JP2006270060A - Light receiving element, receiving module for optical communication and measuring instrument employing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an easy-to-fabricate light receiving element having a small dark current in which middle infrared light in the range of 1.65-3.0 μm can be received by forming a light receiving layer of good crystallinity with little defect on an InP substrate. <P>SOLUTION: A GaInNAsP light receiving layer, a GaInNAsSb light receiving layer or a GaInNAsPSb light receiving layer having a band gap in the range of 1.65-3.0 μm is provided directly or through an InP buffer layer on an InP substrate, an InP window layer or an InAlAs window layer is provided or not provided thereon, a p-region is formed by diffusing zinc selectively while masking and a p-electrode is attached. Since lattice matching can be attained between the GaInNAsP, GaInNAsSb or GaInNAsPSb and the InP substrate (degree of mismatch is ±0.2% or less), a graded layer for varying the lattice constant gradually is not required. The InP substrate may be an n-InP or SI-InP substrate. A light receiving element of middle infrared light in the range of 1.65-3.0 μm can thereby be fabricated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light receiving element using a group 3-5 compound semiconductor and having sensitivity in a near infrared region, particularly a wavelength region of 1.65 μm to 3.0 μm, and an optical communication module or measurement system using the same.

  A light emitting element and a light receiving element for optical communication using a quartz fiber use a signal light having a wavelength of 1.55 μm because the absorption of the quartz optical fiber is minimized in the vicinity of 1.55 μm. Further, since signals with different wavelengths are required for transmission and reception, 1.3 μm signal light is used as another wavelength. This is also less absorbed by the quartz optical fiber. These are optical communication signal lights. The light receiving element is formed by epitaxially growing an n-InP buffer layer, an InGaAs light receiving layer, and an n-InP window layer on an n-InP substrate, and a mask is attached on the window layer to form zinc. Are selectively diffused to form a p region, and an n electrode and a p electrode are used.

Since the InGaAs light receiving layer is a ternary system, the mixed crystal ratio is determined depending on the condition of lattice matching with InP. For example, a composition such as In _0.53 Ga _0.47 As is used for the light receiving layer. Although the band gap is also determined by the light receiving layer composition, the InGaAs light receiving layer for optical communication can receive light in the range of 1 μm to 1.6 μm. A conventional InGaAs light-receiving layer for optical communication having a wide band gap cannot receive near-infrared light and mid-infrared light of 1.65 μm or more.

  Light from 1.65 [mu] m to 3.0 [mu] m may be absorbed in water or used to test food sugar content. FIG. 6 is a graph showing the wavelength dependence of the water absorption rate. The horizontal axis represents the wavelength (nm) of light. The vertical axis represents the water absorption rate. There is almost no absorption up to 1200 nm. Absorption of about 40% appears at 1400 nm. However, the absorption decreases again from 1600 nm to 1700 nm. There is an absorption peak at 1900 nm. It decreases at 2000 nm and the absorption continues to increase between 2200 nm and 2800 nm. The absorption is close to 100% at 2800 nm to 3100 nm. The absorption peak at 1400 nm is low and insufficient.

  Therefore, if it is desired to measure the concentration of water, mid-infrared light of 1900 nm or 2800 nm to 3100 nm is suitable. If there is a light source that emits mid-infrared light and a light-receiving element that receives light, a sensor for mid-infrared light should be manufactured.

  If a sensor that measures moisture concentration can be made, it has various uses. For example, soil moisture can be easily measured. Alternatively, the amount of water in the garbage can be measured in the garbage disposal apparatus. Because it is based on optical means, it is easier to install and easier to measure than a sensor that measures the amount of water by changing capacitance or dielectric capacitance.

  Organic substances have many bonds such as C—H, O—H, and N—H, and create vibration levels, and thus have a number of absorption peaks in the mid-infrared range. The concentration of the organic substance can also be obtained by emitting light having a wavelength matched with the peak and receiving the transmitted light on the organic substance. For example, polyethylene terephthalate (PET) has an absorption spectrum as shown in FIG. There is strong absorption at 1650 nm, 2140 nm, and 2300 nm. The concentration of PET should be able to be measured using these absorption peaks.

JP-A-9-166717

  Patent Document 1 uses an InGaAsP light receiving layer as a light receiving element in order to place a laser and a light receiving element (PD) on the same straight line in a transmission / reception module for optical communication. Since InGaAsP is a quaternary system, the composition can be freely changed, and a light receiving element having a sensitivity of only 1 μm to 1.4 μm is produced. The absorption edge wavelength is set to 1.4 μm using an InGaAsP mixed crystal having a larger band gap than InGaAs.

  Unlike the InGaAs light receiving layer, since the sensitivity region is narrow, the laser light can be transmitted in front of the laser generating 1.55 μm. There is an advantage that a duplexer and a multiplexer are unnecessary. It has been described to show that a photodiode having a light receiving layer of a quaternary system is known. Although it does not fall within the wavelength range of 1.65 μm to 3 μm, which is the wavelength range of the present invention, it is listed here because it contains arsenic As and phosphorus P as Group 5.

T.A. Murakami, H .; Takahasi, M .; Nakayama, Y .; Miura, K. et al. Takemoto, D.H. Hara, “InxGa1-xAs / InAsyP1-y detector for near infrared (1-2.6 μm)”, Conference Processings of Indium Phosphate and Related Materials ”

  As a photodetector having sensitivity in the near-infrared wavelength region of 1.65 μm or more, a light receiving element using InGaAs or GaInNAs as a light receiving layer has been proposed. The substrate is an InP substrate or a GaAs substrate.

Non-Patent Document 1 proposes a photodiode for mid-infrared light in which the light receiving layer is InGaAs. Even if InGaAs is used, the composition matching the InP substrate (for example, In 53%, Ga 47%) has a sensitivity of only 1 to 1.6 μm as described above. Non-Patent Document 1 produced In _0.82 Ga _0.18 As with a lower band gap by reducing the Ga ratio and increasing the In ratio. Since the In ratio is high, the band gap is narrow and mid-infrared light can be received. It is claimed that by changing the composition, it is possible to produce an InGaAs-based light receiving element having a sensitivity of up to 2.6 μm. However, when the In ratio is increased, lattice matching with InP of the substrate is lost.

