WO2016113935A1 - 深紫外led及びその製造方法 - Google Patents
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Definitions
- the present invention relates to AlGaN-based deep ultraviolet LED technology.
- FIG. 22 is a cross-sectional view showing an example of the structure of a general conventional deep ultraviolet LED.
- the light emitted from the quantum well layer 5 is emitted upward (in the air) through the barrier layer 4, the n-type AlGaN layer 3, the AlN buffer layer 2, and the sapphire substrate 1.
- some light is totally internally reflected by the refractive index difference between the n-type AlGaN layer 3, the AlN buffer layer 2, the sapphire substrate 1, and the air, and the Al (or Au) reflective electrode layer 11.
- most is absorbed by the p-type GaN contact layer 9 and the Ni layer 10 and disappears internally.
- the light emitted from the quantum well layer 5 and propagated downward is also absorbed by the p-type GaN contact layer 9 and the Ni layer 10, and most of the light is lost.
- Patent Document 1 it is disclosed that a concavo-convex structure is provided on the surface and side surfaces of a sapphire substrate to suppress internal total reflection and improve the light extraction efficiency by about 20%.
- the photonic crystal periodic structure is generally formed at the interface between two structures having different refractive indexes, and is generally an unevenness mainly composed of a pillar structure or a hole structure. It is known that total reflection is suppressed by prohibiting the presence of light in the region where the periodic structure is formed, and that this is used to contribute to improvement of light extraction efficiency (Patent Document 2). reference).
- Non-Patent Document 1 the p-type GaN contact layer that absorbs deep ultraviolet light is replaced with a transparent p-type AlGaN contact layer that is transparent to deep ultraviolet light, and the thickness of the Ni layer is as thin as about 1 nm. Thus, it has been reported that the light extraction efficiency has been improved by 1.7 times.
- JP 2014-68010 A Japanese Patent No. 5315513
- OPTRONICS 2014.2 NO. 386, issued on February 10, 2014, 56 (general review), improving the light extraction efficiency of AlGaN deep ultraviolet LEDs by making the device transparent, pp. 58-66.
- Patent Document 1 a part of the suppression of absorption of light propagating in the upper direction (substrate side) in FIG. 22 is improved.
- Non-Patent Document 1 Although the absorption of light propagated in the lower direction (reflecting electrode side) in FIG. 22 has been improved, the reflectance of the Ni (1 nm) / Al reflecting electrode is about 70%, The problem that light is absorbed a little has not been solved.
- An object of the present invention is to further suppress the absorption of light propagating in the vertical direction and further improve the light extraction efficiency in the deep ultraviolet LED.
- a deep ultraviolet LED having a design wavelength ⁇ , an Al reflective electrode layer, an ultrathin Ni layer (about 1 nm) for ohmic contact, and a wavelength ⁇
- a transparent p-type AlGaN contact layer in this order from the opposite side of the substrate in this order, at least in the thickness direction of the transparent p-type AlGaN contact layer, or from the transparent p-type AlGaN contact layer to the ultrathin film Ni
- a photonic crystal periodic structure provided in a range in a thickness direction including an interface with the Al reflective electrode layer including a layer, and the photonic crystal periodic structure has a photonic band gap
- a deep ultraviolet LED is provided.
- This photonic crystal periodic structure is a cylindrical hole (refractive index of 1.0) having a large refractive index difference with respect to a refractive index of 2.60 at a wavelength of 265 nm of the transparent p-type AlGaN contact layer, and has a photonic band gap.
- the TE light having the wavelength ⁇ is reflected, and the effect becomes prominent in proportion to the size of the photonic band gap.
- the closer the distance from the quantum well layer to the photonic crystal periodic structure the larger the solid angle and the more prominent the reflection effect.
- TM light passes through the photonic crystal periodic structure and passes through the ultra-thin Ni layer and the Al reflective electrode layer, but since the TM light does not open the photonic band gap, its transmittance is small and the Al reflective electrode layer has a low transmittance. Absorption is significantly suppressed. Therefore, the light propagating in the lower direction, that is, the reflection electrode layer can be reflected almost completely.
- the reflection structure can ignore the absorption of deep ultraviolet light
- various methods for improving the light extraction efficiency seen in blue LEDs and white LEDs and the effects thereof can be utilized to the maximum.
- an uneven structure such as a photonic crystal (PhC) is provided at the interface having a refractive index to suppress internal total reflection and improve the light extraction efficiency, or by removing the sapphire substrate and
- a deep ultraviolet LED manufacturing method in which a design wavelength is ⁇ , an Al reflective electrode layer, an ultrathin Ni layer, and a transparent p-type AlGaN contact layer are formed on a substrate.
- a step of preparing a laminated structure containing in this order from the opposite side, and the Al including the ultra-thin Ni layer at least in the thickness direction of the transparent p-type AlGaN contact layer or from the transparent p-type AlGaN contact layer For forming a photonic crystal periodic structure provided in a range in the thickness direction including the interface with the reflective electrode layer, or for forming an uneven structure such as a photonic crystal at an interface having a different refractive index, or For forming an uneven structure such as a photonic crystal on the light extraction surface of the semiconductor layer obtained by peeling the sapphire substrate, or encapsulating the entire LED element with resin.
- the light extraction efficiency of the deep ultraviolet LED can be dramatically improved.
- FIG. 1A It is sectional drawing which shows one structural example of deep-UV LED by the 1st Embodiment of this invention. It is sectional drawing which shows one structural example of deep-UV LED by the modification of FIG. 1A. It is an image figure which shows the mode of TE light and TM light which injected into the photonic crystal (hole). It is a figure which shows the relationship between PBG of TE light, and R / a. It is a figure which shows the relationship between PBG of TM light, and R / a. It is sectional drawing which shows the calculation model of conventional type deep ultraviolet LED. It is sectional drawing which shows the calculation model of deep-UV LED provided with the transparent p-type AlGaN contact layer.
- FIG. 4E It is a figure which shows the example which provided the 2nd photonic crystal periodic structure on the sapphire substrate of the structure of FIG. 4E.
- (A) is sectional drawing
- (b) is a top view.
- FIG. 1 It is a figure which shows the relationship between PBG of TM light and R / a in the 2nd photonic crystal periodic structure (pillar structure) by 2nd Embodiment.
- FIG. 14A is a cross-sectional view of a deep ultraviolet LED in which a photonic crystal is provided at two locations in a structure based on a transparent p-type AlGaN contact layer / ultra-thin Ni layer, and further provided with nano-PSS-derived AlN coupled pillars.
- b) is a plan view. It is a bird's-eye view of deep UV LED of FIG. 14A.
- Deep UV LED with a photonic crystal (hole) and nano-PSS-derived AlN-coupled pillar as the light extraction surface in a transparent p-type AlGaN contact layer / ultra-thin Ni layer base structure with a sapphire substrate peeled off and a support substrate attached
- FIG. Cross-sectional view of deep ultraviolet LED encapsulating resin after providing first and second photonic crystals on transparent p-type AlGaN contact layer / ultra-thin Ni layer base structure, and further providing nano-PSS-derived AlN bond pillar structure It is.
- FIG. 18B is a cross-sectional view of a deep ultraviolet LED in which a resin is sealed in the structure of FIG. 18A.
- FIG. 19B is a cross-sectional view of a deep ultraviolet LED encapsulating resin in the structure of FIG. 19A. It is a detailed view of a photonic crystal formation process using a two-layer resist. It is a figure which shows the cross-sectional SEM image in a photonic crystal formation process. It is sectional drawing which shows the structure of the conventional general deep ultraviolet LED.
- the structure of an AlGaN deep ultraviolet LED having a design wavelength ⁇ of 265 nm is shown in FIG.
- the AlGaN-based deep ultraviolet LED according to the present embodiment includes, in order from the top of the figure, a sapphire substrate 1, an AlN buffer layer 2, an n-type AlGaN layer 3, a barrier layer 4, and a quantum well.
- the photonic crystal periodic structure 100 is provided in the range of the thickness direction of the transparent p-type AlGaN layer 8a, and the photonic crystal periodic structure 100 is a circular hole (columnar structure, hole) 101 (h ) And having a photonic band gap, it is a reflective photonic crystal periodic structure that reflects light of wavelength ⁇ .
- the reflective photonic crystal periodic structure 100 has a cylindrical shape or the like and has a refractive index smaller than that of the transparent p-type AlGaN contact layer 8a.
- a columnar structure 101 (h) having a cross-section of a circle with a radius of R has a hole structure formed in a triangular lattice shape with a period a along the x and y directions.
