Single crystal GaN substrate, method of growing same and...

Details Single crystal GaN substrate, method of growing same and... Single crystal GaN substrate, method of growing same and...

2002-09-19

2003-12-23

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

C438S022000, C438S024000, C438S042000

Reexamination Certificate

active

06667184

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a single crystal gallium nitride (GaN) substrate for producing blue light emitting diodes (LEDs) and blue light laser diodes (LDs) composed of group 3-5 nitride type semiconductors, a method of growing a single crystal gallium nitride substrate, and a method of producing a single crystal gallium nitride substrate.
This application claims the priority of Japanese Patent Applications No.2001-284323 filed on Sep. 19, 2001 and No.2002-230925 filed on Aug. 8, 2002, which are incorporated herein by reference.
Blue light emitting diodes (LEDs) based upon the group 3-5 nitride type semiconductors (InGaN, GaN) have been manufactured, sold and used on a large scale. Almost all the practical nitride type LEDs are made upon insulating sapphire (&agr;-Al
2
O
3
) substrates. Sapphire belongs to trigonal symmetry group (a=b=c, &agr;, &bgr;, &ggr;<120,≠90). GaN films and InGaN films are heteroepitaxially grown on a sapphire three rotationally symmetric plane for producing LEDs. On-SiC GaN type LEDs having a silicon carbide SiC substrate have been proposed and used on a small scale. On-sapphire LEDs made upon sapphire substrates have very high dislocation density of 10
9
to 10
10
cm
−2
. Despite great many dislocations, on-sapphire LEDs do not degenerate and enjoy a long lifetime.
Since low-cost techniques of manufacturing sapphire have been established, sapphire substrates are easily produced and are sales on the market at an inexpensive price. Sapphire is chemically stable, physically sturdy and rigid. Sapphire crystal plates have been most suitable for substrates of blue light emitting device chips. Sapphire will be favorably used as a substrate for making blue light LEDs and LDs in future.
Sapphire has, however, some drawbacks as a substrate. Sapphire lacks natural cleavage. Sapphire is an insulator. Lack of natural cleavage incurs a problem of chip-division. A device-fabricated sapphire wafer is cut and separated into individual device chips by mechanical dicing. The mechanical dicing lowers the yield and enhances the cost.
Insulating sapphire cannot lead electric current. A sapphire substrate cannot be an n-type substrate which carries an n-electrode at the bottom as a cathode. Then, InGaN-type LEDs are made by piling a thick n-GaN film on the insulating sapphire substrate, epitaxially growing n-GaN, n-InGaAs, p-GaN films, etching away a peripheral part of the epitaxial films from the top p-GaN film to the lowest n-GaN film, forming an n-electrode upon an exposed region of the n-GaN film, and forming a p-electrode on the top p-GaN film. Thus, on-sapphire devices must have a wide double-stepped shape. The intermediate n-GaN requires an extra area of a chip. Twice wirebondings are required for connecting n- and p-electrodes formed on upper layers with two lead pins. Extra etching and extra wirebonding increase steps and time of fabrication. The upper n-electrode curtails an effective area of a light emitting region. The extra area and the extra steps enhance the cost.
The above is drawbacks of sapphire as a substrate of an LED. Additional weak points appear as a substrate of an LD (laser diode). An LD requires a set of resonator mirrors for reflecting light reciprocally and amplifying light power by repetition of stimulation. Sapphire lacks cleavage. Resonator mirrors cannot be fabricated on on-sapphire LDs by cleavage. The resonator mirrors should be formed by mechanical polishing or etching which requires much time. The further weak point of the on-sapphire LDs is extremely high dislocation density. GaN, InGaN or AlGaN films grown on sapphire substrates have many dislocations of more than 10
9
cm
−2
. Despite high density of dislocations, InGaN LEDs emanate blue, green light with high efficiency and a long lifetime. But in the case of InGaN-laser diodes (LDs), excess high density of current flowing at a narrow area will degenerate LDs. Sapphire substrates have been the most prevalent substrates for InGaN LEDs till now. Sapphire, however, will not necessarily the most suitable substrates for InGaN-LDs in future.
2. Description of Related Art
The most suitable substrate for nitride type (InGaN) LDs and LEDs should be a GaN single crystal substrate which allows InGaN, GaN, AlGaN films to grow homoepitaxially. But, immaturity of crystal growth technology forbids device makers from obtaining wide, high quality GaN single crystal wafers till now. If high quality, wide GaN single crystal wafers can be manufactured, GaN single crystal wafers will be the optimum substrates for the nitride type LDs. GaN has advantages over sapphire. First of all, GaN has natural cleavage. Cleavability facilitates wafer-to-chip separation and enhances yield of the process. Resonator mirrors can be formed by the natural cleavage. An n-type GaN substrate has electric conductivity. The n-GaN substrate allows an LD or an LED to have an n-electrode at the bottom of a chip. The bottom n-electrode simplifies the device structure and widens the area of a light emanating region. There is no lattice misfit between the substrate and epi-films, which reduces the possibility of incurring inner stress and distortion. The lattice fitting will ensure a long lifetime for nitride type LDs.
However, it is impossible to make a melt of gallium nitride (GaN), since heating does not convert GaN polycrystals into a melt but sublimes GaN polycrystals into vapor. Thus, Czochralski method and Bridgman method which a melt polycrystal material into a melt, cool a part of the melt and make a large single crystal bulk solid at a thermal equilibrium, are unavailable for making a GaN single crystal. Somebody says that it may be possible to make a single crystal GaN bulk by heating under ultrahighpressure which forbids GaN from subliming. But, the allegation has not been confirmed. Even the ultrahighpressure would make a GaN melt, very small GaN crystals would be made by the melt of GaN. Such tiny crystal is no use for making a large diameter wafer of GaN.
A new method of making a thick GaN film on a foreign material substrate (e.g., sapphire) by vapor phase epitaxial growth method was proposed. It is an extension of a film growth method. However, a sapphire substrate which is chemically stable and physically rigid cannot be eliminated after the GaN film has been grown on the sapphire substrate. Thus, sapphire is not pertinent for the substrate for growing GaN films for the purpose of obtaining a freestanding GaN crystal. Recently trials have been done for eliminating sapphire substrates from grown GaN films by a laser. However, the separation of the sapphire substrates from the GaN films is difficult even by high power lasers.
Instead of the sapphire substrate, another candidate which can be eliminated from grown GaN films would be a GaAs substrate. A (111) plane of GaAs has three-fold rotation symmetry. A C-plane GaN film would be grown in vapor phase along c-axis on the (111) GaAs substrate. However, it is found that thick GaN is not grown upon a GaAs substrate. Perhaps differences of lattice constants and thermal expansions between GaAs and GaN cause the difficulty of growing thick GaN on the GaAs substrate. The lattice misfit and the thermal distortion induce large inner stress which forbids a GaN film from growing to a thick crystal. A breakthrough was required for making a thick GaN crystal in vapor phase.
The inventors of the present invention contrived a GaAs-based epitaxial lateral overgrowth method (ELO) for making low-dislocation GaN crystals by preparing a GaAs substrate, making an ELO mask having many small regularly-populated windows on the GaAs substrate, and growing GaN films by a vapor phase growing method on the ELO-masked GaAs substrate. The inventors had filed a series of patent applications based on the GaAs-based ELO methods for making GaN crystal bulks.
{circle around (1)} Japanese Patent Application No.9-298300
{circle around (2)} Japanese Patent Application No.10-9008
{circle around (3)} Japanese Patent Appli

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