JP2508625B2 - Semiconductor laser - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor laser, particularly a compound semiconductor laser.

[Outline of Invention]

According to the present invention, at least one of the cladding layers provided adjacent to the active layer has a superlattice structure made of a compound thin film semiconductor containing only two elements to improve heat dissipation.

[Conventional technology]

In the compound semiconductor laser, the laser characteristics and the lifetime are greatly affected by how efficiently the heat generated in the light emitting portion, that is, the active layer, is dissipated to the outside.

Various types of compound semiconductor lasers have been proposed, but basically, the structure is such that a clad layer for confining carriers and light to the active layer is provided adjacent to the active layer. It This clad layer is composed of a compound semiconductor layer of a ternary or quaternary mixed crystal having a larger energy bandgap and a smaller refractive index than the active layer and having a lattice matching. For example, 3 for AlGaAs
In the case of the original semiconductor laser, as shown in FIG.
On the main surface 1 of the As substrate (1), a first clad layer (2) made of an AlGaAs layer, an active layer (3) made of a GaAs layer, and the same composition as, for example, the first clad layer (2). A second cladding layer (4) consisting of a semiconductor layer, and further on top of this, for example
The GaAs cap layer (5) and MOCVD (Metalor)
gnic Chemical Vapor Deposition) or MBE (Molecul
ar Beam Epitaxy) and the like. (6) and (7) show electrodes which are ohmic-deposited on the other main surface of the GaAs substrate (1) and the cap layer (5), respectively. The electrode (7) is designed to have a function as a heat sink by itself, or is soldered to a heat sink (9), that is, a heat radiator provided separately from the electrode, by a solder layer (8).

Thus, the clad layers (2) and (4) are made of, for example, Al.
It is composed of a ternary mixed crystal of GaAs to confine light and carriers to the active layer of GaAs. From the viewpoint of heat conduction, the thermal resistance increases as the number of constituent elements of the mixed crystal increases. As a result, the cladding layer made of ternary mixed crystal AlGaAs has low thermal conductivity. And this trend is GaInAsP
It becomes more remarkable as the number of constituent elements of the mixed crystal increases, as in the case of a four-element or AlGaInP-based four-element semiconductor laser.

When the thermal conductivity of the clad layer adjacent to the active layer is low as described above, the heat generated from the active layer during operation is not effectively dissipated, so that the temperature rise of the active layer becomes large and the oscillation is impaired. In addition, dislocations are generated in the active layer and crystals in the vicinity of the active layer, and metamorphosis due to crystal growth occurs, resulting in stability of laser characteristics and shortening of life.

In the compound semiconductor, when the ternary mixed crystal is formed by adding the constituent elements to the binary compound, the decrease of the thermal conductivity, that is, the increase of the thermal resistance is caused by, for example, P.D. Electronics, 67 10 161-168 (PDMaycock; Solid-State El
ectronics, Pergamon Press) as reported, and there is a system whose thermal conductivity decreases by more than one digit depending on the constituent elements.

Further, it is known that heat in the compound semiconductor is mainly propagated by phonons, and scattering of the phonons due to various causes is a factor that reduces the thermal conductivity. As one cause of this phonon scattering, for example, in the case of III-V group compound semiconductors,
There is an effect due to so-called disordering in which two kinds of elements randomly occupy the group III site.

Among the phonons, LA phonons, which have a large group velocity and a long mean free path, are mainly involved in heat conduction.

[Problems to be solved by the invention]

The present invention is intended to reduce the scattering of LA phonons in the cladding layer of the above-mentioned semiconductor laser, improve the thermal conductivity, and improve the laser characteristics and life.

[Means for solving problems]

For example, as shown in FIG. 1, the present invention sequentially forms a first clad layer (1) on a substrate (11) made of, for example, GaAs via a GaAs buffer layer (not shown).
2), the active layer (13), the second cladding layer (14) and the cap layer (15) are continuously epitaxially grown by MOCVD, MBE or the like. Reference numerals (16) and (17) denote electrodes that are ohmic-deposited on the other main surface of the substrate (11) and the cap layer (15), respectively. The electrode (17) itself has a function as a heat sink, or is soldered to another heat sink (19) by a solder layer (18) as shown.

In the present invention, the first and second clad layers (12) are used in a semiconductor laser with a clad layer of ternary system or more.
And (14) or the first clad layer (12) on the heat sink side is a semiconductor layer having a superlattice structure of a repetitive periodic laminated structure of the lowest dimensional binary compound thin film semiconductor layers.
For example, in an AlGaAs ternary semiconductor laser, a lower layer of AlAs and GaAs is formed by periodic superposition of thin film semiconductors by binary n and m atomic layers (AlAs) _n (GaA
s) _{Let m be} a superlattice structure. The composition of the clad layer having the superlattice structure is required so that the average composition of the whole is effective for confining carriers and light in the active layer (13) in the original mixed crystal such as AlGaAs. The required refractive index difference and energy band gap difference with respect to the active layer (13), that is, a composition having a smaller refractive index and a larger energy band gap than the active layer (3). The composition is selected so that the LA phonons of the binary compound forming the clad layer of the superlattice structure have overlaps to enhance the thermal conductivity.

