JP6087414B2 - Manufacturing method of nitride semiconductor epitaxial wafer for transistor - Google Patents

Description

  The present invention relates to a method for manufacturing a nitride semiconductor epitaxial wafer for a transistor.

  As a conventional technique, nitride semiconductors containing indium (In), gallium (Ga), aluminum (Al), nitrogen (N), etc. are visible from ultraviolet light by controlling the composition ratio of these group III elements. Development has been promoted and put to practical use as a material for innovative high-efficiency light-emitting devices that cover most areas of light.

  Nitride semiconductors also have high saturation electron velocities and high breakdown voltage, so in the future, they are expected to be used as dream electronic device materials that achieve orders of magnitude higher efficiency and higher output in the high frequency range. ing.

  In Patent Document 1, in order to obtain an internal electric field of a nitride semiconductor in a non-destructive and non-contact manner, the nitride semiconductor is irradiated with sound waves to generate piezoelectric polarization, and the nitride semiconductor is irradiated with probe light. A nitride semiconductor evaluation method is disclosed in which the modulation spectrum of probe light reflected or transmitted at is measured and the internal electric field of the nitride semiconductor is determined based on the period of the Franz-Keldish oscillation appearing in the modulation spectrum.

  Patent Document 2 discloses a group III nitride-based field effect transistor in which a dielectric layer having a high proportion of donor electrons is formed on an AlGaN barrier layer in order to neutralize traps in the AlGaN barrier layer. .

JP 2009-4706 A JP-T-2004-517461

  A nitride semiconductor field effect transistor has a structure in which a source electrode, a drain electrode, and a gate electrode are provided on a nitride semiconductor epitaxial wafer. When a high frequency signal is sent to the transistor and a voltage is applied to the drain electrode, the nitride semiconductor field effect transistor is supplied from the source electrode. Electrons flow through the channel layer in the nitride semiconductor epitaxial wafer and flow to the drain electrode, thereby driving at a high output. Further, by applying a voltage to the gate electrode, a current flowing between the source and drain of the wafer (hereinafter also referred to as “Ids”) is controlled.

  A field effect transistor is a device that drives a high output by sending a current to a drain electrode by sending a high-frequency signal. However, even if the signal is stopped and a high output drive is not performed (idle state), The drain current of is flowing. Although it depends on the specifications of various transistors, a drain current of about 50 mA / mm always flows even in an idle state.

  In the conventional nitride semiconductor field effect transistor, the Ids value immediately after switching from the high output state to the idle state (that is, immediately after stopping the signal from the state in which the high frequency signal is transmitted) is the Ids value of the idle state before the high output ( In other words, there is a problem that the Ids value is lower than a prescribed idle state. Since the fluctuation of the Ids value in the idle state immediately after the high output also affects the gain of the transistor, it is necessary to reduce the decrease rate of the Ids value in the idle state immediately after the high output and stabilize the Ids value in the idle state. is there.

  Accordingly, an object of the present invention is to provide a method of manufacturing a nitride semiconductor epitaxial wafer for a nitride semiconductor field effect transistor that stabilizes the Ids value in an idle state before and after high-power driving and suppresses fluctuations in gain. It is in.

  In order to achieve the above object, one embodiment of the present invention provides the following method for manufacturing a nitride semiconductor epitaxial wafer for a transistor.

[1] a step of cleaning the surface of a substrate made of silicon carbide ;
Placing the substrate having a cleaned surface in an ammonia gas atmosphere at a first temperature of 1100 to 1200 ° C. for 30 seconds or less; and
Epitaxially growing a nitride semiconductor layer having a surface shape with a positive skewness Rsk while heating at a second temperature on the substrate;
Cooling the epitaxial wafer on which the nitride semiconductor layer is formed to a third temperature lower than the second temperature while maintaining a gas mixed atmosphere of H ₂ / NH ₃ ratio ≦ 4;
Have a epitaxially growing a GaN layer on the nitride semiconductor layer,
A method for producing a nitride semiconductor epitaxial wafer for a transistor , wherein the photoresponse characteristic of the GaN layer is 0.1 or more in a state where no electron supply layer is formed on the GaN layer .
However, in this case, the light response characteristic means that when the measurement atmosphere of the lateral current value of the GaN layer is changed from a light non-irradiated state to a light irradiated state with a photon energy of 1.98 eV, It is represented by the ratio of the lateral current value in the light irradiation state (1.98 eV) to the directional current value.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the nitride semiconductor epitaxial wafer for nitride semiconductor field effect transistors which stabilized the Ids value of the idle state before and behind high output drive, and suppressed the fluctuation | variation of the gain can be provided. .

