Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2001-06-15
2002-11-05
Thompson, Craig (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
Reexamination Certificate
active
06475820
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
The present document is based on Japanese Priority Document JP 2000-182037, filed in the Japanese Patent Office on Jun. 16, 2000, the entire contents of which being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for growing a semiconductor layer made of a nitrogen-containing (also referred to as “nitride-base” hereinafter) III-V group compound, and a method for fabricating semiconductor light emitting elements using such method for growing a semiconductor layer.
2. Description of the Related Art
Nitride-base III-V group compound semiconductor typified by gallium nitride (GaN) (also referred to as “GaN-base semiconductor” hereinafter) is a promising material for a light emitting element capable of emitting light in a green to blue spectral region, and even in a ultraviolet region.
In particular, such GaN-base semiconductor has attracted a great deal of attention since a light emitting diode (LED) using thereof was put into practical use. A semiconductor laser using such GaN-base semiconductor has also been reported as successful, and is expected to be applied to an optical pick-up of a device (also referred to as an optical disk device) such as a DVD (digital versatile disk) such that performing read (reproduction) or write (recording) operation to or from an optical recording medium (also referred to as an optical disk) which optically stores information.
FIG. 4
is a perspective view showing such GaN-base semiconductor light emitting element (laser diode LD) having a general constitution fabricated on a sapphire substrate.
On a sapphire substrate
11
, a GaN-base semiconductor layer which includes an active layer
16
having a multiple quantum well structure is stacked, all of which compose a semiconductor stack
10
.
In such semiconductor stack
10
, a p-type cladding layer and an n-type cladding layer are formed so as to sandwich the active layer
16
, where a p-electrode
10
a
and an n-electrode
10
b
are formed so as to be respectively connected to such cladding layers. Since the sapphire substrate
11
is an insulating material, a semiconductor layer which is connected to the n-type cladding layer or an extended portion
10
c
of such n-type cladding layer per se is formed on the sapphire substrate
11
so as to projected out from the semiconductor stack
10
, and further thereon such n-electrode
10
b
is formed.
When a predetermined voltage is applied from a power source B between such p-electrode
10
a
and the n-electrode
10
b
, the active layer
16
within the semiconductor stack
10
emits laser light L.
FIG. 5A
is a sectional view showing in more detail a portion of the foregoing semiconductor stack
10
.
In such constitution, a buffer layer
12
typically made of GaN is formed on the sapphire substrate
11
, and further thereon an n-type GaN layer (contact layer)
13
of approx. 5.0 &mgr;m thick, an n-type AlGaN layer (cladding layer)
14
of approx. 0.5 &mgr;m thick, an n-type GaN layer (guide layer)
15
of approx. 0.1 &mgr;m thick, the active layer (light emitting layer)
16
having a multiple quantum well (MQW) structure typically made of GaInN, a p-type AlGaN layer (cap layer)
17
of approx. 0.02 &mgr;m, a p-type GaN layer (guide layer)
18
of approx. 0.1 &mgr;m thick, a p-type AlGaN layer (cladding layer)
19
of approx 0.5 &mgr;m thick, and a p-type GaN layer (contact layer)
20
of approx. 0.1 &mgr;m thick are stacked in this order.
As for the layers described in the above, an n-type impurity (donor impurity) to be doped into the n-type layers can be silicon (Si) or the like, and a p-type impurity (acceptor impurity) to be doped into the p-type layers can be magnesium (Mg), zinc (Zn) or the like.
FIG. 5B
is a schematic view showing a potential profile of such active layer
16
having a multiple quantum well structure.
Such quantum well structure is attained by a constitution of the active layer
16
in which layers individually having an indium (In) content of 2% and 8%, which differ in the potential, are alternatively stacked with each other.
A method for fabricating such GaN-base semiconductor light emitting element (laser diode LD) will be explained.
Fabrication of a light emitting element or the like using the GaN-base semiconductor requires such GaN-base semiconductor to be grown on a substrate made of sapphire, SiC or the like in a multi-layered manner. Typical methods for growing the GaN-base semiconductor include the metal-organic chemical vapor deposition (MOCVD) process and the molecular beam epitaxy (MBE) process, where the former is advantageous on the practical basis and is widely used since it does not require a high degree of vacuum.
In the above-mentioned MOCVD process, a substrate to be processed is placed in an MOCVD reaction chamber (reactor) typically made of quartz glass, to which ammonia (NH
3
) as a nitrogen source and other source materials such as gallium (Ga), aluminum (Al) and indium (In) depending on a GaN-base semiconductor to be grown are supplied together with a carrier gas, while being heated by, for example, an RF coil surrounding such reaction chamber, to thereby grow the GaN-base semiconductor on such target substrate housed in the reaction chamber.
A method for growing the multiple GaN-base semiconductor layers whose constitution is shown in
FIG. 5A
will be explained referring to
FIGS. 6A and 6B
.
As shown in
FIG. 6A
, the sapphire substrate
11
having the c-plane exposed on the surface thereof is subjected to thermal cleaning, and further thereon the buffer layer
12
typically made of GaN, the n-type GaN layer (contact layer)
13
of approx. 5.0 &mgr;m thick, the n-type AlGaN layer (cladding layer)
14
of approx. 0.5 &mgr;m thick and the n-type GaN layer (guide layer)
15
of approx. 0.1 &mgr;m thick are stacked by crystal growth in this order.
An n-type impurity (donor impurity) available for the doping into the foregoing n-type layers in the above process is represented by silicon (Si).
Next, as shown in
FIG. 6B
, the active layer (light emitting layer)
16
having a multiple quantum well (MQW) structure made of GaInN is formed through crystal growth by the MOCVD process on the n-type GaN layer
15
.
The p-type AlGaN layer (cap layer)
17
of approx. 0.02 &mgr;m, the p-type GaN layer (guide layer)
18
of approx. 0.1 &mgr;m thick, the p-type AlGaN layer (cladding layer)
19
of approx. 0.5 &mgr;m thick, and the p-type GaN layer (contact layer)
20
of approx. 0.1 &mgr;m thick are then formed on the active layer
16
in this order through crystal growth by the MOCVD process, to thereby obtain a structure shown in FIG.
5
A.
The p-Type impurities (acceptor impurity) available for the doping into the foregoing p-type layers in the above process include magnesium (Mg) and zinc (Zn).
In the successive process steps, the extended portion
10
c
of the n-type cladding layer as shown in
FIG. 4
is formed by etching, electrodes
10
a
,
10
b
are formed, and end planes for allowing laser oscillation are formed for example by etching, to thereby obtain a desired laser diode.
Such laser diode LD is produced by a crystal growth method such as the MOCVD process, and thus generally employs a sapphire substrate.
There is, however, a large lattice mismatch between the sapphire substrate and the GaN layer, which results in a large number of threading dislocation introduced into the semiconductor stack composed of the GaN layers, and ruins reliability of the obtained element.
A method for obtaining a high-quality crystal area with less threading dislocation has thus been proposed in which a GaN layer of approx. 1 to 3 &mgr;m thick (typically 2 &mgr;m) is grown on the sapphire substrate, etching the GaN layer so as to leave such GaN layer on the sapphire substrate projected in a ridge form, and a new GaN layer is laterally grown from the side planes of the individual ridges, to thereby deflect and converge the threading dislocation.
Such growth method for the semiconductor layer will be explained
Asatsuma Tsunenori
Hashimoto Shigeki
Nakajima Hiroshi
Sonnenschein Nath & Rosenthal
Thompson Craig
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