JP3775259B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitride semiconductor (In_XAl_YGa_1-XYN, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).
[0002]
[Prior art]
A laser element in which an active layer is doped with Si is described in JP-A-7-297494. This publication discloses that the threshold current is reduced by doping Si into a thick active layer made of GaN.
[0003]
However, as described in the above publication, an element structure having a thick active layer with a single layer thickness of, for example, 0.1 μm or more has a weak output and is very difficult to cause laser oscillation. A laser element having an active layer having a multiple quantum well structure in which a well layer and a barrier layer having a single film thickness in the vicinity of 100 angstroms are stacked is described, for example, in JP-A-8-64909. . This publication describes a laser element having an active layer having a multiple quantum well structure in which a well layer is doped with Zn. By doping a very small amount of Zn into a well layer, an acceptor-like structure is obtained. It has been shown that a simple impurity level is formed to reduce the threshold current. Furthermore, JP-A-6-268257 discloses In._XGa_1-XN well layer and In_YGa_1-YA light-emitting element having an active layer having a multiple quantum well structure in which a barrier layer made of N is stacked is shown. Further, this publication may be doped with an n-type impurity or a p-type impurity. Are listed.
[0004]
It is known that doping the n-type and p-type impurities in the active layer and forming impurity levels in the band gap increases the light emission output of the light emitting element and lowers the threshold current of the laser element. It has been.
[0005]
[Problems to be solved by the invention]
By the way, the present applicant recently announced a 410 nm laser oscillation at a room temperature with a nitride semiconductor at a pulse current (for example, Jpn.J.Appl.Phys.Vol35 (1996) pp.L74-76). The announced laser device is a so-called electrode stripe type laser device, and has an active layer of a multiple quantum well structure in which non-doped InGaN is stacked.
[0006]
However, the nitride semiconductor laser is still only pulsed and has a threshold current of 1 to 2A. In order to continuously oscillate the nitride semiconductor, it is necessary to further reduce the threshold current.
[0007]
Accordingly, an object of the present invention is to increase the light emission output of a laser element made of a nitride semiconductor and further reduce the threshold current to achieve continuous oscillation at room temperature.
[0008]
  In a nitride semiconductor laser device having an active layer having a multiple quantum well structure in which a well layer made of a nitride semiconductor containing indium and a barrier layer made of a nitride semiconductor having a larger band gap than the well layer are stacked.The active layer is made of In. _X Ga _1-X N (0 < X ≦ 1) well layer and In _{X '} Ga _{1-X '} N (0 < X '≦ 1, X ' < X ) Barrier layer is laminated,The well layer of the active layer has a region with a large In composition and a region with a small In composition, andN-type impurity is doped in the barrier layerA nitride semiconductor laser device characterized by the above.
[0009]
The nitride semiconductor laser element according to claim 1, wherein the n-type impurity is Si.
[0010]
  The Si concentration in the barrier layer is 1 × 10 ¹⁸ / Cm ³ ~ 1x10 ²² / Cm ³ The nitride semiconductor laser element according to claim 1 or 2, wherein
[0011]
  4. The nitride semiconductor laser element according to claim 1, further comprising a p-type nitride semiconductor layer containing Al in contact with the active layer. 5.
[0012]
  5. The nitride semiconductor laser device according to claim 1, wherein the n-type contact layer in contact with the n-electrode includes an n-type nitride semiconductor containing Al and Ga. 6.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention. As a basic structure, a buffer layer 2, an n-type contact layer 3, an n-type cladding layer 4, an active layer 5 having a multiple quantum well structure doped with impurities, and a first p-type layer are formed on a substrate 1. 6, a second p-type layer 7, a third p-type layer 8, and a p-type contact layer 9 are stacked in order, and the n-type contact layer has a striped negative electrode. The p-type contact layer is provided with a positive electrode.
[0021]
The spectrum when each pulse current is passed through this laser element is shown in FIG. In FIG. 3, (a) shows an emission spectrum at 280 mA (immediately after the threshold), (b) shows 295 mA, (c) shows 320 mA, and (d) shows an emission spectrum at 340 mA. (B), (c), and (d) show spectra during oscillation.
