JP2008177624A5 - - Google Patents

Description

Semiconductor laser light source device provided with gallium nitride based semiconductor laser element

  The present invention relates to a gallium nitride based semiconductor light emitting device, and more particularly to a gallium nitride based semiconductor laser device having a quantum well structure active layer made of a nitride semiconductor containing at least indium and gallium.

  Gallium nitride based semiconductors (GaInAlN) are used as semiconductor materials such as semiconductor laser elements (LD) and light emitting diode elements (LEDs) having an emission wavelength in the ultraviolet to green wavelength region. The blue LD using the gallium nitride semiconductor is described in, for example, Applied Physics Letters, vol. 69, No. 10, p. 1477 to 1479 (Non-Patent Document 1), and a cross-sectional view thereof is shown in FIG. Show.

In FIG. 10, 101 is a sapphire substrate, 102 is a GaN buffer layer, 103 is an n-GaN contact layer, 104 is an n-In _0.05 Ga _0.95 N layer, 105 is an n-Al _0.05 Ga _0.95 N clad layer, 106 is an n- A GaN guide layer, 107 is an active layer having a multiple quantum well structure composed of an In _0.2 Ga _0.8 N quantum well layer and an In _0.05 Ga _0.95 N barrier layer, 108 is a p-Al _0.2 Ga _0.8 N layer, and 109 is a p-GaN guide layer. , 110 is a p-Al _0.05 Ga _0.95 N clad layer, 111 is a p-GaN contact layer, 112 is a p-side electrode, 113 is an n-side electrode, and 114 is a SiO ₂ insulating film. Here, the multi-quantum well structure active layer 107 is composed of a total of nine layers including five 3 nm thick In _0.2 Ga _0.8 N quantum well layers and four 6 nm thick In _0.05 Ga _0.95 N barrier layers. Well layers and barrier layers are alternately formed.

  In addition, Japanese Patent Application Laid-Open No. 8-316528 (Patent Document 1) similarly describes a blue LD using a gallium nitride-based semiconductor, and these are all multi-quantum having five or more quantum well layers. A well structure active layer was used.

On the other hand, a blue LED using a gallium nitride based semiconductor is described in, for example, the above-mentioned Japanese Patent Application Laid-Open No. 8-316528, and a cross-sectional view thereof is shown in FIG. In FIG. 11, 121 is a sapphire substrate, 122 is a GaN buffer layer, 123 is an n-GaN contact layer, 124 is an n-Al _0.3 Ga _0.7 N second n-type cladding layer, and 125 is an n-In _0.01 Ga _0.99 GaN first n-type. Cladding layer 126, 3 nm thick In _0.05 Ga _0.95 N single quantum well structure active layer, 127 p-In _0.01 Ga _0.99 GaN first p-type cladding layer, 128 p-Al _0.3 Ga _0.7 N second p-type cladding layer 129 is a p-GaN contact layer, 130 is a p-side electrode, and 131 is an n-side electrode. Thus, in the blue LED using the gallium nitride semiconductor, an active layer having only one quantum well layer has been used.

  However, the conventional blue LD and blue LED element have the following problems.

  First, for blue LD, the oscillation threshold current value is as high as 100 mA or more, and it is necessary to significantly reduce the oscillation threshold current value for practical use for information processing such as an optical disk. Further, when an LD is used for an optical disk, it is necessary to inject a high frequency current having a frequency of about 300 MHz into the LD and to modulate the optical output at the same frequency in order to prevent a data read error due to noise during data read. The conventional blue LD has a problem that a data read error occurs because the optical output is not modulated even when a high frequency current is injected.

Also, in order to provide blue LED elements for a wider range of applications, such as those that have already been put to practical use with respect to blue LEDs, such as large color displays that can display brightly even with a wide viewing angle, It is desired to realize LEDs with higher brightness by improvement.
Applied Physics Letters, vol. 69, No. 10, pp. 1477 to 1479. JP-A-8-316528

The present invention has been made in view of the circumstances as described above, and solves the problems in the gallium nitride semiconductor light emitting device, and includes a semiconductor laser light source including a gallium nitride semiconductor laser device having good laser oscillation characteristics. An object is to provide an apparatus .

