JP2555282B2 - Semiconductor laser device and method of manufacturing the same - Google Patents

Description

The present invention relates to a semiconductor laser having a current constriction effect and an optical waveguide effect, and more particularly to a semiconductor laser device using an InGaAlP-based semiconductor material and the same. It relates to a manufacturing method.

(Prior Art) In recent years, as a semiconductor manufacturing method for a semiconductor laser used as a light source for optical information, a chemical vapor deposition method using an organic metal (hereinafter referred to as a MOCVD method) is used instead of the liquid phase growth method which has been conventionally used. (Abbreviated) is being developed. The MOCVD method is excellent in uniformity over controllability of composition and film thickness, and is considered to occupy an important position in future laser manufacturing technology. Recently, visible semiconductor lasers using InGaAlP formed on a GaAs substrate by MOCVD have been receiving attention.

FIG. 4 is a cross-sectional view showing a schematic structure of a semiconductor laser (Japanese Patent Application No. 61-42933) previously proposed by the present inventors, which is self-aligned by utilizing selective growth by the MOCVD method. It was created by forming a current confinement structure and an optical waveguide structure. In the figure, 41 is an n-GaAs substrate, 42 is an n-GaAs buffer layer, 43 is an n-InGaAlP clad layer, and 44 is In _0.5 Ga.
_0.5 P active layer, 45 p-In _0.5 Ga _0.25 Al _0.25 P clad layer, 46 p-GaAs contact layer, 47 n-GaAs current blocking layer, and 48, 49 electrode layers.

Here, the Al composition of the In _1-xy Ga _x Al _y P clad layer is selected to have y <0.35 having a direct transition band gap, which is due to the following circumstances.

FIG. 5 shows the relationship between the band gap of In _1-xy Ga _x Al _y P lattice-matched to the GaAs substrate and the Al composition y. The bang gap increases linearly with the Al composition in the direct transition region y <0.35, but is almost constant in the indirect transition region y> 0.35. On the other hand, the mobility of holes, which dominates the series resistance of the laser device, decreases linearly as the Al composition y increases, as shown in FIG. It is desirable that the clad layer of the laser device has a large bandgap value and a high hole mobility. Therefore, when this mixed crystal system is used as a clad layer of the laser, it has a direct transition type bandgap. The composition was usually chosen.

This element has excellent design freedom and reproducibility, but the performance of the elements created so far is that the resonator length is 250μ.
The threshold value is about 90 mA, where m, stripe width 5 μm, and active layer thickness 0.1 μm. This value is significantly higher than the threshold current of 25 mA predicted from the threshold current density of about ² KA / cm ² estimated by the wide stripe type laser produced by the same double hetero wafer, and therefore It is suspected that there is a large leakage current. As a result of detailed investigation of the cause, p-type In having a stripe-shaped convex portion
_A large number of defects exist at the interface between the _0.5 Ga _0.25 Al _0.25 P clad layer 45 and the n-type GaAs current blocking layer 47 regrown by the MOCVD method, and the leak current at the pn junction due to these defects. Was extremely large in both the forward and reverse directions, and it was found that the current blocking function was insufficient.

The leakage current is particularly large on the side surface of the stripe-shaped convex portion, and this large leakage current causes an abnormal lateral gain distribution, which makes the mode unstable and causes noise due to fluctuations in the optical output. Was increasing.

(Problems to be Solved by the Invention) As described above, in the conventional device, a large amount of current leaks to areas other than the stripe portion, leading to an increase in threshold value and instability of mode output.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a low noise visible semiconductor laser device having stable mode characteristics made of an InGaAlP semiconductor.

Another object of the present invention is to provide a method of manufacturing a semiconductor laser device which achieves the above object.

[Structure of the Invention] (Means for Solving Problems) The gist of the present invention is to increase the Al composition of the p-type InGaAlP-based cladding layer and increase the carrier concentration of the n-type GaAlAs-based current blocking layer. The carrier confinement is realized not by using a P-n junction but by using a barrier due to band discontinuity at the heterojunction interface between the two.

The present inventors measured the relationship between the capacitance and voltage of the Schottky junction formed on In _1-xy Ga _x Al _y P on GaAs with respect to samples of various Al compositions to obtain the values of GaAs and InGaAlP. We investigated the Al composition dependence of the energy discontinuity in the electron band. As a result, the valence band of InGaAlP is located on the lower energy side than the valence body of GaAs, and the difference is that as the Al content increases, it goes from the direct transition region to the indirect transition region. As shown, from about 0.3 eV between GaAs and InGaP, Ga
It was found that As and InAlp increase linearly up to about 0.7 eV.

