JP3830858B2 - Ultra-high-speed, wide-wavelength optical saturable absorber semiconductor, semiconductor device using the same, and waveguide-type optical-optical switch - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is an optical semiconductor device (for example, an optical-optical switch) that performs an optical logic / optical switch operation that can be used for an optical switch / optical repeater that is expected to constitute an optical communication / optical information processing system, or As a semiconductor thin film having optical saturable absorption characteristics constituting a saturable absorption mirror, which is a main component in a laser oscillator that generates an ultrashort optical pulse, and a semiconductor thin film having ultrafast performance over a wide wavelength band, and The present invention relates to an optically saturable absorbing semiconductor device, an optical-optical switch, an optically saturable absorbing semiconductor mirror, and the like that are used at an ultra-high speed and a wide wavelength band.
[0002]
[Prior art]
Conventionally, prior art documents in this field include the following.
[0003]
[1] Japanese Patent Laid-Open No. 7-36065
[2] M.M. Haiml et al. , Appl. Phys. Lett. , Vol. 74, p. 1269 (1999)
[3] J. L. Oudar, et. al. Phys. Rev. Lett. , Vol. 55 (19), p. 2074 (1985)
[4] W.M. H. Knox et. al. Phys. Rev. Lett. , Vol. 56 (11), p. 1191 (1986)
[5] P.I. W. Schoenlein et. al. , Appl. Phys. Lett. , Vol. 51 (18), p. 1442 (1987)
[6] Japanese Patent Application 2001-273313 (unpublished)
[7] T.M. Asano et al. , Appl. Phys. Lett. , Vol. 77, pp. 19-21 (2000)
[8] Takeshi Kamiya et al., “Applied Physics”, vol. 70, pp. 1271 (2001)
The light absorption saturation characteristic (or saturable absorption characteristic) of a semiconductor thin film is an effect that a large amount of carriers are generated by irradiating strong light, thereby reducing the light absorption coefficient. The intensity density of light that causes a decrease in the absorption coefficient is referred to as saturation excitation density Is.
[0004]
FIG. 1 is a diagram for explaining transient characteristics of light absorption saturation. When strong control light is incident, the material becomes transparent and weak signal light is transmitted. When the control light is cut off, the absorption is recovered with the time constant τ, so that the transmitted signal light is gradually weakened.
[0005]
In FIG. 1, as shown in FIG. 1B, strong light called control light is applied for a very short time t. ₀ The time change of the transmission amount with respect to weak light called the signal light shown to Fig.1 (a) when only irradiating is shown. The horizontal axis represents the delay time of the signal light pulse with respect to the control light pulse. Time t ₀ In the meantime, since the control light is present, the absorption coefficient is decreased and the transmission amount of the signal light is increased. However, as shown in FIG. 1C, after the control light disappears, the excited carriers have a time constant τ. Therefore, the amount of transmitted signal light also attenuates with a time constant τ. This time constant τ is referred to as absorption saturation recovery time, and the smaller the time constant τ, the more excellent the high-speed response.
[0006]
Conventionally, optical saturable absorption characteristics in semiconductors include band filling effects caused by high density carriers generated by strong photoexcitation density occupying the density of states, or bandgap renormalization effects that are many-body effects caused by high density carriers. Furthermore, it is caused by a phase space filling effect based on Wannier excitons.
[0007]
In any case, since the effect exhibited by the carrier excited at the band edge is used, the wavelength of the operating light is limited to the band edge wavelength. In this case, electrons excited from the valence band to the bottom of the conduction band must go through a spontaneous emission process in order to recombine with holes and return to the original state. For this reason, the order is 1 ns (nanosecond). Took time.
[0008]
As an attempt to reduce the lifetime of the electrons by introducing carrier traps, an attempt is made to grow a semiconductor thin film at a temperature lower than usual and intentionally introduce crystal defects in the film to serve as an electron trap. (Refer to prior art document [1]).
[0009]
Furthermore, by introducing an acceptor impurity such as Be as a dopant into such a low-temperature MBE growth film, it has become possible to realize a fast response of about 1 ps (picosecond) (prior art document [1]). , [2]).
[0010]
[Problems to be solved by the invention]
However, all of the above-described conventional techniques are limited to a limited wavelength band of a band gap wavelength or a wavelength band shorter by 10 to 20 nm (in terms of photon energy, the band gap energy or the vicinity of 20 meV). .
[0011]
In addition, in order to introduce lattice defects intentionally trapped in a crystal, not only a fast response component but also a slow response component is often included in the time response characteristics (prior art document [2 〕reference).
[0012]
On the other hand, when a semiconductor thin film is excited using a short optical pulse having a short wavelength corresponding to a photon energy higher than the band gap energy, absorption saturation occurs within an initial time of 0.1 to 0.2 ps. It was found. This is a phenomenon in which only the density of states corresponding to the excited energy of the carriers is saturated, and is a phenomenon that occurs before the energy is relaxed and the thermal distribution state is reached.
[0013]
This phenomenon is called spectrum hole burning (SHB) (refer to prior art documents [3] and prior art documents [4]). This SHB phenomenon is a phenomenon that exhibits an ultra-high-speed response, but in reality, it is accompanied by a slow response with a long time constant ranging from 1 ps to 1 ns (= 1000 ps) (see the prior art document [5]), which is removed. It was a major technical challenge.
[0014]
In view of the above situation, the present invention eliminates the slow response component associated with the ultrafast response associated with the spectral hole burning phenomenon, and eliminates the slow response component over the entire wide wavelength range instead of the narrow wavelength range. It is an object of the present invention to provide an optically saturable absorbing semiconductor having a truly ultra-high speed and a wide wavelength band, having no slow response over a wide wavelength range, a semiconductor device using the same, and a waveguide type optical-optical switch. And
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[1] An optical semiconductor thin film made of a group III-V compound semiconductor manufactured on a semiconductor substrate, and the growth temperature of this thin film crystal is in the range of 150 ° C. to 450 ° C. The group V element and group III Element beam intensity ratio V / III is in the range of 2 to 200 Including a large amount of lattice defects due to excessive group V elements The thin film crystal shows only a steep response with a time constant of about 1 ps in the transient characteristic of absorption saturation at the band edge wavelength, The slow response is small enough to be ignored.
