JP2008244036A - Semiconductor crystal and semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor crystal and a semiconductor device having a nitride-based compound semiconductor composite layer with excellent characteristics. <P>SOLUTION: This semiconductor device has a structure wherein the crystal growth of an AlGaN buffer layer 2' is carried out on an SiC substrate 1 in order to improve the bond between the substrate 1 and a buffer layer, a GaN buffer layer 2 is grown on the AlGaN buffer layer 2' by 2 μm, an In<SB>0.25</SB>Al<SB>0.05</SB>Ga<SB>0.70</SB>N channel layer 3 is grown on the GaN buffer layer 2 by 30 nm, and then an In<SB>0.10</SB>Al<SB>0.80</SB>Ga<SB>0.10</SB>N barrier layer 4 is grown on the In<SB>0.25</SB>Al<SB>0.05</SB>Ga<SB>0.70</SB>N channel layer 3 by 25 nm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a semiconductor crystal and a semiconductor device, and more particularly to a nitride compound semiconductor crystal and a semiconductor device using the same.

  When configuring a high electron mobility transistor (HEMT) capable of high-speed and high-power operation with a nitride-based compound semiconductor, conventionally, AlGaN is used for a barrier layer that supplies carriers, GaN is used for a channel layer where electrons travel, The AlGaN / GaN HEMTs used were often used. The reason for this is that due to the lattice distortion resulting from lattice mismatch between AlGaN and GaN, it is possible to generate a two-dimensional electron gas in the GaN channel layer due to the piezoelectric polarization effect and the spontaneous polarization effect, even if AlGaN is not specifically doped. On the other hand, from the standpoint of transistor fabrication, GaN and AlGaN are relatively easy materials for crystal growth among nitride-based compound semiconductors.

  With the progress of crystal growth technology, InGaN has improved crystal quality that can be used in the field of optical devices such as LEDs, and by applying this to the HEMT channel layer, an attempt to create a HEMT with higher performance than AlGaN / GaN HEMT Has been made. Theoretically, the InGaN channel may increase the sheet carrier density of the two-dimensional electron gas of HEMT than the GaN channel, and if realized, the characteristics of the device can be improved (for example, Non-Patent Document 1 below). reference).

However, it has been found from trial manufacturing that there are still problems in the crystal quality of InGaN itself and the formation of a steep hetero interface with other layers in order to use InGaN for electronic devices. In general, nitride-based compound semiconductors often perform crystal growth at a high temperature exceeding 1000 ° C., whereas InGaN crystal growth must suppress re-evaporation of In from the crystal surface during crystal growth. Therefore, it is necessary to grow at a low temperature, and defects and transitions are likely to occur in the crystal.
“MOCVD-grown InGaN-channel HEMT structures with electron mobility of over 1000cm2 / Vs”, Naoya Okamoto et al., Elsevier, Journal of Crystal Growth, 2004, Vol. 272, pages 278-284. “High quality InAlN / GaN HEMT epi grown by MOCVD on Si substrate”, Noriyuki Watanabe et al., Shingaku Technical Report, ED2006-233, MW2006-186 (2007-1), pages 183-187.

  In the present invention, the above problem, that is, the crystal growth of InGaN, must be performed at a low temperature in order to suppress the re-evaporation of In from the crystal surface. Therefore, defects and transitions are likely to occur in the crystal. The present invention has been made in view of the problem that an electronic device with good characteristics cannot be obtained, and the problem to be solved by the present invention is a semiconductor crystal and a semiconductor having a nitride compound semiconductor composite layer with excellent characteristics To provide an apparatus.

In order to solve the above-mentioned problem, in the present invention, as described in claim 1,
A first layer containing In, Al, Ga and N, and a second layer containing Al and N, wherein the potential of the conduction band edge of the second layer is the conduction band edge of the first layer. The semiconductor crystal is characterized by being higher than the potential of.

In the present invention, as described in claim 2,
A semiconductor crystal having a buffer layer on a substrate, a channel layer on the buffer layer, and a barrier layer on the channel layer, wherein the buffer layer contains Ga and N, and the channel layer is In And Al, Ga and N, the barrier layer contains Al and N, the potential of the conduction band edge of the buffer layer is higher than the potential of the conduction band edge of the channel layer, and the conduction band edge of the barrier layer The semiconductor crystal is characterized by being lower than the potential of.

In the present invention, as described in claim 3,
3. The semiconductor crystal according to claim 1, wherein In incorporation is increased by addition of Al in the first layer or the channel layer. 4.

