JP2005019654A - Quantum dot semiconductor element and its manufacturing method, and quantum dot semiconductor laser, optical amplification element, photoelectric transducer, optical transmitter, optical relay and optical receiver using the quantum dot semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prolong the wavelengths in a function wavelength such as oscillation wavelength, amplification wavelength and photoelectric transducing wavelength in a quantum dot semiconductor element. <P>SOLUTION: A method of manufacturing the quantum dot semiconductor element includes the steps of forming a quantum dot 23a in a substrate 21, introducing nitrogen onto a surface of the quantum dot 23a, and forming a cap layer 25 for protecting the quantum dot 23a having the surface onto which nitrogen is introduced. By such manufacturing method, the quantum dot semiconductor element in which nitrogen is introduced onto the surface of the quantum dot is formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a quantum dot semiconductor device, and more particularly to a quantum dot semiconductor device having a longer working wavelength. And it is related with the manufacturing method of this quantum dot semiconductor element. Furthermore, the present invention relates to a quantum dot laser, an optical amplification element, a photoelectric conversion element, an optical transmitter, an optical repeater, and an optical receiver using the quantum dot semiconductor element.
[0002]
[Prior art]
In recent years, with the aim of further development of electronics technology, devices utilizing quantum mechanical effects have been actively researched and developed. Quantum dots are known as one of materials that utilize this quantum mechanical effect. A quantum dot is a minute particle made of a semiconductor, a metal, or the like, and is a very fine potential box that gives a carrier a three-dimensional quantum confinement. Quantum dots exhibit the unique properties of zero-dimensional electron systems. For example, in a quantum dot semiconductor laser using quantum dots as an active layer of a semiconductor laser, a very sharp emission spectrum, improved temperature characteristics at the oscillation threshold current, reduced oscillation threshold current, increased modulation bandwidth, etc. A very attractive characteristic improvement in application can be realized.
[0003]
In the early days of this quantum dot, a technique for finely processing a semiconductor crystal using an electron beam lithography technique or a focused ion beam was used. However, due to problems such as limitations of nanoscale fine processing and damage. Recently, self-organization during crystal growth has been used exclusively. As a method for producing the quantum dots, there is a SK (Stranski-Krastanow) method using crystal growth of a lattice mismatched material.
[0004]
The SK method is a method for growing a second semiconductor having a lattice constant larger than the first lattice constant on a first semiconductor base substrate having a first lattice constant. The second semiconductor grows in a two-dimensional layer at the beginning of the growth, but when the thickness of the second semiconductor exceeds a certain threshold, it grows into a nanoscale three-dimensional island on the two-dimensional layer. To do. This three-dimensional island is a quantum dot, and the two-dimensional layer is called a wetting layer. Such growth is believed to be due to the bonding force and lattice mismatch between the underlying substrate and the growth layer. In the initial stage of growth, the second semiconductor has a strong bonding force with the first semiconductor base substrate and grows two-dimensionally flat. When further growth is performed, the first semiconductor base substrate and the growth layer of the second semiconductor The strain energy due to lattice mismatch increases, and finally a three-dimensional island-like growth occurs. Note that the three-dimensional island has a property of becoming smaller as the lattice mismatch between the first semiconductor base substrate and the second semiconductor growth layer is larger (see, for example, Patent Document 1).
[0005]
On the other hand, in recent years, communication traffic has increased rapidly due to the rapid spread of the Internet, and in response to this, an optical communication system capable of transmitting a large amount of communication signals at high speed has been developed. An optical communication system generally includes a light source unit that emits light, an external modulation unit that modulates light output from the light source unit according to information to be transmitted and outputs it as an optical signal, and light that amplifies the modulated optical signal An optical transmitter that outputs an amplified optical signal with an amplifying unit, a transmission path that includes an optical fiber that transmits light output from the optical transmitter, and an optical amplifying unit that amplifies the light transmitted through the optical fiber And an optical receiver including a receiving unit that receives the amplified light. If the transmission distance is long, an optical repeater having an optical amplifying unit for amplifying light to compensate for transmission loss is interposed in the transmission line as necessary. There is also an optical transmitter that directly modulates the light source unit without providing an external modulation unit.
[0006]
[Patent Document 1]
JP 2000-196198 A
[0007]
[Problems to be solved by the invention]
By the way, in the current optical communication system, an optical fiber made of silica glass is mainly used, and the transmission loss of the optical fiber is small in the wavelength band of 1.3 μm or the wavelength of 1.55 μm. These wavelength bands are used as bands. For this reason, a quantum dot semiconductor device corresponding to the oscillation wavelength 1.3 μm band or the wavelength 1.55 μm band has been demanded, and the wavelength in the working wavelength such as the oscillation wavelength, the amplification wavelength, and the photoelectric conversion wavelength in the quantum dot semiconductor device is increased. Is desired.
[0008]
In particular, quantum dot semiconductor lasers are expected to improve various characteristics based on the above-described unique properties of the zero-dimensional electron system, and are expected to improve the performance of optical transmitters incorporating them. In the current optical communication system, a rare earth element-doped optical fiber amplifier is used as an optical amplifier. This rare earth element-doped optical fiber amplifier is larger than other components because a long optical fiber is required to obtain a predetermined amplification factor. As a result, miniaturization of optical transmitters, optical repeaters, and optical receivers incorporating rare earth element-doped optical fiber amplifiers is hindered.
