JP2004109594A - Waveguide type semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a waveguide type semiconductor element of which the impurity concentration diffusing into a waveguide layer is low and the series resistance is low. <P>SOLUTION: The waveguide type semiconductor element (2) is provided with a p-type cladding layer (18), a waveguide layer (6) and a n-type cladding layer (4). The waveguide type semiconductor element (2) is provided with an impurity layer (8) in contact with the waveguide layer (6) in at least a gap out of gaps between the p-type cladding layer (18) and the waveguide layer (6) and between the n-type cladding layer (4) and the waveguide layer (6). For example, when the impurity layer (8) is disposed between the p-type cladding layer (18) and the waveguide layer (6), the impurity concentration of the impurity layer (8) is ≥1/10 to ≤1/2 of the impurity concentration of the p-type cladding layer (18). Owing to the presence of the impurity layer (8), the impurity concentration diffused from the p-type cladding layer (18) to the waveguide layer (6) is reduced. Also the impurity concentration of the impurity layer (4) has a lower limit and the waveguide type semiconductor element (2) with the low series resistance is provided. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a waveguide-type semiconductor device applied to an optical communication system, and more particularly, to a waveguide-type semiconductor device in which impurity diffusion into a waveguide layer is reduced.
[0002]
[Prior art]
2. Description of the Related Art An electro-absorption (EA) optical modulator is a waveguide-type semiconductor device having a PIN junction formed by sandwiching a light absorption layer, which is an intrinsic semiconductor, between a p-type semiconductor and an n-type semiconductor. is there. In the EA optical modulator, output light can be modulated by modulating a voltage applied to a PIN junction. The main parameters representing the performance of the EA optical modulator include the extinction ratio and the photoelectric flow. The extinction ratio indicates the magnitude of the intensity change of the light output from the EA optical modulator, and the photoelectric flow rate indicates the amount of photocurrent generated by light absorption in the light absorbing layer. The higher the extinction ratio, the better the performance of the EA optical modulator, and the better the photoelectric flow.
[0003]
As a parameter affecting the extinction ratio and the photoelectric flow rate, there is an impurity concentration diffused in the light absorption layer. These impurities are mainly diffused from the p-type semiconductor layer and the n-type semiconductor layer sandwiching the light absorption layer in the manufacturing process of the EA optical modulator. The impurities diffused in the light absorbing layer are ionized and remain in the light absorbing layer, and thereby, the electric field intensity distribution in the light absorbing layer becomes uneven when a voltage is applied. This variation in the electric field intensity increases the tail of the absorption spectrum in the light absorption layer, causing problems such as a decrease in the extinction ratio, an increase in the absorption loss, and an increase in the photocurrent. Therefore, in order to obtain an EA optical modulator having good characteristics, it is important to suppress impurity diffusion from the p-type semiconductor layer and the n-type semiconductor layer sandwiching the light absorption layer.
[0004]
For example, in order to suppress the diffusion of impurities from the p-type semiconductor layer to the light absorption layer, the conventional EA optical modulator has a higher impurity concentration between the light absorption layer and the p-type cladding layer than the p-type cladding layer. (See, for example, Patent Document 1).
[0005]
[Patent Document 1]
JP 2001-21851 A (page 4, see FIG. 1)
[0006]
[Problems to be solved by the invention]
In the EA optical modulator disclosed in Patent Document 1 described above, the range of the impurity concentration of the impurity diffusion preventing layer is not particularly considered. However, if the impurity concentration of the impurity diffusion preventing layer is too low, there is a problem that the series resistance of the EA optical modulator increases and the frequency characteristics deteriorate.
[0007]
Further, in the EA optical modulator disclosed in Patent Document 1, etc., since a p-type InGaAsP clad layer having a high impurity concentration exists between the impurity diffusion preventing layer and the light absorbing layer, the impurity diffusion to the light absorbing layer is prevented. However, there is a problem that even if the impurity concentration of the impurity diffusion preventing layer is reduced, impurity diffusion into the light absorbing layer must be recognized to some extent.