[Action]

According to the present invention described above, the cladding layers (12) and (1
Since both or one of the clad layers (12) has a superlattice structure to reduce the dimension of the mixed crystal, the scattering due to the disordering in the mixed crystal described above is reduced, and the thermal conductivity here is increased. .

However, at the heterojunction, which is the interface between different types of semiconductors, when the LA phonons propagating in one crystal propagate into the other crystal, there is an overlap in the LA phonon dispersion relationship between the two crystals. Without it, LA phonons are scattered and cannot propagate. This is because momentum and energy cannot be conserved. Therefore, since there are many heterojunctions in a superlattice of different kinds of thin crystals that are periodically overlapped, it is necessary to consider the propagation of LA phonons. In the superlattice (AlAs) _n (GaAs) _m system by the periodic superposition of
Since the LA phonon dispersion functions of GaAs and GaAs almost overlap each other, there is no particular problem with respect to the LA phonon scattering described above. In other systems as well, the dispersion curves of LA phonons in the different semiconductor layers forming the superlattice structure are, for example, the curves (2
Although different as shown in 1) and (22), the so-called zone folding (zo
Each of the dispersion curves is folded into 1 / (n + m) by ne folding, and the state after the zone folding of the dispersion curves (21) and (22) is shown in the left end of the figure with a chain line and a broken line, respectively. It is shown that there is an overlap at (n + m) = 8, which means that LA phonons can propagate without being scattered.
Therefore, in the semiconductor laser according to the present invention, the thermal conductivity is increased in the monoatomic layer, that is, n + m = 2 to n + m = 8 in the cladding layer adjacent to the active layer (13), and the heat is efficiently transferred to, for example, the heat sink (9). The heat is released by.

〔Example〕

 An example of the present invention will be described in detail with reference to FIG.

The substrate (11) is composed of a GaAs substrate of one conductivity type, for example, N type.

The first cladding layer (12) has the same conductivity type as that of the substrate (11), and has a cycle formed by repeating and stacking a thin film semiconductor layer of an n-atom layer of AlAs and a thin film semiconductor layer of a m-atom layer of GaAs. A superlattice structure of (AlAs) _n (GaAs) _m (2 ≦ n + m ≦ 8) is obtained by the selective superposition.

 The active layer (13) is composed of a GaAs layer.

The second cladding layer (14) is the first cladding layer (12)
The superlattice has the same composition and structure as the above, but has a conductivity type different from that of the first cladding layer (12), for example, P type.

The cap layer (15) is composed of a P-type GaAs layer having the same conductivity type as the first cladding layer (14).

Then, as shown by the broken line, the central portion is left in a stripe shape from the cap layer (15) side, and the second portion is selectively provided on the left and right sides thereof.
By implanting ions such as protons to a depth reaching the clad layer (14) of, for example, a current confinement region (20) having a high specific resistance.
After that, the electrodes (16) and (17) are adhered, and the electrode (17) side is soldered to the heat sink (19) by the solder layer (18).

In the example described above, the present invention is applied to an AlGaAs ternary semiconductor laser, but a quaternary semiconductor such as GaIn is used.
It can also be applied to an AsP semiconductor laser, in which case (GaAs) _n (InP) _m ▲ (Gap) ^' _n ▼ ▲
The cladding layer is composed of a superlattice structure of (InAs) ^' _m ▼ (in no particular order). When applied to an AlGaInP-based semiconductor laser, a superlattice structure of a binary compound thin film semiconductor of (AlP) _l (GaP) _m (InP) _n is used.

〔The invention's effect〕

As described above, according to the present invention, at least one of the clad layers adjacent to the active layer is made to have a superlattice structure of the lowest dimensional binary thin film semiconductor to enhance its thermal conductivity. 13) and the temperature rise in the vicinity of it can be effectively avoided, thereby improving the laser characteristics, stability, and lifetime, and further, for example, continuous oscillation at room temperature in the short wavelength region of AlGaInP system is possible. Enables the construction of various semiconductor lasers.

[Brief description of drawings]

1 is an enlarged schematic sectional view of an example of a semiconductor laser according to the present invention, FIG. 2 is a dispersion curve diagram of LA phonons used for explaining the present invention, and FIG. 3 is an enlarged schematic sectional view of a conventional semiconductor laser. It is a figure. (11) is a substrate, (12) and (14) are first and second cladding layers, (13) is an active layer, (15) is a cap layer, and (16).
And (17) are electrodes, and (20) is a current constriction region.

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Masao Ikeda Inventor Masao Ikeda 6-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Within Sony Corporation (56) References JP-A-60-130878 (JP, A) JP-A-SHO 57-152178 (JP, A) JP 61-244086 (JP, A) JP 61-234587 (JP, A) JP 61-131414 (JP, A)

Claims (1)

(57) [Claims] 1. A clad layer having a superlattice structure, wherein at least one of the clad layers provided in contact with the active layer is a superlattice structure clad layer formed by periodic stacking of only thin film semiconductors of a binary compound. , Its average composition has a required band gap difference and refractive index difference with respect to the active layer,
Carrier and light confinement effect can be achieved in the active layer 3
The composition is selected to be similar to that of the mixed crystal of the original system and above, and the zone-folding dispersion curve of the cladding layer with the superlattice structure is folded to 2 ≦ n + m ≦ 8, which results in the dispersion of LA phonons. A semiconductor laser characterized by being formed by overlapping so as to increase thermal conductivity.