KR900000021B1 - Semiconductor laser - Google Patents

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Description

Semiconductor laser

1 is a cross-sectional view showing an embodiment of a semiconductor laser device according to the present invention.

2 is a diagram showing an energy band of a quantum well structure.

3 is a diagram showing experimental results of a relaxation vibration frequency (fr).

4 to 6 are cross-sectional views of the multi-quantum well structure showing the present invention.

7 to 10 are cross-sectional views of a semiconductor laser showing another embodiment of the present invention.

* Explanation of symbols for main parts of the drawings

3: active layer 4: barrier layer

11: undoped GaAs well layer 12: P = Ga 0.7 Al 0.3 As barrier layer

13: n-Ga0.55 Al0.45As cladding layer 14: P-Ga0.55 Al0.45As cladding layer

15: undoped Ga 0.7 Al 0.3 As barrier layer 16: P = Ga 0.7 Al 0.3 As barrier layer

17: n-Ga 0.7 Al 0.3 As barrier layer 18: n-GaAs substrate

19: n-GaAs buffer layer 20: multiple quantum well active layer

21: n-GaAs light absorption layer

22: P-Ga0.55 Al0.45As buried cladding layer

23 P-GaAs gap layer 24 P-side electrode

25 n-side electrode 26 n-Ga0.65 Al 0.35 As light guide layer

27: disordered layer of quantum well 28: SiO ₂ film

29: n-Ga0.55Al0.45As buried clad layer 30: Zn diffusion region

31 Insulating GaAs substrate 32 Undoped Ga0.55Al0.45As

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly, to a structure of a semiconductor laser characterized by a high modulation speed and a small threshold current.

The modulation rate of the semiconductor laser element is proportional to the frequency limit in the modulation of the semiconductor laser element. Therefore, in order to speed up a semiconductor laser element, it is necessary to make the frequency limit in direct modulation of the said semiconductor laser element as high as possible.

Normally, the frequency limit in direct modulation of semiconductor lasers is about 5 kHz, but it is theoretically predicted that the frequency limit will be increased if a so-called quantum well type laser device having a thickness of the active layer is smaller than the magnitude of the electromagnetic wave flux in the crystal. . [Y, ARAKAWA et al .: Applied physics letters, 45,950 (1984)]. On the other hand, even in the conventional semiconductor laser device, it is experimentally confirmed that the doping of the active layer with a high concentration increases the frequency limit. [C, B, SU et al .: Applied physics letters, 46,344 (1985)]. However, in any of these cases, the frequency limit of the direct modulation is around 10 kHz unless other special studies are conducted.

Further, increasing the impurity concentration of the P-type active layer increases the modifiable frequency of the semiconductor laser according to C, B, SU, etc., described in the notices 162 to 163 of the ninth laser conference.

This is attributable to the increase of the gain coefficient with respect to the increase of the injection cage due to the doping of impurities. However, this method has a problem that the life of the carrier of the active layer is shortened, the threshold current increases, and the luminous efficiency is lowered.

An object of the present invention is to provide a semiconductor laser capable of eliminating the drawbacks of the prior art and having a low threshold current, a high luminous efficiency, and capable of directly modulating at 10 dB or more.

The frequency limit for direct modulation of semiconductor lasers is largely the relaxation vibration frequency (fr). The relaxation oscillation frequency fr is generated from the phase shift in the fluctuation of light and electrons. However, as a viable method for increasing the relaxation oscillation frequency fr, the relaxation oscillation frequency fr is a gain for the increase in the carrier density? N. A ratio Δg / Δn of the increment Δg, that is, a method of increasing the differential gain may be considered. In so-called quantum well type laser devices in which the active layer of the semiconductor rearer device is made thinner and smaller than the magnitude of the wave velocity of free electrons in the crystal, differential gain is reported in the above-mentioned documents of Y and ARAK-AWA.

On the other hand, it is reported by the C, B, and SU that the increase of the impurities (fr) in the active layer of the conventional semiconductor laser element is increased, but this is also because the differential gain increases due to the high concentration of impurities. I can think of it.

