Visible light surface emitting laser device

Abstract: A visible light surface emitting laser device includes a GaAs substrate (1) having a (100) oriented surface as a main surface; a double heterojunction structure including at least an AlGaInP first cladding layer (2), an undoped GaInP active layer (4) epitaxially grown on the first cladding layer (2) under such a condition that natural superlattice will be formed, and an AlGaInP second cladding layer (5) formed on the active layer (4) and having opposite conductivity type to that of the first cladding layer (2); a groove (18) penetrating the substrate (1); and regions (7) formed by performing diffusion of impurities having opposite conductivity type to that of the second cladding layer (5) into regions other than a region opposite to the groove (18) from the second cladding layer (5) side so as not to reach the first cladding layer (2), in which the natural superlattice is disordered (8). Therefore, a current confinement structure and a light confinement structure can be realized in the active layer. As a result, a visible light surface emitting laser device that oscillates at low threshold current and that controls the transverse mode can be realized.

Description: FIELD OF THE INVENTION The present invention relates to a visible light surface emitting laser device using AlGaInP system material. BACKGROUND OF THE INVENTION Figure 7 is a cross-sectional view showing a surface emitting laser disclosed in "Room Temperature Pulsed Oscillation of GaAlAs/GaAs Surface Emitting Junction Laser Grown by MBE" by K. Iga, T. Nishimura, K. Yagi, T. Yamaguchi, and T. Niina, Japanese Journal of Applied Physics, Vol.25 (1986), pp.924 - 925. In figure 7, reference numeral 30 designates a (100) oriented n type GaAs substrate. An n type GaAs layer 31 is disposed on a main surface of the substrate 30. An n type Al 0.3 Ga 0.7 As cladding layer 32 is disposed beneath the n type GaAs layer 31. A p type GaAs active layer 33 is disposed beneath the n type cladding layer 32. A p type Al 0.3 Ga 0.7 As cladding layer 34 is disposed beneath the active layer 33. A circular mesa groove 43 penetrates the substrate 30 and the n type GaAs layer 31. The p type cladding layer 34 has a circular mesa projection at a region opposite to the mesa groove 43. A p type Al 0.1 Ga 0.9 As layer 42 is disposed beneath the circular mesa projection of the p type cladding layer 34. A SiO 2 film 35 is disposed beneath the p type cladding layer 34 so as to cover the side surface and the circumference of the top surface of the circular mesa projection. A circular SiO 2 film 36 which is a high reflectivity film is disposed at the center of the top surface of the circular mesa projection. A Au/Zn/Au electrode 45 covers the lowest part of the laser device and is connected to the ring-shaped surface 41 of the p type Al 0.1 Ga 0.9 As layer 42. A SiO 2 film 37 is disposed on the other main surface of the substrate 30. A Au/Sn electrode 38 is disposed on the SiO 2 film 37 and on the side surface of the circular mesa groove 43. A Au coated mirror 44 is disposed on the bottom of the mesa groove 43. An active region 39 is formed in a region in the active layer 33 just under the mesa groove 43. A laser light 40 is emitted from the bottom of the mesa groove 43 on which the Au coated mirror is provided. Operation principle of this surface emitting laser will be described. Electrons and holes supplied from the electrodes 38 and 45 are injected into the active layer 33. These electrons and holes serving as carriers are efficiently confined in the active layer 33 by the cladding layers 32 and 34 and then they are combined to generate light equivalent to the energy band gap of the active layer 33. The generated light increases with the increase in the current level and repeats reflection and amplification between the Au coated mirror 44 on the bottom of the mesa groove 43 and the circular SiO 2 film 36 which is a high reflectivity film on the top surface of the circular mesa projection. When the current exceeds a certain value (threshold current), the gain exceeds the loss, generating a laser oscillation. Thus, the laser light 40 is emitted from the bottom of the mesa groove 43. In a case where the prior art surface emitting laser is formed using AlGaInP system material, of course the surface emitting laser structure can be realized, but the current confinement into the active layer is not sufficient in the prior art structure. In addition, since the AlGaInP system material has a higher thermal resistance than that of the AlGaAs series material, it is difficult to generate a laser oscillation and even if the laser oscillation is generated, the temperature characteristic is very poor. In order to perform the current confinement, a structure is generally employed in which only the active region (light emitting region) is remained in a column shape by etching and the side surface of the