JP2011211239A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device using a group III-V nitride semiconductor including ohmic electrodes small in contact resistance.SOLUTION: This semiconductor device includes a first group III-V nitride semiconductor layer 12 and a second group III-V nitride semiconductor layer 13 having a large bandgap relative to that of the first group III-V nitride semiconductor layer 12, a third group III-V nitride semiconductor layer 21 having a p-type conductivity, and first ohmic electrodes 14, all of which are sequentially formed on a substrate 11. The lower part of each ohmic electrode 14 is formed to pierce the second group III-V nitride semiconductor layer 13 and the third group III-V nitride semiconductor layer 21 and to reach a region on the lower side than a two-dimensional electron-gas layer of the first group III-V nitride semiconductor layer 12.

Description

  The present invention relates to a semiconductor device using a group III-V nitride semiconductor, and relates to a group III-V nitride semiconductor device including an ohmic electrode having a low contact resistance.

The III-V nitride semiconductor is aluminum (Al) or boron whose general formula is represented by B _w Al _x Ga _y In _z N (w + x + y + z = 1; 0 ≦ w, x, y, z ≦ 1) (B) refers to a compound semiconductor composed of a compound of gallium (Ga) or indium (In) and nitrogen (N).

  The III-V nitride semiconductor has advantages such as a high breakdown voltage based on a large band gap, a high electron saturation speed and a high electron mobility, and a high electron concentration at the heterojunction. Research and development are underway for the purpose of application to high-speed devices for millimeter waves and the like. In particular, a heterojunction structure in which III-V nitride semiconductor layers having different band gaps are stacked, or a quantum well structure or superlattice structure in which a plurality of these are stacked can control the degree of modulation of electron concentration in the device. Therefore, it is used as a basic structure of a device using a III-V nitride semiconductor.

  As a semiconductor device using a group III-V nitride semiconductor having a heterojunction structure, for example, there is a heterojunction field effect transistor (HFET) (see, for example, Patent Document 1).

  The HFET is, for example, an operation layer made of gallium nitride (GaN) sequentially formed on a substrate, a barrier layer made of undoped aluminum gallium nitride (AlGaN), and a source electrode and a drain electrode formed on the barrier layer. And a gate electrode.

  AlGaN has a larger band gap than GaN, so the heterojunction interface between the working layer and the barrier layer requires electrons from the difference in spontaneous polarization and difference in piezoelectric polarization between AlGaN and GaN, and is required in the barrier layer. Accordingly, electrons derived from the n-type impurity doped, electrons derived from other uncontrollable defects in the semiconductor layer, etc. accumulate at a high concentration to form a two-dimensional electron gas layer (2DEG). The 2DEG layer operates as a channel carrier of the field effect transistor.

  Further, if a cathode (ohmic) electrode and an anode electrode are formed on a group III-V nitride semiconductor layer laminated so as to form a heterojunction interface, the 2DEG layer operates as a channel carrier of the diode. A barrier diode (SBD) is obtained (see, for example, Patent Document 2).

  In order to apply a semiconductor device using a group III-V nitride semiconductor as a high-voltage element for power or a high-speed element for the millimeter wave band, it is necessary to reduce the contact resistance of the ohmic electrode portion and lower the on-resistance. Desired. However, in the conventional HFET, SBD, etc., the source / drain electrode or the cathode electrode is formed on the undoped AlGaN layer. For this reason, since electrons must reach the 2DEG layer beyond the potential barrier of the undoped AlGaN layer, the contact resistance increases.

  As a method for reducing the contact resistance, for example, there is a recess ohmic structure in which a recess is formed in the outermost barrier layer, an ohmic contact layer is formed in the formed recess, and an ohmic electrode is formed on the ohmic contact layer. It is known (for example, refer to Patent Document 3). In addition, a method for reducing contact resistance by introducing an impurity having conductivity into the surface of a barrier layer is known (see, for example, Patent Documents 4 and 5).

JP 2002-16245 A JP 2004-31896 A JP 2001-102565 A JP 2004-56146 A JP 2004-111910 A

  However, the potential barrier of the barrier layer remains in the conventional semiconductor device having the recess ohmic structure. In addition, the etching at the time of forming the recess portion causes etching damage to the semiconductor layer, or the carrier concentration of the 2DEG layer decreases due to the etching damage, so that the contact resistance cannot be sufficiently reduced. is there.

  Further, it is difficult to determine the etching stop position of the recessed portion that has been dug, and there is a problem that the manufacturing process of the semiconductor device becomes complicated and the yield decreases.

