WO2022219731A1 - 窒化物半導体紫外線発光素子及びその製造方法 - Google Patents

窒化物半導体紫外線発光素子及びその製造方法 Download PDF

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WO2022219731A1
WO2022219731A1 PCT/JP2021/015380 JP2021015380W WO2022219731A1 WO 2022219731 A1 WO2022219731 A1 WO 2022219731A1 JP 2021015380 W JP2021015380 W JP 2021015380W WO 2022219731 A1 WO2022219731 A1 WO 2022219731A1
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layer
region
algan
mole fraction
type layer
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French (fr)
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光 平野
陽祐 長澤
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Soko Kagaku Co Ltd
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Soko Kagaku Co Ltd
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Priority to TW110144622A priority patent/TWI907573B/zh
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10H—INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
    • H10H20/00—Individual inorganic light-emitting semiconductor devices having potential barriers, e.g. light-emitting diodes [LED]
    • H10H20/80—Constructional details
    • H10H20/81—Bodies
    • H10H20/822—Materials of the light-emitting regions
    • H10H20/824—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP
    • H10H20/825—Materials of the light-emitting regions comprising only Group III-V materials, e.g. GaP containing nitrogen, e.g. GaN

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  • the present invention relates to a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are vertically laminated, and a manufacture thereof. Regarding the method.
  • nitride semiconductor light emitting devices in which a light emitting device structure composed of a plurality of nitride semiconductor layers is formed by epitaxial growth on a substrate such as sapphire.
  • the nitride semiconductor layer is represented by the general formula Al 1-xy Ga x In y N (0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0 ⁇ x+y ⁇ 1).
  • a light-emitting element structure of a light-emitting diode has a double heterostructure in which an active layer made of a nitride semiconductor layer is sandwiched between two cladding layers of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer.
  • the active layer is an AlGaN-based semiconductor
  • the AlN mole fraction also referred to as the Al composition ratio
  • the bandgap energy can be adjusted to the bandgap energy (approximately 3.4 eV and approximately 6.2 eV) that can be taken by GaN and AlN.
  • an ultraviolet light emitting device having an emission wavelength of about 200 nm to about 365 nm can be obtained.
  • an ultraviolet light emitting device having an emission wavelength of about 200 nm to about 365 nm can be obtained.
  • the recombination of carriers (electrons and holes) in the active layer causes the bandgap energy Luminescence occurs.
  • a p-electrode is provided on the p-type nitride semiconductor layer, and an n-electrode is provided on the n-type nitride semiconductor layer.
  • the active layer is an AlGaN-based semiconductor
  • the n-type nitride semiconductor layer and the p-type nitride semiconductor layer sandwiching the active layer are composed of an AlGaN-based semiconductor with a higher AlN mole fraction than the active layer.
  • a p-type nitride semiconductor layer with a high AlN mole fraction is placed on the uppermost layer of the p-type nitride semiconductor layer.
  • a p-type contact layer capable of good ohmic contact with a p-electrode made of an AlGaN-based semiconductor (specifically, p-GaN). Since the p-type contact layer has a smaller AlN mole fraction than the AlGaN-based semiconductor forming the active layer, the ultraviolet rays emitted from the active layer toward the p-type nitride semiconductor layer are absorbed by the p-type contact layer. , cannot be effectively extracted to the outside of the device. For this reason, a general ultraviolet light emitting diode whose active layer is an AlGaN-based semiconductor employs an element structure as schematically shown in FIG. is effectively extracted to the outside of the device (see, for example, Patent Documents 1 and 2 below).
  • a general ultraviolet light emitting diode has a template 102 formed by depositing an AlGaN semiconductor layer 101 (for example, an AlN layer) on a substrate 100 such as a sapphire substrate.
  • a semiconductor layer 103, an active layer 104, a p-type AlGaN-based semiconductor layer 105, and a p-type contact layer 106 are deposited in order, and a part of the active layer 104, the p-type AlGaN-based semiconductor layer 105, and the p-type contact layer 106 is , the n-type AlGaN-based semiconductor layer 103 is removed by etching until it is exposed, and the n-electrode 107 is formed on the exposed surface of the n-type AlGaN-based semiconductor layer 103, and the p-electrode 108 is formed on the surface of the p-type contact layer 106, respectively.
  • the active layer is made to have a multiple quantum well structure, and an electron blocking layer is provided on the active layer. .
  • compositional modulation occurs due to Ga segregation (segregation associated with mass transfer of Ga), and localized AlN moles extending in an oblique direction with respect to the clad layer surface It has been reported that a layered region with a low fraction is formed (see, for example, Patent Document 3 and Non-Patent Documents 1 and 2 below).
  • An ultraviolet light emitting element composed of an AlGaN-based semiconductor is fabricated on a substrate such as a sapphire substrate, for example, by a well-known epitaxial growth method such as an organometallic compound vapor phase epitaxy (MOVPE) method.
  • MOVPE organometallic compound vapor phase epitaxy
  • Drift in crystal growth equipment is caused by changes in the effective temperature of the crystal growth site due to deposits on trays, chamber walls, etc. For this reason, in order to suppress the drift, conventionally, the growth history is examined, and an experienced person slightly changes the set temperature or the composition of the raw material gas, or the growth schedule for a certain period is fixed, cleaning, etc. Although we have devised ways such as performing maintenance in the same way for a certain period of time, it is difficult to completely eliminate drift.
  • the inventors of the present application have found a locally low AlN mole fraction formed by Ga segregation in the clad layer composed of an n-type AlGaN semiconductor layer and providing a low-resistance current path to the active layer.
  • a ternary mixed crystal such as AlGaN is a crystalline state in which Group 3 elements (Al and Ga) are randomly mixed, and can be approximated by a "random configuration". explained in a simple way.
  • the covalent bond radius of Al differs from the covalent bond radius of Ga, the higher the symmetry of the atomic arrangement of Al and Ga in the crystal structure, the more stable the structure generally.
  • AlGaN-based semiconductors with a wurtzite structure can have two types of arrangement: random arrangement without symmetry and stable symmetric arrangement.
  • a state appears in which the symmetrical arrangement is dominant.
  • metalastable AlGaN in which the AlGaN composition ratio (the composition ratio of Al, Ga, and N) is represented by a predetermined integer ratio, a symmetric arrangement structure of Al and Ga appears.
  • Fig. 1 shows a schematic diagram of one unit cell (two monolayers) in the c-axis direction of AlGaN.
  • white circles indicate sites where Group 3 element atoms (Al, Ga) are located, and black circles indicate sites where Group 5 element atoms (N) are located.
  • ML a monoatomic layer
  • one unit cell is denoted as 2ML.
  • the site planes of the group 3 elements (A3 plane, B3 plane) and the site planes of the group 5 elements (A5 plane, B5 plane) indicated by hexagons in FIG. 1 are both parallel to the (0001) plane.
  • Each site on the A3 plane and the A5 plane (generically called the A plane) has six sites at each vertex of the hexagon and one site at the center of the hexagon.
  • Each site on the A plane overlaps in the c-axis direction, and each site on the B plane overlaps in the c-axis direction.
  • the atoms (N) at one site on the B5 plane are composed of atoms (Al, Ga) at three sites on the A3 plane located above the B5 plane and one site on the B3 plane located below the B5 plane.
  • Site atoms (Al, Ga) form a 4-coordinate bond
  • the atom (Al, Ga) at one site of the B3 plane is the atom (N) at one site of the B5 plane located above the B3 plane.
  • 4-coordinate bonds with atoms (N) at three sites on the A5 plane located below the B3 plane Therefore, as shown in FIG. It does not overlap with each site in the c-axis direction.
  • FIG. 2 illustrates the positional relationship between each site on the A plane and each site on the B plane as a plan view of the A3 plane and the B3 plane viewed from the c-axis direction.
  • the black circles and white circles in FIG. 2 distinguish whether the site surface is the A3 surface or the B3 surface.
  • each of the six vertices of a hexagon is shared by two other adjacent hexagons, and the central site is not shared with any other hexagon, so within one hexagon:
  • FIG. 3 illustrates an arrangement structure of only one of the A3 plane and the B3 plane.
  • Ga is indicated by a large black circle
  • Al is indicated by a small black circle.
  • the Al 1 Ga 5 N 6 of 1) above has one of the A3 plane and the B3 plane of the Al 1 Ga 2 N 3 arrangement structure of 2) above, and the other of the GaN arrangement structure (the site of the group 3 element is It can take a 2ML unit symmetric arrangement structure with all Ga).
  • the Al 1 Ga 5 N 6 of 5) above has one of the A3 plane and the B3 plane of the Al 2 Ga 1 N 3 arrangement structure of 4) above, and the other AlN arrangement structure (the site of the group 3 element is A symmetric arrangement structure of 2ML units with all Al) can be taken.
  • Al3Ga9N12 Al1Ga3N4 )
  • 7 Al5Ga7N12
  • 8 Al7Ga5N12
  • the Al 1 Ga 3 N 4 of 6) above has one of the A3 plane and B3 plane having the Al 1 Ga 1 N 2 arrangement structure of 3) above, and the other having the GaN arrangement structure (group 3 element site can take a symmetrical array structure of 2 ML units where all are Ga).
