JP5662494B2 - Photonic crystal device - Google Patents

Description

  The present invention relates to a device such as a laser oscillation element in which a photonic crystal is used for a resonator.

  In recent years, attempts have been made to reduce the power consumption of the entire CMOS chip by integrating a photonic crystal laser with low power consumption on a CMOS chip and realizing optical communication on the chip. In a photonic crystal laser, laser oscillation is performed by combining a photonic crystal technique capable of confining light and a gain of light by a compound semiconductor.

  For laser oscillation, it is necessary to drive the laser by applying energy to the element. In general, it is known that energy is given by light and current in a semiconductor laser. However, since a laser integrated on a chip is driven by an electronic circuit, a laser driven by current is required. Conventionally, various photonic crystal lasers realized by current driving have been reported. For example, in Non-Patent Documents 1 to 3, trapezoidal p-type and n-type doping regions are formed in a direction from the resonator toward the wafer surface, and a pn junction formed by the configuration of these doping regions is formed by the resonator. Has been. The photonic crystal lasers of Non-Patent Documents 1 to 3 generate a pair of electrons and holes by injecting a current into the pn junction, and oscillate.

C. M. Long et al., Lateral current injection photonic crystal membrane light emitting diodes, J. Vac. Sci. Technol. B, Vol. 28, No. 2, pp. 359 -364, 2010 B. Ellis et al., Ultralow-threshold electrically pumped quantum dot photonic-crystal nanocavity laser, Nature Photonics, Vol. 5, pp. 297 -300, 2011 S. Matsuo et al., Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser, Optics Express, Vol. 20, pp. 3773 -3780, 2012

  In the conventional photonic crystal laser, there is a leakage current that flows outside the active layer particularly in a high bias state. This leakage current is disadvantageous in that it is small under a low bias condition near the threshold value but increases under a high bias condition.

  In order to realize optical communication such as on-chip optical communication, it is necessary to suppress a leakage component of current and to flow current efficiently to the active layer.

  The present invention has been made in the above situation, and an object thereof is to provide a photonic crystal device that can suppress a leakage component of current and improve the efficiency of laser driving.

A photonic crystal device for achieving the above object includes a photonic crystal in which air holes are regularly arranged in a semiconductor slab, and a line defect waveguide formed in a part of the photonic crystal. An active layer formed in a part of the line defect waveguide; and an electrode structure for applying a bias to the active layer in a plane direction different from the extending direction of the line defect waveguide ; And a groove structure formed in both side regions of the active layer so as to be within the range of the line defect waveguide and the width of the line defect waveguide .

  The depth, width, or length of the groove structure may be different. For example, a structure that penetrates the slab or digs a groove to the center of the slab is conceivable. The width of the groove structure may be wider or narrower than the line defect waveguide. The length of the groove structure may be changed so that the gap between the groove structure and the active layer ranges from 0 to a value several times the photonic crystal lattice constant.

  According to the present invention, the leakage component of current can be suppressed and the efficiency of laser driving can be improved.

It is a figure which shows the structure of the conventional photonic crystal device. In the conventional photonic crystal device, it is a figure for demonstrating the density distribution of an electron and a hole in case a bias voltage is 3V. It is a figure which shows the structural example of the photonic crystal device of 1st Embodiment. In the photonic crystal device of 1st Embodiment, it is a figure for demonstrating the density distribution of an electron and a hole in case a bias voltage is 3V. It is a figure for comparing the mode distribution of light in a 1st embodiment with the mode distribution of a conventional example. It is a figure for demonstrating the photonic band structure of the groove part in 1st Embodiment compared with the waveguide structure of a prior art example. It is a figure which shows an example of the relationship between the width | variety of a groove | channel, and Q value in the groove part in 1st Embodiment. It is explanatory drawing for demonstrating the manufacturing method of the photonic crystal device of 1st Embodiment. In the photonic crystal device of a 1st embodiment, it is a figure showing an example of relation between injection current and output power. It is a figure which shows the structural example of the photonic crystal device of 2nd Embodiment. It is a figure which shows the structural example of the photonic crystal device of 3rd Embodiment.

