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UV photodetectors in 6HSiC
Sensors and Actuators
UV photodetectors
in 6H-SiC
M. M. Anikin*, A. N. Andreev, and V. E. Chelnokov A.F. Ioff
91
A, 33 (1992) 91-93
Physico-Technical
Institute,
Academy
S. N. Pyatko, of Sciences,
N. S. Savkina,
St. Petersburg
A. M. Strelchuk,
A. L. Syrkin
(Russia)
Abstract have fabricated two types of UV photodetector in 6H-Sic: UV photodiodes using Sic Schottky-barrier structures; Sic photodiodes using shallow p-n junctions. The monochromatic sensitivity of the Schottky-barrier photodetectors (at a wavelength of 215 nm) is 0.15 A/W, and of the p-n junction photodetectors (at a wavelength of 225 nm) 0.13 A/W. We
Silicon carbide photodetectors are highly sensitive to ultraviolet radiation (200-400 nm) while being practically insensitive in the visible spectral range. We have developed two types of UV photodetectors in 6H-Sic: (i) UV photodiodes using Sic Schottky-barrier structures (SBS); (ii) Sic photodiodes using shallow p-n junctions. In Schottky barriers the space-charge region lies on the surface, i.e., they lack a passive semiconductor layer where some of the incident radiation is wasted. Therefore, the photosensitivity of a Schottkybarrier photodiode can serve as a reference for sensitivity estimates of junction photodiodes. The present work on 6H-SiC SBS UV photodiodes relies on a technology of Au-Sic Schottky barriers that we developed earlier [ 11. The Schottky barriers were formed on the surface of n-type conduction epitaxial layers, with uncompensated N,-N =5x 10’6-11 donor concentration, 1017cmP3, grown by an open sublimation method [2]. n-Sic-6H substrates were of (OOOl)Siorientation. The structures had an area S = 3 x 10e3 cm*. The barriers showed low leakage currents, N 10-‘” A, up to a breakdown voltage of lOO- 170 V at room temperature, and could be operated up to 573 K where the leakage current rose to -lo-*A (Fig. l(b)). Forward current-
voltage characteristics for voltages such that qV exceeded the cp,-barrier height, were linear. The ohmic resistance of the diodes, the sum of the semiconductor bulk resistance and contact resistance, was 2-3 51 at room temperature (Fig. l(a)). Forward current-voltage characteristics for AuSic-SBS structures in a low-current range are given in Fig. 2(a). At voltages kT/q 4 V < (p,Jq and temperatures of 293-520 K these were exponential Z= I,, exp(qV//ISkT). Exponential parts of the characteristics extended over 5-6 orders in current (from
o
*Author to whom correspondence should be. addressed.
09%4247/92/$5.00
423456
8
Fig. 1. Current-voltage characteristics of Schottky diodes: a, forward current; b, reverse current.
@ 1992 -
Elsevier Sequoia. All rights reserved
92
f-293; a- 3/7; 3 -340, 4 - 373; f-395; 6-423,
Fig. 2. a, forward current-voltage characteristics of Schottky diodes in a low-current range; b, temperature dependence of ideality factor.
10m9 to low3 A). The ideality factor, p, remained constant at 1.05- 1.07 throughout the temperature range investigated (Fig. 2(b)). In the structures grown, practically no evidence was found of the presence of an interfacial, layer between the metal and the semiconductor. Their properties are close to those of an ideal SBS. UV photodectors using these structures were made by vacuum deposition of a thin, semitransparent Au layer. With a surface area of lo-* cm2, the capacitance of the structures was about 630 pF. For a reverse voltage of 1 V the leakage current was 10-l’ A. The photosensitivity spectral variation of Au-Sic-6H structures in the shortcircuit mode of operation is shown in Fig. 3. The monochromatic photosensitivity in the best samples, at a wavelength of 215 nm, reached 0.15 A/W, which corresponds to the quantum efficiency of 0.8 electron/photon. Photodetectors with p-n junctions are based on n+-p-n structures grown by an open sublimation method [2]. Structures having an area of 3 x 10e3 cm2 had a breakdown voltage of 300 V, and an operating temperature range of 300-800 K. Two types of photodetectors incorporating Sic p-n structures have been developed. (i) Photodiodes with a uniformly doped illuminated n-layer. (ii) Photodiodes with a built-in field in the illuminated n-layer. The built-in field was produced by doping the n-layer nonuniformly during growth. The thickness of the epitaxial n-layer was h, = 1.0-2.0 pm. Ohmic contacts to the p-layer were deposited by vacuum evaporation of W with
Fig. 3. Quantum efficiency dependence of Schottky diode photodetectors vs. wavelength of the incident light.
