JP5449819B2 - Nitride semiconductor thick film substrate - Google Patents

Description

The present invention relates to a nitride semiconductor substrate , and more particularly to a nitride semiconductor thick film substrate for growing a light emitting device structure including a plurality of nitride semiconductor layers .

  Conventionally, nitride semiconductors have been used or studied for light-emitting devices and high-power devices in view of their characteristics. For example, when a light-emitting device is manufactured using a nitride semiconductor, a light-emitting device capable of emitting any single color light within a wide color range from purple to orange can be obtained by adjusting the composition of the semiconductor. it can. In recent years, blue light emitting diodes and green light emitting diodes have been put to practical use by utilizing the characteristics of nitride semiconductors, and blue-violet semiconductor lasers have been developed as semiconductor laser devices. When forming the nitride semiconductor film, a substrate such as sapphire, SiC, spinel, Si, or GaAs is used as the substrate.

  For example, when sapphire is used as a substrate, a GaN or AlN buffer layer is formed in advance at a relatively low temperature of 500 to 600 ° C. before epitaxial growth of the GaN film, and then the substrate is heated to a high temperature of 1000 to 1100 ° C. It is known that when a GaN film is epitaxially grown by heating, a GaN film having a good surface and a structurally and electrically favorable crystal can be obtained. In addition, when using SiC as a substrate, it is known that a thin AlN film may be used as a buffer layer when epitaxially growing a GaN film.

However, when a substrate other than a nitride semiconductor such as GaN is used in this way, the nitride semiconductor film in the resulting nitride semiconductor film has a difference in thermal expansion coefficient and lattice constant from the nitride semiconductor film grown thereon. Have a number of crystal defects. These defects are classified into edge dislocations and screw dislocations, and the total dislocation density is about 1 × 10 ⁹ to 1 × 10 ¹⁰ cm ⁻² . It is known that these defects trap the carriers and thus impair the electrical characteristics of the film. In addition, it is known that the life characteristics of a laser device that causes a large current to flow are deteriorated.

  Therefore, in order to obtain a nitride semiconductor film with reduced lattice defects and good electrical characteristics, the thickness of GaN or the like can be increased using hydride vapor phase epitaxy (H-VPE), high-pressure synthesis, or sublimation. Attempts have been made to form films and use them as nitride semiconductor substrates. When a nitride semiconductor thick film is used as a substrate, the electrical resistance increases due to a decrease in defect density and a decrease in residual carrier concentration in the thick film. Therefore, the resistance is reduced by doping impurities. .

Japanese Patent Laid-Open No. 9-312417 Japanese Patent Laid-Open No. 11-1399

  However, when the GaN thick film is grown by the H-VPE method or the like as in the prior art, there is a problem in the crystal quality and electrical characteristics of the obtained GaN thick film (hereinafter referred to as “thick GaN thick film”). Unless otherwise specified, a thick film means a film having a thickness of 20 μm or more). For example, when a GaN thick film substrate formed by doping a predetermined high concentration of impurities is used for a nitride semiconductor laser device, the threshold voltage of the laser device decreases, but the threshold current density is reversed. It tends to increase. In addition, the surface morphology of the GaN thick film substrate becomes very bad, and the quality of the crystal is not good.

On the other hand, when a GaN thick film substrate formed by doping a predetermined low concentration impurity is used in a nitride semiconductor laser device, the surface morphology of the GaN thick film substrate is good and the threshold current density of the laser device is also low. However, the threshold voltage tends to increase. For this reason, a nitride semiconductor substrate (for example, a GaN thick film substrate) that does not impair the electrical characteristics of the nitride semiconductor light emitting device is desired. In view of the situation in the prior art as described above, the present invention directly aims at forming a nitride semiconductor thick film substrate desirable for growing a light emitting device structure including a plurality of nitride semiconductor layers. It is intended to provide a nitride semiconductor light emitting device having excellent characteristics with.

  In order to solve the above-mentioned problems, it is necessary that the surface of the nitride semiconductor thick film used as the substrate is in electrical good contact with the light emitting element structure formed by epitaxial growth. It is important to appropriately control the growth conditions so as to improve the properties.

According to the present invention, the nitride semiconductor thick film substrate for growing a light emitting device structure including a nitride semiconductor layer of the multiple, the substrate has a first and second opposed major surfaces, the substrate The first main surface is formed of a first layer region having a high impurity concentration of 3 × 10 ¹⁸ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less , and the substrate is 3 × 10 ¹⁸ cm ⁻³ or less 1 × 10. ¹⁷ cm also viewed including the ^-3 or more and less the second layer region of the lower first layer region a low impurity concentration, the first main surface of the substrate in terms of for growing a light emitting device structure thereon In addition, a substrate region partially exposed on the first main surface side after the formation of the light emitting element structure is used as a region for forming an electrode .

The substrate region for forming the electrode on the first main surface side of the substrate may be either the first layer region or the second layer region of the substrate that is partially exposed.

  The nitride semiconductor substrate preferably has n-type conductivity. That is, it is preferable that the first layer region of the nitride semiconductor substrate includes a conductivity determining impurity for n-type. As the impurity contained in the nitride semiconductor substrate, silicon, germanium, oxygen, carbon, sulfur, selenium, or tellurium can be used.

The nitride semiconductor substrate preferably includes a layer region having the lowest impurity concentration of 8 × 10 ¹⁸ cm ⁻³ or less. The first layer region of the nitride semiconductor substrate preferably has a thickness of 10 μm or less.

  ADVANTAGE OF THE INVENTION According to this invention, the nitride semiconductor light-emitting device which has the outstanding characteristic using the nitride semiconductor thick film board | substrate can be provided.

