JP6916510B2 - Manufacturing method of optical device - Google Patents

Description

The present invention relates to an optical device and a method for manufacturing the optical device.

Conventionally, as semiconductor optical devices, semiconductor lasers and LEDs having a near-infrared wavelength having a quantum well as a light emitting layer have been widely used. The quantum well is obtained by heteroepitaxial growth of semiconductor crystals having different band gaps. For example, an InGaAs crystal thin film is grown on a GaAs crystal substrate, and a GaAs crystal is further grown on the GaAs crystal substrate. Quantum wells are disclosed, for example, in Patent Document 1.

The quantum well is a semiconductor thin film structure formed by a two-dimensional crystal growth method, and has a quantum confinement effect in the one-dimensional direction only in the thickness direction of the thin film. The quantum well has a constant film thickness, and the emission wavelength or absorption wavelength determined by the film thickness is also constant (narrow band). In two-dimensional crystal growth, the crystal atomic layers are sequentially laminated, so that the interface of the thin film has a step-and-terrace structure. The step-and-terrace structure is a structure having a flat terrace portion which is an atomic alignment portion and a step portion having a height of about one atomic layer. The shape of the interface of the thin film formed by two-dimensional crystal growth is called a two-dimensional structure. The interface, which is a two-dimensional structure, is characterized by having an atomic layer level flatness.

JP-A-2009-170775

The present inventors have found a novel quantum structure capable of having a wider band than a quantum well. The optical device according to one aspect of the present invention includes a semiconductor thin film, the semiconductor thin film having an average film thickness of less than the critical film thickness and having an interface having a three-dimensional structure.

According to the experiments of the present inventors, by making the interface of the semiconductor thin film a three-dimensional structure instead of a two-dimensional structure like a quantum well, the film thickness of the thin film is distributed with variation in the plane, so that light emission occurs. Wavelengths or absorption wavelengths are distributed, resulting in broader emission or light absorption.

Here, the three-dimensional structure is a shape formed by three-dimensional growth or a shape similar to a shape formed by three-dimensional growth. Conventional quantum structures formed by three-dimensional growth include, for example, quantum dots and quantum dashes. The quantum dots and quantum dashes do not have a thin film, and the entire quantum structure is a three-dimensional structure. Specifically, the quantum dots have a quantum confinement effect in the three-dimensional direction including not only the thickness direction but also the in-plane direction, and have dot-shaped particles. The quantum dash has a quantum confinement effect in the two-dimensional direction, and has protrusions in the shape of elongated dot particles in the quantum dots. The three-dimensional structure is characterized by having little or no flat portion due to the atomic layer, as the two-dimensional structure has.

The quantum structure found by the present inventors does not have a three-dimensional structure as a whole like quantum dots and quantum dashes, but has a semiconductor thin film such as a quantum well, but the interface of the thin film is three. It has a dimensional structure.

The three-dimensional structure of the thin film interface can be obtained even if the film thickness of the thin film is equal to or higher than the critical film thickness, but the average film thickness of the thin film should be less than the critical film thickness in order to prevent defects from occurring in the thin film. preferable. The critical film thickness is the film thickness at which a misfit transition occurs. When the film thickness is equal to or higher than the critical film thickness, a misfit transition occurs due to strain due to lattice mismatch, and a defect occurs in the thin film. The defect becomes a non-emission center and reduces the emission intensity. Here, the average film thickness is the average of the film thicknesses measured at a plurality of locations at the interface of the three-dimensional structure. From the viewpoint of easily obtaining the three-dimensional structure of the interface, the average film thickness of the thin film is preferably less than the critical film thickness and close to the critical film thickness.

Another aspect of the present invention is a method of manufacturing an optical device. In the embodiment, the manufacturing method includes a growth step of growing a semiconductor thin film having a crystal structure having a crystal structure different from that of the substrate on the substrate. In the growth process, a three-dimensional structure grows at the interface of the semiconductor thin film due to strain caused by the semiconductor thin film having a different lattice constant from that of the substrate. The average film thickness of the semiconductor thin film is less than the critical film thickness.

It is a schematic diagram of a thin film having an interface of a three-dimensional structure. It is sectional drawing of the semiconductor optical device. It is sectional drawing of the growth substrate used in an experiment. It is a two-dimensional AFM image of the interface of the In _0.34 Ga _{0.66 As thin film.} It is a three-dimensional AFM image of the interface of the In _0.34 Ga _{0.66 As thin film.} It is a figure which shows the emission wavelength of the light emitting element which has In _0.34 Ga _{0.66 As thin film.} It is a two-dimensional AFM image of the interface of the In _0.3 Ga _{0.7 As thin film.} It is a three-dimensional AFM image of the interface of the In _0.34 Ga _{0.7 As thin film.} It is a figure which shows the emission wavelength of the light emitting element which has In _0.3 Ga _{0.7 As thin film.} It is a two-dimensional AFM image of the interface of an In _0.2 Ga _{0.8 As thin film.} It is a three-dimensional AFM image of the interface of an In _0.2 Ga _{0.8 As thin film.} It is an AFM image of the interface of an In _0.5 Ga _{0.5 As thin film.} It is an AFM image of the interface of In _0.28 Ga _{0.72 As thin film.} It is a figure which shows the emission wavelength of the light emitting element which has In _0.28 Ga _{0.72 As thin film.} It is a graph which shows the relationship between a growth temperature and a PL peak wavelength. It is a graph which shows the relationship between the growth temperature and PL peak intensity. It is a graph which shows the relationship between the growth temperature and FWHM. It is a graph which shows the relationship between a growth temperature and a PL peak wavelength. It is a graph which shows the relationship between the growth temperature and PL peak intensity. It is a graph which shows the relationship between the growth temperature and FWHM. It is a graph which shows the relationship between the wavelength and PL peak intensity. It is a figure which shows the relationship between the composition ratio of In _x Ga _{1-x As, and the film thickness.}

[1. Outline of the embodiment]
The optical device according to the embodiment has a semiconductor thin film. The semiconductor thin film functions as a light emitting layer or a light absorbing layer. Hereinafter, the semiconductor thin film is simply referred to as a thin film. The thin film has an average film thickness of less than the critical film thickness. The thin film has an interface with a three-dimensional structure. The three-dimensional structure is, for example, a shape having quantum dot-shaped or quantum dash-shaped protrusions at the interface. The average film thickness is the average of the film thicknesses measured at a plurality of locations at the interface of the three-dimensional structure. The average film thickness can be obtained, for example, by cleaving a sample having a thin film, measuring the thickness of the thin film at a plurality of locations on an SEM image of a cross section, and obtaining the average of the measured values.

