WO2008007522A1

WO2008007522A1 - Semiconductor element, optical switching element and quantum cascade laser element

Info

Publication number: WO2008007522A1
Application number: PCT/JP2007/062237
Authority: WO
Inventors: Songbek Che; Akihiko Yoshikawa; Yoshihiro Ishitani
Original assignee: National University Corporation Chiba University
Priority date: 2006-06-19
Filing date: 2007-06-18
Publication date: 2008-01-17
Also published as: JP5388274B2; JPWO2008007522A1

Abstract

Provided are novel semiconductor element, optical switching element and quantum cascade laser. The semiconductor element includes a well layer having a GaN well layer and an InN well layer, and a superlattice layer having a pair of AlGaN barrier layers sandwiching the well layer. In the semiconductor element, increase of an electron-hole recombination probability (transition probability) between a conduction band and a valence band by wave function control, and an emission wavelength control are performed. The semiconductor element has, as an active layer, a stacked structure composed of the well layer, which has the GaN well layer and the InN well layer, and the pair of AlGaN barrier layers sandwiching the well layer.

Description

Specification

Semiconductor device, optical switching device and quantum cascade laser device

[0001] The present invention relates to a semiconductor device, an optical switching device, and a quantum cascade laser, and more particularly to a device utilizing intersubband transition.

Background art

[0002] Discrete energy levels exist in well layers such as quantum well structures and superlattice structures formed in semiconductor devices, and light absorption and electron transition occur between these levels. This is generally called intersubband transition.

[0003] Transition between subbands (hereinafter referred to as "ISBT") has several characteristics different from band transition between conduction band and valence band. One is that the transition energy of ISBT is variable depending on the quantum well and superlattice structure. The other is that the transition rate of the transitioned electrons is much faster than in the case of interband transitions.

[0004] In the case of normal interband transition, the relaxation rate is about nsec order. The relaxation rate of ISBT is ~ psec to fsec order, and it can be applied to future terabit-class ultrafast optical switches. Expected.

[0005] There are three typical basic structures of ultrafast optical switches using ISBT. GaNZAIN, CdSZBeTe, and InGaAsZAlAs. Among them, the GaNZAIN superlattice, which is a nitride-based ISBT structure, has the largest conduction band offset (-3 eV) among these three, and is reported to have the fastest and relaxation rate (fsec order). (See, for example, Patent Documents 1 and 2 and Non-patent Document 1). Particularly, in Non-Patent Document 1 below, the shortest ISBT wavelength has been reported to date. In this paper, 100 periods of GaNlnmZAlN4nm superlattice were grown using molecular beam epitaxy (hereinafter referred to as “MBE” t) method, and an ISBT wavelength of 1.08 m was observed.

[0006] On the other hand, as described above, the ISBT can easily control the transition wavelength only by changing the quantum well width or the periodic structure of the superlattice. An example of such a device that utilizes light emission between subbands is a quantum cascade laser. Ordinary semiconductor lasers have conduction bands and valence charges. Laser oscillation occurs due to the inversion distribution between the subbands, but the cascade laser forms an inversion distribution between discrete levels in the conduction band of the quantum well and causes laser oscillation. As described above, the quantum cascade laser can control the lasing wavelength simply by changing the quantum well structure. The range is limited by the band offset of the quantum well. In other words, it is impossible to obtain laser oscillation with energy larger than the band offset amount.

. Incidentally, an oscillation wavelength of 3.4 to 24 m has been obtained so far.

In other words, the GaNZAIN superlattice currently has the fastest electron relaxation rate and is considered to be able to achieve the largest conduction band offset, so the basic structure of ultrafast optical switches and quantum cascade laser devices This is a very promising structure with the possibility of further shortening the intersubband transition wavelength.

Patent Document 1: Japanese Patent Laid-Open No. 8-179387

Patent Document 2: JP 2001-108950 A

Non-Patent Document 1: K. Kishino et al., APL., Vol. 81, 1234-1236 (2002) Disclosure of the Invention

Problems to be solved by the invention

[0009] The GaNZAIN superlattice structure has a very large conduction band offset, and is suitable for shortening the ISBT T wavelength. The degree of lattice mismatch between GaNZAIN is as large as 2.4%. Lamination is difficult. Assuming coherent growth on GaN, the A1N barrier layer is subject to tensile strain. This tensile strain causes cracks in the A1N layer, making it difficult to maintain crystal quality.

[0010] When considering application to optical devices such as quantum cascade lasers using ISBT, the short wavelength of the I SBT wavelength is also very important. The GaNZAIN superlattice has observed 1.08 m ISBT absorption, but the GaN well layer thickness needs to be further reduced to make the wavelength shorter than this. However, in this case, the ratio of the A1N layer of the entire superlattice increases, so that cracks are more likely to occur and it becomes difficult to maintain the crystal quality. The GaN well layer thickness is also less than lnm, making it difficult to control. In other words: Shortening the wavelength below L m is difficult with the current GaNZAIN superlattice structure.

By the way, in general, an ultrafast optical switch or a quantum cascade laser structure is a quantum well structure. An optical element structure in which a large number of layers are stacked. For example, an ISBT ultrafast optical switch has a quantum well structure of about 100 cycles to minimize the optical energy required for switching and increase the amount of light absorbed in the ISBT region. The GaNZAIN superlattice structure has a very large conduction band offset and is suitable for shortening the wavelength of ISBT. However, as described above, the lattice mismatch between GaNZAIN is as large as 2.4%. Many stacks are difficult. And assuming that it grows coherently on GaN, the A1N barrier layer is subject to tensile strain. When the number of periods in the superlattice is increased, cracks easily occur, the crystal quality deteriorates rapidly, and ISBT absorption cannot be observed.

