JP7556401B2 - How a transistor is manufactured - Google Patents

Description

The present invention relates to a method for manufacturing a transistor using a nitride semiconductor.

GaN channel HEMTs are high-speed, high-voltage electronic devices that are already in practical use and being used as high-power wireless communication devices.

In order to further improve the performance of GaN channel HEMTs (improving high frequency characteristics), technology has been investigated and reported for fabricating devices using N-polarity surfaces (by growing crystals in the -c-axis direction). N-polarity HEMTs reverse the polarity of GaN, and form a back barrier made of AlGaN, followed by a GaN channel, from the substrate side. This eliminates the trade-off that previously existed between barrier layer thickness and sheet carrier density, making it possible to shorten the gate length.

In addition, N-polarity HEMTs can form source and drain electrodes without damaging the sheet carriers in the source and drain regions, which reduces the source and drain resistance. Due to these advantages, N-polarity HEMTs have been reported to achieve a relatively low on-resistance and high power characteristics and power efficiency compared to conventional Ga-polarity HEMTs.

In order to further improve the performance of such N-polarity HEMTs, it is effective to increase the Al composition of the back barrier layer. By increasing the Al composition of the back barrier layer, it becomes possible to increase the polarization difference with the GaN channel layer and increase the sheet carrier density. In fact, it has been reported that a high sheet carrier density of 3×10 ¹³ cm ^-2 or more can be achieved in Ga-polarity HEMTs by using a very thin AlN barrier.

However, the crystal growth of AlN is not easy. Since AlN has a lower equilibrium vapor pressure than GaN, the optimal growth conditions shift to a lower V/III ratio, lower pressure, and higher temperature. Such growth conditions place a heavy load on the crystal growth apparatus. In particular, the low V/III ratio and high temperature growth conditions in which the supply of group V raw materials is small and the supply of carrier gas _H2 is relatively large cause significant damage to the growth chamber and heater. For these reasons, the crystal growth of AlN requires the design of a special heater and growth apparatus with high temperature specifications. For this reason, the crystal growth of AlN requires some kind of capital investment, which increases the cost.

On the other hand, it is also possible to grow GaN and AlN in a growth apparatus that grows GaN-based materials without using any special equipment to fabricate the above-mentioned HEMT. However, in this case, the following problems arise.

First, Al is mixed in during GaN growth. When AlN, GaN, AlGaN, etc. are also grown in a crystal growth device for GaN-based materials, deposits from the Al or Ga raw materials may adhere to or remain in the growth chamber or gas line. These deposits and residues deposited at low temperatures will re-evaporate when exposed to a relatively high temperature environment for AlN growth, and will be mixed in as impurities during GaN or AlN growth. In particular, when the growth temperature is set to a high temperature to form a high-quality AlN layer, high-temperature growth may occur from the GaN layer stage before the barrier layer. In this case, there is a risk of Al being mixed in as an impurity during GaN growth. Such problems can be suppressed by keeping the growth temperature of GaN and AlN as constant as possible and not setting it higher than the set temperature for the buffer layer or thermal cleaning.

The second problem is the contamination of Ga during AlN growth. In the case of high-temperature growth, Ga evaporates easily and is therefore rarely contaminated during AlN growth. However, in a situation where the growth temperature is limited to suppress Al contamination into GaN as described above, Ga cannot be sufficiently evaporated and purged, and Ga is contaminated during AlN growth, resulting in the formation of AlGaN. In such a situation, it is not possible to form an AlN thin film with the desired characteristics.

In order to prevent Ga from being mixed into AlN, it is necessary to clean the growth chamber. Cleaning the growth chamber here means cleaning the parts inside the equipment used for crystal growth, especially the growth chamber where crystal growth is performed, as much as possible and replacing them with unused parts. For example, the growth chamber, gas lines, susceptors and heaters, quartz parts, vacuum parts such as O-rings, flanges, and gaskets, vacuum equipment such as dry pumps, and quartz windows for observing the growth state using a growth state observation device. Residues (Ga and its compounds, etc.) formed by the reaction of raw materials during past crystal growth have accumulated on these parts, and these evaporate during crystal growth and are mixed into the AlN growth. By cleaning the residues on the above-mentioned parts as much as possible, it is possible to suppress Ga from being mixed into the AlN layer.

