JP7271101B2 - Nitride semiconductor epi substrate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a substrate structure for improving device characteristics in a nitride semiconductor epitaxial substrate for use in power devices capable of achieving high frequency and high output power.

In a high electron mobility transistor (HEMT) using a nitride semiconductor, particularly a gallium nitride-based compound semiconductor substrate, a technique for improving electrical characteristics by interposing a so-called spacer layer between an electron transit layer and an electron supply layer. It has been known.

In Patent Document 1, in a nitride-based III-V group compound semiconductor device having a heterostructure, a first binary compound semiconductor layer forming a channel layer and a barrier layer forming a barrier layer, in which the composition ratio of Al and Ga is A ternary mixed crystal semiconductor layer made of constant AlGaN, and a second binary compound semiconductor layer interposed between the first binary compound semiconductor layer and the ternary mixed crystal semiconductor layer, A technique is disclosed in which the first binary compound semiconductor layer is GaN, and the second binary compound semiconductor layer is AlN having a layer thickness of 1 molecular layer or more and 4 molecular layers or less.

Patent Document 2 discloses a structure in which an electron supply layer made of a group III element nitride essentially containing Ga, a spacer layer, and a channel layer are joined in this order in a lattice-matched manner, and There is disclosed a compound semiconductor device in which a spacer layer is made of an AlGaN layer, and the AlN mixed crystal ratio of the region of the spacer layer in contact with the channel layer is higher than that of the remaining region.

Japanese Patent Application Laid-Open No. 2004-200711 JP-A-2003-229439

In the invention of Patent Document 1, when AlN is used for the spacer layer, that is, the second binary compound semiconductor layer described above, AlN has an extremely large bandgap of 6.2 eV, so the layer thickness becomes too thick. Considering that current injection from the barrier layer to the channel layer is hindered and the heterostructure does not function, the film thickness is set to 1 molecular layer or more and 4 molecular layers or less, while maintaining the sharpness of the junction interface. , the tunnel effect enables sufficient carrier transport.

In the invention of Patent Document 2, the AlN mixed crystal ratio of the boundary region with the channel layer, where the most remarkable piezoelectric field effect can be expected for the 2DEG layer rather than the entire AlGaN spacer layer, is selectively increased with respect to the remaining region. We are trying to solve this problem. That is, by increasing the AlN mixed crystal ratio only in the boundary region instead of the entire spacer layer, specifically, by increasing the AlN mixed crystal ratio while keeping the thickness of the spacer layer to the extent that lattice relaxation does not occur. , greatly increases the piezoelectric field application effect on the channel layer. Further, by increasing the AlN mixed crystal ratio in the boundary region, the conduction band bottom energy level Ec on the spacer layer side can be increased, the conduction band discontinuity value can be increased, and the spontaneous polarization effect can be enhanced. As a result, the triangular potential can be formed deeper and narrower on the channel layer side than in a structure in which the spacer layer has a uniform composition. We are working on output.

As described above, any of the above inventions can be said to be a useful technology in the HEMT structure, but in particular, it is difficult to say that the reduction of the thickness of the spacer layer has been sufficiently studied, and there is room for further improvement. it is conceivable that.

SUMMARY OF THE INVENTION In view of such problems, it is an object of the present invention to provide a nitride semiconductor epitaxial substrate that has a structure having a thin spacer layer and is suitable for a nitride semiconductor device with higher performance.

In the nitride semiconductor epitaxial substrate of the present invention, a channel layer, a spacer layer, and an electron supply layer are laminated in this order, and the channel layer is GaN and the spacer layer is Al _a Ga _1-a N (0<a<0.5). ), the electron supply layer is Al _x In _y Ga _1-xy N (0<x+y≦1), and the spacer layer has a layer thickness of two molecular layers or less.

By having such a structure, there has conventionally been a problem that the current collapse suppressing effect was not sufficiently obtained due to the presence of the spacer layer, but the nitride semiconductor epitaxial substrate can significantly improve this problem. can.

In addition, a nitride semiconductor HEMT using such a nitride semiconductor epitaxial substrate has better electrical characteristics, which is preferable.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to provide a nitride semiconductor epitaxial substrate that significantly overcomes the problem that a sufficient current collapse suppression effect cannot be obtained due to the presence of a spacer layer. Semiconductor HEMTs can exhibit excellent electrical properties.