  • Al 5 Ga 7 N 12 of 7) above has the arrangement structure of Al 1 Ga 2 N 3 of 2) above on one of the A3 plane and the B3 plane, and the arrangement structure of Al 1 Ga 1 N 2 of 3) above on the other.
  • a symmetric array structure of 2ML units can be taken.
  • Al 7 Ga 5 N 12 in 8) above has the arrangement structure of Al 1 Ga 1 N 2 in 3) above on one of the A3 plane and the B3 plane, and the arrangement structure of Al 2 Ga 1 N 3 in 4) on the other.
  • a symmetric array structure of 2ML units can be taken.
  • Al 3 Ga 1 N 4 in 9) above has one of the A3 plane and B3 plane having the Al 1 Ga 1 N 2 arrangement structure of 3) above, and the other having the AlN arrangement structure (all sites of group 3 elements are Al ) can have a 2ML unit symmetric array structure.
  • each of the AlGaN composition ratios 1), 5) to 9 although not specifically exemplified, for example, by synthesizing different symmetrical arrangement structures in the same plane on the A3 plane and the B3 plane described above, , Al 1 Ga 2 N 3 , Al 1 Ga 1 N 2 , and Al 2 Ga 1 N 3 in 2) to 4) above, each of the A3 plane and the B3 plane has the same symmetric arrangement structure of Al and Ga. considered to be obtained.
  • each metastable AlGaN having the AlGaN composition ratios 1), 5) to 9) is formed in the c-axis direction in units of 1 ML in the same manner as the metastable AlGaN having the AlGaN composition ratios 2) to 4).
  • the metastable AlGaN shown in the above 1) to 9) has a symmetrical arrangement of Al and Ga atoms, and is energetically stable AlGaN.
  • AlGaN in order to grow AlGaN while maintaining a certain crystal quality, it is necessary to grow the crystal at a high temperature of 1000° C. or higher.
  • Ga is assumed to move around at 1000° C. or higher even after the atoms reach sites on the crystal surface.
  • Al unlike Ga, Al easily adsorbs to the surface, and although it is thought that Al moves somewhat after entering the site, it is strongly restricted. Therefore, even if it is metastable AlGaN, Al 1 Ga 5 N 6 of the above 1) has a high composition ratio of Ga. It is considered that the disorder causes the atomic arrangement of Al and Ga to become nearly random, and the stability described above is lowered compared to other metastable AlGaN.
  • the peak emission wavelength ( ⁇ p) and the absorption edge wavelength ( ⁇ ae) determined by the average AlN mole fraction of the absorption spectrum of the n-type AlGaN semiconductor layer, if the wavelength difference ( ⁇ p ⁇ ae) is 10 nm or more, light emission from the active layer absorption in the n-type AlGaN semiconductor layer is suppressed.
  • the transmittance (ratio of incident light intensity I0 and transmitted light intensity I, I/I0) when light emitted from the active layer is transmitted through the n-type AlGaN semiconductor layer in the depth direction is the optical path length of n-type AlGaN. Attenuates exponentially with respect to the thickness of the system semiconductor layer. While the thickness of the n-type AlGaN-based semiconductor layer is as large as about 1 to 4 ⁇ m, the thickness of each layer in the depth direction of the layered region is as short as about 20 nm on average.
  • the absorption edge wavelength in the layered region is the absorption of the entire n-type AlGaN semiconductor layer. Although it shifts slightly to longer wavelengths than the edge wavelength ⁇ ae, the optical density of the layered region (the common logarithm of the reciprocal of the transmittance) is smaller than that of the entire n-type AlGaN semiconductor layer, so the wavelength is longer than the absorption edge wavelength ⁇ ae. Absorption at the side becomes very limited.
  • the average AlN mole fraction of the n-type AlGaN-based semiconductor layer should be about 4% higher than the AlN mole fraction of the metastable AlGaN. set.
  • the AlGaN composition ratio of metastable AlGaN is an integer ratio
  • the AlN mole fraction can take discrete values of about 8.33%. Therefore, the setting range of the average AlN mole fraction of the n-type AlGaN-based semiconductor layer is similarly a discrete range about 4% higher than the metastable AlGaN.
  • the absorption edge wavelength ⁇ ae determined by the average AlN mole fraction of the n-type AlGaN-based semiconductor layer is different from the peak emission wavelength ⁇ p. If the wavelength difference ( ⁇ p ⁇ ae) of 10 nm or more cannot be secured by using the above method, part of the emission spectrum (especially the part distributed on the short wavelength side) is absorbed in the n-type AlGaN semiconductor layer, and the external quantum efficiency decreases. lead to decline.
  • the discrete AlN mole fraction that can be taken by metastable AlGaN is increased by one step (approximately 8.33%) so that the wavelength difference ( ⁇ p ⁇ ae) becomes Similarly, the average AlN mole fraction of the n-type AlGaN-based semiconductor layer must be increased stepwise so as to be 10 nm or more.
  • the contact resistance between the n-electrode and the n-type AlGaN-based semiconductor layer and the bulk of the current flowing through the n-type AlGaN-based semiconductor layer increase. Due to the higher resistivity, the parasitic resistance of the current path between the n-electrode and the active layer becomes higher, reducing the wall plug efficiency.
  • the contact resistance between the n-type AlGaN-based semiconductor layer and the n-electrode is the AlN mol of the n-type AlGaN-based semiconductor layer.
  • the contact resistance can be adjusted to 0.01 ⁇ cm 2 or less by appropriately selecting the heat treatment temperature, and the forward voltage Vf is practically problematic. It becomes a level that does not exist (see Patent Document 4).
  • the wavelength difference ( ⁇ p- ⁇ ae) exceeds 10 nm in order to avoid a decrease in external quantum efficiency, the contact resistance and bulk resistivity will rather increase, resulting in a decrease in wall plug efficiency. Things can happen. Therefore, in order to avoid a decrease in the external quantum efficiency with respect to the target peak emission wavelength ⁇ p, if the average AlN mole fraction is set one step higher (about 8.33%), the wavelength difference ( ⁇ p - ⁇ ae) changes from less than 10 nm to more than 10 nm, and the wall plug efficiency decreases, the metastable AlGaN dominates in the layered region so that the wavelength difference does not significantly exceed 10 nm. It is preferable to set an average AlN mole fraction without forming . As a result, it is possible to prevent a decrease in external quantum efficiency and at the same time suppress a decrease in wall plug efficiency according to the target peak emission wavelength ⁇ p.
  • the present invention has been made in view of the above-mentioned problems, and aims to form metastable AlGaN locally in a layered region having a low AlN mole fraction in an n-type AlGaN semiconductor layer. By suppressing it, it is possible to prevent the deterioration of the luminous efficiency caused by the dominant formation of metastable AlGaN in the layered region.
  • the present invention provides a nitride semiconductor comprising a light-emitting element structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked vertically.
  • the n-type layer is composed of an n-type AlGaN semiconductor, the active layer disposed between the n-type layer and the p-type layer has a quantum well structure including one or more well layers made of an AlGaN-based semiconductor;
  • the p-type layer is composed of a p-type AlGaN semiconductor, each of the semiconductor layers in the n-type layer, the active layer, and the p-type layer is an epitaxial growth layer having a surface on which a multi-stepped terrace parallel to the (0001) plane is formed; wherein the n-type layer has a layered region with a locally low AlN mole fraction dispersed within the n-type layer;
  • Each extending direction of the layered region on a first plane perpendicular to the upper surface of the n-type layer has a portion that is inclined with respect to a line of intersection between the upper surface of the n-type layer and the first plane.
  • a first average AlN mole fraction Xna1 over the entire depth direction of the n-type layer is (n ⁇ 0.25)/12 ⁇ Xna1 ⁇ (n+0.25)/12 is within the range of
  • There is a region of n/12 A first feature of the present invention is that an intermediate AlGaN region having an AlN mole fraction of (n-0.5)/12 is formed in the layered region.
  • the third average AlN mole fraction Xna3 over the region from the upper end of the n-type layer to the specific depth is (n ⁇ 0.25)/12 ⁇ Xna3 ⁇ (n+0.25)/12 It is preferable to be within the range of
  • the second average AlN mole fraction Xna2(d) is, over the entire depth direction of the n-type layer, (n ⁇ 0.25)/12 ⁇ Xna2(d) ⁇ (n+0.25)/12 It is preferable to be within the range of
  • the present invention provides a nitride semiconductor comprising a light-emitting element structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked vertically.
  • a method for manufacturing an ultraviolet light emitting device The n-type layer of an n-type AlGaN semiconductor is epitaxially grown on a base portion including a sapphire substrate having a main surface inclined at a predetermined angle with respect to the (0001) plane.
  • the active layer having a quantum well structure including one or more well layers made of an AlGaN-based semiconductor is epitaxially grown, and a multistep terrace parallel to the (0001) plane is formed on the surface of the well layer.
  • the integer n is 6 or 7
  • a first average AlN mole fraction Xna1 over the entire depth direction of the n-type layer is (n ⁇ 0.25)/12 ⁇ Xna1 ⁇ (n+0.25)/12 is within the range of
  • a method for manufacturing a nitride semiconductor ultraviolet light emitting device is provided, the first feature of which is the formation of the n-type layer.