The three-dimensional structure has a shape having protrusions, and the density of the protrusions ^{is preferably 10 10} pieces / cm ² or more, and more preferably 10 ¹² pieces / cm ² or less.

The interface preferably has an RMS surface roughness of 1 nm or more, and more preferably has an RMS surface roughness of 2 nm or more. The interface preferably has an RMS surface roughness of 4 nm or less.

The central wavelength of the light emitting band of the optical device is preferably 950 nm or more, and more preferably 1000 nm or more.

The central wavelength of the light emitting band of the optical device is preferably 1200 nm or less, and more preferably 1100 nm or less.

The thin film preferably comprises In _x Ga _1-x A _x (x is the composition ratio of indium). x is preferably 0.25 or more, more preferably 0.28 or more, and even more preferably 0.3 or more. x is preferably 0.45 or less, more preferably 0.42 or less, more preferably 0.4 or less, and even more preferably 0.38 or less.

The average film thickness of the thin film is preferably 3 nm or more, and more preferably 4 nm or more. The average film thickness of the thin film is preferably 9 nm or less, more preferably 8 nm or less.

The method for manufacturing an optical device according to an embodiment includes a growth step of growing a thin film having a crystal structure having a lattice constant different from that of the substrate on the substrate. In the growth process, a three-dimensional structure grows at the interface of the thin film due to the strain caused by the thin film having a different lattice constant from that of the substrate. The average film thickness of the thin film is less than the critical film thickness.

The method for manufacturing an optical device according to the embodiment _{includes a growth step of growing a thin film of In x} Ga _1-x As (x is an indium composition ratio) on a substrate. The substrate has a different lattice constant from _{In x} Ga _{1-x As.} x is 0.25 or more and 0.45 or less. The growth temperature of the thin film is preferably 500 ° C. or higher, and preferably 550 ° C. or lower. The growth temperature is preferably a temperature at which at least one of thermal diffusion and re-evaporation of indium occurs.

[2. Details of the embodiment]

[2.1 Semiconductor thin film]
FIG. 1 shows an InGaAs thin film 13 formed on a GaAs crystal substrate 11. The interface 15 on the upper side of the thin film 13 does not have a flat portion like a step-and-terrace structure, and has a three-dimensional structure. Like the quantum well, the thin film 13 has a quantum confinement effect in the one-dimensional direction only in the thickness direction of the thin film. The thin film 13 emits light or absorbs light at a wavelength depending on its thickness. As shown in FIG. 1, the thin film 13 having a three-dimensional structure at the interface 15 has a non-uniform film thickness, and as a result, the bandwidth of light is widened.

[2.2 Light emitting element]

FIG. 2 shows a semiconductor optical device (quantum device) 100 including an active layer 10 having a plurality of thin films 13. The device 100 is, for example, a light emitting element. The light emitting element 100 includes ^{a buffer layer 23 formed on the n +} -GaAs substrate 25, an n-type clad layer 21 formed on the buffer layer 23, an active layer 10 formed on the clad layer 21, and an active layer. It has a p-type clad layer 31 formed on the clad layer 31 and a buffer layer 23 formed on the clad layer 31. In FIG. 2, the buffer layer 23 is n ⁺ -GaAs, the clad layer 21 is n-Al _0.35 Ga _0.65 As, and the clad layer 31 is p-Al _0.35 Ga _0.65 As. , The buffer tank 33 is n ⁺ −GaAs. The light emitting element 100 has electrodes and the like (not shown).

The active layer 10 has a multiple structure such as a multiple quantum well, and has a structure in which each of the plurality of thin films 13 produced is sandwiched between the barrier layers 11. In FIG. 2, the barrier layer 11 is a GaAs and the thin film 13 is an InGaAs thin film.

At least one of the plurality of thin films 13 has an interface 15 with the upper barrier layer 11 having a three-dimensional structure as shown in FIG. The direction in which crystal growth proceeds is upward. As shown in FIG. 2, in the active layer 10, all the interfaces 15 of the plurality of thin films 13 may have a three-dimensional structure, or the interface 15 between the thin film having the interface of the two-dimensional structure and the interface 15 of the three-dimensional structure may be formed. It may have a structure in which the thin film 13 to have is mixed.

The optical device 100 may be, for example, a light emitting device for an optical coherence tomography (OCT) or a light emitting device for near-infrared division multiplexing (WDM). The optical device 100 may be a light receiving element. The optical device 100 may be, for example, a near-infrared optical detector. The optical device 100 may be a solar cell.

The semiconductor crystal layer shown in FIG. 2 is manufactured by, for example, a molecular beam epitaxy method (MBE) method. MBE growth is a method in which a crystal is grown on a crystal substrate 25 by epitaxial growth. MBE growth is performed by the MBE growth device. The MBE growth apparatus supplies raw materials from the raw material supply mechanism to the crystal substrate 25 installed in the substrate holder in the ultra-high vacuum chamber. A substrate heater is installed in the vacuum chamber to heat the substrate 25.

[2.3 Experiment]
In the experiment, In, Ga, and As atoms were supplied on the GaAs crystal substrate, and In _x Ga _1-x As was epitaxially grown. The experimental results are shown below. In the experiment, a sample having the cross-sectional structure of the growth substrate shown in FIG. 3 was prepared. Further, in the experiment described below, unless otherwise specified, raw materials (In, Ga, As) having a thickness of the _{In x} Ga _{1-x As thin film 13 of 7 nm are supplied.}

Here, the raw material having a thickness of T [nm] is a raw material having a thickness of T [nm] when the interface (surface) 15 forms a thin film having a two-dimensional structure. Is. In the following experiments, the interface 15 may have a three-dimensional structure, in which case the thickness of the grown thin film is not constant. When a raw material having a thickness of T [nm] is supplied and the interface 15 has a three-dimensional structure, the average film thickness of the thin film is T [nm]. However, as will be described later, since the raw material may be re-evaporated during the thin film growth process, the film thickness may be thinner than the amount of the raw material supplied. When the raw material is re-evaporated, the average film thickness of the thin film becomes less than T [nm] even if an amount of the raw material having a thickness of T [nm] is supplied.

