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Single crystal diamond UV detector with a groove-shaped electrode structure and enhanced sensitivity
Accepted Manuscript Title: Single crystal diamond UV detector with a groove-shaped electrode structure and enhanced sensitivity Authors: Kang Liu, Bing Dai, Victor Ralchenko, Yuanqin Xia, Baogang Quan, Jiwen Zhao, Guoyang Shu, Mingqi Sun, Ge Gao, Lei Yang, Pei Lei, Jiaqi Zhu, Jiecai Han PII: DOI: Reference:
S0924-4247(16)30914-1 http://dx.doi.org/doi:10.1016/j.sna.2017.01.027 SNA 10037
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-11-2016 13-1-2017 23-1-2017
Please cite this article as: Kang Liu, Bing Dai, Victor Ralchenko, Yuanqin Xia, Baogang Quan, Jiwen Zhao, Guoyang Shu, Mingqi Sun, Ge Gao, Lei Yang, Pei Lei, Jiaqi Zhu, Jiecai Han, Single crystal diamond UV detector with a grooveshaped electrode structure and enhanced sensitivity, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.01.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Single crystal diamond UV detector with a groove-shaped electrode structure and enhanced sensitivity Kang Liua#, Bing Daia#, Victor Ralchenkoa,b,c, Yuanqin Xiad, Baogang Quane, Jiwen Zhaoa, Guoyang Shua, Mingqi Suna, Ge Gaoa, Lei Yanga, Pei Leia, Jiaqi Zhua,f*, Jiecai Hana a
Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080 ,
P.R. China b
General Physics Institute RAS, Vavilov str. 38, Moscow 119991, Russia
c
National Research Nuclear University MEPhI, Moscow 115409, Russia
d
National Key Laboratory of Science and Technology on Tunable Laser, 92 Xidazhi Str., Harbin
150080, P.R. China e
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy
of Sciences, Beijing 100190, P. R. China f
Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education,
Harbin 150080, P. R. China # These two authors contributed equally to this work. Kang Liu, ([email protected]); Bing Dai, ([email protected]) Corresponding author: Jiaqi Zhu ([email protected])
Graphical Abstract
PRIME NOVELTY Statement We designed, simulated and fabricated an ultraviolet radiation detector on single crystal CVD diamond with a 3D electrode structure. This resulted in a significant enhancement of photocurrent in comparison with the device with conventional planar structure.
Highlights
● A UV detector on single crystal diamond with 3D electrodes is designed and fabricated
● Ti-Au electrodes in grooves etched in diamond, improve uniformity of electric field
● The 3D detector demonstrated 30 - 50% better sensitivity compared to a planar device
Abstract An ultraviolet (UV) detector on single crystal chemical vapor deposition (CVD) diamond with three-dimensional (3D) metal electrodes, is designed, and fabricated with the aim to improve collection efficiency of photo-induced carriers. Interdigitated Ti/Au electrodes were formed within 10 µm deep groove etched in diamond to ensure a more uniform field pattern in the diamond bulk. In the preliminary tests the groove-shaped 3D device performance was compared to a detector with a conventional planar structure on the same material at applied electric field of 1 V/µm. It is found that the responsivity of the 3D electrode structure in the spectral range of 220 – 280 nm is up to 50% higher than that for the planar structure.
