Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Fluid growth from gaseous state combined with preceding...
Reissue Patent
2002-09-18
2004-10-05
Whitehead, Jr., Carl (Department: 2813)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Fluid growth from gaseous state combined with preceding...
Reissue Patent
active
RE038613
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for growing a nitride compound semiconductor. More particularly, it relates to a new and improved method for growing a GaN or other nitride III-V compound semiconductor.
GaN, AlGaN, GaInN and other nitride III-V compound semiconductors have band gaps ranging from 1.8 eV to 6.2 eV, and they theoretically may be used to provide light emitting devices capable of emitting infrared to ultraviolet light and for this reason, these metal nitride compound semiconductors are being studied by engineers.
To fabricate light emitting diodes (LED) or semiconductor lasers using nitride III-V compound semiconductors, it is necessary to stack layers of AlGaN, GaInN, GaN, etc. and to sandwich a light emitting layer (active layer) between an n-type cladding layer and a p-type cladding layer.
To grow a p-type GaN layer, for example, by metal organic chemical vapor deposition or other vapor phase growth in a conventional technique, trimethylgallium (TMG, Ga(CH
3
)
3
) as Ga source, ammonia (NH
3
) as N source, and cyclopentadienyl magnesium (CP
2
Mg) as a p-type dopant, for example, are supplied onto a heated substrate, such as sapphire substrate, SiC substrate or GaAs substrate, in hydrogen (H
2
) carrier gas or mixed gas containing H
2
and nitrogen (N
2
), to grow a Mg-doped GaN layer by heat decomposition reaction. Since the Mg-doped GaN layer has a high resistance immediately after the growth, it is subsequently annealed in a vacuum or in an inert or inactive gas to change it into a p-type semiconductor layer. It is considered that the change into a p-type occurs because Mg in GaN is activated and releases carriers.
However, the carrier concentration of the p-type GaN layer obtained in the above-mentioned process is around only 3×10
17
cm
−3
, and the resistance still remains undesirably high. Therefore, the use of a nitride III-V compound semiconductor in a semiconductor laser presents some difficulties. A first problem attendant on the use of a p-type GaN layer as a contact layer for the p-side electrode arises because the p-type GaN layer has a high resistance and a large voltage loss may occur in the p-type GaN layer when the laser is operated with a high electric current. A second problem arises because of a low carrier concentration of the p-type GaN layer, which causes a contact resistance as high as 10
−2
cm
2
between the p-type GaN layer and the p-side electrode. This causes a voltage loss of approximately 10 V along the interface between the p-type GaN layer and the p-side electrode while the semiconductor laser is operated in a typical inrush current density, 1 kA/cm
2
, and causes a deterioration in laser characteristics. A third problem is that the need for an annealing step for activating the impurity after growth of the Mg-doped GaN layer causes an increase in the number of steps required to perform the manufacturing process.
The above problems concerning p-type GaN also apply to fabrication of a p-type layer of any nitride III-V compound semiconductor or, more generally, a nitride compound semiconductor, other than GaN.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method for growing a nitride compound semiconductor, which provides fabrication of a p-type nitride compound semiconductor with a high carrier concentration, which does not need to be annealed after growth of the semiconductor to activate the p-type impurity therein.
After considerable research directed to solving these conventional technical problems, it has now been discovered that a p-type nitride compound semiconductor with a high carrier concentration may be obtained by preventing generation of active hydrogen during formation of the semiconductor and that it is very important to select an appropriate nitrogen source material and an appropriate carrier gas.
In accordance with an embodiment of the invention, a carrier gas other than H
2
is used preferably because H
2
, when caught into grown crystal, inactivates Mg used, for example, as a p-type impurity. Suitable carrier gases which may be employed in accordance with this embodiment are inert or non-reactive carrier gases, such as, argon (Ar), helium (He) and Nitrogen (N
2
), for example. From the economical viewpoint, N
2
is most inexpensive.