However, the incorporation of Ga into AlN cannot be completely suppressed by simply cleaning the growth chamber. This is because Ga raw material is used in the process of growing each layer of the HEMT structure. In a typical GaN channel HEMT structure with an AlN barrier, a back barrier layer made of AlGaN, a channel layer made of GaN, and a top barrier layer made of AlN are stacked in this order on a substrate (see non-patent document 1).

In the HEMT structure, the channel layer is made of a material with a smaller band gap and higher mobility than the top barrier layer and back barrier layer, and GaN is the most well-known channel layer material. Therefore, a very thin barrier layer made of AlN with a thickness of about several nm is formed on the channel layer made of GaN. The order of crystal growth is growth of AlGaN, growth of GaN, and growth of AlN. Therefore, growth of GaN or AlGaN is performed before growth of AlN, where the inclusion of Ga is a problem.
As described above, even if the growth chamber has been cleaned, since GaN or AlGaN is grown immediately before the growth of the AlN layer, residues that accumulate in the growth chamber during these growth processes evaporate during the growth of AlN and become mixed into the AlN layer.

K. Shinohara et al., "Self-Aligned-Gate GaN-HEMTs with Heavily-Doped n+-GaN Ohmic Contacts to 2DEG", International Electron Devices Meeting, vol. 12, 13384135, pp. 617-620, 2012.

As mentioned above, to grow high-quality AlN, it is necessary to realize high temperature, low pressure, and low V/III ratio conditions, and to achieve this, special specifications must be applied to the crystal growth apparatus, which increases costs. On the other hand, to grow AlN using a general crystal growth apparatus that grows GaN-based materials, it is necessary to clean the parts of the growth apparatus involved in crystal growth as much as possible in order to suppress the inclusion of Ga in the AlN barrier during low-temperature growth. However, even if cleaning is performed at a cost, if a layer containing Ga, such as GaN or AlGaN, is grown in a process prior to the growth of AlN, the impurities containing Ga that remain in the apparatus during the growth of these will evaporate and be mixed in when the AlN layer is grown. As a result, it is not possible to obtain an AlN barrier with the desired characteristics.

As such, conventional technology had the problem that it was not easy to manufacture transistors with the desired characteristics using AlN.

The present invention has been made to solve the above problems, and aims to make it possible to manufacture transistors with desired characteristics using AlN.

The method for manufacturing a transistor according to the present invention includes a first step of cleaning a crystal growth apparatus; a second step of, after the first step, growing AlN in the -c-axis direction using the crystal growth apparatus to form an AlN layer on the substrate without growing a compound semiconductor containing Ga in the crystal growth apparatus; a third step of, following the second step, growing a nitride semiconductor different from the AlN layer in the -c-axis direction using the crystal growth apparatus to form a channel layer that forms a heterojunction on the AlN layer; a fourth step of forming a source electrode and a drain electrode on the channel layer; and a fifth step of forming a gate electrode on the channel layer.

As described above, according to the present invention, after cleaning the crystal growth apparatus, an AlN crystal is grown in the -c-axis direction without growing a compound semiconductor containing Ga, thereby forming an AlN layer on a substrate, and a nitride semiconductor different from the AlN layer is grown in the -c-axis direction to form a heterojunction channel layer, thereby making it possible to manufacture a transistor using AlN with the desired characteristics.

FIG. 1A is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention. FIG. 1B is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention. FIG. 1C is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention. FIG. 1D is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention. FIG. 1E is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention. FIG. 1F is a cross-sectional view showing a state of a transistor in the middle of a process for explaining a method of manufacturing a transistor according to an embodiment of the present invention.