  • the second average AlN mole fraction Xna2(d) is set in the depth direction of the n-type layer across the (n ⁇ 0.25)/12 ⁇ Xna2(d) ⁇ (n+0.25)/12 It is preferable to form the n-type layer so as to be within the range of
  • the AlGaN-based semiconductor is represented by the general formula Al 1-x Ga x N (0 ⁇ x ⁇ 1), and the bandgap energies that GaN and AlN can take are the lower and upper limits, respectively.
  • a trace amount of impurities such as a group 3 element such as B or In or a group 5 element such as P may be contained within the range.
  • a GaN-based semiconductor is basically a nitride semiconductor composed of Ga and N, but it may contain trace amounts of impurities such as group 3 elements such as Al, B or In or group 5 elements such as P.
  • You can AlN-based semiconductors are nitride semiconductors basically composed of Al and N, but contain trace amounts of impurities such as group 3 elements such as Ga, B or In or group 5 elements such as P. You can Therefore, in the present application, the GaN-based semiconductor and the AlN-based semiconductor are each part of the AlGaN-based semiconductor.
  • n-type or p-type AlGaN semiconductors are AlGaN semiconductors doped with Si, Mg, or the like as donor or acceptor impurities.
  • an AlGaN-based semiconductor not specified as p-type or n-type means an undoped AlGaN-based semiconductor, but even an undoped AlGaN-based semiconductor does not include a small amount of donor or acceptor impurities that are unavoidably mixed.
  • the first plane is not an exposed surface specifically formed in the manufacturing process of the n-type layer or a boundary surface with another semiconductor layer, but an imaginary plane extending vertically in parallel within the n-type layer. is a flat surface.
  • an AlGaN-based semiconductor layer, a GaN-based semiconductor layer, and an AlN-based semiconductor layer are semiconductor layers made of an AlGaN-based semiconductor, a GaN-based semiconductor, and an AlN-based semiconductor, respectively.
  • the first average AlN mole fraction Xna1 is n/12, which is the discrete AlN mole fraction of metastable AlGaN where the integer n is 6 or 7. is controlled within a range of ⁇ 0.25/12 ( ⁇ about 2.08%) around , a first metastable AlGaN region (Al n Ga 12-n N 12 ) having an AlN mole fraction of n/12 is stably formed. Furthermore, the second average AlN mole fraction Xna2(d) is also distributed in the vicinity of n/12.
  • a second metastable AlGaN region (Al n ⁇ 1 Ga 13 -n N 12 ) is not predominantly formed, instead an intermediate AlGaN region (Al n-0.5 Ga 12.5 ⁇ n N 12 ) are formed.
  • the target peak emission wavelength ⁇ p is determined by the AlN mole fraction is within a certain wavelength range where it is difficult to simultaneously avoid a decrease in external quantum efficiency and a decrease in wall-plug efficiency due to the selection of discrete values of about 8.33%.
  • an intermediate AlGaN region is formed within the layered region and a first metastable AlGaN region is formed within the n-type body region to avoid a reduction in external quantum efficiency while at the same time reducing wall plug efficiency. can be suppressed.
  • the nitride semiconductor ultraviolet light emitting device having the first characteristic is manufactured, so metastable AlGaN is dominant in the layered region. A decrease in luminous efficiency due to the formation can be prevented.
  • the present invention provides that the second average AlN mole fraction Xna2(d) in a region from the upper end of the n-type layer to the specific depth, Xna2(d) ⁇ n/12
  • the present invention provides a nitride semiconductor ultraviolet light emitting device having a second feature that it is within the range of:
  • the present invention is characterized in that, in the first step, the second average AlN mole fraction Xna2(d) is from the upper end of the n-type layer to the specific depth. in the area Xna2(d) ⁇ n/12
  • a method for manufacturing a nitride semiconductor ultraviolet light emitting device characterized in that the n-type layer is formed so as to fall within the range of:
  • the contact resistance between the n-electrode and the n-type layer, and The bulk resistivity with respect to the current flowing through the upper layer portion of the n-type layer can be further suppressed, and the deterioration of the wall plug efficiency can be further suppressed while avoiding the deterioration of the external quantum efficiency.
  • Xna2(d) ⁇ n/12 It is preferable to be within the range of
  • the second average AlN mole fraction Xna2(d) is the specific depth of the n-type layer.
  • absorption of light emitted from the active layer can be further suppressed in a region deeper than the specific depth of the n-type layer, and a decrease in external quantum efficiency can be further suppressed.
  • the peak emission wavelength is set to a predetermined value within the range of 280 nm to 315 nm when the integer n is 7, and the integer n is 6, it is preferably set to a predetermined value within the range of 300 nm to 330 nm.
  • the peak emission wavelength of the nitride semiconductor ultraviolet light emitting element is within the range of 300 nm to 330 nm when the integer n is 6, so that the peak emission wavelength is a predetermined value within the range of 280 nm to 315 nm when the integer n is 7. It is preferable to form the active layer so as to have a predetermined value.
  • the integer n when the integer n is 7, for a particular peak emission wavelength set to a predetermined value within the range of 280 nm to 315 nm, when the integer n is 6:
  • a specific peak emission wavelength set to a predetermined value within the range of 300 nm to 330 nm. can be done.
  • the specific peak emission wavelength is within the specific wavelength range described above, and the formation of metastable AlGaN predominantly in the layered region reduces the emission efficiency (external quantum efficiency or wall plug efficiency). It is the peak emission wavelength at which degradation can occur.
  • the active layer has a multiple quantum well structure including two or more well layers, and an AlGaN-based quantum well layer is disposed between the two well layers.
  • a barrier layer composed of a semiconductor is preferably present.
  • the well layer made of an AlGaN-based semiconductor and the barrier layer made of an AlGaN-based semiconductor are alternately formed. It is preferable to form the active layer having a multiple quantum well structure including two or more well layers by epitaxial growth.
  • the active layer has a multiple quantum well structure, and an improvement in luminous efficiency can be expected compared to the case where there is only one well layer.
  • the nitride semiconductor ultraviolet light emitting device further includes a base portion including a sapphire substrate, and the sapphire substrate has a main surface inclined at a predetermined angle with respect to the (0001) plane. and the light emitting element structure is formed above the main surface, and each semiconductor layer from the main surface of the sapphire substrate to the p-type layer is formed in a multi-step shape parallel to the (0001) plane It is preferably an epitaxially grown layer having a terraced surface.
  • epitaxial growth can be performed using a sapphire substrate having an off-angle so that multi-stepped terraces appear on the surface of each layer from the main surface of the sapphire substrate to the surface of the active layer. It is possible to realize a nitride semiconductor ultraviolet light emitting device having the above characteristics.
  • the nitride semiconductor ultraviolet light-emitting device or the method for manufacturing the nitride semiconductor ultraviolet light-emitting device having the first or second feature at a specific peak emission wavelength, AlN moles are locally present in the n-type AlGaN-based semiconductor layer.
  • the embodiment that utilizes metastable AlGaN in the layered region of the n-type layer and the embodiment that does not utilize the metastable AlGaN are adjusted to the target peak emission wavelength. By using them properly according to need, it is possible to increase the degree of freedom in setting the peak emission wavelength while preventing a decrease in luminous efficiency.
  • FIG. 2 is a diagram schematically showing the wurtzite crystal structure of AlGaN;
  • FIG. 2 is a plan view showing the positional relationship between each site on the A plane and each site on the B plane viewed from the c-axis direction of the wurtzite crystal structure shown in FIG. 1 ;
  • FIG. 2 is a cross-sectional view of essential parts schematically showing an example of the structure of the nitride semiconductor ultraviolet light emitting device according to the first embodiment
  • FIG. 5 is a cross-sectional view of a main part schematically showing an example of the lamination structure of the active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4
  • FIG. 6 is a diagram schematically showing a more detailed structure of the inclined area IA shown in FIG. 5;
  • the emission wavelength of the quantum well structure composed of the AlGaN well layer and the AlGaN barrier layer, the film thickness of the well layer, and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 50%.
  • a graph showing the relationship.
  • a graph showing the relationship between 4 is a graph showing the relationship between the emission wavelength of a quantum well structure composed of GaN well layers and AlGaN barrier layers, the film thickness of the well layers, and the AlN mole fraction of the barrier layers.
  • FIG. 5 is a plan view schematically showing an example of the structure of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4 when viewed from the upper side of FIG. 4; 7 is a graph showing an example of changes in second average AlN mole fraction Xna2(d) at depth d from the upper end of the n-type layer according to depth d; SEM image showing the main part of the cross section of the sample piece measured for the AlN mole fraction by the CL method.
  • FIG. 14 is a diagram showing first and second CL spectra on six Y-coordinates on the measurement cross section of the sample piece shown in FIG.
  • FIG. 2 is a cross-sectional view of a main part schematically showing an example of the structure of a nitride semiconductor ultraviolet light emitting device according to a second embodiment
  • FIG. 17 is a cross-sectional view of a main part schematically showing an example of a layered structure of a main part including an active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 16
  • FIG. 2 is a cross-sectional view of a main part schematically showing an example of an element structure of a general ultraviolet light emitting diode;
  • a nitride semiconductor ultraviolet light emitting device (hereinafter simply referred to as "light emitting device") according to an embodiment of the present invention will be described based on the drawings.