- the columnar structure 101 (h) has a structure that does not reach the interface between the transparent p-type AlGaN contact layer 8 a and the electron block layer 7. This is because if the photonic crystal periodic structure 100 is not left about 50 nm, the electron block layer 7 may be damaged by dry etching.
- the photonic crystal periodic structure 100 has a photonic band gap, and a transparent p-type AlGaN contact layer 8a having different refractive indexes and air are formed as two structures at the bottom surface.
- the dielectric constants ⁇ 1 and ⁇ 2 of each structure corresponding to R / a which is the ratio of the radius R of the circular hole to the period a, the design wavelength ⁇ , and the refractive indexes n 1 and n 2 of the two structures are given as follows.
- the photonic band structure of TE light and TM light is analyzed using the plane wave expansion method. Specifically, it is input to Maxwell's wave equation expressed by the following equations (2) and (3), and its eigenvalue is calculated.
- E ′
- ⁇ relative dielectric constant
- G reciprocal lattice vector
- k wave number
- ⁇ frequency
- c speed of light
- E electric field.
- a first photonic band (a photonic band gap can be confirmed by obtaining a photonic band structure of TE light in a range of 0.20 ⁇ R / a ⁇ 0.40 in steps of 0.01 with R / a as a variable.
- the photonic band gap between 1 st PB) and the second photonic band (2 nd PB) is PBG1
- the photonic band between the seventh photonic band (7 th PB) and the eighth photonic band (8 th PB) With the gap as PBG4, the relationship between each PBG and R / a is obtained. The result is shown in FIG. 3A.
- the photonic band structure of TM light is obtained, the PBG between 1 st PB and 2 nd PB is PBG1, the PBG between 3 rd PB and 4 th PB is PBG2, and the PBG between 5 th PB and 6 th PB is PBG3.
- a steep change in the density of states in the vicinity of the photonic band gap and a sharp peak in other frequency regions are due to the group velocity being zero.
- the representative symmetry point at which the group velocity becomes zero is a standing wave by changing the propagation direction of light by the diffraction of Bragg at the M point.
- the steep change rate of the state density is almost proportional to the size of the photonic band gap.
- the relationship between the size of the photonic band gap and the reflection / transmission effect, and the light extraction efficiency (LEE) increase / decrease rate in the deep ultraviolet LED are obtained by analysis by the FDTD method, and the diameter of the photonic crystal that maximizes the LEE increase / decrease rate d, the period a and the depth h are obtained.
- Step S01 A ratio (R / a) between the period a, which is a periodic structure parameter, and the radius R of the structure is provisionally determined.
- Step S02 The average refractive index n av is calculated from the respective refractive indexes n 1 and n 2 of the first structure and these and R / a, and this is substituted into the Bragg condition equation, and the period a and radius for each order m Get R.
- Step S03 The photonic band structure of TE light is analyzed by a plane wave expansion method using dielectric constants ⁇ 1 and ⁇ 2 of each structure obtained from R / a, wavelength ⁇ , and refractive indexes n 1 and n 2 .
- Step S04 The R / a at which the PBG between the first photonic band and the second photonic band of TE light is maximized is determined by repeated analysis by changing the value of R / a of the provisional determination.
- the wavelength ⁇ is obtained by simulation analysis using the FDTD method in which the individual period a and radius R according to the order m of the Bragg condition and the depth h of an arbitrary periodic structure are used as variables. Calculate the light extraction efficiency for.
- Step S06 By repeatedly performing the simulation by the FDTD method, the order m of the Bragg condition that maximizes the light extraction efficiency with respect to the wavelength ⁇ , and the period a, the radius R, and the depth h of the periodic structure parameter corresponding to the order m are determined. To do.
- the depth has a depth h that is greater than or equal to the period a as shown in FIG.
- R / a is changed as a variable in 0.01 steps to obtain the diameter d and period a at each R / a, and the photonic crystal is designed.
- the calculation model of Table 1 was created and the LEE increase / decrease rate was determined by the FDTD method.
- output 1 is an output in a structure without a photonic crystal (Flat structure)
- output 2 is an output in a structure having a photonic crystal periodic structure, and the output is obtained by a radiation pattern (far solution).
- the LEE increase / decrease rate was compared between the output of the entire LED element and the output in the axial direction (angle 5 ° to 20 °) (see Table 2).
- a near-field monitor was installed at the interface between the Al reflective electrode in Table 1 and the transparent p-type AlGaN contact layer. The purpose is to detect light leaking without being completely reflected by the photonic crystal. Then, the output of the photonic crystal structure corresponding to each R / a with respect to a structure without a photonic crystal (Flat structure) was compared and obtained by an increase / decrease rate. (See FIG. 24) As the R / a increases, the rate of increase or decrease of light leaked without being reflected by the photonic crystal decreases. This can suppress light absorption / disappearance in the Al reflective electrode. As a result, LEE increases as R / a increases.
- the calculation model analyzed by the FDTD method has a design wavelength of 265 nm and a degree of polarization of 0.07. Examples of specific structures are shown in FIGS. 4A to 4F. Table 3 shows the thickness of each structure used.
- FIG. 4A is a diagram showing a specific example of the LED having the conventional structure shown in FIG. 22, and in order from the top of the figure, the sapphire substrate 1, the AlN buffer layer 2, the n-type AlGaN layer 3, the barrier layer 4, the quantum The well layer 5, the barrier layer 6, the electron block layer 7, the p-type AlGaN layer 8, the p-type GaN contact layer 9, the Ni layer 10, and the Al reflective electrode 11.
- FIG. 4B shows a structure in which a transparent p-type AlGaN contact layer 8a that does not absorb deep ultraviolet light is provided.
- the structure from the sapphire substrate 1 to the electron block layer 7 is the same as that in FIG. 4A.
- the transparent p-type AlGaN contact layer 8a and the Al reflective electrode 11 are formed.
- the ultra-thin Ni layer (1 nm) was omitted due to the limitation of calculation resources.
- the decrease in output per 1 nm of Ni layer thickness which was obtained by another analysis for reference, was 7%.
- FIG. 4C shows the same structure as FIG. 4B from the sapphire substrate 1 to the transparent p-type AlGaN contact layer 8a in order to estimate the reduction in output due to absorption when the Ni layer 10 is as thick as 10 nm.
- the underlying structure is a Ni layer 10 and an Al reflective electrode 11.
- the circular hole 101 (h) is located from the p-type AlGaN layer 8 to the interface between the p-type GaN contact layer 9 and the Ni layer 10 and has a depth of 300 nm.
- the circular hole 101 (h) is located from the transparent p-type AlGaN contact layer 8a to the interface of the Al reflective electrode 11, and has a depth of 300 nm.
- the circular hole 101 (h) is located at the interface between the transparent p-type AlGaN contact layer 8a and the transparent p-type AlGaN contact layer 8a and the Ni layer 10 and has a depth of 300 nm.
- the output value was obtained in the far field for each of the above structures.
- the output magnification of the output value in the other new structure was obtained with respect to the output value in the conventional structure of FIG. 4A. Furthermore, due to the limitation of calculation resources, regarding the output magnification of the structure of FIG. 4B and the structure of FIG. 4E in which the output magnification is calculated by omitting the ultrathin Ni layer of 1 nm, the output decrease in the ultrathin Ni layer of 1 nm in another analysis is 7%. The value corrected by subtracting the magnification corresponding to is shown. Then, the light extraction efficiency (LEE) of the conventional structure of FIG. 4A was set to 10%, and LEEs of other structures were obtained by multiplying by the correction magnification (see Table 4).
- LEE light extraction efficiency
- the LEE is 1.69 times, and a value substantially equal to 1.7 times described in Non-Patent Document 1 is obtained.
- 4D a structure in which PhC is added to the conventional structure
- FIG. 4E a structure in which PhC is added to the transparent p-type AlGaN contact layer
- FIG. 4F PhC is added to the Ni layer 10 nm and the transparent p-type AlGaN contact layer.
- the TM light is slightly transmitted through PhC and absorbed in the p-type GaN contact layer and the Ni layer 10 nm as shown in FIG. 4F, the reflection effect by the photonic crystal periodic structure is not perfect. I understand.
- the structure of FIG. 4E in which a photonic crystal periodic structure is provided in the thickness direction of the transparent p-type AlGaN contact layer based on the transparent p-type AlGaN contact layer and the ultrathin Ni layer, is unique to deep ultraviolet LEDs. Output reduction due to the absorption of deep UV light can be almost completely suppressed.
