JP2991707B1 - Semiconductor optical switch - Google Patents

Abstract

An all-optical semiconductor optical switch for controlling and distributing an optical signal corresponds to an optical communication system of terabits / second or more. SOLUTION: An InGaAs quantum well layer 22 having a well width smaller than the well layers 21a and 21b is arranged between two InGaAs quantum well layers 21a and 21b having the same well width.
A semiconductor optical device comprising a GaAs substrate 11 provided with a plurality of coupled triple quantum well structures 20 each having an AlAs quantum barrier layer 23 narrower than the quantum well layers 21a and 22 and between the quantum well layers 21b and 22. A switch that irradiates light that resonates with the energy of one inter-subband transition of the coupled triple quantum well structure 20 and changes the light absorption coefficient at the same inter-subband transition or another inter-band transition energy. Let it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical switch for controlling and distributing an optical signal, and more particularly to a semiconductor optical switch utilizing a transition between sub-bands in a conduction band of a semiconductor quantum well structure.

[0002]

2. Description of the Related Art In recent years, research and development of digital optical communication technology has been increasingly performed. This is because optical signal transmission is superior to electric signal transmission in terms of transmission speed, interference between signals, and the like. However, in the method of converting an optical signal into an electric signal, since the processing speed is limited by an optical-to-electronic conversion or an electric signal processing device having a low response speed, the practical use of an all-optical switch capable of performing higher-speed signal processing is practical. Is desired. Further, realization of an optical non-linear material having an ultra-high-speed response and high optical non-linearity for this purpose is expected.

Conventionally known optical switches include those using the Faraday effect and the Kerr effect in an electro-optic crystal, those using the nonlinear optical effect in an optical fiber, and those using the absorption coefficient and refractive index change in a semiconductor. ing. Among them, those using a semiconductor (especially a compound semiconductor) can be easily miniaturized, can be monolithically integrated with other semiconductor elements such as a semiconductor laser, an optical modulator, and a light receiving element. The adoption of a quantum structure such as a quantum well or the like has an advantage that the efficiency of optical switching can be increased.

A conventional semiconductor optical switch utilizing absorption due to inter-band transition has a problem that the switch-off time is limited by the inter-band recombination time (several nanoseconds) of the actual excited carriers. Was. On the other hand, the transition time between the sub-bands in the conduction band of the semiconductor quantum well structure has a relaxation time of several picoseconds or less, and the switching-off speed can be more than 1000 times faster than the inter-band transition.

[0005] However, InG reported in the past has been reported.
It is extremely difficult to realize intersubband transition at a wavelength of 1.55 μm or less, which is a wavelength band of optical communication, using a strained quantum well (on a GaAs substrate) or a strained quantum barrier (on an InP substrate) of aAs / AlAs. Deep quantum well (1.5e
V or more). In order to obtain the band discontinuity that enables this, an extremely large amount of strain is required, and it has been difficult to form a high quality quantum well. In addition, the width of the quantum well must be reduced to about several atomic layers, and there is a problem that it is difficult to form a thin film having sufficient optical nonlinearity.

Here, consider a single quantum well layer having two quantum levels (subbands: E1 and E2) as shown in FIG. In this structure, it is necessary to narrow the quantum well width in order to shorten the transition wavelength between subbands. At this time, the well width dependence of the inter-subband transition between E1 and E2 is as shown in FIG. That is, as the well width becomes smaller, the energy between sub-bands becomes larger, and the transition wavelength between sub-bands becomes shorter.

However, since the quantum levels E1 and E2 both increase with a decrease in the well width, in order to realize an intersubband transition in the near infrared region, E2 is lower than the energy height of the barrier and E1 Is required to be sufficiently lower than the point X of the barrier layer. Therefore, 1.5, which is the wavelength band of optical communication, is used.
In order to realize a transition between sub-bands in μm, a material system having a large band discontinuity that enables an extremely high barrier height is required, and it is difficult to realize such a system.