[0016]
[2] In a semiconductor thin film crystal made of a group III-V compound semiconductor manufactured on a semiconductor substrate by using molecular beam epitaxy, the growth temperature is in the range of 150 ° C. to 450 ° C. And the group III element beam intensity ratio V / III is in the range of 2 to 200 Including a large amount of lattice defects due to excessive group V elements And the dopant of Be is 1 to 50 × 10 ¹⁸ cm ^-3 In a transient characteristic of light absorption saturation by degenerate pump-probe measurement at a photon energy corresponding to the band gap of the thin film crystal or a photon energy higher than the band gap of the thin film crystal, a single value of 1 ps or less is used. A characteristic having a time constant is exhibited, or the relative magnitude of a secondary response having a larger time constant associated with the main response is 10% or less.
[0017]
[3] A semiconductor thin film, and a low reflection film and a high reflection film arranged so as to sandwich the semiconductor thin film. Signal light is incident from the low reflection film side and reflected by the high reflection film side. The planar thin film optical-optical switch having a configuration that emits light from the light source and controls on / off of signal light by on / off of control light uses the semiconductor thin film crystal described in [2] above as the semiconductor thin film. To do.
[0018]
[4] A semiconductor thin film and an optical waveguide having a double heterostructure configured so that the semiconductor thin film is sandwiched between other semiconductor thin films. The signal light and the control light propagate through the waveguide, and the control light is turned on. In the waveguide type optical-optical switch that controls on / off of signal light by turning on / off, the semiconductor thin film crystal described in [2] is used as the semiconductor thin film.
[0019]
[5] having a semiconductor thin film, a low reflection film and a high reflection film arranged so as to sandwich the semiconductor thin film, light is incident from the low reflection film side, and passes through the semiconductor thin film and is reflected by the high reflection film The semiconductor thin film passes through the semiconductor thin film again and is emitted from the low reflection film side. When the light intensity is low, the semiconductor thin film exhibits large absorption, and when the light intensity is high, the semiconductor thin film In the light reflecting mirror exhibiting small absorption, the semiconductor thin film crystal described in [2] is used as the semiconductor thin film.
[0020]
[6] A semiconductor thin film and two low reflection films arranged so as to sandwich the semiconductor thin film, and light enters from the low reflection film side, passes through the semiconductor thin film, and exits from the other low reflection film When the light intensity is low, the semiconductor thin film exhibits a large absorption, and when the light intensity is high, the semiconductor thin film exhibits a small absorption. The semiconductor thin film crystal described is used.
[0021]
[7] Growth temperature of 150 to 450 by MBE method using a semiconductor epitaxial wafer having a first double heterostructure for an optical waveguide provided with a groove having a depth of 1 to 10 μm in width and reaching the substrate by etching. At 50 ° C., dopant such as Be or C is 1 to 50 × 10 ¹⁸ cm ^-3 The second double heterostructure including the semiconductor according to claim 1 added at a concentration of 2 is regrown so as to be embedded in the groove, and the band gap energy of the core layer of the second double heterostructure is The optical waveguide is selected so as to be smaller than the band gap energy of the core layer of the first double hetero structure and includes a part of the groove, and an optical waveguide is provided in a direction perpendicular to the groove.
[0022]
[8] A semiconductor epitaxial wafer having a first double heterostructure for an optical waveguide provided with a groove having a depth of 1 to 10 μm and reaching the substrate by etching, and a growth temperature of 150 to 450 ° C. by MBE. 1 to 50 × 10 dopants such as Be or C ¹⁸ cm ^-3 A second double heterostructure epitaxial wafer comprising the semiconductor of claim 1 added at a concentration of about 1 to 10 μm in width to the substrate and having a thickness substantially the same as the thickness of the first double heterostructure And using a second semiconductor epitaxial wafer provided with a mesa type path having a height to reach, the wafers are superposed and bonded by heat treatment so that the mesa type path is filled with the groove, In the semiconductor wafer in which the substrate of the second semiconductor epitaxial wafer is removed by etching, the band gap energy of the core layer of the second double hetero structure is smaller than the band gap energy of the core layer of the first double hetero structure. And including a part of the groove, and an optical waveguide is provided in a direction perpendicular to the groove. .
[0023]
The present invention uses a thin film crystal grown at a low temperature by the MBE method and containing a large amount of lattice defects (antisite As) due to an excessive group V element as a semiconductor thin film crystal used as a photo-saturable absorbing semiconductor, and a dopant for this. By adding Be as a complex defect with this lattice defect, an ultrafast component with a time constant of 1 ps or less is obtained in the transient response characteristic of absorption saturation at the band edge wavelength of this thin film crystal or a wavelength slightly shorter than that. The slow response component is suppressed to an allowable minimum.
[0024]
The standard growth temperature in the MBE method, which is one method for producing a GaAs thin film crystal that is representative of a compound semiconductor, is 450 ° C. to 800 ° C., but other conditions are also standard within this range. A thin film crystal grown within a certain range has few crystal defects and is suitable for manufacturing an electronic device or an optoelectronic device. When such semiconductors are irradiated with intense light, high-density carriers are generated and occupy a density of states corresponding to this energy.
[0025]
As a result, only the light absorption coefficient corresponding to this energy is reduced, and light of this energy passes through the semiconductor thin film crystal. This effect is referred to as a spectrum hole burning (SHB) effect (see the prior art documents [3] and [4]).
[0026]
FIG. 2 shows an experimental example in which the SHB phenomenon was actually measured (refer to the prior art document [4]). The sample is a semiconductor thin film crystal having a GaAs-AlGaAs quantum well structure. When a high-intensity femtosecond ultrashort light pulse having a higher energy than the band gap is incident on this, the transmission spectrum of the weak light transmitted through the sample is measured over time using a white light pulse. is there.
[0027]
The energy width of the incident ultrashort light pulse is 20 meV. Looking at the transmission spectrum, the transmission increases at photon energies that match (or slightly lower) the energy of the incident ultrashort light pulse, and looks like a mountain on the spectrum (shown as a shadow in the figure). This mountain disappears in about 200 fs.