[0009]
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a quantum dot semiconductor element in which the wavelength of the acting light is increased and a method for manufacturing the quantum dot semiconductor element. Furthermore, it aims at providing the quantum dot semiconductor laser, optical amplification element, photoelectric conversion element, optical transmitter, optical repeater, and optical receiver using such a quantum dot semiconductor element.
[0010]
[Means for Solving the Problems]
In order to achieve the above-described object, the quantum dot semiconductor device according to the present invention introduces nitrogen into the surface of the quantum dot. And the manufacturing method of such a quantum dot semiconductor element has the process of forming a quantum dot in a substrate, the process of introducing nitrogen into the surface of the quantum dot, and the cap which covers the quantum dot which introduced nitrogen into the surface Forming a layer. In the quantum dot semiconductor device described above, the quantum dots are InAs quantum dots formed on a predetermined surface of GaAs.
[0011]
In the quantum dot semiconductor device having such a configuration, by introducing nitrogen into the surface of the quantum dot, the working wavelength can be increased as compared with a quantum dot semiconductor device that is not introduced.
[0012]
And in the above-mentioned quantum dot semiconductor element, the cap layer which coat | covers the said quantum dot which introduce | transduced nitrogen on the surface is further provided. The quantum dot semiconductor device having such a configuration can protect the quantum dots by providing a cap layer, and further stack the quantum dot layer on the cap layer to make the quantum dot layer multi-layered. it can.
[0013]
Furthermore, in the above-described quantum dot semiconductor device, the cap layer is one of InGaAs, GaAsN, or InAsSb. Since the quantum dot semiconductor device having such a configuration uses InGaAs, GaAsN, or InAsSb for the cap layer, the working wavelength can be further increased.
[0014]
In addition, the quantum dot semiconductor laser according to the present invention uses any of the quantum dot semiconductor elements described above for the active layer. The optical amplifying element according to the present invention uses any of the above-described quantum dot semiconductor elements for the amplifying layer. The photoelectric conversion element according to the present invention uses any of the above-described quantum dot semiconductor elements for the photoelectric conversion layer.
[0015]
The quantum dot semiconductor laser, the optical amplifying element, and the photoelectric conversion element having such a configuration are obtained by converting the quantum dot semiconductor element into which nitrogen is introduced into the active layer of the quantum dot semiconductor laser, the amplification layer of the optical amplification element, and the photoelectric conversion of the photoelectric conversion element. Since each layer is used, the working wavelength can be made longer than when a quantum dot semiconductor element into which nitrogen is not introduced is used, and the characteristics can be improved based on the unique properties of the zero-dimensional electron system.
[0016]
Further, according to the present invention, light including a light source unit that emits light, a modulation unit that modulates light of the light source unit according to information to be transmitted, and an optical amplification unit that amplifies the light modulated by the modulation unit In the transmitter, the light source unit includes a quantum dot semiconductor laser using any one of the above quantum dot semiconductor elements in an active layer. In the present invention, an optical transmitter including a light source unit that emits light, a modulation unit that modulates light from the light source unit according to information to be transmitted, and an optical amplification unit that amplifies the light modulated by the modulation unit The optical amplifying unit includes an optical amplifying element using any of the above-described quantum dot semiconductor elements in the amplifying layer.
[0017]
Since the optical transmitter having such a configuration uses a quantum dot semiconductor laser or an optical amplifying element using a quantum dot semiconductor element into which nitrogen is introduced, it is compared with a case where a quantum dot semiconductor element into which nitrogen is not introduced is used. Thus, the working wavelength can be increased. In particular, the optical transmitter can be reduced in size by using an optical amplifying element using a quantum dot semiconductor element into which nitrogen is introduced.
[0018]
In the present invention, in the optical repeater including an optical amplifying unit interposed in the transmission path and amplifying the light, the optical amplifying unit uses any of the above-described quantum dot semiconductor elements in the amplifying layer. Is provided. Since the optical repeater having such a configuration uses an optical amplifying element using a quantum dot semiconductor element into which nitrogen is introduced, the working wavelength is longer than that in the case of using a quantum dot semiconductor element into which nitrogen is not introduced. The wavelength can be changed. In addition, the optical repeater can be reduced in size.
[0019]
In the present invention, in an optical receiver comprising an optical amplifying unit for amplifying light and a receiving unit for receiving the light amplified by the optical amplification, the optical amplifying unit includes any one of the quantum dots described above in the amplifying layer. An optical amplification element using a semiconductor element is provided. In the present invention, in an optical receiver comprising an optical amplifying unit for amplifying light and an optical receiving unit for receiving the light amplified by the optical amplification, the optical receiving unit is connected to any of the photoelectric conversion layers described above. A photoelectric conversion element using such a quantum dot semiconductor element is provided.
[0020]
Since the optical receiver having such a configuration uses an optical amplifying element or a photoelectric conversion element using a quantum dot semiconductor element into which nitrogen is introduced, the working wavelength is compared with that in the case of using a quantum dot semiconductor element into which nitrogen is not introduced. Can be made longer. In particular, by using an optical amplifying element using a quantum dot semiconductor element into which nitrogen is introduced, the optical receiver can be downsized.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described below with reference to the drawings. First, the manufacturing apparatus of a nitrogen introduction | transduction quantum dot semiconductor element is demonstrated.
[0022]
FIG. 1 is a schematic configuration diagram of a growth chamber in an apparatus for manufacturing a nitrogen-introduced quantum dot semiconductor device by molecular beam epitaxy.