Moreover, in this invention, as described in Claim 4,
The buffer layer is In _a Al _b Ga _1-ab N (0 ≦ a, 0 ≦ b, a + b <1), and the channel layer is In _v Al _w Ga _1-vw N (0 < v, 0 <w, v a + w <1), wherein wherein said barrier layer is an _{in x Al y Ga 1-x} -y N (0 ≦ x, 0 <y, x + y ≦ 1) The semiconductor crystal according to Item 2 or 3 is formed.

In the present invention, as described in claim 5,
5. The semiconductor crystal according to claim 4, wherein a and b in the In _a Al _b Ga _1-ab N satisfy 0 <a ≦ 0.05 and 0 <b ≦ 0.05. A semiconductor crystal is formed.

In the present invention, as described in claim 6,
5. The semiconductor crystal according to claim 4, wherein the buffer layer is GaN.

In the present invention, as described in claim 7,
7. The semiconductor crystal according to claim 4, wherein v and w in the In _v Al _w Ga _1-vw N satisfy 0 <v <0.30 and 0 <w <0.32. The semiconductor crystal characterized by this is formed.

In the present invention, as described in claim 8,
8. The semiconductor crystal according to claim 4, wherein _{y in the} In _x Al _y Ga _1-xy N satisfies 0.49 <y ≦ 1. .

In the present invention, as described in claim 9,
9. The semiconductor crystal according to claim 4, wherein x and y in the In _x Al _y Ga _1-xy N satisfy x + y = 1 and 0 ≦ x ≦ 0.18. A semiconductor crystal is formed.

In the present invention, as described in claim 10,
10. The semiconductor crystal according to claim 2, wherein the substrate is a silicon substrate, and the plane orientation is (111) and a plane orientation equivalent thereto.

In the present invention, as described in claim 11,
A semiconductor device having the semiconductor crystal according to claim 1 is formed.

  That is, for example, there is provided a HEMT structure composed of InAlGaN as a material suitable for a channel layer of InGaN-based HEMT and InAlGaN having a composition different from that of the channel layer as a barrier layer material suitable for combining with it, particularly InAlN.

  For example, by using InAlGaN containing Al in the channel layer, a high-quality InAlGaN channel / InAl (Ga) N barrier crystal can be obtained, whereby a semiconductor crystal having a nitride compound semiconductor composite layer with excellent characteristics and A semiconductor device can be provided.

  Hereinafter, the best mode for carrying out the present invention will be described by taking as an example a HEMT structure in which the buffer layer is GaN, the channel layer is InAlGaN, and the barrier layer is InAlGaN or InAlN. It is not limited to this.

  FIG. 1 shows an example of the basic configuration of the present invention. In the figure, a GaN buffer layer 2, an InAlGaN channel layer 3 as a first layer, and an InAlGaN barrier layer 4 as a second layer are formed on a substrate 1 in this order. In this figure, portions not necessary for the description of the present invention are omitted.

From the study of InAlN crystal growth, we found that the presence of Al is deeply involved in the In incorporation efficiency into the crystal. In InGaN crystal growth, the In incorporation efficiency can be achieved by adding a small amount of Al. The knowledge that can be improved. Here, the composition formula of InAlGaN is In _v Al _w Ga _1-vw N (0 <v, 0 <w, v + w <1).

  When growing InGaN containing no Al, the re-evaporation of In, which is once taken into the crystal from the crystal surface, becomes an obstacle to increasing the In composition in the crystal. It is the temperature at the time of crystal growth that has the greatest effect on the re-evaporation of In. When the growth temperature reaches 600 ° C. or higher, In re-evaporation begins to occur. It will only be captured.

In the growth of Group V materials, generally nitride-based compound semiconductors, re-evaporation can be suppressed only slightly by increasing the amount of ammonia supplied, but on the other hand, unintentional impurity contamination (mainly carbon) increases in the crystal, and dislocations, etc. The crystal defects increase. On the other hand, when the crystal composition is In _v Al _w Ga _1-vw N as in the present invention, In once taken into the crystal is affected by Al having a large binding energy, and thus re-evaporates. Since it becomes difficult, the conditions under which growth is possible are expanded as compared with normal InGaN growth.

  However, the addition of Al also has the effect of increasing the potential at the conduction band edge. The use of InGaN instead of GaN for the HEMT channel layer has a lower potential at the conduction band edge than GaN, resulting in an increase in the concentration (sheet carrier density) of the two-dimensional electron gas generated in the channel. It is because it can do.

Here, with respect to the barrier layer on the channel layer, the effect of the present invention is sufficiently exerted even with a conventionally used AlGaN barrier layer, but in order to enhance the effect, the composition of the barrier layer is changed to In. _{From x} Al _y Ga _1-xy N (0 ≦ x, 0 <y, x + y ≦ 1), the composition of the AlGaN barrier layer that is normally used (generally, the Al composition y is less than 0.3) It is also effective to generate more two-dimensional electron gas on the channel layer side by increasing the Al composition y and enhancing the spontaneous polarization effect in the crystal. At this time, it is required from the operating principle of the HEMT that the potential at the conduction band edge of the barrier layer needs to be higher than that of the channel layer.