1 is a schematic cross-sectional view showing a nitride semiconductor laser device according to a reference example closely related to the present invention. It is a graph which shows the relationship between the impurity concentration in the growth termination area | region of a GaN thick film board | substrate, and the threshold voltage of the laser apparatus containing the board | substrate. Is a graph showing the relationship between the SiH ₄ introduced amount and concentration of impurities contained in the in the substrate for growing the GaN thick film substrate. It is a graph which shows the relationship between the impurity concentration in a GaN thick film board | substrate, and the threshold voltage of the laser apparatus containing the board | substrate. It is a graph which shows the relationship between the impurity concentration in a GaN thick film board | substrate, and the threshold current density of the laser apparatus containing the board | substrate. It is a graph which shows the relationship between the impurity concentration in a GaN thick film board | substrate, and the surface roughness of the board | substrate. FIG. 6 is a schematic cross-sectional view showing a nitride semiconductor laser device according to another reference example closely related to the present invention. It is a schematic sectional view showing a nitride semiconductor laser device according to the real施例of the present invention. It is typical sectional drawing which shows the nitride semiconductor laser apparatus by the other Example of this invention. It is a figure which shows an example of the impurity concentration distribution in the nitride semiconductor substrate used for the laser apparatus by this invention. It is a figure which shows the other example of impurity concentration distribution in the nitride semiconductor substrate used for the laser apparatus by this invention. It is a figure which shows the other example of impurity concentration distribution in the nitride semiconductor substrate used for the laser apparatus by this invention. It is a figure which shows the relationship between the effect of this invention, and the impurity concentration range in a thick film board | substrate. It is a figure which shows the other example of impurity concentration distribution in the nitride semiconductor substrate by this invention. It is a figure which shows the other example of impurity concentration distribution in the nitride semiconductor substrate by this invention. It is a figure which shows the other example of impurity concentration distribution in the nitride semiconductor substrate by this invention.

( Reference Example 1)
In Reference Example 1 closely related to the present invention, as a method for growing a nitride semiconductor thick film, an H-VPE method, a high-pressure synthesis method, or a sublimation method is conceivable. As a method of making it, H-VPE method is the most preferable. In this reference example , a nitride semiconductor light emitting device using a thick GaN film grown on a sapphire substrate by the H-VPE method as a nitride semiconductor substrate will be described.

First, a sapphire substrate having a (0001) plane was cleaned, and a non-doped GaN film having a thickness of about 2 μm was grown as an underlayer by MOCVD (metal organic vapor phase deposition) method according to the following procedure. That is, the cleaned sapphire substrate was introduced into the MOCVD apparatus and cleaned at a high temperature of about 1100 ° C. in a hydrogen (H ₂ ) atmosphere. Thereafter, at a relatively low temperature of 600 ° C., while introducing H ₂ as a carrier gas at a flow rate of 10 l / min, 5 l / min NH ₃ and 20 μmol / min trimethyl gallium (TMG) were introduced to obtain a thickness of about A 20 nm GaN buffer layer was grown. Thereafter, the supply of TMG was once stopped and the temperature was raised again to about 1050 ° C., and then about 100 μmol / min TMG was introduced to grow a non-doped GaN film having a thickness of about 3 μm in one hour. Thereafter, the supply of TMG and NH ₃ was stopped and cooled to room temperature, and the sapphire substrate on which the non-doped GaN underlayer was grown was taken out of the MOCVD apparatus. The buffer layer is not limited to GaN, and an AlN film or a GaAlN film may be formed by selectively using trimethylaluminum (TMA), TMG, and NH ₃ .

Next, when a thick film is formed on the non-doped GaN base layer formed on the sapphire substrate by the above method, a thickness of about 0.2 μm and 7 μm are used so that no cracks are generated in the thick film. A stripe-shaped growth suppression film having a width of 10 μm and a spacing of 10 μm was formed, and a selective growth (with lateral epitaxial growth) was performed thereon by the H-VPE method to grow a flat GaN thick film. In this reference example , as the growth suppression film, a SiO ₂ film deposited by electron beam evaporation (EB method) was etched using photolithography.

Hereinafter, a method of growing the GaN thick film 102 as shown in FIG. 1 by the H-VPE method will be described. A sapphire substrate on which a non-doped GaN underlayer including a stripe-like growth suppression film formed by the above method was grown was introduced into the H-VPE apparatus. The substrate was heated to about 1050 ° C. while introducing N ₂ carrier gas and NH ₃ at a flow rate of 5 l / min. Thereafter, 100 cc / min of GaCl was introduced onto the substrate to start the growth of the GaN thick film 102. GaCl was produced by reacting HCl gas with metallic Ga held at about 800 ° C. By supplying an impurity gas through a doping line that is singly connected to the vicinity of the substrate, it was possible to arbitrarily perform impurity doping during film growth. In this reference example , 200 nmol / min monosilane (SiH ₄ ) is supplied simultaneously with the start of film growth (SiH ₄ impurity concentration: about 3.8 × 10 ¹⁸ cm ⁻³ ) to grow a low Si-doped GaN layer region 102b. 3 minutes before the end of film growth, the flow rate of SiH ₄ is increased to 500 nmol / min (SiH ₄ impurity concentration: about 8.0 × 10 ¹⁸ cm ⁻³ ) to make the high Si-doped GaN layer region 102 a about 5 μm. Grown to a thickness of.

Note that Si doping is not limited to SiH ₄ , and other raw materials such as monochlorosilane (SiH ₃ Cl), dichlorosilane (SiH ₂ Cl ₂ ), or trichlorosilane (SiHCl ₃ ) may be used. The film was grown for 3 hours by the above method, and a GaN thick film having a total thickness of about 350 μm was grown. Thereafter, the sapphire substrate was removed by polishing, and a GaN thick film substrate 102 was obtained.

  A light emitting element structure was formed on the GaN thick film substrate 102 obtained as described above by MOCVD. Hereinafter, an example in which a blue-violet laser element structure is formed on the GaN thick film substrate 102 will be described.

First, the GaN thick film substrate 102 was introduced into the MOCVD apparatus, and the temperature was raised to about 1050 ° C. while introducing N ₂ and NH ₃ at a flow rate of 5 l / min. When the temperature reaches about 1050 ° C., 100 μmol / min TMG and 10 nmol / min SiH ₄ are introduced under a carrier gas of H ₂ to grow the n-type GaN buffer layer 103 to a thickness of about 4 μm. It was. Thereafter, the flow rate of TMG was adjusted to 50 μmol / min, 40 μmol / min TMA was introduced, and the n-type Al _0.1 Ga _0.9 N cladding layer 104 was grown to a thickness of 0.5 μm. Thereafter, the supply of TMA was stopped, the flow rate of TMG was adjusted to 100 μmol / min, and the n-type GaN light propagation layer 105 was grown to a thickness of 0.1 μm.