  Hereinafter, a plurality of embodiments of the present invention will be described. The photonic crystal device according to each embodiment includes a photonic crystal in which air and semiconductor materials, which are media having different refractive indexes, are regularly arranged, and outputs light using the photonic crystal. This is a semiconductor laser.

<Conventional example>
First, the configuration of a conventional photonic crystal device 100 will be described with reference to FIG. FIG. 1 is a perspective view showing a configuration example of a conventional photonic crystal device 100.

  As shown in FIG. 1, a conventional photonic crystal device 100 is formed on an InP substrate 101 as a whole, and a sacrificial layer 102 and a buried growth layer 103 are sequentially formed on the substrate 101.

  The sacrificial layer 102 has an air gap structure 201.

  The buried growth layer 103 includes a two-dimensional photonic crystal slab having a plurality of air holes in the xz plane, and an active layer 106 embedded in an InP thin film is formed in the photonic crystal. Yes. In FIG. 1, the y direction represents the normal direction of the xz plane.

  The n-type region R1 is formed by doping the embedded growth layer 103 with a group IV or group VI element, and the p-type region R2 is formed by doping the buried growth layer 103 with a group II or group IV element. . The region R1 and the region R2 have a structure in which electrons and holes can be injected from the n-type electrode 104 and the p-type electrode 105, respectively.

  For example, the defect region 107 is formed as an optical waveguide in which defects are formed linearly along the z-axis direction and light passes therethrough. In the defect region 15, the above-described active layer 106 is formed.

  In the photonic crystal device 100, a leakage current (indicated by broken arrows in the left and right directions in FIG. 1) flowing outside the resonator is shown.

  2A and 2B are diagrams for explaining the concentration distribution of electrons and holes when the bias voltage is 3 V in the photonic crystal device 100, where FIG. 2A is an electron concentration distribution, and FIG. The distribution of pore concentration is shown.

  2 (a) and 2 (b) show an example in which the density level for indicating the density corresponding to the density is displayed between “0” and “2” as the density becomes higher.

In FIG. 2A and FIG. 2B, the highest concentration is a white portion (lightness level of “0”) corresponding to the position of the active layer 106 shown in FIG. The concentration is 2 × 10 ¹⁸ cm ⁻³ or more.

  On the other hand, in the example shown in FIG. 2A and FIG. 2B, the n-type region R1 and the p-type region R2 are doped in the region outside the resonator where the leakage current flows despite the doping. Also has some electron and hole concentrations.

<First Embodiment>
The conventional example has been described above with reference to FIGS. 1 and 2. However, in order to improve the laser driving efficiency, it is preferable to suppress the leakage current and lower the electron concentration and hole concentration in the aforementioned region, that is, the region outside the resonator where the leakage current flows.

  FIG. 3 is a diagram illustrating a configuration example of the photonic crystal device 10 in the present embodiment. This embodiment is a photonic crystal device 10 provided with a groove (groove structure) 17 in place of the defect region 107 shown in FIG.

  As shown in FIG. 3, the photonic crystal device 10 is formed on an InP substrate 11 as a whole, and a sacrificial layer 12 and a buried growth layer 13 are formed on the substrate 11 in this order. For example, an InP substrate is used as the substrate 11. In general, since the wavelength of light used for optical communication is in the 1.3 to 1.5 μm band, InP-based substrates having a band gap in the wavelength band are widely used.

  The sacrificial layer 12 is configured to have an air gap structure 21. For example, InGaAs is used as the material of the sacrificial layer 12.