In order to achieve the above object, a semiconductor laser light source device including the gallium nitride based semiconductor laser element of the present invention includes:
A lower cladding layer made of a nitride semiconductor;
A quantum well structure active layer formed on the lower cladding layer and made of a nitride semiconductor containing at least indium and gallium;
An upper cladding layer formed on the quantum well structure active layer, made of a nitride semiconductor, and having a flat portion and a ridge portion;
An insulating film formed on the flat portion and on a side surface of the ridge portion;
An electrode formed on the insulating film and on the upper surface of the ridge portion and electrically connected to the ridge portion;
The quantum well structure active layer is formed of two quantum well layers and one barrier layer sandwiched between them.
Each quantum well layer has a layer thickness of 10 nm or less,
The quantum well structure active layer and the gallium nitride semiconductor laser element characterized that you have formed an oscillation unit,
A drive circuit for injecting current into the gallium nitride based semiconductor laser device;
With
The light output from the gallium nitride based semiconductor laser element is modulated,
The current injected into the gallium nitride based semiconductor laser element is high-frequency modulated, and the modulation frequency of this current is 300 MHz or more .

  As described above, in the gallium nitride based semiconductor laser device according to the present invention, the number of quantum well layers in the multiple quantum well structure active layer made of a nitride semiconductor containing at least indium and gallium is set to 2, and the layers of each quantum well layer By setting the thickness to 10 nm or less, electrons and holes were uniformly distributed in all the quantum well layers. As a result, the electrons and holes injected into the quantum well layer are efficiently recombined, so that the light emission efficiency is improved and the oscillation threshold current value can be reduced. In addition, electrons and holes are effectively injected into the quantum well layer where electrons and holes have disappeared due to recombination, so the density of electrons and holes existing in the quantum well layer is effectively modulated. As a result, the optical output can also be modulated, and a gallium nitride semiconductor laser element that can be used for an optical disk and does not generate an error when reading data has been realized.

  Further, a gallium nitride based semiconductor laser device according to the present invention in which the number of quantum well layers in the active layer having a multi-quantum well structure made of a nitride semiconductor containing at least indium and gallium is 2 and driving for injecting a high frequency current up to about 1 GHz By combining the circuits, a semiconductor laser light source device that does not generate an error when reading data for an optical disk can be provided.

  A configuration of a gallium nitride based semiconductor light emitting device according to the present invention includes a quantum well structure active layer made of a nitride semiconductor containing at least indium and gallium sandwiched between a cladding layer made of a nitride semiconductor and / or a guide layer, The quantum well structure active layer includes two quantum well layers and one barrier layer sandwiched between them.

  In finding the present invention as described above, the inventor conducted a detailed investigation on the cause of the above-described problems in the conventional device, and found the following.

  First, with regard to the blue LD, the InGaN material used as the quantum well layer has a large effective mass of both electrons and holes and a large number of crystal defects, which greatly increases the mobility of electrons and holes. And the electrons and holes are not uniformly distributed in all the quantum well layers of the active layer having the multiple quantum well structure. That is, electrons are injected only to about two quantum well layers on the n-type cladding layer side for injecting electrons, and holes are injected only to about two quantum well layers on the p-type cladding layer side for injecting holes. . Accordingly, in a multi-quantum well structure active layer having five or more quantum well layers, since the ratio of electrons and holes existing in the same quantum well layer is small, the efficiency of light emission due to recombination of electrons and holes is reduced. As a result, the threshold current value of laser oscillation is increased.

  In addition, since the mobility of electrons and holes is small in this way, the movement of electrons and holes between quantum well layers becomes slow, and electrons and holes are newly admitted into the quantum well layer where electrons and holes disappear due to recombination. Holes are not injected, and the electrons and holes injected into the quantum well layer adjacent to the cladding layer continue to exist in the quantum well layer as they are. Therefore, even if the injection current is modulated, the density of electrons and holes existing in the quantum well layer is not modulated. For this reason, even if a high frequency current is injected, the light output is not modulated.

  Therefore, in the present invention, by setting the number of quantum well layers in the active layer having a multiple quantum well structure made of a nitride semiconductor containing at least indium and gallium to two, electrons and holes are uniformly distributed in all the quantum well layers. It was made to distribute. As a result, the light emission efficiency was improved and the oscillation threshold current value could be reduced. In addition, since electrons and holes are effectively injected into the quantum well layer where electrons and holes have disappeared due to recombination, the density of electrons and holes existing in the quantum well layer by injection of high-frequency current is achieved. As a result, the light output can also be modulated.

  Thus, in uniformly distributing the electrons and holes in all the quantum well layers, if the layer thickness of the quantum well layer is too thick, the distribution of the electrons and holes uniformly is inhibited, The thickness of the quantum well layer is preferably 10 nm or less.

  Further, similarly, since the distribution of electrons and holes is hindered if the barrier layer is too thick, the thickness of the barrier layer is preferably 10 nm or less.