FIG. 1 is a cross-sectional view showing a schematic configuration example of a nitride semiconductor field effect transistor according to a second embodiment of the present invention. FIG. 2 shows the elapsed time after the signal stop of the nitride semiconductor field effect transistors according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention, and the Ids in the idle state after high output (after signal stop). It is a graph which shows the relationship with a source-drain current) value rate. FIG. 3 is a graph showing optical response characteristics of the GaN layer of the nitride semiconductor field effect transistors according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention (Ids value in the 1.98 eV light irradiation state / light non-irradiation state). It is a graph which shows the relationship between the Ids (source-drain current) value decreasing rate immediately after the high output of a nitride semiconductor field effect transistor (at the time of zero seconds after signal stop). FIG. 4 is a graph showing a measurement result of a source-drain current (Ids) of the nitride semiconductor field effect transistor according to Example 1 of the present invention. FIG. 5 is a graph showing measurement results of source-drain current (Ids) of the nitride semiconductor field effect transistor of Comparative Example 1. FIG. 6 is a graph showing measurement results of source-drain current (Ids) of the nitride semiconductor field effect transistor of Comparative Example 2.

[First Embodiment]
The nitride semiconductor epitaxial wafer for a transistor according to the first embodiment of the present invention is a nitride semiconductor epitaxial wafer for a transistor in which a nitride semiconductor layer and a GaN layer serving as an electron transit layer are epitaxially grown in this order on the substrate. And the optical response characteristic of the said GaN layer is 0.1 or more.

  However, in this case, the light response characteristic is that the measurement atmosphere of the lateral current value of the GaN layer is changed from a light non-irradiation state (hereinafter also referred to as “dark state”) to a light irradiation state with a photon energy of 1.98 eV. The ratio of the lateral current value in the light irradiation state (1.98 eV) to the lateral current value in the light unirradiated state.

  When measuring the lateral current of the GaN layer of the nitride semiconductor epitaxial wafer for a transistor, the measurement is performed without forming the AlGaN layer. In the measurement in this state, since the two-dimensional electron gas induced by the piezoelectric effect of the AlGaN layer is not generated, only the leak component of the GaN layer can be measured.

  We have repeatedly studied nitride semiconductor epitaxial wafers for transistors. As a result, the optical response characteristics of the GaN layer and the field effect transistor fabricated using this nitride semiconductor epitaxial wafer for transistors are immediately after high output (at zero seconds after signal stop). The field effect transistor is composed of a nitride semiconductor epitaxial wafer for a transistor having a GaN layer having a photoresponse characteristic of 0.1 or more of the GaN layer serving as an electron transit layer. The Ids value decrease rate immediately after the high output is small, the Ids value in the idle state before and after the high output drive is stabilized, and the field effect transistor is obtained in which the gain fluctuation is suppressed.

[Second Embodiment]
The nitride semiconductor field effect transistor according to the second embodiment of the present invention includes a nitride semiconductor layer, a GaN layer functioning as an electron transit layer, and an AlGaN layer functioning as an electron supply layer in this order on a substrate. In a field effect transistor in which a source electrode, a drain electrode and a gate electrode are formed on the AlGaN layer by epitaxial growth, the transistor is in an on-wafer state in which it is not packaged, and the gate electrode is negatively biased to The optical response characteristic of the GaN layer in the off state is 0.1 or more.
However, the light response characteristic in this case means that the measurement atmosphere of the current value between the source electrode and the drain electrode is a light non-irradiation state when the light non-irradiation state is changed to a light irradiation state with a photon energy of 1.98 eV. Is expressed as a ratio of the current value between the source electrode and the drain electrode in the light irradiation state (1.98 eV) to the current value between the source electrode and the drain electrode.