FIG. 2 is a schematic sectional view showing one mode of the nitride semiconductor epitaxial substrate of the present invention; FIG. 10 is a cross-sectional view obtained by observing electron supply layer 4/spacer layer S/channel layer 3 in the vicinity of electron supply layer 4/spacer layer S/channel layer 3 according to Comparative Example 1 and Example 1, and FIG. A bright-field STEM image (left) and an HAADF-STEM image (right) of cross-sectional STEM observation at a lower magnification than in FIG.

Hereinafter, the present invention will be described in detail with reference to the drawings as well. In the nitride semiconductor epitaxial substrate of the present invention, a channel layer 3, a spacer layer S and an electron supply layer 4 are laminated in this order, the channel layer 3 being GaN and the spacer layer being Al _a Ga _1-a N (0<a <0.5), the electron supply layer ₄ is _{AlxInyGa1 -xyN} (0<x+y≤1), and the spacer layer S has a layer thickness of two molecular layers or less.

FIG. 1 is a schematic cross-sectional view showing one embodiment of the nitride semiconductor epitaxial substrate of the present invention. It should be noted that all the drawings shown in the present invention are schematic simplifications and exaggerated shapes for the sake of explanation, and the shapes, dimensions and ratios of details differ from the actual ones.

The nitride semiconductor substrate Z shown in FIG. 1 has a base substrate 1 on which a buffer layer 2, a channel layer 3, a spacer layer S and an electron supply layer 4 are sequentially formed. Although not shown, a HEMT can be formed by adding electrodes and, if necessary, a cap layer.

INDUSTRIAL APPLICABILITY The present invention exhibits particularly favorable characteristics in a HEMT that has a heterointerface and uses a two-dimensional electron gas (2DEG) generated in the vicinity of the interface as a current path. Therefore, the base substrate 1 and the buffer layer 2 are not particularly limited in their materials, physical properties, structures, and manufacturing methods, and widely known techniques can be applied.

Examples of the base substrate 1 include silicon single crystal, silicon carbide, sapphire, gallium nitride (GaN), and the like. By the way, among these materials, compared to silicon carbide, sapphire, etc., which have higher insulating properties, silicon single crystal tends to be disadvantageous in terms of vertical breakdown voltage, but it is easy to increase the diameter and reduce cost. In the present invention, a nitride semiconductor substrate using a silicon single crystal is exemplified because it can be said to be suitable in that it can be made.

As the buffer layer 2, for example, a buffer layer structure disclosed in Japanese Patent No. 5159858 or Japanese Patent No. 5188545 can be applied. Specifically, the first layer is composed of AlN with a thickness of 50 to 200 nm and the second layer is composed of AlGaN with a thickness of 100 to 300 nm, or an Al _x Ga _1-x N single crystal layer (0.6 ≤ x≦1.0) and an Al _y Ga _1-y N single crystal layer (0≦y≦0.5) are alternately laminated in this order from the substrate side, and the Al _x Ga _1-x N single crystal layer ( 0.6≦x≦1.0) contains 1×10 ¹⁸ to 1×10 ²¹ atoms/cm ³ of carbon, and the Al _y Ga _1-y N single crystal layer (0≦y≦0.5 ), a multilayer buffer layer containing 1×10 ¹⁷ to 1×10 ²¹ atoms/cm ³ of carbon can be applied. Furthermore, if a high-resistance buffer layer, for example, a GaN layer having a carbon concentration of 1×10 ¹⁸ to 3×10 ¹⁸ atoms/cm ³ and a thickness of about 100 to 200 nm is provided in contact with the channel layer 3, this This is particularly preferable because the effect of improving the breakdown voltage in the vertical direction by the GaN layer is exhibited.

In the present invention, the channel layer 3 is made of a nitride semiconductor of a first group 13 element, and the electron supply layer 4 is made of a nitride semiconductor of a first group 13 element and a second group 13 element. Group 13 elements include gallium (Ga), aluminum (Al) and indium (In). In the present invention, the first group 13 element is any one element of Ga, Al and In, the second group 13 element is any one element of Ga, Al and In, It is an element other than the 13th group element of 1. A combination in which the first Group 13 element is Ga and the second Group 13 element is Al is preferable in terms of a high degree of freedom in substrate design.