Below, a method for manufacturing a transistor according to an embodiment of the present invention will be described with reference to Figures 1A to 1F.

First, a crystal growth apparatus for forming (growing) a nitride semiconductor containing AlN, which will be described later, is cleaned (step 1). This includes cleaning the gas lines that supply raw materials to the crystal growth apparatus, the growth chamber in which the crystal growth takes place, and the susceptor on which the substrate is placed, as well as replacing the inside of the gas lines and growth chamber with an inert gas. The crystal growth apparatus is an apparatus that performs crystal growth using methods such as metal-organic chemical vapor deposition (MOCVD or MOVPE), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE).

For example, the above-mentioned cleaning can be performed by cleaning the parts inside the crystal growth apparatus that come into contact with the raw material gas as much as possible and replacing these parts with unused parts. The above-mentioned parts include, for example, the growth chamber, gas lines, susceptors, heaters, quartz components, vacuum parts such as O-rings, flanges, and gaskets, vacuum devices such as dry pumps, and quartz windows for observing the growth state using a growth state observation device. Residues (such as Ga and Ga compounds) formed by reactions of the raw materials during previous crystal growth have accumulated on these parts, and these evaporate during crystal growth and become mixed into the AlN growth. By cleaning these parts as much as possible, it is possible to suppress Ga mixing into the AlN layer.

As described above, after cleaning the crystal growth apparatus, AlN is grown in the -c axis direction using the crystal growth apparatus without growing a compound semiconductor containing Ga in the crystal growth apparatus, forming an AlN layer 102 on the substrate 101 as shown in Figure 1A (step 2). As described above, after cleaning, the AlN layer 102 is formed without growing a compound semiconductor containing Ga, so that Ga is not mixed into the AlN layer 102, and the desired characteristics can be obtained.

Here, the substrate 101 can be made of a material on which AlGaN can be crystal-grown, such as _sapphire ( _Al2O3 ), SiC, or AlN. Alternatively, a so-called template substrate can be used, in which an AlGaN layer 121 made of AlGaN with a main surface having an N polarity is formed on the substrate 101 made of AlN and with a main surface having an N polarity (group V polarity). In this case, the AlN layer 102 is crystal-grown in contact with the top of the AlGaN layer 121.

The AlGaN layer 121 can be grown on the substrate 101 by a crystal growth method such as MOCVD, MBE, or HVPE. In forming the AlGaN layer 121 made of AlGaN with an N-polarity plane as the main surface orientation, a predetermined buffer layer or substrate pretreatment process may be required, and these configurations may be introduced. For example, when the substrate 101 is made of sapphire, the AlGaN layer 121 can be formed on a nitride layer formed by nitriding the main surface of the substrate 101. Also, a buffer layer can be formed on the substrate 101 under so-called low-temperature conditions, and the AlGaN layer 121 can be formed on the buffer layer.

In addition, when the substrate 101 is made of AlN, the substrate 101 is prepared with the N-polarity plane as the main surface orientation, and the AlGaN layer 121 is directly epitaxially grown on the substrate 101, so that the surface of the AlGaN layer 121 is an N-polarity plane. In order to remove the effects of defects, oxide films, and impurities remaining on the surface of the substrate 101, the AlGaN layer 121 can be formed via a relatively thick buffer layer. It is important that the main surface of the AlGaN layer 121 is an N-polarity plane. It is also important that the Al composition of the top layer of the AlGaN layer 121 is 0.8 or more, as described below.

Here, the growth of the AlN layer 102 and the nitride semiconductor layer described later can be performed in the crystal growth apparatus used to form the AlGaN layer 121. In this case, it is important that the above-mentioned cleaning (first step) is performed after the formation of the AlGaN layer 121. In addition, the crystal growth apparatus used to grow the AlN layer 102 and the crystal growth apparatus used to form the AlGaN layer 121 can be different. In this case, the formation of the AlGaN layer 121 can be performed chronologically after cleaning the crystal growth apparatus used to grow the AlN layer 102.

