Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-11-05
2004-10-19
Trinh, Michael (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S103000, C438S069000, C438S795000, C438S799000
Reexamination Certificate
active
06806503
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a light-emitting diode and a laser diode capable of emitting ultraviolet light through current injection.
BACKGROUND ART
High-density recording media have been significantly developing in line with advances in information technologies. For example, read-write media in an optical recording system have been shifted from compact disks to digital video disks (DVDs) capable of recording in higher density. The reading and writing operations in such optical disks are performed through the medium of light. This implies the possibilities of higher recording density by use of light having a shorter wavelength.
From this point of view, as a semiconductor laser or laser diode (hereinafter referred to as “LD”), GaAlAs infrared-LDs for use in compact disks and GaInAlP infrared-LDs for use in DVDs have come into practical use. Further, various researches are carrying out toward the practical use of other LDs, such as a GaN light-emitting diode capable of emitting blue light having shorter wavelength.
Light-emitting diodes (hereinafter referred to as “LEDs”) are predominantly used as displays, and practical applications of GaAs, GaP and GaN LEDs have open the way for three-color display. Researches of an ultraviolet LED are also carrying out forward applications to a backlight for liquid crystal displays or a light source for bactericidal devices or ultraviolet-cure resins.
Zinc oxide (hereinafter referred to as “ZnO”) is known as one of luminescent materials emitting light having a shorter wavelength than that of GaN. ZnO is widely used as green-color fluorescent materials, for example, in low-energy-electron-impact type electroluminescence (EL) devices, and researches are also carrying out forward application to a transparent conductive film for solar cells by taking advantage of its high electrical conductivity and an optical transparency in a visible wavelength range.
It is known that ZnO is a direct transition type semiconductor having a band gap of about 3.38 eV at room temperature, and exhibits fluorescence in an ultraviolet wavelength range (about 350 nm at room temperature) by exciting with ultraviolet light. If light-emitting diodes or laser diodes can be fabricated using ZnO, such diodes would be able to use as a pumping source of fluorescent materials or high-density recording media.
DISCLOSURE OF INVENTION
(Problem to be Solved by the Invention)
Generally, it is required to join a p-type semiconductor to an n-type semiconductor to fabricate a light-emitting diode or laser diode. While an n-type ZnO thin-film can be fabricated without any difficulty, the technique of fabricating p-type ZnO thin-film involves many challenges. In fact, the first article concerning this technique was just reported in 1999 by Kawai, Osaka University, Japan. This article describes that a p-type ZnO thin-film can be achieved by preparing a target made of a sintered material containing Ga substituted for a part of Zn in ZnO and forming a film through a pulsed laser deposition (PLD) method under N
2
O gas so as to increase hole concentration of the film based on a co-doping effect.
However, any other research organizations have not been able to verify that the ZnO thin-film according to the above technique exhibits p-type semiconductor characteristics, at the time this application was filed. ZnO inherently tends to transform readily into an n-type semiconductor, and hardly fabricated as a stable p-type semiconductor. This complicates fabrication of LEDs to be actuated by current injection to its p-n junction.
It has not been reported any diode formed by joining an n-type ZnO semiconductor to a p-type ZnO semiconductor. SrCu
2
O
2
is one of p-type semiconductors suitable for joining to the n-type ZnO semiconductor. SrCu
2
O
2
is described as an indirect-transition type semiconductor having a band gap of about 3.2 eV at room temperature. To the contrary, calculations of its energy band suggest that it is a direct-transition type semiconductor. In addition, SrCu
2
O
2
exhibits p-type conductivity by adding K
+
ions (Kudo, Yanagi, Hosono, Kawazoe, APL, 73, 220 (1998)).
The article of Kudo et al. describes as follows.
The carrier concentration and mobility of a SrCu
2
O
2
thin-film fabricated through the PLD method are 1×10
−3
cm
−3
and 0.5 cm
2
/Vs, respectively. It has a pyramidal quadratic system (space group: 141/a) and a lattice constant of a=b=0.5480 nm and c=0.9825 nm. While the lattice matching between a ZnO (0001) surface and a SrCu
2
O
2
(112) surface is 19%, SrCu
2
O
2
can be heteroepitaxially grown on ZnO because quintuple of the lattice constant of SrCu
2
O
2
is approximately equal to sextuple of the lattice constant of ZnO. Further, they can be formed as a single crystal phase if a substrate has a temperature of 200° C. or more.
Kudo et al. confirmed that diode characteristics were yielded by forming a n-type ZnO film on a SrCu
2
O
2
film (Kudo, Yanagi, Hosono, Kawazoe, Yano, APL, 75, 2851). However, a ZnO film having desirable crystallinity could not be obtained because in the fabrication process of Kudo et al., the SrCu
2
O
2
film is formed on a substrate and then the ZnO film is formed on the SrCu
2
O
2
film. Specifically, for assuring the desirable crystallinity in a ZnO film, the substrate must be heated up to 500° C. or more, which leads to vanished diode characteristics. As a result, Kudo et al. could not confirm any luminescence from the diode.
CuAlO
2
and CuGaO
2
are also p-type semiconductors suitable for joining to the n-type ZnO semiconductor. CuAlO
2
discovered and reported by H. Kawazoe et al. (Nature, vol.389, p.939 (1997)) is a semiconductor having a so-called delafossite-type crystal structure and exhibiting p-type conductivity. CuAlO
2
has a band gap of 3.1 eV or more, and may provide a thin film having a resistivity of 1&OHgr;.
CuGaO
2
is also a semiconductor having a so-called delafossite-type crystal structure and exhibiting p-type conductivity. It is conceivable that these p-type transparent semiconductors have adaptability to fabrication of diodes, but there has not been any actual case of fabrication of diodes or light-emitting diodes from these materials.
(Means for Solving Problems)
The present invention provides a light-emitting diode comprising an n-type ZnO layer having desirable crystallinity and a p-type semiconductor layer selected from the group consisting of SrCu
2
O
2
, CuAlO
2
and CuGaO
2
. The p-type layer is formed on the n-type ZnO layer to provide a p-n junction allowing the ZnO layer to emit ultraviolet light.
The present invention further provides a method for producing a light-emitting diode. This method comprises the steps of forming an n-type ZnO layer on a transparent substrate having a temperature which allows the ZnO layer to be formed with desirable crystallinity, and forming on the ZnO layer a p-type semiconductor layer selected from the group consisting of SrCu
2
O
2
, CuAlO
2
and CuGaO
2
.
More specifically, according to a first aspect of the present invention, there is provided an ultraviolet-light-emitting diode comprising an n-type ZnO layer formed on a transparent substrate and exhibiting only intrinsic luminescence in the vicinity of a band gap thereof, and a p-type semiconductor layer selected from the group consisting of SrCu
2
O
2
, CuAlO
2
and CuGaO
2
. The p-type layer is formed on the n-type ZnO layer to provide a p-n junction therebetween.
In the light-emitting diode according to the first aspect of the present invention, the transparent substrate may be a single crystal substrate. This single crystal substrate may have an atomically flat yttria-stabilized zirconia (YSZ) (111) surface.
The light-emitting diode according to the first aspect of the present invention may further include a transparent electrode inserted between the transparent substrate and the ZnO layer. The transparent electrode serves as an electrode for the ZnO layer.
The light-emitting diode may include a Ni layer formed on the p-type semiconductor laye
Hirano Msahiro
Hosono Hideo
Kawamura Kenichi
Orita Masahiro
Ota Hiromichi
Japan Science and Technology Agency
Soward Ida M.
Trinh Michael
Westerman Hattori Daniels & Adrian LLP
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