JPH08330678A - Semiconductor laser - Google Patents

Abstract

PURPOSE: To improve injection concentration of current into an active layer by embedding a stripe light emitting part by a solid embedding material with high electrical resistance. CONSTITUTION: A light emitting part 2 comprises a first clad layer 2a, an active layer 2c made of compound semiconductor having a general formula of InXGaYAl1- X- YN (where, 0<=x<=1, 0<=y<=1), and a second clad layer 2c in sequence from the substrate side. A stripe light emitting part is embedded by a solid embedding material with a high electrical resistance. Thus, a current can be allowed to entirely flow in the part 2 in a vertical penetration direction. In addition, since the embedding material in the light emitting part functions as stripe structure, a current injection efficiency for the part 2, that is, laser outputting can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an edge emitting semiconductor laser, and more particularly to a semiconductor laser having a light emitting portion made of an InGaAlN compound semiconductor.

[0002]

2. Description of the Related Art A semiconductor laser capable of emitting short wavelength light ranging from blue light to ultraviolet light has been demanded in response to a demand for increasing data density in fields such as communication and recording. As a semiconductor material for such a laser, a compound semiconductor composed of a group III element and nitrogen, among which In _x Ga is particularly preferable.
_y Al _1-xy N, a compound semiconductor having a general formula of (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) (hereinafter, the compound semiconductor is referred to as InG
Attention is focused on a compound semiconductor of aAlN type). As a material for a substrate used when forming the semiconductor laser, a sapphire crystal which has been easily available is conventionally used. On the other hand, generally, semiconductor lasers are classified into a gain waveguide type and a refractive index waveguide type in which a waveguide structure is internally provided with a refractive index distribution. By the way, InGaAl
In the case of N-based compound semiconductors, the latter index-guided type, which is easy to increase the current injection density, is currently the mainstream. FIG. 3 shows a conventional structure of a refractive index guided InGaAlN based semiconductor laser using a sapphire crystal as a substrate. In the figure, the light emitting portion 2 having the double hetero structure is laminated on one surface of the sapphire crystal substrate 10a with the buffer layer 10b interposed therebetween. The light emitting section 2 includes a lower clad layer 20a, an active layer 20c, and an upper clad layer 20b. The optical resonator is composed of front and rear surfaces in the figure, and the laser light L is emitted in the direction of the thick arrow.

[0003]

By the way, the sapphire crystal is electrically insulating, and it is not possible to provide an electrode, for example, an n-side electrode, on the back surface of the substrate 10a. Therefore, as shown in FIG. 3, the upper surface of the lower clad layer 20a is exposed,
N-side electrodes 40a and 40b are provided on the upper surface thereof. The p-side electrode 50 is provided on the upper surface of the upper clad layer 20b. Further, the side wall of the light emitting unit 20 is in direct contact with air. Index-guided type I having the above structure
The nGaAlN semiconductor laser has the following various problems. (1) The current based on the voltage applied between the p and n electrodes is not in the direction perpendicularly penetrating the substrate 10a but in FIG.
As indicated by a dashed arrow therein, it flows laterally in the lower cladding layer 20a. Therefore, the density of current injection into the active layer 20c is not sufficient. (2) The side wall of the light emitting section 20 is in contact with air, and due to the excessive difference in the refractive index between the constituent material of the active layer 20c and the air, the light confinement effect is also excessive and the higher-order mode is activated. easy. (3) Generally, so-called upside-down mounting in which the p-side electrode 50 side is welded to a heat sink is performed in order to cope with heat generation of the light emitting portion due to current injection. In this case, the heat of the active layer is radiated to the heat sink via the upper clad layer 20b and the p-side electrode 50.
However, in the conventional structure shown in FIG. 3, since both electrodes and electric wiring therefor exist only on one surface of the substrate, the p-side electrode 5
In order to weld the heat sink only to 0, it is necessary to take measures to prevent a short-circuit accident, which complicates the upside-down process.

A main object of the present invention is to provide an InGaAlN-based semiconductor laser having an improved current injection density in the active layer. Another object of the present invention is to provide a refractive index guided InGaAlN based semiconductor laser capable of suppressing the generation of higher order modes. Yet another object of the present invention is to provide an InGaAlN-based semiconductor laser that is easy to mount upside-down and has excellent heat dissipation. Other subjects of the present invention will be easily understood by those skilled in the art from the following detailed description and examples.

