in the green region centered at 2.49 eV. Current-voltage characteristics of implanted and homoepitaxial p−n diodes were also determined.

ITO ∕ Ti ∕ Au Ohmic contacts on n-type ZnO

The specific contact resistivity and chemical intermixing of indium-tin-oxide (ITO)∕Ti∕Au Ohmic contacts on n-type bulk ZnO substrates are reported as a function of annealing temperature up to 450°C. A minimum contact resistivity of 4.6×10−6Ωcm2 was obtained at 50°C and the value remained <10−5Ωcm2 for all temperatures up to 450°C, suggesting that the Ti is an effective diffusion barrier which prevents formation of Au–In solid solutions. Optical microscopy of the contacts shows roughening of the morphology after annealing at 450°C. Indium outdiffusion to the surface of the contact stack is significant by 350°C, and Zn outdiffusion is evident by 450°C. Both the In and underlying Ti become oxidized after these higher temperature treatments.

ZnO light-emitting diode grown by plasma-assisted metal organic chemical vapor deposition

We report a breakthrough in fabricating ZnO homojunction light-emitting diode by metal organic chemical vapor deposition. Using NO plasma, we are able to grow p-type ZnO thin films on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 1016–1017cm−3 and mobility of 1–10cm2V−1s−1. Room-temperature photoluminescence spectra reveal nitrogen-related emissions. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I=40mA and defect-related emissions in the blue-yellow spectrum range.

Electrical properties of undoped bulk ZnO substrates

Undoped bulk ZnO crystals obtained from Tokyo Denpa show either resistive behavior [(5×104)−(3×105) Ohm cm) or low n-type conductivity (n ⋍1014 cm−3) with mobilities in the latter case of 130–150 cm2/V sec. The variation in resistivity may be related to the thermal instability of Li that is present in the samples. The Fermi level is pinned by 90-meV shallow donors that are deeper than the 70 meV and hydrogen-related 35-meV shallow donors in Eagle Pitcher and Cermet substrates. In all three cases, 0.3-eV electron traps are very prominent, and in the Tokyo Denpa material they dominate the high-temperature capacitance-frequency characteristics. The concentration of these traps, on the order of 2×1015 cm−3, is about 20 times higher in the Tokyo Denpa ZnO compared to the two other materials. The other electron traps at Ec −0.2 eV commonly observed in undoped n-ZnO are not detected in conducting Tokyo Denpa ZnO samples, but they may be traps that pin the Fermi level in the more compensated high-resistivity samples.

CBE growth of high‐quality ZnO epitaxial layers

AbstractFurther improvements on the recently reported novel approach to zinc oxide Chemical Beam Epitaxy (CBE) are presented. Hydrogen peroxide is employed as a very efficient novel oxidant. ZnO layers with a thickness from 100 nm to 600 nm were grown on c‐sapphire using a MgO buffer. PL‐mapping as well as conductivity mapping shows a good uniformity across the 2 inch ZnO‐on‐sapphire epiwafers. The measured surface roughness for the best layers is as low as 0.26 nm. HRXRD measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO. The FWHM of the HRXRD (0002) rocking curves measured for the 2 inch ZnO‐on‐sapphire wafers is as low as 27 arcsec with a very high lateral homogeneity across the whole wafer. Plane view HRTEM observations reveal the very good quality of the ZnO films. The results indicate that CBE is a suitable technique to fabricate ZnO of very high structural quality, which can eventually be used as an alternative to bulk ZnO substrates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphology and Optical Properties of ZnO Nanorods Grown by Catalyst-assisted Vapor Transport on Various Substrates

ABSTRACTZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk ZnO(0001) substrates. Morphology studies showed that ZnO nanorods grew mostly perpendicularly to the GaN substrate surface, whereas a more random directional distribution was found for nanorods on Si. Optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN substrates. Raman spectroscopy revealed biaxial strain in the nanorod samples grown on Si. Conductive atomic force microscopy was applied to study I-V spectra of individual nanorods.

Ti ∕ Au n-type Ohmic contacts to bulk ZnO substrates

Electron-beam-deposited Ti∕Au ohmic contacts on undoped (n∼1017cm−3) bulk ZnO substrates exhibited as-deposited specific contact resistivity of 3×10−4Ωcm2, regardless of the polarity (Zn face or O face) of the ZnO substrate. The annealing environment (air or N2) also had no significant effect on contact properties. The specific contact resistivity slightly decreased after annealing at 300 °C but started to increase above 350 °C. The measurement temperature dependence of specific contact resistivity revealed that the dominant current transport mechanism is field emission even in the moderately doped ZnO. As the annealing temperature increased, some voids were observed on the metal surface, possibly due to reaction of Ti∕Au metallization and the evaporation of the oxygen from the ZnO substrate.

Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces

Bulk ZnO substrates were rapid thermal annealed in either air or N2 at temperatures up to 500°C. The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after 200°C anneals in N2 or 300°C in air. The Schottky barrier height, ΦB, obtained from Pt∕Au diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from 0.35eV in control samples to ∼0.30eV in those annealed at 400°C and an increase in the saturation current density (from 4Acm−2 in the control samples to 17–37Acm−2 in the annealed samples). These results show that the ZnO surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of ZnO devices. At much higher annealing temperatures (1000°C), the surface completely decomposes to leave metallic Zn droplets.

GaN epitaxy on thermally treated c-plane bulk ZnO substrates with O and Zn faces

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga2ZnO4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O- and Zn-face ZnO.