In the case of InGaAs for mid-infrared light, for example, in order to have a sensitivity of up to 2.6 μm, there is a problem that the lattice mismatch with InP is large and crystal defects are generated, resulting in an increase in dark current. Therefore, Non-Patent Document 1 provides an InAs _y P _1-y graded layer in which, for example, 12 to 20 layers of y are gradually changed between the InP substrate and the InGaAs light receiving layer for lattice relaxation. Each thickness is 1 μm. Therefore, a graded layer having a thickness of 12 μm to 20 μm is interposed between the InP substrate and the InGaAs light receiving layer so as to reduce lattice mismatch. Therefore, it is claimed that the dark current is sufficiently small. The dark current is stated to be 20 μA to 35 μA.

FIG. 8 shows the structure of a photodiode proposed by Non-Patent Document 1. On the n ⁺ -InP substrate 52, 12 to 20 graded layers InAs _y P _1-y 53 with slightly different mixed crystal ratios y are laminated. Further, an InAs _0.6 P _0.4 layer 54 is provided thereon. A light-receiving layer _thereon, and by further growing a window layer of _InAs 0.6 _{P 0.4} 56 provided _{n-In 0.82 Ga} 0.18 As55. A mask such as SiN is provided on the InAs _0.6 P _0.4 window layer and zinc is diffused to provide a p region at the center of the element, and a pn junction is formed in the middle of the light receiving layer. Further, a p-electrode 58 is formed in the p-type region. An n-electrode 60 is attached to the bottom surface of the InP substrate. Since the In _0.82 Ga _0.18 As light receiving layer must be lattice-matched, the window layer cannot be InP.

Japanese Patent Laid-Open No. 9-219563 “Semiconductor optical device and application system using it”

  On the other hand, a light receiving element using GaInNAs as a light receiving layer has also been proposed. For example, Patent Document 2 proposes such a light emitting element and a light receiving element. This is proposed for receiving mid-infrared light of 1.7 μm to 5 μm. Group 3 is a quaternary system consisting of Ga and In and Group 5 is composed of N and As. It is stated that light having a wavelength of 1.7 μm or more can be received because a band gap of 0.73 eV or less can be produced.

FIG. 9 is a cross-sectional view of a photodiode using a light receiving layer having an absorption edge wavelength of 2076 nm proposed by Patent Document 2. In FIG. An n-InP layer, a non-doped GaInNAs layer, a p-InP layer, and a p-GaInAs layer are formed on the n-InP substrate by epitaxial growth. Further, both sides of the element are etched away until reaching the n-InP substrate to form a convex shape. A p-electrode is formed so as to cover the surface with SiO ₂ and further make a hole at the top to make ohmic contact with p-GaInAs. An n electrode is formed in a ring shape on the bottom surface of the n-InP substrate. The light receiving layer is GaInNAs.

Non-Patent Document 1 suppresses lattice mismatch between the InP substrate and In _0.82 Ga _0.18 As by interposing a large number of InAs _y P _1-y graded layers. It is difficult and expensive to epitaxially grow a large number of graded layers with y gradually increasing. In addition, the dark current is not sufficiently low. It shows a dark current three orders of magnitude greater than the dark current of a photodiode for optical communication composed of a normal InP substrate and an InGaAs light receiving layer epitaxially grown thereon.

  Further, since InAsP is used for the window layer instead of InP, the sensitivity in a short wavelength region of 1.5 μm or less is greatly reduced due to the window effect corresponding to the composition. Furthermore, since very high stress is inherent in the crystal, it is liable to break during the process, resulting in poor mass productivity.

  A light receiving element using GaInNAs of Patent Document 2 as a light receiving layer has not been realized yet. The reason is that crystal growth of GaInNAs itself is technically difficult. In particular, it is difficult to achieve good crystallinity with a bulk crystal of 0.2 μm or more. Patent Document 2 says that GaInNAs and the InP substrate are lattice-matched, but it is difficult to grow a good GaInNAs thin film even if the lattice-matching conditions are satisfied. The condition necessary for successful growth of the thin film crystal on the InP substrate is not only the lattice matching condition. Even if the GaInNAs quaternary system is aligned with InP, when grown on InP, a large number of defects are generated in GaInNAs and a high-quality single crystal thin film is not obtained.

  In other words, there is still no light-receiving element with excellent sensitivity that functions in the mid-infrared region and a low dark current.

  The present invention proposes a photodiode in which a light-receiving layer (light absorption layer) having a composition of GaInNAsP, GaInNAsSb, or GaInNAsPSb is formed on an InP substrate. An InP substrate can be used as the substrate, and mid-infrared light of 1.67 μm to 3.0 μm can be received. Proposing to realize a light-receiving layer with improved crystal matching with InP by forming a light-receiving layer (light absorption layer) in which GaInNAs is added with phosphorus P or antimony Sb or both P and Sb. To do.

  That is, nonuniformity and defects in the crystal composition are remarkable only with GaInNAs proposed by Patent Document 2 and the like, but by adding P and Sb, the nonuniformity in crystal composition and the generation of defects can be counteracted. When only the lattice constants are compared, even though the GaInNAs disclosed in Patent Document 2 is the same as the InP substrate, the GaInNAs does not grow well on the InP substrate. In the present invention, a small amount of P or Sb is added to make a thin single crystal that is firm and has good consistency. By adding P or Sb, the band gap has an effect of increasing the wavelength, and by adding these, a high-quality single crystal can be grown on the InP substrate with good consistency. The gist of the present invention is there. It is effective to add P and Sb separately, but both P and Sb may be added.

  The preferable Sb content in the light receiving layer composition is 0.1 at% to 20 at%. If the Sb content exceeds 20 at%, lattice mismatch appears and defects are generated. The occurrence of defects cannot be suppressed when the content is 0.1 at% or less.

  The preferable P content in the light receiving layer composition is 0.01 at% to 10 at%. If the P content exceeds 10 at%, there is a drawback that the emission intensity becomes weak. If the P content is 0.01% or less, defects increase. The contents of Sb and P in the light receiving layer composition when both P and Sb are added are 0.1 at% to 20 at% and 0.01 at% to 10 at%, respectively. Even if it adds Sb of 0.1 at% or less and P of 0.01 at% or less, the effect does not appear.