Thereafter, the supply of TMG is stopped, H ₂ as a carrier gas is replaced with N ₂ , the temperature is lowered to 700 ° C., 10 μmol / min of trimethylindium (TMI) is introduced as an indium raw material, and 15 μmol / min of TMG was introduced and an In _0.05 Ga _0.95 N barrier layer (not shown) having a thickness of 4 nm was grown. Thereafter, the supply amount of TMI was increased to 50 μmol / min, and an In _0.2 Ga _0.8 N quantum well layer (not shown) having a thickness of 2 nm was grown. The formation of such barrier layers and well layers was alternately repeated to form a multiple quantum well (MQW) layer 106 including four barrier layers and three well layers. When the formation of the MQW layer 106 is completed, the supply of TMI and TMG is stopped, the temperature is raised again to 1050 ° C., the carrier gas is changed from N ₂ to H ₂ again, and 50 μmol / min TMG, 30 μmol / min TMA and 10 nmol / min biscyclopentadienylmagnesium (Cp ₂ Mg) as a p-type doping raw material were supplied to grow a p-type Al _0.2 Ga _0.8 N carrier block layer 107 having a thickness of 20 nm. Thereafter, the supply of TMA was stopped, the supply amount of TMG was adjusted to 100 μmol / min, and a p-type light propagation layer 108 having a thickness of 0.1 μm was grown. Thereafter, the TMG supply rate is adjusted to 50 μmol / min, 40 μmol / min TMA is introduced, a 0.4 μm thick p-type Al _0.1 Ga _0.9 N cladding layer 109 is grown, and finally the TMG supply rate is The supply of TMA was stopped by adjusting to 100 μmol / min, and the growth of the light emitting device structure was completed by growing the p-type GaN contact layer 110 having a thickness of 0.1 μm. Thereafter, the supply of TMG and Cp ₂ Mg was stopped and cooled to room temperature, and the light emitting element structure formed on the thick film substrate 102 was taken out from the MOCVD apparatus.

The average value Ra of the surface roughness of the outermost surface of the film included in the laser element structure was about 8 nm, indicating good flatness. Thereafter, using a dry etching apparatus, the p-type GaN contact layer 110 was left in a stripe shape having a width of 5 μm, and the p-type Al _0.1 Ga _0.9 N guide layer 109 was etched to form an optical waveguide. Thereafter, an Au / Pd electrode 111 was formed on the p-type GaN contact portion 110, and an Al / Ti electrode 101 was formed on the back surface of the GaN thick film substrate 102. Finally, processing was performed using a cleaving or dry etching method so that the resonator length was about 1 mm to form a mirror end face.

As described above, a laser device having a blue-violet emission wavelength as shown in FIG. 1 can be manufactured using a nitride semiconductor. The laser device fabricated by the present embodiment has a threshold current density of the threshold voltage and 1 kA / cm ² to about 5V required for oscillation, their thresholds neighborhood condition As a result of a life test of about 1000 hours below, no characteristic deterioration was observed. In Reference Example 1, after the GaN thick film 102 was grown on the sapphire substrate, the sapphire substrate was peeled off and the nitride semiconductor light emitting device structure was formed on the GaN thick film substrate 102. Needless to say, the sapphire substrate may be peeled after the nitride semiconductor light emitting device structure is formed on the GaN thick film 102 without peeling. Further, not only the GaN thick film substrate but also a thick film substrate made of another nitride semiconductor (Al _x Ga _y In _1-xy N: 0 ≦ x ≦ 1, 0 ≦ y ≦ 1) is used in the present reference example. The same effect as 1 is obtained. Further, the same effect as in Reference Example 1 can be obtained even when a material in which a part of nitrogen element (about 10% or less) of nitride semiconductor constituent elements is replaced with P, As, or Sb is used for the thick film substrate. It is done.

( Reference Example 2)
In the plurality of laser devices according to Reference Example 2 closely related to the present invention, the SiH ₄ doping concentration when the GaN thick film substrate was formed by H-VPE was variously changed for each device. Was the same as in Reference Example 1.

First, a sapphire substrate on which a non-doped GaN underlayer including a stripe-like growth suppression film is formed by the same method as in Reference Example 1 is introduced into an H-VPE apparatus, and N ₂ carrier gas and NH ₃ are each 5 l / min. The substrate was heated to about 1050 ° C. while introducing at a flow rate of. Thereafter, 100 cc / min of GaCl was introduced onto the substrate to start the growth of the GaN thick film 102. At the start of film growth, the impurity doping concentration is 5 × 10 ¹⁷ cm ⁻³ , 1 × 10 ¹⁸ cm ⁻³ , 3 × 10 ¹⁸ cm ⁻³ , 5 × 10 ¹⁸ cm ⁻³ , and 8 × 10 ¹⁸ cm. ^-3 by supplying SiH ₄ gas to ^-3 , and growing the SiH ₄ flow rate at various rates 3 minutes before the growth of the GaN film having a total thickness of 350 μm is completed. The doping concentration in 102a was increased. However, in any case, the concentration in the growth termination region 102a was set to be approximately 2 × 10 ¹⁹ cm ⁻³ or less. Thereafter, a laser device was fabricated by the same method as in Reference Example 1.

In the graph of FIG. 2, the values of the threshold voltages of the various laser devices manufactured in Reference Example 2 are shown. That is, in this graph, the vertical axis indicates the threshold voltage, and the horizontal axis indicates the impurity concentration in the growth termination region 102a of the GaN thick film substrate. Reference numerals 201; 202; 203; 204; and 205 are SiH ₄ doping concentrations of 5 × 10 ¹⁷ cm ⁻³ , 1 × 10 ¹⁸ cm ⁻³ , and 3 × 10 ¹⁸ cm, respectively, at the start of growth of the GaN thick film substrate. ⁻³ , 5 × 10 ¹⁸ cm ⁻³ , and 8 × 10 ¹⁸ cm ⁻³ .