In accordance with an embodiment, a nitrogen source should be used that does not release H
2
. This does not mean that the nitrogen source cannot contain a hydrogen
radical
group
(—H). When both a methyl
radical
group
(—CH
3
) and a hydrogen
radical
group
(—H) are contained in a single molecule, there is a high probability that they join and form stable methane (CH
4
). Therefore, an essential condition of molecules to be used as a nitrogen source material is that the number of hydrogen
radicals
groups
should be equal to or less than the number of methyl
radicals
groups
, per molecule of the nitrogen source. For example, trimethylhydrazine, ((CH
3
)
2
N—NH(CH
3
)) is decomposed as
(CH
3
)
2
N—NH(CH
3
)→N(CH
3
)
2
+N(CH
3
)→2N+C
2
H
6
+CH
4
(1)
and does not release hydrogen.
A nitrogen source material having ethyl (—C
2
H
5
)
radicals
groups
instead of methyl
radicals
groups
behaves somewhat differently. For example, a possible decomposition reaction of diethyl amine (HN(C
2
H
5
)
2
) is
HN(C
2
H
5
)
2
→NH
3
+2C
2
H
4
(2)
Another possible decomposition reaction is:
HN(C
2
H
5
)
2
→NC
2
H
5
+C
2
H
6
→NH+C
2
H
6
+C
2
H
4
(3)
These are decomposition reactions that release hydrogen. In Equations (2) and (3), ethylene is produced. The decomposition and formation of ethyl
radicals
groups
into ethylene is called &bgr;-elimination decomposition which is very liable to occur in organic metal compounds such as triethylgallium (TEG, Ga(C
2
H
5
)
3
). However, it is believed that &bgr;-elimination decomposition is unlikely to occur in ethyl compounds containing group V elements such as As and N (for example, Appl. Organometal Chem. vol. 5, 331(1991)). For these materials, the decomposition reaction proceeds as follows:
HN(C
2
H
5
)
2
→N+C
2
H
6
+C
2
H
5
(4)
and the reaction does not release hydrogen. Therefore, ethyl
radicals
groups
may be regarded the same as methyl
radicals
groups
, and also other alkyl
radicals
groups
may be regarded the same as methyl
radicals
groups
.
Aromatic ring hydrocarbons having phenyl
radicals
groups
(C
6
H
5
—) are hydrocarbons which do not exhibit &bgr;-elimination. Aromatic ring hydrocarbons are very stable, and need a large energy for their decomposition. Amines having a phenyl
radical
group
coupled to nitrogen, e.g. phenylmethylamine (C
6
H
5
—NH(CH
3
)), is decomposed in accordance with the reaction formula:
C
6
H
5
—NH(CH
3
)→C
6
H
5
—N+CH
4
→N+CH
4
+C
6
H
5
(5)
and does not release hydrogen. In this respect, phenyl
radicals
groups
may be regarded equivalent to methyl
radicals
groups
. The same also applies to higher order aromatic ring hydrocarbons, such as naphthalene. In general, however, they are disadvantageous because of a high vapor pressure.
Taking the above factors into account, nitrogen source materials suitable for obtaining p-type III-V compound semiconductors with a higher carrier concentration are NR
3
, NHR
2
, etc. as amine compounds, RN═NR, HN═NR, etc. as azo compounds, R
2
N—NH
2
, R
2
N—NHR, R
2
N—NR
2
, RHN—NRH, RHN—NR′H, RHN—NR′
2
etc. as hydrazine compounds, and R—N
3
, etc. as azide compounds, where R is an alkyl
radical
group
or a phenyl
radical
group
(—C
6
H
5
), and H is a hydrogen
radical
group
(—H).
In accordance with the present invention, in an embodiment, the group III element materials used for the growth a nitride III-V compound semiconductor also do not release hydrogen. Trimethylgallium (TMG, Ga(CH
3
)
3
), for example, does not release hydrogen during decomposition, and therefor
Asatsuma Tsunenori
Kawai Hiroji
Nakamura Fumihiko
Jr. Carl Whitehead
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
Thompson Craig
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