  On the other hand, with respect to the blue LED, the current-light output characteristic of the element that is currently put into practical use tends to saturate as current is injected as shown in FIG. In the conventional blue LED, there is only one quantum well active layer, and both injected electrons and holes are present in this one quantum well layer, but when the injection amount is increased, the InGaN that forms the quantum well layer Since the effective mass of electrons and holes of the semiconductor material is large, the distribution of the injected electrons and holes in the momentum space is increased, and the light emission efficiency is lowered. Therefore, as in the present invention, by setting the number of quantum well layers in the active layer having a multiple quantum well structure made of a nitride semiconductor containing at least indium and gallium to two, the injected electrons and holes are two quantum wells. Since it is divided into layers, the density of electrons and holes existing per quantum well layer is reduced, and the distribution of electrons and holes in the momentum space can be reduced. As a result, the tendency to saturate in the current-light output characteristics was improved, and a higher-intensity gallium nitride-based LED element was realized by improving the light output.

  Hereinafter, it demonstrates in detail according to a specific example.

(First embodiment)
FIG. 1 is a sectional view showing a gallium nitride based semiconductor laser device according to a first embodiment of the present invention, and FIG. 2 is an enlarged sectional view of a portion A in FIG.

In this figure, 1 is a sapphire substrate having a c-plane as a surface, 2 is a GaN buffer layer, 3 is an n-GaN n-type contact layer, 4 is an n-Al _0.1 Ga _0.9 Nn-type cladding layer, and 5 is an n-GaN guide layer. , 6 is a multiple quantum well structure active layer composed of two layers of In _0.2 Ga _0.8 N quantum well layer 14 and one layer of In _0.05 Ga _0.95 N barrier layer 15, 7 is an Al _0.2 Ga _0.8 N evaporation prevention layer, and 8 is p-GaN guide layer, 9 p-Al _0.1 Ga _0.9 Np-type cladding layer, 10 p-GANP type contact layer, 11 p-side electrode, the 12 n-side electrode, 13 is an SiO ₂ insulating film. Incidentally, the n-Al _0.1 Ga _0.9 Nn-type cladding layer is an example of the lower cladding layer, p-Al _0.1 Ga _0.9 Np-type cladding layer is an example of the upper cladding layer.

  In the present embodiment, the surface of the sapphire substrate 1 may have other plane orientations such as a-plane, r-plane, and m-plane. Further, not only a sapphire substrate but also a SiC substrate, spinel substrate, MgO substrate, Si substrate, or GaAs substrate can be used. In particular, an SiC substrate is easier to cleave than a sapphire substrate, so that there is an advantage that it is easy to form a laser resonator end face by cleavage. The buffer layer 2 may be made of another material such as AlN or AlGaN ternary mixed crystal as long as it can epitaxially grow a gallium nitride based semiconductor thereon.

The n-type cladding layer 4 and the p-type cladding layer 9 may be an AlGaN ternary mixed crystal having an Al composition other than n-Al _0.1 Ga _0.9 N. In this case, when the Al composition is increased, the energy gap difference and the refractive index difference between the active layer and the cladding layer are increased, carriers and light are effectively confined in the active layer, and the oscillation threshold current is further reduced and the temperature characteristics are improved. I can plan. Further, if the Al composition is reduced to such an extent that the confinement of carriers and light is maintained, the mobility of carriers in the cladding layer increases, so that there is an advantage that the element resistance of the semiconductor laser element can be reduced. Further, these clad layers may be quaternary or more mixed crystal semiconductors containing a small amount of other elements, and the n-type clad layer 4 and the p-type clad layer 9 may not have the same mixed crystal composition.

  If the energy gap of the guide layers 5 and 8 is a material having a value between the energy gap of the quantum well layer constituting the multiple quantum well structure active layer 6 and the energy gap of the clad layers 4 and 9, GaN However, other materials such as a ternary mixed crystal such as InGaN or AlGaN or a quaternary mixed crystal such as InGaAlN may be used. Further, it is not necessary to dope the donor or acceptor over the entire guide layer, only a part of the multiple quantum well structure active layer 6 side may be undoped, and further the entire guide layer may be undoped. In this case, there is an advantage that the number of carriers present in the guide layer is reduced, light absorption by free carriers is reduced, and the oscillation threshold current can be further reduced.

The composition of the two In _0.2 Ga _0.8 N quantum well layers 14 and the one In _0.05 Ga _0.95 N barrier layer 15 constituting the multi-quantum well structure active layer 6 can be set according to the required laser oscillation wavelength. When the oscillation wavelength is desired to be increased, the In composition of the quantum well layer 14 is increased. When the oscillation wavelength is desired to be decreased, the In composition of the quantum well layer 14 is decreased. Further, the quantum well layer 14 and the barrier layer 15 may be a quaternary or higher mixed crystal semiconductor containing a small amount of other elements in the InGaN ternary mixed crystal. Further, the barrier layer 15 may simply use GaN.