active region is buried by AlGaInP system material having high resistance or pn junction. In this case, it is difficult to perform regrowth on the AlGaInP containing Al which is easily oxidized. In addition, when the AlGaInP system material is regrown on the substrate having step difference, there arises a problem such that an abnormal growth is induced by the remarkable difference in growth speed depending on the surface orientation. In Patents Abstracts of Japan, Volume 13, No. 348 (E-799) and the corresponding JP-A-1108789 there is described a surface emission semiconductor laser device comprising a semiconductor substrate and an active layer having a superlattice or quantum well structure formed onto the substrate. A Zn diffusion layer is formed up to a first cladding layer of the device. The quantum well structure is formed from GaAs and AlGaAs. The device claims to achieve low threshold currents. SUMMARY OF THE INVENTION An object of the present invention is to provide an AlGaInP visible light surface emitting laser having an improved laser characteristic. Other object and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only. The invention is disclosed in claim 1. Various embodiments of the invention are disclosed in the dependent claims. According to a first aspect of the present invention, a visible light surface emitting laser device which emits laser light in a direction substantially perpendicular to a main surface of a substrate, comprising: a GaAs substrate having a (100) oriented surface as a main surface; a double heterojunction structure including at least an AlGaInP first cladding layer having an opposite conductivity type of that of said substrate, an undoped GaInP active layer epitaxially grown on said first cladding layer under such a condition that a natural superlattice will be formed, and an AlGaInP second cladding layer formed on said active layer and having opposite conductivity type to that of said first cladding layer; a groove or recess penetrating said substrate; and regions in which the natural superlattice is disordered, formed by performing diffusion of impurities having opposite conductivity type to that of said second cladding layer from the second cladding layer side into the regions other than the region opposite to said groove or recess so as not to reach said first cladding layer, whereby current confinement to the active layer can be performed effectively. Thus, the GaInP active layer having natural superlattice structure is put between the AlGaInP system cladding layers, and regions in which the natural superlattice is disordered by impurity diffusion are formed in the active layer thereby to provide an effective refractive index difference. Therefore, a current confinement structure and a light confinement structure can be provided in the active layer, resulting in a visible light surface emitting laser that oscillates at low threshold current and that controls the transverse mode. According to a second aspect of the present invention, a visible light surface emitting laser device which emits laser light in a direction substantially perpendicular to a main surface of a substrate according to claim 1, comprising: an n type GaAs substrate having a (100) oriented surface as a first main surface; an n type semiconductor layer formed on said first main surface of the substrate; a double heterojunction structure formed on said n type semiconductor layer and including a p type AlGaInP first cladding layer, an undoped GaInP active layer epitaxially grown on said p type cladding layer under such a condition that a natural superlattice will be formed, and an n type AlGaInP cladding layer formed on said active layer; a groove or recess penetrating said substrate; regions in which the natural superlattice is disordered, formed by performing diffusion of first p type impurities from the n type AlGaInP cladding layer side into the regions other than the region opposite to said groove or recess so as not to reach said p type cladding layer, whereby current confinement to the active layer can be performed effectively; and a p type region formed by performing diffusion of second p type impurities from the second main surface of the substrate opposite to said first main surface, such that the diffusion front beneath said groove or recess penetrates said n type semiconductor layer and reaches into said p type cladding layer. Thus, Zn is diffused into a part of the n type semiconductor layer to form a p type current path selectively, whereby current confinement to the active layer can be performed effectively. As a result, a visible light surface emitting laser device oscillating at still lower threshold current can be realized. According to a further aspect of the present invention, AlGaAs having lower thermal resistance than that of AlGaInP is used for a part of the cladding layer. Therefore, a visible light surface emitting laser device oscillating at temperature characteristic can be realized. