CN111965510A - Method and system for testing intrinsic photoconductivity of high-resistivity wide-bandgap semiconductor material - Google Patents
Method and system for testing intrinsic photoconductivity of high-resistivity wide-bandgap semiconductor material Download PDFInfo
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Abstract
A method for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material comprises the following steps: preparing a photoconductivity test sample in which a solid electrode serving as an anode and a hollow electrode serving as a cathode and covered with a transparent conductive film are respectively formed on the two side surfaces in the thickness direction of a substrate made of a high-resistivity wide-bandgap semiconductor material, and connecting the photoconductivity test sample to a test circuit; applying a laser beam to the transparent conductive film in a direction perpendicular thereto through a diaphragm; adjusting the aperture of the diaphragm, and acquiring a voltage peak value at two ends of a capacitor connected in a test circuit and a voltage peak value of a current detector for detecting current change caused by a photon-generated carrier generated by a photoelectric conductivity test sample under the irradiation of a laser beam by using an oscilloscope under each aperture; calculating the minimum on-state resistance of the photoconductivity test sample under each aperture according to the voltage peak value of the two ends of the capacitor and the voltage peak value of the current detector; and fitting the plurality of groups of apertures and the minimum on-state resistance to obtain the intrinsic photoconductivity of the substrate.
Description
Technical Field
The invention relates to the technical field of testing of semiconductor materials and devices, in particular to a method and a system for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material.
Background
High voltage photoconductive switches are a device of great interest for high pulse power systems such as accelerators, radars and high power supplies. In high resistivity wide bandgap semiconductor materials, for example, vanadium compensated semi-insulating 4H silicon carbide (SiC) has high dark resistivity (greater than 10 eV) due to its wide bandgap (3.26eV)11Omega cm), high critical field intensity (3MV/cm), high electron saturation velocity (2.0X 10)7cm/s) and high thermal conductivity (4.9W/cm. cndot.), is an ideal material for preparing epitaxial photoconductive test samples with compact structure and high pressure resistance.
Multiple research works prove the feasibility and the superiority of the photoconductive switch, and are expected to promote the application of the silicon carbide photoconductive switch in the fields of accelerators, radars and the like. For high power output systems, the switch resistance should be as low as possible, but so far the best performing switch has only reached an on-resistance of 1 Ω, which is directly related to the conductivity of the silicon carbide single crystal material under optical excitation. Conventional test methods currently only give the total on-resistance of the device in the test circuit, but may include contact resistance and impedance in the test circuit in addition to the silicon carbide substrate. Therefore, a method for accurately measuring intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material is needed.
Disclosure of Invention
The problems to be solved by the invention are as follows:
aiming at the problems in the prior art, the invention aims to provide a method and a system for testing the intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material, which can be simply and reliably realized.
The technical means for solving the problems are as follows:
the invention provides a method for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material, which comprises the following steps of:
1) preparing a photoconductivity test sample, wherein solid electrodes serving as anodes and hollow electrodes serving as cathodes and covered with transparent conductive thin films are respectively formed on the two side surfaces in the thickness direction of a substrate made of a high-resistivity wide-bandgap semiconductor material, and the photoconductivity test sample is connected into a test circuit;
2) applying a laser beam to the transparent conductive film through a diaphragm in a direction perpendicular to the transparent conductive film;
3) adjusting the aperture of the diaphragm, and acquiring a voltage peak value at two ends of a capacitor connected in the test circuit and a voltage peak value of a current detector for detecting current change caused by a photon-generated carrier generated by the photoconductivity test sample under the irradiation of the laser beam by using an oscilloscope under each aperture;
4) calculating the minimum on-state resistance of the photoconductivity test sample under each aperture according to the voltage peak value of the two ends of the capacitor and the voltage peak value of the current detector;
5) and fitting multiple groups of the apertures and the minimum on-state resistance to obtain the intrinsic photoconductivity of the substrate.
According to the present invention, the intrinsic photoconductivity of the silicon carbide substrate can be obtained by using the area of the laser excitation region as a measurement variable by fitting a plurality of sets of diaphragm apertures and the corresponding minimum on-resistance under each aperture by changing the aperture of the diaphragm to the area of the laser trigger region (laser spot area) on the substrate according to the present invention. The light path building and the electrical connection are removed, and the whole test system only comprises a pulse laser, a silicon carbide photoconductivity test sample, an oscilloscope and the like, so that the test method is simple and reliable.