Keywords: UV detector; single crystal diamond; 3D electrodes; photoresponse
1. Introduction Due to wide band gap, high breakdown voltage, high carrier mobility and high radiation tolerance, single crystalline (SC) diamond becomes one of most interesting candidates for next generation of photon detectors for the spectral range from UV to X-rays. Diamond is particularly suitable for solar blind UV detectors due to its band gap of 5.45 eV that ensure only a very weak photoelectric response to visible light. The potential applications of the UV and VUV detectors include, in particular, space research, such as LYRA [1] and Solar Orbiter [2] projects, UV laser lithography [3], laser beam imaging [4], monitoring of fast pulsed sources, like UV excimer laser beams [5,6], for diagnostics of thermonuclear plasma [7] and laser plasma [8]. Photorestive diamond UV detectors contain metal or
carbon electrodes to collect the charge induced by ionizing radiation. Commonly, two basic geometries of electrodes are used: (i) planar electrode structures [9-11] deposited on one side of diamond film or plate, and (ii) metal-diamond-metal sandwich structures with the contacts placed on two opposite sides [6,12,13]. The planar electrodes are typically formed as interdigitited finger-type system of metal strips with a gap of the order of tens microns [3,6], and they can be easily prepared by a photolithography. The penetration depth d for incident UV light is small enough, rapidly decreasing with the wavelength decrease. Below the fundamental absorption edge (λ = 225 nm), the penetration depth amounts to d ≈ 11 µm at wavelength λ = 215 nm [14], d ≈ 3 µm at λ = 210 nm, reducing further to d ≈ 0.3 µm at λ = 180 nm [15,16]. The formed charge carriers drift to the electrodes within a thin layer, of the order of penetration depth d, close to the diamond surface. However, the electric field E for the in-plane electrode geometry is not uniform, reaching a maximum on the surface, where various defects in diamond are present, in particular, those induced by polishing (the depth of the polishinginduced defected layer can be of the order of 1 µm [17]). The charge trapping by the subsurface defects is one of the reasons, why the collection efficiency of the detector can be less than 100%. To homogenize the E-field spatial distribution Forneris et al. [18] suggested to form buried (threedimensional) electrodes in epitaxial CVD film, while preserving the interdigitited geometry. They have fabricated the 3D detector with Cr electrodes deposited inside parallel 6 μm deep grooves, produced by ion etching, that under 1MeV proton micro-beam irradiation demonstrated an improved performance: change collection efficiency increased to 0.85 compared 0.55 to that for common superficial electrode configuration. The geometrical arrangement of the electric field lines due to the 3D patterning of the electrodes results in a shorter travel path for the excess charge carriers, thus contributing to a more efficient charge collection mechanism. However, such 3D diamond sensor has not been used for UV detection, to our best knowledge. With a similar idea a UV detector with buried tungsten finger-type electrodes on diamond was reported recently by Liu et al. [19]. However, in spite of a certain improvement observed in the detector sensitivity, in view of very shallow electrodes, with depth less than 120 nm, the impact on the E-field distribution was negligible in fact. Just a side comment, we note here, that there is another approach to build 3D particle or X-ray detectors, based on laser local graphitization of diamond bulk to produce long ordered vertical pillars immersed in the diamond [20-22]. These graphitic electrodes are designed to collect the charge at a large depth, a few hundred micrometers, that is not necessary for UV detector performance. In spite of encourages results, still there are problems with the laser-produced graphitic pillars, such as microcracks formation around diamond-graphite interface [23], and incomplete transformation of diamond to graphite, resulting in too high resistivity of the electrode [24]. In the present work we fabricated, tested and simulated an UV detector with buried groove-shaped metal electrode structure on a single crystal CVD diamond, and compared its performance with that for a sensor with conventional planar structure, prepared on the same material. A significant increase in photoresponsivity for the 3D detector was measured. 2. Experimental 2.1. Design