[0022]
(A) shows the spectrum immediately after oscillation, and in this state, it can be seen that many small emission peaks (Fabry-Perot mode) appear before and after the main emission peak in the vicinity of about 404.2 nm, and that the state is immediately after laser oscillation. . This is the so-called longitudinal mode spectrum. When the current value is increased, as shown in (b), the spectrum becomes a single mode, and laser oscillation near 404.2 nm is exhibited. The following is a feature of the present invention. When the current is further increased, as shown in (c), 403.3 nm (3.075 eV), 403.6 nm (3.072 eV), 403.9 nm (3.070 eV). ), 404.2 nm (3.068 eV), and 404.4 nm (3.066 eV), in addition to the main emission peak, emission peaks with high intensity appear irregularly at intervals of 1 meV to 100 meV. Further, in (d), a new peak clearly appears in addition to the peak. These spectral intervals are not constant and are clearly different from the longitudinal mode spectra.
[0023]
In general, in the case of a semiconductor laser, when laser oscillation occurs, many small emission peaks due to the longitudinal mode of laser light appear before and after the main emission peak. The emission spectrum in this case consists of emission peaks at approximately equal intervals. In the red semiconductor laser, the interval between the emission peaks is about 0.2 nm. For a blue semiconductor laser, it is about 0.05 nm (1 meV) or less (however, the longitudinal mode of the blue semiconductor laser was measured for the first time by the present applicant at a resonator length of 600 μm). That is, the behavior of a normal laser element is shown in the states of FIGS. However, in the case of the laser element of the present invention, as shown in (c) and (d), there are clearly many peaks that are not equally spaced apart from the emission peak due to the longitudinal mode of the conventional laser element. This can also be seen by comparing the spectra according to the current values in FIG. In the laser device of the present invention, the output is increased by the appearance of such an emission spectrum.
[0024]
The reason why the output of the laser element increases when such a peak occurs is not clear, but the following may be considered, for example. When the active layer has a quantum well structure, the thickness of the well layer is adjusted to 100 angstroms or less, preferably 70 angstroms or less, and most preferably 50 angstroms or less. On the other hand, the barrier layer is also adjusted to 150 angstroms or less, preferably 100 angstroms or less. In the light emitting device of the present invention, when such a thin film having a single film thickness of several tens of angstroms is stacked, neither the well layer nor the barrier layer grows with a uniform film thickness, and the uneven layer is overlapped. Are also in an overlapping state. FIG. 2 is a schematic cross-sectional view showing an enlarged state of the interface between the active layer 5 and the cladding layer in the laser element of FIG. As shown in FIG. 2, when a double heterostructure in which such an uneven active layer is sandwiched between cladding layers having a band gap larger than that of the active layer is realized, electrons and holes injected into the active layer are formed in the recesses. Is also confined, and confined in both the vertical and horizontal directions together with the vertical direction of the cladding layer. For this reason, the carriers are confined in a quantum box or quantum disk made of three-dimensional InGaN having a difference in roughness of about 10 to 70 angstroms, and a quantum effect different from the conventional quantum well structure appears. . Accordingly, light emission based on a large number of quantum levels is observed even at room temperature, and a large number of light emission peaks are observed at intervals of 1 meV to 100 meV in the emission spectrum. Another reason is that, since carriers are confined in a small quantum box made of three-dimensional InGaN, the exciton effect appears prominently and many emission peaks are observed.
[0025]
In addition, in-plane non-uniformity of the In composition can be considered as one of the reasons for the large number of irregularities in the InGaN well layer. That is, since a region with a large In composition and a region with a small In composition can be formed in a single well layer, many irregularities are generated on the surface of the well layer. InGaN is a material in which a mixed crystal is difficult to grow, and InN and GaN tend to phase-separate. For this reason, a region having a non-uniform In composition is formed. Then, electrons and holes are localized in the region having a high In composition, and exciton light emission or biexiton light emission is performed, so that the output of the laser is improved and a large number of peaks are formed. In particular, in a laser element, this bi-exciton laser oscillation causes a large number of peaks to appear in the same manner as the quantum disk and quantum box, and the threshold value of the laser element decreases due to the large number of peaks, and the output is improved. Exciton is a pair in which electrons and holes are bonded together by a weak Coulomb force.
