JPH0521894A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser capable of stabilized lasing at high temperature with room temperature oscillation wavelength of 680 [nm] or lower. CONSTITUTION:Quantum well layers 5, 7, 9 comprise Ga0.43In0.57P with a 0.43 Ga composition ratio x, and are made thinner such that no crystal growth is produced. Barrier layers 6, 8 are spaced away through the respective quantum well layers 5, 7, 9, and constitute an active layer together with the quantum well layers 5, 7, 9. Optical confinement layers 4, 10 comprise AlGaInP and hold the active layer therebetween. An optical waveguide formed with the active layer and the optical confinement layers 4, 10 is formed on a GaAs substrate and held between p, n cladding layers. The oscillation wavelength of the laser is of 680nm or longer at room temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser which outputs visible light, and more particularly to a semiconductor laser which operates even when the element temperature becomes high.

[0002]

2. Description of the Related Art An AlGaInP / GaInP visible light semiconductor laser having a light confinement layer of AlGaInP and a quantum well layer of GaInP is a laser expected as a light source for optical information processing equipment. However, the current visible light semiconductor laser has a high oscillation threshold current and a large thermal resistance. Therefore, the deterioration of operating characteristics in a high temperature environment is remarkable. This has been a major factor limiting the fields to which semiconductor lasers can be applied.

Conventionally, various attempts have been made to improve the temperature characteristics of such semiconductor lasers. For example,
Doping impurities into the p-type cladding layer at high concentration,
Attempts have been made to improve the temperature characteristics by devising a heat sink. Attempts have also been made to optimize the thickness of the active layer and coat the end faces of the resonator with high reflectance to reduce the threshold current of laser oscillation and improve the temperature characteristics. This attempt is based on
Presented on pages 1177-1178.

Extended Abstracts of the 22nd (1990 In
ternational) Conference on SolidState Devices and M
aterials, Sendai, 1990 `` High Temperature Operation
of Index-Guided AlGaInP Semiconductor Lasers ''

[0005]

However, in any of the above-mentioned conventional measures, the maximum element temperature at which stable laser oscillation can be performed, that is, the maximum oscillation temperature is 130 ° C. at most.

An object of the present invention is to provide a semiconductor laser which can stably oscillate even when the element temperature is higher.

[0007]

According to the present invention, in a semiconductor laser formed on a GaAs substrate, the optical waveguide has a Ga _x In _1-x P composition ratio x of Ga being less than 0.48.
And an active layer composed of a plurality of quantum well layers each having a thickness that does not cause crystal defects and barrier layers separating the quantum well layers, and AlGaInP sandwiching the active layers.
And a light confinement layer made of, and having an oscillation wavelength of 680 nm or more at room temperature.

[0008]

Since the Ga composition ratio x is less than 0.48, the band gap difference between the active layer and the cladding layer becomes large, and the effect of confining carriers in the quantum well layer increases. Moreover, since a plurality of quantum well layers are formed, carriers are effectively captured.

Also, the lattice constant of the quantum well layer becomes larger than that of GaAs, compressive stress is applied to the quantum well layer, the band structure of its valence band changes, and its density of states decreases. Therefore, the threshold current of laser oscillation decreases, and laser oscillation is performed with a small carrier amount.

Therefore, even in a high temperature environment, the amount of carriers overflowing from the quantum well layer is suppressed.

[0011]

1A is a sectional view showing the structure of a visible light semiconductor laser having a double hetero structure according to an embodiment of the present invention.

The semiconductor substrate 1 is a GaAs semiconductor substrate cut out with the (100) crystal plane as the main surface, and the semiconductor substrate 1 is doped with Si. On this GaAs semiconductor substrate 1, a temperature of 700 [° C.] and a pressure of 60 [Tor
r] environment under metal-organic vapor phase epitaxy (MOVPE
Method), the following thin film is epitaxially grown to form a semiconductor laser. That is, the buffer layer 2, the n-type cladding layer 3, and the light confining layer 4 are sequentially stacked on the semiconductor substrate 1. The material of the buffer layer 2 is GaAs doped with Si, and the material of the cladding layer 3 is Se.
(Al _0.7 Ga _0.3 ) _0.5 In _0.5 P,
The material of the light confinement layer 4 is non-doped (Al _0.4 G
a _0.6) is _0.5 In _0.5 P.

Further, an active layer is formed on the light confinement layer 4. This active layer is composed of three quantum well layers 5, 7 and 9 in which carriers are confined (or recombined) and barrier layers 6 and 8 which separate these quantum well layers. The material of each quantum well layer 5, 7, 9 is Ga _0.43
In _0.57 P, and the Ga composition ratio x is less than 0.48 (x
X = 0.43 in Ga _x In _1-x P of <0.48)
Is set to. The thickness of each of these layers is set to be sufficiently thin that crystal defects do not occur, that is, 100 angstroms. The material of each of the barrier layers 6 and 8 is (Al _0.4 Ga _0.6 ) _0.5 In _0.5 P, and the thickness of each of these layers is 80 Å. A light confinement layer 10 is further formed on the active layer with the same material and the same thickness as the light confinement layer 4, and the active layer is vertically sandwiched by the respective light confinement layers 4 and 10. The light confinement layers 4 and 10 and the active layer form an optical waveguide through which laser light propagates.

