JP2865325B2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Industrial application field) The present invention relates to a refractive index guide type semiconductor laser device.

(Prior art) Metal-organic vapor phase epitaxy (MOCVD) and molecular beam epitaxy (MBE) have come into widespread use as semiconductor laser epitaxial growth methods instead of conventional liquid-phase epitaxial growth (LPE). . These growth methods are much more excellent in mass productivity and controllability than LPE, but have a problem that they are not suitable for growing a thick buried region around the active region of a refractive index guide type semiconductor laser. This is due to the fact that the growth rate is slow, and it is difficult to grow a flat buried layer having a deep step. As a laser structure suitable for such a growth method, there is a current constriction structure utilizing a diffusion potential difference between a hetero pn junction of a laser active layer and a pn junction of a cladding layer. However, this type of laser has a problem that the voltage applied to the cladding layer pn junction at the time of high injection exceeds the diffusion potential and the cladding layer pn junction turns on, causing a large leak current to flow. Hereinafter, this problem will be specifically described.

FIG. 9 is a cross-sectional view of a semiconductor laser having a current constriction structure by such a diffusion potential difference. This semiconductor laser is an n-type semiconductor having a thickness of 1 μm and a carrier density of 1 × 10 ¹⁸ cm ⁻³ laminated on an n-type InP substrate 1 having a carrier density of 3 × 10 ¹⁸ cm ^−3.
InP buffer / cladding layer (first semiconductor layer) 2 having a width of 1.5 μm and a thickness of 0.1 μm formed in stripes thereon
And the undoped _{In 1-x Ga x As y} P 1-y active layer composed of (wavelength 1.3μm equivalent composition) (third semiconductor layer) 3, it is stacked on the active layer 3 of the first semiconductor layer 2 A p-type InP cladding layer (second semiconductor layer) 6 having a thickness of 1 μm and a carrier density of 1 × 10 ¹⁸ cm ⁻³ , a thickness of 0.5 μm formed on the p-type InP layer 6,
A p-type _{In 1-x Ga x As y} P 1-y contact layer 7 of the carrier density 5 × 10 ¹⁸ cm ^-3, AuGe electrode 8 provided on the lower portion of the substrate 1
And an AuZn / Ti / Pt / Au electrode 9 provided on the contact layer 7. In order to reduce the leakage current, a mesa structure 10 having a width of 16 μm is formed above the n-type cladding layer 6 to reduce the area of the buried heterojunction. Although not explicitly shown in the figure, the resonator end face is formed by cleavage, and the resonator length is 400 μm. The emission side of the cleavage surface is coated with a low reflectance coating with a reflectance of 0.1%, and the other cleavage surface is coated with a multi-layer high reflectance coating with a reflectance of 90%. The semiconductor laser chip is mounted and bonded in a suitable package. The oscillation wavelength is 1.3 μm.

When the current injection level is low, the double hetero pn junction 2-3-6 including the active layer 3 is not
The current is effectively confined in the active layer pn junction due to the diffusion potential difference from the pn junction 2-6. However, as the current injection level is increased, the voltage applied to the pn junction increases, so that the leakage current of the homo pn junction starts to increase rapidly when the applied voltage exceeds the diffusion potential of the homo pn junction. This characteristic is shown by a dotted line in FIG. Since the leak current rapidly increases from the vicinity of the injection current of 150 mA, the optical output is saturated at around 140 mW. As described above, the conventional semiconductor laser in which the current is narrowed by the diffusion potential difference has a problem that a large leak current flows at the time of high injection and a large optical output cannot be obtained.

(Problems to be Solved by the Invention) As described above, a conventional semiconductor laser that performs current narrowing by a diffusion potential difference has a problem that a large leak current flows during high injection and a large optical output cannot be obtained.

An object of the present invention is to reduce a leakage current at the time of high injection in a semiconductor laser that performs current narrowing by a diffusion potential difference,
An object of the present invention is to provide a semiconductor laser capable of obtaining a large optical output.

[Means for Solving the Problems] The present invention provides a first semiconductor layer (first cladding layer) of a first conductivity type and a second semiconductor layer (second cladding layer) of a second conductivity type. ) And a third semiconductor layer (active layer) having a narrower band gap than the first and second semiconductor layers and a third semiconductor layer via the first and second semiconductor layers. A semiconductor laser device having a buried heterostructure provided with a means for injecting a current, in a region between the first semiconductor layer and the second semiconductor layer and outside a stripe-shaped active layer (buried region), Thickness with a wider forbidden band than the first semiconductor layer and a different lattice constant from the first semiconductor layer 25
An undoped fourth semiconductor layer (pseudomorphic layer) having a thickness of nm or less is formed, and a fourth conductive layer having a narrower forbidden band width than the fourth semiconductor layer between the fourth semiconductor layer and the second semiconductor layer. A semiconductor laser device in which five semiconductor layers are formed, and the second semiconductor layer and the fifth semiconductor layer form a pn junction.

