rus
Issue #4/2016
V.Luchinin
Russian electronic components for extreme conditions: silicon carbide industry founded by LETI
Russian electronic components for extreme conditions: silicon carbide industry founded by LETI
The most popular technology when creating SiC-based devices using the absolutely prevalent epitaxial technology is changing the type of adding impurity in the reactor without its "decompression", directly during
the epitaxial growth. The modern epitaxial reactor owned by LETI enables this process, including
the automatic loading of substrates.
the epitaxial growth. The modern epitaxial reactor owned by LETI enables this process, including
the automatic loading of substrates.
Теги: electronic components microelectronics nanoelectronics silicon carbide карбид кремния микроэлектроника наноэлектроника электронная компонентная база
There is no doubt that the development of silicon carbide industry is crucial for Russia as one of the priorities for solving the problems of ECB import substitution and ensuring the parity in technologies that define the country’s scientific and technological competitiveness and security. Let us have a look at some competitive solutions of extreme SiC-based ECB developed by LETI.
ECB in power impulse high-voltage electronics
In the modern foreign semi-conductor industry, silicon carbide has firmly occupied the niche of a material used in power high-voltage electronics. In the local practice, the producers have also announced the manufacture of silicon-carbide power modules. However, the basic electronic components are still imported from abroad. Therefore, these technological solutions cannot be regarded as import substitution.
As for the creation of a real domestic SiC ECB, it is worthy to note the design and development work carried out under the order of the Ministry of Industry and Trade of the Russian Federation, "Development and expansion of switching elements production with nanosecond and picosecond switching periods and working voltages of 30–3000 V" (encoded as "Apparatus 10"). The basic design solution was developed by the Center of Microtechnology and Diagnostics at Saint-Petersburg Electrotechnical University (LETI), and the main result of the work was organizing mass production of SiC-based power high-voltage electronic devices by OJSC "Svetlana" in Russia.
Drift step recovery diodes (DSRD) proposed by a group of Soviet scientists from Ioffe Institute [1] and implemented for the first time using silicon have until now been the fasted semiconductor switches. The switching time is determined by the processes occurring in the diode base, which creates an electron-hole plasma and depends in the first place on the base thickness and the saturated drift velocity of the main charge carriers. The other important parameters in the implementation of high-voltage high-current impulse switchers are electric field disruption voltage, doping level and distribution of admixtures in the base, as well as the lifespan of non-equilibrium charge carriers in the diode base. The latter is associated with the fact that under a short impulse bidirectional injection in the base, the carriers would be able to recombine until they are ejected from the base during the switching.
Based on the above parameters, band gap width and, of course, thermal conductivity, which determines the thermal advantages of the material and, consequently, the increase of the timing frequency of impulse sequences, the silicon carbide is better than silicon by an order of magnitude and is second only to diamond.
The samples of DSRD created using n+-p-p+ epitaxial technology (Fig.1) and implemented with two types of protection against disruption (of trench and meso-structure types) have the working voltages of up to 2 kV and sub-nanosecond periods of switching (600–900 ps) with the voltage impulse growth rate (dU/dt) of 3–5 V/ps. The direct fall of voltage on the diode structure is not more than 3 V, and the reverse current with the chip area of 4 mm2 is 10–8 A. The increase of working voltage by more than 10 kV is achieved by the formation of a high-voltage assembly with the preservation of the sub-nano-second switching periods of the device [2].
ECB in power transistor electronics
An important milestone in the development of the domestic technology of semiconductor SiC was the creation in Saint-Petersburg Electrotechnical University of a field-effect transistor with insulated shutter (MOSFET) based on SiC as a basic amplifying element of ECB for extreme operational modes and conditions.
In the short-term, power MIS-transistors based on 4H polytype SiC may occupy their niche in the sphere of power electronic systems due to their combined characteristics of high operational disruption voltages, low resistance in the active area, high density of the switching power and low losses during the switching, as well as a sufficiently high working frequency and temperature.
The cross-sectional view of the designed and implemented vertical MIS-transistor is shown in Fig.1a. The active part of the device is the lowly doped n--layer with the p-, n+- and p+-areas formed therein. The p+-area plays the role of a contact with the p-area and is formed with the purpose of aligning the potential between the p+-area and the source for neutralization of the parasitic n-p-n transistor. In the open state, when the shutter is applied positive voltage relative to the source, the current flows through the contact to the source, n+-area, transistor channel, J-FET area delineated by two p-n-transitions, lowly doped drift area, highly doped substrate and the contact to the source.