In the present invention, the solid electrode may be formed on one surface of the substrate in the thickness direction, the ring-shaped hollow electrode having an outer diameter equal to the diameter of the solid electrode may be formed on the other surface of the substrate opposite to the one surface, and the transparent conductive thin film may be covered on the hollow electrode. The resistivity of the area irradiated by laser is reduced due to the existence of photon-generated carriers, the rest area keeps a semi-insulating state, and the photoconductivity test sample of the structure can be directly triggered by irradiating the substrate with laser through the transparent conductive film.
In the present invention, in step 1), the solid electrode is a metal thin film with high light reflection characteristics prepared by depositing Al, Ag, Au, Pt, Ti, or Ni, and the hollow electrode is a metal thin film prepared by depositing Ni, Ti, Au, or Pt. Therefore, the solid electrode not only acts as an anode of the photoconductivity test sample, but also can reflect the laser beam which enters and penetrates through the substrate back to the inside of the substrate by virtue of the high light reflection characteristic, thereby improving the utilization rate of the laser pulse.
In the present invention, in step 1), the transparent conductive thin film may be a highly light-transmitting and highly conductive thin film made of aluminum-doped zinc oxide, gallium oxide, indium tin oxide, graphene, or a transparent metal film, and the material thereof is preferably aluminum-doped zinc oxide. Therefore, the transparent conductive film can conduct photon-generated carriers by virtue of good conductive performance of the transparent conductive film, and laser beams can penetrate through the film and vertically enter the substrate.
In the present invention, in step 2), the wavelength of the laser light may be 355nm to 1064 nm. By applying laser pulses with different wavelengths to the direction vertical to the plane of the transparent conductive film, the transient intrinsic photoconductivity of picoseconds to nanoseconds in the high-resistivity wide-bandgap semiconductor material can be detected.
In the present invention, in step 3), the aperture of the aperture may be set to five or more values within a range from 0.5mm to the inner diameter of the hollow electrode. Thereby, the minimum on-resistance for different laser trigger areas can be obtained by using the novel measurement method of the variable aperture method.
In the present invention, the substrate is preferably made of high-resistivity high-purity silicon carbide, vanadium-doped silicon carbide or unintentionally doped silicon carbide, and has a resistivity ranging from 1e3 Ω · m to 1e11 Ω · m and a thickness ranging from 0.2mm to 5 mm.
The invention also provides a test system for the intrinsic photoconductivity test method of the high-resistivity wide-bandgap semiconductor material, which comprises the following steps: a laser for emitting the laser beam; the diaphragm with variable aperture for adjusting the diameter of the laser beam; a test circuit connected to at least the capacitor, the current detector and the sample for measuring optical conductivity, the diaphragm being located between the laser and the sample for measuring optical conductivity; and the oscilloscope is used for acquiring the voltage signal in the test circuit.
In the present invention, the test circuit may include: a power source; a circuit protection resistor; a capacitor for storing electrical energy; a load resistance; the photoconductivity test sample; and the current detector; the power supply, the circuit protection resistor and the capacitor are connected in series to form a closed loop; the photoconductivity test sample is connected with the current detector in series and then is connected with two ends of the capacitor in parallel; and the two ends of the capacitor are also connected in parallel with a high-voltage probe for measuring voltage. The test circuit is simple in structure, and can effectively extract the parameters to be tested of the photoconductivity test sample.
In the present invention, the resistance value of the load resistor may be 10 Ω to 50 Ω, the internal resistance of the current detector is less than 1 Ω, and the voltage value of the power supply is 10V to 5000V.
The invention has the following effects:
the invention can realize the transient intrinsic photoconductivity variable aperture test of the high-resistivity wide-bandgap semiconductor material in the picosecond to nanosecond level by a simple and reliable method, has the advantages of simple whole test system, simple control, visual result and high stability, and fills the blank that no method for testing the transient photoconductivity of the high-resistivity semiconductor material in the picosecond to nanosecond level exists at present. The invention has important potential application value in the field of semiconductor photoelectric property characterization, and has important significance in the fields of national defense, leading-edge science and technology and the like, such as research and application of light guide switch devices.
