JP3881467B2 - Surface emitting laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface emitting laser that emits light in a direction perpendicular to a substrate surface.
[0002]
[Prior art]
Currently, surface emitting lasers are actively developed for applications such as parallel interconnection using optical signals. This takes advantage of the feature that the surface emitting laser elements are easily arrayed one-dimensionally and two-dimensionally. However, in order to take advantage of these features, the light output and element resistance of each laser element must be uniform.
This surface-emitting laser has a structure in which both sides of an active layer are sandwiched between p-type and n-type distributed Bragg reflectors (DBR), and emits laser light in the direction in which the reflectors are stacked. Currently, a 0.85 μm band surface emitting laser using a GaAs-based material has been put into practical use. In particular, a GaAs quantum well that can oscillate more efficiently is used for the active layer. In addition, the DBR, ^which optical length is ^laminated lambda / 4 of the Al _x Ga _1-x As and Al _y Ga ₁ -y ^{As the (x> y) are alternately used.}
[0003]
The structure of such a surface emitting laser will be described. As shown in FIG. 4, an n-type DBR 403 is formed on a substrate 401 made of n-type GaAs via a buffer layer 402 made of n-type GaAs. Yes. An active layer 405 is formed on the DBR 403 through a lower spacer 404 made of non-doped AlGaAs. A p-type DBR 407 is formed on the active layer 405 via an upper spacer 406 made of non-doped AlGaAs. On the DBR 407, an upper electrode 409 having an ohmic contact is formed via an ohmic contact layer 408 made of GaAs doped with impurities at a high concentration. A lower electrode 410 that is also ohmic-bonded is formed on the back surface of the substrate 401. Then, the isolation insulating layer 411 is formed for current confinement and element isolation by filling the insulating material after forming the trench.
[0004]
Here, as described above, the DBRs 403 and 407 have a structure in which Al _x Ga _1-x As having a low refractive index and Al _y Ga _1-y As having a high refractive index are alternately stacked. . As the low refractive index Al _x Ga _1-x As, AlAs with X = 1, Al _0.9 Ga _0.1 As with X = 0.9, or the like is used. Further, as Al _y Ga _1-y As on the high refractive index side, generally y> 0.15 is set. In this way, the bandgap of Al _y Ga _1-y As, can be larger than the photon energy of the oscillation light, it is possible to prevent the absorption of the laser oscillation light.
[0005]
Further, in order to obtain laser oscillation, it is necessary to set the reflectivity of the DBRs 403 and 407, which are reflecting mirrors, to 99% or more. Therefore, the DBR 407 on the laser emission side has a structure in which about 20 sets of Al _x Ga _1-x As and Al _y Ga _1-y As are stacked. The DBR 403 on the opposite side has a structure in which about 30 sets are stacked.
Further, when a current is passed through the DBRs 403 and 407, in order to prevent an increase in electrical resistance due to band discontinuity at the heterointerface between Al _x Ga _1-x As and Al _y Ga _1-y As, the impurity concentration Of 10 ¹⁸ cm ⁻³ or more. In addition, an AlGaAs layer having an intermediate band gap or a graded layer in which the Al composition ratio is gradually changed may be inserted at the interface with the DBRs 403 and 407 having different band gaps.
[0006]
By the way, as described above, the upper electrode 409 needs to be formed on the outermost surface of the DBR 407 by ohmic junction in order to allow current to flow thereover. For this purpose, an ohmic contact layer 408 made of GaAs doped with impurities at a high concentration is provided on the DBR 407. This is for the following reason. That is, the high refractive index AlGaAs constituting the DBR is easily oxidized because it contains Al. This is because it is very difficult to satisfactorily ohmic-join the metal electrode on the oxidized state.
Here, since the ohmic contact layer 408 is also used as one layer of the reflecting mirror, the thickness of the layer is set to about 60 nm in order to set the optical length to λ / 4.
[0007]
[Problems to be solved by the invention]
However, the conventional surface emitting laser has the following problems.