The transistor cell has a hexagonal shape (Fig.1b) with a channel length of 1 µm and a J-FET area width of 3 µm, and the width of shutter overlap area and n+-area of the source is 1 µm. The transistor was formed from an array of hexagonal cells and "floating" rings around the perimeter of the device, which provides a smooth field gradient in the periphery, thus avoiding a surface disruption.
The key problem in the creation of a high-performance power MIS-transistor based on 4H SiC with low channel resistance is the electric characteristics of the sub-shutter dielectric and the boundaries of the 4H-SiC/SiO2 section, which worsen the working characteristics of the device [3].
The SiC-based power MIS-transistor (Fig.1c) was created on a lowly doped epitaxial layer with a width of 11 µm and nitrogen concentration of about 7 · 1015 cm–3, grown on a commercial highly doped 4-inch SiC (0001) n-type substrate with the the angle of disorientation of 4°. The P-area with a depth of 1 µm and rectangular shape of admixture distribution with an aluminum concentration of 1·1018 cm-3 was formed using the method of ion implantation. The same method was used to implant n+- and p+-areas with a depth of 220 and 300 nm with a phosphorus and aluminum concentration of 6 · 1019 cm–3 and 1 · 1020 cm–3, respectively.
The sub-shutter dielectric was provided by application of a two-layer system of silicon nitride (5 nm) and silicon dioxide (45 nm) by the method of plasma enhanced chemical vapor deposition (PECVD) with the further oxidation in the atmosphere of dry oxygen under the temperature of 1150°C for 1 hour. The polysilicon shutter doped by phosphorus with a width of 450 nm was formed by the method of low-pressure chemical vapor deposition (LPCVD).
The analysis of the basic characteristics of the created transistor structures (Fig.2) showed that the voltage the switched-on transistor is 1.8 V (Vds=2 V, Ids=10 µA), and the transistor resistance in the switched-on state Rds spec(on) at a room temperature calculated from the angle of the output characteristics with Vgs=20 V is
29 mΩ · cm2. The density of the switching current at a room temperature is higher than 120 A/cm2 with Vds=5 V.
The transistor disruption voltage at a room temperature was 900 V. It may be increased by increasing the "floating" rings with the purpose of creating a smooth gradient of the potential on the periphery of the device, and corresponding increase of the width and decrease of the admixture concentration in the drift area. The results of comparing the main characteristics of the created SiC MIS-transistor to its analogs are shown in Fig.3.
It is noteworthy that currently power MIS-transistors based on SiC have a considerably lower resistance in the switched-on state than its silicon counterparts. However there are a number of problems which considerably limit the further decrease of their resistance, in particular, low mobility of charge carriers, which is mostly due to the high density of surface states at the boundary of the 4H-SiC/SiO2 section. The low mobility of charge carriers in the transistor channel also results in a a significant limitation of its working frequency range.
ECB in the auto-emission high-frequency electronics
The contemporary ECB development stage is characterized by the revival of vacuum micro- and nano-electronics with the purpose of achieving ultra-high frequencies of a giga- and teraherz range, ensuring a high result of "multiplication of generated power by frequency" in lamps with millimeter and sub-millimeter frequency ranges, resolving the tasks of switching and generation in ultra-short impulse electronics and X-ray devices. The vacuum auto-emission electronics is also characterized by high radiation and temperature resistance.
The most important element of a vacuum device is its sources of electrons. The effective cathodes with field emission have until now been under intensive research. Silicon carbide may be regarded as a promising material for auto-emission electronics, in the first place, due to the extreme values of the critical field strength for avalanche breakdown, thermal conductivity and mechanical strength. The additional advantages of SiC are its resistance to chemical and radiation exposure.
Such circumstances made it possible to forecast the creation of SiC-based micro-cathodes with field emission, which combine a high current density of the emission, stability of emission characteristics and acceptable low values of the electric field strength at the emission start.