MEANS TO SOLVE THE PROBLEM The present inventors made the active layer or quantum which were undoped conventionally in order to make modulation (fr) of the laser element whose thickness of an active layer smaller than the magnitude | size of the wave velocity of free electron in crystal | crystallization more high, and to speed up modulation. When the active layer is composed of two or more active layers, such as a well type laser, the impurity may be supplied to those active layers or the barrier layer between the active layers, and the impurity concentration is higher than the carrier density injected into the active layer during laser oscillation. We found that we needed to supply. In addition, it was found that when the donor is supplied as the impurity type at this time, the two-dimensional property of the electrons is lost and the derivative gain tends to be small, so that the acceptor is more effective. In a multi-quantum well type laser device in which a plurality of thin active layers are formed by sandwiching a barrier layer, carriers generated by impurities doped in the barrier layer are doped in the active layer.

In this case, it has been found that the band tail formed by the impurity doping does not lose the two-dimensional properties of electrons and holes, and the derivative gain does not decrease, so that modulation can be speeded up. 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 magnitude of the wave velocity of free electrons in the crystal. When the active layer or the active layer has two or more active layers, the barrier layer has a larger band gap than the active layer. By doping impurities with a density greater than the carrier density injected into the active layer, the frequency limit is increased by increasing the fr of the quantum well type laser device, and the modulation speed is increased.

As one of methods for reducing the threshold current of a semiconductor laser, there is a method in which the active layer has a multi-quantum well structure. This is due to the increase in the proportion of carriers that contribute to the gain due to the stepped density of states due to the two-dimensionality of the carriers. In addition, it is generally known that the gain of the increase of the injection carrier increases in the multi-quantum well structure as compared with the conventional double heterostructure. By doping to P-type or n-type of the multiple quantum well active layer of this multiple quantum well semiconductor laser, it is expected that the high speed modulation characteristics will be greatly improved.

However, when the doping is performed over the entire multi quantum well layer, band tailing occurs in the well layer in which the carrier is locally present, and there is a concern that the two-dimensionality of the carrier is lost.

Therefore, the present inventors have found that if the doping is not performed on the well layer in which the carrier is present, and selectively doping only to the barrier layer, the two-dimensional properties of the carrier are not lost and the high-speed modulation characteristics can be improved. ). However, in the multi-quantum well structure, the wave function of electrons and holes penetrates into the barrier layer, so that the atomic layer of the layer contacting the well layer is undoped, and the center region of the barrier layer is P-type or An n-type structure was invented (FIG. 5). In addition, when selective doping is performed in the center region of the barrier layer, as shown in FIG. 6, it has been found that the P-type cladding layer is n-type and the n-type cladding layer is P-type. As a result, the active layer is doped to both P and n, and the increase in gain for the injection carrier is greatly increased, and a large improvement in high-speed modulation can be expected.

As a result, in a semiconductor laser in which a well layer including at least a clad layer and a well layer and a barrier layer is formed on a semiconductor substrate, the thickness of the well layer is equal to or less than the electron wavelength of the electron and the barrier layer is wider than the well layer. In a semiconductor laser having a P-type and an n-type cladding layer having a prohibition band wider than the well layer, a multi-quantum well active layer having alternately overlapped with a barrier layer, with respect to the stacking direction of each semiconductor layer such as a multi-quantum well active layer The semiconductor laser having a high modulation rate was obtained by spatially varying the conductivity types of the layers forming the multi-quantum well active layer.

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 size of the wave flux of free electrons in the crystal, and when the active layer or two or more active layers have an active layer, By doping the barrier layer with an impurity having a density greater than the carrier density injected into the active layer, the (fr) of the quantum well type laser device is made high so that direct modulation with a frequency limit of 20 kHz or more, that is, much more than 10 kHz is possible. The semiconductor laser device can be significantly increased in speed, and can be applied as a light source for an optical operation circuit and ultra-high speed optical communication.