First, the GaAs layer (ohmic contact layer), which is the uppermost layer for forming an ohmic electrode, absorbs laser light, so that there is a problem of reducing the light intensity of laser light that can be output. Further, since the laser beam emitted in this way is absorbed, the temperature of the surface on which the GaAs layer is formed rises. For this reason, the surface of the element is easily oxidized, and the reliability of the element is impaired.
[0008]
Further, in the above-described element, a GaAs layer that is not originally an etching target may be thinned to some extent in etching processing for forming the upper electrode 409 or the like. Thus, if the GaAs layer is etched, this constitutes a part of the DBR, so that the reflectivity of the DBR decreases. In particular, since the GaAs layer is disposed as the uppermost layer, the difference in refractive index between the GaAs layer and air is large, and the reflectance at this surface is large. L For this reason, the variation in the thickness of the GaAs layer becomes a variation in the phase of the reflected light, which greatly affects the reflectivity of the DBR. Thus, when the reflectivity of the DBR decreases, the threshold current for laser oscillation increases, and in the worst case, the laser does not oscillate.
[0009]
The present invention has been made to solve the above problems, and an object of the present invention is to enable a surface emitting laser to oscillate more stably.
[0010]
[Means for Solving the Problems]
A surface emitting laser according to the present invention includes a first distributed Bragg reflector having a first conductivity type formed on a substrate made of GaAs, and an active layer formed on the first distributed Bragg reflector. A second distributed Bragg reflector having the second conductivity type formed on the active layer, and a protective layer made of InGaP having the second conductivity type formed on the second distributed Bragg reflector; An ohmic contact layer made of GaAs partially formed on the protective layer, an upper electrode made of metal formed by ohmic contact with the ohmic contact layer, and a metal formed by ohmic contact on the back surface of the substrate and a lower electrode made of a first distributed Bragg reflector has a different aluminum composition than the first semiconductor layer made of AlGaAs of a first conductivity type as the first semiconductor layer The second semiconductor layer made of AlGaAs of the first conductivity type having a higher refractive index than that of the first semiconductor layer is alternately stacked, and the second distributed Bragg reflector is made of the third semiconductor made of AlGaAs of the second conductivity type. a fourth semiconductor layer are stacked alternately made of AlGaAs of a second conductivity type third higher refractive index than the semiconductor layer of a different aluminum composition than the semiconductor layer and the third semiconductor layer, the outermost A third semiconductor layer is disposed in the upper layer, and the optical lengths of the first to fourth semiconductor layers and the protective layer are formed to ¼ of the wavelength of oscillation light obtained from the active layer , and the ohmic contact layer is The optical length is formed to be an integral multiple of 1/2 of the above wavelength .
Since it comprised in this way, the protective layer which consists of InGaP will be arrange | positioned at the end surface from which a laser is radiate | emitted.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a cross-sectional view showing a configuration of a surface emitting laser according to an embodiment of the present invention. The configuration of this surface emitting laser will be described. An n-type DBR (distributed Bragg reflector) 103 is formed on a substrate 101 made of n-type GaAs via a buffer layer 102 made of n-type GaAs. . Since the surface cleanliness of the substrate 101 has deteriorated due to exposure to the atmosphere once, the buffer layer 102 is formed, and the DBR 103 is subsequently formed in the same apparatus.
[0012]
The DBR 103 includes a 69 nm-thick low refractive index layer made of Al _0.9 Ga _0.1 As made n-type by doping Si, and a film made of Al _0.15 Ga _0.85 As made n-type by doping Si. High refractive index layers having a thickness of 62 nm are alternately stacked. In addition, 37 sets of the low refractive index layer and the high refractive index layer are laminated. The optical length of each of the low refractive index layer and the high refractive index layer is 1/4 of the laser oscillation wavelength λ.
[0013]
An active layer 105 is formed on the DBR 103 via a lower spacer 104 made of non-doped Al _0.5 Ga _0.5 As. The active layer 105 has a multiple quantum well structure composed of an 8 nm thick quantum well layer made of non-doped GaAs and a 10 nm thick barrier layer made of non-doped Al _0.3 Ga _0.7 As.
A p-type DBR 107 is formed on the active layer 105 via an upper spacer 106 made of non-doped Al _0.5 Ga _0.5 As.
