JPH1164899A - Nonlinear optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize intersubband absorption which can form a well layer of multiple quantum well structure with good flatness and has high light confinement efficiency and enough strength. SOLUTION: A nonlinear optical device is provided with a GaN buffer layer 2 formed on a sapphire substrate 1, a AlGaN clad layer 3 formed on the buffer layer 2, a GaN intermediate layer 4 formed on the clad layer 3, the multiple quantum well structure 5 which is constituted by alternatively laminating GaN well layers and AlGaN barrier layers on the intermediate layer 4, constitutes a part of a waveguide and further functions as a light absorbing layer by intersubband transition and a laser amplifier section 6 formed on the multiple quantum well structure 5. In the nonlinear optical device, thickness of the barrier layer of the multiple quantum well structure 5 is made to be critical film thickness or below, thickness of the intermediate layer 4 is made to be 15 times the critical film thickness or above and thickness of the clad layer 3 is made to be the critical film thickness or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear optical device such as an optical switch, an optical modulator, and a wavelength conversion element.
In particular, the present invention relates to a nonlinear optical device having a multiple quantum well structure functioning as a light absorption layer by intersubband transition.

[0002]

2. Description of the Related Art In recent years, with the development of optoelectronics-related technologies such as semiconductor lasers, low-loss optical fibers, optical fiber amplifiers, and high-speed integrated circuits, it has become possible to transmit a large amount of information of 10 gigabits per second over long distances. I have. However, in the coming multimedia era, ordinary end users will be able to use a large amount of information, such as high-definition video information, in real time, so it is necessary to build an infrastructure that can transmit and process even larger amounts of information. Will be needed.

[0003] In order to transmit and process a large amount of information by utilizing the broadband characteristics of an optical fiber, optical frequency multiplexing (optical FDM) is required.
It is considered appropriate to use technology or optical time division multiplexing (optical TDM) technology. In order to realize large-scale and efficient optical FDM networks and optical TDM networks, we have developed optical devices with new functions such as compact and optically efficient wavelength converters and optically controlled ultra-high-speed nonlinear optical switches. It is urgent to do it.

[0004] As such an element, a non-linear optical device utilizing light absorption accompanying transition between sub-bands of electrons can be considered. By utilizing the inter-subband absorption, the response speed can be increased and the nonlinearity can be increased.

[0005] The intersubband absorption layer needs to operate at a wavelength around 1.55 µm used in optical communication.
As for inter-subband absorption, absorption at this wavelength has been reported using InGaAs / AlAs and a quantum well layer formed on an InP substrate (JHSmet et al., Appl.
Phys. Lett., Vol. 64, pp 986-987 (1997)). However, in the case of this material system, the height of the energy barrier is relatively small, and when combined with the laser amplifier as described above, electrons cross over the barrier, and the effect of the intersubband absorption layer is not sufficiently achieved. It is feared that it will not be demonstrated. GaN, preferably with a larger band gap
It is required to form an intersubband absorption layer using a nitride semiconductor such as AlN or AlN.

However, on a sapphire substrate, for example,
When an attempt is made to form a quantum well structure using GaN as a well layer and Al _0.6 Ga _0.4 N as a barrier layer on a substrate grown with aN as a buffer layer, a favorable heterointerface is formed due to a different lattice constant. It can easily be inferred that it is difficult.

Further, if an AlGaN cladding layer having a small refractive index is provided adjacent to the multiple quantum well structure for confining light, the following problem occurs. That is, in order to realize sufficient light confinement, the cladding layer needs to have a sufficient thickness, but in this case, it is considered that the critical thickness is exceeded. For this reason, it can be easily inferred that the above-described multiple quantum well grown on the cladding layer has an adverse effect.

[0008] These problems lead to the inability to achieve the expected absorption intensity due to the transition between sub-bands, and must be solved in order to realize a nonlinear optical device utilizing absorption between sub-bands. It is.

[0009]

As described above, conventionally, when a non-linear device is produced by utilizing the transition between sub-bands in a multiple quantum well structure, a good quantum well and a good cladding layer can be formed due to a difference in lattice constant. There is a problem that it is difficult to obtain the expected intersubband absorption intensity.

The present invention has been made in view of the above circumstances, and an object of the present invention is to form a well layer having a multiple quantum well structure with good flatness, to achieve high light confinement efficiency, and It is an object of the present invention to provide a nonlinear optical device capable of realizing inter-subband absorption with a high intensity.

