JPH058568B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a GaInAsP layer having a diffraction grating.
This invention relates to a vapor phase growth method for epitaxially growing an InP layer.

[Technical background of the invention and its problems]

It is an optical device with a diffraction grating.
DFB (distributed feedback) lasers use feedback by diffraction of light on a diffraction grating, whereas conventional Fabry-Perot lasers use both cleavage planes as mirrors. Therefore, the oscillation wavelength is determined by the period of the diffraction grating and the effective refractive index, and the rate of change in the oscillation wavelength due to temperature fluctuation is as small as 1 [Å/degree] or less.
Therefore, DFB lasers have better controllability and stability than Fabry-Perot lasers, and are attracting attention as the mainstream of future semiconductor lasers, especially semiconductor lasers that require single mode oscillation.

A DFB laser includes a diffraction grating with a period corresponding to the oscillation wavelength, and it is necessary to grow the crystal while preserving this diffraction grating on the diffraction grating. As an example, to explain a heterojunction DFB laser of GaInAsP mixed crystal and InP crystal, the composition is 1.3 [μm]
In the case of a band laser, the period of the diffraction grating is about 2000 [Å] when first-order diffracted light is used.

DFB laser structures include one in which a diffraction grating 71a is provided on an InP substrate 71 as shown in FIG. 7a, and one in which a diffraction grating 75a is provided on a GaInAsP layer 75 as shown in FIG. 7b. A manufacturing method for the structure shown in FIG. 7b will be explained. First, an N-InP cladding layer 7 is formed on an N-InP substrate 71.
2. Sequentially grow an N-GaInAsP active layer 73, a P-InP buffer layer 74, and a P-GaInAsP light guide layer 75, and form a diffraction grating 75 on the surface of the light guide layer 75.
Provide a. Next, a P-InP cladding layer 78 and a P-GaInAsP contact layer 79 are grown.
After that, the DFB laser is completed by attaching electrodes.

However, this type of method has the following problems. That is, when growing the InP cladding layer 78 on the P-GaInAsP optical guide layer 75 having the diffraction grating 75a, the diffraction grating 75a may be deformed or disappear. The deformation or disappearance of the diffraction grating becomes a major factor that causes characteristic deterioration such as a decrease in the laser oscillation threshold.

As a method to prevent the disappearance of the diffraction grating provided on the InP layer, in the case of LPE (liquid phase epitaxial) method,
This is done by covering a wafer with a diffraction grating with a GaAs cover or a GaAsP cover. However, when vapor phase growth is performed in a system with two or more growth chambers, the wafer must be moved between the growth chambers, so using the above-mentioned cover complicates the system and also requires the use of a diffraction grating. Since the active layer must be grown on a finely uneven surface, morphology and crystal quality are likely to deteriorate. In other ways,
In some cases, an InP layer with a diffraction grating is exposed on the surface and held in a gas flow containing phosphine in a hydrogen atmosphere. According to this method,
In the case of a second-order diffraction grating (period: 4500 Å, depth 1500 Å), heat treatment at 620 [℃] for 90 minutes is said to require a minimum of 10 ^-3 atm of phosphine to preserve the diffraction grating. . However, the condition of the oxide film on the surface is different due to the pretreatment of the diffraction grating, etc., and there is a drawback that reproducibility is poor.

In addition, as a method to prevent the disappearance of the diffraction grating provided on the GaInAsP layer, the same method as for the InP layer can be used.
There are methods that use a GaAs cover, GaAsP cover, or InP cover, and methods that apply phosphorus pressure or arsenic pressure in a hydrogen-based phosphine or arsine atmosphere.
However, compared to a diffraction grating made of an InP layer, it is extremely difficult to suppress thermal deformation of a diffraction grating made of a GaInAsP layer.

[Purpose of the invention]

The object of the present invention is to provide GaInAsP with diffraction gratings.
It is an object of the present invention to provide a vapor phase growth method that can epitaxially grow an InP layer on the layer without causing deformation of the diffraction grating, and is suitable for manufacturing a DFB laser.

[Summary of the invention]

The gist of the present invention is to grow an InP layer thin enough to embed the diffraction grating at a temperature that does not cause thermal deformation of the diffraction grating, and then raise the temperature and grow the InP layer again under well-controlled conditions. There is a particular thing.