  • the contents of the invention are schematically shown with emphasis on the essential parts for easy understanding of the explanation, so the dimensional ratio of each part does not necessarily correspond to the actual element. They do not have the same dimensional ratio.
  • the light-emitting element is a light-emitting diode.
  • the light-emitting device 1 of the first embodiment includes a base portion 10 including a sapphire substrate 11, a plurality of AlGaN-based semiconductor layers 21 to 24, a p-electrode 26, and an n-electrode 27. and a structure 20 .
  • the light emitting element 1 is mounted (flip-chip mounted) with the side of the light emitting element structure 20 (upper side in FIG. 4) facing a base for mounting (such as a submount). The direction is the base portion 10 side (lower side in FIG. 4).
  • the direction perpendicular to the main surface 11a of the sapphire substrate 11 (or the upper surfaces of the underlying portion 10 and the AlGaN semiconductor layers 21 to 24) is referred to as the "vertical direction” (or the “vertical direction”).
  • the direction from the base portion 10 to the light emitting element structure portion 20 is defined as the upward direction, and the opposite direction is defined as the downward direction.
  • a plane parallel to the vertical direction is referred to as a "first plane".
  • a plane parallel to the main surface 11a of the sapphire substrate 11 (or the upper surfaces of the underlying portion 10 and the AlGaN-based semiconductor layers 21 to 24) is referred to as a "second plane”, and a direction parallel to the second plane is referred to as a "second plane.” lateral direction”.
  • the underlying portion 10 includes a sapphire substrate 11 and an AlN layer 12 directly formed on the principal surface 11 a of the sapphire substrate 11 .
  • the sapphire substrate 11 has a main surface 11a inclined at an angle (off angle) within a certain range (for example, about 0.3° to 6°) with respect to the (0001) plane, and has a multistep shape on the main surface 11a. This is a slightly inclined substrate with exposed terraces.
  • the AlN layer 12 is composed of AlN crystal epitaxially grown from the main surface of the sapphire substrate 11 , and this AlN crystal has an epitaxial crystal orientation relationship with respect to the main surface 11 a of the sapphire substrate 11 .
  • the AlN crystal is grown such that the C-axis direction ( ⁇ 0001> direction) of the sapphire substrate 11 and the C-axis direction of the AlN crystal are aligned.
  • the AlN crystal forming the AlN layer 12 may be an AlN-based semiconductor layer that may contain a small amount of Ga or other impurities.
  • the film thickness of the AlN layer 12 is assumed to be approximately 2 ⁇ m to 3 ⁇ m.
  • the structure of the base portion 10, the substrates to be used, and the like are not limited to those described above.
  • an AlGaN-based semiconductor layer having an AlN mole fraction equal to or higher than the AlN mole fraction of the AlGaN-based semiconductor layer 21 may be provided between the AlN layer 12 and the AlGaN-based semiconductor layer 21.
  • the AlGaN-based semiconductor layers 21 to 24 of the light-emitting element structure 20 are composed of an n-type cladding layer 21 (n-type layer), an active layer 22, an electron block layer 23 (p-type layer), and a p-type layer in this order from the underlying portion 10 side. It has a structure in which the type contact layer 24 (p-type layer) is epitaxially grown in order and laminated.
  • the AlN layer 12 of the base portion 10 epitaxially grown in order from the main surface 11a of the sapphire substrate 11, the n-type cladding layer 21 of the light emitting element structure portion 20, each semiconductor layer in the active layer 22, and the electron block.
  • the layer 23 has a surface formed with multi-stepped terraces parallel to the (0001) plane derived from the main surface 11a of the sapphire substrate 11 by step-flow growth.
  • the p-type contact layer 24 of the p-type layer since it is formed by epitaxial growth on the electron blocking layer 23, a similar multi-stepped terrace can be formed, but a similar multi-stepped terrace is not necessarily formed. It does not have to have a surface.
  • the active layer 22, the electron blocking layer 23, and the p-type contact layer 24 in the light-emitting element structure 20 are laminated on the second region R2 on the upper surface of the n-type cladding layer 21.
  • the etched portion is removed by etching or the like and formed on the first region R1 on the upper surface of the n-type cladding layer 21 .
  • the upper surface of the n-type cladding layer 21 is exposed in the second region R2 except for the first region R1.
  • the upper surface of the n-type clad layer 21 may differ in height between the first region R1 and the second region R2.
  • the upper surface is separately defined in the first region R1 and the second region R2.
  • n-type cladding layer 21 composed of an n-type AlGaN semiconductor
  • multi-stepped terraces parallel to the (0001) plane are formed on the growth surface in the epitaxial growth process, and Ga, which is easy to mass transfer, moves between adjacent terraces.
  • Ga which is easy to mass transfer, moves between adjacent terraces.
  • the inclined region is formed by extending obliquely upward.
  • the layered regions 21a having a low AlN mole fraction are locally dispersed uniformly in the n-type cladding layer 21.
  • Each extending direction of the layered region 21a on the first plane has a portion that is inclined with respect to the line of intersection between the upper surface of the n-type cladding layer 21 and the first plane.
  • the formation of the Ga-enriched n-type region in the graded region that is, mass transfer of Ga from the terrace region to the graded region causes In part, the density of Al can be relatively increased to form an Al-enriched n-type region in which the AlN mole fraction is higher than the average AlN mole fraction.
  • the layered region 21a extends in an oblique direction with respect to the surface of the n-type cladding layer 21, and the bandgap energy is locally reduced, so that carriers are easily localized. and functions as a low-resistance current path.
  • a region other than the layered region 21a in the n-type cladding layer 21 is called an n-type body region 21b.
  • the first average AlN mole fraction Xna1 over the entire depth of the n-type cladding layer 21 is within the range represented by the following inequality (1).
  • n in the following formula (1) is an integer of 6 or 7. (n ⁇ 0.25)/12 ⁇ Xna1 ⁇ (n+0.25)/12 (1)
  • the third average AlN mole fraction Xna3 over the region from the upper surface of the n-type cladding layer 21 to the specific depth dx is within the range represented by the following inequality (2) preferably in (n ⁇ 0.25)/12 ⁇ Xna3 ⁇ (n+0.25)/12 (2)
  • the above relationship is satisfied for each of the specific depths dx.
  • the second average AlN mole fraction Xna2(d) is within the range represented by the following inequality of formula (3) throughout the depth direction of the n-type cladding layer 21 is more preferable. (n ⁇ 0.25)/12 ⁇ Xna2(d) ⁇ (n+0.25)/12 (3)
  • Xna2(d) is preferably within the range represented by the following inequality (4) in the region from the upper surface of the n-type cladding layer 21 to the specific depth dx. Furthermore, in the above preferred embodiment in which Xna2(d) is within the range represented by formula (3), Xna2(d) is the following in a region from the upper surface of the n-type cladding layer 21 to a specific depth dx: It is more preferable to be within the range represented by the inequality of formula (5). Xna2(d) ⁇ n/12 (4) (n ⁇ 0.25)/12 ⁇ Xna2(d) ⁇ n/12 (5)
  • Xna2(d) is within the range represented by formula (4) or formula (5)
  • Xna2(d) is expressed by the following formula (6) in a region deeper than the specific depth dx ), more preferably within the range represented by the inequality of formula (7).
  • n/12 ⁇ Xna2(d) (6) n/12 ⁇ Xna2(d) ⁇ (n+0.25)/12 (7)
  • the first average AlN mole fraction Xna1 is the AlN mole fraction ( n / 12 ) is controlled within a range of ⁇ 0.25/12 ( ⁇ about 2.08%) around the reference, the first metastable AlGaN region is uniform in the n-type body region 21b. is formed in In the region where the second average AlN mole fraction Xna2(d) is lower than the AlN mole fraction (n/12) of the first metastable AlGaN region, the Al-enriched n-type region of the n-type body region 21b A first metastable AlGaN region is formed therein.
  • the AlN mole fraction is one step (approximately 8.33%) smaller than that of the first metastable AlGaN region, and the AlGaN composition ratio is Al n ⁇ 1 Ga 13-n N 12 with an integer ratio.
  • a second metastable AlGaN region can be formed with the mass transfer of Ga during the formation of the layered region 21a.
  • the second metastable AlGaN region is not predominantly formed, but instead an intermediate AlGaN region in which the AlN mole fraction is intermediate between the first and second metastable AlGaN regions. (Al n-0.5 Ga 12.5-n N 12 ) is formed.
  • the thickness of the n-type cladding layer 21 is assumed to be about 1 ⁇ m to 2 ⁇ m, which is the same as the thickness used in general nitride semiconductor ultraviolet light emitting devices. , 2 ⁇ m to 4 ⁇ m.
  • the active layer 22 includes two or more well layers 220 composed of AlGaN-based semiconductors (excluding AlN-based semiconductors) and one or more layers composed of AlGaN-based semiconductors (excluding GaN-based semiconductors) or AlN-based semiconductors. It has a multiple quantum well structure in which barrier layers 221 are alternately laminated. It is not always necessary to provide the barrier layer 221 between the bottom well layer 220 and the n-type cladding layer 21 . In addition, in this embodiment, the barrier layer 221 is not provided between the uppermost well layer 220 and the electron blocking layer 23. An AlGaN layer or an AlN layer may be provided.