In the framework of the conducted studies and developments, four technological routes for the creation of SiC-based auto-emission structures were proposed and implemented:
• formation of auto-emission spikes by the method of a sharply focused ion beam (Fig.4a);
• formation of topologically ordered arrays of auto-emission spikes by the method of reactive ion-plasma etching with metal catalyst (Fig.4b);
• formation of topologically ordered two-level auto-emission micro-sized matrix of pedestals with nano-sized spikes using a two-stage technology, which combines the processes of photolithography, reactive ion-plasma etching and catalyst micro-masking (Fig.4c);
• formation of hetero-structured two-staged matrices of auto-emitters based on carbide silicon and nano-crystalline diamond (Fig.4d).
The basis of all the auto-emission structures were mono-crystals of the n-type substrate 6H-SiC with the specific resistance of 0.05 Ω·cm. When describing the general characteristics of the results, detailed in [4–5], it should be noted that currently the implementation is stable when based on SiC micro-cathodes with field emission having the following basic parameters:
• emission start voltage of 10–15 V/µm;
• emission current density of up to 10 A/cm2;
• auto-emission spikes density of 5·108 cm–2;
• stability of work in the vacuum from 10–6 to 10–9 mm Hg.
The results of formation hetero-structured cathodes "silicon carbide – diamond" are of a particular interest [6], because they showed their temporal stability during the increase voltage at the emission start. Besides, this hetero-composition, combining the materials with a considerably different width of the band gap, can be regarded as a basic structure for solid-state auto-emission devices, which do not use vacuum, thus creating certain complexities for encapsulation.
ECB in the ultraviolet optoelectronics
The ultraviolet photo-electronics area was formed by the early XXI century due to a high practical interest to ultraviolet-range optoelectronic devices that had been paid for many years.
It is apparent that UV-range photo-detectors must be characterized by low darkness currents, high sensitivity and high performance, as well as working parameter stability. Currently, the basic materials of UV-photometry are silicon carbide, nitrides of gallium, aluminum and their solid solutions. Different optoelectronic products have been developed and are available on the market, such as photo-detectors functioning in the wavelength ranges from 180 to 400 nm. Although the first-choice materials for ultraviolet photo-electronics are GaN and AlN-GaN, the silicon carbide still occupies its stable niche in the following areas:
• high temperature UV-photo-
electronics;
• control of radiation from powerful UV-excimer lasers, UV-lamps and other sources (it is known that SiC- and diamond-based photo-detectors are the most stable to a prolonged UV-radiation exposure);
• registration of germicidal UV-radiation;
• control of ignition, flame detectors, electric spark detectors.
Currently, the most popular types of SiC-based UV-photo-detectors are photo-diodes with a minuscule p-n transition and diodes with Schottky barrier. The micro- and nano-electronic department of Saint-Petersburg Electrotechnical University developed prototypes of such photo-detection structures. The structure of the above-mentioned photo-detectors is based on epitaxial structures of 4H-SiC, obtained by the method of vapor-phase epitaxy of carbide silicon on n+ 4H-SiC substrata with a diameter of 76–100 mm, with a specific resistance of at most 0.025 Ω∙cm, with a width of 350 µm and direction deviation of 0001 – 4°. In this case, the number of the layers, their widths, conductivity types and concentration of the doping admixture are determined by the type of the photo-detection structure. At the same time, the widths and levels of doping the basic areas were chosen based on the maximum working voltage Urev. ≤15 V, which provides the full depletion of n (p) – layers. In other words, the width of the spatial charge area W is equal to the epitaxial layer thickness d (1).
The photo-detectors based on a diode with Schottky barrier are made in the form of a vertical structure with a solid semi-transparent electrode or with a cellular type electrode. The manufacture of this type UV-photo-detector implies using an n-n+ type 4H-SiC epistructure with an epitaxial layer having a width of d = 5 µm and nitrogen doping level of 6–7 ∙ 1014 cm–3.
The manufactured samples of photo-detectors with Ni-SiC Schottky barrier are characterized by a potential barrier of 1.20–1.25 eV, and the Pt-SiC structures – 1.00–1.05 eV. In all the types of structures, the reserves darkness currents at voltages of 0.5 to 1 V were at most 50 pA. According to the results of capacitance-voltage measurements, it was determined that the saturation of the C-U characteristics occurs at voltages Urev. ≥ 8 V, which corresponds to the full depletion of the basic area of the photo-detector.
