JP5334501B2 - Nitride semiconductor device - Google Patents

Description

  The present invention relates to a nitride semiconductor device such as a nitride semiconductor laser, and more particularly to a nitride semiconductor device in which an operating voltage is reduced.

  Nitride semiconductors composed of GaN, AlN, InN and mixed crystals thereof have already been put into practical use as materials for blue light-emitting diodes (LEDs) and blue-violet laser diodes (LDs), and also have good heat resistance and environmental resistance. Taking advantage of this, the use as a semiconductor material for electronic devices is also increasing.

  By the way, in a nitride semiconductor, it is common to produce a p-type nitride semiconductor by doping Mg. Since this nitride semiconductor has a deep acceptor level, the activation rate of the acceptor is very low, and it is difficult to increase the hole concentration.

  For this reason, it is not easy to form an ohmic electrode having a low resistance with respect to the p-type nitride semiconductor. Increasing the contact resistance between the p-type contact layer and the p-type electrode causes an increase in operating voltage in the nitride semiconductor device. Furthermore, an increase in the operating voltage generates heat at the p-type electrode and easily deteriorates the p-type electrode, thus lowering the reliability.

  To solve the problem of reducing the contact resistance between the p-type contact layer and the p-type electrode, a two-layer structure having different p-type impurity concentrations of a layer having a high p-type impurity concentration and a layer having a low p-type impurity concentration from the p-type electrode side. Japanese Patent Application Laid-Open No. H10-228473 discloses a technique of using a p-type contact layer.

  In Patent Document 1, in order to obtain ohmic contact with the p-type electrode, it is effective to dope the acceptor impurity at a high concentration in the layer that is in direct contact with the p-type electrode.

  In addition, when the acceptor impurity is doped at a high concentration, the hole concentration decreases conversely. Therefore, by providing a layer doped with an acceptor impurity at a concentration that maximizes the hole concentration, lowering the hole injection efficiency. It shows how to prevent it.

  On the other hand, Patent Document 2 shows that when this two-layer structure is used for the p-type contact layer, the threshold voltage increases as the thickness of the layer having a high acceptor impurity concentration is increased. This is because a layer having a high acceptor impurity concentration has a low hole concentration, and therefore the resistance of the layer becomes high. Therefore, in Patent Document 2, it is desirable that the film thickness of the layer having a high acceptor impurity concentration is 50 nm or less.

  Further, Patent Documents 3 and 4 show means for forming p-type InGaN in the p-type contact layer. Since the band gap of InN is very narrow with respect to around 0.65 eV and 3.4 eV of GaN, InGaN which is a mixed crystal can obtain a narrower band gap than GaN. Further, InGaN can achieve a high hole concentration even when Mg is used as a p-type impurity. For this reason, formation of a low-resistance ohmic electrode can be expected.

  The prior art document information related to the invention of this application includes the following.

JP-A-8-97471 Japanese Patent Laid-Open No. 10-242587 JP-A-8-97468 JP-A-9-289351

  However, there is still a strong demand for reducing the operating voltage of nitride semiconductors, and further reduction in the contact resistance between the p-type contact layer and the p-type electrode is desired.

  Accordingly, an object of the present invention is to provide a nitride semiconductor device in which the operating voltage is reduced by realizing the formation of an ohmic electrode having a lower resistance than in the prior art.

The present invention was devised to achieve the above object, and the invention of claim 1 has a p-type contact layer on a nitride semiconductor layer, and the p-type contact layer is in order from the p-type electrode side. in the nitride semiconductor device constituted by a p-type first contact layer and the p-type second contact layer, the first contact layer is p-type impurities have 1.4 × 10 ²⁰ cm ^-3 free consisting Mg an in _x Ga _1-x consists N (x = 0.07), the p-type second contact layer with a GaN containing p-type impurities 7.0 × 10 ¹⁹ cm ^-3 consisting of Mg, the first contact The nitride semiconductor device has a thickness of 15 nm and a thickness of the second contact layer of 50 nm .

