JP2010180111A - Self-support substrate and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-quality GaN-based semiconductor self-support substrate for a substrate giving sufficient electronic device characteristics. <P>SOLUTION: The self-support substrate comprises a GaN-based semiconductor and is characterized by that, when a Schottky diode is formed by directly using Ni as a metal electrode on the surface of the self-support substrate, the ideality factor n-value in the current-voltage characteristics is 1 or more and 1.3 or less. Preferably, when the Schottky diode is formed, a current value under application of -5 V reverse voltage is not more than 50 times as the theoretical current value calculated as a sum of the calculated values by a thermal-field emission model and by a thermoelectronic emission model. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a free-standing substrate made of a GaN-based semiconductor and a method for manufacturing the same. More specifically, the present invention relates to a free-standing substrate made of a GaN-based semiconductor that is useful as a high-quality GaN-based free-standing substrate, and a manufacturing method thereof.

  In recent years, silicon carbide (SiC) and gallium nitride (GaN) have been attracting attention as materials for power devices, high-frequency devices, and high-temperature operation devices that are used in inverters mounted on automobiles and the like that require a large withstand voltage. These materials have already been applied to devices that emit visible light, but their large forbidden band width and withstand voltage are attracting attention and are expected as electronic device applications. Another aspect of the reason that these materials are attracting attention is that they are composed of non-toxic elements having a large Clarke number like silicon (Si), which is a typical semiconductor material.

  A Schottky diode is expected as one of the power devices described above. The Schottky diode itself is an important component in a power supply circuit and the like as a high-speed switching diode, but is an important device for evaluating the potential of a material as an electronic device. In fact, a power device using SiC that has already been commercialized is a Schottky diode.

In particular, GaN has a wider forbidden band than SiC and high electron mobility. In addition, it has excellent physical properties such as band engineering that replaces Al and In with Ga sites, and is expected as a power device material. Has been.
Nitride semiconductor Schottky diodes represented by GaN have problems such as large leakage current, poor ideal factor n value in current-voltage characteristics, low breakdown voltage, high on-resistance, and short device life. Improvement of electronic device characteristics is desired.

Patent Document 1 refers to this cause. Impurities and dislocations in gallium nitride semiconductors act as acceptors, and in regions with an electron concentration of less than 5 × 10 ¹⁶ cm ⁻³ , impurities and dislocations with respect to donor concentration Since the ratio increases, it is pointed out that when forming a gallium nitride semiconductor having an electron concentration lower than this electron concentration, the acceptor concentration caused by impurities and dislocations cannot be ignored with respect to the donor concentration. That is, in a gallium nitride semiconductor having an electron concentration of less than 5 × 10 ¹⁶ cm ⁻³ , a gallium nitride semiconductor having a desired carrier concentration cannot be grown unless the impurity concentration and the number of dislocations are controlled. It is pointed out that no Schottky diode characteristics can be obtained.

In order to solve this problem, in Patent Document 1, a semiconductor layer exhibiting good electronic device characteristics has a group III nitride support base and an electron carrier concentration of less than 5 × 10 ¹⁶ cm ⁻³ , A first gallium nitride-based epitaxial layer provided on a group III nitride substrate, a donor dopant is added to the first gallium nitride-based epitaxial layer, and the gallium nitride-based epitaxial layer is 2 × A nitride semiconductor having a carbon concentration of less than 10 ¹⁶ cm ⁻³ and a dislocation density of less than 1 × 5 × 10 ⁸ cm ^{−2 in the} first gallium nitride based epitaxial layer is disclosed.

In Patent Document 1, as a group III nitride support substrate, a template in which a gallium nitride region is formed on a sapphire substrate (threading dislocation density is 1 × 10 ⁸ cm ⁻³ ) and a GaN wafer (threading dislocation density is 1 × 10 ⁶ cm). ^-3 ), metal organic vapor phase epitaxy (MOCVD)
It is disclosed that good electronic device characteristics can be obtained by forming an epitaxial layer by the method. For example, the ideal factor in the current-voltage characteristics of the Schottky diode formed on the epitaxial layer, the so-called n value, is a good value of 1.03 to 1.05, and GaN is also a potential as an electronic device material. It is confirmed that it is high.

  On the other hand, when manufacturing a GaN-based material as a free-standing substrate such as a Schottky diode substrate or a light-emitting diode (LD or LED) crystal growth substrate, an epitaxial layer is further formed on the GaN wafer as in Patent Document 1 by MOCVD. Use of the formed one is not always effective in the industry in terms of process complexity and cost. Therefore, it is desired to improve the quality of a free-standing substrate made of a GaN-based semiconductor.

Patent Document 2 describes a high-quality and high-quality gallium nitride-based material and a method for manufacturing the same, but improves the electronic device characteristics of the aforementioned Schottky diode, the device characteristics of the light-emitting diode, and the like. In order to contribute to the practical use of these, further quality improvement of the GaN-based semiconductor free-standing substrate has been demanded.
Non-Patent Document 1 shows an example in which a GaN free-standing substrate processed by various CMP (Chemical Mechanical Polishing) is directly processed into a Schottky diode, but the n value is 1.5 or more, and the barrier height is originally It is much smaller than expected from Ni / GaN, resulting in a very large reverse leakage current.

  This is because the crystallinity, surface state, etc. of a conventional free-standing substrate made of a GaN-based semiconductor have reached a certain level of quality as a substrate for epitaxial growth, but it is still unsatisfactory from the viewpoint of using it as an electronic device. It is enough. When using a self-supporting substrate directly in a device, it is necessary to clarify the requirements for crystallinity and surface condition, and to establish a crystal growth process, processing process, and surface treatment process that can meet these requirements, and further research and development are required. Met.

JP 2007-149985 A JP 2007-277077 A Applied Surface Science 255 (2008), pages 3085-3089.

    As described above, GaN-based semiconductors have excellent potential as materials for electronic devices such as Schottky diodes and light-emitting diodes. However, in silicon where supply of over 90% is made as a semiconductor material, most of the conventional electronic devices are formed on a self-supporting substrate so that an epitaxial layer is not used. The method of forming an epitaxial layer on the GaN wafer by MOCVD as in No. 1 has room for improvement in terms of process complexity and cost. On the other hand, the GaN-based semiconductor free-standing substrate itself has a problem that sufficient characteristics cannot be obtained.

  In view of the above problems, an object of the present invention is to provide a high-quality GaN-based semiconductor free-standing substrate for a substrate that can obtain sufficient electronic device characteristics.

As a result of intensive studies, the present inventors have considered that the Schottky diode is an important device for evaluating the potential for use as an electronic device. Focused on the index. Then, the present inventors have found a self-standing substrate made of a GaN-based semiconductor having high quality characteristics and a method for manufacturing the same.

That is, the gist of the present invention resides in the following [1] to [15].
[1] A free-standing substrate made of a GaN-based semiconductor, and when a Schottky diode is formed directly on the surface of the free-standing substrate using Ni as a metal electrode, an ideal factor n value in current-voltage characteristics is 1 or more and 1.3 or less A self-supporting board characterized by
[2] The self-standing substrate according to [1], wherein when a Schottky diode is formed using Ni as a metal electrode directly on the surface of the self-standing substrate, a current value when a reverse voltage of −5 V is applied is a thermal electric field. A self-supporting substrate characterized by being 50 times or less of a theoretical current value calculated as a sum of calculated values of an emission model and a thermal electron emission model.

[3] A free-standing substrate made of a GaN-based semiconductor, and when a Schottky diode is formed directly on the surface of the free-standing substrate using Ni as a metal electrode, the current value when a reverse voltage of −5 V is applied is a thermal field emission model And a self-standing substrate characterized in that it is not more than 50 times the theoretical current value calculated as the sum of the calculated values of the thermal electron emission model.
[4] The self-supporting substrate according to any one of [1] to [3], wherein the damage depth of the surface measured by observing the lattice disturbance in the cross-sectional lattice image observation by TEM is 10 nm or less.

[5] The peak intensity ratio of the ion mass spectrum of Si / Ga measured by TOF-SIMS at a primary ion Au, a primary ion acceleration voltage of 25 kV, a scanning area of 200 μm square, and a secondary ion integration time of 150 seconds is 0.01 or less. The self-supporting substrate according to [1] to [4].
[6] The self-supporting substrate according to any one of [1] to [5], wherein an RMS value at 1 × 1 μm ² is 1 nm or less measured in an area excluding the peripheral 1 mm by AFM (Atomic Force Microscopy).

[7] The self-supporting substrate according to [1] to [6], wherein the dislocation density is 5 × 10 ⁶ cm ⁻² or less.
[8] The above [1] to [7], wherein the GaN-based semiconductor is made of Al _x In _y Ga _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, [x + y] <1). ] Free standing substrate.
[9] The self-supporting substrate according to [8], wherein x + y <0.5.
[10] The self-supporting substrate according to [1] to [9], wherein the thermal conductivity at room temperature is 250 (W / m · K) or more.

