JP2000031599A - Nitride based iii-v compound semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive-index guide-type GaN-based semiconductor laser, i.e., a nitride-based III-V compound semiconductor laser, which prevents the generation of a higher mode without narrowing a ridge width and which is oscillated in a basic mode. SOLUTION: A ridge shape is constituted. GaN-based buried layers 2 which absorb oscillation light are formed on both sides in the widthwise direction of its ridge. A material for the buried layers 2 or their crystal state or the like is selected in such a way that the absorption coefficient α of the buried layers 2 is set at α>=104 cm-1 which can obtain Δn>=10-3 as a value by which a refractive index difference Δn formed between the ridge part near an active layer and its both sides can obtain a refractive-index guide effect.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a nitride III-
The present invention relates to a group V compound semiconductor laser, a so-called GaN-based semiconductor laser, and particularly to a refractive index-guided GaN-based semiconductor laser.

[0002]

2. Description of the Related Art GaN-based semiconductors can be selected from a wide range of energy band gaps, so that short-wavelength oscillation semiconductor lasers can be obtained. Therefore, recently, this type of GaN-based semiconductor laser has been receiving attention as a short-wavelength laser such as a light source for optical data recording. on the other hand,
Among the semiconductor lasers, the refractive index guide type semiconductor laser has an advantage that a stable transverse mode up to a high output is easily obtained in the oscillation light.

In a conventional refractive index guide type GaN-based semiconductor laser, a buried layer made of an insulating layer such as SiO ₂ is formed on both sides in the width direction of a ridge formed along the resonator length direction. A configuration is adopted in which a refractive index difference Δn from a portion is created (Appl
ied Physics Letters vol.72 pp2014).

[0004]

However, the refractive index guide type GaN-based semiconductor laser described above has a low heat dissipation effect because the insulating layer such as SiO ₂ constituting the buried layer has low thermal conductivity. Since the temperature inside the laser element easily rises, the temperature characteristics are poor, and there are problems in reliability and life.

The buried layer SiO ₂ is made of GaN
Since it is transparent to the oscillation laser light of the system laser, in the transverse mode, the higher-order mode gain of the first-order mode or higher is not so small, and the ridge width that does not satisfy the cut-off condition of the higher-order mode is high. During output driving, higher order modes are easily established. In order to suppress the occurrence of such higher-order modes, it is necessary to make the ridge width narrower than the cutoff condition. However, when the ridge width becomes narrower, the contact area of the electrode with respect to the ridge width becomes smaller.
In other words, the series resistance of the laser diode increases, and problems such as a decrease in laser characteristics, that is, an increase in drive voltage and a decrease in life due to this occur.

In order to avoid such a problem, the ridge width is selected to be large. In this case, a higher-order mode is set as described above. That is, FIG.
As shown by the (current) -L (light output) characteristics, a kink occurs in the light output characteristics. And this higher mode is
If it occurs in the operating range, it becomes unstable, and the higher-order mode causes noise in, for example, optical recording and reproduction.

In the present invention, a refractive index guide type GaN system, that is, a nitride system II, in which a higher order mode is avoided and a fundamental mode oscillation is performed without reducing the ridge width.
A group IV compound semiconductor laser can be obtained.

[0008]

SUMMARY OF THE INVENTION According to the present invention, a nitride system I is provided.
The II-V compound semiconductor laser has a ridge type configuration,
On both sides in the width direction of the ridge, a GaN-based buried layer that absorbs oscillation laser light is formed. The absorption coefficient α of this buried layer is such that the refractive index difference Δn formed between the ridge portion and both sides near the active layer can obtain Δn ≧ 10 ^−3, which is the value at which the refractive index guiding effect is obtained. ≧ 10 ⁴ cm ^-1
Next, the material, crystal state, impurity concentration, and the like of the buried layer are selected.

As described above, in the present invention, by arranging the GaN-based buried layers arranged on both sides of the ridge to absorb the oscillating light, the condition for cutting off higher-order modes higher than the first-order mode is satisfied. The ridge width that can be used is increased.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention constitutes a ridge type semiconductor laser, and has a refractive index guide type in which a GaN-based buried layer for absorbing oscillation laser light is formed on both sides of the ridge in the width direction. Semiconductor laser. In the present invention, the buried layer is formed in the vicinity of the active layer.
The refractive index difference Δn formed on the ridge and on both sides of the ridge is a value at which the refractive index guiding operation is obtained, and the light absorption coefficient α for the oscillating laser beam capable of obtaining Δn ≧ 10 ⁻³ is α ≧ 10 ⁴
The material is selected so as to indicate cm ⁻¹ , or a crystalline state, an impurity concentration, or the like.