[0008]
An object of the present invention is to provide a waveguide-type semiconductor device having a low impurity concentration that diffuses into a waveguide layer and a low series resistance.
[0009]
[Means for Solving the Problems]
A waveguide semiconductor device according to the present invention includes a p-type cladding layer, a waveguide layer, and an n-type cladding layer. The waveguide-type semiconductor element may include the waveguide layer at least one of between the p-type cladding layer and the waveguide layer and between the n-type cladding layer and the waveguide layer. An impurity layer in contact is provided. When the impurity layer is provided between the p-type cladding layer and the waveguide layer, the impurity concentration is equal to or more than one-tenth and three-minutes of the impurity concentration of the p-type cladding layer. When provided between the n-type cladding layer and the waveguide layer, the impurity concentration is not less than one-tenth and three-minutes of the impurity concentration of the n-type cladding layer. 1 or less.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 is a sectional view of a waveguide type semiconductor device according to the present embodiment. This semiconductor element has a buried type structure, and has a stripe-shaped waveguide layer at the center thereof. This semiconductor device is used as an EA modulator. In FIG. 1, the EA modulator 2 includes an n-type InP (indium phosphide) substrate 4. On the n-type InP substrate 4, a stripe-shaped waveguide layer (light absorbing layer) 6 and a first p-type InP clad layer 8 are sequentially stacked. The light absorption layer 6 is an InGaAsP (indium gallium arsenide phosphorus) multiple quantum well (MQW) light absorption layer. The light absorbing layer 6 and the first p-type InP cladding layer 8 are buried on the n-type InP substrate 4 by semi-insulating semiconductor layers 10 provided on both sides thereof. Here, the semi-insulating semiconductor layer 10 includes, for example, semi-insulating layers 12 and 14 of InP or the like doped with Fe (iron) and an n-type InP hole strap layer 16 doped with S (sulfur). A second p-type InP cladding layer 18 is laminated on the first p-type InP cladding layer 8 and the semi-insulating semiconductor layer 10, and a p-type InP cladding layer 18 is formed on the p-type InP cladding layer 18. An InGaAs (indium gallium arsenide) contact layer 20 is stacked. An electrode 22 and an electrode 24 are provided on the surfaces of the p-type InGaAs contact layer 20 and the n-type InP substrate 4, respectively.
[0011]
Usually, an optical modulator for optical communication used in a wavelength band of 1.3 μm to 1.6 μm uses a metal-organic vapor phase epitaxy method to form a crystal based on In, Ga, As, and P on an InP substrate. Manufactured by growing. When the metal organic chemical vapor deposition method is used, Zn (zinc) is generally used as an impurity for a p-type semiconductor. Zn has the advantage that the desired impurity concentration can be easily obtained. Also in the EA optical modulator according to the present embodiment, Zn is used as an impurity (acceptor) for a p-type semiconductor. Note that Zn has a disadvantage that it is relatively easily diffused, but in the optical modulator according to the present embodiment, Zn diffusion to the light absorption layer can be sufficiently reduced.
[0012]
In the waveguide type semiconductor device 2 according to the present embodiment, the first p-type InP clad layer 8 is provided between the light absorption layer 6 and the second p-type clad layer 18. The first p-type InP cladding layer 8 has a lower Zn concentration than the second p-type InP cladding layer 18, and the Zn concentration is 1/10 or more of that of the second p-type InP cladding layer 18. 1/3 or less. The first p-type InP cladding layer 8 plays a role in preventing Zn from diffusing from the p-type semiconductor layer into the light absorbing layer 6. Hereinafter, the structure of the EA optical modulator 2 according to the present embodiment will be described in comparison with the structure of the optical modulator having no first p-type InP cladding layer 8 with reference to FIG. FIG. 2 shows a laminated structure including an n-type substrate, a light absorption layer, a p-type cladding layer, and a p-type contact layer. Here, the laminated structure of FIG. 2A shows a structure in which the central portion of the optical modulator 2 shown in FIG. 1 is extracted. In FIG. 2A, the electrodes 22, 24 and the semi-insulating semiconductor layer 10 shown in FIG. 1 are omitted in order to particularly clarify the first p-type InP cladding layer 8 (FIG. 1). . FIG. 2B shows a structure similar to that of FIG. 2A in an optical modulator having no first p-type InP cladding layer 8. Each of the stacked structures shown in FIGS. 2A and 2B includes an n-type InP substrate 4.