The occurrence of defects can be suppressed by adding a very small amount of P or Sb to GaInNAs. The light-receiving layer formed of GaInNAs containing P or Sb in the upper range has a degree of lattice mismatch (| Δa / a |) with the InP substrate of 0.2% or less. That can not be formed on an InP substrate as the non-patent document 1, _In 0.82 _{Ga 0.18} As absorption layer as described in FIG. 8, an InP substrate and _{an In} 0.82 _{Ga 0.18} As absorption layer An InAs _y P _1-y graded layer (about 12 to 20 layers) for matching had to be provided.

  Although the graded layer can be formed by the MBE method or the like, it is necessary to change the mixed crystal ratio y accurately and quantitatively, so that the production is not easy and the cost is increased. In the present invention, such a graded layer for lattice matching between the substrate and the light receiving layer (light absorbing layer) is not necessary. This is because the light receiving layer is lattice-matched with the substrate in the range of ± 0.2% from the beginning. Since no graded layer is required, manufacturing is easy, cost is low, and yield is improved.

In addition, the window layer need not be InAs _0.6 P _0.4 as shown in FIG. In Non-Patent Document 1, since the light-receiving layer and InP do not match, the window layer must also be InAs _0.6 P _0.4 . This is because the light receiving layer is not aligned with InP. However, since the GaInNAsP, GaInNAsSb, or GaInNAsPSb light-receiving layer is aligned with the InP substrate in the present invention, InP can also be used as the window layer. Alternatively, In _0.52 Al _0.48 As that is lattice-matched with InP and has a slightly shorter band gap wavelength than InP can be used as the window layer. The band gap of In _0.52 Al _0.48 As is 0.86 μm while the band gap of InP is 0.92 μm. Since the InP window layer can be used, it is not necessary to make InAs _0.6 P _0.4, which is difficult to control the mixed crystal ratio, as the window layer, and the window layer can be easily manufactured.

In addition, if InAsP is used as the window layer, there is absorption, and the sensitivity is lowered at a wavelength of 1.5 μm or less. However, in the present invention, since the light receiving layer is aligned with the InP substrate, InP or In _0.52 Al _0.48 As can be used as the window layer, and there is no absorption at 1.5 μm or less, and a short wavelength region of 1.5 μm or less. There will be no reduction in sensitivity.

  The present invention proposes a photodiode fabricated on an InP substrate using GaInNAsP or GaInNAsSb or GaInNAsPSb in which P or Sb or both P and Sb are added to GaInNAs as a light-receiving layer. Even if the lattice constants match, GaInNAs has a non-uniform crystal composition and defects when viewed microscopically, but by adding a little Sb or P or both, the crystal composition becomes uniform and defects are generated. Can be suppressed. It has been experimentally found that a good light-receiving layer crystal can be obtained because the substrate and the light-receiving layer are matched.

  The inventors have also found that the use of a tilted substrate improves the N incorporation efficiency and improves the crystal uniformity. If an off-angle InP substrate whose surface is slightly inclined from the (100) plane is used instead of the (100) plane InP substrate having the normal (100) plane as the surface, the nitrogen N uptake efficiency increases and the light receiving layer and window layer The crystal uniformity can be improved. A desirable inclination angle (off angle; off angle) of the InP substrate is 0.2 ° to 20 ° from (100).

FIG. 11 shows a structure according to an example of the light receiving element (GaInNAsP light receiving layer) of the present invention. An n-InP layer 3, a GaInNAsP light-receiving layer 4, and an n-InP window layer 5 are provided on the n-InP substrate 2 by epitaxial growth. The n-InP layer 3 is a buffer layer, which may be doped n-type or non-doped. Even if it is not doped, it becomes n-type. The n-InP window layer 5 may also be n-type doped or non-doped. Even if it is not doped, it becomes n-type. A protective film 7 (SiN, SiO ₂ ) is provided on the n-InP window layer 5, an opening is provided in the center, and a mask is formed to selectively diffuse zinc from the opening to form a p region 6. A pn junction 9 can be formed in the middle of the GaInNAsP light receiving layer 4. A p-electrode 8 is provided on the p-region 6. An n-electrode 10 is provided on the bottom surface of the n-InP substrate 2. The light receiving layer is a ternary system. Although it is a complicated layer, since a small amount of P is contained in GaInNAs, the lattice constant is approximated to the InP substrate, and the lattice matching is excellent. It is possible to change the mixed crystal ratio of the light receiving layer so as to have sensitivity in a wavelength range of 1.65 μm to 3 μm.

FIG. 12 shows a structure according to another example of the light receiving element (GaInNAsSb light receiving layer) of the present invention. On the n-InP substrate 2, an n-InP layer 3, a GaInNAsSb light-receiving layer 24, and an n-InP window layer 5 are provided by epitaxial growth. The n-InP buffer layer 3 may be doped undoped or n-type. Even if it is not doped, it becomes n-type. As in the light receiving element of FIG. 11, a protective film 7 (SiN, SiO _2, etc.) is provided on the n-InP window layer 5 and an opening is provided in the center to selectively diffuse zinc from the opening as a mask. Region (p region) 6 is formed. A pn junction 9 can be formed in the middle of the GaInNAsSb light receiving layer 24. A p-electrode 8 is provided on the p-region 6. An n-electrode 10 is provided on the bottom surface of the n-InP substrate 2. The light receiving layer is a GaInNAsSb ternary system. Since a small amount of Sb is contained in GaInNAs, the compatibility with the InP substrate is improved and the matching is excellent. It is possible to change the mixed crystal ratio of the light receiving layer so as to have sensitivity in a wavelength range of 1.65 μm to 3 μm.