The thickness of the channel layer 3 and the electron supply layer 4 is not particularly limited. Further, the electron supply layer 4 may be doped with various elements. Various elements include, for example, carbon, phosphorus, magnesium, silicon, iron, oxygen, and hydrogen.

In the present invention, a spacer layer S is provided between the channel layer 3 and the electron supply layer 4 . This spacer layer S is also basically for obtaining the same function as the spacer layer in the prior art, and is intended to achieve both high output and improved electron mobility by increasing the concentration of the two-dimensional electron gas in the HEMT. purpose.

In the present invention, the spacer layer S is Al _a Ga _1-a N (0<a<0.5), the electron supply layer 4 is Al _x In _y Ga _1-xy N (0<x+y≦1), and the spacer layer S has a layer thickness of two molecular layers or less, and in particular, the spacer layer S has a layer thickness of two molecular layers or less.

Such a form is specified by observing a cross section of the nitride semiconductor substrate Z with a scanning transmission electron microscope (STEM) and performing elemental analysis with an energy dispersive X-ray spectroscopy (EDS). FIG. 2 is a cross-sectional view of electron supply layer 4/spacer layer S/channel layer 3 near the electron supply layer 4/spacer layer S/channel layer 3 in Comparative Example 1 and Example 1 observed by STEM. The results obtained by EDS analysis are shown. EDS is measured by an EDS measuring device attached to the STEM device.

Note that the values obtained by EDS are only reference values due to the principle of measurement, but they accurately reflect the abundance ratios of different elements. It is possible to roughly know exactly what kind of distribution is being done.

The spacer layer S has a composition of Al _a Ga _1-a N (0<a<0.5). A certain amount of Al must be present in order to function as a spacer layer, that is, to improve mobility. If the channel layer 3 is made of GaN as in the present invention, the mobility improvement effect is manifested from an Al composition a of about 0.1, and the effect increases as a increases.

By the way, since the spacer layer S is in direct contact with the channel layer 3 made of GaN, the difference in lattice constant with GaN, which increases in proportion to a, increases the dislocation density due to the strain generated at the interface. do. This increased dislocation density is considered to be one of the causes of deterioration of current collapse.

Further, when the layer is formed by the metal organic chemical vapor deposition method (MOCVD method), the spacer layer S having a composition of Al _a Ga _1-a N (0<a<0.5) contains more carbon than GaN. It can be said that the current collapse is aggravated due to the large amount of the current.

Furthermore, the thinner the spacer layer S having a composition of Al _a Ga _1-a N (0<a<0.5) is, the more lattice-matched it is with the GaN of the channel layer 3, so defects are less likely to occur. It is effective in suppressing current collapse. However, if the thickness is the same, the larger the value of a, the larger the difference in lattice constant from GaN.

In summary, when the spacer layer S is made of Al _a Ga _1-a N (0<a<0.5), from the viewpoint of the current collapse suppression effect, the thinner the thickness and the smaller the value of a, the better. It can be said that the mobility cannot be improved unless a is equal to or greater than a predetermined value.

Based on the above viewpoints, differences from the prior art will be described in detail below.

Patent Document 1 exemplifies a spacer layer made of AlN having a layer thickness of 1 molecular layer or more and 4 molecular layers or less. In other words, the current injection inhibition from the barrier layer to the channel layer, which is a drawback of AlN, is dealt with by reducing the layer thickness. However, when AlN and GaN are in direct contact with each other, dislocations are generated at the interface and the AlN layer has a high carbon concentration.

In Patent Document 2, in order to make the Al atomic ratio of the spacer layer in contact with the channel layer higher than that of the remainder, and furthermore, to enhance the effect of suppressing alloy scattering, the portion in contact with the channel layer is preferably AlN, and the AlN layer The thickness is preferably several atomic layers. In other words, it can be said that in Patent Document 2 as well, AlN is preferable for the portion in contact with the channel layer, and there is a technical idea of coping with the demerits of using AlN by reducing the layer thickness.

