JP2679974B2 - Semiconductor laser device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a quantum well semiconductor laser that oscillates at an unprecedented low threshold current, and more particularly to a semiconductor laser for an optoelectronic integrated circuit or for an optical integrated circuit. It is a thing. [Prior Art] As a light source for a future electro-optical integrated circuit (OEIC) or an optical integrated circuit (OIC), a semiconductor laser that oscillates with a low threshold current, that is, a semiconductor laser with low power consumption is desired. To date, a method of making the active layer into a quantum well type and using the quantum size effect to reduce the threshold current has been proposed by Sugimoto et al., IEICE Technical Report, OQE85-78, Vol.
Published on page 5. However, with this method, the threshold current is about 8 mA, which is improved to only about half of 20 mA of the conventional double hetero structure semiconductor laser. [Problems to be Solved by the Invention] Regarding the above-mentioned conventional technique, the device structure of the particle well active layer is almost optimized, and in the conventional quantum well active layer, the above threshold current (about 8 mA) or less It is difficult to lower the threshold of. However, such a threshold current value is still unsuitable as a light source for OEICs in the future, and further lower threshold currents are required for multi-functionalization and high integration of OEICs. It was An object of the present invention is to provide an unprecedented low threshold current semiconductor laser (<3 mA), and further to provide a highly functional and highly integrated light source for OEIC. [Means for Solving the Problems] In order to obtain a semiconductor laser that oscillates at a low threshold current, the present inventor has basically adopted a semiconductor laser having a quantum well structure as an active layer of a carrier injection type semiconductor laser. Considering that, all or part of the quantum well active layer consisting of a well layer having a thickness equal to or less than the Dow-Broe wavelength of electrons and a barrier layer having a band gap larger than that of the well layer (for example, only the well layer or only the barrier layer). , Or both the well layer and the barrier layer) are doped with a high concentration (> 10 ¹⁸ cm ⁻³ ) of impurities to artificially control the electron density and the hole density. It was found that electrons and / or holes (or both) exist in the quantum well active layer at the time of bias, and as a result, a low threshold current can be obtained. The quantum well active layer has a multiple quantum well structure in which well layers and barrier layers are alternately stacked, or the molar ratio of Al in the barrier layer is gradually changing.
IN-SCH type (Graded-Index-Separate-Confinement-H
The effect is remarkable when an eterostructure) structure is used. The present inventor creates a gain spectrum analysis model of an impurity-doped quantum well active layer and applies it to a multiple quantum well structure. The calculation results are shown in FIG. In this calculation, the Al molar ratio (X _W ) of the well layer was 0, the Al molar ratio (X _B ) of the barrier layer was 0.2, and the well layer thickness was 5 nm. It has been found that increasing the doping concentration in both P-doping and n-doping lowers the threshold carrier density required for oscillation and lowers the threshold current. 2 × 10 ¹⁸ cm, especially for n-doping
^-3 or more, and with p-doping of 4 × 10 ¹⁸ cm ^-3 or more, the threshold carrier density is reduced to about half that of the undoped multiple quantum well structure, which is 1/4 or less of the conventional double hetero structure. It turned out to be. However, since the rapidly its crystallinity when the doping concentration of 1 × 10 ¹⁹ cm ^-3 or more is reduced, as the doping concentration of 1 × 10 ¹⁹ cm ^-3
I knew that was the limit. At this time, the threshold carrier density is reduced to about 1/20 in n-doping and 1/6 in p-doping as compared with the conventional double hetero structure. It was also found that Mg, Be, Si, Se, etc., which have small diffusion in the solid phase, are effective as impurities. [Operation] As described above, it can be explained as follows that the threshold current is lowered when the quantum well active layer is heavily doped with impurities. When impurities are doped at a high concentration, there are a large number of quantized carriers in the well layer even without bias, that is, even when carriers are not implanted. Considering the case of P doping, the quantum levels in the valence band are occupied by holes emitted from the acceptor. The gain that contributes to the laser oscillation is obtained by subtracting the net optical absorption. Therefore, when the quantized holes are present in the valence band as described above, the net optical absorption is reduced. As a result, oscillation occurs at a low injection electron density. The same can be explained with n-doping. [Example] Next, an example of the present invention will be described with reference to the drawings. First
1A and 1B are sectional views showing an embodiment of a semiconductor laser device according to the present invention. FIG. In FIG. 1, n-type Ga _{1 is formed} on the n-type GaAs substrate crystal _1.
