Semiconductor device having reflecting layer

Abstract: The present invention provides a semiconductor surface emitting diode having a reflecting layer formed of different semiconductor materials for reflecting incident light to optical wave interference, an active layer formed by epitaxy for producing said incident light, and a light-emitting surface through which light produced by the active layer is emitted. The reflective layer includes semiconductor films having different compositions superposed on each other, and the composition of said reflective layer changes continuously or in steps, in the direction of the thickness of said reflecting layer, at least at each interphase between the adjacent semiconductor films.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor device, a surface emitting diode and an array of surface emitting diodes, which use a light reflecting layer consisting of unit semiconductors whose thickness continuously varies in a direction of thickness of the reflecting layer. Discussion of the Prior Art A light emitting diode is widely used for optical data communication equipment, displays and sensors. Such light emitting diode includes an active layer for producing light, which is formed on a semiconductor substrate by epitaxy, such as liquid phase epitaxy or vapor phase epitaxy. As one type of light emitting diode, there is known a surface emitting diode adapted to emit light produced by an active layer, from a light emitting surface which is formed substantially parallel to the active layer. The optical output of a light emitting diode is determined by the internal quantum efficiency upon conversion of an electric energy into an optical energy, and the external quantum efficiency upon emission of light outside the diode. In the surface emitting diode, the reflecting layer is disposed on one side of the active layer which is remote from the light emitting surface, so that the reflecting layer reflects a portion of the light produced by the active layer, by optical wave interference known as Bragg reflection, back toward the active layer, in order to increase the external quantum efficiency for thereby improving the optical output of the diode. The reflecting layer is generally a laminar structure consisting of a plurality of unit semiconductors which are superposed on each other and each of which consists of two or more semiconductor films having different compositions. The specific wavelength of light to be reflected by the reflecting layer is determined by the particular refractive indices of the semiconductor films. Such a reflecting layer is also used in a semiconductor laser. An example of a unit semiconductor of the reflecting layer consists of a AlAs semiconductor film and a Al x Ga 1-x As semiconductor film. Thicknesses T A and T G of these AlAs and Al x Ga 1-x As semiconductor films are calculated according to the following equations (1) and (2), respectively: T A = λB/4n A T G = λB/4n G where, λB: center wavelength of light to be reflected n A : refractive index of the AlAs semiconductor film n G : refractive index of the Al x Ga 1-x As semiconductor film The thickness of the unit semiconductor is therefore equal to T A + T G . There is also known a surface emitting diode array which has a common substrate, a plurality of active layers formed on the common substrate, and a plurality of light emitting surfaces each formed on one side of the corresponding active layer which is remote from the substrate. This surface emitting diode array is used as a light source for an image-wise exposing head for a printer, and a display device. Generally, the surface emitting diode array is fabricated by forming p-n junctions on the common substrate, by vapor or liquid phase epitaxy or other epitaxial crystal growth techniques. The wavelength of the light that can be reflected by the reflecting layer by optical wave interference is limited to a specific wavelength range which satisfies the condition of the optical wave interference. That is, the wavelength range of the light reflected by the reflecting layer is relatively narrow, and depends upon the thickness (calculated according to the above equations) and refractive index of each unit semiconductor. Accordingly, even small amounts of variation in the thickness and change in the composition of the unit semiconductors will cause deviation of the wavelength range of the light reflected by the reflecting layer, from the nominal wavelength of the light produced by the active layer, whereby the optical output of the diode is lowered. This means considerable difficulty in fabricating the diode so as to strictly meet the requirements. In the case of an infrared emitting diode using a GaAs active layer, for example, the ultraviolet radiation produced by the active layer has a wavelength range of about ±35nm whose center is 880nm. To completely cover this wavelength range, the thickness of the reflecting layer should be extremely accurately controlled. Further, it is difficult to precisely control the epitaxial growth on a large substrate, so as to assure constant thickness of the reflecting layer over its entire surface area. Thus, the yield ratio