As shown in FIG. 2, when the impurity concentration in the growth termination region 102a of the GaN thick film substrate 102 is 1 × 10 ¹⁸ cm ⁻³ or more, a decrease in threshold voltage is observed. However, this effect is noticeable when the impurity concentration of the growth termination region 102a is in the range of 3 × 10 ¹⁸ cm ⁻³ or more. On the other hand, if the thick substrate is doped with an excessive impurity concentration, crystal defects increase in the epitaxial layer grown thereon, and the impurity concentration in the growth termination region 102a of the thick substrate is preferably 3 × 10 ^18. cm ^-3 or more preferably in the 1 × 10 ¹⁹ cm ^-3 in the range.

Further, when the other region 102b excluding the growth termination region 102a in the GaN thick film substrate 102 has an impurity concentration of 8 × 10 ¹⁸ cm ⁻³ or less, the threshold voltage due to the growth termination region 102a having a high impurity concentration. The effect of the decrease ratio becomes remarkable. However, if the impurity concentration in the GaN thick film substrate excluding the growth termination region 102a exceeds 3 × 10 ¹⁸ cm ⁻³ , the threshold current density tends to increase (see FIG. 5). If the resistance is 10 ¹⁷ cm ⁻³ or less, the resistance of the substrate itself becomes too high. Therefore, the impurity concentration in the other region 102b excluding the growth end region 102a of the GaN thick film substrate 102 is 3 × 10 ¹⁸ cm ⁻³ or less and 1 × It is preferably in the range of 10 ¹⁷ cm ⁻³ or more.

In the reference example 2, an example in which Si is doped as an impurity has been described. However, it is known that the same tendency as in the case of Si is exhibited even when other impurities that cause n-type conductivity are used.

(Comparative Example 1)
Also in the plurality of laser devices according to Comparative Example 1, the SiH ₄ doping concentration when the GaN thick film substrate was formed by H-VPE was variously changed for each device. However, Comparative Example 1 differs from Reference Examples 1 and 2 in that the doping concentration set at the start of the growth of the GaN thick film substrate is kept constant until the end of the growth.

That is, during the growth of the GaN thick film substrate by the H-VPE method, the SiH ₄ flow rate was maintained at various constant values within the range of 10 nmol / min to 1000 nmol / min. In this way, a plurality of types of GaN thick film substrates having different impurity concentrations were formed, and the characteristics of the laser device structures fabricated on the substrates were measured. As shown in FIG. 3, the SiH ₄ supply amount and the impurity concentration in the GaN thick film substrate are in a proportional relationship, and the impurity concentration when the SiH ₄ supply amount is 1000 nmol / min is 1.6 × 10 ^19. cm ^-3 . FIG. 4 shows the relationship between the impurity concentration in the GaN thick film substrate thus formed and the threshold voltage of the laser device structure formed thereon, and the relationship between the impurity concentration in the substrate and the threshold current density. Is shown in FIG. 5, and the relationship between the impurity concentration and the average surface roughness of the substrate is shown in FIG.

As shown in FIG. 4, as the impurity concentration in the GaN thick film substrate increases, the oscillation threshold voltage of the obtained laser device decreases. One reason for this is that the resistance of the GaN thick film substrate decreases due to the influence of impurities. However, the more important reason is that the threshold voltage is lowered by reducing the Schottky barrier generated in the contact region between the GaN thick film substrate and the lowermost layer of the laser element structure formed by MOCVD on the GaN thick film substrate. It has been clarified from observations by the present inventors by an electron beam excitation current method (EBIC method). Lasing threshold voltage is converged to a value in the order of 5V in a range impurity concentration of about 5 × 10 ¹⁸ cm ^-3 or more GaN thick film substrate. On the other hand, referring to FIG. 5, when the impurity concentration of the GaN thick film substrate is about 3 × 10 ¹⁸ cm ⁻³ or more, the laser oscillation threshold current density tends to increase. This is because the average surface roughness Ra of the substrate begins to increase as the impurity concentration in the GaN thick film substrate exceeds about 3.2 × 10 ¹⁸ cm ⁻³ as shown in FIG. It is thought to be caused. That is, if the substrate surface roughness increases, the unevenness at the interface of each layer included in the laser element structure formed thereon increases, and the dispersion of light in the propagation layer 105 that propagates the laser light increases. This is considered to cause an increase in threshold current density. In Reference Examples 1 and 2, the above problem can be avoided by increasing the impurity concentration only in the growth termination region 102 a of the GaN thick film substrate 102.

For example, in FIG. 4, FIG. 5, and FIG. 6, the broken line indicates the characteristic value in the laser device manufactured in Reference Example 1 (that is, in FIG. 1, the impurity concentration of the highly Si-doped GaN region 102a is about 8.0 × 10 ¹⁸ cm ⁻³ and the impurity concentration of the low Si-doped GaN region 102b is about 3.8 × 10 ¹⁸ cm ⁻³ ). In the laser device using the GaN thick film substrate having a constant impurity concentration over the entire thickness as in the comparative example 1, the characteristics in the substrate that can realize the same or higher characteristics as compared with the laser device of the reference example 1 It can be seen that there is no constant amount of impurities.

( Reference Example 3)
In Reference Example 3 closely related to the present invention, a laser device in which the conductivity type of each layer in Reference Example 1 was reversed was manufactured. That is, in this reference example , a GaN thick film substrate was formed by the H-VPE method in the same manner as in Reference Example 1, but organometallic biscyclopentadienyl magnesium (Cp ₂ Mg) was introduced as an impurity. The amount of impurity Mg introduced was increased 3 minutes before the end of the film growth, and a GaN thick film having p-type conductivity with a total thickness of 350 μm was formed. The obtained GaN thick film substrate is introduced into an MOCVD apparatus, and a p-type GaN buffer layer, a p-type Al _0.1 Ga _0.9 N cladding layer, a p-type propagation layer, an InGaN-MQW active layer, a carrier block layer, an n-type propagation layer. Then, an n-type Al _0.1 Ga _0.9 N cladding layer and an n-type contact layer were grown sequentially. Thereafter, a laser device was manufactured through the same process as in Reference Example 1.