  When measuring the source-drain current of a field effect transistor, the measurement is performed in an on-wafer state where the transistor is not packaged and the transistor is off. In this state of measurement, when the gate voltage is applied negatively, the two-dimensional electron gas region is depleted and there is no electron in this part, so it is possible to measure only the leakage component of the GaN layer. is there.

  As a result of repeated studies on nitride semiconductor field effect transistors, the optical response characteristics of the GaN layer functioning as an electron transit layer, and the Ids value decrease rate immediately after high output of the nitride semiconductor field effect transistor (at zero seconds after signal stop) When the optical response characteristic of the GaN layer is 0.1 or more, the Ids value decrease rate immediately after the high output is small, and the Ids value in the idle state before and after the high output drive is stabilized, Gain fluctuation can be suppressed.

  In order to evaluate the optical response characteristics of the GaN layer, in the case of the nitride semiconductor epitaxial wafer for transistors according to the first embodiment, the lateral current of the GaN layer is measured and the electric field according to the second embodiment is measured. In the case of the effect transistor, the current between the source and the drain is measured, but both measure only the leakage component of the GaN layer in the state where the two-dimensional electron gas is not generated, which is a similar index.

  FIG. 1 shows a schematic configuration example of a nitride semiconductor field effect transistor according to a second embodiment of the present invention.

  The nitride semiconductor field effect transistor 100 is a GaN-based high electron mobility transistor (HEMT), and has, for example, a silicon carbide (SiC) substrate as the substrate 101, and is formed on the substrate 101 made of SiC. An epitaxial growth of an aluminum nitride (AlN) layer as the nitride semiconductor layer 102, a gallium nitride (GaN) layer 103 that functions as an electron transit layer, and an aluminum gallium nitride (AlGaN) layer 104 that functions as an electron supply layer in this order The source electrode 106, the drain electrode 107, and the gate electrode 108 are formed on the AlGaN layer 104.

  As the SiC substrate, a polytype 4H or polytype 6H semi-insulating SiC substrate is used. Here, the numbers 4H and 6H indicate the repetition period in the c-axis direction, and H indicates a hexagonal crystal. The substrate 101 is preferably a semi-insulating SiC substrate in order to reduce parasitic capacitance and obtain good high frequency characteristics, but may be a conductive SiC substrate, sapphire substrate, silicon substrate, GaN substrate, or the like.

  The nitride semiconductor layer 102 is formed of, for example, aluminum nitride (AlN), has a shape of a surface 102a with a positive skewness Rsk, and functions as a nucleation layer. The skewness Rsk indicates a surface roughness curve, and is a physical quantity (anonymous number) obtained by dividing the cube average of the height deviation Z (x) at the reference length by the cube of the root mean square. The shape of the surface 102a having a positive skewness Rsk indicates that there is a sharply raised convex portion, and the shape of the surface 102a having a negative skewness Rsk indicates that there is a concave portion that is sharply depressed below. ing. In order to reduce the decrease rate of the Ids value immediately after high output of the nitride semiconductor field effect transistor, the skewness Rsk of the surface 102a of the nitride semiconductor layer 102 is preferably positive, and more preferably 0.5 or more.

  The GaN layer 103 functions as an electron transit layer. On the AlGaN layer 104 side of the GaN layer 103, a two-dimensional electron gas generated by a piezo effect in the AlGaN layer 104 due to a difference in lattice constant between the GaN layer 103 and the AlGaN layer 104 (an effect of generating an electric field by distorting the crystal). 105 exists.

  The AlGaN layer 104 functions as an electron supply layer and induces a piezo effect in the GaN layer 103.

  The source electrode 106 has, for example, a multilayer structure of titanium and aluminum. The drain electrode 107 has a multilayer structure of titanium and aluminum, for example. The gate electrode 108 has a multilayer structure of nickel and gold, for example.

(Control of skewness of nitride semiconductor layer surface)
It is considered that the skewness of the surface 102a of the nitride semiconductor layer 102 is related to the supply amount molar ratio (V / III ratio) of the group V material and the group III material during the growth of the nitride semiconductor layer 102. In order to make the skewness Rsk positive, the V / III ratio is preferably 1000 to 8000.