[0013]
Referring to FIG. 2B, in the optical modulator without the first p-type InP cladding layer 8 for comparison, the InGaAsP MQW light absorption layer 6 and the p-type A portion of the (Zn-doped) InP cladding layer 18 is grown (first epitaxial growth: 1st epi.), And then a current blocking layer 10 (not shown) is grown (second epitaxial growth: 2nd epi.). Finally, the rest of the p-type InP cladding layer 18 and the p-type (Zn-doped) InGaAs contact layer 20 are grown (third epitaxial growth: 3rd epi.). On the other hand, in the waveguide type semiconductor device 2 according to the present embodiment, as shown in FIG. 2A, an InGaAsP MQW light absorbing layer 6 and a low-concentration p A portion of the type (Zn-doped) cladding layer 8 and the p-type (Zn-doped) InP cladding layer 18 are grown (1st epi.), And then a current blocking layer 10 (not shown) is grown (1st epi.). 2nd epi.), And finally, the remaining p-type InP cladding layer 18 and a p-type (Zn-doped) InGaAs contact layer 20 are grown (3rd epi.).
[0014]
In the laminated structure according to the present embodiment, a low-concentration p-type InP cladding layer 8 having a lower Zn concentration than the p-type InP cladding layer 18 is provided between the InGaAsP MQW light absorbing layer 6 and the p-type InP cladding layer 18. By providing this, the concentration of Zn diffused from the p-type InP cladding layer 18 to the InGaAsP MQW light absorption layer 6 is reduced. This is based on an analysis result by a numerical simulation. According to the result of the numerical simulation, the Zn concentration in the light absorption layer largely depends on the Zn concentration in the p-type semiconductor layer stacked directly thereon. Thus, in order to reduce the Zn concentration in the light absorbing layer, it is necessary to lower the Zn concentration in the p-type semiconductor layer immediately above the light absorbing layer. In the EA optical modulator 2 according to the present embodiment, since the low-concentration p-type InP cladding layer 8 having a lower Zn concentration than the p-type InP cladding layer 18 is stacked directly on the light absorption layer, the EA optical modulator 2 is directly above the light absorption layer. The Zn concentration diffused into the light absorbing layer is smaller than that of the conventional stacked structure in which the p-type InP clad layer 18 is stacked. Here, the Zn concentration of the p-type InP cladding layer 8 is preferably lower than the Zn concentration of the p-type InP cladding layer 18 in order to lower the Zn concentration diffused into the light absorbing layer as compared with the conventional laminated structure. Is 1/3 or less of the Zn concentration of the p-type InP cladding layer 18. On the other hand, if the Zn concentration in the p-type semiconductor layer immediately above the light absorbing layer is too low, a problem occurs in that the series resistance of the semiconductor element increases. Therefore, in the EA optical modulator 2 according to the present embodiment, the Zn concentration of the low-concentration p-type InP cladding layer 8 stacked directly on the light absorption layer is set to 1/10 of the Zn concentration of the p-type InP cladding layer 18. Set to at least 1/3 or less.