[0012] It should be noted that if the number of cycles is reduced, it is possible to suppress the generation of cracks and maintain the crystal quality. This is preferable because the amount of force absorption decreases, making it difficult to obtain the ONZOFF ratio necessary for optical switching. Absent. When manufacturing and designing a quantum cascade laser structure, it is necessary to provide a multi-stage light emitting layer and an electron injection layer, and the same problem arises. Limiting the number of periods of the superlattice is a major demerit in the structural design.

In view of the above problems, an object of the present invention is to provide a novel semiconductor device, an optical switching device using the same, and a quantum cascade laser.

Means for solving the problem

[0014] The inventors of the present invention have made extensive studies on the above-mentioned problems. As a result, the AlGa N barrier layer is introduced into the well layer by introducing a layer that generates a compressive strain that eliminates the tensile strain applied to the AlGaN barrier layer. It was possible to prevent the generation of cracks, and when a layer having an InN force was used for this layer, the inventors realized that the ISBT wavelength could be shortened, and the present invention was completed.

That is, a semiconductor device according to one means of the present invention includes a well layer having a GaN well layer and an InN well layer, and a superlattice layer having a pair of AlGaN barrier layers sandwiching the well layer. In the conventional GaNZAIN superlattice ISBT structure, the generation of cracks due to the tensile stress of the A1N layer (AlGaN layer) is a problem, and it is difficult to increase the number of cycles. Can do. In other words, tensile stress is applied to the AlGaN barrier layer, but compressive stress is applied to the InN layer by providing the InN layer (the lattice mismatch between InNZGaN is about + 11%). It is possible to form a strain compensation structure that cancels the tensile strain in the A1N layer by providing it in the layer. Rack generation can be suppressed.

[0016] Furthermore, the InN well layer has the smallest bandgap energy (approximately 0.63 eV) among nitride semiconductors, and the conduction band offset amount from GaN is approximately 1.7 eV, which is the difference from A1N. Can be estimated to be about 3.5 eV. In other words, this semiconductor device can achieve a value about twice the conduction band offset of the GaNZAIN superlattice, and can further shorten the intersubband transition wavelength (less than 1 μm). . Such a large band offset can be realized by using a nitride semiconductor, particularly InN.

[0017] In this means, although not limited, the thickness of the InN well layer in the well layer is at least one of a thickness of 2 molecules or less and a thickness of 0.6 nm or less. It is more preferable that the thickness is not more than 1 molecule or not more than 0.3 nm in that the effect of the invention can be ensured. Here, “single molecule thickness” refers to the length of one half of one side of the crystal unit cell in the growth direction. Although not limited, for example, in the growth of a nitride semiconductor crystal in the C-axis direction, the length becomes 1Z2 of the C-axis length. In other words, it corresponds to 2.9A for InN and 2.6A for GaN. Our experiments show that the critical thickness of InN is 1 to 2 molecules in InNZGaN heterostructures. In other words, when growing a GaNZAIN superlattice layer, if an InN2 molecular thickness is grown in the GaN well layer, more preferably one molecular thickness, the lattice relaxation due to the lattice mismatch between InNZGaN is prevented and the InN ultrathin film layer becomes coherent. Grows in the GaN well layer. In addition, this single-molecule-thick InNZGaN well layer has a compressive strain due to In N, and becomes a strain opposite to that of the A1N barrier layer, so that a strain compensation structure can be formed. Accumulation and cracking can be prevented, and poor crystal quality can be prevented. For example, according to a simple estimate, in the case of a single molecular thickness InNZGaNZAIN superlattice based on the GaN lattice, it is possible to form a strain compensation structure by setting the A1N layer thickness to about 1.3 nm.

[0018] On the other hand, in general, A1N has a lattice mismatch degree of 2.4% (tensile strain) with respect to GaN. InN is + 11% (compressive strain) with respect to GaN. Since A1N has an even greater degree of lattice mismatch of 13% or more, it is practically difficult to produce a high-quality superlattice structure. In addition, InN is thermally unstable compared to other nitride semiconductors. However, there is also a problem that the optimum growth temperature differs greatly compared to GaN and A1N. Specifically, the critical growth temperature on the high-temperature side of InN single crystals is about 500 ° C, whereas GaN and A1N usually grow at a high temperature of 650 ° C to 1000 ° C or higher. These heterostructures, quantum well structures and superlattice structures are also difficult to form. However, we have experimentally confirmed that the thermal properties of ultra-thin InN layer growth are different from Balta InN. Normally, when the growth temperature is 500 ° C or higher (in this case, it is assumed that the growth is on the group III polar surface), the crystal growth hardly occurs due to the thermal instability 'dissociation of InN. However, in the case of an ultra-thin InN layer, the InN layer is less than 2 molecules thick, so unlike the case of Balta, the InN ultra-thin film can be formed without thermal decomposition even at higher temperatures. It is done. The results of our studies to date have shown that it is possible to form an ultra-thin InZ thin-film layer with pseudo-lattice matching up to 650 ° C and the formation of a ZGaN multiple quantum well structure. It is predicted that formation is possible by controlling the feed ratio of VZIII group raw materials. In other words, according to this, it is possible to produce a GaNZAIN superlattice ISBT structure in which InN having a thickness of less than two molecules is introduced at a high temperature of 650 to 700 ° C. It is possible to realize intersubband transition in the wavelength region. In this means, one form of the thickness of the In N well layer is preferably 2 molecular thickness or less, but may be a fractional molecular thickness. “Fractional molecular thickness” refers to a state in which InN layers are not uniformly present in a planar shape, but InN having a thickness of about 1 molecule is dispersed in an island shape. Even in such an InN fractional molecular thickness ZGaN well layer ZAIGaN barrier layer, the above-described intersubband transition wavelength can be shortened and a strain compensation structure can be formed.