In the laser device fabricated in Reference Example 3, a decrease in the threshold voltage of 3 to 5 V was confirmed by increasing the impurity concentration in the growth termination region of the p-type thick film substrate. However, when a p-type substrate is used, the threshold voltage is 20 V or higher no matter how the impurity concentration is changed, and the better effect is obtained when an n-type GaN thick film substrate is used as in Reference Example 1. Did not appear. This is presumably because a good contact interface is difficult to form because the resistance value of the p-type substrate itself is higher than that of the n-type substrate.

( Reference Example 4)
In Reference Example 4, which is closely related to the present invention , as in Reference Example 1, GaN thick film growth is started by the H-VPE method, and at the same time, a predetermined flow rate of SiH ₄ is introduced to grow an n-type GaN thick film. It was. However, the impurity is changed from SiH ₄ to germanium (Ge) or oxygen (O) 3 minutes before the end of the film growth, and the impurity concentration in the growth termination region is increased, and the n-type GaN including the impurity concentration change at a thickness of 350 μm. A thick film was formed. The obtained GaN thick film substrate was introduced into an MOCVD apparatus, and thereafter a laser apparatus was fabricated by the same method as in Reference Example 1.

In the laser device of Reference Example 4 obtained in this way, a reduction of about 1 to 20 V in threshold voltage was confirmed as compared with the laser device manufactured by the method of Comparative Example 1 in which the impurity concentration in the substrate was not changed. . In addition, by appropriately controlling the impurity concentration in the substrate, the threshold voltage and the threshold current density could be reduced to low values of about 5 V and about 1 kA / cm ² , respectively.

( Reference Example 5)
Reference Example 5 closely related to the present invention is only in the case where high impurity concentration regions are provided at two locations of the growth start region 102c and the growth termination region 102a of the thick film substrate 102 as shown in FIG. Different from Reference Example 1.

The laser device of Reference Example 5 oscillates at a threshold voltage of about 5 V and a threshold current density of about 1 kA / cm ² by optimizing the impurity concentration in the thick film substrate 102 and has a long lifetime. The characteristics are shown. Note that the impurity concentration of the highly Si-doped GaN region 102c to which the electrode 101 is bonded is preferably equal to or less than the impurity concentration of the highly Si-doped GaN region 102a serving as a base for epitaxial growth. FIG. 10 shows an example of the impurity concentration distribution in the thickness direction of the nitride semiconductor substrate used in Reference Example 5.

  It is considered that by increasing the impurity concentration in both the growth start region 102c and the growth termination region 102a, the carrier flow from the electrode 101 to the light emitting portion 106 becomes smooth and the threshold voltage is reduced.

(Example 1 )
8 and 9 are schematic cross-sectional views of the laser device according to the first embodiment of the present invention . This is the laser device basically according to Example 1 is prepared by the same process as in Reference Example 1, it differs in that the position of the electrode 101 as compared to the reference example 1 has been changed. That is, in the laser device of FIG. 8, after the laser element structure is formed on the thick film substrate 102, a part of the high impurity concentration region 102a in the thick film substrate 102 is exposed by reactive ion etching, and the exposed portion An n-type Al / Ti electrode 101 is formed thereon. On the other hand, in the laser device of FIG. 9, a part of the low impurity concentration region 102b in the thick film substrate 102 is exposed by the same etching, and the Al / Ti electrode 101 is formed on the exposed portion. Each of the laser devices as shown in FIGS. 8 and 9 had a threshold voltage and a threshold current density almost equal to those of the laser device of Reference Example 1, and had good life characteristics. . In particular, in the laser device of FIG. 8, it is not always necessary to dope the low Si-doped GaN region 102b with impurities, which results in good surface flatness of the thick film substrate 102 and threshold current density. Reduction was easy.

In the above-described examples and reference examples , the H-VPE method and the MOCVD method are used as the crystal growth method. However, other thick film growth methods may be used, and the laser device structure may be MBE (molecular beam epitaxy). ) Method or the like. However, when the nitride thick film substrate formed by the H-VPE method was used, the effects described in the above-described examples were confirmed particularly remarkably. This is probably because chlorine (Cl) taken into the nitride semiconductor thick film substrate during film growth by H-VPE contributes to the effect of the present invention.

Moreover, the raw material to be used is not limited to TMG, TMA, TMI, NH ₃ , and Cp ₂ Mg, and any raw material capable of growing a desired compound layer can be used. Further, the impurity for the n-type GaN thick film substrate is not limited to Si, O, or Ge, and carbon (C), selenium (Se), sulfur (S), tellurium (Te), or the like can also be used. .

  Furthermore, although the thickness of the high impurity concentration region 102a is about 5 μm in the above embodiment, it may be a thick film that does not cause undesired unevenness on the surface of the thick film substrate. The appropriate thickness of the high impurity concentration region 102a varies depending on the impurity concentration, but is desirably 50 μm or less in order to obtain the effects of the present invention, and is 10 μm or less and within a range of 2 nm. What is more desirable is clear from experimental results. Further, when growing a nitride thick film substrate, a method of highly doping (δ doping) the surface region (thickness of about 50 nm or less) was also effective.

11 and 12 show impurity concentration profiles in the thickness direction of other nitride semiconductor substrates in which the effect of the present invention has been confirmed. FIG. 11 shows a case where three regions having different impurity concentration gradients exist in the GaN thick film substrate 102, and the impurity concentration increases toward the surface on which the laser element structure is formed by epitaxial growth. . If the impurity concentration region on the surface side where epitaxial growth is performed is the first region, the impurity concentration of the first region is higher than the impurity concentration of the second region located below the first region.

The GaN thick film substrate 102 of FIG. 12 includes four impurity concentration regions of an A region, a B region, a C region, and a D region in order from the side where epitaxial growth is performed, and the C region has the highest impurity concentration. The A region on the surface side where epitaxial growth is performed has a higher impurity concentration than the B region and the D region located therebelow.

Even in the case of a nitride semiconductor substrate having an impurity concentration profile other than those shown in FIGS. 11 and 12 , when the A region, B region, C region, etc. are sequentially formed from the surface side where epitaxial growth is performed, A The effect of the present invention was obtained when a region having a lower impurity concentration exists below the region.

  Further, although the laser device has been described as the light emitting device in the above embodiments, the present invention can reduce the driving voltage and improve the flatness of the device surface even when manufacturing a light emitting diode (LED) device. it can.