[0023]
In general, molecular beam epitaxy (MBE) is 10 ^-10 -10 ^-11 In an ultra-high vacuum of about Torr, the substrate is heated to a growth temperature, and the cell containing the raw material of the element to be epitaxially grown is heated to evaporate the raw material and supply it to the substrate in the form of a molecular beam. This is a crystal growth method for epitaxial growth.
[0024]
The apparatus for producing a nitrogen-introduced quantum dot semiconductor element has a growth chamber 1 for growing each layer on a substrate 21, an exchange chamber (load lock chamber) for exchanging a sample, and an exchange chamber to a growth chamber 1, as in the well-known MBE apparatus. And a plurality of chambers such as a transfer chamber for transferring a sample such as the substrate 21. In FIG. 1, a growth chamber 1 includes a chamber 10, an indium cell (hereinafter abbreviated as “In cell”) 11, a gallium cell (hereinafter abbreviated as “Ga cell”) 12, and an arsenic cell (hereinafter “ Abbreviated as “As cell”) 13, nitrogen plasma cell 14, substrate holder 15, manipulator 16, RHEED electron gun 17, and RHEED screen 18.
[0025]
The chamber 10 is a housing for separating the outside from the environment, and the internal pressure of the chamber 10 can be set to an ultra-high vacuum by using a vacuum pump such as a rotary pump or a turbo molecular pump.
[0026]
The substrate holder 15 is a member made of, for example, Mo (molybdenum) that supports the substrate 21, and is supported by the chamber 10 by the manipulator 16. A resistance heater (not shown) is attached to the tip of the manipulator 16 near the substrate holder 15, and the substrate 21 is heated by the resistance heater through the manipulator 16 and the substrate holder 15. A thermocouple is built in the substrate holder 15, and the substrate temperature is detected by this thermocouple. Further, from the viewpoint of more accurately monitoring the substrate temperature of the substrate 21, an infrared radiation thermometer (not shown) is further provided.
[0027]
The In cell 11, the Ga cell 12, and the As cell 13 are Knudsen-cell type molecular beam sources. The In cell 11 is a substrate 21 in which In is evaporated as a molecular beam by heating a crucible containing indium (hereinafter abbreviated by the element symbol “In”) with a resistance wire to evaporate In at a high temperature and opening the shutter 11a. To supply. Similarly, the Ga cell heats a crucible containing gallium (hereinafter abbreviated as the element symbol “Ga”) with a resistance wire to evaporate Ga at a high temperature, and opens the shutter 12a to make Ga a molecular beam. Supply to the substrate 21. The As cell evaporates at a high temperature by heating arsenic (hereinafter abbreviated as element symbol “As”), and supplies As to the substrate 21 as a molecular beam by opening the shutter 13a. Control of molecular beams of In, Ga, and As is performed by opening / closing the shutter (opening time) and the heating temperature of the crucible.
[0028]
The nitrogen plasma cell 14 mainly generates atomic nitrogen (nitrogen plasma) by applying an RF (Radio Frequency) electric field to nitrogen gas, and supplies the atomic nitrogen to the substrate 21 by opening the shutter 14a. Here, as an apparatus for generating atomic nitrogen, there is an ECR (Electron Cyclotron Resonance) plasma cell, and an ECR plasma cell can be used. However, it is relatively difficult to generate nitrogen ion species that damage quantum dots. The RF plasma cell used in this embodiment is preferable because it can be easily controlled. The atomic nitrogen molecular beam is controlled by opening / closing the shutter (opening time) and RF power.
[0029]
FIG. 2 is a diagram showing a spectrum of nitrogen plasma. The horizontal axis of FIG. 2 indicates the wavelength (Wavelength) in nm units, and the vertical axis indicates the intensity (Intensity).
[0030]
In the nitrogen plasma cell 14, 1 ^st positive N ₂ 2 ^st positive N ₂ Excited nitrogen molecule called atomic nitrogen (Atomic N) ₂ ) And nitrogen ions are generated, and it is desirable to suppress the generation of nitrogen ions because they damage the crystal. Therefore, the nitrogen plasma cell 14 has nitrogen ions (light emission lines 391, 428 nm) and 2 as shown in FIG. ^st positive N ₂ (316-400 nm) Almost no series, 1 based on atomic nitrogen (emission lines 743.0, 818.3, 865.1 nm) and neutral nitrogen radicals ^st positive N ₂ The RF power is controlled to be relatively low so that the molecular beam source includes the (550 to 800 nm) series. Therefore, in this embodiment, atomic nitrogen and 1 ^st positive N ₂ Is considered to contribute to the addition of nitrogen to the surface of the quantum dots.
[0031]
The RHEED electron gun 17 is a device that irradiates an electron beam of about 10 keV to 40 keV, and the RHEED screen 18 is a device that projects a diffraction image of the electron beam reflected and diffracted by the substrate 21. These RHEED electron gun 17 and RHEED screen 18 are provided to constitute an RHEED device.
[0032]
RHEED (Reflection High Energy Electron Diffraction) is a method in which an electron beam is incident on a substrate surface at a shallow angle and an electron beam diffracted by a surface atomic arrangement is projected onto a fluorescent screen. In RHEED, since an electron beam is incident at a shallow angle, the diffraction image well represents the characteristics of the surface two-dimensional grating, and has a characteristic that the growth process is not disturbed. For this reason, RHEED is the most suitable method for observing the surface atomic arrangement, and the surface atomic arrangement can be observed during the growth process.