Here, as described above, when a template substrate on which the AlGaN layer 121 is formed is used, the surface of the AlGaN layer 121 has an oxide film, defects, and surface contamination due to exposure to the outside of the crystal growth apparatus. For this reason, the AlN layer 102 can function as a buffer layer in addition to the function as a barrier layer. In this case, the thickness of the AlN layer 102 is set depending on the amount of oxide film, defects, and contamination on the surface of the AlGaN layer 121 described above. For example, in a typical interface state or contamination situation, the thickness of the AlN layer 102 can be set to about 50 nm or more to avoid these effects.

As mentioned above, the thickness of the AlN layer 102 is set taking into account the contamination state of impurities on the surface of the AlGaN layer 121 due to exposure to the atmosphere, avoiding the influence of the interface between the epitaxially grown layer and the substrate, and taking into account the critical film thickness. For this reason, the thickness of the AlN layer 102 cannot be set in general. However, the critical film thickness of AlN on AlGaN can be calculated using the method of "Matthew and Blakeslee" (Reference 1). According to a rough calculation, the critical film thickness of AlN on AlGaN with an Al composition of 0.9 is 107 nm. The critical film thickness of AlN on AlGaN with an Al composition of 0.8 is 47 nm.

Therefore, the lower the Al composition of the AlGaN layer 121, the more the maximum thickness of the AlN layer 102 is limited. On the other hand, as mentioned above, in order to make the AlN layer 102 function as a buffer layer and reduce the effects of defects and impurities on the surface of the AlGaN layer 121, it is preferable that the thickness of the AlN layer 102 is about 50 nm. For these reasons, the Al composition of the top layer of the AlGaN layer 121 can be set to 0.8 or more.

Next, following the formation of the AlN layer 102 described above, a nitride semiconductor different from the AlN layer 102 is epitaxially grown in the -c-axis direction using the crystal growth apparatus used to form the AlN layer 102, thereby forming a channel layer 103 that forms a heterojunction on the AlN layer 102, as shown in Figure 1B (third step). Furthermore, AlGaN is then epitaxially grown on the channel layer 103 to form a top barrier layer 104.

The channel layer 103 can be made of, for example, GaN. The thickness of the channel layer 103 is set within a range that does not exceed the critical film thickness and does not deteriorate the crystal quality. The larger the Al composition of the AlGaN layer 121, the smaller the critical film thickness of the channel layer 103 made of GaN. When the Al composition is 0.8, the critical film thickness of the channel layer 103 is about 9.6 nm. Therefore, if the thickness of the channel layer 103 is set to 10 nm or less, it is better because the critical film thickness is not exceeded and a deterioration in crystal quality is not caused. Note that if the introduced defects have a small effect on the characteristics of the transistor to be created, the critical film thickness can be exceeded. Note that the channel layer 103 is not limited to GaN, and can also be made of nitride semiconductors such as InGaN, AlGaN, InAlN, and InAlGaN.

The top barrier layer 104 is introduced to suppress gate leakage and improve pinch-off characteristics, but is not necessary. For example, instead of the top barrier layer 104, a layer that serves as a barrier after insulating the gate electrode, such as an insulating layer (dielectric layer) of SiN, Al ₂ O ₃ , SiO ₂ , etc., can be formed. The top barrier layer 104 can also be combined with the above-mentioned insulating layer. If this combination improves the pinch-off characteristics and provides good characteristics, these may be applied. The top barrier layer 104 is not limited to AlGaN, and can be composed of InAlN, AlN, InAlGaN, and combinations of these materials as long as the above-mentioned purpose is achieved and the crystal quality is not deteriorated.

Next, as shown in Fig. 1C, the top barrier layer 104 is patterned, and subsequently, a portion of the channel layer 103 around the patterned top barrier layer 104 is removed in the thickness direction. Next, n-type doped n ⁺ -GaN is regrown on the channel layer 103 that has been thinned and exposed around the top barrier layer 104, to form contact layers 105 and 106 as shown in Fig. 1D. The contact layers 105 and 106 have a higher impurity concentration than the channel layer 103. The contact layers 105 and 106 are formed in contact with the channel layer 103, sandwiching the top barrier layer 104 therebetween.

