JP4864270B2 - Method for forming ohmic electrode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming an ohmic electrode, and more particularly to a method for forming an ohmic electrode used as an electrode in a semiconductor device such as a field effect transistor (FET).
[0002]
[Prior art]
In recent years, FETs using an GaN crystal layer as an electron transit layer using an AlGaN / GaN heterojunction have been actively developed.
[0003]
FIG. 14 is a diagram showing an example of the element structure of an AlGaN / GaN heterojunction FET.
The AlGaN / GaN heterojunction FET 100 is formed on a substrate 101 such as sapphire, a GaN electron transit layer 102, Al_zGa_1-zIt has a structure in which N electron supply layers 103 (Al composition ratio z is in the range of 0 to 1) are sequentially stacked. Al_zGa_1-zA gate electrode 104 is formed on the N electron supply layer 103. Also Al_zGa_1-zA source electrode 105 and a drain electrode 106 are formed on the N electron supply layer 103 with the gate electrode 104 interposed therebetween. In addition, Al_zGa_1-zA passivation film 107 is formed on the exposed surface of the N electron supply layer 103.
[0004]
GaN used in the AlGaN / GaN heterojunction FET 100 and the like illustrated in FIG. 14 is a material having a wide band gap, a high breakdown electric field strength, and a large saturation electron velocity, and is used as a high voltage operation and high output device material. Attention has been paid. At present, power devices for mobile phone base stations are required to operate at a high voltage of 40 V or higher, and FETs having a structure like the AlGaN / GaN heterojunction FET 100 are very promising.
[0005]
In general, a low-resistance ohmic electrode is indispensable for increasing the efficiency of an FET. GaN-based semiconductors are known to have weak surface pinning, unlike GaAs, Si, and the like that have been widely used. Therefore, when forming an ohmic electrode on a GaN-based semiconductor, a design guideline different from the case of forming it on GaAs, Si or the like (see, for example, Patent Document 1) is required.
[0006]
Until now, in GaN-based semiconductors, an ohmic electrode is usually formed by selecting the type of metal and optimizing the annealing conditions for alloying according to the combination of the metals (for example, non-patent literature) 1). Conventionally, for example, in the case of an ohmic electrode for n-GaN, a metal having a small work function such as Al or a metal having good adhesion such as Ti that reacts with GaN is often used.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 5-003214 (paragraph numbers [0018] to [0023], FIG. 1)
[Non-Patent Document 1]
Kensuke Kasahara, 9 others, “Low-resistance contact formation and K-band high-power characteristics in AlGaN / GaN HJFET”, IEEJ Transaction C, Vol. 122, No. 1, p29-35
[0008]
[Problems to be solved by the invention]
However, in the formation of ohmic electrodes on GaN-based semiconductors, before the alloying by annealing, a clear design guideline on how to combine and form the ohmic electrode metal to be alloyed by annealing. Was lacking.
[0009]
For this reason, the original potential of the metal cannot be extracted, and there are cases where the contact resistance cannot be lowered sufficiently because an unalloyed region remains in the electrode even after annealing. Furthermore, a high annealing temperature is required to promote alloying, which may be one of the causes of reducing the efficiency and yield of semiconductor processes.
[0010]
The present invention has been made in view of these points, and an object thereof is to provide a method of forming an ohmic electrode for forming a low-resistance ohmic electrode by alloying with good uniformity by annealing at an optimum temperature. .
[0011]
[Means for Solving the Problems]
  In order to solve the above problems, the present invention provides a method of forming an ohmic electrode that can be realized by the flow shown in FIG. The ohmic electrode forming method of the present invention is formed by alloying a plurality of types of metals stacked on a semiconductor layer by annealing, and in the method of forming an ohmic electrode used as an electrode of a semiconductor device, the plurality of types of metals Consists of any combination of Nb and Al or V and Zr,The semiconductor layer in which the ohmic electrode is formed is i-Al whose Al composition ratio z is in the range of 0 to 1. _z Ga _1-z N layer or i-GaN layer,The atomic ratio of the plurality of types of metals stacked before annealing is set to the composition ratio of an alloy formed from the plurality of types of metals after annealing, and the plurality of types of metals are formed at a film thickness ratio that is the atomic number ratio. It is formed by stacking on the semiconductor layer and annealed at a temperature capable of forming the alloy.
[0012]
  According to such a method for forming an ohmic electrode, in forming the ohmic electrode, the atomic ratio of the plurality of kinds of metals stacked on the semiconductor layer before annealing is determined by the composition of the alloy formed after annealing.RatioSet (step S3), each metal is laminated and formed at a film thickness ratio that is the atomic ratio (step S4). By annealing this (step S5), the tendency for each metal to be alloyed without excess or deficiency or to be alloyed without excess or deficiency increases, and an alloy having a composition ratio assumed before annealing is formed with good uniformity. Become so.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
First, the outline of the present invention will be described.
FIG. 2 is a diagram schematically showing the state before and after annealing when forming an alloy.
