Semiconductor device

Abstract

To provide a semiconductor element which is used as an SOA having quantum dots, in which the flatness of gain in a wavelength band is secured when used with a prescribed or greater amount of current injection.SOLUTION: A semiconductor element 1 comprises a quantum dot group 4 that is formed by laminating a plurality of quantum dot layers 5 differing in a center wavelength at which gain is maximum. The quantum dot group 4 is constituted such that the whole or part of the plurality of quantum dot layers 5 have their center wavelength sequentially shifting in the direction of the lamination thereof. Assuming that the quantum dot layer whose center wavelength is longest among the plurality of quantum dot layers 5 is a longest wavelength layer 5A, the quantum dot layer whose center wavelength is shortest is a shortest wavelength layer 5B, and a group including the longest wavelength layer 5A and consisting of some of the plurality of quantum dot layers 5 that are laminated from the longest wavelength layer 5A toward the shortest wavelength layer 5B is a longest wavelength layer group, the gain of the quantum dot group 4 in the center wavelength of the longest wavelength layer 5A or the longest wavelength layer group is larger than the gain in the respective center wavelengths of the other quantum dot layers 5.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a semiconductor device having quantum dots.

Background Art

[0002] Conventionally, as a semiconductor device having quantum dots, for example, the one described in Patent Document 1 has been proposed. The semiconductor device described in Patent Document 1 is used for a semiconductor optical amplifier (SOA), and includes an active layer having a composite quantum dot in which a plurality of quantum dots are stacked and a side barrier layer in contact with the side surface of the composite quantum dot. Note that SOA is an abbreviation for Semiconductor Optical Amplifier.

[0003] This semiconductor device is assumed to be used in an optical communication system or the like, and the number of stacked quantum dots constituting each quantum dot layer and the magnitude of the strain of the side barrier layer are designed so as to have a flat gain band corresponding to the shift amount of the gain spectrum in its operating temperature range.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, applying this type of semiconductor device to applications such as ranging sensors, for example, in-vehicle LiDAR, has been studied. In this case, the semiconductor device of the SOA is arranged, for example, in the vicinity of a laser light source having a wavelength in a predetermined near-infrared region and is used to amplify the laser light from the laser light source. Note that LiDAR is an abbreviation for Light Detection And Ranging.

[0006] Here, as ranging methods in LiDAR, for example, the TOF method and the FMCW method are known. TOF is an abbreviation for Time Of Flight, and the TOF method calculates the distance based on the time from emitting light externally until receiving the reflected light reflected by an external object. FMCW is an abbreviation for Frequency Modulated Continuous Wave, and the FMCW method continuously modulates the frequency of the laser light emitted externally and calculates the distance based on the frequency shift of the reflected light reflected by an external object. Hereinafter, for simplicity of explanation, the FMCW-based LiDAR is simply referred to as "FMCW-LiDAR".

[0007] In the case of in-vehicle FMCW-LiDAR, the semiconductor element as an SOA is required to have the characteristic of obtaining a constant gain over a wide wavelength band in a wide temperature range of the in-vehicle environment (for example, -40°C to 85°C, etc.). Quantum dots are known to have a small threshold change and temperature robustness even in such a wide temperature range. Therefore, in order to ensure the gain in the wavelength band of the laser light used, it has been studied to configure a semiconductor element as an SOA using a plurality of quantum dot groups with different operating wavelengths. In addition, for an SOA for in-vehicle use, compared with the case of communication use, in order to obtain a large gain in the wavelength band used, it is necessary to use it with a current injection amount equal to or more than a predetermined value.

[0008] However, as a result of the study by the present inventors, when the current injection amount is increased to ensure a gain equal to or more than a predetermined value, a difference occurs in the amount of gain increase between the short-wavelength side and the long-wavelength side in the wavelength band used, and it has been found that the flatness of the gain is impaired. For example, in the semiconductor element described in Patent Document 1, when the current injection amount is increased to obtain a gain equal to or more than a predetermined value required for in-vehicle use, the gain increase on the short-wavelength side becomes relatively larger than that on the long-wavelength side, and the flatness of the gain cannot be ensured.

[0009] In view of the above points, an object of the present invention is to ensure gain flatness in a wavelength band when a semiconductor device used as an SOA having quantum dots is used with a current injection amount of a predetermined value or more.

Means for Solving the Problems

[0010] To achieve the above object, the semiconductor device according to claim 1 is a semiconductor device in which a plurality of quantum dot layers (5) having different wavelengths at which gain is maximized are stacked, and a part or all of the plurality of quantum dot layers are provided with a quantum dot group (4) having a configuration in which wavelengths are sequentially shifted along the stacking direction of the quantum dot layers. Among the plurality of quantum dot layers, the quantum dot layer having the longest wavelength is defined as the longest wavelength layer, and the quantum dot layer having the shortest wavelength is defined as the shortest wavelength layer. A group consisting of a part of the plurality of quantum dot layers stacked from the longest wavelength layer toward the shortest wavelength layer including the longest wavelength layer is defined as the longest wavelength layer group. The quantum dot group has a gain at the wavelength of the longest wavelength layer or the longest wavelength layer group that is larger than the gain at the wavelength of each of the other quantum dot layers. <,> The plurality of quantum dot layers each have quantum dots, and the quantum dots are crystal-grown so as to cause strain. The emission wavelengths are different for each quantum dot layer. The energy levels of the ground states of the quantum dots in each quantum dot layer are different from the higher-order levels, which are higher in energy than the ground states of the quantum dots in other quantum dot layers on the side of the longest wavelength layer than the quantum dot layer in question. 。