- FIG. 4E is suitable as a base structure (template) for devising the structure for improving the light extraction efficiency described in various embodiments described later.
- the absorption of light propagating in the vertical direction can be suppressed in the deep ultraviolet LED, and the light extraction efficiency can be improved by 5 times or more compared to the conventional structure.
- the deep ultraviolet LED according to the second embodiment of the present invention has irregularities on the other light extraction surface in addition to the reflective photonic crystal periodic structure provided in the transparent p-type AlGaN contact layer in the first embodiment.
- a structure and a photonic crystal are provided to further improve the light extraction efficiency.
- FIG. 5 is a cross-sectional view showing an example of such a structure.
- the second photonic crystal periodic structure 110 is provided in addition to the first photonic crystal periodic structure 100 located in the range from the transparent p-type AlGaN contact layer 8 a to the interface of the Al reflective electrode 11.
- the second photonic crystal periodic structure 110 is provided on the back surface of the sapphire substrate 1, and the second photonic crystal periodic structure 110 transmits light having a wavelength ⁇ by having a photonic band gap. It is a transmissive photonic crystal periodic structure.
- the second photonic crystal periodic structure 110 includes a columnar structure 111 such as sapphire having a refractive index larger than that of the surrounding air. It is a perforated (pillar) structure formed in a triangular lattice shape with a period a along the x direction and the y direction.
- a columnar structure 111 such as sapphire having a refractive index larger than that of the surrounding air. It is a perforated (pillar) structure formed in a triangular lattice shape with a period a along the x direction and the y direction.
- FIG. 6 is an image diagram showing a state of TM light incident on a photonic crystal (pillar).
- the electric field of TM light tends to stay in a dielectric spot that exists perpendicularly between pillar structure rods (pillars) 111 (p), and the average refractive index n av , period a, and design wavelength ⁇ are Bragg conditions. If the above condition is satisfied, it can be understood that the electric field surface is scattered by Bragg diffraction, that is, TM light is transmitted to the periodic structure surface in the present embodiment.
- TM light An effective way to know the physical properties of photonic crystals by TM light is to obtain and analyze a photonic band (PB) structure from the plane wave expansion method.
- PB photonic band
- the eigenvalue equation of TM light is derived from the Maxwell equation as follows.
- E ′
- ⁇ relative dielectric constant
- G reciprocal lattice vector
- k wave number
- ⁇ frequency
- c speed of light
- E electric field.
- PB photonic band
- the ratio of the period a to the radius R (R / a) in the photonic crystal periodic structure 110 is a value determined so as to improve the light transmission effect based on the photonic band of TM light.
- FIG. 8B is a flowchart showing a processing example of a calculation simulation for determining the photonic crystal periodic structure 110 according to the second embodiment of the present invention.
- Step S1 In step S1, R / a (R: radius, a: period) is changed, for example, in 0.01 steps within a range of 0.20 ⁇ R / a ⁇ 0.40.
- Step S2 Since the scattered wave that satisfies the Bragg condition corresponds to one of the photonic bands (PB), the period a that transmits the design wavelength ⁇ is related by the Bragg equation.
- the focused photonic band is a scattered wave (k + G) that satisfies the Bragg condition.
- n av 1.435.
- FIG. 9 is a diagram showing the relationship between the PBG of TM light and R / a in the second photonic crystal structure (pillar structure) 110.
- the photonic band gap (PBG) between 1 st PB-2 nd PB and 3 rd PB-4 th PB is defined as PBG1 and PBG2, respectively, and the relationship between R / a and PBG is shown in FIG.
- ⁇ PhC is the wavelength in the photonic crystal (PhC).
- the reason for selecting the second photonic band (2 nd PB) and the fourth photonic band (4 th PB) is that PBG1 and PBG2 are 0.20 ⁇ R / a ⁇ 0 as shown in FIG. This is because the second photonic band (2 nd PB) and the fourth photonic band (4 th PB) generate standing waves at each symmetry point, and then change the light propagation direction.
- FIGS. 10A and 10B These principles will be described with reference to FIGS. 10A and 10B.
- the second photonic band (2 nd PB) results in a standing wave at each point of symmetry R / a, is that the R / a of closest or meet at a wavelength 265nm and a point in a vacuum.
- FIG. 12A shows a photonic band structure of wavelength ⁇ 3 (order) and wave number in vacuum related to the second photonic band (2 nd PB).
- a standing wave does not occur at any R / a where 0.20 ⁇ R / a ⁇ 0.40.
- the phase is increased in proportion to the order, and the phase becomes the same at R / a, and a standing wave is generated.
- There is a standing wave with 5 and 6 antinodes in a period length of m 3.
- the fourth photonic band (4 th PB) of all R / a obtained in step S4 is obtained.
- FIG. 12B Multiplied by 5 integers is shown in FIG. 12B, and multiplied by 6 integers is shown in FIG. 12C.
- FIG. 13A shows a photonic band structure of wavelength and wave number in vacuum for the second photonic band (2 nd PB).
- FDTD method finite time domain difference method
- Step S8 Compare the output of the entire LED element and the output in the axial direction (angle 5 ° to 20 °), and distribute light in the axial direction from R / a and order m, which have a large increase / decrease rate of light extraction efficiency (LEE). R / a and order m excellent in properties are selected. Therefore, the diameter, period, and depth, which are parameters for optimizing the photonic crystal, are determined.
- step S7 The calculation result of step S7 is shown in Table 11 and FIG.
- Po (W) in Table 11 indicates the output of the entire LED element
- Po ( ⁇ 20 °) indicates the output in the axial direction.
- FIG. 25 is a graph showing the angle distribution of the output and the light distribution of the LED. From the above results, since the LEE and the axial output of the photonic crystal corresponding to each R / a show high values, the above optimization method is appropriate.
- the deep ultraviolet LED according to the present embodiment includes a reflective photonic crystal structure (first photonic crystal periodic structure) provided on a transparent p-type AlGaN contact layer and a transmissive photonic crystal periodic structure provided on the back surface of a sapphire substrate ( In addition to the second photonic crystal periodic structure), a periodic structure (uneven structure) described below is added to improve the light extraction efficiency.
- FIG. 14A is a cross-sectional view showing a configuration example of the deep ultraviolet LED according to the present embodiment
- FIG. 14B is a perspective view (bird's eye view) thereof.
- the nano-PSS and the coupled pillar periodic structure 220 are truncated cone structures formed in a triangular lattice shape with a period a along the x and y directions. It is.
- the surface of the sapphire substrate 1 (the lower surface in FIG. 14A) has, for example, a nano PSS (Patterned Sapphire Substrate) periodic structure (triangular pyramid shape or conical hole) 220a having a period of about 1 ⁇ m.
- Such a concave structure can be formed by processing the surface by a wet etching method using a mask pattern such as a photoresist formed on the surface of the sapphire substrate 1.
- an AlN film is epitaxially grown by about several ⁇ m in the nano-PSS periodic structure 220a using the CVD method or the like. Then, the concave structure is filled with the AlN film, and an AlN-coupled pillar 220b having a hexagonal frustum made of AlN is selectively formed in the thickness direction thereon. Ultimately, it becomes a flat epi film.
- the crystallinity of the quantum well layer 5 is higher than that of the conventional case. And the internal quantum efficiency (IQE) of the deep ultraviolet LED is improved.
- the deep ultraviolet light emitted from the quantum well layer 5 propagates through the formed hexagonal frustum AlN coupling pillar 220b as a waveguide and enters the sapphire substrate 1, Total internal reflection at the interface between the sapphire substrate 1 and the nano-PSS periodic structure 220a is suppressed, and the light extraction efficiency is improved.
- a calculation model to be analyzed by the FDTD method has a design wavelength of 265 nm and a polarization degree of 0.07. Specific structural examples are shown in FIGS. 15A to 15C, respectively.
- Table 5 shows the film thickness of each structure.
- the diameter / period / depth of the photonic crystal (pillar) provided on the back surface of the sapphire substrate was 258 nm / 369 nm / 300 nm.
- FIG. 15A shows a structure in which a transparent p-type AlGaN contact layer 8a that does not absorb deep ultraviolet light is provided, and a photonic crystal (pillar) periodic structure formed on the back surface of the sapphire substrate 1 in order from the top of the drawing ( (Second photonic crystal periodic structure) 110, sapphire substrate 1, nano-PSS (triangular pyramid-shaped) periodic structure 220a formed on the surface of sapphire substrate 1, AlN coupled pillar 220b, n-type AlGaN layer 3, barrier layer 4, The quantum well layer 5, the barrier layer 6, the electron blocking layer 7, the transparent p-type AlGaN contact layer 8 a, the photonic crystal (hole) periodic structure (first photonic crystal periodic structure) 100, and the Al reflective electrode 11.