  For example, the case where the nitride semiconductor layer 102 is formed of aluminum nitride (AlN) will be described. Al atoms decomposed from trimethylaluminum (TMA) supplied to the growth surface or substrate surface (hereinafter referred to as “growth surface”) react with N atoms on the growth surface to form AlN. In the initial growth stage of the AlN layer, AlN crystals grow on the surface of the substrate 101. When the V / III ratio is in the range of 1000 to 8000, Al atoms decomposed from TMA supplied to the growth surface are in a state of being easily moved on the surface of the growth surface. Easily moving on the growth surface is considered to be a state where the AlN crystal that is the starting point of growth is reached and the growth is easy to proceed. In this case, since the AlN crystal formed in the initial stage of growth grows from the starting point so that the AlN crystal grows in a convex shape, the skewness of the formed AlN layer becomes positive.

  When the AlN layer is grown at a V / III ratio higher than 8000, for example 10,000, it is considered that the process proceeds in the same manner as described above until an AlN crystal is formed on the growth surface in the initial stage of growth. Since the / III ratio is as high as 10,000, the movement of Al atoms on the growth surface is suppressed. As a result, when AlN crystals are uniformly formed on the surface of the growth surface and the growth proceeds, a plurality of peaks made of AlN crystals are formed on the growth surface. Then, grain boundaries (boundary: generated when a plurality of flat peaks made of AlN are combined) and defect portions (portions in which crystal growth is inhibited) remain during the growth of the AlN layer. As a result, AlN It is considered that the skewness of the layer becomes negative.

(Quality of GaN layer)
In the case of an epitaxial wafer, the GaN layer 103 has an optical response characteristic in a state where the AlGaN layer 104 is not laminated, or in the case of a transistor, in an unpackaged on-wafer state and in a state where the transistor is off. It is 0.1 or more. The closer the photoresponse characteristic is to 1, the better the characteristic. In order for the GaN layer 103 to have such a light response characteristic, it is necessary to control the growth conditions of the GaN layer 103 described later.

(Manufacturing method of embodiment)
Next, an example of a method for manufacturing the nitride semiconductor field effect transistor 100 will be described.

(1) Cleaning of substrate surface The substrate 101 is heated in a hydrogen (H ₂ ) atmosphere not containing ammonia (NH ₃ ) at a predetermined temperature, for example, 1100 to 1200 ° C., for a predetermined time (for example, 5 to 10 minutes). put. By this heat treatment, the surface of the substrate 101 is cleaned. The atmosphere may be a mixed gas (H ₂ / N ₂ ) of hydrogen (H ₂ ) and nitrogen (N ₂ ).

(2) NH ₃ gas treatment step Then, NH ₃ gas treatment where the substrate is placed 101 NH ₃ above gas atmosphere predetermined temperature (first temperature), for example 1100 to 1200 ° C. for 30 seconds or less while maintaining Perform the process. In the NH ₃ gas treatment process, the quality of the nitride semiconductor layer 102 is improved by supplying nitrogen atoms that are easily desorbed in the subsequent formation step of the nitride semiconductor layer 102. When the processing time of the NH ₃ gas processing step exceeds 30 seconds, for example, when the substrate 101 is formed of SiC, a SiNx layer is remarkably formed on the surface, and attention is paid to the formation of the nitride semiconductor layer 102. is necessary.

(3) Formation Step of Nitride Semiconductor Layer Next, the nitride semiconductor layer 102 has a V / III ratio during growth of 1000 to 8000 in order to form a surface shape with a positive skewness Rsk. The growth temperature of the nitride semiconductor layer 102 is set to a temperature higher than the growth temperature of the GaN layer 103, for example, 1000 ° C. to 1100 ° C. (second temperature), for example, 1100 ° C. to 1200 ° C.

(4) Cooling Step of Epitaxial Wafer Next, a predetermined temperature (third temperature) lower than the second temperature in a H ₂ / NH ₃ gas atmosphere with an H ₂ / NH ₃ ratio ≦ 4, for example, 1000 ° C. to 1100 Cool the epitaxial wafer to 0C. As for the cooling condition of the epitaxial wafer, when the H ₂ gas fraction exceeds 80% (that is, when the H ₂ / NH ₃ ratio> 4), the nitride semiconductor layer 102 is gradually etched back to maintain the skewness Rsk value. Care must be taken because it will not be possible.

