JPH05129724A - Multiple quantum well type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a multi-longitudinal mode which is controlled accurately and having low quantum noise level by a method wherein at least two well layers are provided and the difference between the maximum thickness and the minimum thickness is brought to a fixed value. CONSTITUTION:An active layer region has three well layers, and the these well layers are formed in LZ1=120 angstrom, LZ2=125 angstrom and LZ3=130 angstrom in thickness. To be more precise, the difference between the maximum thickness and the minimum thickness of the well layers is 10 angstrom. The thickness of a barrier layer is LB1=LB2=LB3=LB4=60 angstrom. The well layers consists of Al0.12Ga0.88As, and the barrier layers consists of Al0.30Ga0.70As. As the well layers have different thicknesses, their energy levels are also different with each other, and the main oscillation is generated in the lowest energy level. However, as there is an energy level directly above, the longitudinal mode is brought to a multimode state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-quantum well semiconductor laser capable of obtaining multimode longitudinal modes.

[0002]

2. Description of the Related Art In recent years, research on quantum well semiconductor lasers,
Development is active. Quantum well type semiconductor laser is the thickness of the active layer is reduced to the extent of the spread of the electron wave function,
It is a semiconductor laser in which gain and optical loss characteristics have a quantum size effect. The quantum well semiconductor laser has features such as low threshold current density, high differential quantum efficiency, and low temperature dependence, and can obtain a narrow spectral line width. Quantum well type semiconductor lasers are classified into single quantum well type semiconductor lasers and multiple quantum well type (hereinafter also referred to as MQW) semiconductor lasers. Conventionally, the MQW semiconductor laser has a plurality of potential well layers and barrier layers in the active layer region, and the thickness of the well layers is constant. That is, the development and research of MQW semiconductor lasers having a single longitudinal mode are being carried out exclusively.

Mode hopping noise observed in a relatively low frequency region in a semiconductor laser for an optical disk deteriorates the image quality of a video disk, for example. Mode hopping noise is noise generated when a longitudinal mode of a semiconductor laser jumps to the next mode, that is, mode hopping, because a plurality of modes compete with each other. In particular, when this mode hopping noise is combined with the return light induced noise caused by the return light from the optical disc, the image quality is significantly deteriorated.

Methods for reducing the mode hopping noise include, for example, making a longitudinal mode into a single mode by using a diffraction grating or a compound resonator, or making a longitudinal mode into a multimode. Further, in order to reduce the return light induced noise, it is known that it is effective to make the longitudinal mode multimode and reduce the coherence of the laser light.

[0005]

As a method of making the longitudinal mode multi-mode, there are known a method of making the multi-mode by gain guiding and a method of making the multi-mode by transient characteristics such as pulsation generation or high frequency superposition. There is.

However, in these methods of increasing the longitudinal mode into multiple modes, there is a problem that the quantum noise level is higher by one digit or more than the quantum noise level in the single longitudinal mode. In addition, these methods also have a problem that it is difficult to obtain the multimode longitudinal mode in an accurately controlled state.

Therefore, it is an object of the present invention to provide a semiconductor laser which can be controlled accurately and which can obtain a multimode longitudinal mode having a low quantum noise level.

[0008]

The above object has at least two well layers, and the difference between the maximum thickness and the minimum thickness of the well layers is equal to or more than a certain value. This can be achieved with a semiconductor laser.

[0009]

When the composition of the well layer and the barrier layer forming the active layer region is constant, the emission wavelength of the semiconductor laser becomes longer as the thickness of the well layer becomes thicker. The quantum well type semiconductor laser of the present invention is a multiple quantum well type having at least two well layers, and the difference between the maximum thickness of the well layers and the minimum thickness of the well layers is a certain value or more. Therefore, the quantum well semiconductor laser of the present invention has an energy level corresponding to at least the maximum thickness and the minimum thickness of the well layer, and as a result,
The vertical mode is multimode.

In the present specification, the term "multimode" refers to the intensity of the main peak having the maximum intensity among the emission intensities of 10.
When it is 0%, it means a state in which one or more subpeaks having an emission intensity of 50% or more are present. Therefore, "the difference between the maximum thickness and the minimum thickness of the well layer is equal to or more than a certain value" means that in the MQW semiconductor laser, one or more subpeaks of the emission intensity are present in each well layer. Refers to the existence of differences between thicknesses.

The constant value is such that the minimum thickness of the well layer is 1.
In the case of 00 angstrom or more, it is desirable that it is 10 angstrom. When the constant value is about 10 Å, the difference between the emission wavelength of the main peak and the emission wavelength of the sub-peak is, for example, about 1.5 nm, although it depends on the composition of the compound semiconductor forming the active layer. Below 10 Å, the longitudinal mode tends to be single mode. If this fixed value is too large,
The MQW semiconductor laser has two emission wavelength peaks. If the minimum thickness of the well layer is less than 100 Å, the constant value is preferably about 8 Å.