of the known diode is considerably lowered due to local thickness variation of the reflecting layer. It is also noted that where each unit semiconductor of the reflecting layer consists of two superposed semiconductor films having different compositions, different lattice constants of the two semiconductor films may cause crystal defects due to lattice mismatch, resulting in an increase in the number of crystal defects such as dislocation of the active layer formed by epitaxy on the reflecting layer. The dislocation of the active layer may grow under heat during operation of the relevant semiconductor device (e.g., surface emitting diode), leading to so-called "dark line deterioration" of the active layer, thereby shortening the expected service life of the device. The use of the two semiconductor films has another drawback that the discontinuity of bands at the interface of the two semiconductor films increases an electrical resistance value, requiring the device to be operated with a higher voltage, as compared with a device which does not have a reflecting layer. JP-A-1 200 678 discloses a light-emitting diode having the reflecting layer disposed on one side of the active layer which is remote from the light emitting surface so that the reflecting layer reflects a portion of the light produced by the active layer back towards the active layer. The reflection layer is constituted by a GaAs/AlAs superlattice layer where p-GaAs layers and p-AlAs layers have been laminated alternately. The alternately laminated GaAs and AlAs semiconductor films have a composition which abruptly changes at the interface of the films. The conventional surface emitting diode has only one active layer, and is not capable of emitting lights having different center wavelengths. For permitting the diode to emit radiations having respective different center wavelengths, it is considered to provide the diode with two or more active layers that are superposed on each other. In this arrangement, however, a radiation produced by one of the active layers is more or less absorbed by another active layer, and a radiation is produced by this active layer upon absorption of the radiation produced by the above one active layer. Consequently, the output level of the emitted light having the desired wavelength is not sufficiently high. The conventional surface emitting diode array may suffer from difference in the intensity of the radiations emitted by the individual diodes. This difference may result in undesired variation in the density of images printed by a print head if the diode array is used as a light source of the print head for image-wise exposing a recording medium or a photosensitive medium (such as a photoconductive drum). To solve this problem, it is considered to use suitable resistors in the driver circuit for the diode array, so that the radiations emitted by the individual diodes have the same intensity. However, this solution inevitably makes the print head complicated. Moreover, the solution requires an increased power consumption to assure output uniformity of all the diodes, since the resistors are used to lower the original output values of the diodes except one diode whose original output value is the smallest, to the smallest output value. The conventional surface emitting diode array also suffers from reduction in the intensity of the emitted radiations, because portions of the radiations produced by the diodes are absorbed by the substrate or other elements. This problem is serious particularly where the surface area of the light emitting surface for each diode is extremely small, as in the case of a surface emitting diode array used as a light source for a print head as indicated above, wherein multiple diodes are arranged on the substrate. If the intensity of the emitted radiations is not sufficiently high, the amount of image-wise exposure of a recording or photosensitive medium is insufficient, whereby the resolution of printed images such as characters is accordingly lowered. EP-A-0 430 041 shows a semiconductor device comprising a light reflecting layer consisting of three wave interference-type reflecting layers capable of reflecting waves of respective wavelength bands. DE-OS-2 629 356 relates to an arrangement for the simultaneous modulation and demodulation of signals having different wavelengths and generally refers to a double-diode which may be constructed of different materials or of the same material having a different concentration. The Japanese Journal of Applied Physics, Extended Abstracts of the 21 st Conference on Solid State Devices and Materials, Tokyo, JP, 28-30 August 1989, pp. 429-432 discloses a Bragg reflector consisting of ten pairs of AlAs/GaAs quarter-wavelength stacks in the construction of a surface emitting diode. SUMMARY OF THE INVENTION It is the object of the invention is to provide a surface emitting diode whose active layer is free from crystal defects and which has prolonged service life and is operated with a comparatively low voltage. The object indicated above may be accomplished according to the present invention, which provides a semiconductor surface emitting diode having a