[2.3.1 In _0.34 Ga _0.66 As 7nm]

4 and 5 show observation images of the interface (surface) 15 of _{the In 0.34} Ga _0.66 As thin film 13 produced at various substrate temperatures (growth temperatures). The observation images shown in FIGS. 4 and 5 were obtained by an atomic force microscope (AFM). FIG. 4 is a two-dimensional image of the interface 15, and FIG. 5 is a three-dimensional image of the interface 15.

4 (a) and 5 (a) show the interface 15 of the thin film 13 grown at 437 ° C. 437 ° is a relatively low growth temperature suitable for growing a conventional quantum well having a two-dimensional interface 15. The interface 15 shown in FIGS. 4 (a) and 5 (a) has a flatness due to an atomic layer and has a two-dimensional structure. The RMS surface roughness of the interface 15 shown in FIGS. 4 (a) and 5 (a) was 0.360 nm. Since the surface roughness of 0.360 nm is equivalent to the thickness of the monolayer (ML), the interface 15 shown in FIGS. 4 (a) and 5 (a) has a two-dimensional structure. It is confirmed that it is a step-and-terrace structure.

4 (b) and 5 (b) show the interface 15 of the thin film 13 grown at 469 ° C. The interface 15 shown in FIGS. 4 (b) and 5 (b) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 1.62 nm. The density of protrusions was 1.3 × 10 ¹¹ pieces / cm ² . The density of protrusions was determined by observing an AFM image and counting the number of protrusions per unit area.

4 (c) and 5 (c) show the interface 15 of the thin film 13 grown at 489 ° C. The interface 15 shown in FIGS. 4 (c) and 5 (c) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 1.66 nm. The density of the protrusions was 1.0 × 10 ¹¹ pieces / cm ² .

4 (d) and 5 (d) show the interface 15 of the thin film 13 grown at 505 ° C. The interface 15 shown in FIGS. 4 (d) and 5 (d) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 1.66 nm. The density of protrusions was 4.4 × 10 ¹⁰ pieces / cm ² .

4 (e) and 5 (e) show the interface 15 of the thin film 13 grown at 510 ° C. The interface 15 shown in FIGS. 4 (e) and 5 (e) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the indicated interface 15 was 2.98 nm. The density of protrusions was 3.5 × 10 ¹⁰ pieces / cm ² .

4 (f) and 5 (f) show the interface 15 of the thin film 13 grown at 520 ° C. The interface 15 shown in FIGS. 4 (f) and 5 (f) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 3.69 nm. The density of protrusions was 2.4 × 10 ¹⁰ pieces / cm ² .

4 (g) and 5 (g) show the interface 15 of the thin film 13 grown at 526 ° C. The interface 15 shown in FIGS. 4 (g) and 5 (g) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 3.38 nm. The density of protrusions was 2.1 × 10 ¹⁰ pieces / cm ² .

4 (h) and 5 (h) show the interface 15 of the thin film 13 grown at 538 ° C. The interface 15 shown in FIGS. 4 (h) and 5 (h) is a three-dimensional structure having a large number of particle-like protrusions without having flatness due to the atomic layer. The RMS surface roughness of the interface 15 was 2.95 nm.

At a relatively low growth temperature of 437 ° C., the interface 15 undergoes two-dimensional film growth, whereas at a growth temperature of 469 ° C. or higher, the interface 15 loses its flatness and the interface 15 has 3 It can be seen from FIGS. 4 and 5 that the dimension is growing. The protrusions in the three-dimensional structure obtained in the experiment have a conventional quantum dot-like shape. However, the ideal quantum dot structure is an isotropic shape or a target shape with facets on the side surface, but the three-dimensional structure obtained in the experiment is non-uniform in shape, which is considered to be due to thermal diffusion of In. The sex is observed, and the size of the protrusions is not uniform and varies. The protrusions at the interface 15 obtained at the growth temperature of 538 ° C. include those that are elongated. The elongated protrusions have a conventional quantum dash-like shape.

The interface 15 grown at 437 ° C. is flat because it grows in a two-dimensional film, and no large roughness is observed. On the other hand, when the interface 15 grows three-dimensionally, it exceeds the height of one atomic layer. Many protrusions are generated in. When the interface 15 grows three-dimensionally, for example, ^{a protrusion density of 10 10} pieces / cm ² or more can be obtained. The protrusion density is preferably 10 ¹² pieces / cm ² or less, and more preferably 10 ¹¹ pieces / cm ² or less.

As shown in FIGS. 4 and 5, the size of the protrusions increases as the growth temperature increases. It is considered that this is because the protrusions are greatly grown due to the thermal diffusion of In. Further, it is considered that the existence of elongated protrusions at the interface 15 grown at 538 ° is caused by the connection between the protrusions due to the thermal diffusion of In.

The RMS surface roughness of the interface 15 grown in a two-dimensional film at 437 ° C. is a very small value of 0.360 nm, which is equivalent to 1 ML, whereas the RMS surface roughness of the interface 15 grown in three dimensions is It will be 1 nm or more, which is equivalent to several ML. The RMS surface roughness of the interface 15 is preferably 2 nm or more. The RMS surface roughness of the interface 15 is preferably 4 nm or less, more preferably 3 nm or less.

The RMS surface roughness of the interface 15 tends to increase as the growth temperature is increased. This is considered to be due to the growth of each protrusion. However, when the growth temperature is 538 ° C., the RMS surface roughness is smaller than when the growth temperature is 526 ° C. It is considered that this is because the heat diffusion of In causes the protrusions to be connected to each other and the roughness is reduced. Further, it is considered that the cause is the re-evaporation of In.

FIG. 6 has _{an In 0.34} Ga _0.66 As thin film 13 obtained at each of the four growth temperatures of _{T 1} = 437 ° C, T ₂ = 489 ° C, T ₃ = 526 ° C, and T _{4 = 538 ° C.} Indicates the emission wavelength of the light emitting element. In FIG. 6, the vertical axis represents the emission intensity (Norm.PL intensity), and the horizontal axis indicates the emission wavelength and photon energy. The emission intensity in FIG. 6 is normalized, and the actual emission intensity differs depending on the temperature. The relationship between the growth temperature and the emission intensity will be described later.