  Next, a method for manufacturing the gallium nitride based semiconductor laser will be described with reference to FIGS. In the following explanation, the case where the MOCVD method (metal organic vapor phase epitaxy) is used is shown. However, any growth method capable of epitaxially growing GaN may be used. It is also possible to use other vapor phase growth methods such as

First, a GaN buffer layer 2 is formed on a sapphire substrate 1 having a c-plane as a surface, which is installed in a predetermined growth furnace, using trimethylgallium (TMG) and ammonia (NH ₃ ) as raw materials at a growth temperature of 550 ° C. Grow 35 nm.

Next, the growth temperature is increased to 1050 ° C., and a 3 μm thick Si-doped n-GaN n-type contact layer 3 is grown using TMG, NH ₃ , and silane gas (SiH ₄ ) as raw materials. Further continued trimethylaluminum (TMA) was added to raw materials, the growth temperature to grow a thickness of 0.7μm of Si-doped n-Al _0.1 Ga _0.9 Nn type cladding layer 4 remain 1050 ° C.. Subsequently, TMA is removed from the raw material, and the Si-doped n-GaN guide layer 5 having a thickness of 0.05 μm is grown with the growth temperature kept at 1050 ° C.

Next, the growth temperature is lowered to 750 ° C., and TMG, NH ₃ , and trimethylindium (TMI) are used as raw materials, and an In _0.2 Ga _0.8 N quantum well layer (thickness 5 nm) 14 and an In _0.05 Ga _0.95 N barrier layer are formed. A multiple quantum well structure active layer (total thickness 15 nm) 6 is formed by sequentially growing (thickness 5 nm) 15 and an In _0.2 Ga _0.8 N quantum well layer (thickness 5 nm) 14. Further, TMG, TMA, and NH ₃ are used as raw materials to grow an Al _0.2 Ga _0.8 N evaporation preventing layer 7 having a thickness of 10 nm with the growth temperature kept at 750 ° C.

Next, the growth temperature is again increased to 1050 ° C., and Mg-doped p-GaN guide layer 8 having a thickness of 0.05 μm is formed using TMG, NH ₃ , and cyclopentadienyl magnesium (Cp ₂ Mg) as raw materials. To grow. The TMA added to the raw material continues further, the growth temperature to grow a Mg-doped p-Al _0.1 Ga _0.9 Np-type cladding layer 9 having a thickness of 0.7μm remain 1050 ° C.. Subsequently, TMA is removed from the raw material, and a 0.2 μm thick Mg-doped p-GaN p-type contact layer 10 is grown with the growth temperature kept at 1050 ° C. to complete a gallium nitride epitaxial wafer.

  Thereafter, the wafer is annealed in a nitrogen gas atmosphere at 800 ° C. to reduce the resistance of the Mg-doped p-type layer.

Further, using normal photolithography and dry etching techniques, etching is performed from the outermost surface of the p-GaN p-type contact layer 10 in a stripe shape having a width of 200 μm until the n-GaN n-type contact layer 3 is exposed. Next, using the same photolithography and dry etching techniques as described above, a p-GaN p-type contact layer is formed on the remaining surface of the p-GaN p-type contact layer 10 so as to form a ridge structure in a 5 μm wide stripe. 10 and the p-Al _0.1 Ga _0.9 Np-type cladding layer 9 are etched.

Subsequently, an SiO ₂ insulating film 13 having a thickness of 200 nm is formed on the side surface of the ridge and the surface of the p-type layer other than the ridge. A p-side electrode 11 made of nickel and gold is formed on the surfaces of the SiO ₂ insulating film 13 and the p-GaN p-type contact layer 10, and the surface of the n-GaN n-type contact layer 3 exposed by etching is made of n of titanium and aluminum. The side electrode 12 is formed to complete a gallium nitride based LD wafer.

  Thereafter, the wafer is cleaved in a direction perpendicular to the ridge stripe to form a laser cavity end face, and further divided into individual chips. Then, each chip is mounted on the stem, and each electrode and the lead terminal are connected by wire bonding to complete a gallium nitride semiconductor laser element.

  The blue LD device manufactured as described above has a laser characteristic of an oscillation wavelength of 430 nm and an oscillation threshold current of 40 mA, and the light output is sufficiently modulated by injection of a high-frequency current of 300 MHz or more and a maximum frequency of about 1 GHz. Was confirmed. As a result, when the blue LD element of this example was used for an optical disk, a data read error could be prevented and a blue LD element usable for an optical disk could be realized.