  • the electron block layer 23 is composed of a p-type AlGaN semiconductor.
  • the p-type contact layer 24 is composed of a p-type AlGaN semiconductor or a p-type GaN semiconductor.
  • the p-type contact layer 24 is typically composed of p-GaN.
  • FIG. 5 schematically shows an example of a laminated structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in the active layer 22 .
  • FIG. 5 illustrates a case where the well layers 220 and the barrier layers 221 each include three layers. Three layers of a barrier layer 221 and a well layer 220 are laminated in this order on the n-type cladding layer 21, and the electron blocking layer 23 is positioned on the well layer 220 of the uppermost layer.
  • FIG. 6 schematically shows, for example, a stepped structure (macrostep structure) appearing on the surface of the inclined region IA of one well layer 220. As shown in FIG.
  • the well layer 220 is composed of an AlGaN-based semiconductor and the AlN mole fraction is not 0%
  • the mass of Ga from the terrace region TA to the inclined region IA is Due to the migration, a Ga-enriched well region 220a having an AlN mole fraction lower than the average AlN mole fraction Xwa in the well layer 220 is formed in the graded region IA.
  • the density of Al may be relatively increased to form an Al-enriched well region in which the AlN mole fraction is higher than the average AlN mole fraction Xwa.
  • the average AlN mole fraction Xwa of the well layer 220 is, for example, Ga-enriched
  • the AlN mole fraction difference between the inclined region IA and the terrace region TA in the well layer 220 is 4% or less, which can suppress the occurrence of double emission peaks due to the AlN mole fraction difference.
  • the barrier layer 221 when the barrier layer 221 is composed of an AlGaN-based semiconductor (excluding an AlN-based semiconductor), the barrier layer 221 also has an AlN mole fraction in the graded region IA equal to the average of the barrier layer 221.
  • a Ga-enriched barrier region 221a having a lower AlN mole fraction Xba is formed.
  • an Al-enriched barrier region having a higher AlN mole fraction than the average AlN mole fraction Xba of the barrier layer 221 is formed in a part of the terrace region TA, similar to the well layer 220. It's okay to be there.
  • the average AlN mole fraction Xba of the barrier layer 221 is approximately , Xb0+2% to Xb0+8%. As a result, about 2% or more is ensured as the AlN mole fraction difference between the Ga-enriched barrier region 221a of the barrier layer 221 and the terrace region TA.
  • the AlN mole fraction of the terrace region TA of the barrier layer 221 is 1% or more higher than the AlN mole fraction of the Ga-enriched barrier region 221a within a range of approximately 51% to 90%, It is set to be higher, preferably 2% or more, more preferably 4% or more.
  • the AlN mole fraction difference between the Ga-enriched barrier region 221a and the terrace region TA in the barrier layer 221 is set to 4 to 5% or more. Although it is preferable to do so, the effect of carrier localization can be expected even with a concentration of about 1 to 2%.
  • the Ga-enriched EB region 23a having a lower AlN mole fraction than the average AlN mole fraction Xea of the electron blocking layer 23 is formed in the gradient region IA. Furthermore, in one embodiment, similarly to the well layer 220, an Al-enriched EB region having a higher AlN mole fraction than the average AlN mole fraction Xea of the electron blocking layer 23 is formed in a part of the terrace region TA. It's okay to be.
  • the AlN mole fraction of the terrace region TA of the electron blocking layer 23 is generally within the range of 69% to 90%, and is 20% or more, preferably 25% or more, more preferably 25% or more than the AlN mole fraction of the terrace region of the well layer 220. is set to be higher than 30%. Furthermore, the AlN mole fraction of the Ga-enriched EB region 23a of the electron blocking layer 23 is 20% or more, preferably 25% or more, more preferably 30%, more than the AlN mole fraction of the Ga-enriched well region 220a of the well layer 220. It is set to be higher than %.
  • the average AlN mole fraction Xea of the electron block layer 23 is , Xe0+2% to Xe0+8%. Thereby, about 2% or more is ensured as the AlN mole fraction difference between the Ga-enriched EB region 23a of the electron block layer 23 and the terrace region TA.
  • the AlN mole fraction locally present in the graded region IA of the barrier layer 221
  • the Ga-enriched barrier region 221a with a low AlN mole fraction carriers are easily localized. , carriers are more likely to be localized. Therefore, from the n-type cladding layer 21 side through the layered region 21a, from the electron blocking layer 23 side through the Ga-enriched EB region 23a, the Ga-enriched well region 220a of the well layer 220 is efficiently charged. , and the recombination of carriers (electrons and holes) in the well layer 220 can improve the luminous efficiency.
  • the film thickness of the well layer 220 including the terrace region TA and the inclined region IA, is set within a range of, for example, 3ML to 14ML according to the target value of the peak emission wavelength ⁇ p of the light emitting element 1.
  • the film thickness of the barrier layer 221 is set within a range of 6 nm to 8 nm, for example, including the terrace area TA and the inclined area IA.
  • the film thickness of the electron blocking layer 23 is set within a range of, for example, 15 nm to 30 nm (optimum value is about 20 nm), including the terrace area TA and the inclined area IA.
  • the AlN mole fraction and film thickness of the well layer 220 are
  • the AlN mole fraction of the barrier layer and the electron blocking layer 23 adjacent to the well layer 220 is the peak emission of the light-emitting device 1. It is set according to the target value of the wavelength ⁇ p.
  • the barrier layers and the electron block layer 23 adjacent to the well layer 220 are set according to the target value of the peak emission wavelength ⁇ p of the light emitting device 1 .
  • FIG. 7 shows the quantum well structure model in which the well layer 220 and the barrier layer 221 are made of AlGaN, and the film thickness of the well layer is varied within the range of 3ML to 14ML or 4ML to 14ML.
  • It is a graph of the simulation result of the emission wavelength (corresponding to the peak emission wavelength) obtained by As a condition of the above simulation, it is assumed that the Ga-enriched well region 220a of the well layer 220 is dominated by metastable AlGaN having an AlGaN composition ratio of an integer ratio, and the Ga-enriched well region 220a of the well layer 220 is 50% (1/2), which is the AlN mole fraction of metastable AlGaN, in FIG.
  • metastable AlGaN having an AlGaN composition ratio of Al 1 Ga 1 N 2 , Al 5 Ga 7 N 12 , or Al 1 Ga 2 N 3 is present in the Ga-enriched well region 220 a of the well layer 220 .
  • the film thickness of the well layer 220 is set within the range of 3 ML to 14 ML, and the AlN mole fraction of the Ga-enriched barrier region 221a of the barrier layer 221 is set to 66. It can be seen that the peak emission wavelength can be set within the range of 246 nm to 328 nm by adjusting each within the range of 7% to 100%.
  • FIG. 10 shows that the AlN mole fraction of the barrier layer is 66.7% (AlGaN) and 100% (AlN) for the quantum well structure model in which the well layer is GaN and the barrier layer is AlGaN or AlN.
  • 2 is a graph of simulation results of emission wavelengths (corresponding to peak emission wavelengths) obtained by changing the film thickness of the well layer within the range of 4 ML to 10 ML. From FIG. 10, it can be seen that the emission wavelength varies within the range of approximately 270 nm to 325 nm.
  • the film thickness of the well layer 220 is within the range of 4 ML to 10 ML, and the Ga-enriched barrier region 221a of the barrier layer 221 is It can be seen that by adjusting the AlN mole fraction within the range of 66.7% to 100%, the peak emission wavelength can be set within the range of 270 nm to 325 nm.
  • the first average AlN mole fraction Xna1 is within the range shown by the above formula (1), that is, within the range of about 56.3% to about 60.4%.
  • the absorption edge wavelength ( ⁇ ae) of the n-type cladding layer 21 determined by the average AlN molar fraction Xna1 of 1 is located within the range of about 263 nm to about 269 nm. Therefore, if the target value of the peak emission wavelength ⁇ p is within the above range of 280 nm to 315 nm, the wavelength difference ( ⁇ p ⁇ ae) between the peak emission wavelength ( ⁇ p) and the absorption edge wavelength ( ⁇ ae) is ensured to be 10 nm or more.
  • the first average AlN mole fraction Xna1 is within the range shown by the above formula (1), that is, within the range of about 47.9% to about 52.1%.
  • the absorption edge wavelength ( ⁇ ae) of the n-type cladding layer 21 determined by the average AlN molar fraction Xna1 of 1 is located within the range of about 276 nm to about 283 nm. Therefore, if the target value of the peak emission wavelength ⁇ p is within the above range of 300 nm to 330 nm, the wavelength difference ( ⁇ p ⁇ ae) between the peak emission wavelength ( ⁇ p) and the absorption edge wavelength ( ⁇ ae) is sufficiently 10 nm or more.
  • the lower limit of the target value from 300 nm to about 293 nm.
  • the contact resistance between the n-type cladding layer 21 and the n-electrode 27 formed on its exposed surface is higher than when the AlN mole fraction is less than 50%, but higher than when the AlN mole fraction is 60%. is low, and a significant increase in contact resistance is suppressed.
  • the first average AlN mole fraction Xna1 and the second average AlN mole fraction Xna2(d) are set to Since it is not set unnecessarily high, an unnecessary increase in bulk resistivity is also suppressed.