[0011]

[Means for Solving the Problems]

(Structure) In order to solve the above problem, the present invention employs the following structure. That is, the present invention has a structure in which a plurality of nitride-based semiconductor layers are stacked on a substrate, a part of a waveguide is formed in the nitride-based semiconductor layer, and the nitride-based semiconductor layer functions as a light absorption layer by intersubband transition. In the nonlinear optical device having the multiple quantum well structure described above, the barrier layer of the multiple quantum well structure is made of AlGaN, and a cladding layer made of AlGaN is formed between the multiple quantum well structure and the substrate. An intermediate layer having a larger lattice constant than the cladding layer is formed between the well layer and the cladding layer.

Here, preferred embodiments of the present invention include the following. (1) The barrier layer of the multiple quantum well structure is composed of Al _x Ga ₁ -xN (x
≧ 0.6), and the well layer of the multiple quantum well structure is GaN
That.

(2) The thickness of the barrier layer of the multiple quantum well structure is less than the critical thickness, and the intermediate layer is made of GaN, AlGaN,
Or, it is formed of InGaN and the thickness is 15
And the thickness of the cladding layer must be equal to or greater than the critical thickness.

(Action) According to the experiments of the present inventors, it has become clear that the quality of the quantum well structure depends on the thickness of the barrier layer. In the experiment, a GaN buffer layer was grown on a sapphire (0001) plane substrate by MOCVD, and a multiple quantum well structure having Al _0.6 Ga _0.4 N as a barrier layer and GaN as a well layer was grown thereon. .

When the thickness of the well layer was set to 1.5 nm, the thickness of the barrier layer was set to various values, and the intensity of photoluminescence (PL) from the quantum well layer was examined, as shown in FIG. If the thickness of the barrier layer is 9 nm or less, the wavelength 32
PL emission was observed at around 0 nm, but when the thickness was greater than this, no signal was observed at this wavelength, or the half-width was wide and the intensity was very weak.

When the cross-sectional structures of a sample having a barrier layer thickness of 14 nm and a sample having a thickness of 3 nm were observed with a TEM (transmission electron microscope), the heterojunction interfaces of the sample having a barrier layer thickness of 14 nm were not flat, and While the thickness was also non-uniform, when the barrier layer thickness was 3 nm,
Both the film thickness uniformity was good. That is, the barrier layer has a thickness of 9 nm.
If the thickness exceeds, the interface flatness and the film thickness uniformity deteriorate, and as a result, the subband in the quantum well is not formed, or even if it is formed, the width of the energy level becomes very wide. It is considered that such PL intensity decreased. Therefore, the value of 9 nm is considered to mean the critical film thickness. As a calculation formula for giving the critical film thickness, for example, as described in the literature (Appl. Phys. Lett., Vol. 69, pp2360),

[0017]

(Equation 3) There is. However, a _0, a _s the lattice constant of the corresponding layer and the underlayer, b is the magnitude of Burgers vector, lambda is the angle between the Burgers vector and the interface size, upsilon is Poisson's ratio, h _c is the critical film Shows the thickness. The author of the above document is G
As values for aN, b = 0.3084 nm, λ = π / 3, υ = 0.38 (2)

In our experiments, the layer in question is Al _0.6 Ga _0.4 N, and the values of the above parameters are not known at present, but h _c is calculated using the above parameters for GaN. Then, it becomes 9 nm, which matches the value shown in the experiment. This is presumably because the physical properties of AlGaN and GaN are close to each other, and the difference falls within the range of experimental error.

In order to form an element utilizing the transition between sub-bands in this manner, the thickness of the barrier layer needs to be equal to or less than the thickness defined by the above equations (1) and (2). Can be concluded.

However, when a quantum well structure is formed with such a thin barrier layer, there is a problem that light leaks out and the light cannot be sufficiently confined. In order to avoid this problem, it is necessary to grow an AlGaN cladding layer and grow a quantum well structure thereon. In this regard, the experiments of the present inventors will be described below.

As a cladding layer, Al _0.6 Ga _0.4 N is grown to a thickness of 150 nm exceeding the above critical film thickness.
An intermediate layer is grown, and a 3 nm thick Al _0.6 Ga _0.4
An experiment was conducted by growing a multiple quantum well structure having 30 wells consisting of an N barrier layer and a GaN well layer having a thickness of 1.5 nm. The back side of the sample was mirror-polished to a thickness of 50 μm, cleaved to a size of 8 mm square, and a wavelength of 1 to 6 was measured from the end face.
Near-infrared light of μm was incident, and the transmitted light spectrum was measured by FTIR from the opposite end face. When the thickness of the intermediate layer was examined as a parameter, absorption was observed at a wavelength of about 1.5 μm when the thickness of the intermediate layer was large, but was not observed when the thickness was small. FIG. 4 shows the relationship between the absorption strength and the thickness of the intermediate layer. From this result, the intermediate layer is 1
If it is 35 nm or more, it can be concluded that the absorption intensity is not affected.