reflecting layer formed of different semiconductor materials for reflecting incident light by optical wave interference, an active layer formed by epitaxy for producing the light upon the reflecting layer, and a light emitting surface through which light produced by the active layer is emitted, characterized in that the reflecting layer includes semiconductor films having different compositions superposed on each other, a composition between the adjacent semiconductor films changes continuously in a direction of thickness of the reflecting layer or changes continuously at each interface between the adjacent semiconductor films. In the surface emitting diode constructed according to a preferred embodiment of the invention, the composition of the reflecting layer changes continuously only at each interface between the adjacent semiconductor films, whereby the difference in the lattice constant between the adjacent films is reduced to accordingly mitigate the lattice mismatch, which causes crystal defects of the reflecting layer, thereby reducing the crystal defects of the active layer which is formed by epitaxy on the reflecting layer. As a result, the so-called dark line deterioration caused by the dislocation due to the crystal defects of the active layer is minimized, whereby the service life of the diode can be elongated. Further, the change in the composition of the reflecting layer at the interfaces of the semiconductor films is effective to mitigate the discontinuity of bands of the reflecting layer, thus lowering the electrical resistance of the reflecting layer, permitting the diode to be operated with a reduced voltage applied thereto. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features and advantages of the present invention will become more apparent by reading the following detailed description of presently preferred embodiments of the invention, when considered in conjunction with the accompanying drawings, in which: Fig. 1 is an elevational view in cross section of a still further embodiment of a surface emitting diode of this invention; Fig. 2 is a graph for explaining a change in the composition of a reflecting layer of the diode of Fig. 41; Fig. 3 is a graph indicating a current-optical output relationship of the diode of Fig. 41, as compared with those of comparative diodes; Fig. 4 is a graph indicating a current-voltage relationship of the diode of Fig. 41, as compared with those of comparative diodes; Fig. 5 is a graph indicating a change in the composition of the reflecting layer of the diode of Fig. 41; Referring next to Fig. 1, there is shown a surface emitting diode 410. The diode 410 has an n-GaAs substrate 412, and the following layers formed on the substrate 412 in the order of description: n-GaAs/n-AlAs reflecting layer 414; n-AlGaAs clad layer 416; p-GaAs active layer 418; p-AlGaAs clad layer 420; and p-GaAs cap layer 422. The clad layer 416, active layer 418 and clad layer 420 constitute a double heterostructure. The cap layer 422 has a top surface which serves as a light emitting surface 424, on which a positive electrode 426 is formed. A negative electrode 428 in the form of a layer is formed on a lower surface of the substrate 412. With a forward voltage applied between the electrodes 426, 428, the active layer 418 of the double heterostructure produces light, which is emitted from the light emitting surface 424 of the cap layer 422. The reflecting layer 414 functions to reflect, by optical wave interference, a portion of the light which is produced by the active layer 418 and travels toward the substrate 412. The light reflected by the reflecting layer is also emitted from the light emitting surface 424, so that the optical output of the diode 410 is increased. The layers 416, 418, 420 and 422 formed by MOCVD have approximate thickness values of 2 nm, 0.1 nm, 2 nm and 0.1 nm, respectively. The reflecting layer 414 consists of 15 unit semiconductors superposed on each other. Each unit semiconductor consisting of an n-GaAs semiconductor film having a thickness of about 61nm and an n-AlAs semiconductor film having a thickness of about 73nm. These two semiconductor films of the 15 unit semiconductors are alternately superposed to form a laminar structure. The composition n-Al x Ga 1-x As at the interface of the two n-GaAs and n-AlAs semiconductor films continuously change as indicated by solid line in Fig. 2, by continuously changing a proportion of the material gases which are introduced into a MOCVD furnace during formation of the reflecting layer 414. The thickness values of the n-GaAs and n-AlAs semiconductor films of the n-GaAs/n-AlAs reflecting layer 414 are determined by the desired refractive indices of these films, the wavelength of the light produced by the p-GaAs active layer 418, and the other factors. Since the composition n-Al x Ga 1-x As at the interface of the n-GaAs and n-AlAs films of the reflecting layer 414 continuously changes, a lattice mismatch due to a difference in the lattice constant between the adjacent two semiconductor crystal films is mitigated to minimize the crystal defects of the n-GaAs/n-AlAs reflecting layer 414, whereby the crystal defects of the