FIG. 16 shows a structure according to an example of a third type light receiving element (GaInNAsPSb light receiving layer) of the present invention. On the n-InP substrate 2, a non-doped InP layer 13, a GaInNAsPSb light-receiving layer 14, an InAlAs window layer 25, and an InGaAs cap layer 26 are provided by epitaxial growth. The non-doped InP layer 13 is a buffer layer. Even if it is not doped, it becomes n-type. The n-InAlAs window layer 25 may also be n-type doped or non-doped. Even if it is not doped, it becomes n-type. The InGaAs cap layer 26 may also be n-type doped or non-doped. Even if it is not doped, it becomes n-type. A protective film 7 (SiN, SiO ₂ ) is formed on the InGaAs cap layer 26, and an opening is provided in the central portion to serve as a mask to selectively diffuse zinc from the opening to form the p region 6. A pn junction 9 is formed in the middle of the GaInNAsPSb light receiving layer 14. A p-electrode 8 is provided on the p-region 6. An n-electrode 10 is provided on the bottom surface of the n-InP substrate 2. The light receiving layer is a 6-element system. Although it is a complicated layer, since GaInNAs contains a small amount of P and Sb, the lattice constant is approximated to the InP substrate, and the lattice matching is excellent. It is possible to change the mixed crystal ratio of the light receiving layer so as to have sensitivity in a wavelength range of 1.65 μm to 3 μm.

11, FIG. 12, and FIG. 16 exemplify top-illuminated photodiodes in which light enters from the top surface, but back-illuminated types can also be used. In the case of the back-illuminated type, the n-electrode is formed in a ring shape to form a central opening so that light enters the bottom surface of the substrate from the opening.
The InP substrate of the present invention is not necessarily an n-InP substrate. A semi-insulating substrate SI-InP can also be used.

In FIG. 13, an n-InP layer 32, an n-InP buffer layer 33, a GaInNAsP34, and an n-InP window layer 35 are epitaxially grown on an iron (Fe) -doped SI-InP substrate 30, and a protective mask (SiN, SiO ₂ 37) and zinc is selectively diffused to form a Zn diffusion region (p region) 36. A pn junction 39 is formed in the middle of GaInNAsP34. Both sides of the InP window layer 35, GaInNAsP34, and n-InP buffer layer 33 are removed by etching to expose the n-InP layer 32. An n electrode 40 is provided on the n-InP layer 32. A p electrode 38 is attached to the p region 36. The exposed part is covered with a protective film 37. Light enters from the back surface of the SI-InP substrate 30. Although the structure is complex, since the transparency of iron-doped SI-InP is higher than that of n-InP (for example, S-doped), the sensitivity can be further increased.

FIG. 14 shows an epitaxial growth of an n-InP layer 32, an n-InP buffer layer 33, a GaInNAsSb 44, and an n-InP window layer 35 on an iron (Fe) -doped SI-InP substrate 30, and a protective mask (SiN, SiO ₂ 37) and zinc is selectively diffused to form a Zn diffusion region (p region) 36. A pn junction 39 is formed in the middle of GaInNAsSb44. Both sides of the InP window layer 35, GaInNAsSb 44, and n-InP buffer layer 33 are removed by etching to expose the n-InP layer 32. An n electrode 40 is provided on the n-InP layer 32. A p electrode 38 is attached to the p region 36. The exposed part is covered with a protective film 37. Light enters from the back surface of the SI-InP substrate 30.

In the present invention, since GaInNAsP, GaInNAsSb, or GaInNAsPSb is used as a light-receiving layer (light absorption layer), it can be well aligned with the InP substrate. Even if there is some lattice mismatch, it can be suppressed to within ± 0.2%. Since there is no mismatch with the InP substrate, a crystal with good crystallinity and few defects is obtained. The light receiving element has little dark current and excellent sensitivity. A preferable concentration of Sb is 0.1 at% to 20 at%, and a preferable concentration of P is 0.01 at% to 10 at%. “at%” refers to the ratio among group 5 elements. It is not the ratio to the whole atom. It is a percentage expression of the mixed crystal ratio. GaInNAs and GaInNAsSb, GaInNAsP, and GaInNAsPSb of the present invention can also be seen by the difference in surface roughness of the epitaxially grown crystal surface. In order to investigate the effect of the present invention, an epitaxially grown thin film as shown in FIG. 17 was formed and the surface state was observed. An InAlAs buffer layer having a thickness of 0.3 μm was provided on an iron-doped InP substrate, and a sample was prepared by epitaxially growing GaInNAs (conventional example) and GaInNAsSb (present invention) having a thickness of 1 μm thereon.
Specific compositions are Ga _0.47 In _0.53 N _0.018 As _0.982 (conventional example) and Ga _0.47 In _0.53 N _0.018 As _0.932 Sb _0.05 (invention). It is. This is obtained by replacing a part of As atoms in the conventional example with Sb. Irregularities on the surface of both samples were observed with an atomic force microscope (AFM).
FIG. 18 shows the surface of Ga _0.47 In _0.53 N _0.018 As _0.982 (conventional example). The roughness of the surface can be seen well from micrographs. The unevenness of the surface along one straight line is shown in a graph. The horizontal axis is the dimension along the line segment and indicates a length of 5 μm. The vertical axis represents minute irregularities on the surface, and the full scale is ± 25 nm. Many irregularities of about 10 nm to 5 nm are observed. There are as many as ± 15 nm irregularities. The surface is extremely rough and the surface roughness is large.
FIG. 19 shows the surface of Ga _0.47 In _0.53 N _0.018 As _0.932 Sb _0.05 (invention). It can be seen well from the micrograph that the surface flatness is excellent. The unevenness of the surface along one straight line is shown in a graph. The horizontal axis is the dimension along the line segment and shows a length of 0.7 μm. The vertical axis represents minute irregularities on the surface, and the full scale is ± 2.5 nm. Note that it is 1/10 of the scale of FIG. It can be seen that although there are irregularities of about 0.3 nm to 0.1 nm, it is extremely smooth and flat. The surface roughness is about 1/30 or less of the conventional example of FIG. In the sample of FIG. 19, the other conditions are the same except that 5 at% of Sb is added. Therefore, the surface condition is improved by the effect of Sb addition.

The InAs _y P _1-y graded layer for relaxing the lattice mismatch between the substrate and the light receiving layer as described in Non-Patent Document 1 using InGaAs as the light receiving layer (FIG. 8) is unnecessary. Since the graded layer is produced with as many as 20 layers, it requires delicate control and takes time. Since the present invention does not require such a mechanism for lattice matching, the structure can be simplified and the manufacturing cost can be reduced as compared with Non-Patent Document 1.