  • the p-electrode 26 is composed of a multilayer metal film such as Ni/Au, and is formed on the upper surface of the p-type contact layer 24 .
  • the n-electrode 27 is composed of, for example, a multilayer metal film such as Ti/Al/Ti/Au, and is formed on a part of the exposed surface in the second region R2 of the n-type cladding layer 21 .
  • the p-electrode 26 and the n-electrode 27 are not limited to the multilayer metal films described above, and the electrode structure such as the metals constituting each electrode, the number of layers, and the order of layers may be changed as appropriate. FIG.
  • FIG. 11 shows an example of the shape of the p-electrode 26 and the n-electrode 27 viewed from above the light emitting element 1 .
  • a line BL existing between the p-electrode 26 and the n-electrode 27 indicates a boundary line between the first region R1 and the second region R2, and includes the active layer 22, the electron blocking layer 23, and the p-type electrode. It coincides with the outer peripheral side wall surface of the contact layer 24 .
  • the first region R1 and the p-electrode 26 have a comb shape as an example in plan view.
  • the visual shape, arrangement, and the like are not limited to those illustrated in FIG. 11 .
  • the AlN layer 12 included in the base portion 10 and the nitride semiconductor layers 21 to 24 included in the light emitting element structure portion 20 are epitaxially grown in order on the sapphire substrate 11 by the metal-organic compound vapor phase epitaxy (MOVPE) method. lamination.
  • MOVPE metal-organic compound vapor phase epitaxy
  • the n-type cladding layer 21 is doped with, for example, Si as a donor impurity
  • the electron block layer 23 and the p-type contact layer 24 are doped with, for example, Mg as an acceptor impurity.
  • the surfaces of the AlN layer 12, the n-type cladding layer 21, the active layer 22 (well layer 220 and barrier layer 221), and the electron block layer 23 are provided with multistep terraces parallel to the (0001) plane.
  • the main surface 11a is inclined with respect to the (0001) plane at an angle (off angle) within a certain range (for example, about 0.3 ° to 6 °), A slightly inclined substrate is used in which a multi-stepped terrace is exposed on the main surface 11a.
  • a growth rate at which multi-stepped terraces are likely to appear (specifically, for example, growth temperature, raw material gas and carrier
  • the growth rate is achieved by appropriately setting various conditions such as gas supply amount and flow rate. Since these conditions may vary depending on the type and structure of the film forming apparatus, several samples are actually produced in the film forming apparatus to specify these conditions.
  • the growth starting point of the layered region 21a is set in the stepped portion (inclined region) between the multi-stepped terraces formed on the upper surface of the AlN layer 12 immediately after the growth is started by the mass transfer of Ga.
  • the growth temperature, the growth pressure, and the donor impurity concentration are set so that the layered region 21a can grow obliquely upward due to the segregation associated with the mass transfer of Ga, following the epitaxial growth of the n-type cladding layer 21. selected.
  • the growth temperature is preferably 1050° C. or higher at which mass transfer of Ga easily occurs and 1150° C. or lower at which good n-type AlGaN can be prepared.
  • the growth temperature is preferably 1050° C. or higher at which mass transfer of Ga easily occurs and 1150° C. or lower at which good n-type AlGaN can be prepared.
  • the mass of Ga due to excessive migration, the AlN mole fraction tends to fluctuate randomly even in metastable AlGaN, so it becomes difficult to stably form a metastable AlGaN region with an AlN mole fraction of 50% to 58.3%. there is a possibility.
  • the growth pressure 75 Torr or less is preferable as a favorable AlGaN growth condition, and 10 Torr or more is realistic and preferable as the control limit of the film forming apparatus.
  • the donor impurity concentration is preferably about 1 ⁇ 10 18 to 5 ⁇ 10 18 cm ⁇ 3 .
  • the above growth temperature, growth pressure, and the like are only examples, and optimal conditions may be appropriately specified according to the film forming apparatus to be used.
  • the supply amounts and flow rates of source gases (trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas, and ammonia gas) and carrier gases used in the metal-organic compound vapor phase epitaxy are determined by the first is set as a target value of the average AlN mole fraction Xna1.
  • the second average AlN mole fraction Xna2(d) sandwiches the AlN mole fraction (n/12) of the first metastable AlGaN region as the depth d changes. is changing.
  • the change in the AlN mole fraction Xna2(d) can be realized by positively modulating the supply amount and flow velocity of the raw material gas and carrier gas, or modulating the growth temperature.
  • the natural change in substrate surface temperature that occurs as the n-type cladding layer 21 grows and becomes thicker may be utilized. In this case, the supply amounts and flow velocities of the raw material gas and the carrier gas are set according to the tendency of the substrate surface temperature change.
  • the donor impurity concentration does not necessarily have to be controlled uniformly in the vertical direction with respect to the thickness of the n-type cladding layer 21 .
  • the impurity concentration in the predetermined thin film thickness portion in the n-type cladding layer 21 is lower than the above set concentration, for example, less than 1 ⁇ 10 18 cm ⁇ 3 , more preferably 1 ⁇ 10 17 cm ⁇ 3 or less. It may be a controlled low impurity concentration layer.
  • the thickness of the low impurity concentration layer is preferably greater than 0 nm and approximately 200 nm or less, more preferably approximately 10 nm or more and 100 nm or less, and further preferably approximately 20 nm or more and 50 nm or less.
  • the donor impurity concentration of the low impurity concentration layer may be lower than the set concentration, and may partially include an undoped layer (0 cm ⁇ 3 ). Furthermore, part or all of the low impurity concentration layer preferably exists in an upper layer region with a depth of 100 nm or less downward from the upper surface of the n-type cladding layer 21 .
  • the entire upper surface of the n-type cladding layer 21 is subsequently subjected to metal organic chemical vapor phase epitaxy (MOVPE) or the like.
  • MOVPE metal organic chemical vapor phase epitaxy
  • the active layer 22 (well layer 220, barrier layer 221), electron block layer 23, p-type contact layer 24, etc. are formed by the known epitaxial growth method.
  • the acceptor impurity concentration of the electron block layer 23 is preferably about 1.0 ⁇ 10 16 to 1.0 ⁇ 10 18 cm ⁇ 3
  • the acceptor impurity concentration of the p-type contact layer 24 is, for example, 1.0.
  • About ⁇ 10 18 to 1.0 ⁇ 10 20 cm ⁇ 3 is preferable.
  • the acceptor impurity concentration does not necessarily have to be controlled uniformly in the vertical direction with respect to each film thickness of the electron blocking layer 23 and the p-type contact layer 24 .
  • the well layer 220 was grown under the growth conditions under which the above-described multi-stepped terraces are likely to appear, with the average AlN mole fraction Xwa of the well layer 220 as a target value.
  • the barrier layer 221 is grown with the average AlN mole fraction Xba of the barrier layer 221 as a target value.
  • the average AlN molar fractions Xwa and Xba of the well layer 220 and the barrier layer 221 are as described above, and redundant description is omitted.
  • the average AlN mole fraction Xea of the electron blocking layer 23 is set as a target value.
  • An electron blocking layer 23 is grown.
  • the average AlN mole fraction Xea of the electron blocking layer 23 is as described above, and redundant description is omitted.
  • the growth temperature of the active layer 22 (well layer 220, barrier layer 221), the electron blocking layer 23, and the p-type contact layer is set to T1 for the growth temperature of the n-type cladding layer 21, and T1 for the growth temperature of the active layer 22.
  • T2 the growth temperature of the electron blocking layer 23
  • T4 the p-type contact layer growth temperature
  • the growth temperature T3 of the electron blocking layer 23 can be reduced, for example, by increasing the flow rate of the nitrogen source gas and decreasing the growth rate.
  • the GaN When the growth temperature T3 of the electron blocking layer 23 is raised from the growth temperature T2 of the active layer 22, the GaN is decomposed in the well layer 220 positioned therebelow during the transition process of the growth temperature.
  • the characteristics of the light-emitting element 1 may deteriorate due to decomposition. Therefore, in order to suppress decomposition of the GaN, a film thinner than the barrier layer 221 (for example, 3 nm or less, preferably less than 3 nm) is placed between the uppermost well layer 220 and the electron blocking layer 23 to prevent the decomposition of the GaN. , 2 nm or less) with a higher AlN mole fraction than the barrier layer 221 and electron block layer 23 .
  • the active layer 22 (well layer 220, barrier layer 221), the electron blocking layer 23, the p-type contact layer 24, etc. on the entire upper surface of the n-type cladding layer 21 in the manner described above, next, By a well-known etching method such as reactive ion etching, the second regions R2 of the nitride semiconductor layers 21 to 24 are selectively etched until the upper surface of the n-type cladding layer 21 is exposed. A portion of the second region R2 on the upper surface is exposed.
  • a p-electrode 26 is formed on the p-type contact layer 24 in the unetched first region R1 by a well-known film formation method such as an electron beam evaporation method, and an n electrode in the etched first region R2 is formed.
  • An n-electrode 27 is formed on the mold cladding layer 21 .
  • heat treatment may be performed by a known heat treatment method such as RTA (rapid thermal annealing).