The invention of claim 2 has a p-type contact layer on the nitride semiconductor layer, and the p-type contact layer is composed of a p-type first contact layer and a p-type second contact layer in order from the p-type electrode side. In the nitride semiconductor device, the first contact layer is made of In _x Ga _1-x N (x = 0.07) containing 1.4 × 10 ²⁰ cm ^{−3 of a} p-type impurity made of Mg , and the p-type The second contact layer contains p-type impurities composed of Mg of 4.0 × 10 ¹⁹ cm ⁻³ and has an In composition ratio lower than that of the p-type first contact layer, In _y Ga _1-y N (y = 0.02). ), The first contact layer has a thickness of 15 nm, and the second contact layer has a thickness of 50 nm .

  According to the present invention, since the contact resistance between the p-type contact layer and the p-type electrode can be reduced as compared with the conventional case, the operating voltage of the nitride semiconductor device can be lowered.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view of a nitride semiconductor device according to a preferred embodiment of the present invention.

As shown in FIG. 1, a nitride semiconductor device 1 includes an n-type GaN buffer layer 3, an n-type AlGaN cladding layer 4, an n-type GaN guide layer 5, and a non-doped layer on a Si-doped n-type GaN substrate 2. InGaN active layer 6, p-type GaN guide layer 7 doped with Mg, p-type AlGaN cladding layer 8, p-type In _y Ga _1-y N (0 ≦ y <0.2) second contact layer 9, p-type In _x In this structure, Ga _1-x N (0 <x ≦ 0.2) first contact layers 10 are sequentially stacked.

A p-type electrode 11 made of a Pd / Au laminate is formed on the surface of the p-type In _x Ga _1-x N first contact layer 10, and an n-type made of a Ti / Au laminate is formed on the back surface of the n-type GaN substrate 2. Electrode 12 is formed.

Furthermore, the p-type AlGaN cladding layer 8, the p-type In _y Ga _1-y N second contact layer 9, and the p-type In _x Ga _1-x N first contact layer 10 have a ridge shape, and the ridge side surface and p The upper surface of the type AlGaN cladding layer 8 is covered with an insulating film 13.

  The structural conditions (or reasons for limiting the numerical values) of the p-type first and second contact layers desirable for forming a low-resistance ohmic electrode on the epitaxial wafer having the above-described configuration are shown below.

First, the reason why the In composition of the p-type In _x Ga _1-x N first contact layer 10 in contact with the p-type electrode 11 is 0 <x ≦ 0.2 (that is, 20% or less) is that InGaN is immiscible. This is because it is difficult to grow a crystal with a high In composition.

  Further, since InGaN needs to be grown at a lower temperature as the In composition becomes higher, it is difficult to obtain a high-quality crystal. In other words, in the In composition region where crystal growth is possible at a relatively high temperature, it is possible to obtain a higher hole concentration than GaN, but if the growth temperature is lowered to increase the In composition, the crystal quality deteriorates. This is because the problem that the hole concentration is lowered occurs.

Next, the Mg concentration of the p-type In _x Ga _1-x N first contact layer 10 can be controlled in the range of 1.0 × 10 ²⁰ cm ^{−3 to} 2.0 × 10 ²⁰ cm ^−3. It is desirable and more effective when the Mg concentration is 1.4 × 10 ²⁰ cm ⁻³ .

FIG. 2 shows the relationship between the Mg concentration of the p _- type In _x Ga _1-x N first contact layer 10 and the contact resistance of the p-type electrode 11.

As shown in FIG. 2, in the range where the Mg concentration is 1.4 × 10 ²⁰ cm ⁻³ or less, the contact resistance decreases as the concentration is increased, and the Mg concentration exceeds 1.4 × 10 ²⁰ cm ^−3. Then, the contact resistance increases as the concentration increases. When it is attempted to obtain a contact resistance of about 10 ⁻⁴ Ωcm ² or less, the optimum Mg concentration of the p-type In _x Ga _1-x N first contact layer 10 is found to be in the above-mentioned range.

