JP6944569B2 - Epitaxial substrates for semiconductor devices and semiconductor devices - Google Patents

Description

The present invention relates to a semiconductor device, and more particularly to a semiconductor device configured by using a self-supporting substrate made of semi-insulating GaN.

Nitride semiconductors have a wide bandgap of direct transition type, a high dielectric breakdown electric field, and a high saturated electron velocity. Therefore, they are used as semiconductor materials for light emitting devices such as LEDs and LDs and high-frequency / high-power electronic devices. It's being used.

As a typical structure of a nitride electronic device, there is a high electron mobility transistor (HEMT) structure in which AlGaN is laminated as a "barrier layer" and GaN is formed as a "channel layer". This takes advantage of the feature that a high concentration of two-dimensional electron gas is generated at the AlGaN / GaN laminated interface due to the large polarization effect (spontaneous polarization effect and piezo polarization effect) peculiar to the nitride material.

Nitride electronic devices are generally made using commercially available dissimilar material substrate substrates such as sapphire, SiC, and Si. However, there is a problem that a large number of defects occur in the GaN film heteroepitaxially grown on these dissimilar material substrates due to the difference in the lattice constant and the coefficient of thermal expansion between GaN and the dissimilar material substrate. There is.

On the other hand, when the GaN film is homoepitaxially grown on the GaN substrate, defects due to the above-mentioned differences in the lattice constant and the coefficient of thermal expansion do not occur, and the GaN film exhibits good crystallinity.

Therefore, when a nitride HEMT structure is formed on a GaN substrate, the mobility of the two-dimensional electron gas existing at the AlGaN / GaN stacking interface is improved. The characteristics can be expected to improve.

However, commercially available GaN substrates produced by the hydride vapor phase growth method (HVPE method) generally exhibit an n-type conduction type due to oxygen impurities incorporated into the crystal. .. The conductive GaN substrate serves as a leakage current path between the source and drain electrodes when the HEMT element is driven with a high voltage. Therefore, in order to manufacture a HEMT device, it is desirable to use a semi-insulating GaN substrate.

In order to realize a semi-insulating GaN substrate, it is known that it is effective to dope elements that form deep acceptor levels, such as transition metal elements (for example, Fe) and group 2 elements (for example, Mg), into GaN crystals. Has been done.

It is already known that a high-quality semi-insulating GaN single crystal substrate can be realized by selecting zinc element (Zn) from Group 2 elements (see, for example, Patent Document 1). Further, a high resistance layer doped with iron (Fe), which is a transition metal element, is formed on the substrate, and an intermediate layer having a high effect of incorporating Fe is formed between the high resistance layer and the electron traveling layer. As a result, a mode for preventing Fe from entering the electron traveling layer is already known (see, for example, Patent Document 2).

It has already been made to fabricate a HEMT structure on a semi-insulating GaN substrate or on a substrate with a semi-insulating GaN film and evaluate various characteristics (see, for example, Non-Patent Documents 1 to 3). ).

When the nitride film is epitaxially grown on the semi-insulating GaN substrate, a silicon (Si) element may be incorporated from the outside at the interface between the semi-insulating GaN substrate and the nitride film (nitride epitaxial film). Since the silicon (residual silicon) acts as a donor element, a conductive layer is formed at the nitride film / substrate interface. Since this conductive layer acts as a leak path for the drain-source current in the HEMT element, it causes a decrease in pinch-off characteristics and a decrease in breakdown voltage.

Japanese Patent No. 5039813 Japanese Unexamined Patent Publication No. 2013-74211

Yoshinori Oshimura, Takayuki Sugiyama, Kenichiro Takeda, Motoaki Iwaya, Tetsuya Takeuchi, Satoshi Kamiyama, Isamu Akasaki, and Hiroshi Amano, "AlGaN / GaN Heterostructure Field-Effect Transistors on Fe-Doped GaN Substrates with High Breakdown Voltage", Japanese Journal of Applied Physics, vol.50 (2011), p.084102-1-p.084102-5. V. Desmaris, M. Rudzinski, N. Rorsman, PR Hageman, PK Larsen, H. Zirath, TC Rodle, and HFF Jos, "Comparison of the DC and Microwave Performance of AlGaN / GaN HEMTs Grown on SiC by MOCVD With Fe- Doped or Unintentionally Doped GaN Buffer Layers ", IEEE Transactions on Electron Devices, Vol.53, No.9, pp.2413-2417, September 2006. M. Azize, Z. Bougrioua, and P. Gibart, "Inhibition of interface pollution in AlGaN / GaN HEMT structures regrown on semi-insulating GaN templates", Journal of Crystal Growth vol.299 (2007) p.103-p.108 ..

The present invention has been made in view of the above problems, and an object of the present invention is to provide an epitaxial substrate for a semiconductor element in which a leakage current is suppressed and a withstand voltage is high.

In order to solve the above problems, in the first aspect of the present invention, the epitaxial substrate for a semiconductor element is a semi-insulating self-supporting substrate made of Zn-doped GaN and a group 13 adjacent to the self-supporting substrate. A buffer layer made of nitride, a channel layer made of Group 13 nitride adjacent to the buffer layer, and a Group 13 nitride provided on the opposite side of the channel layer from the buffer layer. A second region comprising a barrier layer composed of the self-supporting substrate and the buffer layer, and a part of the first region composed of the self-supporting substrate and the buffer layer is a second region containing ^{Si at a concentration of 1 × 10 17} cm ^{-3 or more.} The region 2 is a diffusion region of Zn from the self-supporting substrate containing Zn at a concentration of 1 × 10 ¹⁷ cm ^-3 or more, which exists including the interface between the self-supporting substrate and the buffer layer.