- the calculation was performed by omitting the ultrathin Ni layer (1 nm) due to the limitation of calculation resources.
- FIG. 15B is a structure for observing a decrease in output due to absorption when the thickness of the Ni layer 10 is increased to 10 nm with respect to the structure shown in FIG. 15A. From the top of the drawing, the steps up to the electronic block layer 7 are the same as in FIG. 15A.
- the subsequent structure is a transparent p-type AlGaN contact layer 8 a, a photonic crystal (hole) periodic structure 100, a Ni layer 10, and an Al reflective electrode 11.
- FIG. 15C shows a structure having a p-type GaN contact layer 9 that absorbs deep ultraviolet light and a Ni layer 10 (10 nm). From the top of the drawing, the structure up to the electron blocking layer 7 is the same as that shown in FIG. 15A. is there.
- the subsequent structure is a p-type AlGaN layer 8, a p-type GaN contact layer 9, a photonic crystal (hole) periodic structure 100, an Ni layer 10, and an Al reflective electrode 11.
- the output magnification of the output value in the other structure relative to the output value in the conventional structure of FIG. 4A was obtained. Further, due to the limitation of calculation resources, the output magnification of the structure of FIG. 15A in which the output magnification was calculated by omitting the ultrathin Ni layer 1 nm was set to a magnification corresponding to 7% output reduction in the ultrathin Ni layer 1 nm in another analysis. Corrected by subtracting. Then, the light extraction efficiency (LEE) of the conventional structure of FIG. 4A was set to 10%, and LEEs of other structures were obtained by multiplying by the correction magnification (see Table 6).
- LEE light extraction efficiency
- FIG. 15A A structure in which a transparent p-type AlGaN contact layer / photonic crystal (hole) periodic structure / ultra thin film Ni layer (1 nm) is mounted as shown in FIG. 15A, a photonic crystal (pillar) periodic structure and nano-PSS on the back surface of the sapphire substrate.
- the light extraction efficiency increased by another 2% from 25% to 27% of the structure of FIG. 4E.
- the structure based on transparent p-type AlGaN contact layer / photonic crystal (hole) periodic structure / ultra thin film Ni layer (1 nm) can suppress the absorption of deep ultraviolet light. Therefore, for example, it can be used as a base for various structural improvements for improving the light extraction efficiency such as the nano-PSS-derived AlN bonded pillar structure.
- the deep ultraviolet LED according to the fourth embodiment of the present invention is based on the deep ultraviolet LED (FIG. 15A) having the nano-PSS-derived AlN-bonded pillar structure in the third embodiment. ing. Then, after forming a deep ultraviolet LED comprising transparent p-type AlGaN contact layer 8a / photonic crystal (hole) periodic structure 100 / ultra-thin film Ni layer (1 nm) 10a, a support substrate 31 is pasted on the Al reflective electrode layer 11 side. Then, the sapphire substrate 1 is peeled off and the AlN coupled pillar 220b is used as a light extraction surface.
- the structure include an AlN coupled pillar 220b, an n-type AlGaN layer 3, a barrier layer 4, a quantum well layer 5, a barrier layer 6, an electron block layer 7, a transparent p-type AlGaN contact layer 8a, a photonic crystal (Hole) Periodic structure 100, ultrathin Ni layer 10a, Al reflective electrode layer 11, and support substrate 31.
- an AlN coupled pillar 220b an n-type AlGaN layer 3, a barrier layer 4, a quantum well layer 5, a barrier layer 6, an electron block layer 7, a transparent p-type AlGaN contact layer 8a, a photonic crystal (Hole) Periodic structure 100, ultrathin Ni layer 10a, Al reflective electrode layer 11, and support substrate 31.
- the first feature is that the sapphire substrate 1 is peeled off. LED light is extracted from the four surfaces of the back surface and the side wall of the sapphire substrate 1.
- the ratio of internal disappearance due to total internal reflection on the four surfaces of the side wall of deep ultraviolet light emitted and propagated in the quantum well layer 5 is large. Therefore, when the sapphire substrate 1 is peeled off, the depth (thickness) of the portion constituted by the semiconductor excluding the sapphire substrate 1 is about several ⁇ m, and the surface area of the four side walls constituted thereby is the front (surface). Smaller than the surface area. Therefore, the internal disappearance is negligible.
- the second feature is that since the AlN coupled pillar 220b is exposed, deep ultraviolet light is directly emitted from the AlN coupled pillar 220b into the air.
- This AlN coupled pillar 220b has a great effect as a waveguide, and light is extracted from the LED in a condensed form from the front, so the axial light extraction efficiency (5 ° to 20 °) is remarkably improved. Is done.
- the third feature is that by sticking the support substrate 31 having excellent thermal conductivity, the external emission efficiency of heat is improved and the lifetime of the deep ultraviolet LED is extended.
- Such a deep ultraviolet LED according to the fourth embodiment will be described more specifically.
- the calculation model analyzed by the FDTD method has a design wavelength of 265 nm and a degree of polarization of 0.07, and has a structure similar to that shown in FIG.
- the calculation model is, in order from the top, an AlN coupled pillar 220b (4 ⁇ m), an n-type AlGaN layer 3 (1.4 ⁇ m), a barrier layer 4 (10 nm), a quantum well layer 5 (10 nm), The barrier layer 6 (10 nm), the electron blocking layer 7 (40 nm), the transparent p-type AlGaN contact layer 8a (350 nm), the Al reflective electrode layer 11 (210 nm), and the support substrate 31 (10 ⁇ m).
- the total film thickness is 16,030 nm, which is the same film thickness as the model of each embodiment described above.
- the output value was obtained in the far field, and the output magnification of the output value in the other structure of each embodiment with respect to the output value in the conventional LED structure shown in FIG. 4A was obtained.
- the output magnification of the LED structure shown in FIG. 16 in which the output magnification was calculated by omitting the ultra-thin Ni layer 1 nm due to the limitation of calculation resources corresponds to an output reduction 7% in the ultra-thin Ni layer 1 nm in a separate analysis. The magnification to be corrected is subtracted.
- the light extraction efficiency (LEE) of the conventional LED structure shown in FIG. 4A was set to 10%, and LEEs of other LEE structures were obtained by multiplying the correction magnification (see Table 7A).
- Table 7A is a table showing characteristics of the structure of FIG. 15A and the structure of FIG.
- the LED structure of FIG. 16 according to this embodiment has the highest light extraction efficiency of 27%.
- the magnification of the light extraction efficiency in the on-axis direction is 6.7 times higher than that of a conventional deep ultraviolet LED having a p-type GaN contact. This value is larger than the value in FIG. 15A.
- peeling the sapphire substrate 1 suppresses deterioration of the light extraction efficiency due to total internal reflection at the side wall of the substrate 1, and improves the light extraction efficiency in the axial direction due to the waveguide effect of the AlN coupled pillar 220b.
- the photonic crystal (hole) periodic structure 100 included in the LED structure shown in FIG. 16 also contributes to the light extraction efficiency in the axial direction. Therefore, in order to verify the waveguide effect of the single AlN coupled pillar 220b, the structure of FIGS. 4A and 4B and the structure of FIG. 16 excluding the photonic crystal (hole) periodic structure 100 (“AlN coupled pillar LED”).
- Table 7B shows the results of direct comparison of the on-axis output magnifications of the LED structures by creating a calculation model and analyzing by the FDTD method.
- FIG. 4A is a structure of a conventional LED having a p-type GaN contact layer
- the structure of FIG. 4B is an LED structure in which the p-type GaN contact layer is replaced with a transparent p-type AlGaN contact layer.
- FIG. 23 is a diagram showing the light distribution of a conventional LED, a transparent p-type AlGaN contact layer LED, and an AlN coupled pillar LED.
- Table 7B shows output values obtained by adding all outputs from 5 ° to 90 ° in FIG. 23 (the horizontal direction is 90 ° and the vertical direction is 0 °).
- the axial power factor of the AlN-coupled pillar in the AlN-coupled pillar LED is 4.9 times that of the conventional LED structure, and that of the transparent p-type AlGaN contact layer-based structure. was 2.6 times higher.
- the deep ultraviolet LED according to the present embodiment is outside the deep ultraviolet LED structure described in the third embodiment and the fourth embodiment.
