Low temperature solution
growth is an attractive method for the
preparation of nanostructured semiconductor materials with a wide
range of applications from optoelectronics to chemical sensing. Despite
the widespread application of low temperature solution growth, basic
phenomena taking place during the growth are still under debate. The
growth is mostly carried out in batch reactors, which are largely
scalable and convenient for applied research and industrial applications.
The batch reactors are filled with reactants and sealed, and there
is no further inflow of the reactants during the growth. As the growth
proceeds, the reactants are depleted, and the growth velocities decrease.
Conventionally, the growth process is analyzed in static conditions,
where the gradual depletion of the reactants in time is neglected.
We analyzed time evolution of the growth of ZnO nanorod arrays on
conventional sol–gel seed layers and on GaN substrates patterned
by focused ion beam lithography. The focused ion beam lithography
allows for precise control of the distances between the nanorods in
the arrays. We show that for short growth times the growth is reaction
limited, while for longer times the growth regime depends on the distance
between the nanorods and changes from reaction limited to diffusion
limited as the distance between the nanorods decreases. Under diffusion
limited growth conditions, the nanorod height depends on the position
within the pattern. The nanorods at the edge of the hexagonal pattern
with 19 nanorods in diameter are significantly taller than the nanorods
in the center. These experimental observations are validated by the
solution of the diffusion equation by a finite element method.
One dimensional ZnO nanostructures prepared by favorable and simple solution growth methods are at the forefront of this research. Vertically oriented ZnO nanorods with uniform physical properties require high-quality seed layers with a narrow size distribution of the crystallites, strong c-axis orientation, and low surface roughness and porosity. It has been shown that high quality seed layers can be prepared by the sol–gel process. The sol–gel process involves three essential steps: preparation of the sol, its deposition by dip coating, and thermal treatment comprising preheating and annealing. We put emphasis on the investigation of the heat treatment on the properties of the seed layers and on the vertical alignment of the nanorods. It was demonstrated that for the vertical alignment of the nanorods, the preheating step is crucial and that the temperatures reported in the literature have been too low. With higher preheating temperatures, conditions for the vertical alignment of the nanorods were achieved in both investigated annealing atmospheres in air and in argon.
We study the effect of thermal annealing on the electrical properties of the nanoscale p-n heterojunctions based on single n-type ZnO nanorods on p-type GaN substrates. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and on the substrates locally patterned by focused ion beam lithography. Electrical properties of single nanorod heterojunctions are measured with a nanoprobe in the vacuum chamber of a scanning electron microscope. The focused ion beam lithography provides a uniform nucleation of ZnO, which results in a uniform growth of ZnO nanorods. The specific configuration of the interface between the ZnO nanorods and GaN substrate created by the focused ion beam suppresses the surface leakage current and improves the current-voltage characteristics. Further improvement of the electrical characteristics is achieved by annealing of the structures in nitrogen, which limits the defect-mediated leakage current and increases the carrier injection efficiency.
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