(5) Formation of GaN layer and AlGaN layer Next, the GaN layer 103 is formed on the nitride semiconductor layer 102, and the AlGaN layer 104 is formed on the GaN layer 103. Through the above steps, a nitride semiconductor epitaxial wafer for GaN-based HEMT is formed. The growth temperature (third temperature) of the GaN layer 103 and the AlGaN layer 104 is, for example, 1000 ° C. to 1100 ° C.

(6) Formation of electrodes Next, the source electrode 106, the drain electrode 107, and the gate electrode 108 are formed on the AlGaN layer 104. An element isolation trench is formed around the HEMT element by dry etching. Note that element isolation may be performed by providing an insulating region by ion implantation. The nitride semiconductor field effect transistor (GaN-based HEMT) 100 is formed through the above steps.

In controlling the quality of the GaN layer 103, among the above steps, (2) NH ₃ gas treatment step, (3) nitride semiconductor layer formation step, and (4) epitaxial wafer cooling step are particularly important. It is.

(Manufacturing method of Example 1)
Next, an example of the manufacturing method of the nitride semiconductor field effect transistor according to the first embodiment of the present invention will be described. First, a polytype 4H or polytype 6H semi-insulating SiC substrate is prepared as the substrate 101.

Next, the SiC substrate is introduced into a metal organic chemical vapor deposition (MOCVD) apparatus, and is set in a H ₂ / N ₂ mixed gas flow atmosphere containing no NH ₃ at a set temperature of 1175 ° C. for 5 minutes. Heat treatment. This heating cleans the surface of the SiC substrate.

Next, ammonia gas is introduced into a reactor, which is a reaction furnace of the MOCVD apparatus, for 15 seconds under the condition of an H ₂ / NH ₃ ratio of 1 while maintaining the temperature at 1175 ° C. (NH ₃ gas treatment process).

  Next, ammonia gas and trimethylaluminum (TMA) are used as raw materials for the nitride semiconductor layer 102 while keeping the temperature at 1175 ° C. (second temperature) so that the V / III ratio becomes 5000. And a 30 nm-thickness AlN layer having a surface shape with a positive skewness Rsk is formed (nitride semiconductor layer forming step).

Next, the reactor of the MOCVD apparatus is set to an H ₂ / NH ₃ ratio of 3 and cooled to a set temperature of 1050 ° C. (third temperature) in an H ₂ / NH ₃ gas mixed atmosphere (epitaxial wafer cooling step). .

  Next, a GaN layer 103 having a thickness of 1200 nm is formed at a temperature of 1050 ° C. (third temperature) using ammonia gas and trimethylgallium (TMG) as raw materials.

  Next, the same MOCVD apparatus is used on the GaN layer 103 to form an AlGaN layer 104 having a film thickness of 30 nm and an Al composition of 18%, for example, at a temperature of 1050 ° C. using ammonia gas, TMA, and TMG. To do. Through the above steps, a nitride semiconductor epitaxial wafer for GaN-based HEMT is formed.

  A source electrode 106, a drain electrode 107, and a gate electrode 108 are formed on the AlGaN layer 104 by using a photolithography technique. The nitride semiconductor field effect transistor (GaN-based HEMT) 100 is formed through the above steps.

  Next, characteristic evaluation of the nitride semiconductor field effect transistor formed as described above will be described with reference to FIGS.

  FIG. 2 shows signals of nitride semiconductor field effect transistors according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention (details of Examples 2 and 3 and Comparative Examples 1 to 3 will be described later). It is a graph which shows the relationship between the elapsed time after a stop, and the Ids value rate after a high output (after a signal stop). In this figure, the y-axis is obtained by dividing the Ids value in the idle state after high output of the nitride semiconductor field effect transistor (after signal stop) by the specified Ids value in the idle state (Ids value in the idle state before high output). Value, that is, the Ids value rate after high output (after signal stop), and the elapsed time after the x-axis is stopped from the signal, that is, the elapsed time after switching from the high output state to the idle state. From this figure, it can be seen that when the transistor is switched from the high output state to the idle state, the Ids value rate decreases especially immediately after the high output (at the time of zero seconds after the signal is stopped).