On the other hand, in the present invention, the thickness of the spacer layer S and the Al atomic ratio of the spacer layer S in contact with the channel layer 3 are further studied to make the spacer layer S thinner. Based on this, the relationship between the Al atomic ratio of the spacer layer S and the effect on the HEMT was studied from a new point of view.

That is, focusing on the fact that the presence of the spacer layer S contributes to the deterioration of the current collapse, the Al atomic ratio of the spacer layer S made of AlGaN is lowered compared to the spacer layers described in Patent Documents 1 and 2. In addition, by reducing the thickness, AlGaN and GaN forming the channel layer 3 are more lattice-matched, so defects are less likely to occur and there is an effect of suppressing collapse. The inventors have found a phenomenon that there is an optimum value that does not significantly impair the mobility improvement effect of , and have arrived at the present invention.

In the present invention, the inventors have found that the above effects can be obtained by configuring the spacer layer S to have a layer thickness of two molecular layers or less. Since the spacer layer S must always exist, the lower limit is one molecular layer. On the other hand, it has been found that the thickness of the spacer layer S is disadvantageous in that the current collapse is worsened if it is too thick.

In the present invention, the Al atomic ratio in the spacer layer S is preferably evaluated by STEM-EDS. However, it is currently extremely difficult to accurately evaluate the Al atomic ratio with a thickness of 1 to 2 molecules. Therefore, even if STEM-EDS is used, highly accurate quantification cannot be expected. It is possible.

In FIG. 2, Comparative Example 1 and Example 1 are compared. In Comparative Example 1, in which the spacer layer S is thick, the existence of the spacer layer S can be clearly confirmed from the photograph. On the other hand, although the layer thickness is thin and the resolution is not clear, even in Example 1, it is possible to confirm the gradation to the extent that it can be discriminated. In Example 1, the EDS graph of FIG. 2 does not seem to show any difference in the Al atom ratio between the spacer layer S and the electron supply layer 4, but the numerical data analysis results show that the Al atoms in the spacer layer S The ratio was 20%, and the Al atomic ratio of the electron supply layer 4 was 15%.

FIG. 3 is a diagram showing a brighter image observed at a lower magnification than in FIG. 2 so that the boundary near the interface between the spacer layer S and the electron supply layer 4 in the cross-sectional view of Example 1 shown in FIG. 2 can be seen more clearly. A field-of-view STEM image (left) and an HAADF-STEM image (right). Since the HAADF image has a Z-contrast, the more light atoms there are in the analysis region (here, the higher the Al composition of AlGaN), the darker it appears. From FIG. 3 , three layers having different contrasts can be confirmed in order from the bottom: the brightest (GaN layer), the darkest (spacer layer), and the intermediate brightness (AlGaN electron supply layer). Thus, the presence of the spacer layer having a higher Al composition ratio than the AlGaN electron supply layer can be clearly confirmed in the channel layer 3 portion of the present invention.

In the present invention, when the Al atomic ratio of the spacer layer S is higher than the Al atomic ratio of the electron supply layer 4, compared with the case where there is no difference between the Al atomic ratio of the spacer layer S and the electron supply layer 4, It can be said that this is preferable in that the original mobility improvement effect of the spacer layer can be more retained.

The Al atomic ratio of the electron supply layer 4 is preferably between 10% and 30%. That is, in Al _x In _y Ga _1-xy N (0<x+y≦1), x is preferably 0.1 to 0.3, y is 0 to 0.9, and x+y≦1. The thickness of the electron supply layer 4 is also not particularly limited, but is appropriately designed between 10 nm and 60 nm.

The profile shape due to the Al atomic ratio in the spacer layer S is obtained by the MOCVD method immediately after the channel layer 3 is formed. It can be obtained appropriately by optimizing the timing and the like.

A preferred mode for obtaining the spacer layer S and the electron supply layer 4 according to the present invention by the MOCVD method is to form a channel made of GaN using a Ga source gas and an N source gas in a reactor of a vapor phase growth apparatus. After forming the layer 3, in forming the spacer layer and the electron supply layer, the Ga raw material gas, the N raw material gas and the Al raw material gas are used to form the spacer layer at a furnace pressure of 200 to 300 hPa for 5 to 10 seconds. After forming S, the electron supply layer is formed by rapidly changing the supply ratio of the raw material gases while maintaining the film formation temperature during the formation of the spacer layer.