(Operation) The first semiconductor layer and the second semiconductor layer are cladding layers, which have a larger band gap than the third semiconductor layer (active layer), and play a role of injecting electrons and holes into the active layer, and emit light to the active layer. It also has the role of confining. Current confinement in the active layer is achieved by the diffusion potential difference between the double hetero pn junction formed by the active layer and the cladding layer and the buried pn junction formed between the second semiconductor I layer and the fifth semiconductor layer. Is done. When the current injection level is low, the leak current does not flow because the buried pn junction does not turn on. As the current injection level increases, the embedded pn
The voltage applied to the junction increases and a leak current starts to flow. At this time, the leak current increases rapidly in the conventional semiconductor laser, but in the semiconductor laser of the present invention, the fourth semiconductor layer acts as a barrier, so that the increase in the leak current can be suppressed.

In order to use the fourth semiconductor layer as a barrier, the forbidden band width of the fourth semiconductor layer needs to be sufficiently larger than the forbidden band widths of the first and fifth semiconductor layers. When a semiconductor layer is used, it is difficult to obtain a sufficient barrier height. However, if the thickness of the semiconductor layer is made sufficiently thin even if lattice matching cannot be achieved, the fourth semiconductor layer can be elastically deformed without introducing defects such as dislocations and non-radiative recombination centers that adversely affect device characteristics. Can be distorted. Such a thin film layer elastically distorted is called a pseudomorphic layer. Although the upper limit of the thickness of this layer varies depending on the material, assuming a general compound semiconductor, it is possible to distort without forming a fatal defect up to about 25 nm. In order to obtain a sufficient barrier effect, it is necessary to have a thickness such that a tunnel current does not flow. In the present invention, a pseudomorphic layer having a large forbidden band width is used as the fourth semiconductor layer.

FIG. 1 is a schematic sectional view of a semiconductor laser according to a first embodiment of the present invention. This semiconductor laser has a thickness of 1 nm laminated on an n-type InP substrate 1 having a carrier density of 3 × 10 ¹⁸ cm ^−3.
μm, an n-type InP buffer / cladding layer (first semiconductor layer) 2 having a carrier density of 1 × 10 ¹⁸ cm ⁻³ and a width of 1.5 μm
Undoped I 0.1μm thick formed in stripes
_{n 1-x Ga x As y} P 1-y outer portion of the active layer composed of (wavelength 1.3μm equivalent composition) and (third semiconductor layer) 3, the active layer 3 a first cladding layer 2 Undoped pseudomorphic In _0.5 Ga _0.5 P layer (fourth semiconductor layer) 4 having a thickness of 0.09 μm and a carrier density of 1 × 10 4, laminated on the pseudomorphic layer 4. ¹⁸ cm ^-3 n-type In
P layer (fifth semiconductor layer) 5, active layer 3, and fifth semiconductor layer 5
Having a thickness of 1 μm and a carrier density of 1 × 10 ¹⁸
cm ⁻³ p-type InP cladding layer (second semiconductor layer) 6 and p-type I
formed on the nP layer 6, thickness of 0.5 [mu] m, p-type carrier density of _{^{5 × 10 18 cm -3 In 1}} -x Ga x As y P 1-y contact layer 7
And an AuGe electrode 8 provided below the substrate 1 and AuZn / Ti / Pt / Au electrodes 9a and 9b provided on the p-type contact layer 7.
Consists of In order to reduce the leakage current, the mesa 10 having a width of 16 μm is formed above the n-type cladding layer 2 to reduce the area of the buried heterojunction.

This semiconductor chip is formed by MOCVD three times. In the first growth, the n-type clad layer 2 and the active layer 3 are grown, and the active layer 3 is etched in a stripe shape with a sulfuric acid / hydrogen peroxide-based etchant using the SiO ₂ film as a mask. Then, using the SiO ₂ film as a mask, a pseudomorphic layer 4 and a fifth semiconductor layer 5 are grown in a portion without the active layer 3 (2).
Second growth). Since the step of this selective growth is small, abnormal growth does not occur near the step. Next, the SiO ₂ film is removed, and the whole is formed with the p-type cladding layer 6 and the p-type contact layer 7.
(3rd growth). After the mesa 10 is formed by etching, the entire surface is covered with a SiN _x film 11, and the mesa 10 is formed.
An opening is formed in the SiN _x film 11 on 10 and an AuZn electrode 9a is formed by lift-off. Furthermore, Ti / Pt / Au pad 9b and AuGe on the back
The laser chip is formed by depositing and cleaving the electrodes 8 respectively.