That is, the present invention provides a vapor phase growth method for growing an InP layer on a GaInAsP layer having a diffraction grating, in which the wafer having the GaInAsP layer as the top layer is heated to the growth temperature. is held in a hydrogen atmosphere at a temperature of 400 to 500 [°C] at which the diffraction grating does not undergo thermal deformation and an InP layer is deposited on the diffraction grating, and then at least the diffraction grating is held at the same temperature as the wafer holding temperature. A first InP layer having a thickness necessary for filling the wafer is epitaxially grown, and then the wafer is heated to a temperature higher than the growth temperature, and a second InP layer is epitaxially grown at that temperature. This is the method.

〔Effect of the invention〕

According to the present invention, the diffraction grating on the GaInAsP layer
Since InP is grown at a low temperature until it is filled with the InP layer, the diffraction grating does not deform during this time. Once the diffraction grating is embedded with InP, it is grown under normal growth conditions (a temperature higher than the above growth temperature, conditions that yield high quality crystals and a fast growth rate).
Even if the InP layer is grown in this way, the diffraction grating will not be deformed. Therefore, without thermally deforming the diffraction grating on the GaInAsP layer, it is possible to
It becomes possible to grow an InP layer in a vapor phase. Therefore, when applied to the manufacture of a DFB laser, it is possible to improve device characteristics such as lowering the oscillation threshold.

[Embodiments of the invention]

Hereinafter, details of the present invention will be explained with reference to illustrated embodiments.

6a to 6c show the schematic structure of a hydride vapor phase growth apparatus used in the method of one embodiment of the present invention, in which FIG. 6a is a longitudinal cross-sectional view, FIG. 6b is a cross-sectional view, and FIG. Figure c is an enlarged cross-sectional view taken along arrow AA in figure a. In the figure, 30 is a reaction tube made of quartz glass or the like, and a reaction chamber inside this reaction tube 30 is defined by a partition plate 31.
It is symmetrically partitioned vertically. An exhaust port 34 is provided near the right end of the reaction tube 30. Further, 35 is a sample to be subjected to vapor phase growth, 36 is an operating rod for smoothly moving the sample 35 between the upper chamber 32 and the lower chamber 33, and 37 and 38 are resistance heating furnaces. Note that the region heated by the heating furnace 37 (the region in which the raw metal etc. are accommodated) is defined as a high temperature region, and the region heated by the heating furnace 38 (in the region in which the sample is accommodated) is defined as a low temperature region.

Gallium metal 41 and indium metal 42, which are group raw materials, are housed on the left side (gas introduction side) of the upper chamber 32. Carrier gas introduction pipes 43 and 44 for flowing hydrogen chloride gas into the metals 41 and 42 are connected to the left end of the reaction tube 30, respectively. Further, the upper chamber 32 is connected to an introduction pipe 45 for flowing phosphine and arsine, which are group V raw materials, and hydrogen sulfide as an N-type doping gas, and an introduction pipe 46 for flowing diethylzinc as a P-type doping gas. . Also, in the lower chamber 33, corresponding to the above-mentioned 41 to 46, gallium metal 51, indium metal 52, hydrogen chloride gas introduction pipes 53 and 54, and introduction pipes for flowing phosphine, arsine, and hydrogen sulfide as an N-type doping gas. 55, an introduction pipe 56 through which diethylzinc, a P-type doping gas, flows is provided.

When growing an InP layer using this equipment, indium metal 42,52 diluted with hydrogen is used at 9%.
of hydrogen chloride is passed through the V group raw material introduction pipe 45,5.
5 to 10% phosphine gas diluted with hydrogen. Then, an InP layer is grown by a chemical reaction between the indium chloride that has reacted with the indium metals 42 and 52 and the thermally decomposed phosphine.
When growing a GaInAsP layer, as in the case of growing an InP layer, hydrogen chloride gas is supplied to the gallium metals 41 and 51 and the indium metals 42 and 52 independently.
10 [%] phosphine gas diluted with hydrogen and arsine gas are flowed from No.55. InP layer growth and
A hydrogen carrier gas was used for both growth of the GaInAsP layer, with a total flow rate of 1.5 [/min].

Next, a method for manufacturing a DFB laser using the above apparatus will be explained.

