Method of fabricating nitride semiconductor device

Details Method of fabricating nitride semiconductor device Method of fabricating nitride semiconductor device

2003-01-31

2003-12-23

Mulpuri, Savitri (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

C438S046000, C438S483000

Reexamination Certificate

active

06667185

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method of fabricating a nitride semiconductor device such as a semiconductor laser diode expected to be applied to the fields of optical information processing and the like.
Recently, a nitride semiconductor of a group III-V compound, that is, a compound including nitride (N) as a group V element, is regarded as a promising material for a short-wavelength light emitting device due to its large energy gap. In particular, a gallium nitride-based compound semiconductor (Al
x
Ga
y
In
z
N, wherein 0≦x, y, z≦1 and x+y+z=1) has been earnestly studied and developed, resulting in realizing a practical blue or green light emitting diode (LED) device. Furthermore, in accordance with capacity increase of an optical disk unit, a semiconductor laser diode lasing at a wavelength of approximately 400 nm is earnestly desired, and a semiconductor laser diode using a gallium nitride-based semiconductor is to be practically used.
Now, a conventional gallium nitride-based semiconductor laser diode will be described with reference to a drawing.
FIG. 11
shows the sectional structure of the conventional gallium nitride-based semiconductor laser diode showing laser action. As is shown in
FIG. 11
, the conventional semiconductor laser diode includes a buffer layer
302
of gallium nitride (GaN), an n-type contact layer
303
of n-type GaN, a first cladding layer
304
of n-type aluminum gallium nitride (AlGaN), a first light guiding layer
305
of n-type GaN, a multiple quantum well (MQW) active layer
306
including gallium indium nitride layers having different composition ratios of indium (Ga
1−x
In
x
N/Ga
1−y
In
y
N, wherein 0<y<x<1), a second light guiding layer
307
of P-type GaN, a second cladding layer
308
of p-type AlGaN and a p-type contact layer
309
of p-type GaN successively formed on a substrate
301
of sapphire by, for example, metal organic vapor phase epitaxial growth (MOVPE).
An upper portion of the second cladding layer
308
and the p-type contact layer
309
are formed into a ridge with a width of approximately 3 through 10 &mgr;m. A lamination body including the MQW active layer
306
formed on the semiconductor substrate
301
is etched so as to expose part of the n-type contact layer
303
, and the top face and the side faces of the etched lamination body are covered with an insulating film
310
. In a portion of the insulating, film
310
above the p-type contact layer
309
, a stripe-shaped opening is formed, and a p-side electrode
311
in ohmic contact with the p-type contact layer
309
through the opening is formed over a portion of the insulating film
310
above the ridge. Also, on a portion of the n-type contact layer
303
not covered with the insulating film
310
, an n-side electrode
312
in ohmic contact with the n-type contact layer
303
is formed.
In the semiconductor laser diode having the aforementioned structure, when a predetermined voltage is applied to the p-side electrode
311
with the n-side electrode
312
grounded, optical gain is generated within the MQW active layer
306
, so as to show laser action at a wavelength of approximately 400 nm. The wavelength of the laser action depends upon the composition ratios x and y or the thicknesses of the Ga
1−x
In
x
N and Ga
1−y
In
y
N layers included in the MQW active layer
306
. At present, the semiconductor laser diode having this structure has been developed to show continuous laser action at room temperature or more.
It is generally well known that the growth temperature for growing a nitride semiconductor crystal by the MOVPE is changed in accordance with the composition ratio of a group III element introduced into gallium nitride (GaN).
It is reported that, in growing a semiconductor of, for example, gallium indium nitride (GaInN), nitrogen (N
2
) is preferably used as a material carrier gas with the growth temperature for the semiconductor set to approximately 800° C. (Applied Physics Letters, Vol. 59, pp. 2251-2253, 1991).
On the other hand, it is also known that the first and second cladding layers
304
and
308
and the first and second light guiding layer
305
and
307
not including indium are preferably grown at a growth temperature of 1000° C. or more with hydrogen (H
2
) used as a carrier gas.
The fabrication processes for these semiconductor layers are disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-196757 or 6-177423.
The outline of the processes will now be described with reference to FIG.
11
.
First, with hydrogen introduced onto a substrate
301
, the principal plane of the substrate
301
is subjected to a heat treatment at a temperature of approximately 1050° C. Then, after lowering the substrate temperature to approximately 510° C., ammonia (NH
3
) and trimethylgallium (TMG), that is, mutually reactive gases, are introduced onto the substrate
301
, so as to grow a buffer layer
302
. Thereafter, with the introduction of TMG stopped, the substrate temperature is increased to approximately 1030° C., and TMG and monosilane (SiH
4
) are introduced onto the substrate
301
with hydrogen used as a carrier gas, thereby successively growing an n-type contact layer
303
, a first cladding layer
304
and a first light guiding layer
305
, whereas trimethylaluminum (TMA) is additionally introduced as a group III material gas in growing the first cladding layer
304
.
Next, the introduction of the material gases is stopped, the substrate temperature is lowered to approximately 800° C., and the carrier gas is changed to nitrogen. Subsequently, trimethylindium (TMI) and TMG are introduced onto the substrate
301
as the group III material gases, thereby growing a MQW active layer
306
.
Then, the introduction of the group III material gases is stopped, the substrate temperature is increased to approximately 1020° C., and a group III material gas, that is, TMG and TMA if necessary, and cyclopentadienylmagnesium (Cp
2
Mg) including a p-type dopant are introduced onto the substrate
301
, thereby successively growing a second light guiding layer
307
, a second cladding layer
308
and a p-type contact layer
309
.
After growing the MQW active layer
306
, as a protection film for the active layer in increasing the temperature from 800° C. to 1020° C., a semiconductor layer of GaN is formed according to the description of Japanese Laid-Open Patent Publication No. 9-186363 or a semiconductor layer of Al
0.2
Ga
0.8
N is formed according to description of, for example, Japanese Journal of Applied Physics (Vol. 35, pp. L74-L76, 1996).
In general, the vapor phase epitaxial growth is conducted in an atmosphere of reduced pressure lower than the atmospheric pressure, the atmospheric pressure or increased pressure lower than approximately 1.5 atm.
A technique to suppress defects from occurring on an interface between a substrate and gallium nitride by growing gallium nitride on a substrate of sapphire by selective growth or the like is recently tried. It is reported with respect to this technique that gallium nitride with a flat face and high crystal quality can be obtained by conducting the vapor phase epitaxial growth under reduced pressure in particular.
As described so far, as a characteristic of growth of a gallium nitride-based semiconductor, different carrier gases are used in growing a layer including indium, namely, the MQW active layer
306
, and layers not including indium, such as the first cladding layer
304
and the first light guiding layer
305
. In general, nitrogen is used for growing the former layer and hydrogen is used for growing the latter layers.
Accordingly, in the fabrication of a semiconductor laser diode, particularly in forming a multilayer structure including double heterojunction layers sandwiching an active layer by the vapor phase epitaxial growth, it is necessary to change the carrier gas before and after forming the active layer. Also, the substrate temperature is changed at the same time. In changing the carrier gas, the i

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