The optimized electrode geometry should match the UV photons penetration depth, groove depth and interdistance between the grooves (stripe electrodes) to provide, ideally, a uniform electric field in between. The conventional in-planar structure is enough to collect carriers generated by light at wavelengths shorter than 210 nm with small photons penetration depth (d < 3 µm). However, it is desirable to improve detection efficiency to the UV light with λ > 210 nm with larger penetration depth, yet still close to cut-off wavelength for absorption (λ ≈ 225 nm). In contrast, due to deeper position of electrodes inside of diamond, the groove-shaped structure can provide a more uniform electric field inside of photo-sensitive block, spreading the field to deeper layers, more carriers generated by a portion of light deeper penetrated in diamond could be collected, as depicted schematically in Fig. 1. The electron-hole pairs excited by photons are separated by an electric field in diamond bulk and are collected on the electrodes, thus improving the efficiency of UV photons detection. In case of the planar electrodes the field rapidly reduces with depth (in the particular device at a depth of about ten of microns) and the generated carriers will recombine and lost for detection. In order to provide the E-field high enough at depths of a few tens micrometers, buried interdigital electrodes were designed and compared with conventional planar electrodes, prepared on the same diamond substrate, as shown schematically in Fig. 2. The in-plane finger-type parallel electrodes on diamond top surface are separated by a serpentine folded photosensitive area, uncovered with the metal. The width of the electrodes, that may absorb the incident radiation (those photons are lost for sensing) is made smaller than that of the uncoated area. In case of 3D electrodes the grooves are formed between diamond ridges with virgin surface, and metalized both on bottom and side walls. Both types of the electrode arrays have the same width and period. To rationalize the design, the penetration depth for particular single crystal diamond sample was measured firstly, and distributions of electrical field for planar and groove-shaped detect structure were calculated. Then, for comparison purpose, the groove-shaped detector together with conventional planar electrodes was fabricated on the same diamond plate to ensure identical quality of the material. 2.2. Diamond characterization A high-quality single crystal diamond was produced by chemical vapor deposition (CVD) with a microwave plasma enhanced CVD system (PlASSYS, SSDR150) in CH4-H2 gas mixture. A synthetic type Ib high pressure-high temperature (HPHT) diamond plate with (100) orientation was used as the substrate for the diamond epitaxy. After deposition, laser cutting and mechanical polishing the CVD diamond crystal with dimensions 3.0×3.0×0.3 mm3 was obtained. The Raman spectrum taken at excitation wavelength of 532 nm demonstrated the sharp 1st order Raman peak of diamond at around 1332 cm-1 with width (FWHM) of Δν = 5.9 cm-1 (Fig. 3a). A weak side band at
1410 cm-1 and a rising background on the right side of the spectrum are originated from neutral nitrogen-vacancy NV0 color center (zero phonon line at 575 nm) formed due to a nitrogen impurity in the sample. The substitutional nitrogen (which is the paramagnetic impurity defect) concentration of ~700 ppb was assessed from electron spin resonance (ESR) measurements. While this nitrogen content is of two orders of magnitude higher than in the most pure diamond that could be produced by CVD method, it was still to realize the UV sensors and judge on the advantage of the 3D electrodes. The optical transmission spectrum T(λ) was measured by UV-Visible Spectrophotometer (TU1810, PERSEE, China) in the wavelength range of 200 ~ 800 nm (Fig. 3b). The absorption α(λ) and penetration depth d(λ) = α-1 spectra were calculated from T(λ) taking into account the dispersion of diamond refractive index and reflection spectral variations [16]. The plot for penetration depth against wavelength is shown in inset in Fig. 3b, indicating a high transmission in the visible spectrum and effective absorption in the range of 220 ~ 240 nm, close to the diamond band gap of 5.45 eV. The penetration depth ranges from tens to hundreds microns in this part of UV spectrum, being in accordance with data by Nebel et al. [14]. 2.3. Simulation of electric field distribution The simulation of electric field pattern E(x, z) in diamond for the two electrode configuration has been carried out using a finite element software package ANSYS at the applied electric field V/Lgap = 1 V/µm, where V is the bias voltage and Lgap is the gap between the electrodes. The structure reproduced the real fabricated device with proper material conductivity, dielectric permittivity and identical electrode parameters: the metal strip width of 10 µm and the gap Lgap =30 µm. The 3D geometry implied the same dimensions as the planar one, but the electrodes are placed on bottom and on sidewalls of parallel 10 µm deep grooves etched in diamond. The calculated 2D field distributions E(x, z) for the two designs are displayed in Fig. 4a. For the planar electrodes the field E(x, z) becomes very weak at depth z more than 10 µm, reducing to almost zero for depth > 30 µm. Thus, the electric field can drive charge carriers effectively induced by photons only with penetration depth of ~10 µm or less, while the charge carriers generated deeper would not drift to the electrodes. In contrast, the field extends to deeper layers in case of groove-shaped electrode structure (Fig. 4b). The simulation reveals a strong and rather uniform electric field with depth within depth range of 10 µm. The field became weaker at depth z > 20 µm, and drops almost to zero for depth z> 40 µm, thus the better charge collection efficiency is expected for the groove-shaped electrode structure, as the charge from a larger volume is collected in this case.