  Non-Patent Document 1 uses InGaAs as a light-receiving layer and lattice mismatch with InP, so that an InP window layer cannot be provided on the light-receiving layer (FIG. 8). Although InAsP is used as the window layer, it absorbs wavelengths shorter than 1.5 μm. In the present invention, the window layer can be InP. InP can pass light of 1.5 μm or less.

  Non-Patent Document 1 says that the dark current is 20 μA to 30 μA (@ 1 V, 300 K), but it is not a small value as the dark current. Since there is no lattice mismatch in the present invention, there are few defects and the dark current can be about 2 to 5 nA. The low dark current is important in photodiodes. The present invention is an excellent light receiving element because the dark current is about 1/1000 to 1/10000 of Non-Patent Document 1.

  Patent Document 2 is a light receiving element for mid-infrared light in which a light-receiving layer of GaInNAs is provided on an InP substrate or a GaAs substrate, but as described above, it is difficult to grow a crystal of GaInNAs on an InP substrate. GaInNAs having a good crystal structure with a thickness of 2 μm or more has not been obtained yet. Even if the theoretical lattice constant of GaInNAs is the same as the lattice constant a of the InP substrate, it does not grow well on the InP substrate (to a thickness of 0.2 μm or more).

  The present invention makes it possible to grow high-quality crystals more easily by adding P or Sb or both P and Sb to GaInNAs. In addition to the coincidence of lattice constants, it is necessary to improve the uniformity of the crystal composition seen microscopically and suppress the generation of defects. It is considered that P and Sb satisfy it.

[Example 1 (GaInNAsP light-receiving layer / InP substrate: just substrate, off-angle substrate: MOVPE method: FIGS. 1 and 3)]

(1) A light receiving element having a structure as shown in FIG. 1 was manufactured.
The electrode / layer structure is from top to bottom,
p-electrode 8 / SiN film 7
Non-doped InP window layer 5 (d = 1.5 μm) / Zn diffusion region 6
n-GaInNAsP4 (Si doped; n = 3 × 10 ¹⁵ cm ⁻³ , d = 1 μm)
Non-doped InP buffer layer 3 (d = 2 μm)
n-InP substrate 2 (S-doped)
n-electrode 10
It is like that.

Non-doped InP3 (2 μm thickness), GaInNAsP4 (carrier concentration 3 × 10 ¹⁵ cm ⁻³ , 1 μm thickness) doped with a small amount of Si, and non-doped InP window layer 5 (1.5 μm thickness) on a 2-inch S-doped n-InP substrate 2 Epitaxial growth was performed by the MOVPE method. The epitaxial growth temperature is 520 ° C.

  The raw materials are trimethylindium (TMIn), triethylgallium (TEGa), tertiarybutylarsine (TBAs), tertiarybutylphosphine (TBP), dimethylhydrazine (DMHy), and tetraethylsilane (TESi).

  Two types of InP substrates were used: a (100) just substrate and an off-angle substrate tilted 10 ° from (100) in the [111] direction.

  The degree of lattice mismatch between the GaInNAsP light-receiving layer and the InP substrate is 0.1%. The lattice mismatch could always be ± 0.2% or less. The growth was interrupted at the stage where an InP buffer layer and a GaInNAsP layer were grown on the InP substrate, and photoluminescence was examined.

  FIG. 3 shows a PL (photoluminescence) spectrum of the GaInNAsP layer. The horizontal axis represents wavelength (μm), and the vertical axis represents photoluminescence intensity (arbitrary scale). The solid line is obtained by epitaxially growing InP / GaInNAsP / InP on a 10 ° off-angle InP substrate. A broken line is obtained by epitaxially growing InP / GaInNAsP / InP on a (001) just InP substrate. The photoluminescence of GaInNAsP was weak at a wavelength of 2.4 μm or less, but started to increase at about 2.45 μm, and showed a peak at a wavelength of 2.6 μm. It can be seen that the band gap of the light receiving layer is about 0.48 eV.

  The result was that the photoluminescence intensity was stronger on the off-angle substrate tilted by 10 degrees in the [111] direction than on the (100) just substrate. The GaInNAsP provided on the off-angle substrate has a photoluminescence intensity about 6 times that of the GaInNAsP provided on the just substrate. This means that the light receiving layer grown on the off-angle substrate has better crystallinity.

  A PIN type photodiode was fabricated using a wafer (InP substrate / InP buffer layer / GaInNAsP / InP window) epitaxially grown on an off-angle InP substrate tilted by 10 degrees in the [111] direction. Zn was selectively diffused using SiN as a mask to form a pn junction in GaInNAsP.

  The light receiving diameter is 300 μmΦ. The dark current was 3 nA (@ 2 V, 300 K), and good results were obtained. This is the dark current of Non-Patent Document 1 having an InGaAs light receiving layer (about 1 / 10,000 of 20 μA to 30 μA). That means that GaInNAsP has good crystallinity and few defects. It is an excellent n-photodiode for mid-infrared light.

[Example 2 (GaInNAsSb light-receiving layer / InP substrate: just substrate; off-angle substrate: MBE method: FIGS. 2 and 4)]
A light receiving element having a structure as shown in FIG. 2 was manufactured. The electrode / layer structure is from top to bottom,

p-electrode 8 / SiN film 7
Non-doped InP window layer 5 (d = 1.5 μm) / Zn diffusion region 6
n-GaInNAsSb24 (Si-doped; n = 3 × 10 ¹⁵ cm ⁻³ , d = 1 μm)
Non-doped InP buffer layer 3 (d = 1 μm)
n-InP substrate 2 (S-doped)
n-electrode 10
It is like that.

Non-doped InP (1 μm thickness), non-doped GaInNAsSb (carrier concentration 3 × 10 ¹⁵ cm ⁻³ , 1 μm thickness), and non-doped InP window layer (1.5 μm thickness) were epitaxially grown on a 2-inch S-doped InP substrate by MBE. The growth temperature is 490 ° C.