[0028]
FIG. 3 illustrates such a process using a band diagram. Within the initial 200 fs time, only the density of states corresponding to the excited carrier energy is saturated. However, the carrier eventually undergoes energy relaxation and changes to a hot carrier having a Boltzmann distribution. As energy relaxation to the crystal lattice further progresses, the carrier temperature approaches the lattice temperature, but the number of carriers is governed by the recombination process, so a time of several ns called the spontaneous emission lifetime is required until the carriers disappear. Cost.
[0029]
FIG. 4 shows a time response waveform measured using an ultrashort light pulse (pump light pulse) which is incident excitation light and a weak light pulse (probe light pulse) having the same energy as this. In FIG. 4, the horizontal axis indicates the time delay of the probe light pulse with respect to the pump light pulse. The steep time response based on SHB ends at the initial 200 fs, and the slow response eventually dominates (see prior art document [5]).
[0030]
By the way, as shown in FIG. 5, when the wavelength of light is selected so that the energy of the excited light is 100 meV or more (expressed in terms of wavelength, 810 nm or less) than the semiconductor band gap energy (corresponding wavelength is 865 nm), It is known that the slow response component is greatly reduced, only the steep transient response component based on the initial SHB becomes dominant, and an extremely fast absorption recovery process of several hundred fs (femtosecond) or less can be obtained. (Refer to prior art document [6]).
[0031]
There, it is shown that the slow response component becomes small enough to be ignored in the wavelength range of 800 nm to 750 nm in terms of wavelength. The lower limit of the true wavelength is not 750 nm, and a truly fast absorption recovery process without such a slow response is realized up to a much shorter wavelength. However, a slow response component occurs within 100 meV from the band edge (expressed in terms of wavelength: 865 nm to 810 nm), and in this range, it is difficult to say that it is truly ultrafast.
[0032]
The present invention uses a thin film crystal grown at a low temperature by the MBE method and containing a large amount of lattice defects (antisite As) due to an excessive group V element as a semiconductor thin film crystal used as a photo-saturable absorbing semiconductor. By adding Be and forming a composite defect of the lattice defect and Be, the transient response characteristic of the absorption saturation at the band edge wavelength is limited to an ultrafast component with a time constant of 1 ps or less, and the accompanying slowness It is characterized by using a semiconductor thin film that suppresses the response component to an allowable minimum. This makes it not only a band edge wavelength, but also a very high speed with no slow response component in a wide wavelength range corresponding to the inside of the band. It will be described with reference to the following embodiment that a good response can be obtained.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0034]
Examples of the present invention will be described with respect to a GaAs thin film crystal which is the most basic and representative material of a compound semiconductor.
[0035]
FIG. 6 is a diagram showing a layer structure of a semiconductor thin film showing an embodiment of the present invention.
[0036]
In this figure, 10 is a GaAs substrate, 11 is an Al formed on the GaAs substrate 10. _0.3 Ga _0.7 As etch stop layer, 12 is a GaAs operation layer formed on the etch stop layer 11, and 13 is Al formed on the GaAs operation layer 12. _0.3 Ga _0.7 It is an As cap layer.
[0037]
As shown in FIG. 6, a GaAs thin film crystal is formed on a GaAs substrate 10 using an MBE method at a growth temperature of 250 ° C. _0.3 Ga _0.7 As etch stop layer 11, GaAs operation layer (700nm) 12, Al _0.3 Ga _0.7 As cap layer 13 was grown sequentially. In the GaAs operation layer 12, Be is 2 × 10 6 as an acceptor impurity. ¹⁹ cm ^-3 Is doped at a concentration of
[0038]
Next, FIG. 7 shows an absorption spectrum (αL vs. wavelength λ) measured with weak light after removing the substrate to the etch stop layer 11 by chemical etching.
[0039]
Next, an example in which the absorption saturation recovery time response characteristic of this GaAs thin film is measured with respect to the wavelength λ at the band edge by a pump-probe method using a wavelength variable titanium sapphire laser oscillating at a pulse width of 200 fs and repeatedly oscillating at 80 MHz. It is shown in FIG. The probe light pulse is one in which the intensity of the pump light pulse is 1/10 with respect to the pump light pulse, and therefore the wavelength is the same.
[0040]
In FIG. 8, the horizontal axis represents the delay time t of the probe light pulse with respect to the pump light pulse, and the vertical axis represents the intensity change ΔT of the probe light transmitted through the GaAs thin film with reference to the transmission intensity T at t <0. It is. When the pump light is incident at the instant t = 0, the transmittance of the probe light increases instantaneously. This is the SHB effect described in FIG. When the pump light is extinguished, the transmittance of the probe light decreases with a single time constant τ = 0.7 ps, and no slow response component having other time constants is observed.
[0041]
FIG. 9 shows absorption recovery transient response characteristics measured with various wavelengths of light having energy above the band edge using a GaAs thin film having such characteristics. 9A is an absorption recovery transient response characteristic diagram measured with light having a wavelength of 820 nm (time constant τ is 0.8 ps), and FIG. 9B is an absorption recovery transient response characteristic diagram measured with light having a wavelength of 790 nm. (Time constant τ is 0.8 ps), FIG. 9C is an absorption recovery transient response characteristic diagram (time constant τ is 0.4 ps) measured with light having a wavelength of 750 nm.
[0042]
As is clear from FIG. 9, although the time constant of the initial response slightly changes as the wavelength becomes shorter, only an ultrafast response with a time constant of 0.3 to 0.8 ps over a wide range up to the wavelength λ = 750 nm. appear.
[0043]
In the case of the transient response characteristic shown in FIG. 5 described above, it is for a GaAs thin film crystal grown at a normal growth temperature (400 to 700 ° C.) and not doped. In FIG. When disappears, the transmittance of the probe light first attenuates rapidly (absorption is recovered rapidly), but the attenuation process gradually changes with time.
[0044]
The time constant τ of this slow decay process is about 100 to 500 ps. This slow decay process is thought to be due to the annihilation process of band edge excitons. The magnitude of this slow decay process decreases as the wavelength of light λ becomes shorter, and decreases to a level that can be ignored for the first time at a wavelength of 800 nm or less. This wavelength is energy that is 100 meV or more higher than the band edge in terms of photon energy.
