JP2009152491A - Nitride semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor element in which contact resistance between a p-side GaN layer and an electrode can be reduced. <P>SOLUTION: A mask 11 for selective growth is formed on a sapphire substrate 1 as a substrate for growth, an undoped GaN layer 3, an n-type GaN layer 4, an active layer 5, and a p-type GaN layer 6 are laminated in order on an AlN buffer layer 2, and a mixed doped GaN layer 6a is formed on the p-type GaN layer 6. The mixed doped GaN layer 6a has GaN doped with a p-type impurity and an n-type impurity at the same time, and concentrations thereof are each ≥1×10<SP>19</SP>cm<SP>-3</SP>. Further, the mixed doped GaN layer 6a is ≤25 nm in thickness. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor device with reduced contact resistance between a p-type GaN layer and an electrode.

  Development of semiconductor devices called gallium nitride compound semiconductors, so-called III-V group nitride semiconductors (hereinafter referred to as nitride semiconductors), is active. Nitride semiconductors are used in blue LEDs used as light sources for lighting, backlights, etc., LEDs used in multicoloring, LDs, and the like. Since it is difficult to manufacture a bulk single crystal in a nitride semiconductor, GaN is grown on a heterogeneous substrate such as sapphire or SiC by using MOCVD (metal organic chemical vapor deposition). A sapphire substrate is particularly used as a growth substrate because it is excellent in stability in a high-temperature ammonia atmosphere in an epitaxial growth process.

  When manufacturing a nitride semiconductor by MOCVD, for example, an organometallic compound gas is supplied as a reaction gas into a reaction chamber in which a sapphire substrate is installed as a growth substrate, and the crystal growth temperature is about 900 ° C. to 1100 ° C. An epitaxial layer of a GaN semiconductor crystal is grown on the sapphire substrate while being held at a high temperature.

  However, the growth constant such as a sapphire substrate and GaN have different lattice constants, so in the GaN-based semiconductor layer grown on the growth substrate, there exist dislocations (lattice defects) extending vertically from the substrate. Yes. This dislocation does not occur only in the GaN on the growth substrate but propagates to the nitride semiconductor layer formed on the GaN, thereby causing stacking faults. These lattice defects adversely affect the device such as pn junction leakage, shortening of carrier life, and generation of cracks due to stress generated between semiconductor layers.

Thus, selective growth or selective lateral growth (ELO: Epitaxial Lateral Overgrowth) is well known as a method for reducing such dislocations. For example, as disclosed in Patent Document 1, the ELO is one in which a selective growth mask is formed on a growth substrate and a GaN-based semiconductor layer is crystal-grown in an opening.
JP 2000-68599 A

  However, when the GaN-based semiconductor layer is crystal-grown using ELO as in the above prior art, the crystallinity in the stacking direction of the GaN-based semiconductor layer can be increased, but the contact between the surface of the GaN-based semiconductor layer and the metal Resistance increases. Among the GaN-based semiconductor layers, since the contact resistance with the metal is somewhat small, the electrode is provided on the p-type GaN contact layer.

  However, when ELO is used, there is a tendency that the contact resistance between the p-type GaN contact layer and the electrode becomes larger and the operating voltage increases. Even when ELO is not used, the ohmic contact between the p-type GaN and the electrode is difficult to obtain. Usually, the contact resistance is reduced by annealing treatment or the like, and the p-side contact layer that is easy to obtain the ohmic contact. Is required.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a nitride semiconductor device that can reduce the contact resistance between a p-side GaN layer and an electrode.

In order to achieve the above object, the invention according to claim 1 is doped with a p-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more and an n-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more with a film thickness of 25 nm or less. The nitride semiconductor device is characterized in that the mixed doped GaN layer is formed on the p-type GaN layer, and an electrode is formed on the mixed doped GaN layer.

  The invention according to claim 2 is the nitride semiconductor device according to claim 1, wherein the p-type impurity is Mg and the n-type impurity is Si.

  The invention according to claim 3 is characterized in that the p-type GaN layer and the mixed doped GaN layer are formed by selective growth, and the nitride according to any one of claims 1 and 2 It is a semiconductor element.