[11] A method for manufacturing the self-supporting substrate according to [1] to [10],
(1) A growth process for growing a GaN-based semiconductor, (2) a polishing process, and (3) a rubbing cleaning process.

[12] A method of manufacturing a self-supporting substrate according to [1] to [10], comprising a growth step of growing a GaN-based semiconductor by an HVPE (Hydride Vapor Phase Growth Growth) method. Production method.
[13] In the growth step,
A carrier gas containing H ₂ gas, GaCl gas, and NH ₃ gas are supplied to the reaction chamber, the growth temperature is set to 900 ° C. or higher and 1200 ° C. or lower, and the growth pressure is set to 8.08 × 10 ⁴ Pa or higher and 1.21 ×. 10 ⁵ Pa or less, GaCl gas partial pressure of 1.0 × 10 ² Pa to 1.0 × 10 ⁴ Pa and NH ₃ gas partial pressure of 9.1 × 10 ² Pa to 2.0 × The method for producing a self-supporting substrate according to [11] or [12], wherein the pressure is 10 ⁴ Pa or less.

[14] The method for manufacturing a self-supporting substrate according to [11] to [13], further including a polishing step of setting a polishing rate to 1000 nm / hour or less after the growth step.
[15] The method for manufacturing a self-supporting substrate according to [12] to [14], including a rubbing cleaning step after the growth step.

  According to the present invention, it is possible to provide a high-quality GaN-based semiconductor free-standing substrate for a substrate that can obtain sufficient electronic device characteristics. For example, not only a GaN-based material Schottky diode can be put into practical use, but also a field-effect transistor in a GaN-based material having a Schottky barrier as a gate can be put into practical use. In particular, it is not necessary to perform further epitaxial growth on a support substrate made of a GaN-based material, and excellent characteristics can be exhibited even if a Schottky diode is directly formed on the surface. Further, for example, it can be used as a substrate for crystal growth of a semiconductor light emitting element such as an LED or LD, thereby enabling practical use of the semiconductor light emitting element as a power device.

It is a figure for demonstrating the measurement principle of a laser flash method. It is a figure which shows schematic structure of the HVPE apparatus used for manufacture of the support base | substrate which consists of a GaN-type thick film material in this invention. It is a figure which shows the result of the surface roughness measurement by AFM. It is sectional drawing for demonstrating the structure of the Schottky diode obtained in the Example. It is an example of the current-voltage characteristic (forward characteristic) of the Schottky diode of this invention formed on the self-supporting GaN crystal substrate whose effective donor density | concentration is 1 * 10 < ¹⁶ > atoms / cm < ³ >. The two solid lines show the slopes when the n value is n = 1 and n = 2, respectively, as a reference. It is an example of the current-voltage characteristic (reverse direction characteristic) of the Schottky diode of this invention formed on the self-supporting GaN crystal substrate whose effective donor density | concentration is 1 * 10 < ¹⁶ > atoms / cm < ³ >. The thick solid line represents the theoretical value, that is, the sum of the thermal field emission current and thermionic emission current considering the barrier drop due to image power. The dotted line is a value obtained by multiplying the theoretical value by 10 to make it easy to understand that the actually measured value is 10 times or less of the theoretical value. It is a figure which shows the example of n value and barrier height of the Schottky diode of this invention formed on the self-supporting GaN crystal substrate whose effective donor density | concentration is 1 * 10 < ¹⁶ > atoms / cm < ³ >. The figure which shows the result (current-voltage characteristic) which evaluated the reverse direction characteristic to -200V about the diode of this invention formed on the self-supporting GaN crystal substrate whose effective donor density | concentration is 1 * 10 < ¹⁶ > atoms / cm < ³ >. It is. The thick solid line indicates the theoretical value, ie, the sum of the thermal field emission current and thermionic emission current considering the barrier drop due to image power, and the dotted line indicates that the measured value is 10 times or less of the theoretical value. To make it easier to understand, the theoretical value is multiplied by 10. The solid line per Schottky diode of the present invention, the reverse voltage -5V and -50 V, the sum of the thermal electron emission current in consideration of the barrier lowering by the thermal-field emission current and imaging force calculates, with respect to the effective donor concentration N _d And plotted. White circles and black circles are measured values of current values at reverse voltages of -5 V and -50 V, respectively.

Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and various modifications can be made without departing from the scope of the invention.
[1] A self-supporting substrate made of a GaN-based semiconductor The GaN-based semiconductor constituting the present invention is a III-V group compound semiconductor material containing GaN as a main component, and the group III element generally contains the most Ga. , Part of which is replaced by Al or In, and the group V element is N. Accordingly, when expressed by a general formula, it becomes Al _x In _y Ga _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, [x + y] <1). The majority of the group III elements of the GaN-based material is preferably Ga, that is, x and y preferably satisfy the inequality [x + y] <0.5. Further, it goes without saying that such GaN-based materials include GaN-based materials doped with a small amount of impurities or p-type or n-type impurities intentionally added for adjusting the conductivity type.

  The self-supporting substrate of the present invention is made of a GaN-based semiconductor, and does not include a substrate made of a GaN-based semiconductor composed of a plurality of epitaxially grown layers using the GaN-based semiconductor as a supporting substrate. That is, the free-standing substrate refers to a thick film material layer from which a base substrate has been peeled, or a wafer obtained by cutting the thick film material layer with a wire saw or the like, and does not include an epitaxial layer formed thereafter. In addition, the thick film material layer which does not peel a base substrate can also show the effect of this invention. In this case, it is necessary to form a thick film material layer having an appropriate thickness on the base substrate in order to obtain material characteristics necessary for exhibiting the effects of the present invention. The thickness of the base substrate is usually 10 μm or more, preferably 30 μm or more, and more preferably 50 μm or more.

[2] Schottky Diode Characteristics Needless to say, the device characteristics of a Schottky diode are significantly affected by the interface characteristics between a metal and a semiconductor where a Schottky junction is formed. As a factor that impairs device characteristics, damage or contamination due to polishing of the semiconductor surface, or unevenness may be considered.

When forward voltage is applied to a Schottky diode that has many impurity levels due to damage or contamination in the semiconductor region, carrier recombination occurs in the metal / semiconductor interface region, resulting in an ideal factor in the forward current of the Schottky diode. The (n value) deviates from the ideal value of 1 and approaches 2, and the saturation current value of the reverse current increases.
In addition, the unevenness of the metal / semiconductor interface causes local electric field concentration, and increases the tunneling current component of the forward current. However, when the tunneling current component is large, the n value is larger than 2. There is a case. In addition, the effect of electric field concentration is significant in the reverse current characteristics, and tunneling and local avalanche multiplication are generated even when a low voltage is applied, which increases the reverse current.

The occurrence of carrier recombination and tunneling current when a forward voltage is applied causes diode loss. In addition, an increase in the reverse current component is a cause of impairing the rectification characteristics of the Schottky diode. That is, in order to obtain a Schottky diode with good device characteristics, it is extremely important to improve the quality of the semiconductor surface.
By applying such a principle, the present invention is characterized in that it has the following characteristics (surface state, surface impurity level, dislocation density, thermal conductivity) as a self-standing substrate made of a GaN-based semiconductor.

That is, the self-supporting substrate of the present invention has excellent electronic device characteristics when a Schottky diode is formed directly on the surface using Ni as a metal electrode. Each characteristic will be described in detail below.
Of the following values, measured values (for example, n value and current value when measured after forming a Schottky diode described later) and theoretical values (for example, calculated values of thermal field emission model, thermionic emission) The model calculated value and the theoretical current value calculated as the sum of these values are the values when the temperature of the freestanding substrate is assumed to be 16 ° C.

[2-1] Ideal Factor n Value in Current-Voltage Characteristics When a Schottky diode is formed directly on the surface of the self-standing substrate of the present invention using Ni as a metal electrode, the ideal factor n value in current-voltage characteristics is 1 or more. It is characterized by being 1.3 or less. By having such characteristics, it is possible to obtain a high-quality free-standing substrate that has excellent crystallinity and can withstand practical use as a substrate wafer. In addition, n value is so preferable that it is close to 1, and a preferable upper limit is 1.2 or less, More preferably, it is 1.1 or less.

The Schottky diode is formed as follows. First, the free-standing substrate of the present invention is cleaned. Specifically, for example, SPM treatment effective in silicon technology, hydrochloric acid treatment, aqua regia treatment, and HF treatment are performed, and the wafer is thoroughly washed with pure water and blown with nitrogen gas to remove pure water from the wafer and dry it. . Next, prior to forming the Schottky electrode, an ohmic electrode is first formed. On the back surface of the free-standing substrate, Ti is deposited to 20 nm and Al is deposited to 200 nm, and this is heat-treated in N ₂ at 750 ° C. for 3 minutes to form an ohmic electrode.