Therefore, by using InGaAlP having a composition having an indirect transition type band gap that has not been conventionally used for the above-mentioned reason as a cladding layer, it is possible to form a heterojunction having an unprecedented large band discontinuity. A new device structure used for current confinement is conceivable. Further, it has been proved that if the Al composition is set to y ≧ 0.4, a barrier of 0.6 eV or more is obtained, and the leak current can be sufficiently suppressed regardless of the defects at the interface of the pn junction.

The present invention has been made paying attention to such a point.
-Type GaAs substrate, an n-type clad layer, an active layer, and a p-type clad layer having stripe-shaped protrusions, and an InGaAlP-based double heterojunction structure formed on the GaAs substrate, and this double heterojunction structure And a GaAlAs-based current blocking layer formed on at least a part of the convex portion of the p-type cladding layer on the above-mentioned portion, and forming the p-type cladding layer in a semiconductor laser device for performing optical waveguide and current constriction. In
The Al composition of _1-xy Ga _x Al _y P is set to y ≧ 0.4.

Further, in the present invention, in the method of manufacturing a semiconductor laser device having the above structure, particularly when the GaAlAs-based n-type current blocking layer is grown and formed by the MOCVD method, the substrate is previously heated in a source gas atmosphere of phosphorus and isodium. Is a method for cleaning the substrate surface.

(Operation) With the above structure according to the present invention, discontinuity in the valence band and p- between the n-type current blocking layer and the p-type cladding layer are caused.
An extremely high barrier is formed in the valence band due to the built-in potential of the n-junction, and the leakage of holes that are canria can be made extremely small. For this reason,
When it is difficult to form a good pn junction and sufficient current blocking capability cannot be expected with a normal heterojunction, as in the case where an n-GaAlAs current confinement layer is formed by regrowth on p-InGaAlP. Also in the above, good current confinement is realized by the barrier formed in the valence band of p-InGaAlP. Therefore, the threshold value is reduced and the mode is stabilized. In particular, p-In _1-xy Ga _x Al _y P Al composition y of the cladding layer
When is larger than 0.4, the laser operation becomes intermittent, the oscillation spectrum width becomes wide, and the effect of suppressing the instability of the operation due to the returning light is obtained. This effect is
This is extremely advantageous when applied to a light source such as a video disc player, which requires low noise operation.

Further, according to the manufacturing method of the present invention, it is possible to clean the surface of InGaAlP having a high Al composition, which is usually difficult, and to re-grow other crystals thereon. That is, oxygen is strongly attached to the surface of InGaAlP once exposed to the atmosphere, and good crystal growth cannot be expected on this surface as it is. This difficulty becomes more serious as the Al content increases. As a means for surface cleaning, thermal cleaning is generally performed by heating to 750 to 800 ° C. in a group V element atmosphere that has been used in GaAlAs. However, the In
Since GaAlP has a higher evaporation rate of In than other group III elements, the composition and lattice constant near the surface change during heating, and the crystallinity of the regrown layer is significantly degraded. However, the heating in the atmosphere of In and P according to the present invention effectively suppresses the evaporation of In as well as the group V element, so that the above difficulties can be solved and an InGaAlP clad layer having a high Al composition can be formed. It is possible to form a GaAlAs-based current blocking layer having good crystallinity.

(Examples) The details of the present invention will be described below with reference to illustrated examples.

FIG. 1 is a sectional view showing a schematic structure of a semiconductor laser according to one embodiment of the present invention. In the figure, reference numeral 11 denotes an n-GaAs substrate, on which an n-GaAs buffer layer 12 and n-In
A GaP buffer layer 13 is formed. The n-InAlP cladding layer 14, the InGaP active layer 15 and the p-InG are formed on the buffer layer 13.
A double heterojunction structure portion composed of aAlP-based clad layers 16, 17 and 18 is formed. Here, the cladding layer 17 has a low Al composition and acts as an etching stop layer described later. Further, the clad layer 18 is processed in a stripe shape, whereby a stripe-shaped rib is formed in the p-clad layer.

A p-InGaP contact layer 19 and a p-GaAs contact layer 20 are formed on the clad layer 18. On the side surface of the double heterojunction structure and the contact layer 20, n-GaAs is formed.
The current blocking layer 21 is formed. A p-GaAs contact layer 22 is formed on the contact layer 20 and the current blocking layer 21. Then, a metal electrode 23 is attached to the upper surface of the contact layer 22, and a metal electrode 24 is attached to the lower surface of the substrate 11.