2.4. Device fabrication The groove-shaped structure was fabricated according to geometry and dimensions used for the simulation described above. A planar structure, with same electrode width and electrode interval as for groove-shaped structure, has also been prepared on a neighbor area of the same diamond sample. The specific fabrication processes are shown in Fig. 4. The sample was first cleaned in a boiling 1:1:1 nitric, perchloric and sulphuric acid bath to remove surface contamination. The groove-shaped detector structure was firstly fabricated (Fig. 5a). The aluminum film was evaporated and photoresist layer was spun on the diamond surface in sequence, then the pattern of interdigitated electrodes was transferred by photolithography and wet etching of Al. Next, the diamond crystal was placed in an inductively coupled plasma (ICP) reactive-ion etching (RIE) system (Plasma Lab 80 Plus, Oxford Instruments) and etched for 30 min with 30 sccm flow rate of oxygen at pressure of 10 mTorr and 100 W bias power. For the first 2 min, 700 W ICP power was applied, then the etching continued for 28 min at 1,000 W ICP power to produce the groove-shaped structures. Finally, the titanium film (20 nm thick) capped with gold (100 nm) were deposited as electrodes by vacuum evaporation, and the Al mask was removed by sodium hydroxide wet etching. The planar UV detector structure was fabricated according to the process sequence shown in Fig. 5b. The resist (AZ6130, Shipley) was spun on the diamond surface, followed by the pattern of interdigitated electrodes transfer using a photolithography. Next, a Ti film capped with gold was deposited as electrode by vacuum evaporation, and the resist was removed by acetone wet etching. In order to form a good Ohmic contact, an annealing was carried out in vacuum at 400°C for 1 h to form the carbide TiC layer. A step profilometer (Dektak XT, Bruker) employed to measure etching morphology and depth, revealed the etched grooves in form of inverted trapezoid with depth of ≈9.2 µm and inclined sidewalls. The groove depth of about 10 µm has been chosen a compromise between the desire to obtain a really deep groove (several tens microns) and the restriction imposed by the resulting groove shape. Indeed, the side-walls of the groove was not strictly vertical, so the bottom width is narrower that the exit of the groove (see Fig. 6b,c). For the 9 µm deep groove the bottom narrows by approximately 2 times, and for deeper grooves it could completely shrink. The similar morphology of the groove electrodes was revealed with white light interferometry (Fig. 6a-c). The width of the top and bottom of the inverted trapezoid was about 10 µm and 4 µm, respectively, indication the side-wall slope of 17° with respect to vertical. The presence of Ti/Au layer both of side-walls and bottom of the grooves was confirmed with Energy Dispersive X-Ray Spectroscopy (EDX) as shown in Fig. 6d.
3. Device test 3.1. Dark current and I-V characteristics The I-V characteristics of the devices were measured by Keithley 4200-SCS Parameter Analyzer to evaluate the dark current and photocurrent, as a function of the applied voltage in the range of -30 V to 30 V. The maximum electric field E of ±1 V/µm was close to that corresponding to a velocity saturation of charge carriers in diamond [25]. The dark current revealed almost identical behavior, symmetric to polarity, for the two structures, as presented in Fig. 7. This indicates no significant change in surface conductivity has been introduced in the process of the detector fabrication. The photocurrent Iph was measured upon illumination by monochromatic light obtained with a monochromator coupled to a Xe lamp (LE-SP-MON-1000XTHP, LEOPTICS). The dependence of the photocurrent on bias voltage Iph(V) for the planar and groove-shaped electrodes, exited at wavelength λ = 220 nm, is shown in Fig. 8. The latter system demonstrates an enhanced sensitivity: it generates Iph ~ 28 nA at E = 1 V/µm, the 36% higher compared to Iph ~ 20.5 nA for the planar electrode system. No saturation in the photocurrent for both electrode configurations was observed within the voltage range used. Fig. 8 shows a slight non symmetric behavior, which may be ascribed from mechanism of trapping and releasing for carriers. For positive bias, photo generated carriers will be trapped by some kinds of capture centers, when the bias reversed, trapped carriers will be released.
The spectral responsivity R = Iph/Pi of the detectors was also measured, where Pi is the incident optical power, defined as the power density of incident light multiplied by photosensitive area of 0.489 mm2 for each particular structure. The incident optical power was measured by HAMAMATSU S1226-44BQ photodiode exposed to illumination with the same optical area. The measured responsivities R for wavelengths of 220, 240, 280, 350 and 400nm are compared in Table. 1 for the two devices at maximum bias V = +30 V. A clear positive effect in the response with the groove-shaped structure takes place for the entire wavelength range explored. The maximum Rmax = 0.107 A/W was achieved at λ = 220 nm for groove-shaped structure, the 36% better than for the inplane electrodes. The responsivity monotonically reduces with wavelength as a result of optical absorption decrease. Even larger positive effect, up to 57%, was observed for the 3D structure at longer wavelengths of 240 and 280 nm, such benefit being comparable with charge collection increase for the diamond proton detector with similar 3D electrodes as reported by Forneris et al. [18].