In the second aspect of the present invention, in the epitaxial substrate for a semiconductor device according to the first aspect, the buffer layer is made of GaN, the channel layer is made of GaN, and the barrier layer is made of AlGaN.

A third aspect of the present invention is the epitaxial substrate for a semiconductor element according to the first aspect, wherein the buffer layer is a multilayer buffer layer or two or more in which two or more Group 13 nitride layers having different compositions are laminated. It is a composition gradient buffer layer composed of a Group 13 nitride containing the Group 13 elements of No. 13 and the abundance ratio of the Group 13 elements changes in the thickness direction, the channel layer is made of GaN, and the barrier layer is made of AlGaN. bottom.

In a fourth aspect of the present invention, the semiconductor element is a semi-insulating self-supporting substrate made of Zn-doped GaN, a buffer layer made of Group 13 nitride adjacent to the self-supporting substrate, and the buffer. A channel layer made of Group 13 nitride adjacent to the layer, a barrier layer made of Group 13 nitride provided on the opposite side of the channel layer from the buffer layer, and the barrier layer. A gate electrode, a source electrode, and a drain electrode provided above are provided, and a part of the first region composed of the self-supporting substrate and the buffer layer contains Si at ^{a concentration of 1 × 10 17} cm ^-3 or more. The second region is from the self-supporting substrate containing Zn at a concentration of 1 × 10 ¹⁷ cm ^-3 or more, which exists including the interface between the self-supporting substrate and the buffer layer. It is a diffusion region of Zn.

In the fifth aspect of the present invention, in the semiconductor device according to the fourth aspect, the buffer layer is made of GaN, the channel layer is made of GaN, and the barrier layer is made of AlGaN.

A sixth aspect of the present invention is the semiconductor element according to the fourth aspect, wherein the buffer layer is a multilayer buffer layer or two or more Group 13 in which two or more Group 13 nitride layers having different compositions are laminated. It is a composition gradient buffer layer made of a Group 13 nitride containing an element and the abundance ratio of the Group 13 element changes in the thickness direction, the channel layer is made of GaN, and the barrier layer is made of AlGaN.

According to the first to sixth aspects of the present invention, the leakage current at the time of driving the semiconductor element can be reduced, and the withstand voltage (element voltage) of the semiconductor element can be improved.

It is a figure which shows typically the cross-sectional structure of the HEMT element 20. It is a figure which shows the density profile of the Zn element and the Si element in the vicinity of the interface of a GaN buffer layer and a GaN substrate in Example 1. FIG. It is a figure which shows the density profile of the Zn element and the Si element in the vicinity of the interface of a GaN buffer layer and a GaN substrate in the comparative example 1. FIG. It is a figure which shows the density profile of Zn element and Si element, and the secondary ion signal profile of Al element in the depth direction from the surface of the barrier layer 4 in Example 7. FIG.

The group numbers in the periodic table shown in this specification are based on the group numbers 1 to 18 according to the revised version of the Inorganic Chemistry Naming Method by the International Union of Pure Applied Chemistry (IUPAC) in 1989. , Group 13 refers to aluminum (Al), gallium (Ga), indium (In), etc., and Group 14 refers to silicon (Si), germanium (Ge), tin (Sn), lead (Pb), etc. , Group 15 refers to nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and the like.

<Overview of epitaxial substrate and HEMT element>
FIG. 1 schematically illustrates a cross-sectional structure of a HEMT element 20 as an embodiment of a semiconductor device according to the present invention, which includes an epitaxial substrate 10 as an embodiment of an epitaxial substrate for a semiconductor element according to the present invention. It is a figure which shows.

The epitaxial substrate 10 includes a self-supporting substrate 1, a buffer layer 2, a channel layer 3, and a barrier layer 4. Further, the HEMT element 20 is provided with a source electrode 5, a drain electrode 6, and a gate electrode 7 on the epitaxial substrate 10 (on the barrier layer 4). The thickness ratio of each layer in FIG. 1 does not reflect the actual one.

The self-supporting substrate 1 is ^{a GaN substrate having a (0001) plane orientation doped with Zn of 1 × 10 18} cm ^-3 or more, and has a specific resistance at room temperature of 1 × 10 ² Ωcm or more and exhibits semi-insulating properties. The size of the self-supporting substrate 1 is not particularly limited, but considering ease of handling and the like, it is preferable to have a thickness of about several hundred μm to several mm. The self-supporting substrate 1 can be manufactured by, for example, the flux method.

The formation of the self-supporting substrate 1 by the flux method is roughly performed by using a melt containing metal Ga, metal Na, metal Zn, and C (carbon) in a growing container (alumina crucible) arranged so as to be horizontally rotatable in a pressure-resistant container. The GaN single crystal formed on the seed substrate is separated from the seed substrate by keeping the inside of the growth vessel at a predetermined temperature and a predetermined pressure while introducing nitrogen gas in a state where the growth container is horizontally rotated. Obtained by. As the seed substrate, a so-called template substrate obtained by forming a GaN thin film on a sapphire substrate by the MOCVD method or the like can be preferably used.

The buffer layer 2 is a layer made of Group 13 nitride, which is formed (adjacent) on one main surface of the self-supporting substrate 1. The buffer layer 2 may be a single layer composed of one group 13 nitride as a whole, or may be a multilayer buffer layer composed of two or more group 13 nitride layers having different compositions. An example of the single layer is a GaN buffer layer made entirely of GaN. Examples of the multilayer buffer layer include a configuration in which a GaN layer is laminated on _{an Al a} Ga _{1-a N layer (0 <a ≦ 1).} Alternatively, the buffer layer 2 is a composition gradient buffer composed of Group 13 nitrides containing 2 or more Group 13 elements (for example, Ga and Al), and the abundance ratio (mole fraction) of each element changes in the thickness direction. It may be provided as a layer. The buffer layer 2 is formed to have a thickness of about 50 nm to 1000 nm. In the present embodiment, the buffer layer 2 is different from the so-called low temperature buffer layer formed at a low temperature of less than 800 ° C., at a temperature similar to or higher than the formation temperature of the channel layer 3 and the barrier layer 4. It is formed at a high temperature.

