JP2007123913A - Method for manufacturing epitaxial wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an epitaxial wafer having a crystalline, excellent AlGaInP base DH (double hetero) structure on a GaP substrate, and to provide an LED of high luminance using the wafer. <P>SOLUTION: In a method for manufacturing an epitaxial wafer having a double hetero structure of a GaP buffer layer, an AlGaInP first stage lower clad layer, an AlGaInP lower clad layer as an active layer, an AlGaInP active layer, and an AlGaInP upper clad layer on a GaP compound semiconductor substrate, after the GaP buffer layer is grown on the GaP compound semiconductor substrate and the AlGaInP first stage lower clad layer is grown thereon, the surface of the AlGaInP first stage lower clad layer is polished and then the epitaxial growth of the AlGaInP lower clad layer, the GaInP active layer, and the AlGaInP upper clad layer is caused to occur. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a wafer formed by an epitaxial growth method, and more particularly to an epitaxial wafer having a heterojunction with a lattice mismatch and a light emitting diode manufactured using the wafer.

2. Description of the Related Art In recent years, attention has been paid to a technique for manufacturing various devices by epitaxially growing an active layer having a lattice constant different from that of a substrate on a semiconductor substrate. As the material for the lattice-mismatched, is used as a material for a light-emitting diode in the visible light region (LED) and laser diodes _{(LD) (Al X Ga 1} -X) Y In 1-Y P (0 ≦ X ≦ 1, 0 ≦ Y ≦ 1). Especially in this material system, application as LED and LD which have a DH structure using the said material on a GaAs substrate is performed actively (for example, refer patent document 1). FIG. 3 shows a wafer structure when a GaAs substrate is used. In this case, there is no difference in lattice constant between the GaAs substrate and the (Al _x Ga _1-x ) _0.51 In _0.49 P epitaxial layer (lattice matching system), and a good quality epitaxial wafer is obtained. On the other hand, the use of GaAs _{substrate, (Al X Ga 1-X} ) of the light emitted in the _Y an In DH structure portion of _1-Y P, which is emitted to the substrate side is absorbed by the GaAs substrate For this reason, there is a drawback in that it cannot be taken out to the surface side and the luminous efficiency does not increase. Therefore, for the purpose of reducing the light absorption by the substrate and increasing the light emission efficiency, research has been conducted using GaP that is transparent to the emitted visible light as the substrate. However, when using GaP as the substrate, due to the difference in lattice constant between the GaP substrate and the _{(Al X Ga 1-X)} Y In 1-Y P ( lattice mismatch), (Al _X Ga _1-X ) _Y In _1-Y P The crystallinity of the epitaxial layer has been a problem.

As another example, using a GaAs substrate, a y ≠ 0.51 not allow configuration matching _{(Al X Ga 1-X)} Y In 1-Y P based visible LED material using short wavelength And a visible light LED that uses a GaAs _Y P _1-Y material with 0 <y <1 as an active layer on a GaAs substrate.

Here, regarding the difference in lattice constant, the lattice mismatch occurs when a Ga _0.65 In _0.35 P active layer having a composition that emits yellow to orange light having an emission wavelength of about 590 nm when an LED is fabricated is epitaxially grown. The degree is about 2.7%. Conventionally, in order to alleviate the crystallinity degradation caused by the lattice mismatch, a predetermined composition from the lattice constant of the GaP substrate (Al _X Ga _1-X) continuously lattice constant to the lattice constant of the _Y In _1-Y P Inserting an epitaxial layer to be changed (referred to as a lattice constant changing layer or a composition gradient layer) (see, for example, Patent Document 2), or inserting a special buffer layer before growing a layer having a different lattice constant ( For example, see Non-Patent Document 1.).

Here, the degree of lattice mismatch is defined by the following equation (1).
Lattice mismatch degree = (lattice constant of epitaxial layer−lattice constant of substrate) × 100 / (lattice constant of substrate) (1)
However, it is actually difficult to improve the crystallinity of the DH structure portion having a different lattice constant only by providing a composition gradient layer or a buffer layer having a special growth condition.
JP-A-2-257777 JP-A-3-203316 Appl. Phys. Lett. 53 (5). 1988

  When using a GaP substrate that is transparent to the emitted visible light, or when using a light emitting layer having a different lattice constant using a GaAs substrate, a composition gradient layer, which has been conventionally used, It is difficult to grow a DH structure portion having a lattice constant different from that of a substrate, having a good surface state, and good crystallinity only by inserting a buffer layer having a suitable growth condition. When there is a lattice mismatch, the composition is different from that of the peripheral portion and the growth rate is increased, so that raised protrusions called hillocks are formed on the crystal surface distributed in an island shape. Further, when such an epitaxial wafer is used, a high-brightness LED has not been obtained yet.