The central wavelength λ ₁ of the emission band by the In _0.34 Ga _0.66 As thin film 13 obtained at the growth temperature T ₁ is around 1100 nm. Its bandwidth Δλ ₁ is 27 nm. Here, the bandwidth is the full width at half maximum (FWHM), and so on. _{At a growth temperature of T 1} = 437 ° C. in which the interface 15 has a two-dimensional structure _{, the produced In 0.34} Ga _0.66 As thin film 13 has a two-dimensional interface similar to that of a normal quantum well structure. Bandwidth is relatively narrow.

The central wavelength λ ₂ of the emission band by the In _0.34 Ga _0.66 As thin film 13 obtained at the growth temperature T ₂ is shifted to around 1160 nm (shift to the long wavelength side; red shift). This is because the red shift is due to the segregation of In due to the relatively high growth temperature and the concentration distribution of In. Moreover, the bandwidth Δλ ₂ is relatively wide. This is because the interface 15 has a three-dimensional structure, so that the film thickness of the thin film 13 is not constant and a distribution occurs. Since light emission is performed at a wavelength that depends on the thickness of the thin film, if the film thickness becomes non-uniform, the bandwidth of the light emission wavelength increases. Further, it is considered that the wide band is also contributed by the fact that the concentration distribution of In is generated by the diffusion and re-evaporation of In, and the random In composition ratio on the interface 15 is generated.

The central wavelength λ ₃ of the emission band by the In _0.34 Ga _0.66 As thin film 13 obtained at the growth temperature T ₃ is shifted to around 1180 nm (shift to the short wavelength side; blue shift). At a high growth temperature, not only thermal diffusion of In but also re-evaporation occurs, so that the concentration of In decreases. A decrease in the concentration of In causes a shortening of the wavelength. Therefore, the blue shift is due to the high growth temperature that causes the re-evaporation of In. Further, the bandwidth Δλ ₃ is wider than the bandwidth Δλ _{2 at} the growth temperature T _2. This is due to the increase in film thickness non-uniformity of the thin film 13 and the occurrence of the In concentration distribution.

The central wavelength λ ₄ of the emission band by the In _0.34 Ga _0.66 As thin film 13 obtained at the growth temperature T ₄ is further blue-shifted to around 1020 nm. This, than in the case of the growth temperature T _3, because the re-evaporation of In is increased. Further, the bandwidth Δλ ₄ is even wider than the bandwidth Δλ _{3 at} the growth temperature T _3. This is due to the increase in film thickness non-uniformity of the thin film 13 and the occurrence of the In concentration distribution. The spectral shape of the light emitted by the In _0.34 Ga _0.66 As thin film 13 obtained at the growth temperature T _{4 shows a smooth monomodity.}

From the above experimental results, it can be seen that the thin film 13 having the interface 15 having a three-dimensional structure has a wider emission bandwidth than the quantum well 13 having the interface 15 having a two-dimensional structure. Further, it can be seen that the central wavelength can be changed depending on the temperature at which the three-dimensional structure is grown.

[2.3.2 In _0.3 Ga _0.7 As 7 nm]

7A and 7B show observation images (AFM images) of the interface 15 of _{the In 0.3} Ga _0.7 As thin film 13 prepared at various growth temperatures. FIG. 7A is a two-dimensional image of the interface 15, and FIG. 7B is a three-dimensional image of the interface 15.

7A and 7B (a) show the interface 15 of the thin film grown at 415 ° C, and FIGS. 7B and 7B (b) show the interface 15 of the thin film grown at 440 ° C. Shown. It can be seen that at the growth temperatures of 415 ° C and 440 ° C, the interface 15 has a two-dimensional structure.

7A (c) and 7B (c) show the interface 15 of the thin film 13 grown at 484 ° C. 7A (d) and 7B (d) show the interface 15 of the thin film 13 grown at 516 ° C. It can be seen that at the growth temperatures of 484 ° C and 516 ° C, the interface 15 has a three-dimensional structure. From the AFM image, it can be seen that the higher the growth temperature, the larger the particle-like protrusions grow.

7A (e) and 7B (e) show the interface 15 of the thin film 13 grown at 536 ° C. It can be seen that at the growth temperature of 536 ° C., the interface 15 has a two-dimensional structure. If the growth temperature is too high, more re-evaporation of the In raw material occurs and the In composition decreases, so that it is considered that a sufficient film thickness for three-dimensional growth could not be obtained.

FIG. 8 shows _{In 0.3} Ga _0. obtained at each of the five growth temperatures of _{T 11} = 415 ° C, T ₁₂ = 440 ° C, T ₁₃ = 484 ° C, T ₁₄ = 516 ° C, and T _{15 = 536 ° C. 7} The emission wavelength of the light emitting element having the As thin film 13 is shown. In FIG. 8, the vertical axis represents the emission intensity (Norm.PL intensity), and the horizontal axis indicates the emission wavelength and photon energy. The emission intensity in FIG. 8 is normalized, and the actual emission intensity differs depending on the temperature.

In _0.3 Ga _0.7 As has a _{lower In concentration than In 0.34} Ga _0.66 As, so that the emission wavelength is shorter. The interface 15 of the thin film 13 obtained at the growth temperature T ₁₁ = 415 ° C, the thin film 13 obtained at the growth temperature T ₁₂ = 440 ° C, and the thin film 13 obtained at the growth temperature T ₁₅ = 536 ° C has a two-dimensional structure. Therefore, the band is relatively narrow.

On the other hand, _{the thin film 13 obtained at the growth temperature T 13} = 484 ° C. and the thin film 13 obtained at the growth temperature T ₁₄ = 516 ° C. have a wide band because the interface 15 has a three-dimensional structure. .. The resulting central wavelength lambda ₁₃ of the light emitting zone by a thin film 13 at the growth temperature _{T 13,} located near 1030 nm, bandwidth [Delta] [lambda] ₁₃ is 63 nm.

_{The central wavelength λ 14} of the emission band due to the thin film 13 obtained at the growth temperature T ₁₄ = 516 ° C. is blue-shifted to around 1010 nm due to the decrease in In concentration due to the re-evaporation of In. Further, the bandwidth Δλ ₁₄ is 75 nm, and the bandwidth is wider than the bandwidth _{Δλ 13 at the growth temperature T 13} due to the increase in the film thickness non-uniformity of the thin film 13.