[0045]
On the other hand, in the semiconductor thin film of the present invention shown in FIG. 9, the temporal change in transmittance is dominant in the initial steep increase and steep attenuation at any wavelength in the wavelength range shorter than the band edge wavelength. The constant is 0.8 ps or less. Thus, an extremely high-speed response characteristic with a negligible response component that is negligible can be obtained.
[0046]
An embodiment for producing a semiconductor thin film crystal with a low slow response will be described.
[0047]
FIG. 10 shows an MBE growth condition in which a growth temperature Tg is set to 360 ° C., a beam intensity ratio of As to Ga (V / III ratio) is set to 4, and a Be doping concentration [Be] is set to 0 to 2 × 10. ¹⁹ cm ^-3 The transient response characteristics of the absorption saturation recovery measured at the wavelength of the band edge are shown for the GaAs thin film crystal produced by changing in the above range. [Be] = 0 is a non-doped thin film.
[0048]
As [Be] increases, the response becomes steep, [Be] = 4-8 × 10 ¹⁸ cm ^-3 Thus, the time constant τ decreases to about 2 ps, and the slow response decreases. Raise [Be] to 2 × 10 ¹⁹ cm ^-3 In the case of, the initial response time constant τ decreases to 1.2 ps, but thereafter, a slow response with a long time constant occurs. Thus, the optimum [Be] changes sensitively depending on the growth temperature Tg and the V / III ratio.
[0049]
In FIG. 11, [Be] is 2 × 10 ¹⁹ cm ^-3 The transient response characteristic of absorption saturation recovery measured at the wavelength of the band edge is shown for a GaAs thin film crystal manufactured by changing the V / III ratio to 4 and changing Tg to 560, 360, and 280 ° C. As the growth temperature Tg decreases, the response becomes steep. At the growth temperature Tg = 280 ° C., the time constant τ decreases to 0.25 ps, and the slow response component also decreases.
[0050]
Thus, the embodiment shown in FIGS. 6 to 9 uses the GaAs semiconductor thin film crystal selected through the above-described process.
[0051]
As described above, the present invention is characterized by using a semiconductor thin film crystal grown at a low temperature as the photo-saturable absorbing material. The semiconductor thin film crystal was manufactured using the low temperature growth conditions in the MBE method. However, the semiconductor thin film crystal is not limited to this, and is manufactured using metal organic chemical vapor deposition (MOCVD) or other methods. However, it goes without saying that the same effect can be obtained.
[0052]
Also, Be is used as a dopant, and the concentration is 2 × 10. ¹⁹ cm ^-3 In this case, the dopant may be another acceptor or donor such as C (carbon), and the concentration is 0.1 to 5 × 10 6. ¹⁹ cm ^-3 As long as it is in the range, it can be anywhere.
[0053]
Furthermore, even if it is not low temperature growth and it is not Be dope, if the lifetime of a carrier can be shortened by methods, such as a proton or radiation irradiation, it may be sufficient. The point is that only a steep response having a time constant of about 1 ps can be obtained in the transient characteristic of absorption saturation at the band edge wavelength, and other slow responses should be negligibly small.
[0054]
Further, although GaAs has been described as a representative semiconductor thin film crystal, the present invention is not limited to this and can be applied to any semiconductor. Furthermore, the present invention is equally applicable to an artificial crystal structure such as a semiconductor quantum well structure, a quantum wire structure, or a quantum dot structure.
[0055]
FIG. 12 is a system configuration diagram of a WDM-OTDM system using an optical-optical switch manufactured using the semiconductor thin film of the present invention as an optical saturable absorbing medium as a pulse separation device (DEMUX) on the receiving side.
[0056]
In this figure, # 11 to # 14, # 21 to # 24, and # 31 to # 34 are transmitters, 101 is a switch, 102A to 102C are lasers, 103 is an optical multiplexer, 104 is an optical fiber, and 105 is optical. A duplexer 106 is a pulse separator (DEMUX) on the receiving side.
[0057]
As described above, when the semiconductor thin film of the present invention is used as the optical saturable absorbing medium, an unprecedented ultrafast response of several hundred femtoseconds or less can be realized in the recovery process of optical absorption saturation. Ultra-high-speed optical-optical switch (refer to prior art documents [1] and [6]), which is a key device of a pulse separation device (DEMUX: Demultiplexer) 106 on the receiving side of divisional multiplexing (OTDM) transmission Can be realized.
[0058]
In addition, since the operating wavelength range extends over a wide range of 120 nm or more from the band edge wavelength, wavelength separation is performed on the receiving side in the WDM-OTDM method using both the WDM (Wavelength Division Multiplexing) method and the OTDM method. If the optical-optical switch using the optically saturable absorption semiconductor thin film crystal of the present invention is used in the system shown in FIG. Since the semiconductor thin film of the present invention having the same composition and structure for each wavelength can be used without manufacturing the composition by changing the composition separately, a great convenience can be brought about in the system configuration.
[0059]
Furthermore, since the present invention provides a semiconductor optical nonlinear medium having extremely high-speed response, its application range is not limited to a planar optical-optical switch. Needless to say, the present invention can be applied to waveguide type optical-optical switches and other optical-optical switches.
[0060]
FIG. 13 shows a waveguide type optical-optical switch using the semiconductor thin film crystal of the present invention as an optical saturable absorbing medium.
[0061]
As shown in this figure, a semiconductor thin film crystal 21 and an optical waveguide having a double heterostructure configured to sandwich the semiconductor thin film crystal 21 between other semiconductor thin film crystals 23 are provided. If the semiconductor thin film crystal is used, the signal light Ps and the control light Pc propagate through the optical waveguide, and the on / off of the signal light Ps is controlled by the on / off of the control light Pc. It can be applied to an ultra-high-speed optically saturable absorbing semiconductor device that is ultra-fast and has no slow response over a wide wavelength range.
[0062]
FIG. 14 shows a mode in which a highly reflective film is provided on one surface of a semiconductor thin film having the structure shown in FIG. 6 and this is used as an optically saturable absorber semiconductor mirror (SESAM: Semiconductor Saturable Absorber Mirror) used as one reflector of a laser resonator. It is an example of a synchronous laser oscillator.
[0063]
In this figure, 31 is a laser resonator, 32 is one reflecting mirror, 33 is a laser diode, 34 is an antireflection film, 35 and 37 are lenses, and 36 is a beam splitter. The SESAM light reflecting mirror of the present invention is used as a laser. It is used as one reflecting mirror 32 of the resonator 31.