Drawings
FIG. 1 is a schematic diagram of a system for testing intrinsic photoconductivity of a high resistivity wide bandgap semiconductor material according to one embodiment of the present invention;
FIG. 2 is a schematic diagram of the construction of a photoconductivity test specimen used in the test system of FIG. 1;
description of the symbols:
1. a laser;
2. a diaphragm;
3. a photoconductivity test sample;
4. a current detector;
5. a capacitor;
6. a circuit protection resistor;
7. a load resistance;
8. a power source;
9. a substrate (silicon carbide substrate);
10. a solid electrode;
11. a hollow electrode;
12. a transparent conductive film.
Detailed Description
The present invention is further described below in conjunction with the following embodiments and the accompanying drawings, it being understood that the drawings and the following embodiments are illustrative of the invention only and are not limiting thereof.
Disclosed herein is a high resistivity wide bandgap semiconductor intrinsic photoconductivity test system (hereinafter referred to as a test system) that is simple, easy to implement, and highly stable. FIG. 1 is a schematic diagram of a test system according to an aspect of the present invention.
As shown in fig. 1, the test system of the present embodiment includes a laser 1, an aperture 2, a test circuit, and an oscilloscope not shown. The laser 1 is a pulsed laser, which is mainly used for emitting laser pulses. The diaphragm 2 is a diaphragm with a variable aperture, is located between the laser 1 and a photoconductivity test sample 3, and is mainly used for controlling the diameter of a laser pulse (laser beam) emitted by the laser 1, so as to adjust the area of a laser trigger region on the photoconductivity test sample 3, specifically, the diameter of the laser trigger region is equal to the aperture of the diaphragm 2 and is changed along with the aperture size of the diaphragm 2.
The test circuit is connected with a photoconductivity test sample 3, and is mainly used for testing various parameters of the photoconductivity test sample 3 during transient conduction under the irradiation of laser pulses. The test circuit comprises a photoconductivity test sample 3, a current detector 4, a capacitor 5, a circuit protection resistor 6, a load resistor 7 and a power supply 8. The capacitor 5, the circuit protection resistor 6 for protecting the circuit and the power supply 8 are connected in series to form a closed loop, the photoconductivity test sample 3, the current detector 4 and the load resistor 7 for voltage division are connected in series and then connected in parallel to two ends of the capacitor 5, and a high-voltage probe (not shown) for measuring voltage is also connected in parallel to two ends of the capacitor 5. The photoconductivity test sample 3 generates photon-generated carriers under the irradiation of laser beams to conduct a circuit, and the blocking state is recovered after the laser beams are removed. The current detector 4 is mainly used for detecting current change caused by photo-generated carriers generated by the photoelectric conductivity test sample 3 under the irradiation of the laser beam in a circuit, and the resistance value of the current detector 4 can be a value smaller than 1 Ω such as 0.025 Ω, 0.05 Ω, and the like. The capacitor 5 is mainly used for storing electric energy and can be charged and discharged quickly when the circuit is conducted. In the test circuit of the present embodiment, the capacitance of the capacitor 5 is 22nF, the resistance of the circuit protection resistor 6 is 1.68M Ω, the resistance of the load resistor is 10 Ω to 50 Ω, and the voltage of the power supply 8 may be 10V to 5000V.
In addition, the test system adopts an oscilloscope which is not shown in the figure to collect data signals in the test circuit, and specifically, the oscilloscope is mainly used for collecting voltage signals of the high-voltage probe and voltage signals of the current detector 4.
Fig. 2 is a schematic structural view of a photoconductivity test specimen 3 used in the test system shown in fig. 1. As shown in fig. 2, the photoconductivity test sample 3 includes a substrate 9, a solid electrode 10, a hollow electrode 11, and a transparent conductive thin film 12.
The substrate 9 is made of a high-resistivity wide-bandgap semiconductor material, preferably high-purity silicon carbide, vanadium-doped silicon carbide or unintentionally doped silicon carbide, and has a resistivity in the range of 1e3 Ω · m to 1e11 Ω · m. In the present embodiment, the substrate 9 is a silicon carbide substrate made of a high-resistivity silicon carbide single crystal material, and may be a square or circular thin plate having a thickness of 0.2mm to 5mm and a side length or diameter of 5mm to 50 mm.
