JP2005136085A - Method for manufacturing gain-coupled dfb semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a gain-coupled DFB semiconductor laser, using simple processes. <P>SOLUTION: An active layer having multiple quantum well structure is arranged on a substrate, and further, an upper cladding layer overlaying the active layer is arranged. A plurality of portions, in the upper surface of the upper cladding layer which portions are arranged along the optical axis of the active layer, are exposed to inductively coupled plasma, and gain of the active layer is periodically modulated along the optical axis of the active layer. Gain grating is formed into the active layer by this method, so that the gain joint type DFB layer is obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a gain-coupled distributed feedback (DFB) semiconductor laser.

The gain-coupled DFB laser is superior in single-mode property than the refractive index-coupled DFB laser. Non-Patent Document 1 below discloses various gain-coupled DFB semiconductor lasers.
Arthur J. Lowery et al., “Performance Comparison of Gain-Coupled and Index-Coupled DFB Semiconductor Lasers”, IEEE JOURNAL OF QUANTUM ELECTRONICS, 1994 September, Vol. 30, No. 9, pp. 2051-2063

  An object of the present invention is to manufacture a gain coupled DFB semiconductor laser by a simple process.

  The present invention relates to a method for manufacturing a gain-coupled DFB semiconductor laser. In this method, an active layer having a multiple quantum well structure is provided on a substrate, an upper clad layer covering the active layer is provided, and an upper surface of the upper clad layer is disposed along the optical axis of the active layer. Exposing the plurality of portions to inductively coupled plasma to periodically modulate the gain of the active layer along the optical axis of the active layer.

  When the present inventors expose the upper cladding layer to the inductively coupled plasma for a very short time, the gain of the active layer located under the upper cladding layer increases, and when the upper cladding layer is exposed to the inductively coupled plasma for a longer time. The phenomenon that the gain of the active layer decreases was discovered. Therefore, if a plurality of portions periodically arranged along the optical axis of the active layer on the upper surface of the upper cladding layer are exposed to inductively coupled plasma, the gain of the active layer is periodically changed along the optical axis of the active layer. Can be modulated. By forming a gain grating in the active layer in this way, a gain coupled DFB laser can be manufactured. In this method, a gain coupled DFB laser is manufactured by a simple process without requiring a complicated process of growing a multiple quantum well structure on an uneven surface. This method does not require the active layer to be exposed because the active layer is not directly exposed to the inductively coupled plasma. Therefore, even if the active layer is made of a material that easily oxidizes, a gain-coupled DFB laser can be satisfactorily manufactured.

  The step of exposing the plurality of portions of the upper cladding layer to inductively coupled plasma includes forming a mask on the upper cladding layer having openings periodically arranged along the optical axis of the active layer, and forming the upper cladding layer on the mask. It may be exposed to inductively coupled plasma. Since the portion of the active layer covered by the mask is not affected by inductively coupled plasma, its gain does not change. On the other hand, the gain of the portion of the active layer located under the mask opening is increased or decreased by inductively coupled plasma. As a result, the gain of the active layer is periodically modulated in accordance with the periodic arrangement of the mask openings.

  Exposing the plurality of portions of the upper cladding layer to the inductively coupled plasma includes exposing the portions to the inductively coupled plasma for a first time to obtain a gain of the portion of the active layer covered by the plurality of portions. It may be reduced. In this case, the gain-reduced portion is periodically arranged in the active layer, thereby forming a gain grating.

  The active layer may be made of InGaAsP or AlGaInAs, and the upper cladding layer may be made of InP.

The process of exposing multiple portions of the upper cladding layer to inductively coupled plasma generates inductively coupled plasma from a gas containing hydrogen and adds hydrogen to the active layer at a concentration that varies periodically along the optical axis of the active layer. May be. The gas may be a mixed gas of CH ₄ mixed gas or _CH 4 in _{H 2, H 2} and Cl _2. In this case, the gain of the active layer is periodically modulated according to the periodic hydrogen concentration distribution along the optical axis of the active layer, and a gain grating is formed.