Further, in the epitaxial substrate 10 according to the present embodiment, Zn doped in the self-supporting substrate 1 is diffused to at least the buffer layer 2. This point will be described later.

The channel layer 3 is a layer formed (adjacent) on the buffer layer 2. The channel layer 3 is formed to have a thickness of about 50 nm to 5000 nm. Further, the barrier layer 4 is a layer provided on the side opposite to the buffer layer 2 with the channel layer 3 interposed therebetween. The barrier layer 4 is formed to have a thickness of about 2 nm to 40 nm.

The barrier layer 4 may be formed adjacent to the channel layer 3 as shown in FIG. 1, in which case the interface between the two layers is a heterojunction interface. Alternatively, a spacer layer (not shown) may be provided between the channel layer 3 and the barrier layer 4. In this case, the region from the interface between the channel layer 3 and the spacer layer to the interface between the barrier layer 4 and the spacer layer is heterogeneous. It becomes the junction interface region.

In each case, the channel layer 3 is formed of GaN, and the barrier layer 4 is AlGaN (Al _x Ga _1-x N, 0 <x <1) to InAlN (In _y Al _1-y N, 0 <y <. It is a preferable example that it is formed in 1). However, the combination of the channel layer 3 and the barrier layer 4 is not limited to this.

The formation of the buffer layer 2, the channel layer 3, and the barrier layer 4 is realized by, for example, the MOCVD method. In the layer formation by the MOCVD method, for example, when the buffer layer 2 and the channel layer 3 are formed of GaN and the barrier layer 4 is formed of AlGaN, the organic metal (MO) raw material gas for Ga and Al (MO) raw material gas ( Using a known MOCVD furnace configured to be capable of supplying TMG, TMA), ammonia gas, hydrogen gas, and nitrogen gas into the reactor, the self-supporting substrate 1 placed in the reactor is heated to a predetermined temperature. At the same time, it can be carried out by sequentially depositing GaN crystals and AlGaN crystals generated by the gas phase reaction between the organic metal raw material gas corresponding to each layer and the ammonia gas on the self-supporting substrate 1.

The source electrode 5 and the drain electrode 6 are metal electrodes having a thickness of about ten and several tens of nm to one hundred and several tens of nm, respectively. The source electrode 5 and the drain electrode 6 are preferably formed as, for example, a multilayer electrode made of Ti / Al / Ni / Au. The source electrode 5 and the drain electrode 6 have ohmic contact with the barrier layer 4. The source electrode 5 and the drain electrode 6 are preferably formed by a vacuum deposition method and a photolithography process. In order to improve the ohmic contact between the two electrodes, it is preferable to perform heat treatment for several tens of seconds in a nitrogen gas atmosphere at a predetermined temperature between 650 ° C. and 1000 ° C. after forming the electrodes.

The gate electrode 7 is a metal electrode having a thickness of about ten and several tens of nm to one hundred and several tens of nm. The gate electrode 7 is preferably configured as, for example, a multilayer electrode made of Ni / Au. The gate electrode 7 has a Schottky contact with the barrier layer 4. The gate electrode 7 is a preferable example of being formed by a vacuum vapor deposition method and a photolithography process.

<Method of manufacturing epitaxial substrate and HEMT element>
(Making a self-supporting substrate)
First, a procedure for manufacturing the self-supporting substrate 1 by the flux method will be described.

First, a c-plane sapphire substrate having a diameter similar to the diameter of the self-supporting substrate 1 to be manufactured is prepared, and a GaN low-temperature buffer layer is formed on the surface thereof at a temperature of 450 ° C. to 750 ° C. to a thickness of about 10 nm to 50 nm. A film is formed, and then a GaN thin film having a thickness of about 1 μm to 10 μm is formed at a temperature of 1000 ° C. to 1200 ° C. by the MOCVD method to obtain a MOCVD-GaN template that can be used as a seed substrate.

Next, using the obtained MOCVD-GaN template as a seed substrate, a Zn-doped GaN single crystal layer is formed by using the Na flux method.

Specifically, first, the MOCVD-GaN template is placed in the alumina crucible, and then 10 g to 60 g of metal Ga, 15 g to 90 g of metal Na, and 0.4 g to metal Zn are placed in the alumina crucible. Fill 5 g and 10 mg to 500 mg of C, respectively.

The alumina crucible is placed in a heating furnace, the temperature inside the furnace is set to 800 ° C. to 950 ° C., the pressure inside the furnace is set to 3 MPa to 5 MPa, the heating is performed for about 20 hours to 400 hours, and then the mixture is cooled to room temperature. After cooling, remove the alumina crucible from the furnace. By the above procedure, a brown GaN single crystal layer is deposited on the surface of the MOCVD-GaN template with a thickness of 300 μm to 3000 μm.

The GaN single crystal layer thus obtained is polished with diamond abrasive grains to flatten the surface thereof. As a result, a Lux-GaN template in which a GaN single crystal layer is formed on the MOCVD-GaN template can be obtained. However, polishing is performed within a range in which the total thickness of the nitride layer in the Lux-GaN template is maintained at a value sufficiently larger than the target thickness of the self-supporting substrate 1 to be finally obtained.