  An object of the present invention is to solve the above-described conventional problems and to realize a semiconductor device using a group III-V nitride semiconductor having an ohmic electrode having a low contact resistance.

  In order to achieve the above object, the present invention has a semiconductor device having an ohmic electrode in direct contact with a two-dimensional electron gas layer.

  Specifically, a semiconductor device according to the present invention according to the present invention includes a first group III-V nitride semiconductor layer formed on a substrate and having a two-dimensional electron gas layer, and a first group III-V. A second group III-V nitride semiconductor layer formed on the nitride semiconductor layer and having a band gap larger than that of the first group III-V nitride semiconductor, and a lower portion thereof is a second group III-V nitride. An ohmic electrode formed through the physical semiconductor layer and reaching a region below the two-dimensional electron gas layer in the first group III-V nitride semiconductor layer, and the first group III-V nitride semiconductor layer And an impurity doped layer formed by introducing a conductive impurity in a portion in contact with the ohmic electrode in the second III-V nitride semiconductor layer.

  According to the semiconductor device of the present invention, the first III-V group nitride semiconductor layer includes an ohmic electrode formed so as to fill an opening reaching a lower part of the two-dimensional electron gas layer. Therefore, the ohmic electrode and the two-dimensional electron gas layer are in direct contact. In particular, the contact surface between the electrode and the semiconductor layer is provided with an impurity doped layer into which an impurity having conductivity is introduced, so that the electrode and the two-dimensional electron gas layer are in surface contact instead of point contact or line contact. It will be. Accordingly, since electrons can reach the two-dimensional electron gas layer without exceeding the potential barrier of the barrier layer, the contact resistance can be greatly reduced.

  In the semiconductor device of the present invention, the second group III-V nitride semiconductor layer preferably has a stacked structure in which a plurality of group III-V nitride semiconductor films are stacked.

  In the semiconductor device of the present invention, two ohmic electrodes are formed at a distance from each other, and a gate electrode is provided in a region between the two ohmic electrodes on the second group III-V nitride semiconductor layer. Is preferably formed. With such a configuration, a field effect transistor including an ohmic electrode having a small contact resistance can be realized.

  The semiconductor device of the present invention further includes a third group III-V nitride semiconductor layer formed on the second group III-V nitride semiconductor layer, and the ohmic electrode further includes a third group III-V. It is preferably formed so as to penetrate the group nitride semiconductor layer. With such a configuration, the contact resistance can be greatly reduced even in a semiconductor device having a cap layer.

  In this case, the third group III-V nitride semiconductor layer preferably has a stacked structure in which a plurality of group III-V nitride semiconductor films are stacked.

  In this case, two ohmic electrodes are formed at a distance from each other, and a gate electrode is formed in a region between the two ohmic electrodes on the second group III-V nitride semiconductor layer. It is preferable.

  In this case, the third group III-V nitride semiconductor layer has a gate recess portion that exposes the second group III-V nitride semiconductor layer in a region between the two ohmic electrodes. It is preferable that the gate recess is formed.

  In this case, the semiconductor device further includes a fourth group III-V nitride semiconductor layer formed between the gate electrode and the third group III-V nitride semiconductor layer and having p-type conductivity, Preferably, the fourth group III-V nitride semiconductor layer is in ohmic contact.

  In the semiconductor device of the present invention, an anode electrode formed at a position different from the ohmic electrode on the second group III-V nitride semiconductor layer and in Schottky contact with the second group III-V nitride semiconductor layer is provided. Furthermore, it is preferable to provide. With such a configuration, a Schottky barrier diode having a cathode electrode with low contact resistance can be realized.

  In the semiconductor device of the present invention, the ohmic electrode has an opening that penetrates the second group III-V nitride semiconductor layer and reaches a lower side than the two-dimensional electron gas layer in the first group III-V nitride semiconductor layer. It is preferable that the wall is inclined so that the width of the opening is wider toward the top. With such a configuration, it becomes easy to form the ohmic electrode by vapor deposition and lift-off, and a highly reliable semiconductor device can be realized.

  In the semiconductor device of the present invention, the conductive impurity is preferably silicon.

  In the semiconductor device of the present invention, the lower portion of the ohmic electrode is preferably formed to a depth of 10 nm or more than the two-dimensional electron gas layer in the first III-V group nitride semiconductor layer. With such a configuration, the contact resistance can be reliably reduced. In addition, when the opening is formed by etching, it is not necessary to strictly control the etching stop position, which facilitates the manufacture of the semiconductor device.