[0014]
When forming the ohmic electrode, the metal A and the metal B are laminated before annealing, and the A_xB_yAssume that an alloy is formed. In FIG. 2, the annealing conditions (temperature, time, atmosphere, etc.) are constant.
[0015]
First, the atomic ratio A / B between the metal A layer and the metal B layer formed by lamination before annealing is determined by the A to be formed after annealing._xB_yIt is assumed that the alloy composition ratio A / B is smaller than x / y. For example, the metal A layer is formed thinner than the metal B layer, and A_xB_yFor example, the metal A layer does not contain enough atoms to satisfy the alloy composition ratio x / y. In such a case, after annealing, for example, A_xB_yThe alloy is partially formed in the layer, and the remaining metal B that is not alloyed remains,_xB_yThe alloy is formed unevenly.
[0016]
In addition, the atomic ratio A / B before annealing is A after annealing._xB_yThe case where it becomes larger than the composition ratio x / y of an alloy is assumed. For example, the metal A layer is formed thicker than the metal B layer, and A_xB_yThis is the case, for example, when the number of atoms equal to or more than the number of atoms satisfying the alloy composition ratio x / y is included in the metal A layer. In such a case, after annealing, for example, A_xB_yAs the alloy is formed in the layer, the remaining metal A that is not alloyed remains in the lower part of the layer, and as a whole A_xB_yThe alloy is formed unevenly.
[0017]
On the other hand, the atomic ratio A / B before annealing is A after annealing._xB_yWhen the metal A layer and the metal B layer are formed with a film thickness ratio A / B corresponding to the atomic ratio A / B, which is equal to the composition ratio x / y of the alloy, this is the optimum condition. , A in the layer_xB_yAn alloy is formed uniformly. Thus, an ohmic electrode with low contact resistance can be obtained by forming a uniform alloy.
[0018]
However, the atomic ratio A / B is A_xB_yEven when the alloy composition ratio x / y is smaller or larger, the annealing temperature is increased to increase A_xB_yIt may be possible to form an electrode that includes an alloy of a different form than the alloy. However, in a semiconductor device such as an FET, the annealing temperature should not be too high in order to suppress the thermal influence on the portion other than the ohmic electrode. Therefore, in order to form a low-resistance ohmic electrode under a lower temperature annealing condition, as described above, the atomic ratio A / B before annealing is A after annealing._xB_yThe metal A layer and the metal B layer are formed at a film thickness ratio A / B that is equal to the alloy composition ratio x / y.
[0019]
Note that the atomic ratio A / B at the time of stacking is strictly A_xB_yNot only when the composition ratio x / y of the alloy is equal, but the atomic ratio A / B is_xB_yThe layers may be stacked at a film thickness ratio A / B that is set to a ratio that approximates the composition ratio x / y of the alloy and that has an atomic ratio A / B. Also in this case, an alloy with good uniformity can be formed.
[0020]
FIG. 1 is a diagram showing a flow of forming an ohmic electrode according to the present invention.
In forming the ohmic electrode, first, a metal used as the ohmic electrode is selected (step S1). For example, when an ohmic electrode is formed from a Ti—Al alloy, Ti and Al are selected as the metals. In addition, the metal selected by this step S1 shall be 2 or more types. In addition, the one kind of metal selected here includes an already alloyed metal.
[0021]
After the selection of metals, an alloy formed from those metals is specified (step S2). In this case, for example, a conventionally known phase diagram (state diagram) is used, and an alloy that is easily formed or easily formed is specified in consideration of an annealing temperature and the like. For example, in the case of a Ti-Al alloy, generally TiAl_ThreeIt is known that an alloy is likely to be formed (for example, see “Journal of the Japan Institute of Metals, Vol. 64, No. 2 (2000) p85-94”).
[0022]
After the alloy is specified, the atomic number ratio of the metal to be laminated is set to the composition ratio of the alloy to be formed or a ratio approximate thereto (step S3). For example, TiAl_ThreeSince the alloy has a composition ratio Ti / Al = 1/3, the atomic ratio Ti / Al is set to a ratio approximate to the composition ratio Ti / Al, such as 1/3 or 0.3.
[0023]
After the atomic number ratio is set, the respective metals are laminated and formed at a film thickness ratio that provides the atomic number ratio (step S4). That is, a film thickness or a film thickness ratio such that the number of atoms corresponding to the set atomic ratio is included in each layer to be stacked is obtained, and each metal is stacked by using this.
[0024]
Finally, annealing is performed at an annealing temperature capable of forming the alloy specified in step S2 (step S5), and the laminated metal is alloyed before annealing. As the annealing temperature at this time, it is preferable to set a temperature at which the alloy can be formed at the lowest possible temperature among the temperatures at which the predetermined alloy can be formed. This is because, when the annealing temperature is high, alloying can be promoted, but on the other hand, for example, each semiconductor layer constituting the semiconductor device forming the ohmic electrode may have a thermal influence on portions other than the ohmic electrode.