Furthermore, since the inter-subband relaxation is mainly caused by electron-phonon scattering, the inter-subband relaxation time becomes longer with an increase in the inter-subband energy.
At 55 μm, it is 2 to 3 picoseconds, so there is a problem that it is difficult to cope with large-capacity communication of terabit / sec or more.

[0009]

As described above, in the conventional semiconductor optical switch based on the intersubband transition, 1.5 times
There is a problem that it is difficult to shorten the wavelength to the wavelength band of optical communication of about 5 μm and to achieve both high speed and high nonlinearity.

The present invention has been made in view of the above circumstances, and has as its object to perform high-speed optical modulation or optical switching by utilizing the inter-subband transition of a quantum well. It is an object of the present invention to provide a semiconductor optical switch that can cope with an optical communication system of more than 1 / sec.

[0011]

(Structure) In order to solve the above-mentioned problem, the present invention employs the following structure.
That is, the present invention provides a semiconductor optical switch for controlling and distributing an optical signal, wherein a second quantum well layer having a width different from the width of the first quantum well layer is provided between two first quantum well layers having the same width. And at least one coupled quantum well structure in which a quantum barrier layer narrower than the well layers is disposed between the first and second quantum well layers, on the single crystal substrate. Irradiating light that resonates with the energy of one inter-subband transition of the conduction band of the structure to change the absorption coefficient, refractive index, or optical gain of light at the same energy of the inter-subband transition; It is characterized in that the light absorption coefficient, the refractive index, or the optical gain at the energy of the transition is changed.

Here, preferred embodiments of the present invention include the following. (1) The single crystal substrate is a compound semiconductor substrate or a sapphire substrate. (2) Two or more coupled quantum well structures are provided, and adjacent coupled quantum well structures are separated by a barrier layer wider than the coupled quantum well structure.

(3) The first and second quantum well layers are made of InGa
As, and the quantum barrier layer is made of AlAs, AlGaAs,
It is made of AlAsSb or AlGaAsSb. (4) In (3), the energy difference between all the sub-bands in the conduction band is 36 meV or more.

(5) The first and second quantum well layers are made of GaN or InGaN, and the quantum barrier layers are made of AlN or AlG.
aN. (6) In (5), the energy difference between all the sub-bands in the conduction band is 88 meV or more.

(7) The control light incident on the coupled quantum well structure resonates at one inter-subband transition energy, and the signal light is substantially equal to the same inter-subband transition energy. (8) The control light incident on the coupled quantum well structure resonates with one inter-subband transition energy, and the signal light is substantially equal to the other inter-subband transition energy. (9) A waveguide including a coupled quantum well structure is provided.

(Operation) In order to perform optical modulation or optical switching by transition between subbands of a quantum well, first, control light that resonates at the transition wavelength between subbands is incident. At this time, the absorption coefficient at the inter-subband absorption peak decreases with the light input intensity as shown in FIG. Accordingly, a change in the absorption coefficient or the refractive index occurs near the absorption peak, and an optical modulator or an optical switch can be realized by manufacturing a waveguide structure including the quantum well. If the energy interval between the sub-bands is increased, the wavelength of the transition between the sub-bands can be shortened, and if the relaxation time between the sub-bands is increased, the speed of the optical modulation or the optical switch can be increased. .

As a method for shortening the intersubband transition to the 1.5 μm band, there is known a method using an asymmetric coupled double quantum well structure as shown in FIG.
Sung, et.al. Electron Lett. 33,818 (1997)). As shown in FIG. 8, two quantum wells of different widths (the wider first
Quantum well QW1 and narrow second quantum well QW2)
Are combined, the number of sub-bands in each quantum well becomes two (E1, E2, E3, E4). In FIG. 8, E1 to E4 are indicated by n = 1 to 4.

However, in such a coupled double quantum well structure, since the wave functions of the odd-numbered subbands are unevenly distributed in the wide quantum wells and the wave functions of the even-numbered subbands are unevenly distributed in the narrow quantum wells, the odd-numbered subbands are unevenly distributed. The overlap of the electron wave functions between and the even-numbered subbands becomes smaller. Not only does the absorption intensity due to the inter-subband transition of 1-4 decrease, but also the inter-subband relaxation rate of 4-3 decreases. When this structure is used in an optical switch, the switching efficiency is also switched off. Speed also decreases. For this reason, there is a problem that it is not suitable for application to an ultra-high-speed and high-efficiency switch.