[0019] In this means, although not limited, it is preferable that the well layer has a plurality of GaN well layers, and the InN well layer is sandwiched between the plurality of GaN well layers. . By doing so, the GaN well layer is arranged between the AlGaN barrier layer and the InN well layer, and the above tensile stress and compressive stress can be surely offset.

[0020] Further, although not limited in this means, the well layers may have the same number of GaN well layers and In N well layers. In this case, the force that the AlGaN barrier layer and the InN well layer are stacked adjacent to each other. Ga between the other AlGaN barrier layer and the InN well layer N can be present and the effect in this means can be achieved.

[0021] Further, although not limited to this means, the well layer and the pair of AlGaN barrier layers sandwiching the well layer have a one-cycle quantum well structure, and the AlGa in this quantum well structure

It is also preferable that a plurality of quantum well structures are stacked in common with the N barrier layer. By doing so, it is possible to reduce the number of steps for manufacturing a semiconductor element and to have an advantage that the design becomes easy.

[0022] In this means, the well layer and the pair of AlGaN barrier layers sandwiching the well layer may have a single-period quantum well structure, and a plurality of quantum well structures may be stacked via a spacer layer. preferable. By doing so, accumulation of strain energy in the AlGaN barrier layer can be prevented, and cracks can be more easily suppressed.

[0023] Further, although not limited to this means, the well layer has subbands, and the optical wavelength corresponding to the energy in the subband interval can be controlled to 2 m or less.

[0024] Further, this means is not limited, but has a subband in the well layer, and utilizes a light absorption transition in the subband interval and a fast relaxation process via an electron-phonon interaction. Thus, it is also preferable to function as an ultrafast optical switch.

[0025] Further, although not limited in this means, the superlattice layer is electrically controlled by adjusting the period of the well layer and the AlGaN barrier layer of the superlattice layer, which has subbands in the well layer. It is also preferable to provide a child injection layer and a light emitting layer to function as a quantum cascade laser.

[0026] Further, an optical switching element according to another means of the present invention comprises a substrate, a nother layer, a cladding layer, a superlattice layer, and a cap layer, which are laminated in order, and the superlattice layer comprises: A well layer having a GaN well layer and an InN well layer and a pair of AlGaN barrier layers sandwiching the well layer are provided. In the conventional GaNZAIN superlattice ISBT structure, the generation of cracks due to the tensile stress of the A1N layer (AlGaN layer) is a problem, and it is difficult to increase the number of cycles. be able to. In other words, the tensile force is applied to the AlGaN barrier layer. By providing the InN layer, compressive stress is applied to the InN layer (the lattice mismatch between InNZGaN is approximately + 11%). By providing it in the GaN layer, it is possible to form a strain compensation structure that cancels the tensile strain in the A1N layer. Even if the period is increased, the generation of cracks can be suppressed.

In addition, a quantum cascade laser device according to another means of the present invention includes a substrate, a buffer layer, a first contact layer, a first cladding layer, a superlattice layer, a second cladding layer, a second cladding layer, The contact layers are sequentially stacked, and the superlattice layer has a well layer having a GaN well layer and an InN well layer, and a pair of AlGaN barrier layers sandwiching the well layer.

The invention's effect

As described above, the present invention can provide a novel semiconductor device, an optical switching device using the same, and a quantum force scaled laser.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention can be expressed as many different embodiments and examples, and is not limited to the present embodiments and examples.

FIG. 1 is a schematic diagram of a semiconductor element (ISBT optical switch) according to the present embodiment. As shown in FIG. 1, the semiconductor optical device 1 according to the present embodiment is configured by sequentially stacking a buffer layer 3, a cladding layer 4, a superlattice layer 5, and a cap layer 6 on a substrate 2. The semiconductor element according to this embodiment can be used as an optical switching element.

[0031] The substrate 2 serves as a basis for growing the noffer layer 3, the clad layer 4, the superlattice layer 5 and the like, and is not limited to, for example, a sapphire substrate, a SiC substrate, a GaN barrier, A1 N-balta substrates can be used, and other substrates with GaN and A1N crystal layers deposited by hydride vapor phase epitaxy (HV PE), metal organic chemical vapor deposition (MOCVD), etc. be able to. Note that the polarity of the surface of the substrate 2 and the polarity of the layer laminated on it by the surface treatment (nitriding treatment) are determined. The surface polarity can be either a group III polar surface or a group V polar surface. It is not limited. Substrate 2 is formed using an R-plane sapphire substrate, GaN grown in the A-plane or M-plane direction, or a selective growth technique to reduce the effect of the internal electric field (11-22) or (1-101) Surface GaN film can be used.

[0032] The noffer layer 3 is for growing the superlattice layer 5 with good crystal quality, and various types can be adopted without being limited as long as it exhibits the above functions. For example, a GaN crystal, A1N crystal, AlGaN mixed crystal, or the like can be suitably used.