  Summarizing the above, focusing on the conduction band edge, in order to efficiently generate the two-dimensional electron gas that is important in the HEMT structure, the conduction band edge is formed in the buffer layer, channel layer, and barrier layer in the HEMT structure. Must have a potential of “channel layer <buffer layer <barrier layer”. Here, the inequality sign “<” represents the potential relationship (low <high) of the conduction band edge potential of each layer.

Here, the buffer layer is required to satisfy the above potential height relationship and not to generate a two-dimensional electron gas in the channel layer. Therefore, a composition region having a small spontaneous polarization or piezoelectric polarization must be used, and in the In _a Al _b Ga _1-ab N buffer layer (0 ≦ a, 0 ≦ b, a + b <1), a and It is desirable that b is in the range of 0 ≦ a ≦ 0.05 and 0 ≦ b ≦ 0.05. Ultimately, the GaN buffer layer (a = b = 0) is used.

Then, when the composition satisfying the condition of “channel layer <buffer layer <barrier layer” is obtained from the In composition dependence of the conduction band edge potential of InGaN and the conduction band edge potential of AlN based on the Vegard law, In the GaN buffer layer and the In _v Al _w Ga _1- vw N channel layer, v and w are 0 <v <0.30 and 0 <w <0.32. Although the In composition v can be 0.30 or more in terms of crystal growth, it is difficult to use a composition of substantially 0.30 or more because phase separation tends to occur. The upper limit of the Al composition is automatically determined from the limit value of the In composition. When the Al composition exceeds 0.32, the conduction band edge of the In _v Al _w Ga _1-vw N channel layer is that of the GaN buffer layer. It will exceed.

Next, regarding the GaN buffer layer and the In _x Al _y Ga _1-xy N barrier layer, when y is 0.49 <y ≦ 1, the potential condition of the conduction band edge is satisfied, and the Al composition y It is possible to generate a two-dimensional electron gas more effectively than AlGaN having a ratio of less than 0.3. A particularly preferred composition is such that the relationship between x and y is x + y = 1 and 0 ≦ x ≦ 0.18. At this time, the spontaneous polarization in the barrier layer becomes larger than that of AlGaN and is close to AlN, so that a two-dimensional electron gas can be generated more effectively.

  The effect of the present invention can be exhibited by using any substrate (for example, GaN, sapphire, SiC, Si, ZnO, etc.) on which a nitride compound semiconductor can be grown. In order to manufacture a HEMT having a high output, it is preferable to combine with a Si (111) substrate that can be obtained at a low cost as a substrate (for example, see Non-Patent Document 2). In general, a nucleation layer or the like is inserted between the substrate and the buffer layer. However, as long as a good buffer layer can be formed in this portion, the effect of the present invention is not reduced even if an arbitrary structure is used. .

  As a method for crystal growth, generally, a metal-organic chemical vapor deposition method (MOCVD) that is excellent in supplying nitrogen atoms of a group V material is used for the growth of a nitride-based compound semiconductor, but the structure of the present invention can be realized. As a growth method, any method such as halogenated vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), gas source MBE, or the like can be used.

[Embodiment 1]
As a first embodiment of the invention, as shown in FIG. 2, as a basic device structure, a GaN buffer layer / In _0.25 Al _0.05 Ga _0.70 N channel layer / Although an In _0.10 Al _0.80 Ga _0.10 N barrier layer is used, the present invention is not limited to this structure, and can be applied to the material system described in the claims.

The SiC substrate 1 is washed with a diluted hydrofluoric acid solution to obtain a clean surface. Next, the cleaned SiC substrate 1 is loaded into the MOCVD apparatus, and the temperature is raised in a hydrogen atmosphere to perform thermal cleaning. After cleaning, first, an AlGaN buffer layer 2 ′ is crystal-grown to improve the connection between the substrate 1 and the buffer layer, and then the GaN buffer layer 2 is grown by 2 μm. Next, after growing the In _0.25 Al _0.05 Ga _0.70 N channel layer 3 by 30 nm, the In _0.10 Al _0.80 Ga _0.10 N barrier layer 4 is grown by 25 nm. Usually, when an InGaN layer having an In composition of 0.25 is intended to be grown, phase separation is likely to occur and the surface is likely to be rough, so that a good heterointerface cannot be obtained. However, in the structure of this embodiment, a steep heterogeneity is obtained. An interface can be obtained.