According to such a method, a nitride semiconductor layer having a thickness of 1 to 2 molecules can be formed by MOCVD without using a technique such as molecular vapor deposition.

As described above, the nitride semiconductor epitaxial substrate of the present invention has the effect of improving the mobility of the two-dimensional electron gas and increasing the speed of the transistor in the same manner as a conventional substrate having a spacer layer. It is a structure capable of reducing the on-resistance of the AlGaN/GaN-HEMT device while suppressing the deterioration of the current collapse characteristics due to .

EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited to the following examples.

[Common experimental conditions]
A silicon single crystal substrate having a diameter of 6 inches, a thickness of 1000 μm, a p-type resistivity of 0.01 Ωcm, and a plane orientation of (111) was prepared as a base substrate 1 . After cleaning this by a known substrate cleaning method, it is set in an MOCVD apparatus, and after heating and gas replacement, heat treatment is performed at a growth temperature of 1000° C. for 15 minutes under conditions of a 100% hydrogen atmosphere and a furnace pressure of 135 hPa. Then, the natural oxide film on the surface of the underlying substrate 1 was removed, and silicon atomic steps were developed on the surface.

Subsequently, an AlN single crystal having a thickness of 70 nm was formed using trimethylaluminum (TMAl) and ammonia (NH ₃ ) as raw material gases. Next, the growth temperature was adjusted to 1000° C., the furnace pressure was adjusted to 60 hPa, and trimethylgallium (TMG), TMAl, and NH ₃ were used as raw material gases to form an Al _0.1 Ga _0.9 N single crystal layer with a thickness of 300 nm. . Next, using TMG, TMAl, and NH ₃ as raw material gases, an AlN single crystal layer with a thickness of 5 nm and an Al _0.1 Ga _0.9 N single crystal layer with a thickness of 30 nm are alternately laminated to form a multilayer with a layer thickness of about 2450 nm. formed a structure. The buffer layer 2 was formed on the underlying substrate 1 as described above.

On the buffer layer 2, a GaN single crystal layer having a thickness of 3000 nm was stacked as the channel layer 3 at a growth temperature of 1030° C. and a reactor pressure of 200 hPa.

A spacer layer S (Al _a Ga _1-a N) was formed on the channel layer 3 under the conditions described later in Example 1 and Comparative Example 1, respectively.

Then, on the spacer layer S, an Al _0.18 Ga _0.82 N single crystal layer with a thickness of 24 nm is formed as the electron supply layer 4 by adjusting the growth temperature to 1000° C. and the furnace pressure to 200 hPa, and further as a cap layer. A 4 nm GaN layer was formed. Through the steps described above, a nitride semiconductor epitaxial substrate for evaluation was obtained. The thickness and carbon concentration of each layer formed by vapor phase epitaxy were controlled by adjusting the flow rate and supply time of source gas, substrate temperature, and other known growth conditions.

[Example 1]
A spacer layer S was formed on the channel layer 3 by introducing TMG, TMAl and NH ₃ as material gases for 1.5 seconds at a growth temperature of 1030° C. and a furnace pressure of 200 hPa. As a result of evaluation from FIG. 2, the layer thickness was about 0.25 nm (one molecular layer).

[Comparative Example 1]
A spacer layer S was formed on the channel layer 3 by introducing TMG, TMAl and NH ₃ as material gases for 10 seconds at a growth temperature of 1030° C. and a furnace pressure of 50 hPa. In this case, the Al content in the spacer layer S becomes considerably higher than that in Example 1, and a high Al concentration region having an Al composition ratio exceeding 50% is formed in the thickness direction. The layer thickness was about 1 nm (4 molecular layers).

With respect to the obtained nitride semiconductor epitaxial substrates of Example 1 and Comparative Example 1, the spacer layer S and part of the channel layer 3 and the electron supply layer 4 adjacent thereto were subjected to cross-sectional observation and elemental analysis. The conditions are shown below.