Next, the seed substrate is separated from the Lux-GaN template by irradiating the seed substrate with laser light while scanning at a scanning speed of 0.1 mm / sec to 100 mm / sec by the laser lift-off method. As the laser light, for example, it is preferable to use a third harmonic of Nd: YAG having a wavelength of 355 nm. In this case, the pulse width may be 1 ns to 1000 ns and the pulse period may be about 1 kHz to 200 kHz. At the time of irradiation, it is preferable to appropriately condense the laser light to adjust the light density. Further, it is preferable to irradiate the laser beam while heating the Lux-GaN template at a temperature of about 30 ° C. to 600 ° C. from the side opposite to the seed substrate.

After separating the seed substrate, the surface of the obtained laminated structure on the side peeled from the seed substrate is polished. As a result, a self-supporting substrate 1 made of GaN in which ^{Zn is doped at a concentration of 1 × 10 18} cm ^{-3 or more can be obtained.}

(Manufacturing of epitaxial substrate)
Subsequently, the production of the epitaxial substrate 10 by the MOCVD method will be described. In the epitaxial substrate 10, the buffer layer 2, the channel layer 3, and the barrier layer 4 are laminated in this order under the following conditions in a state where the self-supporting substrate 1 is placed on a susceptor provided in the reactor of the MOCVD furnace. Obtained by doing. However, the buffer layer 2 exemplifies a case where a single GaN buffer layer, a multilayer buffer layer containing Ga and Al as Group 13 elements, or a composition gradient buffer layer is formed. The formation temperature means the susceptor heating temperature.

Further, in the present embodiment, the group 15/13 gas ratio is the total of group 13 (Ga, Al, In) raw materials TMG (trimethylgallium), TMA (trimethylaluminum), and TMI (trimethylindium). It is the ratio (molar ratio) of the supply amount of aluminum, which is a raw material of Group 15 (N), to the supply amount. The Al raw material gas / Group 13 raw material gas ratio when the barrier layer 4 is formed of AlGaN is the ratio (molar ratio) of the supply amount of the Al raw material to the supply amount of the entire Group 13 (Ga, Al) raw materials. The In raw material gas / Group 13 raw material gas ratio when the barrier layer 4 is formed by InAlN is the ratio (molar ratio) of the supply amount of the In raw material to the supply amount of the entire Group 13 (In, Al) raw materials. Both are determined according to the desired composition of the barrier layer 4 (Al molar ratio x or In composition ratio y).

Buffer layer 2:
Formation temperature = 1000 ° C to 1200 ° C;
Reactor pressure = 15 kPa to 105 kPa;
Carrier gas = hydrogen;
Group 15 / Group 13 gas ratio = 250-10000;
Al raw material gas / Group 13 raw material gas ratio = 0 (in the case of GaN buffer layer);
Al raw material gas / Group 13 raw material gas ratio = 0 to 1 depending on the position in the thickness direction (in the case of a multilayer buffer layer or a composition gradient buffer layer).

Channel layer 3:
Formation temperature = 1000 ° C to 1150 ° C;
Reactor pressure = 15 kPa to 105 kPa;
Carrier gas = hydrogen;
Group 15 / Group 13 gas ratio = 1000-10000.

Barrier layer 4 (when formed with AlGaN):
Formation temperature = 1000 ° C to 1200 ° C;
Reactor pressure = 1 kPa to 30 kPa;
Group 15 / Group 13 gas ratio = 5000-20000;
Carrier gas = hydrogen;
Al raw material gas / Group 13 raw material gas ratio = 0.1 to 0.4.

Barrier layer 4 (when formed with InAlN):
Formation temperature = 700 ° C to 900 ° C;
Reactor pressure = 1 kPa to 30 kPa;
Group 15 / Group 13 gas ratio = 2000-20000;
Carrier gas = nitrogen;
In raw material gas / Group 13 raw material gas ratio = 0.1 to 0.9.

(Manufacturing of HEMT element)
The fabrication of the HEMT element 20 using the epitaxial substrate 10 can be realized by applying a known technique.

For example, after performing element separation processing by etching to remove the boundary portion of each element to about 50 nm to 1000 nm using a photolithography process and the RIE method, the surface of the epitaxial substrate 10 (the surface of the barrier layer 4) is subjected to element separation processing. _{A SiO 2} pattern layer is obtained _{by forming a SiO 2} film having a thickness of 50 nm to 500 nm and then etching and removing _{the SiO 2} film at the planned formation location of the source electrode 5 and the drain electrode 6 by photolithography.

Next, the source electrode 5 and the drain electrode 6 are formed by forming a metal pattern made of Ti / Al / Ni / Au at the planned formation locations of the source electrode 5 and the drain electrode 6 by using a vacuum vapor deposition method and a photolithography process. Form. The thickness of each metal layer is preferably 5 nm to 50 nm, 40 nm to 400 nm, 4 nm to 40 nm, and 20 nm to 200 nm, respectively.

Then, in order to improve the ohmic properties of the source electrode 5 and the drain electrode 6, heat treatment is performed for 10 seconds to 1000 seconds in a nitrogen gas atmosphere at 600 ° C. to 1000 ° C.

Subsequently, a photolithography process is used _{to remove the SiO 2} film at the location where the gate electrode 7 is to be formed from the _{SiO 2 pattern layer.}

Further, the gate electrode 7 is formed by forming a Schottky metal pattern made of Ni / Au at a portion where the gate electrode 7 is to be formed by using a vacuum vapor deposition method and a photolithography process. The thickness of each metal layer is preferably 4 nm to 40 nm and 20 nm to 200 nm.

The HEMT element 20 is obtained by the above process.