Conclusions In summary, a diamond UV detector, with groove-shaped electrode structure, was designed to improve the photoresponse as compared to convention planar electrode geometry. The dimensions of the electrode grid, in particular the groove depth, were rationalized from consideration of UV photons penetration depth in diamond and simulation of electric field distribution for the two electrode geometries. The placing the electrodes deeper in diamond significantly improved the uniformity of the electric field, in which the photo-induced charge carriers drift to the collecting electrodes. As a result, the responsivity of the groove-shaped electrode structure in the spectral range of 220 – 280 nm increased up to 50% compared to that for the planar structure. These results are considered as the preliminary ones. A further improvement in sensitivity could be possible due to minimization of area of electrodes on the groove bottom to provide to avoid photon absorption by the metal electrodes, and due to a better quality diamond with a lower nitrogen impurity concentration.
Acknowledgments This work was financially supported by "the Fundamental Research Funds for the Central Universities"(HIT.NSRIF.2015040), "National Science Fund for Distinguished Young Scholars" (51625201), "National Natural Science Foundation of China" (51372053), "Innovative research group of National Natural Science Foundation of China" (11421091), "National Key Laboratory Funds" (914C490106150C49001) and "International Science & Technology Cooperation Program of China" (2015DFR50300). We are grateful to Wei Wang (HIT) for his work in simulation of electric field, and to Yufeng Zhang (HIT) for assistance in device fabrication.
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Author Biography Kang Liu was born in 1986. He received his M.Sc. in mechanics from Harbin Institute of Technology, China, in 2014, and he is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. His current research mainly involves the optoelectronic applications of wide bandgap semiconductor. Bing Dai, born in 1984, obtained his PhD degree in quantum engineering at Nagoya University, Japan, in 2013. He has currently focused on quantum devices such as high density MRAM and diamond devices consisting SiV color center, NV center. Victor Ralchenko is the head of Diamond Material Laboratory at General Physics Institute of Russian Academy of Sciences (GPI RAS) in Moscow, Russia and a professor at Harbin Institute of Technology, P.R. China. He received his MS in Quantum Electronics in 1976 at Moscow Institute of Physics and Technology, and PhD in Physics in 1989 at GPI RAS. His current research interests include CVD diamond growth, characterization and treatment, and applications of diamond materials in electronics and photonics. Yuanqin Xia was born in 1968. He received his M.Sc. in Physical Electronics from Harbin Institute of Technology, China, in 1994, and obtained his PhD in Physical Electronics from the same university in 2001. He is now working as a professor in Harbin Institute of Technology, focusing Optoelectronics technology. Baogang Quan was born in 1976. He obtained his M.Sc. in materials science and Engineering from Harbin Institute of Technology, China, in 2003, and received his PhD in condensed matter physics from Institute of physics, Chinese Academy of Sciences, China, in 2006. He is working as a engineer in Institute of physics, Chinese Academy of Sciences, He is mainly engaged in micro/nanofabrication technologies. Jiwen Zhao, born in 1994, obtained bachelor's degree in composite materials and engineering in Harbin Institute of Technology, and continued to complete his doctorate degree in material science and engineering in HIT. The major theme in his research is the diamond growth and applications of diamond in thermotics. Guoyang Shu was born in 1992. He received his M.Sc. in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. His current research focusing on crystal growth kinetics for diamond. Mingqi Sun was born in 1990. she is a candidate of PhD program in Harbin Institute of Technology, China, majoring in materials science and Engineering. Her current research topic is about developing thermal interface materials based on diamond. Ge Gao was born in 1992. She received his M.Sc. in mechanics from Harbin Institute of Technology, China, in 2016, and She is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. Her subject concerns Micro mechanical properties of ceramics. Lei Yang was born in 1983. He received his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is working as a engineer in Harbin Institute of Technology, researching measurement based on XPS.
Pei Lei was born in 1986. He received his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is a now working as a researcher at Harbin Institute of Technology, focusing optical and electrical properties of inorganic thin film. Jiaqi Zhu, born in 1974, has obtained his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2004. Now, he is a professor working in Harbin Institute of Technology. His current interests concern optical and electrical properties of inorganic thin film. Jiecai Han was born in 1966, he is a professor working in Harbin Institute of Technology, and was elected academician of the Chinese Academy of Sciences in 2015. He Mainly engaged in the mechanics of composite materials. At the same time, he was vice president of Harbin Institute of Technology.