  The method induces the entire top surface of the upper cladding for a second time shorter than the first time after the step of providing the upper cladding layer and before exposing the portions of the upper cladding layer to inductively coupled plasma. The method may further comprise the step of exposing to the coupled plasma and increasing the gain of the entire active layer. Such an increase in gain improves the optical properties of the active layer and reduces the current threshold of a gain coupled DFB laser manufactured by this method.

  The method of the present invention can manufacture a gain-coupled DFB semiconductor laser in a simple process.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a cross-sectional view showing the structure of a gain-coupled DFB (Distributed Feedback) semiconductor laser 1 according to this embodiment. The gain-coupled DFB laser 1 includes an n-type cladding layer 12, a multiple quantum well (MQW) active layer 14, a p-type cladding layer 16, and a p-type contact layer that are sequentially provided on an n-type InP substrate 10. 18. A p-type electrode 20 is provided on the upper surface of the p-type contact layer 18. An n-type electrode 22 is provided on the lower surface of the substrate 10. Further, the gain-coupled DFB laser 1 has end faces (facets) 24 and 26 perpendicular to the main surface (upper surface and lower surface) of the MQW active layer 14. These end faces 24 and 26 are mirror surfaces formed by crystal cleavage and constitute a laser resonator. The direction perpendicular to the end faces 24 and 26 is called the longitudinal direction, and the direction perpendicular to it is called the transverse direction. The optical axis 15 of the MQW active layer 14 is parallel to the vertical direction. When a current exceeding the threshold value is injected into the DFB laser 1 through the p-type electrode 20 and the n-type electrode 22, a single mode laser beam 28 is generated in the MQW active layer 14 and emitted from the DFB laser 1. The

  The MQW active layer 14 has a multiple quantum well-separated confinement hetero (MQW-SCH) structure. The MQW active layer 14 includes a multiple quantum well composed of a plurality of wells and a plurality of barriers stacked alternately. These wells and barriers are made of InGaAsP. In addition, a separate confinement heterostructure (SCH) layer is provided above and below the multiple quantum well. In other words, the MQW active layer 14 has a structure in which a lower SCH layer, a multiple quantum well, and an upper SCH layer are sequentially stacked from the n-type cladding layer 12 toward the p-type cladding layer 16. The upper SCH layer and the lower SCH layer are both made of InGaAsP and have the same band gap and refractive index. This refractive index is lower than the refractive indices of the wells and barriers in the multiple quantum wells.

  The n-type cladding layer 12 and the p-type cladding layer 16 are both made of InP and have a lower refractive index than the upper and lower SCH layers in the MQW active layer 14. The n-type cladding layer 12 may be integrated with the substrate 10. In other words, the active layer 14 may be directly provided on the upper surface of the substrate 10, and a portion of the substrate 10 adjacent to the MQW active layer 14 may function as the n-type cladding layer 12. The p-type contact layer 18 is deposited on the upper surface of the p-type cladding layer 16. The p-type contact layer 18 has a lower refractive index than the p-type cladding layer 16.

  As shown in FIG. 1, in the MQW active layer 14, a plurality of photoluminescence degradation portions 14 a and a plurality of normal portions 14 b are alternately arranged along the optical axis 15 of the MQW active layer 14. The photoluminescence degradation part 14a is a part in which the photoluminescence intensity is degraded by an ICP process to be described later. On the other hand, the normal part 14b is not subjected to the ICP process, and therefore the photoluminescence intensity is not deteriorated. Therefore, the photoluminescence degradation part 14a has a lower gain than the normal part 14b.

  Both the photoluminescence degradation part 14 a and the normal part 14 b are periodically arranged along the optical axis 15 of the MQW active layer 14. Therefore, the gain of the MQW active layer 14 is periodically modulated along the optical axis 15, thereby forming a gain grating. The laser 1 including the active layer 14 having a gain grating operates as a gain-coupled DFB laser.