- a transparent resin structure is formed by an encapsulation process or the like.
- 17A and 17B are cross-sectional views showing examples of LED structures.
- an encapsulating resin 41 is provided outside the structure of FIG. 15B.
- a photonic crystal (pillar) periodic structure 110 is formed on the back surface of the sapphire substrate 1 from the sapphire substrate 1 side to the front surface side (the lower side in the figure).
- nano-PSS (triangular pyramid-shaped) periodic structure 220a AlN coupled pillar 220b, n-type AlGaN layer 3, barrier layer 4, quantum well layer 5, barrier layer 6, electron blocking layer 7, transparent p A type AlGaN contact layer 8a, a photonic crystal (hole) periodic structure 100, an ultrathin Ni layer 10a, an Al reflective electrode 11, and an encapsulating resin 41.
- the LED structure shown in FIG. 17B is the same structure as FIG. 16, and the AlN coupled pillar 220b, the n-type AlGaN layer 3, the barrier layer 4, the quantum well layer 5, the barrier layer 6, The electron block layer 7, the transparent p-type AlGaN contact layer 8 a, the photonic crystal (hole) periodic structure 100, the ultrathin film Ni layer 10 a, the Al reflective electrode 11, the support substrate 31, and the encapsulating resin 51.
- the difference in the refractive index between the semiconductor layer and air is large on the four side walls of the sapphire substrate 1, and total internal reflection occurs at the interface.
- the effects of total internal reflection on the side surface of the sapphire substrate 1 are mitigated by enclosing transparent resins 41 and 51 having a refractive index intermediate between air and side walls in a position surrounding the outside of the deep ultraviolet LED structure.
- the light extraction efficiency can be improved.
- the influence of internal disappearance due to total internal reflection on the four surfaces of the side wall of the sapphire substrate 1 of the deep ultraviolet light emitted and propagated in the quantum well layer 5 is great.
- the surface area of the side wall 4 surface having a semiconductor portion depth of about several ⁇ m is smaller than the front surface area, so that the internal disappearance is negligible.
- a calculation model analyzed by the FDTD method has a design wavelength of 265 nm and a degree of polarization of 0.07.
- the specific structure is the same as that shown in FIGS. 15B and 16 except for the ultra-thin Ni layer (1 nm) 10a that is omitted due to the limitation of calculation resources.
- Table 8 shows the film thickness of each structure.
- the output value was obtained in the far field, and the output magnification of the output value in the other structure (FIGS. 17A and 17B) with respect to the output value of the deep ultraviolet LED in the conventional structure of FIG. 4A was obtained.
- the values in Table 8 were corrected by subtracting a magnification corresponding to 7% output reduction of the ultrathin Ni layer 1 nm, which was omitted in the calculation model analyzed by the FDTD method.
- the value of the light extraction efficiency in the axial direction (5 ° to 20 °) is also shown.
- the light extraction efficiency (LEE) of the conventional structure deep ultraviolet LED of FIG. 5A was set to 10%, and the LEE of other structures (FIGS. 17A and 17B) was obtained by multiplying by the correction magnification (see Table 9).
- both FIGS. 17A and 17B show the highest light extraction efficiency of 31%. It was confirmed that encapsulating the entire deep ultraviolet LED with a transparent resin alleviates total internal reflection and improves light extraction efficiency.
- the entire deep ultraviolet LED is encapsulated with a transparent resin, thereby reducing internal total reflection and improving light extraction efficiency.
- the deep ultraviolet LED according to the sixth embodiment of the present invention has a package structure in which an Al reflective film 61 is provided on the outer side wall of the deep ultraviolet LED described in the third and fifth embodiments. Some improve the light extraction efficiency.
- 18A and 18B are cross-sectional views showing an example of the structure. 18A shows a structure corresponding to FIG. 14A, and FIG. 18B shows a structure corresponding to FIG. 17A.
- the light emitted to the outside of the LED is designed to be reflected by the Al reflecting film 61 in the upper direction of the drawing. Therefore, the light extraction efficiency from the axial direction is remarkably improved.
- a calculation model analyzed by the FDTD method has a design wavelength of 265 nm and a polarization degree of 0.07.
- the structure used for the analysis is shown in a cross-sectional structure in FIGS. 19A and 19B corresponding to FIGS. 18A and 18B.
- the film thickness of the specific structure is the same as the structure of FIG. 17A shown in Table 8.
- FIGS. 19A and 19B differs from the actual structure of FIGS. 18A and 18B due to the limitation of calculation resources, and the Al reflective film (thickness 200 nm) 61a provided on the side wall portion is opposite to the LED semiconductor interface A vertically standing structure was adopted.
- the output monitor for detecting the output was placed only in the upper part, the output value was obtained in the far field, and the output magnification of the upper output value in the other structure with respect to the upper output value in the conventional structure in FIG. 4A was obtained. Further, correction was made by subtracting a magnification corresponding to 7% output reduction of the ultra-thin Ni layer 1 nm, which was omitted in the model. Furthermore, the light extraction efficiency in the on-axis direction (5 ° to 20 °) was added. Then, the light extraction efficiency (LEE) of the conventional structure of FIG. 4A was set to 10%, and LEEs of other structures were obtained by multiplying by the correction magnification (see Table 10).
- the output magnification in the axial direction (5 ° to 20 °) is 7.1 to 7.7 times that of the conventional structure, which is a significant improvement.
- the reflectivity of deep ultraviolet light in the Al reflective electrode and Al reflective film is about 90%, this result is highly efficient compared to 80% of commercially available blue / white LEDs.
- the base structure that can be realized by the device according to each embodiment of the present invention, in particular, the appropriate arrangement of the photonic crystal periodic structure according to the first and second embodiments, the third to third
- the seventh embodiment of the present invention shows that the photonic crystal periodic structure, nano-PSS periodic structure, etc. described in each of the above embodiments can be processed using a transfer technique based on the nanoimprint lithography method. It is.
- Nanoimprinting has an excellent technique for transferring a photonic crystal pattern of a mold in a large area to an organic resist spin-coated on a substrate. Also, if a resin film mold is used, transfer is possible even if the substrate is warped by several hundred microns.
- the organic imprinting resist for nanoimprinting does not necessarily have a sufficient etching selectivity with respect to the material that is the pattern formation portion in order to emphasize fluidity. Further, the pattern size of the mold does not match the pattern formation portion size after etching. In order to solve this problem, a process using a two-layer resist is performed as follows.
- a transfer technique using a two-layer resist method is used in which a lower layer resist having a high etching selectivity is coated on a structure to be processed, and an upper layer resist having fluidity and oxygen resistance is coated thereon.
- a mold for transfer and a resin film for the mold. More specifically, as an example, an organic lower layer resist is spin-coated on the substrate surface on which the periodic structure is formed.
- a silicon-containing upper resist is spin-coated on the lower resist surface.
- the periodic structure is transferred onto the upper resist surface using a nanoimprint lithography method using a mold.
- the upper resist to which the periodic structure has been transferred is exposed to oxygen plasma to impart oxygen resistance, and the remaining upper resist remaining in the nanoimprint transfer is removed.
- the organic lower resist is etched with oxygen plasma to form a mask for dry etching of the substrate.
- the substrate is dry-etched with ICP plasma.
- the above steps 1) to 6) are a transfer technique using a two-layer resist method on a substrate.
- the thickness of the lower layer resist is changed to be about 1.5 times the depth of the periodic structure on the mold (an example of a sapphire substrate).
- the etching depth can be obtained on the transfer object.
- the upper resist By changing the oxygen plasma conditions at the time of forming the mask of the lower layer resist by, the size of about 30% can be adjusted with respect to the diameter of the periodic structure on the mold.
- FIG. 20 is a diagram illustrating an example of a manufacturing process of a periodic structure according to the present embodiment.
- a transfer technique based on a nanoimprint lithography method using a two-layer resist having both characteristics of fluidity and etching selectivity is used.
- a photonic crystal periodic structure having a fine pattern of nm order was transferred to a sapphire substrate as an example.
- a mold for accurately reproducing the periodic structure optimized in each of the above implementations on a sapphire substrate is created.
- a resin mold can be used so as to follow the warp of the sapphire substrate 81.
- an organic lower layer resist 83 having a large etching selectivity is spin-coated on the sapphire substrate 81 with a thickness g.
- the thickness g is selectively determined according to the etching selectivity of the lower layer resist 83 with respect to the sapphire substrate 81.
- a silicon-containing upper layer resist 85 having fluidity and oxygen resistance function is spin-coated at a predetermined thickness on the surface of the lower layer resist 83 (FIG. 20A).