<Uneven distribution of Si and diffusion of Zn>
In the HEMT element 20 manufactured by the above procedure and conditions, when the region consisting of the self-standing substrate 1 and the buffer layer 2 is defined as the first region, Si is 1 in a part of the first region. It has a second region contained at a concentration of × 10 ¹⁷ cm ^{-3 or higher.} Since Si is not intentionally contained in the process of manufacturing the HEMT element 20, particularly in the process of producing the self-supporting substrate 1 and forming the buffer layer 2 adjacent to the self-supporting substrate 1, the second one is used. The inclusion of Si at the above-mentioned concentration in the region is presumed to be that Si taken in from the outside during the process remained after the formation of the HEMT element 20. More specifically, the second region includes an interface between the free-standing substrate 1 and the buffer layer 2. However, it is not formed inside the self-supporting substrate 1.

In addition, in the HEMT element 20 according to the present embodiment, Zn doped in the self-supporting substrate 1 is diffused to at least the buffer layer 2. ^{Moreover, Zn exists at a concentration of 1 × 10 17} cm ^-3 or more in the entire range of the second region described above (the minimum concentration in the second region is 1 × 10 ¹⁷ cm ^-3 ). Exists like).

In the HEMT element 20 according to the present embodiment, by satisfying the condition of the concentration, the leakage current at the time of driving is reduced and a high withstand voltage (high element voltage) is realized.

On the other hand, it has been confirmed that the leakage current is large and the withstand voltage is low in the HEMT element in which the minimum value of the Zn concentration in the second region is ^{less than 1 × 10 17} cm ^-3.

This is because Si taken in from the outside can act as a donor element, and when Zn diffusion that satisfies the above-mentioned concentration conditions does not occur, this action acts as a leakage path for the drain-source current. layer is formed inside the HEMT device, where there is a possibility that such reduction in the degradation or breakdown voltage of the pinch-off characteristics occur, there is Zn so that the minimum value of the density in the second region is at 1 × 10 ¹⁷ cm ^-3 In this case, it is considered that the presence of Zn inhibits the action of Si as a donor element.

That is, according to the present embodiment, it is possible to obtain a semiconductor device in which the leakage current at the time of driving is reduced and the withstand voltage (element voltage) is improved.

(Example 1)
[Preparation of Zn-doped GaN single crystal substrate by flux method]
A GaN low temperature buffer layer of 30 nm is formed on the surface of a c-plane sapphire substrate having a diameter of 2 inches and a thickness of 0.43 mm at 550 ° C., and then a GaN thin film having a thickness of 3 μm is formed at 1050 ° C. by the MOCVD method. Then, a MOCVD-GaN template that can be used as a seed substrate was obtained.

Using the obtained MOCVD-GaN template as a seed substrate, a Zn-doped GaN single crystal layer was formed by using the Na flux method.

Specifically, first, a MOCVD-GaN template is placed in an alumina crucible, and then 30 g of metal Ga, 45 g of metal Na, 1 g of metallic zinc, and 100 mg of carbon are filled in the alumina crucible. bottom. The alumina crucible was placed in a heating furnace, heated at a furnace temperature of 850 ° C. and a furnace pressure of 4.5 MPa for about 100 hours, and then cooled to room temperature. When the alumina crucible was taken out from the furnace after the cooling was completed, a brown GaN single crystal layer was deposited on the surface of the MOCVD-GaN template to a thickness of about 1000 μm.

The GaN single crystal layer thus obtained is polished with diamond abrasive grains to flatten the surface thereof, and the total thickness of the nitride layer formed on the base substrate is 900 μm. bottom. As a result, a Lux-GaN template in which a GaN single crystal layer was formed on the MOCVD-GaN template was obtained. When the Lux-GaN template was visually inspected, no cracks were confirmed.

Next, the seed substrate was separated from the Lux-GaN template by irradiating the seed substrate with laser light while scanning at a scanning speed of 30 mm / sec by the laser lift-off method. As the laser light, a third harmonic of Nd: YAG having a wavelength of 355 nm was used. The pulse width was about 30 ns and the pulse period was about 50 kHz. At the time of irradiation, the laser light was condensed into a circular beam having a diameter of about 20 μm so that the light density was about 1.0 J / cm. Further, the laser beam irradiation was performed while heating the Lux-GaN template from the side opposite to the seed substrate at a temperature of about 50 ° C.

After separating the seed substrate, the surface of the obtained laminated structure on the side peeled off from the seed substrate was polished to obtain a Zn-doped GaN free-standing substrate having a total thickness of 430 μm.

The crystallinity of the obtained Zn-doped GaN substrate was evaluated using an X-ray locking curve. The half width of (0002) surface reflection was 120 seconds, and the half width of (10-12) surface reflection was 150 seconds, showing good crystallinity.

[Manufacture of epitaxial substrate by MOCVD method]
Subsequently, an epitaxial substrate was produced by the MOCVD method. Specifically, a GaN layer as a buffer layer, a GaN layer as a channel layer, and an AlGaN layer as a barrier layer were laminated and formed on the Zn-doped GaN substrate in this order according to the following conditions. In the present embodiment, the group 15/13 gas ratio is the ratio (molar ratio) of the supply amount of the group 15 (N) raw material to the supply amount of the group 13 (Ga, Al) raw material. The Al raw material gas / Group 13 raw material gas ratio is a ratio (molar ratio) of the supply amount of the Al raw material to the supply amount of the entire Group 13 (Ga, Al) raw materials.

GaN buffer layer:
Formation temperature = 1150 ° C;
Reactor pressure = 15 kPa;
Group 15 / Group 13 gas ratio = 1000;
Thickness = 600 nm.

GaN channel layer:
Formation temperature = 1050 ° C;
Reactor pressure = 15 kPa;
Group 15 / Group 13 gas ratio = 1000;
Thickness = 3000 nm.

AlGaN barrier layer:
Formation temperature = 1050 ° C;
Reactor pressure = 5 kPa;
Group 15 / Group 13 gas ratio = 12000;
Al raw material gas / Group 13 raw material gas ratio = 0.25;
Thickness = 25 nm.