The GaN-based buried layer has an energy band gap for absorbing the oscillation laser light at the band edge, that is, a GaN-based material having a band gap smaller than the energy band gap of the active layer corresponding to the energy of the oscillation laser light. layer, for example, (Al _X Ga _y I
n _{1−X −y} ) N, 0 ≦ x ≦ 1 InGaN, AlGaI
nN.

[0012] Alternatively, the GaN-based buried layer is formed of a GaN-based material layer having a light absorption effect, for example, having reduced crystallinity, without relying on band edge absorption. As the GaN-based material, for example, (Al _X Ga _y
In _{1−X −y} ) N, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x +
It is composed of, for example, GaN, AlGaN or the like with y ≦ 1.

Next, a semiconductor laser according to the present invention will be illustrated, but the present invention is not limited to this example. FIG.
1 shows a schematic sectional view of an example of a semiconductor laser according to the present invention. In this example, the buried layers 2 arranged on both sides of the ridge 1 are made of InGaN that absorbs the oscillating laser light due to band edge absorption. This semiconductor laser will be described together with an example of a manufacturing method thereof with reference to schematic sectional views in main steps of FIGS.

As shown in FIG. 2, on a C-plane sapphire substrate, for example, as a substrate 11, a buffer layer of a first conductivity type, in this example, an n-type GaN layer, and an n-side electrode described later are contacted. The contact layer 12 is epitaxially grown. Subsequently, on this, for example, an AlGaN layer as the same n-type first cladding layer 13, a GaN layer as the same n-type first guide layer 14, and further, for example, Ga _1-x In _x N Ga _1-y In _y N (x ≠
y), an active layer 15 having a multiple quantum well structure, a second conductivity type protecting this active layer 15, in this example a p-type protective layer
6, a similar p-type GaN layer as the second guide layer 17, a similar p-type AlGaN layer as the second cladding layer 18, and a p-type GaN layer as the contact layer 19. MOCVD (Metalo
It is grown epitaxially by the rganic Chemical Vapor Deposition method.

Next, as shown in FIG.
9, a stripe-shaped mask layer 20 extending in a direction perpendicular to the plane of the drawing is formed on the ridge forming portion. The mask layer 20 is, for example, entirely deposited, CVD
SiO ₂ is formed by a (Chemical Vapor Deposition) method, and pattern etching is performed by photolithography using, for example, a hydrofluoric acid-based etchant to form a stripe having a required width.

As shown in FIG. 4, by using the mask layer 20 as an etching mask, RIE (Reactive Ion Etching) crosses the contact layer 19 which is not covered by the mask layer 20 and is exposed to the outside. The ridge 1 is formed under the mask layer 20 by etching to a depth that enters the layer 18. That is, the grooves 21 are formed on both sides of the ridge 1.

As shown in FIG. 5, the buried layer 2 is
To MOCVD or MBE (Molecular Beam Epitax)
y: formed by a molecular beam epitaxy method or the like, and the mask layer 20 is removed.

On the contact layer 19 and the buried layer 2,
Forming an etching mask layer (not shown) formed on the buried layer 2 having an opening formed on one side of the ridge 1, for example;
For example, as shown in FIG. 6, etching is performed by RIE to a position where the contact layer 12 is exposed to the outside.

As shown in FIG. 1, one electrode of a metal electrode, in this example, an n-side electrode 22 is ohmic-coated on the contact layer 12, and the contact layer 19 of the ridge 1 extends from the contact layer 19 to the buried layer 2. , And the other electrode made of a metal electrode, in this example, the p-side electrode 23 is applied in ohmic contact. Thus, the semiconductor laser according to the present invention is obtained. Needless to say, this semiconductor laser can be mass-produced by forming a large number of laser elements on the common substrate 11 at the same time and forming chips as usual.

The configuration shown in FIG.
m GaN-based laser was constructed. In this case, the buried layer 2 has a band gap smaller than the band gap of the active layer 15, that is, a band gap smaller than the band gap corresponding to the energy of the oscillation wavelength, and is a buried layer that absorbs oscillation laser light.

FIG. 7 shows that the buried layer 2 is made of In _0.2 Ga _0.8 N
This shows the relationship between the thickness d of the second cladding layer 18 under the buried layer 2 and the refractive index difference Δn, where Δn = 3 × 10 ⁻³ at the thickness d = 1500 °, d
= 1000 °, Δn = 5 × 10 ⁻³ is obtained. According to this, it is understood that a GaN-based laser having a refractive index guide type configuration is configured. That is, the refractive index guide is usually Δn
≧ 1 × 10 ⁻³ , and more certainly Δ ≧ 3 × 10 ⁻³ , so that d ≦
It can be seen that a refractive index guide type laser can be obtained at 1500 °.