Figure Captions Fig. 1. Collecting carriers with planar (left) and buried (right) electrode structures. Fig. 2. Schematic view of UV detector geometries (a) with groove-shaped electrode (right) and the in-plane electrode structure (left) fabricated on the same diamond sample. The groove’s bottom and sidewalls are metalized. Sketch of the electrical connections of (b) the planar detector and (c) the groove-shaped structure detector. Fig. 3. Raman spectrum (a) of diamond sample. The band at 1410 cm-1 is due to photoluminescence signal from nitrogen-vacancy NV0 center. Optical transmission spectrum (b). Inset: the penetration depth spectrum for wavelengths 220 ~ 300 nm. Fig. 4. Simulation of electric field distributions for planar structure (a) and groove-shaped structure (b) of finger-type electrodes. The black horizontal line marks the depth of 10 µm. The color bars show the electric filed E strength, which changes from zero to 1.5 V/µm. Note that the field is weak at this depth for the planar electrodes configuration. Fig. 5. Schematics of device fabrication processes for groove-shaped(a) and planar(b) structures. Fig. 6. White light interferometer image (a); a pseudo-color (b) and gray (c) presentations of a part of the pattern with a groove at a higher magnification. Elemental composition of the metal electrode on the bottom and side-wall of the groove as measured by EDX spectroscopy (d). Fig. 7. The dark current vs bias voltage for planar (circles) and groove-shaped (squares) electrode structures. Fig. 8. The measured photocurrent vs bias voltage for planar and groove-shaped structures exited by 220 nm light.
Figr-1
Figr-2
Figr-3
Figr-4
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Figr-8
Table. 1. Responsivities Rgr and Rpl for groove-shaped structure and planar structure, respectively, and their ratio at bias V = +30 V. Wavelength, λ (nm) Rgr (A/W)
Rpl (A/W) Rgr/Rpl
220
240
280
350
400
0.107±0.00
0.032±0.00
0.022±0.00
0.013±0.00
0.005±0.00
5
5
3
2
2
0.078±0.00
0.021±0.00
0.014±0.00
0.009±0.00
0.004±0.00
5
5
3
2
2
1.37
1.52
1.57
1.44
1.25
S0924-4247(16)30914-1 http://dx.doi.org/doi:10.1016/j.sna.2017.01.027 SNA 10037
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
14-11-2016 13-1-2017 23-1-2017
Please cite this article as: Kang Liu, Bing Dai, Victor Ralchenko, Yuanqin Xia, Baogang Quan, Jiwen Zhao, Guoyang Shu, Mingqi Sun, Ge Gao, Lei Yang, Pei Lei, Jiaqi Zhu, Jiecai Han, Single crystal diamond UV detector with a grooveshaped electrode structure and enhanced sensitivity, Sensors and Actuators: A Physicalhttp://dx.doi.org/10.1016/j.sna.2017.01.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Single crystal diamond UV detector with a groove-shaped electrode structure and enhanced sensitivity Kang Liua#, Bing Daia#, Victor Ralchenkoa,b,c, Yuanqin Xiad, Baogang Quane, Jiwen Zhaoa, Guoyang Shua, Mingqi Suna, Ge Gaoa, Lei Yanga, Pei Leia, Jiaqi Zhua,f*, Jiecai Hana a
Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080 ,
P.R. China b
General Physics Institute RAS, Vavilov str. 38, Moscow 119991, Russia
c
National Research Nuclear University MEPhI, Moscow 115409, Russia
d
National Key Laboratory of Science and Technology on Tunable Laser, 92 Xidazhi Str., Harbin
150080, P.R. China e
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy
of Sciences, Beijing 100190, P. R. China f
Key Laboratory of Micro-systems and Micro-structures Manufacturing, Ministry of Education,
Harbin 150080, P. R. China # These two authors contributed equally to this work. Kang Liu, ([email protected]); Bing Dai, ([email protected]) Corresponding author: Jiaqi Zhu ([email protected])
Graphical Abstract
PRIME NOVELTY Statement We designed, simulated and fabricated an ultraviolet radiation detector on single crystal CVD diamond with a 3D electrode structure. This resulted in a significant enhancement of photocurrent in comparison with the device with conventional planar structure.
Highlights
● A UV detector on single crystal diamond with 3D electrodes is designed and fabricated
● Ti-Au electrodes in grooves etched in diamond, improve uniformity of electric field
● The 3D detector demonstrated 30 - 50% better sensitivity compared to a planar device
Abstract An ultraviolet (UV) detector on single crystal chemical vapor deposition (CVD) diamond with three-dimensional (3D) metal electrodes, is designed, and fabricated with the aim to improve collection efficiency of photo-induced carriers. Interdigitated Ti/Au electrodes were formed within 10 µm deep groove etched in diamond to ensure a more uniform field pattern in the diamond bulk. In the preliminary tests the groove-shaped 3D device performance was compared to a detector with a conventional planar structure on the same material at applied electric field of 1 V/µm. It is found that the responsivity of the 3D electrode structure in the spectral range of 220 – 280 nm is up to 50% higher than that for the planar structure.