FIGS. 1a to 1c are cross-sectional views showing the DFB laser manufacturing process. As the substrate 11, a tin-doped
An InP substrate with an angle of 3° in the <011> direction from the 100 plane was used. Carrier concentration is 1-3x
10 ¹⁸ [cm ^-3 ]. This InP substrate 11 is mounted on a substrate holder at the tip of the operating rod 36 in the reaction tube 30, and the temperature of the furnace is set to 820 [°C] in the high temperature range and 670 °C in the low temperature range.
Raise the temperature to [℃]. In order to suppress evaporation of phosphorus from the InP substrate 11, 15 [cc/min] of phosphine gas is flowed as soon as the temperature of the low temperature region where the substrate 11 is placed exceeds 350 [°C]. Then, as shown in FIG. 1a, an N-InP cladding layer 1 is formed on the substrate 11.
2 (carrier concentration ~7×10 ¹⁷ cm ^-3 , film thickness 3 ~
5 μm), undoped GaInAsP active layer 13 (carrier concentration ~1×10 ¹⁶ cm ^-3 , composition 1.3 μm, film thickness
0.1 μm), P-InP buffer layer 14 (carrier concentration ~7×10 ¹⁷ cm ^-3 , film thickness 0.05 μm), P-GaInAsP light guide layer 15 (carrier concentration ~7×10 ¹⁷ cm ^-3 , composition 1.1 ~ 1.2 μm, film thickness 0.2 μm) and P-InP protective layer 16 (carrier concentration 7×10 ¹⁷ cm ^-3 , film thickness ~0.5 μm)
grow sequentially. Here, the uppermost P-InP protective layer 16 is grown to protect the surface of the GaInAsP optical guide layer 15.

Next, the sample is taken out from the reaction tube 30 and P
- Remove the InP protective layer 16 by wet etching with hydrochloric acid to expose the clean surface of the GaInAsP light guide layer 15. Thereafter, as shown in FIG.
The diffraction grating 15a with a depth of 300 to 600 [Å] is
Formed by beam interference exposure method.

Next, the sample is put into the reaction tube 30 again, and is provided with a diffraction grating 15a as shown in FIG.
P-InP layer 17,1 on GaInAsP light guide layer 15
8 (carrier concentration 2×10 ¹⁸ cm ^-3 , film thickness 1.5 μm) was grown by the method described below, and then P was deposited on top of it.
-GaInAsP contact layer 19 (carrier concentration 5
×10 ¹⁸ cm ^-3 , composition 1.3 μm, film thickness 0.5 μm). After this, electrodes are formed on the lower surface of the N-InP substrate 11 and the upper surface of the contact layer 19, thereby completing the DFB laser.

Here, the InP crystal growth on the diffraction grating 15a will be explained in detail.

The present inventors have developed a GaInAsP layer (composition 1.15~
A diffraction grating (period: 2000 Å, depth: 300 to 600 Å) placed on a 1.18 µm surface was heat-treated in a hydrogen atmosphere without applying phosphorous or arsenic pressure, and the relationship between the heat treatment temperature and the degree of deformation of the diffraction grating was investigated. Examined. The total flow rate of hydrogen carrier gas was fixed at 1.5 [/min], and the heat treatment time was selected between 0 and 100 minutes. The depth of the diffraction grating before heat treatment and the depth of the diffraction grating after heat treatment are respectively
d ₀ , d ₁ and the degree of thermal deformation Δ of the diffraction grating is expressed in % as Δ = (d ₁ /d ₀ ) × 100. When the heat treatment time is changed, the heat treatment temperature T [℃] and the diffraction grating The relationship between the degree of preservation Δ[%] and the degree of preservation is shown in FIG. Diffraction gratings can be completely preserved at temperatures below 500 [°C] with heat treatment of 40 minutes, which is the minimum time required to grow crystals on the diffraction gratings. The surface of the diffraction grating is hardly deformed at heat treatment temperatures below 550 [°C], but when the temperature exceeds 550 [°C], the surface becomes rough due to the evaporation of arsenic and phosphorus. It was also found that at a heat treatment temperature of 600 [°C], the surface became noticeably rough in 20 minutes or more, and at a heat treatment temperature of 650 [°C] or higher, the surface became rough already in 10 minutes. different compositions
Period formed on GaInAsP layer (1.15-1.3μm)
Figure 4 a and b show the degree of deformation of the 2000 [Å] and 4000 [Å] diffraction gratings at 500 [℃] and 550 [℃] with the heat treatment time fixed at 40 minutes. . a is the period of the diffraction grating 2000 [Å], b is the period
The case of 4000 [Å] is shown. From this figure, it can be seen that the diffraction grating is easier to preserve in GaInAsP, which has a longer wavelength composition. Furthermore, it can be seen that the larger the period of the diffraction grating, the easier it is to store.

In the method of this embodiment, InP crystal growth on a diffraction grating is performed as follows using the above experimental results.