[0033]
From this feature, in this embodiment, RHEED is used as an apparatus for monitoring (monitoring) the surface in the growth process. In this embodiment, the growth of each layer is not controlled on the basis of the substrate temperature, but the growth is controlled while directly confirming the structure of the substrate surface by RHEED (in-situ observation). Each layer can be grown. In this embodiment, as will be described later, for example, the buffer layer is grown at a substrate temperature of about 550 ° C., but this substrate temperature is a reference, and the growth is controlled based on the diffraction image of RHEED. .
[0034]
Next, the manufacturing method of a nitrogen introduction | transduction quantum dot semiconductor element is demonstrated.
[0035]
FIG. 3 is a flowchart showing a method for manufacturing a nitrogen-introduced quantum dot semiconductor device. FIG. 4 is a schematic cross-sectional view of a nitrogen-introduced quantum dot semiconductor element in each manufacturing process. FIG. 5 is a diagram showing an RHEED pattern in the [−110] direction in the 2.5 ML InAs growth process. FIG. 6 is a diagram showing an RHEED pattern in the [−110] direction in a 1.0 ML InAs growth process. FIG. 7 is a diagram showing an RHEED pattern in the [−110] direction in the heterostructure growth process for 2.5 ML InAs.
[0036]
First, the first cleaning of the substrate 21 is performed (S11). This first cleaning is performed to remove contaminants attached to the substrate 21 before the substrate 21 is placed in the chamber 10. In this embodiment, the first cleaning is performed by boiling the substrate 21 with acetone and methanol, removing the oxide film with hydrofluoric acid, ₂ O (water): H ₂ O ₂ (Hydrogen peroxide): H ₂ SO ₄ (Sulfuric acid) It was performed by etching with a solution prepared at a ratio of 1: 1: 3.
[0037]
Here, the substrate 21 will be described. The substrate 21 is made of a III-V semiconductor that can have a band gap corresponding to the 1.3 μm band or the 1.55 μm band. The 1.3 μm band and the 1.55 μm band are communication wavelength bands in the current optical communication system. B (boron), Al (aluminum), Ga (gallium) and In (indium) are used as group III elements, and N (nitrogen), P (phosphorus), As (arsenic) and group V elements are used. Sb (antimony) is used. In the present embodiment, the substrate 21 is GaAs.
[0038]
Further, the surface structure of the substrate 21 will be described. The surface reconstruction structure on the GaAs (001) surface is divided into a Ga-rich surface and an As-rich surface, and the surface reconstruction structure on the As-rich surface is (2 × 4) α from the higher substrate temperature. There are (2 × 4) β, (2 × 4) γ, and c (4 × 4). The coverage of As is 2/4, 3/4, 4/4, and 7/4 in order, which indicates that As is higher, the As is more easily desorbed. In (2 × 4) α, two As dimers (As dimers) exist in the [−110] direction, and in (2 × 4) β, three As dimers exist in the [−110] direction, In (2 × 4) γ, one of the two As dimers exists in the [110] direction and the other in the [−110] direction, and in c (4 × 4), one As dimer is [110]. Exists in the direction. On the other hand, a Ga-rich surface can be obtained by desorbing As, and the surface reconstruction structure in that case is (4 × 2). In this embodiment, nitrogen can be accurately grown from various experimental results, and therefore the surface reconstruction structure is (2 × 4) γ. Therefore, in this embodiment, the (21 × 4) γ GaAs (001) substrate 21 is used.
[0039]
Next, the substrate 21 is put into the chamber 10 of the growth chamber 1, and the substrate 21 is subjected to the second cleaning (S12). This second cleaning is performed for removing an oxide film or the like on the surface of the substrate 21 after the substrate 21 is placed in the chamber 10. In the present embodiment, this second cleaning is 3.0 × 10. ^-6 The substrate 21 was heated at 600 ° C. for 20 minutes in an As atmosphere of Torr. The removal of the oxide film and the like was confirmed by referring to the RHEED pattern.
[0040]
Next, the buffer layer 22 is grown on the substrate 21 in order to reduce crystal defects and flatten the surface at the atomic level (S13). In this embodiment, in order to bring out the surface of a good surface reconstruction structure (2 × 4) γ with few defects, 3.0 × 10 ^-6 A GaAs buffer layer 22 was grown to a thickness of 160 nm at a substrate temperature of about 550 ° C. in an As atmosphere of Torr (see FIG. 4A). It is grown while confirming that the surface reconstruction structure is (2 × 4) γ by referring to the RHEED pattern. 3.0 × 10 after buffer layer growth ^-6 When the surface is observed in the As atmosphere of Torr, the [110] direction pattern of RHEED shows the rods of (00), (± 1/20), (± 10), and in the [−110] direction, As can be seen from FIGS. 5A, 6A, and 7A, streaky (00), (0 ± 1/4), (0 ± 2/4) in the RHEED pattern. , (0 ± 3/4), (0 ± 1) rods can be confirmed, and the strength of the (0 ± 2/4) rod is weak, so that the surface reconstruction structure of the buffer layer 22 is (2 × 4) It can be confirmed that it is γ.
[0041]
Next, a thin film 23 of a predetermined material is grown on the buffer layer 22 in order to form quantum dots by the SK method (S14). In this embodiment, at a substrate temperature of about 473 ° C., 3.0 × 10 ^-6 The InAs thin film 23 was grown to a predetermined film pressure by opening the shutter of the In cell 11 for a predetermined time in the As atmosphere of Torr (see FIG. 4B). The growth rate of the InAs thin film 23 by the In cell 11 is about 86 s / ML (molecular layer) when the temperature of the In cell 11 is 640 ° C.