[0005]

Therefore, the semiconductor laser of the present invention has a conductive substrate, one electrode provided on one surface side thereof, and a double hetero structure provided on the other surface side of the substrate. A striped light-emitting portion and the other electrode provided on the light-emitting portion, and the light-emitting portion includes a first clad layer sequentially from the substrate side and an In _x Ga _y Al layer.
An active layer made of a compound semiconductor having a general formula of _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1) and a second clad layer are provided, and the stripe-shaped light-emitting portion is high. It is embedded with a solid embedding material having electric resistance. In a preferred embodiment of the present invention, the solid embedding material has a refractive index of 5 from the effective refractive index n _a of the active layer at the laser oscillation wavelength.
A material having a low effective refractive index n _b of × 10 ^{-1 to} 5 × 10 ^-4 is used.

[0006]

The above-mentioned structure provides the following remarkable effects. (A) When a conductive substrate is used, it is possible to dispose two electrodes so as to sandwich the substrate from above and below, and by doing so, a current is passed in a direction that vertically penetrates the entire surface of the light emitting portion. be able to. In addition, by embedding the light emitting portion with a solid embedding material having a high electric resistance capable of functioning as a stripe structure, the efficiency of current injection into the light emitting portion, and thus the laser output, can be increased. (B) said solid filling material, in addition to the high electrical resistance, 5 × 10 ^-1 ~ than the effective refractive index n _a of the active layer of the laser oscillation wavelength
When the effective refractive index n _b is also lower by 5 × 10 ⁻⁴ , an appropriate light confinement effect can be obtained, unlike the case where the light emitting portion is surrounded by air as in the conventional structure shown in FIG. It is possible to suppress the occurrence of higher-order modes. (C) Due to the fact that the electrodes can be placed above and below the conductive substrate and the embedded structure, the entire upper surface of the other electrode formed on the second cladding layer has a problem of short-circuit accident, which is a problem in the case of FIG. Without this, upside-down mounting can be easily performed, and heat dissipation is further improved.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
FIG. 1 is a perspective view of an embodiment of the semiconductor laser of the present invention. FIG. 2 is a perspective view of another embodiment of the semiconductor laser of the present invention. FIG. 3 is a perspective view of a conventional InGaAlN based semiconductor laser. In FIGS. 1 and 2, the same parts are indicated by the same numbers.

[0008]

EXAMPLE In FIG. 1, an n-side electrode 4 is formed on the lower surface of a conductive substrate 1, and a light emitting portion 2 is formed on the upper surface of the substrate 1 via a buffer layer 1a. The light emitting portion 2 is an active layer 2c.
Is sandwiched between the n-type first clad layer 2a and the p-type second clad layer 2b. The entire light emitting portion is formed as a single striped structure in the center of the substrate 1 as shown in the drawing, and both side walls of the striped structure have a high electrical resistance and the specific effective refractive index n _b . It is embedded with the solid embedding material 3a, 3b. The upper surface of the light emitting part 2 and the solid filling materials 3a and 3b
And the upper surface thereof form one coplanar surface, and the p-side electrode 5 is formed on the entire coplanar surface. Similar to FIG. 3, the optical resonator is composed of front and rear surfaces in the figure, and the laser light is emitted in the direction of the thick arrow L. In addition, the current passage is formed in a direction penetrating perpendicularly to all three layers of the light emitting unit 2 as shown by the broken line in FIG. 1 based on the installation location of both electrodes.

Each of the layers of the light emitting portion 2 is made of In _x Ga _y.
It is formed of at least one compound semiconductor having a general formula of Al _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1), and emits light of short wavelength ranging from blue light to ultraviolet light. Can be released. Among InGaAlN-based compound semiconductors, an example of a combination of a clad layer material and an active layer material particularly suitable for emitting light from blue light to ultraviolet light is given by the notation of clad layer material / active layer material: GaAlN / InGaN, Ga
AlN / GaN, GaAlN / GaAlN, InGaA
1N / InGaN, InGaAlN / GaN, InGa
AlN / GaAlN, InGaAlN / InGaAlN
Are exemplified. Of these combinations, GaAlN
The composition ratio of each of the materials such as / GaAlN in which the cladding layer material and the active layer material are composed of the same element is different. The composition ratio of each is determined such that the bandgap of the cladding layer is larger than the bandgap of the active layer.

Among the cladding layer materials, Ga _1-x Al _x N
The compound semiconductor having the general formula of (where 0 ≦ x ≦ 0.5) is particularly preferable from the viewpoint of increasing the output of the laser of the present invention and decreasing the oscillation threshold current. Generally InG
aAlN-based compound semiconductor, especially Ga _1-x Al _x N
When using a compound semiconductor as the cladding layer material,
As an n-type dopant, it is easy to control the carrier concentration.
i is preferred. On the other hand, as the p-type dopant, for example, M
g or Zn is used. These p-type dopants cannot change the InGaAlN-based compound semiconductor to p-type just by being doped, but merely change the property to a high resistance body. Therefore, since the semiconductor is made p-type,
After the doping, electron beam irradiation or heat treatment in an inert atmosphere such as nitrogen is performed.