  The buried growth layer 13 includes a two-dimensional photonic crystal slab having a plurality of air holes (photonic crystal holes). The active layer 16 is formed by embedding, for example, an InP thin film in the photonic crystal. Has been. The buried growth layer 13 is formed of a compound (III-V group mixed crystal) having a higher thermal conductivity than the core layer region including the active layer 16 and a larger band gap than the core layer region. For example, InP is used as the material of the buried growth layer 13.

  The active layer 16 has a quantum well structure and is formed of a compound (III-V mixed crystal) that excites light by carrier injection. For example, InGaAsP is used as the material of the active layer 16.

  In this embodiment, the plurality of air holes are regularly arranged at regular intervals in the xz plane. For example, the interval between adjacent air holes is adjusted to a size required by λ / N, where λ is the operating wavelength of the photonic crystal device and N is the refractive index of the buried growth layer 13. In the example of FIG. 3, the interval between adjacent air holes is set to 422 nm, for example, but may be changed as appropriate.

  The buried growth layer 13 has a groove portion 17 as a region where no air hole is formed. The groove portion 17 is formed as a region in which defects are formed linearly along, for example, the z-axis direction and current is not passed through.

  The depth of the groove portion 17 reaches, for example, from the surface of the buried growth layer 13 to a depth position adjacent to the buried heterostructure. The active layer 16 described above is formed in a region sandwiched between the groove portions 17.

  In FIG. 3, the n-type region R1 is formed by doping the embedded growth layer 13 with a group IV or group VI element, and the p-type region R2 is formed by doping the embedded growth layer 13 with a group II or group IV element. Formed by. The regions R1 and R2 have a structure in which electrons and holes can be injected from the n-type electrode 14 and the p-type electrode 15, respectively. In FIG. 3, the regions R1 and R2 are indicated by hatching.

  4A and 4B are diagrams for explaining the concentration distribution of electrons and holes when the bias voltage is 3 V in the photonic crystal device 10, wherein FIG. 4A is an electron concentration distribution, and FIG. The distribution of pore concentration is shown.

  Similar to those shown in FIGS. 2 (a) and 2 (b), in FIGS. 4 (a) and 4 (b), as the density increases, the light and shade levels for indicating the density corresponding thereto are set. It is displayed between “0” and “2”.

4 (a) and 4 (b), similarly to those shown in FIGS. 2 (a) and 2 (b), the concentration is highest at the position of the active layer 16 shown in FIG. Corresponding white portion (lightness level of “0”). That is, the white portion has a density of 2 × 10 ¹⁸ cm ⁻³ or more.

  On the other hand, in the example shown in FIGS. 4 (a) and 4 (b), the leakage current shown by the broken line in FIG. 1 flows in comparison with that shown in FIGS. 2 (a) and 2 (b). The electron concentration and hole concentration in the region that has been reduced are low.

  4 (a) and 4 (b), it was found that the carrier concentration in the active region increased by 20% compared to those in FIGS. 2 (a) and 2 (b). This means that at the same bias voltage (= 3 V), the efficiency of carrier injection into the active layer is higher in the photonic crystal device 10 of this embodiment than in the conventional example. In other words, the leak current is reduced in the photonic crystal device 10.

  5A and 5B are diagrams for explaining the magnetic field distribution of the fundamental mode in the photonic crystal device 10 in comparison with the conventional one, where FIG. 5A shows the configuration of the conventional photonic crystal device 100, and FIG. The magnetic field distribution in the fundamental mode of the conventional example, (c) shows the configuration of the photonic crystal device 10 shown in FIG. 3, and (d) shows the magnetic field distribution in the fundamental mode of this embodiment. 5A and 5C are top views of the photonic crystal device shown in FIGS. 1 and 3, respectively.

  The basic mode magnetic field distribution shown in FIGS. 5B and 5D is obtained by plotting magnetic field components calculated by the finite difference time domain method (FDTD method). The magnetic field distribution is a magnetic field distribution on the xz plane.

  FIG. 5 shows an example in which the lattice constant of the photonic crystal is 422 nm and the width of the groove 17 is 300 nm.

