JPH04241487A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a multiple quantum well semiconductor laser for long-wave long-band optical communication low in a vibration threshold, excellent in high temperature operating characteristics and capable of high speed modulating. CONSTITUTION:A multi-layer barrier layer made of a quantum well 4 with tensile distortion applied and two barrier layers 3 with the well 4 interposed is used as a multiple quantum well active layer. The lowest order hall sub band of this quantum well 4 is a light hall sub band 12, while the lowest order electron in the quantum well 4 is higher than that of the quantum well active layer 5 and energy of the hall sub band is lower than that of the layer 5, respectively. Thickness of the barrier layer is such that tunnel time constants of electrons and halls from the quantum well 4 to the quantum well active layer 5 are 10 picoseconds or lower. A carrier with current implanted is efficiently trapped in the quantum well 4 and the quantum well active layer 5 in the multi- layer barrier layer, and the carrier in the quantum well 4 in the multi-layer barrier layer is implanted into the quantum well active layer 5 by high speed tunneling.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] Semiconductor lasers that oscillate at wavelengths from 1.3 μm to 1.6 μm are extremely important as light sources for optical fiber communications, and research and development of semiconductor lasers that can perform high-speed modulation of 10 Gbit/sec or more are expected. has been done. The present invention relates to a semiconductor laser capable of ultra-high-speed modulation.

0002

[Prior art] 1.3 μm with low optical fiber loss
InX Ga1-X As, InX Ga1-X lattice-matched to the InP substrate, which oscillates at a wavelength of 1.6 μm from
AsY P1-Y-based long-wavelength semiconductor lasers are used in actual optical fiber communication systems, but in recent years, many research institutes are considering using a quantum well structure in the active layer to improve laser characteristics. It is being done. For example, Kitamura et al., IEEE Photonics Technology Letters
ology Letters) vol. 1.2, NO. 5, 3
10-311 (1990). FIG. 3 shows the layer structure of a conventional multi-quantum well active layer long-wavelength semiconductor laser crystal, and FIG. 4 shows a band structure diagram near the quantum well active layer of the laser crystal. In FIG. 3, the semiconductor laser crystal includes an n-type InP substrate 14, an n-type InP cladding layer 15, In0.76Ga0.24As0.5
P0.5 Guide layer 16, In0.53G with a layer thickness of 7 nm
a0.47As quantum well layer 17 and In0 layer thickness 20 nm
．． 76Ga0.24As0.5 P0.5 Barrier layer 1
A multi-quantum well active layer in which In0.7 is laminated alternately and In0.7
6Ga0.24As0.5 P0.5 guide layer 19,
p-type InP cladding layer 20, p-type In0.53Ga0.
47As cap layer 21. All of these semiconductor layers are lattice matched to the n-type InP substrate 14. Here, the number of quantum wells was set to four. FIG. 4 shows the conduction band edge 22 around the multi-quantum well active layer of this semiconductor laser.
, the energy of the valence band edge 23 is shown in the layer thickness direction. An electronic subband edge 24 and a heavy hole subband edge 25 are formed in the quantum well layer 17 by carrier confinement. A multi-quantum well semiconductor laser with this structure oscillates at a wavelength of 1.55 μm with low optical fiber loss, oscillates at a lower threshold than conventional bulk active layer semiconductor lasers, and can operate at high temperatures. Became.

0003

[Problems to be Solved by the Invention] The current direct modulation band of such a semiconductor laser is currently several GHz;
It is expected that a band of about 20 GHz can be obtained by reducing parasitic capacitance. However, in order to obtain a wider band, there are problems caused by the layer structure of the laser, one of which is the problem of relaxation of carriers from the barrier layer or guide layer to the subbands within the quantum well. Electrons and holes injected into the guide layer and the multi-quantum well active layer region diffuse through the guide layers 16 and 19 and the barrier layer 18 and move.
Optical phonons and acoustic phonons are trapped in the quantum well 17 and released to relax the inside of the quantum well layer, but the slowest of these processes is diffusion in the guide layer and barrier layer. In order to shorten this diffusion time, it is effective to make the guide layer and barrier layer thinner to make the diffusion region smaller. However, if the barrier layer is made thinner, adjacent quantum wells will couple, and the effective mass small electron subband 2 of
4 expands energetically and becomes close to the bulk state. As a result, the advantage of the quantum well structure that the density of states becomes energetically discrete is lost. In the layer structure shown in this conventional example, the lower limit of the barrier layer thickness is about 7 to 10 nm since the above-mentioned subbands are discrete. In this way, there is a trade-off between increasing the speed of carrier injection into the quantum well and the quantum well effect.

