Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-03-18
2004-08-10
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S002000
Reexamination Certificate
active
06773504
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an oxygen doping method into a gallium nitride crystal and an oxygen-doped n-type gallium nitride single crystal substrate for producing light emitting diodes (LEDs), laser diodes (LDs) or other electronic devices of groups 3 and 5 nitride semiconductors. Nitride semiconductors means GaN, InGaN, InAlGaN and so on which are grown as thin films on a sapphire substrate. An activation layer is a GaInN layer. Other parts are mainly GaN layers. Thus, the light emitting diodes based upon the nitride semiconductors are represented as GaN-LEDs or InGaN-LEDs which mean the same LEDs.
This application claims the priority of Japanese Patent Application No. 2001-113872 filed on Apr. 12, 2001 which is incorporated herein by reference.
2. Description of Related Art
Light emitting devices making use of nitride semiconductors have been put on the market as blue-light LEDs. At present, all of the available nitride light emitting devices are made upon sapphire substrates. An epitaxial wafer is obtained by growing a GaN film, a GaInN film and so forth upon a C-plane single crystal sapphire substrate heteroepitaxially. A unique n-dopant for GaN, AlInGaN, or InGaN thin films is silicon (Si). Silicon acts as an n-impurity in GaN by replacing a gallium site. A series of wafer processes produces GaInN-LEDs on the on-sapphire epitaxial wafer. A lattice constant of sapphire (&agr;-Al
2
O
3
) is different from that of gallium nitride. Despite the large lattice misfit, a sapphire substrate allows gallium nitride to grow heteroepitaxially on it. The on-sapphire GaN includes great many dislocations. In spite of the many dislocations, the GaN films on sapphire are stable and endurable.
Sapphire is a crystal of a trigonal symmetry group. C-plane of sapphire has quasi-three fold rotation symmetry. Gallium nitride belongs to hexagonal symmetry. C-plane of gallium nitride has perfect three-fold rotation symmetry. Since the symmetry groups are different for GaN and sapphire, any other planes than C-plane of sapphire cannot grow a GaN crystal. Thus, the GaInN-LEDs in use include sets of c-axis grown InGaN, InGaAlN or GaN thin films grown on C-planes of sapphire substrates.
All the GaN or GaInN thin films heteroepitaxially grown on the sapphire substrates are C-plane growing crystals. Sapphire substrates cannot make non-C-plane growing GaN crystals at all. Since sapphire has been a unique seed crystal for growing GaN until recently, it has been impossible to make a non-C-plane GaN crystal. At present, all the GaInN-LEDs and GaInN-LDs on the market consist of a pile of C-plane grown GaN, InGaN or AlInGaN thin films grown on C-plane sapphire substrates.
Large lattice misfit between sapphire and gallium nitride induces plenty of dislocations in a gallium nitride crystal grown on a sapphire substrate. Gallium nitride has rigidity as high as ceramics. The rigidity maintains the framework of crystals for a long time. Inherent dislocations in GaN crystals of LEDs do not enlarge by current injection unlike GaAs crystals. Since the dislocations do not increase, the GaN crystals on sapphire do not degrade. In spite of the great many dislocations, GaN-LEDs enjoy a long life time, high reliability and good reputation.
Sapphire substrates, however, have some drawbacks. Sapphire is a very rigid, hard crystal. Sapphire lacks cleavage. Sapphire is an insulator. Rigidity, non-cleavage and insulation are weak points of sapphire. When a plenty of device units have been fabricated upon a sapphire substrate wafer by wafer processes, the device-carrying sapphire wafer cannot be divided by natural cleavage like silicon wafers. The sapphire wafer should be mechanically cut and divided into individual device chips. The mechanical dicing step raises the cost.
The non-cleavage is not a serious obstacle for making LEDs (light emitting diodes) on sapphire substrates, since an LED has no resonator mirror. In the case of producing LDs (laser diodes) on sapphire substrates, the non-cleavage is a fatal drawback. A laser diode (LD) requires two mirrors at both ends of an active (stripe) layer as a resonator for amplifying light by injected current. It is convenient to form resonator mirrors by natural cleavage in a laser diode, because natural cleaved planes are endowed with flatness and smoothness. On-sapphire LDs prohibit natural cleavage from making resonator mirrors. Flat, smooth mirrors should be made on both ends of the laser chips by a vapor phase etching method, e.g., RIE (reactive ion etching), which is a difficult operation. Mirror-polishing should be done chip by chip after the wafer process has finished. Mirror-finishing of the resonators by the RIE is a main reason raising the cost of manufacturing the on-sapphire GaInN-LDs.
Another drawback results from the fact that sapphire is an insulator. Insulation prevents on-sapphire LEDs and LDs from having an n-electrode on the bottom. Sapphire forces LEDs and LDs to have extra n-type layers upon an insulating substrate but below an active layer. The n-electrode is formed by partially etching away a p-layer and the active layer, revealing the extra n-layer and depositing an n-electrode alloy on the n-layer. Both a p-electrode and the n-electrode are formed on the top surface of the LED or LD. Since electric current flows in the horizontal direction, the n-layer should have a sufficient thickness. It takes much time to eliminate a part of the p-layer and form an ohmic n-electrode on the revealed n-layer. An increase of the steps and time enhances the cost of the on-sapphire LEDs. Both the n-electrode and the p-electrode occupy a wide area on the top of the LED, which raises a necessary area of the LED. On-sapphire GaInN-LEDs which are prevailing cannot conquer the above drawbacks yet.
A gallium nitride (GaN) single crystal substrate would be an ideal substrate which has a probability of solving the drawbacks. Since thin films of GaN or GaInN are epitaxially deposited upon a substrate for producing blue light LEDs and LDs, a GaN bulk single crystal would eliminate the problem of lattice misfitting between the deposited films and the substrate. If an n-type bulk single crystal GaN substrate can be produced, an n-electrode can be formed on the bottom of the n-type GaN substrate. An allocation of a p-electrode at the top and an n-electrode at the bottom facilitates to produce LEDs, to bond the LEDs on packages, and to wirebond the LEDs to wiring patterns on the packages. The bottom n-electrode enables an LED to reduce the chip size.
Another advantage results from cleavability of a bulk GaN single crystal. A device-produced GaN wafer can be divided into stripe arrays of individual device (LED or LD) chips by natural cleavage. However, cleavage planes (1-100), (01-10) and (-1010) are parallel to three sides of an equilateral triangle defined upon a C-plane (0001) of GaN. The GaN crystal has not a square set of cleavage planes but a triangle set of cleavage planes. Square device (LED or LD) chips are produced by cutting a device-carrying GaN wafer partially by natural cleavage and partially by mechanical dicing.
Furthermore, an LD (laser diode) chip can produce resonator mirrors by natural cleavage. Replacement of the RIE by the natural cleavage reduces the cost of making GaInN-type blue light laser diodes (LDs).
However, there is no mineral containing gallium nitride single crystals. No attempt of making a wide, bulk GaN single crystal substrate artificially has succeeded until recently. Since a GaN single crystal substrate was inaccessible, it was not possible to make GaInN type LEDs or LDs on a single crystal GaN substrate until recently.
Recently, vapor epitaxial methods which can grow a GaN single crystal on a foreign material substrate have been proposed and improved. The methods are described as follows.
(1) Metallorganic Chemical Vapor Deposition Method (MOCVD)
The most prevailing method for making GaN crystals is a Metallorganic Chemical Vapor Deposition Method (MOCVD). The MOCVD produces a GaN crystal by placin
Motoki Kensaku
Ueno Masaki
Anderson Matthew
Norton Nadine G.
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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