[0026]
Furthermore, the threshold current can be reduced by doping the active layer with n-type impurities and / or p-type impurities. By doping these impurities, the exciton localized in the region with a large In composition in the active layer is now localized in a deeper impurity level, and the exciton emission When the effect becomes significant, the threshold value decreases.
[0027]
What has been described about the active layer of the laser element of the present invention is shown in an easily understandable manner in the energy band diagram of FIG. 6A shows an energy band of an active layer having a multiple quantum well structure, and FIG. 6B shows an enlarged energy band of the single well layer of FIG. 6A. As described above, in-plane nonuniformity of the In composition in the well layer means that there are InGaN regions having different band gaps in a single InGaN well layer width as shown in FIG. Accordingly, electrons in the conduction band once fall into an InGaN region having a large In composition, and recombine with holes in the valence band, thereby releasing hν energy. This localizes electrons and holes in the well layer-wide region with a large In composition, forms localized excitons, and helps to lower the laser threshold. The lower threshold and higher output are due to the effect of this localized exciton. Further, by doping this well layer with an n-type impurity such as Si or a p-type impurity such as Zn, a level of an impurity level can be further formed between the conduction band and the valence band. In FIG. 6B, the level is shown by Si and Zn. When an impurity is doped, an energy level at an impurity level is formed. Therefore, the electrons fall to a deeper level, and the holes move to the level of the p-type impurity, where the electrons and holes recombine and release a smaller energy of hν ′. This means that electrons and holes are further localized, and this localized exciton effect lowers the laser threshold. The reason why a large number of peaks appear is that, in addition to the localized excitons, light is emitted between a large number of quantum levels due to the effect of the quantum box confined in three dimensions.
[0028]
Examples of the n-type impurity include Si, Ge, Sn, Se, and S. Examples of the p-type impurity include Zn, Cd, Mg, Be, and Ca. By doping these impurities into the active layer, particularly preferably into the well layer, light emission at the impurity level is caused between the quantum levels, the energy level between the bands is reduced, and the threshold is lowered. Can do. Needless to say, both n-type and p-type impurities may be doped.
[0029]
Particularly preferably, doping the n-type impurity, particularly Si or Ge, can increase the emission intensity and reduce the threshold current. FIG. 4 is a diagram showing the ratio of Si concentration doped into the well layer and the rate of decrease in threshold current. Specifically, in a laser element having an active layer having a multiple quantum well structure in which a well layer made of InGaN having an average film thickness of 30 angstroms and an average film thickness of 70 angstrom barriers are stacked, the well layer is doped with Si. The ratio of the decrease in the threshold value of the laser element at this time is shown, and each point shown in the figure shows the actual Si concentration. As shown in this figure, by doping Si, the threshold current can be reduced by nearly 50% at maximum. Therefore, the preferred Si concentration is 1 × 10¹⁸/cm³~ 1x10²²/cm³And more preferably 5 × 10¹⁸/cm³~ 2x10²¹/cm³, Most preferably 1 × 10¹⁹/cm³~ 1x10²¹/cm³It is. Although this figure is shown for Si, it was confirmed that the same tendency was observed for other n-type impurities, Ge, Sn, and the like.
[0030]
FIG. 5 is a graph showing the Mg concentration doped in the well layer and the ratio of the reduction rate of the threshold current. This is also a laser element having an active layer having a multiple quantum well structure in which a well layer made of InGaN having an average film thickness of 30 angstroms and an average film thickness of 70 angstrom barriers are laminated, and the well layer is doped with Mg. The ratio of the decrease in the threshold value of the laser element is shown, and each point shown in the figure indicates the actual Mg concentration. As shown in this figure, the threshold current can be reduced by nearly 25% by doping Mg. The preferred Mg concentration is 1 × 10¹⁷/cm³~ 1x10²²/cm³And more preferably 1 × 10¹⁸/cm³~ 2x10²¹/cm³, Most preferably 1 × 10¹⁸/cm³~ 1x10²¹/cm³It is. In addition, although this figure showed about Mg, it confirmed that there existed the same tendency also about other p-type impurities, Zn, Cd, Be, etc.