A solid electrode 10 as an anode is formed on one surface of the substrate 9 in the thickness direction, and the solid electrode 10 is a thin film having a high light reflecting property prepared by depositing a metal having Al, Ag, Au, Pt, Ti, Ni, or the like, and has a diameter smaller than the side length or diameter of the substrate 9, and may have a thickness of more than 20 nm. Therefore, the solid electrode 10 not only functions as an anode of the photoconductivity test sample 3, but also reflects the laser beam incident on and penetrating through the substrate 9 back to the inside of the substrate 9 by virtue of its highly reflective property, thereby improving the utilization rate of the laser pulse.
A hollow electrode 11 is formed on the other surface of the substrate 9 in the thickness direction, i.e., the surface on the opposite side of the solid electrode 10, the hollow electrode 11 is a thin film prepared by depositing a metal such as Ni, Ti, Au, Pt, etc., and the hollow electrode 11 is formed in a ring shape having an outer diameter equal to the diameter of the solid electrode 10, a ring width of 0.5mm to 2mm, and a thickness in the range of preferably 3nm to 200nm, more preferably 5nm to 100 nm.
The hollow electrode 11 is covered with a transparent conductive film 12 having a diameter larger than the inner diameter of the hollow electrode 11 and smaller than the outer diameter of the hollow electrode 11, i.e., the transparent conductive film 12 is disposed to be electrically connected to the hollow electrode 11 and constitutes a cathode of the photoconductivity test sample 3 together with the hollow electrode 11. The transparent conductive film 12 can be a film with good transparent conductive performance prepared by using materials such as aluminum-doped zinc oxide, gallium oxide, indium tin oxide, graphene or a nano-thickness transparent metal film. When the transparent conductive film 12 is prepared by using materials such as aluminum-doped zinc oxide, gallium oxide, indium tin oxide, graphene and the like, the thickness range can be 20nm to 2000nm due to the good light transmission of the transparent conductive film. When the transparent conductive film 12 is made of a metal, the thickness of the transparent metal film is generally 6nm or less, and the material may be, for example, a metal having good conductivity such as Au, Ag, Pt, or Cu. In this embodiment, the aluminum-doped zinc oxide transparent conductive film is preferable, and the thickness is preferably 20nm to 2000nm, more preferably 100nm to 350 nm.
The transparent conductive film 12 can conduct photo-generated carriers by virtue of good conductivity, and can enable laser beams to penetrate through the film to be vertically incident into the substrate due to high light transmittance. In the structure of the substrate 9, if the transparent conductive film 12 is not provided, when the spot diameter is smaller than the inner diameter of the hollow electrode 11, the photogenerated carriers are not electrically connected with the hollow electrode 11, and the oscilloscope cannot obtain an effective current signal, and if the transparent conductive film 12 is replaced by a non-transparent conductor, the laser beams with different diameters cannot vertically enter the substrate.
In the present embodiment, the resistivity of the laser-irradiated region is decreased by the presence of photogenerated carriers, and the remaining region is kept in a semi-insulating state. According to the structure of the photoconductivity test sample 3, laser beams can be utilized to irradiate the surface of the substrate 9 through the transparent conductive thin film 12 for direct triggering, and meanwhile, the area of a laser triggering area can be adjusted by adjusting the aperture of the diaphragm 2 under the same laser power density, so that the area of the laser triggering area is used as a measurement variable, the transient minimum on-state resistance (the minimum on-state resistance for short) of a test circuit under different triggering areas is obtained by changing the area of the laser triggering area, the minimum on-state resistance and the area of the triggering area are fitted to obtain the intrinsic resistivity of the substrate 9, and the intrinsic photoconductivity of the substrate 9 is obtained.
Based on the test system, the invention provides a method for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material (hereinafter referred to as a test method). The main steps of the test method of the present invention are described below.
1) A photoconductivity test sample 3 was prepared. Specifically, a solid electrode 10 is prepared on one side surface of a sheet-like substrate 9, a hollow electrode 11 is prepared on the surface on the opposite side of the above one side surface, and a transparent conductive film 12 is covered on the hollow electrode 11. The prepared photoconductivity test sample 3 was connected to a test circuit, and the side of the hollow electrode 11 covered with the transparent conductive film 12 was directed to the diaphragm 2.