  As the InP substrate, a (100) just InP substrate and an off-angle InP substrate inclined by 15 degrees from (100) in the [1-11] direction were used. After the epitaxial growth to the GaInNAsSb layer, the surface photoluminescence was measured. The result is shown in FIG. The horizontal axis is the wavelength (μm). The vertical axis represents photoluminescence intensity (arbitrary scale). The solid line is the photoluminescence of GaInNAsSb grown on the off-angle substrate, and the broken line is the photoluminescence grown on the (100) just substrate. The solid line is a low value up to about 2.8 μm, but starts to increase rapidly from around 2.86 μm. It has a peak at 2.95 μm.

  This GaInNAsSb has a band gap wavelength of 2.95 μm. The band gap is 0.42 eV. GaInNAsSb provided on an off-angle substrate has a peak height of 2.95 μm about 9 times higher than GaInNAsSb grown on a (100) just substrate. It is better to provide it on an off-angle substrate because it has better crystallinity.

  In order to investigate the influence of the off-angle (off-angle, tilt angle) of the substrate on the GaInNAsSb light-receiving layer (light absorption layer), several InP substrates with different off-angles were prepared, and on the off-angle InP substrate, A sample on which an InP buffer layer and a GaInNAsSb light-receiving layer (Sb concentrations were 1 at% and 5 at%) was produced under the same conditions. The results are shown in FIGS. The horizontal axis is the off angle (degree) of the InP substrate, and the vertical axis is the photoluminescence intensity (arbitrary scale). In the case of Sb concentration of 1 at% (black circle mark)) and in the case of Sb concentration of 5 at% (black triangle mark), there are 9 types of samples, and 9 dots are marked on the graph. Measured and averaged over multiple samples.

[1. When Sb concentration is 1 at% (black circle in FIG. 5)]
(100) Just, the photoluminescence (PL) is 0.1, the off angle is 2 degrees, and the photoluminescence is 0.25. Photoluminescence increases to 1.0 at an off angle of 5 degrees. The off angle is 10 degrees and the maximum value is 1.2. The off angle is 13 degrees and the PL is 1.0. When the off angle is 15 degrees, the PL drops to 0.9. The off angle is 20 degrees and the PL is 1.15. When it exceeds 20 degrees, PL starts to decrease. The off angle is 25 degrees and the PL drops to 0.3. At an off angle of 33 degrees, PL decreases to 0.05.
From this measurement, it is understood that when the Sb concentration is 1 at%, the off angle of the desired InP substrate is 5 degrees to 20 degrees. This result is for the GaInNAsSb light-receiving layer, but a similar result was obtained for the GaInNAsP light-receiving layer.

[2. When Sb concentration is 5% (black triangle mark in FIG. 15)]
(100) Just and photoluminescence (PL) is 0.1. This is the same as when Sb = 1 at%. The off-angle is 0.2 degree, and the photoluminescence becomes strong at 0.9. The off-angle is 2 degrees and the photoluminescence rises to 1.3. The off-angle is 5 degrees and the photoluminescence is 1.0. The off-angle is 10 degrees and the photoluminescence is 0.9. The off angle is 13 degrees and the PL is 1.0. When the off angle is 15 degrees, the PL drops to 0.9. The off angle is 20 degrees and the PL drops to 0.85. When it exceeds 20 degrees, PL starts to decrease. The off angle is 25 degrees and the PL drops to 0.25. At an off angle of 33 degrees, PL decreases to 0.05.
From this measurement, it is understood that when the Sb concentration is 5 at%, the desired off-angle of the InP substrate is 0.2 degrees to 20 degrees. This result is for a GaInNAsSb light-receiving layer (Sb = 5 at%), but the same result was obtained when the Sb concentration was 20 at% or less. In the GaInNAsP light-receiving layer (P concentration 0.01 at% to 10 at%), similar results were obtained by increasing the P concentration to 2 at%.
Furthermore, similar results were obtained even in the case of GaInNAsPSb (P concentration 0.01 at% to 10 at%, Sb concentration 0.1 at% to 20 at%).
[Upper limit of Sb concentration and P concentration]
When the Sb concentration exceeds 20 at%, photoluminescence is hardly obtained. Further, the crystal purity was lowered so that the carrier concentration in non- ^doping exceeded 1 × 10 ¹⁷ cm ^−3, and only a crystal purity of a level that could not be used for the light receiving element was obtained.
If the concentration of P exceeds 10 at%, the epi surface becomes cloudy and photoluminescence is hardly obtained, and cannot be used for a light receiving element.

  In this example, the degree of lattice mismatch between the GaInNAsSb light receiving layer and the InP substrate is 0.15%. Since the mismatch is 0.2% or less, a graded layer is unnecessary and there are few crystal defects.

  Zn was selectively diffused using SiN as a mask on an epitaxial wafer obtained by epitaxially growing InP / GaInNAsSb / InP on an off-angle substrate (off-angle 15 °) to form a pn junction in GaInNAsSb. The light receiving diameter was 200 μmΦ. The dark current was 2 nA @ 2V, and good results were obtained. This is a minute dark current of about 1/1000 of Non-Patent Document 1. It can be seen that this is an excellent mid-infrared photodiode.

[Example 3 (GaInNAsSb light-receiving layer / InP substrate :; off-angle substrate: photodiode)]
Using the GaInNAsSb / InP light-receiving element produced in Example 2, a reception module for optical communication was produced. The light receiving diameter is 200 μm. The dark current obtained was 3 nA @ 5V. Furthermore, the wavelength dependence of sensitivity was investigated. The result is shown in FIG. The horizontal axis represents the wavelength (Wavelength: nm). The vertical axis represents the relative sensitivity (Relativistic Sensitivity: dB). The wavelength dependence of the sensitivity of a normal InGaAs / InP light receiving element used in optical communication was also examined.

  Ordinary InGaAs / InP is different from InGaAs / InP for mid-infrared for Non-Patent Document 1 described above. This is a general-purpose (no graded layer) InGaAs / InP photodiode for ordinary optical communication using the conventional 1.3 μm band and 1.55 μm band.

  InGaAs / InP used in normal optical communication has sensitivity from 950 nm, and good sensitivity from 1300 nm to 1550 nm. However, the sensitivity of an ordinary InGaAs photodiode for optical communication is drastically lowered when it exceeds 1620 nm. There is almost no sensitivity at 1640 nm. This is because the band gap of the InGaAs light receiving layer is as large as about 0.76 eV.

