Semiconductor laser device

Abstract: A semiconductor laser device includes a first conductivity type cladding layer (102), an active layer (103), and a second conductivity type cladding layer (105), disposed on a first conductivity type substrate (101). A multiquantum barrier (104) comprising superlattice semiconductor layers is disposed between the active layer (103) and either of the cladding layers. The active layer includes a layer having a crystal composition in which a lattice constant of the layer is different from a lattice constant of the first conductivity type substrate by 0.1 percent or more so that a strain may be applied to the layer. Therefore, a threshold current is significantly reduced, resulting in a high power semiconductor laser capable of operating at a high temperature.

Description: The present invention relates to semiconductor laser devices comprising III-V and II-VI compound semiconductor materials, that operate at a high temperature with a low threshold current. Generally, a semiconductor laser employs a double heterojunction structure in which an active layer is sandwiched by cladding layers having larger energy band gaps than the active layer. When a forward bias voltage is applied to the double heterojunction structure, carriers are injected into the active layer from the cladding layers and effectively confined in the active layer due to a potential barrier caused by a difference in energy band gaps between the active layer and the cladding layer, resulting in carrier recombinations that produce laser light. However, if the potential barrier is small, carriers in the active layer, especially electrons having a small effective mass, overflow into the cladding layers, whereby a threshold current unfavorably increases, resulting in deterioration in laser characteristics. In order to suppress the overflow of carriers, a multiquantum barrier (hereinafter referred to as MQB) is proposed by K. Iga, H. Uenohara, and F. Koyama in Electronics Letters, Vol. 22 (1986), p. 1008. The MQB has a multiquantum well structure comprising 10-odd angstroms wells (1nm=10 angstroms) and 10-odd angstroms barriers and utilizes a phenomenon in which an electron wave is reflected at an interface between the quantum wells. When the MQB is inserted between an active layer and a cladding layer, electrons having energies higher than a potential barrier formed between the active layer and the cladding layer are reflected by a quantum mechanical effect of the MQB, whereby the overflow of electrons into the cladding layer is suppressed. This manner is schematically shown in figure 9. In figure 9, reference character Δ Ec designates a difference in energy band gaps between the active layer and the cladding layer at a conduction band side and reference character Δ Ue designates an increase in the effective potential barrier due to the MQB. In this way, electrons are reflected by the potential barrier caused by the MQB even when the electrons have energies higher than Δ Ec and effectively confined in the active layer. Figure 8 is a cross-sectional view showing a semiconductor laser incorporating an MQB, disclosed in a conference digest of 12th IEEE International Semiconductor Laser Conference, 1990, p. 21, PD-10, by K. Kishino, A. Kikuchi, Y. Kaneko, and I. Nomura. In figure 8, there are successively epitaxially grown on an n type GaAs substrate 21 an n type GaInP buffer layer 22, an n type AlInP cladding layer 23, a superlattice cladding layer 24, a GaInP active layer 25, a superlattice cladding layer 26, an MQB 27 comprising GaInP wells and AlInP barriers, a p type AlInP cladding layer 28, and a p type GaInP cap layer 29. Preferably, these layers are grown by a gas source MBE method. The superlattice cladding layers 24 and 26 decrease non-radiative recombinations of carriers at the interfaces between the active layer 25 and the cladding layers 24 and 26 to improve the laser characteristics. In this prior art, the MQB is applied to a semiconductor laser for the first time and a reduction in the threshold current and an improvement in the temperature characteristic are achieved by the carrier overflow suppressing effect of the MQB. In addition, Electronics Letters, 16th January 1992, Vol. 28, No. 2, p.150 discloses a semiconductor laser including an InGaP/InGaAlP multiquantum well active layer and an InGaP/InGaAlP MQB inserted between an InGaAlP guide layer on the active layer and a p type InGaP cladding layer. In this laser structure, a high temperature (90 C) CW (continuous wave) operation is realized. In the conventional semiconductor laser devices incorporating the MQBs, effects of the MQBs are logically and experimentally confirmed. However, a threshold current density of each laser is not sufficiently reduced by only incorporating the MQB. In addition, the conventional MQBs are not optimized to suppress an overflow of holes. In an