[0025]
Thus, before annealing, the alloy to be formed is specified, and when the metal constituting the alloy is laminated, the atomic ratio is approximated to the specified alloy composition ratio or the composition ratio. The ratio is set, and the layers are formed at a film thickness ratio that matches the set atomic ratio. Thereby, an alloy can be formed with good uniformity and a low-resistance ohmic electrode can be formed. Furthermore, since the optimum annealing temperature for the formation of the alloy can be set, the annealing temperature does not become too high, and the efficiency of the semiconductor process and the yield are improved.
[0026]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
First, a case where an ohmic electrode is formed by stacking Ti and Al on an AlGaN / GaN heterojunction FET will be described.
[0027]
FIG. 3 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET when Ti and Al are stacked to form an ohmic electrode.
In the AlGaN / GaN heterojunction FET 10 shown in FIG. 3, a GaN electron transit layer 12 having a thickness of 1 μm is formed on a sapphire substrate 11, and a non-doped Al having a thickness of 20 nm is formed thereon._zGa_1-zN (i-Al_zGa_1-zN) An electron supply layer 13 (Al composition ratio z is in the range of 0 to 1) is formed. This non-doped Al_zGa_1-zIn the formation region of the source electrode and the drain electrode on the N electron supply layer 13, a Ti layer 14 and an Al layer 15 are laminated with a predetermined film thickness, respectively. A gate electrode 16 is formed between the source electrode and drain electrode formation regions, and a passivation film 17 is formed between the gate electrode 16 and the source electrode and between the gate electrode 16 and the drain electrode. The
[0028]
A normal MOVPE (Metal Organic Vapor Phase Epitaxy) method or the like can be used to form the AlGaN / GaN heterojunction FET 10. First, the AlGaN / GaN heterojunction FET 10 is formed on the sapphire substrate 11 on the GaN electron transit layer 12, non-doped Al._zGa_1-zAfter the N electron supply layer 13 is formed, Ti and Al are vapor-deposited, and the Ti layer 14 and the Al layer 15 are stacked. After the Ti layer 14 and the Al layer 15 are stacked, annealing is performed and these are alloyed to form the source electrode and the drain electrode of the AlGaN / GaN heterojunction FET 10. The gate electrode 16 is formed before or after annealing for alloying the Ti layer 14 and the Al layer 15, and the passivation film 17 is formed after annealing.
[0029]
FIG. 4 is a diagram showing an outline of the Al—Ti phase diagram. In FIG. 4, the horizontal axis indicates the Al composition (atom%), and the vertical axis indicates the temperature (° C.).
In the case of combining Ti and Al, from FIG. 4, it is shown that the Al composition is about 75% and the temperature is about 500 ° C._ThreeAlloys are easily formed. TiAl as ohmic electrode_ThreeWhen an alloy is formed, the composition ratio is Ti / Al = 1/3. That is, the ohmic electrode formed after annealing of the Ti layer 14 and the Al layer 15 has a uniform TiAl_ThreeIn order to form an alloy, it is only necessary to include a number of atoms having the same ratio as the composition ratio Ti / Al = 1/3. Therefore, the atomic ratio Ti / Al between the Ti layer 14 and the Al layer 15 to be laminated before annealing is expressed as TiAl._ThreeThe alloy composition ratio Ti / Al may be set, and the Ti layer 14 and the Al layer 15 may be laminated to have a film thickness corresponding to the composition ratio.
[0030]
However, as described above, the atomic ratio Ti / Al to be set is not strictly limited to TiAl formed after annealing._ThreeThe composition ratio of the alloy may not be the same as Ti / Al._ThreeYou may make it set to the ratio approximated to the composition ratio Ti / Al of an alloy. In that case, TiAl_ThreeThe film thicknesses of the Ti layer 14 and the Al layer 15 are set in accordance with the atomic ratio Ti / Al set to a ratio approximate to the alloy composition ratio Ti / Al.
[0031]
Where Ti is the density a₁= 4.54 g / cm^Three, Atomic number b₁= 22, while Al is the density a₂= 2.69 g / cm^Three, Atomic number b₂= 13. The film thickness of the Ti layer 14 is c₁The thickness of the Al layer 15 is c₂Then, the atomic ratio Ti / Al is (a₁× c₁/ B₁) / (A₂× c₂/ B₂) And can be simplified. In the case of Ti and Al, a₁/ B₂And a₂/ B₂In this case, the film thickness ratio of the Ti layer 14 and the Al layer 15 is Ti / Al = c.₁/ C₂However, it can be considered that it corresponds to the atomic ratio Ti / Al almost as it is.
[0032]
Therefore, in the AlGaN / GaN heterojunction FET 10 shown in FIG. 3, the atomic ratio Ti / Al set to a composition ratio Ti / Al or a ratio close thereto is used as the film thickness ratio Ti between the Ti layer 14 and the Al layer 15. / Al and each layer may be stacked.