The sample shown in FIG. 1b was placed in the reaction tube 30, and with only hydrogen carrier gas flowing, the temperature of the furnace was raised to a high temperature range of 720 [°C] and a low temperature range of 500 [°C]. About 30 years ago the profile became steady.
The chamber holding the sample after minutes (e.g. upper chamber 32)
An InP growth gas is flowed into a growth chamber different from the above (for example, the lower chamber 33). That is, hydrogen chloride (9% diluted with hydrogen) is added to the indium metal 52 at 17 [cc/min], and phosphine (10% diluted with hydrogen) is added to the V group raw material introduction pipe 55.
15 [cc/min], diethyl zinc (200ppm diluted with hydrogen)
Introduce 10 [cc/min] of The hydrogen chloride gas sufficiently reacts with the indium metal 52, and the growth chamber (lower chamber 3
After about 10 minutes when the gas composition in 3) becomes steady, remove the sample.
At the same time, phosphine was transferred to the growth chamber (lower chamber 33) where the InP growth gas was flowing, and phosphine was added to the chamber (upper chamber 32) where the sample had been waiting in a hydrogen atmosphere until then.
[cc/min] Sink, add a large amount and immediately drain the chamber (upper chamber 3).
2) Make sure that the inside is filled with phosphine. In this state, as shown in FIG.
grow to a certain extent. As a result, the GaInAsP layer 15
The upper diffraction grating 15a will be buried with an InP thin film layer 17. Note that about 2 minutes is sufficient for the growth time of the InP thin film layer 17 under the above epitaxial conditions.

Next, the sample was moved to a chamber (upper chamber 32) in which phosphine had been flowed in advance, and the phosphine flow rate was adjusted.
The temperature was reduced to 15 [cc/min] and the temperature of the furnace was further increased from the high temperature range of 820 [℃] to the low temperature range of 670 [℃]. After about 20 minutes when the temperature stabilized, the sample was placed in the chamber where it had been waiting in the phosphine. Growth chamber (lower chamber 33) different from (upper chamber 32)
Flow InP growth gas through. That is, hydrogen chloride (9% hydrogen dilution) was added to indium metal 52 at 25 [cc/min].
23 [cc/min] of phosphine (10% diluted with hydrogen) and diethyl zinc (200 ppm) were introduced from the V group raw material introduction pipe 55.
Flow 30 [cc/min] of diluted hydrogen. After about 10 minutes when the composition of the growth gas becomes steady, the sample is moved to the growth chamber (lower chamber 33) where the InP growth gas is flowing.
A P-InP layer (second InP layer) 18 is grown as shown in FIG. Under this growth condition, the growth rate of InP is
It is 0.12 [μm/min].

Here, while the P-InP layer is grown in two steps, the first InP layer 17 is grown to completely cover the diffraction grating 15a and prevent thermal deformation, and its film thickness is It is sufficient that the area is filled. However, the growth temperature is the diffraction grating 15
It is necessary to carry out the process at a level that does not cause thermal deformation (a), but as the temperature decreases, the growth rate of the InP layer decreases and the surface becomes rough. The growth of the InP layer uses the reaction between indium metal and hydrogen chloride.
In order for this reaction to occur, the high temperature area of the furnace is
Since it is difficult to lower the temperature below 700 [℃], InP
Reducing the layer growth temperature means increasing the temperature difference between the hot and cold regions of the furnace.
Although the surface of the InP layer grown below 520 [°C] is rough, the ~0.1 [μm] InP thin film 1 is formed on the diffraction grating 15a.
After growing InP layer 18 at 670 [℃]
[μm] When grown, the surface morphology is good and there is no problem with crystallinity. The growth temperature of the first InP layer 17 can be 400 [°C] or higher, and the growth rate of the InP layer when the growth gas, that is, the flow rate of hydrogen chloride and phosphine flowing through the indium metal, is each 1000 [ppm]. Figure 5 shows the growth temperature. Second
It is important that the InP layer 18 grown for the second time has good crystallinity and is grown under conditions that provide good film thickness controllability and reproducibility.