2) Laser light is applied to the transparent conductive film 12 through the diaphragm 2 in a direction perpendicular to the plane of the transparent conductive film 12 by a pulse laser 1. The pulse width of the laser pulse is picosecond to nanosecond, the wavelength can be 355nm to 1064nm, and the laser power density is 100W/cm2～10MW/cm2。
3) Under the same laser power density, the diameter of the laser beam penetrating through the transparent conductive film 12 is adjusted by adjusting the aperture of the diaphragm 2 (i.e. changing the area of the laser trigger area on the substrate 9), and voltage signals of a plurality of groups of high-voltage probes under different apertures and voltage signals of the current detector 4 are acquired by using an oscilloscope. In the present embodiment, the aperture diameter of the aperture stop 2 is preferably set to five or more values within a range from 0.5mm to the inner diameter of the hollow electrode 11.
4) The minimum on-state resistance under each aperture is calculated by utilizing the peak data of the voltage of the high-voltage probe and the voltage of the current detector 4 acquired by the oscilloscope, and the three satisfy the formula (1):
Rtest＝VHV·(RCVR/VCVR)-RCVR，
wherein R istestIs the minimum on-resistance, RCVRIs the resistance, V, of the current probe 4HVIs the voltage peak, V, of the high voltage probeCVRIs the voltage peak of the current probe 4.
5) The intrinsic resistivity of the substrate 9 is obtained by fitting the data of the aperture (which can also be converted to the area of the laser trigger region) of the plurality of sets of diaphragms 2 to the minimum on-resistance under the aperture.
In particular, the minimum on-resistance RtestContains the inherent on-resistance of the substrate 9 and the induced impedance of the test circuit, so the apparent minimum on-resistance of the test can be expressed as equation (2):
Rtest＝RZ+RSiC，
wherein R isSiCIs the intrinsic on-resistance, R, of the substrate 9ZIs the inductive impedance of the test circuit.
In addition, the intrinsic on-resistance R of the substrate 9SiCThe relationship between the thickness of the substrate and the area of the laser trigger region is expressed as formula (3):
RSiC＝ρSiC·D/S，
where ρ isSiCIs the lowest intrinsic on-state resistivity of the substrate 9, i.e. the intrinsic resistivity of the substrate 9, under the trigger of the laser pulse; d is the thickness of the substrate 9; s is the laser trigger area.
For the test circuit with unchanged external conditionsInductive impedance RZCan be considered as a constant C. Thus, substituting formula (3) for formula (2), the minimum on-resistance RtestCan be represented by equation (4):
Rtest＝ρSiC·D/S+C。
for each aperture d of the diaphragm 2, there is a uniquely determined laser trigger region area S and a minimum on-resistance R at that aperture calculated by equation (1)testTherefore, in order to obtain the intrinsic resistivity ρ of the substrate 9 by fitting multiple sets of data according to equation (4) using the laser trigger region area S as a measurement variableSiC，ρSiCThe inverse of (a) is the intrinsic photoconductivity of the substrate 9.
The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.
Example 1
The optical path is set up and connected to the test circuit as shown in fig. 1. In the test circuit, the capacitance of the capacitor 5 is 22nF, and the resistance R of the current probe 4CVR0.025 omega, the resistance R of the circuit protection resistor 6L1.68M Ω, a resistance value of the load resistor 7 is 10 Ω, and a voltage V of the power supply 8DCIs 2000V.
1) In this embodiment, the substrate 9 is made of silicon carbide single crystal material. A solid electrode 10 having a diameter of 6mm and a thickness of 200nm, which is deposited from Ag, is formed on one surface of a substrate 9 having a thickness of 0.5mm and a size of 10mm × 10mm, a hollow electrode 11 having an inner diameter of 5mm, an outer diameter of 6mm and a thickness of 150nm, which is deposited from a Ni-Ti alloy, is formed on the other surface, and an aluminum-doped zinc oxide transparent conductive film 12 having a diameter of 5.5mm and a thickness of 120nm is covered on the hollow electrode 11. The prepared photoconductivity test sample 3 is connected to a test circuit.
2) A laser pulse is applied to the substrate 9 through the diaphragm 2 in a direction perpendicular to the plane of the transparent conductive film 12 by a pulse laser 1. In this embodiment, the laser wavelength emitted by the pulse laser 1 is 532nm, and the laser power density is 1MW/cm2。
3) The aperture of the aperture 2 is adjusted to 1mm, 1.5mm, 2mm, 2.5mm, 4mm, and 5mm, respectively, while maintaining the laser power density, thereby changing the spot area of the laser light irradiated on the substrate 9 through the transparent conductive film 12. Acquiring the voltage V of the high-voltage probe under the six apertures by using an oscilloscopeHVAnd the voltage V of the current detector 4CVR。
4) According to the voltage V of the high-voltage probe collected by the oscilloscopeHVAnd the voltage V of the current detector 4CVRThe minimum on-resistance R of each of the six apertures is calculated by the formula (1) as the peak value of (A)test。
5) Fitting the six apertures d with the minimum on-resistance R under each aperture by the formula (4)testObtaining the intrinsic resistivity p of the substrate 9SiCAnd the intrinsic photoconductivity of the substrate 9 is further determined. In this example ρSiC0.2 Ω · m, the intrinsic photoconductivity of the substrate 9 is therefore 5S/m.