[0033]
FIG. 5 is a graph showing the relationship between the annealing temperature and the contact resistivity. In FIG. 5, the horizontal axis represents the annealing temperature (° C.), and the vertical axis represents the contact resistivity (Ωcm).²) Respectively. FIG. 5 shows the atomic number ratio Ti / Al of the Ti layer 14 and the Al layer 15 in the AlGaN / GaN heterojunction FET 10 shown in FIG. 3 as 0.15, 0.3, 0.5, and 0.75. The graph shows the annealing temperature dependence of the contact resistivity when the film thickness is changed, that is, when the film thickness ratio Ti / Al is changed at those ratios to form a laminate. Here, the annealing time at each annealing temperature is 30 seconds.
[0034]
At the atomic ratio Ti / Al = 0.3, the contact resistivity is lowest at all measured annealing temperatures, and the contact resistivity can be kept low even under low temperature annealing conditions of 500 ° C. in particular. In the temperature range higher than the temperature of 500 ° C., it is recognized that the contact resistivity tends to increase as the annealing temperature increases.
[0035]
When the atomic ratio Ti / Al = 0.15, the contact resistivity is higher at all measured annealing temperatures than when the atomic ratio Ti / Al = 0.3. In this case, it is recognized that the contact resistivity decreases as the annealing temperature increases up to the vicinity of the melting point of Al, and thereafter increases as the annealing temperature increases.
[0036]
When the atomic ratio Ti / Al = 0.5, Schottky is used at relatively low temperature annealing conditions, and when the atomic ratio Ti / Al = 0.75, all measured annealing temperatures are used. It becomes a contact.
[0037]
Therefore, the film thickness ratio Ti / Al of the Ti layer 14 and the Al layer 15 is set to the atomic ratio Ti / Al = 0.3, that is, TiAl._ThreeWhen the alloy composition ratio Ti / Al = 1/3, the lowest contact resistivity is exhibited under the lowest temperature annealing conditions (annealing temperature 500 ° C.).
[0038]
FIG. 6 is a diagram showing current-voltage characteristics when alloyed at an annealing temperature of 500 ° C., (a) is the atomic ratio Ti / Al = 0.15, (b) is the atomic ratio Ti / Al = 0.3, (c) shows the case of atomic number ratio Ti / Al = 0.5, and (d) shows the case of atomic number ratio Ti / Al = 0.75. In FIG. 6, the horizontal axis represents voltage (2 V / div), and the vertical axis represents current (1 mA / div).
[0039]
When the Ti layer 14 and the Al layer 15 are annealed at an atomic number ratio of Ti / Al = 0.15, 0.3, 0.5, 0.75, an annealing temperature of 500 ° C., and an annealing time of 30 seconds, FIG. As shown in (b), good ohmic characteristics are obtained when the atomic ratio Ti / Al = 0.3, that is, when the film thickness ratio Ti / Al = 0.3. In the case of other atomic ratio Ti / Al (or film thickness ratio Ti / Al), as shown in FIGS. 6A, 6C, and 6D, ohmic characteristics cannot be obtained.
[0040]
From the measurement results shown in FIG. 5 and FIG. 6, in the formation of the ohmic electrode, the atomic ratio Ti / Al of the Ti layer 14 and the Al layer 15 laminated before annealing is changed to TiAl formed after annealing._ThreeThe composition ratio Ti / Al of the alloy or a ratio approximate to the composition ratio Ti / Al is set, and each layer is formed with a film thickness ratio Ti / Al that becomes the atomic ratio Ti / Al, so that TiAl is formed after annealing._ThreeAn alloy can be formed with good uniformity.
[0041]
Conventionally, when an ohmic electrode is to be formed using a combination of Ti and Al, annealing is generally performed at a temperature of 600 ° C. or higher. At an annealing temperature close to the melting point of Al (about 660 ° C.), In some cases, deterioration of morphology due to agglomeration of Al becomes a problem. However, as described above, the film thickness ratio Ti / Al is formed as TiAl._ThreeIt is possible to form an ohmic electrode that is annealed at a lower temperature than the conventional annealing temperature of 500 ° C. and has a low resistance by optimally setting and stacking according to the alloy composition ratio Ti / Al. Become.
[0042]
Further, in the AlGaN / GaN heterojunction FET 10 shown in FIG. 3, a layer in which an ohmic electrode is formed (non-doped Al_zGa_1-zThe N-electron supply layer 13) is a non-doped layer and is generally, for example, n-Al_zGa_1-zIt can be said that it is a layer in which ohmic contact is difficult to be obtained as compared with a doped layer such as an N layer (z = 0 to 1). According to the present invention, it is possible to form a low-resistance ohmic electrode not only for a doped layer that is relatively easy to obtain ohmic contact, but also for a layer that is difficult to obtain such ohmic contact.