Here, the total film thickness of the lower spacer 104 and the upper spacer 106 is 88 nm. The optical lengths of the lower spacer 104, the active layer 105, and the upper spacer 106 as a whole are designed to be equal to the desired oscillation wavelength.
[0014]
The DBR 107 is made of a low-refractive index layer having a thickness of 69 nm made of Al _0.9 Ga _0.1 As made p-type by doping C and Al _0.15 Ga _0.85 As made p-type by doping C. High refractive index layers having a thickness of 62 nm are alternately stacked. Moreover, 23 sets of the low refractive index layer and the high refractive index layer are laminated | stacked. The optical length of each of the low refractive index layer and the high refractive index layer is 1/4 of the laser oscillation wavelength λ.
[0015]
In this embodiment, a protective layer 108 having a thickness of 64 nm made of p-type InGaP doped with Zn as an impurity is provided on the DBR 107. Here, the optical length of the protective layer 108 was set to ¼ of the laser oscillation wavelength λ.
In addition, an upper electrode 110 is formed on the protective layer 108 via an ohmic contact layer 109 and ohmic-bonded thereto. The ohmic contact layer 109 is made of p-type GaAs doped with C in order to make an ohmic contact, and has a thickness of 236 nm. The upper electrode 110 is made of AuZnNi. The ohmic contact layer 109 is disposed only under the region where the upper electrode 110 is formed.
[0016]
Further, an isolation insulating layer 121 made of polyimide is formed for current confinement to the region where the upper electrode 110 is formed and for element isolation. A region separated by the isolation insulating layer 121 becomes one element region. For example, if the isolation insulating layer 121 is formed in a lattice shape on the substrate 101, one surface emitting laser element is formed in the lattice, and as a result, a plurality of surface emitting lasers are arrayed in the substrate 101. The state formed above is obtained.
As shown in FIG. 1, an ohmic-junction lower electrode 111 made of AuGeNi is formed on the back surface of the substrate 101.
[0017]
Next, the outermost surface of the laser emitting portion of the surface emitting laser described above will be described. As shown in FIG. 2 (a), the outermost surface of the laser emitting portion is laminated with a high refractive index layer 107b, a low refractive index layer 107a, a high refractive index layer 107b, a low refractive index layer 107a, and a protective layer 108. On top of this, an upper electrode 110 is formed via an ohmic contact layer 109. Here, the refractive index in each of these layers is as shown in FIG. 2 (b). Below the protective layer 108 made of InGaP, which is the uppermost layer, a low refractive index made of Al _0.9 Ga _0.1 As having a smaller refractive index than this. There is a rate layer 107a. Therefore, the refractive index of the protective layer 108 to the DBR 107 including the air layer on the protective layer 108 is changed alternately in magnitude.
[0018]
By the way, for example, the upper electrode 110 and the ohmic contact layer 109 are patterned by forming a GaAs film and an AuZnNi film constituting the ohmic contact layer 109 on the protective layer 108 and then patterning them. Is going on.
Here, the processing of these grooves and patterns is performed using a known photolithography technique and etching technique.
In the conventional case shown in FIG. 4, there is an ohmic contact layer 408 used as one layer of a reflecting mirror under the upper electrode 409, and this upper surface is exposed when the upper electrode 409 is processed. Therefore, when the upper electrode 409 is processed, the ohmic contact layer 408 is slightly etched, and the film thickness may decrease. For this reason, the problem that the reflectance of a reflective mirror falls may generate | occur | produce.
[0019]
However, in this embodiment, when the upper electrode 110 is formed, a GaAs layer serving as the ohmic contact layer 109 exists below. This layer is not used as a part of the reflecting mirror. A protective layer 108 used as part of the reflector is underneath the GaAs layer. Therefore, the protective layer 108 used as a part of the reflecting mirror is not affected at all when the upper electrode 110 is formed. That is, according to this embodiment, the reflectance of the reflecting mirror does not decrease when the upper electrode 110 is formed.