[0011] This semiconductor device includes a quantum dot group in which a part or all of a plurality of quantum dot layers having different wavelengths at which gain is maximized are stacked such that their wavelengths are sequentially shifted. And, in the quantum dot group, the gain of the longest wavelength layer, which is the quantum dot layer having the longest wavelength, or the gain of a plurality of layers including this is larger than the gain of each of the other quantum dot layers. That is, this semiconductor device has a configuration in which the gain of the quantum dot layer having the longest wavelength among the quantum dot groups is relatively larger than the gain of the corresponding wavelength in the other short wavelength side quantum dot layers. Therefore, even when used with a current injection amount of a predetermined value or more, it is possible to suppress a relative decrease in the gain on the long wavelength side with respect to the gain on the short wavelength side in the entire quantum dot layer group, and to ensure a flat gain in the wavelength band.

[0012] Note that the reference numerals in parentheses attached to each component etc. indicate an example of the correspondence relationship between the component etc. and the specific components etc. described in the embodiments described later.

Brief Description of the Drawings

[0013]

Figure 1

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Figure 12

Modes for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are identical or equivalent to each other will be described with the same reference numerals.

[0015] (First Embodiment) The semiconductor element 1 of the first embodiment will be described with reference to the drawings.

[0016] 〔Basic Configuration〕 As shown in FIG. 1, for example, the semiconductor element 1 of this embodiment includes a semiconductor substrate 2, an n-clad layer 3, a quantum dot group 4 formed by laminating a plurality of quantum dot layers 5, and a p-clad layer 6, and these are laminated in this order. The semiconductor element 1 is suitably applied to, for example, an SOA used in in-vehicle FMCW-LiDAR, but of course, it can also be adopted for other applications.

[0017] The semiconductor substrate 2 is made of, for example, a semiconductor material such as n-type GaAs, and an n-clad layer 3 is laminated as a lower clad layer on the (100) plane. An electrode (not shown) is formed on, for example, the surface of the semiconductor substrate 2 opposite to the n-clad layer 3, and a voltage can be applied to the quantum dot group 4.

[0018] The n-clad layer 3 is made of, for example, a semiconductor material such as n-type AlGaAs and is laminated by an arbitrary film formation method such as the MBE (abbreviation for Molecular Beam Epitaxy) method.

[0019] The quantum dot group 4 is an active layer formed by laminating a plurality of quantum dot layers 5.

[0020] Hereinafter, for the convenience of explanation, as shown in FIG. 1, the total number of the plurality of quantum dot layers 5 is set as n (n: an integer greater than 0), and the layer closest to the semiconductor substrate 2 side among the plurality of quantum dot layers 5 is referred to as the "first layer 5-1". Further, along the stacking direction of the plurality of quantum dot layers 5, they are referred to as "second layer 5-2", "third layer 5-3", "fourth layer 5-4", ··· "nth layer 5-n". Furthermore, among the directions along the stacking direction of the plurality of quantum dot layers 5, the side closer to the semiconductor substrate 2 is defined as "down", and the opposite side is defined as "up". Another quantum dot layer 5 located downward as seen from a certain quantum dot layer 5 may be referred to as a "lower layer", and another quantum dot layer 5 located upward may be referred to as an "upper layer".

[0021] The quantum dot group 4 is configured such that, as it goes from the first layer 5-1 to the nth layer 5-n, the central wavelength at which the maximum gain of the quantum dots is obtained sequentially shifts to the short wavelength side. Hereinafter, among the plurality of quantum dot layers 5, the layer with the longest central wavelength is referred to as the "longest wavelength layer 5A", and the layer with the shortest central wavelength is referred to as the "shortest wavelength layer 5B". In this embodiment, for the quantum dot group 4, the end on the n-clad layer 3 side among the plurality of quantum dot layers 5 is the longest wavelength layer 5A, and the end on the p-clad layer 6 side is the shortest wavelength layer 5B. That is, in this embodiment, the quantum dot group 4 is configured such that the central wavelength of the quantum dots shifts to the short wavelength as it goes toward the upper layer.

[0022] Note that the "long wavelength" and "short wavelength" at the central wavelength of the quantum dot layer 5 are relative. For example, when applied to the SOA of in-vehicle FMCW-LiDAR, at room temperature, the central wavelength of the longest wavelength layer 5A is 1250~1600nm, and the central wavelength of the shortest wavelength layer 5B is 1100nm~1450nm, etc., but it is not limited thereto.

[0023] The quantum dot group 4 is formed by repeating the formation of the quantum dot layer 5 having quantum dots. Note that the quantum dot group 4 is formed by, for example, but not limited to, repeatedly stacking quantum dot layers 5 with a thickness of ten-odd nm to several tens of nm.