Below, the formation method of the photo-luminescence degradation part 14a is demonstrated. The photoluminescence degradation part 14a can be formed using inductively coupled plasma (Inductively Coupled Plasma: ICP). The inventor has developed a mixed gas of CH ₄ and H ₂ (hereinafter “CH ₄ / H ₂ gas”) or a mixed gas of CH ₄ , H ₂ and Cl ₂ (hereinafter “CH ₄ / H ₂ / Cl ₂ gas”). It was found that when the InP / InGaAsP-MQW / InP structure was exposed to an inductively coupled plasma (ICP) generated from “)” for a very short time, etching was not performed and the photoluminescence intensity of MQW increased. The present inventor has also found that when the exposure time is increased, etching is started and the photoluminescence intensity rapidly decreases. H was diffused and implanted in the portion where the photoluminescence intensity rapidly decreased. This H is considered to function as a non-radiative recombination center. Furthermore, the present inventor has also found that the photoluminescence intensity of a portion protected by an etching mask such as SiN does not change. Therefore, if the MQW active layer 14 is exposed to ICP for an appropriate time while protecting the normal portion 14b with a mask, the photoluminescence degradation portion 14a can be formed.

Thus, the gain of the InGaAsP-MQW active layer 14 can be increased or decreased by ICP processing. Such a change in photoluminescence intensity does not occur in the ECR plasma treatment even when the same CH ₄ / H ₂ or CH ₄ / H ₂ / Cl ₂ gas is used. Further, the higher the ICP power, the greater the rate of decrease in photoluminescence intensity after the start of etching, and the higher the H injection efficiency. Here, the ICP power means high-frequency power supplied to an induction coil used for generating inductively coupled plasma (ICP). In order to reduce the photoluminescence intensity to 1/10 with an etching amount within 0.5 μm, the ICP power is desirably 1000 W or more. Even when the photoluminescence intensity is increased, the ICP power is desirably 1000 W or more. However, the higher the ICP power, the shorter the processing time. This is to prevent excessive etching of the p-type cladding layer 16.

  The change in the photoluminescence intensity of the InGaAsP-MQW active layer 14 is basically determined according to the relationship between the ICP power and the processing time. However, it is also affected by sample conditions such as the thickness of the layer interposed from the surface of the sample to be processed to the MQW active layer 14.

As described above, it is considered that hydrogen added to the MQW active layer 14 by inductively coupled plasma works as a non-radiative recombination center. Therefore, if H is added to the MQW active layer 14 at a concentration that periodically changes along the optical axis 15, a gain grating is formed in the MQW active layer 14. When the concentration of H added to the MQW active layer 14 is 5 × 10 ¹⁷ atoms / cc, the photoluminescence intensity of the MQW active layer 14 is reduced to 1/3. Accordingly, it is desirable that H is added to the photoluminescence degradation portion 14a at a concentration of 5 × 10 ¹⁷ atoms / cc or more. Conversely, in the normal part 14b, it is desirable that the concentration of H is less than 5 × 10 ¹⁷ atoms / cc.

  Hereinafter, an example of the method for manufacturing the gain-coupled DFB laser according to the above embodiment will be specifically described with reference to FIGS. First, as shown in FIG. 2, an MQW active layer 34 and a p-type InP cladding layer 36 are sequentially formed on an n-type InP substrate 30 by metal organic vapor phase epitaxy (OMVPE). . In this example, the upper part of the n-type InP substrate 30 adjacent to the MQW active layer 34 functions as an n-type cladding layer. In the MQW active layer 34, five layers of InGaAsP wells and six layers of InGaAsP barriers are alternately stacked. The well has a bandgap wavelength of 1.31 μm and the barrier has a bandgap wavelength of 1.1 μm. Above and below the MQW active layer, SCH layers made of InGaAsP are provided. The clad layer 36 is deposited on the upper surface of the upper SCH layer, and its thickness is 2 μm.

As shown in FIG. 3, a SiN film 31 having a thickness of 1000 mm is formed on the upper surface of the cladding layer 36 by plasma enhanced chemical vapor deposition (PE-CVD). Next, as shown in FIG. 4, a grating (lattice) -like resist pattern 39 is formed on the SiN film 31 by ordinary photolithography. The resist pattern 39 extends perpendicular to the paper surface of FIG. 4, that is, perpendicular to the optical axis 35 of the MQW active layer 34. The pitch of the resist pattern 39 is 2100 mm. Using the resist pattern 39 as a mask, parallel plate type reactive ion etching (RIE) using CF ₄ is performed on the SiN film 31. As a result, a SiN grating pattern 31a is formed as shown in FIG.