  Therefore, after growing the composition gradient layer or buffer layer, the surface is polished and flattened so that dislocations, defects, and surface irregularities caused by lattice mismatch do not affect the subsequent crystal growth layer. Thus, it is an object of the present invention to produce a DH structure portion with good crystallinity and thus a high-brightness LED.

That is, the present invention relates to the following.
(1) A method of manufacturing an epitaxial wafer having a double heterostructure of a GaP buffer layer, an AlGaInP first-stage lower cladding layer, an AlGaInP lower cladding layer, an AlGaInP active layer, and an AlGaInP upper cladding layer as an active layer on a GaP compound semiconductor substrate. Then, after the GaP buffer layer is grown on the GaP compound semiconductor substrate and the AlGaInP first stage lower cladding layer is grown thereon, the surface of the AlGaInP first stage lower cladding layer is polished, A method of manufacturing an epitaxial wafer for light-emitting diodes, comprising epitaxially growing the AlGaInP lower cladding layer, the GaInP active layer, and the AlGaInP upper cladding layer sequentially.
(2) An AlGaInP composition gradient layer whose lattice constant continuously changes from the lattice constant of the GaP compound semiconductor substrate to the lattice constant of the active layer is grown between the GaP buffer layer and the AlGaInP first-stage lower cladding layer. The method for producing an epitaxial wafer for light-emitting diode use according to (1), wherein

The present invention is transparent GaP substrate to visible light, after growing the a lattice mismatch _{(Al X Ga 1-X)} Y In 1-Y buffer layer or gradient composition layer of P, polished the surface The effect of lattice mismatch is eliminated by flattening the surface and then performing epitaxial growth again to obtain an active layer structure with good crystallinity. Moreover, if such an epitaxial wafer is used, LED with higher brightness than the conventional one can be obtained. In the present invention, when GaAs is used as the substrate, the same effect can be obtained when GaAsP-based epitaxial growth is performed and when an AlGaInP-based DH structure is epitaxially grown.

On the GaP substrate, a GaP buffer layer is first epitaxially grown in order to reduce the influence of crystal defects on the substrate. The crystal growth method is generally based on the MOVPE method. On its active layer was adjusted to the composition such that emission at 590~610nm which are intended in the present invention _{(Al X Ga 1-X)} Y In 1-Y P (0.18 ≦ X ≦ 0.30, Y = 0.51 cladding layer composition having the same composition buffer layer adapted _{to) (Al X Ga 1-X} ) Y in 1-Y P (X ≧ 0.70, Y = 0.51) is grown. The lattice constant of this layer is different from the lattice constant of the GaP substrate, and there is a lattice mismatch of about 2.7%. Further, a composition gradient layer is inserted before growing such buffer layers having different lattice constants, and a layer having a lattice mismatch of about 2.7% is finally grown by gradually changing the lattice constant. good. In order to alleviate the influence of this lattice mismatch, the surface of the buffer layer is polished. The thickness to be polished may be about 1/2 to 2/3 of the buffer layer. The polishing of the crystal surface is performed by the same polishing machine as that for polishing the normal substrate surface. For example, the buffer layer is grown about 3 μm, and is polished by about 2 μm to make the remaining film thickness about 1 μm. After polishing, it is introduced again into the MOVPE apparatus and crystal growth of the active layer structure portion is performed.

When performing crystal growth once again, it is first 2.0μm about growth polished before grown buffer layer and the same composition in the _{(Al X Ga 1-X)} Y In 1-Y P as a lower cladding layer of the DH structure portion. The buffer layer and the lower cladding layer are doped n-type with H ₂ Se as a dopant, and the carrier concentration is about 1 × 10 ¹⁸ cm ⁻³ . Thereafter, the active layer is grown. The film thickness is set to about 1.0 μm, and the composition is set so that the target emission wavelength is between 590 to 610 nm. This layer is doped p-type with DEZn as a dopant. The carrier concentration is about 5 × 10 ¹⁷ cm ⁻³ . Next, an upper cladding layer is grown. The composition is the same as that of the lower cladding layer, but it is doped p-type using DEZn as a dopant. The carrier concentration is about 5 × 10 ¹⁷ cm ⁻³ . The film thickness is about 2 μm which is the same as the lower cladding layer. On the DH structure portion thus obtained, GaInP having the same composition as the active layer is grown as a contact layer. This layer is also doped p-type to the same extent as the upper cladding layer, and the film thickness is about 0.5 μm.