[0015]
In the EA light modulator according to the present embodiment, by reducing the concentration of the impurity diffused into the light absorption layer, the tail of the absorption spectrum in the light absorption layer is eliminated, and the performance of the EA light modulator is improved. FIG. 3 schematically shows the difference in absorption spectrum between the EA light modulator having no low-concentration p-type cladding layer and the light absorption layer of the EA light modulator according to the present embodiment. FIGS. 3A and 3B show absorption spectra in the light absorption layer of the EA light modulator according to the present embodiment and the EA light modulator without the low-concentration p-type cladding layer, respectively. When the incident light wavelength of the EA optical modulator (operating wavelength) and lambda _a, when the bias voltage (modulation voltage) when the zeros (solid line), was applied bias voltage (V _{bi =} -2 V) of (dotted line) The difference between the absorption coefficients is larger in the EA optical modulator according to the present embodiment (FIG. 3A) than in the EA optical modulator without the low-concentration p-type cladding layer (FIG. 3B). Become. Thus, if the EA optical modulator according to the present embodiment is used, the extinction ratio (10 log (I _max / I _max ) obtained from the maximum (I _max ) and the minimum (I _min ) of the output light intensity of the EA modulator is used. _min )) can be increased.
[0016]
In the optical modulator according to the present embodiment, Zn is used as the impurity (acceptor) for the p-type semiconductor, but Be (beryllium) or Mg (magnesium) may be used. Further, as the impurity (donor) for the n-type semiconductor, Si (silicon), S (sulfur), Sn (tin), or the like can be used.
[0017]
Hereinafter, as an example, the Zn concentrations of the p-type InP cladding layer 18 and the low-concentration p-type InP cladding layer 8 in FIG. 2 are 1 × 10 ¹⁸ (cm ⁻³ ) and 1 × 10 ¹⁷ (cm ⁻³ ), respectively. The results of the study in the case of
FIG. 4 shows the result of calculating the transition of the Zn concentration distribution in the manufacturing process of the EA optical modulator according to the present embodiment and the EA optical modulator device having no low-concentration p-type cladding layer. In the graph of FIG. 4, the horizontal axis indicates the position in the depth direction near the light absorption layer and the p-type semiconductor layer on the light absorption layer in the EA optical modulator, and the vertical axis indicates the position at a certain position in the depth direction. Shows the Zn concentration. The value in the depth direction on the horizontal axis increases from the p-type semiconductor layer toward the light absorption layer. FIG. 4A shows the calculation result of the EA optical modulator according to the present embodiment, and FIG. 4B shows the calculation result of the EA optical modulator having no low-concentration p-type cladding layer. Referring to FIG. 4B, in the EA optical modulator having no low-concentration p-type cladding layer, after the first epitaxial growth (solid line: 1st epi.), It is already 1 × 10 ¹⁷ (cm ⁻³ ). The above Zn diffuses into the light absorbing layer, and a large amount of Zn diffuses into the light absorbing layer 6 from the bottom of the p-type cladding layer 18. Further, as the crystal growth process proceeds (broken line: 2nd epi.), The Zn diffusion extends to the deep portion of the absorption layer 6, and after the third epitaxial growth (dashed line: 3nd epi.), The deep portion of the light absorption layer 6 Up to 2 × 10 ¹⁷ (cm ⁻³ ) or more Zn is diffused. On the other hand, with reference to FIG. 4A, in the EA optical modulator according to the present embodiment, the Zn concentration of the light absorbing layer 6 is such that the light absorbing layer 6 as a whole has no EA having a low concentration p-type cladding layer. It is lower than the Zn concentration in the same region in the optical modulator. In the EA optical modulator according to the present embodiment, after the third epitaxial growth, 2 × 10 ¹⁷ (cm ⁻³ ) of Zn diffused in the EA optical modulator having no low-concentration p-type cladding layer. In the deep part, only diffusion of about 4 × 10 ¹⁶ (cm ⁻³ ) is observed. In the EA modulator according to the present embodiment, the diffusion of Zn from the low-concentration p-type InP layer 8 into the light absorbing layer 6 is suppressed to be lower than that of the EA optical modulator having no low-concentration p-type cladding layer. You can see that it is.
[0018]
FIG. 5 shows the results of measuring the extinction ratio characteristics of each of the EA optical modulator according to the present embodiment and the EA optical modulator having no low-concentration p-type cladding layer. In the graph of FIG. 5, the horizontal axis represents the bias voltage applied to the EA optical modulator, and the vertical axis represents the extinction ratio. According to FIG. 5, in the EA optical modulator (thin line) having no low-concentration p-type cladding layer, the extinction ratio of 0 V / −3 V is about 20 dB, whereas the EA optical modulator according to the present embodiment is used. In the (bold line), the extinction ratio of 0 V / −3 V is about 24 db. Thus, it is understood that the extinction ratio is improved by about 20% when the EA optical modulator according to the present embodiment is used.