n-AlGaAs clad layer 416, p-GaAs active layer 418 and p-AlGaAs clad layer 420 which are formed on the reflecting layer 414 by epitaxy are reduced. Accordingly, the surface emitting diode 410 is protected against the so-called dark line deterioration due to the growth of the dislocation caused by the crystal defects, during operation of the diode. Thus, the life expectancy of the diode 410 is prolonged. The continuous change in the composition at the interface of the n-GaAs and n-AlAs semiconductor films of the reflecting layer 414 is also effective to mitigate the discontinuity of bands at the interface, as compared with that in a conventional reflecting layer whose unit semiconductor consists of an n-GaAs film and an n-AlAs semiconductor film which are superposed on each other, without an interposed composition. Accordingly, the electrical resistance of the reflecting layer is lowered, and the required operating voltage of the diode is accordingly reduced. Experiments were conducted on the surface emitting diode 410 according to the present embodiment, and two comparative examples, i.e., comparative examples 1 and 2. The comparative example 1 is a surface emitting diode using a reflecting layer which does not have a continuously varying composition interposed between the adjacent n-GaAs and n-AlAs semiconductor films. The comparative example 2 does not have any reflecting layer. In the comparative example 1, the composition of the semiconductor material abruptly changes at the interface of the two adjacent films, as indicated by dashed lines in Fig. 2. Figs. 3 and 4 respectively show a current-output relationship and a current-voltage relationship, which were measured on the specimens. It will be understood from Figs. 3 and 4 that the instant diode 410 exhibited substantially no difference in the current-output characteristic with respect to the comparative example 1, and provided considerably improved optical output as compared with the comparative example 2. It will be further understood that the present diode 410 exhibited considerably improved current-voltage characteristic as compared with the comparative example 1, and was operated with a low voltage almost equal to that of the comparative example 2 which does not have a reflecting layer. In the illustrated embodiment, the composition of the n-GaAs/n-AlAs reflecting layer continuously changes only at and near the ends of the nominal thicknesses of the GaAs and AlAs films, as indicated by the solid line in Fig. 2. However, the composition of the reflecting layer, i.e., ratio x of the composition Al x Ga 1-x As may continuously change throughout the entire thickness of the reflecting layer, in a cyclic manner, as indicated by solid line in Fig. 5. Further, the ratio x may change in steps in a cyclic manner. As in the preceding embodiment, the p-GaAs active layer 418 may be replaced by any other compound semiconductor material, such as Gap, InP and InGaAsP, for providing a double or single heterostructure or a homostructure for a surface emitting diode or laser. While the surface emitting diode 410 has the n-GaAs/n-AlAs reflecting layer 414, the semiconductor materials and the manner of changing the composition may be suitably selected, depending upon the desired refractive indices of the semiconductor materials and the wavelength of the light produced by the active layer. The diode 410 may also be formed by epitaxy other than MOCVD, for example, by molecular beam epitaxy. While the present invention have been described in its preferred embodiments, for illustrative purpose only, by reference to the accompanying drawings, it is to be understood that the invention may be embodied with various changes, modifications and improvements, which may occur to those skilled in the art, within the scope of the invention defined by the following claims.

Claims: A semiconductor surface emitting diode having a reflecting layer formed of different semiconductor materials for reflecting incident light by optical wave interference, an active layer formed by epitaxy for producing said incident light, and a light emitting surface through which light produced by the active layer is emitted, wherein said reflecting layer includes semiconductor films having different compositions superposed on each other, characterized in that the composition between the adjacent semiconductor films changes continuously in a direction of thickness of said reflecting layer. A semiconductor surface emitting diode according to claim 1, wherein the composition changes continuously only at each interface between said adjacent semiconductor films. A semiconductor surface emitting diode according to claim 1, wherein the composition of said reflecting layer (414) changes continuously in a cyclic manner throughout the thickness of said reflecting layer.

Inventors: HIROTANI MASUMI, KATO TOSHIHIRO, SAKA TAKASHI, SUSAWA HIROMOTO, YAMAUCHI NORIKATSU, HIROTANI, MASUMI, KATO, TOSHIHIRO, SAKA, TAKASHI, SUSAWA, HIROMOTO, YAMAUCHI, NORIKATSU
Applicants: DAIDO STEEL CO LTD, YAMAUCHI NORIKATSU, DAIDO TOKUSHUKO KABUSHIKI KAISHA, YAMAUCHI, NORIKATSU
Application year: 1991
Main classification: H01L33
IPC codes: H01L33