  When an LED is manufactured using the epitaxial wafer manufactured as described above, it is important how good the crystallinity of the DH structure portion as a light emitting portion is despite the influence of lattice mismatch. Therefore, once the surface is polished after growing the buffer layer as in the present invention, the uneven portion of the island-grown surface is flattened due to the effect of lattice mismatch, and the generated crystal defects are difficult to propagate to the subsequent layers. Therefore, it is possible to epitaxially grow a DH structure portion with good crystallinity. As a result, it is possible to obtain higher luminance than the lattice-mismatched LED that has been obtained conventionally.

Next, an example of the present invention will be described in detail for each of the two cases with and without a composition gradient layer. This time, an epitaxial wafer having a DH structure using GaInP as an active layer was fabricated on a GaP substrate. The GaP substrate used is an n-type substrate doped with S. In this example, crystal growth was performed under reduced pressure by the MOVPE method. The raw material gases used were TMA, TMG, TMI, and PH ₃ (100% gas), and ultra-high purity H ₂ gas was used as the carrier gas.
Example 1
First, the example about the case where there is no composition gradient layer is shown. A cross-sectional structure of the wafer is shown in FIG. For crystal growth, a GaP buffer layer 7 was first grown at 730 ° C. by 0.5 μm. At this time, Se was used as a dopant to perform n-type doping. Subsequently, an (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P first stage lower cladding layer 6 having the same composition as the lower cladding layer 5 in the DH structure portion was grown by about 3 μm. This layer was also doped with Se as a dopant to about 1 × 10 ¹⁸ cm ^{−3 in the same} manner as the GaP buffer layer 7. The lattice constant with this composition is larger than that of GaP, and the degree of lattice mismatch is about 2.7%. Therefore, the surface state after this layer was grown by 3 μm was poor, and there were many protrusions raised because the composition was different from the peripheral part called hillock and the growth rate was faster than the peripheral part due to island growth. The size of this hillock was about 20 × 30 μm and the height was about 6 μm after 3 μm epitaxial growth. The density in the wafer plane was about 1000 to 3000 cm ⁻² .

Here, the first-stage crystal growth was stopped, and the wafer was taken out. The removed wafer was polished by a normal polishing machine for polishing the GaAs substrate. The polish was fixed to a smooth glass or ceramic surface plate, and was chemically and mechanically polished using a polish solution mainly composed of SiO ₂ ultrafine particles having an outer diameter of about 0.5 μm and sodium hypochlorite. At this time, the load was suppressed to about 50 g / cm ² in order to reduce processing damage. The polishing rate varies depending on the apparatus state, but by setting an appropriate value depending on the apparatus, the unevenness of the surface after polishing can be suppressed to a maximum value of 0.002 μm. This time, the polishing rate was 0.05 μm / min, and the surface roughness after polishing was 0.5 μm or less. As the polishing amount, about 2 μm of about 3 μm of the (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P growth layer was polished to leave 1 μm. Although the surface of this wafer showed hillocks in some places, it was flat.

Again, crystal growth of the DH structure portion was performed with the MOVPE apparatus. (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P grown before polishing was grown again by 1 μm as the lower cladding layer 5. Subsequently, an active layer 4 having a composition of Ga _0.65 In _0.35 P was grown by 1 μm. The degree of lattice mismatch between the GaP substrate 8 and the active layer 4 is that the lattice constants of GaP and Ga _0.65 In _0.35 P are 5.450 angstroms and 5.597 angstroms, respectively. %. The active layer was doped p-type using DEZn as a dopant, and its carrier concentration was 5 × 10 ¹⁷ cm ⁻³ . On the active layer, (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P having the same composition as the lower cladding layer was grown as the upper cladding layer 3 by 2 μm. Similarly to the active layer 4, the upper cladding layer 3 was also doped p-type with DEZn as a dopant. Similarly, the carrier concentration was set to 5 × 10 ¹⁷ cm ⁻³ . Further, in order to facilitate the electrode formation, a p-type Ga _0.65 In _0.35 P layer having the same composition as that of the active layer 4 was finally grown as the contact layer 2. For comparison, under the same conditions, an epitaxial wafer having the same structure was fabricated by growing the lower cladding layer 5 of (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P 2 μm from the beginning and subsequently growing the active layer 4 without polishing the wafer surface. Then, LEDs were fabricated and their light emission characteristics were compared.

