High Quality Hydrothermal ZnO Crystals

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wnloaded from https://www.cambridge.org/core. IP address: 80.82.77.83, on 25 May 2018 at 20:46:07, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S109257830000260X

day runs, but growth was anisotropic: the ratio of growth rates between the fast (C+) and slow (C-) growth directions was 3 to 1. Wafers were sliced and the C+ and C- planes were identified by etching in nitric acid, following the method developed by Mariano and Hanneman. 3 Surface properties were analyzed by : double axis x-ray rocking curve with a high resolution four circle x-ray diffractometer; room temperature PL with a 0.25m grating monochrometer, ccd detector, and a 275nm, 5ns pulsed variable wavelength laser source; PL at 4 K with a 1.26m grating spectrometer and at 2.1 K with a high resolution 4.0m grating spectrometer, both equipped with an RCA C31034A photomultiplier and employing a HeCd laser. For this study, two different surface conditions or surface treatments were studied: a mechanical polish provided by a commercial service, and a chemomechanical polish performed by Eagle-Picher Corporation. We also compared our hydrothermal ZnO (grown at AFRL/Hanscom) to vapor-phase grown ZnO provided by Eagle-Picher. For this comparison, low temperature PL was used to evaluate the presence or absence of excitonic peaks. RESULTS AND DISCUSSION The crystal growth characteristics of hydrothermal ZnO are dictated by anisotropy between the two opposite surfaces of the basal plane. The opposite sides of a basal plane wafer have different atomic arrangements at their surfaces due to this anisotropy. That is, the C+ side comprises a Zn-rich layer, and the C- side comprises an O-rich layer. The resulting electronic charge distribution is illustrated in Fig. 1, adapted from the work of Mariano and Hanneman. 3 “Zn - Surface”

ZnO Single Crystal Zn

[0001]

BULK

BULK

O

O

Zn

Zn

O O

Zn

Zn

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BULK

[0001]

Zn O

O

Zn

O Zn

Zn O

Zn

O

O

“O - Surface”

Figure 1. Electronic Charge Distribution of Zinc Oxide. Typically, seeds were slices cut parallel to the basal plane to maximize the yield of (0001) wafers. The growth habit of hydrothermal ZnO crystals grown on basal plane seeds is shown in Fig. 2.4 For crystal growth in the [0001] direction or C+ sector, the growth rate was faster and the crystal color is clear to light yellow-green. For the [ 0001 ] direction or C- sector, the growth rate was about 1/3 as fast and the color is typically dark green. Slices were cut as shown from the light green and dark green sectors to distinguish between their chemical and electronic properties. Because of the different growth rates, we speculate that the different colors are due to a difference in impurity incorporation between the two sectors.

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