In order to investigate the reason, ones prepared by growing GaN on AlGaN by 90 nm and those grown by 200 nm were prepared, and their surfaces were observed by SEM (scanning electron microscope). The surface flatness was very poor when the GaN thickness was 90 nm, whereas it was almost flat when the GaN thickness was 200 nm. When the thickness exceeds the critical film thickness, the growth becomes an island shape, and as the growth progresses, the island expands, and the adjacent islands are united to recover flatness again. From this, it can be explained as follows that the absorption intensity is affected by the thickness of the intermediate layer.

Since the AlGaN cladding layer is sufficiently thick, the lattice can be considered to be relaxed. Therefore, the GaN intermediate layer grown thereon has an AlG
Although it is lattice-matched to aN, if it exceeds the critical film thickness (9 nm), the flatness deteriorates as in the case of the multiple quantum well structure. Even if the multiple quantum well structure is grown on the intermediate layer having poor flatness, the well layer of the multiple quantum well structure naturally has poor flatness. Therefore, a sub-band level cannot be formed, or even if a sub-band level can be formed, the width of the level is widened, and the absorption intensity between sub-bands is significantly reduced. However, when the thickness of the intermediate layer is further increased, the flatness is improved, and strong subband absorption can be obtained.
This experimental result can be interpreted that the thickness of the intermediate layer in which the intersubband absorption intensity is not affected is 15 times the critical thickness.

When an experiment was conducted by changing the intermediate layer to InGaN or AlGaN, the same tendency was observed in each case. The film thickness itself was not affected by the difference in lattice constant from the cladding layer and the absorption intensity between subbands was not affected, but in any case, if the film thickness was at least 15 times the critical film thickness or more, The inter-absorption intensity was not affected.

[0025]

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 diagram schematically illustrating a cross-sectional configuration of a nonlinear optical device according to an embodiment of the present invention.

In FIG. 1, (0) of the sapphire substrate 1
001) GaN with a thickness of 2 μm on the surface by MOCVD
A buffer layer 2, a cladding layer 3 made of Al _0.6 Ga _0.4 N having a thickness of 130 nm, an intermediate layer 4 made of GaN having a thickness of 150 nm, an Al _0.6 Ga _0.4 N barrier layer having a thickness of 3 nm, and Si having a thickness of 1.5 nm. 2 × 10 ¹⁸ cm ^-3 doped Ga
A multiple quantum well structure 5 having 30 wells composed of N well layers and an optical amplifier 6 are sequentially stacked.

In this structure, the optical waveguide 7 is formed above a part of the buffer layer 2. As the optical amplifier 6, M
It may be formed by growing a laser amplifier using a nitride semiconductor cascade laser having a QW structure, or by directly attaching a laser amplifier grown on an InP substrate on the multiple quantum well structure 5. May be.

An example of the laser amplifier 6 is shown in FIG. This has already been proposed by the present inventors (Japanese Patent Application No. 8-244785), and one layer is formed on the contact layer 81.
N-type GaN layer 61 having a thickness of 00 nm, n-type A having a thickness of 170 nm in which the ratio of Al is continuously changed from 0 to 0.46
lGaN graded layer 62 and undoped AlG
An aN / undoped GaN quantum well layer 63 is stacked.

Specifically, the quantum well layer 63 is made of Al _0.67
Ga _0.33 N barrier layer 63a, GaN well layer 63b, Al
_0.67 Ga _0.33 N barrier layer 63 c, GaN well layer 63 d, A
l _0.78 Ga _0.22 N barrier layer 63e, GaN well layer 63f,
Al _0.78 Ga _0.22 N barrier layer 63 g, GaN well layer 63
h, Al _0.89 Ga _0.11 N barrier layer 63i, GaN well layer 6
3j, Al _0.89 Ga _0.11 N barrier layer 63 k, GaN well layer 63 l, AlN barrier layer 63 m, GaN well layer 63 n, A
The 1N barrier layers 63o are sequentially stacked.

Each of the layers 63a to 63o has a thickness of 7 monolayers. In the barrier layer, the lattice constant changes with a change in the Al composition, and the thickness becomes the same or slightly smaller in the stacking order. The energy barrier of each well layer is
They are equal or larger in the stacking order.

On the quantum well layer 63, the ratio of Al is set to 0.1.
An n-type AlGaN grading layer 64 having a thickness of 125 nm continuously changed from 12 to 0.46 is formed. These AlGaN / GaN quantum well layers 63 and AlG
25 combinations with the aN grading layer 64
Pairs are sequentially stacked. And the 25th set of AlGa
On the N / GaN quantum well layer 63, a 100 nm thick n
A GaN layer 65, an n-type AlAs cladding layer 66, and an n-type GaN contact layer 82 are formed.