According to the present invention, a mixed doped GaN layer doped with a p-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more and an n-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more with a film thickness of 25 nm or less is a p-type GaN layer. Since the electrode is formed on the mixed doped GaN layer, the contact resistance between the electrode and the GaN semiconductor is lowered, and the current easily flows through the p-type GaN layer.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an example of a cross-sectional structure of a nitride semiconductor device of the present invention. FIG. 1 shows a state in which a plurality of nitride semiconductor elements are formed on one wafer.

The nitride semiconductor constituting the nitride semiconductor element is formed by a known MOCVD method or the like. Here, the nitride semiconductor represents an AlGaInN quaternary mixed crystal and is called a so-called group III-V nitride semiconductor, and Al _x Ga _y In _z N (x + y + z = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1). The GaN-based semiconductor is a semiconductor composed only of GaN or a semiconductor containing GaN as a constituent material, and 0 <y ≦ 1 in the AlGaInN quaternary mixed crystal.

A selective growth mask 11 is formed on a sapphire substrate 1 which is a growth substrate, and an AlN buffer layer 2 is formed in a region where a portion of the selective growth mask 11 has been removed. On the AlN buffer layer 2, an undoped GaN layer 3, an n-type GaN layer 4, an active layer 5, and a p-type GaN layer 6 are sequentially stacked, and each of these semiconductor layers is formed by MOCVD. The active layer 5 has a multiple quantum well structure including a barrier layer made of GaN and a well layer made of In _X1 Ga _{1 -X1} N (0 <X1). A sapphire substrate or the like is used as the growth substrate, but any substrate having a hexagonal crystal structure (wurtzite structure) may be used.

  A mixed doped GaN layer 6 a is formed on the p-type GaN layer 6. In the mixed doped GaN layer 6 a, the basic material constituting the semiconductor layer is the same GaN layer as the p-type GaN layer 6, and therefore the hatching is the same as that of the p-type GaN layer 6 in the drawing. The difference between the p-type GaN layer 6 and the mixed doped GaN layer 6a is a doped impurity. For example, Mg (magnesium) is used as the p-type impurity in the p-type GaN layer 6, but Si (silicon), which is an n-type impurity, is simultaneously doped in the mixed doped GaN layer 6 a in addition to this Mg. .

That is, the mixed doped GaN layer 6a is doped with p-type impurities and n-type impurities, and the impurity concentration thereof is 1 × 10 ¹⁹ cm ⁻³ or more. Here, the impurity concentration means the atomic concentration of the dopant. As will be described later, the mixed doped GaN layer 6a has a film thickness of 25 nm (250 mm) or less.

The AlN buffer layer 2 to the mixed doped GaN layer 6a are selectively grown. An insulating film is used for the selective growth mask 11, and as the insulating film, SiO ₂ , Si ₃ N ₄ , ZrO ₂ or the like is used. is there.

  A transparent electrode 7 is formed on the mixed doped GaN layer 6a, and a p-electrode 8 is formed on the transparent electrode 7. An n electrode 9 is provided on the exposed n-type GaN layer 4 by mesa etching from the mixed doped GaN layer 6a. The transparent electrode 7 is made of ZnO, ITO or the like, the p electrode 8 is made of a metal multilayer film such as Pd / Au, and the n electrode 9 is made of Al or the like.

  On the other hand, FIG. 2 shows a structure in which the p-electrode 8 is provided directly on the mixed doped GaN layer 6a without the transparent electrode 7 shown in FIG. The constituent materials of the p electrode 18 and the n electrode 19 in this case are the same as those in FIG.

As described above, the mixed doped GaN layer 6a doped with a p-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more and an n-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more with a film thickness of 25 nm or less is used as a p-type GaN layer. 6 and an electrode is formed on the mixed doped GaN layer 6a. The p-type GaN layer 6 is doped with p-type impurities at 1 × 10 ¹⁹ cm ⁻³ or more. Therefore, in FIG. 1, the contact resistance between the transparent electrode 7 and the GaN semiconductor is lowered, and in FIG. 2, the contact resistance between the p electrode 18 and the GaN semiconductor is lowered, and the current easily flows through the p-type GaN layer 6.

