Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
2001-01-05
2003-12-02
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S094000, C257S103000
Reexamination Certificate
active
06657300
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to the manufacture of III-V light emitting and laser diodes, particularly towards improving the characteristics of the electrical contact to the p-type portion of the diode.
BACKGROUND
Gallium nitride (GaN) compounds have wavelength emissions in the entire visible spectrum as well as part of the UV.
FIG. 1
illustrates a typical GaN-based light emitting diode (LED). Currently, most GaN-based LEDs are epitaxially grown on a sapphire or silicon carbide (SiC) substrate. A double hetero-structure that includes a nucleation layer, n-type layer, active region, p-type AlGaN layer, and a p-type layer of GaN is formed on the substrate. In general, the ability to fabricate ohmic contacts to the p-type layer is desirable for the realization of reliable light emitting diodes and laser diodes. Ohmic contacts to p-type GaN are difficult to achieve because the attainable hole concentration is limited for Mg-doped III-nitride based semiconductors. In addition, many light-emitting diodes and vertical cavity surface-emitting laser diodes use thin, transparent metal contacts. The choice of metals is limited and metal layers need to be thin, e.g. <15 nm, to reduce light absorption. Because there is poor lateral current spreading in p-type GaN, the metal layers typically cover nearly the entire device area.
P-type conductivity for GaN is achieved by doping with Mg, which substitutes for gallium in the GaN lattice and acts as an acceptor (Mg
Ga
). Mg
Ga
introduces a relatively deep acceptor level into the band gap of GaN. As a consequence, only ~1% of the incorporated Mg acceptors are ionized at room temperature. To illustrate, a Mg concentration ([Mg]) of ~5e19 cm
−3
is needed to achieve a room temperature hole concentration of ~5e17 cm
−3
. Further, Mg-doped GaN requires a post-growth activation process to activate the p-type dopants. The post-growth activation process may be, for example, thermal annealing, low-energy electron-beam irradiation, or microwave exposure. For conductivity-optimized Mg-doped GaN layers, [Mg]<5e19 cm
−3
, the acceptor concentration (N
A
) is about equal to the atomic Mg concentration and the resistivity can be around 1 &OHgr;cm or less. These layers may be referred to as “p-type conductive layers”. Increasing the Mg content beyond approximately 5e19 cm
−3
does not translate to higher acceptor concentration. Typically, a reduction of N
A
is observed when the [Mg] exceeds a certain maximum concentration and the layer becomes resistive.
SUMMARY
P-type layers of a III-nitride-based light-emitting device are optimized for formation of an Ohmic contact with metals. In some embodiments, a p-type transition layer is formed between a p-type conductivity layer and the metal contact. The p-type transition layer may be a GaN layer with a resisitivity greater than 7 ohm-centimeters, a III-nitride layer, a III-nitride layer with added As or P, or a superlattice with alternating highly doped or elemental dopant sublayers and lightly doped or undoped sublayers.
In some embodiments, the p-type layer is continuous with varying levels of dopant. The concentration of dopant in the region of the p-type layer adjacent to the p-contact is greater than the concentration of dopant in the region of the p-type layer adjacent to the active region. The p-type layer may also have a varying composition, for example of Al or In or both.
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Camras Michael D.
Chen Changhua
Chen Xiaoping
Christenson Gina L.
Goetz Werner K.
Chen Xiaoping
Crane Sara
Leiterman Rachel V.
Lumileds Lighting U.S. LLC
Patent Law Group LLP
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