As a constituent material of the active layer 2c, a compound semiconductor having the general formula of In _1-x Ga _x N (where 0.7 ≦ x ≦ 0.96) is selected from the compound semiconductors having the above general formula. Is particularly preferably used without dopant.

In the combination of the clad layer material and the active layer material, it is preferable that the active layer is designed to have a refractive index higher than that of the clad layer, whereby light is efficiently confined in the active layer. .

The double-hetero structure of the light-emitting portion 2 has a typical structure of three layers shown in FIG.
It may be uantum well) or MQW (Multi Quantum Well). Further, an SCH (Separate Confinm) of a general structure in which an active layer sandwiched by upper and lower clad layers has a light emitting layer sandwiched by upper and lower optical waveguide layers.
ent Heterostructure). In the case of SCH, when discussing the effective refractive index difference Δ with the solid embedding material, the effective refractive index n _a of the waveguide layer in the active layer is taken into consideration.

The entire shape of the light emitting section 2 is a stripe structure having a constant width of l _{1 as} shown in the figure between both end faces constituting the optical resonator. The position of the stripe structure has a constant width of l _{2 in} the width direction of the substrate 1
Is preferably near the center of the entire width of. Generally, the substrate width l ₂ is
The width l _{1 of the} stripe structure is about 0.05 to 50 μm, and particularly 2 μm to about 100 to 1000 μm.
About 10 μm is preferable.

The substrate 1 serves as a basis for forming an InGaAlN-based semiconductor layer and has conductivity.
Any material that can form an electrode on the surface may be used. For example, a crystal body, preferably GaN, SiC, ZnO, or the like, can be mentioned, and a single crystal of GaN and SiC can be particularly mentioned. By the way, the reflective surface of the optical resonator cannot be formed by cleavage when a conventional sapphire substrate is used, so reactive ion etching (Reactive Ion Etching)
The conductive substrate 1 is formed by G in the present invention.
The use of a cleavable single crystal material such as aN or SiC has an advantage that ideal mirror surfaces can be easily formed on both end faces of the light emitting portion by utilizing the cleavability. The cleaved surface may be coated with one or more suitable dielectric thin films to control its reflectance.

The buffer layer 1a is made of GaN, and is provided as necessary as a layer for improving the crystal quality of the InGaAlN-based semiconductor layer when forming it on the crystal substrate 1. In the embodiment of FIG. 1, the substrate 1 has n conductivity type, but it may be p type. At this time, the conductivity types of the first and second cladding layers are also changed accordingly.

The following method is exemplified as a method of forming the light emitting portion 2 in a stripe structure. First, the conductive substrate 1
If necessary, a buffer layer 1a is formed thereon, and the first cladding layer 2a (for example, GaAlN) of the same conductivity type as that of the substrate and the active layer 2c (for example, undoped I
nGaN) and the second clad layer 2b (for example, GaAlN) having a conductivity type different from that of the substrate are sequentially grown. If necessary to obtain p-type conductivity, heat treatment after crystal growth,
Post-processing such as electron beam irradiation processing is performed. Next, a mask is formed on the upper surface of the light emitting portion by photolithography,
By etching by RIE, the entire light emitting portion 2 has a stripe structure.

Filling materials 3a and 3b are airtightly deposited on the entire surfaces of both side walls of the thus formed stripe structure so as to have the same height as that of the second cladding layer 2b. As the deposition method, a known film forming method may be used, and among these, CVD, sputtering and the like are preferable. When an InGaAlN compound semiconductor, which will be described later, is used as the filling material, it is preferably formed by a method similar to the method of forming the layers 2a and 2c.

The solid embedding materials 3a and 3b are composed of electrodes 4 and
The current based on the voltage applied between 5 and 5 is indicated by the dotted line in FIG.
Electric resistance for efficiently injecting into the active layer 2c through the path shown
Suppresses the function and generation of higher modes from the active layer 2c
Function. Its electrical resistance is low
Also 10^FourΩcm or more, preferably 10^FiveΩcm or more, special
Preferably 10⁶Ωcm or more. Also the material 3
From the function of suppressing the generation of higher modes,
Is the effective refractive index n of the active layer at the laser oscillation wavelength._aThan 5
× 10^-1~ 5 × 10^-FourLow effective refractive index n_bHaving. n
_a-N_bThat is, the effective refractive index difference Δ is preferably 1 × 10^-2
~ 5 × 10^-3Degree, particularly preferably 5 × 10 ^-2~ 5 x 1
0^-3It is a degree.