[0064]
As shown in FIG. 15, because of the saturable absorption characteristic of this reflecting mirror, the laser is mode-locked and can generate an optical pulse with a very narrow time width. Moreover, since the SESAM of the present invention has a wide wavelength band characteristic, if the laser itself is tunable in a wide range, an optical pulse can be produced in a wide wavelength range by using the SESAM of the present invention.
[0065]
FIG. 16 is a diagram showing an application example in which the optically saturable absorbing semiconductor thin film of the present invention is used as an optical nonlinear absorbing semiconductor.
[0066]
In this figure, 41 is a laser resonator, 42 is an optical nonlinear absorption semiconductor of the present invention, 43 is a laser diode, 44 is an antireflection film, 45, 46, 47 and 48 are lenses, and 49 is a reflecting mirror. After removing the substrate of the semiconductor thin film crystal having the structure shown in FIG. 1, the laser is mode-locked by the saturable absorption characteristic of the thin film crystal only by arranging it as the optical nonlinear absorption semiconductor 42 inside the laser resonator 41. Therefore, it becomes possible to generate an optical pulse with a very narrow time width. If the laser itself is tunable over a wide wavelength range, an optical pulse can be produced over a wide wavelength range even with such a configuration.
[0067]
Next, the waveguide type optical-optical switch of the present invention and the manufacturing method thereof will be described.
[0068]
This embodiment mainly provides a waveguide type monolithic semiconductor optical-optical switch in which a semiconductor single crystal doped with Be or another dopant, which is buried and grown by low temperature MBE regrowth, has a light absorption saturation region.
[0069]
Conventionally, as a phenomenon having a time constant τ of picoseconds or less, a phenomenon using the exciton absorption saturation observed at the band edge in a semiconductor quantum well structure (see JP-A-7-325275), and also a quantum A device using transition between subbands of a conduction band in a well structure (see prior art document [7]) has been proposed. However, the former is effective only for light having an electric field component in the plane of the thin film, and the latter is effective only for light waves having an electric field component perpendicular to the thin film surface. Therefore, considering an optical-optical switch having an optical waveguide structure, the former is effective for TE waves, and the latter is effective for TM waves, and has strong polarization dependence. Become. This is unsuitable for use as a device on the receiving side of optical fiber communication that requires polarization independence. In order to eliminate the polarization dependence when using the former mechanism, a surface type optical-optical switch that inputs light perpendicularly to a thin film has been proposed. For example, FIG. 17 shows an example of a module configuration using a planar all-optical switch. In this figure, the planar light-optical switch has a structure in which diffraction gratings are formed on both surfaces of a glass platform, and an MQW semiconductor saturable absorbing semiconductor is pasted thereon. P and S indicate control light and signal light. A is an all-optical switch, and B is an output port (see prior art document [8]).
[0070]
As described above, the surface type optical-optical switch is disclosed, but from the viewpoint of device construction, there is a strong demand for a waveguide type monolithic device in which light propagates in a plane. Furthermore, any of the above light absorption saturation phenomena uses a phenomenon between band edges or quantum levels, and therefore, the energy (wavelength) of light at which the device operates is limited, and one device has a wide wavelength band. It is not possible to work across.
[0071]
The wavelength band of the conventional example is 30 nm at most, and if the wavelength is different, another device having different constituent materials must be prepared.
[0072]
In order to guide the light propagating through the optical fiber to the semiconductor device or to guide the light emitted from the semiconductor device to the optical fiber, it is necessary to make the semiconductor device a waveguide type. This is because the optical path after being introduced into the device is determined by the waveguide structure and is not disturbed by mechanical vibration or the like. A good example is the combination of a semiconductor laser diode or semiconductor laser amplifier and an optical fiber. There are TE polarization and TM polarization in the light wave propagating through the waveguide, and a device that responds equally to both of these is required. Many of the devices conventionally proposed use a quantum well structure, and this structure has a remarkable anisotropy in the stacking direction and the stacking in-plane direction. In particular, devices utilizing the light absorption saturation phenomenon related to excitons generated at the band edge of the quantum well structure and the light absorption saturation phenomenon accompanying intersubband transition of the conduction band are accompanied by significant anisotropy.
[0073]
In order to solve this problem, if the operating part of the waveguide type device is a quantum well structure, it uses a property related to the inside of the band, or a semiconductor film having a much thicker thickness than the quantum well structure. It is inevitable to use (bulk thin film crystal). Next, the devices that have been proposed in the past use the exciton level or the transition between quantum levels generated at the band edge of the quantum well structure, and the light involved in this has a specific energy (wavelength). There is a drawback that the operating wavelength band is narrow. As a result, WDM involving a large number of wavelengths involves the complexity of having to prepare devices made of different materials for each wavelength.
[0074]
As a means for solving these problems, first, the operation part showing the light absorption saturation, which is the central part of the device, is composed of a bulk thin film crystal. In particular, bulk semiconductor thin-film crystals grown by the low temperature growth MBE method and doped with an appropriate amount of Be or other dopants have light absorption of 1 picosecond or less from the band edge energy Eg2 to the photon energy above 600 meV. The saturation characteristics are as described with reference to FIGS. Moreover, since this is a bulk semiconductor crystal, this phenomenon is characterized by no anisotropy.
[0075]
On the other hand, the semiconductor structure constituting the waveguide has a normal double heterostructure. If the band gap energy Eg1 of the semiconductor of the core layer is selected so that Eg1> Eg2, Eg2 <E <Eg1. A light wave having photon energy E satisfying the above relationship can propagate through the semiconductor waveguide with low loss, and when it reaches an operating region exhibiting light absorption saturation characteristics, it can receive strong absorption there.
[0076]
The operation principle and manufacturing method of this device will be described in detail below with reference to the drawings and examples.
[0077]
FIG. 18 shows a structure in which the waveguide type semiconductor optical switch of the present invention is integrated in two channels.
[0078]
In this figure, 51 and 52 are the first channel control light and signal light input ports, 53 is the first channel signal light output port, 54 is the operating region indicating light absorption saturation, and λ1c and λ2c are the first. , Control light for the second channel, λ1s, λ2s indicate the signal light for the first and second channels.