The film thickness of the p-type In _x Ga _1-x N first contact layer 10 is preferably set in the range of 2 nm to 20 nm. This is because it is difficult to grow InGaN having a film thickness of less than 2 nm with a desired composition and a desired Mg concentration, and when the film thickness is greater than 20 nm, dislocation occurs due to lattice mismatch with the lower layer. As a result, the crystal quality deteriorates and the contact resistance increases.

On the other hand, the p-type In _y Ga _1-y N second contact layer 9 has InGaN (0 <y <0.2) or GaN having a smaller In composition than the p-type In _x Ga _1-x N first contact layer 10. It is desirable that (y = 0).

Further, the Mg concentration of the p-type In _y Ga _1-y N second contact layer 9 is equal to or lower than the Mg concentration of the p-type In _x Ga _1-x N first contact layer 10 and the p-type AlGaN cladding layer 8 It is desirable that the Mg concentration be equal to or higher.

This is because when the Mg concentration of the p _- type In _y Ga _1-y N second contact layer 9 is higher than the Mg concentration of the p-type In _x Ga _1-x N first contact layer 10, the contact resistance is remarkably increased. Because it ends up. Further, when the Mg concentration of the p _- type In _y Ga _1-y N second contact layer 9 is lower than the Mg concentration of the p-type AlGaN cladding layer 8, the hole concentration is lowered, and the contact resistance is also increased. Because.

According to the nitride semiconductor device 1 having the above configuration, the contact resistance between the p-type contact layer (p-type In _x Ga _1-x N first contact layer 10) and the p-type electrode 11 can be reduced as compared with the conventional case. Therefore, the operating voltage can be lowered.

As a modification of the nitride semiconductor device 1, as shown in FIG. 3, there is no n-type GaN guide layer 5, p-type GaN guide layer 7, and insulating film 13, and n-type and p-type AlGaN cladding layers 4 and 8 are n. Type and p-type GaN cladding layers 4 'and 8', p-type GaN cladding layer 8 ', p-type In _y Ga _{1 -y} N second contact layer 9, p-type In _x Ga _{1 -x} N first contact The same effect can be obtained even in the nitride semiconductor device 30 having a structure in which the layer 10 is not ridge-shaped.

Example 1
In Example 1, a nitride semiconductor laser using the nitride semiconductor device 1 of FIG. 1 was produced.

  First, an n-type GaN substrate 2 by Si doping is placed in a reactor of a MOVPE (metal organic vapor phase epitaxy) apparatus, and the temperature of the n-type GaN substrate 2 is increased to 900 ° C. while supplying ammonia and hydrogen as a carrier gas. Heated.

  Then, after raising the temperature of the n-type GaN substrate 2 to 900 ° C., trimethylgallium (TMG) and silane gas are supplied, and then the temperature is raised to 1050 ° C. while the (0001) surface of the n-type GaN substrate 2 is raised. An n-type GaN buffer layer 3 by Si doping was grown thereon, then trimethylaluminum (TMA) was supplied while maintaining the temperature at 1050 ° C., and an n-type AlGaN cladding layer 4 by Si doping was laminated.

  Thereafter, the supply of TMA was stopped, and an n-type GaN guide layer 5 was grown by Si doping. Here, the supply of TMG and silane gas was stopped, and the temperature was lowered to 800 ° C. At that time, the carrier gas is switched from hydrogen to nitrogen while ammonia is supplied, and when the temperature reaches 800 ° C., TMG and trimethylindium (TMI) are supplied, and a non-doped InGaN active layer (multi-quantum well (MQW)) 6 grew. At this time, TMI prepared two supply lines having different supply amounts, and alternately switched them to grow well layers and barrier layers having different In compositions.

Next, the supply of TMI and TMG is stopped, the temperature is raised again to 1050 ° C., the carrier gas is switched from nitrogen to hydrogen, TMG and Cp ₂ Mg (cyclopentadienyl magnesium) are supplied, and p by Mg doping The p-type AlGaN cladding layer 8 by Mg doping was laminated by supplying the type GaN guide layer 7 and TMA.