[0031]
【Example】
Hereinafter, a method of obtaining the laser element having the structure shown in FIG. 1 using the MOVPE method will be described. FIG. 1 shows one structure of the laser element of the present invention, and the laser element of the present invention is not limited to this structure. In the present invention, In_XGa_1-XN, Al_YGa_1-YGeneral formulas such as N merely indicate the composition formula of the nitride semiconductor, and even if different layers are indicated by the same formula, these layers never show the same composition.
[0032]
[Example 1]
A substrate 1 having a sapphire A surface as a main surface is prepared, and this substrate 1 is placed in a reaction vessel of a MOVPE apparatus, and then TMG (trimethylgallium) and ammonia are used as source gases at a temperature of 500 ° C. A buffer layer 2 made of GaN is grown on the surface of 1 to a thickness of 200 angstroms. In addition to the A plane, the substrate 1 can use sapphire having a plane orientation such as a C plane and an R plane. In addition to sapphire, a spinel 111 plane (MgAl₂O₄), A known substrate made of a single crystal such as SiC, MgO, Si, ZnO, or GaN is used. The buffer layer 2 is provided to alleviate the lattice mismatch between the substrate and the nitride semiconductor. Usually, GaN, AlN, AlGaN, etc. are grown with a film thickness of 1000 angstroms or less. When a close substrate or a lattice-matched substrate is used, it may not be grown depending on factors such as the growth method and growth conditions, and can be omitted. However, when a substrate having a lattice constant different from that of a nitride semiconductor such as sapphire or spinel is used, the buffer layer 2 is formed at a temperature of 200 ° C. or more and 900 ° C. or less as described in JP-A-4-29723. When grown, the crystallinity of the next nitride semiconductor layer grown at a high temperature is dramatically improved.
[0033]
Subsequently, the temperature is raised to 1050 ° C., TMG, TMA (trimethylaluminum), ammonia, and SiH as donor impurities are used as source gases.₄Using (silane) gas, the n-type contact layer 3 made of Si-doped Al0.3Ga0.7N is grown to a thickness of 4 μm.
[0034]
The n-type contact layer 3 also functions as an optical confinement layer. By making the n-type contact layer 3 an n-type nitride semiconductor containing Al and Ga, preferably n-type AlYGa1-YN (0 <Y <1), the refractive index difference from the active layer can be increased, and optical confinement can be achieved. It acts as a cladding layer as a layer and a contact layer for injecting current. Further, this contact layer is made of Al._YGa_1-YBy setting N, the light emission of the active layer can be made difficult to spread in the n-type contact layer, so that the threshold value is lowered. The n-type contact layer 3 is made of Al_YGa_1-YIn the case of N, it is desirable to have a structure in which the Al mixed crystal ratio on the substrate side is small and the Al mixed crystal ratio on the active layer side is large, that is, a composition gradient structure. With this structure, an n-type contact layer with good crystallinity can be obtained, so that the crystallinity of the nitride semiconductor stacked on the n-type contact layer with good crystallinity is also improved. As a result, the threshold value is lowered and the reliability of the element is remarkably improved. In addition, since the Al mixed crystal ratio on the active layer side is large, the refractive index difference from the active layer is also large, which effectively acts as a light confinement layer. The n-type contact layer 3 may be GaN. In the case of GaN, the ohmic characteristics with the n electrode are very excellent. When the contact layer is GaN, it is necessary to provide an optical confinement layer made of AlGaN between the GaN contact layer and the active layer. The film thickness of the n-type contact layer 3 is desirably adjusted to 0.1 μm or more and 5 μm or less. When the thickness is 0.1 μm or less, it is difficult to act as a light confinement layer, and when the electrodes are provided on the same surface side, a precise etching rate must be controlled, which is disadvantageous. On the other hand, if it is thicker than 5 μm, cracks tend to be easily formed in the crystal.
[0035]
Subsequently, the temperature is maintained at 1050 ° C., and the n-type cladding layer 4 made of Si-doped n-type GaN is grown to a thickness of 500 Å using TMG, ammonia, and silane gas as source gases.