  FIG. 3 shows changes in the threshold current value and the maximum modulation frequency of the injection current that can modulate the optical output when the number of quantum well layers is changed from 1 to 5 in the gallium nitride semiconductor laser device. A graph showing is shown. The structure of each semiconductor laser is the same as that of the first embodiment of the present invention except that the number of quantum well layers is different and the number of barrier layers is different depending on the number of quantum well layers. It is the same as the element. As can be seen from this figure, the oscillation threshold current is low and the optical output can be sufficiently modulated even by injection of a high frequency current of 300 MHz or higher, for example, about 1 GHz at the maximum frequency, the number of quantum well layers is 2. This is only the gallium nitride based semiconductor laser device according to the first embodiment of the present invention.

  In the present embodiment, the quantum well layer 14 and the barrier layer 15 constituting the multiple quantum well structure active layer 6 are both 5 nm thick. However, these layer thicknesses are not necessarily the same and are different. It doesn't matter. In addition, in order to uniformly inject electrons and holes into the two quantum well layers, if each layer thickness of the quantum well layer 14 and the barrier layer 15 is set to 10 nm or less, it is not limited to this embodiment, and other layer thicknesses may be used. The same effect can be obtained.

  FIG. 4 shows a change in the maximum modulation frequency of the injection current capable of modulating the optical output when the thickness of the barrier layer is changed in the gallium nitride semiconductor laser device having two quantum well layers. A graph diagram is shown. The structure of the semiconductor laser at this time is the same as that of the gallium nitride based semiconductor laser device according to the first embodiment except that the thickness of the barrier layer is different. From this figure, it can be seen that if the thickness of the barrier layer is 10 nm or less, the light output can be sufficiently modulated even by injection of a high-frequency current of 300 MHz or more and a maximum of about 1 GHz. This is also the case with the quantum well layer. If the layer thickness of the quantum well layer is 10 nm or less, the light output can be sufficiently modulated even by injection of a high-frequency current of 300 MHz or more and a maximum of about 1 GHz. confirmed.

In this embodiment, the Al _0.2 Ga _0.8 N evaporation preventing layer 7 is formed so as to be in contact with the active layer 6 having the multiple quantum well structure, but this evaporates while the quantum well layer 14 is raised in growth temperature. This is to prevent this from happening. Accordingly, any material that protects the quantum well layer 14 can be used as the evaporation preventing layer 7, and AlGaN ternary mixed crystals or GaN having other Al compositions may be used. Further, Mg may be doped into the evaporation preventing layer 7, and in this case, there is an advantage that holes are easily injected from the p-GaN guide layer 8 and the p-Al _0.1 Ga _0.9 Np type cladding layer 9. Furthermore, when the In composition of the quantum well layer 14 is small, the quantum well layer 14 does not evaporate even if the evaporation preventing layer 7 is not formed. Therefore, even if the evaporation preventing layer 7 is not particularly formed, the gallium nitride of this embodiment The characteristics of the semiconductor laser device are not impaired.

  In this embodiment, the ridge stripe structure is formed to confine the injection current, but other current confinement techniques such as an electrode stripe structure may be used. In the present embodiment, the cavity end face of the laser is formed by cleavage. However, since the sapphire substrate may be hard and difficult to cleave, the cavity end face can also be formed by dry etching.

  Furthermore, since sapphire which is an insulator is used as a substrate in this embodiment, the n-side electrode 12 is formed on the surface of the n-GaN n-type contact layer 3 exposed by etching. If GaAs or the like is used for the substrate, the n-side electrode 12 may be formed on the back surface of the substrate. Also, the p-type and n-type configurations may be reversed.

(Second embodiment)
FIG. 5 is a circuit diagram showing a semiconductor laser device and a driving circuit according to the second embodiment of the present invention. The semiconductor laser device 16 shown in FIG. 5 uses a gallium nitride based semiconductor laser device having two quantum well layers obtained in the first embodiment of the present invention. The high-frequency drive circuit 17 is configured by using normal semiconductor components, and is a semiconductor laser drive circuit for modulating an injection current to the semiconductor laser 17 at a high frequency to modulate an optical output. In this embodiment, the modulation frequency of the injected current is 300 MHz. In the gallium nitride semiconductor laser device obtained in the first example, the maximum modulation frequency of the injected current was 1 GHz or more, and the light output could be sufficiently modulated even at a frequency of 300 MHz. When this embodiment is used as a light source for an optical disk, the optical output of the semiconductor laser is sufficiently modulated, so that the coherency of the laser light can be reduced, and noise due to the return light of the laser light from the disk surface is reduced. I was able to. As a result, it is possible to read data from the optical disc without error.