  FIG. 3 shows the optical response characteristics of the nitride semiconductor field effect transistors according to Examples 1 to 3 and Comparative Examples 1 to 3 of the present invention and the Ids value decrease rate immediately after high output (at zero seconds after signal stop). It is a graph which shows a relationship. In the figure, the y-axis represents the Ids value decrease rate (= 1-Ids value rate) immediately after high output (at the time of zero seconds after signal stop), the x-axis represents the photoresponse characteristics of the GaN layer, and the field effect transistor high The correlation between the Ids value decrease rate immediately after output and the photoresponse characteristics of the GaN layer is shown.

  4 to 6 show measurement results of Ids values of the GaN layers of Examples and Comparative Examples. Ids values at an applied voltage of 1 V to 100 V in a state in which no light is irradiated onto the GaN layer and light having a photon energy of 1.98 eV. It is a graph of the Ids value in the applied voltages 1V-100V when changing to an irradiation state. The ratio of the Ids value in the light irradiation state to the Ids value in the dark state is the light response characteristic, and the x axis in FIG. 3 shows the light response characteristic at the applied voltage of 20V. The light response characteristic is represented by (1.9s evidence value in light irradiation state) / (ids value in light non-irradiation state) at each applied voltage as described above.

  It can be said that the Ids value decrease rate indicated by the y-axis in FIG. 3 is a good field effect transistor in which the Ids value in the idle state before and after high output is more stable as the value is smaller, and the gain variation is suppressed. .

  In the experiment leading to the present invention, as shown in FIG. 3, the optical response due to light irradiation with photon energy of 1.98 eV of the off-current flowing through the GaN layer has a correlation with the Ids value decrease rate immediately after the high output of the field effect transistor. I discovered that. That is, as can be seen from FIG. 3, when the optical response characteristic of the GaN layer is as good as 0.1 or more, the Ids value decrease rate immediately after the high output of the field effect transistor is reduced, and the idle state before and after the high output drive is reduced. It has been found that the Ids value of can be stabilized.

  This phenomenon is caused by irradiating light having a sufficiently low energy with respect to a band gap of 3.39 eV of GaN, and electrons are excited from the valence band to immediately charge deep level electron traps. This is considered to be due to the effect that the flow of a minute current flowing is inhibited. Since there is some causal relationship between the electron trap and the Ids value decrease rate immediately after the high output of the field effect transistor, it is understood that the above correlation is obtained.

  In this embodiment, when the Ids value decrease rate immediately after the high output with respect to the Ids value in the specified idle state is 0.3 or less, the Ids value in the idle state before and after the high output is stable, and the field effect transistor Judge that the fluctuation of the gain is suppressed.

  The characteristics of the field effect transistor manufactured in Example 1 will be described. FIG. 4 shows the Ids value measurement result of the GaN layer of Example 1. That is, FIG. 4 shows the current value between the source and the drain when the nitride semiconductor field effect transistor 100 is in an on-wafer state in which the gate electrode 108 is negatively biased and the transistor is in an off state. The measurement atmosphere is changed from a dark state to a light irradiation state (1.98 eV).

  The photoresponse characteristics, which is the ratio of the Ids value in the light irradiation state (1.98 eV) to the Ids value in the dark state, is 0.1 or more in all of the source-drain voltage 1 V to 100 V, and the source-drain voltage It was 0.52 when (hereinafter also referred to as “Vds”) was 20V.

  As shown in FIGS. 2 and 3, the transistor manufactured in Example 1 has an Ids value rate after high output of 0.92 at zero seconds after signal stop, that is, an Ids value decrease rate immediately after high output is 0. It can be seen that immediately after the high output, an amount of current that is not so different from the specified Ids amount in the idle state (the Ids amount in the idle state before the high output) flows.

Example 2 is a nitride semiconductor according to the same method as in Example 1 except that the H ₂ / NH ₃ ratio of the H ₂ / NH ₃ mixed gas was set to 4 in the cooling process of the epitaxial wafer after the AlN layer was formed. A field effect transistor (GaN-based HEMT) 100 was manufactured. Similar to the first embodiment, the skewness Rsk on the surface of the AlN layer is positively controlled.