- a mold pattern (resin mold) 87 and 89 is transferred to the upper resist 85 using a nanoimprint apparatus (FIG. 20B).
- the upper resist 85 to which the mold patterns 87 and 89 have been transferred is exposed to oxygen plasma to impart oxygen resistance, and the remaining upper resist remaining in the nanoimprint transfer is removed. (FIG. 20 (c)). Thereby, the upper resist pattern 85a is formed.
- the organic lower resist 83 is etched with oxygen plasma to form a pattern mask 85b for dry etching the sapphire substrate 81 (FIG. 20D).
- the diameter d 1 on the sapphire substrate 81 side of the pattern mask shown in FIG. 20E can be finely adjusted within a range of about 30% of d 1 by adjusting the oxygen plasma conditions.
- the sapphire substrate 81 is dry-etched with ICP plasma through a pattern mask, and the periodic structure 81a optimized according to each embodiment of the present invention can be formed on the sapphire substrate 81 (FIG. 20 (e)). ).
- the shape after etching is a trapezoidal shape of d 1 ⁇ d 2 as shown in FIG. 20F, and the side wall angle depends on the etching selectivity of the organic lower layer resist. If the thickness g of the organic underlayer resist is changed, the depth of the photonic crystal periodic structure formed on the sapphire substrate 81a after the dry etching can be easily increased by about 1.5 times the depth of the mold. It can be.
- the diameter of the periodic structure can be easily changed about 30%. Therefore, it eliminates the manufacturing time of the mold and contributes to cost reduction, which is a great merit in terms of manufacturing cost of the semiconductor light emitting device.
- FIGS. 21A to 21C are actual SEM photographs (nanoimprint process phC pillar cross-section SEM) when the steps of FIGS. 20B, 20E, and 20F are performed. Are shown as “nanoimprint”, “pattern mask formation” and “dry etching / ashing”, respectively.
- processing and control capable of producing a clean periodic structure are software processing by CPU (Central Processing Unit) or GPU (Graphics Processing Unit), ASIC (Application Specific Integrated Circuit) or FPGA ProGraFed (GProGraGiFraGiGraGiFraGiGiFraGiGiFraGiGiFraG It can be realized by hardware processing.
- Each component of the present invention can be arbitrarily selected, and an invention having a selected configuration is also included in the present invention.
- a program for realizing the functions described in the present embodiment is recorded on a computer-readable recording medium, and the program recorded on the recording medium is read into a computer system and executed to execute processing of each unit. May be performed.
- the “computer system” here includes an OS and hardware such as peripheral devices.
- the “computer system” includes a homepage providing environment (or display environment) if a WWW system is used.
- the “computer-readable recording medium” means a storage device such as a flexible disk, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory in a computer system serving as a server or a client in that case, and a program that holds a program for a certain period of time are also included.
- the program may be a program for realizing a part of the above-described functions, and may be a program that can realize the above-described functions in combination with a program already recorded in the computer system. At least a part of the functions may be realized by hardware such as an integrated circuit.
- the present invention can be used for deep ultraviolet LEDs.
- a period of the photonic crystal periodic structure
- R radius of the periodic structure
- h processing depth of the periodic structure
- 1 ... sapphire substrate 2 ... AlN buffer layer, 3 ... n-type AlGaN layer, 4 ... barrier layer, 5 Quantum well layer 6 Barrier layer 7
- Electron blocking layer 8 p-type AlGaN layer 8a Transparent p-type AlGaN contact layer 10 Ni layer 10a Ultra-thin Ni layer 11 Al reflective electrode layer , 31 ... support substrate, 41, 51 ... encapsulating resin, 61, 71 ... Al reflective film, 100 ... first (reflective) photonic crystal periodic structure, 101 (h) ... circular hole (columnar structure (hole)) 110 ... second photonic crystal periodic structure, 111 (p) ... pillar, 220 ... nano PSS and coupled pillar periodic structure, 220a ... nano PSS periodic structure, 220b ... AlN coupled pillar.
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Abstract
Description
本発明の第1の実施の形態に係る深紫外LEDの一例として、設計波長λを265nmとするAlGaN系深紫外LEDの構造を図1A(a)に示す。図1A(a)に示すように、本実施の形態によるAlGaN系深紫外LEDは、図の上から順番に、サファイア基板1、AlNバッファー層2、n型AlGaN層3、バリア層4、量子井戸層5、バリア層6、電子ブロック層7、透明p型AlGaNコンタクト層8a、極薄膜Ni層10a、Al反射電極層11、を有する。そして、透明p型AlGaN層8aの厚さ方向の範囲内に、フォトニック結晶周期構造100を設けており、かつ、フォトニック結晶周期構造100は、円孔(柱状構造体,ホール)101(h)を設け、フォトニックバンドギャップを有することにより波長λの光を反射する反射型フォトニック結晶周期構造である。