[0024] In the present embodiment, the quantum dot group 4 includes the longest wavelength layer 5A, and a plurality of layers from the longest wavelength layer 5A to the shortest wavelength layer 5B are defined as the "longest wavelength layer group". Excluding the longest wavelength layer group, the center wavelength of each quantum dot layer 5 is different.

[0025] For example, in the quantum dot group 4, the quantum dots in the longest wavelength layer group are InAs, and the quantum dots in the other quantum dot layers 5 are In x Ga (1-x) As (0 < x < 1). By this configuration, excluding the longest wavelength layer group, the emission wavelengths of each quantum dot layer 5 are different. The quantum dot group 4 is configured such that, for example, the quantum dots other than the longest wavelength layer group have different compositions or lattice constants for each quantum dot layer 5, so that the emission wavelength thereof is sequentially shifted. Note that the quantum dots are configured such that, for example, they are crystal-grown so as to cause strain, and the emission wavelengths of each quantum dot layer 5 are different.

[0026] In the present embodiment, in the quantum dot group 4, all of the plurality of quantum dot layers 5 constituting the longest wavelength layer group have the same center wavelength. In other words, in the quantum dot group 4, the total volume of the quantum dots having the longest center wavelength is larger than the total volume of the quantum dots in each of the other different quantum dot layers 5. Thereby, when a high current is injected, it is possible to obtain a gain of a predetermined value or more, that is, a high output, in the wavelength band of the quantum dot group 4, while ensuring the smoothness of the gain. Details thereof will be described later. Note that the number of quantum dot layers 5 constituting the longest wavelength layer group is two or three or more, but may be changed as appropriate.

[0027] The p-clad layer 6 is made of a semiconductor material such as p-type AlGaAs, and is formed on the quantum dot group 4 by an arbitrary film formation method such as the MBE method. On the surface of the p-clad layer 6 opposite to the quantum dot group 4, an electrode (not shown) is formed, which forms a pair with an electrode (not shown) formed on the semiconductor substrate 2, and a voltage can be applied to the quantum dot group 4 by these pair of electrodes.

[0028] The above is the basic configuration of the semiconductor device 1 of the present embodiment. The semiconductor device 1 obtains a gain of a predetermined value or more in a wide wavelength range by sequentially changing the height of the quantum dots in the quantum dot group 4 along the stacking direction, and the gain is smoothed by the longest wavelength layer group during high current injection.

[0029] 〔Effect by the longest wavelength layer group〕 Next, the gain smoothing by the longest wavelength layer group will be described with reference to the drawings.

[0030] First, the problems in the semiconductor device of the comparative example without the longest wavelength layer group will be described. The semiconductor device of the comparative example is different from the semiconductor device 1 of the present embodiment in that all of the plurality of quantum dot layers 5 constituting the quantum dot group 4 have different center wavelengths of gain and do not have a quantum dot layer 5 with the same center wavelength. The semiconductor device of the comparative example is configured such that the gain spectra of the plurality of quantum dot layers 5 overlap, so that, for example, as shown in FIG. 2, the gain in the wavelength band is smoothed at the electron injection amount for optical communication system applications.

[0031] Note that "low injection" in FIG. 2 means the electron injection amount (for example, a current amount of less than 10 mA) when assuming an optical communication system application, and "high injection" means the electron injection amount (for example, a current amount of 10 mA or more) when assuming an in-vehicle LiDAR application. "High injection" or "high electron injection" in this specification means the electron injection amount (for example, but not limited to, 10 mA or more) in applications that require high output such as in-vehicle LiDAR.

[0032] Here, in the quantum dot SOA, when the temperature changes, the gain changes even with the same electron injection amount, and between room temperature (for example, 25 ° C) and high temperature (for example, 85 ° C), the gain with respect to the electron injection amount and the wavelength band in which a gain of a predetermined value or more can be obtained change. For example, in the semiconductor device of the comparative example, if the wavelength band in the case of room temperature and low injection is defined as WR1 and the wavelength band in the case of high temperature and low injection is defined as WR2 as shown in FIG. 2, the range is WR1> WR2, and the gain is room temperature> high temperature.

[0033] Therefore, in order to secure a desired gain equal to or greater than a predetermined value, i.e., a target gain, in the entire wavelength band, it is conceivable to increase the amount of electron injection and increase the gain of each quantum dot layer 5. However, as a result of the study by the present inventors, in the semiconductor device of the comparative example, as shown in FIG. 2, when the injection is high at both room temperature and high temperature, the gain increase on the short-wavelength side is relatively larger than the gain increase on the long-wavelength side, and it has been found that the gain smoothness is impaired in the wavelength bands WR1 and WR2.

[0034] Specifically, in the semiconductor device of the comparative example, when the amount of electron injection was changed within the range of 3 mA to 15 mA at 25°C, for example, as shown in FIG. 3, although the gains at 1220 nm and 1285 nm both increased, the degrees of increase were significantly different. For example, the gain at 1220 nm was smaller than the gain at 1285 nm at 3 mA, but became comparable to the gain at 1285 nm at 6 mA, and became larger than the gain at 1285 nm when it reached 9 mA or more, while the difference from the gain at 1285 nm widened.

