1. Field of the Invention
The present invention relates to improvements of a surface-light-emitting device including a light-generating layer and a light resonator constituted by two multi-film reflecting layers between which the light-emitting layer is interposed.
2. Discussion of Related Art
There is known a surface-light-emitting device including a semiconductor substrate and a plurality of semiconductor layers which are grown on the semiconductor substrate. The semiconductor layers comprise a light-generating layer (active portion), and a light resonator constituted by two multi-film reflecting layers (DBR: distributed-Bragg reflectors) which are located on the opposite sides of the light-generating layer, to reflect a light generated by the light-generating layer. The semiconductor layers further comprise an outermost semiconductor layer which is located on one of the opposite sides of a laminar semiconductor structure consisting of the above-indicated semiconductor layers, which one side is remote from the substrate. The outermost semiconductor layer has a light-emitting surface from which the light generated by the light-generating layer and resonated by the light resonator is emitted. Examples of such a surface-light-emitting device include a resonant-cavity light-emitting diode and a surface-light-emitting laser.
JP-A-2000-299492 discloses a light-emitting diode of quantum-well type, as an example of the surface-light-emitting device. This light-emitting diode includes a plurality of semiconductor layers laminated on a substrate. The semiconductor layers comprise a light-generating layer of a quantum-well structure having a thickness not larger than the wavelength (100 xc3x85, i.e., 10 nm) of the electron wave, and a light resonator consisting of two reflecting layers which are located on the respective opposite sides of the light-generating layer and which reflect a light generated by the light-generating layer. By applying an electric current between a pair of electrodes formed on the respective opposite surfaces of a laminar semiconductor structure consisting of the semiconductor layers, the light generated by the light-generating layer is emitted from one of the opposite surfaces of the laminar semiconductor structure which is remote from the substrate. In the light-emitting diode disclosed in the above-identified publication, an electron wave within the light-generating layer and an optical wave within the light resonator are coupled together, so that the light-generating layer generates a light only in a resonance mode. This phenomenon so-called xe2x80x9ca cavity QED effectxe2x80x9d permits the emitted light to have a high degree of directivity and a narrow line width, and prevents total reflection of the light by the crystal surface, assuring an advantage of a high degree of external quantum efficiency.
A known surface-light-emitting device as described above includes, for instance, an n-GaAs semiconductor substrate, and a first multi-film reflecting layer, a light-generating layer and a second multi-film reflecting layer which are successively grown on the substrate. The first multi-film reflecting layer consists of a multiplicity of n-(Al)GaAs/Al(Ga)As films superposed on each other such that the (Al)GaAs films and the Al(Ga)As films are alternately arranged. The ratios of the elements indicated in the parentheses are suitably selected, and may be zero. The second multi-film reflecting layer consists of a multiplicity of p-(Al)GaAs/Al(Ga)As films superposed on each other such that the (Al)GaAs films and the Al(Ga)As films are alternately arranged. The parentheses have the same meaning as described just above. Where the wavelength of the light to be emitted from the surface-light-emitting device is as short as about six hundred and some tens of nanometers (nm), the concentrations of Al of the (Al)GaAs films must be made relatively high in order to reduce the light absorptance. However, an increase in the Al concentration undesirably reduces or substantially zeros the refractive index at the boundaries of the alternately arranged films, leading to difficulty in obtaining a sufficiently high degree of reflectivity of the multi-film reflecting layers. Another problem encountered in the known surface-light-emitting device relates to a change of a gas used to form the first multi-film reflecting layer (formed on the side of the substrate) and the light-generating layer (active portion of the device). Described in detail, epitaxial growth of the AlGaInP/GaInP light-generating layer on the first multi-film reflecting layer is initiated by changing the gas from arsine AsH3 which has been used for growing the As-based material of the first multi-film reflecting layer, to phosphine PH3 to be used for the epitaxial growth of the light-generating layer on the first reflecting layer. However, this arsine-to-phosphine change of the gas takes a relatively long time, causing an increased surface roughness of the first reflecting layer, and oxidization of Al, making it difficult to form a high-quality active portion on the first reflecting layer, that is, the light-generating layer having a high quality on the first reflecting layer.
