JP3138822B2 - High-speed nonlinear optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A non-linear optical device that changes the transmittance of operating light that is constantly irradiated by turning on and off operating light applied from the outside, sacrificing electron confinement In order to increase the speed by shortening the tunneling time of holes, the semiconductor layer with a large energy band gap constituting the quantum barrier is replaced with a semiconductor layer with a small energy band gap constituting the narrow quantum well. In a TBQ structure sandwiched between a semiconductor layer having a small energy band gap and a semiconductor layer having a small energy band gap forming a wide quantum well, a semiconductor layer having a large energy band gap forming the quantum barrier is formed by a semiconductor which serves as a barrier to both electrons and holes. A quantum barrier for holes, the width of the quantum barrier for holes It is configured to be narrower than the width of the quantum barrier.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in high-speed operation characteristics of a non-linear optical device that changes the transmittance of operating light that is constantly irradiated by turning on and off externally applied operating light.

In recent years, it has a TBQ structure (Tunneling Bi-Quantum well) in which both sides of a quantum barrier are sandwiched by quantum wells having different widths. It is required that an optical-optical switch, an optical bistable device, an optical-optical memory, etc., using a non-linear optical device that changes the transmittance can operate at an ultra-high speed.

[Conventional technology]

FIG. 6 is a band structure diagram of a conventional optical device having a non-linear TBQ structure.

In this figure, 1 is a narrow quantum well, 2 is a quantum barrier, 3 is a wide quantum well, 4 is operated light, 5 is operating light, 6 is a hole, and 7 is an electron.

As shown in this figure, in the conventional nonlinear optical device having the TBQ structure, a semiconductor layer having a large band gap constituting the quantum barrier 2 is replaced with a semiconductor layer having a small energy band gap constituting the quantum well 1 having a small width. A superlattice structure sandwiched between a layer and a semiconductor layer having a small energy band gap and constituting a wide quantum well 3 is formed by laminating a plurality of periods.

Now, in this nonlinear optical device, a narrow quantum well 1
It is assumed that the operated light 4 having a wavelength λ in the vicinity of the exciton level is incident.

In this state, when operating light 5 having a strong wavelength such that excitons or free electrons are formed by excitation into a narrow quantum well, the formation of excitons by the operated light is hindered, and the exciton is formed in the superlattice layer. The absorption saturation of the operating light occurs, the operating light is hardly absorbed, and the transmittance of the operating light increases.

The above-described light / optical switch and the like are configured by utilizing the operation of controlling the transmittance of the operated light by the operating light.

The change in the absorptance of the superlattice layer due to the excitation of the excitons in the narrow quantum well 1 or the excitation of the free electrons by the irradiation of the operating light is originally extremely fast.

The excited free electrons 7 tunnel through the quantum barrier 2 and fall into the wide quantum well 3, so that the absorption of the superlattice layer can be recovered to the initial state very quickly.

FIG. 7 is a graph showing a change characteristic of transmittance of a superlattice due to irradiation of operating light of a conventional nonlinear optical device.

This figure shows a temporal change in the transmittance of the operated light having a wavelength near the exciton level when a light pulse of 100 fs is used as the operating light.

According to this figure, the rise of the transmittance is extremely fast due to the irradiation of the operation light, but the change of the transmittance with respect to the operation light is slow.

This is because the electron 7 in the narrow quantum well 1 easily tunnels into the wide quantum well 3 through the quantum barrier 2, and the electron concentration in the narrow quantum well 1 suddenly falls due to the turning off of the operation light 5, but the hole This is because 6 has a larger effective mass than electrons, so it is difficult to tunnel through the quantum barrier, and its life in the narrow well 1 becomes longer.

As described above, since the hole tunneling time is long, it takes a long time to recover the transmittance.

[Problems to be solved by the invention]

As described above, in the conventional wired optical device, when a high-frequency response is made, the peak of the transmittance is set to “on”, and the point at which the fast absorption recovery determined by the tunneling time of electrons is completed is set to “off”. However, as is clear from FIG. 7, since the transmittance of the electrons after the passage of the tunneling time is still quite high, "ON / OFF"
The "off" ratio is sacrificed.

Therefore, in order to enable high-frequency response while keeping the "on / off" ratio high, the tunneling time of holes that controls the slow component of the absorption recovery process must be shortened.

When the thickness of the quantum barrier 2 for electrons and holes is reduced, the tunneling time of electrons and holes can be shortened, but the confinement of excitons weakens, and the operating light decreases. The efficiency of the change in the intensity of the operated light upon incidence (peak height at the time of ON) becomes small.

Accordingly, it is an object of the present invention to shorten the hole tunneling time and speed up the nonlinear optical device without sacrificing confinement for electrons.