Keywords: UV detector; single crystal diamond; 3D electrodes; photoresponse
1. Introduction Due to wide band gap, high breakdown voltage, high carrier mobility and high radiation tolerance, single crystalline (SC) diamond becomes one of most interesting candidates for next generation of photon detectors for the spectral range from UV to X-rays. Diamond is particularly suitable for solar blind UV detectors due to its band gap of 5.45 eV that ensure only a very weak photoelectric response to visible light. The potential applications of the UV and VUV detectors include, in particular, space research, such as LYRA [1] and Solar Orbiter [2] projects, UV laser lithography [3], laser beam imaging [4], monitoring of fast pulsed sources, like UV excimer laser beams [5,6], for diagnostics of thermonuclear plasma [7] and laser plasma [8]. Photorestive diamond UV detectors contain metal or
carbon electrodes to collect the charge induced by ionizing radiation. Commonly, two basic geometries of electrodes are used: (i) planar electrode structures [9-11] deposited on one side of diamond film or plate, and (ii) metal-diamond-metal sandwich structures with the contacts placed on two opposite sides [6,12,13]. The planar electrodes are typically formed as interdigitited finger-type system of metal strips with a gap of the order of tens microns [3,6], and they can be easily prepared by a photolithography. The penetration depth d for incident UV light is small enough, rapidly decreasing with the wavelength decrease. Below the fundamental absorption edge (λ = 225 nm), the penetration depth amounts to d ≈ 11 µm at wavelength λ = 215 nm [14], d ≈ 3 µm at λ = 210 nm, reducing further to d ≈ 0.3 µm at λ = 180 nm [15,16]. The formed charge carriers drift to the electrodes within a thin layer, of the order of penetration depth d, close to the diamond surface. However, the electric field E for the in-plane electrode geometry is not uniform, reaching a maximum on the surface, where various defects in diamond are present, in particular, those induced by polishing (the depth of the polishinginduced defected layer can be of the order of 1 µm [17]). The charge trapping by the subsurface defects is one of the reasons, why the collection efficiency of the detector can be less than 100%. To homogenize the E-field spatial distribution Forneris et al. [18] suggested to form buried (threedimensional) electrodes in epitaxial CVD film, while preserving the interdigitited geometry. They have fabricated the 3D detector with Cr electrodes deposited inside parallel 6 μm deep grooves, produced by ion etching, that under 1MeV proton micro-beam irradiation demonstrated an improved performance: change collection efficiency increased to 0.85 compared 0.55 to that for common superficial electrode configuration. The geometrical arrangement of the electric field lines due to the 3D patterning of the electrodes results in a shorter travel path for the excess charge carriers, thus contributing to a more efficient charge collection mechanism. However, such 3D diamond sensor has not been used for UV detection, to our best knowledge. With a similar idea a UV detector with buried tungsten finger-type electrodes on diamond was reported recently by Liu et al. [19]. However, in spite of a certain improvement observed in the detector sensitivity, in view of very shallow electrodes, with depth less than 120 nm, the impact on the E-field distribution was negligible in fact. Just a side comment, we note here, that there is another approach to build 3D particle or X-ray detectors, based on laser local graphitization of diamond bulk to produce long ordered vertical pillars immersed in the diamond [20-22]. These graphitic electrodes are designed to collect the charge at a large depth, a few hundred micrometers, that is not necessary for UV detector performance. In spite of encourages results, still there are problems with the laser-produced graphitic pillars, such as microcracks formation around diamond-graphite interface [23], and incomplete transformation of diamond to graphite, resulting in too high resistivity of the electrode [24]. In the present work we fabricated, tested and simulated an UV detector with buried groove-shaped metal electrode structure on a single crystal CVD diamond, and compared its performance with that for a sensor with conventional planar structure, prepared on the same material. A significant increase in photoresponsivity for the 3D detector was measured. 2. Experimental 2.1. Design