On the other hand, it has been proposed to form the first and second (n- and p-) multi-film reflecting layers consisting of (Al)GaInP/Al(Ga)InP films. The reflecting layers according to this proposal exhibit a high degree of reflectivity even where the light to be emitted has a considerably short wavelength. In addition, the epitaxial growth of the AlGaInP/GaInP light-generating layer on the first reflecting layer does not require a change from arsine AsH3 to phosphine PH3, before the epitaxial growth of the light-generating layer on the first reflecting layer. Accordingly, the light-generating layer serving as the active portion of the surface-light-emitting device can be advantageously given a high degree of crystallinity. However, the use of the p-(Al)GaInP/Al(Ga)InP second multi-film reflecting layer gives rise to a relatively large degree of carrier blockage, and an accordingly high electric resistance, and results in reduced enclosure of electrons, so that the operating efficiency of the surface-light-emitting device is deteriorated where the device is designed to emit a light having a relatively short wavelength. Another problem arises from the use of the (Al)GaInP/Al(Ga)InP films for the first and second (n- and p-) multi-film reflecting layers, where a mesa structure is formed by selective etching effected on the laminar semiconductor structure grown on the semiconductor substrate. That is, it is extremely difficult to practice a wet-etching process to form the mesa structure. Although the application of the wet-etching process is not impossible, the yield ratio in the production of the surface-light-emitting device is considerably low where the wet-etching process is used to form the mesa structure. In addition, the difficulty to form the mesa structure by the wet-etching process leads to difficulty to form a current-blocking region by subjecting the peripheral portion of the mesa structure to an oxidizing treatment.
The present invention was made in view of the prior art described above. It is therefore a principal object of the present invention to provide a surface-light-emitting device which includes multi-film reflecting layers having a high degree of reflectivity with respect to a relatively short wavelength of light, and a light-generating layer having a high degree of crystallinity, and which has a low electric resistance and exhibits a high operating efficiency. It is an optional object of this invention to provide a surface-light-emitting device including a laminar semiconductor structure which is formed on a semiconductor substrate and which has a mesa structure formed by wet-etching and having a current-blocking region formed by an oxidizing treatment.
The principal object indicated above may be achieved according to the principle of the present invention, which provides a surface-light-emitting device including a semiconductor substrate, and a laminar semiconductor structure consisting of a plurality of semiconductor layers formed by epitaxial growth on the semiconductor substrate, the laminar semiconductor structure including a light-generating layer, and two multi-film reflecting layers between which the light-generating layer is interposed and which cooperate to constitute a light resonator for reflecting a light generated by the light-generating layer, the laminar semiconductor structure having a light-emitting surface at one of opposite ends thereof remote from the semiconductor substrate, the light generated by the light-generating layer being emitted from the light-emitting surface, characterized in that the two multi-film reflecting layers consist of a first multi-film reflecting layer formed principally of AlGaInP on the semiconductor substrate, and a second multi-film reflecting layer formed principally of AlGaAs on one of opposite sides of the light-generating layer which is remote from the semiconductor substrate.
In the present surface-light-emitting device, the first multi-film reflecting layer formed on the semiconductor substrate, which is required to have a comparatively high degree of reflectivity, is formed principally or essentially of AlGaInP, while the second multi-film reflecting layer located on the side of the light-emitting surface is formed principally or essentially of AlGaAs. The first multi-film reflecting layer formed principally of AlGaInP has a large difference between refractive index values of adjacent ones of the alternately arranged films, and a low degree of light absorptance, so that the first multi-film reflecting layer exhibits a high reflectivity, even where the light to be emitted has a relatively short wavelength. In addition, the formation of the light-generating layer (formed principally of AlGaInP/GaInP, for example) by epitaxial growth on the first multi-film reflecting layer does not require a change of a gas to form the light-generating layer after the first reflecting layer, so that the light-generating layer is given a high degree of crystallinity. Further, the first multi-film reflecting layer formed principally of AlGaInP has a lower electric resistance than a reflecting layer formed principally of AlGaAs, and the light-generating layer is interposed between the first reflecting layer formed principally of AlGaInP and the second reflecting layer formed principally of AlGaAs on the side of the light-emitting layer, so that a carrier-enclosing region for enclosing the carrier can be suitably formed upstream of the barriers as seen in the direction of movement of the carrier, assuring a high degree of operating efficiency of the device.