[0019]
FIG. 6 shows the results of measuring the extinction ratio characteristics and the photocurrent characteristics of each of the EA optical modulator according to the present embodiment and the EA optical modulator having no low-concentration p-type cladding layer. The horizontal axis of the graph in FIG. 6 indicates the extinction ratio, and the vertical axis indicates the photoelectric flow rate. According to FIG. 6, in the EA optical modulator having no low-concentration p-type cladding layer, the photoelectric flow is about 12 mA to 16 mA (open circle), whereas in the EA optical modulator according to the present embodiment, The photoelectric flow rate is about 6 mA to 8 mA (filled circles). In the EA optical modulator according to the present embodiment, the photoelectric flow is reduced by half as compared with the EA optical modulator having no low-concentration p-type cladding layer. This is because, in the EA optical modulator according to the present embodiment, the amount of Zn diffused into the light absorbing layer 6 is smaller than that in the EA optical modulator having no low-concentration p-type cladding layer, and the light due to the tail of the absorption spectrum is reduced. It means that the current is decreasing.
[0020]
As described above, in the EA optical modulator according to the present embodiment, between the light absorption layer and the p-type clad layer having a high impurity concentration, the low-concentration p-type clad layer having a lower impurity concentration than the p-type clad layer is provided. Is provided, the concentration of impurities diffused into the light absorption layer can be reduced. As a result, an EA optical modulator having excellent characteristics can be obtained. In addition, since the impurity concentration of the low-concentration p-type cladding layer is limited to 1/10 or more of the impurity concentration of the high-concentration p-type cladding layer, the series resistance of the EA optical modulator can be suppressed to a small value and a good frequency can be obtained. Properties can be obtained.
[0021]
In the EA optical modulator according to the present embodiment, the light absorption layer is laminated on the n-type substrate. After the n-type cladding layer is laminated on the semi-insulating substrate, A light absorption layer may be stacked thereon.
[0022]
In the optical modulator according to the present embodiment, by providing a low-concentration p-type cladding layer in contact with the light-absorbing layer between the light-absorbing layer and the high-concentration p-type cladding layer, Although the concentration of the impurity diffused into the light absorption layer is reduced, the n-type substrate (or high-concentration) is provided between the light absorption layer and the n-type substrate (or n-type cladding layer) in contact with the light absorption layer. By providing a low-concentration n-type cladding layer having an impurity concentration lower than that of the n-type cladding layer (1/10 to の of the impurity concentration of the n-type substrate or the high-concentration n-type cladding layer), It is also possible to reduce the concentration of impurities diffused from the light absorption layer (or from the high concentration n-type cladding layer). Further, in the optical modulator according to the present embodiment, both the low-concentration p-type cladding layer and the low-concentration n-type cladding layer may be provided.
[0023]
Embodiment 2 FIG.
FIG. 7 shows a structure of a semiconductor device (hereinafter, referred to as an EA / LD device) in which an EA optical modulator and a semiconductor laser (LD) according to the first embodiment are integrated. 7A is a perspective view, and FIG. 7B is a partially enlarged cross-sectional view of a portion surrounded by a broken line A in FIG. Referring to FIG. 7A, the EA / LD device 50 includes a light modulation unit 52 and a semiconductor laser unit 54. The light modulation section 52 and the semiconductor laser section 54 both have a common n-type InP substrate 4. On the n-type InP substrate 4, a stripe-shaped light absorption layer 6 is provided in the light modulation section 52, and a stripe-shaped laser active layer 60 is provided in the semiconductor laser section 54. The light absorbing layer 6 and the laser active layer 60 are joined in abutting manner. Electrodes 62 and 64 are provided on the surface of the EA / LD device 50. In the EA / LD device 50, as shown in FIG. 7B, a low-concentration p-type InP cladding layer 8 is provided between the light absorbing layer 6 and the p-type InP cladding layer 18 in the light modulation section 52. The structure provided is adopted. The light absorbing layer 6 and the laser active layer 60 are respectively buried from both sides thereof with the semi-insulating semiconductor layer 10. When a voltage is applied between predetermined electrodes of the EA / LD device 50, laser oscillation occurs in the semiconductor laser unit 54. The laser light output from the semiconductor laser unit 54 is directly input to the light absorption layer 6 of the light modulation unit 52. In the light modulation unit 52, a voltage is applied between the electrodes different from that during laser oscillation to perform light modulation, and the light modulated in the light modulation unit 52 is output from the end face of the EA / LD device 2. (Arrow B).
