A nitride semiconductor has been practically developed as a material of highly luminous blue and pure green LED to fabricate light sources of a full color LED display, a traffic signal, and an image scanner. Those LED devices has a basic structure comprising a substrate of sapphire, a buffer layer made of GaN, n-contact layer made of GaN doped with Si, an active layer made of a single quantum well (SQW) structure of InGaN or made of a multiple quantum well (MQW) structure containing InGaN, a p-cladding layer made of AlGaN doped with Mg, and a p-contact layer made of GaN doped with Mg, subsequently formed thereon. Such LED device has an excellent characteristics that at the forward current of 20 mA, the light emission wavelength is 450 nm, the output is 5 mW, and the external quantum efficiency is 9.1% in case of the blue LED, and the light emission wavelength is 520 nm, the output is 3 mW, and the external quantum efficiency is 6.3% in case of the green LED.
Such nitride semiconductor light emitting device adapts a double hetero-structure with an active layer of a single quantum well or a multiple quantum well structure having a well layer of InGaN.
Also in such nitride semiconductor light emitting device, since the multiple quantum well structure has a plurality of mini-bands which emit light efficiently even with a small current, the device characteristics such as the output of the multiple quantum well structure is expected to be improved as compared with that of the single quantum well structure.
A publication of, for example, the Japanese Laid Open patent publication of H10-135514 discloses a LED device comprising an active layer including a light emitting layer of the multiple quantum well structure with a barrier layer of undoped GaN and a well layer of undoped InGaN, and also including cladding layers with greater bandgap than that of the barrier layer of the active layer, in order to improve the light emission efficiency and the light emission output.
However where the active layer is formed of the multiple quantum well structure, since the total thickness of such active layer is thicker than that of the single quantum well structure, the serial resistivity along the vertical direction becomes high, and in turn, in case of LED device, the Vf (the forward voltage) tends to increase.
A publication of, for example, the Japanese Laid Open patent publication of H9-298341 discloses a technology to reduce the Vf, that is, a laser device comprising a p-side beam waveguide layer and a contact layer which are made of a superlattice structure having an InAlGaN layer over the active layer. This technology is based upon the idea that where, p-side nitride semiconductor layers containing In are formed of the superlattice structure, the carrier concentration of the p-region layers increases and the threshold current of the laser device decreases. However since a quaternary compound such as InAlGaN generally has a poor crystallinity, further it is difficult to make the nitride semiconductor containing In to be p-type, practically such LED device or LD device can be hardly fabricated.
As described above, although the multiple quantum well structure has been expected favorable to increase the luminous output because the light emitting output can be expected to be highly improved, it has been difficult to realize such expected effect of the multiple quantum well structure the active layer of.
It is to be noted that, as for the LD device, the present applicant has announced that a nitride semiconductor laser device with an active layer is successfully fabricated to achieve the continuous oscillation of ten thousands hours or more firstly in the world. (ICNS' 97 Subscript, Oct. 27-31, 1997, P444 to 446, and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L1568 to 1571, Part 2, No. 12A, 1 Dec. 1997)
However the LED devices used for the light source for illumination such as for the outside display exposed to direct sunshine require the lower Vf and the higher light emission efficiency than those of the conventional LED devices. Also the LD devices used for the light source such as for an optical pick-up require further improvement a lower threshold current to have a longer life.
Recently an another publication of, for example, the Japanese Laid Open patent publication of H8-97468 suggests that in light emitting device made of the nitride semiconductor, instead of a conventional p-contact layer on which a GaN p-electrode is formed; a contact layer made of InGaN of bandgap less than the GaN is grown, so that a barrier between the p-contact layer and the p-electrode is reduced thereby obtaining a good ohmic contact therebetween.
However it is difficult to grow the good crystalline layer of InGaN having less defects, thus the satisfactorily low ohmic contact can be hardly obtained as expected. Further there is an another problem that such contact resistivity of the grown InGaN layers is unstable because of the divergence of the crystallinity of the InGaN layer. Therefore the conventional nitride semiconductor device including a p-contact layer made of InGaN can hardly achieve the satisfactorily low and stable operating voltage and the high light emitting output. As the result, there is a problem, where the LED device comprises the contact layer made of InGaN, that the forward voltage at the forward current of 20 mA falls within a range of 3.4V through 3.8V which is not sufficiently low and also has a great divergence.
In addition, since the device made up of the nitride semiconductor, in its structure, the device may be easily damaged by an electrostatic voltage of 100V which is much lower than that people can feel, a sufficient attention should be paid in handling the device. Therefore in order to enhance the reliability of the nitride semiconductor device, it has been desired to further improve its electrostatic withhold voltage.