mλ/nav=2a (1)
(ステップS01)
周期構造パラメータである周期aと構造体の半径Rの比(R/a)を仮決定する。
第1の構造体のそれぞれの屈折率n1とn2、及びこれらとR/aから平均屈折率navを算出し、これをブラッグ条件の式に代入し、次数mごとの周期aと半径Rを得る。
R/a及び波長λ並びに前記屈折率n1、n2から得られる各構造体の誘電率ε1及びε2を用いた平面波展開法により、TE光のフォトニックバンド構造を解析する。
TE光の第一フォトニックバンドと第二フォトニックバンド間のPBGが最大となるR/aを、前記仮決定のR/aの値を変えて繰り返し行う解析により決定する。
PBGを最大にするR/aについて、ブラッグ条件の次数mに応じた個別の周期a及び半径R、並びに、任意の周期構造の深さhを変数として行うFDTD法によるシミュレーション解析により、前記波長λに対する光取出し効率を求める。
FDTD法によるシミュレーションを繰り返し行うことにより、波長λに対する光取出し効率が最大となるブラッグ条件の次数mと、その次数mに対応する周期構造パラメータの周期a、半径R、及び、深さhを決定する。
(1):mλ/nav=2a)の次数mを決定する。
nav=[n2 2+(n1 2-n2 2)(2π/30.5)(R/a)2]0.5=1.848 (4)
次に、本発明の第2の実施の形態について説明する。
ステップS1において、0.20≦R/a≦0.40の範囲において、R/a(R:半径、a:周期)を、例えば0.01ステップで変化させる。
ブラッグの条件を満たす散乱波は各フォトニックバンド(PB)の何れかに相当するので、設計波長λを透過させる周期aをブラッグの式で関連付けする。ここで、着目するフォトニックバンドはブラッグの条件を満たす散乱波(k+G)である。
ステップS3においては、ステップS2で決定したR/a、波長λ、屈折率n1、n2から誘電率ε1、ε2を求め、平面波展開法によるTM光のフォトニックバンド(PB)構造を得る。PBG1、PBG2の最大値に対応する次数がm=3~4であるR/aを最適化の候補とする。
ブラッグの条件を満たす第二フォトニックバンド(2ndPB)と第四フォトニックバンド(4thPB)の縦軸(ωa/2πc)を真空中の波長λVに換算し、次数m=1においてλVとka/2πのフォトニックバンド構造を得る。縦軸はωa/2πc=a/λPhCと変換できる。但し、λPhCはフォトニック結晶(PhC)中の波長である。従って、λV=λ1=a1/(ωa/2πc)×nav、また、ブラッグの式、すなわち、1×λV/nav=2a1よりa1=λv/2navと導出される。
最初に次数m=3で決定されるR/aについて検討する。図11Aは、次数m=3で決定されるR/aについて示す図であり、R/a=0.35(次数m=1)の第二フォトニックバンド(2ndPB)が定在波を生じる条件を示す図である。図11Bは、次数m=3で決定されるR/aについて示す図であり、R/aの第四フォトニックバンド(4thPB)が定在波を生じる条件を示す図である。
次数m=4では、λ4=a4/(ωa/2πc)×nav、a4=4λV/2navとなる。第二フォトニックバンド(2ndPB)に関する真空中波長と波数のフォトニックバンド構造を図13Aに示す。各対称点における真空中の波長×4=1060nmに最も近接するR/aは次数m=1と同様にM点(R/a=0.28)、K点(R/a=0.35)となる。また、あるR/aの第四フォトニックバンド(4thPB)が定在波を生じる条件は、m=1における入射波長の6整数倍、7整数倍、8整数倍である。そこで各対称点における真空中の波長×4=1060nmに点で接するか最接近するR/aを求めると、6整数倍ではΓ点(R/a=0.40)、M点(R/a=0.35)である(図13B)。7整数倍ではΓ点(R/a=0.23)M点(R/a=0.20)K点(R/a=0.36)である(図13C)。8整数倍ではΓ点(該当なし)、M点(該当なし)、K点(R/a=0.27)となり(図13D)、何れも最適化の候補となる。
ステップS3からステップS6までで得られた最適化候補である次数mとR/aに対応するフォトニック結晶を有限時間領域差分法(FDTD法)で計算する。深さに関しては次数m=3~4において最も大きい周期aの0.5倍以上の任意の値を選択する。
LED素子全体の出力比較と軸上方向(角度5°~20°)の出力比較を行い、光取出し効率(LEE)増減率が大きいR/aと次数mの中から、軸上方向の配光性に優れたR/a及び次数mを選択する。従って、フォトニック結晶最適化のパラメータである、直径、周期、深さが決定される。
次に、本発明の第3の実施の形態による深紫外LEDについて図面を参照しながら説明を行う。
本発明の第4の実施の形態に係る深紫外LEDは、図16に示すように、第3の実施の形態におけるナノPSS由来AlN結合ピラー構造を備えた深紫外LED(図15A)をベースにしている。そして、透明p型AlGaNコンタクト層8a/フォトニック結晶(ホール)周期構造100/極薄膜Ni層(1nm)10aからなる深紫外LEDを作成した後に、Al反射電極層11側に支持基板31を貼り付けた後に、サファイア基板1を剥離してAlN結合ピラー220bを光取出し面とした構造である。
次に、本発明の第5の実施の形態について図面を参照しながら詳細に説明する。
次に、本発明の第6の実施の形態について詳細に説明する。
以下、本発明の第7の実施の形態ついて詳細に説明する。
1)加工対象の構造体に対しエッチング選択比の大きい下層レジストをコートし、その上に流動性と酸素耐性を有する上層レジストとコートする、二層レジスト法を用いた転写技術を用いる。
2)また、転写には金型を用い、金型には樹脂フィルムを用いることも可能である。より具体的には、周期構造を形成する基板面上にこの基板に対しエッチング選択比の大きい、一例として、有機下層レジストをスピンコートする。次に、流動性と酸素耐性機能を有する、一例として、シリコン含有上層レジストを下層レジスト面上にスピンコートする。3)次に、上層レジスト面上に金型を用いたナノインプリントリソグラフィー法を用いて、周期構造を転写する。
4)次に、周期構造が転写された上層レジストを酸素プラズマに曝し、酸素耐性を付与するとともに、ナノインプリント転写において残存した上層レジストの残膜を除去する。
5)次に、酸素耐性を有した上層レジストをマスクとして、有機下層レジストを酸素プラズマでエッチングし、基板のドライエッチングのためのマスクを形成する。
6)最後に、このマスクをエッチングマスクとして、基板をICPプラズマでドライエッチングする。
Claims (27)
- 設計波長をλとする深紫外LEDであって、
反射電極層と、極薄膜金属層と、透明p型AlGaNコンタクト層とを、基板とは反対側からこの順で有し、
前記透明p型AlGaNコンタクト層の厚さ方向の範囲内に設けられた第1のフォトニック結晶周期構造を有し、
前記第1のフォトニック結晶周期構造は、
空気と前記透明p型AlGaNコンタクト層との周期構造を有する第1の構造体からなり、かつ、
前記第1のフォトニック結晶周期構造はフォトニックバンドギャップを有する
ことを特徴とする深紫外LED。 - 前記第1のフォトニック結晶周期構造は、
さらに、厚さ方向に前記反射電極層の範囲まで延長して設けられていることを特徴とする請求項1に記載の深紫外LED。 - 前記フォトニックバンドギャップはTE偏光成分に対して開いており、かつ、
波長λ、前記第1の構造体の周期a及び前記第1の構造体を構成する2つの材料の平均屈折率navがブラッグ条件を満たし、かつ、
当該ブラッグ条件の次数mは2<m<5の範囲にあり、かつ、
前記第1の構造体の深さhを前記周期aの2/3以上とすることを特徴とする請求項1又は2に記載の深紫外LED。 - 前記第1のフォトニック結晶周期構造は、ナノインプリントリソグラフィー法による転写技術を用いて形成されたものであることを特徴とする請求項1から3までのいずれか1項に記載の深紫外LED。
- 前記第1のフォトニック結晶周期構造は、流動性の高いレジストとエッチング選択比の高いレジストによる2層レジスト法を用いたドライエッチングを用いて形成されたものであることを特徴とする請求項4に記載の深紫外LED。
- 前記第1のフォトニック結晶周期構造のパラメータは、
周期構造パラメータである周期aと前記第1の構造体の半径Rの比(R/a)を仮決定するステップと、
前記第1の構造体のそれぞれの屈折率n1とn2、及びこれらと前記R/aから平均屈折率navを算出し、これをブラッグ条件の式に代入し、次数mごとの周期aと半径Rを得るステップと、
前記R/a及び波長λ並びに前記屈折率n1、n2から得られる各構造体の誘電率ε1及びε2を用いた平面波展開法により、TE光のフォトニックバンド構造を解析するステップと、
TE光の第一フォトニックバンドと第二フォトニックバンド間のPBGが最大となるR/aを、前記仮決定のR/aの値を変えて繰り返し行う解析により決定するステップと、
前記のPBGが最大となるR/aについて、ブラッグ条件の次数mに応じた個別の周期a及び半径R、並びに、任意の周期構造の深さhを変数として行う有限時間領域差分法(FDTD法)によるシミュレーション解析により、前記波長λに対する光取出し効率を求めるステップと、
前記FDTD法によるシミュレーションを繰り返し行うことにより、前記波長λに対する光取出し効率が最大となるブラッグ条件の次数mと、その次数mに対応する周期構造パラメータの周期a、半径R、及び、深さhを決定するステップと、
を有するパラメータ計算方法により求めたものであることを特徴とする請求項1から5までのいずれか1項に記載の深紫外LED。 - さらに、
前記基板の裏面(側)に異なる屈折率を持つ2つの構造体からなる第2のフォトニック結晶周期構造を有し、
前記第2のフォトニック結晶周期構造は、
空気と基板の媒質との周期構造を有する第2の構造体からなることを特徴とする請求項1から6までのいずれか1項に記載の深紫外LED。 - 前記第2のフォトニック結晶周期構造は、
真空中の設計波長λVと周期構造のパラメータである周期aと半径Rはブラッグ条件を満たし、R/aが、0.20から0.40までの範囲において、TM光のフォトニックバンド構造において2個のフォトニックバンドギャップを第四フォトニックバンド以内に有し、かつ、
前記フォトニックバンドギャップはTM光に対して開くが故に透過効果が大きくなり、かつ、
前記R/aは、次数m=3又は4において、各フォトニックバンドギャップの最大値に対応する値であり、または、