[0024]
In the EA / LD device according to the present embodiment, the structure of the light modulation section having excellent light modulation characteristics is employed, so that the performance of the EA / LD device itself can be improved.
[0025]
【The invention's effect】
According to the waveguide type semiconductor device of the present invention, the semiconductor device includes a p-type cladding layer, a waveguide layer, and an n-type cladding layer, and is provided between the p-type cladding layer and the waveguide layer and between the n-type cladding layer and the waveguide layer. An impurity layer in contact with the waveguide layer is provided on at least one of the layers, and when the impurity layer is provided between the p-type cladding layer and the waveguide layer, the impurity concentration of the impurity layer is The concentration is one-tenth or more and one-third or less of the concentration, and when provided between the n-type cladding layer and the waveguide layer, the impurity concentration is one-tenth of the impurity concentration of the n-type cladding layer. Since it is not less than one-third and less than one-third, it is possible to realize a waveguide-type semiconductor element having a low impurity concentration diffused into the waveguide layer and a low series resistance.
[Brief description of the drawings]
FIG. 1 is a sectional view of an EA modulator according to an embodiment.
FIG. 2 is a diagram comparing a partial structure of the EA optical modulator according to the present embodiment with a similar structure of an optical modulator having no low-concentration p-type cladding layer.
FIG. 3 is a diagram showing an absorption spectrum in the light absorption layer of the EA light modulator according to the present embodiment and the EA light modulator having no low-concentration p-type cladding layer.
FIG. 4 is a diagram showing a transition of a Zn concentration distribution in a process of manufacturing a device of the EA optical modulator according to the present embodiment and an EA optical modulator having no low-concentration p-type cladding layer.
FIG. 5 is a diagram showing the extinction ratio characteristics of the EA optical modulator according to the present embodiment and the EA optical modulator having no low-concentration p-type cladding layer.
FIG. 6 is a diagram showing extinction ratio characteristics and photocurrent characteristics of the EA optical modulator according to the present embodiment and an EA optical modulator having no low-concentration p-type cladding layer.
FIG. 7 is a diagram showing a structure of a semiconductor device in which an EA optical modulator and a semiconductor laser according to the present embodiment are integrated.
[Explanation of symbols]
2 optical modulator, 4 n-type InP substrate, 6 InGaAsP MQW light absorption layer, 8 low-concentration p-type InP cladding layer, 10 semi-insulating semiconductor layer, 18 p-type InP cladding layer, 20 p-type InGaAs contact layer, 22, 24
electrode.

Claims (1)

In a waveguide type semiconductor device comprising a p-type cladding layer, a waveguide layer and an n-type cladding layer,
An impurity layer in contact with the waveguide layer is provided between at least one of the p-type cladding layer and the waveguide layer and between the n-type cladding layer and the waveguide layer,
The impurity layer,
When provided between the p-type cladding layer and the waveguide layer, the impurity concentration thereof is 1/10 or more and 1/3 or less of the impurity concentration of the p-type cladding layer;
When provided between the n-type cladding layer and the waveguide layer, the impurity concentration thereof is one-tenth or more and one-third or less of the impurity concentration of the n-type cladding layer. Semiconductor device.

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