Next, as shown in FIG. 1E, a source electrode 107 and a drain electrode 108 are formed on the channel layer 103 via the contact layer 105 and the contact layer 106 (step 4). The source electrode 107 and the drain electrode 108 are adjusted in material, thickness, forming conditions, and heat treatment conditions to form ohmic contact with the contact layer 105 and the contact layer 106. For example, a layered structure such as Ti/Al/Ni/Au is generally deposited by electron beam deposition or the like, and then heat-treated in a nitrogen atmosphere to form the source electrode 107 and the drain electrode 108 that are in ohmic contact with the contact layer 105 and the contact layer 106.

Next, as shown in FIG. 1F, a gate electrode 109 is formed on the channel layer 103 via a top barrier layer 104 (step 5). The gate electrode 109 is formed by selecting a material that forms a Schottky contact with the top barrier layer 104 made of AlGaN, and by selecting growth conditions that form the Schottky contact. For example, the gate electrode 109 can be made of a metal such as Ni.

By using the above manufacturing method, a transistor (N-polarity HEMT) is obtained in which a heterojunction channel layer 103 is formed on the AlN layer 102, which serves as a barrier layer (back barrier layer). In addition, each nitride semiconductor layer of this transistor grows in the -c-axis direction from the substrate 101 side, so that the main surface is N-polar.

As described above, according to the present invention, after cleaning the crystal growth apparatus, an AlN crystal is grown in the -c-axis direction without growing a compound semiconductor containing Ga, forming an AlN layer on a substrate, and a nitride semiconductor different from the AlN layer is grown in the -c-axis direction to form a heterojunction channel layer, thereby making it possible to manufacture a transistor with the desired characteristics using AlN.

It is to be understood that the present invention is not limited to the embodiments described above, and that many modifications and combinations can be implemented by a person having ordinary knowledge in the art within the technical concept of the present invention.

[Reference 1] J. W. Matthews, A. E. Blakeslee, "Defects in epitaxial multilayers: I. Misfit dislocations", Journal of Crystal Growth Volume 27, pp. 118-125, December 1974.

101...substrate, 102...AlN layer, 103...channel layer, 104...top barrier layer, 105...contact layer, 106...contact layer, 107...source electrode, 108...drain electrode, 109...gate electrode, 121...AlGaN layer.

Claims (3)

A first step of cleaning a crystal growing apparatus;
a second step of forming an AlN layer on the substrate by growing AlN in a −c-axis direction using the crystal growth apparatus without growing a compound semiconductor containing Ga using the crystal growth apparatus after the first step;
a third step of growing a nitride semiconductor different from the AlN layer in a −c-axis direction by the crystal growth apparatus following the second step to form a channel layer that forms a heterojunction on the AlN layer;
a fourth step of forming a source electrode and a drain electrode on the channel layer;
and a fifth step of forming a gate electrode on the channel layer .
The substrate has an AlGaN layer formed thereon, the AlGaN layer being made of AlGaN with N-polarity on a main surface thereof,
the second step includes growing the AlN layer on and in contact with the AlGaN layer;
The uppermost layer of the AlGaN layer has an Al composition of 0.8 or more,
The AlN layer has a thickness of 50 nm or more.
A method for manufacturing a transistor comprising the steps of: 2. The method of claim 1 , further comprising the steps of:
the first step includes cleaning a gas line for supplying raw materials to the crystal growth apparatus, a growth chamber for performing crystal growth, and a susceptor for placing the substrate thereon, and replacing the inside of the gas line and the growth chamber with an inert gas. 3. The method for producing a transistor according to claim 1, further comprising the steps of:
The method for manufacturing a transistor, wherein the nitride semiconductor is any one of GaN, InGaN, AlGaN, InAlN, and InAlGaN.

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