As the solid embedding material, inorganic oxides (the numbers in parentheses below are approximate refractive indices of each substance) such as SiO ₂ (1.4 to 1.5), MgO (1.
7), Al ₂ O ₃ (1.7 to 1.76), P ₂ O
₅ (1.47), B ₂ O ₃ (1.48 to 1.64), Z
nO (2.0), GeO ₂ (1.65), ZrO
Such as ₂ (2.1), and _{In x Ga y Al 1-xy} N
(Where, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1) compound semiconductors such as GaN, InGaN, GaAlN
And so on. Above all, there is an advantage that various effective refractive indexes n _b can be obtained from the above compound semiconductors by controlling the composition thereof. For example, the band gap and effective refractive index can be produced as designed by controlling X and Y therein. In addition, among them, the undoped material in which the amount of impurities is reduced as much as possible or the material doped with acceptor impurities such as Mg and Zn not only has preferable high electric resistance, but also the material itself is easily manufactured. In particular, the latter doped body can be easily made into a high electric resistance body only by doping. As a specific example of the compound semiconductor, I doped with 1 × 10 ¹⁹ cm ^{−3 of} Mg is used.
n _0.02 Ga _0.98 N (room temperature, high electrical resistance of the effective refractive index 2.53 at an oscillation wavelength of 410nm), Ga _{0. 98} Al of Mg doping _{0. 04} N (high electrical in the same effective refractive index 2.50 Resistor) and so on.

The active layer 2c is formed of, for example, In with n _a of 2.54.
In GaN, the clad layer 2a and the clad layer 2b
As described above, it is preferable that the effective refractive index of the solid embedding materials 3a and 3b is less than 2.54 when n _b is formed of, for example, GaAlN having a value of 2.45. For example, SiO ₂ , Zn
_{O, ZrO 2, In 0.02 Ga} 0.98 N, Ga 0.98 Al 0. 04
There are N etc. Of these, materials other than SiO ₂ are particularly preferable because the effective refractive index difference Δ is in an appropriate range.

[0022] With reference to the case where the light emitting unit 2 is a SCH structure having a waveguide layer, a conductor-wave layer n _a is 2.52
In the example in which the clad layer 2a and the clad layer 2b are formed of GaAlN with n _b of 2.45, the filling materials 3a and 3b have an effective refractive index of less than 2.52, for example, Ga _0.98 Al ₀ of said Mg doping. ₀₄ N is suitable.

For the upper and lower electrodes, well-known materials and structures may be used for the upper and lower electrodes. For example, Au is used for the p-side electrode 5 and Al, In, etc. for the n-side electrode 4.

Another embodiment shown in FIG. 2 is a filling material 3a, 3 which shares a low effective refractive index n _b and a high electric resistance.
FIG. 1 in which b is embedded in the entire side walls of the stripe of the light emitting unit 2.
Different from the above embodiment, solid embedding materials 3c and 3d having high electric resistance are embedded only in both side walls of the second cladding layer 2b. At that time, the first clad layer 2a
In each of the active layers 2c, the insides of the alternate long and short dash lines 6 and 6 and the regions 7 and 7 beyond it are made of the same material and are integrally connected. When a voltage is applied between the electrodes 4 and 5, due to the presence of the burying materials 3c and 3d having high electric resistance, only the width l ₁ portion (that is, the light emitting portion 2) within the alternate long and short dash lines 6 and 6 in FIG. A current is injected, but no current is injected into the regions 7, 7. Thus, the area surrounded by the alternate long and short dash lines 6 and 6 substantially functions as the stripe side wall of the light emitting section 2.

In the embodiment of FIG. 2, the filling material 3c,
When 3d has not only a high electric resistance but also an effective refractive index n _b lower than the effective refractive index n _a of the active layer 2c,
Even if the inside and outside of the alternate long and short dash lines 6 and 6 in the active layer 2c and the first cladding layer 2a are formed of the same material, the portions outside the alternate long and short dash lines 6 and 6, that is, the regions 7 and 7.
In the material (1), the effective refractive index n _b decreases due to the presence of the low-refractive-index embedding material in close proximity, and the effective refractive index difference Δ is within the above range. Thus, it has a function of preventing the occurrence of the higher-order mode.