  In this embodiment, the modulation frequency of the injection current is set to 300 MHz. However, if the frequency is such that the noise due to the return light of the laser beam from the disk surface can be reduced by reducing the coherency of the laser beam, the maximum is possible. The nitride semiconductor laser may be driven at other modulation frequencies up to a frequency of about 1 GHz.

(Third embodiment)
FIG. 6 is a circuit diagram showing a semiconductor laser device and a driving circuit according to the third embodiment of the present invention. The semiconductor laser device 18 shown in FIG. 6 uses the gallium nitride semiconductor laser device having two quantum well layers obtained in the first embodiment of the present invention. By adjusting the stripe width and the etching depth of the p-Al _0.1 Ga _0.9 Np-type cladding layer 9, a self-oscillation type in which the light output is modulated even when a constant current that is not modulated is injected. It is a semiconductor laser. Here, the stripe width was set to 3 μm, and the remaining film thickness upon etching of the p-Al _0.1 Ga _0.9 Np type cladding layer 9 was set to 0.2 μm. The stripe width and the remaining film thickness at the time of etching are not limited to the values in this specific example, but the stripe width is 1 to 5 μm, and the remaining film thickness of the p-type cladding layer 9 is 0.05. It should just be thru | or 0.5 micrometer. The modulation frequency of the optical output in the self-excited oscillation type gallium nitride semiconductor laser device thus produced was 800 MHz.

  In the gallium nitride based semiconductor laser device according to the third embodiment, the density of electrons and holes existing in the quantum well layer is easily modulated by using two quantum well layers. Therefore, not only the light output is modulated by modulating the injection current to modulate the density of electrons and holes, but also the injection of a constant current that is not modulated modulates the density of electrons and holes, resulting in a light output. A modulated self-oscillation type semiconductor laser can be easily manufactured, and the optical output can be modulated at a high frequency.

  The constant current drive circuit 19 is configured using normal semiconductor components, and is a semiconductor laser drive circuit for injecting a constant current. When this embodiment is used as a light source for an optical disk, the optical output of the semiconductor laser is sufficiently modulated, so that the coherency of the laser light can be reduced, and noise due to the return light of the laser light from the disk surface is reduced. We were able to. As a result, it is possible to read data from the optical disc without error.

The gallium nitride based semiconductor laser device 18 used in the third embodiment adjusts the stripe width when forming the ridge structure and the depth for etching the p-Al _0.1 Ga _0.9 Np type cladding layer 9. The self-oscillation type semiconductor laser is used, but a self-oscillation type semiconductor laser may be used by installing a supersaturated absorption layer in the vicinity of the active layer as used in ordinary GaAs semiconductor lasers. .

(Reference example)
FIG. 7 is a cross-sectional view showing a gallium nitride based semiconductor light-emitting diode element as a reference example, and FIG. 8 is an enlarged cross-sectional view of a portion B in FIG.

In this figure, 21 is a sapphire substrate having a c-plane as a surface, 22 is a GaN buffer layer, 23 is an n-GaN n-type contact layer, 24 is an n-Al _0.1 Ga _0.9 Nn-type cladding layer, and 25 is an n-GaN guide layer. , 26 is a multiple quantum well structure active layer comprising two layers of In _0.2 Ga _0.8 N quantum well layer 34 and one layer of In _0.05 Ga _0.95 N barrier layer 35, 27 is an Al _0.2 Ga _0.8 N evaporation prevention layer, and 28 is A p-GaN guide layer, 29 is a p-Al _0.1 Ga _0.9 Np-type cladding layer, 30 is a p-GaN p-type contact layer, 31 is a p-side electrode, and 32 is an n-side electrode.

  In the present light-emitting diode element, the surface of the sapphire substrate 21 may have other plane orientations such as a-plane, r-plane, and m-plane. Moreover, not only a sapphire substrate but a SiC substrate, a spinel substrate, a MgO substrate, or a Si substrate can also be used. In particular, since the SiC substrate is easier to cleave than the sapphire substrate, there is an advantage that the operation of dividing the LED element into chips can be easily performed. The buffer layer 22 may be made of another material such as AlN or AlGaN ternary mixed crystal as long as it can epitaxially grow a gallium nitride based semiconductor thereon.

n-type cladding layer 24 and p-type cladding layer 29, AlGaN3-element mixed crystal and having the Al composition other than n-Al _0.1 Ga _0.9 N, or simply by using GaN. In this case, when the Al composition is increased, the energy gap difference between the active layer and the cladding layer is increased, and carriers are effectively confined in the active layer, so that the temperature characteristics can be improved. Further, if the Al composition is reduced to such an extent that carrier confinement is maintained, the mobility of carriers in the cladding layer increases, and thus there is an advantage that the element resistance of the light emitting diode element can be reduced. Further, these cladding layers may be quaternary or more mixed crystal semiconductors containing other elements in minute amounts, and the n-type cladding layer 24 and the p-type cladding layer 29 may not have the same mixed crystal composition.

