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Long-wavelength semiconductor light emitting device and its manufacturing methodRelated Patent Categories: Coherent Light Generators, Particular Active Media, SemiconductorLong-wavelength semiconductor light emitting device and its manufacturing method description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060120423, Long-wavelength semiconductor light emitting device and its manufacturing method. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION DATA [0001] This application is divisional of U.S. patent application Ser. No. 10/917900, filed Aug. 13, 2004 and allowed Dec. 23, 2005, and which is incorporated herein by reference to the extent permitted by law. This application claims the benefit of priority to Japanese Patent Application No. JP2003-295091, filed Aug. 19, 2003, which also is incorporated herein by reference to the extent permitted by law. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a long-wavelength semiconductor light emitting device and its manufacturing method, and more particularly to those using GaInNAs-system semiconductors (simply called GaInNAs semiconductors hereinbelow) as materials of the active layer. [0004] 2. Description of the Related Art [0005] Long-wavelength semiconductor light emitting devices using GaInNAs semiconductors as materials of the active layers can cover the emission wavelength region from 1.3 to 1.55 .mu.m depending upon the mixing ratio of In and N in GaInNAs, and can be realized by using inexpensive GaAs substrates. Furthermore, these GaInNAs long-wavelength semiconductor light emitting devices permit large diffraction index differences An among layers of materials in lattice matching with substrates. Therefore, these materials make it possible to fabricate excellent distributed Bragg reflectors (DBR), and there has been a movement toward their applications to vertical cavity surface emitting lasers (VCSEL), which are hopeful as a form of optical communication lasers. Therefore, these GaInNAs long-wavelength semiconductor light emitting devices have been remarked for years as the next-generation optical communication semiconductor lasers substituting expensive GaInAsP long-wavelength semiconductor light emitting devices using InP substrates. [0006] When a GaInNAs well layer is formed on an AlGaAs layer by metal organic chemical vapor deposition (MOCVD), the GaInNAs well layer catches Al therein by approximately 0.1% even though tri-methyl aluminum (TMA) is not supplied intentionally during its growth, and this aluminum adversely affects the static characteristics of the GaInNAs semiconductor laser. However, it has been reported that a GaInNAs well layer grown on a GaAs layer will not take Al therein (Photonics West 2003 Session No. 4995-08, herein below referred to as Non-patent Literature 1). [0007] Additionally, there are some other proposals about techniques for manufacturing GaInNAs semiconductor lasers by the use of GaNAs layers as barrier layers of active layers (Japanese Patent Laid-open Publication No. JP-H10-145003-A, referred to as Patent Literature 1; Photonics West 2003 Session No. 4994-18, referred to as Non-patent Literature 2; and Photonics West 2003 Session No. 4994-33, referred to as Non-patent Literature 3). [0008] In the above-introduced GaInNAs long-wavelength semiconductor light emitting devices, an AlGaAs layer in lattice matching with GaAs as its substrate is used as a clad layer. However, during the growth of the AlGaAs layer, tri-methyl aluminum or tri-ethyl aluminum (TEA) used as the source material of Al reacts in vapor phase with di-methyl hydrazine (DMHy) used as the source material of N, and produces reaction products (adducts). The Inventors confirmed by observation using a transmission electron microscope that the products of the vapor phase reaction produced during growth of the AlGaAs layer as a barrier layer of the active layer, for example, degrades the sharpness of the interface with the active layer. [0009] Furthermore, the Inventors prepared a trial GaInNAs long-wavelength semiconductor laser and analyzed it by secondary ion mass spectroscopy (SIMS). As a result, they found a large amount of Al in portions of well layers and barrier layers even when any Al source material is not supplied intentionally into the reaction vessel during the growth of peripheral layers of the active layer (well layers, barrier layers and guide layers). Although the Inventors are not sure how the layers catch Al, they can presume that exposure of the substrate to an atmosphere containing a mixture of Al or its source material and an N source material invites the intrusion of Al and will cause a quality degradation of the active layer. This is a serious problem. A report of a research institute (Agilent Technologies) also remarks this issue of Al as inviting serious adverse influences to the static characteristics of GaInNAs semiconductor lasers (Non-patent Literature 1). [0010] To overcome this problem, AGILENT proposes to first grow an n-type clad layer; then remove the substrate out of the reaction vessel of the growth apparatus; next clean the interior of the reaction vessel; and thereafter resume the growth of the active layer (Non-patent Literature 1). Taking account of defects departing from the interface of the layer grown after interruption of the growth, which will adversely affect the reliability, as well as an increase of the manufacturing cost by the need of the double-step growth, a new technique is invoked, which can prevent intake of the Al impurity in one step of crystal growth. In addition, for realization of practical GaInNAs long-wavelength semiconductor lasers, it remains unclear whether or not the Al impurity has to be removed completely from the active layer. OBJECTS AND SUMMARY OF THE INVENTION [0011] It is therefore an object of the present invention to provide a long-wavelength semiconductor light emitting device having excellent characteristics and a long lifetime, as well as its manufacturing method, with which an active layer of a good quality can be obtained because of a sufficiently low concentration of Al impurity in the active layer. [0012] The Inventors made vigorous researches to accomplish the above-mentioned object as abstracted below. [0013] As a technique for solving the problems in the prior techniques, the Inventors have found that the concentration of Al impurity contained in the active layer can be reduced to 1.times.10.sup.19 cm.sup.