[0036]
The n-type clad layer 4 functions as a buffer layer when InGaN is grown on the light guide layer on the n-layer side and the active layer, and can also grow n-type InGaN in addition to n-type GaN. When growing with a buffer layer, it is desirable to grow with a film thickness of 0.05 μm or less. Further, when the contact layer 2 is grown with GaN as described above, the n-type cladding layer 4 needs to be grown with AlGaN in order to function as an optical confinement layer. In the case of an AlGaN layer, it is desirable that the film thickness be grown to a thickness of 0.01 μm to 0.5 μm. If it is thinner than 0.01 μm, it will hardly act as a light confinement layer, and if it is thicker than 0.5 μm, it tends to crack easily in the crystal.
[0037]
Next, the temperature is set to 750 ° C., and the active layer 5 doped with Si using TMG, TMI (trimethylindium), ammonia, and silane gas as the impurity gas is grown. The active layer 5 is made of 1 × 10 Si first.²⁰/cm³A well layer made of In0.2Ga0.8N doped at a concentration of 25 .ANG. Next, the silane gas is stopped and a barrier layer made of non-doped In0.01Ga0.95N is grown to a thickness of 50 angstroms at the same temperature only by changing the molar ratio of TMI. This operation is repeated 13 times. Finally, a well layer is grown to grow an active layer 5 having a multiple quantum well structure with a total film thickness of 0.1 μm.
[0038]
The active layer 5 has a multiple quantum well structure in which at least the well layer includes a nitride semiconductor containing In. The multiple quantum well structure is a stack of a well layer and a barrier layer. In the case of the present invention, if the well layer is composed of a nitride semiconductor containing In, the barrier layer has a band gap larger than that of the well layer. If is large, it is not particularly necessary to contain In. Preferably, In_XGa_1-XA well layer made of N (0 <X ≦ 1), and In_{X '}Ga_{1-X '}The barrier layer is made of N (0 ≦ X ′ <1, X ′ <X). Since the ternary mixed crystal InGaN has better crystallinity than the quaternary mixed crystal, the light emission output is improved. In addition, the barrier layer has a larger band gap energy than the well layer, and is stacked to form a well + barrier + well +... + Barrier + well layer (or vice versa) to form a multiple quantum well structure. To do. The thickness of the well layer is preferably adjusted to 70 angstroms or less, more preferably 50 angstroms or less. The thickness of the barrier layer is also preferably adjusted to 150 angstroms or less, more preferably 100 angstroms or less. If the well layer is thicker than 70 angstroms or the barrier layer is thicker than 150 angstroms, the output of the laser element tends to decrease. Thus, when the active layer is MQW in which InGaN is stacked, it is possible to realize a high-power LD between about 365 nm and 660 nm by light emission between quantum levels. As a particularly preferred embodiment, when both layers are InGaN, InGaN is softer than GaN and AlGaN crystals. Therefore, since the thickness of the first p-type AlGaN layer can be increased, laser oscillation can be realized. Further, the n-type impurity may be doped into the well layer as in this embodiment, may be doped into the barrier layer, or may be doped into both the well layer and the barrier layer.
[0039]
The thickness of the active layer 5 is such that the n-type contact layer 3 is made of Al._YGa_1-YIn the case of N, it is desirable to grow with a film thickness of 200 angstroms or more, more preferably 300 angstroms or more. This is because by growing the active layer made of MQW thickly, the vicinity of the outermost layer of the active layer acts as a light guide layer. That is, the n-type contact layer 3 and the third p-type layer 8 function as a light confinement layer, and the vicinity of the outermost layer of the active layer functions as a light guide layer. The upper limit of the thickness of the active layer is not particularly limited, but it is usually desirable to adjust it to 0.5 μm or less.
[0040]
Next, TMG, TMA, ammonia, and Cp2Mg (cyclopentadienylmagnesium) as the p-type impurity are used as source gases, and the first p-type layer 6 made of Mg-doped p-type Al0.2Ga0.8N is formed to 100 angstroms. Grow with film thickness.