Then, the supply of TMA is stopped, the p-type In _y Ga _1-y N second contact layer 9 is grown by Mg doping, the supply of TMG and Cp ₂ Mg is then stopped, the carrier gas is switched from hydrogen to nitrogen, The temperature was lowered to 800 ° C., and when the temperature reached 800 ° C., TMI, TMG, and Cp ₂ Mg were supplied again, and the p-type In _x Ga _1-x N first contact layer 10 by Mg doping was laminated. After the crystal growth of the p-type In _x Ga _1-x N first contact layer 10 was completed, the supply of TMI, TMG, Cp ₂ Mg, and ammonia was stopped, and the temperature was lowered to room temperature with only nitrogen as a carrier gas being supplied.

  After returning the temperature to room temperature, the n-type GaN substrate 2 was taken out from the reaction furnace, and annealed in a nitrogen atmosphere at 600 ° C. for 20 minutes using an annealing apparatus to activate p-type impurities.

Thereafter, the p-type AlGaN cladding layer 8, the p-type In _y Ga _1-y N second contact layer 9, and the p-type In _x Ga _1-x N first contact layer 10 are removed by the RIE apparatus except for the central portion. A ridge shape was obtained by etching halfway through the AlGaN cladding layer 8.

Next, the entire surface is covered with an insulating film 13, and further, the insulating film 13 on the ridge is removed by photolithography, and Pd / Au is formed on the exposed p-type In _x Ga _1-x N first contact layer 10 surface. A p-type electrode 11 by lamination was formed, and an n-type electrode 12 by Ti / Au lamination was formed on the back surface of the n-type GaN substrate 2.

  Finally, although not shown, a laser resonator was formed by cleavage or the like to form a nitride semiconductor laser element.

At this time, the p-type In _x Ga _1-x N first contact layer 10 in contact with the p-type electrode 11 has a film thickness of 15 nm, an Mg concentration of 1.4 × 10 ²⁰ cm ⁻³ , and an In composition of x = 0. 07 (ie 7%) InGaN.

The p-type In _y Ga _1-y N second contact layer 9 has a film thickness of 50 nm and an Mg concentration of 7.0 × 10 ¹⁹ cm ⁻³ (y = 0 (ie, the In composition is 0%)). Consists of. Further, the Mg concentration of the p-type AlGaN cladding layer 8, which is the lower layer of the p _- type In _y Ga _1-y N second contact layer 9, was 2.0 × 10 ¹⁹ cm ⁻³ .

In the semiconductor laser fabricated in Example 1, the contact resistance between the p-type In _x Ga _1-x N first contact layer 10 and the p-type electrode 11 and the operating voltage were sufficiently lower than those in the past.

(Example 2)
In Example 2, a nitride semiconductor light emitting diode using the nitride semiconductor element 30 having the structure shown in FIG. 3 was produced.

  This nitride semiconductor light-emitting diode was epitaxially grown using metal organic vapor phase epitaxy (MOVPE method).

  First, the n-type GaN substrate 2 by Si doping was placed in a furnace, and the temperature of the n-type GaN substrate 2 was raised while supplying ammonia and hydrogen as a carrier gas. When the temperature is raised to 900 ° C., TMG and silane gas are supplied, and further, the n-type GaN buffer layer 3 is formed by Si doping while the temperature is raised to 1050 ° C., and the n-type GaN cladding layer 4 is formed by Si doping when the temperature reaches 1050 ° C. 'Grow up.

  Next, the supply of TMG and silane gas was stopped, the temperature was lowered to 800 ° C., and the carrier gas was switched from hydrogen to nitrogen while supplying ammonia. Thereafter, when the temperature reached 800 ° C., TMG and TMI were supplied, and a non-doped InGaN active layer (multi-quantum well (MQW)) 6 was grown.

At this time, TMI prepared two lines with different supply amounts, and alternately switched them to grow well layers and barrier layers having different In compositions. Thereafter, the supply of TMI and TMG is stopped, the temperature is raised again to 1050 ° C., the carrier gas is switched from nitrogen to hydrogen, TMG and Cp ₂ Mg are supplied, and a p-type GaN cladding layer 8 ′ is grown by Mg doping. I let you.