[0042]
Here, in order to examine the formation conditions of the quantum dots, the predetermined time was set to 86 seconds and 210 seconds, and two types of InAs thin films 23 having a predetermined film pressure of 1.0 ML and 2.5 ML were formed. When the film pressure of the InAs thin film 23 is 1.0 ML, only streaky (00) and (0 ± 1) rods are visible in the [−110] direction RHEED pattern, as shown in FIG. A quantum dot is not formed in a two-dimensional layered structure having only the two-dimensional wetting layer 23a. On the other hand, when the film pressure in the InAs thin film 23 is 2.5 ML, the rods of (00) and (0 ± 1) are formed in the RHEED pattern in the [−110] direction as shown in FIGS. 5 (C) and 7 (B). Has changed to an arrow pattern, it can be seen that quantum dots 23b having a three-dimensional island structure are formed on the two-dimensional wetting layer 23a. The lattice constants of GaAs and InAs are 5.623 angstroms and 6.058 angstroms, respectively, and about 7% of strain energy resulting from these differences is accumulated, resulting in the formation of quantum dots with a three-dimensional island structure. Is done. Note that the RHEED patterns after 10 seconds during the growth are shown in FIGS. 5B and 6B, respectively, under the conditions of 2.5 ML and 1.0 ML.
[0043]
Next, nitrogen is introduced into the InAs surface (S15). This is one feature of the present invention. In this embodiment, at a substrate temperature of about 473 ° C., 3.0 × 10 ^-6 Nitrogen atoms were introduced into the InAs surface by applying a 224 W high-frequency electric field to the nitrogen plasma cell 14 in the As atmosphere of Torr and opening the shutter 14a for 8 seconds from the generation of the nitrogen plasma. As a result, as shown in FIG. 4C, a layer 24 relating to nitrogen atoms is formed on the quantum dots 23b and the wetting surface 23a. As can be seen by comparing FIG. 7B and FIG. 7C, the RHEED pattern after introduction of nitrogen for 8 seconds is substantially the same as the RHEED pattern after growth of InAs 210 seconds and damages the quantum dots 23b. It can be seen that nitrogen is introduced. That is, the quantum dots are maintained even after the introduction of nitrogen. The nitrogen atoms introduced to the surface of InAs are considered to be chemically adsorbed on the surface (As and atomic nitrogen on the surface layer of InAs are exchanged to form an InAsN nitride layer on the surface layer of InAs). However, nitrogen is introduced into the surface of InAs without damaging the quantum dots, and the working wavelength in the nitrogen-introduced quantum dot semiconductor element becomes longer than that in the case where a layer relating to nitrogen is not formed, as will be described later.
[0044]
Next, a GaAs cap layer is grown to cover the layer 24 related to nitrogen (S16). In this embodiment, a GaAs cap layer 25 is grown by 50 nm at a substrate temperature of about 473 ° C. in an As atmosphere of 3.0 × 10 −6 Torr (see FIG. 4D). FIG. 7D shows the RHEED pattern in the [−110] direction in this case. By providing the cap layer, the quantum dot in which the layer 24 relating to nitrogen is formed can be protected, and by further overlapping the quantum dot layer on the cap layer, the quantum dot layer can be multilayered.
[0045]
Through these manufacturing steps, the surface reconstruction (2 × 4) γ GaAs layers 21 and 22, the InAs layer 23 that forms InAs quantum dots, the nitrogen-related layer 24, and the GaAs cap layer 25. A nitrogen-introduced quantum dot semiconductor device comprising:
[0046]
FIG. 8 is a diagram showing an emission spectrum of photoluminescence. The horizontal axis in FIG. 8 indicates the wavelength in nm, and the vertical axis indicates the intensity. The measurement was performed at a measurement temperature of 4K and an excitation light source with an oscillation wavelength of 488 nm and 20 mW Ar. ⁺ The sample was emitted using a laser and the emission was detected using a GaInAs CCD.
[0047]
Interband edge energy E in semiconductor crystals _g Irradiation with excitation light having a larger energy may generate light called photoluminescence. This is light emitted when carriers are generated by excitation light and transition to an excited state to perform recombination. When a high-purity crystal is irradiated with excitation light at an extremely low temperature, a sharp emission peak appears in the energy region near the band edge (near-band-edge). On the other hand, when electrons and holes are recombined, the emission spectrum becomes unique to the impurities and lattice defects through a process of coupling through impurities and lattice defects present in the crystal. The emission intensity is determined by many factors such as the intensity and energy of excitation light, the impurity concentration, the concentration of non-radiative recombination centers, and the measurement temperature. By analyzing the emission spectrum of this photoluminescence, information on the band structure, impurities, lattice defects, and quantum properties can be obtained. When the semiconductor device is applied to a semiconductor laser, an optical amplification device, or a photoelectric conversion device, the emission wavelength and amplification The working wavelength such as wavelength or conversion wavelength is known.
[0048]
In FIG. 8, the emission spectrum of A is the case of InAs quantum dots (InAs QDs) in which nitrogen is not introduced, and the emission spectrum of B is the case of InAs quantum dots (InAs QDs + nitrogen) into which nitrogen is introduced. As can be seen from the emission spectra of A and B, it can be seen that the emission peak at 4K is shifted from about 1060 nm to about 1190 nm by about 130 nm and lengthened. Thus, the nitrogen-introduced quantum dot semiconductor element has a longer working wavelength than when no nitrogen-related layer is formed.