バルクＺｎＯ基板の特性とホモエピタキシャル成長

Correlation between the quality of bulk ZnO substrates and the properties of homoepitaxial ZnO films grown by metalorganic vapor phase epitaxy (MOVPE) has been investigated. Two kinds of commercial wafers grown by different methods have been employed for the investigation. One of them exhibited (i) mosaic structure revealed by xray diffraction (XRD), and (ii) room-temperature photoluminescence (PL) featured by strong LO phonon replicas of free exciton emission. This reflects a dominant phonon scattering by defects, and/or emissions from impurities or defects. The other substrate showed (i) fine single crystal from XRD, and (ii) free exciton emission from the PL data at room temperature. PL peak position of the homoepitaxial film on the former substrate was lower than that of the free exciton emission. For the homoepitaxial film on the latter, (i) the PL peak at room temperature appeared at the energy position of free exciton emission, and (ii) the surface morphology was smoother than that of the film on the former substrate. We found that crystal quality of substrates significantly influenced the ZnO homoepitaxial films, and therefore it is a key to choose a high quality bulk ZnO substrate for homoepitaxial growth to make high quality epilayers contributing to conductivity control and device applications.

Optical Properties of Bulk and Epitaxial ZnO for Waveguide Applications

ABSTRACTThis paper investigates the optical properties of bulk and epitaxial ZnO layers. High quality undoped and doped bulk ZnO crystals have been produced by melt growth techniques in addition to ZnO thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on silicon, sapphire and the bulk ZnO substrates. This work focuses on investigating the suitability of bulk and epitaxial ZnO for waveguide applications using various spectroscopic techniques. The photoluminescence showed the dominance of strong and narrow band due to the band edge emissions for undoped ZnO. Ultraviolet-visible transmission data revealed the variation of the bandgap with different doping elements. Raman spectra showed a narrow and strong peak, corresponding to the E2 mode at 438 cm-1, characteristic of the ZnO crystallinity. A broad 2LO peak appeared near 1150 cm-1 due to the coupling between LO phonons and free carriers. A clear variation in refractive index with doping was observed by spectroscopic ellipsometery suggesting that ZnO could be used for waveguide applications.

MOVPE growth of ZnO using various oxygen precursors

The II–VI compound semiconductor ZnO is already widely used for various applications such as UV resistive coatings, gas sensors and surface acoustic wave devices. Recently, the optoelectronic properties of ZnO are attracting increasing interest, which has been initiated by first reports on p-type doping. ZnO appears especially favorable for such applications because of its large exciton binding energy of about 60 meV, its low-power thresholds for optical pumping at room temperature and the availability of bulk ZnO substrates. Here, we report on the metal-organic vapor phase epitaxy growth of ZnO layers using diethylzinc as the zinc source and iso-propanol or tertiary-butanol as the oxygen precursors. The choice of an appropriate oxygen precursor is of great importance to minimize parasitic prereactions in the gas phase, that have a negative influence on layer quality. Epitaxial growth is conducted in a water-cooled, RF-heated horizontal quartz glass reactor. Growth rates up to 2.6 μm have been accomplished. The influence of the growth parameters as substrate temperature, reactor pressure, gas flow rates, and especially the impact of the II/VI-ratio on growth rates and quality of the epitaxial material will be discussed.

Growth mode control of ZnO toward nanorod structures or high-quality layered structures by metal-organic vapor phase epitaxy

In metal-organic vapor phase epitaxy (MOVPE), ZnO growth modes either for nanorod structures or for high-quality flat layers have been successfully controlled by varying the growth conditions. The nanorod structures of ZnO, e.g., 10−50 nm in diameter and 0.5−1 μm in height, were successfully grown on sapphire substrates at lower temperatures (e.g., 500°C). On the other hand, the uniform layers were formed at higher growth temperatures (e.g., 800°C). The best solution, at this stage, for flat epilayers with two-dimensional growth is the homoepitaxial growth on bulk ZnO substrates. The root mean square (RMS) roughness of the surface was minimized as 2 nm and the full-width at half-maximum of low-temperature (9 K) photoluminescence was as narrow as 0.5 meV, indicating the successful formation of high-quality ZnO grown by MOVPE.

MOCVD Growth of ZnO on Different Substrate Materials

ZnO layers have been grown by metal-organic chemical vapor deposition using diethylzinc and iso-propanol. Heteroepitaxial growth on c-plane sapphire and GaN templates as well as homoepitaxial growth on bulk ZnO substrates has been carried out at a growth rate of 0.95 μm/h. The photoluminescence at 5 K of all samples shows strong near band edge emission with line widths of 5–12 meV and phonon replicas can be identified, indicating the good optical quality. The full widths at half maximum of the θ – 2θ scans are between 100 and 270 arcsec depending on the substrate. The results suggest that the substrate material is limiting the quality of the ZnO layers.

Effects of oxygen plasma condition on MBE growth of ZnO

We report growth condition optimizations of ZnO films by plasma-assisted molecular beam epitaxy. Effects of oxygen plasma conditions on film quality were evaluated by photoluminescence, X-ray characterization and scanning electron microscopy. From the growth experiments on c-plane sapphire and bulk ZnO ( +c, −c ) substrates, it was suggested that the surface morphology is strongly related with the growth direction of ZnO parallel to its c-axis, and was found to be controllable by optimization of oxygen plasma conditions.