Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-07-05
2003-07-01
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S014000, C257S110000, C257S022000
Reexamination Certificate
active
06586762
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride semiconductor device which uses a nitride semiconductor (In
X
Al
Y
Ga
1-X-Y
N, 0≦X, 0≦Y, X+Y≦1) used in light emitting devices such as light emitting diode device (LED) and laser diode device (LD), light receiving devices such as solar cell and optical sensor or electronic devices such as transistor and power devices, and particularly to a nitride semiconductor device comprising nitride semiconductor layer which includes In.
2. Description of the Prior Art
Recently semiconductor laser devices which use nitride semiconductor have been receiving increasing demands for the applications in optical disk systems such as DVD which are capable of recording and reproducing a large amount of information with a high density. Accordingly, vigorous research efforts are being made in the field of semiconductor laser device which uses nitride semiconductor. Because of
0
the capability to oscillate and emit visible light over a broad spectrum ranging from ultraviolet to red, the nitride semiconductor laser, device is expected to have wide applications such as light sources for laser printer and optical network, as well as the light source for optical disk systems. The applicant of the present invention reported a laser which successfully underwent over ten thousand hours of operation under the conditions of continuous oscillation at a wavelength of 405 nm with output power of 5 mW at the room temperature.
Light emitting devices and light receiving devices which use nitride semiconductor have such a structure as a nitride semiconductor which includes In-is used for the active layer and, accordingly, it is important to form a better active region in the active layer in order to improve the device characteristics.
In the prior art, n-type nitride semiconductors doped with n-type impurities have beer used for the active layer of the nitride semiconductor device. Particularly in the case of a device of quantum well structure, the n-type nitride semiconductors doped with n-type impurities have been used in the well layer and, the barrier layer.
In order for light emitting devices which employ nitride semiconductors to have applications in wide fields, they must be further improved in the device characteristics, particularly in the device lifetime.
It is essential to have a longer lifetime and a higher output power for the laser devices which use nitride semiconductors in order to be used as the light source for reading or writing information in high-density optical disk systems described above and have further applications. Other classes of the nitride semiconductor device are also required to have a longer lifetime and a higher output power, and light emitting devices are required to have a higher output power of light emission.
Weak reverse withstanding voltage of the devices using nitride semiconductor, which has been a problem in the part art, has a high probability of leading to destruction of the device during handling in the manufacturing process and mounting on an end product, and is therefore one of the most important problems.
The present invention has been made in consideration of the problems described above, and aims at obtaining a nitride semiconductor device which has excellent device characteristics including the threshold current density and has longer device lifetime and high output power.
SUMMARY OF THE INVENTION
(1) A light emitting device according to the present invention is a type of nitride semiconductor device having a structure where an active layer of a quantum well structure, which comprises a well layer made of a nitride semiconductor that includes In, and a barrier layer made of a nitride semiconductor, is sandwiched by a p-type nitride semiconductor layer and an n-type nitride semiconductor layer, wherein the light emitting device according to the present invention is characterized in that the above active layer has a first barrier layer, that is arranged in a position nearest to the above p-type nitride semiconductor layer, and a second barrier layer, that is different from the first barrier layer, as the above barrier layer and is characterized in that the above first barrier layer does not substantially include an n-type impurity while the above second barrier layer includes an n-type purity. Here, though, barrier layers, other than the first barrier layer and the second barrier layer among the barrier layers in the active layer, are not particularly limited, in the case of usage as a laser device or as a light emitting device of high power, they are preferably doped with an n-type impurity or are not doped with any impurities.
Though, in a conventional multiple quantum well-type (hereinafter referred to as MQW-type) nitride semiconductor device, all the barrier layers are, in general, doped with an n-type impurity, such as Si, in order to enhance light emission efficiency by increasing the initial electron concentration in the active layer, a nitride semiconductor device of the present invention has a barrier layer, that is doped with an n-type impurity in the same manner as in the prior art, while an n-type impurity is not substantially included only in the first barrier layer that is nearest to the p-type nitride semiconductor layer. In such a structure, characteristics with respect to the device lifetime and the reverse withstanding voltage of the nitride semiconductor device can be improved.
Though the mechanism where the lifetime characteristic is improved is not necessarily evident, it can be inferred that, for one reason, the fact that the lifetime of the carriers has become longer than in the prior art contributes to this mechanism. Conventionally a barrier layer, that is doped with an n-type impurity, is arranged on the side of the p-type layer so that diffusion of the p-type impurity from the p-type layer occurs to quite a great degree and, thereby, a barrier layer, that includes an n-type impurity and a p-type impurity, is provided, which causes the lowering of the lifetime of the carriers. According to the present invention, since the first barrier layer is not doped with an n-type impurity, n-type and p-type impurities can be prevented from coexisting in the same barrier layer.
In addition, among barrier layers in the active layer, the barrier layer arranged on the side of the p-type layer (first barrier layer) does not substantially include an n-type impurity so as to have a function different from that of the barrier layer (second barrier layer), which has an n-type impurity, in the active layer. That is to say, by having the second barrier layer, the carriers injected from the n-type layer into the active layer are increased and the carriers that reach deep into the active layer (to the p-type layer side) are increased so that the injection efficiency of the carriers can be increased while, by having the first barrier layer, a barrier layer, in which an n-type impurity is not included, is arranged as a barrier layer nearest to the p-type layer in the active layer so that it becomes possible to increase the injection of the carriers from the p-type layer and also to improve the efficiency.
In the case that an n-type impurity is included in the first barrier layer, the injection of the carriers from the p-type layer tends to be blocked. In particular, the diffusion distance of the carriers from the p-type layer tends to be short in comparison with the carriers from the n-type layer and, therefore, when the first barrier layer, which corresponds to the entrance for the injection of the carriers from the p-type layer to the active layer, has an n-type impurity, the injection of the carriers from the p-type layer is negatively affected to a serious degree. As shown in
FIG. 14
, it is understood that the device lifetime is suddenly lowered as the n-type impurity concentration in the first barrier layer is increased.
Accordingly, by providing the first barrier layer in the active layer, it is observed that a great number of holes can be provid
Nelms David
Nguyen Thinh
Nichia Corporation
Nixon & Vanderhye
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