The present inventors have solved this problem and proposed a structure which enables an ultra-high-speed and high-efficiency semiconductor optical switch by transition between sub-bands in the 1.5 μm band. A subwell structure (C-TQW) was considered.
That is, an asymmetrically-coupled double quantum well structure (AC-DQW) including the first quantum well layer QW1 and the second quantum well layer QW2 having a different width from the first quantum well layer QW1 is further provided.
Another third quantum well layer QW3 having the same width as 1 is coupled. In this case, the number of subbands was two in each quantum well layer, which is six (E1, E2, E3, E4, E5, E
6). In the drawing, E1 to E6 are indicated by n = 1 to 6.

This makes it possible for the wave function of the electrons leaking out to the barrier layer in the coupled double quantum well structure composed of QW1 and QW2 to leak out to the third quantum well layer QW3. As a result, compared to AC-DQW,
The overlap of wave functions between different subbands increases,
It is possible to increase the light absorption intensity between subbands. Further, the energy difference between all sub-bands is larger than the energy of LO phonon (InGaAs type and G
36 meV or more in aAs system, InGaN system and GaN
By designing the structure so as to be 88 me or more, it is possible to increase the relaxation rate between adjacent subbands and to speed up the absorption recovery.

FIG. 10 shows two types of coupled quantum well structures ((a) AC) capable of absorbing 1.55 μm between sub-bands.
-DQW, (b) C-TQW) in comparison of absorption spectra. As shown in this figure, the value of C-TQW is twice or more as large as that of AC-DQW. FIG. 11 shows the above two types of structures.
100 resonating at 1.55 μm intersubband transition wavelength
FIG. 4 shows a transient response waveform of absorption recovery after a femtosecond (half width) optical pulse is incident. FIG. As shown in this figure, the absorption-recovery rate is higher in C-TQW than in AC-DQW.
It can be seen that speeding up is more than three times faster.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. FIG. 1 is a schematic diagram showing a basic configuration of a semiconductor optical switch according to one embodiment of the present invention.
On the semiconductor substrate 11, two first quantum well layers 21
(21a, 21b), a second quantum well layer 22 interposed therebetween, and a quantum triple layer well structure (23a, 23b) separating the quantum well layers 21 and 22 are formed. I have. Then, barrier layers 13 (13a, 13b) are formed on both sides of the coupled triple quantum well structure, and an optical guide layer 14 is formed on the barrier layer 13b opposite to the substrate.

Each of the layers 13, 14, 21, 22, 23 is made of a compound semiconductor and is formed by continuous growth by a growth method such as the MBE method. The energy band of the conduction band of the triple quantum well structure 20 is as shown in FIG.

In the figure, reference numeral 31 denotes control light, 32 denotes signal light, and 33 denotes switched signal light. The control light 31 has a wavelength that resonates with one of the inter-subband transition energies of the coupled triple quantum well structure 20.
Is a wavelength approximately equal to the same inter-subband transition energy or another inter-subband transition energy.

In such a structure, the first quantum well layers 21a and 21b are doped with an n-type impurity.
Electrons are accumulated in lower-order subbands (mainly first and second) of the conduction band. By irradiating light here, electrons are excited from lower-order subbands to higher-order subbands, and intersubband light absorption occurs. At this time, an intersubband light absorption spectrum having an absorption peak corresponding to an intersubband transition such as 1-6, 2-5, 3-6 which is an allowable transition is obtained.