  In contrast, the sensitivity of the GaInNAsSb / InP photodiode of the present invention can be maintained up to 1700 nm. This is suitable for reception in the L band band that needs to maintain high sensitivity up to 1625 nm. That is, the photodiode of the present invention is also useful for long wavelength optical communication. As described above, since GaInNAsSb of Example 2 has an absorption edge at 2.9 μm, it has sensitivity from 1700 nm to 2900 nm. Since optical communication using a quartz fiber does not use such a long wavelength, different applications are possible with respect to sensitivity in that range.

[Example 4 (GaInNAsSb light-receiving layer / InP substrate; substrate: photodiode; soil moisture measurement sensor)]
The moisture content of the soil was measured using the GaInNAsSb / InP light-receiving element produced in Example 2. As shown in FIG. 6, water has absorption peaks at 1450 nm and 1950 nm. Absorption is also large at 2900 nm to 3100 nm. So far, there has been a technique to measure soil moisture by changing capacitance, but there is still no technique to measure it optically. This is because there is no excellent sensor. In order to measure moisture, the absorption light may be measured by applying light having an absorption spectrum peak peculiar to water as described above.

  Using the two water absorption spectra of 1450 nm and 1950 nm, the transmitted light was received by the GaInNAsSb / InP light receiving element of the present invention, and a calibration curve was prepared and calculated. As a result, the difference from the amount of water determined from the weight of water evaporated by actually heating the soil to 1000 ° C. was within 0.3%. That is, the amount of water could be quantified with an accuracy within 0.3%.

[Example 5 (GaInNAsSb light-receiving layer / InP substrate; off-angle substrate: photodiode; PET sorting sensor)]
A plastic separation system was manufactured using the GaInNAsSb / InP light-receiving element manufactured in Example 2. The absorption spectrum of polyethylene terephthalate (PET) is as shown in FIG. The horizontal axis represents wavelength (nm) and the vertical axis represents spectral intensity.
There is a need to sort PET from various plastics. Measuring light of one wavelength cannot distinguish between PET and other materials. Several selected wavelengths are used to measure the absorption or transmission of light at that wavelength with several photodiodes. It was possible to select PET from other substances by applying a neural network to recognize the spectral intensity ratio.

[Example 6 (GaInNAsSb light-receiving layer / InP substrate; off-angle substrate: photodiode; fruit sugar content sensor)]
The sugar content of the apple was measured using the GaInNAsSb / InP light-receiving element produced in Example 2. As a result of investigating the correlation between sugar content estimated from absorption due to sugar content of apples observed up to 1 to 2 μm and sugar content measured by the conventional destructive inspection method, sugar content (10-20%) can be measured with accuracy of ± 1% The result was obtained.

[Example 7 (GaInNAsSb light-receiving layer / InP substrate: 0.2 degree off substrate, just substrate: MOVPE method: FIG. 16)]
(1) A light receiving element having a structure as shown in FIG. 16 was manufactured.
The electrode / layer structure is from top to bottom,
p-electrode 8 / SiN film 7
In _0.53 Ga _0.47 As cap layer 26 (n = 5 × 10 ¹⁵ cm ⁻³ , d = 0.02 μm)
Si-doped In _0.52 Al _0.48 As window layer 25 (n = 5 × 10 ¹⁵ cm ⁻³ , d = 0.6 μm) / Zn diffusion region 6
n-Ga _0.47 In _0.53 N _0.018 As _0.9315 P _0.0005 Sb _0.05 layer 14 (non-doped; n = 8 × 10 ¹⁵ cm ⁻³ , d = 2 μm)
Non-doped InP buffer layer 13 (d = 1 μm)
n-InP substrate 2 (S-doped)
n-electrode 10
It is like that. This light receiving layer is GaInNAsPSb with 0.05 at% P and 5 at% Sb.

On a 2-inch S-doped n-InP substrate 2 (0.2 degrees off in the [11-1] direction from the (100) plane), a non-doped InP (1 μm thickness) layer 13, a non-doped GaInNAsPSb (carrier concentration 8 × 10 ¹⁵ cm ^{− 3} , 2 μm thickness) 14, Si-doped In _0.52 Al _0.48 As window layer (carrier concentration 5 × 10 ¹⁵ cm ⁻³ , 0.6 μm thickness) 25, In _0.53 Ga _0.47 As cap layer ( (Carrier concentration 5 × 10 ¹⁵ cm ⁻³ , 0.02 μm thickness) 26 was epitaxially grown by MBE. The epitaxial growth temperature is 490 ° C. Zn was selectively diffused using the SiN film as a diffusion mask. TiPt was used for the p electrode and AuGeNi was used for the n electrode.
A pin type PD having a light receiving diameter of 200 μmφ was produced. The dark current at a reverse bias voltage of 1 V is 1 nA, which is sufficiently small. The PD was good.

Sectional drawing of the light-receiving element for mid-infrared light which concerns on Example 1 of this invention which provided the GaInNAsP light-receiving layer on the n-InP board | substrate.

Sectional drawing of the light receiving element for mid-infrared light based on Example 2 of this invention which provided the GaInNAsSb light-receiving layer on the n-InP substrate.

6 is a graph showing the photoluminescence measurement results of the GaInNAsP light-receiving layer of Example 1. The horizontal axis represents wavelength (μm), and the vertical axis represents photoluminescence intensity (arbitrary scale).

The graph which shows the photoluminescence measurement result of the GaInNAsSb light receiving layer of Example 2. The horizontal axis represents wavelength (μm), and the vertical axis represents photoluminescence intensity (arbitrary scale).

An InP buffer layer is provided on an (100) just substrate and an off-angle substrate inclined by 2 to 33 degrees in the [111] direction or [11-1] direction from the (100) plane, and a GaInNAsSb light receiving layer ( The graph which shows the measurement result of the photoluminescence intensity | strength of the sample which provided Sb density | concentration 1at%.

The graph which shows the wavelength dependence of the light absorption rate by water. The horizontal axis represents wavelength (nm), and the vertical axis represents absorption rate.

The graph which shows the wavelength dependence of the light absorption rate by a polyethylene terephthalate. The horizontal axis represents wavelength (nm), and the vertical axis represents absorption rate.