  In the semiconductor device of the present invention, it is preferable that the ohmic electrode has a protruding portion protruding from the upper surface of the second group III-V nitride semiconductor layer, and the length of the protruding portion is 1 μm or less. With such a configuration, it is possible to suppress an increase in the sheet resistance of the two-dimensional electron gas layer due to the influence of the projecting portion and an increase in contact resistance.

  According to the semiconductor device of the present invention, it is possible to realize a semiconductor device using a group III-V nitride semiconductor including an ohmic electrode having a small contact resistance.

1 is a cross-sectional view showing a semiconductor device according to a first embodiment of the present invention. It is sectional drawing which shows the ohmic electrode part of the semiconductor device which concerns on the 1st Embodiment of this invention. It is a graph which shows the correlation with the length of the overhang | projection part of the ohmic electrode of the semiconductor device which concerns on 1st of this invention, and contact resistance. 2 is a graph showing current-voltage characteristics of the semiconductor device according to the first embodiment of the present invention. It is sectional drawing which shows the semiconductor device which concerns on the 2nd Embodiment of this invention. It is a graph which shows the correlation with the depth of the opening part, and contact resistivity in the semiconductor device which concerns on the 2nd Embodiment of this invention. It is sectional drawing which shows the semiconductor device which concerns on the 3rd Embodiment of this invention. It is sectional drawing which shows the semiconductor device which concerns on the 4th Embodiment of this invention. It is a graph which shows the current-voltage characteristic of the semiconductor device which concerns on the 4th Embodiment of this invention.

(First embodiment)
A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a cross-sectional configuration of the semiconductor device according to the present embodiment. As shown in FIG. 1, the semiconductor device of this embodiment is a heterojunction field effect transistor (HFET). On the substrate 11, an operation layer 12 made of undoped GaN and a barrier layer 13 made of undoped Al _x Ga _(1-x) N (0 <x ≦ 1) having a larger band gap than GaN are stacked. Has been. Since the operation layer 12 and the barrier layer 13 form a heterojunction interface, a two-dimensional electron gas (2DEG) layer is generated in a region near the heterojunction interface in the operation layer 12.

  A gate electrode 16 that is a Schottky electrode is formed on the barrier layer 13, and ohmic electrodes 14 that are source and drain electrodes are formed on both sides of the gate electrode 16. A surface protective film 17 made of silicon nitride (SiN) is formed so as to cover the gate electrode 16 and the ohmic electrode 14.

  In the HFET of this embodiment, the ohmic electrode 14 is formed so that the base portion penetrates the barrier layer 13 and reaches a region below the 2DEG layer in the operation layer 12. Specifically, a conductive material is embedded in an opening formed so as to penetrate the barrier layer 13 and dig into the operation layer 12. The depth of the opening in which the conductive material is embedded should be deeper than that of the 2DEG layer, and if it is deeper than the 2DEG layer by 10 nm or more, an ohmic electrode having a lower resistance can be obtained. Further, as will be described later, the contact resistance value becomes substantially constant by making the depth of the opening 10 nm or more deeper than the 2DEG layer. Therefore, when the opening is formed by etching, the etching stop position is set. Need not be strictly controlled. As a result, the semiconductor device can be easily manufactured.

Further, an n-type impurity doped layer 18 into which an n-type dopant made of silicon or the like is introduced is formed in a portion of the operation layer 12 and the barrier layer 13 that is in contact with the ohmic electrode 14. As described above, the contact resistance can be further reduced by forming the impurity doped layer 18 in the portion in contact with the ohmic electrode 14 in the operation layer 12 and the barrier layer 13. The concentration of silicon introduced into the impurity doped layer 18 may be about 1 × 10 ¹⁹ cm ⁻³ .

  The ohmic electrode 14 and the 2DEG layer can be brought into direct contact with each other over a wide area by embedding the ohmic electrode 14 in the opening and introducing an n-type dopant into the interface between the ohmic electrode 14 and the operation layer 12 and the barrier layer 13. Therefore, the contact resistance can be reduced. In order to reduce the contact resistance, it is ideal that the ohmic electrode 14 is formed so that the width of the ohmic electrode 14 is completely coincident with the opening and does not protrude on the barrier layer 13.

  FIG. 2 shows an enlarged cross section of the ohmic electrode portion and shows the resistance generated between the ohmic electrode 14 and the 2DEG layer. The contact resistance Rc of the ohmic electrode 14 includes a resistance Rce where the ohmic electrode and the 2DEG layer are in direct contact, a resistance Rco where the ohmic electrode is in contact with the 2DEG layer via the barrier layer 13, and the resistance of the 2DEG layer It depends on the sheet resistance Rs.