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view showing a visible light surface emitting laser device in accordance with a first embodiment of the present invention; Figures 2(a) to 2(f) are cross-sectional views in major process steps for fabricating the laser device of figure 1; Figure 3 is a cross-sectional view showing a visible light surface emitting laser device in accordance with a second embodiment of the present invention; Figure 4 is a cross-sectional view showing a visible light surface emitting laser device in accordance with a third embodiment of the present invention; Figure 5 is a cross-sectional view showing a visible light surface emitting laser device in accordance with a fourth embodiment of the present invention; Figures 6(a) and 6(b) are cross-sectional views in major process steps for fabricating the laser device of figure 4; and Figure 7 is a cross-sectional view showing a surface emitting laser device in accordance with the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 is a cross-sectional view showing a structure of a visible light surface emitting laser device in accordance with a first embodiment of the present invention. In figure 1, reference numeral 1 designates a (100) oriented n type GaAs substrate. A first n type AlGaInP cladding layer 2 is disposed on a first main surface of the substrate 1. A p type AlGaInP cladding layer 3 is disposed beneath the first n type AlGaInP cladding layer 2. An undoped GaInP active layer 4 is disposed beneath the p type AlGaInP cladding layer 3. A second n type AlGaInP cladding layer 5 is disposed beneath the active layer 4. A circular mesa groove 18 penetrates the substrate 1. The second n type cladding layer 5 has a circular mesa projection 19 at a region opposite to the mesa groove 18. An n type GaAs layer 6 is disposed beneath the circular mesa projection 19 of the second n type cladding layer 5. A high reflectivity film 22 is disposed on the top surface of the circular mesa projection 19. An n side electrode 24 is disposed beneath the second n type cladding layer 5 so as to cover the mesa projection 19 having the high reflectivity film 22. A p side electrode 25 is disposed on the other main surface, i.e., a second main surface of the substrate 1. An emitting surface reflecting film 23 is disposed on the bottom of the mesa groove 18. In addition, reference numerals 7 and 9 designate Zn diffused regions and numeral 8 designates a region in which the natural superlattice is disordered by the diffusion of Zn. Reference numeral 21 designates a light emitting region and numeral 26 designates emitted laser light. Figures 2(a) to 2(f) are cross-sectional views in major process steps for fabricating the visible light surface emitting laser device of figure 1. In these figures, the same reference numerals as those shown in figure 1 designate the same or corresponding parts. As shown in figure 2(a), A first n type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 2 of 0.8 micron thickness, a p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 3 of 0.2 micron thickness, an undoped Ga 0.5 In 0.5 P active layer 4 of 2 to 3 microns thickness, a second n type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 5 of 1 micron thickness and an n type GaAs contact layer 6 of 0.5 micron thickness are successively grown on the (100) oriented n type GaAs substrate 1 by for example MOCVD (Metal Organic Chemical Vapor Deposition). Here, it is necessary to epitaxially grow the GaInP active layer 4 under such a condition that natural superlattice will be formed therein and energy band gap thereof will be about 670 to 680 nm when it is converted into wavelength, by a reason described later. The growth is carried out at substrate temperature of 725°C and V/III ratio of 500. Then, as shown in figure 2(b), a mesa projection 19 is formed using ordinary photolithography and etching technique. Then, Zn diffusion is performed from the wafer surface using solid phase or vapor phase diffusion method so as to form a non-diffused region in cylindrical shape or in truncated conical shape at the center of the element, whereby Zn diffused regions 7 not reaching the p type cladding layer 3 is formed as shown in figure 2(c). The diameter of the Zn non-diffused region at the top part of the mesa projection 19 is about 20 microns. Thereafter, as shown in figure 2(d), a circular high reflectivity film 22 having a diameter of about 10 microns is formed on the center of the Zn non-diffused region at the top surface of the mesa projection 19 and, further, an n side electrode 24 is formed on the entire surface of the wafer. Then, as shown in figure 2(e), a circular mesa groove 18 penetrating the substrate 1 is formed from the other main surface, i.e., second main surface of the substrate 1. This circular mesa groove 18 is opposed to the mesa projection 19 and a diameter of its