  In the case of the photonic crystal device 100 shown in FIG. 5A, the basic mode has a distribution as shown in FIG. In this case, the Q value indicating the strength of resonance was 3700.

  On the other hand, in the case of the photonic crystal device 10 shown in FIG. 5C, the basic mode has a distribution as shown in FIG. In this case, the Q value indicating the strength of resonance is 4500, which is larger than the conventional one. That is, in this embodiment, it was found that the optical confinement of the resonator is increased by about 21% compared to the conventional one. It was also found that current can flow efficiently through the buried heterostructure even under high bias conditions. In the example shown in FIG. 5D, non-patent literature (S. Matsuo et al., 20-Gbit / s directly modulated photonic crystal nanocavity laser with ultra-low power consumption, Optics Express, Vol. 19, No. 3, pp. 2242 -2250, 2011), a profile similar to the mode disclosed was obtained.

  In the photonic crystal device 100 shown in FIG. 5A, two waveguides are formed in parallel. These waveguides function as a directional coupler, and the output power of the output waveguide. It is thought that the variation of the will increase. This is because the influence that the output power of the output waveguide depends on the wavelength cannot be ignored.

  On the other hand, since only one waveguide is formed in the photonic crystal device 10 of this embodiment, the waveguide does not function as a directional coupler. In this respect, according to the photonic crystal device 10, since the variation in output power is reduced, the output efficiency is improved and the yield is improved.

  The structure of the groove portion 17 can be formed simultaneously with the etching of the photonic crystal hole, so that it is not necessary to add a manufacturing process for forming the groove portion 17.

  Next, the photonic band structure of the aforementioned groove portion 17 will be described with reference to FIG. FIG. 6 is a view for explaining the photonic band structure of the groove portion 17 in comparison with the waveguide structure of the conventional example.

  In FIG. 6, the solid line indicates the photonic band structure of the conventional waveguide 107 (see FIG. 1), and the broken line indicates the photonic band structure of the groove 17 having a width of, for example, 300 nm.

  FIG. 6 shows the relationship between the wave number calculated according to the two-dimensional plane wave expansion method and the normalized frequency as a photonic band structure.

  A region indicated by a vertical line in FIG. 6 represents a band gap (forbidden band) of the photonic crystal. In this band gap, if there is no defect in the waveguide or the like, there will be no photon. However, according to FIG. 6, there are two solid lines. It can be seen that the mode exists.

  On the other hand, according to FIG. 6, since there is no broken line in the band gap, it can be seen that there is no mode in the band gap in the photonic band structure of the groove portion 17. This clarified that the waveguide having the groove portion 17 has a photonic band gap and has an effect of confining light.

  FIG. 7 is a diagram illustrating an example of the relationship between the width of the groove portion 17 and the Q value. The Q value represents the resonance Q value of the laser of the photonic crystal device.

  The Q value takes a value as shown in FIG. 7 as the width of the groove portion 17 changes. As a result, even if the width of the groove portion 17 changes between 0 to 500 nm, the Q value may deteriorate. I knew it was n’t there. That is, the groove portion 17 functions as a resonator regardless of the width of the groove portion 17.

[Method of manufacturing photonic crystal device]
Next, a manufacturing method of the photonic crystal device 10 in the present embodiment will be described with reference to FIG.

  FIG. 8 is an explanatory diagram for explaining a manufacturing method of the photonic crystal device 10.

  First, as shown in FIG. 8A, an InGaAs sacrificial layer 12, an InP layer 131, a carrier confinement layer 162, an InGaAsP active layer 16, a carrier confinement layer 161, and an InP layer 132 are grown on the InP substrate 11 in order. Let The MOVPE method is used as an example of the crystal growth method.

  Next, as shown in FIG. 8B, the carrier confinement layer 162, the InGaAsP active layer 16, the carrier confinement layer 161, and the InP layer 132 are removed by lithography and etching, and the active layer 16 shown in FIG. A portion other than the portion forming the core layer region is exposed.