subsequent alloying at T = 1600 “C [ 31. Mesas were formed using reactive-ion plasma etch [4]. The uncompensated donor concentration in the structures derived from the slope of C- V characteristics ranged from 2 x lOI to 1 x lOI cmm3. The space-charge region width, at zero bias, in different structures varied from 0.1 to 0.45 pm. A sum of the diffusion lengths of electrons, L,,and holes L, in p- and n-regions, respectively, was L, + L, = 0.1-0.5 pm, as determined by registering the short-circuit photocurrent as a function of the space-charge region width under quasi-uniform excitation. Measured spectral characteristics of the SIC pn structures are shown in Fig. 4 where we compare plots of the short-circuit photocurrent versus wavelength of the incident light for two structures: one with the uncompensated donor concentration decreasing from 3 x 1017cmm3 at the illuminated
Fig. 4. Dependence of the short-circuit photocurrent of p-n junction photodiode vs. wavelength. 1, Structure with built-in field; 2, structure without built-in field.
93
Fig. 5. Spectral variation of the quantum efficiency of p-n ode at different thicknesses of the illuminated n-layer.
photodi-
surface to 9 x 1OL6cmP3 at the space-charge boundary (Fig. 4.(l)), which corresponds to the built-in field strength of 0.8 x 10’ V/cm in a region adjacent to the p-n junction. The other structure was uniformly doped to the uncompensated donor concentration of 3 x 10” cme3 (Fig. 4.(2)). Figure 4 shows that incorporation of the driving field through compensation enhances the efficiency of the photodetectors at the maximum by a factor of 2.5-3.0. Ratios of short circuit photocurrents at the maximum are equal to ratios of the respective sums W + LP +L,, in structures with the builtin field L, being the drift-diffusion length. Next, in successive runs of reactive ion-plasma etch, the illuminated n-layer was etched to a thickness approximately equal to the space-charge width in the n-layer. The spectral variation of the quantum efficiency of the Sic-6H p-n structures in the range 200-400 nm at different thicknesses of the illuminated n-layer are given in Fig. 5. It is seen
that with decreasing n-layer thickness the photosensitivity maximum shifts markedly to the short waves. The effect of photosensitivity enhancement due to the driving field, most pronounced in the long-wave part of the UV range, becomes insignificant. It is also seen that the sensitivity to UV radiation reaches a maximum when the n-layer has about the same thickness as that of the spacecharge layer ( -0.2 pm for the structure presented in Fig. 5). In this case, the photosensitivity maximum occurs at a wavelength of 225 nm. The photosensitivity at the maximum, 0.13 A/W, corresponds to the quantum efficiency of 0.7 electron/photon. The p-n structures investigated, having an area of 1.7 x lo-’ cm2, had a leakage current of about lo-‘* A at 1 V of reverse voltage. The calculated fall and rise times of the photodetectors depended on the RC of the structures (R-resistance, C-capacitance). These times were lo-* s (area lo-’ cm’). The obtained results have demonstrated that Sic photodiodes are highly sensitive detectors of UV radiation, having low leakage currents, and can be used as UV detectors operating at high temperatures. References 1 M. M. Anikin, A. N. Andrew, A. A. Lebedev, S. N. Pyatko, M. G. Rastegaeva, N. S. Savkina, A. M. Strel’chuk, A. L. Syrkin and V. E. Chelnokov, Fiz. Tehn. Polupr., 25 (1991) 328. 2 M. M. Anikin, V. A. Dmitriev, N. B. Guseva and A. L. Syrkin, Izu. Akad. Nuuk SSSR, Neorg. Mater., IO (1984) 1768. 3 M. M. Anikin, M. G. Rastegaeva, A. L. Syrkin and I. V. Chujko, Ext. Abar., 3rd ht. Conf: Sic (ICACSC ‘90), pp. 5-6. 4 A. L. Syrkin, I. V. Popov and V. E. Chelnokov, Sou. Tech. Phys. Lat., 12 (1986) 99.