bottom is about 20 microns. Thereafter, Zn diffusion is performed from the second main surface of the substrate 1 to form a Zn diffused region 9 whose diffusion front beneath the circular mesa groove 18 reaches into the p type cladding layer 3, as shown in figure 2(f). Then, a high reflectivity film 23 comprising multilayer dielectrics is formed on the bottom of the circular mesa groove 18 and a p side electrode 25 is formed on the second main surface of the substrate 1, completing the surface emitting laser device of figure 1. This laser device operates as follows. In this first embodiment of the present invention, Zn diffused region 9 is formed in the surface region of the n type GaAs substrate 1 at the electrode side and on the side surface of the circular mesa groove 18, and the diffused region 9 beneath the circular mesa groove 18 penetrates the n type AlGaInP cladding layer 2 and reaches into the p type AlGaInP cladding layer 3. On the other hand, regions 8 in which the natural superlattice is disordered are formed in the active layer 4 by the Zn diffusion performed from the n type GaAs layer 6 side. Accordingly, when a forward direction bias is applied between the p side electrode 23 and the n side electrode 24, current is efficiently injected into a region in the active layer 4 opposite to the mesa groove 18 as shown by the arrow in figure 1, whereby light emission and recombination of carriers occur in that region. The generated light repeats reflection and amplification between the emitting surface reflecting film 23 formed on the bottom of the mesa groove 18 and the high reflectivity film 22 formed on the top surface of the mesa projection 19, resulting in laser oscillation. Thus, the laser light 26 is emitted from the mesa groove 18. In the regions 8 in the GaInP active layer 4, the natural superlattice in GaInP is disordered by the diffusion of Zn and the energy band gap is increased. The energy band gap is about 650 nm when it is converted into wavelength. As described above, the energy band gap of the undoped GaInP active layer 4 is about 670 to 680 nm when it is converted into wavelength and there arises a difference in energy band gaps between the non-diffused region and the regions 8 in which the natural superlattice is disordered by the Zn diffusion. That is, the refractive index drops in the Zn diffused regions 8. Therefore, the light generated in the Zn non-diffused region in the active layer 4 is confined in this region, whereby the transverse mode can be stabilized. As described above, in the laser structure of the first embodiment of the present invention, current is efficiently injected into the active layer 4 from the Zn diffused region 9 and the transverse mode of the light generated in the light emitting region 21 is stably controlled by the difference in refractive indices between the Zn diffused region 8 and the light emitting region 21. The current confined by the Zn diffused region 9 spreads in transverse direction in the p type AlGaInP cladding layer 3, so that the current confinement effect is lowered a little in the p type AlGaInP cladding layer 3. In the first embodiment of the present invention, the cladding layer 3 is thinned down to 0.2 micron, whereby the lowering of the current confinement effect is reduced. In order to reduce the lowering of the current confinement effect, the thinner cladding layer 3 is better. However, since the diffusion front of the Zn diffused region 9 beneath the mesa groove 18 is required to be stopped in the cladding layer 3, if the cladding layer 3 is too thin, it is difficult to control the diffusion front. The cross-section of the light emitting region 21, i.e., the Zn non-diffused region in the active layer, vertical to the layer thickness direction is not restricted to circular, it may be square. When it is circular like the first embodiment of the present invention, the configuration of the emitted beam is also circular, so that this emitted beam can be easily treated. Description is given of a second embodiment of the present invention. Figure 3 is a cross-sectional view showing a visible light surface emitting laser device in accordance with the second embodiment of the present invention, which is obtained by simplifying the structure of the first embodiment shown in figure 1. In this second embodiment, Zn diffusion is performed without forming the mesa projection 19 in figure 1. The production process is the same as that of the first embodiment except for that the mesa 19 is not formed in this second embodiment. When the diffusion is carried out deeper, the control of the diffusion front becomes more difficult. Therefore, in this second embodiment, it is difficult to control the Zn diffusion performed from the n type GaAs layer 6 side. However, since an etching process is omitted, the production process can be simplified. The principle of operation and the effect of this second embodiment are the