(Consideration of relationship between effect of the present invention and impurity concentration range)
Relationship between effects and the impurity concentration range of recognized present invention from Example 1 and 2 and Comparative Example 1, closely related to the present invention described above will be described with reference to FIG. 13. In FIG. 13 , the impurity concentration ranges of the I region and the II region are shown. Here, the I region represents an impurity concentration range in which the effect of reducing the threshold voltage can be obtained. As described in Comparative Example 1, the threshold voltage reduction effect is due to the reduction of the Schottky barrier generated in the contact region between the substrate and the light emitting element structure due to the addition of impurities. The impurity concentration at which this effect appears was 1 × 10 ¹⁸ / cm ³ or more. In particular, when the impurity concentration is in the range of 3 × 10 ¹⁸ / cm ³ or more and 1 × 10 ¹⁹ / cm ³ or less, the threshold voltage reduction effect is remarkably exhibited. However, if the impurity concentration exceeds 1 × 10 ¹⁹ / cm ³ , crystal defects increase, which is not preferable.

On the other hand, the region II represents an impurity concentration range in which the effect of reducing the threshold current density can be obtained. As described in Comparative Example 1, the effect of reducing the threshold current density is that the surface roughness of the nitride semiconductor substrate is improved by optimizing the impurity concentration, and within the light propagation layer 105 of the light emitting device structure. This is due to a decrease in light dispersion. The impurity concentration at which this effect appears was 8 × 10 ¹⁸ / cm ³ or less. In particular, when the impurity concentration is in the range of 1 × 10 ¹⁷ / cm ³ or more and 3 × 10 ¹⁸ / cm ³ or less, the effect of reducing the threshold current density is prominent. However, if the impurity concentration is less than 1 × 10 ¹⁷ / cm ³ , the electrical resistance increases, which is not preferable.

  Based on the above results, when the nitride semiconductor substrate includes two or more layer regions having different impurity concentrations corresponding to the I region and the II region, a light emitting device (laser device) manufactured using the substrate ) Easily satisfies the effects of the low threshold voltage and the low threshold current density. However, in order to obtain the effect of the low threshold voltage, the light emitting device structure must be formed in contact with the I region. This is because the threshold voltage is reduced because the Schottky barrier generated in the contact region between the substrate and the light emitting element structure is reduced.

  Therefore, when the nitride semiconductor substrate does not include a plurality of layer regions having different impurity concentrations (that is, when the nitride semiconductor substrate has a certain uniform impurity concentration), a light emitting device (laser device) manufactured using the substrate is The effects of the low threshold voltage and the low threshold current density cannot be satisfied simultaneously (see Comparative Example 1).

  The effects of the above-described invention are limited to the description of the threshold voltage reduction effect and the threshold current density reduction effect. However, as a result of further studies by the present inventors, the nitride semiconductor substrate includes two or more different impurity concentration layer regions, so that not only the above-described effects but also the lattice distortion between the substrate and the light emitting device structure is reduced. It has been newly found that there is a function to make it. The effect of relaxing the lattice distortion will be described with reference to FIGS. 1, 4, and 5. FIG. Here, for simplification of description, it is assumed that the nitride semiconductor substrate includes only a two-layer region of an A layer region having a high impurity concentration and a B layer region having a lower impurity concentration. The same explanation can be made when the nitride semiconductor substrate includes three or more different impurity concentration layer regions.

In the laser device of Reference Example 1 (FIG. 1), a high Si-doped GaN region 102a (SiH ₄ impurity concentration: 8.0 × 10 ¹⁸ / cm ³ ) as the A region and a low Si-doped GaN region 102b as the B region. Light emitting element structures 103 to 111 are formed on a nitride semiconductor substrate 102 made of (SiH ₄ impurity concentration: 3.8 × 10 ¹⁸ / cm ³ ).

  FIG. 4 shows the relationship between the uniform impurity concentration in the nitride semiconductor substrate in Comparative Example 1 and the threshold voltage of a light emitting device (laser device) manufactured including the substrate. FIG. 5 shows the relationship between the uniform impurity concentration in the nitride semiconductor substrate in Comparative Example 1 and the threshold current density of a light emitting device (laser device) manufactured including the substrate.

The broken lines in FIGS. 4 and 5 indicate the electrical characteristics obtained with the laser device of Reference Example 1. The threshold voltage value indicated by the broken line in FIG. 4 is a little compared with that of the light emitting device (laser device) including the substrate having the same impurity concentration (8.0 × 10 ¹⁸ / cm ³ ) as the whole A region. It is low. Further, the threshold current density indicated by the broken line in FIG. 5 is also lower than that of the light emitting device including the substrate having the same impurity concentration (3.8 × 10 ¹⁸ / cm ³ ) as the whole B region. . For these reasons, a light emitting device (laser device) using a nitride semiconductor substrate having different impurity concentration regions in the A region and the B region has a low threshold voltage effect due to the A region and a low threshold current due to the B region. It can be seen that the density effect is not just an addition.

This is considered to be due to the fact that the lattice strain between the nitride semiconductor substrate and the light emitting element structure is relaxed by including different impurity concentration regions such as the A region and the B region. It has been clarified by the present inventors that the effect of relaxing the lattice distortion appears in proportion to the impurity concentration, and becomes remarkable when the impurity concentration is 1 × 10 ¹⁸ / cm ³ or more. However, if the impurity concentration exceeds 1 × 10 ¹⁹ / cm ³ , the crystallinity of the substrate is lowered. Therefore, the preferable impurity concentration at which a lattice strain relaxation effect can be obtained is 1 × 10 ¹⁸ / cm ³ or more and 1 × 10 ¹⁹ / cm ^3. Within a range of cm ³ or less.

  The reason why the threshold voltage and the threshold current density can be further reduced by the lattice strain relaxation effect will be described below.

  The nitride semiconductor laser device of FIG. 1 is laminated in the order of B region / A region / light emitting element structure. The A region 102a and the B region 102b included in the substrate 102 have different impurity concentrations, but both of them are mainly made of GaN. On the other hand, the light-emitting element structure is similarly composed of a nitride semiconductor, but includes a layer made of AlGaN having a smaller lattice constant or InGaN having a larger lattice constant than GaN. Therefore, it is conceivable that lattice distortion occurs due to lattice mismatch between the substrate (including the A region and the B region) and the light emitting element structure.

