Active solid-state devices (e.g. – transistors – solid-state diode – Semiconductor is an oxide of a metal or copper sulfide
Reexamination Certificate
2002-03-08
2004-02-24
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Semiconductor is an oxide of a metal or copper sulfide
C257S040000, C438S085000, C438S086000
Reexamination Certificate
active
06696700
ABSTRACT:
This invention relates to p-type transparent thin film oxides.
Transparent semiconductors are special because they allow visible light to pass through but absorb ultraviolet (UV) radiation. Although n-type transparent semiconductors such as ZnO, SnO
2
, ITO (indium tin oxide) have been well developed, unfortunately very few p-type transparent semiconductors have been reported, and their conductivity is far below that of the n-type semiconductors. N-type transparent semiconductors can have a conductivity of 10
3
S·cm
−1
while also benefiting from transmittance values of up to 80%. This unbalanced development is a barrier to the success of transparent p-n junctions or devices, which makes significant applications impossible.
P-type transparent semiconductors are difficult to create due to strong localization of the upper edge of the valence band to oxide ions. Up to now, only very limited p-type transparent conducting oxide (TCO) thin films have been fabricated. Most of these films show either low conductivity or low transmittance. ZnO doped with nitrogen has a low conductivity of 10
−2
S·cm
−1
and has a brown color that permits poor light-transmittance. Another p-type oxide is In
2
O
3
—Ag
2
O, which has a conductivity of 10
3
S·cm
−1
with a low average transmittance of 20% in the visible range.
The most successful p-type TCO was reported by Kawazoe et al. (Nature 389, 939, 1997) to be a CuAlO
2
thin film with a conductivity of 0.95×10
−1
S·cm
−1
and a transmittance ranging from 27% to 52% in the visible light range. This film, CuAlO
2
of delafossite structure, was prepared from a bulk CuAlO
2
target by the laser ablation technique. Unfortunately, laser ablation is not a method employed by industry. Such a technique is successful in research, but not suitable for industrial mass production due to the small deposition area of the film on the substrate.
SUMMARY OF THE INVENTION
This invention provides a transparent Cu—Al—O semi-conducting film having a p-type conductivity greater than 0.95×10
−1
S·cm
−1
.
This invention also relates to a process for preparing a Cu—Al—O film having p-type conductivity, comprising:
a) carrying vapors of organo-copper and organo-aluminum precursors into a CVD chamber with an inert gas flow;
b) reacting and depositing the vapors on a substrate, preferably a light-transmitting substrate, through a chemical vapor deposition process.
Transparent semiconductors are special because they allow visible light to pass through but absorb ultraviolet (UV) radiation. Transparent p-n junctions may permit novel devices such as functional windows and ultraviolet-light emitting diodes.
DETAILED DESCRIPTION OF THE INVENTION
According to one aspect of the invention, there is a p-type transparent semiconductor formed on a substrate, the semiconductor comprising copper, aluminum, and oxygen, preferably fabricated by chemical vapor deposition of organo-copper and organo-aluminum precursors. In one preferred embodiment the organo-copper and organo-aluminum precursors are used in equimolar amounts.
Chemical Vapor Deposition (CVD) systems, including Metal Organic Chemical Vapor Deposition (MOCVD) systems, can be used for the film deposition of compounds upon the substrate. Through the variation of process parameters, the film obtained can be nano-structured, amorphous, polycrystalline or single crystalline, of which nano-structured is preferred. The MOCVD growth mechanism proceeds by the decomposition of organometallic precursors, such as organo-copper and organo-aluminum, with reactive gases, such as oxygen, at a heated surface of the substrate, such as quartz, on which they are to be deposited. The substrate surface is preferably heated. Good results have been achieved with substrate temperatures of 745 and 830° C. The gas distribution unit can include a plasma-generating electrode system for providing plasma-enhanced deposition. The metals deposit on the surface of the substrate, forming the desired compound and the undesired by-products are pumped away in a gaseous form. The film was successfully deposited at a rate of 2 nm/minute.
According to one aspect of the invention, the precursors are copper and aluminum salts comprising ligands of formula I:
where X is Cu or Al and n is 2 when X is Cu and 3 when X is Al, where R
1
, R
2
, R
3
, R
4
, R
5
and R
6
are independently selected from hydrogen, CH
3
, and fluorine. Preferred ligands include acetylacetonate (acac) where R
1
-R
6
each represent hydrogen, dipivaloylmethanate (dpm) where R
1
-R
6
each represent CH
3
, trifluoroacetylacetonate (tfa) where R
1
-R
3
each represent fluorine and R
4
-R
6
each represent hydrogen, pivaloyltrifluoroacetonate (pta) where R
1
-R
3
represent fluorine and R
4
-R
6
represent CH
3
, and hexafluoroacetylacetonate (hfa) where R
1
-R
6
each represent fluorine. Of these, acac and dpm are particularly preferred.
In another aspect, this invention relates to a process for preparing a p-type transparent semiconductor comprising copper, aluminum, and oxygen, comprising:
a) carrying vapors of organo-copper and organo-aluminum precursors into a CVD chamber with an inert gas flow;
b) reacting and depositing the vapors on a substrate, preferably a light transmitting substrate, through a chemical vapor deposition process.
The deposition is preferably carried out in a Plasma Enhanced Metal Organic Chemical Vapor Deposition (PE-MOCVD) apparatus. Oxygen is also supplied as reactive gas.
In one embodiment of this invention, the p-type transparent semiconductor film is further annealed in air, after the deposition. Annealing the semiconductor material leads to an increase in the conductivity, and to an increase in the transmittance of visible light of the material.
The morphology and the thickness of a fabricated p-type transparent semiconductor material can be examined using a field-emission scanning electron microscope, for example with a Phillips XL30 FEG-SEM. The estimated thickness values can be verified with the help of a stylus apparatus, such as an Alpha-step 500 surface profiler. The thickness of the semiconductor fabricated according to this invention can be controlled to be such as 100 nm or 250 nm.
The chemical composition of a film can be determined by energy dispersive X-ray spectroscopy (EDX) calibrated using samples of known composition. It is possible that the ratio of Cu to Al in the semiconductor will be different from the original ratio of Cu to Al present in the precursors, due to the differences in the vaporization speed of the organo-copper and the organo-aluminum precursors, and the differences in the deposition rates of the relevant species. A copper to aluminum molar ratio of 1:1 would be required for a pure CuAlO
2
material. The molar ratio of copper to aluminum in the transparent semiconductor of the invention is preferably lower than 1.3:1, more preferably lower than 1.2:1, and particularly preferably 1:1.
Transmission electron microscopy (TEM) can be used to determine the nature of copper found in a transparent semiconductor, TEM analysis of a semiconducting film produces an image and a diffraction pattern that contains a plurality of rings. These rings can be used to deduce lattice spacing values, which can then be matched with corresponding values for known copper aluminum oxides and copper oxides (see Table III, below), From the list of possible copper containing compounds obtained, a further restriction of possible compounds can be achieved by comparing the low-index planes of the copper aluminum oxides and copper oxides with the rings of the diffraction pattern.
In addition to the identification of phases, the TEM image can give information on the size of the crystallites in the semiconductor. In one embodiment of the invention, the crystallites were found to be below 10 nm. Such nanoscale particles and the small sample thickness result in X-ray powder diffraction (technique used to determine crystal composition) signals which are too weak for analysis.
Other analysis techniques such as XF
Gong Hao
Huang Lei
Wang Yue
Fahmy Wael
Klarquist & Sparkman, LLP
Le Thao X.
National University of Singapore
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