[Means for solving the problem]

In the high-speed nonlinear optical device according to the present invention, a semiconductor layer having a large energy bandgap forming a quantum barrier, a semiconductor layer having a small energy bandgap forming a narrow quantum well, and a quantum well having a wide width are formed. TB sandwiched between semiconductor layers with small energy band gap
In the Q structure, a semiconductor layer having a large energy band gap forming the quantum barrier is constituted by a double layer of a semiconductor material layer serving as a barrier to both electrons and holes, and a semiconductor material layer serving as a barrier only to electrons. Then, a configuration is adopted in which the width of the quantum barrier for holes is smaller than the width of the quantum barrier for electrons.

[Action]

In the TBQ structure of the present invention, the width of the quantum barrier layer for holes is configured to be smaller than the width of the quantum barrier layer for electrons.

FIG. 1 is a band structure diagram for explaining the principle of the nonlinear optical device of the present invention.

In this figure, the width of the quantum barrier for holes is smaller than the width for electrons.

The quantum levels of electrons and holes in quantum well 1 are
The width of the quantum well is set so that the energy is higher than that of the quantum well 3.

The quantum level energy Δ based on the bottom of the conduction band of the semiconductor having a small energy band gap in the quantum well.
E is approximately an integer when n is an integer, m is an effective mass of electrons in a semiconductor having a small energy band gap, h is a Planck constant, and L is a width of a low energy band gap layer.
ΔE _n = h ² n ² / 8mL ²

When such a band structure can be realized, the tunnel time of holes can be shortened without weakening the confinement of electrons.

FIG. 2 is a characteristic diagram of the transmittance of the nonlinear optical device of the present invention.

As shown in this figure, the rise of the transmittance due to the irradiation of the operating light is as fast as that of the nonlinear optical device according to the prior art, and when the operating light is cut off,
The holes generated by the ionization of the excited excitons have a narrow quantum barrier width for holes, so they tunnel through the quantum barrier as quickly as electrons and immediately dissipate. Descend.

〔Example〕

 Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 3 is a band structure diagram of the nonlinear optical device according to the embodiment of the present invention.

In this figure, 2a is the same as that used in the above-described conventional example except that 2a is a semiconductor layer that serves as a quantum barrier for both electrons and holes, and 2b is a semiconductor layer that serves as a quantum barrier for only electrons. is there.

In this embodiment, the semiconductor layer forming the quantum barrier 2 is composed of a semiconductor layer 2a serving as a quantum barrier for both electrons and holes, and a semiconductor layer 2b serving as a quantum barrier for only electrons. ing.

As described above, when the width of the quantum barrier for holes is smaller than the electron barrier for electrons, the tunnel time of holes can be reduced without weakening the confinement of electrons, as described above. Recovery of the absorption rate can be accelerated.

FIG. 4 is a diagram showing the relationship between the composition of the semiconductor according to the present invention and the energy band gap.

An example of a method for designing the nonlinear optical device of the present invention will be described with reference to FIG.

In this example, the structure is based on three types of semiconductor mixed crystal systems having a band lineup as shown in FIG.

In other words, among materials constituting a TYPE-I heterojunction confined in a semiconductor having a small band gap for both electrons and holes, In _0.53 Ga _0.47 As having a small band gap is used.
Is used as a material for the narrow quantum well 1 and In _0.52 Al _0.48 As having a large band gap is used as a material for forming the quantum barrier 2a for both electrons and holes.

Furthermore, as a material used for the quantum barrier 2b for electrons only, there is almost no discontinuity in the conduction band with respect to the material constituting the quantum barrier 2a for both electrons and holes, and it is discontinuous in the valence band and wide. It is necessary to select a material system that forms a staggered junction (TYPE-II) with respect to the material that forms the quantum well 3.

In _0.52 Al _0.48 As and GaAs _0.51 Sb inserted in Fig. 4
A method for selecting a material used for the quantum barrier 2b for only electrons will be described with reference to a diagram showing a relationship between a composition ratio of _0.49 and an energy band gap. In the following description with reference to FIG. 4, in the mixed crystal of InAlGaAsSb, the composition of InAlAs is 1-x, and the composition of GaAsSb is x.

In the figure, the valence band of In _0.52 Al _0.48 As (x = 0.0, and thus without GaAsSb) that constitutes the quantum barrier 2a for both electrons and holes is In _0.53 that constitutes the narrow quantum well 1
0.21 eV lower than the valence band of Ga _0.47 As, GaAs _0.51 Sb
The valence band of _0.49 (x = 1.0 and therefore no InAlAs) is 0.74 eV higher than the valence band of In _0.52 Al _0.48 As (x = 0.0 and therefore no GaAsSb).