In the embodiment shown in FIG. 2, the second cladding layer 2 is used.
Only b can be manufactured by the same etching process and buried deposition method as in the embodiment of FIG. However, the following method, which does not require the etching treatment described below, is particularly preferable because it is easy to manufacture. In order to simplify manufacturing, only the second cladding layer 2b is striped. The InGaAlN-based material has a property of being a high resistance substance only by adding a p-type impurity, and exhibits a p-type conductivity type by performing an appropriate post-treatment, but this property is simplified. To use. That is,
The crystal substrate 1 and the first cladding layer 2a are n-type, and the second cladding layer 2b is p-type impurity such as M.
g and the like are added, and post-treatment is applied only to the regions within the chain lines 6 and 6 to make them p-type. By this method, the upper layer of the active layer 2 is a flat layer entirely made of GaAlN, but only the region within the dashed-dotted lines 6 and 6 in the central portion functions as a p-type cladding layer, and both sides thereof have high resistance. It becomes a substance and functions as a substantial filling material 3c, 3d. As a post-treatment method for imparting the p-type conductivity type, a well-known method may be used, but an electron beam irradiation treatment capable of selective conduction control is preferable.

In the formation of the stripe structure by the above method, processing steps such as etching are omitted as compared with the embodiment of FIG. 1 and the manufacturing cost can be reduced. On the other hand, in the embodiment of FIG. 1, since the entire side wall of the stripe structure is surrounded by a material having a high electric resistance and a low refractive index, it is a high performance laser.

The present invention can be modified in various ways other than the embodiment shown in FIGS. The solid filling material may be provided at an arbitrary position in the light emitting unit 2, for example, on each side surface of the first clad layer, the active layer, or the second clad layer.
The use of a solid fill material having both high electrical resistance and low index of refraction, as in the embodiment of FIG. 1, is particularly preferred, as it makes the laser of the present invention easier to manufacture.

[0029]

Industrial Applicability The semiconductor laser of the present invention is capable of suppressing the generation of higher-order modes, improving the current injection density in the active layer, facilitating upside-down mounting, and excellent in heat dissipation. Therefore, it is suitable for fields such as communication and recording in which there is a strong demand for increasing the data density.

[Brief description of drawings]

FIG. 1 is a perspective view of an embodiment of a semiconductor laser of the present invention.

FIG. 2 is a perspective view of another embodiment of the semiconductor laser of the present invention.

FIG. 3 is a perspective view of a conventional InGaAlN based semiconductor laser.

[Explanation of symbols]

 1 Conductive Substrate 1a Buffer Layer 1a 2 Light Emitting Section 2a n-type First Cladding Layer 2b p-type Second Cladding Layer 2c Active Layers 2c 3a and 3b Solid Embedding Material 4 n-side Electrode 5 p-side Electrode

Claims (9)

[Claims] 1. A conductive substrate, one electrode provided on one surface side thereof, a stripe-shaped light emitting portion having a double hetero structure provided on the other surface side of the substrate, and an upper portion of the light emitting portion. Of the other electrode provided on the substrate, and the light emitting portion has a first cladding layer sequentially from the substrate side and an In _x Ga _y A
l _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1) is composed of an active layer made of a compound semiconductor and a second clad layer. A semiconductor laser embedded with a solid embedding material having high electric resistance. 2. A solid filling material, a semiconductor laser according to claim 1, further comprising an effective refractive index n _a more 5 × 10 ^-1 ~5 × 10 ^-4 low effective refractive index n _b of the active layer of the laser oscillation wavelength . 3. The semiconductor laser according to claim 1, wherein in the striped light emitting portion, the second cladding layer is filled with a solid filling material. 4. The semiconductor according to claim 2, wherein in the stripe-shaped light emitting portion, all of the first cladding layer, the active layer and the second cladding layer are filled with a solid filling material. laser. 5. The semiconductor laser according to claim 2, wherein the double hetero structure is an SCH structure. 6. The semiconductor laser according to claim 2, wherein the electrode on the second clad layer extends over substantially the entire upper surface of the semiconductor laser with the same plane. 7. The crystal substrate and the first cladding layer both have an n-type conductivity, and the second cladding layer is made of a p-type doped InGaAlN-based compound semiconductor and has a light emitting portion. 4. The semiconductor laser according to claim 3, wherein only the relevant portion is of p-type conductivity type, and the rest is non-p-type and has a high electric resistance. 8. The active layer is InGaN and the clad layer is Ga.
The semiconductor laser according to claim 2, which is made of AlN. 9. The semiconductor laser according to claim 2, wherein the crystal substrate having conductivity is made of GaN or SiC.