Although not explicitly shown in the figure, the resonator end face is formed by cleavage, and the resonator length is 400 μm.
The emission side of the cleavage surface is coated with a low reflectance coating with a reflectance of 0.1%, and the other cleavage surface has a reflectance of 90%.
% Multi-layer high reflectivity coating. The semiconductor laser chip is mounted and bonded in a suitable package. The oscillation wavelength is 1.3 μm, and the oscillation threshold is 12.4 mA.

Next, the effect of the laser buried portion will be described in comparison with a conventional example. FIG. 2 (a) shows 1.
FIG. 2B is a band diagram of the buried portion when a voltage of 2 V is applied, and FIG. 2B is a band diagram of the buried portion when a voltage of 1.2 V is applied to the conventional semiconductor laser. φ
_{e and} φ _h are pseudo-Fermi potentials for electrons and holes, respectively. Pseudomorphic InGaP layer 4 forms a major barrier in the conduction band E _c, the barrier of the valence band E _v is small. Although the buried pn junction is turned on by either laser, in the case of this embodiment (a), the pseudomorphic layer 4 acts as a barrier against electrons, so that in this portion,
There is a voltage drop of 0.25V. Accordingly, the voltage applied to the buried pn junction 5-6 (V _pn ≒ φ _e −φ _h ≒ 0.95V) is the voltage applied to the conventional buried pn junction 2-6 (V _pn ≒ φ _e −
It is smaller in comparison with the φ _h ≒ 1.20V). Therefore,
The fifth semiconductor layer can be regarded as a barrier to holes.
For this reason, the current flowing through the buried pn junction of the semiconductor laser of this embodiment is greatly reduced as compared with the case of the conventional laser. FIG. 3 compares the current versus light output characteristics of the semiconductor laser of this embodiment and the semiconductor laser of the conventional example. In the conventional semiconductor laser (dotted line), the leak current rapidly increases from around 150 mA, so that the optical output is saturated at around 140 mW. On the other hand, in the semiconductor laser of the present invention (solid line), although there is a slight leakage current, the increase is suppressed by the presence of the pseudomorphic barrier layer 4, so that a high output operation of 200 mW or more is possible. As a result, power consumption and generation of heat can be reduced, and reliability is improved.

In this structure, the p-type buried layer inside the buried structure based on the pnpn thyristor-like structure is replaced with a pseudomorphic layer 4.
It can also be considered as a configuration replaced by. But pnpn
In general, the structure must be formed with a thick buried layer.
Well suited for growth methods like CVD and MBE. Further, the thyristor-like structure loses the current suppressing function once it is turned on, whereas the embedded structure of the present invention operates the current suppressing mechanism even after turning on.

A semiconductor laser according to a second embodiment of the present invention will be described with reference to FIG. In this embodiment, a plurality (for example, two layers) of pseudomorphic barrier layers 4 are provided on one side of a pn junction formed by the p-type cladding layer 6 and the n-type InP layer 5b. That is, the n-type InP layer 5a is formed on the pseudomorphic layer 4a.
Is formed, and the pseudomorphic layer 4b is formed on the n-type InP layer 5a.
Is formed, and an n-type InP layer 5b is formed on the pseudomorphic layer 4b.
Are formed. Even in this case, the same effect as in the previous embodiment can be obtained.

A semiconductor laser according to a third embodiment of the present invention will be described with reference to FIG. In this embodiment, the pseudomorphic barrier layer 4 is provided on both sides of the pn junction formed by the p-type cladding layer 6b and the n-type InP layer 5. That is, the n-type InP layer 5 is formed on the pseudomorphic layer 4a,
A p-type cladding layer 6b is formed thereon, and a pseudomorphic layer 4b is formed on the p-type cladding layer 6b. Even in this case, the same effect as in the previous embodiment can be obtained.

Referring to FIG. 6, a semiconductor laser according to a fourth embodiment of the present invention will be described. If care is taken not to disturb the carriers flowing through the active layer 3, the pseudomorphic layer 4 can be extended to a part of the active layer 3. For example, the pseudomorphic-InGaP layer 4 is provided between the active layer 3 and the p-type cladding layer 6 as shown in FIG. Since the barrier is low, the influence on carriers flowing through the active layer 3 can be reduced.