Next, an embodiment of the present invention will be described with reference to the drawings.

Example 1

1 is a cross-sectional view showing a first embodiment of a semiconductor laser device according to the present invention, FIG. 2 is a diagram showing an energy band of a quantum well structure, and FIG. 3 is a diagram showing experimental results of a relaxation vibration frequency fr. . In FIG. 1, thereby growing the n-type ㎓ substrate (1) an organic metal vapor phase growth method by the n-type (MOCVD method), Ga ₁ -x AlxAs on the clad layer (X = 0.45) (2) . It grows a multi-quantum well structure on it. The multi-quantum well layer includes a P-type Ga _1- yAlyAs active layer (y = 0,0.2, thickness 3-15 nm) (3) and an undoped Ga _1- z AlzAs barrier layer (Z> y, thickness 3-20 nm) ( 4) alternately grown 2 to 10 layers. Next, the P-type Ga _1- x AlxAs layer 5 and the P-type GaAs layer 6 are grown, and the P-side electrode Cr-Au (7) and the n-side electrode AuGe Ni-Au (8) are deposited to form an element. Cut. Doping at least 1 × 10 ¹⁸ Cm ⁻³ or more of P-type impurity to the active layer 3 increases the differential gain, thereby increasing the frequency limit from the conventional 10 Hz to 20 Hz. Since the lattice defect becomes large when the concentration of the impurity to dope exceeds 1x10 ¹⁹ Cm ^- _e , the impurity concentration may be limited to 1x10 ¹⁸ Cm ^-3 . Doping Zn also causes disordered diffusion due to diffusion, which may result in the loss of quantum structure. Therefore, Mg, Be, or the like is more preferable.

Example 2

Another embodiment of the present invention will be described with reference to FIG. The n-type Ga _1- x AlxAs cladding layer 2 is grown on the n-type GaAs substrate 1 by the organometallic vapor phase growth method. In this embodiment, the multi-quantum well structure formed on the cladding layer 2 includes an undoped Ga ₁ -yAlyAs active layer (y = 0 to 0.2, thickness of 3 to 5 nm) (3), and a P-type Ga _1- . The zAlzAs barrier layer (Z> y, thickness 3-20nm) is alternately grown 2 to 10 layers.

When the P-type impurity of 1 × 10 ¹⁸ Cm ⁻³ or more is doped into the barrier layer 4, the generated holes are generally doped into the active layer 3. The energy band band at this time is shown in FIG.

As shown in the drawing, the high-density holes 9 are present in the active layer 3, and thus, similarly to the above-described embodiment, the differential gain is increased and the frequency limit is increased. 3 shows the experimental results of the relaxation vibration frequency when the barrier layer 4 is doped with 3x10 Cm ^-3 Mg.

3 shows the relaxation oscillation frequency (fr) on the vertical axis of the quadratic root of the light output (P) normalized to the polyhedral limiting light output (Pc) on the horizontal axis, but the data of the conventional quantum well type laser device represented by the broken line ( Compared with 10), in this embodiment, as shown by the solid line 11, the frequency limit is improved to 20 Hz or more.

In this embodiment, since the impurity doping is not directly doped in the active layer 3, a band tail by impurity doping is not formed, and in the quantum well type structure, the two-dimensional properties of electrons and holes are not impaired. For this reason, the differential gain by a quantum well structure does not fall, and it becomes possible to speed up direct modulation further. As the P-type impurity, Mg, Be, etc. are effective similarly to the said Example.

In the above embodiment, not only P-type impurities but also n-type impurities Si, Te, Se, and the like are effective. In each of the above embodiments, if the barrier layer is InP and the active layer is InGaAsP, and the impurities are doped in the same manner, any of the same effects can be obtained. In addition, although both embodiments selectively doped, both the active layer and the barrier layer may be doped together.

