A Novel Technique for Growth of Lithium-free ZnO Single Crystals
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A Novel Technique for Growth of Lithium-free ZnO Single Crystals Shaoping Wang, Aneta Kopec, Andrew G. Timmerman Fairfield Crystal Technology, 8 South End Plaza, New Milford, CT 06776, USA ABSTRACT A ZnO single crystal is a native substrate for epitaxial growth of high-quality thin films of ZnO-based Group II-oxides (e.g. ZnO, ZnMgO, ZnCdO) for variety of devices, such as UV and visible-light emitting diodes (LEDs), UV laser diodes and solar-blind UV detectors. Currently, commercially available ZnO single crystal wafers are produced using a hydrothermal technique. The main drawback of hydrothermal growth technique is that the ZnO crystals contain large amounts of alkaline metals, such as Li and K. These alkaline metals are electrically active and hence can be detrimental to device performances. In this paper, results from a recently developed novel growth technique for ZnO single crystal boules are presented. Lithium-free ZnO single crystal boules of up to 1 inch in diameter was demonstrated using the novel technique. Results from crystal growth and materials characterization will be discussed. INTRODUCTION ZnO is a versatile wide band gap semiconductor with a great potential for semiconductor device applications. ZnO single crystal is a native substrate for epitaxial growth of high-quality thin films of ZnO-based Group II-oxides (e.g. ZnMgO, ZnCdO) for variety of semiconductor devices [1-4]. ZnO is unique because it has a very high excitation binding energy (60meV) enabling stability at high device operating temperatures, and it is highly resistant to radiation damage even compared to GaN. ZnO-based (LEDs and laser diodes) can emit light in a wide spectrum from UV to visible light. ZnO also has a large photo-response and a high photoconductivity, which leads to high-performance solar-blind UV sensors/detectors. ZnO’s attractive piezoelectric properties lead to high-performance ZnO-based surface acoustic wave (SAW) devices. Currently, commercially available ZnO single crystal wafers of 2 inches and 3 inches in diameter (and of (0001) orientation) are produced using a hydrothermal technique [5, 6]. Because of its ability of mass-producing large batches of ZnO single crystals, hydrothermal growth technique is a strong contender for commercial volume production of ZnO single crystals for wafers and substrates for semiconductor device applications. However, there are several drawbacks in hydrothermal growth technique for ZnO. The main drawback of hydrothermal growth technique is that the ZnO crystals contain large amounts of alkaline metals, such as Li, and K, (from LiOH and KOH used in hydrothermal growth). These alkaline metal elements are electrically active and hence can be detrimental to device performances. In fact, because Li impurities may act as an n-type dopant (when they are interstitials in a ZnO crystal lattice) or a ptype dopant (when they substitute Zn in ZnO crystals), hydrothermal-grown ZnO substrates with a high Li concentration (usually at about 5x1018cm-3 or higher) can significantly affect doping in
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