Piscataway, New Jersey 08854. Feng Yan and Bing Guan. Detector Systems Branch, NASA-Goddard Space Flight Center/MEI Technologies, Greenbelt Road,.
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Highly sensitive visible-blind extreme ultraviolet Ni/ 4H-SiC Schottky photodiodes with large detection area Jun Hu, Xiaobin Xin, and Jian H. Zhao SiCLAB, Department of Electrical and Computer Engineering, Rutgers University, 94 Brett Road, Piscataway, New Jersey 08854
Feng Yan and Bing Guan Detector Systems Branch, NASA-Goddard Space Flight Center/MEI Technologies, Greenbelt Road, Greenbelt, Maryland 20771
John Seely Space Science Division, Naval Research Laboratory, Washington, DC 20375
Benjawan Kjornrattanawanich Universities Space Research Association, National Synchrotron Light Source, Beamline X24C, Brookhaven National Laboratory, Upton, New York 11973 Received January 31, 2006; accepted March 24, 2006; posted March 27, 2006 (Doc. ID 67622)
Ni/ 4H-SiC Schottky photodiodes of 5 mm⫻ 5 mm area have been fabricated and characterized. The photodiodes show less than 0.1 pA dark current at −4 V and an ideality factor of 1.06. A quantum efficiency (QE) between 3 and 400 nm has been calibrated and compared with Si photodiodes optimized for extreme ultraviolet (EUV) detection. In the EUV region, the QE of SiC detectors increases from 0.14 electrons/photon at 120 nm to 30 electrons/photon at 3 nm. The mean energy of electron-hole pair generation of 4H-SiC estimated from the spectral QE is found to be 7.9 eV. © 2006 Optical Society of America OCIS codes: 040.5160, 260.7200.
UV, vacuum ultraviolet (VUV), and extreme ultraviolet (EUV) 共5 – 380 nm兲 photodetectors have been widely used in the field of astronomy, EUV lithography, x-ray microscopy, and plasma physics. Many applications require detectors with high responsivity, ultralow dark current, and good radiation hardness. Blindness to visible solar radiation is also vital, especially for solar study in the EUV region, where the signals are relatively weak and the out-of-band signals, such as the visible components of the solar background, are often many orders of magnitude higher than the EUV signal of interest. In recent years, tremendous progress in epitaxial growth technologies and processing of wide bandgap semiconductors, including SiC, has been made, which allows the fabrication of p-i-n,1 avalanche,2,3 metal-semiconductor-metal,4 and Schottky barrier5,6 devices for UV detection. The 6H-SiC p-i-n photodetector was found to have EUV rsponsivity in the range of 0.01– 0.10 A / W and 100% charge collection efficiency [internal quantum efficiency (QE)] near the p-i-n junction.7 The 4H-SiC with a lower cutoff wavelength at 385 nm has been proved to be a very promising candidate for the development of visible-blind UV detectors. In this Letter we report the fabrication of 5 mm⫻ 5 mm Ni/ 4H-SiC Schottky photodiodes with extremely low leakage current and the measurement of a photoresponse in the 3 – 400 nm wavelength range with synchrotron radiation and other laboratory sources. 0146-9592/06/111591-3/$15.00
Our Schottky photodiodes were fabricated on a 2 in. 4H-SiC wafer with a 5 m lightly doped n epilayer on an n+ substrate purchased from Cree, Inc. Device passivation was first accomplished by growing a thin layer 共50 nm兲 of thermal oxide in wet oxygen at 1050° C for 3 h followed by another 200 nm plasma-enhanced chemical vapor deposition of SiO2 and 300 nm plasma-enhanced chemical vapor deposition of Si3N4. After the oxide was removed from the back of the wafer, Ni alloy was sputtered and then annealed at 1050° C for 5 min in 5% H2 foaming gas to form the n-type ohmic contact. The optical window was defined by the inductively coupled plasma etching of Si3N4 and wet etching of SiO2 on the top n layer by using photo resist as the etching mask, and then a semitransparent Ni film of thickness 4.5 nm was deposited in the window as the Schottky contact. Finally, a thick Ni/ Ti/ Au overlay contact was sputtered on the periphery of the Ni Schottky film for wire bonding. The cross-sectional view is shown in the inset of Fig. 1. The dark current–voltage characteristics of typical Ni/ 4H-SiC Schottky photodiodes at room temperature are shown in Fig. 1. The ideality factor was determined to be around 1.06, and the Schottky barrier height was approximately 1.58 eV. The obtained barrier height is very close to the ideal barrier height 共1.66 eV兲 given by Itoh and Matsunami8 共b = 0.7M − 1.95兲, indicating the excellent Schottky contact between Ni and 4H-SiC. © 2006 Optical Society of America
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Fig. 1. The cross-sectional view and I – V characteristics of the 5 mm⫻ 5 mm Ni/ 4H-SiC Schottky photodiode.
