JP2005191477A - Epitaxial wafer for high electron mobility transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an epitaxial wafer for an HEMT which is more excellent in electrical property than uniform dope and planar dope by inserting a thin layer of a high carrier concentration instead of a planar dope technique which hardly shows characteristics in a compound semiconductor mainly containing nitrogen formed by an MOVPE method. <P>SOLUTION: In an epitaxial wafer for an HEMT wherein a high purity GaN layer or an InGaN layer is formed as a channel layer 3 on a substrate 1 by an MOVPE method, and an AlGaN layer is further formed as an electron supply layer 4, the portion 6 of the electron supply layer 4 is made a layer having a carrier concentration of 1×10<SP>19</SP>cm<SP>-3</SP>or more by doping control, and the remaining portions 5, 7 of the electron supply layer 4 are made layers having a lower carrier concentration. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a structure for realizing high electron mobility at the same time while maintaining a high sheet carrier concentration in an epitaxial wafer for a high electron mobility transistor (HEMT) of a GaN-based compound semiconductor thin film.

  The MOVPE (organometallic vapor phase epitaxy) method is a technique for epitaxially growing a thin film crystal of a III-V compound semiconductor on the surface of a crystal substrate. Crystal growth is performed by feeding a carrier gas containing a plurality of group III or group V source gases into a heated crystal substrate in the reactor and thermally decomposing these source gases on the crystal substrate.

  One of devices using a heterojunction manufactured by stacking different kinds of crystals by this growth method is a high electron mobility transistor called HEMT. The HEMT is basically an electric field in which an electrode is provided on a selectively doped structure consisting of a high-purity layer and a layer having a lower electron affinity and a function of supplying electrons by doping. This is an effect transistor. Electrons accumulate two-dimensionally on the high purity layer side of the interface between these layers. Since the electrons are formed in the high purity layer, the mobility is very high, and the electrons are controlled by the electric field effect caused by the bias voltage applied to the gate electrode.

  A compound semiconductor device mainly containing arsenic or phosphorus in group V has a growth technique called planar doping in which the electron supply layer is two-dimensionally doped in order to achieve higher mobility while maintaining a high sheet carrier concentration. . This is because, even at the same sheet carrier concentration, the Coulomb scattering effect is reduced and the electron mobility is increased when the spatial distance between the doped portion and the channel portion is separated from that in which the electron supply layer is uniformly doped.

Although not directly related to the present invention, an n-type GaInN layer is provided on an n-type AlGaN electron supply layer, and this is used as an etching stop layer (see, for example, Patent Document 1).
JP 2003-229413 A

  However, the planar doping technique has the following problems.

  A compound semiconductor mainly containing nitrogen in group V requires a high temperature of 1000 ° C. or higher in order to grow by the MOVPE method. Therefore, unless the group III source and group V source are always supplied during growth, group III and group V atoms are re-desorbed from the surface of the growing thin film, resulting in deterioration of the surface state and crystals of the epitaxial wafer. The generation of atomic vacancies in the device causes deterioration of device characteristics.

  On the other hand, in order to dope two-dimensionally, it is necessary to stop the supply of Group III raw materials during doping. For this reason, crystal degradation occurs at the same time as the planar doping, so that a great improvement in device performance cannot be expected.

  Therefore, the object of the present invention is to solve the above-mentioned problems and insert a thin layer having a high carrier concentration instead of the planar doping technique which is difficult to produce characteristics in a compound semiconductor mainly containing nitrogen grown by the MOVPE method. An object of the present invention is to realize an HEMT epitaxial wafer having more excellent electrical characteristics than uniform doping or planar doping.

  In order to achieve the above object, the present invention is configured as follows.

The epitaxial wafer for a high electron mobility transistor according to the invention of claim 1 is formed by forming a high purity GaN layer or an InGaN layer as a channel layer on a substrate by a MOVPE method, and further forming an AlGaN layer as an electron supply layer. In the epitaxial wafer for an electron mobility transistor, a part of the electron supply layer is a high carrier concentration layer having a high carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or more by doping control, and other than the high carrier concentration layer in the electron supply layer This portion is a low carrier concentration layer having a carrier concentration lower than that of the high carrier concentration layer.