[0020]
In the pattern formation of the ohmic contact layer 109, a predetermined mask pattern is used for the GaAs film, and wet etching with an etchant of H ₂ SO ₄ + H ₂ 0 ₂ + H ₂ 0 is used. Since this etching solution does not etch InGaP at all, the protective layer 108 is not affected when the ohmic contact layer 109 is processed.
The formation of the ohmic contact layer 109 and the upper electrode 110 is the last process of the surface emitting laser. Therefore, the protective layer 108 is protected by the GaAs film that becomes the ohmic contact layer 109 until the end. That is, according to this embodiment, the protective layer 108 can hold the film thickness at the time of film formation without changing to the end.
[0021]
By the way, in the manufacture of the surface emitting laser, the wafer is inspected after epitaxial growth of each layer at a stage before the metal film to be the upper electrode is formed. In this inspection, light is generally irradiated from the upper surface of the wafer, and the reflection spectrum at that time is measured. The quality of the inspection is determined based on whether or not the result of the reflection spectrum measurement is a predetermined value.
[0022]
FIG. 3 is a characteristic diagram showing the reflection spectrum of the wafer epitaxially grown up to the InGaP film of the protective layer 108 in the first embodiment described above. In FIG. 3, the range of 0.82 to 0.88 μm is the DBR stopband, and it can be seen that a reflectance close to 100% is obtained in this region. In addition, the reflectance is small in a very narrow wavelength region at the center of the stop band. This is the resonance wavelength of the resonator structure in this embodiment. This resonance wavelength is the wavelength of the oscillating laser. Therefore, if the half-value width of the decrease in the resonance wavelength and the reflection spectrum is as designed by these inspections, the wafer to be inspected continues the element formation process.
[0023]
In the above-described wafer inspection stage, the GaAs layer serving as the ohmic contact layer 109 is present over the entire wafer (substrate 101), and therefore affects the reflection spectrum to be measured. Here, as described above, in this embodiment, the film thickness of the ohmic contact layer 109 is 236 nm. In other words, the optical length of the GaAs film was made equal to the oscillation wavelength λ. This is because the optical length is an integral multiple of λ / 2 of the oscillation wavelength.
[0024]
In general, if the optical length of the GaAs layer is an integral multiple of λ / 2, the light reflected at the boundary between the air above the layer and the GaAs layer and the DBR below the protective layer 108 are reflected. The phase of the emitted light is the same. On the other hand, the light reflected at the interface between the GaAs layer and the protective layer 108 therebelow has the opposite phase. However, since the difference in refractive index between the GaAs layer and the protective layer 108 thereunder is small, the influence of the reflected light is almost negligible.
Accordingly, even when the GaAs film serving as the ohmic contact layer is attached to the entire region, the reflection spectrum is almost the same as that in the inner case. That is, even when the GaAs film serving as the ohmic contact layer is attached to the entire area, the above-described inspection is not hindered.
[0025]
In addition, since the ohmic contact layer 109 can be formed as thick as 236 nm, even if the alloy ohmic contact with the upper electrode 110 is sufficiently thick, it does not reach the protective layer 108. Therefore, the upper electrode 110 can be joined with the resistance sufficiently lowered. In addition, since the contact resistance of the upper electrode 110 with ohmic contact can be made uniform, variations in element characteristics such as light output can be suppressed.
In the above-described embodiment, the isolation structure (current confinement structure) using the isolation insulating layer 121 is described, but the present invention is not limited to this. Needless to say, a current confinement structure by introducing impurities by ion implantation or a current confinement structure by selective oxidation of the AlAs layer may be used.