[0079]
First, consider only one channel. The signal light λ1s and the control light λ1c are input to the input waveguide from separate ports 52 and 51, respectively. This waveguide eventually overlaps and the signal light and control light overlap and enter the operating region 54 that exhibits light absorption saturation. The waveguide between them is a double hetero waveguide having a core having a band gap energy Eg1, and the loss of the waveguide is extremely small because the photon energy E of the signal light and the control light is smaller than that of Eg1.
[0080]
Since the operating region 54 has a double hetero structure having a bulk semiconductor thin film crystal having a band gap energy of Eg2 as a core, the signal light and the control light incident on the operating region are strongly absorbed. When the intensity of the control light is strong, the absorption coefficient for weak signal light is reduced by the hole burning effect. For this reason, the signal light passes through the operation region and is output to the output port 53. Since the output waveguide is made of the same material as the input waveguide, the signal light reaches the output port 53 without receiving any loss. When the control light is extinguished, the absorption coefficient of the operation region 54 is restored to the original large absorption coefficient with a time constant τ, so that the signal light is greatly attenuated and does not reach the output port 53.
[0081]
As described in detail with reference to FIGS. 8 and 9, this absorption recovery process is only a high-speed process with a time constant of 1 picosecond or less, and a slow process with a time constant of 1 picosecond or more is suppressed. In addition, since the operating region is composed of a bulk semiconductor having a minimum dimension of 1 micron or more, there is no anisotropy. That is, the high-speed light absorption saturation recovery process occurs equally for the TE polarized light and the TM polarized light, and therefore does not depend on the polarization state of the incident signal light. Further, the photon energy E of the input light that can achieve a high-speed light absorption saturation recovery process may be any energy as long as it is Eg2 or more, and therefore satisfies the relationship of Eg2 <E <Eg1 that can propagate through the input waveguide without loss. If the input light has the photon energy E, the high-speed light absorption saturation recovery process can be realized.
[0082]
Because of this wide wavelength band, the waveguide type semiconductor optical switch of the present invention has a large number of channels such as a second channel mainly operating at the wavelength λ2 in addition to the first channel mainly operating near the wavelength λ1. Can be integrated as shown in FIG.
[0083]
The present invention is of a waveguide type, is very easy to couple with input / output optical fibers, and has a wide wavelength band, so that it can be integrated to process a large number of channels having different wavelengths. The benefits are great from a practical point of view.
[0084]
A method for manufacturing a waveguide type semiconductor optical-optical switch according to the present invention will be described below with reference to the drawings.
[0085]
FIG. 19 is a device manufacturing process diagram according to the buried regrowth method of the waveguide type semiconductor optical-optical switch of the present invention.
[0086]
First, as shown in FIG. 19A, a sectional layer structure of an epitaxial wafer having a normal double hetero structure as a starting point is shown.
[0087]
In this figure, 60 is a GaAs substrate, 61 and 63 are layers called first cladding layers, and the composition thereof is A1yGa1-yAs. 62 is a layer called a 1st core layer, The composition is A1xGa1-xAs. That is, the first cladding layers 61 and 63 sandwich the first core layer 62 from above and below.
[0088]
Next, after covering the surface with an oxide film or a nitride film, as shown in FIG. 19B, a groove with a width W (extends in a direction perpendicular to the paper surface) by photolithography and dry etching or wet etching techniques. 64 is formed at a depth to reach the substrate 60. Here, the width W is about 1 to 10 microns.
[0089]
After forming the groove 64, as shown in FIG. 19C, a dopant such as Be or C is added at 1 to 50 × 10 6 at a growth temperature of 150 to 450 ° C. by the MBE method. ¹⁸ cm ^-3 The second double heterostructure 70 added at the concentration is regrown (buried regrowth) so as to fill the groove 64. The layer structure is in the order of the second cladding layer 71 (composition is A1zGa1-zAs), the second core layer 72 (composition is A1wGa1-wAs), and the second cladding layer 73 (composition is A1zGa1-zAs). Here, it is necessary to select w <x so that the band gap energy Eg2 of the second core layer 72 is smaller than the band gap energy Eg1 of the first core layer 62. Polycrystal grows on the oxide film (nitride film) other than the groove 64 by MBE regrowth, but only the polycrystal is removed by an appropriate selective etching method.
[0090]
A normal waveguide forming technique is used to make a waveguide structure as shown in FIG. 18 using this wafer, but it is considered that detailed description is not required.
[0091]
FIG. 20 is a device manufacturing process diagram by a wafer bonding method, which is another method for manufacturing a waveguide type semiconductor optical-optical switch showing an embodiment of the present invention.
[0092]
First, as shown in FIG. 20A, a first double heteroepitaxial wafer in which a groove 84 is formed is prepared. 80 is a GaAs substrate, 81 and 83 are layers called a first cladding layer, and its composition is A1yGa1-yAs. 82 is a layer called a first core layer, and its composition is A1xGa1-xAs. That is, the first cladding layers 81 and 83 sandwich the first core layer 82 from above and below.
[0093]
Separately, as shown in FIG. 20B, a second cladding layer 91 (A1zGa1-zAs), a second core layer 92 (A1wGa1-wAs), and a second cladding layer 93 (A1zGa1-) are formed on a GaAs substrate 90. zAs) in the order of the second double heterostructure epitaxial wafer (its epitaxial thickness is equal to that of the first double heteroepitaxial wafer) and has a width W ′ (W ′ <W) up to the substrate 90 Provide a mesa that is high enough to reach.
[0094]
Next, as shown in FIG. 20C, the second epitaxial wafer shown in FIG. 20B is turned upside down so that the mesa portion fits into the groove 84 of the first epitaxial wafer. The wafers are stacked and then subjected to heat treatment to perform wafer bonding. Then, the structure shown in FIG. 19D can be produced by etching the substrate 90 of the second epitaxial wafer. The process of producing the waveguide structure shown in FIG. 18 using this wafer is a normal process, and no special explanation seems necessary.
[0095]
As described above, the waveguide-type semiconductor optical-optical switch of the present invention utilizes light absorption saturation based on the hole burning effect in a bulk semiconductor thin film crystal and its recovery process, and is ultrafast and has a wide wavelength. It has the characteristics of having bandwidth and polarization independence.