alternative approach to obtaining a laser device with high kink level, high temperature operation and acceptable threshold current density, EP-A-0456429 proposes the introduction of an active layer with a lattice constant different by 0.3% or more relative to that of the semiconductor substrate. This is said to reduce the band gap of the active layer and increase a band gap difference between the active layer and a cladding layer. According to the invention, there is provided a semiconductor laser device including a first conductivity type substrate, and a first conductivity type cladding layer, an active layer, and a second conductivity type cladding layer, disposed on said substrate, said active layer including a layer having a crystal composition in which a lattice constant of said layer is different from a lattice constant of said substrate by 0.1 percent or more so that a compressive strain may be applied to said layer; characterised by a multiquantum barrier, comprising supperlattice semiconductor layers, disposed between said active layer and either of said cladding layers, said multiquantum barrier being constituted by alternating semiconductor layers having large energy band gaps and semiconductor layers having small energy band gaps and at least one of the semiconductor layers having a crystal composition in which a lattice constant of said layer is different from a lattice constant of the substrate by 0.1 percent or more so that a compressive strain may be applied to said layer. Embodiments of the invention provide a semiconductor laser device that reduces a threshold current density and suppresses not only overflow of electrons but also overflow of holes. In further embodiments of the present invention, a semiconductor laser device includes an active layer to which a compressive strain is applied and a multiquantum barrier disposed between an n type cladding layer and the active layer. The multiquantum barrier is formed by alternating semiconductor layers having large energy band gaps and semiconductor layers having small energy band gaps and a compressive strain is applied to at least one of the semiconductor layers. Therefore, a threshold current is significantly reduced and a height of a barrier for confining holes is increased. Other objects 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, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a)-1(c) illustrates a schematic diagram showing a first example of a semiconductor laser device; Figure 2 is a graph showing a relation between a threshold current density and a strain applied to an active layer; Figures 3(a)-3(b) are cross-sectional views of a second example of a semiconductor laser device; Figure 4 is a cross-sectional view showing a third example of a semiconductor laser device; Figures 5(a)-5(b) are cross-sectional views showing a fourth example of a semiconductor laser device, this example being in accordance with an embodiment of the present invention; Figure 6 is a graph showing a relation between a height of a barrier for confining holes and an effective mass of the holes; Figures 7(a)-7(b) are cross-sectional views showing a fifth example of a semiconductor laser device which may be modified to embody the present invention; Figure 8 is a cross-sectional view showing a semiconductor laser device incorporating an MQB in accordance with the prior art; and Figure 9 is a schematic diagram of the MQB in accordance with the prior art. DETAILED DESCRIPTION OF EXAMPLES AND EMBODIMENTS Although the present invention is applicable to all III-V and II-VI compound semiconductor laser devices, a description will be given of an AlGaInP semiconductor laser device producing visible light. Figure 1(a) is a cross-sectional view showing an AlGaInP laser device producing visible light. In figure 1(a), reference numeral 101 designates an n type GaAs substrate. An n type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 102 1.5 microns thick is disposed on the substrate 101 and a Ga x In 1-x P active layer 103 is disposed on the cladding layer 102. An MQB 104 is disposed on the active layer 103. As shown in figure 1(b), the MQB 104 has a superlattice multilayer structure comprising twenty layers, i.e., a p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P buffer layer 104a 226 angstroms thick, ten p type Ga 0.5 In 0.5 P well layers 104b each 11.5 angstroms thick, and nine p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P barrier layers 104c each 17 angstroms thick. A p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer 105 1.5 microns thick is disposed on the MQB 104 and a p type GaAs layer 106 0.5 micron thick is disposed on the cladding layer 105. Regarding the crystal composition, (Al y Ga 1-y ) x In 1-x P latticematches with GaAs when x = 0.5 and 0 < y < 1. In this example, x is not equal to 0.5 in the Ga x In 1-x P active layer 103, so that it may not