Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-04-08
2004-09-21
Nguyen, Hoang V. (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06795471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a nitride compound semiconductor laser (hereafter, may be simply referred to device) having a plurality of crystal layers of group III nitride compound semiconductor expressed by the formula (Al
x
Ga
1-x
)
1-y
In
y
N (0≦x≦1, 0≦y≦1), to which carriers are supplied from electrodes. More specifically, this invention relates to a group III nitride compound semiconductor laser that can emit laser light of wavelengths ranging from ultra-violet to blue and to a method for manufacturing the same.
2. Description of Related Art
A number of possible structures for semiconductor laser have been proposed over the years. Many of them include structures for limiting the area for current injection in the direction parallel to the pn junction, namely transverse direction, and those for confining light generated in the active layer in the transverse direction. Those structures typically break down into two types: the ridge-type, namely mesa-stripe type; and the inner stripe type, namely the internal current flow restriction type.
The ridge-type semiconductor laser has the so-called ridge-type structure in which a stripe-shape narrow bump is formed in a region above a p-type guide layer and a p-side electrode is formed on this bump. The device of this type needs a high-precision process of the ridge structure. This process consists of numerous steps and makes it difficult to improve the manufacturing yield of the device. This is because the dimensions of the ridge structure significantly influence the threshold current for oscillation and light-beam quality.
Meanwhile, Japanese Patent Kokai No. Hei. 11-261160 discloses a group III nitride compound semiconductor laser of an inner stripe type that has a pair of clad layers, an active layer sandwiched between the clad layers and a current constricting layer having a stripe-shape aperture serving as the path for current over the active layer. This current constricting layer is a highly resistant layer that is fabricated by heating an amorphous or poly-crystalline nitride compound semiconductor layer and then crystallizing this layer. This current constricting layer is made of GaN containing impurities at least 1×10
20
cm
−3
. The light is confined in the transverse direction by utilizing the light absorption effect relevant to the impurity energy states in this layer.
However, the clad layer over the aperture of the current constricting layer is regrown on the uneven (bumpy) underlying layer. As a result, when the p-type nitride compound semiconductor containing group II Mg as an acceptor impurity is regrown on the current constricting layer, the distribution of Mg concentration is not uniform in the semiconductor layer of the aperture and then its electric performance deteriorates.
In the case of nitride compound semiconductors, the satisfactory p-type conduction is realized when the Mg concentration is within a very limited range. Thus if there are fluctuations in the distribution of Mg concentration, the p-type conduction properties deteriorate.
In particular, when a p-type clad layer, which is usually Al
x
Ga
1-x
N:Mg (0.05≦x≦0.20), is regrown, an inhomogeneous distribution of Mg in the semiconductor layer of the aperture exerts seriously negative effects. That is, a potential barrier to an injection of carriers (in this case, holes) is developed unless the clad layer itself is a uniform p-type layer since the band gap of the clad layer is larger than that of the guide layer. Besides, the series resistance of the device increases due to the rise in the bulk resistance of the p-type AlGaN. Namely, if the semiconductor layer filling the aperture is the Mg-doped AlGaN clad layer, a degradation of the p-type conduction in this layer directly affects the current-voltage properties of the resulting device.
Operating current and voltage can be lowered in the inner stripe type laser since it provides both the restriction of current injection area and the light confinement in the transverse direction at the same time. It shows good performance in controlling the transverse mode of light and may be manufactured at a high productivity. Compared with the ridge type laser, the inner stripe type laser shows better performance in heat dissipation, providing a long life of use and high reliability. Despite these merits, as the aforementioned problems have not been solved, the inner stripe type semiconductor laser using group III nitride compound semiconductor is not popular yet. Only the ridge type group III nitride compound semiconductor laser has been successfully commercialized so far.
OBJECT AND SUMMARY OF THE INVENTION
This invention has been made to solve the problem that the conventional inner stripe type nitride compound semiconductor laser has poor current-voltage properties. An object of the present invention is, therefore, to provide an inner stripe type nitride compound semiconductor laser that can be driven at low current and voltage, easy to manufacture and stable during operation at the transverse mode of light.
The present invention provides a group III nitride compound semiconductor laser that has a pair of opposing guide and clad layers sandwiching an active layer and a current constricting layer located intermediate in a p-type guide layer.
The current constricting layer is made of AlN deposited at low temperatures between 400-600° C. and has a stripe-shape aperture that restricts the area through which current is injected to the active layer. Namely, the nitride compound semiconductor laser according to the present invention is a nitride compound semiconductor laser having a plurality of crystal layers made of a group III nitride compound semiconductor expressed by the formula (Al
x
Ga
1-x
)
1-y
In
y
N (where 0≦x≦1, 0≦y≦1), the plurality of crystal layers comprising an active layer-side guide layer which is adjacent to the active layer in the crystal layers of the group III nitride compound semiconductor and made of Al
x′
Ga
1-x′-y′
In
y′
N (where 0≦x′≦1, 0≦y′≦1), a current constricting AlN layer which is deposited on said guide layer and has a stripe-shape aperture, an electrode-side guide layer which is made of Al
x″
Ga
1-x″-y″
In
y″
N (where 0≦x″≦1, 0≦y″≦1) and deposited filling the aperture of the current constricting layer, and a clad layer which is made of Al
u
Ga
1-u-v
In
v
N (where 0≦u≦1, 0≦v≦1) and deposited on the electrode-side guide layer. The current constricting layer can block current effectively in the regions other than the aperture because the electric resistance of the AlN film deposited at low temperatures (400-600° C.) is very high.
In the nitride compound semiconductor laser according to the present invention, if the band gaps of the active layer-side guide layer, the electrode-side guide layer and the clad layer are represented by Eg
1
, Eg
2
and Eg
3
, respectively, their relations are Eg
1
≦Eg
2
≦Eg
3
.
The guide layer on the active layer side and the guide layer on the electrode side may have the same composition, Al
x
Ga
1-x-y
In
y
N (where 0≦x≦1, 0≦y≦1).
A semiconductor layer of Al
z
Ga
1-z
N (where 0.05≦z≦0.3) may be formed immediately above the active layer of the device in order to protect the active layer and prevent the overflow of electrons.
The film thickness of the current constricting AlN layer is 100-800 Å, preferably 200-600 Å in the present invention. Since the refractive index of the AlN layer is approximately 2.15 and smaller than those of the other regions, an effective step of the refractive index is provided that can confine light in the horizontal direction (transverse direction) parallel to the pn junction in the vicinity of the aperture of the current constricting layer.
If the AlN film becomes thinner than the lower limit, 100 Å, it becomes difficult to effectively confine the light in the transverse direction. The light co
Ito Atsuya
Kimura Yoshinori
Miyachi Mamoru
Ota Hiroyuki
Takahashi Hirokazu
Nguyen Hoang V.
Nguyen Tuan N.
Pioneer Corporation
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