The leakage current was measured by using the Keithley 595 Quasistatic CV measurement system from 0 to − 5 V. The measurement was carried out in a lightproof microprobe chamber. The instrument specification at 20 pA range has 14 counts ±1.5% reading accuracy and 1 fA resolution. The leakage current was less than 0.1 pA at −4 V. The dynamic resistance at 0 basis, dV / dI was estimated to be 艌3 ⫻ 1013 ⍀. The photoresponse measurements in the wavelength range from 3 to 380 nm were performed at the National Synchrotron Light Source beamline X24C at Brookhaven National Laboratory.9 The monochromator with interchangeable gratings having 150, 600, and 2400 g / mm provided dispersed x-ray and EUV radiation with a resolving power of 500. The overall range of coverage of the gratings is ⬃1.2 nm through the visible. Thin metal filters attenuated the higher harmonics from the monochromator while passing the x-ray or the EUV bandpass of interest. Removing the metal filter and setting the monochromator grating to the zero diffraction order allowed the photodiode to be illuminated by broadband radiation consisting of all wavelengths greater than ⬃1 nm. The photoresponse spectra from 200 to 400 nm were also measured in air by using a xenon lamp light source with a near UV monochromator, and in all cases the beam size underfilled the detector. The photoresponse spectra of the Ni/4H-SiC Schottky photodiodes were characterized at 0 V bias. The measured external QE of two representative photodiodes is shown in Fig. 2. For comparison, the QE of an AXUV Si photodiode from International Radiation Detectors, Inc., is also shown. The QE of our SiC Schottky photodiodes is ⬍0.1% at wavelengths longer than 380 nm because the incident photon energy is less than the bandgap of 4H-SiC. The QE increases gradually as the wavelength decreases and reaches the local maximum of 65% at 275 nm. The QE between 230 and 295 nm is higher than 50%. At wavelengths shorter than 230 nm, the QE rapidly decreases as the wavelength decreases and then starts rising at ⬃160 nm except for the singular wavelength of 120 nm, where the QE of both SiC and Si photodiodes reach their minima. The QE exceeds 100% at
wavelengths shorter than 50 nm and finally reaches 30 electrons/photon at 3 nm. The Ni/4H-SiC Schottky photodiodes show excellent visible blindness. The QE at the wavelength of 390 nm is 0.034%. Comparing with the peak QE of about 65% at 275 nm, the rejection ratio of UV to visible light is 2 ⫻ 103. The QE calibrated at 442 nm by using a 20 mW He–Cd laser was 6.45⫻ 10−5. The QE at 600 and 700 nm was estimated by projecting the W-halogen lamp light through 600 and 700 nm bandpass filters 共bandwidth= 10 nm兲. The QE is as low as 10−5. The actual QE at 600 and 700 nm should be substantially lower than 10−5, considering the contribution of the UV light leaking through the optical filters. To describe the variation of QE with wavelength, the internal QE of the Ni/4H-SiC Schottky photodiode was investigated in the 159– 400 nm wavelength range. The Fresnel coefficients of reflectance and transmittance at the vacuum-Ni and Ni–SiC interfaces were calculated by using the complex index of refraction of the layer materials, Ni and SiC.10,11 The radiation losses due to the reflectance and absorption of the 4.5 nm Ni semitransparent film from 230 to 300 nm varies between 35% and 50%. At zero voltage, the depletion width is about 1.7 m, and the corresponding internal QE between 230 and 300 nm is close to 100%. The decrease of the QE between the wavelength 230 and 160 nm is attributed to the dead-zone effect introduced by the surface recombination, which becomes significant when the penetration depth starts to be comparable with the dead zone. In this wavelength range, a large portion of the photons is absorbed in the dead zone, where the photon-generated carriers are recombined and have no contribution to the photoresponse. The depth of the dead zone has been estimated based on the internal QE. At 200 nm, the external QE is 33%. Given that about 56% incident photons are reflected or absorbed by the 4.5 nm Ni semitransparent film at 200 nm, the internal QE is estimated to be 75%. According to the optical properties of SiC reported by Cobet et al.,11 the penetration depth of 4H-SiC at this wavelength is 11 nm,
Fig. 2. (Color online) Photoresponse spectra in external QE from 3 – 400 nm of two Ni/ 4H-SiC Schottky photodiodes and a Si AXUV-100 photo detector.