[0049]
FIG. 9 is a diagram showing the temperature dependence of the emission spectrum of photoluminescence. The horizontal axis in FIG. 9 indicates the wavelength in nm, and the vertical axis indicates the intensity. The emission spectrum was examined at each temperature of 4K, 6K, 8K10K, 15K, 20K, 30K, 40K, 60K, 80K, 100K, and 150K. FIG. 10 is a diagram showing the temperature characteristics of the emission peak. The horizontal axis in FIG. 10 indicates the temperature in K units, and the vertical axis indicates the wavelength in nm units. The broken line in FIG. 10 is a fitting function based on the measurement result indicating the temperature characteristic of the emission peak.
[0050]
As can be seen from FIG. 9, the nitrogen-introduced quantum dot semiconductor device according to the present embodiment is caused by a relatively strong light emission (emission peak) of about 1200 nm and a two-dimensional wetting layer due to the quantum dots at temperatures up to 150K. There is a broad emission of about 1000 nm to about 1100 nm. Further, as can be seen from FIGS. 9 and 10, it can be seen that the emission peak shifts to the longer wavelength side as the temperature rises. The temperature characteristic of the luminescence peak is expressed as in Equation 1 from the value of each luminescence peak at each temperature.
Eg (T) = Eg (0) −αT ² / (Β + T)
α = 4.99 × 10 ^-4 eV / K, β = 319K (1)
From this equation, the emission peak at a room temperature of 27 ° C. (300 K) is predicted to be about 1300 nm.
[0051]
The nitrogen-introduced quantum dot semiconductor device according to the present invention can be applied to a quantum dot laser, an optical amplification device, an optical signal processing device, a photoelectric conversion device, and the like. Then, next, the case where the nitrogen introduction | transduction quantum dot semiconductor element which concerns on this invention is used for the active layer of a semiconductor laser, the amplification layer of an optical amplification element, or the conversion layer of a photoelectric conversion element is demonstrated. The optical signal processing element is a highly functional / multifunctional element that can not only amplify light but also perform modulation and the like.
[0052]
FIG. 11 is a diagram illustrating a configuration of a nitrogen-introduced quantum dot semiconductor laser or a nitrogen-introduced quantum dot optical amplifier.
[0053]
In FIG. 11, a nitrogen-introduced quantum dot semiconductor laser 100 is a Fabry-Perot type semiconductor laser, and is formed on a semiconductor substrate 112, a semiconductor cladding layer 113 formed on the substrate 112, and the cladding layer 113. The active layer 114, the semiconductor clad layer 114 formed on the active layer 114, the oxide insulating layer 116 formed on the clad layer 114 leaving the clad layer 114 in a stripe shape, the insulating layer 116 and the stripe The metal ohmic electrode 117 formed on the clad layer 114 left in the shape and the metal ohmic electrode 111 formed on the opposite side of the surface of the substrate 112 on which the clad layer 113 is formed. The active layer 114 includes a buffer layer, a quantum dot layer into which nitrogen is introduced, and a cap layer, as in the above embodiment.
[0054]
The nitrogen-introduced quantum dot semiconductor laser 100 having such a configuration outputs laser light from the end face of the active layer 114 when current is injected. The wavelength of the output laser beam is longer than that of an active layer quantum dot semiconductor laser including a buffer layer, a quantum dot layer, and a cap layer.
[0055]
In order to improve the emission intensity, the active layer 114 may be formed by stacking a plurality of layers including a quantum dot layer into which nitrogen is introduced and a cap layer. Further, the active layer 114 may be configured by stacking a plurality of layers including a quantum dot layer and a cap layer into which nitrogen is introduced, and a layer including a normal quantum dot layer and a cap layer into which nitrogen is not introduced.
[0056]
On the other hand, the nitrogen-introduced quantum dot optical amplifier 120 has the same configuration as the nitrogen-introduced quantum dot semiconductor laser 100 except that the active layer 114 shown in FIG. The amplification layer 124 includes a buffer layer, a quantum dot layer into which nitrogen is introduced, and a cap layer, as in the above-described embodiment.
[0057]
In the nitrogen-introduced quantum dot light amplifying element 120 having such a configuration, as shown by a broken line in FIG. 11, when laser light is input from one end face of the amplification layer 124, the input laser light is amplified by stimulated emission and the amplification layer 124. Is output from the other end face of the. The wavelength of the laser light to be amplified is longer than that of the quantum dot light amplifying element of the amplification layer configured to include the buffer layer, the quantum dot layer, and the cap layer.
[0058]
In order to improve the amplification factor, the amplification layer 124 may be configured by stacking a plurality of layers each including a quantum dot layer into which nitrogen is introduced and a cap layer. Further, the amplification layer 114 may be configured by stacking a plurality of layers composed of a quantum dot layer and a cap layer into which nitrogen is introduced, and a layer composed of a normal quantum dot layer and a cap layer into which nitrogen is not introduced.
[0059]
FIG. 12 is a diagram illustrating a configuration of a nitrogen-introduced quantum dot photoelectric conversion element. In FIG. 12, the nitrogen-introduced quantum dot light amplifying element 130 includes a semiconductor substrate 132, a photoelectric conversion layer 133 formed on the substrate 132, a transparent electrode 134 formed on the photoelectric conversion layer 133, and a substrate 132. And a metal ohmic electrode 131 formed on the opposite side of the surface on which the photoelectric conversion layer 133 is formed. The photoelectric conversion layer 133 includes a buffer layer, a quantum dot layer into which nitrogen is introduced, and a cap layer, as in the above-described embodiment. The transparent electrode is, for example, ITO.