Example 3

7 is described in detail using.

an n-type n-type crystal on a GaAs substrate (18) GaAs buffer layer (19), an n-type Ga ₁ -xAlxAs greater the clad layer (13) (X = 0.45) , is even undoped GaAs well layers 11 70Å thick layer 5, Barrier layer 4 formed of P-Ga0.7x Al0.3As layer 16 subjected to Mg doping of 2x10 ¹⁸ (Cm ^-3 ) having a thickness of 20 kV sandwiched by 10 knots of undoped Ga0.7AL0.3As layer 15 The multi-quantum well active layer 20, the P-type Ga _1- x AlxAs cladding layer 14, and the n-type GaAs light absorbing layer 21, which are alternately formed layers, are sequentially formed by MOCVD. Picture completely remove the center of the n-type GaAs layer 21 by the etching process, a stripe shape, and forming a stripe groove in the width 1~15㎛ to expose the surface of the P-type Ga ₁ -xAlxAs clad layer 14 . Next, the P-type Ga _1- x AlxAs clad layer 22 (x = 0.43) and the p-type GaAs gap layer 23 are formed by MOCVD. Thereafter, after forming the P-side electrode 24 and the n-side electrode 25, a laser device having a resonant field of about 300 µm was obtained by cleavage.

At this time, when the thickness of the P-type Ga0.55Al0.45As layer 14 is 0.1 to 0.5 µm, the waveguide structure becomes a refractive index waveguide to stabilize the transverse mode during high-speed modulation.

The prototype device has a wavelength of 830 nm, and continuously oscillates at room temperature with a threshold current of 10 to 25 mA, the oscillation spectrum exhibits a vertical single mode, and the light output is stable to no kink up to 70 mW. When small-signal direct modulation was performed by biasing at an optical output of 60 mW, a good characteristic that the modulation frequency was up to 15 Hz (3 dB down) was obtained. In addition, it was found that the life of a constant light output operation with a light output of 60 mW at 70 ° C. was not significantly deteriorated even after 2000 hours, and the reliability was high. In addition, in the multi-quantum well structure, in addition to the above, the molar ratio W of Al in the Ga _1- w wAlWAs well layer is 0 to 0.2 thickness of 30 to 150 Pa, the number is 2 to 10, and the molar ratio B of Al in the Ga _aB Al _B As barrier layer is 0.2 to In all combinations of 0.5 (stage B> W), the thickness of the undoped barrier layer on both legs and the thickness of the P-type barrier layer in the center and the thickness of the central portion of the P-type barrier layer were 5 to 50 µs, the same shift modulation characteristics were obtained. .

Example 4

Description will be made using FIG.

The n-type GaAlAs cladding layer 13, the n-type GaAlAs light guide layer 26, the multi-quantum well active layer 20 and the P-type GaAlAs cladding layer as shown in FIG. 7 on the n-type GaAs substrate 18 (14), the P-type GaAs gap layer 23 is formed by the MOCVD method sequentially. The photo-etching process removes the P-type GaAs gap layer so that it remains 1-15 µm wide, and Si is deposited until the multi-quantum well active layer 20 is observed in the region other than the stripe-shaped P-type GaAs gap layer 23. Ion implantation.

After that, an SiO ₂ film 28 other than a stripe-shaped P-type GaAs gap layer was deposited, and then the P-side electrode 24 and the n-side electrode 25 were formed. A laser device was obtained.

Also in this embodiment, the same characteristics as in the embodiment of FIG. 7 can be obtained, and in addition to the active layer structure, all of the ranges shown in the embodiment of FIG. 7 are applicable and similar characteristics can be obtained.

Example 5

Description will be made using FIG.