  As in Example 1, when the gate electrode 108 is negatively biased in the on-wafer state and the transistor is in the off state, the current value between the source and the drain is measured from the dark state to the light irradiation state (1.98 eV). Measurements were made while changing the atmosphere. Also in Example 2, the photoresponse characteristics of the GaN layer were 0.1 or more for all Vds values between 1V and 100V, and 0.3 when the Vds value was 20V.

  As shown in FIGS. 2 and 3, the transistor manufactured in Example 2 has an Ids value rate after high output at the time of zero seconds after signal stop of 0.85, that is, an Ids value decrease rate immediately after high output is 0. It can be seen that immediately after the high output, an amount of current that is not so different from the Ids value in the specified idle state flows.

In Example 3, the gas introduction time was changed to 25 seconds in the NH ₃ gas treatment process for the substrate. The formation process of the AlN layer was the same as that in Example 1, and the skewness Rsk on the surface of the AlN layer became positive. In other steps, a nitride semiconductor field effect transistor (GaN-based HEMT) 100 was manufactured in the same manner as in Example 1.

  As in Example 1, when the gate electrode 108 is negatively biased in the on-wafer state and the transistor is in the off state, the current value between the source and the drain is measured from the dark state to the light irradiation state (1.98 eV). Measurements were made while changing the atmosphere. Also in Example 3, the photoresponse characteristics of the GaN layer were 0.1 or more in all cases where the Vds value was between 1V and 100V, and 0.14 when the Vds value was 20V.

  As shown in FIGS. 2 and 3, the transistor manufactured in Example 3 has an Ids value rate after high output at the time of zero seconds after signal stop of 0.87, that is, an Ids decrease rate immediately after high output is 0.13. It can be seen that immediately after the high output, an amount of current that is not so different from the Ids value in the specified idle state flows.

(Comparative example)
5 and 6 show the optical response characteristics of Comparative Examples 1 and 2, respectively. Comparative Examples 1 and 2 are on-wafer states in which a transistor formed without applying the embodiment of the present invention is not packaged, and the gate electrode is negatively biased and the transistor is off. The measurement atmosphere is changed from the dark state to the light irradiation state (1.98 eV).

(Comparative Example 1)
In Comparative Example 1, the NH ₃ gas treatment process of the SiC substrate was set to 25 seconds, and the H ₂ / NH ₃ ratio of the H ₂ / NH ₃ mixed gas was set to 5 in the cooling process of the epitaxial wafer after forming the AlN layer. It is. As apparent from FIG. 5, the ratio of H ₂ / NH ₃ in the cooling process of the epitaxial wafer was set to> 4 so that the nitride semiconductor layer 102 was gradually etched back and the skewness Rsk value could not be maintained. As a result, it can be seen that the optical response characteristic of the GaN layer is less than 0.1 over almost the entire Vds value of 1V to 100V, and 0.09 when the Vds value is 20V. .

  As shown in FIGS. 2 and 3, the transistor manufactured in Comparative Example 1 has an Ids value rate after high output of 0.64 at the time of zero seconds after signal stop, that is, an Ids decrease rate immediately after high output is 0.36. Since the reduction rate exceeds 0.3 which is the reference, it is considered that the gain of the transistor becomes unstable.

(Comparative Example 2)
In Comparative Example 2, the NH ₃ gas treatment process of the SiC substrate is set to 35 seconds longer than 30 seconds, and the H ₂ / NH ₃ ratio of the H ₂ / NH ₃ mixed gas is set in the cooling process of the epitaxial wafer after forming the AlN layer. 4. As is apparent from FIG. 6, it is considered that the formation of the AlN layer was hindered by performing the NH ₃ gas treatment process of the SiC substrate for longer than 30 seconds. Due to the influence, almost the entire Vds value of 1 V to 100 V was observed. It can be seen that the light response characteristic of the GaN layer is less than 0.1, and is 0.05 when the Vds value is 20V.

