JP4906023B2 - GaN-based semiconductor device - Google Patents

Description

【００２６】
表１から次のことが明らかである。
（１）実施例装置は、オン抵抗がコンタクト層を積層した従来例に比べて低下しており、またソース・ドレイン間の電流も大幅に大きくなっている。
（２）実施例装置において、領域４Ａの厚みが１５nmより厚くなると、領域４Ａの半導体層の抵抗は高くなり、また１nmより薄くなると、歪みが小さくなって２次元電子ガス層の効果が弱くなり、結果的に半導体層の抵抗が高くなっている。このようなことから領域４Ａの厚みは１〜１５nmに設定することが好ましい。
【００２７】
【発明の効果】
以上の説明で明らかなように、本発明のＧａＮ系半導体装置は、上層におけるソース電極とドレイン電極の形成領域の厚みを薄くすることにより動作時のオン抵抗は小さくなり、大電流動作が可能になっている。
また、この装置は、従来のように選択成長でコンタクト層を形成することが不要になるので、高い生産性の下で製造することができる。
【図面の簡単な説明】
【図１】本発明の装置の１例Ａを示す断面図である。
【図２】装置Ａの製造に用いる層構造Ａ₀を示す断面図である。
【図３】層構造Ａ₀の上層に領域４Ａと領域４Ｂが形成されている層構造Ａ₁を示す断面図である。
【図４】ソース電極とドレイン電極が形成された層構造Ａ₂を示す断面図である。
【図５】従来のＧａＮ系ＨＥＭＴ構造例を示す断面図である。
【符号の説明】
１ 半絶縁性基板
２ バッファ層
３ 第１のアンドープ材料から成る下層
４ 第２のアンドープ材料から成る下層
４Ａ 上層４におけるソース電極とドレイン電極の形成領域
４Ｂ 上層４における領域４Ａ以外の領域
５ コンタクト層
６ ２次元電子ガス層
７ 保護膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a GaN-based semiconductor device, and more particularly, a HEMT structure made of a GaN-based semiconductor material, and a GaN-based semiconductor device capable of operating at a large current because the on-resistance during operation is greatly reduced as compared with the prior art. About.
[0002]
[Prior art]
GaN-based semiconductor materials such as GaN, InGaN, AlGaN, and AlInGaN have large band gap energy compared to, for example, GaAs-based materials, and have high heat resistance and excellent high-temperature operation. Research and development of electronic devices such as field effect transistors (FETs) and high mobility transistors (HEMTs) are underway.
[0003]
An example of a GaN-based HEMT structure is shown in FIG. In this HEMT structure, on a semi-insulating substrate 1 such as a sapphire substrate, for example, a buffer layer 2 made of GaN, an undoped GaN layer 3, and an undoped AlGaN layer that is much thinner than the undoped GaN layer 3 A layer structure is formed by sequentially laminating 4.
[0004]
On the undoped AlGaN layer 4, a gate electrode G, a source electrode S, and a drain electrode D are arranged in a plane. In that case, the gate electrode G is formed directly on the undoped AlGaN layer 4.
However, the source electrode S and the drain electrode D are generally made of n-AlGaN in which, for example, Si, which is an n-type impurity, is once doped at a high concentration in the formation region of these electrodes in the surface of the undoped AlGaN layer 4. A contact layer 5 is formed and disposed on the contact layer 5. The reason is to realize a large current operation by reducing the resistance between these electrodes and the undoped AlGaN layer 4 to lower the on-resistance during operation.
[0005]
In some cases, the source electrode S and the drain electrode D are formed directly on the undoped AlGaN layer 4, but in this case, a high resistance is caused between these electrodes and the undoped AlGaN layer 4 to operate at a large current. Therefore, as described above, a structure in which the contact layer 5 is interposed between the two is usually used.
In the case of the HEMT structure shown in FIG. 5, the band gap energy of undoped GaN is smaller than the band gap energy of undoped AlGaN. Undoped GaN is a single crystal, but undoped AlGaN is a mixed crystal of AlN and GaN. Therefore, a piezo electric field is generated at the heterojunction interface between the two layers due to the piezoelectric effect based on crystal distortion, and the two-dimensional electron gas layer 6 is formed immediately below the junction interface between the two layers.
[0006]
The electron gas concentration in the two-dimensional electron gas layer formed at the heterojunction interface of the GaN-based material is about 5 × 10 ¹⁸ to 1 × 10 ²⁰ / cm ³ , and this value is formed of, for example, a GaAs-based material. Compared with the electron gas concentration of the two-dimensional electron gas layer being about 5 × 10 ¹⁷ to 1 × 10 ¹⁸ / cm ³ , the concentration is higher by one digit or more.