Next, ultrasonic cleaning is performed in methanol to form a Schottky electrode. Ni is formed on the entire surface by vapor deposition as a Schottky barrier metal. Next, a Schottky electrode pattern having a diameter of 100 nmΦ is formed using a normal photolithography process. Metals other than the electrode pattern masked with resist are selectively etched at room temperature using a solution obtained by diluting a commercially available FeCl ₃ aqueous solution (FeCl ₃ .4H 2 O: H 2 O = 2: 3) with ultrapure water to about 10 to 14 times. . The reason why the diameter of the Schottky electrode is set to 100 μmφ is that pattern formation is easy, and since there are about 100 to 300 dislocations in this, average information of the support substrate can be obtained. It is.

[2-2] Current value when reverse voltage is applied Further, in addition to the above characteristics, the self-standing substrate of the present invention has a reverse direction when a Schottky diode is formed directly on the surface with Ni as a metal electrode. The current value when a voltage of -5 V is applied is not more than 50 times, preferably not more than 10 times, more preferably not more than 3 times the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermionic emission model. It is a feature. By having such characteristics, it is possible to obtain a high-quality free-standing substrate that has excellent crystallinity and can withstand practical use.

The formation method of the Schottky diode is the same as that described above in [2-1].
Further, as a more preferable aspect of the self-supporting substrate of the present invention, the aforementioned Schottky diode is formed, and the current value at the time of applying the reverse voltage of −50 V is calculated as the sum of the calculated values of the thermal field emission model and the thermoelectron emission model. The theoretical current value is 50 times or less, preferably 10 times or less, and more preferably 3 times or less.

Further, as a particularly preferable aspect of the self-standing substrate of the present invention, the current value at the time of applying a reverse voltage of −200 V is 50 times or less of the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermal electron emission model It is preferably 10 times or less, more preferably 3 times or less.
In addition to the fact that no self-supporting substrate having such characteristics has existed so far, it has not been performed by those skilled in the art to confirm the characteristics of the self-supporting substrate using such an evaluation method. In the present invention, grasping the characteristics by such simple and accurate evaluation and commercializing a self-supporting substrate satisfying the characteristics has a very high technical significance in providing a high-quality self-supporting substrate having excellent electrical characteristics. Is.

[3] Other characteristics Schottky diode characteristics are significantly affected by the interface characteristics between metal and semiconductor. Possible causes of damage to electronic device characteristics include damage due to surface polishing, contamination, and unevenness.
When a forward voltage is applied to a Schottky diode that has many defect levels due to damage or contamination in the semiconductor region, current transport that differs from the ideal state occurs near the metal / semiconductor interface, resulting in the forward current of the Schottky diode. The ideal factor (n value) at 1 greatly deviates from the ideal value of 1, and the current value of the reverse current also increases.

In addition, the unevenness of the metal / semiconductor interface causes local electric field concentration and increases the local tunneling current component of the forward current. When the tunneling current component is large, the n value is a value larger than 2. It may become. In addition, the effect of electric field concentration is significant in the reverse current characteristics, and local tunneling and local avalanche multiplication are generated even when a low voltage is applied, causing the reverse current to increase.
An increase in the n value when a forward voltage is applied leads to an increase in on-resistance and causes diode loss. In addition, an increase in the reverse current component impairs the rectification characteristics of the Schottky diode, which also causes a loss in a circuit using the diode. For this reason, the present inventors have found that it is important to reduce damage, contamination, or unevenness due to polishing of the semiconductor surface or the like for the formation of a good Schottky diode.

Therefore, in the self-supporting substrate of the present invention, it is preferable that the surface damage depth, impurity concentration, and surface microscopic shape have the following characteristics.
[3-1] Surface Damage Depth In the self-supporting substrate of the present invention, the surface damage depth is usually 10 nm or less, preferably 5 nm or less, more preferably 3 nm or less. The damage depth can be measured by observing lattice distortion in cross-sectional lattice image observation with a TEM (Transmission Electron Microscope). In the substrate surface layer observed by the cross-sectional TEM image, an incomplete crystal region and an amorphous layer, which are clearly different in lattice arrangement from the complete crystal portion, can be confirmed. The thickness of these two parts is measured as a damage layer.

If the damage on the surface is too deep, the interface characteristics between the metal and the semiconductor are impaired when the device is formed.
In addition, there is a high possibility that impurities in the region where the lattice image observed by the TEM is disturbed are taken in along with the damage in the polishing or cleaning process.
[3-2] Impurity Concentration The self-supporting substrate of the present invention is obtained by TOF-SIMS (time-of-flight secondary ion mass spectrometer, Time of Flight-Secondary Ion Mass Spectrometer), primary ion Au, primary ion acceleration voltage 25 kV, scanning region. The peak intensity ratio of the ion mass spectrum of Si / Ga measured at 200 μm square and secondary ion integration time of 150 seconds is usually 0.01 or less, more preferably 0.005 or less. That is, the concentration of impurities such as Si that are easily adhered in the manufacturing process is extremely low. If the Si / Ga intensity ratio is too large, the electronic device characteristics may be significantly reduced. Further, in some cases, it can be measured by the intensity ratio of SiO ₂ / Ga under the same conditions. In that case, the peak intensity ratio of the ion mass spectrum of SiO ₂ / Ga is usually 0.1 or less, more preferably Is 0.05 or less.

[3-3] Microscopic shape of the surface The self-supporting substrate of the present invention has a very flat surface and little unevenness, and is measured by an AFM (Atomic Force Microscopy) in a region excluding the peripheral 1 mm. The RMS value at 1 × 1 μm ² is usually 1 nm or less, preferably 0.5 nm or less, more preferably 0.2 nm or less.

If the surface unevenness difference is too large, the adhesion of impurities in subsequent processes may be promoted. In addition, it may be difficult to remove adhering impurities in the cleaning process during manufacturing.
The reason why the RMS value is measured in the region excluding the 1 mm periphery of the self-supporting substrate (wafer) of the present invention is that the unevenness of the surface of the wafer periphery tends to increase due to other processing.

[3-4] Dislocation density The self-supporting substrate of the present invention has a dislocation density of usually 1 × 10 ⁷ cm ⁻² or less from the viewpoint of reducing current leakage paths and resistance derived from dislocations and obtaining good electrical characteristics. Preferably it is 5 × 10 ⁶ cm ⁻² or less, more preferably 3 × 10 ⁶ cm ⁻² or less. The dislocation density can be obtained by calculating the density of dark spots usually observed in a CL (cathode luminescence) image.
[3-5] Thermal conductivity As described above, GaN-based electronic devices are expected as power devices. This is because the band gap is wide and the operation at high temperature is easy, but it is necessary to operate without increasing the temperature as much as possible for the stable operation and long life of the device.

  For this reason, even in a GaN-based device, it is necessary to efficiently dissipate heat when operating at high power. For example, even if a Schottky diode that is an element of a power device as shown in the present invention has ideal characteristics, there is a loss due to the electrical resistance of the ohmic electrode or the GaN bulk itself, and a high current In operation, heat is generated.

  Therefore, the power device is devised to make the operating region as close to the heat sink material as possible. In order to bring the operation region closer to the heat sink material, it is necessary to cut the back surface of the semiconductor substrate on which the device operation region is formed, that is, to make the semiconductor substrate thinner. At this time, the thinning of the thickness of about 10 μm is a major factor that significantly reduces the device manufacturing yield, and can be said to be a foothold for reducing the cost of the power device.

Therefore, if a semiconductor material with high thermal conductivity is used in the semiconductor material for power devices, thinning is unnecessary, and the device yield can be improved. The self-supporting substrate made of the GaN-based semiconductor of the present invention is preferably a material suitable for power device applications having high thermal conductivity.
From this point of view, the self-standing substrate of the present invention has a thermal conductivity at room temperature (25 ° C.) of usually 250 W / m · K or more, preferably 300 W / m · K or more, more preferably 345 W / m · K or more. It is preferable that it is a type | system | group semiconductor.

The thermal conductivity can be evaluated by a laser flash method. In general, in order to directly determine the thermal conductivity, it is necessary to prepare a large sample and perform measurement for a long time. On the other hand, in the laser flash method, the thermal conductivity can be measured in a short time using a small sample.
FIG. 1 is a diagram for explaining the measurement principle of the laser flash method. In this method, the surface of a disk-shaped sample S having a diameter of about 10 mm and a thickness of about 1 to 5 mm is irradiated with a laser beam having a pulse width of several hundred μs. This is a measurement method for calculating the thermal diffusivity from the change in the temperature of the back surface of the sample S after heating uniformly. Specifically, the thermal diffusivity α is measured by the laser flash method, and the thermal conductivity λ is calculated by the following formula (1) from the density p and the specific heat capacity C _p obtained by other methods.

λ = α × ρ × C _p (1)
In the laser flash method, the thermal diffusivity is calculated from the temperature change of the back surface of the sample S after the surface of the disk-shaped sample S having a diameter of about 10 mm and a thickness of about 1 to 5 mm is uniformly heated by a laser beam having a pulse width of several hundred μs. The measurement method to be calculated. According to the theoretical solution assuming an adiabatic condition, the back surface temperature of the sample S after pulse heating rises and converges to a constant value as the temperature distribution in the sample S becomes uniform. The laser flash method can measure a small sample in a short time, the analysis method is simple, and measurement from room temperature to high temperature of 200 ° C or higher is possible. Widely used as a static measurement method.