lattice-match with GaAs. More specifically, when x > 0.5, since a lattice constant of the Ga x In 1-x P is smaller than GaAs, a tensile strain is applied to the active layer 103 in the layer structure shown in figure 1(a). When x < 0.5, since the lattice constant of the Ga x In 1-x P is larger than GaAs, a compressive strain is applied to the active layer 103. In figure 1(c), a height of an effective barrier for confining electrons Ue increases because of the MQB 104. More concretely, the height of the effective barrier Ue is twice as high as a difference in energy band gaps Δ Ec (∼150 meV) between the active layer 103 and the p type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P buffer layer 104a at the conduction band side. A description is given of laser characteristics of the semiconductor laser device on the basis of a result of an experiment. When the laser characteristics were examined using a Ga x In 1-x P active layer 150 angstroms thick while varying x in a range of 0.5 to 0.45, a strain of 0 to 0.45 % was found. Figure 2 is a graph showing a relation between the strain applied to the active layer 103 and a threshold current density of the laser. As shown in figure 2, when the MQB is provided or the strain is applied to the active layer, the threshold current density is about 3 KA/cm², which means that the threshold current density is not sufficiently reduced. However, when the provision of the MQB and the application of strain to the active layer are combined, the threshold current density is significantly reduced to about 1.2 KA/cm². In this first example, the active layer 103 is only 150 angstroms thick to increase the output power of the laser. Generally, when the active layer is thin, light is not sufficiently confined in the active layer and a greater part of the light overflows into the cladding layer. Therefore, a light density at a laser output facet of the laser device decreases and a destruction of the facet caused by a high power operation is avoided, so that a high output laser device is realized. However, when the active layer is thin, its volume is small and the carriers in the active layer, for example, electrons easily climb over the light confining barrier Δ Ec shown in figure 1(c) and flow into the cladding layer. This overflow of electrons causes a significant increase in the threshold current. In order to avoid this, it is effective to increase the barrier for confining electrons by the MQB. However, as is evident from the result of the experiment shown in figure 2, the threshold current density is not sufficiently reduced by only providing the MQB. On the other hand, it is well known that the threshold current density is reduced by applying a strain to the active layer. For example, when a compressive strain is applied to the active layer, an effective mass of holes decreases, i.e., an effective density of state of a valence band decreases, so that a threshold carrier concentration, at which laser oscillation begins, is lowered, resulting in a reduction in the threshold current density. However, as shown in figure 2, even when a strain is applied to the active layer, the threshold current density is as high as 2.8 KA/cm², which means that the threshold current density is not sufficiently reduced. Although it is not shown in figure 2, in a laser structure having an active layer 150 angstroms thick, if no MQB is provided and no strain is applied to the active layer, no lasing oscillation occurs. Therefore, by only providing the MQB or applying the strain to the active layer, the threshold current density is reduced to a certain extent. In the first example, since the laser structure includes both the MQB and the strained active layer having a crystal composition in which a lattice constant is different by 0.1 percent or more from a lattice constant of the substrate, the threshold current density is significantly reduced as shown in figure 2. Figure 3(a) is a cross-sectional view showing a semiconductor laser device incorporating an MQB in accordance with a second example. In figure 3(a), a strained active layer 200 is disposed on an n type cladding layer 102. The active layer 200 comprises (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P barrier layers 201 each 80 angstroms thick and Ga A In 1-A P well layers (A = 0.5) 202 each 100 angstroms thick. The same reference numerals as those shown in figure 1 designate the same parts. While in the first example the active layer 103 is a single crystal layer, in this second example the active layer 200 has a multiquantum well structure comprising the barrier layers 201 and the well layers 202 as shown in figure 3(b). In this example, strains are applied to the well layers 202, whereby the whole active layer 200 has a strain and the same effect as described in the first example is obtained. While in the above-described second example strains are applied to