[0012]

EXAMPLE A multiple quantum well type semiconductor laser as shown in the schematic view of FIG. 2 was produced by MOCVD, for example. This semiconductor laser comprises an n + GaAs substrate 10, a 0.5 μm thick GaAs buffer layer 12, an Al _0.47 Ga _0.53 As n-type cladding layer 14 of 1.5 μm thick, and an active layer region 1.
6, a p-type cladding layer 18 made of Al _0.47 Ga _0.53 As and having a thickness of 1.5 μm, and made of GaAs and having a thickness of 0.5 μm.
It is composed of the p + type cap layer 20. M shown in FIG.
The QW semiconductor laser is a refractive index guided semiconductor laser that utilizes the disordering of the MQW structure due to Zn diffusion. Two
2 is a Zn diffusion region, 24 is a SiO ₂ layer, 26 is a lower electrode, and 28 is an upper electrode.

In the conventional multiple quantum well type semiconductor laser, the active layer region has, for example, three well layers each having a thickness of 120 Å and four well layers each having a thickness of 60 Å.
It consists of two barrier layers. That is, the thickness of the well layer is constant, and the MQW semiconductor laser has a single longitudinal mode. On the other hand, in the MQW semiconductor laser of the present invention, as shown in FIG. 1, the active layer region has three well layers, and the thickness of these well layers is L _Z1 = 120 Å,
L _Z2 = 125 Å and L _Z3 = 130 Å. That is, the difference between the maximum thickness and the minimum thickness of the well layer is 10 Å. The thickness of the barrier layer is L
_B1 = _LB2 = _LB3 = _LB4 = 60 angstroms.
The well layer is made of Al _0.12 Ga _0.88 As, and the barrier layer is made of Al.
Consisting _0.30 Ga _{0. 70} As. The thickness of the well layer can be examined by TEM observation of the cross section of the semiconductor laser.

As a result of the different thicknesses of the well layers, the energy levels are different in each well layer, causing a major oscillation at the lowest energy level. However, since there is an energy level just above, the longitudinal mode becomes multimode.

In the above-mentioned embodiment, the oscillation wavelength of the maximum intensity peak is 780 nm, and the oscillation wavelengths of the sub-peaks having the intensity of 50% or more are 779.5 nm and 78.
It is 0.5 nm.

When the MQW semiconductor laser is manufactured by MOCVD, the thickness of the well layer can be accurately controlled on the order of several atoms. Therefore, the difference between the maximum thickness and the minimum thickness of the well layer can be accurately and easily controlled.

Although the MQW semiconductor laser of the present invention has been described based on the preferred embodiment, the present invention is not limited to this embodiment.

The quantum well semiconductor laser of the present invention is made of Al
Not only GaAs type, but also AlGaInP type or Al
It can be composed of a GaInAs-based compound semiconductor.

The multi-quantum well type semiconductor laser of the present invention includes not only a one-dimensional quantum well structure but also two-dimensional and three-dimensional quantum well structures.
A semiconductor laser having a three-dimensional quantum well structure (quantum wire and quantum box laser) can be included, and further a surface emitting semiconductor laser, a GRIN-SCH semiconductor laser, a lateral current injection MQW semiconductor laser, a multi-stripe semiconductor laser, etc. Can be included. Further, the MQW semiconductor laser of the present invention can have a superlattice buffer.

[0020] The thickness of the schematically illustrated well layer in FIG. _{1 L Z1, L Z2, L} Z3 ··· not only satisfies the conditions of _{L Z1 <L Z2 <L Z3} , for example, L _Z1 It is possible to set various conditions such as = L _Z2 <L _Z3 L _Z2 <L _Z1 <L _Z3 . In any case, the difference between the maximum thickness and the minimum thickness of the well layer may be a certain value (for example, 10 angstrom) or more.

[0021]

According to the present invention, the thickness of the well layer can be accurately and easily controlled. Therefore, it is possible to obtain a low-noise multi-quantum well semiconductor laser having a multimode longitudinal mode having an accurate oscillation wavelength while taking advantage of the characteristics of the quantum well semiconductor laser. As a result, for example, mode popping noise and return light induced noise can be effectively reduced, and it can be an effective light source for an optical disc.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an active layer region of a multiple quantum well type semiconductor laser of the present invention.

FIG. 2 is a schematic view of a multiple quantum well type semiconductor laser of the present invention.

[Explanation of symbols]

10 n + GaAs substrate 10 12 GaAs buffer layer 14 n-type clad layer 16 active layer region 18 p-type clad layer 20 p + type cap layer 22 Zn diffusion region 24 SiO ₂ layer 26 lower electrode 28 upper electrode

Claims (2)

[Claims] 1. A multi-quantum well type semiconductor laser having at least two well layers, wherein the difference between the maximum thickness and the minimum thickness of the well layers is a certain value or more. 2. The multi-quantum well semiconductor laser according to claim 1, wherein the constant value is 10 Å.