Oxide films and process for preparing same

Details Oxide films and process for preparing same Oxide films and process for preparing same

1999-11-12

2002-01-29

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S699000, C428S697000, C428S702000, C428S469000, C428S471000

Reexamination Certificate

active

06342313

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention is directed to oxide films, such as zinc oxide (ZnO) films, for use in electrically excited devices such as light emitting devices (LEDs), laser diodes (LDs), field effect transistors (FETs), photodetectors, and transducers. More particularly, this invention is directed to oxide films containing a p-type dopant for use in LEDs, LDs, FETs, and photodetectors wherein both n-type and p-type materials are required, for use as a substrate material for lattice matching to other materials in such devices, and for use as a layer for attaching electrical leads.
For some time there has been interest in producing wide band gap semiconductors to produce green/blue LEDs, LDs and other electrical devices. Historically, attempts to produce these devices have centered around zinc selenide (ZnSe) or gallium nitride (GaN) based technologies. However, these approaches have not been entirely satisfactory due to the short lifetime of light emission that results from defects, and defect migration, in these devices.
Recently, because ZnO has a wide direct band gap of 3.3 eV at room temperature and provides a strong emission source of ultraviolet light, ZnO thin films on suitable supporting substrates have been proposed as new materials for light emitting devices and laser diodes. Undoped, as well as doped ZnO films generally show n-type conduction. Impurities such as aluminum and gallium in ZnO films have been studied by Hiramatsu et al. who report activity as n-type donors (
Transparent Conduction Zinc Oxide Thin Films
Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci. Technol. A 16(2), March/April 1998). Although n-type ZnO films have been available for some time, the growth of p-type ZnO films necessary to build many electrical devices requiring p-n junctions has to date been much slower in developing.
Minegishi et al. (
Growth of P
-
Type ZnO Films
by Chemical Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recently reported on the growth of nitrogen doped ZnO films by chemical vapor deposition and on the p-type conduction of ZnO films at room temperature. Minegishi et al. disclose the growth of p-type ZnO films on a sapphire substrate by the simultaneous addition of NEW in carrier hydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10 mol % was used, secondary ion mass spectrometry (SIMS) confirmed the incorporation of nitrogen into the ZnO film, although the nitrogen concentration was not precisely confirmed. Although the films prepared by Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear to incorporate a small amount of nitrogen into the ZnO film and convert the conduction to p-type, the resistivity of these films is too high for application in commercial devices such as LEDs or LDs. Also, Minegishi et al. report that the carrier density for the holes is 1.5×10
16
holes/cm
3
. The combined effect of the low carrier density for holes and the high value for the resistivity does not permit this material to be used in commercial light emitting devices or laser diodes.
Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producing thin films on substrates by doping IIB-VIA semiconductors with group VA free radicals for use in electromagnetic radiation transducers. Specifically, Park et al. describe ZnSe epitaxial thin films doped with nitrogen or oxygen wherein ZnSe thin layers are grown on a GaAs substrate by molecular beam epitaxy. The doping of nitrogen or oxygen is accomplished through the use of a free radical source which is incorporated into the molecular beam epitaxy system. Using nitrogen as the p-type dopant, net acceptor densities up to 4.9×10
17
acceptors/cm
3
and resistivities less than 15 ohm-cm were measured in the ZeSe film. The combined effect of the low value for the net acceptor density and the high value for the resistivity does not permit this material to be used in commercial devices such as LEDs, LDs, and FETs.
Although some progress has recently been made in the fabrication of p-type doped oxide films which can be utilized in the formation of p-n junctions, a need still exists in the industry for oxide films which contain higher net acceptor concentrations and possess lower resistivity values.
SUMMARY OF THE INVENTION
Among the objects of the present invention, therefore, are the provision of an oxide film containing a high net acceptor concentration on a substrate; the provision of a process for producing oxide films containing p-type dopants; the provision of a process for producing p-n junctions utilizing an oxide film containing a p-type dopant; the provision of a process for producing homoepitaxial and heteroepitaxial p-n junctions utilizing an oxide film containing a p-type dopant; and the provision of a process for cleaning a substrate prior to growing a film on the substrate.
Briefly, therefore, the present invention is directed to a ZnO film on a substrate wherein the film contains a p-type dopant. The film has a net acceptor concentration of at least about 10
15
acceptors/cm
3
, a resistivity less than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and about 50 cm
2
/Vs.
The invention is further directed to a process for growing a p-type ZnO film containing arsenic on a GaAs substrate. The GaAs substrate is first cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. After cleaning, the temperature of the substrate in the chamber is adjusted to between about 300° C. and about 450° C. and the excimer pulsed laser is directed onto a polycrystalline ZnO crystal to grow a film on the substrate. The temperature of the substrate coated with the film in the deposition chamber is then increased to between about 450° C. and about 600° C. and the substrate is annealed for a time sufficient to diffuse arsenic atoms into the film so as to produce a net acceptor concentration of at least about 10
15
acceptors/cm
3
in the film.
The invention is further directed to a process for growing a p-type zinc oxide film on a substrate. The substrate is first cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. After cleaning the substrate, the temperature of the substrate in the chamber is adjusted to between about 300° C. and about 450° C., and a p-type zinc oxide film is grown on the substrate by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant to grow a p-type zinc oxide film containing a net acceptor concentration of at least about 10
15
acceptors/cm
3
.
The invention is further directed to a process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 10
15
acceptors/cm
−3
. A substrate is loaded into a pulsed laser deposition chamber and cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. The temperature of the substrate in the deposition chamber is then raised to between about 300° C. and about 450° C. Subsequently a p-type ZnO film having a net acceptor concentration of at least about 10
15
acceptors/cm
3
is grown on the substrate by directing an excimer laser onto a pressed ZnO powder pellet containing the p-type dopant. Finally an n-type film is grown on top of the p-type film by directing an excimer laser beam onto a pressed ZnO pellet containing the n-type dopant.
The invention is further directed to a process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 10
15
acceptors/cm
−3
. A substrate is loaded into a pulsed laser deposition chamber and cleaned to ensure that the film will have a reduced number of defects and will properly adhere to the substrate. The temperature of the substrate in the deposition chamber is then raised to between about 300° C. and about 450° C. Subsequently an n-type film is grown on the substrate by directing an excimer pulsed

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