Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – Using an energy beam or field – a particle beam or field – or...
Reexamination Certificate
2001-05-07
2003-03-04
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
Using an energy beam or field, a particle beam or field, or...
C428S642000, C428S658000
Reexamination Certificate
active
06527858
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a p-type ZnO single crystal that has a resistivity lower than 100&OHgr;·cm, and a method for producing the same.
BACKGROUND ART
Heretofore, as to single crystal thin films of ZnO (zinc oxide) doped with an impurity, although low-resistance single crystal thin films (for example, of a resistivity lower than 100 &OHgr;·cm) have been obtained, as n-type ZnO single crystals, only high-resistance single crystal thin films (for example, of a resistivity of 100&OHgr;·cm or higher) have been obtained as p-type ZnO single crystals.
If a p-type ZnO single crystal thin film having a low resistance could be synthesized, semiconductor devices having a low resistance would be manufactured as injection-type light-emitting diodes, semiconductor lasers, or solar cells by combining with an n-type ZnO single crystal thin film having a low resistance. For example, a semiconductor laser diode that can oscillate in an ultraviolet region, which is required for high-concentration recording or for transmitting a large quantity of information, can be manufactured with ZnO.
Low-resistance n-type ZnO single crystals have been produced easily by doping B (boron), Al (aluminum), Ga (gallium), or In (indium). However, as to p-type ZnO single crystals, only those of high-resistance have been reported.
As to p-type ZnO single thin film, for example, Kasuga et al. of the Engineering Department of Yamanashi University reported at the 59th Meeting of the Applied Physics Society of Japan (Speech No. 17p-YM-8, Japanese Journal of Applied Physics, Part 2 (1 Nov. 1997) Vol. 36, No. 11A, p. 1453). This p-type ZnO single thin film was obtained by doping N (nitrogen). The resistivity of the obtained p-type ZnO thin film is as high as 100&OHgr;·cm, and is not suitable for practical use. Also, this p-type ZnO single thin film has problems such as that the electric conductivity inverses from p-type to n-type after annealing. Furthermore, the reproducibility of the experiment is poor.
Japanese Patent Laid-Open No. 10-53497 discloses a method for producing a p-type ZnSe (zinc serenide) single crystal having a low resistance. ZnSe is a II-VI compound semiconductor like ZnO. In this method, In (indium), an n-type dopant, is doped together with N to increase the solubility of N in ZnSe to 10
18
cm
−3
or more. However, a p-type ZnO single crystal having a low resistance cannot be obtained even if the same method is applied to ZnO. The reason is as follows:
Firstly, although both ZnSe and ZnO are II-VI compounds semiconductors, these have different crystal structures. ZnSe has a sphalerite structure, while ZnO has a wurtzite structure. Accordingly, the doping characteristics of p-type dopants are different. The solubility of N in ZnSe. is more than 10
18
cm
−3
, while the solubility of N in ZnO is less than 10
17
cm
−3
.
Also, it is reported (by T. Onodera, Associated Professor of Hokkaido University, H. Tabata, Associated Professor of Osaka University, et al.) that, while the solubility of Li (lithium) in ZnSe is low and Li is unstable in ZnSe crystals, the solubility of Li in ZnO is nearly 10%. However, only insulators have been obtained using p-type ZnO single crystals containing Li as the p-type dopant.
Secondly, due to difference in sizes between Se and O, the stabilities of ZnSe crystals and ZnO crystals are different, when these crystals are doped together with N, a p-type dopant, and In, an n-type dopant. In case of ZnSe, the radius of Se atoms to be substituted is larger than the radius of N atoms. In case of ZnO, the radius of O atoms to be substituted is almost the same as the radius of N atoms. Also, the radius of In atoms is about 15% larger than the radius of Zn atoms to be substituted.