  The SiN pattern 31 a has a plurality of openings 31 b arranged at equal intervals along the optical axis of the MQW active layer 34. These openings 31b have the same planar shape. A plurality of portions 36a on the upper surface of the p-type cladding layer 36 are exposed from the opening 31b. These portions 36a are arranged at equal intervals along the optical axis of the MQW active layer 34, similarly to the openings 31b.

  Next, as shown in FIG. 6, the sample 33 is subjected to ICP processing using the SiN pattern 31a as a mask. FIG. 13 is a schematic diagram showing the configuration of an inductively coupled plasma reactive ion etching (ICP-RIE) apparatus 50 that performs this ICP process. The sample 33 is carried into the chamber 52 of the ICP-RIE apparatus 50. The sample 33 is placed on the lower electrode 54 installed in the chamber 52. An upper electrode 56 is installed above the lower electrode 54 and the sample 33. The upper electrode 56 is grounded. A high frequency voltage of 13.56 MHz is applied to the lower electrode 54 from a high frequency power supply 55. An induction coil 58 is wound around the side surface of the chamber 52. High frequency power of 2 MHz is supplied to the induction coil 58 from the inductively coupled plasma power source 60, and a DC bias electric field is generated in the chamber 52 accordingly.

In this example, CH ₄ / H ₂ / Cl ₂ gas is used as a source gas for inductively coupled plasma. When CH ₄ / H ₂ / Cl ₂ gas is introduced into the chamber 52 from the gas inlet 62, an inductively coupled plasma (ICP) 64 is generated by a high-frequency electric field between the lower electrode 54 and the upper electrode 56. The ions 65 in the ICP 64 are accelerated by the bias electric field generated by the induction coil 58 and reach the sample 33. In this manner, the exposed portion 36a on the upper surface of the p-type cladding layer 36 is exposed to the ICP 64.

In this example, the exposed portion 36a is exposed to the ICP 64 for 30 seconds. The output of the inductively coupled plasma power source 60, that is, the ICP power is 2500 W, the output of the high frequency power source 55 is 50 W, the flow rates of CH ₄ , H ₂ and Cl ₂ are 6, 6 and 3 sccm, respectively, and the pressure in the chamber 52 is 0.5 Pa. To do. As a result, the photoluminescence intensity of the portion 34a of the MQW active layer 34 that is not covered by the SiN pattern 31a (that is, the portion covered by the exposed portion 36a) is the photoluminescence intensity of the portion 34b covered by the SiN pattern 31a. 1/10. In this way, the photoluminescence degradation portion 34a and the normal portion 34b are periodically formed in the MQW active layer 34, and the gain of the MQW active layer 34 is periodically modulated.

  Next, the SiN pattern 31a is removed as shown in FIG. 7, and an SiN stripe 48 is formed on the upper surface of the p-type cladding layer 36 as shown in FIG. Subsequently, using the SiN stripe 48 as a mask, a mesa stripe structure is formed by reactive ion etching or wet etching using Br-methanol. Thereafter, current confinement buried layers 43 and 44 are sequentially formed on the surface of the mesa stripe structure exposed by etching. The buried layer 43 is made of p-type InP, and the buried layer 44 is made of n-type InP. Next, a p-type InP clad layer 37 having a thickness of 0.1 μm covering the p-type clad layer 36 and the buried layer 44 is formed, and a p-type InGaAs contact layer 38 having a thickness of 0.5 μm is formed on the clad layer 37. Form.

  Thereafter, as shown in FIG. 10, a trench 45 for element isolation is formed around the mesa stripe structure, and an insulating film 47 covering the contact layer 38 and the trench 45 is formed as shown in FIG. In the film 47, a contact hole 49 is formed at a position located directly above the MQW active layer. As shown in FIG. 12, the p-type electrode 40 is formed so as to fill the contact hole 49, and the n-type electrode 42 is formed on the lower surface of the InP substrate 30. In this way, the gain coupled DFB laser 1a is obtained.