  • the light emitting element 1 is flip-chip mounted on a base such as a submount, and then sealed with a predetermined resin (for example, a lens-shaped resin) such as silicone resin or amorphous fluorine resin.
  • a predetermined resin for example, a lens-shaped resin
  • silicone resin or amorphous fluorine resin can be used in any situation.
  • the cross-sectional structure of the AlGaN-based semiconductor layers 21 to 24 of the light-emitting device 1 manufactured in the above manner is as follows.
  • a sample piece having a vertical (or substantially vertical) cross section can be processed with a focused ion beam (FIB) and observed by an HAADF-STEM image of the sample piece.
  • a HAADF-STEM image provides a contrast proportional to the atomic weight, and heavy elements are displayed brightly. Therefore, regions with a low AlN mole fraction are displayed relatively brightly.
  • HAADF-STEM images are more suitable for observing differences in AlN mole fractions than normal STEM images (bright field images).
  • composition analysis within specific semiconductor layers in the AlGaN-based semiconductor layers 21 to 24 is performed using the above sample piece by energy dispersive X-ray spectroscopy (cross-sectional TEM-EDX) or CL (cathode luminescence) method.
  • cross-sectional TEM-EDX energy dispersive X-ray spectroscopy
  • CL cathode luminescence
  • a sample for composition analysis of the n-type cladding layer 21 is prepared, and a sample piece having a cross section perpendicular (or substantially perpendicular) to the upper surface of the n-type cladding layer 21 is processed with a focused ion beam (FIB) from the sample, A sample piece for measurement was produced.
  • FIB focused ion beam
  • the sample was prepared by forming the n-type cladding layer 21 and AlN molar ratio higher than that of the n-type cladding layer 21 on the underlayer 10 composed of the sapphire substrate 11 and the AlN layer 12 according to the manufacturing procedure of the n-type cladding layer 21 and the like.
  • a fractional AlGaN layer, an AlGaN layer for protecting the surface of the sample, and a protective resin film were deposited in order.
  • a sapphire substrate 11 having a main surface at an off-angle with respect to the (0001) plane was used, and an underlayer 10 in which a multi-stepped terrace was exposed on the surface of an AlN layer 12 was used.
  • the dose of donor impurities (Si) was controlled so that the donor impurity concentration was approximately 3 ⁇ 10 18 cm ⁇ 3 .
  • FIG. 12 shows measured values of the second average AlN mole fraction Xna2(d) by the RBS analysis method.
  • the AlN mole fraction Xna2(d) varies within the range of about 57.8% to about 59.9% according to the change in depth d, satisfying the above formula (3).
  • the first average AlN mole fraction Xna1 is calculated to be about 58.4% from Xna2(d) shown in FIG. They are slightly higher, but they are roughly the same. As will be described later, there is one specific depth dx, which is approximately 1061 nm.
  • the third average AlN mole fraction Xna3 is calculated to be about 57.9% from Xna2(d) shown in FIG. Is pleased.
  • the second average AlN mole fraction Xna2(d) is constant at about 57.8% in the upper layer region with a depth d (nm) of 0 nm to about 750 nm, and As the depth d (nm) increases from about 750 nm to about 1900 nm, it gradually increases from about 57.8% to about 59.9%, and the first metastable It is equal to the AlN mole fraction (approximately 58.3%) of the AlGaN region. Therefore, in the example shown in FIG. 12, the specific depth dx is approximately 1061 nm.
  • a He 2+ ion beam (beam diameter: 2.2 mm) is vertically irradiated from the upper surface side of the n-type cladding layer 21 of the sample at an acceleration voltage of 2.3 MeV. Since the range is as large as 300 nm, the film thickness to be analyzed should be greater than 300 nm.
  • FIG. 13 is a scanning electron microscope (SEM) image showing the main part including the n-type cladding layer 21 on the cross section of the sample piece to be measured.
  • the measurement range of the sample piece (the range of the incident point of the electron beam irradiated for measurement) is 6.5 mm in each of the X direction (horizontal direction parallel to the second plane) and Y direction (vertical direction orthogonal to the second plane).
  • the incident points of the electron beams are set in a grid pattern of 121 mesh ⁇ 41 mesh with 25 ⁇ m and 2.2 ⁇ m.
  • the mesh spacing is about 52 nm in the X direction and about 55 nm in the Y direction.
  • An electron beam with a beam diameter of 50 nm (diameter) was irradiated once to the grid-shaped electron beam incident point within the measurement range of the sample piece, and the CL spectrum at each incident point was measured.
  • FIG. 14 shows the 121 A first CL spectrum (solid line) and a second CL spectrum (dashed line) derived in the following manner are shown for the CL spectra.
  • the first and second CL spectra of six Y-coordinates are displayed on the same graph so as to be mutually identifiable with their respective origins shifted in the direction of the vertical axis.
  • the horizontal axis of FIG. 14 indicates the wavelength (nm).
  • the first CL spectrum of each Y coordinate shown in FIG. 6 to 7 points or more of CL spectra shifted to the same wavelength on the wavelength side were extracted, and the extracted CL spectra were averaged for calculation. Therefore, the measurement area related to the first CL spectrum contains more layer regions 21a than other measurement areas of the same Y coordinate.
  • the measurement region for the second CL spectrum includes more n-type body regions 21b (in particular, Al-enriched n-type regions) than other measurement regions with the same Y coordinate.
  • the first CL spectrum includes the CL spectrum from the layered region 21a, the CL spectrum from the first metastable AlGaN region in the n-type body region 21b, and the Al-enriched n-type in the n-type body region 21b. This is because the CL spectrum from the region is included, and in particular the two previous CL spectra are primarily included, resulting in a composite spectrum of these.
  • the Al-enriched n-type region in the n-type body region 21b is formed in the layered region 21a ( Ga - enriched n -type region) is formed along with the formation of the Al
  • a second intermediate AlGaN region (Al n+0.5 Ga 11.5 ⁇ n N 12 ) may also be formed within the enriched n-type region.
  • the second intermediate AlGaN region has an AlN mole fraction in the first metastable AlGaN region and a third subregion in which the AlN mole fraction is one step (about 8.33%) higher than that of the first metastable AlGaN region. It is located intermediate to the stable AlGaN region (Al n+1 Ga 11-n N 12 ).
  • the signal intensity In11(Y) at the wavelength ⁇ s1 (about 266 nm) corresponding to the AlN mole fraction (about 58.3%) of the first metastable AlGaN region is the maximum signal.
  • an n-type body region having an intensity of about 67% to about 96% of In0(Y) and formed predominantly by a first metastable AlGaN region within the measurement region for the first CL spectrum at each Y coordinate; 21b is included.
  • the signal intensity In1m (Y) at the wavelength ⁇ sm (about 273 nm) corresponding to the AlN mole fraction (about 54.2%) in the middle AlGaN region is the maximum signal intensity In0 (Y ), and the middle AlGaN region (Al n-0.5 Ga 12.5-n N 12 ) is dominant in the measurement region for the first CL spectrum of each Y coordinate. It can be seen that the formed layered region 21a is included.
  • the Y coordinate of the first CL spectrum is the middle Limited to the Y coordinate in the region, the influence from the active layer 22 existing above the n-type cladding layer 21 and the underlayer 10 existing below the n-type cladding layer 21 (in one embodiment, the top layer preferably eliminates the influence from AlN12).
  • the signal intensity In12 (Y) at the wavelength ⁇ s2 (about 279 nm) corresponding to the AlN mole fraction (50%) of the second metastable AlGaN region is the maximum signal intensity In0 ( Y) is about 13% to about 40%, and the second metastable AlGaN region exists, but is not predominantly formed, within the measurement region of the first CL spectrum of each Y coordinate. I understand.
  • the signal intensity In12(Y) is less than 50% of the maximum signal intensity In0(Y), it means that the second metastable AlGaN region is not dominantly formed in the layered region 21a. can be determined.
  • the wavelength ⁇ 1(Y) indicating the maximum signal intensity In1(Y) exists within the range of about 258 nm to about 261 nm, and each wavelength ⁇ 1(Y)
  • the AlN mole fraction Xn1(d) at the corresponding depth d lies in the range of about 61.5% to about 63.5%, both of which are within the second average AlN mole fraction Xna2(d ). Therefore, it can be seen that an Al-enriched n-type region is formed in the n-type body region 21b with the mass transfer of Ga during the formation of the layered region 21a. In the vicinity of the upper end of the n-type cladding layer 21, the AlN mole fraction Xn1(d) is about 61.5%.
  • the signal intensity In21(Y) at the wavelength ⁇ s1 (about 266 nm) corresponding to the AlN mole fraction (about 58.3%) of the first metastable AlGaN region is the maximum signal.
  • an n-type body region having an intensity of about 55% to about 84% of In1(Y) and formed predominantly by a first metastable AlGaN region within the measurement region for the second CL spectrum at each Y coordinate; 21b is included.
  • the signal intensity In21(Y) at the wavelength ⁇ s1 (about 266 nm) corresponding to the AlN mole fraction (about 58.3%) of the first metastable AlGaN region is , are the same, it can be seen that the first metastable AlGaN region is uniformly formed in the n-type body region 21b.