Next, the supply of TMG and Cp ₂ Mg is stopped, the carrier gas is switched from hydrogen to nitrogen, the temperature is lowered to 800 ° C., and then TMI, TMG and Cp ₂ Mg are supplied, and p-type In _y Ga by Mg doping is used. _{A 1-y} N second contact layer 9 and a p-type In _x Ga _1-x N first contact layer 10 by Mg doping were laminated.

After the crystal growth of the p-type In _x Ga _1-x N first contact layer 10 was completed, the supply of TMI, TMG, Cp ₂ Mg, and ammonia was stopped, and the temperature was lowered to room temperature with only nitrogen as a carrier gas being supplied.

After the temperature returned to room temperature, the n-type GaN substrate 2 was taken out from the reaction furnace, and annealed in a nitrogen atmosphere at 600 ° C. for 20 minutes using an annealing apparatus to activate p-type impurities. Thereafter, a p _- type electrode 11 made of Pd / Au laminate is formed on the surface of the p-type In _x Ga _1-x N first contact layer 10, and an n-type electrode 12 made of Ti / Au laminate is formed on the back surface of the n-type GaN substrate 2. Formed.

At this time, the p-type contact layer has a structure in which the p-type In _x Ga _1-x N first contact layer 10 has a thickness of 15 nm, an Mg concentration of 1.4 × 10 ²⁰ cm ⁻³ , and an In composition x = 0. The p-type In _y Ga _1-y N second contact layer 9 is 50 nm thick, the Mg concentration is 4.0 × 10 ¹⁹ cm ⁻³ , and the In composition y = 0. 02 (ie 2%) of InGaN. Further, the Mg concentration of the p-type GaN clad layer 8 ′, which is the lower layer of the p _- type In _y Ga _1-y N second contact layer 9, was 2.0 × 10 ¹⁹ cm ⁻³ .

  The contact resistance and operating voltage of the light emitting diode produced in Example 2 were sufficiently lower than those in the past.

1 is a schematic cross-sectional view of a nitride semiconductor device according to a preferred embodiment of the present invention. It is a measurement figure which shows the relationship between Mg density | concentration of a 1st contact layer, and the contact resistance of an electrode. It is a cross-sectional schematic diagram of the nitride semiconductor light-emitting diode produced in Example 2 of the present invention.

Explanation of symbols

First nitride semiconductor device 9 p-type In _y Ga _1-y N second contact layer 10 p-type In _x Ga _1-x N first contact layer 11 p-type electrode

Claims (2)

In a nitride semiconductor device having a p-type contact layer on a nitride semiconductor layer, wherein the p-type contact layer includes a p-type first contact layer and a p-type second contact layer in order from the p-type electrode side.
Made from the first contact layer is made of Mg p-type impurity 1.4 × 10 ²⁰ cm ^-3 free has been _{In x Ga 1-x N (} x = 0.07),
With pre-Symbol p-type second contact layer made of GaN and the p-type impurity contained 7.0 × 10 ¹⁹ cm ^-3 consisting of Mg,
A nitride semiconductor device, wherein the first contact layer has a thickness of 15 nm, and the second contact layer has a thickness of 50 nm . In a nitride semiconductor device having a p-type contact layer on a nitride semiconductor layer, wherein the p-type contact layer includes a p-type first contact layer and a p-type second contact layer in order from the p-type electrode side.
Made from the first contact layer is made of Mg p-type impurity 1.4 × 10 ²⁰ cm ^-3 free has been _{In x Ga 1-x N (} x = 0.07),
Before Symbol p-type second contact layer is a p-type impurity 4.0 × 10 ¹⁹ cm ^-3 contain consisting Mg, an In an In composition ratio lower than the p-type first contact layer _y Ga _1-y N _(y = 0.02 ) , and
A nitride semiconductor device, wherein the first contact layer has a thickness of 15 nm, and the second contact layer has a thickness of 50 nm .

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