[0035] Also, in the semiconductor device of the comparative example, although the gain in the wavelength band of 1160 nm to 1360 nm at 85°C is generally smaller than that at 25°C, the tendency of the gain increase when the amount of electron injection was increased within the range of 4 mA to 20 mA was the same as that at 25°C. For example, in the semiconductor device of the comparative example, as shown in FIG. 4, at 85°C, the gain at 1250 nm was comparable to the gain at 1320 nm at 4 mA, but became slightly larger than the gain at 1320 nm at 8 mA. And the gain at 1250 nm became even larger than the gain at 1320 nm when it reached 12 mA or more, while the difference from the gain at 1320 nm widened.

[0036] As described above, in the semiconductor device of the comparative example, although the gain in the entire wavelength band can be increased by increasing the amount of electron injection to a predetermined level or more at both room temperature and high temperature, the gain on the short-wavelength side becomes relatively larger than that on the long-wavelength side, and the gain smoothness is impaired.

[0037] Therefore, the semiconductor device 1 of the present embodiment is provided with a longest wavelength layer group, that is, two or more longest wavelength layers 5A having the same center wavelength which is the longest wavelength, in the quantum dot group 4, and is configured such that a gain of a wavelength corresponding to the longest wavelength layer 5A can be obtained relatively more in advance. That is, the semiconductor device 1 is configured such that, in anticipation of the fact that the increase in its gain is larger on the short wavelength side than on the long wavelength side, the gain corresponding to the longest wavelength layer 5A is larger than that on the short wavelength side during low electron injection, and the smoothness of the gain can be ensured during high electron injection. As a result, as shown in FIG. 5 for example, the semiconductor device 1 can achieve both a gain equal to or higher than a predetermined target gain and a flat gain at that time while widely securing the wavelength band WR3.

[0038] According to the present embodiment, the quantum dot group 4 is formed in which the gain on the long wavelength side is relatively increased in advance by the longest wavelength layer group, and at the time of high current injection, the semiconductor device 1 can secure the smoothness of the gain while obtaining a gain equal to or higher than a predetermined value, that is, a high output, in a wide wavelength band.

[0039] (Second Embodiment) The semiconductor device 1 of the second embodiment will be described with reference to the drawings.

[0040] The semiconductor device 1 of the present embodiment is different from the first embodiment in that, as shown in FIG. 6 for example, the arrangement of the quantum dot group 4 is reversed, that is, the longest wavelength layer 5A is arranged at the end on the p-clad layer 6 side, and the shortest wavelength layer 5B is arranged at the end on the n-clad layer 3 side. In the present embodiment, this difference will be mainly described.

[0041] In the present embodiment, the arrangement direction of the plurality of quantum dot layers 5 in the quantum dot group 4 is reversed compared to the first embodiment. In the quantum dot group 4, the first layer 5-1 at the end on the n-clad layer 3 side is the shortest wavelength layer 5B, and the quantum dot layers 5 are stacked such that the center wavelength of each quantum dot layer 5 sequentially shifts to the long wavelength side toward the p-clad layer 6. In other words, in the quantum dot group 4, the nth layer 5-n adjacent to the p-clad layer 6 is the longest wavelength layer 5A.

[0042] By arranging the quantum dot group 4 in the above-described array, in this embodiment, the longest wavelength layer 5A is located in the region closest to the p-clad layer 6 that supplies holes having a larger effective mass and a smaller mobility than electrons. That is, in the quantum dot group 4, the quantum dot layer 5 closer to the longest wavelength layer 5A has a larger supply amount of holes from the p-clad layer 6. As a result, in the quantum dot group 4, the gain on the longest wavelength layer 5A side becomes relatively larger than the gain on the shortest wavelength layer 5B side, and while the gain in the wavelength band when the current injection amount is increased becomes equal to or more than a predetermined value, the gain is smoothed.

[0043] Note that in this embodiment, the quantum dot group 4 is configured such that the central wavelengths at which the maximum gain is obtained in each quantum dot layer 5 are all different, and does not have a plurality of quantum dot layers 5 having the same central wavelength.

[0044] Also according to this embodiment, similar to the first embodiment, the semiconductor element 1 can obtain the effect of ensuring a gain of a predetermined value or more in the wavelength band and smoothing the gain when a high current is injected.

[0045] (Third Embodiment) The semiconductor element 1 of the third embodiment will be described with reference to the drawings.

[0046] The semiconductor element 1 of this embodiment is different from the first embodiment in that, for example, as shown in FIG. 7, the quantum dot group 4 has a dislocation D generated in the quantum dot layer 5 close to the p-clad layer 6. In this embodiment, this difference will be mainly described.

[0047] Similar to the first embodiment, in the quantum dot group 4, the first layer 5-1 at the end on the n-clad layer 3 side is the longest wavelength layer 5A, and the nth layer 5-n at the opposite end is the shortest wavelength layer 5B. However, in this embodiment, a dislocation D has occurred in the group of quantum dot layers 5 on the shortest wavelength layer 5B side. That is, in the quantum dot group 4, for example, the shortest wavelength layer 5B, or a plurality of quantum dot layers 5 including the shortest wavelength layer 5B and extending from the shortest wavelength layer 5B toward the longest wavelength layer 5A side have a lower crystallinity than the other quantum dot layers 5 on the longest wavelength layer 5A side.