Next, a more specific configuration example of the semiconductor optical switch of the present embodiment will be described. FIG. 2 is a cross section showing the element structure of the semiconductor optical switch of the same configuration example.
The semiconductor substrate 11 is made of semi-insulating GaAs, and each layer is formed by growing a GaAs buffer layer 12 on the substrate 11. The first quantum well layers 21a and 21b constituting the coupled triple quantum well structure 20 have a carrier concentration of about 1 × 1
It is n-type In _0.3 Ga _0.7 As of 0 ¹⁸ cm ⁻³ , and its well width is 2.8 nm. The second quantum well layer 22 sandwiched between them has an n-type I type having a carrier concentration of about 1 × 10 ¹⁸ cm ^−3.
n _0.3 Ga _0.7 As, and the well width is 2.5 nm. Further, each of the quantum well layers 21a, 22 and 21b,
The quantum barrier layers 23a and 23b between the layers 22 are made of non-dove Al.
As, and the barrier width is 0.85 nm.

The barrier layers 13a and 13b provided on both sides of the coupled triple quantum well structure 20 are non-dove AlAs,
The thickness was set to 10 nm in order to efficiently confine electrons in the triple quantum well structure 20. In order to enhance the efficiency as an optical switch, this combined triple quantum well structure 20 is
00 cycles were formed. The light guide layer 14 has Al _0.5
Ga _0.5 As was used, and its thickness was 2 μm. Further, a thickness of 50 mm is provided between the light guide layer 14 and the barrier layer 13.
A GaAs cap layer 15 of about nm was provided.

At this time, as shown in FIG. 3, the absorption peak wavelength on the short wavelength side was about 1.55 μm, and the absorption peak wavelength on the long wavelength side was about 2.2 μm. The absorption coefficient at 1.55 μm was about 1500 cm ⁻¹ or more, and a value sufficient for application to an optical switch was obtained. Also, the energy difference between the six subbands was 36 meV or more, which was larger than the LO phonon energy.

FIG. 4 shows an optical pulse (wavelength λ ₁₄ = 1.55 μm, pulse width 100 fs, pulse energy −400 fj / μm ² ) that resonates with the 1-6 intersubband transition wavelength.
FIG. 9 is a diagram showing a transient response waveform of an absorption coefficient at 1.55 μm when the light is irradiated. As can be seen from this figure, the half width of the absorption recovery time is about 750 femtoseconds, and it is possible to switch a terabit-class optical signal.

As described above, according to the present embodiment, GaAs
A coupled triple quantum well in which a narrow InGaAs quantum well layer 22 is sandwiched between wide InGaAs quantum well layers 21 on a substrate 11 and an AlAs quantum barrier layer 23 narrower than these is arranged between each well layer. By providing a plurality of structures 20 and irradiating light that resonates with the energy of one intersubband transition of the conduction band of the coupled triple quantum well structure 20, and changing the light absorption coefficient at the same energy of the intersubband transition Therefore, an ultra-high-speed and high-efficiency semiconductor optical switch suitable for an optical communication system of terabits / second or more can be realized.

The present invention is not limited to the above embodiment. In the embodiment, only the optical response of the absorption coefficient and the optical gain has been described. However, from the well-known Kramers-Kronig relational expression, an optical pulse response similar to the absorption coefficient can be obtained for the refractive index, and the absorption coefficient and the optical It is also possible to realize an optical switch using not only the gain but also the change in the refractive index.

In the embodiment, the well width of the second quantum well layer is made narrower than that of the first quantum well layer. Conversely, the well width of the second quantum well layer is reduced to the first quantum well layer. It may be wider than the well layer.

The materials of the substrate, the quantum well layer, the quantum barrier layer and the like can be appropriately changed according to the specifications. For example, when InGaAs is used as the quantum well layer, the quantum barrier layer is made of A
Not limited to lAs, AlGaAs, AlAsSb, or A
lGaAsSb can be used. When GaN or InGaN is used for the quantum well layer, AlN or AlGaN can be used for the quantum barrier layer. The single crystal substrate is not limited to GaAs, but may be any compound semiconductor substrate on which a quantum well layer and a barrier layer constituting a quantum well structure can be grown, and furthermore, a sapphire substrate may be used. . In addition, various modifications can be made without departing from the scope of the present invention.