Here, in the application of the equation (1), the density of GaN is 6.15 (g / cm ³ ) and the specific heat is 40.8 (J / mol · K) (Barin, I., O. Knaeke, and
O. Kubasehewski, Thermochemical Property
s of Inorganic Substrates, Springer-Verlag, Berlin, 1977).

Thermal diffusivity measurements can be corrected using standard samples. For example, polycrystalline alumina (diameter 10 mm, thickness 1 mm) available from the Fine Ceramic Center Foundation can be used as the standard sample.
As an algorithm for calculating the thermal diffusivity α from the change in the back surface temperature of the sample S, the t1 _{/ 2} method was used. In the t _1/2 method, the thermal diffusivity α is calculated from the time required to reach half of the transient temperature rise on the back surface of the sample S according to the equation (2). Here, d is the thickness of the sample S.

α = 0.1388d ² / t _1/2 (2)
The GaN-based semiconductor of the present invention having the above-described thermal conductivity can be manufactured, for example, by the method for manufacturing a free-standing substrate of the present invention described later.
When measuring the thermal conductivity of the support substrate made of the GaN-based thick film material used in the present invention, first, a plate-like evaluation sample of 10 mm × 10 mm × 1 mm is obtained by polishing and molding both surfaces of the support substrate. Next, a gold film having a thickness of about 200 nm is formed on both surfaces of the sample for evaluation, and a carbon film (thickness of 1 μm or less) is further formed on the laser irradiation surface side, which is used as a sample for measuring thermal conductivity. .

And, using a fully automatic laser flash method thermal constant measuring device TC-7000 available from ULVAC-RIKO, Inc. and a standard material TD-AL for thermal diffusivity measurement available from the Fine Ceramics Center, The thermal conductivity at room temperature was determined.
[4] Manufacturing method The manufacturing method of the self-supporting substrate of the present invention includes:
(1) It has a growth process for growing a GaN-based semiconductor.

Preferably, after the growth step, (2) polishing step (3) rubbing cleaning step.
Hereinafter, each process is explained in full detail.

[4-1] Growth Process The self-supporting substrate of the present invention is an HVPE (Hydride Vapor Phase Epita).
(xial Growth) method, Na flux method, solvothermal method, etc., and a growth step of performing crystal growth by a known GaN bulk growth method. Among these, those having a growth step of growing a GaN-based semiconductor by the HVPE method are preferable.

Although there is no particular limitation on the growth method of the GaN-based semiconductor in the HVPE method, it is preferable to use the growth method described in Patent Document 1 from the viewpoint of obtaining a high-quality crystal. Hereinafter, the growth process by the HVPE method will be described using the growth method disclosed in Patent Document 1 as a specific example.
FIG. 2 is a diagram showing a schematic configuration of an HVPE apparatus suitable for a method for manufacturing a GaN-based semiconductor (hereinafter referred to as “GaN-based material” as appropriate) of the present invention. The HVPE apparatus 100 can be configured as, for example, a vertical HVPE apparatus. The vertical HVPE device is easier to form a laminar flow than the horizontal HVPE device, so it can form a high-quality and highly uniform epitaxially grown film with good reproducibility, and is advantageous for batch processing (simultaneous growth of many sheets). Have

The HVPE apparatus 100 includes a reaction chamber 10, a substrate support unit 30 disposed in the reaction chamber 10, and a heater 20. Carrier gas (G1), GaCl gas (G2), and NH ₃ gas (G3) can be supplied into the reaction chamber 10. As the carrier gas (G1), H ₂ gas and N ₂ gas can be supplied. Moreover, GaCl gas (G2) is produced | generated by making Ga and HCl react, for example.
In the growth process for growing the GaN-based material, a carrier gas G1, a GaCl gas G2, and an NH ₃ gas G3 are supplied from the introduction chamber 40 to the base substrate in the reaction chamber 10.

The temperature (growth temperature) of the base substrate is usually 900 ° C. or higher, preferably 950 ° C. or higher, more preferably 1000 ° C. or higher, and usually 1200 ° C. or lower, preferably 1150 ° C. or lower, more preferably 1100 ° C. or lower.
The pressure (growth pressure) in the reaction chamber 10 is usually 8.08 × 10 ⁴ Pa or higher, preferably 9.09 × 10 ⁴ Pa or higher, more preferably 9.60 × 10 ⁴ Pa or higher. It is 21 × 10 ⁵ Pa or less, preferably 1.11 × 10 ⁵ Pa or less, more preferably 1.06 × 10 ⁵ Pa or less.

Among these, the partial pressure of GaCl gas (G2) is usually 1.0 × 10 ² or more, preferably 2.0 × 10 ² or more, more preferably 4.0 × 10 ² or more, and usually 1.0 × 10 ² or more. It is 10 ⁴ Pa or less, preferably 5.6 × 10 ³ Pa or less, more preferably 4.0 × 10 ³ Pa or less.
The partial pressure of the NH ₃ gas (G3) is usually 9.1 × 10 ² or more, preferably 1.5 × 10 ³ or more, more preferably 2.0 × 10 ³ Pa or more, and usually 2.0 × 10 ⁴ Pa or less, preferably 1.5 × 10 ⁴ Pa or less, more preferably 1.0 × 10 ⁴ Pa.

Furthermore, the carrier gas (G1) is, in the case where the other of the _{H 2} gas further comprises a _{N 2} gas, gamma = _{[H 2} gas partial pressure] / [(a _{H 2} gas partial pressure) of + _{(N 2} gas For example, γ is usually 0.6 or more, preferably 0.8 or more, more preferably 0.9 or more, and usually less than 1.
In order to impart conductivity to the GaN-based material or gallium nitride material, it is preferable to intentionally dope silicon (in the case of n-type). On the other hand, when a high-purity material is required, it is preferable that the impurities are not intentionally doped (undoped). Further, a sample that is not intentionally doped with impurities tends to have a higher thermal conductivity than a sample that is intentionally doped with impurities, which is preferable. Examples of the impurity include oxygen, silicon, carbon, and hydrogen. The preferable impurity concentration of the sample not intentionally doped is as follows.

In a sample not intentionally doped with oxygen as an impurity, the oxygen concentration is preferably less than 1 × 10 ¹⁷ atoms / cm ³ , more preferably less than 5 × 10 ¹⁶ atoms / cm ^3. More preferably, it is less than × 10 ¹⁶ atoms / cm ³ . However, the SIMS detection lower limit of oxygen is 2 × 10 ¹⁶ atoms / cm ³ .

In the sample not intentionally doped with carbon as an impurity, the carbon concentration is preferably less than 5 × 10 ¹⁶ atoms / cm ³ , more preferably less than 3 × 10 ¹⁶ atoms / cm ^3. More preferably, it is less than × 10 ¹⁶ atoms / cm ³ . However, the SIMS detection lower limit of carbon is 1 × 10 ¹⁶ atoms / cm ³ .

In a sample not intentionally doped with hydrogen as an impurity, the hydrogen concentration is preferably 5 × 10 ¹⁷ atoms / cm ³ or less, more preferably 3 × 10 ¹⁷ atoms / cm ³ or less. More preferably, it is less than × 10 ¹⁷ atoms / cm ³ . However, the SIMS detection lower limit of hydrogen is 1 × 10 ¹⁷ atoms / cm ³ .

In the sample not intentionally doped with silicon as an impurity, the silicon concentration is preferably 1 × 10 ¹⁷ atoms / cm ³ or less, more preferably 5 × 10 ¹⁶ atoms / cm ³ or less. more preferably × at ¹⁰ 15 atoms / cm ³ or less, and most preferably ^{1 × 10 15 (atoms / cm} 3) or less. However, the SIMS detection lower limit of silicon is 1 × 10 ¹⁵ atoms / cm ³ .

The greatest feature of the GaN-based thick film material grown under such conditions is that the thermal conductivity at 25 ° C. is as extremely high as 250 W / m · K or more. Such a feature of high thermal conductivity has already been described in Patent Document 2 (the specification of Japanese Patent Application No. 2006-0667907, which is the basis of the priority).
Moreover, when the GaN-type thick film material grown on the above-mentioned conditions was evaluated by cathode luminescence (CL), the dislocation density was 1 × 10 ⁷ cm ⁻² or less. Furthermore, the half width of the X-ray rocking curve in the (002) plane was 300 arcsec or less, and the half width of the X-ray rocking curve in the (102) plane was 500 arcsec or less, and the crystallinity was good.