[0043]
The AlGaN / GaN heterojunction FET has a structure in which an n-GaN layer or a non-doped GaN (i-GaN) layer is formed under the gate electrode in order to stably maintain the gate breakdown voltage at a high value such as 100 V or higher. It is also possible. Even when an ohmic electrode is formed on such an n-GaN layer or an i-GaN layer, the above-mentioned method for forming an ohmic electrode can be applied.
[0044]
FIG. 7 is a diagram showing an example of the element structure of an AlGaN / GaN heterojunction FET in which an n-GaN layer is formed. However, in FIG. 7, the same elements as those shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0045]
An AlGaN / GaN heterojunction FET 20 shown in FIG. 7 is formed by using an ordinary MOVPE method on an SiC substrate 21 with an intentionally undoped GaN electron transit layer 22 and an intentionally undoped Al._0.25Ga_0.75N layer 23, n-Al_0.25Ga_0.75The N electron supply layer 24 and the n-GaN layer 25 have a structure in which they are sequentially deposited. Here, Al_0.25Ga_0.75N is generally used, but in general, the Al composition ratio z is in the range of 0 to 1._zGa_1-zN can be used.
[0046]
Here, the intentionally undoped GaN electron transit layer 22 has a film thickness of 3 μm, and is intentionally undoped Al._0.25Ga_0.75The N layer 23 is formed with a film thickness of 3 nm. n-Al_0.25Ga_0.75The N electron supply layer 24 has a thickness of 20 nm and a Si doping concentration of 2 × 10.¹⁸cm^-3It is formed with. Further, the n-GaN layer 25 is formed with a film thickness of 10 nm or less, for example, with a film thickness of 5 nm, for example, with a Si doping concentration of 2 × 10.¹⁸cm^-3It is formed with.
[0047]
A Ti layer 14 and an Al layer 15 are stacked in the source electrode and drain electrode formation regions on the n-GaN layer 25, and alloys thereof are formed by subsequent annealing. The gate electrode 16 is formed between the source electrode and the drain electrode before or after the annealing, and the passivation film 17 is formed after the annealing.
[0048]
In the AlGaN / GaN heterojunction FET 20 formed in this way, the gate electrode 16 and n-Al_0.25Ga_0.75By forming the n-GaN layer 25 between the N electron supply layers 24, the gate breakdown voltage is improved. On the other hand, for the source electrode and drain electrode formed by the Ti layer 14 and the Al layer 15, the distance to the intentionally undoped GaN electron transit layer 22 is increased, thereby preventing the tunneling of electrons. , It becomes difficult to obtain ohmic characteristics.
[0049]
The above ohmic electrode formation method can also be applied to the n-GaN layer 25 of such an AlGaN / GaN heterojunction FET 20, that is, the atomic ratio Ti of the Ti layer 14 and the Al layer 15 to be laminated. / Al is formed after annealing TiAl_ThreeThe composition ratio Ti / Al of the alloy is set to a ratio that approximates the composition ratio Ti / Al, and the respective layers are laminated and formed at a film thickness ratio Ti / Al that is the atomic ratio Ti / Al. This allows TiAl after annealing_ThreeAn alloy can be formed with good uniformity.
[0050]
In this way, TiAl on the n-GaN layer 25 with good uniformity._ThreeBy forming an alloy, a low-resistance ohmic electrode can be formed. Furthermore, since annealing can be performed at a lower temperature than in the prior art, it is possible to increase the efficiency of the semiconductor process and improve the yield.
[0051]
As described above, the ohmic electrode forming method of the present invention can be used to form various FETs regardless of the material of the layer disposed under the ohmic electrode to be formed and whether or not the layer contains a dopant. It is possible to apply.
[0052]
In the above description, the formation of the ohmic electrode when Ti and Al are combined has been described as an example. However, the same applies to the case where the ohmic electrode is formed using a combination of other metals such as Al and Nb and V and Zr. is there.
[0053]
First, formation of an ohmic electrode combining Al and Nb will be described.
FIG. 8 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET when an ohmic electrode is formed by laminating Al and Nb. However, in FIG. 8, the same elements as those shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0054]
The AlGaN / GaN heterojunction FET 30 shown in FIG. 8 forms an Nb layer 31 instead of the Ti layer 14 of the AlGaN / GaN heterojunction FET 10 shown in FIG. 1, and the Nb layer 31 and the Al layer 15 are non-doped Al._zGa_1-zIt has a structure in which it is stacked on the N electron supply layer 13. The other structure is the same as that of the AlGaN / GaN heterojunction FET 10 shown in FIG. 1, and the formation method thereof is also the same.
[0055]
FIG. 9 is a diagram schematically showing the Al—Nb phase diagram. In FIG. 9, the horizontal axis represents the Al composition (atom%), and the vertical axis represents the temperature (° C.).