[0048] Hereinafter, for the sake of simplicity of explanation, a group composed of a plurality of quantum dot layers 5 including the shortest wavelength layer 5B and extending from the shortest wavelength layer 5B toward the longest wavelength layer 5A side is referred to as the "shortest wavelength layer group" for convenience.

[0049] In the present embodiment, the shortest wavelength layer group is stacked after the longest wavelength layer group, and thus is disposed at a position farther from the semiconductor substrate 2. That is, the shortest wavelength layer group is located at a position farther from the semiconductor substrate 2 in the film formation process of the quantum dot group 4, that is, at a position where the strain of crystal growth in the quantum dot layer 5 accumulates, and has lower crystallinity than the longest wavelength layer group. As a result, the quantum dot group 4 has more crystal defects in the quantum dot layer 5 on the shortest wavelength layer 5B side and is in a state where dislocations D occur, and the gain on the shortest wavelength layer 5B side is smaller than the gain on the longest wavelength layer 5A side. Therefore, when the quantum dot group 4 ensures a gain of a predetermined value or more in the wavelength band at high current injection, the gain is smoothed.

[0050] Note that in the present embodiment, the center wavelengths in each quantum dot layer 5 of the quantum dot group 4 are all different, and the quantum dot group 4 does not have a plurality of quantum dot layers 5 having the same center wavelength. Further, the quantum dot group 4 has a stacked structure in which the center wavelength of the quantum dots in the quantum dot layer 5 sequentially shifts to the short wavelength side toward the upper layer on the p-clad layer 6 side. The quantum dot group 4 is configured to cause dislocations D in the shortest wavelength layer group by setting the number of stacked layers n of the quantum dot layers 5 to a predetermined value or more (for example, 20 or more, although not limited).

[0051] Also according to the present embodiment, similar to the first embodiment, the semiconductor element 1 can obtain the effect of ensuring a gain of a predetermined value or more in the wavelength band and smoothing the gain at high current injection.

[0052] (Fourth Embodiment) The semiconductor element 1 of the fourth embodiment will be described with reference to the drawings.

[0053] As shown in FIG. 8, for example, the semiconductor device 1 of this embodiment is different from the first embodiment in that the quantum dot layer 5 on the shortest wavelength layer 5B side has a smaller number of quantum dots in the layer than the quantum dot layer 5 on the longest wavelength layer 5A side. In this embodiment, this difference will be mainly described.

[0054] Similar to the first embodiment, in the quantum dot group 4, the first layer 5-1 at the end on the n-clad layer 3 side is the longest wavelength layer 5A, and the nth layer 5-n at the opposite end is the shortest wavelength layer 5B. However, in this embodiment, an intentional difference is provided in the quantum dot density of the quantum dot layer 5. The "quantum dot density" here means the ratio occupied by the quantum dots in the quantum dot layer 5 in one quantum dot layer 5.

[0055] In this embodiment, for the quantum dot group 4, for example, the number of quantum dots in the longest wavelength layer 5A or the quantum dot group with the longest wavelength is made larger than the number of quantum dots in other quantum dot layers 5, so that its quantum dot density is relatively large. In other words, similar to the first embodiment, in the quantum dot group 4, the total volume of the quantum dots with the longest center wavelength is larger than the total volume of the quantum dots in each of the other quantum dot layers 5. As a result, the gain on the longest wavelength layer 5A side of the quantum dot group 4 is larger than the gain on the shortest wavelength layer 5B side. Thereby, when the quantum dot group 4 ensures a gain of a predetermined value or more in the wavelength band during high current injection, the gain is smoothed.

[0056] Note that in this embodiment, in the quantum dot group 4, the center wavelengths at which the maximum gain is obtained in each quantum dot layer 5 are all different, and it has a configuration without a plurality of quantum dot layers 5 having the same center wavelength. Further, the quantum dot group 4 has a stacked structure in which the center wavelength of the quantum dots in the quantum dot layer 5 sequentially shifts to the short wavelength side toward the upper layer on the p-clad layer 6 side.

[0057] Also according to this embodiment, similar to the first embodiment, the semiconductor device 1 can obtain the effect of ensuring a gain of a predetermined value or more in the wavelength band and smoothing the gain during high current injection.

[0058] (Fifth Embodiment) The semiconductor element 1 of the fifth embodiment will be described with reference to the drawings.

[0059] As shown in FIG. 9, for example, the semiconductor element 1 of this embodiment is different from the first embodiment in that the quantum dot group 4 is composed of a first group 41 having the largest gain, a second group 42 having the second largest gain after the first group 41, and a third group 43 having the smallest gain. In this embodiment, this difference will be mainly described.

[0060] Similar to the first embodiment, in the quantum dot group 4, the first layer 5-1 is the longest wavelength layer 5A and the nth layer 5-n is the shortest wavelength layer 5B. However, in this embodiment, the first group 41, the second group 42, and the third group 43 are stacked in this order from the n-clad layer 3 side. The first group 41, the second group 42, and the third group 43 are all composed of a plurality of quantum dot layers 5, but the materials and composition ratios of the quantum dots are different.