[Evaluation 1 to STEM observation]
Each nitride semiconductor epitaxial substrate was cleaved in the diameter direction, fragments were sampled from the vicinity of the center of the main surface, and thinned by FIB (Focused Ion Beam) method to obtain a sample for measurement. This sample was observed by STEM (Scanning Transmission Electron Microscope). The apparatus used was JEM-ARM200F manufactured by JEOL Ltd., and the acceleration voltage was set to 200 kV. In addition, elemental analysis was performed with an EDS measuring device (energy dispersive X-ray spectrometer) (JED-2300T) attached to the STEM used, and the measurement conditions were an acceleration voltage of 200 kV, a beam diameter of 0.1 nmφ, and an energy resolution of about 140 eV. and

[Evaluation 2 to EDS analysis]
After the STEM observation, EDS measurement was performed by irradiating 100-point beams along a line in a range of 20 nm width for the electron supply layer 4/spacer layer S/channel layer 3 vicinity. The beam interval was 0.2 nm, and the measurement time per point was 1 second.

Here, one point near the center of the main surface of the nitride semiconductor epitaxial substrate was sampled to specify the form of the spacer layer S. As shown in FIG. Since film formation by the MOCVD method has high film formation accuracy, it can be said that this is sufficient, but if necessary, the number of samples may be further increased. You can sample from a point.

[Evaluation 3 to electron mobility]
Next, the same nitride semiconductor epitaxial substrate that was subjected to STEM observation and EDS analysis was subjected to Hall effect measurement by the vanDer Pauw method to evaluate electron mobility. First, the substrate was diced into chips of 7 mm square, and Ti/Al electrodes with a diameter of 0.25 mm were formed on the four corners of the electron supply layer 4 of each chip by vacuum deposition. An alloying heat treatment was then performed at 600° C. for 5 minutes in an _N2 atmosphere. Then, the Hall effect was measured using HL5500PC manufactured by ACCENT.

[Evaluation 4 to current collapse]
Evaluation of current collapse characteristics was performed as follows. First, grooves for the recess gate region and the element isolation region were formed by dry etching on each of the nitride semiconductor epitaxial substrates for evaluation prepared above, and an Au electrode as a gate electrode, a source electrode and a source electrode were formed on the electron supply layer 4 side. An Al electrode was formed as a drain electrode, and an Al electrode was formed as a rear surface electrode on the rear surface of the underlying substrate by vacuum deposition. Evaluation was performed by applying a constant stress voltage between the source and drain electrodes in the OFF state of the HEMT device, and calculating a constant called a collapse factor from the ratio of the amount of conduction current in the ON state before and after the application. . The closer the value of the collapse factor to 1.0, the smaller the conduction loss of the device. When the collapse factor is from 0.7 to 1.0: ◯, from 0.5 to less than 0.7: Δ, and less than 0.5: x.

As a result, the electron mobility of Example 1 was only about 90% lower than that of Comparative Example 1, which can be said to be a difference that does not pose a serious problem in practice. On the other hand, the current collapse was ◯ in Example 1 and Δ in Comparative Example 1.

From the above results, it can be said that in Example 1, both the effect of improving the electron mobility and the effect of suppressing current collapse are obtained. On the other hand, in Comparative Example 1, compared with Example 1, the electron mobility was approximately the same or higher, but the current collapse suppressing effect was inferior.

From this, the nitride semiconductor epitaxial substrate of the present invention can significantly obtain the effect of suppressing current collapse without significantly impairing the electron mobility. It can be said that it is also possible to make an optimum design according to individual requirements, such as wanting a certain degree of spacer layer insertion effect.

Z nitride semiconductor substrate 1 base substrate 2 buffer layer 3 channel layer S spacer layer 4 electron supply layer

Claims (2)

In a nitride semiconductor epitaxial substrate in which a channel layer, a spacer layer, and an electron supply layer are laminated in this order,
The channel layer is GaN, the spacer layer is _{AlaGa1 -aN} (0<a<0.5), and the electron supply layer is _{AlxInyGa1 -xyN (} 0<x+y≤1 and x is 0.1 to 0.3 ),
The nitride semiconductor epitaxial substrate, wherein the spacer layer has a layer structure of two molecular layers or less in thickness , and the a is larger than the x . A nitride semiconductor HEMT using the nitride semiconductor epitaxial substrate according to claim 1 .

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