UV photodetectors
in 6H-SiC
M. M. Anikin*, A. N. Andreev, and V. E. Chelnokov A.F. Ioff
91
A, 33 (1992) 91-93
Physico-Technical
Institute,
Academy
S. N. Pyatko, of Sciences,
N. S. Savkina,
St. Petersburg
A. M. Strelchuk,
A. L. Syrkin
(Russia)
Abstract have fabricated two types of UV photodetector in 6H-Sic: UV photodiodes using Sic Schottky-barrier structures; Sic photodiodes using shallow p-n junctions. The monochromatic sensitivity of the Schottky-barrier photodetectors (at a wavelength of 215 nm) is 0.15 A/W, and of the p-n junction photodetectors (at a wavelength of 225 nm) 0.13 A/W. We
Silicon carbide photodetectors are highly sensitive to ultraviolet radiation (200-400 nm) while being practically insensitive in the visible spectral range. We have developed two types of UV photodetectors in 6H-Sic: (i) UV photodiodes using Sic Schottky-barrier structures (SBS); (ii) Sic photodiodes using shallow p-n junctions. In Schottky barriers the space-charge region lies on the surface, i.e., they lack a passive semiconductor layer where some of the incident radiation is wasted. Therefore, the photosensitivity of a Schottkybarrier photodiode can serve as a reference for sensitivity estimates of junction photodiodes. The present work on 6H-SiC SBS UV photodiodes relies on a technology of Au-Sic Schottky barriers that we developed earlier [ 11. The Schottky barriers were formed on the surface of n-type conduction epitaxial layers, with uncompensated N,-N =5x 10’6-11 donor concentration, 1017cmP3, grown by an open sublimation method [2]. n-Sic-6H substrates were of (OOOl)Siorientation. The structures had an area S = 3 x 10e3 cm*. The barriers showed low leakage currents, N 10-‘” A, up to a breakdown voltage of lOO- 170 V at room temperature, and could be operated up to 573 K where the leakage current rose to -lo-*A (Fig. l(b)). Forward current-
voltage characteristics for voltages such that qV exceeded the cp,-barrier height, were linear. The ohmic resistance of the diodes, the sum of the semiconductor bulk resistance and contact resistance, was 2-3 51 at room temperature (Fig. l(a)). Forward current-voltage characteristics for AuSic-SBS structures in a low-current range are given in Fig. 2(a). At voltages kT/q 4 V < (p,Jq and temperatures of 293-520 K these were exponential Z= I,, exp(qV//ISkT). Exponential parts of the characteristics extended over 5-6 orders in current (from
o
*Author to whom correspondence should be. addressed.
09%4247/92/$5.00
423456
8
Fig. 1. Current-voltage characteristics of Schottky diodes: a, forward current; b, reverse current.
@ 1992 -
Elsevier Sequoia. All rights reserved
92
f-293; a- 3/7; 3 -340, 4 - 373; f-395; 6-423,
Fig. 2. a, forward current-voltage characteristics of Schottky diodes in a low-current range; b, temperature dependence of ideality factor.
10m9 to low3 A). The ideality factor, p, remained constant at 1.05- 1.07 throughout the temperature range investigated (Fig. 2(b)). In the structures grown, practically no evidence was found of the presence of an interfacial, layer between the metal and the semiconductor. Their properties are close to those of an ideal SBS. UV photodectors using these structures were made by vacuum deposition of a thin, semitransparent Au layer. With a surface area of lo-* cm2, the capacitance of the structures was about 630 pF. For a reverse voltage of 1 V the leakage current was 10-l’ A. The photosensitivity spectral variation of Au-Sic-6H structures in the shortcircuit mode of operation is shown in Fig. 3. The monochromatic photosensitivity in the best samples, at a wavelength of 215 nm, reached 0.15 A/W, which corresponds to the quantum efficiency of 0.8 electron/photon. Photodetectors with p-n junctions are based on n+-p-n structures grown by an open sublimation method [2]. Structures having an area of 3 x 10e3 cm2 had a breakdown voltage of 300 V, and an operating temperature range of 300-800 K. Two types of photodetectors incorporating Sic p-n structures have been developed. (i) Photodiodes with a uniformly doped illuminated n-layer. (ii) Photodiodes with a built-in field in the illuminated n-layer. The built-in field was produced by doping the n-layer nonuniformly during growth. The thickness of the epitaxial n-layer was h, = 1.0-2.0 pm. Ohmic contacts to the p-layer were deposited by vacuum evaporation of W with
Fig. 3. Quantum efficiency dependence of Schottky diode photodetectors vs. wavelength of the incident light.