[0041]
The first p-type layer 6 is composed of a p-type nitride semiconductor containing Al, preferably ternary mixed crystal or binary mixed crystal Al._YGa_1-YIt is desirable to grow N (0 <Y ≦ 1). Further, this AlGaN is desirably formed thinner than a third p-type layer 8 described later, and is preferably adjusted to 10 angstroms or more and 0.5 μm or less. By forming the first p-type layer 6 in contact with the active layer 5, the output of the element is remarkably improved. This is presumed to be due to the action of suppressing decomposition of InGaN in the active layer during the growth of the first p-type layer 6, but the details are unknown. The first p-type layer 6 is preferably grown to a thickness of 10 angstroms to 0.5 μm or less, but may be omitted.
[0042]
Next, the temperature is set to 1050 ° C., and the second p-type layer 7 made of Mg-doped p-type GaN is grown to a thickness of 500 Å using TMG, ammonia, and Cp 2 Mg.
[0043]
The second p-type layer 7 functions as a light guide layer or a buffer layer on the p-layer side, and preferably a binary mixed crystal or a ternary mixed crystal In_YGa_1-YN (0 ≦ Y <1) is grown. The second p-type layer 7 acts as a light guide layer when grown when the active layer is thin. Further, since the first p-type layer 6 is made of AlGaN or the like, this layer acts like a buffer layer, and the third p-type layer 8 to be grown next can be grown with good crystallinity without cracks. In other words, if AlGaN with a large band gap is laminated directly on AlGaN, cracks are likely to occur in AlGaN with a large band gap grown later, so that cracks are less likely to occur through this second p-type layer 7. is doing. The second p-type layer 7 is usually preferably grown to a thickness of about 100 Å to 0.5 μm, but can be omitted.
[0044]
Next, the temperature is raised to 1050 ° C., TMG, TMA, ammonia, Cp 2 Mg is used as the source gas, and the third p-type layer 8 made of Mg-doped Al 0.3 Ga 0.7 N is formed to a film thickness of 0.3 μm. Grow in.
[0045]
The third p-type layer 8 is made of a nitride semiconductor containing Al, preferably binary mixed crystal or ternary mixed crystal Al._YGa_1-YN (0 <Y ≦ 1) is grown. The third p-type layer 8 functions as an optical confinement layer, and is desirably grown at a film thickness of 0.1 μm to 1 μm. Preferably, the third p-type layer 8 is a p-type nitride semiconductor containing Al, such as AlGaN. Acts as a confinement layer. This third p-type layer can also be grown because the active layer 5 is made of a nitride semiconductor containing In. That is, since the active layer containing InGaN acts like a buffer layer, AlGaN can be easily grown in a thick film. On the other hand, it is difficult to grow a nitride semiconductor containing Al on the nitride semiconductor layer containing Al with a thick film that directly becomes a light confinement layer.
[0046]
Subsequently, a p-type contact layer 9 made of Mg-doped p-type GaN is grown to a thickness of 0.5 μm using TMG, ammonia, and Cp 2 Mg at 1050 ° C.
[0047]
The p-type contact layer 9 is a layer for injecting current, and a p-type nitride semiconductor (In_XAl_YGa_1-XYN, 0.ltoreq.X, 0.ltoreq.Y, X + Y.ltoreq.1). In particular, when p-type GaN doped with InGaN, GaN, and Mg is used, a p-type layer with the highest carrier concentration is obtained. Good ohmic contact with the positive electrode can be obtained, and the threshold current can be reduced. As the material of the positive electrode, a metal or alloy having a relatively high work function such as Ni, Pd, Ir, Rh, Pt, Ag, Au, or the like tends to provide an ohmic property.
[0048]
The nitride semiconductor laminated wafer as described above is taken out of the reaction vessel and selectively etched from the uppermost p-type contact layer 9 as shown in FIG. 1 to expose the surface of the n-type contact layer 3 and expose it. After forming striped electrodes on the surfaces of the n-type contact layer 3 and the p-type contact layer 9, the wafer is cleaved from the R surface of the sapphire substrate to form a bar and orthogonal to the striped electrode. The resonant surface of the laser is formed in the direction of the laser, and the resonator length is 600 μm. Thereafter, in accordance with an ordinary method, a reflecting mirror made of a dielectric multilayer film is formed on the resonance surface, and then the wafer is divided at a position parallel to the striped electrode to form a laser chip. When this laser chip was placed on a heat sink and pulse oscillation with a forward current of 320 mA was attempted, laser oscillation with emission peaks at irregular positions as shown in FIG. 3C was exhibited, and the active layer was doped with impurities. The threshold current was reduced by 50% and the output was improved by 30% compared to a laser element that was not.