As a result of measuring the crystal in this embodiment by ordinary hole measurement, it was found that the mobility was 900 cm ² / v · s or more, which was the same mobility as the GaN channel, and a high quality crystal was obtained.

[Embodiment 2]
As a second embodiment of the invention, as shown in FIG. 3, a GaN buffer layer / In _0.15 Al _0.02 Ga _0.83 N on the Si (111) substrate 1 is used as a basic element structure. Although a channel layer / In _0.17 Al _0.83 N barrier layer is used, the present invention is not limited to this structure, and can be applied in the material system described in the claims.

The Si (111) substrate 1 is washed with the RCA method or a hydrofluoric acid solution to obtain a clean surface. Next, the cleaned silicon substrate is loaded into the MOCVD apparatus, and the temperature is raised in a hydrogen atmosphere to perform thermal cleaning. After the cleaning, the AlN buffer layer 2 ″ is first crystal-grown, and then the AlGaN buffer layer 2 ′ is grown while decreasing the Al composition sequentially from the side closer to the silicon substrate, and then the grown GaN buffer layer 2 Next, the GaN buffer layer 2 is grown by 1 μm, and the In _0.15 Al _0.02 Ga _0.83 N channel layer 3 is grown by 10 nm, and then In _0.17 Al _0.83 N The barrier layer 4 is grown to 30 nm, and when the channel layer is made of GaN, the sheet carrier density was about 1.3 × 10 ¹³ cm ⁻² , but this was changed to In _0.15 Al _0.02. The Ga _0.83 N channel layer was improved to 2.1 × 10 ¹³ cm ⁻² , and the mobility was 900 cm ² / v as a result of measurement by normal hole measurement. It was found that high-quality crystals were obtained with mobility equal to or greater than that of GaN channels.

  Thus, by adopting a HEMT structure composed of InAlGaN as a material suitable for the channel layer of InGaN-based HEMT and InAlGaN having a composition different from that of the channel layer as a suitable barrier layer material to be combined therewith, in particular, an InAlN structure. An InGaN-based material can be used for the channel layer in the structure, and the HEMT sheet carrier density can be increased.

It is a figure explaining the HEMT structure to which this invention is applied. It is a figure explaining the example which produces the HEMT structure to which this invention is applied on a SiC substrate. It is a figure explaining the example which produces the HEMT structure to which this invention is applied on Si (111) board | substrate.

Explanation of symbols

  1: substrate, 2, 2 ', 2 ": buffer layer, 3: channel layer, 4: barrier layer.

Claims (11)

A first layer containing In, Al, Ga and N, and a second layer containing Al and N;
A semiconductor crystal, wherein the potential of the conduction band edge of the second layer is higher than the potential of the conduction band edge of the first layer. A semiconductor crystal having a buffer layer on a substrate, a channel layer on the buffer layer, and a barrier layer on the channel layer;
The buffer layer includes Ga and N;
The channel layer includes In, Al, Ga, and N;
The barrier layer comprises Al and N;
A semiconductor crystal, wherein a potential of a conduction band edge of the buffer layer is higher than a potential of a conduction band edge of the channel layer and lower than a potential of a conduction band edge of the barrier layer.   3. The semiconductor crystal according to claim 1, wherein in the first layer or the channel layer, In incorporation is increased by addition of Al. The buffer layer is In _a Al _b Ga _1-ab N (0 ≦ a, 0 ≦ b, a + b <1),
The channel layer is In _v Al _w Ga _1-vw N (0 <v, 0 <w, v + w <1),
4. The semiconductor crystal according to claim 2, wherein the barrier layer is In _x Al _y Ga _1-xy N (0 ≦ x, 0 <y, x + y ≦ 1). 5. The semiconductor crystal according to claim 4, wherein a and b in the In _a Al _b Ga _1-ab N satisfy 0 <a ≦ 0.05 and 0 <b ≦ 0.05. Semiconductor crystal.   5. The semiconductor crystal according to claim 4, wherein the buffer layer is GaN. 7. The semiconductor crystal according to claim 4, wherein v and w in the In _v Al _w Ga _1-vw N satisfy 0 <v <0.30 and 0 <w <0.32. A semiconductor crystal characterized by 8. The semiconductor crystal according to claim 4, wherein _{y in the} In _x Al _y Ga _1-xy N satisfies 0.49 <y ≦ 1. 9. The semiconductor crystal according to claim 4, wherein x and y in the In _x Al _y Ga _1-xy N satisfy x + y = 1 and 0 ≦ x ≦ 0.18. Semiconductor crystal.   10. The semiconductor crystal according to claim 2, wherein the substrate is a silicon substrate, and the plane orientation is (111) and a plane orientation equivalent thereto.   A semiconductor device comprising the semiconductor crystal according to claim 1.

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