[0060]
In the nitrogen-introduced quantum dot photoelectric conversion element 130 having such a configuration, when light is incident on the photoelectric conversion layer 133 via the transparent electrode 134, the light is converted into a current according to the intensity by the photoelectric conversion layer 133, It is taken out from the ohmic electrode 131 and the transparent electrode 134. And the wavelength of the light photoelectrically converted becomes a long wavelength compared with the quantum dot photoelectric conversion element of the photoelectric conversion layer comprised including a buffer layer, a quantum dot layer, and a cap layer.
[0061]
In particular, the nitrogen-introduced quantum dot photoelectric conversion element 130 uses an InAs quantum dot formed on the GaAs substrate and nitrogen is introduced into the surface of the quantum dot for the photoelectric conversion layer 133. It becomes an element that photoelectrically converts light. For this reason, when the nitrogen introduction | transduction quantum dot photoelectric conversion element 130 is utilized for a solar cell, since infrared light can be photoelectrically converted, the conversion efficiency of the whole solar cell can be improved. In addition, when the nitrogen-introduced quantum dot photoelectric conversion element 130 is used for a solid-state imaging element, an imaging element that photoelectrically converts infrared light can be manufactured.
[0062]
In order to improve the conversion efficiency, the photoelectric conversion layer 133 may be configured by stacking a plurality of layers including a quantum dot layer into which nitrogen is introduced and a cap layer. Alternatively, the photoelectric conversion layer 133 may be configured by stacking a plurality of layers including a quantum dot layer and a cap layer into which nitrogen is introduced and a normal quantum dot layer and a cap layer into which nitrogen is not introduced.
[0063]
FIG. 13 is a diagram illustrating a configuration of an optical communication system. In FIG. 13, an optical communication system 200 receives an optical transmitter 201 that transmits an optical signal, a transmission path 202 that transmits an optical signal output from the optical transmitter 201, and an optical signal that is transmitted through the transmission path 202. An optical repeater 204 (204-1,..., 204-n) that is configured to include an optical receiver 203 that amplifies one or a plurality of optical signals in consideration of transmission loss of the optical signal. Provided inside.
[0064]
The optical transmitter 201 includes a laser light source (hereinafter abbreviated as “LD light source”) 211 that outputs laser light, an external modulator 212 that modulates laser light emitted from the LD light source according to information to be transmitted, An optical amplifier 213 that amplifies the laser light modulated by the external modulator 212, an optical coupler 214 that branches the laser light amplified by the optical amplifier 213 into two, and one of the laser lights branched by the optical coupler 214 is received. And a control circuit 216 for controlling the LD light source 211, the external modulator 212, and the optical amplifier 213, and the other laser beam branched by the optical coupler 214 is transmitted to the optical transmitter. 201 output. Based on the output of the light receiving element 215, the control circuit 216 adjusts the gain of the optical amplifier 213 so that laser light having a constant intensity is output from the optical transmitter 201. The LD light source 211 includes, for example, a nitrogen-introduced quantum dot semiconductor laser according to the present invention that emits laser light, and a drive circuit that drives the nitrogen-introduced quantum dot semiconductor laser. A cooling unit for cooling the dot semiconductor laser is further provided.
[0065]
In the above description, the laser light is modulated by the external modulator 212. However, the laser light may be modulated by direct modulation that modulates the drive current of the laser light source. In this case, the drive circuit is an embodiment corresponding to the modulation unit.
[0066]
The optical repeater 204 includes an optical amplifier 241 that amplifies laser light (optical signal) from the transmission path 202. The optical repeater 204 may further include a dispersion compensating optical fiber that compensates for chromatic dispersion as necessary.
[0067]
The optical receiver 203 includes an optical amplifier 231 that amplifies laser light (optical signal) from the transmission path 202, a light receiving element 232 that receives and photoelectrically converts the laser light amplified by the optical amplifier 231, and a light receiving element 232. And a demodulator 233 that demodulates information based on the output.
[0068]
The optical amplifiers 213, 231, and 241 are configured to include, for example, the nitrogen-introduced quantum dot optical amplifier according to the present invention, and further include a cooling unit that cools the nitrogen-introduced quantum dot optical amplifier as necessary. The light receiving elements 215 and 232 are configured to include, for example, the nitrogen-introduced quantum dot photoelectric conversion element according to the present invention, and further include a cooling unit that cools the nitrogen-introduced quantum dot photoelectric conversion element as necessary.
[0069]
In the optical communication system 200 having such a configuration, the optical signal output from the optical transmitter 201 is transmitted through the transmission line 202 while being compensated for transmission loss by the optical repeater 204 and received by the optical receiver 203. In particular, the performance of the optical transmitter 201 is improved by using a nitrogen-introduced quantum dot semiconductor laser, and the optical transmitter 201, the optical receiver 203, and the optical repeater 204 have optical amplifiers 213 and 231, respectively. 241 can be miniaturized by using a nitrogen-introduced quantum dot light amplifying element.
[0070]
In this embodiment, the MBE method is used for the growth of each layer. However, the present invention is not limited to this, and a semiconductor such as a laser vapor deposition method or MOCVD (Metal Organic Chemical Vapor Deposition) is used. Growth manufacturing technology can be used.
[0071]
In the present embodiment, InAs quantum dots are formed. However, the present invention is not limited to this. From the viewpoint of increasing the wavelength, a substance containing a material that reduces the band gap of InAs, for example, InAsN is used. Quantum dots may be formed.