Example 2
The optical path is set up and connected to the test circuit as shown in fig. 1. In the test circuit, the capacitance of the capacitor 5 is 22nF, and the resistance R of the current probe 4CVR0.0249 omega, the resistance R of the circuit protection resistor 6L1.68M Ω, a resistance value of the load resistor 7 is 50 Ω, and a voltage V of the power supply 8DCIs 4000V.
1) In this embodiment, the substrate 9 is made of silicon carbide single crystal material. A solid electrode 10 having a diameter of 10mm and a thickness of 100nm, which is deposited from Pt, is formed on one surface of a substrate 9 having a thickness of 0.8mm and a size of 15mm, a hollow electrode 11 having an inner diameter of 8mm, an outer diameter of 10mm and a thickness of 200nm, which is deposited from a Ni-Ti-Au alloy, is formed on the other surface, and an aluminum-doped zinc oxide transparent conductive film 12 having a diameter of 9mm and a thickness of 350nm is covered on the hollow electrode 11. The prepared photoconductivity test sample 3 is connected to a test circuit.
2) A laser pulse is applied to the substrate 9 through the diaphragm 2 in a direction perpendicular to the plane of the transparent conductive film 12 by a pulse laser 1. In this embodiment, the laser wavelength emitted by the pulse laser 1 is 1064nm, and the laser power density is 2.34MW/cm2。
3) The aperture of the aperture 2 is adjusted to 1mm, 2mm, 3mm, 4mm, 5mm, and 6mm, respectively, while maintaining the laser power density, thereby changing the laser spot area irradiated on the substrate 9 through the transparent conductive film 12. Acquiring the voltage V of the high-voltage probe under the six apertures by using an oscilloscopeHVAnd the voltage V of the current detector 4CVR。
4) According to the voltage V of the high-voltage probe collected by the oscilloscopeHVAnd the voltage V of the current detector 4CVRThe minimum on-resistance R of each of the six apertures is calculated by the formula (1) as the peak value of (A)test。
5) Fitting the six apertures d with the minimum on-resistance R under each aperture by the formula (4)testObtaining the intrinsic resistivity p of the substrate 9SiCAnd the intrinsic photoconductivity of the substrate 9 is further determined. In this example ρSiC0.8 Ω · m, the intrinsic photoconductivity of the substrate 9 is therefore 1.25S/m.
The method and the system for testing the transient intrinsic photoconductivity variable aperture applied to the high-resistivity silicon carbide single crystal material in picosecond to nanosecond magnitude are described above, but the method and the system can also be applied to high-resistivity GaN, high-resistivity ZnO and high-resistivity Ga2O3And testing the transient intrinsic photoconductivity of the high-resistivity AlN, the high-resistivity diamond and other semiconductor materials in a picosecond to nanosecond order.
According to the invention, by designing a photoconductivity test sample with a transparent conducting layer structure, adjusting the area of a laser trigger area of a semiconductor by changing the aperture of a diaphragm, combining a theoretical formula of transient on-resistance obtained by taking the area of a laser excitation area as a measurement variable, a material resistance formula and a test circuit for detecting transient photocurrent signals, which are discovered by the inventor of the application for the first time, and adopting the fitting of actually measured photocurrent data and the theoretical formula, the method for simply and reliably measuring the intrinsic photoconductivity of the high-resistivity silicon carbide single crystal material is realized. The invention is not interfered by the parasitic inductance or the parasitic capacitance of the test circuit, and can effectively extract the intrinsic photoconductivity of the substrate. Meanwhile, the light path building and the electrical connection are removed, and the whole test system only comprises a pulse laser, a silicon carbide photoconductivity test sample, an oscilloscope and other components, so that the test method is overall simple, simple to control, visual in result and high in stability.