[0096]
In the above description, a single channel device is used. However, since it is a planar waveguide device, a plurality of channel devices can be provided on a semiconductor wafer and operated at different wavelengths.
[0097]
In the above description of the manufacturing method, an A1GaAs material is used as an example, but it goes without saying that the present invention can be applied equally to InGaA1As and other semiconductor materials without being particular about this.
[0098]
As described above, according to the present invention, a semiconductor optical-optical switching device having a waveguide structure is provided, which not only facilitates coupling with an optical fiber, but also enables multi-channel integration. It brings about immeasurable effects such as excellent mechanical stability.
[0099]
In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
[0100]
【The invention's effect】
As described above in detail, according to the present invention, the following effects can be obtained.
[0101]
(A) By removing the slow response component associated with the ultra-fast response associated with the spectral hole burning phenomenon, and removing the slow response component over a wide wavelength range instead of a narrow wavelength range, a wide wavelength range It is possible to provide an optically saturable absorption semiconductor device having a truly ultra-high speed and a wide wavelength band that has no slow response.
[0102]
(B) If the semiconductor thin film crystal of the present invention is used as an optical saturable absorption medium, an unprecedented ultrafast response of several hundred femtoseconds or less can be realized in the recovery process of optical absorption saturation, and optical time division multiplexing is performed. It is possible to realize an ultra-high speed optical-optical switch that is a key device of a pulse separation device on the reception side of transmission. Moreover, since the operating wavelength range extends over a wide range of 120 nm or more from the band edge wavelength, after wavelength separation is performed on the receiving side in the method using both the optical wavelength division multiplexing method and the optical time division multiplexing method. By using as an optical-optical switch for DEMUX, the semiconductor thin film of the present invention having the same composition and structure at each wavelength is used without manufacturing the semiconductor by changing the composition of the semiconductor separately for each wavelength. Therefore, a great convenience is brought about in the system configuration.
[0103]
(C) It can be applied to a waveguide type optical-optical switch and other optical-optical switches.
[0104]
(D) It is not limited to an optical-optical switch, and has a wide application field as a saturable absorbing semiconductor. For example, in the case of the optical saturable absorption semiconductor mirror (SESAM) used for one of the reflectors of the laser resonator, the laser is mode-locked due to the saturable absorption characteristics of the reflector, and the light with a very narrow time width is used. Pulses can be generated. Moreover, since the SESAM of the present invention has a wide wavelength band characteristic, if the laser itself is tunable in a wide range, an optical pulse can be produced in a wide wavelength range by using the SESAM of the present invention.
[0105]
(E) By simply arranging the semiconductor thin film crystal of the present invention inside the laser resonator, the laser is mode-locked due to the saturable absorption characteristic of the thin film crystal, and an optical pulse with a very narrow time width is generated. Is possible. If the laser itself is tunable over a wide wavelength range, an optical pulse can be produced over a wide wavelength range even with such a configuration.
[0106]
(F) The waveguide-type semiconductor optical-optical switch of the present invention utilizes light absorption saturation based on the hole burning effect in a bulk semiconductor thin film crystal and its recovery process, and is ultrafast, wide wavelength band, Has wave independence.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining transient characteristics of light absorption saturation.
FIG. 2 is a diagram showing an experimental example in which a spectrum hole burning phenomenon is actually measured.
FIG. 3 is a band diagram illustrating a spectrum hole burning phenomenon.
FIG. 4 is a diagram showing an actual time response waveform related to a spectrum hole burning phenomenon (by a degenerate pump-probe method).
FIG. 5 is an absorption saturation transient response characteristic diagram of a non-doped, MBE-grown GaAs thin film crystal at normal temperature by a degenerate pump-probe method measured at various wavelengths.
FIG. 6 is a diagram showing a layer structure of a semiconductor thin film showing an example of the present invention.
7 is a diagram showing an absorption spectrum measured with weak light after removing the substrate of the epitaxial wafer having the structure of FIG. 6;
FIG. 8 is an absorption recovery transient response characteristic diagram of a band edge wavelength measured using a degenerate pump-probe method according to an embodiment of the present invention.
FIG. 9 is an absorption recovery transient response characteristic diagram at various wavelengths measured using a degenerate pump-probe method, showing an example of the present invention.
FIG. 10 shows an MBE growth condition in which a growth temperature Tg is set to 360 ° C., a beam intensity ratio of As to Ga (referred to as V / III ratio) is set to 4, and a Be doping concentration [Be] is set to 0 to 2 × 10. ¹⁹ cm ^-3 FIG. 5 is a transient response characteristic diagram of absorption saturation recovery measured at a band edge wavelength for a GaAs thin film crystal manufactured by changing the range of.
FIG. 11: [Be] is 2 × 10 ¹⁹ cm ^-3 FIG. 4 is a transient response characteristic diagram of absorption saturation recovery measured at a band edge wavelength for a GaAs thin film crystal manufactured by changing the growth temperature Tg to 560, 360, and 280 ° C. while keeping the V / III ratio constant at 4;
FIG. 12 is a diagram showing an optical wavelength division multiplexing / optical time division multiplexing (WDM-OTDM) transmission system and a pulse demultiplexer (DEMUX) showing an application example of the present invention.
FIG. 13 is a diagram showing a waveguide type optical-optical switch showing an application example of the present invention.
FIG. 14 is a diagram showing a mode-locked laser oscillator using an optically saturable absorption semiconductor mirror SESAM showing an application example of the present invention.
15 is a diagram showing an autocorrelation waveform of the optical pulse generator of FIG. 14 showing an application example of the present invention.
FIG. 16 is a diagram showing a mode-locked laser oscillator using an optically saturable absorbing semiconductor according to an application example of the invention.
FIG. 17 is a diagram showing a module configuration example using a surface-type all-optical switch.
FIG. 18 is a configuration diagram of a waveguide type semiconductor optical single optical switch according to the present invention.
FIG. 19 is a manufacturing process sectional view of the waveguide type semiconductor optical-optical switch of the present invention (regrowth method).
FIG. 20 is a manufacturing process sectional view of the waveguide type semiconductor optical-optical switch of the present invention (wafer bonding method).