Further, in this GaN-based laser, when the thickness d of the second cladding layer 18 under the buried layer 2 is 1000 °, the width W of the ridge 1 and the distance from the light emitting end of the laser are measured. FIG. 8 shows the relationship between the outgoing light and the spread angle θ _{H in} the surface direction. This spread angle θ _H is, for example, DVD
(Digital video disk) requires θ _H ≧ 8 ° in order to make the spot shape close to a circle. Therefore, from FIG. 8, the ridge width for θ _H = 8 ° is 2.3 μm. Become.

In the above-mentioned GaN-based laser, the buried layer 2 is made of a material layer having an energy band gap smaller than the energy band gap of the active layer corresponding to the energy of the oscillation laser light. Although the structure of the buried layer 2 is the same as that of FIG. 1, the material of the buried layer 2 has a larger energy band gap than the energy band gap of the active layer corresponding to the energy of the oscillating laser light, that is, the band edge absorption is smaller. It can also be made of a material not made, for example, GaN. In this case, in the step of forming the buried layer 2, the formation conditions are selected and the film is formed as a layer having low crystallinity. The absorption coefficient is set to 1 × 10 ⁴ cm ⁻¹ or more by increasing the doping concentration or selecting the feed ratio of each of the group III and group V materials.

FIG. 9 shows that the GaN of the buried layer 2 in the GaN-based laser having this configuration is oscillated by selecting the above-mentioned temperature, the supply ratio of each of the group III and group V materials, the impurity doping amount, and the like. Laser light (wavelength 410n
This shows the relationship between each value and the refractive index difference Δn when the absorption coefficient α for m) is changed. As is apparent from this, in order to obtain Δn ≧ 10 ⁻³ , the absorption coefficient α
Is 1 × 10 ⁴ cm ⁻¹ . Also,
In practice, this absorption coefficient may be 5 × 10 ⁵ cm ^-1 or less, preferably 1 × 10 ⁵ cm ^-1 or less.

The above-mentioned contact layer 19 preferably has a thickness of about 0.3 μm. The contact layer 19 is formed on the ridge 1 as in the above-described example.
It is not limited to the case of forming only on the
As shown in the schematic sectional view of FIG.
5 is formed over the ridge 1 and the buried layer 2.
By contacting the p-side electrode 23 thereon, the contact resistance can be further reduced. 10, parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

As described above, according to the present invention, since a refractive index guide type laser is formed, the transverse mode can be stabilized even at a high output. Further, in the configuration of the present invention, the buried layer 2 is a buried layer having absorptivity to the oscillation laser light. Since the off condition can be relaxed and the ridge width can be increased, the series resistance of the laser diode can be reduced. That is, since, for example, AlGaN constituting the ridge has a relatively high resistivity, the ability to increase the ridge width can reduce the operating voltage, improve heat generation, and extend the life.

Further, since the buried layer 2 is made of the same material as the GaN-based material, ie, the laser body, that is, the laser driver and the constituent material, there is no large difference in the thermal expansion coefficient between the two. Therefore, thermal distortion due to ambient temperature and heat generated during operation can be effectively avoided, and operation can be stabilized and life can be extended.

Then, as shown in FIG. 10, the contact layer 25 and the electrode 2 which is in ohmic contact with the contact layer 25 are formed.
3 can extend from over the ridge 1 to a position extending over the buried layer 2.
Can be effectively introduced into the ridge 1.

In the example described above, the SCH (Separate Confinement Heteros) provided with the first and second guide layers is used.
However, the present invention is not limited to the above example. For example, a semiconductor laser having a so-called DH (Double Hetero) structure without these guide layers can be used. Index-guided nitride-based III-V compound semiconductor laser, wherein the first conductivity type is p-type and the second conductivity type is n-type. Can be. However, in the case where a part of the contact layer 12 shown in FIG. 6 is exposed, considering that the surface of the contact layer 12 is likely to be n-type due to a defect on the surface, the first conductivity type is n-type and the second conductivity type is p-type. It can be.

[0030]

As described above, the nitride-based III-V compound semiconductor laser according to the present invention has a refractive index guide structure, but the buried layers disposed on both sides of the ridge are provided with an oscillating laser. By using a buried layer that absorbs light, the cutoff condition of the higher-order mode in the transverse mode can be relaxed and the ridge width can be made larger than in the case of a buried layer that transmits light. As a result, the series resistance of the laser diode can be reduced. That is, for example, AlGa
Since N has a relatively high resistivity, the fact that the ridge width can be widened can reduce operating voltage, improve heat generation,
The service life can be extended.