  However, since the A region 102a has a higher impurity concentration than the B region 102b, the A region 102a has a strong function of relaxing the lattice distortion as described above, and the lattice distortion between the light emitting element structure and the B region causes the A region to be reduced. Relaxed through. The relaxation of the lattice strain provides an effect of a low threshold current density accompanying a decrease in crystal defects in the light emitting device structure and an effect of a low threshold voltage accompanying a decrease in carrier traps caused by crystal defects.

  Therefore, as shown in FIGS. 4 and 5, both the A region and the B region having two different impurity concentrations are compared with the light emitting device using the nitride semiconductor substrate composed of only the A region or only the B region. The threshold voltage and the threshold current density are lower in the light emitting device using the nitride semiconductor substrate containing.

  That is, it can be seen that when the nitride semiconductor substrate includes at least two different impurity concentration regions, the lattice distortion between the substrate and the light emitting element structure is relaxed, and the threshold voltage and the threshold current density are lowered. .

Next, the effect of relieving lattice distortion by the nitride substrate shown in FIG. 12 will be described. In FIG. 12 , the nitride semiconductor substrate 102 is composed of four different impurity concentration regions. These four different impurity concentration regions are referred to as an A region, a B region, a C region, and a D region in order from the surface side where epitaxial growth is performed. That is, a light emitting element structure (laser element structure) is formed so as to be in contact with the A region. Here, it is assumed that the A region and the C region correspond to the I region in FIG. 13 , and the B region and the D region correspond to the II region. Under this assumption, the effect of the plurality of impurity concentration regions included in the nitride semiconductor substrate of FIG. 12 on the light emitting device (laser device) will be described.

Since the impurity concentration range of the A region belongs to the I region, the Schottky barrier generated in the contact region with the light emitting element structure is reduced, and a low threshold voltage effect is exerted on the light emitting device. Further, since the A region has a higher impurity concentration than the B region and the D region, it functions as a buffer layer that relaxes lattice distortion between the nitride semiconductor substrate and the light emitting element structure. The B region and the D region have an effect of a low threshold current density on the light emitting device because these impurity concentration ranges belong to the II region. The C region has almost no low threshold voltage effect on the light emitting device because its impurity concentration range belongs to the I region but is not in direct contact with the light emitting device structure. However, since the C region has the highest impurity concentration compared to the other regions, it can greatly contribute as a buffer layer that relaxes the lattice distortion between the nitride semiconductor substrate and the light emitting device structure. In this manner, the electrical characteristics of the light emitting device using the nitride semiconductor substrate of FIG. 12 can be improved.

Next, a plurality of different impurity concentration regions included in the nitride semiconductor substrate are the third region shown in FIG. 13 (within the impurity concentration range of 1 × 10 ¹⁸ / cm ³ to 8 × 10 ¹⁸ / cm ³ . ) Will be described. As shown in FIG. 13 , the III region represents an impurity concentration range in which the effects of both the I region and the II region can be obtained. Strictly speaking, within the range of 3 × 10 ¹⁸ / cm ³ or more and 8 × 10 ¹⁸ / cm ³ or less of the III region, the effect of the I region is dominant, and 1 × 10 ¹⁸ / cm ³ or more. In the range of 3 × 10 ¹⁸ / cm ³ or less, the effect of the region II is dominant. When the entire substrate has a constant and uniform impurity concentration (when there is one impurity concentration region), the impurity concentration at which the effects of the threshold voltage and the threshold current density are most efficiently obtained is 3 × 10 ¹⁸ / cm. ³ . However, in this case, since the substrate does not have a plurality of different impurity concentration regions, the above-described lattice strain relaxation effect cannot be obtained. Further, as described above, the thickness of the high impurity concentration region is preferably 50 μm or less, and more preferably 10 μm or less. Therefore, the thickness of the substrate having a uniform impurity concentration is also limited to 50 μm or less or 10 μm or less. It must be.

Next, if none of the plurality of different impurity concentration regions included in the nitride semiconductor substrate belongs to region III will be described with reference to FIG. 14. FIG. 14 represents the impurity concentration distribution in the nitride semiconductor substrate. The regions having a plurality of impurity concentrations are referred to as an A region and a B region in order from the surface side where epitaxial growth is performed. That is, a light emitting element structure (laser element structure) is formed so as to be in contact with the A region. As can be seen from FIGS. 13 and 14 , both the A region and the B region correspond to the III region.

The effects of the plurality of impurity concentration regions constituting the nitride semiconductor substrate of FIG. 14 on the light emitting device are as follows. First, since the impurity concentration of the A region belongs to the III region, both the effect of reducing the threshold voltage and the effect of reducing the threshold current density are expected. However, since the A region is in contact with the light emitting element structure and has a higher impurity concentration than the B region, the region A mainly acts to reduce the threshold voltage. In addition, since the A region has a higher impurity concentration than the B region, it can also serve as a buffer layer that relaxes lattice distortion between the nitride semiconductor substrate and the light emitting element structure. On the other hand, since the impurity concentration range of the B region also belongs to the III region, both the reduction effect of the threshold voltage and the reduction effect of the threshold current density are expected. However, since the B region has an impurity concentration lower than that of the A region, the region B mainly acts so as to reduce the threshold current density.

Next, a case where the nitride semiconductor substrate is composed of four different impurity concentration regions will be described with reference to FIG. 15. FIG. 15 represents the impurity concentration distribution in the nitride semiconductor substrate. A plurality of regions having a plurality of impurity concentrations are referred to as an A region, a B region, a C region, and a D region in order from the surface side where epitaxial growth is performed. That is, a light emitting element structure (laser element structure) is formed so as to be in contact with the A region. Here, the A region corresponds to the III region, the B region and the D region correspond to the II region, and the C region corresponds to the I region.