-XAlxAs cladding layer 2 (x = 0.45) and 4 × 1 with a thickness of 8 nm
N-doped GaAS well layer 31 doped with ¹⁸ cm ⁻³ Se,
N-doped or undoped or 4 × 10 ¹⁸ cm ^-3 Se-doped
Alternating 5 Ga _0.8 Al _0.2 As barrier layers 32 with a doping thickness of 3 nm
Multi-quantum well active layer 3 stacked layer by layer and P-type Ga ₁ -x
An AlxAs cladding layer 4 (x = 0.45) and an n-type GaAs current constriction layer 5 are sequentially formed by MOCVD. By the photo-etching process, the n-type GaAs layer 5 is completely removed and the p-type Ga ¹ -x
A thin stripe having a width of 1 to 15 μm exposing the surface of the AlAs cladding layer 4 is formed. Next, p-type Ga ₁ −xA was formed by MOCVD.
lxAs cladding layer 6 (x = 0.45), p-type GaAs cap layer 7
To form Then, after forming the p-type electrode 8 and the n-side electrode 9, a semiconductor laser device having a cavity length of about 300 μm was obtained by the cleavage method. At this time, in order to keep the transverse mode of the laser light stable by making the optical waveguide a refractive index waveguide type, the condition of the thickness d ₄ of the P-type cladding layer 4 was 0.1 <d ₄ <0.7 μm. In the above embodiment, the threshold current value is 1 at the oscillation wavelength of 830 nm.
It oscillated continuously at room temperature at ~ 2mA, the oscillation spectrum showed a longitudinal single mode, and the stability of transverse mode was confirmed up to an optical output of 20mW. At 90 ° C, the optical output is 20 mW, and the service life at constant light output is also
No significant deterioration was observed after 5000 hours, and it was revealed that the reliability was high. This is the well layer as described above.
This is because the threshold current density was remarkably lowered by the multiple quantum well in which 31 was heavily N-doped. Second Embodiment Another embodiment according to the present invention will be described with reference to FIG. On the n-type GaAs substrate 1, the n-type GaAlAs cladding layer 2, the molar ratio of Al gradually changes from 0.45 to 0.2.
A GaAlAs barrier layer 103, a 6 nm thick 6 × 10 ¹⁸ cm ⁻³ Mg-doped GaAs well layer 101, and a 0.1 μm thick p-GaAlAs layer in which the Al molar ratio gradually changes from 0.2 to 0.45. A GRIN-SCH active layer 10 made of a barrier layer 102 is formed, and a p-type GaAlAs cladding layer 4 is further grown on the GRIN-SCH active layer 10 and then photoetching is performed to reach the n-type GaAs substrate 1 so as to remain in a stripe shape having a width of 1 to 5 μm. Etching, then p
-Type GaAlAs layer 12 and n-type GaAlAs layer 13 are grown, and Zn diffusion region 11 is formed.
Is provided. After that, a p-side electrode 8 and an n-side electrode 9 were formed, and then a cleavage method was used to obtain a laser element having a cavity length of about 300 μm. In this embodiment, the structure of the semiconductor laser is BH (Buri
ed Heterostructure) type, so there is no reactive current that does not contribute to oscillation, so the threshold current can be made even lower, continuous oscillation at room temperature is achieved with a threshold current of 0.5 to 1.5 mA, and a wavelength of 800 nm. Vertical single mode is shown. Also, 1
At 00 ° C, a light output of 10 mW at constant light output was not observed, and no remarkable deterioration was observed even after 6000 hours, and a highly reliable device was obtained. Third Embodiment Another embodiment according to the present invention will be described with reference to FIG.