same as those of the first embodiment. More specifically, current is efficiently injected into the active layer 4 from the Zn diffused region 9 as shown by the arrow in figure 3 and transverse mode of the light generated in the light emitting region 21 can be stably controlled by the difference in the refraction indices between the Zn diffused region 8 and the light emitting region 21. Description is given of a third embodiment of the present invention. Figure 4 is a cross-sectional view showing a visible light surface emitting laser device in accordance with the third embodiment of the present invention. In figure 4, reference numeral 10 designates an n type Al x Ga 1-x As (0.7 < x < 0.9) cladding layer and numeral 11 designates a second n type Al x Ga 1-x As (0.7 < x < 0.9) cladding layer. In this third embodiment, the temperature characteristics of the laser devices according to the first and second embodiments are improved. Generally, AlGaInP has high thermal resistance and this causes deterioration in the temperature characteristic of laser device. In this third embodiment, AlGaAs having lower thermal resistance than that of AlGaInP is used for a part of the cladding layer and AlGaInP/AlGaAs double cladding layer structure is employed. Figures 6(a) and 6(b) are cross-sectional views in major process steps for fabricating the laser device of figure 4. As shown in figure 6(a), a first n type Al x Ga 1-x As (0.7 < x < 0.9) cladding layer 10 of 0.8 micron thickness, a p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 3 of 0.2 micron thickness, an undoped Ga 0.5 In 0.5 P active layer 4 of 2 to 3 microns thickness, an n type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 5' of 0.2 micron thickness, a second n type Al x Ga 1-x As (0.7 < x < 0.9) cladding layer 11 of 0.8 micron thickness and an n type GaAs contact layer 6 of 0.5 micron thickness are successively grown on the (100) oriented n type GaAs substrate 1 by for example MOCVD. The GaInP active layer 4 is epitaxially grown under such a condition such that natural superlattice will be formed and the energy band gap will be about 670 to 680 nm when it is converted into wavelength, similarly as in the first embodiment. The growth is carried out at the substrate temperature of 725°C and the V/III ratio of 500. Then, as shown in figure 6(b), a mesa projection 19 is formed using ordinary photolithography and etching technique. Here, if a selective etching is performed using an etchant having different etching rates for AlGaAs and for AlGaInP, the n type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 5' can be used as an etch-stop layer, whereby the mesa projection 19 can be formed with high controllability. The process steps after the formation of the mesa projection 19 are the same as figures 2(c) to 2(f). This laser device will operate as follows. The fundamental laser oscillating operation is the same as those of the first and second embodiments. Since AlGaAs has lower thermal resistance than that of AlGaInP, the heat generated by the laser operation is not confined in the element but effectively radiated to a heat sink which is provided adjacent to the upper part or the lower part of the laser device in a practical use. As a result, deterioration in laser characteristic caused by the heat generated with the laser operation can be reduced. Figure 5 is a cross-sectional view showing a visible light surface emitting laser device in accordance with a fourth embodiment of the present invention. In this fourth embodiment, Zn diffusion is performed without forming the mesa projection 19 of the third embodiment. Also in this case, the same effects as described above can be obtained. As is evident from the foregoing description, in accordance with a first aspect of the present invention, an active layer having natural superlattice structure is put between AlGaInP cladding layers, and effective refraction index difference is provided in the active layer by impurity diffusion. Therefore, a current confinement structure and a light confinement structure can be realized in the active layer. As a result, a visible light surface emitting laser device that oscillates at low threshold current and that controls the transverse mode can be realized. In accordance with a second aspect of the present invention, Zn is diffused into a part of n type semiconductor layer to form a p type current path selectively, whereby current confinement into the active layer can be performed effectively. As a result, a visible light surface emitting laser device which oscillates at still lower threshold current can be realized. In accordance with a third aspect of the present invention, AlGaInP/AlGaAs double cladding layer structure is employed using AlGaAs having lower thermal resistance than that of AlGaInP. Therefore, an AlGaInP system visible light surface emitting laser device having an improved temperature characteristic is realized.

