JPH0712103B2 - Semiconductor laser device - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a quantum well type semiconductor laser device capable of particularly high-speed modulation.

BACKGROUND OF THE INVENTION The high speed modulation of a semiconductor laser device is proportional to the frequency limit in the modulation of the semiconductor laser device. Therefore, in order to increase the speed of the semiconductor laser device, it is necessary to increase the frequency limit of direct modulation of the semiconductor laser device as much as possible. Normally, the frequency limit for direct modulation of semiconductor lasers is about 5 GHz, but recently, when a so-called quantum well type laser device in which the thickness of the active layer is smaller than the size of the electron wave packet in the crystal, the frequency limit becomes higher Forecast [Y, ARAKAWA et al .: Applied
Physics Letters, 45,950 (1984)). On the other hand, it has been experimentally confirmed that even in the conventional semiconductor laser device, if the active layer is heavily doped with impurities, the frequency limit becomes high (C, B, SU et al .: Applied Physics Letters, 46, 344). (1985)). However, in any case, unless otherwise devised, the frequency limit of the above direct modulation is around 10 GHz.

[Object of the Invention]

An object of the present invention is to obtain a semiconductor laser device having a high flux capable of performing direct modulation whose frequency limit exceeds 10 GHz.

[Outline of Invention]

It is the relaxation oscillation frequency fr that determines the frequency limit of direct modulation of the semiconductor laser. The relaxation oscillation frequency fr is generated due to the phase shift due to the fluctuations of light and electrons. As a promising method for increasing fr, the ratio Δg / of the increase Δg of gain to the increase Δn of carrier density Δg /
A method of increasing Δn, that is, the differential gain can be considered. In the so-called quantum well type laser device in which the active layer of the semiconductor laser device is thinned to be smaller than the wave packet size of free electrons in the crystal, the differential gain is increased as described above in Y, AR.
Reported in AKAWA et al. On the other hand, if impurities are heavily doped in the active layer of a conventional semiconductor laser device,
It has been reported by C, B, SU et al. that the fr is increased, which is also considered to be due to the increase of the differential gain due to the high concentration of impurities.

The inventors of the present invention have confirmed that the thickness of an active layer such as a quantum well laser device is smaller than the wave packet size of free electrons in the crystal.
In order to further increase fr and speed up the modulation, in the case where the active layer is composed of two or more active layers which are conventionally undoped or a quantum well type laser, those active layers are used. Alternatively, it suffices to introduce impurities into the barrier layer between the active layers, and regarding the impurity concentration,
It was found that it is necessary to introduce impurities at a higher concentration than the carrier density injected into the active layer during laser oscillation. At this time, if a donor is introduced as an impurity type,
It was found that the acceptor is more effective because the two-dimensionality of the electron is lost and the differential gain tends to be small.
In a multi-quantum well type laser device in which a plurality of thin active layers are provided with a barrier layer sandwiched in the middle, carriers generated by impurities doped in the barrier layer are trapped in the active layer. In this case, it was found that the two-dimensionality of electrons and holes is not lost due to the band-tilt formed by the impurity doping, and the differential gain is not lowered, so that the speed of modulation can be increased. That is, the semiconductor laser device according to the present invention is a semiconductor laser device in which the thickness of the active layer is smaller than the wave packet size of free electrons in the crystal, and when the active layer has two or more active layers, the thickness of the active layer is large. A barrier layer having a larger bandgap and substantially confining the carriers injected into the active layer is doped with impurities that generate carriers with a density higher than the carrier density injected into the active layer. As a result, the fr of the quantum well laser device is increased to increase the frequency limit and speed up the modulation.