Assuming that the relationship between the composition ratio and the energy band gap obeys the Vegard rule, when x is around 0.3, In _0.52 Al
The valence band of the mixed crystal system of _0.48 As and GaAs _0.51 Sb _0.49 coincides with the valence band of the quantum well.

On the other hand, the conduction band was originally In _0.52 Al _0.48 As.
Since there is only a difference of 0.01 eV between GaAs _0.51 Sb _0.49 , there is almost no difference between In _0.52 Al _0.48 As and the mixed crystal system.

From the above considerations, as the configuration of the superlattice in this embodiment, the quantum well layers 1 and 3 have In _0.53 Ga _0.47 As, the quantum barrier layer
2a is In _0.52 Al _0.48 As, and quantum barrier layer 2b is (In _0.372 Ga _0.284
It can be concluded that Al _0.344 ) (As _0.861 Sb _0.139 ) should be used.

FIG. 5 is a structural view of one embodiment of the nonlinear optical device of the present invention.

In this figure, as described above, except that 8 is an etching stopper, 9 is a buffer layer, 10 is a substrate, and 11 is a cap layer.

In this embodiment, the quantum well layer 1 has a thickness of 20 atomic layers, the quantum well layer 3 has a thickness of 30 atomic layers, the quantum barrier layer 2a has a thickness of 5 atomic layers, and the quantum barrier layer 2b has a thickness of 5 atomic layers. A superlattice structure having a structure of 15 atomic layers as one cycle was grown on the (100) InP substrate 10 for 50 cycles by the MBE method.

In addition, as the buffer layer 9, In _0.53 Ga _0.47 As is 300 nm,
20 nm of In _0.52 Al _0.48 As as an etching stopper layer 8,
In _0.53 Ga _0.47 As was grown as the cap layer 11 to a thickness of 10 nm.

The tunneling probability of electrons or holes in the quantum well is given by exp (−k · L). When m is the effective mass, E is the height of the quantum barrier, and L is the width of the quantum barrier, k =
(2 mE) ^1/2 / h, the tunneling probability of holes without weakening the confinement of electrons by narrowing only the quantum barrier layer for holes having a larger effective mass than electrons as in this example. Can be increased, and as a result, the absorption recovery of excitons becomes faster.

In the above embodiment, the quantum barrier layers for both electrons and holes are arranged closer to the narrow well layer, but may be arranged farther from the narrow well layer.

Further, in the above embodiment, the combination that lattice-matches to the InP substrate is shown. However, if each semiconductor layer is thinner than the critical film thickness at which dislocation or the like occurs, the lattice does not completely match, and even in a system having strain. Thus, the effects described in the above embodiments can be obtained.

〔The invention's effect〕

According to the present invention, in the TBQ structure, the tunneling time of holes can be shortened without weakening the confinement of electrons, and the rate of decrease in transmittance can be increased, so that the response can be maintained while keeping the "on / off" ratio high. Speed can be increased, which greatly contributes to higher performance of the nonlinear optical device, particularly to higher speed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a band structure diagram for explaining the principle of a nonlinear optical device of the present invention, FIG. 2 is a transmittance characteristic diagram of the nonlinear optical device of the present invention, FIG. 3 is a band structure diagram of the nonlinear optical device according to the embodiment of the present invention, FIG. 4 is a diagram showing the relationship between the composition of the semiconductor according to the present invention and the energy band gap, and FIG. 5 is a nonlinear optical device of the present invention. Structural drawing of one embodiment of the device, sixth
The figure is a band structure diagram of a conventional optical device of TBQ structure nonlinear type,
FIG. 7 is a graph showing a change characteristic of transmittance of a superlattice due to irradiation of operating light of a conventional nonlinear optical device. 1 ... a narrow quantum well, 2 ... a quantum barrier, 2a ... a semiconductor layer that serves as a quantum barrier for both electrons and holes, 2b ...
A semiconductor layer serving as a quantum barrier only for electrons, 3 ... a wide quantum well, 4 ... operated light, 5 ... operating light, 6 ...
Holes, 7 electrons, 8 etching stoppers, 9
Buffer layer, 10 ... substrate, 11 ... cap layer

──────────────────────────────────────────────────続 き Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/355 501 EPAT (QUESTEL) JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A semiconductor layer having a large energy band gap constituting a quantum barrier, a semiconductor layer having a small energy band gap constituting a narrow quantum well, and a semiconductor having a small energy band gap constituting a wide quantum well. In a TBQ structure sandwiched between layers, a semiconductor layer having a large energy band gap forming the quantum barrier is composed of a semiconductor material layer serving as a barrier to both electrons and holes, and a semiconductor material layer serving as a barrier only to electrons. Wherein the width of the quantum barrier for holes is smaller than the width of the quantum barrier for electrons.