When the residual impurity concentration was measured by secondary ion mass spectrometry (SIMS), the oxygen concentration was less than 5 × 10 ¹⁷ atoms / cm ³ and the silicon concentration was 5 × 10 ¹⁷ for a sample to which no impurities were intentionally added. The atoms / cm ³ or less, the carbon concentration was less than 1 × 10 ¹⁷ atoms / cm ³ , and the hydrogen concentration was less than 1 × 10 ¹⁸ atoms / cm ³ . The lower limit of detection of each element under the SIMS measurement conditions is as follows: oxygen is 2 × 10 ¹⁶ atoms / cm ³ , silicon is 1 × 10 ¹⁵ atoms / cm ³ , carbon is 1 × 10 ¹⁶ atoms / cm ³ , and hydrogen is 1 × 10 ¹⁷ atoms / cm ³

  In the growth step, as described above, a GaN-based material is grown on the base substrate by the HVPE method. As the base substrate, either a semiconductor substrate or a dielectric substrate can be used, but a semiconductor substrate is preferably used. For example, the base substrate preferably has a lattice constant close to the crystal layer of the GaN-based material to be grown thereon, the lattice constant is 0.30 nm to 0.36 nm in the a-axis direction, and the c-axis direction It is particularly preferable to use a compound semiconductor substrate having a thickness of 0.48 nm to 0.58 nm.

The base substrate is preferably a substrate having a crystal structure belonging to a cubic system or a hexagonal system. As a cubic substrate, Si, GaAs, InGaAs, GaP, InP, ZnSe, ZnTe, CdTd, or the like can be used. As a hexagonal substrate, sapphire, SiC, GaN, spinel, ZnO, or the like can be used. be able to.
An off-substrate can also be used as the base substrate. For example, in the case of a sapphire substrate, a substrate on which the nitride semiconductor crystal layer is grown can be an (ABCD) plane or a plane slightly inclined from the (ABCD) plane. Here, A, B, C, and D represent natural numbers.
The angle of this slight inclination is usually 0 ° to 10 °, preferably 0 ° to 0.5 °, more preferably 0 ° to 0.2 °. For example, a sapphire substrate slightly inclined in the m-axis direction from the (0001) plane can be preferably used. In addition, for example, a (11-20) plane, r (1-102) plane, m (1-100) plane, planes equivalent to these planes, and planes slightly inclined from these planes can be used. . Here, the equivalent plane refers to a plane in which the arrangement of atoms is the same crystallographically when rotated by 90 ° in a cubic system and 60 ° in a hexagonal system.

A GaN-based material may be grown directly on the base substrate according to the present invention. However, after forming the base layer on the base substrate, the GaN-based material is grown on the base layer according to the present invention. You may let them.
The underlayer is formed by, for example, molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), PLD (Pulsed Laser Deposition; J. Cryst. Growth, 237/239 (2002) 1153), HVPE. Etc. can be formed. Among these, the MBE method, the MOCVD method, and the PLD method are preferable, and the MBE method and the MOCVD method are particularly preferable.

By forming such an underlayer, it is possible to improve the crystal state and surface state of the layer made of the GaN-based material grown thereon.
Hereinafter, a procedure for forming a GaN-based material layer on a sapphire base substrate through a base layer will be described as an example. First, a base layer is formed on a sapphire substrate by MBE method, MOCVD method, PLD method, HVPE method or the like. Next, a GaN-based thick film material layer is formed on the underlayer by the HVPE method according to the procedure described above. Ga is reacted with HCl and supplied as GaCl gas into the reaction chamber 10, and a nitrogen source is supplied as NH ₃ gas into the reaction chamber 10. The reaction temperature for producing GaCl by reacting Ga and HCl is, for example, about 850 ° C.

  When the GaN-based material layer thus obtained is peeled from the sapphire base substrate, laser lift-off may be used. Specifically, after the growth of the GaN-based material layer, when the interface between the base substrate and the GaN-based material layer is irradiated with a laser to expose the interface to a high temperature, the group V component (N component) is lost and the group III component ( Ga component) remains. When this group III component (Ga component) is removed with hydrochloric acid or the like, the base substrate can be easily removed. As another method, peeling can be performed using a stress generated between the GaN-based material layer and the base substrate during the temperature drop after crystal growth in the growth apparatus.

If the GaN-based material layer is peeled from the sapphire base substrate after the GaN-based material layer is grown, a bulk substrate made entirely of the GaN-based material can be obtained. For example, a GaN-based material layer grown to a thickness of about several to several tens of mm is peeled off from the support base and cut into a substrate having a thickness of about 0.2 to 0.3 mm to obtain a self-supporting substrate.
In the present invention, after such a growth step, the surface is appropriately flattened by any known mechanical grinding or lapping process.

[4-2] Polishing Step After the above-described growth step, a polishing treatment is performed to reduce crystal irregularities due to surface irregularities and mechanical damage. In general, the polishing treatment is preferably performed by the CMP (Chemical Mechanical Polishing) method after the above-described mechanical grinding and lapping.

Specific examples of the polishing step are shown below. First, the GaN-based semiconductor obtained in the above step is affixed to a disk-shaped polishing block having a flat surface with wax, and the GaN-based semiconductor is pressed against a rotating polishing surface plate while being applied with a polishing liquid, and then polished. Is done. On the surface of the polishing surface plate, a non-woven fabric or foamable resin called an abrasive cloth is installed.
Here, in the present invention, the polishing rate is usually 1000 nm / hour or less, preferably 500 nm / hour or less, more preferably 100 nm / hour or less. If the polishing rate is too fast, surface damage due to polishing increases, and the unevenness of the surface increases. This surface unevenness may promote the adhesion of impurities in the subsequent processes. In addition, it may be difficult to remove adhering impurities in the rubbing cleaning process described later.

  Further, as a condition for polishing, in order to obtain a preferable polishing rate, it is preferably performed under acidic conditions, and the pH is usually 0.5 or more, preferably 0.8 or more, usually 2 or less, preferably 1.5 or less, Preferably it is 1.2 or less. If the pH is too low or too high, a preferable polishing rate cannot be obtained. As the abrasive particles, silica is preferably used, but acidic colloidal silica is particularly preferably used. The particle size of the abrasive particles is usually 10 nm or more, preferably 20 nm or more, usually 200 nm or less, preferably 150 nm or less, more preferably 100 nm or less. Further, similarly to the description regarding the pH, in order to obtain a preferable polishing rate, it is preferable that the slurry of the abrasive particles contains an oxidizing agent. Examples of the oxidizing agent include hydrogen peroxide water, nitric acid, peracetic acid, and the like. From the experimental results, hydrogen peroxide water is empirically preferable. The content of the oxidizing agent is usually 1% by weight or more, preferably 3% by weight or more, more preferably 5% by weight or more, and usually 50% by weight or less, preferably 40% by weight or less, based on the entire polishing liquid. More preferably, it is 30 weight% or less. If the content of the oxidizing agent is too much or too little, a preferable polishing rate cannot be obtained.

[4-3] Rub Cleaning Process In the method for manufacturing a self-supporting substrate of the present invention, it is preferable to perform a cleaning process in order to remove impurities on the surface of the GaN-based semiconductor wafer attached by the above-described growth process and polishing process. In particular, when the abrasive particles that easily adhere to the polishing step contain Si, the electronic device characteristics are easily affected. Therefore, it is preferable to perform scrub cleaning in order to reduce the concentration of the Si impurity.

The rubbing cleaning is a method in which, after the polishing step, a cleaning liquid is supplied instead of the polishing liquid and the deposits are physically scraped off.
As the cleaning liquid, an optimum one is selected according to the object to be removed and the crystallinity of the surface of the free-standing substrate.
Usually, a chemical solution having an effect of removing deposits such as a surfactant is used. For example, when an abrasive particle containing Si is to be removed, an alkaline solution is preferable, and an aqueous KOH solution is particularly preferable. The concentration of KOH is usually 1% by weight or more, preferably 3% by weight or more, more preferably 5% by weight, usually 50% by weight or less, preferably 40% by weight or less, more preferably 30% by weight or less. If the alkali concentration is too high, the surface roughness may be deteriorated by the etching action, and if it is too low, a sufficient cleaning effect cannot be obtained.

In addition, it is preferable to change the cleaning liquid depending on the object to be removed and to perform several stages of cleaning, since impurities can be further reduced.
Since the rubbing cleaning is usually performed following the above-described polishing step, a nonwoven fabric or a foamable resin is used for the polishing cloth.
[4-4] Post-process After the rubbing cleaning process, as a finishing process, a self-supporting substrate as a product can be obtained through optional processes such as wax cleaning, pure water cleaning, and drying.

Wax cleaning is a process used to fix the self-supporting substrate to the apparatus in the polishing process and the rubbing cleaning process, and to remove the wax adhering to the surface. Usually, washing | cleaning by immersing in an organic solvent is mentioned. As the organic solvent, alcohol can usually be used. For example, isopropyl alcohol can be used in accordance with the properties of the wax.
The pure water cleaning is a process for finally removing impurities. Usually, it is washed with running water using pure water.

  In the drying, liquid such as pure water attached to the self-supporting substrate is finally removed. From the viewpoint of uniform drying, it is preferable to perform spin drying.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, they are for the purpose of explaining the present invention, and are not intended to limit the present invention to these embodiments.
[1] Reference example 1
A sapphire substrate having a surface of (0001) face and a thickness of 430 μm and a diameter of 2 inches was prepared as a base substrate, and this was washed with an organic solvent as a pretreatment. Thereafter, a base GaN layer having a thickness of 2 μm was grown on the base substrate by an MOCVD apparatus.