[0033] The clad layer 4 is for confining electrons efficiently in the superlattice layer 5 and minimizing the optical power required for optical switching, and various types can be adopted without limitation. For example, AlGaN mixed crystal, GaN crystal, A1N crystal, etc. can be used preferably.

The superlattice layer 5 includes a pair of GaN well layers 51 la and b, and a well layer 51 having an InN well layer 512 sandwiched between the pair of GaN well layers and a pair of AlGaN sandwiching the well layer 51. The barrier layers 52a and 52b and the quantum well structure 53, which is also a force, are formed as one cycle, and this is repeated a plurality of times through the spacer layer 54. FIG. 2 shows a schematic cross-sectional view of the superlattice layer 5 according to the present embodiment.

[0035] The pair of GaN well layers 51la and b are layers including a GaN crystal doped with an impurity such as Si. The composition of the GaN well layers 511a, b may include other group III elements such as In or A1 even if the composition is only a GaN layer. However, in this case, the ratio of elements other than Ga is preferably less than 0.5 with respect to the total amount of Group III elements. The thickness of the GaN well layers 511a and b is not limited, but is preferably 0.5 nm or more and 5 nm or less, for example, more preferably 0.5 nm or more and 2 nm or less. In the semiconductor device according to the present embodiment, the total thickness of the pair of GaN well layers 51 la and b and the InN well layer 512 (the distance between the pair of AlGaN barrier layers 52 a and b) is changed. It is possible to control the energy between subbands, and it is not limited, but it is preferable that the total is within the range of 1 nm or more and 2 Onm or less. It is more preferable that the energy is 1 nm or more and 5 nm or less. Yes. Note that in the transition between subbands, absorption occurs due to the transition of electrons to the first level force of the subband and the second level. Therefore, by using an n-type semiconductor layer, a large number of electrons exist in the first level. Allows intersubband absorption. Therefore, impurities such as Si are doped to control the n-type semiconductor.

The InN well layer 512 is a layer that can obtain a strain compensation effect due to an increase in the conduction band offset amount and a compressive strain opposite to that of the AlGaN barrier layer 511. The thread of the InN well layer 512 may be a layer composed only of InN, or may contain other group elements such as Ga. However, in this case, the percentage of elements other than In is 0 Preferably less than 1. The thickness of the InN well layer is not limited, but is preferably 2 molecular thickness or less, more preferably 1 molecular thickness or less. This is because InN crystals and GaN crystals usually have a lattice mismatch of about 11%! / Although they can grow without causing lattice relaxation by making the InN well layer 53 sufficiently thin. This is because it is considered possible. “Molecular thickness” here means the length of the molecule formed by In and N atoms. Although not limited, for example, in the case of a crystal growing in the c-axis direction, it means half the length of the c-axis in the crystal lattice. As described above, the maximum thickness (hereinafter referred to as “critical film thickness”) of In N grown on GaN that does not cause lattice relaxation is 1 to 2 molecular thickness. If this can be realized, defects due to lattice relaxation can be prevented and a superlattice layer with high crystal quality can be realized. Conversely, if an InN layer is grown at a thickness greater than this, misfit dislocations will occur in the crystal, and there is a concern that the crystal quality will be greatly degraded. Here, the thickness of the InN well layer may be a fractional molecular thickness. “Fractional molecular thickness” refers to a state in which InN with a thickness of about 1 molecule is dispersed and formed in islands, in which the InN layer does not exist evenly. Even in such an InN fractional molecular thickness ZGaN well layer ZAIGaN barrier layer, the above-described intersubband transition wavelength can be shortened and a strain compensation structure can be formed.

[0037] The pair of AlGaN barrier layers 511a and 51 lb are mixed crystal layers of A1N crystal and GaN crystal. Specifically, the composition of the A1 GaN barrier layer can be expressed as AlGaN, and x is a force that is larger than 0 and takes a value of 1 or less, preferably 0.5 or more and 1 or less. The thickness of the pair of AlGaN barrier layers 51 la and 51 lb is not limited, but is preferably 1 nm or more and 50 nm or less, more preferably 1 nm or more and 5 nm or less.

[0038] Spacer layer 54 is a layer having a function of preventing the occurrence of cracks accompanying an increase in the number of superlattice periods, and is not limited, but is preferably composed of, for example, GaN. Further, the thickness is not limited as long as it exhibits the above function, but it is preferably lOnm or more and 50 nm or less. In this way, strain accumulation is prevented, cracks are less likely to occur as the number of superlattice periods increases, and an ISBT structure with a high absorption amount can be realized, and an ISBT wavelength of 1 μm or less can be realized more easily. Become. The number of layers stacked via the spacer layer 56 is not limited, but is preferably 10 cycles or more. Number of cycles As the layer thickness increases, the amount of absorption is expected to increase due to the increase in the total layer thickness. However, it should be considered that there is concern about the generation of cracks as the layer thickness increases.

[0039] The cap layer 6 is a layer having a function of efficiently confining electrons in the superlattice layer 5 and maintaining the crystal quality, and various types can be adopted without limitation. For example, AlGaN, GaN, A1N, etc. can be suitably used.

Here, FIG. 3 shows a schematic diagram of energy bands in the semiconductor optical functional device. FIG. 3 (a) is a band diagram of the ISBT structure in the GaNZAIN superlattice not including the InN layer, and FIG. 3 (b) is a band diagram of the ISBT structure in the InNZGaNZAIN superlattice according to the present embodiment. The slope of the band in the figure is the result of considering the influence of the internal electric field due to the piezo effect and spontaneous polarization when assuming growth in the C-axis direction.