[0072]
Furthermore, in the present embodiment, GaAs is used for the cap layer, but the present invention is not limited to this. From the viewpoint of increasing the wavelength, In is used. _x Ga _1-x As, InSbAs or GaAs _1-y N _y May be used. In _x Ga _1-x When As is used, the wavelength increases as the value of x increases. GaAs _1-y N _y Is used, the wavelength increases as the value of y increases.
[0073]
【The invention's effect】
As described above, in the quantum dot semiconductor device according to the present invention, the working wavelength can be increased by introducing nitrogen into the surface of the quantum dot as compared with the quantum dot semiconductor device that is not introduced. Further, by applying the quantum dot semiconductor device according to the present invention to a semiconductor laser, an optical amplifying device, and a photoelectric conversion device, the wavelength can be increased and the characteristics can be improved. Further, by using these elements for an optical transmitter, an optical repeater, and an optical receiver in an optical communication system, it is possible to increase the wavelength, improve the performance, and reduce the size of the device.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a growth chamber in an apparatus for producing a nitrogen-introduced quantum dot semiconductor device by molecular beam epitaxy.
FIG. 2 is a diagram showing a spectrum of nitrogen plasma.
FIG. 3 is a flowchart showing a method for manufacturing a nitrogen-introduced quantum dot semiconductor device.
FIG. 4 is a schematic cross-sectional view of a nitrogen-introduced quantum dot semiconductor device in each manufacturing process.
FIG. 5 is a diagram showing an RHEED pattern in a [−110] direction in a 2.5 ML InAs growth process.
FIG. 6 is a diagram showing an RHEED pattern in a [−110] direction in a 1.0 ML InAs growth process.
FIG. 7 is a diagram showing an RHEED pattern in the [−110] direction in a heterostructure growth process for 2.5 ML InAs.
FIG. 8 is a diagram showing an emission spectrum of photoluminescence.
FIG. 9 is a graph showing the temperature dependence of the emission spectrum of photoluminescence.
FIG. 10 is a graph showing temperature characteristics of emission peaks.
FIG. 11 is a diagram showing a configuration of a nitrogen-introduced quantum dot semiconductor laser or a nitrogen-introduced quantum dot optical amplifying element.
FIG. 12 is a diagram showing a configuration of a nitrogen-introduced quantum dot photoelectric conversion element.
FIG. 13 is a diagram illustrating a configuration of an optical communication system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Growth chamber, 10 chamber, 11 Indium cell, 12 Gallium cell, 13 Arsenic cell, 14 Nitrogen plasma cell, 15 Substrate holder, 16 Manipulator, 17 RHEED electron gun, 18 RHEED screen, 21, 112, 132 Substrate, 22 Buffer layer , 23 Thin film, 23a Quantum dot, 23b Wetting layer, 24 Nitrogen layer, 25 Cap layer, 100 Nitrogen introduced quantum dot semiconductor laser, 111, 117, 131 Ohmic electrode, 113, 115 Clad layer, 114 Active layer, 116 Insulating layer , 120 Nitrogen-introduced quantum dot optical amplifying element, 124 Amplifying layer, 130 Nitrogen-introduced quantum dot photoelectric conversion element, 133 Photoelectric converting layer, 134 Transparent electrode, 200 Optical communication system, 201 Optical transmitter, 202 Transmission path, 203 Optical receiver 204 Optical repeater 211 Chromatography The light source, 212 an external modulator, 213,231,241 optical amplifier 214 an optical coupler, 215,232 light receiving element, 216 a control circuit, 233 a demodulation circuit

Claims (13)

A quantum dot semiconductor device, wherein nitrogen is introduced into the surface of the quantum dot. 2. The quantum dot semiconductor device according to claim 1, wherein the quantum dots are InAs quantum dots formed on a predetermined surface of GaAs. The quantum dot semiconductor device according to claim 1, further comprising a cap layer that covers the quantum dots having nitrogen introduced on a surface thereof. 4. The quantum dot semiconductor device according to claim 3, wherein the cap layer is one of InGaAs, GaAsN, and InAsSb. A quantum dot semiconductor laser using the quantum dot semiconductor device according to any one of claims 1 to 4 in an active layer. An optical amplifying element using the quantum dot semiconductor element according to any one of claims 1 to 4 for an amplifying layer. A photoelectric conversion element using the quantum dot semiconductor element according to any one of claims 1 to 4 for a photoelectric conversion layer. In an optical transmitter including a light source unit that emits light, a modulation unit that modulates light of the light source unit according to information to be transmitted, and an optical amplification unit that amplifies the light modulated by the modulation unit,
The light source unit includes the quantum dot semiconductor laser according to claim 5. In an optical transmitter including a light source unit that emits light, a modulation unit that modulates light of the light source unit according to information to be transmitted, and an optical amplification unit that amplifies the light modulated by the modulation unit,
The optical transmitter includes the optical amplifying element according to claim 6. In an optical repeater provided with an optical amplifying unit interposed in the transmission path to amplify light,
An optical repeater comprising the optical amplifying element according to claim 6. In an optical receiver comprising an optical amplification unit that amplifies light and a reception unit that receives light amplified by the optical amplification,
The optical receiver includes the optical amplifying element according to claim 6. In an optical receiver comprising an optical amplifying unit for amplifying light and an optical receiving unit for receiving the light amplified by the optical amplification,
The said optical receiver is provided with the photoelectric conversion element of Claim 7, The optical receiver characterized by the above-mentioned. Forming quantum dots on the substrate;
Introducing nitrogen into the surface of the quantum dots;
And a step of forming a cap layer that covers the quantum dots introduced with nitrogen on the surface thereof.

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