Non-Patent Document 1 (T.Murakami, H.Takahasi, M.Nakayama, Y.Miura, K.Takemoto, D.Hara, "In x Ga 1-x As / InAs y P 1-y detector for near infrared (1 -2.6 μm) ”, a cross-sectional view of a photodiode for mid-infrared light in which an InGaAs light receiving layer is provided on an InP substrate proposed by Conference Processings of Indium Phosphide and Related Materials) via a graded layer.

Sectional drawing of the photodiode for mid-infrared light which provided the GaInNAs light-receiving layer on the InP board | substrate proposed by patent document 2 (Unexamined-Japanese-Patent No. 9-219563 "Semiconductor optical element and its application system").

The graph which shows the wavelength dependence of the sensitivity of the GaInNAsSb / InP photodiode of this invention, and the normal InGaAs / InP photodiode for optical communications. The horizontal axis is wavelength (nm), and the vertical axis is relative sensitivity (Relativistic Sensitivity).

Sectional drawing of the light receiving element for mid-infrared light of this invention which provided the GaInNAsP light-receiving layer via the InP buffer layer on the n-InP substrate.

Sectional drawing of the light receiving element for mid-infrared light of this invention which provided the GaInNAsSb light-receiving layer via the InP buffer layer on the n-InP substrate.

Sectional drawing of the light receiving element for mid-infrared light of this invention which provided the n-InP layer and GaInNAsP on the iron dope SI-InP board | substrate.

Sectional drawing of the light receiving element for mid-infrared light of this invention which provided the n-InP layer and GaInNAsSb on the iron dope SI-InP board | substrate.

An InP buffer layer is provided on an (100) just substrate and an off-angle substrate inclined by 2 to 33 degrees in the [111] direction or [11-1] direction from the (100) plane, and a GaInNAsSb light receiving layer ( The graph which shows the measurement result of the photoluminescence intensity | strength of the sample which provided Sb density | concentration 5at%.

Sectional drawing of the light receiving element for mid-infrared light which concerns on Example 7 of this invention which provided the GaInNAsPSb light-receiving layer on the n-InP substrate.

FIG. 4 is a layer structure diagram of a sample for comparing the surface states of a GaInNAs layer (conventional example) and a GaInNAsSb layer (present invention) epitaxially grown on an InAlAs layer on an iron-doped InP substrate.

An atomic force microscope on the surface of an InAlAs layer provided on an iron-doped InP substrate and a GaInNAs layer (comparative example: Ga _0.47 In _0.53 N _0.018 As _0.982 ) epitaxially grown _thereon ( AFM) Graph of unevenness along a photograph and a straight line.

An InAlAs layer is provided on an iron-doped InP substrate, and a GaInNAsSb layer (example of the present invention: Ga _0.47 In _0.53 N _0.018 As _0.932 Sb _0.05 ) is epitaxially grown _thereon . An atomic microscope (AFM) photograph and a graph of unevenness along a straight line.

Explanation of symbols

2 n-InP substrate 3 InP buffer layer
4 GaInNAsP light-receiving layer (light absorption layer)
5 n-InP window layer 6 Zn diffusion region
7 Protective film (SiN, SiO ₂ )
8 p-electrode
9 pn junction
10 n electrode
13 Non-doped InP layer 14 GaInNAsPSb light receiving layer (light absorption layer)
24 GaInNAsSb absorption layer (light absorption layer)
25 InAlAs window layer 26 InGaAs cap layer 30 SI-InP substrate 32 n-InP
33 n-InP
34 GaInNAsP
35 InP
36 Zn diffusion region 37 Protective film 38 p electrode 39 pn junction 40 n electrode

Claims (16)

A light receiving element comprising: an InP substrate; and a GaInNAsP light receiving layer (light absorption layer) provided on the InP substrate and having a band gap wavelength of 1.65 μm to 3.0 μm. The light receiving element according to claim 1, wherein the concentration of P in the GaInNAsP light receiving layer is 0.01 at% to 10 at%. When the lattice constant of the InP substrate is a and the difference between the lattice constants of GaInNAsP and InP is Δa, GaInNAsP has a lattice matching degree such that | Δa / a | ≦ 0.2% with respect to the InP substrate. The light receiving element according to claim 1. A light receiving element comprising: an InP substrate; and a GaInNAsSb light receiving layer (light absorption layer) provided on the InP substrate and having a band gap wavelength of 1.65 μm to 3.0 μm. The light receiving element according to claim 4, wherein the GaInNAsSb light receiving layer has a Sb concentration of 0.1 at% to 20 at%. When the lattice constant of the InP substrate is a, and the difference between the lattice constants of GaInNAsSb and InP is Δa, GaInNAsSb has a lattice matching degree such that | Δa / a | ≦ 0.2% with respect to the InP substrate. The light receiving element according to claim 4. A light receiving element comprising: an InP substrate; and a GaInNAsPSb light receiving layer (light absorption layer) provided on the InP substrate and having a band gap wavelength of 1.65 μm to 3.0 μm 8. The light receiving element according to claim 7, wherein in the GaInNAsPSb light receiving layer, the concentration of P is 0.01 at% to 10 at% and the concentration of Sb is 0.1 at% to 20 at%. When the lattice constant of the InP substrate is a and the difference between the lattice constant of GaInNAsPSb and InP is Δa, GaInNAsPSb has a lattice constant such that | Δa / a | ≦ 0.2% with respect to the InP substrate. The light receiving element according to claim 7. The light receiving device according to any one of claims 1 to 9, wherein the InP substrate is an off-angle substrate inclined from 0.2 to 20 degrees in a [111] direction or a [11-1] direction from (100). element. The light receiving element according to claim 1, wherein the InP substrate is Fe-doped semi-insulating. The light receiving element according to claim 1, further comprising an InP window layer or an InAlAs window layer on the light receiving layer. A light receiving module for optical communication, comprising the light receiving element according to claim 1. A water concentration measuring instrument using the light receiving element according to claim 1. A sugar content measuring instrument using the light receiving element according to any one of claims 1 to 12. 13. A plastic sorting measuring instrument using the light receiving element according to claim 1.

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