FIG. 9 is a diagram for explaining the operation of the nitride semiconductor device of the present invention formed as described above. FIG. 9A shows an energy band diagram at the contact interface between the p-type GaN layer having the conventional structure and the metal constituting the p-electrode. Here, E _F is the Fermi level, E _C is the level of the conduction band, E _V represents the level of the valence band. In FIG. 9A, since the semiconductor layer is in contact with the metal, a band structure of Schottky contact is obtained. Therefore, since there is a barrier at the contact interface, holes are not easily injected from the metal toward the p-type GaN layer.

On the other hand, FIG. 9B is an energy band diagram at the contact interface when the mixed doped GaN layer 6a is formed between the p-type GaN layer 6 and the metal (p electrode or transparent electrode) having the structure of the present invention. is there. A region having a width t corresponds to the mixed doped GaN layer 6a. Energy band of the drawn E _a by a two-dot chain line represents the curve of the same shape as the E _V in FIG. 9 (a).

First, since the GaN layer and a metal obtained by mixing a p-type impurity and the n-type impurity is in contact, bending of the energy band becomes large in the vicinity of the contact interface than E _V in the case of FIG. 9 (a). When the bending of the energy band increases downward, the barrier at the contact interface increases, so that holes do not easily flow from the metal into the semiconductor layer.

On the other hand, since the n-type impurity is simultaneously doped, the energy level is generated as a new E _D by n-type impurity. This energy level E _D, holes are easily flows from the metal.

Thus, the results of p-type impurity added n-type impurity doped, than the effect due to the expansion of the barrier at the contact interface, when the effect is large due to the energy level of E _D newly generated, the contact resistance is lowered, The operating voltage Vf will also decrease. In order to generate this effect, each doping concentration of the p-type impurity and the n-type impurity is set to 1 × 10 ¹⁹ cm ⁻³ or more.

Further, an excessively large thickness of the mixed-doped GaN layer 6a of the n-type impurity, the energy band structure is changed significantly, it affects to device characteristics, level E _V of the valence band in the vicinity of the contact interface Only the region where the bending of the band is large is sufficient, and the film thickness is desirably 250 mm or less.

A method for manufacturing the nitride semiconductor device of FIG. 1 will be described with reference to FIGS. As shown in FIG. 3, a selective growth mask 11 is formed on the growth substrate 1. For example, a sapphire substrate as a growth substrate 1 is put into an MOCVD (metal organic chemical vapor deposition) apparatus, and the temperature is raised to about 1050 ° C. while flowing hydrogen gas to thermally clean the sapphire substrate. Thereafter, SiO ₂ which is an insulating film is formed as a selective growth mask 11 on the sapphire substrate. Sputtering power is 300 W, sputtering atoms Ar are supplied at 20 cc / min, and sputtering is performed for 20 minutes in an atmosphere with a pressure of 1 Pa to form a SiO ₂ film with a thickness of about 1000 mm on the sapphire substrate.

Next, a resist is formed on the SiO ₂ film in a predetermined pattern by photolithography, and dry etching, for example, CF 4 gas is supplied in a plasma state at a pressure of 3 Pa and a power of 100 W to perform etching. Thereafter, when the resist is removed, the result is as shown in FIG.

Thereafter, the AlN buffer layer 2 is grown as shown in FIG. The AlN buffer layer 2 is a high-temperature AlN buffer layer, has a temperature of 900 ° C. or higher (eg, 900 ° C.), a growth pressure of 200 Torr, hydrogen as a carrier gas, and a flow rate of carrier hydrogen (H ₂ ) of 14 L. The flow rate of TMA (trimethylaluminum) was 20 cc / min, and the flow rate of NH ₃ (ammonia) was 500 cc / min. When the molar ratio of NH ₃ / TMA at this time is calculated, it is about 2600. Under this growth condition, a high-temperature AlN buffer layer 2 having a thickness of 30 mm is formed.