Consequently, when ZnSe is doped with both N and In, since In atoms which are larger than Zn atoms enter in the places of Zn atoms (substitute for Zn atoms), and N atoms which are smaller than Se atoms enter the places of Se atoms (substitute for Se atoms), the crystal lattice of ZnSe is stabilized.
On the contrary, when ZnO is doped with both N and In, since N atoms which have almost the same size as O atoms enter in the places of O atoms (substitute for O atoms), the surroundings of In atoms, which are larger than Zn atoms and enter the places of Zn atoms (have substituted for Zn atoms), expands and results a significant strain of the crystal lattice of ZnO. Accordingly, to reduce the energy increase due to strain, oxygen is released to reduce the concentration. The area from which oxygen is released is called an “oxygen vacancy”. This phenomenon degrades crystallinity.
Consequently, since the order of element configuration at lattice points is disturbed, even if carriers are formed, these carriers are scattered in the crystal by dopants and oxygen vacancies. As a result, even if ZnO is doped with both N and In, the electric resistance of this p-type ZnO is high. In addition, since oxygen vacancies are vacancies of anionic elements, the formation of oxygen vacancies induces an electron-excess state, which may cause inversion to n-type.
As described above, ZnO requires the doping technology different from that for ZnSe, and the development of a novel doping method for incorporating p-type dopants into ZnO and stabilizing the incorporated p-type dopants has been demanded.
The object of the present invention is to provide a p-type ZnO single crystal having a low resistance, for example, a resistivity lower than 100&OHgr;·cm.
DISCLOSURE OF THE INVENTION
The present invention provides a p-type ZnO single crystal comprising a single crystal of zinc oxide (ZnO) that contains a p-type dopant composed of nitrogen (N), and an n-type dopant composed of any one or more selected from a group consisting of boron (B), aluminum (Al), gallium (Ga), and hydrogen (H).
This p-type ZnO single crystal is produced by doping a single crystal of ZnO with a p-type dopant composed of nitrogen (N), and an n-type dopant composed of any one or more selected from a group consisting of boron (B), aluminum (Al), gallium (Ga), and hydrogen (H), in such a manner that the concentration of the p-type dopant is higher than the concentration of the n-type dopant.
The mechanism of obtaining a p-type ZnO single crystal having a low resistance using this doping technology will be described below.
ZnO is a compound in which O, a hard base (electron-pair donor), and Zn, a medium (not hard but not soft) acid (electron-pair acceptor) are combined. In the present invention, therefore, N, an element acting as a hard base (hard basic element) was selected as the p-type dopant which substitutes for O, a hard base. Also, as the n-type dopant that was added together with N, elements acting as hard acids (hard acidic elements), i.e., B, Al, Ga, and H, were selected.
A hard basic element means a small element that has a large electronegativity and is difficult to polarize. A hard acidic element is an element that has electron cloud not easily polarized and includes small cations having a high electric charge. As known as the HSAB Principle (Principle of Hard and Soft Acids and Bases), a hard basic element causes a chemical reaction with a hard acidic element to form a stable compound easily.
In the present invention, by doping ZnO with a hard basic element (p-type dopant) together with a hard acidic element (n-type dopant) in accordance with this HSAB principle, a chemical reaction occurs between the p-type dopant and the n-type dopant, and both dopants become easily incorporated in the ZnO crystal. And, the both dopants incorporated in the ZnO crystal become chemically stable in the ZnO crystal. Furthermore, since the radii of B, Al, Ga, and H atoms are almost the same or smaller than the radius of a Zn atom, the above-described oxygen vacancies become difficult to be produced, thus the effect of stabilizing the crystal lattice can be obtained.
Although S (sulfur), Se (selenium), and Te (tellurium) are included in VI B group elements other than O (oxygen), all of these are soft basic elements. Therefore, in the
Yamamoto Tetsuya
Yoshida Hiroshi
Anderson Matthew
Pennie & Edmonds LLP
Rohm & Co., Ltd.
Utech Benjamin L.
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