The present invention relates to semiconductor light-emitting elements such as, for example, ones suitable for transmission and display use.
In recent years, semiconductor light-emitting elements have been prevalently used for optical communication as well as information display panels. It is essential for these semiconductor light-emitting elements to have high light-emitting efficiency. It is further essential for semiconductor light-emitting elements for optical communication to have a high response speed, and they have recently been extensively developed.
A conventional surface-emitting type LED (light-emitting diode) is not so favorable in high-speed responsiveness, and around 150-200 Mbps is its limit. Thus, a semiconductor light-emitting element called xe2x80x9cresonant cavity type LEDxe2x80x9d has been developed. This resonant cavity-type LED is a semiconductor light-emitting element that controls spontaneous light emission by positioning a light-emitting layer at the loop of a standing wave that is generated within a cavity formed by two mirrors, to thereby realize high-speed response and high efficiency [see JP-A-3-229480, and U.S. Pat. No. 5,226,053].
Particularly, POFs (plastic optical fibers) have recently started to be utilized for high-speed communication that copes with a standard such as IEEE1394 or USB2, and a resonant cavity-type LED has been developed that uses, as a light-emitting layer, an AlGaInP type semiconductor material, which is enabled to emit light with high efficiency at a wavelength of 650 nm that is a low-loss waveguide region of the POF [High Brightness Visible (660 nm) Resonant-Cavity Light-Emitting Diode: IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, No. 12, December 1998]. This resonant cavity-type LED has a multi-quantum well structure, and barrier layers and well layers in the multi-quantum well structure were non-doped. That is, in the multi-quantum well structure, the barrier layers and the well layers had the same impurity concentration. This enables the resonant cavity-type LED to achieve a rise/fall time of about 3 ns, but a further improvement in the rise/fall time is requested because of the necessity of high-speed communication. For example, in order to accommodate to the standard of IEEE 1394S-200, it is required that the rise/fall time be not more than 1.6 ns.
It is an object of the present invention to provide a semiconductor light-emitting element superior in high-speed responsiveness.
In order to achieve the above object, in a semiconductor light-emitting element of the present invention, a first multi-layer reflective film, a quantum well light-emitting layer, and a second multi-layer reflective film are stacked on a semiconductor substrate in this order. The first and the second multi-layer reflective film, with a specified spacing between them, form a resonator within which a standing wave is generated, and the quantum well light-emitting layer is positioned at a loop of the standing wave within this resonator. The quantum well light-emitting layer has at least one well layer and barrier layers that sandwich the at least one well layer therebetween. The barrier layers have an impurity concentration higher than an impurity concentration of the well layer.
Through this specification, composition ratios y and z are independent between compound semiconductors.
According to the semiconductor light-emitting element with the above constitution, since the impurity concentration of the barrier layers is higher than that of the well layer, injected carriers are easily recombined in the barrier layers. This facilitates annihilation of injected carriers when a response signal is off. Thereby, when performing optical communication, for example, the fall time when a response signal is turned off becomes shorter. Accordingly, the response speed can be improved.
As described later, if the impurity concentration of the barrier layers is not more than a certain level, the optical output characteristics are not affected adversely.
In one embodiment, the impurity concentration of the barrier layers is set to 2xc3x971018 cmxe2x88x923 or more, whereby impurities that are to form the nonluminous recombination center increase. As a result, recombination of injected carriers is facilitated, thus making it possible to improve the response speed.
By setting the impurity concentration to 1xc3x971019 cmxe2x88x923 or less, diffusion of impurities to the well layer is prevented, thus making it possible to prevent a fall in the optical output due to the diffusion of impurities.
In one embodiment, the impurity concentration of the barrier layers is set to 5xc3x971018 cmxe2x88x923 or more, whereby impurities that are to form the nonluminous recombination center extremely increase. As a result, recombination of injected carriers is facilitated more, and the response speed can further be improved.
In one embodiment, the impurity concentration is in the well layer is 5xc3x971017 cmxe2x88x923 or less, so that an optical output can be increased without lowering light-emitting efficiency of the well layer, different from the case where high-concentration impurities are used for the well layer.
The impurity concentration of the well layer may be zero.
In one embodiment, the quantum well light-emitting layer is preferably composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
In this case, light emission at a wavelength of about 550 nm to 680 nm can be achieved by the quantum well light-emitting layer.
In one embodiment, the second multi-layer reflective film is preferably composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
In this case, the second multi-layer reflective film becomes transparent to light emission at a wavelength of about 550 nm or more, thus making it possible to take out light having a wavelength of about 550 nm or more efficiently.
In one embodiment, a current confinement layer is preferably formed on the second multi-layer reflective film.
In this case, the density of a current to be injected to a lower part of the current confinement layer increases, thus making it possible to further improve the response speed.
In accordance with a semiconductor light-emitting element of one embodiment, the current confinement layer is preferably composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
In this case, the current confinement layer becomes transparent to light emission at a wavelength of about 550 nm or more, thus making it possible to take out light having a wavelength of about 550 nm or more efficiently.
In accordance with a semiconductor light-emitting element of one embodiment, a diffusion layer is preferably formed on the current confinement layer.
In this case, if the current confinement layer is provided with an opening, for example, a current can uniformly be injected into the opening of the current confinement layer, thus making it possible to control the operation voltage at a low level.
In one embodiment, the current diffusion layer is preferably composed of AlyGazIn1xe2x88x92yxe2x88x92zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
In this case, the current diffusion layer becomes transparent to light emission at a wavelength of about 550 nm or more, thus making it possible to take out light emission at a wavelength of about 550 nm or more efficiently.
In one embodiment, preferably, the at least one well layer and/or the barrier layers are doped with any one of Si, Zn, Mg and Se.
Si, Zn, Mg and Se can easily be added by various crystal-growing methods.
In one embodiment, the semiconductor substrate is a GaAs substrate.