  By the way, in order to form the separated nitride semiconductor elements as shown in FIGS. 1 and 2, it is necessary to make the growth rate in the vertical direction larger than the growth rate in the horizontal direction. For this purpose, the opening of the selective growth mask 11 is formed by removing a part of the insulating film. The shape of the opening is shown in FIG.

  For example, as shown in FIG. 7A, a selective growth mask having a large number of hexagonal openings 11a is used. In this case, a hexagonal element is obtained. The nitride semiconductor has a (0001) orientation wurtzite type (hexagonal) crystal structure (the crystal structure in FIG. 10), and has a crystal polarity in which Ga atoms or N atoms are in the growth surface direction (grow in the c-axis direction). have.

  Therefore, a sapphire substrate or the like having the same wurtzite crystal structure is used as the growth substrate, and the main surface of the sapphire substrate has a C-plane, and a nitride semiconductor is stacked on the main surface. All C planes are in the growth surface direction. In this case, if the mask for selective growth having the hexagonal opening 11a shown in FIG. 6 is used, if the hexagonal side L1 is arranged parallel to the M-plane of the growth substrate, the crystal grown by selective growth is almost all. Since it grows in the vertical direction, it becomes a separated semiconductor layer.

  On the other hand, as shown in FIG. 7B, a selective growth mask 11 having a plurality of stripe-shaped openings may be used. In the selective growth mask shown in FIG. 7B, a ridge stripe-shaped element is formed. In this case, the growth pressure, growth temperature, etc. during selective growth may be set so that the growth rate in the vertical direction is larger than the growth rate in the horizontal direction.

Next, in the MOCVD apparatus, the growth temperature is set to 1020 ° C. to 1040 ° C., the supply of TMA of NH ₃ and TMA is stopped, for example, trimethylgallium (TMGa) is supplied at 20 μmol / min, and the undoped GaN layer 3 Are stacked to a thickness of about 0.5 μm.

Thereafter, silane (SiH ₄ ) is supplied as an n-type dopant gas to grow the n-type GaN layer 4 with a thickness of about 4 μm. Next, the supply of TMGa and silane is stopped, the substrate temperature is lowered between 700 ° C. and 800 ° C. in a mixed atmosphere of ammonia and hydrogen, and triethylgallium (TEGa) is supplied at 20 μmol / min to obtain the active layer 5. The undoped GaN barrier layer is laminated, and trimethylindium (TMIn) is supplied at 200 μmol / min, and the InGaN well layer is laminated. A multiple quantum well structure is formed by repeating the GaN barrier layer and the InGaN well layer. The film thickness of the active layer 5 is formed to be about 0.1 μm, for example.

After the active layer 5 is grown, the growth temperature is increased to 1020 ° C. to 1040 ° C., and trimethylgallium (TMGa), which is a Ga atom source gas, ammonia (NH ₃ ), which is a nitrogen atom source gas, and p-type impurity Mg. CP ₂ Mg (biscyclopentadiethyl magnesium) as a dopant material is supplied, and a p-type GaN layer 6 doped with 1 × 10 ¹⁹ cm ⁻³ or more of Mg (magnesium) is grown to a thickness of about 0.3 μm.

Next, the mixed doped GaN layer 6a is grown. However, since it is the same GaN layer as the p-type GaN layer 6 and only the kind of impurities is different, in addition to the p ₂ impurity source gas CP ₂ Mg, an n-type impurity is added. Also, silane (SiH ₄ ), which is a raw material gas, is also supplied and formed so that Si (silicon) is doped at 1 × 10 ¹⁹ cm ⁻³ or more and Mg (magnesium) is doped at 1 × 10 ¹⁹ cm ⁻³ or more. In this way, the mixed doped GaN layer 6a is grown to a thickness of 250 mm or less. A layered structure as shown in FIG. 5 is obtained by selectively growing the undoped GaN layer 3 to the mixed doped GaN layer 6a.