The p-n transition diode based photo-detectors are made in the form of p+-n-n+ and p+-p-n+ type vertical mesa-epitaxial structures. The height of the mesa varied depending on the type of conductivity of the active basic layer. Thus, when using the p+-n-n+ type structure, the depth of SiC etching by the method of reactive ion-plasma etching (RIPE) had to be more than the width of the p+ emitter area. The materials of the ohmic contacts to the low-ohmic n+-SiC and p+-SiC areas were made from the metal compositions of Ti/Ni and Al/Ti/Ni, respectively.
It was determined that the direct branches of the VAC structures of p+-p-n+ and p+-n-n+ were not different. Practically in all the cases, the voltage of opening the p-n structures was 2.5 V.
The photoelectric characteristics of the samples were studied in the spectral range of 200–500 nm. All the measurements were carried out in the short circuit mode, and for the photo-diodes based on p+-p-n+ structures – in a photodiode mode util Urev. = 10 V. Fig.5 shows the spectral characteristics of 4H-SiC of the photo-detection structures. It also presents the spectral characteristics of the commercial 4H-SiC photo-diode SG01D-18 for comparison.
A p+-n-n+ type photo-receiver demonstrates the highest photosensitivity of S=0.138 A/Wt at a wavelength of 295 nm. At the same time, the maximum value of photo-receiver sensitivity SG01D-18 is also at 295 nm and equals to 0.13 A/Wt.
Conclusion
In the framework of development of scientific research and sample prototyping for a new competitive SiC element base, in 2016 it is planned to complete the creation of a full production line at Saint-Petersburg Electrotechnical University "LETI" for the following devices:
• power electronics (diodes with Schottky barrier, impulse high-voltage DSRD-diodes and MDP-transistors);
• high frequency electronics (vacuum diode with field emission; vacuum diode peaker, auto-emission electromagnetic radiation limiter, photoconductive THZ-band antennas, plasma antennas);
• optoelectronics (hard UV radiation sensors for extreme operation conditions);
• microsystem devices (powerful high-frequency micromechanical switches).
The created technological line must provide for a closed cycle from growth of crystals and creation of epitaxial structures to the formation wafers with crystals – chips in the framework of planar integrated-group technology.
The concentration of knowledge, infrastructural and human resources for the implementation of ambitious projects aimed at creation of a competitive national environment in the area of SiC-based electronics allows forecasting their development with the implementation "knowledge-based economy" concept at Saint-Petersburg Electrotechnical University. Taking into account the mentioned problems and the high level of competencies in Saint-Petersburg Electrotechnical University regarding the specified directions, further development is envisaged for the system of national and international cooperation for the formation of a SiC-based industry in Russia and its professionally oriented human resources.
The general progressive trends in the technology of using carbide silicon as the basic ECB material under extreme operation modes and conditions are shown in Fig.6.
ECB in power impulse high-voltage electronics
In the modern foreign semi-conductor industry, silicon carbide has firmly occupied the niche of a material used in power high-voltage electronics. In the local practice, the producers have also announced the manufacture of silicon-carbide power modules. However, the basic electronic components are still imported from abroad. Therefore, these technological solutions cannot be regarded as import substitution.
As for the creation of a real domestic SiC ECB, it is worthy to note the design and development work carried out under the order of the Ministry of Industry and Trade of the Russian Federation, "Development and expansion of switching elements production with nanosecond and picosecond switching periods and working voltages of 30–3000 V" (encoded as "Apparatus 10"). The basic design solution was developed by the Center of Microtechnology and Diagnostics at Saint-Petersburg Electrotechnical University (LETI), and the main result of the work was organizing mass production of SiC-based power high-voltage electronic devices by OJSC "Svetlana" in Russia.
Drift step recovery diodes (DSRD) proposed by a group of Soviet scientists from Ioffe Institute [1] and implemented for the first time using silicon have until now been the fasted semiconductor switches. The switching time is determined by the processes occurring in the diode base, which creates an electron-hole plasma and depends in the first place on the base thickness and the saturated drift velocity of the main charge carriers. The other important parameters in the implementation of high-voltage high-current impulse switchers are electric field disruption voltage, doping level and distribution of admixtures in the base, as well as the lifespan of non-equilibrium charge carriers in the diode base. The latter is associated with the fact that under a short impulse bidirectional injection in the base, the carriers would be able to recombine until they are ejected from the base during the switching.