Next, after the substrate on which the underlying GaN layer has been grown is placed in the reaction chamber of the HVPE apparatus and the reaction temperature is raised to 1070 ° C., a carrier gas G1 consisting essentially of only H ₂ on the GaN layer, While supplying GaCl gas G2 which is a reaction product of Ga and HCl and NH ₃ gas G3, a GaN layer was grown on the underlying GaN layer for 45 hours. In this growth step, the growth pressure was 1.01 × 10 ⁵ Pa, the partial pressure of GaCl gas G2 was 7.33 × 10 ² Pa, and the partial pressure of NH ₃ gas G3 was 4.67 × 10 ³ Pa. .

Subsequently, the sapphire substrate was removed from the substrate on which the GaN layer was grown, and a free-standing GaN single crystal substrate having a thickness of about 1470 μm was obtained.
When the surface of the GaN crystal obtained by separation from the base substrate was observed with an AFM, it was a rough surface with irregularities exceeding 100 nm.
The surface of the GaN crystal was mechanically ground and lapped in a direction parallel to the c-plane to obtain a substantially flat GaN substrate (wafer). Next, the Ga side surface of the substrate was polished with a polishing liquid. As the polishing liquid, an acidic silica slurry having an average particle size of 80 nm was mixed with 10% by weight of an oxidizing agent (H ₂ O ₂ ) and adjusted so that the polishing rate was 50 nm / hour.

The RSM roughness of the surface obtained by polishing was reduced to less than 0.1 nm within a 1 × 1 μm ² area. As a result of observing the cross-sectional lattice image by TEM, the thickness of the work-affected layer was 3 nm.
[2] Reference example 2
After obtaining a free-standing GaN single crystal substrate having a thickness of about 1470 μm under the same conditions as in the growth method of Example 1, the composition of the above polishing liquid was changed, and polishing was performed at a polishing rate of 2000 nm / hour. The RMS roughness of the obtained surface was less than 0.2 nm within a 1 × 1 μm ² area, but the thickness of the work-affected layer by TEM observation was 100 nm.

From the results of Reference Examples 1 and 2, it was confirmed that the thickness of the work-affected layer due to lattice disturbance in the cross-sectional lattice image could be reduced by passing through the polishing step.
[3] Example 1
A sapphire substrate having a surface of (0001) face and a thickness of 430 μm and a diameter of 2 inches was prepared as a base substrate, and this was washed with an organic solvent as a pretreatment. Thereafter, a base GaN layer having a thickness of 2 μm was grown on the base substrate by an MOCVD apparatus.

Next, after the substrate on which the underlying GaN layer has been grown is placed in the reaction chamber of the HVPE apparatus and the reaction temperature is raised to 1070 ° C., a carrier gas G1 consisting essentially of only H ₂ on the GaN layer, While supplying GaCl gas G2 which is a reaction product of Ga and HCl and NH ₃ gas G3, a GaN layer was grown on the underlying GaN layer for 45 hours. In this growth step, the growth pressure is set to 1.01 × 10 ⁵ Pa and the partial pressure of the GaCl gas G2 is set to 7.06 × 10 6.
² Pa, and the partial pressure of NH ₃ gas G3 was 4.35 × 10 ³ Pa.

Next, the sapphire substrate was removed from the substrate on which the GaN layer was grown, and a free-standing GaN single crystal substrate having a thickness of about 1370 μm was obtained.
The surface of the crystal obtained by separation from the base substrate was mechanically ground and lapped in a direction parallel to the c-plane. Next, the Ga side surface of the substrate was polished with a polishing liquid. As the polishing liquid, an acidic silica slurry having an average particle size of 80 nm was mixed with 10% by weight of an oxidizing agent (H ₂ O ₂ ) and adjusted so that the polishing rate was 50 nm / hour.

Next, the surface of the GaN substrate was scrubbed by rubbing a polishing cloth while supplying acid, alkali, and surfactant as cleaning liquids, and residues such as abrasives were removed. Impurities remaining on the surface are reduced, and the peak intensity ratio of the ion mass spectrum of Si / Ga on the surface by TOF-SIMS (primary ion Au, primary ion acceleration voltage 25 kV, scanning area 200 μm square, secondary ion integration time 150 seconds) Was 0.001. Moreover, the SiO ₂ / Ga peak intensity ratio under the same conditions was 0.04.

This free-standing GaN single crystal substrate is based on the Schottky diode capacitance-voltage measurement.
It was confirmed that the conductivity type was n-type, and the effective donor concentration was 1 × 10 ¹⁶ atoms / cm ³ . In addition, as a result of performing the same measurement on another free-standing GaN single crystal substrate obtained by the same manufacturing method, the conductivity type is n-type, and the effective donor concentration is 8 × 10 ¹⁶ atoms / cm ³ and 1.5 × 10 ^17. atoms / cm ³ .
Residual impurity concentration evaluation, dislocation density measurement, crystallinity evaluation, and thermal conductivity measurement were performed at a place excluding 3 mm around the obtained self-standing GaN single crystal substrate. The impurity concentrations of oxygen, carbon and hydrogen were all below the detection limit of SIMS measurement. It was confirmed that the effective donor concentration of this free-standing GaN single crystal substrate was derived from Si as a residual impurity. Also, the dislocation surface density was confirmed to be 3 × 10 ⁶ cm ⁻² by cathodoluminescence (CL) method and evaluation by AFM.

Furthermore, in order to evaluate the surface quality of the support substrate after the above-mentioned surface treatment, surface roughness measurement by AFM and lattice image observation by a cross-sectional TEM method were performed.
FIG. 3 is a diagram showing the results of surface roughness measurement by AFM, and it can be seen that the irregularities on the surface of the support substrate are in the range of ± 1 nm.
Further, as a result of analyzing the lattice image photographed by the cross-sectional TEM method, at least no disorder (strain) was observed in the crystal lattice of the second atomic layer from the outermost surface of the support substrate.

[4] Reference example 3
After obtaining a free-standing GaN single crystal substrate having a thickness of about 1370 μm under the same conditions as in the growth method of Example 1, washing with an acid, alkali, or surfactant as a washing solution without rubbing the polishing cloth, When impurities remaining on the surface were measured, the intensity ratio of Si / Ga on the surface by TOF-SIMS (acceleration voltage 25 Kv, integration for 150 seconds) was 0.02. Further, the SiO ₂ / Ga peak intensity ratio under the same conditions was 0.16.

From the results of Example 1 and Reference Example 3, it was confirmed that impurities such as Si that easily adhere to the manufacturing process could be reduced through the rubbing cleaning process.
[5] Formation of Schottky Electrode A Schottky electrode was formed directly on the surface (main surface) of the self-standing substrate (hereinafter referred to as “support base”) subjected to the surface treatment of Example 1.

First, the support substrate was cleaned. Specifically, sulfuric acid / hydrogen peroxide mixture (SPM) treatment, hydrochloric acid treatment and aqua regia treatment were performed, followed by HF treatment. Thereafter, the substrate was thoroughly washed with pure water and sprayed with nitrogen gas to remove the pure water on the surface of the supporting substrate and dry it. Note that the cleaning procedure may be partially omitted if there is no concern about contamination or surface alteration in consideration of the wafer storage environment before the process.
Prior to forming the Schottky electrode, an ohmic electrode is first formed. Since the support substrate of this example is a self-standing GaN single crystal substrate, an ohmic electrode was formed on the back surface. The electrode was formed by depositing 20 nm of Ti and 200 nm of Al by heat-treating at about 750 ° C. for 3 minutes in N ₂ to form an ohmic electrode. In the case of a supporting base with a base substrate attached, the ohmic electrode may be formed on a part of the main surface made of a GaN-based free-standing substrate.

Next, after ultrasonic cleaning in methanol, a Schottky electrode was formed. First, Ni was used as the metal for the Schottky barrier, and a circular Schottky electrode was formed on the entire surface of the support base by vapor deposition using a metal mask.
FIG. 4 is a cross-sectional view for explaining the structure of the Schottky diode obtained by the above-described procedure. A Ti / Al ohmic electrode (50) is provided on the back surface of the support base n-GaN (40). A plurality of Ni Schottky electrodes (60) having a diameter of 100 to 300 μm are directly formed on the surface. Note that the diameter of the Schottky electrode is not necessarily 100 to 300 μm, but it is easy to form a pattern and the number of dislocations existing in the range (about 100 to 2700 depending on the size of the electrode). ) To obtain average information of the support substrate, the diameter was set to 100 to 300 μm.