Example of Invention

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing an embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a view showing an energy band of a quantum well structure, and FIG. 3 is a view showing experimental results of relaxation oscillation frequency fr. In FIG. 1, an n-type Ga _1- xAlxAs cladding layer (x = 0.45) 2 is formed on the n-type GaAs substrate 1 by metalorganic vapor phase epitaxy.
And a multiple quantum well structure is grown on it.
The multi-quantum well layer is a p-type Ga _1- yAlyAs active layer (y = 0 to 0.
2 and thickness 3 to 15 nm) 3 and undoped Ga ₁ -zAlzAs barrier layer (z> y, thickness 3 to 20 nm) 4 are alternately grown in 2 to 10 layers. Next, the p-type Ga _1- x AlxAs layer 5 and the p-type Ga
As layer 6 is grown, p-side electrode Cr-Au7 and n-side electrode AuGe
Ni-Au8 was vapor-deposited and separated into elements. Here, the active layer 3
If at least 1 × 10 ¹⁸ cm ⁻³ or more of p-type impurity is doped, the differential gain becomes large and the frequency limit becomes high from the conventional 10 GHz to 20 GHz. The concentration of impurities to be doped is 1 ×
If it exceeds 10 ¹⁹ cm ^-3 , lattice defects become large, so it is better to keep the impurity concentration in the range of 1 × 10 ¹⁸ cm ^-3 . Further, when Zn is doped, disorder due to diffusion may occur and the quantum well structure may disappear, so Mg, Be or the like is preferably used to obtain a larger effect.

Another embodiment of the present invention will be described with reference to FIG. The n-type Ga _1- xAlxAs cladding layer 2 is grown on the n-type GaAs substrate 1 by a metal organic chemical vapor deposition method. In this embodiment, the multiple quantum well structure formed on the cladding layer 2 is
Undoped Ga _1- yAlyAs active layer (y = 0 to 0.2, thickness 3 to 15
nm) 3 and a p-type Ga ₁ -zAlzAs barrier layer (z> y, thickness 3 to 20)
nm) 4 and 2 to 10 layers are alternately grown. Here, when the barrier layer 4 is doped with a p-type impurity of 1 × 10 ¹⁸ cm ⁻³ or more, most of the generated holes are trapped in the active layer 3. The energy band diagram at this time is shown in FIG. As shown in the figure, the holes 9 having a high density are present in the active layer 3, the differential gain becomes large and the frequency limit becomes high as in the above embodiment. FIG. 3 shows the experimental results of the relaxation oscillation frequency when the barrier layer 4 was doped with 3 × 10 ¹⁸ cm ⁻³ of Mg. In FIG. 3, the horizontal axis shows the square root of the optical output P normalized by the end-face breakdown limit optical output Pc, and the vertical axis shows the relaxation oscillation frequency fr. In comparison with the data 10 of FIG.
The frequency limit is improved above GHz.

In this embodiment, since the active layer 3 is not directly doped with impurities, a band tail is not formed by the impurity doping, and the two-dimensionality of electrons and holes is not impaired in the quantum well structure. Therefore, the differential gain due to the quantum well structure does not decrease, and the speed of direct modulation can be further increased. As the p-type impurities, Mg, Be and the like are effective as in the above embodiment. In the case of the above embodiment, not only p-type impurities but also
It is also effective for n-type impurities such as Si, Te, and Se. Further, in each of the above embodiments, the same effect can be obtained by using InP as the barrier layer and InGaAsP as the active layer and doping the same impurities. Although selective doping is performed in both of the examples, the active layer and the barrier layer may be uniformly doped.

〔The invention's effect〕

As described above, the semiconductor laser device according to the present invention is a semiconductor laser device in which the thickness of the active layer is smaller than the wave packet size of free electrons in the crystal, and when the active layer has two or more active layers. A barrier layer having a bandgap larger than the thickness of the active layer and substantially confining the carriers injected into the active layer, is doped with impurities that generate carriers with a density higher than the density of carriers injected into the active layer. By increasing the fr of the quantum well laser device, the frequency limit is 20 GHz or more, that is, 10 G
Direct modulation far exceeding Hz is possible, and the speed of semiconductor laser devices can be greatly increased.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a view showing an energy band of a quantum well structure, and FIG. 3 is a view showing experimental results of relaxation oscillation frequency fr. 3 ... Active layer 4 ... Barrier layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideaki Matsueda, Hideaki Matsueda, 1-280 Higashi Koigakubo, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Toshi Kajimura 1-280, Higashi Koigakubo, Kokubunji, Tokyo Hitachi, Ltd. Central Research Laboratory (56) Reference JP-A-58-216489 (JP, A)

Claims (1)

[Claims] 1. In a semiconductor laser device having an active layer with a thickness smaller than the wave packet of free electrons in a crystal, the active layer, or when the active layer comprises two or more active layers, the active layer or Carriers having a band gap larger than that of the active layer and substantially confining the carriers injected into the active layer are filled with carriers having a density higher than the carrier density injected into the active layer. A semiconductor laser device characterized by being doped with generated impurities.