In this way, if the diffraction grating 15a provided on the GaInAsP optical guide layer 15 is maintained at 500 [°C] in a hydrogen atmosphere and the growth temperature is divided into two stages, the diffraction grating interface can be kept clean. Can be embedded. On the other hand, if 5000 [ppm] each of phosphine and arsine are poured into the GaInAsP layer, held for 40 minutes, and then the InP layer is grown at 500 to 600 [℃], the diffraction grating is preserved with good reproducibility due to thermal deformation. Not only is it not visible, but it also appears near the diffraction grating interface.
Regions having different compositions are formed in both the InP cladding layer and the GaInAsP light guide layer. When this region is formed, the difference in refractive index in the lateral direction becomes small, which becomes a hindrance to lowering the laser oscillation threshold, and the threshold current density in the oxide film stripe structure is as high as 6 [KA/cm ² ], preventing CW oscillation at room temperature. . However, when we prototyped a DFB laser from a wafer grown using the method of this example, we found that it had the same oxide film stripe structure and was CW at room temperature.
It oscillated and a good threshold current density of 2 to 3 [KA/cm ² ] was obtained.

Thus, according to the method of this embodiment, the diffraction grating 15
The InP cladding layers 17 and 18 can be well formed on the GaInAsP optical guide layer 15 having a
Moreover, thermal deformation of the diffraction grating 15a can be prevented. Therefore, the oscillation threshold of the DFB laser can be lowered, and the device characteristics can be significantly improved. Furthermore, since the reproducibility of the diffraction grating is good, there are also advantages such as being able to reduce variations in characteristics between chips and between wafers.

Note that the present invention is not limited to the method of the embodiment described above. For example, the growth temperature for growing the first InP layer is not limited to 500 [°C], but may be appropriately selected within the range of 400 to 500 [°C]. Similarly, the growth temperature when growing the second InP layer can also be changed as appropriate. Furthermore, the structure of the vapor phase growth apparatus is not limited to that shown in FIG. 6, and can be modified as appropriate according to specifications. Furthermore, it can be applied not only to hydride vapor phase epitaxy of an InP layer on a diffraction grating provided on a GaInAsP layer, but also to general vapor phase epitaxy, such as metal-organic pyrolysis vapor phase epitaxy, chloride vapor phase epitaxy, etc. be. others,
Various modifications can be made without departing from the spirit of the invention.

[Brief explanation of drawings]

Figures 1 to 6 are for explaining one embodiment of the method of the present invention, and Figures 1 a to c are
Cross-sectional view showing the manufacturing process of DFB laser, Figure 2a,
b is a cross-sectional view showing the manufacturing process of the first and second InP layers, Fig. 3 is a characteristic diagram showing the relationship between heat treatment temperature and degree of preservation of the diffraction grating, and Figs. 4 a and b are
A characteristic diagram showing the relationship between the composition of GaInAsP and the degree of preservation of the diffraction grating. Figure 5 is a characteristic diagram showing the temperature dependence of the growth rate of the InP layer. Figures 6 a to c are the hydrides used in the method of this example. A cross-sectional view showing the schematic configuration of the vapor phase growth apparatus, Figures 7a and b are conventional DFBs.
FIG. 2 is a cross-sectional view showing the structure of a laser. DESCRIPTION OF SYMBOLS 11... N-InP substrate, 12... N-InP cladding layer, 13... GaInAsP active layer, 14... P-InP buffer layer, 15... P-GaInAsP light guide layer, 15
a...Diffraction grating, 16...P-InP protective layer, 17...P
-InP thin film layer (first InP layer), 18...P-InP layer (second InP layer), 19...P-GaInAsP contact layer, 30... reaction tube, 32, 33... reaction chamber, 35...
Sample, 36... operating rod, 37, 38... resistance heating furnace,
41,51...Gallium metal, 42,52...Indium metal, 43,44,45,46,53,
54, 55, 56...Gas introduction pipe.

Claims (1)

[Claims] 1. In a method for vapor phase growing an InP layer on a GaInAsP layer having a diffraction grating, the wafer having the GaInAsP layer as the uppermost layer is subjected to the diffraction process until the temperature of the wafer is raised to the growth temperature. 400-500 so that the grating does not undergo thermal deformation and an InP layer can be deposited on the diffraction grating
holding in a hydrogen atmosphere at a temperature of [°C];
Next, the first InP film is deposited at the same temperature as the wafer holding temperature to a thickness necessary to at least fill the diffraction grating.
A vapor phase growth method comprising the steps of: epitaxially growing a layer; and then raising the temperature of the wafer to a temperature higher than the growth temperature and epitaxially growing a second InP layer at that temperature. Method. 2 The growth temperature of the second InP layer is set to 600 to 700.
The vapor phase growth method according to claim 1, characterized in that the temperature is set at [°C]. 3. The vapor phase growth method according to claim 1, wherein the diffraction grating is formed in a light guide layer of a DFB laser. 4. The vapor phase growth method according to claim 1, wherein a hydride vapor phase growth apparatus is used as a means for growing the first and second InP layers.