[5-1] Characteristics of Schottky Diode The characteristics of the Schottky diode including the Schottky electrode described above were evaluated.
5 and 6 are diagrams for explaining current-voltage characteristics measured for a number of Schottky diodes fabricated on a free-standing GaN crystal substrate having an effective donor concentration of 1 × 10 ¹⁶ atoms / cm ³ . 5 is a forward characteristic, and FIG. 6 is a reverse characteristic. After the production, a simple screening test is performed, and several defective diodes caused by process defects are excluded from the data.
Of the defective diodes, some diodes exhibited characteristics that increased reverse leakage current and led to breakdown. By CL image observation, a thin dark line considered to be a stacking fault parallel to the c-plane was observed in all these diodes. From this, it was obtained that the stacking fault contributes to the reverse leakage current.
Among the defective diodes, some of the other diodes showed ledge (step-like current-voltage characteristics) in the forward characteristics, and the leakage current was extremely large in the reverse direction. CL images showed thin dark lines in these diodes. This suggests that stacking faults may cause ledge in forward characteristics. Therefore, it is preferable that the self-standing substrate of the present invention has few or no stacking faults or dark lines. These diode data were excluded from the discussion below.

Table 1 shows an ideal factor n value, a barrier height, and −5V, −5V, − which are obtained from experimental data for a Schottky diode formed on a free-standing GaN crystal substrate having an effective donor concentration of 1 × 10 ¹⁶ atoms / cm ^3. It is the result which put together an example of the actual value of the reverse direction current in 50V. Table 1 also shows calculated values of reverse current, which will be described later.

  As shown in Table 1, the ideal factor (n value) obtained from the forward current-voltage characteristics is 1.02 to 1.04. In all the Schottky diodes, an n value very close to the ideal value 1 is obtained. Has been obtained. The Schottky barrier height obtained from the forward current voltage characteristics was 0.92 to 0.94 eV, which is a height necessary for power device application.

FIG. 7 is a diagram showing a distribution of n value and Schottky barrier height of a Schottky diode fabricated on a free-standing GaN crystal substrate having an effective donor concentration of 1 × 10 ¹⁶ atoms / cm ³ .
The reverse current voltage characteristics will be described. As a current transport mechanism of an ideal Schottky diode reverse current without a defect or a transition layer, there are two mechanisms of thermal electron emission (TE) and thermal field emission (TFE). The current value is the sum of these two mechanisms, but one of them is dominant depending on the barrier height, doping density, applied voltage, and the like. Within the scope of this discussion, thermal field emission (TFE) is dominant.

  As shown in FIG. 6 and Table 1, the reverse leakage current of the manufactured diode is extremely small. The leakage current value in the reverse direction is the sum of the thermoelectron emission model and the thermal field emission model using only the Schottky barrier height obtained from the actual measurement value in the forward direction and without using any other fitting parameters. It almost agrees with the calculated theoretical current value. That is, a situation was achieved in which the forward characteristics and the backward characteristics were consistently and quantitatively consistent with the ideal model. In addition, the height of the Schottky barrier at this time is a large value of 0.93 eV, which is a value sufficient for use in an actual device.

From the results shown in FIG. 6 and Table 1, the reverse voltage is 10 times the sum of the calculated values of the thermionic emission current and the thermal electron emission current considering the barrier reduction due to the image power under the condition of at least −50V. You can see that it is below.
Note that the degree of difference between the measured current value and the current value obtained by the model calculation differs for each diode. Considering this fact, the main factors that cause the measured current value to be larger than model calculations are surface contamination that has not been completely removed, defects, and defects such as threading dislocations contained in the crystal. It is believed that there is. Therefore, it is considered that if the manufacturing process is further optimized, at least variations can be suppressed and the entire device can be converged to the currently obtained device having the smallest current value. However, for practical use, it is sufficient that it is 50 times or less, preferably 10 times or less of the theoretical value, and it is considered that practical demands can be fulfilled with this device.

There are diodes whose measured current value is slightly lower than the current value obtained by model calculation, but this is an approximate expression used for the calculation, especially when the voltage is low, the error tends to be large, The reason may be that there are some errors in the physical properties such as effective mass used in the calculation.
Next, a theoretical calculation formula for the forward current of the diode will be described. The forward current-voltage characteristic in an ideal Schottky junction is given by the following equation (1).

Here, J _s is the saturation current density and is given by the following equation (2). Also, ^{A *} is the effective Richardson (Richardson) constant defined by the following formula (3) in the case of GaN, than the electron effective mass ^{_{(m n * = 0.23m n0)}} , A * = 28.9A It is given by cm ⁻² · K ⁻¹ . Note that e is an elementary charge, k is a Boltzmann constant, T is an absolute temperature, and h is a Planck constant.

The forward current-voltage characteristic of an ideal Schottky diode is given by the above equation (1), but the forward characteristic of an actual element is experimentally determined using an ideality factor (n). It is given by the following formula (4).

  The ideal factor n is n = 1 in the most ideal element. The n value becomes larger than 1 when there is a current caused by electrons passing through the Schottky barrier due to a tunnel phenomenon or a tunnel current via a surface defect.

The barrier height φ _B is obtained from the following equation (5) using the above equation (2).

The saturation current density J _s is experimentally obtained by extrapolating the measured IV characteristics to V = 0 in the semilogarithmic graph.

  Next, the theoretical calculation formula of the reverse current will be described below. The reverse current due to thermionic emission (TE) is given by the following equation (6) where the applied voltage is V. This is a current due to electrons that cross the barrier due to thermal energy.

  When the electric field of the semiconductor at the Schottky interface is large, the reduction of the Schottky barrier due to the image force must be taken into consideration. The equation describing thermionic emission (TE) taking this into account is the following equation (7) (see T. Hatakemaya et al., Materials Science Forum Vols. 389-393 (2002) pp. 1169-1172).

  The electric field strength E at the Schottky junction interface is expressed by the following formula (8).

Here, Nd is an effective donor concentration, ε ₀ is a vacuum dielectric constant, ε _s is a relative dielectric constant, and Vd is a diffusion potential.
In the present invention, the theoretical calculation related to thermionic emission (TE) uses the above equation (7) considering the reduction of the Schottky barrier barrier.

On the other hand, based on the thermal field emission (TFE) model, considering the electrons distributed to the higher energy side than the Fermi level due to thermal energy, the current density due to electrons passing through the barrier by the tunnel effect (TEF current density: The approximate expression of J _TEF ) is given by the following expression (9) where the applied voltage is V, the electric field strength at the Schottky junction interface is E, and the effective Richardson constant is A ^* (T. Hatakemaya et al., Materials Science Forum Vols .389-393 (2002) pp.1169-1172).

  However,

  The electric field intensity E at the Schottky junction interface is expressed by the above equation (8).

  When the doping density is very high, the potential barrier formed at the Schottky interface becomes very thin, and a state in which almost Fermi level electrons tunnel. This is called field emission, but as a physical phenomenon, it is only one of the special situations of thermal field emission, and is included in thermal field emission. However, in this situation, the accuracy of the above expression is not high as an approximate expression, but it is preferable to obtain it using the following expression (10) (T. Hatakemaya et al., Materials Science Forum Vols. 389-393 (2002) pp) .1169-1172).

  The thermal field emission current may be calculated using the equations (9) and (10), and the one with the larger value may be adopted.

In the case of the doping density and the Schottky barrier height of the device fabricated this time, the field emission current is extremely small, and the amount of current is negligible with respect to other current components.
In addition, as a physical property constant of GaN necessary for the calculation, a value that is trusted to some extent at the present time was used. Specifically, the effective mass of electrons is 0.23 m ₀ (where m ₀ is the mass of electrons), the relative permittivity is 10.4, and the conduction band effective state density necessary for calculating the diffusion potential is 2.74. Each of × 10 ¹⁸ cm ^-3 was used.

Details of the calculation procedure are described below.
In the semi-logarithmic plot of FIG. 5 showing an actual measurement example (forward current-voltage characteristic) of the current-voltage characteristic (forward characteristic) of the Schottky diode, fitting is performed in a linear region where series resistance can be ignored, and n value is obtained. The Schottky barrier height is obtained from the intercept obtained by extrapolating the straight line to the current axis. This time, since the region of the voltage 0.15V to 0.45V is almost completely a straight line, this region was used.

Next, in order to measure an accurate value of the effective donor concentration, the capacitance-voltage characteristic of this diode is measured. The GaN used this time was cut out from a large crystal, and the doping uniformity in the thickness direction is extremely excellent. Reflecting the uniform doping, the 1 / C ² -V plot gives a clean straight line. It was calculated effective donor density _{N d} from the slope of -5V~0V region of the straight line. Diffusion potential V _d, which can also be determined with this linear extrapolation from the graph, since the error due to extrapolation is slightly larger, by calculation using the effective density of states.
Of course, the result obtained by calculation is almost the same as the value obtained by extrapolation.