  As shown in FIG. 3, when the length of the overhanging portion 14a overhanging the barrier layer 13 of the ohmic electrode 14 is increased, the sheet resistance Rs of the 2DEG layer is increased. For this reason, the total contact resistance Rc increases. Accordingly, the length of the overhanging portion 14a is preferably as short as possible. However, since the overhanging portion 14a cannot be eliminated completely in the process, the thickness is preferably 1 μm or less.

  Moreover, it is preferable that the wall surface of the opening has an inclined shape. The ohmic electrode 14 may be formed by a lift-off method in which after a resist film is selectively formed on the barrier layer 13, a metal material is vapor-deposited and the metal material deposited on the resist film is removed together with the resist film. It is common. By inclining the wall surface of the opening, it is easy to deposit a metal material on the opening, and the adhesion of the ohmic electrode to the wall of the opening can be improved.

  FIG. 4 shows the characteristics of the drain current and drain voltage of the HFET at various bias voltages in comparison with the HFET of this embodiment and the conventional HFET. Under any bias conditions, the HFET of this embodiment has a lower on-resistance and a higher current value than the conventional example.

(Second Embodiment)
The second embodiment of the present invention will be described below with reference to the drawings. FIG. 5 shows a cross-sectional configuration of the semiconductor device according to the second embodiment. In FIG. 5, the same components as those of FIG.

As shown in FIG. 5, the semiconductor device of this embodiment includes a cap layer 21 formed on the barrier layer 13 and made of GaN or Al _y Ga _(1-y) N (0 <y ≦ 1). Yes. The conductivity type of the cap layer 21 may be any of n-type, p-type, and i-type. In this embodiment, the case of the p-type will be described as an example.

  When the cap layer 21 is p-type, an effect of suppressing current collapse can be obtained. However, when the ohmic electrode 14 is formed so as to be in contact with the upper surface of the p-type cap layer 21, the contact resistance greatly increases.

  In the HFET of this embodiment, the ohmic electrode 14 which is a source electrode and a drain electrode penetrates the cap layer 21 and the barrier layer 13 and is formed by digging the operation layer 12 to the lower side of the 2DEG layer. Is formed to fill. Further, an impurity doped layer 18 into which an n-type impurity such as silicon is introduced is formed in a portion of the cap layer 21, the barrier layer 13, and the operation layer 12 that is in contact with the ohmic electrode 14.

FIG. 6 shows the relationship between the depth of the opening and the contact resistivity. As shown in FIG. 6, when the opening depth is 0 nm, that is, when the ohmic electrode 14 is formed so as to be in contact with the upper surface of the cap layer 21, the contact resistivity is about 1 × 10 ⁻³ . . On the other hand, when an opening having a depth of 15 nm reaching the interface between the cap layer 21 and the barrier layer 13 is formed and the ohmic electrode 14 is formed in contact with the upper surface of the barrier layer 13, the contact resistivity is 10 minutes. It decreases by 1 and shows a value of about 0.8 × 10 ⁻⁴ . Further, the contact resistivity is lowered by increasing the depth of the opening, and when the depth of the opening becomes about 10 nm deeper than the 2DEG layer, the contact resistivity is approximately constant at a value of about 1 × 10 ^−5. became.

  Thus, it is clear that the contact resistance value of the ohmic electrode can be greatly reduced by forming the opening and forming the ohmic electrode in the formed opening. In this case, it is preferable that the depth of the opening is 10 nm or more deeper than the 2DEG layer and the base of the ohmic electrode reaches 10 nm or more lower than the 2DEG layer because the contact resistance is further reduced. Further, since the contact resistance value becomes substantially constant by making the depth of the opening 10 nm or more deeper than the 2DEG layer, it is necessary to strictly control the etching stop position when the opening is formed by etching. There is no. As a result, the semiconductor device can be easily manufactured.

  As described above, when the cap layer is formed, the effect of reducing the contact resistance is particularly great. The same effect can be obtained not only when the cap layer is p-type but also when it is n-type or undoped.

(Third embodiment)
The third embodiment of the present invention will be described below with reference to the drawings. FIG. 7 shows a cross-sectional configuration of the semiconductor device according to the third embodiment. In FIG. 7, the same components as those in FIG.