前記R/aは、次数m=3又は4において、前記フォトニックバンド構造の縦軸(ωa/2πc)が真空中の波長λVに換算されたとき、第二フォトニックバンド(2ndPB)の対称点であるΓ点、M点、K点の何れかにおいて真空中の波長λV×mと点で接するか最も接近する値であり、または、
前記R/aは、次数m=3のとき、前記フォトニックバンド構造の縦軸(ωa/2πc)の真空中の波長λV×3が、第四フォトニックバンド(4thPB)を5整数倍と6整数倍した各第四フォトニックバンド(4thPB)上の何れかの対称点と点で接するか最も接近する値であり、または、
前記R/aは、次数m=4のとき、前記フォトニックバンド構造の縦軸(ωa/2πc)の真空中の波長λV×4が、第四フォトニックバンド(4thPB)を6整数倍、7整数倍、8整数倍した各第四フォトニックバンド(4thPB)上の何れかの対称点と点で接するか最も接近する値であり、かつ、
各周期構造パラメータは、選択された各R/aと0.5a以上の深さhからなるフォトニック結晶をFDTD法により計算し、光取出し効率増減率と配光性が最適化されるよう最終決定されたパラメータである請求項7に記載の深紫外LED。 - 前記第2のフォトニック結晶周期構造のパラメータは、
周期構造パラメータである周期aと第2の構造体の半径Rの比(R/a)を変化させる第1ステップと、
前記第2の構造体のそれぞれの屈折率n1とn2、及びこれらと前記R/aから平均屈折率navを算出し、これをブラッグ条件の式に代入し、次数m=3とm=4について、周期aと半径Rを得る第2ステップと、
前記R/a及び前記波長λ並びに前記屈折率n1、n2から得られる各構造体の誘電率ε1及びε2を用いた平面波展開法により、TM光のフォトニックバンド構造を解析する第3ステップと、
TM光の第二フォトニックバンド(2ndPB)と第四フォトニックバンド(4thPB)の縦軸(ωa/2πc)を真空中の波長λVに換算し、次数m=1においてλVとka/2πのフォトニックバンド構造を得る第4ステップと、
次数m=3及びm=4について、TM光の第二フォトニックバンド(2ndPB)と第四フォトニックバンド(4thPB)における各対称点における真空中の波長λV×mと点で接するか最も接近するR/aを求め、最適化の候補とする第5ステップと、
前記第5ステップで選択されたR/aに対応するフォトニック結晶の光取出し効率増減率と配光性を、FDTD法で計算し、深さに関しては次数m=3~4において最も大きい周期aの0.5倍以上の任意の値を選択する第6ステップと、
を有するパラメータ計算方法により求めたものであることを特徴とする請求項8に記載深紫外LED。 - 前記第2のフォトニック結晶周期構造のパラメータは、
周期構造パラメータである周期aと構造体の半径Rの比(R/a)を変化させる第1ステップと、
前記第2の構造体のそれぞれの屈折率n1とn2、及びこれらと前記R/aから平均屈折率navを算出し、これをブラッグ条件の式に代入し、次数m=3とm=4について、周期aと半径Rを得る第2ステップと、
前記R/a及び前記波長λ並びに前記屈折率n1、n2から得られる各構造体の誘電率ε1及びε2を用いた平面波展開法により、TM光のフォトニックバンド構造を解析して得られる2つのフォトニックバンドギャップの最大値に対応する次数m=3及び4であるR/aを最適化の候補とする第3ステップと、
TM光の第二フォトニックバンド(2ndPB)と第四フォトニックバンド(4thPB)の縦軸(ωa/2πc)を真空中の波長λVに換算し、次数m=1においてλVとka/2πのフォトニックバンド構造を得る第4ステップと、
次数m=3及びm=4について、TM光の第二フォトニックバンド(2ndPB)と第四フォトニックバンド(4thPB)における各対称点における真空中の波長λV×mと点で接するか最も接近するR/aを求め、最適化の候補とする第5ステップと、
前記第5ステップで選択されたR/aに対応するフォトニック結晶の光取出し効率増減率と配光性を、FDTD法で計算し、深さに関しては次数m=3~4において最も大きい周期aの0.5倍以上の任意の値を選択する第6ステップと、
光取出し効率(LEE)増減率が大きく、配光性の良いR/a及び次数mを選択し、直径、周期、深さのパラメータが決定される第7ステップと、
を有するパラメータ計算方法により求めたものであることを特徴とする請求項7に記載深紫外LED。 - 前記第2のフォトニック結晶周期構造は、ナノインプリントリソグラフィー法による転写技術を用いて形成されたものであることを特徴とする請求項7に記載の深紫外LED。
- 前記第2のフォトニック結晶周期構造は、流動性の高いレジストとエッチング選択比の高いレジストによる2層レジスト法を用いたドライエッチングを用いて形成されたものであることを特徴とする請求項11に記載の深紫外LED。
- さらに、前記第1のフォトニック結晶周期構造と前記第2のフォトニック結晶周期構造との間に導波路構造を設けたことを特徴とする請求項7に記載の深紫外LED。
- 前記導波路構造は、
前記基板表面に設けられる三角錐形状のナノPSS周期構造と、前記ナノPSS周期構造と厚さ方向に連続して形成された六角錐台の柱状からなるAlN結合ピラー周期構造とを有することを特徴とする請求項13に記載の深紫外LED。 - 請求項13又は14に記載の深紫外LEDにおいて、
前記基板が剥離され、前記反射電極層に支持基板が貼り付けられた深紫外LED。 - 請求項14に記載の深紫外LEDにおいて、
前記ナノPSS周期構造を含む前記基板が除去され、前記AlN結合ピラー周期構造が光取り出し面側に設けられている深紫外LED。 - さらに、
前記深紫外LEDの外側に深紫外光に対して透明な樹脂が設けられ、 前記樹脂の屈折率が空気より大きく前記基板を含む化合物半導体層の屈折率より小さいことを特徴とする請求項13から16までのいずれか1項に記載の深紫外LED。 - さらに、
前記深紫外LEDの側壁の外側にAl反射膜を設け、前記Al反射膜は、前記Al反射膜に到達した深紫外光が反射して前記深紫外LEDの上部方向に伝搬するように形成された構造を有することを特徴とする請求項17に記載の深紫外LED。 - 前記極薄膜金属層は、
厚さが1nm程度であることを特徴とする請求項1から18までのいずれか1項に記載の深紫外LED。 - 深紫外LEDの製造方法であって、
設計波長をλとし、反射電極層と、金属層と、波長λに対し透明なp型AlGaN層とを、基板とは反対側からこの順で含有する積層構造体を準備する工程と、
前記p型AlGaN層の厚さ方向の範囲に設けられたフォトニック結晶周期構造を形成するための金型を準備する工程と、
前記積層構造体上に、レジスト層を形成し、前記金型の構造を転写する工程と、
前記レジスト層をマスクとして順次前記積層構造体をエッチングしてフォトニック結晶周期構造を形成する工程と
を有する深紫外LEDの製造方法。 - 前記積層構造体上にレジスト層を形成し、前記金型の構造を転写する工程は、
前記積層構造体上に、流動性の高い第1のレジスト層と、前記第1のレジスト層に対するエッチング選択比の高い第2のレジスト層と、による2層レジスト法を用いたドライエッチングを形成する工程と、
ナノインプリントリソグラフィー法を用いて前記第1のレジスト層に前記金型の構造を転写する工程と、を有し、
前記レジスト層をマスクとして順次前記積層構造体をエッチングしてフォトニック結晶周期構造を形成する工程は、前記第1のレジスト層と前記第2のレジスト層とを、前記第2のレジスト層が露出するまでエッチングするとともに、前記第1のレジスト層のパターン凸部も合わせてエッチングし、
前記第2のレジスト層をマスクとして順次前記積層構造体をエッチングしてフォトニック結晶周期構造を形成する工程を有することを特徴とする請求項20に記載の深紫外LEDの製造方法。 - 反射電極層と、極薄膜金属層と、透明p型AlGaNコンタクト層とを、基板とは反対側からこの順で有し、
前記透明p型AlGaNコンタクト層側の前記基板表面に設けられる三角錐孔または円錐孔のナノPSS周期構造から結晶成長されたAlN結合ピラー周期構造が平坦なAlN膜となるバッファー層を有することを特徴とする深紫外LED。 - 前記平坦なAlN膜は、n型AlGaN層にコンタクトしていることを特徴とする請求項22に記載の深紫外LED。
- 前記三角錐孔または円錐孔は、ウェットエッチングにより形成されることを特徴とする請求項22又は23に記載の深紫外LED。
- 前記平坦なAlN膜は、前記ナノPSS周期構造の前記三角錐孔または円錐孔から選択的に形成された前記AlN結合ピラー周期構造の終端に形成されることを特徴とする請求項22から24までのいずれか1項に記載の深紫外LED。
- 前記AlN結合ピラー周期構造は、エピタキシャル成長により形成されることを特徴とする請求項25に記載の深紫外LED。
- 請求項22から26までのいずれか1項に記載の深紫外LEDにおいて、前記ナノPSS周期構造を含む前記基板が除去され、前記AlN結合ピラー周期構造を光取り出し面とした深紫外LED。
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US11508874B2 (en) | 2019-11-28 | 2022-11-22 | Seiko Epson Corporation | Light emitting apparatus and projector |
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Publication number | Publication date |
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JP5999800B1 (ja) | 2016-09-28 |
CN107210336B (zh) | 2019-05-10 |
JPWO2016113935A1 (ja) | 2017-04-27 |
EP3246956A4 (en) | 2018-05-23 |
US9929317B2 (en) | 2018-03-27 |
EP3246956A1 (en) | 2017-11-22 |
US20170358712A1 (en) | 2017-12-14 |
TWI608631B (zh) | 2017-12-11 |
KR20170101995A (ko) | 2017-09-06 |
TW201637238A (zh) | 2016-10-16 |
CN107210336A (zh) | 2017-09-26 |
KR101848034B1 (ko) | 2018-04-11 |
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