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Fig. 3. The QE of the 4H-SiC photodiode = sr共h / WSiC兲. Black squares, QE of an ideal SiC photodiode 共sr = 1兲, assuming WSiC = 7.8 eV; open circles, measured external QE of the Ni/ SiC Schottky photodiode 共sr = 0.65兲; open triangles, / sr.
and the corresponding depth of the dead zone is about 3 nm. Based on this calculated depth of dead zone, at 160 nm the penetration depth is only around 4.2 nm, and the calculated QE is about 17%, which agrees very well with the experimental result of 18%. The QE of SiC photodiodes starts to increase at 160 nm. Because this wavelength corresponds to the reported 4H-SiC pair generation energy WSiC 共7.8 eV兲,12 the increase is attributed to the multiple electron–hole pair generation. The QE spectra of Ni/ 4H-SiC photodiodes is quite similar to that of the Si photodiode between 3 and 120 nm but is about a factor of three to five lower than that of Si photodiodes in this range. This difference is mainly due to the difference in pair generation energy for Si 共3.64 eV兲 and 4H-SiC and absorption in the 4.5 nm Ni film on the 4H-SiC device. The mean energy of electron–hole pair generation for 4H-SiC, WSiC, is estimated from the QE spectra of Ni/4H-SiC photodiodes in the EUV region, considering the losses due to the reflection and absorption of the 4.5 nm Ni film and the dead-zone effect. Figure 3 shows the measured QE as a function of photon energy from 100 to 420 eV. In this spectrum region, one photon with high energy can generate multiple electron–hole pairs, and the QE is expressed as = sr共h / WSiC兲, where sr is a dimensionless factor that describes the loss processes. For an ideal SiC detector without any losses, sr = 1. For Ni/4H-SiC Schottky photodiodes the average loss due to the reflection and absorption of the 4.5 nm Ni film and dead-zone space effect is assumed to be about 35%. As a result, around 65% incident photons are contributed to the photocurrent, and the slope of QE versus photon energy should be 0.65/ WSiC. The mean pair generation energy of 4H-SiC, WSiC, is found to be 7.9 eV, which is in excellent consistence with the reported 7.8 eV that was measured in the x-ray spectrum.12 A figure of merit, the spectral detectivity,13 D*, is often used to evaluate the sensitivity of photo detectors. For the SiC Schottky photodiode working at 0 V
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bias, the dark current is very small and the Johnson noise dominates. In this case, D* 1/2 = q / h共R0A / 4kBT兲 , where is the QE, R0 is the dynamic resistance at 0 V, A is the detector area, and is the photon frequency. At 275 nm, where the maximum QE is achieved, the spectral detectivity is 3.4⫻ 1015 cm Hz1/2 / W, and the corresponding noise equivalent power NEP= 共A⌬f兲1/2 / D* is equal to 1.7 ⫻ 10−16 Hz1/2 / W. D* over the wavelength range from 190 to 330 nm is above 1015 cm Hz1/2 / W. As a comparison, D* of the Si AXUV detector is 3 orders lower. The uniformity of our devices is also checked in the UV range. The spot size on the photodiode is less than 1 mm2. The peak-to-peak variation of the QE across the active area of the Ni/ 4H-SiC Schottky photodiodes is about ±2%. In summary, 5 mm⫻ 5 mm Ni/4H-SiC Schottky photodiodes are fabricated. I – V measurement shows very low leakage current of ⬍0.1 pA at −4 V and an ideality factor of 1.06. The measured QE at 0 V bias is higher than 50% from 230 to 295 nm and reaches the peak of 65% at 275 nm, corresponding to an internal QE close to 100%. The rejection ratio of UV to visible light is higher than 1000. The QE in the EUV range is higher than 14%, exceeds 100% at the wavelength less than 50 nm, and finally becomes higher than 30 electrons/photon at 3 nm. The spectral detectivity D* = 3.4⫻ 1015 cm Hz1/2 / W at the wavelength of 275 nm is reported. Ni/4H-SiC Schottky photodiodes can be used as very sensitive radiation-hard detectors for the EUV–VUV–UV range. F. Yan’s e-mail @pop500.gsfc.nasa.gov.
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