  • the signal intensity In2m(Y) at the wavelength ⁇ sm (about 259 nm) corresponding to the AlN mole fraction (62.5%) of the second intermediate AlGaN region is the maximum signal intensity In2 (Y) is about 90% to about 100%
  • a second intermediate AlGaN region (Al n+0.5 Ga 11.5 ⁇ n N 12 ) is within the measurement region for the second CL spectrum of each Y coordinate. It can be seen that the Al-enriched n-type region within the predominantly formed n-type body region 21b is included.
  • the AlN molar fraction uniformly and predominantly formed in the n-type body region 21b is approximately 58.3%. Since the first metastable AlGaN region of and the layered region 21a with a lower AlN mole fraction are exposed, an Al-enriched n-type region with an AlN mole fraction slightly exceeding 60% is partially present. It can be seen that the contact resistance between the surface of the n-type cladding layer 21 and the n-electrode 27 is kept low even with this.
  • FIG. 15 is an HAADF-STEM image showing four measurement regions A to D where the AlN mole fraction in the layered region 21a of the sample piece is measured by cross-sectional TEM-EDX line analysis.
  • composition analysis by the cross-sectional TEM-EDX method, first, in the entire measurement area covering the four measurement areas A to D shown in FIG. direction) and lateral direction (parallel to the second plane), and the detection data (Al and Ga X-ray intensity corresponding to each composition) was obtained.
  • each measurement area A to D is rectangular, and the inclination and size are set for each measurement area so that the extending direction of at least one layered area 21a in the measurement area is orthogonal to the scanning direction of line analysis. ing.
  • the inclinations of the measurement areas A to D are approximately equal to about 20°, but strictly speaking, they are not necessarily the same.
  • the scanning direction of line analysis is the vertical direction, and the direction.
  • the central vertical line shown in each measurement region indicates the scanning direction
  • the x mark on the vertical line indicates the position in the vertical direction of the layered region 21a whose AlN mole fraction is to be measured.
  • the positions of the X marks in the measurement regions A to D roughly correspond to Y coordinates 10, 21, 31, and 36 in the measurement range of the AlN mole fraction by the CL method shown in FIG.
  • the spatial resolution is high.
  • Detection data obtained from a plurality of probe locations aligned in the horizontal direction are accumulated to obtain detection data at each scanning position.
  • horizontal aligned means that the irradiation range of the electron beam probe overlaps with a horizontal line that intersects the vertical line and extends in the horizontal direction at each scanning position.
  • the AlN mole fractions in the layered regions 21a indicated by the crosses in the measurement regions A to D derived based on the cumulative detection data obtained in the manner described above are as follows.
  • the AlN mole fraction of the intermediate AlGaN region (approximately 54.17 %) is subtracted to show the AlN mole fraction difference ⁇ .
  • the p-type layer constituting the light-emitting device structure portion 20 was composed of two layers, the electron blocking layer 23 and the p-type contact layer 24, but in the light-emitting device 2 of the second embodiment, the p-type layer has a p-type clad layer 25 composed of one or more p-type AlGaN semiconductors between the electron block layer 23 and the p-type contact layer 24 .
  • the AlGaN-based semiconductor layers 21 to 25 of the light emitting element structure 20 are composed of an n-type cladding layer 21 (n-type layer), an active layer 22, an electron blocking layer 23 (p-type layer), a p-type cladding layer 25 (p-type layer), and a p-type contact layer 24 (p-type layer) which are epitaxially grown in order and stacked.
  • the base portion 10 and the AlGaN-based semiconductor layers 21 to 24, the p-electrode 26, and the n-electrode 27 of the light-emitting element structural portion 20 in the light-emitting element 2 of the second embodiment are the light emission of any one of the first to third embodiments. Since they are the same as the AlGaN-based semiconductor layers 21 to 24, the p-electrode 26, and the n-electrode 27 of the base portion 10 and the light-emitting device structure portion 20 of the device 1, redundant description will be omitted.
  • the p-type cladding layer 25 is composed of the AlN layer 12 of the underlying portion 10 epitaxially grown in order from the main surface 11a of the sapphire substrate 11, the n-type cladding layer 21 of the light emitting element structure portion 20, and each semiconductor layer in the active layer 22. Like the electron blocking layer 23 , it has a surface on which multi-stepped terraces parallel to the (0001) plane derived from the main surface 11 a of the sapphire substrate 11 are formed.
  • FIG. 17 schematically shows an example of a laminated structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in the active layer 22 .
  • a p-type cladding layer 25 is formed on the electron blocking layer 23 having the laminated structure described with reference to FIG. 5 in the first embodiment.
  • the inclined regions IA inclined with respect to the (0001) plane are formed as described above.
  • the thickness of the p-type cladding layer 25 is adjusted, for example, within the range of 20 nm to 200 nm including the terrace area TA and the inclined area IA.
  • the AlN mole fraction of the terrace region TA of the p-type cladding layer 25 is set within a range of 51% or more and less than the AlN mole fraction of the terrace region TA of the electron blocking layer 23 . Furthermore, the AlN mole fraction of the Ga-enriched p-type region 25 a of the p-type cladding layer 25 is set to be less than the AlN mole fraction of the Ga-enriched EB region 23 a of the electron blocking layer 23 .
  • the AlN mole fraction of the terrace region TA of the p-type cladding layer 25 is 1% or more, preferably 2% or more, more preferably 2% or more, more preferably than the AlN mole fraction of the Ga-enriched p-type region 25a within the above range. It is set to be higher than 4%.
  • the AlN mole fraction difference between the Ga-enriched p-type region 25a of the p-type cladding layer 25 and the terrace region TA is 4-5. % or more is preferable, but the effect of localizing carriers can be expected even at about 1 to 2%.
  • the p-type cladding layer 25 is formed under the same growth conditions as those for the n-type cladding layer 21 and the electron blocking layer 23 described in the first embodiment under the growth conditions under which the above-described multi-stepped terraces are easily exposed.
  • the p-type cladding layer 25 is grown with an average AlN mole fraction Xpa of 25 as a target value.
  • the active layer 22 alternately comprises two or more well layers 220 made of an AlGaN-based semiconductor and one or more barrier layers 221 made of an AlGaN-based semiconductor or an AlN-based semiconductor.
  • the active layer 22 has a single quantum well structure with only one well layer 220 and does not have a barrier layer 221 (quantum barrier layer). It may be configured. It is clear that the effect of the well layer 220 employed in each of the above-described embodiments can be similarly obtained for such a single quantum well structure.
  • an n-type AlGaN semiconductor layer having a higher AlN mole fraction than the n-type cladding layer 21 (hereinafter referred to as "n-type lower (referred to as "stratum”) may be provided.
  • n-type lower referred to as "stratum”
  • the average AlN mole fraction over the entire area in the depth direction where the n-type cladding layer 21 and the n-type underlayer are united may be higher than the range of the above formula (1).
  • the n-type underlayer provided below the n-type cladding layer 21 has a higher AlN mole fraction than the n-type cladding layer 21, light emitted from the active layer is not absorbed.
  • the n-type underlayer has a higher AlN mole fraction than the n-type cladding layer 21, it does not come into contact with the n-electrode 27 and forms a current path between the n-electrode 27 and the active layer. Therefore, the parasitic resistance between the n-electrode 27 and the active layer is not increased, and the wall plug efficiency is not lowered.
  • the n-type underlayer does not substantially function as part of the light emitting element structure 20, it can be said that the provision of the n-type underlayer does not bring any particular advantage or conspicuous disadvantage.
  • the first region R1 and the p-electrode 26 have, as an example, a comb shape in plan view, but the plan view shape is not limited to a comb shape. . Moreover, a planar view shape in which a plurality of first regions R1 are present and each of which is surrounded by one second region R2 may be used.
  • the base portion 10 in which a multi-stepped terrace is exposed on the surface of the AlN layer 12 using the sapphire substrate 11 whose main surface has an off-angle with respect to the (0001) plane is used.
  • the magnitude of the off-angle and the direction in which the off-angle is provided are different from the surface of the AlN layer 12 It may be determined arbitrarily as long as a multi-stepped terrace is exposed at the end and a growth starting point of the layered region 21a is formed.
  • the light emitting element 1 including the underlying portion 10 including the sapphire substrate 11 is illustrated as the light emitting element 1 as illustrated in FIG. A part or all of the layers included in the portion 10) may be removed by lift-off or the like.
  • the substrate forming the underlying portion 10 is not limited to the sapphire substrate.
  • the present invention is applicable to a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked vertically. .
  • nitride semiconductor ultraviolet light emitting device 10 base portion 11: sapphire substrate 11a: main surface of sapphire substrate 12: AlN layer 20: light emitting device structure portion 21: n-type clad layer (n-type layer) 21a: layered region (n-type layer) 21b: n-type body region (n-type layer) 22: Active layer 220: Well layer 220a: Ga-enriched well region 221: Barrier layer 221a: Ga-enriched barrier region 23: Electron blocking layer (p-type layer) 23a: Ga-enriched EB region 24: p-type contact layer (p-type layer) 25: p-type clad layer (p-type layer) 25a: Ga-enriched p-type region 26: p-electrode 27: n-electrode 100: substrate 101: AlGaN-based semiconductor layer 102: template 103: n-type AlGaN-based semiconductor layer 104: active layer 105: p-type AlGaN-based

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