In this structure, the current confinement is performed by the contact layer 20 and the current blocking layer 21, and the optical waveguide is performed by the cladding layer 18 formed in a stripe-shaped mesa. The buffer layer
Reference numeral 13 is for improving the quality of the InGaAlP-based crystal formed on GaAs. The contact layer (intermediate contact layer) 19
Is intended to reduce the electric resistance between the clad layer 18 and the contact 20, and may have a band gap larger than that of the contact layer 20 and smaller than that of the clad layer 18. Further, the band gap of the intermediate contact layer 19 is set to be the same as those of the portions contacting the cladding layer 18 and the contact layer 20, and the cladding layer
You may make it change gradually from 18 to the contact layer 20.

Next, a method of manufacturing the semiconductor laser having the above configuration will be described.

2 (a) to 2 (f) are cross-sectional views showing the steps of manufacturing the laser of the embodiment. First, as a raw material, a methyl group III organic metal (trimethylindium, trimethylgallium, trimethylaluminum) and a group V hydride (arsine,
MOCV under atmospheric pressure using phosphine) and
According to the D method, n of the plane orientation (100) is determined as shown in FIG.
-0.5μ thickness on GaAs substrate 11 (Si-doped, 3 × 10 ¹⁸ cm ^-3 ).
m n-GaAs first buffer layer 12 (Se-doped, 3 × 10 ¹⁸ c
m ⁻³ ), 0.5 μm thick n-InGaP second buffer layer 13 (Se
Doped, 3 × 10 ¹⁸ cm ^-3 ), 1.0 μm thick n-In _0.5 Al _0.5
P first clad layer 14 (Se-doped, 1 × 10 ¹⁸ cm ^-3 ), thickness 0.
1 μm In _0.5 Ga _0.5 P active layer 15, 0.1 μm thick p-In _0.5
Al _0.5 P second cladding layer 16 (Mg-doped, 1 × 10 ¹⁸ cm ^-3 ),
0.01 μm thick p-In acting as an etch stop layer
_0.5 Ga _0.5 P Third cladding layer 17 (Mg-doped, 2 × 10 ¹⁸ c
m ⁻³ ), 1.0 μm thick p-In _0.5 Al _0.5 P fourth cladding layer
18 (Mg-doped, 2 × 10 ¹⁸ cm ⁻³ ), 0.01 μm thick p-In _0.5 Ga _0.5 P first contact layer 19 as an intermediate contact layer
(Mg-doped, 2 × 10 ¹⁸ cm ^-3 ) and 0.5 μm thick p-GaAs
A second contact layer 20 (Mg-doped, 2 × 10 ¹⁸ cm ⁻³ ) was sequentially grown to form a double hetero wafer. Then, the SiO ₂ film 26 was formed on the second contact layer 20 in the form of stripes having a width of 5 μm and a thickness of 0.1 μm by thermal decomposition of silane gas and photo-etching.

Then, as shown in FIG. 2 (b), using the SiO ₂ film 26 as a mask, the second contact layer 20 is etched by a selective etchant of GaAs to expose the first contact layer 19 and a stripe of GaAs with a width of 3 μm. Formed mesas 27.

Then, as shown in FIG. 2C, the first contact layer 19 is processed by using an etchant containing hydrogen bromide or hydrobromic acid, using the GaAs striped mesas 27 as a mask.
And further etching the first contact layer 19 and the fourth cladding layer 18 until the third cladding layer 17 is exposed by a selective etchant for InGaAlP or InAlP,
Striped mesas 28 were formed. Thereafter, if necessary, the third clad layer 17 as an etching stopper layer is treated with an etchant containing hydrogen bromide or hydrobromic acid.
Is removed.

By processing this wafer with a selective etchant of GaAs, the second contact layer 20 is etched to reduce its width, and a stripe-shaped mesa having the shape shown in FIG.
29 formed. The selective etchant of GaAs is 28%
Ammonia water, 35% hydrogen peroxide water and water were mixed at a ratio of 1: 30: 9 and used at 20 ° C. Also, In
The selective etchant for GaAlP or InAlP was sulfuric acid or phosphoric acid, which was used at a temperature of 20 to 130 ° C.