A p-type cladding layer 11, a buffer layer 12 and a contact layer 13 are further epitaxially grown on the light confinement layer 10 in this order. The material of the clad layer 11 is Zn-doped (Al _0.7 Ga _0.3 ) _0.5 In
_0.5 P, the material of the buffer layer 12 is Ga _0.5 In _0.5 P doped with Zn, and the material of the contact layer 13 is Z.
It is GaAs doped with n. In order to connect the semiconductor laser to an external circuit, an electrode 14 made of AuGe / Ni / Au metal is formed on the lower surface of the semiconductor substrate 1, and Ti / P is formed on the upper surface of the contact layer 13.
An electrode 15 made of t / Au metal is formed.

The energy band structure of the conduction band of the optical waveguide portion in such a semiconductor laser is shown in FIG. The electron energy E increases toward the right side of the drawing, and well-shaped potentials are formed corresponding to the quantum well layers 5, 7, and 9. The injected electrons are confined in this well-shaped potential. In this embodiment, three quantum well layers 5, 7 and 9 are provided and multiplexed, so that electrons are effectively trapped in each well and the band filling effect is suppressed. Therefore, even if the device temperature increases, the overflow of electrons from the quantum well layer is suppressed.

Further, as the effective band gap difference ΔE _g between the clad layers 4 and 10 and the active layer is larger, the overflow of carriers from the active layer to the respective clad layers 4 and 10 is suppressed. In order to increase the band gap difference ΔE _g , the Ga composition ratio x in Ga _x In _{1 -x} P forming the quantum well layer may be decreased. In the present embodiment, as described above, the composition ratio x of Ga in each quantum well layer 5, 7, 9 is set as small as 0.43, which is less than 0.48. Therefore, the active layer and each clad layer 4, The barrier height between 10 is high. Therefore, the amount of carriers overflowing from the quantum well layers 5, 7, and 9 due to the increase in device temperature is also suppressed by this.

Further, when the band gap difference ΔE _g becomes large, that is, when the effective band gap of the active layer becomes small, the oscillation wavelength λ becomes long, and when the oscillation wavelength λ becomes long, the maximum oscillation temperature T _max becomes high. FIG. 2 shows the maximum oscillation temperature T _max of a general double-heterostructure visible light semiconductor laser.
Is a graph showing the relationship between the oscillation wavelength λ and the oscillation wavelength λ. From the figure, it is understood that the maximum oscillation temperature T _max increases as the oscillation wavelength λ increases. Therefore, the oscillation wavelength of the semiconductor laser according to the present embodiment is long because the band gap difference ΔE _g is large as described above, and the maximum oscillation temperature T _max can be increased.

Further, in order to increase the band gap difference ΔE _g , the composition ratio x of Ga forming the quantum well layer was reduced as described above. However, as the x decreases, the lattice constant of the quantum well layer becomes GaAs. It becomes larger than the lattice constant of. Therefore, the quantum well layers 5, 7, and 9 in this embodiment, which are set to a small value of x = 0.43, are subjected to compressive stress in the crystal growth in-plane direction. The thickness of each quantum well layer 5, 7, 9 is sufficient to maintain good crystallinity without causing dislocation in each quantum well layer 5, 7, 9 due to the compressive strain based on this lattice mismatch. Must be thin. For example, when the compressive strain amount (Δa / a) is 1%, the thickness must be 200 angstroms or less. The amount of compressive strain applied to each quantum well layer 5, 7, 9 in this embodiment is 0.65%, and the thickness of each well layer is 100 angstroms, which is sufficiently thin. Therefore, each quantum well layer 5, 7, 9 is strained within its own elastic limit, and good crystallinity is maintained without causing dislocation due to lattice misalignment.

Due to the compressive strain stress applied to each quantum well layer 5, 7, 9, the energy band structure of the valence band of each well layer changes and the density of states of the valence band decreases. Therefore, the threshold current density J _th of laser oscillation is lowered. Such a decrease in the threshold current density J _th is theoretically disclosed on page 171 of Document 1 below.

Reference 1 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL.25, NO.2, FE
BRUARY 1989 `` Theoretical Gain of Strained-Layer S
Semiconductor Lasers in the Large Strain Regime ”Further, experimentally, it is disclosed on pages 1375 to 1376 of reference 2 and pages 879 to 880 of reference 3.