After the above layers were formed, the susceptor temperature was lowered to around room temperature, the inside of the reactor was returned to atmospheric pressure, and then the produced epitaxial substrate was taken out.

[Manufacturing of HEMT element]
Next, the HEMT element 20 was manufactured using the epitaxial substrate 10. The HEMT element was designed so that the gate width was 100 μm, the source-gate interval was 1 μm, the gate-drain interval was 10 μm, and the gate length was 1 μm.

First, a photolithography process and a RIE method were used to remove the boundary portion of each element by etching to a depth of about 100 nm.

Next, a SiO ₂ film having a thickness of 100nm was formed on the epitaxial substrate, followed by the source electrode by a photolithography, a SiO ₂ film to be formed location of the drain electrode by etching away the SiO ₂ pattern layer Obtained.

Next, using a vacuum vapor deposition method and a photolithography process, a metal pattern consisting of Ti / Al / Ni / Au (each having a film thickness of 25/200/20/100 nm) is formed at the planned formation locations of the source electrode and the drain electrode. By doing so, a source electrode and a drain electrode were formed. Next, in order to improve the ohmic properties of the source electrode and the drain electrode, heat treatment was performed for 30 seconds in a nitrogen gas atmosphere at 825 ° C.

Then, a photolithography process was used _{to remove the SiO 2} film at the site where the gate electrode was to be formed from _{the SiO 2 pattern layer.}

Furthermore, by using a vacuum vapor deposition method and a photolithography process to form a Schottky metal pattern consisting of Ni / Au (each film thickness is 20/100 nm) at the planned formation location of the gate electrode, the gate electrode can be formed. Formed.

The HEMT device was obtained by the above process.

[SIMS evaluation of HEMT element]
The obtained HEMT element is subjected to elemental analysis in the depth direction by SIMS (secondary ion mass spectrometry) to determine the concentrations of Zn element and Si element in each of the AlGaN barrier layer, GaN channel layer, GaN buffer layer and GaN substrate. Examined.

FIG. 2 is a diagram showing a concentration profile of Zn element and Si element in the vicinity of the interface between the GaN buffer layer and the GaN substrate. From the results shown in FIG. 2, the following can be seen.

(1) The Zn element is doped in a ^{high concentration (1 × 10 19} cm ^{-3) on the GaN substrate.}

(2) A first region RE1 composed of a GaN substrate and a GaN buffer layer, and a second region RE2 in which ^{Si elements are present at a high concentration of 1 × 10 17} cm ^{-3 or more is formed in the vicinity of the interface between the two layers.} The peak concentration of Si element is 6 × 10 ¹⁸ cm ^-3 .

(3) In the GaN buffer layer, the Zn concentration gradually decreases as compared with the Si concentration. That is, in the GaN buffer layer, the Zn element is more significantly diffused than the Si element.

(4) The minimum value of the Zn concentration in the second region RE2 is 5.3 × 10 ¹⁷ cm ^-3 (≧ 1 × 10 ¹⁷ cm ^-3 ).

[Evaluation of electrical characteristics of HEMT elements]
The drain current-drain voltage characteristic (Id-Vd characteristic) of the HEMT element was evaluated in the DC mode using a semiconductor parameter analyzer. The pinch-off threshold voltage was Vg = -3V.

As an index for evaluating the drain current leak amount at the time of pinch-off, the drain voltage Vd = 10V and the drain current Id _{Vd = 10V · Vg = -10V} when the gate voltage Vg = -10V is applied are adopted, and this embodiment. When this was obtained for the HEMT element of the above, it was ^{3 × 10 -7 A.} Id _{Vd = 10V · Vg = -10V} smaller the desirable, if normalized value per a _{Id Vd = 10V · Vg = -10V} ≦ 1 × 10 -5 A / mm at a gate width, the drain current leakage amount is small However, in the case of the HEMT element of this embodiment, the drain current leak amount normalized by the gate width of 100 μm is 3 × 10 ^-6 A / mm, so that it is judged to be sufficiently small.

Subsequently, the device withstand voltage was measured. As an index for evaluating the device withstand voltage, when the drain voltage Vd is gradually increased from 0 V while the gate voltage Vg = -10 V is applied, the drain current Id is 1 × 10 ^-5 A (at a gate width of 100 μm). When standardized, it was ^{decided to adopt a drain voltage Vdb exceeding 1 × 10 -4} A / mm for the first time, and when this was obtained for the HEMT element of this embodiment, it was 850 V. The larger Vdb is, the more desirable it is. If Vdb ≧ 300V, it can be determined that the element withstand voltage is sufficient. Therefore, it is determined that the element withstand voltage of the HEMT element of this embodiment is extremely large.

(Comparative Example 1)
The HEMT device was manufactured under the same conditions as in Example 1 except that the growth conditions of the GaN buffer layer were the following conditions different from those in Example 1.

GaN buffer layer:
Formation temperature = 1050 ° C;
Reactor pressure = 15 kPa;
Group 15 / Group 13 gas ratio = 1000;
Thickness = 600 nm.

FIG. 3 shows the concentration profiles of Zn and Si elements near the interface between the GaN buffer layer and the GaN substrate, which were obtained by performing SIMS measurement on the obtained HEMT element under the same conditions as in Example 1. From the results shown in FIG. 3, the following can be seen.

(1) Similar to Example 1, the Zn element is heavily doped in the GaN substrate.

(2) Similar to Example 1, the first region RE1 composed of the GaN substrate and the GaN buffer layer, and the second region RE2 is formed in the vicinity of the interface between the two layers.

(3) Unlike Example 1, the Zn concentration in the GaN buffer layer decreases relatively sharply as compared with the Si concentration. That is, the diffusion of the Zn element in the GaN buffer layer is suppressed more than the diffusion of the Si element.