The produced LED has a size of 350 × 350 μm, and its characteristic evaluation was performed by luminance measurement using an integrating sphere. The brightness of the LED using each substrate after correcting the visibility with respect to the emission wavelength was 2500 millicandelas when the conventional wafer surface was not polished, whereas the wafer surface was polished. In some cases, the brightness was as high as 3500 millicandelas. The applied current at the time of measurement is 20 milliamperes.
(Example 2)
Next, an example in which a composition gradient layer is inserted will be described. The wafer has the structure shown in FIG. In the same manner as shown in Example 1, the GaP buffer layer 7 was first grown at 730 ° C. by 0.5 μm. Subsequently, Al and In raw materials were added, and the supply amount was continuously changed to grow 2 μm of the composition gradient layer 16 whose composition changed from GaP to (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P. The changes in the raw material supply amount in the composition gradient layer 16 were constant for TM → 25 → 14 SCCM, TMIn 3 → 97 SCCM, and TMAl.

After the composition gradient layer 16 was grown, an n-AlGaInP first-stage lower cladding layer 6 having the same composition layer as the lower cladding layer 5 having a composition of (Al _0.2 Ga _0.8 ) _0.65 In _0.35 P was continuously grown by 3 μm. At this point, the first-stage crystal growth was stopped and the wafer was taken out. The surface condition was better than that of Example 1. This is presumably because the composition gradient layer 16 can slightly relax the influence of the lattice mismatch. In this case, the size of the hillock was about 10 × 20 μm, and the height was about 5 μm. Further, the density in the wafer plane was about 400 to 500 cm ⁻² , which was less than that in Example 1 without the composition gradient layer. The removed wafer was polished in the same manner as in Example 1 so that the remaining film thickness of the first-stage lower cladding layer 6 was 1 μm, and the second-stage crystal growth was performed. About flatness, it was 0.5 micrometer or less like Example 1. FIG.

  The DH structure portion was regrown in the same procedure as in Example 1. In this case, the same conditions were used, and the brightness of the epitaxial wafer without the polishing process and the brightness of the LED were compared. When the composition gradient layer 16 was inserted but not polished, it was 2800 millicandelas, whereas when it was polished and grown in two stages, the brightness was as high as 3700 millicandelas. These results are thought to be because the effect of hillocks and crystal defects generated due to the lattice mismatch could not be propagated to the subsequent crystal growth by polishing the surface of the composition gradient layer 16 after growth.

  After growing a buffer layer having a lattice mismatch with the GaP substrate, once polishing the surface, crystal growth of the DH structure portion is performed again to obtain a DH structure portion with good crystallinity. Such an epitaxial wafer When an LED is manufactured using the above, an unprecedented high brightness LED can be obtained.

It is a figure which shows an example of the structure of the epitaxial wafer by this invention. It is a figure which shows an example of the structure of the epitaxial wafer containing the composition gradient layer by this invention. It is a figure which shows the structure of the epitaxial wafer which uses the conventional GaAs substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode 2 p-GaInP contact layer 3 p-AlGaInP upper cladding layer 4 p-GaInP active layer 5 n-AlGaInP lower cladding layer 6 n-AlGaInP first stage lower cladding layer 7 n-GaP buffer layer 8 S-doped n-GaP Substrate 9 Electrode 12 p-GaAs contact layer 16 n-AlGaInP composition gradient layer 17 n-GaAs buffer layer 18 Si-doped n-GaAs substrate

Claims (2)

A method of manufacturing an epitaxial wafer having a double heterostructure of a GaP buffer layer, an AlGaInP first-stage lower cladding layer, an AlGaInP lower cladding layer, an AlGaInP active layer, and an AlGaInP upper cladding layer on a GaP compound semiconductor substrate, After the GaP buffer layer is grown on the GaP compound semiconductor substrate and the AlGaInP first-stage lower cladding layer is grown thereon, the surface of the AlGaInP first-stage lower cladding layer is polished, and then the AlGaInP An epitaxial wafer manufacturing method for light-emitting diodes, comprising epitaxially growing a lower clad layer, the GaInP active layer, and the AlGaInP upper clad layer sequentially. An AlGaInP composition gradient layer having a lattice constant that continuously changes from the lattice constant of the GaP compound semiconductor substrate to the lattice constant of the active layer is grown between the GaP buffer layer and the AlGaInP first-stage lower cladding layer. The manufacturing method of the epitaxial wafer for light emitting diode uses of Claim 1 characterized by the above-mentioned.

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