After the n ⁺ -GaAs layer 15 is grown on the semi-insulating GaAs substrate 14 by the MOCVD method, a semiconductor layer similar to that of the first embodiment is grown. After that, etching is performed to partially expose the surface of the n ⁺ -GaAS layer 15 to form the p-electrode 8 and the n-electrode 9. After this,
A laser element having a cavity length of about 300 μm was obtained by the cleavage method. Also in this embodiment, the characteristics substantially similar to those of the first and second embodiments are shown. Furthermore, in this embodiment, a semiconductor laser having an ultra-low threshold current is formed on the semi-insulating substrate, and oscillation to the OEIC or the like can be expected. Further, in the above examples, Mg, Se as impurities
Although the case was shown, almost the same effect was obtained by using Si and Be. Further, the width of the well layer is ^{3 to} 10 nm, the p-type impurity concentration is (4 to 10) × 10 ¹⁸ cm ⁻³ , and the n-type impurity concentration is (2 to 10) × 10 ¹⁸ cm ^−3. In the case of, almost the same effect was obtained. Further, in the above embodiments, one of p-type and n-type impurity doping was performed, but both types of doping may be performed. The present invention is not limited to the wavelength of about 0.80 μm shown in the embodiment, and the same result is obtained over the entire range where the continuous oscillation at room temperature can be achieved with a GaAlAs semiconductor laser device having a wavelength of 0.86 to 0.89 μm. The semiconductor laser device according to the present invention can be similarly applied to laser materials other than GaAlAs-based materials, such as InGaAsP-based and InGaP-based materials. Further, the structure of the laser is not limited to the one based on the three-layer waveguide shown in each of the above-described embodiments, but may be a LOC structure in which an optical guide layer is provided adjacent to one side of the active layer, or adjacent to both sides of the active layer. The same can be applied to the SCH structure in which the light guide layer is provided. Similar effects were obtained also in the structures in which the conductivity types were all reversed in the above-mentioned respective examples (structures in which p was replaced by n and n was replaced by p). [Effect of the Invention] As described above, the semiconductor laser device according to the present invention has a threshold current much lower than that of a conventional semiconductor laser by introducing a high concentration impurity into all or part of the quantum well active layer. Since a semiconductor laser device having a threshold current can be formed, a highly reliable laser device can be obtained, which is particularly effective as a light source for an optoelectronic integrated circuit or an optical integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, FIG. 3, and FIG. 4 are cross-sectional views of an embodiment according to the present invention, in which each figure (b) is an enlarged view in a circle of FIG. FIG. 4 is a calculated value of the threshold carrier density with respect to the doping concentration, and is a diagram showing the principle of the present invention. 1 ... n-GaAs substrate, 2 ... n-GaAlAs cladding layer, 3
... Multiple quantum well active layer, 4 ... p-GaAlAs cladding layer, 5 ... n ⁺ -GaAs current confinement layer, 6 ... GaAlAs cladding layer, 7 ... p-GaAs cap layer, 8 ... p electrode, 9 ...
... n electrode, 10 ... GRIN-SCH active layer, 11 ... Zn diffusion region, 12 ... p-GaAlAs embedded cladding layer, 13 ... n-
GaAlAs embedded cladding layer, 14 ... Semi-insulating GaAs substrate,
15 …… n ⁺ −GaAs layer.

Claims (1)

(57) [Claims] A first conductivity type clad layer, a well layer having a thickness equal to or less than the Dow-Ploy wavelength of electrons, and a barrier layer having a forbidden band width larger than that of the well layer are alternately laminated on the first conductivity type clad layer. A well of the multiple quantum well active layer, which has a formed multiple quantum well active layer and a clad layer of a second conductivity type opposite to the first conductivity type formed on the multiple quantum well active layer. At least one of the layer and the barrier layer has a concentration of 4 × 10 ¹⁸ cm ^-3 or more and less than 1 × 10 ¹⁹ cm ^-3 or a p-type impurity or 2 × 10 ¹⁸ cm ^-3 or more and less than 1 × 10 ¹⁹ cm ^-3. 2. A semiconductor laser device having the n-type impurity introduced therein. 2. The semiconductor laser device according to claim 1, wherein the well layer has a thickness of 3 nm to 10 nm. 3. The semiconductor laser device according to claim 1 or 2, wherein the p-type impurity is Mg or Be. 4. The semiconductor laser device according to claim 1, wherein the n-type impurity is Se or Si.