[0061] For example, in the first group 41, the quantum dots are composed of InAs, and the gain at the center wavelength is relatively larger than that of the second group 42 and the third group 43.

[0062] For example, in the second group 42, the quantum dots are composed of In 0.5 Ga 0.5 As, and the gain at the center wavelength is relatively smaller than that of the first group 41 and larger than that of the third group 43.

[0063] For example, in the third group 43, the quantum dots are composed of In 0.75 Ga 0.25 As, and the gain at the center wavelength is relatively the smallest.

[0064] That is, in this embodiment, the quantum dot group 4 has quantum dots with a relatively smaller gain material or composition ratio toward the shortest wavelength layer 5B side, and a larger gain can be obtained in the longest wavelength layer 5A.

[0065] Note that, for the first group 41, the second group 42, and the third group 43, the quantum dot layers 5 closer to the p-clad layer 6, i.e., the upper layers, have a configuration in which the center wavelength shifts toward the shorter wavelength side. Also, in this embodiment, the quantum dot group 4 has a configuration in which the center wavelengths in each quantum dot layer 5 are all different and does not have a plurality of quantum dot layers 5 with the same center wavelength. Also, the number of quantum dot layers 5 constituting the first group 41, the second group 42, and the third group 43 can be appropriately changed.

[0066] Also according to this embodiment, similar to the first embodiment, the semiconductor device 1 can ensure a gain of a predetermined value or more in the wavelength band and smooth the gain when injecting a high current.

[0067] (Sixth Embodiment) The semiconductor device 1 of the sixth embodiment will be described with reference to the drawings. In FIGS. 11 and 12, the direction of change in gain when increasing the current injection amount of the quantum dots in the fourth group 44 described later is indicated by a white arrow. Also, in FIG. 12, the gain spectrum corresponding to the ground state of the quantum dots in the fifth group 45 described later is indicated by a thick line.

[0068] The semiconductor device 1 of this embodiment is different from the first embodiment in that, as shown in FIG. 10 for example, it has a fourth group 44 that also uses the ground level and higher levels of the quantum dots, and a fifth group 45 adjusted corresponding to the energy levels of the quantum dots in the fourth group 44. In this embodiment, this difference will be mainly described.

[0069] Similar to the first embodiment, for the quantum dot group 4, the first layer 5-1 on the n-clad layer 3 side is the longest wavelength layer 5A, and the nth layer 5-n on the p-clad layer 6 side is the shortest wavelength layer 5B. However, in this embodiment, the fourth group 44 is on the longest wavelength layer 5A side, and the fifth group 45 is on the shortest wavelength layer 5B side.

[0070] The fourth group 44 is, for example, a group consisting of a plurality of quantum dot layers 5 in which the quantum dots are made of InAs. The fourth group 44 is adjusted so that, at the time of high current injection, in addition to the gain at the wavelength corresponding to the ground level of the quantum dots, a gain at the wavelength corresponding to the energy level next higher than the ground level can be obtained. Hereinafter, for the sake of simplicity of explanation, the energy level next higher than the ground level is simply referred to as the "higher level".

[0071] Specifically, when the quantum dots are made of InAs, in the fourth group 44, as shown in FIG. 11 for example, the wavelength at which the gain of the higher level is obtained is shifted to the shorter wavelength side by about 80 nm with respect to the wavelength at which the gain of the ground level of the quantum dots in each quantum dot layer 5 is obtained. Also, in the fourth group 44, when the current injection amount is increased, the gain at the wavelength corresponding to the higher level increases more than the gain at the wavelength corresponding to the ground state. Further, in the fourth group 44 and the fifth group 45, as going toward the upper layer, the wavelength corresponding to the ground level of the quantum dots is sequentially shifted to the shorter wavelength side, and the configuration is different from that of the higher level of the quantum dots whose ground level is located in the lower layer than itself. Note that the difference between the wavelength corresponding to the ground level and the wavelength corresponding to the higher level of the quantum dots in each quantum dot layer 5 varies depending on the material and composition ratio of the quantum dots, but is, for example, in the range of 60 nm to 100 nm.

[0072] In each quantum dot layer 5, when the quantum dots increase the current injection amount and all the electrons in the ground state become excited states, then the electrons in the higher energy levels with higher energy levels will subsequently become excited states. At this time, for example, if the ground state of the quantum dots in the quantum dot layer 5 located above the longest wavelength layer 5A is substantially the same as the higher energy level of the quantum dots in the longest wavelength layer 5A, the electrons in the ground state of the upper layer and the electrons in the higher energy level of the lower layer will both be in the excited state. That is, the wavelength corresponding to the higher energy level of the quantum dots in the lower layer and the wavelength corresponding to the ground state of the quantum dots in the upper layer are substantially the same, and the gain at this wavelength becomes relatively too large compared to the gain at other wavelengths, which may damage the gain smoothing. To avoid this, as described above, the quantum dot group 4 is configured such that the ground state of the quantum dots in each quantum dot layer 5 is different from the higher energy levels of other quantum dots.

[0073] Regarding the ground state of the quantum dots in each quantum dot layer 5, for example, in the fourth group 44 and the fifth group 45, while setting the quantum dots to a predetermined composition ratio, by changing the composition ratio of the cap and controlling the size and density of the quantum dots, it can be adjusted.