[2.3.3 In _0.2 Ga _0.8 As 7 nm]

9 and 10 show AFM images of the interface (surface) 15 of _{the In 0.2} Ga _0.8 As thin film 13 produced at various substrate temperatures (growth temperatures). FIG. 9 is a two-dimensional image of the interface 15, and FIG. 10 is a three-dimensional image of the interface 15.

9 (a) and 10 (a) show the interface 15 of the thin film 13 grown at 425 ° C. 9 (b) and 10 (b) show the interface 15 of the thin film 13 grown at 434 ° C. 9 (c) and 10 (c) show the interface 15 of the thin film 13 grown at 483 ° C. 9 (d) and 10 (d) show the interface 15 of the thin film 13 grown at 520 ° C. 9 (e) and 10 (e) show the interface 15 of the thin film 13 grown at 536 ° C.

In _{the case of In 0.2} Ga _0.8 As, the interface 15 had a two-dimensional structure at any of the above growth temperatures, and a three-dimensional structure could not be obtained. Therefore, the obtained In _0.2 Ga _0.8 As thin film 13 is a normal quantum well having a two-dimensional interface. A normal quantum well having a two-dimensional interface has a _{relatively low In composition ratio such as In 0.2} Ga _0.8 As, and is obtained by growing at a relatively low temperature of about 420 ° C. to 500 ° C. According to the experiment, a three-dimensional structure could not be obtained even if the growth temperature of _{In 0.2} Ga _0.8 As, which is the composition of a normal quantum well, was raised to 536 ° C. The reason why the three-dimensional structure cannot be obtained will be described later.

[2.3.4 In _0.5 Ga _0.5 As 7 nm]

FIG. 11 shows an AFM image of the interface (surface) 15 of _{the In 0.5} Ga _0.5 As thin film 13 produced at various substrate temperatures (growth temperatures). 11 (a), (c), and (e) are two-dimensional images of the interface 15, and FIGS. 11 (b), (d), and (f) are three-dimensional images of the interface 15.

11 (a) and 11 (b) show the interface 15 of the thin film 13 grown at 484 ° C. 11 (c) and 11 (d) show the interface 15 of the thin film 13 grown at 517 ° C. 11 (e) and 11 (f) show the interface 15 of the thin film 13 grown at 535 ° C.

In _{the case of In 0.5} Ga _0.5 As, the interface 15 has a three-dimensional structure at any of the above growth temperatures. However, since no light emission was obtained or the light emission was very weak, it is considered that the obtained In _0.5 Ga _0.5 As thin film 13 does not sufficiently function as an optical device.

[2.3.5 In _0.28 Ga _0.72 As 9 nm]

FIG. 12 shows an AFM image of the interface (surface) 15 of _{the In 0.28} Ga _0.72 As thin film 13 produced at various substrate temperatures (growth temperatures). Here, an amount of raw material was supplied so that the thickness of the thin film 13 became 9 nm. 12 (a), (c), and (e) are two-dimensional images of the interface 15, and FIGS. 12 (b), (d), and (f) are three-dimensional images of the interface 15.

12 (a) and 12 (b) show the interface 15 of the thin film 13 grown at 446 ° C. 12 (c) and 12 (d) show the interface 15 of the thin film 13 grown at 486 ° C. 12 (e) and 12 (f) show the interface 15 of the thin film 13 grown at 519 ° C. It can be seen that when the growth temperature is 446 ° C., the interface 15 has a two-dimensional structure, but when the growth temperatures are 486 ° C. and 519 ° C., the interface 15 has a three-dimensional structure. From the AFM image, it can be seen that the higher the growth temperature, the larger the particle-like protrusions grow.

FIG. 13 shows the emission wavelengths of the light emitting device having _{the In 0.28} Ga _0.72 As thin film 13 obtained at each of the three growth temperatures of _{T 21} = 446 ° C, T ₂₂ = 486 ° C, and T _{23 = 519 ° C.} show. In FIG. 13, the vertical axis represents the emission intensity (Norm.PL intensity), and the horizontal axis represents the photon energy. The emission intensity in FIG. 13 is normalized, and the actual emission intensity differs depending on the temperature.

The thin film 13 obtained at the growth temperature T ₂₁ = 446 ° C. has a relatively narrow band because the interface 15 has a two-dimensional structure.

On the other hand, _{the thin film 13 obtained at the growth temperature T 22} = 486 ° C. and the thin film 13 obtained at the growth temperature T ₂₃ = 519 ° C. have a wide band because the interface 15 has a three-dimensional structure. ..
_{The central wavelength λ 22} of the emission band by the thin film 13 obtained at the growth temperature T ₂₂ = 486 ° C. is around 1046 nm, and the bandwidth Δλ ₂₂ is 42 nm. _{The central wavelength λ 23} of the emission band by the thin film 13 obtained at the growth temperature T ₂₃ = 519 ° C. is around 1010 nm, and the bandwidth Δλ ₂₃ is 52 nm.

[Comparison by 2.3.6 In composition]

FIG. 14 shows the relationship between the growth temperature and the PL peak wavelength (center wavelength) for each of the plurality of In composition ratios, and FIG. 15 shows the relationship between the growth temperature and the PL peak intensity for each of the plurality of In composition ratios. The relationship is shown, and FIG. 16 shows the relationship between the growth temperature and the bandwidth (full width at half maximum: FWHM) for each of the plurality of In composition ratios. In FIGS. 14, 15, and 16, x = 0.34 indicates In _0.34 Ga _0.66 As, x = 0.30 indicates In _0.3 Ga _0.7 As, and x = 0.20 indicates In _0.2 Ga _0.8 As. 14 to 16 are measurement results at room temperature.

_{In the case of an In 0.2} Ga _0.8 As thin film having an interface 15 having a two-dimensional structure, as shown in FIG. 12, the center wavelength is less than 1000 nm at any growth temperature, and the center wavelength of 1000 nm or more is I can't get it. In particular, when the growth temperature is 500 ° C. or higher, In re-evaporation increases, the In concentration decreases, and the center wavelength becomes even shorter. Further, as shown in FIG. 14, the full width at half maximum (FWHM) is also small, and the band is narrow.