  According to a second aspect of the present invention, in the epitaxial wafer for a high electron mobility transistor according to the first aspect, the thickness of the high carrier concentration layer which is a part of the electron supply layer is about 0.5 to 10 nm. To do.

According to a third aspect of the present invention, in the epitaxial wafer for a high electron mobility transistor according to the second aspect, the low carrier concentration layer which is a portion other than the high carrier concentration layer of the electron supply layer is undoped or 1 × 10 ¹⁹ cm. It is characterized by a low carrier concentration of ^-3 or less.

  The invention according to claim 4 is characterized in that, in the epitaxial wafer for high electron mobility transistor according to claim 3, the change in the carrier concentration in the depth direction at the interface between the high carrier concentration layer and the low carrier concentration layer is made steep. To do.

  According to the present invention, the following excellent effects can be obtained.

In the epitaxial wafer for a high electron mobility transistor of the present invention, a part of the electron supply layer is a high carrier concentration layer having a high carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or more by doping control, and the high carrier in the electron supply layer is further formed. A portion other than the concentration layer is a low carrier concentration layer having a carrier concentration lower than that of the high carrier concentration layer. Doping is always performed in a growth environment in which a Group III material and a Group V material are supplied, so there is no crystal degradation, and the epitaxial wafer structure has a spatial distance between the doping part and the channel part, so that electron transfer. An increase in degree occurs.

  Therefore, according to the structure of the epitaxial wafer according to the present invention, higher electrical characteristics can be realized than the conventional uniformly doped or planar-doped HEMT epitaxial wafer layer. Thereby, a transistor having excellent gain and noise characteristics can be realized.

  Hereinafter, the present invention will be described based on the illustrated embodiments.

  FIG. 1 schematically shows a cross-sectional structure of an epitaxial wafer for a high electron mobility transistor (HEMT) according to an embodiment of the present invention.

  An undoped AlN buffer layer 2 having a thickness of 100 nm is provided on a SiC (or sapphire substrate) 1, an undoped GaN channel layer 3 having a thickness of 2000 nm is provided thereon, and an AlGaN electron supply layer 4 having a thickness of 25 nm is provided thereon. Yes.

A portion 6 of this electron supply layer 4 is a thin n-type AlGaN layer (high carrier concentration) of about 0.5 to 10 nm (here 3 nm) having a high carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or more by doping control. The remaining lower and upper portions 5 and 7 of the electron supply layer are formed as AlGaN layers (low carrier concentration layers) having a lower carrier concentration. That is, in this embodiment, the electron supply layer 4 is composed of three layers: a lower layer (low carrier concentration layer) 5, an intermediate layer (high carrier concentration layer) 6, and an upper layer (low carrier concentration layer) 7. Of these layers, the lower layer 5 is composed of an undoped Al _0.25 Ga _0.75 N layer having a thickness of 8 nm, and the intermediate layer 6 is composed of an n-type Al _0.25 Ga _0.75 N layer having a carrier concentration of 3 × 10 ¹⁹ cm ⁻³ and a thickness of 3 nm. The upper layer 7 is composed of an undoped Al _0.25 Ga _0.75 N layer having a thickness of 14 nm.

FIG. 4 shows a gas sequence during the growth of this high carrier concentration layer-inserted GaN-HEMT epitaxial wafer. The manufacturing process of the electron supply layer 4 is performed as follows. In other words, trimethylaluminum (TMA) and trimethylgallium (TMG ₎ are used as the group III material and ammonia (NH ₃₎ as the nitrogen material when producing an epitaxial wafer having the above-mentioned cross-sectional structure by metal organic vapor phase epitaxy. However, monosilane (SiH ₄₎ is used as a raw material for the silicon dopant. In the growth of the n-type Al _0.25 Ga _0.75 N layer 6 which is an intermediate layer of the electron supply layer 4, not only SiH ₄ and NH ₃ but also TMA and TMG are flowing, which is a feature of this process. .