(4) The minimum value of the Zn concentration in the second region RE2 is 1.7 × 10 ¹⁵ cm ^-3 (<1 × 10 ¹⁷ cm ^-3 ).

_{When Id Vd = 10V · Vg = -10V} was obtained for the HEMT element under the same conditions as in Example 1, it was 8 × 10 ^{-5 A (8 × 10 -4} A / mm when standardized with a gate width of 100 μm). became. That is, it was found that the drain current leak amount was large, and the HEMT element according to this comparative example did not have sufficient pinch-off characteristics.

Further, when Vdb was obtained under the same conditions as in Example 1, it was 100 V, and a sufficient element withstand voltage could not be obtained.

(Examples 2 to 6, Comparative Examples 2 to 3)
The HEMT device was manufactured under the same conditions as in Example 1 except that the growth conditions (growth temperature, pressure in the reactor, group 15/13 gas ratio, formation thickness) of the GaN buffer layer were different. For the obtained HEMT device, the distribution of Zn concentration and Si concentration in the depth direction was obtained by SIMS measurement, and Id _{Vd = 10V · Vg = -10V} and Vdb were measured.

A list of the obtained results is shown in Table 1 together with the results of Example 1 and Comparative Example 1.

As shown in Table 1, in the case of Examples 1 to 6 prepared under the condition that the minimum value of the Zn concentration in the region RE2 is 1 × 10 ¹⁷ cm ^{-3 or more, the drain current leakage amount is small (} _{id Vd = 10V · Vg = -10V} ≦ 1 × 10 -5 a / mm), and was able to device breakdown voltage to obtain a large (Vdb ≧ 300V) HEMT device. On the other hand, in the case of Comparative Examples 1 to 3 produced under the condition that the minimum value of the Zn concentration in the region RE2 is ^{less than 1 × 10 17} cm ^-3 , the drain current leakage amount is large and the device withstand voltage is high. Only small HEMT devices were obtained.

(Example 7)
The epitaxial substrate 10 and the HEMT element 20 were manufactured under the same conditions as in Example 1 except that the growth conditions of the buffer layer 2 and the channel layer 3 were set to the following conditions different from those in Example 1. Of these, when forming the buffer layer 2, the formation conditions were set in two stages, the first condition and the second condition, and the first condition was switched to the second condition during the formation. This is a composition in which the buffer layer 2 is _{a multilayer buffer layer in which a GaN layer is laminated on an Al a} Ga _1-a N layer (0 <a ≦ 1), or the abundance ratios of Al and Ga in the thickness direction are different. It is intended to be formed as a sloping buffer layer. The total thickness of the buffer layer 2 was set to 110 nm.

Buffer layer (first condition):
Formation temperature = 1050 ° C;
Reactor internal pressure = 5 kPa;
Group 13 raw material gas = Al raw material and Ga raw material;
Group 15 / Group 13 gas ratio = 2000;
Al raw material gas / Group 13 raw material gas ratio = 0.03;
Growth rate = 1 nm / sec;
Growth time = 10 seconds.

Buffer layer (second condition):
Formation temperature = 1050 ° C;
Reactor internal pressure = 10 kPa;
Group 13 raw material gas = Ga raw material;
Group 15 / Group 13 gas ratio = 500;
Growth rate = 1 nm / sec;
Growth time = 100 seconds.

GaN channel layer:
Formation temperature = 1050 ° C;
Reactor internal pressure = 100 kPa;
Group 15 / Group 13 gas ratio = 2000;
Thickness = 900 nm.

FIG. 4 shows the Zn element in the depth direction from the surface (upper surface) of the barrier layer 4 obtained by measuring the obtained HEMT element in the depth direction by SIMS measurement under the same conditions as in Example 1. , The concentration profile of the Si element, and the secondary ion signal profile of the Al element in the depth direction (the distribution of the secondary ion counting rate of the Al element in the depth direction). From the results shown in FIG. 4, the following can be seen.

(1) The Zn element is heavily doped in the GaN substrate.

(2) A first region RE1 composed of a GaN substrate and a buffer layer, and a second region RE2 in which ^{Si elements are present at a high concentration of 1 × 10 17} cm ^{-3 or more is formed in the vicinity of the interface between the two layers.} The peak concentration of Si element is 3 × 10 ¹⁸ cm ^-3 .

(3) The Si concentration has a peak in the second region RE2 and decreases sharply as it approaches the channel layer, whereas the Zn concentration decreases slowly from the buffer layer to the channel layer. That is, the Zn element is more significantly diffused than the Si element. Specifically, the Zn element is diffused in the channel layer from the interface with the buffer layer (the interface with the first region RE1) to the range of 200 to 250 nm.

(4) The minimum value of the Zn concentration in the second region RE2 is 5.3 × 10 ¹⁷ cm ^-3 (≧ 1 × 10 ¹⁷ cm ^-3 ).

(5) The Al element exists in a range wider than the target thickness of 110 nm of the entire buffer layer, and a part of the GaN substrate is also included in the range.

_{When Id Vd = 10V · Vg = -10V} was determined for the HEMT element under the same conditions as in Example 1, 8 × 10 ^{-8 A (8 × 10 -7} A / mm when standardized with a gate width of 100 μm). Met. That is, it was found that the drain current leakage amount was small and the HEMT device according to this embodiment had good pinch-off characteristics.

Further, when Vdb was obtained under the same conditions as in Example 1, it was 1200 V, and a sufficient element withstand voltage was obtained.