subsequent alloying at T = 1600 “C [ 31. Mesas were formed using reactive-ion plasma etch [4]. The uncompensated donor concentration in the structures derived from the slope of C- V characteristics ranged from 2 x lOI to 1 x lOI cmm3. The space-charge region width, at zero bias, in different structures varied from 0.1 to 0.45 pm. A sum of the diffusion lengths of electrons, L,,and holes L, in p- and n-regions, respectively, was L, + L, = 0.1-0.5 pm, as determined by registering the short-circuit photocurrent as a function of the space-charge region width under quasi-uniform excitation. Measured spectral characteristics of the SIC pn structures are shown in Fig. 4 where we compare plots of the short-circuit photocurrent versus wavelength of the incident light for two structures: one with the uncompensated donor concentration decreasing from 3 x 1017cmm3 at the illuminated
Fig. 4. Dependence of the short-circuit photocurrent of p-n junction photodiode vs. wavelength. 1, Structure with built-in field; 2, structure without built-in field.
93
Fig. 5. Spectral variation of the quantum efficiency of p-n ode at different thicknesses of the illuminated n-layer.
photodi-
surface to 9 x 1OL6cmP3 at the space-charge boundary (Fig. 4.(l)), which corresponds to the built-in field strength of 0.8 x 10’ V/cm in a region adjacent to the p-n junction. The other structure was uniformly doped to the uncompensated donor concentration of 3 x 10” cme3 (Fig. 4.(2)). Figure 4 shows that incorporation of the driving field through compensation enhances the efficiency of the photodetectors at the maximum by a factor of 2.5-3.0. Ratios of short circuit photocurrents at the maximum are equal to ratios of the respective sums W + LP +L,, in structures with the builtin field L, being the drift-diffusion length. Next, in successive runs of reactive ion-plasma etch, the illuminated n-layer was etched to a thickness approximately equal to the space-charge width in the n-layer. The spectral variation of the quantum efficiency of the Sic-6H p-n structures in the range 200-400 nm at different thicknesses of the illuminated n-layer are given in Fig. 5. It is seen
that with decreasing n-layer thickness the photosensitivity maximum shifts markedly to the short waves. The effect of photosensitivity enhancement due to the driving field, most pronounced in the long-wave part of the UV range, becomes insignificant. It is also seen that the sensitivity to UV radiation reaches a maximum when the n-layer has about the same thickness as that of the spacecharge layer ( -0.2 pm for the structure presented in Fig. 5). In this case, the photosensitivity maximum occurs at a wavelength of 225 nm. The photosensitivity at the maximum, 0.13 A/W, corresponds to the quantum efficiency of 0.7 electron/photon. The p-n structures investigated, having an area of 1.7 x lo-’ cm2, had a leakage current of about lo-‘* A at 1 V of reverse voltage. The calculated fall and rise times of the photodetectors depended on the RC of the structures (R-resistance, C-capacitance). These times were lo-* s (area lo-’ cm’). The obtained results have demonstrated that Sic photodiodes are highly sensitive detectors of UV radiation, having low leakage currents, and can be used as UV detectors operating at high temperatures. References 1 M. M. Anikin, A. N. Andrew, A. A. Lebedev, S. N. Pyatko, M. G. Rastegaeva, N. S. Savkina, A. M. Strel’chuk, A. L. Syrkin and V. E. Chelnokov, Fiz. Tehn. Polupr., 25 (1991) 328. 2 M. M. Anikin, V. A. Dmitriev, N. B. Guseva and A. L. Syrkin, Izu. Akad. Nuuk SSSR, Neorg. Mater., IO (1984) 1768. 3 M. M. Anikin, M. G. Rastegaeva, A. L. Syrkin and I. V. Chujko, Ext. Abar., 3rd ht. Conf: Sic (ICACSC ‘90), pp. 5-6. 4 A. L. Syrkin, I. V. Popov and V. E. Chelnokov, Sou. Tech. Phys. Lat., 12 (1986) 99.