FIG. 2 shows an epitaxial wafer for GaN-HEMT that has been subjected to planar doping as a comparative example 1. The electron supply layer 4 has an undoped Al _0.25 Ga _0.75 N layer 15 having a thickness of 10 nm, Si planar doped. The layer 16 and three layers of an undoped Al _0.25 Ga _0.75 N layer 17 having a thickness of 15 nm. FIG. 5 shows a gas sequence during growth of the epitaxial wafer for the planar doped GaN-HEMT. In the planar doping, the supply of TMA and TMG is stopped, and only SiH ₄ and NH ₃ are present. There is a difference from the process of FIG.

FIG. 3 shows an epitaxial wafer for uniformly doped GaN-HEMT as Comparative Example 2, and the configuration of the electron supply layer 4 is an n-type Al _0.25 Ga having a carrier concentration of 4 × 10 ¹⁸ cm ⁻³ and a film thickness of 25 nm. _0.75 N layer 18 is formed. FIG. 6 shows a gas sequence during growth of the epitaxial wafer for uniformly doped GaN-HEMT. During the growth of the electron supply layer 4, not only TMA, TMG and NH ₃ but also SiH ₄ is consistently formed. There is a difference from the process of FIGS. 4 and 5 in that it continues to be supplied.

  The main point of the present embodiment is that AlGaN having a high carrier concentration instead of planar doping as shown in FIG. 1 is applied to a uniformly doped or planar doped GaN-HEMT epitaxial wafer having a normal structure as shown in FIGS. The thin layer 6 is inserted into the AlGaN electron supply layer 4. In the technique of the present invention, when the electron supply layer 4 is formed, doping is always performed in a growth environment in which a group III raw material and a group V raw material are supplied. Therefore, there is no crystal degradation, and the epitaxial wafer structure has a doped portion. Since the spatial distance of the channel portion is separated, an increase in electron mobility occurs, and as a result, device characteristics superior to the conventional uniformly doped GaN-HEMT can be expected.

[Example]
As an embodiment of the present invention, three types of epitaxial wafers having a high carrier concentration AlGaN layer insertion structure as shown in FIG. 1, a planar doping structure as shown in FIG. 2, and a uniform doping structure as shown in FIG. Hall effect measurement was performed, and the sheet carrier concentration Ns and electron mobility μ were measured.

  The SiC substrate 1 having a diameter of 2 inches was placed on the susceptor, and the susceptor temperature was heated to 1100 ° C., which is the growth temperature of the thin film. After forming an AlN layer as the buffer layer 2 directly on the substrate by the MOVPE method, a high-purity GaN layer as the channel layer 3 and an AlGaN layer as the electron supply layer 4 were formed.

  The source gas was diluted with hydrogen and introduced into the furnace through the gas inlet. The source gas is trimethylaluminum and ammonia for AlN layer growth, trimethylgallium and ammonia for GaN layer growth, trimethylaluminum, trimethylgallium and ammonia for AlGaN layer growth, and monosilane for the dopant. Using. The gas sequences for growing three types of epitaxial wafers are shown in FIGS.

  The results of comparing the electron mobility at the wafer center are shown in the following table.

以上のように、電子供給層に高キャリア濃度層を挿入することによって電気的特性が向上することが確認された。また、Ｃ−Ｖ測定によりキャリア濃度プロファイルをとったところ、図７のように移動度が高いものは界面に電子がきちんと閉じ込められ、２次元電子ガスのピークが高くなることがわかった。 From Table 1, the epitaxial wafer for GaN-HEMT in which the AlGaN thin layer of this embodiment is inserted as compared with the epitaxial wafer for planar doped GaN-HEMT (FIG. 2) and the uniform doping of the normal structure (FIG. 3). It can be seen that the wafer (FIG. 1) exhibits the lowest sheet resistance, highest sheet carrier concentration, and highest electron mobility.