-3 if the supply of the source material of a group III element is interrupted during the growth of a layer (such as an optical guide layer) anteriorly adjacent to the active layer of a GaInNAs long-wavelength semiconductor light emitting device or immediately before the start of growth of the active layer, and a highly reactive gas such as DMHy is supplied together with a source material of As used as a group V element. The Inventors also found that the Al impurity concentration reduced to this level ensures characteristics acceptable for practical use. This technique is completely different from techniques of Patent Literature 1 and Non-patent Literatures 2 and 3. [0014] FIG. 1 shows correlation between the peak concentration of Al impurity in the GaInNAs active layer obtained by bar check and the slope efficiency. FIG. 2 shows correlation between the characteristic temperature measured after assembly of the laser and the peak concentration of Al impurity in the GaInNAs active layer. FIG. 3 shows correlation the emission intensity (peak intensity) from the GaInNAs active layer obtained by photoluminescence (PL) measurement and the peak concentration of Al impurity in the GaInNAs active layer. Note that the reflectance of the front edge of the laser is 50% and the reflectance of the rear edge is 95%. It is appreciated from FIGS. 1, 2 and 3 that, under the Al impurity concentration equal to or lower than 1.times.10.sup.19 cm.sup.-3, the slope efficiency is approximately equal to or higher than 0.25, and the characteristic temperature is equal to or higher than 150K, which are practically acceptable levels, and that the emission intensity is enhanced as well. It is also appreciated that, under the Al impurity concentration lower than or equal to 5.times.10.sup.18 cm.sup.-3, laser characteristics of a significantly high quality, with the slope efficiency equal to or higher than 0.4 and the characteristic temperature exceeding 200K, can be obtained. [0015] The present invention has been made based on these researches. [0016] According to the first aspect of the invention, there is provided a long-wavelength semiconductor light emitting device using Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z (0<x<1, 0<y<1, 0.ltoreq.z<1) as an active layer thereof, characterized in that the peak concentration of Al impurity contained in the active layer is lower than or equal to 1.times.10.sup.19 cm.sup.-3. [0017] The peak concentration of Al impurity contained in the active layer is preferably lower than or equal to 5.times.10.sup.18 cm.sup.-3. The active layer typically has a single-quantum-well structure or multi-quantum-well structure in which the well layer or layers are made of Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, and the peak concentration of Al impurity contained in the well layer or layers is lower than or equal to 1.times.10.sub.19 cm.sub.-3, or more preferably lower than or equal to 5.times.10.sub.18 cm.sub.-3. [0018] According to the second aspect of the invention, there is provided a manufacturing method of a long-wavelength semiconductor light emitting device using Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z (0<x<1, 0<y<1, 0.ltoreq.z<1) as an active layer thereof and having a peak concentration of Al impurity contained in the active layer, which is controlled to be lower than or equal to 1.times.10.sup.19 cm.sup.-3, comprising: [0019] supplying a highly reactive gas together with a source material of As while the supply of a source material of a group III element is interrupted during the growth of a layer anteriorly adjacent to the active layer or immediately before the growth of the active layer. [0020] As the highly reactive gas, here are usable, for example, nitrogen (N) radicals produced by plasma decomposition of di-methyl hydrazine (DMHy), ammonia (NH.sub.3) or nitrogen (N.sub.2). As the source material of As, arsine (AsH.sub.3) or tertiary-butyl arsine (TBAs), for example, may be used. The layer immediately preceding the active layer is typically an optical guide layer. Conditions for the supply of the highly reactive gas may depend on the form of the reaction furnace used. In general, however, if the flow rate is too low, the supply of the gas is not so effective. If the flow rate is too high, it increases the possibility of undesirably etching the growth layer on the substrate. If the flow time is too short, the supply of the gas is not so effective. If the flow time is too long, it increases the possibility of undesirably etching the growth layer on the substrate. Considering these factors, the gas is preferably supplied at a flow rate from 200 sccm to 4 slm for a length of time from one minute to 30 minutes. [0021] Not only during the growth of the layer anteriorly adjacent to the active layer or immediately before the start of growth of the active layer but also during the growth of the layer posteriorly adjacent to the active layer or immediately after the growth of the active layer, the supply of the source material of the group III element may be interrupted and the highly reactive gas may be supplied together with the source material of the group V element. This is effective for cleaning the surface of the reaction chamber or reaction vessel. Continue reading about Long-wavelength semiconductor light emitting device and its manufacturing method... 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