[0041] As can be seen from Figs. 3 (a) and (b), the GaNZAIN structure has a band offset of about 1.8 eV, whereas the InNZGaNZAIN structure with the InN layer inserted has a larger conduction band. An offset amount (~ 3.5eV) can be obtained. Although the InN well layer is a very thin film, it greatly affects the intersubband level energy. In particular, the first level can be greatly shifted downward, and the ISBT wavelength can be further shortened. Conceivable. In particular, it is possible to cause ISBT absorption up to the near infrared range.

[0042] In addition, since the conduction band offset amount can be sufficiently increased by inserting an InN well layer (-3.5 eV), the layer thickness control of the ISBT superlattice can be performed according to the semiconductor optical functional device according to the present embodiment. Can be easily. For example, the nitride ISBT structure fabricated by metalorganic vapor phase epitaxy cannot cover the 1.55 / zm band, which is more difficult to control strictly than the MBE method, and the shortest wavelength is currently 2 m. Therefore, all of the nitride-based ISBT t structures covering the 1.55 / zm band must be formed by the MBE method, but according to the structure according to this embodiment, the GaN well layer has a thickness of ~ 3 nm and a 1.55 m band. Compared to the case where no InN layer is inserted (2 nm or less), the strictness of layer thickness control is relaxed, and the metal-organic vapor phase epitaxy can realize a nitride-based ISBT structure that covers the 1.55 m band. There is an advantage

[0043] Here, the dependence of the ISBT wavelength on the GaN well layer thickness of the GaNZAIN superlattice and InNZGaNZAIN superlattice was determined by theoretical calculation. The results are shown in Fig. 4. In the figure, The horizontal axis shows the thickness of the GaN well layer, and the vertical axis shows the intersubband transition energy. In this calculation, the thickness of the InN well layer is assumed to be one molecular thickness and is calculated at the center of the GaN well layer.

[0044] As shown in Fig. 4, the GaNZAIN superlattice, InNZGaNZAIN superlattice! /, And the deviation, the ISBT wavelength becomes shorter as the GaN well layer thickness decreases, but the same GaN well layer. In the case of thickness, the InNZGaNZAIN superlattice has a shorter wavelength than the transition energy between subbands. In particular, when the GaN well layer thickness is 1.5 nm for the GaNZAIN superlattice, the ISBT wavelength is about 1.2 ^ πι (1.05 eV), whereas for the InNZGaNZAlGa N superlattice, the ISBT is about 650 nm (l. Wavelength can be obtained. That is, the semiconductor optical device according to the present embodiment can reduce the wavelength to the ISBT wavelength in the visible light region. This is because the structure proposed in the present invention has further shortened the operating wavelength of quantum cascade lasers that have been actively studied mainly on the long wavelength side of 3 m or more (to 1.55 m band and visible light source). It is very promising for expansion.

In FIG. 4, the ISBT wavelength when the stacked structure according to the present embodiment, that is, the InN well layer 512 is located between the GaN well layers 511a and b, more specifically at the center thereof, is calculated. The wave function of the electrons in the subband changes depending on the position of the InN well layer 512 in the GaN well layers 511a and b, and the ISBT wavelength and transition probability can be controlled. In other words, the InN well layer 512 can be placed at positions other than the centers of the GaN well layers 51 la and b, and further transition wavelength control and wave function control can be performed depending on the position. For example, as shown in FIG. 5, a structure in which the InN well layer 512 is not sandwiched between GaN well layers is also possible. In addition, a plurality of InN well layers may be arranged between a pair of AlGaN layers.

[0046] FIG. 6 shows a theoretical prediction of the change of the wave function depending on the position of the InN well layer 512 in the InNZGaNZAIN superlattice. The figure shows the calculation result of the wave function (I Ψ ² I) generated in the conduction band and valence band potentials. Here, the GaN well layer 511 has a thickness of 8 molecules, and the A1 N layer has a thickness of 4 nm. It can be seen that the wave function in the subband in the conduction band and the wave function in the valence band vary greatly depending on the position of the InN well layer. The inter-subband transition wavelength changes depending on the position of the InN well layer, and the transition probability of one electron hole also changes.InN well layers are inserted and shifted from the center position compared to the superlattice. Insert It can be seen that the transition probability is greatly improved in the structure. In other words, it is predicted that the InNZGaNZAIN superlattice is also effective in dramatically improving the transition probability between the conduction band and the valence band.

As described above, the semiconductor element according to the present embodiment can shorten the ISBT wavelength, and can be manufactured easily by forming a distortion compensation structure.

Next, a manufacturing process of the semiconductor optical functional device according to the present embodiment will be described. The manufacturing process of the semiconductor optical functional device according to this embodiment includes at least the step of forming the buffer layer 3 on the substrate 2, the step of forming the cladding layer 4, the step of forming the superlattice layer 5, and the cap layer 6. Forming the process.

[0049] The step of forming the noffer layer 3 is not limited, but, for example, an organic metal growth method.

MBE or pulsed laser deposition can be used. Note that this step can be omitted when the substrate 1 is a GaN buttery or A1N buttery substrate or a substrate on which a GaN or A1N layer is formed.