On the other hand, in the case of the In _0.34 Ga _0.66 As thin film, as shown in FIG. 14, the center wavelength is around 1100 nm at the growth temperature of 437 ° C. or lower in which the interface 15 has a two-dimensional structure. As the growth temperature is increased, the interface 15 has a three-dimensional structure, and the center wavelength is red-shifted (lengthened) up to around 500 ° C. When the growth temperature is further raised above around 500 ° C. (around 510 ° C.), a blue shift (shortening of wavelength) occurs, the central wavelength returns to the original 1000 nm at around 520 ° C., and the central wavelength is 1000 nm at around 540 ° C. It will be in the vicinity. Therefore, by appropriately setting the growth temperature, a desired center wavelength can be obtained even in the range of 1000 nm or more, and a center wavelength of 1000 nm or less can be obtained.

Further, in the case of the In _0.34 Ga _0.66 As thin film, as shown in FIG. 16, when the growth temperature is raised and the interface 15 has a three-dimensional structure, the full width at half maximum (FWHM) becomes large and the bandwidth is widened. There is. Further, in the case of the In _0.34 Ga _0.66 As thin film, as shown in FIG. 15, when the growth temperature exceeds 510 ° C., the PL peak intensity also increases. It is considered that this is because the defect generated in the thin film 13 was repaired by the thermal diffusion of In.

_17, for In _{0.28 Ga 0.72} As film (9 nm), shows the relationship between the growth temperature and the PL peak wavelength (center wavelength), FIG. _18, In _{0.28 Ga 0.72} As film The relationship between the growth temperature and the PL peak intensity is shown, and FIG. 19 shows the relationship between the growth temperature and the bandwidth (full width at half maximum: FWHM) for the _{In 0.28} Ga _{0.72 As thin film.} According to FIG. 17, it can be seen that in the In _0.28 Ga 0.72 As thin film, the emission peak wavelength shifts _{depending on the growth temperature.} According to FIG. 18, in the In _0.28 Ga _0.72 As thin film, it can be seen that the emission intensity is increased by increasing the growth temperature. According to FIG. 19, in the In _0.28 Ga _0.72 As thin film, it can be seen that the bandwidth is widened by increasing the growth temperature. 17 to 19 are measurement results at room temperature.

As described above, the InGaAs thin film 13 having a three-dimensional interface 15 is a semiconductor material having a wavelength of 950 nm or more, a emission center wavelength of around 1000 nm, a wide band, and sufficient brightness. It has characteristics that cannot be obtained with a conventional quantum structure. An optical device having a thin film 13 having such characteristics is suitable, for example, in the field of medical imaging. In the field of medical imaging, for example, wideband characteristics in the 1050 nm band are required, and the thin film 13 having a three-dimensional structural interface 15 has characteristics suitable for medical imaging.

Conventionally, there have been reports that InAs quantum dots having a central wavelength of about 1200 nm to 1300 nm were used to obtain a wide band characteristic with a central wavelength of about 1000 nm by shortening the emission wavelength, but the emission intensity is lowered. There was a problem. On the other hand, in the InGaAs thin film having the interface 15 having a three-dimensional structure, sufficient emission intensity is obtained.

Further, in the case of the In _0.3 Ga _0.7 As thin film, the same characteristics as in the case of the _{In 0.34} Ga _{0.66 As thin film are obtained.} Further, in the case of the In _0.28 Ga _0.72 As thin film, an interface having a three-dimensional structure is obtained. Therefore, the In composition ratio x is preferably 0.25 or more, and from the viewpoint of obtaining better characteristics, the In composition ratio x is preferably 0.3 or more. The In composition ratio x is preferably 0.45 or less, more preferably 0.42 or less, and even more preferably 0.4 or less. The growth temperature is preferably 500 ° C. or higher in order to promote wide banding. The growth temperature is preferably 550 ° C. or lower in order to suppress excessive re-evaporation of the raw material.

[2.3.6 Comparison by film thickness]
FIG. 20 shows the relationship between the wavelength and the PL emission intensity when _{the In 0.34} Ga _0.66 As thin film 13 is grown by different amounts of the supplied raw materials (In, Ga, As). There is. In FIG. 20, 7 nm means that a raw material having a thickness of the thin film 13 of 7 nm is supplied, and 5 nm means that a raw material having a thickness of the thin film 13 of 5 nm is supplied. Yes, 3 nm is a case where the raw material is supplied in an amount that makes the thickness of the thin film 13 3 nm. In the case of 7 nm, the growth temperature was 510 ° C., in the case of 5 nm, the growth temperature was 520 ° C., and in the case of 3 nm, the growth temperature was 560 ° C.

At 3 nm, the thin film 13 grew two-dimensionally, and the interface of the thin film 13 had a two-dimensional structure, and at 5 nm and 7 nm, the interface of the thin film 13 had a three-dimensional structure. In the case of 3 nm, since the film thickness is thin, the central wavelength is shorter than that of 5 nm and 7 nm, the band is narrow, and the emission intensity is low. On the other hand, at 5 nm and 7 nm where a three-dimensional structure can be obtained, the center wavelength is around 1000 nm, the band is widened, and the emission intensity is high.

[2.4 Growth of thin film]
Hereinafter, the growth of a thin film having a three-dimensional structural interface will be described. In the following, for ease of understanding, first, the growth of general quantum wells and quantum dots will be described, and then the growth of a thin film having a three-dimensional structural interface will be described.

When an InGaAs crystal is grown on a GaAs substrate, a crystal having the same crystal structure as the substrate but having a lattice constant different from that of the substrate is grown, distortion due to lattice mismatch occurs. Lattice mismatch affects the crystal growth mode. As the crystal growth mode, for example, Frank-van der Merwe (FM) mode and Stranski-Krastanov (SK) mode are known. The FM mode is a mode in which a crystal layer grows on a substrate in a two-dimensional film. The SK mode is a mode in which a crystal grows three-dimensionally.

When growing a general quantum well, for example, an In _x Ga _1-x As crystal is grown on a GaAs substrate in FM mode. In a general quantum well, a relatively low In composition ratio of about x = 0.2 is often adopted.