After the n-type GaAlAs cladding layer 13 and the multi-quantum well active layer 20 and the P-type GaAlAs cladding layer 14 are grown on the n-type GaAs substrate 18 as in the embodiment of FIG. Etching is carried out to the n-type GaAs substrate 18 so as to remain in a stripe shape having a width of 1 to 5 mu m, and then the P-type GaAlAs buried layer 22 and the n-type GaAlAs layer 29 are grown to form a Zn diffusion region 30. To form. Then, after forming the P side electrode 24 and the n side electrode 25, the laser element of about 300 micrometers of resonator fields was obtained by cleavage method. Also in this embodiment, good high-speed modulation characteristics were exhibited, and since the periphery of the active layer was all surrounded by GaAlAs, there was no lateral diffusion of carriers, more excellent high-speed characteristics, and modulation up to 20 Hz was possible. Also in the active layer structure, all of the ranges shown in the embodiment of FIG. 4 were applicable and the same characteristics could be obtained.

Example 6

Description will be made using FIG.

20 Å 2 × of the undoped GaAlAs layer 32 and the 70 두께 undoped GaAs well layer 11 sandwiched on the insulating GaAs substrate 31 by three layers and the undoped Ga0.7Al0.3As layer 15 having a thickness of 10 Å. Alternately formed two layers of barrier layers formed of n-Ga0.7Al0.3As layer 17 subjected to Se doping of 10 ¹⁸ (cm ^-3 ), again two layers of an undoped GaAs well layer having a thickness of 70 s and a thickness of 10 s Alternating two layers of barrier layer formed of P-Ga0.7Al0.3As layer 15 subjected to Mg dope of 2x10 ¹⁸ (cm ^-3 ) having a thickness of 20 kW sandwiched by undoped Ga0.7Al0.3As layer 15 The multi-quantum well active layer 20 and the undoped GaAlAs layer 32 formed are formed. Thereafter, the growth layer was left in a stripe shape having a width of 1 to 5 mu m, and the P-type GaAlAs buried layer 22 and the n-type GaAlAs buried layer 29 were formed, and then the P electrode layer 24 and the n electrode layer 25 were formed. After that, a laser device having a resonance field of about 300 µm was obtained by cleavage. This laser element has a structure in which carrier is transversely injected into the active layer. In addition, since the multi-quantum well active layer supplies impurities of both Pn to the barrier layer, the increase in gain for the injection carrier is further increased, enabling direct modulation up to 20 dB. The multi-quantum well active layer having both P and n impurities in the barrier layer was applied to the examples shown in FIGS. 7, 8, and 9, and thus high-speed modulation characteristics could be obtained. In each of the above examples, similar results were obtained even when Be was used as the P-type impurity and Si was used as the n-type impurity.

Example 7

n-type GaAlAs cladding layer 13 on n-type GaAs substrate 18, 5 layers of P-Ga0.8Al0.2As well layers doped with 1 × 10 ¹⁷ (cm ⁻¹³ ) Mg, and 2 × 10 ¹⁸ A quantum well layer 40 in which four layers of P-Ga0.7Al0.3As barrier layer doped with Mg of (cm ^-3 ) is formed is formed, and thereon, 1 x 10 ¹⁷ (cm ^-3 ) of Se A quantum well layer 50 including four layers of P-Ga0.8Al0.2As well layers and four layers of P-Ga0.7Al0.3As barrier layers doped with 1 × 10 ¹⁹ (cm ⁻³ ) Se was formed. Thus, the quantum well active layer 20 was formed. The P-type GaAlAs cladding layer 14 and the P-type GaAs gap layer 23 are formed thereon by a sequential MOCVD method, and then have a stripe structure as in Example 4, and the electrodes 24 and 25 are formed. And the like to obtain a semiconductor laser. This laser modulation rate was 15 Hz.

Example 8

An n-type GaAlAs-based cladding layer on an n-type GaAs substrate, 5 layers of an undoped GaAlAs well layer with a film thickness of 40 μs, an undoped Ga0.7Al0.3As with a film thickness of 10 μs, P-Ga0.7Al0.3As and a film thickness of 30 μs After alternately stacking a barrier layer 4 having a thickness of 10 μs of undoped Ga 0.7 Al 0.3 As sequentially laminated thereon, a 10 μm thick undoped Ga 0.7 Al 0.3 As layer and a film thickness of 20 μs of P-Ga 0.7 Al 0 were deposited. A quantum well active layer and a P-type GaAlAs-based cladding layer in which a barrier layer composed of a .3As layer and an undoped GaAlAs well layer were formed were sequentially formed by MOCVD, and the semiconductor laser was fabricated in the same manner as in Example 4. The characteristics of this semiconductor laser were the same as in Example 3.