As described above, according to the present embodiment, Al _0.6
The thickness of the Ga _0.4 N cladding layer 3 is set to 130 nm and its critical thickness or more, and the thickness of the GaN intermediate layer 4 is set to 150 nm.
By setting the thickness of the Al _0.6 Ga _0.4 N barrier layer in the multiple quantum well structure 5 to 3 nm or less and the critical thickness of the multiple quantum well structure 5 or less. The well layer can be formed with good flatness, the light confinement efficiency is high, and a sufficient intensity of intersubband absorption can be realized.

The present invention is not limited to the above embodiment. In the embodiment, GaN is used for the intermediate layer, but InGaN may be used for the intermediate layer. By using the InGaN layer, an effect as a crack prevention layer can be exhibited, whereby the number of wells can be increased, and an element having higher absorption efficiency can be formed. Also,
AlGa having a lower Al composition than the cladding layer as an intermediate layer
N may be used. In this case, since the refractive index is smaller than when GaN is used, the light intensity distribution is relatively biased toward the multiple quantum well layer, and the absorption efficiency is increased.

The Al composition of the cladding layer is not limited to 0.6, but may be changed within a range where the average refractive index of the multiple quantum well layer and the intermediate layer is smaller than the average refractive index, that is, within a range where light can be confined. In addition, without departing from the gist of the present invention,
Various modifications can be made.

[0035]

As described above, according to the present invention, an intermediate layer having a larger lattice constant than the cladding layer is formed between the multiple quantum well structure and the cladding layer.
By optimally setting the thickness of the barrier layer and the thickness of the cladding layer of the multiple quantum well structure, the optical confinement efficiency is high,
It is possible to realize a non-linear optical device having a structure capable of expressing a sufficient intensity of inter-subband absorption.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a nonlinear optical device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration example of a laser amplifier used in the embodiment of FIG. 1;

FIG. 3 is a diagram showing the dependence of PL intensity on the thickness of a barrier layer.

FIG. 4 is a diagram showing the dependence of the intersubband absorption intensity on the thickness of the intermediate layer.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 sapphire substrate 2 GaN buffer layer 3 AlGaN cladding layer 4 GaN intermediate layer 5 AlGaN / GaN multiple quantum well structure 6 amplifying unit 7 optical waveguide

Claims (4)

[Claims] 1. A semiconductor device comprising: a plurality of nitride-based semiconductor layers laminated on a substrate; a part of a waveguide in the nitride-based semiconductor layers; and a function as a light absorption layer by transition between sub-bands In the nonlinear optical device having the multiple quantum well structure, the barrier layer of the multiple quantum well structure is made of AlGaN, and a cladding layer made of AlGaN is formed between the multiple quantum well structure and the substrate. A nonlinear optical device, wherein an intermediate layer having a larger lattice constant than the cladding layer is formed between the well structure and the cladding layer. 2. The multi-quantum well structure barrier layer is formed of Al _x G
2. The nonlinear optical device according to claim 1, wherein a _1-x N (x ≧ 0.6), and the well layer of the multiple quantum well structure is GaN. 3. The barrier layer of the multiple quantum well structure has a thickness of:
The following equation (A _0, a _s the lattice constant of the corresponding layer and the underlying layer, b = 0.
3084 nm, λ = π / 3, υ = 0.38, and h _c is a film thickness or less, and the intermediate layer is made of GaN, AlG
aN or InGaN, the thickness of which is at least 15 times the critical thickness defined by the above equation, and the thickness of the cladding layer is at least the critical thickness defined by the above equation. The nonlinear optical device according to claim 1 or 2, wherein 4. A GaN buffer layer formed on a substrate,
An AlGaN cladding layer formed on the buffer layer, and GaN, AlGaN, or InGaN having a larger lattice constant than the cladding layer formed on the cladding layer.
, A GaN well layer and Al
_{x Ga 1-x N (x} ≧ 0.6) becomes a barrier layer are laminated alternately, thereby constituting a part of the waveguide, and a multiple quantum well structure that serves as a light absorbing layer according to intersubband transitions The thickness of the barrier layer of the multiple quantum well structure is expressed by the following equation: (A _0, a _s the lattice constant of the corresponding layer and the underlying layer, b = 0.
3084 nm, λ = π / 3, υ = 0.38, h _c is a critical thickness or less defined by the following formula, and the thickness of the intermediate layer is 15 times or more the critical thickness defined by the above equation. Wherein the thickness of the cladding layer is equal to or greater than the critical thickness defined by the above equation.