When the source electrode S and the drain electrode D are operated in this HEMT structure, the undoped AlGaN layer 4 functions as an electron supply layer and supplies electrons to the undoped GaN layer 3. The supplied electrons move at high speed by the action of the two-dimensional electron gas layer 6 and travel to the drain electrode D. At this time, by operating the gate electrode G and generating a depletion layer having a desired thickness immediately below it, various modulation operations can be realized in the HEMT structure.
[0007]
[Problems to be solved by the invention]
Incidentally, in the case of the HEMT structure shown in FIG. 5, the on-resistance during operation is smaller than in the case of the HEMT structure in which the source electrode S and the drain electrode D are formed directly on the undoped AlGaN layer 4, but these electrodes and Since the high-resistance undoped AlGaN layer 4 is interposed between the two-dimensional electron gas layers with a predetermined thickness, there is a limit to the reduction in the on-resistance during operation.
[0008]
The contact layer 5 is usually formed by selective growth. However, if a HEMT structure with reduced on-resistance during operation can be manufactured without performing this selective growth, the industrial merit is growing.
The present invention solves the above-mentioned problems in the conventional HEMT structure shown in FIG. 5 and can realize a reduction in on-resistance during operation without forming a contact layer. An object is to provide a semiconductor device.
[0009]
[Means for Solving the Problems]
The inventors of the present invention have a high electron gas concentration in the two-dimensional electron gas layer formed at the heterojunction interface of GaN-based materials. For example, in the case of a heterojunction interface between undoped AlGaN and undoped GaN, As a matter of fact, attention was paid to the phenomenon that the resistance decreases because the carrier of the undoped layer effectively increases due to the seepage effect of the two-dimensional electron gas layer in the undoped GaN.
[0010]
The idea is that if the undoped AlGaN layer is selectively thinned in the region where the source and drain electrodes are to be formed, the region can perform the same function as the conventional contact layer. Through various experiments, the correctness of the idea was confirmed, and the GaN-based semiconductor device of the present invention was developed.
That is, the GaN-based semiconductor device of the present invention is
A layer structure of a lower layer made of a first undoped material and an upper layer made of a second undoped material having a band gap energy larger than that of the first undoped material is formed on the substrate. And
A GaN-based semiconductor device in which a gate electrode, a source electrode, and a drain electrode are formed on the surface of the upper layer,
The thickness of the upper layer in the formation region of the source electrode and the drain electrode is smaller than the thickness in other regions.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
An example A of a GaN-based semiconductor device of the present invention is shown in FIG.
This apparatus A has a layer structure in which a buffer layer 2 made of, for example, GaN, a lower layer 3 made of a first undoped material, and an upper layer 4 made of a second undoped material, which are described later, are sequentially laminated on a semi-insulating substrate 1. Is formed. The entire surface is covered with a protective film 7 such as SiO ₂ .
[0012]
The partial region 4A of the upper layer 4 is thinner than the other region 4B, and the source electrode S and the drain electrode D are formed in the thin region 4A, and the thick other region 4B has a gate. An electrode G is formed.
Both the first undoped material of the lower layer 3 and the second undoped material of the upper layer 4 are GaN-based semiconductor materials. In that case, as the second undoped material and the first undoped material, a material having a larger band gap energy (Eg) than that of the latter is selected and used in combination. As a result, a two-dimensional electron gas layer 6 is formed immediately below the heterojunction interface between the lower layer 3 and the upper layer 4, that is, at the top of the lower layer 3.
[0013]
As the first undoped material of the lower layer 3, GaN (Eg = 3.40 eV) is usually used. At this time, as the second undoped material of the upper layer 4, for example, AlGaN (Eg = 4.16 eV), AlInGaN (Eg = 3.8 eV), AlGaNAs (Eg = 4.5 eV), AlGaNP (Eg = 4.2 eV) AlGaInNAsP (Eg = 4.0 eV), AlGaNAsP, or the like can be used.
[0014]
The first undoped material is not limited to GaN, but may be various GaN-based mixed crystals. In that case, the second undoped material has a GaN-based material having a larger Eg than the mixed crystal. It is necessary to use a semiconductor material.
Here, the thickness of the lower layer 3 is not particularly limited, but is usually set to about 1000 to 4000 nm.