When Nb and Al are combined, Nb_ThreeAl alloy is easily formed. Nb as ohmic electrode_ThreeWhen an Al alloy is formed, the composition ratio is Al / Nb = 1/3. Therefore, the atomic ratio Al / Nb between the Nb layer 31 and the Al layer 15 laminated before annealing is expressed as Nb_ThreeThe composition ratio of the Al alloy is set to Al / Nb = 1/3 or a ratio close thereto, and the layers are laminated at a film thickness ratio of Al / Nb so that the atomic ratio is Al / Nb.
[0056]
Here, Nb has a density of 8.56 g / cm.^ThreeAtomic number 41, while Al has a density of 2.69 g / cm.^ThreeSince the atomic number is 13, the film thickness ratio Al / Nb between the Nb layer 31 and the Al layer 15 almost corresponds to the atomic number ratio Al / Nb as it is. Therefore, in the AlGaN / GaN heterojunction FET 30 shown in FIG. 8, the Nb layer 31 and the Al layer 15 are_ThreeThe Al alloy composition ratio Al / Nb = 1/3 or an atomic number ratio Al / Nb set to a ratio close thereto may be formed to have a film thickness of the same ratio.
[0057]
FIG. 10 is a diagram showing current-voltage characteristics when Al and Nb are laminated and alloyed, where (a) shows the atomic ratio Al / Nb = 0.15, and (b) shows the atomic ratio Al / N /. Nb = 0.3, (c) shows an atomic ratio Al / Nb = 0.5, and (d) shows an atomic ratio Al / Nb = 0.75. In FIG. 10, the horizontal axis represents voltage (2 V / div), and the vertical axis represents current (1 mA / div).
[0058]
The Nb layer 31 and the Al layer 15 shown in FIG. 8 have an atomic ratio of Al / Nb = 0.15, 0.3, 0.5, 0.75, that is, the film thickness ratio Al of the Nb layer 31 and the Al layer 15. / Nb is the ratio thereof, and annealing is performed at an annealing temperature of 500 ° C. and an annealing time of 30 seconds. In this case, when the atomic ratio Al / Nb = 0.3, which is a ratio approximate to the composition ratio Al / Nb = 1/3, that is, when the film thickness ratio Al / Nb = 0.3, FIG. ), Good ohmic characteristics can be obtained. In the case of other atomic number ratio Al / Nb (film thickness ratio Al / Nb), as shown in FIGS. 10A, 10C, and 10D, ohmic characteristics cannot be obtained.
[0059]
N-Al_zGa_1-zOn the doped layer such as the N layer (Al composition ratio z is in the range of 0 to 1), on the n-GaN layer 25 or on the i-GaN layer as shown in FIG. The same applies to the formation of electrodes.
[0060]
Next, formation of an ohmic electrode combining V and Zr will be described.
FIG. 11 is a diagram showing an example of the element structure of an AlGaN / GaN heterojunction FET when V and Zr are stacked to form an ohmic electrode. However, in FIG. 11, the same elements as those shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0061]
In this AlGaN / GaN heterojunction FET 40 shown in FIG. 11, a V layer 41 and a Zr layer 42 are formed in place of the Ti layer 14 and Al layer 15 of the AlGaN / GaN heterojunction FET 10 shown in FIG. Layer 41 and Zr layer 42 are non-doped Al_zGa_1-zIt has a structure in which it is stacked on the N electron supply layer 13. The other structure is the same as that of the AlGaN / GaN heterojunction FET 10 shown in FIG. 1, and the formation method thereof is also the same.
[0062]
FIG. 12 is a diagram showing an outline of the Zr-V system phase diagram. In FIG. 12, the horizontal axis indicates the V composition (atom%), and the vertical axis indicates the temperature (° C.).
When combining V and Zr, V₂Zr alloys are easily formed. V as ohmic electrode₂When forming a Zr alloy, the composition ratio is Zr / V = 1/2. Therefore, the atomic ratio Zr / V between the V layer 41 and the Zr layer 42 laminated before annealing is set to the composition ratio Zr / V = 1/2 or a ratio close thereto, and the atomic ratio Zr / V Each layer is formed by lamination at such a film thickness ratio Zr / V.
[0063]
Here, V is a density of 5.8 g / cm.^ThreeAtomic number 23, while Zr has a density of 6.53 g / cm.^ThreeSince the atomic number is 40, the atomic ratio Zr / V = 16/25 of the V layer 41 and the Zr layer 42 corresponds to the film thickness ratio Zr / V = 1/1. Therefore, in the AlGaN / GaN heterojunction FET 40 shown in FIG. 11, the atomic ratio Zr / V between the V layer 41 and the Zr layer 42 is set to the composition ratio Zr / V = 1/2, and the same ratio as this is set. The V layer 41 and the Zr layer 42 may be stacked and formed at a film thickness ratio Zr / V = 1 / (16 × 2/25) corresponding to the atomic ratio Zr / V. Alternatively, the atomic number ratio Zr / V between the V layer 41 and the Zr layer 42 is set to a ratio that approximates the composition ratio Zr / V = 1/2, and the film is adjusted so as to correspond to the atomic number ratio Zr / V The V layer 41 and the Zr layer 42 may be stacked and formed with the thickness ratio Zr / V.