[0034]

As described above in detail, according to the present invention, a coupling quantum well structure for forming an all-optical semiconductor optical switch is composed of two first quantum well layers having the same width, and A second quantum well layer arranged between the quantum well layers and having a different width from the well layer; and a quantum barrier layer arranged between the first and second quantum well layers and having a width smaller than these well layers. Thereby, it is possible to perform optical modulation or optical switching at high speed by utilizing the intersubband transition of the quantum well,
It is possible to support an optical communication system of terabits / second or more.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a basic configuration of a semiconductor optical switch according to one embodiment.

FIG. 2 is a sectional view showing a specific example of the element structure of the semiconductor optical switch according to the embodiment;

FIG. 3 is a diagram showing intersubband light absorption spectrum characteristics of the embodiment.

FIG. 4 is a diagram showing an optical pulse response waveform of an intersubband optical absorption coefficient according to the embodiment.

FIG. 5 is a diagram showing an energy band structure of a single quantum well.

FIG. 6 is a graph showing well width dependence of inter-subband transition energy of a single quantum well.

FIG. 7 is a diagram showing the light input intensity dependence of intersubband light absorption.

FIG. 8 is a diagram showing an energy band structure of a conduction band of an asymmetrically coupled double quantum well.

FIG. 9 is a diagram showing an energy band structure of a conduction band of a coupled triple quantum well.

FIG. 10 is a diagram showing an intersubband absorption spectrum of an asymmetrically-coupled double quantum well (AC-DQW) and a symmetrically-coupled triple quantum well (C-TQW).

FIG. 11: 1.55 μm of asymmetrically coupled double quantum well (AC-DQW) and symmetrically coupled triple quantum well (C-TQW)
The figure which shows the transient response waveform of absorption recovery between subbands.

[Explanation of symbols]

11 ... GaAs substrate (semiconductor substrate) 12 ... GaAs buffer layer 13 ... AlAs barrier layer 14 ... Al _0.5 Ga _0.5 As optical guide layer 15 ... GaAs cap layer 20 ... coupled triple quantum well structure 21 (21a, 21b) ... InGaAs quantum Well layer (first quantum well layer) 22 ... InGaAs quantum well layer (second quantum well layer) 23 (23a, 23b) ... AlAs quantum barrier layer 31 ... Control light 32 ... Signal light (input) 33 ... Signal light (output)

Claims (5)

(57) [Claims] 1. A second quantum well layer having a width different from that of a first quantum well layer having a width equal to two first quantum well layers, and the well layers being located between the first and second quantum well layers. A coupled quantum well structure with a narrower quantum barrier layer than
At least one is provided on a single crystal substrate, and is irradiated with light that resonates with energy of one inter-subband transition of the conduction band of the coupled quantum well structure, and absorbs light at the same energy of the inter-subband transition, A semiconductor optical switch characterized by changing a refractive index or an optical gain or changing an absorption coefficient, a refractive index, or an optical gain of light at energy of another intersubband transition. 2. The semiconductor device according to claim 1, wherein two or more coupling quantum well structures are provided, and adjacent coupling quantum well structures are separated by a barrier layer wider than said coupling quantum well structure. The semiconductor optical switch as described in the above. 3. The first and second quantum well layers are made of InGaAs.
And the quantum barrier layer is made of AlAs, AlGaAs,
3. The semiconductor optical switch according to claim 1 , wherein the semiconductor optical switch is made of AlAsSb or AlGaAsSb, and the energy difference between all the sub-bands in the conduction band of the coupled quantum well structure is 36 meV or more. 4. The method according to claim 1, wherein the first and second quantum well layers are GaN or I.
nGaN, and the quantum barrier layer is made of AlN or AlG
consists aN, semiconductor optical switch according to claim 1 or 2, wherein the energy difference between all of the subbands in the conduction band of said coupled quantum well structure is not less than 88MeV. 5. The control light incident on the coupled quantum well structure resonates with one inter-subband transition energy, and the signal light incident on the coupled quantum well structure has the same inter-subband transition energy or another inter-subband transition energy. 5. The semiconductor optical switch according to claim 3, wherein the transition energy is substantially equal to the inter-subband transition energy.