[0049]
[Example 2]
In the step of growing the active layer of Example 1, using Zn as the impurity gas instead of silane gas, Zn was 1 × 10¹⁹/cm³A well layer made of In0.2Ga0.8N doped at a concentration of 25 angstroms and a barrier layer made of non-doped In0.01Ga0.95N are grown to a thickness of 50 angstroms. A laser element having a resonator length of 600 μm was obtained in the same manner except that the active layer 5 having the structure was grown. The threshold current was reduced by 25% compared to a laser element in which no impurity was doped in the active layer. The output was improved by 10%.
[0050]
[Example 3]
In the step of growing the active layer of Example 1, Si was used in an amount of 1 × 10 6 by using silane gas and diethyl zinc as impurity gases.²⁰/cm³, And Zn 1 × 10¹⁹/cm³A well layer made of In0.2Ga0.8N doped at a concentration of 25 angstroms and a barrier layer made of non-doped In0.01Ga0.95N are grown to a thickness of 50 angstroms. A laser element having a cavity length of 600 μm was obtained in the same manner except that the active layer 5 having the structure was grown. The threshold current was reduced by 60% compared to a laser element in which no impurity was doped in the active layer. The output was improved by 35%.
[0051]
【The invention's effect】
As described above, the laser device of the present invention has a light emission output improved by having a region with a large In composition and a region with a small In composition in one well layer of the active layer. Further, since the n-type impurity and the p-type impurity are doped in the active layer, the threshold value can be reduced without reducing the light emission output. For this reason, a laser element having a high light emission output and a low threshold value can be realized. In addition, the threshold current may be further reduced by using the laser element of the present invention as a laser element that stabilizes a transverse mode such as a buried hetero type, refractive index guided type, or effective refractive index guided type. .
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention.
2 is an enlarged schematic cross-sectional view showing the vicinity of an active layer of the laser device of FIG.
FIG. 3 is a diagram showing emission spectra when a pulse current is passed through the laser element of the present invention by comparing each current value.
FIG. 4 is a graph showing the relationship between the Si concentration doped in the active layer and the threshold current reduction rate of the laser element.
FIG. 5 is a graph showing the relationship between the Mg concentration doped in the active layer and the reduction rate of the threshold current of the laser element.
FIG. 6 is an energy band diagram of a well layer of the laser device of the present invention.
[Explanation of symbols]
1 ... Board
2 ... Buffer layer
3 ... n-type contact layer
4 ... n-type cladding layer
5 ... Active layer
6: First p-type layer
7: Second p-type layer
8: Third p-type layer
9 ... p-type contact layer

Claims (5)

In a nitride semiconductor laser device having an active layer having a multiple quantum well structure in which a well layer made of a nitride semiconductor containing indium and a barrier layer made of a nitride semiconductor having a larger band gap than the well layer are stacked.
The active _{layer, In X Ga 1-X N} (0 <X ≦ 1) and a well layer made _{of, In X 'Ga 1-X} ' N (0 <X '≦ 1, X'<X) from become barriers Layers are stacked,
One well layer of the active layer has a region with a large In composition and a region with a small In composition,
A nitride semiconductor laser element, wherein the barrier layer is doped with an n-type impurity . The nitride semiconductor laser element according to claim 1, wherein the n-type impurity is Si. The Si concentration in the barrier layer is 1 × 10 ¹⁸ / Cm ³ ~ 1x10 ²² / Cm ³ The nitride semiconductor laser element according to claim 1 or 2, wherein 4. The nitride semiconductor laser element according to claim 1, further comprising a p-type nitride semiconductor layer containing Al in contact with the active layer. 5. 5. The nitride semiconductor laser element according to claim 1, wherein the n-type contact layer in contact with the n-electrode includes an n-type nitride semiconductor containing Al and Ga. 6.

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