[0064]
FIG. 13 is a diagram showing current-voltage characteristics when V and Zr are alloyed. FIG. 13A shows an atomic ratio Zr / V = 0.15, and FIG. 13B shows an atomic ratio Zr / V = 0. .3, (c) shows the case where the atomic ratio Zr / V = 0.5, and (d) shows the case where the atomic ratio Zr / V = 0.75. In FIG. 13, the horizontal axis represents voltage (2 V / div), and the vertical axis represents current (1 mA / div).
[0065]
After the V layer 41 and the Zr layer 42 shown in FIG. 11 are stacked at a film thickness ratio Zr / V corresponding to the atomic ratio Zr / V = 0.15, 0.3, 0.5, 0.75, Annealing is performed at an annealing temperature of 500 ° C. and an annealing time of 30 seconds. In this case, when the film thickness ratio Zr / V is adjusted to the atomic ratio Zr / V = 0.5 with the composition ratio Zr / V = 1/2, as shown in FIG. Is obtained. In the case of other atomic ratios Zr / V, ohmic characteristics cannot be obtained as shown in FIGS. 13 (a), (b), and (d).
[0066]
However, the film thickness ratio Zr / V of the V layer 41 and the Zr layer 42 is not limited to a value corresponding to the atomic ratio Zr / V strictly equal to the composition ratio Zr / V = 1/2. Even when the number ratio Zr / V is set to a ratio approximate to the composition ratio Zr / V = 1/2, ohmic characteristics can be obtained.
[0067]
N-Al_zGa_1-zAn ohmic combination of V and Zr on a doped layer such as an N layer (Al composition ratio z ranges from 0 to 1), an n-GaN layer 25 or an i-GaN layer as shown in FIG. The same applies to the formation of electrodes.
[0068]
Thus, the present invention can be applied to the case where an ohmic electrode is formed using various metals. For example, as a metal laminated for the purpose of forming an ohmic electrode, Ti, V, Zr, Nb, Ta, which are metals that easily react with GaN, and Al, Nb, Mo, Hf, Zr, which are metals having a small work function, are used. , V, or Au, Pt, Pd, Ag, W, Mo, Cu, Ni, etc., which are difficult to oxidize, can be laminated in combination. For example, the general formula A_lB_mC_nWhen an alloy represented by the following formula is formed, an atomic ratio (A: B: C) is set to a composition ratio (l: m: n) of the alloy or a ratio close thereto, and the atomic ratio ( A layer, a B layer, and a C layer may be stacked and formed at a film thickness ratio (A: B: C) such that A: B: C). After the lamination, an ohmic electrode is formed by forming an alloy by annealing at a temperature of about 400 ° C. to 900 ° C.
[0069]
In the above description, the case where an ohmic electrode is formed on an FET formed using a GaN-based material has been described as an example, but the present invention is not limited to this. That is, in addition to a compound semiconductor device formed using a GaN-based material, it can be applied to the formation of an ohmic electrode as an electrode of a compound semiconductor device formed using a GaAs-based material or an InP-based material. .
[0070]
(Additional remark 1) In the formation method of the ohmic electrode formed by alloying the multiple types of metal laminated | stacked on the semiconductor layer by annealing, and being used as an electrode of a semiconductor device,
The atomic number ratio of the plurality of kinds of metals laminated before annealing is set to a composition ratio of an alloy formed from the plurality of kinds of metals after annealing or a ratio approximate thereto,
The plurality of types of metals are stacked on the semiconductor layer at a film thickness ratio that is the atomic ratio,
Annealing at a temperature capable of forming the alloy;
A method for forming an ohmic electrode.
[0071]
(Supplementary Note 2) The semiconductor device in which the ohmic electrode is formed has an Al composition ratio z in the range of 0 to 1._zGa_1-zThe method for forming an ohmic electrode according to claim 1, wherein the method is an AlGaN / GaN heterojunction field effect transistor having an electron supply layer containing N and an electron transit layer containing GaN.
[0072]
(Supplementary note 3) The method for forming an ohmic electrode according to supplementary note 1, wherein the semiconductor device on which the ohmic electrode is formed is a compound semiconductor device formed using a GaAs-based material.
[0073]
(Additional remark 4) The said semiconductor device in which the said ohmic electrode is formed is a compound semiconductor device formed using InP type material, The formation method of the ohmic electrode of Additional remark 1 characterized by the above-mentioned.
[0074]
(Additional remark 5) The said semiconductor layer in which the said ohmic electrode is formed is n-Al whose Al composition ratio z is the range of 0-1._zGa_1-zN layer or i-Al_zGa_1-zThe method for forming an ohmic electrode according to attachment 1, wherein the method is an N layer.
[0075]
(Additional remark 6) The said semiconductor layer in which the said ohmic electrode is formed is an n-GaN layer or an i-GaN layer, The formation method of the ohmic electrode of Additional remark 1 characterized by the above-mentioned.
