Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
2000-11-17
2004-01-13
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C251S096000, C251S097000, C251S200000
Reexamination Certificate
active
06677619
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a device provided with a nitride semiconductor In
x
Al
y
Ga
1−x−y
N (0=x, 0=y, x+y=1) including light emitting devices such as LED (light emitting diode), LD (laser diode) and SLD (super luminescent diode), solar cells, light receiving devices such as optical sensors and electronic devices such as transistors and power devices.
BACKGROUND OF THE INVENTION
Nitride semiconductors have been recently produced as materials used for a high bright blue LED and a pure green LED, a full color LED display and a traffic signal LED. Such LEDs are provided with an active layer of SQW (Single Quantum Well) or MQW (Multi Quantum Well) where the well layer is made of InGaN and positioned between a p-type nitride layer and an n-type nitride layer to form a DH (Double Hetero) structure. The wavelength of the blue or green light emitting from the active layer depends on a ratio of In in the InGaN well layer.
The inventors have first realized laser emitting by using the above nitride materials and reported it in Jpn. J. Appl. Phys. 35(1996)L74 and Jpn. J. Appl. Phys. 35(1996)L217. The laser device comprises the DH structure where the active layer is MQW having InGaN well layers and showed the following data:
Threshold current: 610 mA;
Threshold current density: 8.7 kA/m2;
Wavelength: 410 nm
(pulse width 2 &mgr;m and pulse cycle 2 ms) p The inventors have further improved the laser device and reported it in Appl. Phys. Lett. 69(1996)1477. The laser device comprises a ridge strip structure formed on a part of p-type nitride semiconductor and showed the following data.
Threshold current: 187 mA;
Threshold current density: 3 kA/m2;
Wavelength: 410 nm
(pulse width 2 &mgr;m, pulse cycle 2 ms and duty ratio: 0.1%)
The inventors have first succeeded in CW (Continuous-Wave) Oscillation or Operation at room temperature and reported it in Gijutsu-Sokuho of Nikkei Electronics issued on Dec. 2, 1996, Appl. Phys. Lett. 69(1996) and Appl. Phys. Lett. 69(1996)4056.
The laser diode showed a lifetime of 27 hours at 20° C. under the threshold current density of 3.6 ka/cm
2
the threshold voltage of 5.5 V and the output of 1.5 mW.
On the other hand, the blue and green LED of nitrides showed a forward current (If) of 20 mA and a forward voltage (Vf) of 3.4 to 3.6 V which are higher by 2 V or more than those of red LEDs made of GaAlAs semiconductors. Therefore, further decrease of Vf in the blue and green LED was required. Additionally, there was required an effective LD which can decrease the threshold current and voltage to get a longer lifetime of CW operation at room temperature, because the conventional LD still had a higher threshold current and voltage.
The inventors have gotten the idea that technology of decreasing the threshold in LDs was applicable to LEDs in order to decrease the Vf. Therefore, a first object of the present invention is to decrease the threshold current and voltage of nitride semiconductor LDs and realize a longer lifetime of CW operation.
In the specification, it should be understood that the general formulae: In
x
Ga
1−x
N and Al
y
Ga
1−y
N show chemical atoms which compose of nitride layers and therefore, even if different layers are represented by the same formula, the different layers do not necessarily have the same composition, that is, the same x or y does not mean the same ratio.
DISCLOSURE OF THE INVENTION
According to a first aspect of the present invention, there is provided a nitride semiconductor device comprising a p-type region comprising one or more p-conductivity semiconductor layers of nitride, a n-type region comprising one or more n-conductivity semiconductor layers of nitride and an active layer of a nitride semiconductor which is positioned between said p type region and said n type region, at least one layer of said p type region being a super lattice layer comprising first thin layers of nitride and second thin layers of nitride, said first layers having different composition from those of said second layers and the first and second thin layers being laminated alternately.
The super lattice structure can make the nitride layers improved in crystallinity and then make the nitride layers decreased in resistivity, resulting in smaller resistance of the p-type region and higher power efficiency of the device.
In the present invention, the p-type region means a region comprising one or more nitride semiconductor layers between an active layer and a p-electrode while the n-type region means a region comprising one or more nitride semiconductor layers between the active layer and an n-electrode.
According to a second aspect of the present invention, there is provided a nitride semiconductor device having an active layer made of a nitride semiconductor between the n-type region of one or more nitride semiconductor layers and the p-type region of one or more nitride semiconductor layers, at least one semiconductor layer in the p-type region or the n-type region is a super lattice layer made by laminating first layers and second layers which are made of nitride semiconductor, respectively, and have different constitutions from each other.
The super lattice structure can make the nitride layers improved in crystallinity and then make the nitride layers decreased in resistivity, resulting in smaller resistance of the n-type region and higher power efficiency of the device.
In a preferred embodiment of the first and second nitride semiconductor devices, the super lattice layer is made by laminating first layers which is made of a nitride semiconductor and has a thickness of not more than 100 angstroms and second layers which is made of a nitride semiconductor having different constitutions from the first layer and has a thickness of not more than 100 angstroms.
In order to keep or confine carriers in the active layer, at least one of the first and second layers is preferably made of a nitride semiconductor containing Al, especially Al
Y
Ga
1−Y
N (0<Y≦1).
In a second preferred embodiment of the first and second nitride semiconductor devices, for the super lattice, the first layer is preferably made of a nitride semiconductor represented by the formula In
X
Ga
1−X
N (0≦X ≦1) and the second layer is preferably made of a nitride semiconductor represented by the formula Al
Y
Ga
1−Y
N (0<Y≦1, X=Y≠0). According to the second embodiment, all the nitride layers have a good crystallinity, which results in improving output of the nitride semiconductor device (improvement of power efficiency). In LED or LD devices, the forward voltage (hereinafter referred to Vf) and also the threshold current and voltage can be lowered. In order to form a nitride layer having better crystallinity in the first and second semiconductor device, it is further recommendable that first layers of the super lattice structure are made of a nitride semiconductor represented by the formula In
X
Ga
1−X
N (0≦X<1) and said second layer is made of a nitride semiconductor represented by the formula Al
Y
Ga
1−Y
N (0<Y<1).
In the above first and second semiconductor devices, it is preferable that the first layer and the second layer are made of a nitride semiconductor and have a thickness of not more than 70, especially 40 angstroms, respectively, while said first layer and said second layer have a thickness of not less than 10, especially 5 angstroms, respectively. The thickness within the above range makes it easy to form Al
x
Ga
1−y
N (0<Y≦1), which layer is otherwise difficult to be formed with a good crystallinity. Especially, in case that the super lattice layer can be made as at least one layer of the p-type region between the p-electrode and the active layer and also as at least one layer of the n-type semiconductor region between the n-contact layer for current charging and the active layer, it is recommendable to get better effect that thickness of the first and second layer should be set within the above range.
In the above
Nagahama Shinichi
Nakamura Shuji
Senoh Masayuki
Andújar Leonardo
Flynn Nathan J.
Nichia Chemical Industries Ltd.
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