0004

[Means for Solving the Problems] A semiconductor laser of the present invention has an active layer comprising a first barrier layer, a first quantum well layer,
A single or multiple multilayer film formed by sequentially laminating a second barrier layer, a second quantum well layer, a second barrier layer, a first quantum well layer, and a first barrier layer. and the band gaps of the first and second barrier layers are large enough to form at least one conduction band and valence band subband in the first and second quantum well layers. , and the energy of the lowest conduction band subband edge formed in the second quantum well layer is smaller than the energy of the lowest conduction band subband edge formed in the first quantum well layer, and The energy of the lowest order valence band subband edge of the second quantum well layer is greater than the energy of the lowest order valence band subband edge formed in the first quantum well layer, and the lattice constant of the quantum well layer is smaller than the lattice constant of the substrate, and the lowest order valence band subband of the first quantum well layer is a light hole subband, and the first and second barrier layers The layer thickness is such that tunneling of electrons and holes from the first quantum well layer to the second quantum well layer occurs at a speed on the order of 10 picoseconds or less.

[0005]

[Operation] In the semiconductor laser of the present invention, the multi-quantum well active layer has first and second different quantum wells arranged alternately with barrier layers in between, and the lowest order electronic subband edge of the second quantum well is energy is lower than that of the first quantum well, and the lowest order Hall subband edge of the second quantum well is lower than that of the first quantum well.
Since the energy is higher than that of the second quantum well, the second quantum well becomes the light emitting layer. Coupling between adjacent second quantum wells is prevented by the presence of the first quantum well, and the second quantum well maintains discrete subbands. Electrons and holes diffuse and move through the barrier layer, are trapped in the first and second quantum wells, and relax the interior of each quantum well. Electrons and holes relaxed to the lowest order subband edge of the first quantum well finally relax to the second quantum well by tunneling. Since the first quantum well uses a semiconductor layer with a smaller lattice constant than the substrate,
Tensile strain is applied to the first quantum well, and the valence band edge becomes a light hole band. By making the lowest hole subband in the valence band of the quantum well the light hole subband, it is possible to make hole tunneling from the first quantum well to the second quantum well as fast as electrons. can. If the thickness of the barrier layer is made thin so that the tunnel time constant of electrons and holes is 10 picoseconds or less,
Carriers are injected into the second quantum well, which is the light emitting layer, following the laser current modulation of about several tens of GHz. Further, in this structure, the barrier layer can be made extremely thin without bringing the subband of the second quantum well close to the bulk state, so that the delay in carrier injection caused by carrier diffusion in the barrier layer can be reduced.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view showing an example of the layer structure around the active layer of a semiconductor laser crystal according to the present invention, and FIG. 2 is a band structure diagram around the active layer of the semiconductor laser crystal. In FIG. 1, the semiconductor laser crystal includes an n-type InP substrate 1,
n-type InP cladding layer 2, In0.76G layer thickness 2 nm
a0.24As0.5 P0.5 Barrier layer 3 and layer thickness 1
5nm In0.55Ga0.45As0.8 P0.
2 Quantum well layer A4 and 2 nm layer thickness In0.76Ga0
．． 24As0.5 P0.5 Barrier layer 3 and layer thickness 7 nm
In0.53Ga0.47As quantum well layer B5 and 1
A multilayer film with several periods and a layer thickness of 2 nm In0.76
Ga0.24As0.5 P0.5 Barrier layer 3, layer thickness 15 nm In0.55Ga0.45As0.8 P0
．． 2 Quantum well layer A4, 2 nm layer thickness of In0.76Ga
0.24As0.5 P0.5 Barrier layer 3, p-type In
It is formed by sequentially laminating a P cladding layer 6 and a p-type In0.53Ga0.47As cap layer 7. In FIG. 1, the number of periods of the multilayer film is three. FIG. 2 shows the band structure of the laser crystal around the multiple quantum well active layer. The conduction band edge 8 and the valence band edge 9 have a structure as shown in the figure, and quantum wells A and B have an electron subband and a hole subband, respectively. subband ends 10, 11,
They are shown as Hall subband edges 12,13. Since the quantum well A has a lattice constant smaller than that of the InP substrate 1, tensile strain of about 0.5% is applied, and the valence band edge becomes the light hole band edge. Quantum well layer thickness is 15n
m, the lowest order valence band subband edge is the light hole subband edge 12. In FIG. 2, the electronic subband edge of quantum well B is lower in energy than that of quantum well A, and the hole subband edge of quantum well B is higher in energy than that of quantum well A. Therefore, quantum well B becomes a light emitting layer (active layer). Electrons and holes injected into the multiple quantum well region diffuse through the barrier layer 3 and are trapped in the quantum wells A and B, but since the barrier layer 3 is extremely thin at 2 nm, the electrons and holes injected into the quantum well A move at an extremely high speed. , B is trapped. The trapped carriers relax in quantum wells A and B and reach electron subband edges 10 and 11 and hole subband edges 12 and 13, respectively. The carriers in the quantum well A are then transferred to the barrier layer 3.
tunnel to reach quantum well B. Since the barrier layer 3 has a thickness of 2 nm, the time constant of this tunnel is several picoseconds for electrons. Regarding holes, in the case of a heavy hole, the effective mass is about 10 times larger than that of an electron, so the tunnel time constant is several tens of picoseconds, but in this case, since it is a light hole, the effective mass is that of an electron. It is about the same level,
The tunneling time is also several picoseconds, similar to that of electrons. Therefore, carriers injected into the multi-quantum well active layer region reach the lowest order subband 11 of quantum well B with a time constant of several picoseconds.
Reach 13.