all well layers 202, strain may be applied to only one well layer. Also in this case, the same effect as described above is obtained. In the above-described first and second examples, since the laser includes the MQB and the strained active layer, the threshold current density, at which the lasing oscillation begins, is significantly reduced. However, when a compressive strain is applied to the active layer, an effective mass of holes in the active layer is reduced, whereby the holes are likely to overflow into the cladding layer. Accordingly, if the overflow of holes can be reduced, the threshold current density is further reduced. Figure 4 is a cross-sectional view showing a semiconductor laser device incorporating an MQB in accordance with a third example. In figure 4, an MQB 107 is inserted between an active layer 103 and an n type cladding layer 102. The same reference numarals as in figure 3(a) designate the same parts. While in the first example : the MQB 104 is inserted between the p type cladding layer and the active layer, in this third example, in addition to the MQB 104, an n type MQB 107 is inserted between the n type cladding layer 102 and the active layer 103. The n type MQB 107 increases a barrier for confining holes in the active layer 103, whereby the threshold current density is further reduced. Since the MQB 107 employed in the third example has the same structure as the conventional one, the increase in the barrier for confining holes in the active layer 103 is not sufficient. A description is now given of an MQB producing a higher barrier for confining carriers, especially holes, than the conventional MQB. Figure 5(a) is a cross-sectional view showing a semiconductor laser device incorporating an MQB, in accordance with a fourth example, embodying the present invention. Figure 5(b) is a cross-sectional view showing a structure of the MQB in detail. In the figures, a strained MQB 304 is inserted between an active layer 103 and an n type cladding layer 102. The MQB 304 comprises twenty layers, i.e., a (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P barrier layer 304a 226 angstroms thick, ten Ga z In 1-z P well layers 304b each 17 angstroms thick, and nine (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P barrier layers 304c each 11.5 angstroms thick. In addition, the same reference numerals as those in figure 1 designate the same parts. In the MQB structure of this example, when z is smaller than 0.5 in the Ga z In 1-z P well layer 304b, a compressive strain is applied to the well layer and an effective mass of holes in the well layer decreases. Figure 6 is a graph logically showing how the height of the barrier for confining holes varies with the decrease in the effective mass of holes. In figure 6, reference character ΔEv designates an amount of band discontinuity at a valence band at a heterojunction portion between the Ga x In 1-x P active layer and the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P cladding layer. Reference character Uh designates the height of the effective barrier for confining holes. Reference character m * ho designates an effective mass of holes in the Ga z In 1-z P well layer 304b which lattice-matches with GaAs when z is equal to 0.5. Reference character m * h designates an effective mass of holes in the Ga z In 1-z P well layer 304b when z is smaller than 0.5. In addition, the axis of abscissas shows a ratio of m * h to m * ho and the axis of ordinates shows Uh relative to Δ Ev. As shown in figure 6, when z is equal to 0.5 in the Ga z In 1-z P, i.e., when m * ho is equal to m * h (m * h /m * ho = 1), Uh is about 1.4ΔEv. In other words, when no strain is applied to the Ga z In 1-z P well layer 304b, the height of the barrier for confining holes increases by only 40 percent. However, when a compressive strain is applied to the Ga z In₁₋ z P well layer 304b, i.e., when z is smaller than 0.5, Uh increases with the decrease in the effective mass of holes. For example, when m * h /m * ho is equal to 0.25, i.e., when the effective mass of holes is reduced, by applying the compressive strain, to one fourth of the effective mass of holes in the case where no strain is applied, Uh is approximately equal to 2.4ΔEv, which means that the height of the barrier for confining holes significantly increases. In this way, the strained MQB of this embodiment is effective in increasing the height of the barrier for confining holes. In the semiconductor laser device of this example, the strained MQB 304 is inserted between the active layer 103 and the n type cladding layer 102 and the MQB 104, to which no strain is applied, is inserted between the active layer 103 and the p type cladding layer 105. In this structure, electrons and holes are effectively confined in the active layer by the MQBs 104 and 304 and never overflow into the cladding layers, resulting in a semiconductor laser which