As described above, it was confirmed that electrical characteristics were improved by inserting a high carrier concentration layer into the electron supply layer. Further, when the carrier concentration profile was taken by CV measurement, it was found that the electron with the high mobility as shown in FIG. 7 was properly confined at the interface, and the peak of the two-dimensional electron gas was increased.

  From the above results, it was found that the epitaxial wafer structure invented can realize electrical characteristics exceeding uniform doping and planar doping by growth by the MOVPE method.

  The thickness, concentration, or insertion position of the high carrier concentration layer varies depending on the characteristics required for the target device, and also varies depending on the growth conditions and the growth furnace, so trial and error must be performed.

[Modification]
In the above embodiment, there is only one high carrier concentration layer. However, depending on the target device or process, it is possible to make a plurality of layers, and it is also possible to design each layer to have a different carrier concentration. .

  In the above embodiment, the Al composition of the high carrier concentration layer and the other electron supply layers are the same. However, in order to increase the carrier concentration of the high carrier concentration layer, the Al composition may be lowered to the extent that 2DEG does not accumulate. Is possible.

  In the above embodiment, the channel layer 3 is described as being a high-purity GaN layer. However, the channel layer 3 may be composed of a high-purity InGaN layer.

It is sectional drawing of the epitaxial wafer for GaN-HEMT which made a part of AlGaN electron supply layer based on the Example of this invention high carrier concentration. 6 is a cross-sectional view of a planar-doped GaN-HEMT epitaxial wafer according to Comparative Example 1. FIG. 4 is a cross-sectional structure of a uniformly doped GaN-HEMT epitaxial wafer according to Comparative Example 2. It is a gas sequence figure at the time of epitaxial wafer growth for high carrier concentration layer insertion GaN-HEMT concerning the example of the present invention. 6 is a gas sequence diagram during growth of an epitaxial wafer for planar-doped GaN-HEMT according to Comparative Example 1. FIG. 6 is a gas sequence diagram during growth of an epitaxial wafer for uniformly doped GaN-HEMT according to Comparative Example 2. FIG. It is the figure which showed the profile of the depth direction of the carrier concentration of the epitaxial wafer which concerns on the Example of this invention compared with the comparative examples 1 and 2. FIG.

Explanation of symbols

1 SiC substrate 2 Buffer layer 3 Channel layer 4 Electron supply layer 5 Lower layer (low carrier concentration layer)
6 Intermediate layer (high carrier concentration layer)
7 Upper layer (low carrier concentration layer)

Claims (4)

In an epitaxial wafer for a high electron mobility transistor in which a high-purity GaN layer or an InGaN layer is formed as a channel layer on a substrate by MOVPE, and an AlGaN layer is further formed as an electron supply layer.
A part of the electron supply layer is made a high carrier concentration layer having a high carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or more by doping control, and a portion other than the high carrier concentration layer in the electron supply layer is more than the high carrier concentration layer. An epitaxial wafer for a high electron mobility transistor characterized by having a low carrier concentration layer having a low carrier concentration. The epitaxial wafer of claim 1,
An epitaxial wafer for a high electron mobility transistor, wherein the thickness of the high carrier concentration layer which is a part of the electron supply layer is about 0.5 to 10 nm. The epitaxial wafer according to claim 2, wherein
An epitaxial wafer for a high electron mobility transistor, wherein the low carrier concentration layer, which is a portion other than the high carrier concentration layer of the electron supply layer, is undoped or has a low carrier concentration of 1 × 10 ¹⁹ cm ⁻³ or less. . The epitaxial wafer according to claim 3, wherein
An epitaxial wafer for a high electron mobility transistor characterized by a sharp change in the carrier concentration in the depth direction at the interface between a high carrier concentration layer and a low carrier concentration layer.

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