Further, since the GaN-based material, that is, the laser body, that is, the laser driver, and the constituent material are made of the same material, there is no large difference in the thermal expansion coefficient between the two. Therefore, thermal distortion due to ambient temperature and heat generated during operation can be effectively avoided, and operation can be stabilized and life can be extended.

Further, since the buried layer is made of a GaN-based material having better thermal conductivity than SiO ₂ ,
Since the temperature rise can be effectively avoided even by driving for a long time, the operation can be stabilized, the reliability can be improved, the continuous use can be performed for a long time, and the life can be prolonged.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an example of a semiconductor laser according to the present invention.

FIG. 2 is a schematic cross-sectional view in one step of an example of a semiconductor laser according to the present invention.

FIG. 3 is a schematic cross-sectional view in one step of an example of a semiconductor laser according to the present invention.

FIG. 4 is a schematic cross-sectional view in one step of an example of a semiconductor laser according to the present invention.

FIG. 5 is a schematic cross-sectional view in one step of an example of a semiconductor laser according to the present invention.

FIG. 6 is a schematic cross-sectional view in one step of an example of a semiconductor laser according to the present invention.

FIG. 7 is a diagram illustrating a relationship between a thickness d of a cladding layer and a refractive index difference Δn.

FIG. 8 is a diagram illustrating a relationship between a ridge width W and an emission angle of an emitted laser beam in a plane direction.

FIG. 9 is a diagram showing a relationship between a light absorption coefficient α of GaN of a buried layer and a refractive index difference Δn in a semiconductor laser.

FIG. 10 is a schematic sectional view of another example of the semiconductor laser according to the present invention.

FIG. 11 is a diagram showing light output characteristics of a conventional semiconductor laser.

[Explanation of symbols]

1 ... ridge, 2 ... buried layer, 11 ... substrate,
12 contact layer, 13 first clad layer,
14 ... first guide layer, 15 ... active layer, 16 ...
-Protective layer, 17 ... second cladding layer, 18 ... second
Guide layer, 19 contact layer, 20 mask layer, 21 groove, 22 n-side electrode, 23 p
Side electrode

Claims (8)

[Claims] 1. A ridge-type nitride III-V compound semiconductor laser, comprising: a Ga-absorbing laser beam on both sides in the width direction of the ridge.
A nitride-based III-V compound semiconductor laser, comprising an N-based buried layer. 2. The buried layer according to claim 1, wherein said buried layer has an absorption coefficient of 10 ⁴ cm ⁻¹ or more for oscillation laser light
3. The nitride-based III-V compound semiconductor laser according to claim 1, wherein the nitride-based III-V compound semiconductor laser is formed by an aN-based buried layer. 3. The buried layer has an energy band gap smaller than an energy band gap corresponding to the energy of oscillation laser light,
2. The nitride III-V compound semiconductor laser according to claim 1, wherein the laser is formed by a buried layer that absorbs the oscillation laser light by band edge absorption. 4. The buried layer has an energy band gap smaller than an energy band gap corresponding to the energy of oscillation laser light,
Oscillation laser light is absorbed by band edge absorption (Al
_X Ga _y In _{In 1-X -y) N,} 0 ≦ x ≦ 1 of the nitride III-V compound semiconductor laser according to claim 1, characterized in that it is constituted by the material. 5. The buried layer has an energy band gap larger than an energy band gap corresponding to the energy of the oscillation laser light,
Absorption coefficient for oscillation laser light is 10 ⁴ cm ^-1 or more and 10
^2. The nitride III- according to claim 1, wherein the nitride III- is formed by a GaN-based buried layer of ⁵ cm ^-1 or less.
Group V compound semiconductor laser. 6. The buried layer according to claim 5, wherein the buried layer is formed of a GaN-based buried layer having low crystallinity and having an absorption coefficient of 10 ⁴ cm ⁻¹ or more for oscillation laser light. The nitride-based group III-V compound semiconductor laser according to the above. 7. The buried layer according to claim 5, wherein said buried layer is formed of a GaN-based buried layer having a high impurity concentration and an absorption coefficient of 10 ⁴ cm ⁻¹ or more for oscillation laser light. The nitride-based group III-V compound semiconductor laser according to the above. 8. The buried layer is, (Al _X Ga _y In
_{1-X -y} ) N, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦
6. The method according to claim 5, wherein the first material is made of a material.
3. A nitride-based III-V compound semiconductor laser according to item 1.