Based on the above parameters, band gap width and, of course, thermal conductivity, which determines the thermal advantages of the material and, consequently, the increase of the timing frequency of impulse sequences, the silicon carbide is better than silicon by an order of magnitude and is second only to diamond.
The samples of DSRD created using n+-p-p+ epitaxial technology (Fig.1) and implemented with two types of protection against disruption (of trench and meso-structure types) have the working voltages of up to 2 kV and sub-nanosecond periods of switching (600–900 ps) with the voltage impulse growth rate (dU/dt) of 3–5 V/ps. The direct fall of voltage on the diode structure is not more than 3 V, and the reverse current with the chip area of 4 mm2 is 10–8 A. The increase of working voltage by more than 10 kV is achieved by the formation of a high-voltage assembly with the preservation of the sub-nano-second switching periods of the device [2].
ECB in power transistor electronics
An important milestone in the development of the domestic technology of semiconductor SiC was the creation in Saint-Petersburg Electrotechnical University of a field-effect transistor with insulated shutter (MOSFET) based on SiC as a basic amplifying element of ECB for extreme operational modes and conditions.
In the short-term, power MIS-transistors based on 4H polytype SiC may occupy their niche in the sphere of power electronic systems due to their combined characteristics of high operational disruption voltages, low resistance in the active area, high density of the switching power and low losses during the switching, as well as a sufficiently high working frequency and temperature.
The cross-sectional view of the designed and implemented vertical MIS-transistor is shown in Fig.1a. The active part of the device is the lowly doped n--layer with the p-, n+- and p+-areas formed therein. The p+-area plays the role of a contact with the p-area and is formed with the purpose of aligning the potential between the p+-area and the source for neutralization of the parasitic n-p-n transistor. In the open state, when the shutter is applied positive voltage relative to the source, the current flows through the contact to the source, n+-area, transistor channel, J-FET area delineated by two p-n-transitions, lowly doped drift area, highly doped substrate and the contact to the source.
The transistor cell has a hexagonal shape (Fig.1b) with a channel length of 1 µm and a J-FET area width of 3 µm, and the width of shutter overlap area and n+-area of the source is 1 µm. The transistor was formed from an array of hexagonal cells and "floating" rings around the perimeter of the device, which provides a smooth field gradient in the periphery, thus avoiding a surface disruption.
The key problem in the creation of a high-performance power MIS-transistor based on 4H SiC with low channel resistance is the electric characteristics of the sub-shutter dielectric and the boundaries of the 4H-SiC/SiO2 section, which worsen the working characteristics of the device [3].
The SiC-based power MIS-transistor (Fig.1c) was created on a lowly doped epitaxial layer with a width of 11 µm and nitrogen concentration of about 7 · 1015 cm–3, grown on a commercial highly doped 4-inch SiC (0001) n-type substrate with the the angle of disorientation of 4°. The P-area with a depth of 1 µm and rectangular shape of admixture distribution with an aluminum concentration of 1·1018 cm-3 was formed using the method of ion implantation. The same method was used to implant n+- and p+-areas with a depth of 220 and 300 nm with a phosphorus and aluminum concentration of 6 · 1019 cm–3 and 1 · 1020 cm–3, respectively.
The sub-shutter dielectric was provided by application of a two-layer system of silicon nitride (5 nm) and silicon dioxide (45 nm) by the method of plasma enhanced chemical vapor deposition (PECVD) with the further oxidation in the atmosphere of dry oxygen under the temperature of 1150°C for 1 hour. The polysilicon shutter doped by phosphorus with a width of 450 nm was formed by the method of low-pressure chemical vapor deposition (LPCVD).
The analysis of the basic characteristics of the created transistor structures (Fig.2) showed that the voltage the switched-on transistor is 1.8 V (Vds=2 V, Ids=10 µA), and the transistor resistance in the switched-on state Rds spec(on) at a room temperature calculated from the angle of the output characteristics with Vgs=20 V is
29 mΩ · cm2. The density of the switching current at a room temperature is higher than 120 A/cm2 with Vds=5 V.
The transistor disruption voltage at a room temperature was 900 V. It may be increased by increasing the "floating" rings with the purpose of creating a smooth gradient of the potential on the periphery of the device, and corresponding increase of the width and decrease of the admixture concentration in the drift area. The results of comparing the main characteristics of the created SiC MIS-transistor to its analogs are shown in Fig.3.