oscillates with very low threshold current. In addition, a temperature characteristic of the laser is improved, resulting in a semiconductor laser which operates at a high temperature. In addition, since the strained MQB also increases the height of the barrier for confining electrons, the strained MQB may be inserted between the p type cladding layer and the active layer. Figure 7(a) is a cross-sectional view showing a semiconductor laser device incorporating an MQB, in accordance with a fifth example, and figure 7(b) is a cross-sectional view showing the MQB in detail. In this example, a description is given of an MQB which is effective in improving characteristics of an AlGaInP laser device producing visible light. In figure 7(b), an MQB 404 has a superlattice multilayer structure comprising a plurality of AlGaInP barrier layers 404a and a plurality of AlGaAs well layers 404b. An energy band gap of the barrier layer 404a should always be higher than that of the well layer 404b. For example, when the barrier layer 404a comprises (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P and the well layer 404b comprises GaAs, a difference in energy band gaps between these layers is about 1.6 eV. This is by 1.2 eV larger than the difference in energy band gaps of 0.4 eV between the Ga 0.5 In 0.5 P well layer and the (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P barrier layer shown in figure 1(b). Therefore, the height of the barrier for confining carriers is further increased. Furthermore, another merit of this example resides in that the well layer 404b can be highly doped. Generally, it is difficult to dope impurities into AlGaInP to a high concentration. Accordingly, the MQB comprising only AlGaInP system material has a relatively high resistance, increasing series resistance of the laser. As the result, consumption power of the laser increases. In the MQB structure of this example, since the well layer comprises AlGaAs system material to which impurities can be doped to a high concentration, the resistance of the MQB itself is reduced and the series resistance of the laser is reduced. Thus, the height of the effective barrier caused by the MQB is further increased while the series resistance caused by the MQB is reduced, resulting in a semiconductor laser device producing visible light which consumes less power. According to the fifth example, a semiconductor laser device that confines carriers with high efficiency and reduces the consumption power is realized. While in the fifth example as described so far, no strain is applied to the MQB 404, a strain may be applied to the MQB in accordance with the present invention by adjusting the composition of the barrier layer, whereby the carrier confining effect is further increased.

Claims: A semiconductor laser device including a first conductivity type substrate (101), and a first conductivity type cladding layer (102), an active layer (103), and a second conductivity type cladding layer (105), disposed on said substrate, said active layer including a layer having a crystal composition such that the lattice constant of said layer is different from the lattice constant of said substrate by 0.1 percent or more so that a compressive strain is applied to said layer; characterised by a multiquantum barrier (104;304;404), comprising supperlattice semiconductor layers (104b;104c; 304b;304c;404a;404b), disposed between said active layer and either of said cladding layers, said multiquantum barrier being constituted by alternating semiconductor layers having large energy band gaps and semiconductor layers having small energy band gaps and at least one of the semiconductor layers having a crystal composition such that the lattice constant of said layer is different from the lattice constant of the substrate by 0.1 percent or more so that a compressive strain is applied to said layer. The semiconductor laser device of claim 1, wherein said active layer comprises a plurality of semiconductor layers including at least one layer to which a strain is applied. The semiconductor laser device of claim 1 wherein said multiquantum barrier (304) is disposed between the first conductivity type cladding layer and the active layer and a further multiquantum barrier (104) is disposed between the second conductivity type cladding layer and the active layer. The semiconductor laser device of claim 1 wherein said multiquantum barrier is disposed between an n type cladding layer and the active layer. The semiconductor laser device of claim 1 wherein said multiquantum barrier comprises AlGaAs system material.

Inventors: ARIMOTO SATOSHI, MOTODA TAKASHI, NISHIMURA TAKASHI, ARIMOTO, SATOSHI, MOTODA, TAKASHI, NISHIMURA, TAKASHI, ARIMOTO, SATOSHI, C/O MITSUBISHI DENKI K.K., MOTODA, TAKASHI, C/O MITSUBISHI DENKI K.K., NISHIMURA, TAKASHI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01S3
IPC codes: H01S5