  Next, as shown in FIG. 8C, the InP layer 133 is grown again on both end faces of the carrier confinement layer 162, the InGaAsP active layer 16, the carrier confinement layer 161, and the InP layer 132. A buried heterostructure is formed by a butt joint growth method.

  Next, as illustrated in FIG. 8D, the InP layer 134 is grown over the top surfaces of the InP layers 132 and 133. An n-type region R1 and a p-type region R2 are formed by impurity diffusion and ion implantation.

  Next, as shown in FIG. 8E, a plurality of air holes 20 are formed by dry etching so as to penetrate the InP layers 134, 133, 131 and reach the vicinity of the middle of the InGaAs sacrificial layer 12. In the example of FIG. 8E, the plurality of air holes 20 are regularly arranged at a constant interval (for example, 422 nm) (FIG. 3). Further, the groove portion 17 shown in FIG. 3 is formed by lithography.

  Next, electrodes 14 and 15 are formed on the n-type region R1 and the p-type region R2, respectively. For this formation, for example, a vacuum evaporation method or a lift-off method is used. Then, the InGaAs sacrificial layer 12 is wet-etched using the plurality of air holes 20 to form an air gap 21 between the substrate 11 and the InP layer 131 as shown in FIG. In this way, the photonic crystal device 10 of this embodiment can be manufactured. In FIG. 3, InP layers 131, 132, 133, and 134 correspond to the buried growth layer 13 shown in FIGS.

  In FIG. 8, the lattice constant of the photonic crystal is 430 nm, the length of the buried heterostructure is 2.58 μm, and the thickness of the slab layer 12 by the air gap is 250 nm.

  The relationship between the injection current and the output power of the photonic crystal device 10 manufactured in this way will be described with reference to FIG.

  FIG. 9 is a diagram illustrating an example of the relationship between the injection current and the output power in the photonic crystal device 10.

  In FIG. 9, when the threshold current was 4.9 μA, it was observed that the signal was continuously transmitted at room temperature. That is, the photonic crystal device 10 including the groove portion 17 can continuously oscillate a laser with a small threshold. Furthermore, it has been found that the output light continues to increase to a current value corresponding to at least 40 times the threshold value. This is because the case of the photonic crystal device 10 of the present embodiment is compared with the case of Non-Patent Document 3 in which the output light starts to be saturated at an injection current corresponding to several times the threshold value. This means that the efficiency of current injection is improved under high device conditions.

  As described above, according to the photonic crystal device 10 of the present embodiment, the configuration of the groove portion 17 suppresses the current leakage component and improves the laser driving efficiency.

Second Embodiment
A second embodiment will be described with reference to FIG. FIG. 10 is a diagram illustrating a configuration example of the photonic crystal device 10A in the present embodiment.

  In FIG. 10, the photonic crystal device 10A is different from the first embodiment in that it has an n-type region R11 and a p-type region R21.

  That is, in the photonic crystal device 10A, as in the first embodiment, the InGaAs sacrificial layer 12 having the air gap 21 and the InP buried growth layer 13 having the plurality of air holes 20 are sequentially formed on the InP substrate 11. Has been. The arrangement and interval of the air holes 20 are the same as the arrangement and interval illustrated in FIG. The active layer 16 is embedded in the same manner as the example shown in FIG.

  On the other hand, unlike the first embodiment, the n-type region R11 and the p-type region R21 have different shapes from the regions R1 and R2 illustrated in FIG. In this case, there is an effect that the series resistance of the element can be reduced.

<Third Embodiment>
A third embodiment will be described with reference to FIG. FIG. 11 is a diagram illustrating a configuration example of the photonic crystal device 10B in the present embodiment.

  In FIG. 10, the photonic crystal device 10 </ b> B is different from the first embodiment in that the groove portions 18 and 19 have different lengths.