Then, by an MOCVD method under reduced pressure using trimethylgallium and arsine as raw materials, an n-GaAs current blocking layer 21 (Se-doped, 5 × 10 ¹⁸ cm ⁻³ ) as shown in FIG.
Was grown to a thickness of 0.5 μm. At this time, for the growth, the temperature was raised to 830 ° C. while introducing the diluted phosphine gas, and the InAlP surface was cleaned by waiting for about 5 minutes. After that, the phosphine gas flow was switched to the arsine gas flow, and after waiting for about 1 second, trityl gallium organometallic gas was introduced. As a result, no growth of GaAs was observed on the SiO ₂ film 26, and a wafer having a sectional shape shown in FIG. 2 (e) was obtained.

Then, after removing the SiO ₂ film 26, as shown in FIG. 2 (f), the p-GaAs third contact layer 2 is formed on the entire surface by MOCVD.
2 (Mg-doped, 5 × 10 ¹⁸ cm ⁻³ ) was grown to a thickness of 3 μm. Then, the third contact layer 22 is formed by a normal electrode attaching process.
By depositing the Au / Zn electrode 23 on the upper surface and the Au / Ge electrode on the lower surface of the substrate 11, a laser wafer having the structure shown in FIG. 1 was obtained.

The wafer thus obtained is cleaved and the resonator length is set to 250
When we made a laser element of μm, the threshold current was 40m.
A, good performance with differential quantum efficiency of 20% per side was obtained.
The light output increased linearly to 10 mW or more according to the drive current, and showed good current-light output characteristics without kink. Further, it was found that both the far-field image and the near-field image were unimodal, and that good mode control was performed. Furthermore, the oscillation spectrum width W is significantly wider than that of a normal semiconductor laser,
Therefore, noise induced by reflected light from the outside is significantly reduced.

As described above, according to this example, the current confinement structure and the optical waveguide structure can be formed in a self-aligned manner, and even when InGaAlP, which has a strong demand as a light source for optical information processing, is used, a single basic A semiconductor laser having a transverse mode and a small astigmatism and stable operation even when a large amount of externally reflected light is returned can be realized with high reproducibility and high reliability. In this case, an extremely high barrier (about 0.7 eV) between the n-GaAs21 current blocking layer and the p-InAlP cladding layer 18 in the valence band due to the discontinuity of the valence band and the built-in potential of the pn junction. ) Is formed, and the leakage of holes that are carriers can be made extremely small. For this reason,
Even when it is difficult to form a good pn junction and sufficient current blocking ability cannot be expected with a normal heterojunction, good current confinement is realized by the barrier formed in the valence band of p-InAlP. It Therefore, the threshold value is reduced and the mode is stabilized.

In addition, according to the manufacturing method of the present embodiment, InAl
It is possible to clean the surface of P and regrow the GaAs crystal on it. That is, the InAlP clad layer 18 once exposed to the atmosphere
Oxygen is strongly adhered to the surface of, and it is not possible to expect good crystal growth of the GaAs current blocking layer 21 on this surface as it is. However, in the heating in the atmosphere of In and P according to the present embodiment, evaporation of In is effectively suppressed similarly to the group V element, so that the above-mentioned difficulties can be solved and the InAlP cladding layer 18 is also good. The GaAs current blocking layer 21 having excellent crystallinity can be formed.

The present invention is not limited to the above embodiment. In the embodiment, InAlP is used as the p-type cladding layer and GaAs is used as the n-type current blocking layer. However, In _1-xy Ga _x Al _y P (0.35 ≦ y) is used as the p-type cladding layer and Ga is used as the n-type current blocking layer.
Even when AlAs was used, a similar threshold current was obtained. In addition, the low noise characteristic against the reflected light from the outside is In
_{It was obtained when 1-xy} Ga _x Al _y P (y ≧ 0.4) was used for the p-type cladding layer.

In the embodiment, the p-type fourth clad layer 18 is etched to form stripe ribs, and then the p-type second contact layer 20 is re-etched, but this re-etching step is not always necessary. Further, the second to fourth cladding layers 16 to 18 made of p-InGaAlP system can be formed as a single layer. Furthermore, the p-type first contact layer (intermediate contact layer) 19 does not necessarily have to be used. Further, the third contact layer 22 is omitted and the second contact layer 20 is omitted.
The electrodes may be formed directly on the top.

Further, in the embodiment, the stripe direction is parallel to the [011] crystal axis, but it is needless to say that it can be applied to the case of the [011] direction. In addition, various modifications can be made without departing from the scope of the present invention.

[Advantages of the Invention] As described in detail above, according to the present invention, p-type In _1-xy Ga _x
Since the Al composition of the Al _y P-based cladding layer is set to y ≧ 0.4, the carrier confinement is not at the pn junction but at the heterojunction interface between the p-type cladding layer and the n-type GaAlAs-based current blocking layer. It can be reproduced by utilizing the barrier due to the band discontinuity. Therefore, even when it is difficult to form a good pn junction, good current confinement is performed by the barrier, and the mode stabilization and threshold of the semiconductor laser using the InGaAlP-based semiconductor material are achieved. It is possible to reduce the value.