It is noteworthy that currently power MIS-transistors based on SiC have a considerably lower resistance in the switched-on state than its silicon counterparts. However there are a number of problems which considerably limit the further decrease of their resistance, in particular, low mobility of charge carriers, which is mostly due to the high density of surface states at the boundary of the 4H-SiC/SiO2 section. The low mobility of charge carriers in the transistor channel also results in a a significant limitation of its working frequency range.
ECB in the auto-emission high-frequency electronics
The contemporary ECB development stage is characterized by the revival of vacuum micro- and nano-electronics with the purpose of achieving ultra-high frequencies of a giga- and teraherz range, ensuring a high result of "multiplication of generated power by frequency" in lamps with millimeter and sub-millimeter frequency ranges, resolving the tasks of switching and generation in ultra-short impulse electronics and X-ray devices. The vacuum auto-emission electronics is also characterized by high radiation and temperature resistance.
The most important element of a vacuum device is its sources of electrons. The effective cathodes with field emission have until now been under intensive research. Silicon carbide may be regarded as a promising material for auto-emission electronics, in the first place, due to the extreme values of the critical field strength for avalanche breakdown, thermal conductivity and mechanical strength. The additional advantages of SiC are its resistance to chemical and radiation exposure.
Such circumstances made it possible to forecast the creation of SiC-based micro-cathodes with field emission, which combine a high current density of the emission, stability of emission characteristics and acceptable low values of the electric field strength at the emission start.
In the framework of the conducted studies and developments, four technological routes for the creation of SiC-based auto-emission structures were proposed and implemented:
• formation of auto-emission spikes by the method of a sharply focused ion beam (Fig.4a);
• formation of topologically ordered arrays of auto-emission spikes by the method of reactive ion-plasma etching with metal catalyst (Fig.4b);
• formation of topologically ordered two-level auto-emission micro-sized matrix of pedestals with nano-sized spikes using a two-stage technology, which combines the processes of photolithography, reactive ion-plasma etching and catalyst micro-masking (Fig.4c);
• formation of hetero-structured two-staged matrices of auto-emitters based on carbide silicon and nano-crystalline diamond (Fig.4d).
The basis of all the auto-emission structures were mono-crystals of the n-type substrate 6H-SiC with the specific resistance of 0.05 Ω·cm. When describing the general characteristics of the results, detailed in [4–5], it should be noted that currently the implementation is stable when based on SiC micro-cathodes with field emission having the following basic parameters:
• emission start voltage of 10–15 V/µm;
• emission current density of up to 10 A/cm2;
• auto-emission spikes density of 5·108 cm–2;
• stability of work in the vacuum from 10–6 to 10–9 mm Hg.
The results of formation hetero-structured cathodes "silicon carbide – diamond" are of a particular interest [6], because they showed their temporal stability during the increase voltage at the emission start. Besides, this hetero-composition, combining the materials with a considerably different width of the band gap, can be regarded as a basic structure for solid-state auto-emission devices, which do not use vacuum, thus creating certain complexities for encapsulation.
ECB in the ultraviolet optoelectronics
The ultraviolet photo-electronics area was formed by the early XXI century due to a high practical interest to ultraviolet-range optoelectronic devices that had been paid for many years.
It is apparent that UV-range photo-detectors must be characterized by low darkness currents, high sensitivity and high performance, as well as working parameter stability. Currently, the basic materials of UV-photometry are silicon carbide, nitrides of gallium, aluminum and their solid solutions. Different optoelectronic products have been developed and are available on the market, such as photo-detectors functioning in the wavelength ranges from 180 to 400 nm. Although the first-choice materials for ultraviolet photo-electronics are GaN and AlN-GaN, the silicon carbide still occupies its stable niche in the following areas:
• high temperature UV-photo-
electronics;
• control of radiation from powerful UV-excimer lasers, UV-lamps and other sources (it is known that SiC- and diamond-based photo-detectors are the most stable to a prolonged UV-radiation exposure);
• registration of germicidal UV-radiation;
• control of ignition, flame detectors, electric spark detectors.