  As shown in FIGS. 2 and 3, the transistor manufactured in Comparative Example 2 has an Ids value rate after high output at the time of zero seconds after signal stop of 0.52, that is, an Ids decrease rate immediately after high output is 0.48. The Ids value immediately after the high output is reduced to about half of the defined idle state Ids value, and it is considered that the gain of the transistor becomes unstable.

(Comparative Example 3)
In Comparative Example 3, the NH ₃ gas treatment process of the SiC substrate is set to 35 seconds longer than 30 seconds, and the H ₂ / NH ₃ ratio of the H ₂ / NH ₃ mixed gas is set in the cooling process of the epitaxial wafer after forming the AlN layer. It is set to 5. The NH ₃ gas treatment process for the SiC substrate is performed for longer than 30 seconds, and the H ₂ / NH ₃ ratio> 4 in the cooling process of the epitaxial wafer inhibits the formation of the AlN layer and etch back in the cooling process. Therefore, it is considered that the skewness Rsk on the surface of the AlN layer became negative and the GaN layer was not formed well. For this reason, the field effect transistor of Comparative Example 3 has a light response characteristic of the GaN layer of less than 0.1 over almost the entire Vds value of 1 V to 100 V, and 0.03 when the Vds value is 20 V. It was.

  As shown in FIGS. 2 and 3, the transistor manufactured in Comparative Example 3 has an Ids value rate of 0.32 after high output at the time of zero seconds after signal stop, that is, an Ids value decrease rate immediately after high output is 0. 68, which greatly exceeds the reference 0.3, and the Ids value immediately after the high output is reduced to less than half of the Ids value in the specified idle state, and it is considered that the transistor gain becomes unstable.

  From the results of Examples 1 to 3 and Comparative Examples 1 to 3, the optical response characteristics of the GaN layer and the Ids value decrease rate after high output of the field effect transistor, particularly immediately after high output (at zero seconds after signal stop). Has a correlation, and a field effect transistor composed of a nitride semiconductor epitaxial wafer having a photoresponse characteristic of a GaN layer of 0.1 or more stabilizes the Ids value in an idle state before and after high-power driving, and the field effect transistor Fluctuations in gain can be suppressed.

  In addition, this invention is not limited to the said embodiment and said Example, A various change, improvement, substitution, and a combination are possible. Although the GaN-based HEMT has been described in the above embodiments and examples, the present invention can also be applied to other field effect transistors. Moreover, in the said embodiment and the said Example, although the SiC substrate was used as a substrate material and the case where an AlN layer was used as a nitride semiconductor layer formed between a board | substrate and a GaN layer was demonstrated, this invention is The present invention can also be applied when other nitride semiconductor layers such as AlGaN are used.

DESCRIPTION OF SYMBOLS 100 ... Nitride semiconductor field effect transistor, 101 ... Substrate, 102 ... Nitride semiconductor layer,
102a ... the surface of the nitride semiconductor layer, 103 ... GaN layer, 104 ... AlGaN layer,
105 ... Two-dimensional electron gas, 106 ... Source electrode, 107 ... Drain electrode,
108: Gate electrode

Claims (1)

Cleaning the surface of the substrate made of silicon carbide ;
Placing the substrate having a cleaned surface in an ammonia gas atmosphere for a period of 30 seconds or less while heating at 1100 to 1200 ° C. as a first temperature;
Epitaxially growing a nitride semiconductor layer having a surface shape with a positive skewness Rsk while heating at a second temperature on the substrate;
Cooling the epitaxial wafer on which the nitride semiconductor layer is formed to a third temperature lower than the second temperature while maintaining a gas mixed atmosphere of H ₂ / NH ₃ ratio ≦ 4;
Have a epitaxially growing a GaN layer on the nitride semiconductor layer,
A method for producing a nitride semiconductor epitaxial wafer for a transistor , wherein the photoresponse characteristic of the GaN layer is 0.1 or more in a state where no electron supply layer is formed on the GaN layer .
However, in this case, the light response characteristic means that when the measurement atmosphere of the lateral current value of the GaN layer is changed from a light non-irradiated state to a light irradiated state with a photon energy of 1.98 eV, It is represented by the ratio of the lateral current value in the light irradiation state (1.98 eV) to the directional current value.

Images