The above embodiments are intended to illustrate and not to limit the scope of the invention, which is defined by the claims, but rather by the claims, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.
Claims (10)
1. A method for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material is characterized by comprising the following steps:
1) preparing a photoconductivity test sample, wherein solid electrodes serving as anodes and hollow electrodes serving as cathodes and covered with transparent conductive thin films are respectively formed on the two side surfaces in the thickness direction of a substrate made of a high-resistivity wide-bandgap semiconductor material, and the photoconductivity test sample is connected into a test circuit;
2) applying a laser beam to the transparent conductive film through a diaphragm in a direction perpendicular to the transparent conductive film;
3) adjusting the aperture of the diaphragm, and acquiring a voltage peak value at two ends of a capacitor connected in the test circuit and a voltage peak value of a current detector for detecting current change caused by a photon-generated carrier generated by the photoconductivity test sample under the irradiation of the laser beam by using an oscilloscope under each aperture;
4) calculating the minimum on-state resistance of the photoconductivity test sample under each aperture according to the voltage peak value of the two ends of the capacitor and the voltage peak value of the current detector;
5) and fitting multiple groups of the apertures and the minimum on-state resistance to obtain the intrinsic photoconductivity of the substrate.
2. The method according to claim 1, wherein the conductivity of the intrinsic light of the high resistivity wide bandgap semiconductor material is measured,
in step 1), the solid electrode is formed on one surface of the substrate in the thickness direction, the hollow electrode having a ring shape with an outer diameter equal to the diameter of the solid electrode is formed on the other surface opposite to the one surface, and the transparent conductive thin film is covered on the hollow electrode.
3. The method according to claim 2, wherein the conductivity of the intrinsic light of the high resistivity wide bandgap semiconductor material is measured,
in the step 1), the solid electrode is a metal film with high light reflection characteristics prepared by depositing Al, Ag, Au, Pt, Ti or Ni, and the hollow electrode is a metal film prepared by depositing Ni, Ti, Au or Pt.
4. The method according to claim 2, wherein the conductivity of the intrinsic light of the high resistivity wide bandgap semiconductor material is measured,
in the step 1), the transparent conductive thin film is a thin film with high light transmittance and high conductivity, which is formed by aluminum-doped zinc oxide, gallium oxide, indium tin oxide, graphene or a transparent metal film, and the material of the transparent conductive thin film is preferably aluminum-doped zinc oxide.
5. The method according to claim 1, wherein the conductivity of the intrinsic light of the high resistivity wide bandgap semiconductor material is measured,
in the step 2), the laser wavelength of the laser beam is 355 nm-1064 nm.
6. The method for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material according to claim 1 or 2,
in the step 3), more than five values are selected from the aperture of the diaphragm within the range of 0.5mm to the inner diameter of the hollow electrode.
7. The system for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material according to claim 1, wherein,
the substrate is preferably made of high-resistivity high-purity silicon carbide, vanadium-doped silicon carbide or unintentionally doped silicon carbide, the resistivity range is 1e3 Ω.m-1 e11 Ω.m, and the thickness is 0.2 mm-5 mm.
8. A test system for the method for testing the intrinsic photoconductivity of the high-resistivity wide-bandgap semiconductor material according to any one of claims 1 to 7, characterized by comprising:
a laser for emitting the laser beam;
the diaphragm with variable aperture for adjusting the diameter of the laser beam;
a test circuit connected to at least the capacitor, the current detector and the sample for measuring optical conductivity, the diaphragm being located between the laser and the sample for measuring optical conductivity; and
and the oscilloscope is used for acquiring the voltage signal in the test circuit.
9. The system for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material according to claim 8, wherein,
the test circuit includes:
a power source;
a circuit protection resistor;
the capacitor for storing electrical energy;
a load resistance;
the photoconductivity test sample; and
the current detector;
the power supply, the circuit protection resistor and the capacitor are connected in series to form a closed loop;
the photoconductivity test sample is connected with the current detector in series and then is connected with two ends of the capacitor in parallel;
and the two ends of the capacitor are also connected in parallel with a high-voltage probe for measuring voltage.
10. The system for testing intrinsic photoconductivity of a high-resistivity wide-bandgap semiconductor material according to claim 9, wherein,
the resistance value of the load resistor is 10-50 Ω, the internal resistance of the current detector is less than 1 Ω, and the voltage value of the power supply is 10V-5000V.
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