[0015]
The thickness of the electrode formation region 4A in the upper layer 4 is related to the relationship between the constituent material of the upper layer and the constituent material of the lower layer 3 combined therewith, the level of the electron gas concentration in the two-dimensional electron gas layer 6, the crystal It is determined as appropriate in relation to defects. If the thickness of the formation region 4A is too thin, film defects will occur and the formation of the continuous two-dimensional electron gas layer 6 will not proceed. Conversely, if it is too thick, there will be inconveniences such as increased resistance of the undoped layer. The thickness of the formation region 4A is preferably set to about 1 to 15 nm.
[0016]
If the thickness of the formation region 4A is in the above-described range, the state of the two-dimensional electron gas layer 6 immediately below it is good, and the electron gas concentration is high, so that a sufficient contact layer is formed between the electrode and the lower layer 3. Demonstrate the function.
This apparatus A can be manufactured as follows.
First, a buffer layer 2 made of GaN, for example, undoped GaN is formed on a semi-insulating substrate 1 by a crystal growth method such as gas source molecular beam epitaxy (GSMBE) or metal organic chemical vapor deposition (MOCVD). The lower layer 3, for example, the upper layer 4 made of undoped AlGaN, is sequentially formed, and the layer structure A ₀ shown in FIG. 2 is manufactured.
[0017]
Here, as the substrate 1, a sapphire substrate is usually used, but a substrate of SiC, GaAs, Si, GaN or the like may be used.
Next, for example, SiO ₂ is deposited on the entire surface of the layer structure A ₀ , and then patterned with a resist. Further, by a reactive ion beam etching method (RIBE), a region other than the region where the gate electrode is to be formed is desired. Dry etching is performed to the depth to be achieved.
[0018]
As a result, as shown in FIG. 3, a layer structure A ₁ having an upper layer 4 in which a region 4A having a desired thickness and a region 4B not etched are formed is manufactured.
Next, after removing the resist and the SiO ₂ film of the layer structure A ₁ , for example, SiO ₂ is formed again on the entire surface, and the SiO ₂ film is patterned with the resist, and the source electrode and the drain electrode are to be formed. And an electrode material is deposited thereon by sputtering, for example.
[0019]
As a result, as shown in FIG. 4, the layer structure A ₂ in which the source electrode S and the drain electrode D are formed in the region 4A of the upper layer 4 is manufactured.
As the electrode material, for example, Ta—Si, Al / Ti, Ti / Au, or the like can be used. These are all in ohmic contact with the upper layer 4.
In addition, after the formation of the source electrode and the drain electrode, the layer structure A ₂ is preferably heat-treated in an N ₂ gas atmosphere furnace, for example. This is because the ohmic contact with the upper layer of these electrodes is further improved. At that time, the heat treatment temperature is preferably set to 400 to 800 ° C., and the treatment time is preferably set to 5 to 30 minutes.
[0020]
Finally, for example, SiO ₂ is formed again on the entire surface, and then a gate electrode is formed on the upper region 4B in the same manner as described above, whereby the device A shown in FIG. 1 is manufactured.
Incidentally, in a series of steps described above, the keep further deposited a thin layer such as InGaN or GaN in the surface area 4A of the layer structure A _1, the source electrode S and a drain in the layer structure A ₂ shown in FIG. 4 The ohmic contact between the electrode D and the region 4A is further improved.
[0021]
【Example】
The apparatus A shown in FIG. 1 was manufactured as follows.
On the sapphire substrate 1, radicalized nitrogen (3 × 10 ⁻⁶ Torr), metal Ga (5 × 10 ⁻⁷ Torr), and metal Si (5 × 10 ⁻⁹ Torr) are used, and the growth temperature is 640 ° C. by the GSMBE method. Then, a buffer layer 2 made of n-GaN having a thickness of 50 nm is formed, and further, using metal Ga (1 × 10 ⁻⁶ Torr) and ammonia (5 × 10 ⁻⁵ Torr), a growth temperature of 850 ° C. An undoped GaN layer 3 having a thickness of 1000 nm was formed. On top of that, metal Al (1 × 10 ⁻⁷ Torr), metal Ga (1 × 10 ⁻⁷ Torr), and ammonia (5 × 10 ⁻⁶ Torr) are used, and an undoped layer with a growth temperature of 850 ° C. and a thickness of 30 nm. The AlGaN layer 4 was formed to form the layer structure A ₀ in FIG.
