Chemical-etch-assisted growth of epitaxial zinc oxide
E. J. Adles and D. E. Aspnes
Department of Physics, NC State University, Raleigh, NC 27695-8202∗
(Dated: October 12, 2009)
We use real-time spectroscopic polarimetric observations of growth and a chemical model derived
therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire
by metalorganic chemical vapor deposition. Particulate zinc oxide formed in the gas phase is used
to advantage as the deposition source. Our real-time data provide unequivocal evidence that: a
seed layer is required; unwanted fractions of ZnO are deposited; but these fractions can be removed
by cycling between brief periods of net deposition and etching. The transition between deposition
and etching occurs with zinc precursor concentrations that only differ by 13%. These processes are
understood by considering the chemistry involved.
arXiv:0910.2199v1 [cond-mat.mtrl-sci] 12 Oct 2009
Zinc oxide (ZnO) is a transparent conducting oxide with a room-temperature band gap
of 3.37 eV. It is currently under intense investigation for magneto-optic applications and as
a cheap replacement for optical and optoelectronic devices currently depending on gallium
and indium. [1, 2] Bulk, epitaxial, and nanostructured ZnO have been grown by a vari-
ety of methods including metalorganic vapor phase epitaxy, metalorganic chemical vapor
deposition (MOCVD), molecular beam epitaxy, pulsed laser deposition, and vapor-liquid-
solid processes. [1, 2, 3] Of these processes MOCVD has several advantages, including the
production of high-quality films through fine-tuning of various processing parameters, sim-
pler and less costly equipment, scalability, and higher throughput compared to conventional
physical-vapor-deposition techniques. 
Here we address two issues that cause difficulties in the growth of zinc oxide by MOCVD.
The common zinc precursors diethylzinc (DEZ) and dimethylzinc readily react in the gas
phase with oxidizing species such as O2, NO, and N2O to create particles of sizes of the
order of hundreds of nanometers or larger (the so-called particle problem).  Adverse
effects of particulate ZnO in the gas phase range from deposition of poor-quality material to
the clogging of process lines leading to reactor downtime. Second, deposition of ZnO often
yields granular or columnar structures and surface roughness of a few to a few tens of nm.
 Some of the recent attempts to address these issues include the use of exotic precursors
such as Zn(TTA)2TMEDA and Zn(TMHD)2;[4, 6] alcohols as oxidizing agents; modified
reactor designs that separate precursors and effectively transform the growth process into
alternating layer epitaxy; atmospheric pressure deposition;[8, 9] and variation of the VI/II
In this work we report results obtained by real-time spectroscopic polarimetry that pro-
vide insights into the chemistry and growth of ZnO by MOCVD. We take advantage of
these insights to develop an efficient process where the consequences of the above difficul-
ties are minimized. Minimum modification of our MOCVD reactor is required. We show
that particulate ZnO in the gas phase, when properly managed, provides a useful source for
Our deposition system is a modified Emcore GS-3300 MOCVD reactor with an inte-
grated real-time spectroscopic polarimeter for in-situ analysis and control of growth. The
polarimeter measures the relative reflectance and the complex reflectance ratio from 240 to
840 nm at a 4 Hz rate, allowing growth to be followed on this time scale. Details of the
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