  If the energy gap of the guide layers 25 and 28 is a material having a value between the energy gap of the quantum well layer constituting the multi-quantum well structure active layer 26 and the energy gap of the cladding layers 24 and 29, GaN is used. However, other materials such as a ternary mixed crystal such as InGaN or AlGaN or a quaternary mixed crystal such as InGaAlN may be used. Further, it is not necessary to dope the donor or acceptor over the entire guide layer, only a part on the multiple quantum well structure active layer 26 side may be undoped, and further the entire guide layer may be undoped. In this case, there is an advantage that the number of carriers present in the guide layer is reduced, light absorption by free carriers is reduced, and the light output is further improved. The guide layers 25 and 28 have an advantage of facilitating injection of electrons and holes from the n-type cladding layer 24 and the p-type cladding layer 29 into the multi-quantum well structure active layer 26, respectively. , 28 does not significantly deteriorate the LED element characteristics, so the guide layers 25, 28 may be omitted.

The composition of the two In _0.2 Ga _0.8 N quantum well layers 34 and the one In _0.05 Ga _0.95 N barrier layer 35 constituting the multi-quantum well structure active layer 26 can be set according to the required emission wavelength. When the emission wavelength is desired to be increased, the In composition of the quantum well layer 34 is increased. When the emission wavelength is desired to be decreased, the In composition of the quantum well layer 34 is decreased. Further, the quantum well layer 34 and the barrier layer 35 may be a quaternary or higher mixed crystal semiconductor containing a small amount of other elements in the InGaN ternary mixed crystal. Further, the barrier layer 35 may be simply made of GaN.

  Next, a method for manufacturing the gallium nitride based semiconductor light emitting diode will be described with reference to FIGS. In the following explanation, the case where the MOCVD method (metal organic vapor phase epitaxy) is used is shown. However, any growth method capable of epitaxially growing GaN may be used. It is also possible to use other vapor phase growth methods such as

First, a GaN buffer layer 22 is grown to 35 nm at a growth temperature of 550 ° C. on a sapphire substrate 21 having a c-plane as a surface, which is installed in a predetermined growth furnace, using TMG and NH ₃ as raw materials.

Next, the growth temperature is raised to 1050 ° C., and a 3 μm thick Si-doped n-GaN n-type contact layer 23 is grown using TMG, NH ₃ , and SiH ₄ as raw materials. Further, TMA is added to the raw material, and a 0.3 μm thick Si-doped n-Al _0.1 Ga _0.9 Nn-type cladding layer 24 is grown with the growth temperature kept at 1050 ° C. Subsequently, the TMA is removed from the raw material, and the Si-doped n-GaN guide layer 25 having a thickness of 0.05 μm is grown with the growth temperature kept at 1050 ° C.

Next, the growth temperature is lowered to 750 ° C., and TMG, NH ₃ , and TMI are used as raw materials, and an In _0.2 Ga _0.8 N quantum well layer (thickness 3 nm) 34, an In _0.05 Ga _0.95 N barrier layer (thickness 5 nm). ) 35 and an In _0.2 Ga _0.8 N quantum well layer (thickness 3 nm) 34 are sequentially grown to form a multiple quantum well structure active layer (total thickness 11 nm) 26. Further, TMG, TMA and NH ₃ are used as raw materials, and an Al _0.2 Ga _0.8 N evaporation preventing layer 27 having a thickness of 10 nm is grown with the growth temperature kept at 750 ° C.

Next, the growth temperature is raised again to 1050 ° C., and 0.05 μm thick Mg-doped p-GaN guide layer 28 is grown using TMG, NH ₃ , and Cp ₂ Mg as raw materials. Subsequently, TMA is added to the raw material, and a 0.3 μm thick Mg-doped p-Al _0.1 Ga _0.9 Np type cladding layer 29 is grown with the growth temperature kept at 1050 ° C. Subsequently, TMA is removed from the raw material, and a 0.2 μm thick Mg-doped p-GaN p-type contact layer 30 is grown with the growth temperature kept at 1050 ° C. to complete a gallium nitride based epitaxial wafer.

  Thereafter, the wafer is annealed in a nitrogen gas atmosphere at 800 ° C. to reduce the resistance of the Mg-doped p-type layer.

  Further, using normal photolithography and dry etching techniques, etching is performed from the outermost surface of the p-GaN p-type contact layer 30 to the n-GaN n-type contact layer 23 in a predetermined region for LED element fabrication. .