The effects of the plurality of impurity concentration regions constituting the nitride semiconductor substrate of FIG. 15 on the light emitting device are as follows. First, since the impurity concentration of the A region belongs to the III region, both the reduction effect of the threshold voltage and the reduction effect of the threshold current density are expected. However, since the A region is in contact with the light emitting element structure, it mainly acts to reduce the threshold voltage. In addition, since the A region has a higher impurity concentration than the B region, it can also serve as a buffer layer that relaxes lattice distortion between the nitride semiconductor substrate and the light emitting element structure. On the other hand, the B region and the D region operate so as to produce a reduction effect of the threshold current density on the light emitting device because these impurity concentrations belong to the II region. In addition, although the impurity concentration range of the C region belongs to the I region, it is not in direct contact with the light emitting device structure, so that the lower threshold voltage due to the reduction of the Schottky barrier between the light emitting device structure and the C region. However, it can greatly contribute as a buffer layer that relaxes the lattice distortion between the nitride semiconductor and the light emitting device structure due to having the highest impurity concentration compared to other impurity regions. In this manner, the electrical characteristics of the light emitting device using the nitride semiconductor substrate of FIG. 15 can be improved.

Next, a case where a nitride semiconductor substrate having a continuously changing the impurity concentration distribution will be described with reference to FIG. 16. Here, the profile of the continuous impurity concentration shown in FIG. 16 will be described by dividing it into four appropriate regions. A plurality of regions having a plurality of impurity concentrations are referred to as an A region, a B region, a C region, and a D region in order from the surface side where epitaxial growth is performed. That is, a light emitting element structure (laser element structure) is formed so as to be in contact with the A region. Here, the A region and the C region correspond to the III region, the B region corresponds to the I region, and the D region corresponds to the II region.

The effects of the plurality of impurity concentration regions constituting the nitride semiconductor substrate of FIG. 16 on the light emitting device are as follows. First, since the impurity concentration range of the region A belongs to the region III, both the effect of reducing the threshold voltage and the effect of reducing the threshold current density are expected. However, since the A region is in contact with the light emitting element structure, it mainly acts to reduce the threshold voltage. In addition, since the impurity concentration of the A region decreases from the boundary with the B region toward the interface with the light emitting element structure, the deterioration of crystallinity near the interface between the substrate and the light emitting element structure due to the addition of impurities is reduced. Can also work. Further, by continuously changing the impurity concentration, for example, as shown in FIGS. 14 and 15 , compared with the case where the impurity concentration is changed discontinuously (steeply), the A region and the B region The resistance in the boundary region between them can be kept low.

  On the other hand, the B region has an impurity concentration range belonging to the I region, but is not in direct contact with the light emitting element structure, and therefore has the highest threshold voltage reduction effect compared to other impurity regions. Due to having the impurity concentration, it contributes as a buffer layer that relaxes lattice distortion between the nitride semiconductor substrate and the light emitting element structure. In addition, since the lattice strain can be continuously dispersed by continuously changing the impurity concentration, the lattice strain relaxation effect can be efficiently exhibited.

Since the impurity concentration range of the C region belongs to the III region, both the effect of reducing the threshold voltage and the effect of reducing the threshold current density are expected. However, since the C region is not in direct contact with the light emitting device structure, the C region acts to produce a threshold current density reduction effect rather than a threshold voltage reduction effect. Further, by continuously changing the impurity concentration, the resistance in the boundary region between the B region and the C region can be kept low. Since the impurity concentration range of the D region belongs to the II region, the D region acts to reduce the threshold current density for the light emitting device. In this way, the electrical characteristics of the light emitting device using the nitride semiconductor substrate of FIG. 16 can be improved.

Note that in real施例described above, in the case of forming a light emitting device structure on a nitride semiconductor thick film on a substrate, it may be used after the surface of the substrate is polished course.

  As described above, according to the present invention, a nitride semiconductor light emitting device having excellent characteristics can be provided using a nitride semiconductor thick film substrate.

101 Al / Ti electrode, 102 GaN thick film substrate, 102a High Si doped GaN region, 102b Low Si doped GaN region, 102c High Si doped GaN region, 103 n-type GaN buffer layer, 104 n-type Al _0.1 Ga _0.9 N cladding layer , 105 n-type GaN light propagation layer, 106 multiple quantum well layer, 107 p-type Al _0.2 Ga _0.8 N carrier block layer, 108 p-type light propagation layer, 109 p-type Al _0.1 Ga _0.9 N clad layer, 110 p-type GaN contact Layer, 111 Au / Pd electrode.

Claims (8)

A nitride semiconductor thick film substrate for growing a light emitting device structure including a plurality of nitride semiconductor layers ,
The substrate has opposing first and second main surfaces;
The first main surface of the substrate is formed of a first layer region having a high impurity concentration of 3 × 10 ¹⁸ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less ,
The substrate may also look including a least a second layer region of 3 × 10 ¹⁸ cm ^-3 or less 1 × 10 ¹⁷ cm ^-3 or more and lower than the first layer region low impurity concentration,
A first main surface of the substrate is a surface for growing the light emitting device structure thereon;
A nitride semiconductor thick film substrate, wherein a region of the substrate that is partially exposed on the first main surface side after forming the light emitting device structure is used as a region for forming an electrode . The nitride semiconductor thick film substrate according to claim 1, wherein the substrate region for forming the electrode is the first layer region that is partially exposed. The nitride semiconductor thick film substrate according to claim 1, wherein the substrate region for forming the electrode is the second layer region that is partially exposed. Nitride semiconductor thick film substrate according to any one of claims 1 to 3 wherein the nitride semiconductor substrate, characterized in that it has a n-type conductivity. 5. The nitride semiconductor thick film substrate according to claim 4 , wherein the first layer region of the nitride semiconductor substrate contains a conductivity determining impurity for n-type. Impurities included in said nitride semiconductor substrate, nitride according silicon, germanium, oxygen, carbon, sulfur, to any one of claims 1 to 3, characterized in that one selected from the group consisting of selenium and tellurium Semiconductor thick film substrate . Wherein the nitride semiconductor substrate, a nitride semiconductor according to claim 1, characterized in that it contains the layer region having the lowest impurity concentration of 8 × 10 ¹⁸ cm ^-3 or less 6 Thick film substrate . Nitride semiconductor thick film substrate according to any one of claims 1 to 7 wherein the first layer region of the nitride semiconductor substrate is characterized by having a thickness of 10 [mu] m.

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