(Example 8)
The HEMT element 20 was manufactured under the same conditions as in Example 7, except that the growth conditions for the buffer layer 2 and the channel layer 3 were the following conditions different from those in Example 7. That is, also in this embodiment, when forming the buffer layer 2, the formation conditions are set in two stages of the first condition and the second condition, and the first condition is switched to the second condition during the formation. Further, the total thickness of the buffer layer 2 was set to 350 nm.

Buffer layer (first condition):
Formation temperature = 1050 ° C;
Reactor internal pressure = 5 kPa;
Group 13 raw material gas = Al raw material and Ga raw material;
Group 15 / Group 13 gas ratio = 2000;
Al raw material gas / Group 13 raw material gas ratio = 0.01;
Growth rate = 1 nm / sec;
Growth time = 50 seconds.

Buffer layer (second condition):
Formation temperature = 1050 ° C;
Reactor internal pressure = 10 kPa;
Group 15 / Group 13 gas ratio = 500;
Growth rate = 1 nm / sec;
Growth time = 300 seconds.

GaN channel layer:
Formation temperature = 1050 ° C;
Reactor internal pressure = 100 kPa;
Group 15 / Group 13 gas ratio = 2000;
Thickness = 1700 nm.

With respect to the obtained HEMT element, the concentration profiles of Zn element and Si element in the depth direction from the surface (upper surface) of the barrier layer 4 obtained by performing SIMS measurement under the same conditions as in Example 1, and the depth thereof. From the secondary ion signal profile of the Al element in the vertical direction, the following was found.

(1) The Zn element is doped in a ^{high concentration (1 × 10 19} cm ^{-3) on the GaN substrate.}

(2) A first region RE1 composed of a GaN substrate and a buffer layer, and a second region RE2 in which ^{Si elements are present at a high concentration of 1 × 10 17} cm ^{-3 or more is formed in the vicinity of the interface between the two layers.} The peak concentration of Si element is 4 × 10 ¹⁸ cm ^-3 .

(3) The Si concentration has a peak in the second region RE2 and decreases sharply as it approaches the channel layer, whereas the Zn concentration decreases slowly from the buffer layer to the channel layer. That is, the Zn element is more significantly diffused than the Si element.

(4) The minimum value of the Zn concentration in the second region RE2 is 8.2 × 10 ¹⁷ cm ^-3 (≧ 1 × 10 ¹⁷ cm ^-3 ).

(5) The Al element exists in a range wider than the target thickness of 350 nm of the entire buffer layer, and a part of the GaN substrate is also included in the range.

_{When Id Vd = 10V · Vg = -10V} was obtained for the HEMT element under the same conditions as in Example 1, it was 2 × 10 ^{-7 A (2 × 10 -6} A / mm when standardized with a gate width of 100 μm). there were. That is, it was found that the drain current leakage amount was small and the HEMT device according to this embodiment had good pinch-off characteristics.

Further, when Vdb was obtained under the same conditions as in Example 1, it was 1050 V, and a sufficient element withstand voltage was obtained.

Claims (6)

A semi-insulating self-supporting substrate made of Zn-doped GaN,
A buffer layer made of Group 13 nitride adjacent to the free-standing substrate and
A channel layer made of Group 13 nitride adjacent to the buffer layer and
A barrier layer made of Group 13 nitride, which is provided on the side opposite to the buffer layer with the channel layer interposed therebetween.
With
A part of the first region composed of the free-standing substrate and the buffer layer is a ^{second region containing Si at a concentration of 1 × 10 17} cm ^-3 or more, and the second region is the free-standing substrate. It is a diffusion region of Zn from the self-supporting substrate containing Zn at a concentration of 1 × 10 ¹⁷ cm ^-3 or more, which exists including the interface of the buffer layer.
An epitaxial substrate for a semiconductor element. The epitaxial substrate for a semiconductor device according to claim 1.
The buffer layer is made of GaN and is made of GaN.
The channel layer is made of GaN
The barrier layer is made of AlGaN.
An epitaxial substrate for a semiconductor element. The epitaxial substrate for a semiconductor device according to claim 1.
The buffer layer is composed of a multilayer buffer layer in which two or more Group 13 nitride layers having different compositions are laminated or a Group 13 nitride containing two or more Group 13 elements, and the abundance ratio of the Group 13 elements is in the thickness direction. Is a composition gradient buffer layer that changes in
The channel layer is made of GaN
The barrier layer is made of AlGaN.
An epitaxial substrate for a semiconductor element. A semi-insulating self-supporting substrate made of Zn-doped GaN,
A buffer layer made of Group 13 nitride adjacent to the free-standing substrate and
A channel layer made of Group 13 nitride adjacent to the buffer layer and
A barrier layer made of Group 13 nitride, which is provided on the side opposite to the buffer layer with the channel layer interposed therebetween.
A gate electrode, a source electrode, and a drain electrode provided on the barrier layer, and
With
A part of the first region composed of the free-standing substrate and the buffer layer is a ^{second region containing Si at a concentration of 1 × 10 17} cm ^-3 or more, and the second region is the free-standing substrate. It is a diffusion region of Zn from the self-supporting substrate containing Zn at a concentration of 1 × 10 ¹⁷ cm ^-3 or more, which exists including the interface of the buffer layer.
A semiconductor device characterized by this. The semiconductor device according to claim 4.
The buffer layer is made of GaN and is made of GaN.
The channel layer is made of GaN
The barrier layer is made of AlGaN.
A semiconductor device characterized by this. The semiconductor device according to claim 4.
The buffer layer is composed of a multilayer buffer layer in which two or more Group 13 nitride layers having different compositions are laminated or a Group 13 nitride containing two or more Group 13 elements, and the abundance ratio of the Group 13 elements is in the thickness direction. Is a composition gradient buffer layer that changes in
The channel layer is made of GaN
The barrier layer is made of AlGaN.
A semiconductor device characterized by this.

Images