Referring to FIG. 7, a semiconductor laser according to a fifth embodiment of the present invention will be described. This embodiment is a combination of the third embodiment in FIG. 5 and the fourth embodiment in FIG. That is, n
A p-type cladding layer 6b is formed on the type InP layer 5 and the active layer 3, a pseudomorphic layer 4b is formed on the p-type cladding layer 6b, and a p-type cladding layer 6a is formed on the pseudomorphic layer 4b.
Are formed. Even in this case, the same effect as in the previous embodiment can be obtained.

A semiconductor laser according to a sixth embodiment of the present invention will be described with reference to FIG. In this embodiment, a semiconductor laser is formed on a p-type InP substrate 1. In the buried portion, if the pseudomorphic layer 4 is arranged in the p-type cladding layers 2 and 5 as shown in FIG. 8, the same effect as in the embodiment of FIG. 1 can be expected. In this embodiment, the n-type cladding layer 6c formed on the active layer 3 is surrounded by the p-type cladding layer 5.

The present invention is not limited to the above embodiments, and various modifications are possible. For example, when an optical waveguide layer is formed in contact with the active layer, a distributed feedback semiconductor laser or a distributed Bragg reflection semiconductor laser, a semiconductor laser having a quantum well in the active layer, and a non-reflective coating on both end surfaces are provided. The present invention can be applied to various types of semiconductor lasers, such as a waveform semiconductor laser amplifier, an axially nonuniform injection function laser having a plurality of current injection electrodes (such as a tunable laser), and a bistable laser. In addition, the pseudomorphic barrier layer of the present invention includes a current confinement layer, a window portion of a so-called window structure semiconductor laser in which an active layer near an end face of a resonator is removed, and a bistable laser or an automatic oscillation (pulsation) laser. It can be applied to a saturable absorption layer and the like.

In the above embodiment, the fifth semiconductor layer and the first semiconductor layer are made of the same material, but they need not always be the same. Further, the material is not limited to the InGaAsP / InP system, but can be applied to the InGaAlP / GaAs system, the GaAlAs / GaAs system, and the like. The pseudomorphic barrier layer is not limited to InGaP, and various III-V semiconductors, II-VI semiconductors, SiC, and the like can be used. The growth method is MOCVD, MB
E, hydride vapor phase growth, chloride vapor phase growth, CBE
Various methods such as (chemical beam epitaxy) and MO-MBE are conceivable.

In each embodiment, the same effect can be obtained even if the conductivity type of each semiconductor layer is reversed.

[Effects of the Invention] According to the present invention, since the pseudomorphic layer is provided, a semiconductor laser having a small leak current and capable of high-output operation can be easily obtained with good controllability.

[Brief description of the drawings]

FIG. 1 shows a semiconductor laser according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view taken along a cross section perpendicular to the stripe, FIG. 2 is a band diagram of a current confinement region of the semiconductor laser of the first embodiment and the conventional example, and FIG. FIG. 4 is a sectional view of a semiconductor laser according to a second embodiment of the present invention, FIG. 5 is a sectional view of a semiconductor laser according to a third embodiment of the present invention, and FIG. FIG. 7 is a sectional view of a semiconductor laser according to a fifth embodiment of the present invention; FIG. 8 is a sectional view of a semiconductor laser according to a sixth embodiment of the present invention; FIG. 2 is a sectional structural view of a conventional semiconductor laser. DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate, 2 ... First semiconductor layer, 3 ... Third semiconductor layer, 4 ... Fourth semiconductor layer, 5 ... Fifth semiconductor layer, 6
... second semiconductor layer, 8 ... electrodes, 9 ... electrodes.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (1)

(57) [Claims] A first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type; and the first and second semiconductor layers formed in a stripe shape between the first semiconductor layer and the second semiconductor layer. A third semiconductor layer having a narrower forbidden band width than the second semiconductor layer, formed in a region between the first semiconductor layer and the second semiconductor layer, excluding the stripe-shaped third semiconductor layer. Has a lattice constant different from the lattice constant of the first semiconductor layer has a wider band gap than the first semiconductor layer and has a thickness of 25
undoped fourth semiconductor layer of less than nm, a fifth semiconductor layer of the first conductivity type formed between the fourth semiconductor layer and the second semiconductor layer and having a narrower forbidden band width than the fourth semiconductor layer, Means for injecting a current into the third semiconductor layer via the first semiconductor layer and the second semiconductor layer, wherein the second semiconductor layer and the fifth semiconductor layer form a pn junction. Characteristic semiconductor laser device