  The inventor actually manufactured the DFB laser 1a by the above method and measured its characteristics. As a result, an oscillation threshold value of 10 mA and a side mode suppression ratio of 50 dB were obtained.

  If the entire upper surface of the p-type cladding layer 36 is exposed to the ICP for a very short time before the formation of the SiN film 31 shown in FIG. 3, the photoluminescence intensity of the MQW active layer 34 can be raised. In this case, the exposure time is 10 seconds, the output of the ICP inductively coupled plasma power supply 60 (ie, ICP power) is 1250 W, and the other conditions are the same as those of the ICP process shown in FIG. By this ICP treatment, the photoluminescence intensity of the entire MQW active layer 34 can be doubled. Thereafter, when SiN film formation, patterning, and ICP processing are performed in the same manner as described above, the optical characteristics of the normal portion 34b can be improved, and the current threshold value of the gain-coupled DFB laser 1a can be reduced.

  The present invention has been described in detail based on the embodiments. However, the present invention is not limited to the above embodiment. The present invention can be variously modified without departing from the gist thereof.

In the above example, the active layer is made of InGaAsP. However, the active layer may be composed of other semiconductor materials, such as AlGaInAs. Similar to InGaAsP-MQW, exposure of the InP / AlGaInAs-MQW / InP structure to inductively coupled plasma using CH ₄ / H ₂ gas or CH ₄ / H ₂ / Cl ₂ gas increases or decreases the photoluminescence intensity of the MQW layer. can do. Therefore, even when the active layer is made of AlGaInAs, a gain-coupled DFB semiconductor laser can be manufactured by the same method as described above. AlGaInAs is easily oxidized, but the active layer is not exposed to the atmosphere in the above method, so that oxidation of AlGaInAs can be prevented.

It is sectional drawing which shows the structure of the gain coupling type | mold DFB semiconductor laser of embodiment. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is a perspective view which shows an example of a gain coupling type DFB semiconductor laser manufacturing method. It is a perspective view which shows an example of a gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is sectional drawing which shows an example of the gain coupling type DFB semiconductor laser manufacturing method. It is the schematic which shows an inductively coupled plasma etching apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 and 1a ... Gain coupling | bonding type | mold DFB semiconductor laser, 10 and 20 ... board | substrate, 12 ... n-type clad layer as a lower clad layer, 14 and 34 ... MQW active layer, 14a and 34a ... Photoluminescence degradation part, 14b and 34b ... Normal part, 16 and 36... P-type cladding layer as upper cladding layer, 18 and 38... P-type contact layer, 20 and 40... P-type electrode, 22 and 42.

Claims (7)

Providing an active layer having a multiple quantum well structure on a substrate;
Providing an upper cladding layer covering the active layer;
Exposing a plurality of portions periodically arranged along the optical axis of the active layer, of the upper surface of the upper cladding layer, to an inductively coupled plasma. The step of exposing the plurality of portions to inductively coupled plasma includes forming a mask on the upper clad layer having openings periodically arranged along the optical axis of the active layer, and forming the upper clad layer on the mask Exposure to inductively coupled plasma through
The method of claim 1. Exposing the plurality of portions to the inductively coupled plasma exposes the plurality of portions to the inductively coupled plasma for a first time and reduces a gain of a portion of the active layer covered by the plurality of portions. ,
The method according to claim 1 or 2. The active layer is made of InGaAsP or AlGaInAs,
The upper cladding layer is made of InP;
The method according to claim 1. The step of exposing the plurality of portions to the inductively coupled plasma generates the inductively coupled plasma from a gas containing hydrogen and adds hydrogen to the active layer at a concentration that periodically changes along the optical axis of the active layer. To
The method according to claim 1. Gas containing hydrogen is a mixed gas of CH ₄ mixed gas or _CH 4 in _{H 2, H 2} and Cl _2,
The method of claim 5. After the step of providing the upper cladding layer, prior to the step of exposing the portions to the inductively coupled plasma, the entire upper surface of the upper cladding is exposed to the inductively coupled plasma for a second time shorter than the first time. The method according to claim 1, further comprising increasing the gain of the entire active layer.

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