As shown in FIG. 7, the semiconductor device of this embodiment includes a control layer 22 formed between the gate electrode 16 and the cap layer 21. The control layer 22 is made of GaN having a p-type conductivity or Al _z Ga _(1-z) N (0 <z ≦ 1) and is in ohmic contact with the gate electrode 16.

  Since the control layer 22 has a p-type conductivity and is in ohmic contact with the gate electrode 16, the control layer 22 and the operation layer 12 form a pn junction. Therefore, a depletion layer is formed immediately below the control layer 22 even when no bias is applied to the gate electrode 16. As a result, an HFET having a normal Schottky contact gate electrode without the control layer 22 becomes a normally-on (depletion) type transistor, whereas the HFET of the present embodiment has a normally-off (enhancement) type. It becomes a transistor. In particular, in a power system power supply circuit, a normally-off transistor is indispensable as a switch, and this embodiment is effective for such a use.

(Fourth embodiment)
The fourth embodiment of the present invention will be described below with reference to the drawings. FIG. 8 shows a cross-sectional configuration of the semiconductor device according to the fourth embodiment.

As shown in FIG. 8, the semiconductor device of this embodiment is a Schottky barrier diode (SBD). On the substrate 11, an operation layer 12 made of GaN and a barrier layer 13 made of Al _x Ga _(1-x) N (0 <x ≦ 1) having a larger band gap than GaN are stacked. Since the operation layer 12 and the barrier layer 13 form a heterojunction interface, a 2DEG layer is generated in a region near the heterojunction interface in the operation layer 12.

  An ohmic electrode 14 that is a cathode electrode is formed so as to penetrate the barrier layer 13 and reach the lower side of the 2DEG layer in the operation layer 12, and an anode electrode 19 that is a Schottky electrode is formed so as to surround the ohmic electrode 14. ing. A surface protective film 17 made of silicon nitride (SiN) is formed so as to cover the ohmic electrode 14 and the anode electrode 19.

  Also in this embodiment, an impurity doped layer 18 into which an n-type impurity is introduced is formed in a portion of the barrier layer 13 and the operation layer 12 that is in contact with the ohmic electrode 14. Further, the contact resistance can be further reduced by forming the ohmic electrode 14 so as to reach 10 nm or more below the 2DEG layer.

  FIG. 9 shows the relationship between the anode voltage and the current density by comparing the SBD of this embodiment with the conventional SBD. As shown in FIG. 9, it is clear that the SBD of this embodiment has a higher current density value and a smaller contact resistance value than the conventional SBD.

  In each embodiment, an example in which the barrier layer, the cap layer, and the control layer are formed of a single film has been shown. However, each of the barrier layer, the cap layer, and the control layer has a stacked structure in which a plurality of films are stacked. It may be.

  Note that common materials may be used for the ohmic electrode and the Schottky electrode. For example, a laminated film of titanium (Ti), aluminum (Al), titanium (Ti), and gold (Au) is used for the n-type ohmic electrode. Used, a laminated film of nickel (Ni), platinum (Pt) and gold (Au) is used for the p-type ohmic electrode, and palladium (Pd) or palladium silicon alloy (PdSi) and gold (Au) are used for the Schottky electrode. A stacked film may be used.

  The semiconductor device according to the present invention can realize a semiconductor device using a group III-V nitride semiconductor having an ohmic electrode having a low contact resistance, and is useful as a semiconductor device using a group III-V nitride semiconductor. .

DESCRIPTION OF SYMBOLS 11 Board | substrate 12 Operation | movement layer 13 Barrier layer 14 Ohmic electrode 16 Gate electrode 17 Surface protective film 18 Impurity doped layer 19 Anode electrode 21 Cap layer 22 Control layer

Claims (2)

A first III-V nitride semiconductor layer formed on a substrate and having a two-dimensional electron gas layer;
A second group III-V nitride semiconductor layer formed on the first group III-V nitride semiconductor layer and having a larger band gap than the first group III-V nitride semiconductor;
A third group III-V nitride semiconductor layer formed on the second group III-V nitride semiconductor layer and having a p-type conductivity;
A lower part is formed through the second group III-V nitride semiconductor layer and the third group III-, and is lower than the two-dimensional electron gas layer in the first group III-V nitride semiconductor layer. And a first ohmic electrode reaching the region. A fourth group III-V nitride semiconductor layer formed on the third group III-V nitride semiconductor layer and having p-type conductivity;
2. The semiconductor device according to claim 1, further comprising a second ohmic electrode formed on the fourth fourth group III-V nitride semiconductor layer.

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