Although the structure of the laser is not particularly shown in this embodiment, it may be of a planar type, SAS type, BH type, or DCPBH type.
It can be applied to commonly used structures such as molds.

[0008]

Effects of the Invention Electrons and holes injected into the multiple quantum well region are efficiently trapped from the barrier layer to the two quantum wells. Since the barrier layer is extremely thin at 2 nm, the diffusion time of carriers within the barrier layer is extremely short. quantum well A
The trapped electrons and holes are efficiently injected into the quantum well B by tunneling with a time constant of several picoseconds. Efficient tunneling of holes is achieved because the lowest hole subband of quantum well A is a light hole subband. Moreover, the quantum well B which becomes the light emitting layer
The sub-bandwidth of is as narrow as in the case of a thick barrier layer, and the effect of reducing the density of states due to the quantum well effect is not impaired. As a result, this was not possible in conventional semiconductor lasers. It becomes possible to make the barrier layer thin and to keep the subband width of the quantum well active layer narrow. Therefore, the semiconductor laser of the present invention has both the advantages of a quantum well semiconductor laser in that it has a low oscillation threshold and can operate at high temperatures, and the advantage that high-speed modulation of 10 Gb/s or more is possible.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the layer structure of a semiconductor laser crystal according to an example of the present invention.

FIG. 2 is a band structure diagram of a semiconductor laser crystal according to an example of the present invention.

FIG. 3 is a cross-sectional view showing the layer structure of a conventional semiconductor laser crystal.

FIG. 4 is a band structure diagram of a conventional semiconductor laser crystal.

[Explanation of symbols]

1, 14 n-type InP substrate 2, 15 n-type InP cladding layer 3, 18 In0.76Ga0.24As0.5 P
0.5 Barrier layer 4 In0.55Ga0.45As
0.8 P0.2 Quantum well layer A5, 17 In0.
53Ga0.47As quantum well layer B (active layer) 6, 20 p-type InP cladding layer 7, 21 p-type In0.53Ga0.47As cap layer 8, 22 conduction band edge 9, 23 valence band edge 10, 11, 24 electron Subband end 12 Light hole subband end

Claims (1)

[Claims] 1. A semiconductor laser configured by sandwiching an active layer between a P-type cladding layer and an N-type cladding layer on a semiconductor substrate, wherein the active layer includes a first barrier layer and a first quantum well layer. , a second barrier layer, a second quantum well layer,
The first barrier layer is composed of a single period or a plurality of periods of a multilayer film formed by sequentially laminating a second barrier layer, a first quantum well layer, and a first barrier layer, and the first and second barrier layers has a band gap large enough to form at least one conduction band and valence band subband in the first and second quantum well layers, and the lowest order band gap formed in the second quantum well layer. The energy of the conduction band subband edge of is smaller than the energy of the lowest conduction band subband edge formed in the first quantum well layer, and the energy of the lowest conduction band subband edge of the second quantum well layer is The energy of the band edge is greater than the energy of the lowest valence band subband edge formed in the first quantum well layer, and the lattice constant of the first quantum well layer is greater than the lattice constant of the substrate. The lowest order valence band subband of the first quantum well layer is a light hole subband, and the layer thickness of the first and second barrier layers is from the first quantum well layer to the first quantum well layer. 1. A semiconductor laser having a size such that tunneling of electrons and holes into a quantum well layer of No. 2 occurs at a speed of 10 picoseconds or less.