[0076]
(Appendix 7) The plurality of types of metals are two or more selected from the group consisting of Ti, V, Zr, Nb, Ta, Al, Mo, Hf, Au, Pt, Pd, Ag, W, Cu, and Ni. The method for forming an ohmic electrode according to supplementary note 1, wherein the ohmic electrode is a metal.
[0077]
(Supplementary note 8) The method for forming an ohmic electrode according to supplementary note 1, wherein the plurality of kinds of metals are any combination of a combination of Ti and Al, a combination of Nb and Al, and a combination of Zr and V. .
[0078]
【The invention's effect】
As described above, in the present invention, in forming the ohmic electrode, the atomic ratio of the plurality of kinds of metals stacked on the semiconductor layer before annealing is set to the composition ratio of the alloy formed after annealing or a ratio approximate thereto. The metal is laminated and formed at a film thickness ratio that is the atomic ratio. Thereby, an alloy with good uniformity can be obtained after annealing, and a low-resistance ohmic electrode can be formed.
[0079]
Furthermore, since the annealing temperature can be set according to the alloy to be formed, the efficiency of the semiconductor process and the yield can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a flow of forming an ohmic electrode according to the present invention.
FIG. 2 is a diagram schematically showing a state before and after annealing when forming an alloy.
FIG. 3 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET when an ohmic electrode is formed by stacking Ti and Al.
FIG. 4 is a diagram showing an outline of an Al—Ti phase diagram.
FIG. 5 is a graph showing the relationship between annealing temperature and contact resistivity.
6A and 6B are diagrams showing current-voltage characteristics when alloyed at an annealing temperature of 500 ° C., where FIG. 6A shows an atomic ratio Ti / Al = 0.15, and FIG. 6B shows an atomic ratio Ti / Al. = 0.3, (c) shows an atomic ratio Ti / Al = 0.5, and (d) shows an atomic ratio Ti / Al = 0.75.
FIG. 7 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET in which an n-GaN layer is formed.
FIG. 8 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET when an ohmic electrode is formed by stacking Al and Nb.
FIG. 9 is a diagram schematically showing an Al—Nb phase diagram.
FIGS. 10A and 10B are diagrams showing current-voltage characteristics when Al and Nb are laminated and alloyed, in which FIG. 10A shows an atomic ratio Al / Nb = 0.15, and FIG. 10B shows an atomic ratio Al; /Nb=0.3, (c) shows an atomic number ratio Al / Nb = 0.5, and (d) shows an atomic number ratio Al / Nb = 0.75.
FIG. 11 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET when an ohmic electrode is formed by stacking V and Zr.
FIG. 12 is a diagram showing an outline of a Zr-V system phase diagram.
FIGS. 13A and 13B are diagrams showing current-voltage characteristics when alloying V and Zr, wherein FIG. 13A shows an atomic ratio Zr / V = 0.15, and FIG. 13B shows an atomic ratio Zr / V = 0.3 and (c) show the case where the atomic ratio Zr / V = 0.5, and (d) shows the atomic ratio Zr / V = 0.75.
FIG. 14 is a diagram showing an example of an element structure of an AlGaN / GaN heterojunction FET.
[Explanation of symbols]
10, 20, 30, 40 AlGaN / GaN heterojunction FET
11 Sapphire substrate
12 GaN electron transit layer
13 Non-doped Al_zGa_1-zN electron supply layer
14 Ti layer
15 Al layer
16 Gate electrode
17 Passivation film
21 SiC substrate
22 Intentionally undoped GaN electron transit layer
23 Intentionally undoped Al_0.25Ga_0.75N layers
24 n-Al_0.25Ga_0.75N electron supply layer
25 n-GaN layer
31 Nb layer
41 V layer
42 Zr layer

Claims (2)

In a method for forming an ohmic electrode, which is formed by alloying a plurality of kinds of metals laminated on a semiconductor layer by annealing, and used as an electrode of a semiconductor device,
The plurality of types of metals are composed of any combination of Nb and Al or V and Zr,
Wherein said semiconductor layer ohmic electrode is formed is the i-Al _z Ga _1-z N layer or i-GaN layer Al composition ratio z ranges from 0 to 1,
The atomic number ratio of the plurality of types of metals laminated before annealing is set to the composition ratio of the alloy formed from the plurality of types of metals after annealing,
The plurality of types of metals are stacked on the semiconductor layer at a film thickness ratio that is the atomic ratio,
Annealing at a temperature capable of forming the alloy;
A method for forming an ohmic electrode. The semiconductor device in which the ohmic electrode is formed has an AlGaN / GaN having an electron supply layer containing Al _z Ga _{1 -z} N whose Al composition ratio z is in the range of 0 to 1, and an electron transit layer containing GaN. 2. The method of forming an ohmic electrode according to claim 1, which is a heterojunction field effect transistor.

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