[0050] The step of forming the clad layer 4 is not limited, and a metal organic vapor phase growth method, an MBE method, or a laser beam deposition method can be used. When formed by the MBE method, the substrate temperature varies depending on the material of the layer to be formed and can be adjusted as appropriate.For example, when GaN is used as the cladding layer 4, it may be 600 ° C or higher and 800 ° C or lower. I like it. In the case of forming by the MBE method, it is also preferable to perform organic cleaning and heat treatment before forming the cladding layer. The temperature of this heat treatment also varies depending on the material of the layer to be formed, but when a GaN layer is formed, a preferable range is 800 ° C. or higher and 1000 ° C. or lower.

[0051] The step of forming the superlattice layer 5 further includes the step of forming the AlGaN barrier layer 511, the step of forming the GaN well layer 512, the step of forming the InN well layer 513, and the spacer layer 52. The process of carrying out.

[0052] The step of forming the AlGaN barrier layers 52a and 52b is not limited, but a metal organic vapor phase epitaxy method, an MBE method, or a laser beam deposition method can be used. When forming by the MBE method, the conditions can be appropriately adjusted depending on the material and the like, but are not limited, but it is preferably 600 ° C or higher and 900 ° C or lower, but 600 ° C or higher and 700 ° C or lower. It is more preferable. [0053] The step of forming the GaN well layers 511a and b is not limited, and a metal organic vapor phase epitaxy method, an MBE method, and a laser beam deposition method can be used. When forming by the MBE method, the conditions can be appropriately adjusted depending on the material, etc., and are not limited, but it is preferably 600 ° C or higher and 900 ° C or lower, but 600 ° C or higher and 700 ° C or lower. Is more preferred.

[0054] The process of forming the InN well layer 512 is not limited, but an MBE method or a pulsed laser deposition method can be used. When forming by the MBE method, the conditions can be appropriately adjusted depending on the material and the like, but are not limited, but it is preferably 600 ° C or more and 700 ° C or less. The growth temperature of the InN well layer 512 according to this embodiment is 150 ° C. or more higher than the growth limit temperature of the In-polar InN single film. When the InN layer is very thin, with a thickness of less than 2 molecules, unlike Balta InN, it can be grown at higher temperatures. The process of forming the InN well layer 512 remains a problem in controlling the ultrathin film at the atomic and molecular level, but can also be performed by metal organic vapor phase epitaxy, which is generally used for the growth of nitride semiconductor devices.

[0055] The step of forming the spacer layer 54 is not limited, and a metal organic vapor phase growth method, an MBE method, and a laser laser deposition method can be used. When formed by the MBE method, the substrate temperature varies depending on the material of the layer to be formed and can be adjusted as appropriate.For example, when GaN is used as the cladding layer 4, it may be 600 ° C or higher and 800 ° C or lower. I like it.

[0056] The step of forming the cap layer 6 is not limited, but metal organic vapor phase epitaxy, MBE, and laser laser deposition can be used. When forming by the MBE method, the conditions can be adjusted as appropriate depending on the material, etc., and are not limited, but it is preferably 600 ° C or higher and 900 ° C or lower, and 600 ° C or higher and 700 ° C or lower. More preferably.

[0057] The semiconductor device according to the present embodiment can be manufactured through the above steps.

[Embodiment 2]

Although the present embodiment is different from the first embodiment in the superlattice layer 5, the configuration other than that is the same as that of the first embodiment. FIG. 7 shows a schematic cross-sectional view of the superlattice layer 5 of the semiconductor optical functional device according to the present embodiment. In the present embodiment, the configuration of the different superlattice layer 5 will be described, and the other configurations are the same, and thus the description thereof will be omitted. The superlattice layer 5 according to the present embodiment includes a well layer 51 having a GaN well layer 511a, b and an InN well layer, and a pair of AlGaN barrier layers 52a, b sandwiching the well layer 51. Force common to the point that the quantum well structure 53 has one period Unlike the first embodiment, the AlGaN barrier layer in the adjacent quantum well structure is formed in common without using the spacer layer 52. .

[0060] According to this embodiment, since one spacer is not used, the effect of greatly shortening the growth time can be achieved. Note that the semiconductor optical functional device in the present embodiment can be manufactured by omitting the step of forming the spacer layer.

As described above, a novel semiconductor optical functional device can also be provided by this embodiment.

[0062] (Embodiment 3)

The present embodiment is an embodiment of a quantum cascade laser element, and has the same configuration as that of the first embodiment except that means for injecting electrons into the superlattice layer 5 is provided. Figure 8 shows a schematic cross-sectional view of the quantum cascade laser device according to this embodiment. Here, only different configurations will be described, and the other configurations are the same configurations, and thus description thereof will be omitted.

As shown in FIG. 8, the quantum cascade laser device 11 according to this embodiment includes a buffer layer 3, a contact layer 7a, a cladding layer 4a, a superlattice layer 5, a cladding layer 4b, and a contact layer 7b on a substrate 2. Are sequentially laminated. The contact layers 7a and 7b are connected to the first electrode 8a and the second electrode 8b, respectively.

[0064] The contact layers 7a and 7b are layers that can ensure electrical connection with the first and second electrodes 8a and 8b, and are not limited to, but, for example, GaN, InGaN, or AlGaN. A layer in which Si or the like is implanted as an impurity can be preferably used.

[0065] The first and second electrodes 8a and b are not limited as long as they have electrical conductivity, but are composed of, for example, aluminum (A1), titanium (Ti), gold (Au), or the like. You can use electrode materials!