According to one preferred form of this invention, the semiconductor substrate is formed of n-GaAs, and the first multi-film reflecting layer consists of n-(Al)GaInP films and n-Al(Ga)InP films which are alternately laminated on each other, while the second multi-film reflecting layer consists of p-(Al)GaAs films and p-Al(Ga)As films which are alternately laminated on each other. In this case, the light-generating layer is formed of AlGaInP/GaInP. In the device according to this preferred form of the present invention, the first multi-film reflecting layer consisting of the alternately arranged n-(Al)GaInP films and n-Al(Ga)InP films has a high refractive index and a low degree of light absorptance, and exhibits a high degree of reflectivity even where the light to be emitted has a comparatively short wavelength, and the formation of the light-generating layer of AlGaInP/GaInP by epitaxial growth on the first multi-film reflecting layer of n-(Al)GaInP/Al(Ga)InP does not require a change of the gas to form the light-generating layer on the first reflecting layer, so that the light-generating layer is given a high degree of crystallinity. Further, the first multi-film reflecting layer consisting of the alternately arranged n-(Al)GaInP films and n-Al(Ga)InP films has a lower carrier blockage and a lower electric resistance than a reflecting layer consisting of alternately arranged n-(Al)GaAs films and n-Al(Ga)As films, and the light-generating layer is interposed between the first reflecting layer consisting of the alternately arranged n-(Al)GaInP films and n-Al(Ga)InP films and the second reflecting layer consisting of the alternately arranged p-(Al)GaAs films and p-Al(Ga)As films, so that carrier-enclosing regions Ke and Kh for enclosing the carrier can be suitably formed upstream of the barriers as seen in the direction of movement of the carrier (electrons and holes), assuring a high degree of light-emitting efficiency of the device. Where the laminar semiconductor structure grown on the semiconductor substrate includes a mesa structure formed by etching an end portion of the structure, as described below in detail, an ordinary etching process may be used for easy formation of the mesa structure, since the second multi-film reflecting layer formed of p-(Al)GaAs/Al(Ga)As does not include phosphorus (P). Further, a non-electrically-conductive current-blocking region can be easily formed by oxidizing aluminum (Al) in an exposed portion of the mesa structure which includes the second reflecting layer formed of p-(Al)GaAs/Al(Ga)As.
According to another preferred form of this invention, the laminar semiconductor structure includes a mesa structure which has the light-emitting surface and the second multi-film reflecting layer, the mesa structure being formed by etching an end portion of the laminar semiconductor structure, which end portion is laminated on the light-generating layer, the mesa structure having a smaller cross sectional area taken in a plane parallel to the light-emitting surface, than the other portion of the laminar structure, the second multi-film reflecting layer comprising a generally annular non-electrically-conductive current-blocking region which is formed by oxidizing an exposed portion of the mesa structure which is a peripheral part as seen in the above-indicated plane. In this arrangement, the generally annular non-electrically-conductive current-blocking region can be easily formed by oxidizing the peripheral portion of the mesa structure, since the peripheral portion of the mesa structure is exposed in the radial direction of the mesa structure.
According to a further preferred form of this invention, the light-generating layer includes at least one active layer of quantum-well structure each interposed between adjacent two barrier layers, each active layer and the adjacent two barrier layers being located between the first and second multi-film reflecting layers such that each active layer is aligned with an antinode of the light resonated by the light resonator. In the present arrangement, an electron wave within the light-generating layer and an optical wave within the light resonator are coupled together, so as to provide a so-called xe2x80x9ccavity QED effectxe2x80x9d permitting the light-generating layer to generate a light only in a resonance mode. Accordingly, the light emitted from the light-emitting surface has a high degree of directivity and a narrow line width, without total reflection of the light by the monocrystalline semiconductor surface, assuring a high degree of quantum efficiency and a large optical output.