Currently, the most popular types of SiC-based UV-photo-detectors are photo-diodes with a minuscule p-n transition and diodes with Schottky barrier. The micro- and nano-electronic department of Saint-Petersburg Electrotechnical University developed prototypes of such photo-detection structures. The structure of the above-mentioned photo-detectors is based on epitaxial structures of 4H-SiC, obtained by the method of vapor-phase epitaxy of carbide silicon on n+ 4H-SiC substrata with a diameter of 76–100 mm, with a specific resistance of at most 0.025 Ω∙cm, with a width of 350 µm and direction deviation of 0001 – 4°. In this case, the number of the layers, their widths, conductivity types and concentration of the doping admixture are determined by the type of the photo-detection structure. At the same time, the widths and levels of doping the basic areas were chosen based on the maximum working voltage Urev. ≤15 V, which provides the full depletion of n (p) – layers. In other words, the width of the spatial charge area W is equal to the epitaxial layer thickness d (1).
The photo-detectors based on a diode with Schottky barrier are made in the form of a vertical structure with a solid semi-transparent electrode or with a cellular type electrode. The manufacture of this type UV-photo-detector implies using an n-n+ type 4H-SiC epistructure with an epitaxial layer having a width of d = 5 µm and nitrogen doping level of 6–7 ∙ 1014 cm–3.
The manufactured samples of photo-detectors with Ni-SiC Schottky barrier are characterized by a potential barrier of 1.20–1.25 eV, and the Pt-SiC structures – 1.00–1.05 eV. In all the types of structures, the reserves darkness currents at voltages of 0.5 to 1 V were at most 50 pA. According to the results of capacitance-voltage measurements, it was determined that the saturation of the C-U characteristics occurs at voltages Urev. ≥ 8 V, which corresponds to the full depletion of the basic area of the photo-detector.
The p-n transition diode based photo-detectors are made in the form of p+-n-n+ and p+-p-n+ type vertical mesa-epitaxial structures. The height of the mesa varied depending on the type of conductivity of the active basic layer. Thus, when using the p+-n-n+ type structure, the depth of SiC etching by the method of reactive ion-plasma etching (RIPE) had to be more than the width of the p+ emitter area. The materials of the ohmic contacts to the low-ohmic n+-SiC and p+-SiC areas were made from the metal compositions of Ti/Ni and Al/Ti/Ni, respectively.
It was determined that the direct branches of the VAC structures of p+-p-n+ and p+-n-n+ were not different. Practically in all the cases, the voltage of opening the p-n structures was 2.5 V.
The photoelectric characteristics of the samples were studied in the spectral range of 200–500 nm. All the measurements were carried out in the short circuit mode, and for the photo-diodes based on p+-p-n+ structures – in a photodiode mode util Urev. = 10 V. Fig.5 shows the spectral characteristics of 4H-SiC of the photo-detection structures. It also presents the spectral characteristics of the commercial 4H-SiC photo-diode SG01D-18 for comparison.
A p+-n-n+ type photo-receiver demonstrates the highest photosensitivity of S=0.138 A/Wt at a wavelength of 295 nm. At the same time, the maximum value of photo-receiver sensitivity SG01D-18 is also at 295 nm and equals to 0.13 A/Wt.
Conclusion
In the framework of development of scientific research and sample prototyping for a new competitive SiC element base, in 2016 it is planned to complete the creation of a full production line at Saint-Petersburg Electrotechnical University "LETI" for the following devices:
• power electronics (diodes with Schottky barrier, impulse high-voltage DSRD-diodes and MDP-transistors);
• high frequency electronics (vacuum diode with field emission; vacuum diode peaker, auto-emission electromagnetic radiation limiter, photoconductive THZ-band antennas, plasma antennas);
• optoelectronics (hard UV radiation sensors for extreme operation conditions);
• microsystem devices (powerful high-frequency micromechanical switches).
The created technological line must provide for a closed cycle from growth of crystals and creation of epitaxial structures to the formation wafers with crystals – chips in the framework of planar integrated-group technology.
The concentration of knowledge, infrastructural and human resources for the implementation of ambitious projects aimed at creation of a competitive national environment in the area of SiC-based electronics allows forecasting their development with the implementation "knowledge-based economy" concept at Saint-Petersburg Electrotechnical University. Taking into account the mentioned problems and the high level of competencies in Saint-Petersburg Electrotechnical University regarding the specified directions, further development is envisaged for the system of national and international cooperation for the formation of a SiC-based industry in Russia and its professionally oriented human resources.
The general progressive trends in the technology of using carbide silicon as the basic ECB material under extreme operation modes and conditions are shown in Fig.6.
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