[0074] The fifth group 45 is, for example, a group consisting of a plurality of quantum dot layers 5 composed of In 0.5 Ga 0.5 As. The fifth group 45 is adjusted such that the ground state of the quantum dots is between the ground state and the higher energy level of the quantum dots in the fourth group 44, for example, as shown in FIG. 12. Thereby, the fifth group 45 can obtain the gain at the wavelength located between the ground state and the higher energy level of the quantum dots in the fourth group 44, and at the same time, when injecting a high current, the effect of smoothing the maximum gain in the wavelength band can be obtained.

[0075] Since there is an optimal value for the total number of layers of the quantum dot group 4 that is efficient, simply increasing the number of layers of the quantum dot group 4 does not improve the efficiency, and it does not result in a configuration with a high gain, i.e., high output. Therefore, the quantum dot group 4 needs to be adjusted to obtain high output and gain smoothness at high current injection with a limited total number of layers. Thus, in this embodiment, as described above, the ground level of the quantum dots in the quantum dot layer 5 of the quantum dot group 4 has an energy level different from the higher levels of the quantum dots in the quantum dot layer 5 located in the layer below itself. And the quantum dot group 4 is configured such that the ground level of the quantum dots in the fifth group 45 is between the ground level and the next higher level of the quantum dots in the fourth group 44, so that a gain with a wavelength corresponding to the interval between these levels is obtained and the gain is smoothed.

[0076] Also according to this embodiment, similar to the first embodiment, the semiconductor device 1 can obtain the effect of ensuring a gain of a predetermined value or more in the wavelength band and smoothing it at high current injection. Further, in this embodiment, even with a limited total number of quantum dot layers 5, it is possible to achieve both high output and gain smoothing at high current injection.

[0077] (Other embodiments) Although the present disclosure has been described based on examples, it is understood that the present disclosure is not limited to such examples or structures. The present disclosure also includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, and further other combinations and forms including only one of these elements, more, or less, fall within the scope and spirit of the present disclosure. For example, the semiconductor device 1 can be configured with a free combination of the components of the above embodiments, except for those that are clearly incompatible.

[0078] In addition, in each of the above embodiments, it goes without saying that the elements constituting the embodiment are not necessarily essential, except when explicitly stated as being essential or when considered to be clearly essential in principle. Also, in each of the above embodiments, when numerical values such as the number, numerical value, quantity, range, etc. of the components of the embodiment are mentioned, they are not limited to that specific number, except when explicitly stated as being essential or when clearly limited to a specific number in principle. Further, in each of the above embodiments, when referring to the shape, positional relationship, etc. of the components, etc., they are not limited to that shape, positional relationship, etc., except when explicitly stated or when clearly limited to a specific shape, positional relationship, etc. in principle.

Description of Reference Numerals

[0079] 2 Semiconductor substrate 3 n-clad layer 4 Quantum dot group 5 Quantum dot layer 5A Longest wavelength layer 5B Shortest wavelength layer 6 p-clad layer

Claims

A semiconductor device used in a semiconductor optical amplifier, comprising: a plurality of quantum dot layers (5) having different wavelengths at which the gain is maximized, and a quantum dot group (4) in which some or all of the plurality of quantum dot layers are configured such that the wavelength sequentially shifts along the stacking direction of the quantum dot layers; Among the plurality of quantum dot layers, the quantum dot layer having the longest wavelength is defined as the longest wavelength layer (5A), and the quantum dot layer having the shortest wavelength is defined as the shortest wavelength layer (5B). A group consisting of a part of the plurality of quantum dot layers stacked from the longest wavelength layer toward the shortest wavelength layer including the longest wavelength layer is defined as the longest wavelength layer group; In the quantum dot group, the gain at the wavelength of the longest wavelength layer or the longest wavelength layer group is greater than the gain at the wavelength of each of the other quantum dot layers; The plurality of quantum dot layers have quantum dots; The quantum dots are grown with crystal growth that causes strain, and the emission wavelengths are different for each quantum dot layer; The quantum dot group is a semiconductor device in which the ground levels of the quantum dots in each quantum dot layer are different from the higher energy levels that are next in energy level to the ground levels of the other quantum dots on the side of the longest wavelength layer than the quantum dot layer. Claim 2 The semiconductor device according to claim 1, wherein the quantum dots have different lattice constants for each quantum dot layer due to different constituent materials or composition ratios of the constituent materials. Claim 3 The quantum dots are composed of InAs or In ｘ Ga （１－ｘ） As (0 < x < 1), the semiconductor device according to claim 2. Claim 4 In the quantum dot group, a plurality of the quantum dot layers are stacked on a semiconductor substrate (2); The semiconductor device according to claim 3, wherein the longest wavelength layer is disposed at an end portion closest to the semiconductor substrate in the quantum dot group. Claim 5 The semiconductor device according to claim 4, wherein the total volume of the quantum dots in the longest wavelength layer or the total volume of the quantum dots in the longest wavelength layer group is greater than the total volume of the quantum dots in each of the other quantum dot layers. Claim 6 The semiconductor device according to claim 5, wherein a part of the plurality of quantum dot layers including the longest wavelength layer and extending from the longest wavelength layer toward the shortest wavelength layer have the same wavelength. Claim 7 The semiconductor device according to claim 5, wherein each of the longest wavelength layer or the group of longest wavelength layers has a higher ratio of the quantum dots in the quantum dot layer than the ratio of the quantum dots in the other quantum dot layers.