Example 9

After the n-type cladding layer was formed on the GaAs substrate, five layers of the undoped layer and one atomic layer in contact with the well layer were undoped thereon, and the other four layers of the P-type barrier layer were alternately stacked. A quantum well active layer was formed, a P-type cladding layer was formed thereon, and the semiconductor laser was obtained in the same manner as in Example 4. The modulation rate of this laser was 13 Hz.

Further, for the semiconductor laser of the above embodiment, the effect of the present invention was remarkably produced from varying the doping amount, that is, from 5 x 10 ¹⁷ (cm ^-3 ) (see FIG. 12). When it became 2 * 10 <^19> (cm <^-3> ), crystal defects became large and a semiconductor laser could not be obtained.

Claims (9)

A semiconductor layer group including a multi-quantum well type active layer 3 formed by stacking at least a cladding layer 2, an active layer 3 or a well layer 11 and a barrier layer 4 on a semiconductor substrate. A semiconductor laser in which the thickness of the active layer (3) is smaller than the magnitude of the wave flux of free electrons in the crystal, wherein impurities are injected into the active layer (3). The laser according to claim 1, wherein the impurity is an acceptor. The semiconductor according to claim 1, wherein in the impurity implantation into the multi-quantum well type active layer 3, the impurity density of the barrier layer 4 is larger than that of the well layer (active layer) 3. laser. A plurality of quantum wells in which a plurality of semiconductor layers formed on a semiconductor substrate are alternately overlapped with a well layer 11 having a thickness equal to or less than the electron debroy wavelength and a barrier layer 16 having a larger prohibition band than the well layer 11. In a semiconductor laser having a P-type 14 and an n-type cladding layer 13 having a larger prohibition than the well layer 11 so as to sandwich the active layer 20 and the multi-quantum well active layer 20, the multi-quantum well A semiconductor laser, characterized in that the conductive type of each layer forming the multiple quantum well active layer (20) is spatially different with respect to the stacking direction of the active layer (20). 5. The conductive type according to claim 4, wherein at least one pair of the well layer 11 and the barrier layer 15, which are adjacent to the P-type cladding layer 14 adjacent to and adjacent to the P-type cladding layer 14, is n-type. A semiconductor laser, characterized in that the conductivity type of the other multi-quantum well active layer (20) is P-type. 5. The conductive type according to claim 4, wherein the conductivity type of at least one barrier layer 15 continuously adjacent to the P-type cladding layer 14 in the multi-quantum well active layer 20 is n-type, and other than this region. A semiconductor laser, characterized in that the conductivity type of the barrier layer (15) of the multiple quantum well active layer (20) is P-type, and all the well layers (11) are undoped. 5. The conductive type of claim 4, wherein the conductive type of at least one barrier layer 15 continuously adjacent to the P-type cladding layer 14 in the multiple quantum well layer 20 is n-type or a well layer 11. At least one atomic layer is undoped from the interface in contact, and the conductivity type of the barrier layer 15 of the multi-quantum well active layer 20 other than this region is P-type or at least one atomic layer is removed from the interface in contact with the well layer 11. A semiconductor laser, characterized in that it is a woofer and all the well layers 11 are undoped. The conductive layer of claim 4, wherein at least one atomic layer is undoped from the interface of the barrier layer 15 in contact with the well layer 11, and the barrier layer 15 other than this region is P-type, and the well layer 11 ) Is an undoped semiconductor laser. The p-type impurity is Be, Mg, the n-type impurity is Se, Si, and the impurity concentration is 5 × 10 ¹⁷ (cm ⁻³ ) or more. Semiconductor laser.

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