For a general quantum well, a large In composition ratio x is not always preferable. It is generally considered that when the In composition ratio x becomes large, the distortion due to the lattice mismatch becomes large, defects are likely to occur in the crystal, and the function as a quantum well is likely to be impaired. That is, as the In composition ratio x increases, the critical film thickness (Matthews and Blakeslee critical film thickness) decreases. When the film thickness of the grown crystal exceeds the critical film thickness, a misfit transition occurs due to strain due to lattice mismatch, and defects occur in the crystal. Therefore, in a general quantum well, the film thickness is set to a value sufficiently smaller than the critical film thickness.

However, if the In composition ratio x is low, the critical film thickness becomes large, so that even if the film thickness of the quantum well is sufficiently smaller than the critical film thickness, it is easy to make it relatively thick. In order for the thin film to function as a quantum well, it must be a thin film with a film thickness of 10 and several nm or less at which the quantum effect appears, but if x = 0.2, the critical film thickness is 14 nm or more. Therefore, the film thickness can be set relatively freely in the range of 10 and several nm or less. Since the film thickness can be freely set, it becomes easy to obtain a desired wavelength.

Further, when growing general quantum dots, for example, an InAs crystal (that is, _{x = 1 in an In x} Ga _1-x As crystal) is grown on a GaAs substrate in SK mode. When the quantum dots grow, the crystal grows three-dimensionally. Since InAs has a large lattice mismatch with the GaAs substrate, it grows in the SK mode. Further, the amount of the raw material for growing the quantum dots is much smaller than that of the quantum well, and may be an amount corresponding to, for example, growing a thin film having a thickness of about 2 atomic layers (2ML).

On the other hand, the growth of the thin film 13 having the three-dimensional structural interface 15 is a composite of FM mode and SK mode. The growth of the thin film 13 having the three-dimensional structural interface 15 is FM-mode in that the thin film is formed (even if the interface is not flat). However, the growth of the thin film 13 having the three-dimensional structural interface 15 is different from the FM mode in that it includes three-dimensional growth.

Further, the growth of the thin film 13 having the three-dimensional structural interface 15 is SK mode-like in that it includes three-dimensional growth. However, the growth of the thin film 13 having the three-dimensional structural interface 15 differs from the SK mode in that a larger amount of raw material is supplied than in the case of forming quantum dots.

In the growth of the thin film 13 having the three-dimensional structural interface 15 of the embodiment, the thin film 13 is grown in the FM mode to the vicinity of the critical film thickness within a range not exceeding the critical film thickness, so that the thin film is like a quantum well. Is formed, and the strain due to the lattice mismatch between the substrate and the grown crystal is increased near the interface 15. Moreover, since the growth temperature is relatively high (for example, 480 ° C., more preferably 500 ° C. or higher), a large strain and a high temperature state are combined to cause SK-mode three-dimensional growth at the interface 15. The three-dimensional growth of the interface 15 is caused by the action of a force for relaxing strain in a crystal raw material (particularly In) in a state where thermal diffusion can occur due to high temperature. Further, by increasing the composition ratio of In, the strain can be increased and the growth of the three-dimensional structure can be promoted.

In the growth of the thin film 13 having the three-dimensional structural interface 15, it is possible to suppress the occurrence of defects in the thin film 13 by not exceeding the critical film thickness. _{For example, in the case of In 0.5} Ga _0.5 As shown in FIG. 11, the critical film thickness is about 4 nm, but an amount of raw material exceeding the critical film thickness (raw material having a film thickness of 7 nm) is supplied. Has been done. It is considered that when the critical film thickness is exceeded, the defects in the thin film 13 become remarkable, and even if the thin film 13 has the three-dimensional structural interface 15, the function as an optical device is impaired. Even at In _0.5 Ga _0.5 As, an amount of raw material that does not exceed the critical film thickness, for example, an amount that makes the film thickness 3 nm is supplied so that the average film thickness becomes less than the critical film thickness. By doing so, it is expected that a thin film having a three-dimensional structural interface and functioning as an optical device can be obtained.

Further, as shown in FIG. 20, even in the case of In _0.34 Ga _0.66 As thin film 13, when a raw material having an amount of the thin film 13 having a thickness of 3 nm is supplied, the three-dimensional structural interface 15 is formed. What has not been obtained is that since the film thickness is relatively small, the accumulation of strain that causes a three-dimensional structure is small, and even if the growth temperature is high (560 ° C.), the FM mode is maintained and the two-dimensional structure is formed. It is probable that it has become. On the contrary, the appearance of the three-dimensional structure at 5 nm and 7 nm supports the growth of the three-dimensional structure due to strain.

_{In the In 0.2} Ga _0.8 As thin film 13 shown in FIGS. 9 and 10, since the lattice mismatch with the substrate is originally relatively small, strain accumulation is relatively small even when the film thickness is 7 nm. It is considered that the FM mode was maintained and the structure became a two-dimensional structure even when the growth temperature was raised. Even in a composition in which the strain is small, growth of the three-dimensional structural interface is expected by increasing the film thickness and increasing the accumulation of strain.

From the above, it can be seen that in order to grow a thin film having a three-dimensional structural interface, an appropriate raw material supply amount (film thickness) and an appropriate growth temperature should be set with respect to the composition of the crystal raw material. Here, the raw material supply amount is preferably an amount having a thickness near the critical film thickness, which is less than the critical film thickness determined by the composition of the crystal raw material and the like. By setting the raw material supply amount to a thickness less than the critical film thickness, the average film thickness of the thin film having the three-dimensional structural interface becomes less than the critical film thickness. As the film thickness increases to near the critical film thickness, the accumulation of strain increases and a three-dimensional structural interface can be obtained. The growth temperature is preferably a temperature at which at least one of thermal diffusion and re-evaporation of the raw material (particularly In) occurs.

FIG. 21 and Table 1 below show the results of growing the thin film with different raw material compositions, film thickness (raw material supply amount), and growth temperature. In Table 1, the emission peak wavelength, full width at half maximum, and PL peak intensity show the measurement results at room temperature. In FIG. 21 and Table 1, the film thickness indicates the amount of raw material supplied.