8. The group of quantum dots is sandwiched between an n-clad layer (3) and a p-clad layer (6) in the stacking direction. The longest wavelength layer is disposed at an end portion on the p-clad layer side of the group of quantum dots. The semiconductor device according to any one of claims 1 to 3, wherein the shortest wavelength layer is disposed at an end portion on the n-clad layer side of the group of quantum dots.

9. The semiconductor device according to any one of claims 1 to 3, wherein the group of quantum dots is composed of a material or a composition ratio in which the gain of the quantum dots at the wavelength is smaller than the gain of the quantum dots at the wavelength in the other quantum dot layers, in the quantum dot layer closer to the shortest wavelength layer.

10. A semiconductor device used in a semiconductor optical amplifier, comprising a group of quantum dots (4) in which a plurality of quantum dot layers (5) having different wavelengths at which the gain is maximized are stacked, and a part or all of the plurality of quantum dot layers are configured such that the wavelength sequentially shifts along the stacking direction of the quantum dot layers. Among the plurality of quantum dot layers, the quantum dot layer having the longest wavelength is defined as the longest wavelength layer (5A), the quantum dot layer having the shortest wavelength is defined as the shortest wavelength layer (5B), and a group consisting of a part of the plurality of quantum dot layers stacked from the longest wavelength layer toward the shortest wavelength layer side including the longest wavelength layer is defined as the group of longest wavelength layers. In the group of quantum dots, the gain at the wavelength of the longest wavelength layer or the group of longest wavelength layers is larger than the gain at the wavelength of each of the other quantum dot layers. The plurality of quantum dot layers have quantum dots. The quantum dots are crystal-grown to cause strain, and the emission wavelengths are different for each quantum dot layer. For each quantum dot layer, the lattice constant is different due to different constituent materials or composition ratios of the constituent materials, and the quantum dots are composed of InAs or InxGa(1 - x)As (0 < x < 1). The group of quantum dots is formed by stacking a plurality of the quantum dot layers on a semiconductor substrate (2). The longest wavelength layer is disposed at an end portion closest to the semiconductor substrate in the group of quantum dots. A semiconductor device in which the total volume of the quantum dots in the longest wavelength layer or the total volume of the quantum dots in the longest wavelength layer group is larger than the total volume of the quantum dots in each of the other quantum dot layers.

11. The semiconductor device according to claim 10, wherein the quantum dot groups are such that the ground levels of the quantum dots in each of the quantum dot layers are different from the higher levels having higher energy levels than the ground levels of the other quantum dots on the side of the longest wavelength layer than the quantum dot layer.

12. The quantum dot group includes a first group (44) composed of a plurality of the quantum dot layers adjusted so that gains of the ground level and the higher level of the quantum dots are obtained, and a ground level of the quantum dots is adjusted with respect to the ground level and the higher level of the quantum dots in the first group. It has a second group (45) composed of a plurality of the quantum dot layers, The semiconductor device according to any one of claims 1, 2, 3, and 11, wherein in the second group, the ground level of the quantum dots is between the ground level and the higher level of the quantum dots in the first group.

13. The semiconductor device according to claim 12, wherein in the longest wavelength layer group, the higher level of the quantum dots is on the shorter wavelength side by 60 nm to 100 nm from the ground level.

14. A semiconductor device used in a semiconductor optical amplifier, A quantum dot group (4) is provided in which a plurality of quantum dot layers (5) having different wavelengths at which the gain is maximized are stacked, and a part or all of the plurality of quantum dot layers are configured such that the wavelength sequentially shifts along the stacking direction of the quantum dot layers. Among the plurality of quantum dot layers, the quantum dot layer having the longest wavelength is defined as the longest wavelength layer (5A), the quantum dot layer having the shortest wavelength is defined as the shortest wavelength layer (5B), and a group composed of two or more quantum dot layers stacked from the longest wavelength layer toward the shortest wavelength layer side including the longest wavelength layer is defined as the longest wavelength layer group. In the quantum dot group, the gain at the wavelength of the longest wavelength layer group is larger than the gain at the wavelength of each of the other quantum dot layers. The plurality of quantum dot layers have quantum dots. The quantum dots are crystal-grown to generate strain, and for each quantum dot layer, the emission wavelength is different. For each quantum dot layer, the lattice constant is different due to different constituent materials or different composition ratios of the constituent materials, and they are composed of InAs or InxGa(1 - x)As (0 < x < 1). The group of quantum dots has a plurality of the quantum dot layers stacked on a semiconductor substrate (2). The longest wavelength layer is disposed at an end portion of the group of quantum dots closest to the semiconductor substrate. A semiconductor device in which the total volume of the quantum dots in the longest wavelength layer or the total volume of the quantum dots in the group of longest wavelength layers is larger than the total volume of the quantum dots in each of the other quantum dot layers.

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