Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Utility Patent
1998-12-17
2001-01-02
Tran, Minh Loan (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S103000, C257S745000, C257S751000
Utility Patent
active
06169297
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a metal thin film with an ohmic contact for light emit diodes and a method of producing such a film and, more particularly, to an improvement in a formation of ohmic contact in such a metal thin film during a process of producing blue light emit diodes using p-type gallium nitride (GaN)-based compound semiconductors, the improvement being for a formation of an ohmic contact having excellent thermal and electrical properties.
2. Description of the Prior Art
As well known to those skilled in the art, in order to produce optical devices, such as light emit diodes or laser diodes, it is primarily necessary to provide conditions for a formation of optimal ohmic contact at the junction between a semiconductor layer and a metal layer. Such conditions for the formation of optimal ohmic contact include smooth surface morphology, good thermal stability, simple processibility, low contact resistance, high production yield, good corrosion resistance, and good adhesion to III-V Group compound semiconductors.
One of the important factors, required to accomplish the above-mentioned conditions for a formation of optimal ohmic contact, is a diffusion barrier provided in a metal thin film deposited on a semiconductor layer. Such a diffusion barrier reduces a relative action between a metal capping layer and the semiconductor layer, thus providing a smooth surface morphology and improving the thermal stability of the resulting metal thin film. This effectively reduces a decomposition of the ohmic contact. Such a smooth surface morphology also improves the production yield of optical devices. An ohmic contact, having a low contact resistance, improves the operational efficiency of the resulting optical devices.
A prior art technique for a formation of such an ohmic contact in a metal thin film for optical devices may be referred to in U.S. Pat. No. 5,563,422, entitled “Gallium Nitride-Based III-V Group Compound Semiconductor Device and Method of Producing the Same”. In the above U.S. patent, an electrode, covered with a metal thin film, is used in place of a transparent electrode since some problems occur in an n-type ohmic contact due to nonconductive characteristics of a transparent substrate or a sapphire substrate. However, the technique, disclosed in the above U.S patent, is problematic in that the electrode, covered with the metal thin film, fails to allow light to effectively pass through. In Material Research Society Symposium Proceeding 449, 1061, 1997, T. Kim and et al. reported that a required specific contact resistance Rc for an optimal ohmic contact, formed in an Ni/Cr/Au model through a heat treatment process for 30 minutes and at 500° C., is 8.3×10
−2
&OHgr;cm
−2
. In the same Proceeding 449, 1093, 1197, J. T. Trexler and et al. reported that a required specific contact resistance for an optimal ohmic contact, formed in a Cr/Au model through a heat treatment process for 15 minutes and at 900° C., is 4.3×10
−1
&OHgr;cm
−2
.
In the prior art, there were a plurality of reference documents disclosing a formation of Ni or Pt ohmic contact in a metal thin film for optical devices. However, it is almost impossible to refer to prior art documents disclosing a formation of p-type gallium nitride (GaN) ohmic contact in a metal thin film.
Such a defective ohmic contact, formed in a metal thin film, causes serious problems in a continuous wave mode of gallium nitride light emit diodes (GaN LEDs) and laser diodes (LDs). In an effort to overcome the above-mentioned problems, the inventors of this invention have actively studied a formation of ohmic contact in a metal thin film by interposing a Pt layer between Ni and Au layers or an Ni layer between Pt and Au layers different from a typical formation of P-type ohmic contact in Ni/Au or Ni/Cr/Au models, thus forming an Ni/Pt/Au or Pt/Ni/Au thin film having good thermal, electrical and mechanical properties.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art. An object of the present invention is to provide an ohmic contact metal system, which is formed through a vapor deposition of a metal, such as nickel (Ni) or platinum (Pt) acting as an inter-diffusion barrier in a metal thin film, on a p-type gallium nitride (GaN) semiconductor layer different from typical Ni/Au or Pt/Au models, thus having good thermal, electrical and structural properties.
Another object of the present invention is to provide a new method of producing a metal thin film for LEDs, which forms such a metal thin film on a p-type GaN semiconductor layer through a vapor deposition.
A further object of the present invention is to provide an ohmic contact, which is formed in a metal thin film deposited on a p-type GaN semiconductor layer through either a vapor deposition of a platinum inter-diffusion barrier between nickel and gold layers or a vapor deposition of a nickel inter-diffusion barrier between platinum and gold layers different from typical Ni/Au or Pt/Au models.
Primarily, in order to form a P-type ohmic contact of the present invention, it is necessary to set the doping concentration of the P-type GaN semiconductor to be not lower than 10
18
cm
−3
. The quality of the ohmic contact, formed in a metal thin film, is mainly influenced by the doping concentration of the GaN semiconductor and is classified into three types. That is, when the doping concentration of the GaN semiconductor is lower than 10
17
cm
−3
, the ohmic contact results in a thermionic emission. When the doping concentration of the GaN semiconductor Is ranged from 10
17
cm
−3
to 10
18
cm
−3
, the ohmic contact results in a thermionic field emission. On the other hand, when the doping concentration of the GaN semiconductor is higher than 10
18
cm
−3
, the ohmic contact results in a field emission. When the doping concentration of the GaN semiconductor is increased, there is a flow of carriers in the metal thin film due to a tunneling effect regardless of a metal semiconductor contact barrier. An ohmic contact is thus naturally formed in the metal thin film for optical devices.
Secondarily, such a formation of P-type ohmic contact in a metal thin film requires use of a diffusion barrier, having an excellent thermal property. Such a diffusion barrier is an important factor for a formation of ohmic contact. The diffusion barrier is preferably made of a metal, such as nickel, platinum, or chrome. In a typical method of forcing a P-type ohmic contact, magnesium, used as a p-type dopant, forms an Mg—H compound during the growth of gallium nitride (GaN), thus causing a serious problem during the process of forming a GaN semiconductor layer having a high doping concentration. In such a case, the resulting GaN layer regrettably has a low doping concentration of about 10
17
cm
−3
. It is thus almost impossible to form a desired ohmic contact in the metal thin film due to such a low doping concentration.
A metal thin film with a nickel (Ni) ohmic contact may be formed as several models, such as Ni/Au, Ni/Cr/Au and Ni/Ni/Au films, even when the semiconductor layer has a low doping concentration. However, such models have a high specific contact resistance ranged from about 1×10
−2
ohm-cm
2
to about 8×10
−2
ohm-cm
2
. In addition, the thermal property of such models is regrettably reduced due to a diffusion of nickel, thus forming a poor surface morphology of a resulting metal thin film and deteriorating the production yield of the optical devices using the metal thin film.
However, the method of this invention produces a metal thin film for optical devices using both nickel as a diffusion barrier and platinum as an inter-diffusion barrier between nickel and gold. The metal thin film of this invention thus has a desired ohmic contact having a good thermal property and a smooth surface morphology. The specific contact resistance o
Jang Heung Kyu
Jang Ja Soon
Kim Hyo Keun
Park Seong Ju
Seong Tae Yeon
Greer Burns & Crain Ltd.
Kwangju Institute of Science and Technology
Tran Minh Loan
LandOfFree
Metal thin film with ohmic contact for light emit diodes does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Metal thin film with ohmic contact for light emit diodes, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Metal thin film with ohmic contact for light emit diodes will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2548047