  Using the measured Schottky height and effective donor concentration, together with the basic physical constants of GaN described above, thermionic emission current and thermal field emission current considering the barrier drop due to image power are calculated, and the sum of both is calculated. The reverse current value of an ideal Schottky diode is used. The thermoelectron emission current is obtained from the above equation (7). Moreover, the larger one of the values calculated | required by the said Formula (9) and the said Formula (10) is employ | adopted for the said thermal field emission current. In the present embodiment, the sum of the calculated values according to the equations (7) and (9) is employed as the calculated value of the reverse current. The current value greatly depends on the temperature. The temperature of the wafer stage at the time of diode measurement was recorded, and the temperature of the wafer stage was used for the calculation. Although standard measurement is performed near room temperature, it may be performed at a temperature such as −100 ° C. to 600 ° C. depending on the device application. As an example, the measurement results shown in this example are those obtained when the temperature of the wafer stage (self-standing substrate) is 16 ° C.

FIG. 8 shows the results of evaluating reverse characteristics up to −200 V (current-voltage) for several diodes formed on a free-standing GaN crystal substrate having an effective donor concentration of 1 × 10 ¹⁶ atoms / cm ^3. FIG.
Although the difference from the theoretical value is slightly increased as compared with the low voltage up to -50V, the current value obtained by the TFE model is also within 10 times at -200V. This is the reason why the current value gradually deviates from the theoretical value in the reverse direction, but there are several possible reasons. The first reason is the possibility that impact ionization has occurred to a small extent. This is a GaN original phenomenon that has nothing to do with defects, so it should be included in the theoretical calculation to be compared with the experimental results. However, an accurate value of the electric field dependence of the impact ionization coefficient is necessary for the calculation, There are few reliable reports. The second reason is considered to be the influence of several hundreds of dislocations included in the device. The third reason is that the manufactured Schottky diode is not provided with a guard ring, and thus the contribution of current due to electric field concentration around the electrode can be considered.

To evaluate the quality of the support substrate and the appropriateness of the fabrication process in R & D and trial production, a method that compares the theoretical value with the actual measurement value under a relatively low voltage condition such as -5 V or -50 V is simple and practical. It is.
Note that the above-described electrical characteristics in this example were obtained with a Schottky diode directly formed on a self-standing GaN single crystal substrate having a carrier concentration of 1 × 10 ¹⁶ atoms / cm ^3. Has confirmed that it can also be obtained in a Schottky diode directly formed on a free-standing GaN single crystal substrate having a carrier concentration of 8 × 10 ¹⁶ atoms / cm ³ or 1.5 × 10 ¹⁷ atoms / cm ³ .

  That is, in the Schottky diode in which the main surface of the present invention is directly formed on the main surface of the support base made of a GaN-based freestanding substrate, the ideal factor n value is 1.0 or more, preferably 1.3 or less, preferably 1.2 or less, more preferably 1.1 or less, and a current value when a reverse voltage of −5 V is applied is 50 times or less of the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermionic emission model. , Preferably 10 times or less, more preferably 3 times or less.

Preferably, the current value when a reverse voltage of −50 V is applied is 50 times or less, preferably 10 times or less, more preferably 3 times the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermal electron emission model. Is less than double.
More preferably, the current value when the reverse voltage -200 V is applied is not more than 50 times, preferably not more than 10 times, more preferably not more than the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermionic emission model. 3 times or less.

FIG. 9 shows the reverse voltage − calculated by the theoretical model for the Schottky diode of the present invention.
The current value in 5V and -50V, a diagram has been plotted against the effective donor concentration _{N d,} Schottky barrier height of the opposite direction when the 0.93EV, the theoretical current value at an applied voltage of -5V and -50V It is shown with a solid line. In addition, the white circle and black circle shown in the figure are measured values under the respective reverse voltage conditions. From the results shown in FIG. 9, in the Schottky diode of the present invention, the current value when the reverse voltage of −5 V is applied is almost equal to the current value calculated by the model, and an extremely good Schottky barrier is formed. I understand.

The theoretical current value in the reverse direction largely depends on the Schottky barrier height and the effective donor concentration. As the effective donor concentration increases, the thermal field emission component becomes larger and the reverse current increases. In particular, when the reverse bias voltage is large, the tunnel probability increases, so that the value rapidly increases at -50V compared to -5V. In the region where the effective donor concentration is 10 ¹⁵ cm ⁻³ or less, thermionic emission becomes dominant, and the current value has a gentle dependence on the reverse bias voltage.

As shown in FIG. 9, current measured values (white circles and black circles) at reverse bias voltages −5 V and −50 V of the diodes fabricated this time are all within 10 times the theoretically calculated values. It can be seen that the key barrier is very good.
In addition, even in a diode in which a Schottky electrode is directly formed on the surface of a GaN-based thick film epitaxially grown by MOCVD or MBE on a base substrate, its device characteristics are good, and an ideal factor n value in current-voltage characteristics 1.0 to 1.1, and the current value when reverse voltage −5 V is applied is not more than 50 times the theoretical current value calculated as the sum of the calculated values of the thermal field emission model and the thermionic emission model, preferably 10 times or less, more preferably 3 times or less.

  Conventionally, it has been considered that when a device region is formed directly on a free-standing substrate surface (main surface) made of a GaN-based semiconductor, favorable device characteristics cannot be obtained. Even if a Schottky diode is directly formed without forming an epitaxial layer on the main surface), the ideal factor (n value) is close to 1, and a high Schottky barrier height and low leakage current characteristics are obtained. Therefore, industrial fields that use free-standing substrates composed of high-quality GaN-based semiconductors, such as the field of field-effect transistors in GaN-based materials with Schottky barriers as gates, semiconductors such as LEDs and LDs. In the field as a substrate for crystal growth of a light-emitting element, its applicability is extremely high.

100 HVPE apparatus 10 Reaction chamber 20 Heater 30 Substrate support 40 n-GaN
50 Ti / Al ohmic electrode 60 Cu / Ni Schottky electrode

Claims (15)

  When a Schottky diode is formed by using Ni as a metal electrode directly on the surface of the free-standing substrate made of a GaN-based semiconductor, the ideal factor n value in current-voltage characteristics is 1 or more and 1.3 or less. A self-supporting board characterized by   2. The self-standing substrate according to claim 1, wherein when a Schottky diode is directly formed on the surface of the self-standing substrate using Ni as a metal electrode, a current value when a reverse voltage of −5 V is applied is a thermal field emission model and a heat A self-supporting substrate characterized by being 50 times or less of a theoretical current value calculated as a sum of calculated values of an electron emission model.   When a Schottky diode is formed by using Ni as a metal electrode directly on the surface of the free-standing substrate made of a GaN-based semiconductor, the current value when a reverse voltage of -5 V is applied is a thermal field emission model and a thermoelectron. A self-supporting substrate characterized by being 50 times or less of a theoretical current value calculated as a sum of calculated values of an emission model.   The self-standing substrate according to any one of claims 1 to 3, wherein the damage depth of the surface measured by observing the disorder of the lattice in the cross-sectional lattice image observation by TEM is 10 nm or less. The peak intensity ratio of the ion mass spectrum of Si / Ga measured by TOF-SIMS at a primary ion Au, a primary ion acceleration voltage of 25 kV, a scanning region of 200 μm square, and a secondary ion integration time of 150 seconds is 0.01 or less. Item 5. The self-supporting substrate according to any one of Items 1 to 4. 6. The self-supporting substrate according to claim 1, wherein an RMS value at 1 × 1 μm ² measured by an AFM (Atomic Force Microscopy) in a region excluding a peripheral area of 1 mm is 1 nm or less. The self-supporting substrate according to any one of claims 1 to 6, wherein the dislocation density is 1 x 10 ⁷ cm ^-2 or less. The GaN-based semiconductor is made of Al _x In _y Ga _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, [x + y] <1). A self-supporting substrate as described in 1.   9. The self-supporting substrate according to claim 8, wherein x + y <0.5.   The self-standing substrate according to any one of claims 1 to 9, wherein the thermal conductivity at room temperature is 250 (W / m · K) or more. A method for manufacturing a self-supporting substrate according to any one of claims 1 to 10,
(1) A growth process for growing a GaN-based semiconductor, (2) a polishing process, and (3) a rubbing cleaning process.   A method for manufacturing a self-supporting substrate according to any one of claims 1 to 10, comprising a growth step of growing a GaN-based semiconductor by an HVPE (Hydride Vapor Phase Epitaxy Growth) method. Manufacturing method. In the growth process,
A carrier gas containing H ₂ gas, GaCl gas, and NH ₃ gas are supplied to the reaction chamber, the growth temperature is set to 900 ° C. or more and 1200 ° C. or less,
The growth pressure is set to 8.08 × 10 ⁴ Pa or more and 1.21 × 10 ⁵ Pa or less,
The partial pressure of GaCl gas is set to 1.0 × 10 ² Pa or more and 1.0 × 10 ⁴ Pa or less,
The partial pressure of NH ₃ gas is set to 9.1 × 10 ² Pa or more and 2.0 × 10 ⁴ Pa or less,
The manufacturing method of the self-supporting substrate according to claim 11 or 12.   The method for manufacturing a self-supporting substrate according to any one of claims 11 to 13, further comprising a polishing step of setting a polishing rate to 1000 nm / hour or less after the growth step.   The method for manufacturing a self-supporting substrate according to any one of claims 12 to 14, further comprising a rubbing cleaning step after the growth step.

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