[Explanation of symbols]
10, 60, 80, 90 GaAs substrate
11 Al _0.3 Ga _0.7 As etch stop layer
12 GaAs working layer (700nm thickness)
13 Al _0.3 Ga _0.7 As cap layer
21 Semiconductor thin film crystal
23 Other semiconductor thin film crystals
31, 41 Laser resonator
32 One reflector
33, 43 Laser diode
34, 44 Anti-reflective coating
35, 37, 45, 46, 47, 48 Lens
36 Beam splitter
42 Optical nonlinear absorption semiconductor
49 Reflector
# 11 to # 14, # 21 to # 24, # 31 to # 34 Transmitter Receiver
51, 52 First channel control light and signal light input ports
53 1st channel signal light output port
54 Operating region showing light absorption saturation
λ1c, λ2c Control light for the first and second channels
λ1s, λ2s First and second channel signal light
61, 63, 81, 83 First cladding layer
62,82 First core layer
64,84 Groove
70 Second double heterostructure
71, 73, 91, 93 Second cladding layer
72,92 Second core layer
101 switch
102A to 102C laser
103 Optical multiplexer
104 optical fiber
105 Optical demultiplexer
106 Receiving side pulse separation device (DEMUX)

Claims (8)

In a semiconductor thin film crystal made of a III-V compound semiconductor fabricated on a semiconductor substrate, the growth temperature of the thin film crystal is in the range of 150 ° C. to 450 ° C., and the beam intensity ratio between the group V element and the group III element The film is grown under the condition that V / III is in the range of 2 to 200 and contains a large amount of lattice defects due to an excessive V group element and doped with a dopant of Be or C. In the transient characteristic of absorption saturation, only a steep response with a time constant of about 1 ps is shown, and other slow responses are negligibly small, and there is no slow component over a wide wavelength band. Only a photo-saturable semiconductor that can be obtained . A semiconductor thin film crystal made of a group III-V compound semiconductor manufactured on a semiconductor substrate by using molecular beam epitaxy has a growth temperature in the range of 150 ° C. to 450 ° C. as a growth condition. The element has a beam intensity ratio V / III in the range of 2 to 200 , contains a large amount of lattice defects due to excess group V elements , and is doped with a Be dopant at a concentration of 1 to 50 × 10 ¹⁸ cm ^−3. In addition, a transient characteristic of light absorption saturation measured by a degenerate pump-probe method at a photon energy corresponding to the band gap of the thin film crystal or a photon energy higher than that has a single time constant of 1 ps or less. or a wide wavelength which the main response to relative sub responses with larger time constant than the attendant size is characterized by comprising 10% or less Over a range, from very fast response with no slow component light saturable absorber semiconductor obtained. It has a semiconductor thin film, a low reflection film and a high reflection film arranged so as to sandwich the semiconductor thin film, and signal light is incident from the low reflection film side, reflected by the high reflection film, and emitted from the low reflection film side. 2. A wide wavelength band characterized in that the semiconductor thin film crystal according to claim 2 is used as the semiconductor thin film in a planar optical-optical switch having a configuration and controlling on / off of signal light by on / off of control light. An optically saturable absorber semiconductor device capable of providing only an ultrafast response with no slow component . A semiconductor thin film and an optical waveguide having a double heterostructure configured such that the semiconductor thin film is sandwiched between other semiconductor thin films, and signal light and control light propagate through the waveguide, and control light is turned on / off In a waveguide type optical-optical switch for controlling on / off of signal light, the semiconductor thin film crystal according to claim 2 is used as the semiconductor thin film, and an ultra-high speed without a slow component over a wide wavelength band. Optically saturable absorber semiconductor device that can obtain only response . A semiconductor thin film, and a low reflection film and a high reflection film arranged so as to sandwich the semiconductor thin film, light is incident from the low reflection film side, is reflected by the high reflection film through the semiconductor thin film, When the light intensity is low, the semiconductor thin film absorbs a large amount of light, and when the light intensity is high, the semiconductor thin film absorbs a small amount of absorption. The optically saturable absorbing semiconductor device capable of providing only an ultrafast response with no slow component over a wide wavelength band , wherein the semiconductor thin film crystal according to claim 2 is used as the semiconductor thin film. A structure having a semiconductor thin film and two low reflection films arranged so as to sandwich the semiconductor thin film, light is incident from the low reflection film side, passes through the semiconductor thin film, and is emitted from the other low reflection film 3. The semiconductor thin film according to claim 2, wherein the semiconductor thin film exhibits a large absorption when the light intensity is low, and the semiconductor thin film exhibits a small absorption when the light intensity is high. An optically saturable absorption semiconductor device capable of obtaining only an ultrafast response without a slow component over a wide wavelength band, characterized by using a crystal. Using a semiconductor epitaxial wafer having a first double heterostructure for an optical waveguide provided with a groove having a depth of 1 to 10 μm and reaching the substrate by etching, a growth temperature of 150 to 450 ° C. by MBE method, The second double heterostructure including the semiconductor according to claim 1, wherein a dopant such as Be or C is added at a concentration of 1 to 50 × 10 ¹⁸ cm ⁻³ , and is regrown to be embedded in the groove, A band gap energy of the second double heterostructure core layer is selected to be smaller than a band gap energy of the first double heterostructure core layer, including a part of the groove, Is a waveguide-type optical-optical switch characterized in that optical waveguides are provided at right angles. A semiconductor epitaxial wafer having a first double heterostructure for an optical waveguide provided with a groove having a depth of 1 to 10 μm in width and reaching the substrate by etching, and a growth temperature of 150 to 450 ° C. by MBE, Be or dopants such as C was added at a concentration of 1~50 × 10 ¹⁸ cm ^-3, includes a semiconductor according to claim 1, the second double having substantially the same thickness as the thickness of the heterostructure first double Both heterostructure epitaxial wafers using a second semiconductor epitaxial wafer provided with a mesa type path having a width of 1 to 10 μm and reaching the substrate, so that the mesa type path is filled with the groove. Are bonded together by applying heat treatment to the semiconductor wafer from which the substrate of the second semiconductor epitaxial wafer has been removed by etching. A band gap energy of the second double heterostructure core layer is selected to be smaller than a band gap energy of the first double heterostructure core layer, and includes a part of the groove A waveguide type optical-optical switch characterized in that an optical waveguide is provided in a direction perpendicular to this.

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