  Next, as shown in FIG. 6, mesa etching is performed until the n-type GaN layer 4 is exposed from the mixed doped GaN layer 6 a to form the transparent electrode 7 on the mixed doped GaN layer 6 a, and p on the transparent electrode 7. If the electrode 8 and the n electrode 9 are formed on the exposed surface of the n-type GaN layer 4, the nitride semiconductor device of FIG. 1 is completed. The transparent electrode 7 is made of ZnO, the thickness is 2000 mm, the p electrode 8 is made of a metal multilayer film of Pd / Au, each film thickness is 100 mm / 1250 mm, the n electrode 9 is made of Al, and the thickness is 3000kg.

  On the other hand, in order to make the structure without the transparent electrode 7 of FIG. 2, from the state of FIG. 6, the p electrode 18 is formed as a Pd / Au metal multilayer film, and each film thickness is formed to 100 mm / 1250 mm, and the n electrode 19 is formed. As Al, the thickness is formed to 3000 mm.

For the production of each semiconductor layer, each semiconductor layer such as triethylgallium (TEGa), trimethylgallium (TMG), ammonia (NH ₃ ), trimethylaluminum (TMA), trimethylindium (TMIn), as well as hydrogen or nitrogen as a carrier gas. Necessary gas such as silane (SiH ₄ ) as a dopant gas for n-type, CP ₂ Mg (cyclopentadienylmagnesium) as a dopant gas for p-type By supplying and sequentially growing in a range of about 700 ° C. to 1200 ° C., a semiconductor layer having a desired composition and a desired thickness can be formed with a desired composition.

  A VI characteristic curve of the nitride semiconductor device of FIG. 1 fabricated as described above is indicated by a dotted line Y in FIG. On the other hand, the VI characteristic curve of the nitride semiconductor device having the configuration of FIG. 1 in which the mixed doped GaN layer 6a is eliminated and the p-type GaN layer 6 alone is 0.3 μm thick is shown by a solid line X in FIG. As described above, Si is used for the n-type impurity and Mg is used for the p-type impurity. The film thickness of the mixed doped GaN layer 6a was 200 mm.

  These are nitride semiconductor elements formed in a hexagonal ridge shape by a hexagonal pattern as shown in FIG. 7A, and are elements having a size of 120 μm × 160 μm. As can be seen from the curve Y of the structure of the present invention, the operating voltage Vf is smaller in all forward current regions (If) than in the curve X. For example, when compared at If = 20 mA, Vf = 8.2V is obtained for the curve X, and Vf = 7.5V is obtained for the curve Y. It can be seen that as the forward current increases, the voltage difference between the curves X and Y increases, and the resistance of the device is lower in the structure of the present invention.

Thus, the contact resistance between GaN and the electrode can be reduced by mixing and doping Si as an n-type impurity and Mg as a p-type impurity.

It is a figure which shows an example of the cross-sectional structure of the nitride semiconductor element of this invention. It is a figure which shows an example of the cross-sectional structure of the nitride semiconductor element of this invention. It is a figure which shows one manufacturing process of the nitride semiconductor element of this invention. It is a figure which shows one manufacturing process of the nitride semiconductor element of this invention. It is a figure which shows one manufacturing process of the nitride semiconductor element of this invention. It is a figure which shows one manufacturing process of the nitride semiconductor element of this invention. It is a figure which shows the shape and arrangement example of the mask for selective growth on the substrate for growth. It is a figure which shows the comparison of the VI characteristic of the conventional structure and the structure of this invention. It is a figure which shows the energy band structure explaining the effect | action of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Growth substrate 2 AlN buffer layer 3 Undoped GaN layer 4 N-type GaN layer 5 Active layer 6 p-type GaN layer 6a Mixed doped GaN layer 7 Transparent electrode 8 P electrode 9 N electrode 11 Mask for selective growth

Claims (3)

A mixed doped GaN layer doped with a p-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more and a n-type impurity of 1 × 10 ¹⁹ cm ⁻³ or more with a thickness of 25 nm or less is formed on the p-type GaN layer, A nitride semiconductor device comprising a structure in which an electrode is formed on a mixed doped GaN layer.   2. The nitride semiconductor device according to claim 1, wherein the p-type impurity is Mg and the n-type impurity is Si.   The nitride semiconductor device according to claim 1, wherein the p-type GaN and the mixed doped GaN layer are formed by selective growth.

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