[0066] In the quantum cascade laser device according to the present embodiment, the electron injection layer and the light emitting layer are provided by changing the superlattice period and controlling the subband level position in the conduction band in the above configuration. Thus, laser emission can be performed by forming an inversion distribution between the subband levels. In addition, according to the quantum cascade laser element according to the present embodiment, the electrons that contributed to the light emission can contribute to the light emission again, which is suitable for high external quantum efficiency and high output. It is easy to control by changing the period of the laser, and it is possible to design the laser oscillation wavelength longer to a longer wavelength range (for example, terahertz range) without any material restrictions.

[0067] As described above, a novel quantum cascade laser device can also be provided by this embodiment.

Example

[0068] InNZGaNZAl N superlattice structures were actually created to confirm the effects of the semiconductor elements according to the above embodiments, and ISBT absorption was confirmed. This will be described below.

[0069] (Production of sample 1)

First, the structure of the InN well layer in the GaN well layer was fabricated and evaluated by the MBE method. In this sample, it was an experiment to confirm the formation of the InN layer, and the formation of the AlGaN barrier layer was omitted. Figure 9 shows a cross-sectional TEM image of a single-molecule InNZGaN superlattice structure grown at 600 ° C. According to this cross-sectional TEM image, it was found that a single-molecule-thick InN layer with steep interface roughness at the atomic layer level could be realized, and a 14-nm GaN layer could be realized. Also, no new defects were found here, and it was confirmed that a very high quality superlattice structure was formed. Such ultra-flat 'ultra-thin InN layer thickness control is realized by strict film-thickness' supply rate control. Using these technologies, we confirmed the possibility of producing a single-molecule-thick InNZGa NZAlGaN superlattice structure. did.

[0070] (Production of sample 2)

Next, we evaluated the formation of an InN well layer in a GaN well layer with a structure in which the growth temperature was raised from 600 ° C to 650 ° C. In this sample, the AlGaN barrier layer was not formed as an experiment to confirm the formation of the InN layer. Figure 10 shows a cross-sectional TEM image of a single-molecule InNZGaN superlattice structure grown at 650 ° C. It can be seen that in the sample of this example, the InN layer does not exist uniformly in a planar shape, but an InN layer in which InN having a thickness of about one molecule is dispersed in an island shape is formed. In other words, formation of fractional molecular InN was confirmed. Grows at 600 ° C Compared to the case of Sample 1, the formation of the InN well layer is thought to be due to the difference in growth temperature. Considering that the high temperature critical growth temperature of barium polar InN Balta is 500 ° C, it is considered that thermal decomposition of the In atom and InN and thermal desorption occur at high temperature of 650 ° C. . According to this cross-sectional TEM image, as in the case of FIG. 9, the generation of new defects in the superlattice region was not observed, and it was confirmed that a very high quality superlattice structure was formed. . From the above results, it is possible to introduce an ultra-thin InN well layer even under relatively high temperature growth conditions at 650 ° C! By using these techniques, an InNZGaNZAIN superlattice can be fabricated as in Sample 3. The possibility was confirmed.

[0071] (Production of Sample 3)

In this sample, a Ga-polar GaN film grown on C-plane sapphire by metalorganic vapor phase epitaxy was used as the substrate. This GaN substrate is organically cleaned, then introduced into an MBE device, heat-treated at 820 ° C, and a superlattice structure of InN (single molecule) ZGaN (lnm) / A1N (3nm) at 650 ° C. A1N layer 100 periods of growth were made common, and a Ga-polar GaN layer of 20 nm was formed as a cap layer thereon. Figure 11 shows a schematic diagram of this superlattice structure.

[0072] (X-ray diffraction measurement result of sample 3)

Here, Fig. 12 shows the X-ray diffraction measurement results of sample 3, grown at 650 ° C, InN (single molecule thickness) / GaN (lnm) / A1N (3 nm) superlattice structure. For comparison, the results of the GaN (lnm) / Al N (3 nm) superlattice structure formed under the same conditions as in sample 3 except that InN (one molecular thickness) is inserted for comparison are also shown. I will show you. When the two structures are compared, a satellite peak indicating the formation of a periodic structure can be clearly seen. Here, it can be seen that since the full width at half maximum of the satellite peak is very narrow, a steep interface is formed. Compared to the sample that had been introduced with InN (single molecule thickness), the introduced sample had a stronger satellite peak. This is thought to be due to the improved interface flatness and crystallinity due to the strain compensation effect of introducing the InN well layer (single molecule thickness).

[0073] (Absorption spectrum measurement result)

FIG. 13 is a diagram showing an absorption spectrum of p-polarized light incident on the surface of Sample 3 at an angle of 60 degrees. For comparison, here is also shown the absorption spectrum of the superlattice structure with InN (one molecular thickness) inserted! /, N! / ヽ GaN (lnm) / A1N (3 nm). two Clear ISBT absorption could be observed from the structure. Specifically, in the GaNZAIN superlattice structure, the ISBT absorption wavelength is observed at 1. 48 m. InN (single molecule thickness) ZGaN (lnm) ZAlN (3 nm) superlattice with an InN insertion of around 1.49 μm Successfully confirmed ISBT absorption. This result shows that the InN (single molecule thickness) ZGaNZAIN superlattice structure is sufficiently applicable to ultrafast optical switches and quantum cascade lasers using the ISBT process. In this case, the wavelength is shorter than that of the GaN (lnm) ZAlN (3 nm) superlattice, and the actual thickness of each superlattice is thicker than the design value. There seems to be a problem with layer control (position of single molecule InN, etc.). By optimizing the growth conditions such as the supply rate of the InN layer and improving the single-molecule-thickness InN control technique technology, it is considered possible to reduce the wavelength.