[0026]
【The invention's effect】
As described above, according to the present invention, the first distributed Bragg reflector having the first conductivity type formed on the substrate made of GaAs, and the active formed on the first distributed Bragg reflector. A protective layer comprising: a second distributed Bragg reflector having a second conductivity type formed on the active layer; and a second conductivity type InGaP formed on the second distributed Bragg reflector. An ohmic contact layer made of GaAs partially formed on the protective layer, an upper electrode made of metal formed by ohmic contact with the ohmic contact layer, and an ohmic contact formed on the back surface of the substrate. and a lower electrode made of a metal, the first distributed Bragg reflector, different aluminum sets the first semiconductor layer made of AlGaAs of a first conductivity type as the first semiconductor layer And second semiconductor layers made of AlGaAs of the first conductivity type having a higher refractive index than that of the first semiconductor layer, and the second distributed Bragg reflector is made of AlGaAs of the second conductivity type. becomes the third semiconductor layer and stacked in the third fourth semiconductor layer is alternating composed of different have an aluminum composition of the third of the second conductivity type of a higher refractive index than the semiconductor layer AlGaAs and a semiconductor layer In the uppermost layer, the third semiconductor layer is disposed, and the optical lengths of the first to fourth semiconductor layers and the protective layer are formed to ¼ of the wavelength of the oscillation light obtained from the active layer , The ohmic contact layer was formed so that its optical length was an integral multiple of 1/2 of the above wavelength .
[0027]
Since it comprised in this way, the protective layer which consists of InGaP will be arrange | positioned at the end surface from which a laser is radiate | emitted. That is, there is no GaAs that absorbs laser light at the end face from which the laser is emitted. Therefore, according to the present invention, first, it becomes possible to suppress the temperature rise of the element surface during laser emission, and the laser can oscillate more stably.
Further, in the above configuration, the upper electrode that is in ohmic contact with the GaAs ohmic contact layer partially formed on the protective layer is formed. This partially formed ohmic contact layer is formed by patterning after forming a GaAs layer over the entire protective layer. Here, at the time of this pattern processing, the upper electrode has already been formed. In addition, GaAs pattern processing can have a high selection ratio with InGaP. That is, the protective layer is hardly affected by the change in film thickness during the process of manufacturing the surface emitting laser having the above-described configuration. Therefore, according to the present invention, an element can be formed without reducing the reflectivity of the DBR, so that the laser can oscillate more stably.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of a surface emitting laser according to an embodiment of the present invention.
2 is an explanatory diagram showing a refractive index on the outermost surface of a laser emitting portion of the surface emitting laser of FIG. 1; FIG.
FIG. 3 is a characteristic diagram showing a measurement result of a reflection spectrum for inspection.
FIG. 4 is a cross-sectional view showing a configuration of a conventional surface emitting laser.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... Buffer layer, 103 ... DBR (distributed Bragg reflector), 104 ... Lower spacer, 105 ... Active layer, 106 ... Upper spacer, 107 ... DBR, 108 ... Protective layer, 109 ... Ohmic contact layer, 110: upper electrode, 111: lower electrode, 121: isolation insulating layer.

Claims (2)

A first distributed Bragg reflector having a first conductivity type formed on a substrate made of GaAs;
An active layer formed on the first distributed Bragg reflector;
A second distributed Bragg reflector having a second conductivity type formed on the active layer;
A protective layer made of InGaP of the second conductivity type formed on the second distributed Bragg reflector;
An ohmic contact layer made of GaAs partially formed on the protective layer;
An upper electrode made of metal formed by ohmic contact with the ohmic contact layer;
A lower electrode made of metal formed by ohmic bonding to the back surface of the substrate,
The first distributed Bragg reflector is
Second composed of different have an aluminum composition of the first conductivity type higher refractive index than the first semiconductor layer AlGaAs and the first semiconductor layer made of AlGaAs of a first conductivity type as the first semiconductor layer The semiconductor layers are alternately stacked,
The second distributed Bragg reflector is
A third semiconductor layer made of AlGaAs of the second conductivity type and a fourth semiconductor made of AlGaAs of the second conductivity type having an aluminum composition different from that of the third semiconductor layer and having a higher refractive index than the third semiconductor layer. And the third semiconductor layer is arranged on the uppermost layer,
The optical lengths of the first to fourth semiconductor layers and the protective layer are formed to ¼ of the wavelength of oscillation light obtained from the active layer ,
The surface-emitting laser , wherein the ohmic contact layer has an optical length that is an integral multiple of 1/2 of the wavelength . The surface emitting laser according to claim 1 , wherein
A spacer layer made of AlGaAs disposed so as to sandwich the active layer;
The surface emitting laser according to claim 1, wherein an optical length of the entire active layer and the spacer layer is equal to a wavelength of the light.

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