Method for manufacturing vertical GaN light emitting diodes

Details Method for manufacturing vertical GaN light emitting diodes Method for manufacturing vertical GaN light emitting diodes

2003-07-03

2004-11-16

Cuneo, Kamand (Department: 2829)

Semiconductor device manufacturing: process

Bonding of plural semiconductor substrates

Subsequent separation into plural bodies

C438S113000, C438S455000, C438S460000, C438S462000, C438S977000

Reexamination Certificate

active

06818531

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing vertical GaN light emitting diodes, and more particularly to a method for manufacturing vertical GaN light emitting diodes, from which an insulating sapphire substrate with low thermal conductivity is removed and in which a conductive substrate such as a silicon substrate is installed so as to have improved luminance and reliability.
2. Description of the Related Art
Generally, light emitting diodes (LEDs) are semiconductor elements, which emit light based on the recoupling of electrons and holes, and are widely used as various types of light sources in optical communication and electronic equipment. GaN serves as a compound for manufacturing blue-light emitting diodes.
Frequency (or wavelength) of light emitted from the light emitting diode is functionally related to a band gap of a semiconductor material to be used. When the band gap is small, photons with low energy and a longer wavelength are generated. In order to generate photons with a shorter wavelength, there is required a semiconductor material with a broader band gap.
For example, AlGaInP commonly used in lasers emits light corresponding to visible red light (approximately 600~700 nm). On the other hand, silicon carbide (SiC) and Group III nitride semiconductor materials such as gallium nitride (GaN) with a comparatively broad band gap emit light corresponding to visible blue light or ultraviolet rays. A short wavelength LED has an advantage in increasing a storage space of an optical storage (approximately 4 times as large as that of a general LED emitting red light).
The same as other Group III nitride semiconductor materials for emitting blue light, there is no practical technique for forming a bulk single crystal made of GaN. Accordingly, there is required a substrate suitable for growing a GaN crystal thereon. Sapphire, i.e., aluminum oxide (Al
2
O
3
), is typically used as such a substrate for growing the GaN crystal thereon.
However, a sapphire substrate has an insulating property, thus limiting the structure of a GaN light emitting diode. With reference to
FIG. 1
, the structure of a conventional GaN light emitting diode is will be described in detail.
FIG. 1
is a cross-sectional view of a conventional GaN light emitting diode
10
. The GaN light emitting diode
10
comprises a sapphire substrate
11
and a GaN light emitting structure
15
formed on the sapphire substrate
11
.
The GaN light emitting structure
15
includes an n-type GaN clad layer
15
a
, an active layer
15
b
formed to have a multi-quantum well structure, and a p-type. GaN clad layer
15
c
. Here, the n-type GaN clad layer
15
a
, the active layer
15
b
and the p-type GaN clad layer
15
c
are sequentially formed on the sapphire substrate
11
. The light emitting structure
15
may be grown on the sapphire substrate
11
using MOCVD (metal-organic chemical vapor deposition), etc. Here, in order to improve the lattice matching of the light emitting structure
15
and the sapphire substrate
11
, a buffer layer (not shown) made of AlN/GaN may be formed on the sapphire substrate
11
before the growing of the n-type GaN clad layer
15
a.
The p-type GaN clad layer
15
c
and the active layer
15
b
are removed at designated portions by dry etching so as to selectively expose the upper surface of the n-type GaN clad layer
15
a
. An n-type contact
19
is formed on the exposed upper surface of the n-type GaN clad layer
15
a
, and a p-type. contact
17
is formed on the upper surface of the p-type GaN clad layer
15
c
. A designated voltage is applied to the n-type contact
19
and the p-type contact
17
. Generally, in order to increase a current injection area while not negatively affecting luminance, a transparent electrode
16
may be formed on the upper surface of the p-type GaN clad layer
15
c
before forming the p-type contact
17
on the p-type GaN clad layer
15
c.
As described above, since the conventional GaN light emitting diode
10
uses the insulating sapphire substrate
11
, the two contacts
17
and
19
are formed on the sapphire substrate so that the contacts
17
and
19
are nearly horizontal with each other. Accordingly, when a voltage is applied to the conventional GaN light emitting diode
10
, a current flows over a narrow area from the n-type contact
19
to the p-type contact
17
via the active layer
15
b
in a horizontal direction. Since a forward voltage (V
f
) of the light emitting diode
10
is increased due to this narrow current flow, the current efficiency of the light emitting diode
10
is lowered and an electrostatic discharge effect is weak.
Further, the conventional GaN light emitting diode
10
emits a great amount of heat in proportion to the increase of the current density. On the other hand, the sapphire substrate
11
has low thermal conductivity, thus not rapidly dissipating heat. Accordingly, mechanical stress is exerted between the sapphire substrate
11
and the GaN light emitting structure
15
due to the increased temperature, thus causing the GaN light emitting diode
10
to be unstable.
Moreover, in order to form the n-type contact
19
, a portion of the active layer
15
b
with a size at least larger than that of the contact
19
to be formed must be removed. Accordingly, a tight emitting area is reduced, and the luminous efficiency according to the luminance relative to the size of the diode
10
is lowered.
In order to solve this problem, there is required a vertical light emitting diode. A method for manufacturing the vertical light emitting diode must comprise a step of removing a sapphire substrate from a GaN light emitting structure so as to form a contact layer on upper and lower surfaces of the vertical light emitting diode.
The sapphire substrate may be removed from the GaN light emitting structure using several conventional techniques. Since the sapphire substrate has a high strength, there is a limit to the ability to remove the sapphire substrate from the GaN light emitting structure using mechanical polishing. Further, the removal of the sapphire substrate from the GaN light emitting structure using a laser beam may cause damage to the GaN single crystal plane of the GaN light emitting structure due to the lattice mismatching and the difference of thermal coefficient of expansion (TCE) between the sapphire substrate and the light emitting structure during exposure to the laser beam.
More specifically, when the laser beam is irradiated on the lower surface of the sapphire substrate in order to remove the sapphire substrate from a GaN single crystalline layer, residual stress occurs due to the difference of thermal coefficient of expansion between the sapphire substrate and the GaN single crystalline layer, and the lattice mismatching thereof. That is, the thermal coefficient of expansion of sapphire is approximately 7.5×10
−6
/K, while the thermal coefficient of expansion of GaN single crystal is approximately 5.9×10
−6
/K. In this case, the rate of the lattice mismatching is approximately 16%. In case that a GAN/AlN buffer layer is formed on the sapphire substrate prior to the growing of the GaN single crystalline layer, the rate of the lattice mismatching is several percent (%). Accordingly, when the heat is generated by exposure to the laser beam, large-sized compressive stress is exerted on the surface of the sapphire substrate and large-sized tensile stress is exerted on the surface of the GaN single crystalline layer. Particularly, since the area of the irradiation of the laser beam is narrow (maximally 10 mm×10 mm), the laser beam is repeatedly irradiated on sectional areas of the sapphire substrate so that the laser beam can be irradiated on the entire surface of the sapphire substrate. Thereby, the level of stress becomes more serious, thus excessively damaging the surface of the GaN single crystalline layer.
As a result, the damaged GaN single crystalline plane drastically reduces the electric characteristics of the fin

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