[0074] As described above, according to this example, it can be confirmed that the effect of the semiconductor optical functional device according to the above embodiment can be achieved, and a GaNZA IN superlattice ISBT in which a single-molecule-thick InN layer is introduced. A structure can be realized.

Industrial applicability

As described above, the present invention can be industrially used as a semiconductor optical functional device, an ultrafast semiconductor optical switch, and a quantum cascade laser.

Brief Description of Drawings

FIG. 1 is a schematic sectional view of a semiconductor device according to a first embodiment.

2 is a schematic cross-sectional view of a superlattice layer in a semiconductor device according to Embodiment 1. FIG.

FIG. 3 is an energy band diagram of the semiconductor element according to the first embodiment.

FIG. 4 is a graph showing the dependence of inter-subband transition energy on the well layer thickness of the semiconductor optical functional device according to Embodiment 1 obtained by theoretical calculation.

FIG. 5 is a schematic cross-sectional view of another example superlattice structure in the semiconductor element according to Embodiment 1.

FIG. 6 is a schematic cross-sectional view of a superlattice layer in a semiconductor element according to Embodiment 2.

FIG. 7 is a schematic cross-sectional view of a superlattice layer in a quantum cascade laser device according to a third embodiment.

[Fig. 8] Cross-sectional TEM image of sample 1 superlattice structure (600 ° C) in the example (drawing) Substitute).

FIG. 9 is a cross-sectional TEM image (drawing substitute) of the superlattice structure (650 ° C.) in the semiconductor optical functional device of Sample 2 in the example.

FIG. 10 is a schematic sectional view of a semiconductor optical functional device of Sample 3 in an example.

FIG. 11 is a diagram showing an X-ray diffraction measurement result of the superlattice structure in the semiconductor optical functional device of Sample 3.

FIG. 12 is a diagram showing a result of an X-ray diffraction measurement of a comparative example for sample 3.

FIG. 13 is a diagram showing an absorption spectrum by p-polarized light incident at an angle of 60 degrees with respect to the surface of the semiconductor element of sample 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor device, 2 ... Board | substrate, 3 ... Buffer layer, 4, 4a, 4b ... Cladding layer, 5 ... Superlattice layer, ₆ ... Cap layer, ₇ , _7a , 7b ... Contact layer, 8 ... Electrode 8a ... first electrode, 8b ... second electrode, 51 ... well layer, 52, 52a, 52b- "AlGaN barrier layer, 53 ... quantum well structure, 54 ... spacer layer 511, 511a, 511b- "GaN well layer, 512 ... 512ηΝ well layer

Claims

The scope of the claims

[I] a well layer having a GaN well layer and an InN well layer;

A semiconductor device comprising a superlattice layer having a pair of AlGaN barrier layers sandwiching the well layer.

[2] The semiconductor device according to [1], wherein the InN well layer in the well layer has a thickness of 2 molecules or less.

[3] The semiconductor device according to [1], wherein the thickness of the InN well layer in the well layer is 0.6 nm or less.

4. The semiconductor device according to claim 1, wherein the well layer has a plurality of GaN well layers, and the InN well layer is sandwiched between the plurality of GaN well layers.

5. The semiconductor element according to claim 1, wherein the number of well layers includes the same number of GaN well layers and InN well layers.

[6] The well layer and the pair of AlGaN barrier layers sandwiching the well layer have a single-period quantum well structure, and the plurality of quantum well structures are stacked in common with the AlGaN barrier layer in the quantum well structure. The semiconductor device according to claim 1.

7. The semiconductor device according to claim 1, wherein the well layer and a pair of AlGaN barrier layers sandwiching the well layer have a one-period quantum well structure, and a plurality of the quantum well structures are stacked via a spacer layer. Semiconductor element.

[8] Conduction band by wave function control For the purpose of increasing the recombination probability (transition probability) of electron holes between valence bands and controlling the emission wavelength, the well layer having a GaN well layer and an InN well layer, A semiconductor element having a stacked structure formed of a pair of AlGaN barrier layers sandwiching a well layer as an active layer.

[9] An optical switching element in which a substrate, a nofer layer, a cladding layer, a superlattice layer, and a cap layer are laminated in order,

The superlattice layer is an optical switching element having a well layer having a GaN well layer and an InN well layer, and a pair of AlGaN barrier layers sandwiching the well layer.

10. The semiconductor device according to claim 1, wherein the well layer has subbands, and an optical wavelength corresponding to energy in the subband interval is 2 m or less.

[II] The well layer has a subband, and the light absorption transition and the electric current in the subband interval 2. The optical semiconductor device according to claim 1, wherein a fast relaxation process via a phonon interaction is utilized.

[12] A quantum cascade laser device having a substrate, a nofer layer, a first contact layer, a first cladding layer, a superlattice layer, a second cladding layer, and a second contact layer,

The quantum lattice laser element, wherein the superlattice layer has a well layer having a GaN well layer and an InN well layer, and a pair of AlGaN barrier layers sandwiching the well layer.

[13] The well layer has subbands, and the superlattice layer functions as an electron injection layer and a light emitting layer by adjusting the period of the well layer and the AlGaN barrier layer of the superlattice layer. The semiconductor device according to claim 1.