[0022]
The entire surface of the layer structure A ₀ is coated with a 100 nm thick SiO ₂ film by P-CVD, and then patterned with a resist. A wet process using buffered hydrofluoric acid is applied to areas other than the region 4B where the gate electrode is to be formed. Etching was performed to remove the SiO ₂ film. The exposed undoped AlGaN layer 4 was dry-etched by RIBE to obtain the layer structure A ₁ shown in FIG.
[0023]
At this time, the depth of etching was changed as shown in Table ₁ to produce a layer structure A ₁ with a different thickness of the region 4A.
Then, after removing the resist and SiO ₂ film on the area 4B, by forming a SiO ₂ film again over the entire surface, and the opening of the portion to be a source electrode and a drain electrode by patterning a resist, there Ta- After sputtering Si, lift-off was performed to form a source electrode S and a drain electrode D (FIG. 4). Thereafter, the whole was subjected to heat treatment for 60 minutes in an N ₂ gas atmosphere furnace having a temperature of 1050 ° C.
[0024]
Finally, the SiO ₂ film on the region 4B was opened, and Pt / Au was deposited thereon to form the gate electrode G, whereby the device A shown in FIG.
The obtained device was measured for electrical characteristics, that is, on-resistance of FET and current between source and drain. The results are shown in Table 1.
For comparison, after forming a 50 nm-thick contact layer made of Si-doped n-AlGaN (Si doping concentration 1 × 10 ¹⁹ / cm ³ ) on the layer structure of FIG. A HEMT having a layer structure similar to that of the example except that the drain electrode D was formed was manufactured. The characteristics are shown in Table 1 as a conventional example.
[0025]
[Table 1]

[0026]
From Table 1, the following is clear.
(1) In the device according to the example, the on-resistance is lower than that in the conventional example in which the contact layers are stacked, and the current between the source and the drain is greatly increased.
(2) In the example apparatus, when the thickness of the region 4A is greater than 15 nm, the resistance of the semiconductor layer in the region 4A is increased. When the thickness is less than 1 nm, the strain is reduced and the effect of the two-dimensional electron gas layer is decreased. As a result, the resistance of the semiconductor layer is increased. Therefore, the thickness of the region 4A is preferably set to 1 to 15 nm.
[0027]
【Effect of the invention】
As is apparent from the above description, the GaN-based semiconductor device of the present invention can be operated with a large current by reducing the ON resistance during operation by reducing the thickness of the source electrode and drain electrode formation regions in the upper layer. It has become.
In addition, since this apparatus does not require the contact layer to be formed by selective growth as in the prior art, it can be manufactured with high productivity.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an example A of an apparatus of the present invention.
FIG. 2 is a cross-sectional view showing a layer structure A ₀ used for manufacturing the device A. FIG.
FIG. 3 is a cross-sectional view showing a layer structure A _{1 in} which a region 4A and a region 4B are formed in an upper layer of the layer structure A ₀ ;
FIG. 4 is a cross-sectional view showing a layer structure A ₂ in which a source electrode and a drain electrode are formed.
FIG. 5 is a cross-sectional view showing an example of a conventional GaN-based HEMT structure.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semi-insulating board | substrate 2 Buffer layer 3 Lower layer 4 which consists of 1st undoped material Lower layer 4A which consists of 2nd undoped material Source electrode and drain electrode formation area 4B in upper layer 4 Areas other than area 4A in upper layer 4 Contact layer 6 Two-dimensional electron gas layer 7 Protective film

Claims (4)

The whole is composed of GaN-based semiconductor material,
A layer structure of a lower layer made of a first undoped material and an upper layer made of a second undoped material having a band gap energy larger than that of the first undoped material is formed on the substrate;
A GaN-based semiconductor device in which a gate electrode, a source electrode, and a drain electrode are formed on the surface of the upper layer,
The GaN-based semiconductor device according to claim 1, wherein a thickness of the upper layer in a region where the source electrode and the drain electrode are formed is 1 to 15 nm, and is thinner than a thickness of the upper layer in another region. The GaN-based semiconductor device according to claim 1, wherein the source electrode and the drain electrode are heat-treated at 400 to 800 ° C. The GaN-based semiconductor device according to claim 1, wherein the first undoped material is GaN, and the second undoped material is AlGaN .   The upper layer in the region between the source electrode and the gate electrode and at least one region between the drain electrode and the gate electrode has a region thinner than the thickness of the upper layer in the region where the gate electrode is formed. The GaN-based semiconductor device according to any one of claims 1 to 3.

Images