Reference 2 ELECTRONICS LETTERS 16th August 1990 Vol.26 No.17
`` VERY LOW THRESHOLD CURRENT AlGaInP / Ga _x In _1-x PS
TRAINED SINGLE QUANTUM WELL VISIBLE LASERDIODE ”Reference 3 Appl.Phys.Lett., Vol.58, No.9,4 March 1991“ Effects
of strained-layer structures on the threshold curr
ent density of AlGaInP / GaInP visible lasers ”Since the threshold current density J _th of laser oscillation decreases, the threshold current decreases and laser oscillation is performed with a small carrier amount. Therefore, the amount of carriers overflowing from the quantum well layers 5, 7, and 9 due to the increase in the device temperature is
This is also suppressed.

As described above, according to this embodiment, by adopting the above-mentioned compressive strain quantum well structure, the band gap difference Δ
E _g is increased and the threshold current density J _th is reduced. Furthermore, the carrier is effectively captured by multiplexing the quantum wells. As a result, stable laser operation is performed even when the element temperature rises.

FIG. 3 is a graph showing the results of characteristic evaluation by continuously operating the semiconductor laser according to this embodiment.
This characteristic evaluation was performed on a sample in which each thin layer of the semiconductor laser was formed in a stripe structure, the cavity length L was 500 μm, the stripe width was 5 μm, and the end face coating was not applied. The oscillation wavelength λ of the semiconductor laser at the time of evaluation is room temperature (25
It is 700 [nm] at (° C). The horizontal axis of the figure shows the operating current [mA] of the semiconductor laser, the vertical axis shows the optical output [mW], and the evaluation result is shown with the element temperature [° C] as a parameter. As can be seen from the figure, according to the semiconductor laser of the present embodiment, a light output of 7 [mW] or more can be stably obtained up to an element temperature of 150 ° C. Moreover, each light output is low operating current value. Has been obtained. That is,
Provided is a semiconductor laser having an oscillation wavelength of 680 nm or more at room temperature and capable of performing stable laser operation even at high temperatures. In this characteristic evaluation, the element temperature was 150 ° C.
Although confirmed only up to now, it is considered that stable laser operation can be obtained even at an element temperature of 150 ° C. or higher.

According to the semiconductor laser of the present embodiment, heat is applied to fields in which application of a visible laser is expected, particularly in a multi-beam laser array and an electronic device integrated circuit (OEIC) where the element temperature becomes high. When the semiconductor laser is mounted with the generated laser bonding surface on the side opposite to the heat sink via the substrate, particularly large effects can be obtained when this embodiment is applied.

[0025]

As described above, according to the present invention, G
Since the composition ratio x of a is less than 0.48, the band gap difference between the active layer and the cladding layer becomes large, and the effect of confining carriers in the quantum well layer increases. Moreover,
Since a plurality of quantum well layers are formed, carriers are effectively captured. The lattice constant of the quantum well layer is G
It becomes larger than the lattice constant of aAs, compressive stress is applied to the quantum well layer, and the band structure of its valence band changes,
Its density of states decreases. Therefore, the threshold current of laser oscillation decreases, and laser oscillation is performed with a small carrier amount.

Therefore, even if the device temperature increases, the amount of carriers overflowing from the quantum well layer is suppressed.
As a result, a semiconductor laser that operates stably at high temperature is provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a cross-sectional structure of a semiconductor laser according to an embodiment of the present invention and its conduction band energy band structure.

FIG. 2 is a graph showing the relationship between the maximum oscillation temperature T _max and the oscillation wavelength λ of a general double hetero structure visible light semiconductor laser.

FIG. 3 is a graph showing the relationship between the operating current and the optical output of the semiconductor laser according to this example.

[Explanation of symbols]

1 ... Semiconductor substrate (Si-doped GaAs) 2 ... Buffer layer (Si-doped GaAs) 3 ... N-type cladding layer (Se-doped (Al _0.7 Ga _0.3 ))
_0.5 In _0.5 P) 4, 10 ... Light confinement layer (non-doped (Al _0.4 G
a _0.6 ) _0.5 In _0.5 P) 5,7,9 ... Quantum well layer (non-doped Ga _0.43 In _0.57)
P) 6, 8 ... Barrier layer (undoped (Al _0.4 Ga _0.6 ) _0.5
In _0.5 P) 11 ... P-type cladding layer (Zn-doped (Al _0.7 G
a _0.3 ) _0.5 In _0.5 P) 12 ... Buffer layer (Zn-doped Ga _0.5 In _0.5 P) 13 ... Contact layer (Zn-doped GaAs) 14 ... Electrode (AuGe / Ni / Au) 15 ... Electrode (Ti / Pt / Au)

Claims (1)

Claim: What is claimed is: 1. A semiconductor laser formed on a GaAs substrate, wherein the optical waveguide has a Ga _x In _1-x composition ratio x of Ga is less than 0.48.
An active layer composed of a plurality of quantum well layers made of P and having a thickness that does not cause crystal defects and a barrier layer separating the quantum well layers, and an optical confinement layer made of AlGaInP sandwiching the active layers. A semiconductor laser having an oscillation wavelength of 680 nm or more at room temperature.