Claims: A visible light surface emitting laser device which emits laser light in a direction substantially perpendicular to a main surface of a substrate, comprising: a GaAs substrate (1) having a (100) oriented surface as a main surface; a double heterojunction structure including at least an AlGaInP first cladding layer (3) having an opposite conductivity type of that of said substrate (1), an undoped GaInP active layer (4) epitaxially grown on said first cladding layer (3) under such a condition that a natural superlattice will be formed, and an AlGaInP second cladding layer (5) formed on said active layer (4) and having opposite conductivity type to that of said first cladding layer (3); a groove or recess (18) penetrating said substrate (1); and regions (8) in which the natural superlattice is disordered, formed by performing diffusion of impurities having opposite conductivity type to that of said second cladding layer (5) from the second cladding layer (5) side into the regions (7) other than the region opposite to said groove or recess (18) so as not to reach said first cladding layer (3), whereby current confinement to the active layer (4) can be performed effectively. A visible light surface emitting laser device which emits laser light in a direction substantially perpendicular to a main surface of a substrate according to claim 1, comprising: an n type GaAs substrate (1) having a (100) oriented surface as a first main surface; an n type semiconductor layer (2;10) formed on said first main surface of the substrate (1); a double heterojunction structure formed on said n type semiconductor layer (2;10) and including a p type AlGaInP first cladding layer (3), an undoped GaInP active layer (4) epitaxially grown on said p type cladding layer(3) under such a condition that a natural superlattice will be formed, and an n type AlGaInP (5) cladding layer formed on said active layer (4); a groove or recess (18) penetrating said substrate (1); regions (8) in which the natural superlattice is disordered, formed by performing diffusion of first p type impurities from the n type AlGaInP cladding layer (5) side into the regions (7) other than the region opposite to said groove or recess (18) so as not to reach said p type cladding layer (3), whereby current confinement to the active layer can be performed effectively; and a p type region (9) formed by performing diffusion of second p type impurities from the second main surface of the substrate (1) opposite to said first main surface, such that the diffusion front beneath said groove or recess (18) penetrates said n type semiconductor layer (2;10) and reaches into said p type cladding layer (3). A visible light surface emitting laser device in accordance with claim 2 wherein said n type semiconductor layer (10) comprises AlGaAs. A visible light surface emitting laser device in accordance with claim 2, further comprising: an n type GaAs contact layer (6) formed on said n type AlGaInP cladding layer (5); a p side electrode (23) formed on said second main surface of the substrate; and an n side electrode (24) formed on said n type GaAs (6) contact layer. A visible light surface emitting laser device in accordance with claim 3, further comprising: an n type GaAs contact layer (6) formed on said n type AlGaInP cladding layer (5); a p side electrode (23) formed on said second main surface of the substrate (1); and an n side electrode (24) formed on said n type GaAs contact layer (6). A visible light surface emitting laser device in accordance with claim 2 wherein said first and second p type impurities are Zn. A visible light surface emitting laser device in accordance with claim 4 wherein an n type AlGaAs layer (11) is provided between said n type AlGaInP (5') cladding layer and said n type GaAs contact layer (6). A visible light surface emitting laser device in accordance with claim 2, in which the n type semiconductor layer (2) comprises AlGaInP.

Inventors: ARIMOTO SATOSHI, ARIMOTO, SATOSHI, ARIMOTO, SATOSHI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01S3
IPC codes: H01S5