  Subsequently, a p-side electrode 31 made of nickel and gold is formed on the surface of the p-GaN p-type contact layer 30, and an n-side electrode 32 made of titanium and aluminum is formed on the surface of the n-GaN n-type contact layer 23 exposed by etching. Then, a gallium nitride LED wafer is completed.

  Thereafter, the wafer is divided into individual chips. Then, each chip is mounted on the stem, and each electrode and the lead terminal are connected by wire bonding to complete a gallium nitride based semiconductor light emitting diode element.

  The blue LED element produced as described above has a light emission characteristic of a forward current of 20 mA, an emission wavelength of 430 nm, and an optical output of 6 mW. Further, as shown in FIG. 9, the current-light output characteristics were not saturated even at a high injection current, and the characteristics were improved as compared with the conventional LED element.

  In this blue LED element, the thicknesses of the quantum well layer 34 and the barrier layer 35 constituting the multi-quantum well structure active layer 26 are 3 nm and 5 nm, respectively, but these layer thicknesses are two quantum well layers. In order to uniformly inject electrons and holes, if the thickness of each of the quantum well layer 34 and the barrier layer 35 is 10 nm or less, the same effect can be obtained with other layer thicknesses.

Further, in this blue LED element, the Al _0.2 Ga _0.8 N evaporation preventing layer 27 is formed so as to be in contact with the multiple quantum well structure active layer 26, while the quantum well layer 34 is raised in the growth temperature. This is to prevent evaporation. Accordingly, any material that protects the quantum well layer 34 can be used as the evaporation preventing layer 27, and AlGaN ternary mixed crystals or GaN having other Al compositions may be used. Further, Mg may be doped into the evaporation preventing layer 27, and in this case, there is an advantage that holes are easily injected from the p-GaN guide layer 28 and the p-Al _0.1 Ga _0.9 Np type cladding layer 29. Further, when the In composition of the quantum well layer 34 is small, the quantum well layer 34 does not evaporate even if the evaporation preventing layer 27 is not formed. The characteristics of the diode element are not impaired.

1 is a cross-sectional view showing a semiconductor laser device according to a first embodiment of the present invention. It is sectional drawing to which the A section of FIG. 1 was expanded. It is a graph which shows the quantum well layer number dependence of the threshold current value in 1st Example, and the quantum well layer number dependence of the maximum modulation frequency of the injection current which can modulate an optical output. FIG. 6 is a graph showing the dependence of the maximum frequency of the injection current on the thickness of the barrier layer, which can modulate the optical output. It is a circuit diagram which shows the semiconductor laser element and drive circuit which concern on 2nd Example of this invention. It is a circuit diagram which shows the semiconductor laser element and drive circuit based on 3rd Example of this invention. It is sectional drawing which shows the semiconductor light-emitting diode element as a reference example. It is sectional drawing to which the B section of FIG. 7 was expanded. It is a graph which shows each current - light output characteristic of the semiconductor light emitting diode element of a reference example, and the conventional semiconductor light emitting diode element. It is sectional drawing which shows the structural example of the conventional blue LD. It is sectional drawing which shows the structural example of the conventional blue LED.

3 n-GaN n-type contact layer 4 n-Al _0.1 Ga _0.9 Nn-type cladding layer 5 n-GaN guide layer 4, 24 n-AlGaN n-type cladding layer 6, 25 Multiple quantum well structure active layer 8, 28 p-GaN guide layer 9, 29 p-AlGaN p-type cladding layer 14, 34 InGaN quantum well layer 15, 35 InGaN barrier layer

Claims (1)

A lower cladding layer made of a nitride semiconductor;
A quantum well structure active layer formed on the lower cladding layer and made of a nitride semiconductor containing at least indium and gallium;
An upper cladding layer formed on the quantum well structure active layer, made of a nitride semiconductor, and having a flat portion and a ridge portion;
An insulating film formed on the flat portion and on a side surface of the ridge portion;
An electrode formed on the insulating film and on the upper surface of the ridge portion and electrically connected to the ridge portion;
The quantum well structure active layer is formed of two quantum well layers and one barrier layer sandwiched between them.
Each quantum well layer has a layer thickness of 10 nm or less,
The quantum well structure active layer and the gallium nitride semiconductor laser element characterized that you have to form an oscillation unit,
A drive circuit for injecting current into the gallium nitride based semiconductor laser device;
With
The light output from the gallium nitride based semiconductor laser element is modulated,
A semiconductor laser light source device characterized in that a current injected into the gallium nitride based semiconductor laser element is modulated at a high frequency, and a modulation frequency of the current is 300 MHz or more.