  That is, in the photonic crystal device 10B, an InGaAs sacrificial layer 12 having an air gap 21 and an InP buried growth layer 13 having a plurality of air holes 20 are sequentially formed on an InP substrate 11 as in the first embodiment. Has been. The arrangement and interval of the air holes 20 are the same as the arrangement and interval illustrated in FIG. The active layer 16 is embedded in the same manner as the example shown in FIG.

  On the other hand, unlike the first embodiment, the two groove portions 18 and 19 have different lengths. For example, the groove 18 is formed so as to have a depth corresponding to a position adjacent to the buried heterostructure, and the groove 19 is formed so as to have a length corresponding to a position where a part of the buried heterostructure is notched. Such a groove structure is formed by a method such as additional etching or focused ion beam (FIB) after etching the photonic crystal hole (FIG. 9E) and forming the air gap 21 (FIG. 9F). Can be formed. According to the photonic crystal device 10 </ b> B of FIG. 11, no undoped InP layer remains on the side of the active layer 16, and leakage current flowing on the side of the active layer 16 can be reduced.

<Modification>
The photonic crystal device that can suppress the leakage component of current and improve the efficiency of laser driving has been described above with reference to the first to third embodiments. However, the configuration of other shapes is applied. You can also

  For example, the groove portions 17, 18, and 19 may be formed to have different depths, widths, or lengths. For example, a structure that penetrates the slab or digs a groove to the center of the slab is conceivable. The widths of the groove portions 17 to 19 may be wider or narrower than the optical waveguide. The lengths of the groove portions 17 to 19 may be changed within a range in which the gap between the groove portion and the active layer 16 is from 0 to a value several times the photonic crystal lattice constant.

  In the photonic crystal device of each embodiment, the material of the active layer is InGaAsP. However, for example, InGaAs, InGaAlAs, or the like may be applied. Further, although the quantum well is used as the structure of the active layer, a bulk, a quantum wire, or a quantum dot may be applied. Moreover, although the material of the carrier confinement layer is InGaAsP, for example, InGaAs, InAlAs, GaAs, InP, or the like may be applied.

  Alternatively, although the material of the buried growth layer is InP, for example, InGaAsP, GaAs or the like may be applied. Although the material of the sacrificial layer is InGaAs, for example, InAlAs or the like may be applied. Further, although the case where the InP substrate 11 is used is described, for example, a GaAs, sapphire substrate, silicon substrate, or other semiconductor substrate can be applied.

  The space | interval or arrangement | sequence of an air hole is not restricted to each embodiment, It can change. Alternatively, the width, depth, and arrangement of the groove portions 17, 18, and 19 are not limited to each embodiment, and can be changed. For example, if the photonic crystal is formed in a range excluding a region where the fundamental mode electromagnetic distribution is strong, the grooves 17, 18, and 19 are formed in a predetermined region with respect to the waveguide direction of the optical waveguide. It may be.

  The photonic crystal device has been described by taking the case of a semiconductor laser as an example. However, for example, the photonic crystal device may be applied to a semiconductor optical switch, a modulator, a photodetector, or the like using a change in refractive index.

DESCRIPTION OF SYMBOLS 10 Photonic crystal device 11 Substrate 13 Embedded growth layer 14,15 Electrode 16 Active layer 17, 18, 19 Groove part

Claims (2)

A photonic crystal in which air holes are regularly arranged in a semiconductor slab;
A line defect waveguide formed in a part of the photonic crystal;
An active layer formed in a part of the line defect waveguide ;
An electrode structure for applying a bias to the active layer in a plane direction different from the extending direction of the line defect waveguide ;
A photonic crystal comprising a groove structure formed in both side regions of the active layer so as to be within the range of the line defect waveguide and the width of the line defect waveguide device. 2. The photonic crystal device according to claim 1 , wherein a gap between the groove structure and the active layer is formed in a range from 0 to a value several times the photonic crystal lattice constant. .