Method of forming transparent contacts to a p-type GaN layer

Details Method of forming transparent contacts to a p-type GaN layer Method of forming transparent contacts to a p-type GaN layer

1999-06-08

2001-09-11

Chaudhuri, Olik (Department: 2814)

Semiconductor device manufacturing: process

Coating with electrically or thermally conductive material

To form ohmic contact to semiconductive material

C438S605000

Reexamination Certificate

active

06287947

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to optoelectronic devices, such as light emitting diodes and laser diodes, and relates more particularly to methods of forming contacts to light-emitting layers of optoelectronic devices.
BACKGROUND ART
Optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes, are solid state devices that generate light in response to excitation signals. Traditionally, the most efficient LEDs emit light having a peak wavelength in the red region of the light spectrum. However, a type of LED based on Gallium nitride (GaN) has been developed that can efficiently emit light having a peak wavelength in the blue region of the light spectrum. This LED may provide significantly greater light output than traditional LEDs. Moreover, since blue light has a shorter wavelength than red light, the blue light generated by the GaN-based LED can be readily converted to produce light having a longer wavelength. This efficient conversion increases the likelihood that marketable “white light” LEDs can be fabricated. GaN-based LEDs are also fabricated to generate green light.
In an exemplary known GaN-based LED, a light-emitting semiconductor structure is formed on a sapphire substrate. The semiconductor structure includes an n-type GaN region and a p-type GaN region. These two regions are epitaxially grown. Typically, metalorganic vapor phase epitaxy is used. The p-type GaN may be formed using magnesium (Mg) or zinc (Zn) as a dopant. Other layers may also be included. For example, a buffer layer may be formed between the sapphire substrate and the semiconductor structure to serve as a transition layer that promotes adhesion between the sapphire substrate and the GaN material. The buffer layer may be formed of aluminum nitride (AIN).
In operation, light is generated in response to applying an excitation signal to the p-type GaN region and the n-type GaN region. Thus, ohmic contacts must be formed on these two regions. A concern is that an acceptable low resistance ohmic contact for the p-type GaN material is difficult to fabricate. A number of different structural arrangements and fabrication methods have been tested for forming an acceptable contact region for the p-doped GaN region. Preferably, the contact is light transmissive, so that generated light may escape through the contact. Bi-metal light transmissive contacts are described in Japanese Laid-Open Patent Application (Kokai) Nos. 10-135515 to Shibata and 10-209500 and 10-209493 to Kamimura et al. While other materials are identified in the three references, the preferred materials for forming the contacts are either cobalt (Co) and gold (Au) or nickel (Ni) and Au. A first metal (e.g., Ni or Co) is deposited on the surface of the p-type GaN material. A second metal (e.g., Au) is then deposited on the first metal. The two metals are heat treated in an ambient containing oxygen, causing the first metal to oxidize and causing the second metal to penetrate the first metal and reach the GaN material. If the first metal is Ni, the heat treatment will form a phase of NiO, but the Au remains in the metallic state.
While the known light-transmissive contacts for p-type GaN layers of optoelectronic devices achieve acceptable performance with respect to transmissivity and lateral conductivity along the surfaces of the layers, further improvements are desired. What is needed is a method of forming a light-transmissive contact that has desirable optical and electrical characteristics when applied to a p-type GaN layer.
SUMMARY OF THE INVENTION
A method of forming a light-transmissive contact of a light source having a p-type Gallium nitride (GaN) layer includes, in one embodiment, introducing a selected metal in an oxidized condition, rather than oxidizing the metal only after it has been deposited on the surface of the p-type GaN. Under selected conditions, the oxidized metal contacts provide sufficient lateral conductivity (measured in the parameter V
f
) to negate the conventional requirement of a second highly conductive contact metal, such as gold (Au). Since the second metal tends to adversely affect the optical transparency of the resulting contact, the reduction of the contact structure to a single oxidized metal is desirable.
In this first embodiment in which the first metal is introduced to the p-type GaN surface in an oxidized condition, the oxidation may occur either prior to or during introduction of the material to the p-type GaN surface. However, the preferred method is to reactively evaporate or reactively sputter the metal onto the surface in an oxidizing environment. The metal is preferably nickel (Ni), or a Group II or transition metal. If a second metal is required, a precious metal is preferred, with gold (Au) being the most preferred metal. For example, Ni may be reactively evaporated or sputtered to form NiO, followed by Au evaporation or sputtering deposition and by an anneal. The anneal causes the Au to penetrate the oxidized metal and to fuse to the surface of the p-type GaN layer. If the temperature is sufficiently high (at least 550° C.), the anneal will activate the p-doping of the GaN layer. As an alternative that is consistent with this first embodiment, the first metal can be doped or infused with the second metal to provide the desired optical characteristics (e.g., a window) and the desired electrical characteristics (i.e., an ohmic contact). For example, a magnesium oxide (MgO) doped or infused with Au or silver (Ag) may provide the desired characteristics. The MgO may be co-deposited with the second metal using co-evaporation or premixing. As used herein, references to the oxides, such as NiO and MgO, are intended to represent all phases of the oxides (e.g., NiO
x
) and their stoichiometric deviations. Moreover, the term “layer” is intended to include systems of layers that cooperate to achieve desired characteristics. For example, the p-type GaN layer may be a series of p-doped GaN layers.
When the first metal is deposited as an oxide, such as NiO, and a highly conductive second metal is deposited on the transparent metal oxide, the heat treatment that causes diffusion of the second metal through the transparent metal oxide is typically implemented in a non-reactive environment. (However, there may be applications in which an oxygen-containing environment is advantageous.) An environment of N
2
may be employed. The optical transmissivity of the contact will be inversely dependent upon the cumulative amount of metallic species in the contact material and is dependent upon the transmissivity of the metal species. On the other hand, the lateral conductivity of the contact is directly dependent upon the amount of contiguous metal species in the contact structure. Thus, transmissivity and lateral conductivity are in conflict. That is, a minimal amount of metal is desirable for optical transparency, but a larger amount of metal provides an increase in lateral conductivity. These two considerations result in selection of an optimal range of layer thickness for the two metals, particularly the highly conductive metal (e.g., Au). Preferably, the first metal has a thickness of less than 150 Å, while the second metal has a thickness of less than 100 Å. The optimal thickness may differ depending upon the application. For example, in a typical device operating at 20 mA drive current, a 50 Å thick Au layer provides a V
f
ranging between 3.0 and 3.4 volts with 80% transmissivity at the wavelength of 500 nm. However, for a device in which power efficiency is less of a concern than maximizing luminous output for a given chip area, a V
f
ranging from 4.0 to 4.8 volts may be acceptable, so that the Au layer may have a thickness of 5 to 30 Å.
In a second embodiment, the oxidation occurs only after at least one of the two metals is deposited on the surface of the p-type GaN layer. The selection of the metals for forming the contact structure is fundamentally the same as the selection of materials for the first embodiment. One of the materials is primarily s

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