[Brief description of drawings]

FIG. 1 is a sectional view showing an element structure of a semiconductor laser according to an embodiment of the present invention, FIGS. 2 (a) to (f) are sectional views showing a manufacturing process of the laser of the above embodiment, and FIG. 3 is an Al. FIG. 4 is a characteristic diagram showing the relationship between composition and band discontinuity in the valence band, FIG. 4 is a sectional view showing the device structure of a conventional laser, FIG. 5 is a characteristic diagram showing the relationship between Al composition and band gap, and FIG. The figure is a characteristic diagram showing the relationship between Al composition and hole mobility. 11 ... n-GaAs substrate, 12 ... n-GaAs first buffer layer,
13 ... n-InGaP second buffer layer, 14 ... n-InAlP first buffer layer
Cladding layer, 15 …… InGaP active layer, 16 …… p-InAlP second
Cladding layer, 17 ... p-InGaP third cladding layer (etching stop layer), 18 ... p-InAlP fourth cladding layer, 19 ...
... p-InGaP first contact layer, 20 ... P-GaAs second contact layer, 21 ... n-GaAs current blocking layer, 22 ... p-Ga
As third contact layer, 23, 24 ... current, 26 ... SiO ₂ film, 2
7-29 …… Striped mesa.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Masayuki Ishikawa 1 Komukai Toshiba Town, Saiwai-ku, Kawasaki City Toshiba Research Institute Co., Ltd. (72) Inventor Yukio Watanabe 1 Komukai Toshiba Town, Saiwai-ku Kawasaki City Toshiba Corporation (72) Inventor Motoyuki Yamamoto 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki City

Claims (7)

(57) [Claims] 1. An InGaAlP-based double heterojunction structure formed on an n-type GaAs substrate, an n-type clad layer, an active layer and a p-type clad layer having stripe-shaped protrusions, and an InGaAlP-based double heterojunction structure formed on the GaAs substrate. In a semiconductor laser device including an n-type GaAlAs-based current blocking layer formed on the double heterojunction structure portion except at least a part of the convex portion of the p-type cladding layer, for optical waveguide and current confinement. A semiconductor laser device, wherein the ratio of Al in In _1-xy Ga _x Al _y P forming the p-type cladding layer is set to y ≧ 0.4. 2. The semiconductor laser according to claim 1, wherein at least one p-type intermediate contact layer having a bandgap smaller than that of the clad layer is provided on the p-type clad layer. apparatus. 3. The semiconductor laser device according to claim 1, wherein the carrier concentration of the n-type current blocking layer is higher than the carrier concentration of the p-type cladding layer. 4. The carrier concentration of the p-type cladding layer is 7 ×
The semiconductor laser device according to claim ³ , wherein the n-type current blocking layer has a carrier concentration of more than 10 ¹⁷ cm ^{-3 and} more than 1 × 18 ¹⁸ cm ^-3 . 5. The semiconductor laser device according to claim 2, wherein a p-type contact layer is provided on the intermediate contact layer and the n-type current blocking layer. 6. The intermediate contact layer has a small bandgap near the contact layer, a large bandgap near the p-type cladding layer, and a bandgap gradually changing between them. The semiconductor laser device according to claim 5, wherein 7. An In-type cladding layer comprising an n-type cladding layer, an active layer and a p-type cladding layer on an n-type GaAs substrate.
InGaA with Al ratio of _1-xy Ga _x Al _y P set to y ≧ 0.4
lP-based double heterojunction structure is grown and formed by chemical vapor deposition using an organic metal, and then a p-type contact layer is formed on the p-type clad layer of the double heterojunction structure using an organic metal. A step of growing by vapor phase growth, a step of forming an etching mask on the contact layer, a step of selectively etching the contact layer using the mask, and a partial etching of the p-type cladding layer. Forming a stripe-shaped convex portion on the clad layer, and then using the organic metal at least on the double heterojunction structure portion and on the side surface of the contact layer after the mask is left or after the mask is removed. And a step of growing an n-type GaAlAs-based current blocking layer by vapor phase epitaxy. In growing the current blocking layer, by heating the substrate at the raw material gas atmosphere in advance phosphorus and indium, the method of manufacturing a semiconductor laser device, characterized in that for cleaning of the substrate surface.