In FIG. 21, white circles (S1, S2, S11, S12 and black circles (S3 to S10) indicate a combination of composition and film thickness (raw material supply amount) in which a thin film having a three-dimensional structural interface is obtained. The black circles (S3 to S10) indicate that a relatively high emission intensity was obtained as compared with the white circles (S1, S2, S11, S12). In FIG. 21, the square marks (S13 to S17). ) Indicates a combination of composition and film thickness (raw material supply amount) in which a thin film having a three-dimensional structural interface could not be obtained at the experimental growth temperature. In the square marks (S13 to S17), the experimental growth temperature is , S13 is 425 ° C., 440 ° C., 483 ° C., 520 ° C., 536 ° C., S14 is 500 ° C., S15 is 443 ° C., S16 is 443 ° C., and S17. Is 440 ° C., 480 ° C., 520 ° C., and 540 ° C. In FIG. 21, a cross indicates that a three-dimensional structural interface is obtained, but non-emission or light emission is very weak.

In FIG. 21, a critical film thickness curve (Matthews and Blakeslee critical film thickness curve) is also shown. The critical film thickness curve drawn in FIG. 21 is for reference only and is not exact. In addition, the critical film thickness varies slightly depending on the temperature. In FIG. 21, the film thickness range below the critical film thickness and near the critical film thickness is shown in gray.

As shown in FIG. 21, in the case of a combination of the raw material composition and the film thickness (raw material supply amount) within the gray range (less than the critical film thickness and near the critical film thickness) (in the case of white circles and black circles). When a three-dimensional structural interface is obtained by appropriately setting the growth temperature, and when an amount of raw material having a thickness equal to or greater than the critical film thickness corresponding to the raw material composition is supplied (in the case of x mark), Although a three-dimensional structure can be obtained, non-emission or light emission is very weak. Further, even if the raw material supply amount is less than the critical film thickness, if the amount is much lower than the critical film thickness, the three-dimensional structural interface cannot be obtained and the two-dimensional structural interface is obtained. In particular, even if the composition is such that a three-dimensional structural interface can be obtained depending on the film thickness, such as _{In 0.34} Ga _{0.66 As, if the film thickness is small, the accumulation of strain due to lattice mismatch does not increase.} It is considered that the three-dimensional structural interface does not appear. On the contrary, even if the composition does not have a two-dimensional structural interface in the experiment, it is expected that a three-dimensional structural interface can be obtained by setting the film thickness to the vicinity of the critical film thickness.

Since S13 and S14 (square marks) shown in Table 1 are thin films having a two-dimensional structural interface, the full width at half maximum is 20.0 meV and 26.8 mev, respectively, which are narrow bands. On the other hand, in S1 to S12 (white circles and black circles), a wide band of 30 meV or more can be obtained. From the viewpoint of a wider band, the light emission band is more preferably 40 meV or more, and further preferably 60 meV or more.

The present invention is not limited to the above embodiment. For example, the thin film is not limited to InGaAs, and is not particularly limited as long as it has the same crystal structure as the substrate and lattice mismatch occurs with respect to the substrate. Due to the lattice mismatch with respect to the substrate, distortion that causes a three-dimensional structural interface can be obtained. By utilizing the strain due to lattice mismatch, a thin film having a three-dimensional structural interface can be obtained regardless of the type of raw material of the thin film. A thin film having a three-dimensional structural interface supplies an amount of raw material that is less than the critical film thickness according to the composition of the thin film and forms a film thickness close to the critical film thickness, regardless of the composition of the thin film, and has an appropriate temperature. It is obtained by growing a thin film with.

The thin film may be, for example, InGaN. The thin film preferably contains In, which is prone to thermal diffusion or re-evaporation. The thin film may be SiGe, GaInAS, GaInN, or ZnSSe. Further, the substrate may be Si, GaAs, GaN, or ZnSe.

The combination of [thin film / substrate] may be [SiGe / Si], [GaInAS / GaAs], [GaInN / GaN], or [ZnSSe / ZnSe]. ] May be. The combination of these thin films and the substrate is preferable because they have the same crystal structure and have a large lattice mismatch.

The optical device is not limited to the light emitting element, and may be a light receiving element.

10 Active layer 11 Substrate 13 Semiconductor thin film 15 Interface 100 Optical device

Claims (11)

A method for manufacturing an optical device including an active layer sandwiched between clad layers at the top and bottom.
In _x Ga _1-x As (x is the composition ratio of indium, the value of x is 0.3,0. A growth step of growing a thin film of 34, 0.36 or 0.38) as the active layer is included.
In the growth step, a three-dimensional structure grows at the interface of the thin film due to strain caused by the thin film having a different lattice constant from the substrate, and the average film thickness of the thin film is less than the critical film thickness.
The three-dimensional structure has a shape having quantum dot-shaped or quantum dash-shaped protrusions at the interface.
A method for manufacturing an optical device, wherein when the composition ratio is as follows, the average film thickness is within the film thickness range or the film thickness near the critical film thickness shown on the right.
When x = 0.3, the average film thickness is 7 nm.
When x = 0.34 When the average film thickness is 5 nm or more and 7 nm or less When x = 0.36 When the average film thickness is 3 nm or more and 5 nm or less When x = 0.38 The average film thickness is 3 nm The three-dimensional structure has a shape having protrusions and has a protrusion.
The method for manufacturing an optical device according to claim 1, wherein the density of the protrusions is 10 ¹⁰ pieces / cm ² or more. The method for manufacturing an optical device according to claim ² , wherein the density of the protrusions is 10 ¹² pieces / cm 2 or less. The method for manufacturing an optical device according to any one of claims 1 to 3, wherein the interface has an RMS surface roughness of 1 nm or more. The method for manufacturing an optical device according to any one of claims 1 to 3, wherein the interface has an RMS surface roughness of 2 nm or more. The method for manufacturing an optical device according to claim 4 or 5, wherein the interface has an RMS surface roughness of 4 nm or less. The method for manufacturing an optical device according to any one of claims 1 to 6, wherein the central wavelength of the light emitting band is 950 nm or more. The method for manufacturing an optical device according to claim 7, wherein the center wavelength is 1000 nm or more. The method for manufacturing an optical device according to any one of claims 1 to 8, wherein the central wavelength of the light emitting band is 1200 nm or less. The method for manufacturing an optical device according to claim 9, wherein the center wavelength is 1100 nm or less. The method for manufacturing an optical device according to any one of claims 1 to 10, wherein the growth temperature is 480 ° C. or higher and 550 ° C. or lower.

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