The optimized electrode geometry should match the UV photons penetration depth, groove depth and interdistance between the grooves (stripe electrodes) to provide, ideally, a uniform electric field in between. The conventional in-planar structure is enough to collect carriers generated by light at wavelengths shorter than 210 nm with small photons penetration depth (d < 3 µm). However, it is desirable to improve detection efficiency to the UV light with λ > 210 nm with larger penetration depth, yet still close to cut-off wavelength for absorption (λ ≈ 225 nm). In contrast, due to deeper position of electrodes inside of diamond, the groove-shaped structure can provide a more uniform electric field inside of photo-sensitive block, spreading the field to deeper layers, more carriers generated by a portion of light deeper penetrated in diamond could be collected, as depicted schematically in Fig. 1. The electron-hole pairs excited by photons are separated by an electric field in diamond bulk and are collected on the electrodes, thus improving the efficiency of UV photons detection. In case of the planar electrodes the field rapidly reduces with depth (in the particular device at a depth of about ten of microns) and the generated carriers will recombine and lost for detection. In order to provide the E-field high enough at depths of a few tens micrometers, buried interdigital electrodes were designed and compared with conventional planar electrodes, prepared on the same diamond substrate, as shown schematically in Fig. 2. The in-plane finger-type parallel electrodes on diamond top surface are separated by a serpentine folded photosensitive area, uncovered with the metal. The width of the electrodes, that may absorb the incident radiation (those photons are lost for sensing) is made smaller than that of the uncoated area. In case of 3D electrodes the grooves are formed between diamond ridges with virgin surface, and metalized both on bottom and side walls. Both types of the electrode arrays have the same width and period. To rationalize the design, the penetration depth for particular single crystal diamond sample was measured firstly, and distributions of electrical field for planar and groove-shaped detect structure were calculated. Then, for comparison purpose, the groove-shaped detector together with conventional planar electrodes was fabricated on the same diamond plate to ensure identical quality of the material. 2.2. Diamond characterization A high-quality single crystal diamond was produced by chemical vapor deposition (CVD) with a microwave plasma enhanced CVD system (PlASSYS, SSDR150) in CH4-H2 gas mixture. A synthetic type Ib high pressure-high temperature (HPHT) diamond plate with (100) orientation was used as the substrate for the diamond epitaxy. After deposition, laser cutting and mechanical polishing the CVD diamond crystal with dimensions 3.0×3.0×0.3 mm3 was obtained. The Raman spectrum taken at excitation wavelength of 532 nm demonstrated the sharp 1st order Raman peak of diamond at around 1332 cm-1 with width (FWHM) of Δν = 5.9 cm-1 (Fig. 3a). A weak side band at
1410 cm-1 and a rising background on the right side of the spectrum are originated from neutral nitrogen-vacancy NV0 color center (zero phonon line at 575 nm) formed due to a nitrogen impurity in the sample. The substitutional nitrogen (which is the paramagnetic impurity defect) concentration of ~700 ppb was assessed from electron spin resonance (ESR) measurements. While this nitrogen content is of two orders of magnitude higher than in the most pure diamond that could be produced by CVD method, it was still to realize the UV sensors and judge on the advantage of the 3D electrodes. The optical transmission spectrum T(λ) was measured by UV-Visible Spectrophotometer (TU1810, PERSEE, China) in the wavelength range of 200 ~ 800 nm (Fig. 3b). The absorption α(λ) and penetration depth d(λ) = α-1 spectra were calculated from T(λ) taking into account the dispersion of diamond refractive index and reflection spectral variations [16]. The plot for penetration depth against wavelength is shown in inset in Fig. 3b, indicating a high transmission in the visible spectrum and effective absorption in the range of 220 ~ 240 nm, close to the diamond band gap of 5.45 eV. The penetration depth ranges from tens to hundreds microns in this part of UV spectrum, being in accordance with data by Nebel et al. [14]. 2.3. Simulation of electric field distribution The simulation of electric field pattern E(x, z) in diamond for the two electrode configuration has been carried out using a finite element software package ANSYS at the applied electric field V/Lgap = 1 V/µm, where V is the bias voltage and Lgap is the gap between the electrodes. The structure reproduced the real fabricated device with proper material conductivity, dielectric permittivity and identical electrode parameters: the metal strip width of 10 µm and the gap Lgap =30 µm. The 3D geometry implied the same dimensions as the planar one, but the electrodes are placed on bottom and on sidewalls of parallel 10 µm deep grooves etched in diamond. The calculated 2D field distributions E(x, z) for the two designs are displayed in Fig. 4a. For the planar electrodes the field E(x, z) becomes very weak at depth z more than 10 µm, reducing to almost zero for depth > 30 µm. Thus, the electric field can drive charge carriers effectively induced by photons only with penetration depth of ~10 µm or less, while the charge carriers generated deeper would not drift to the electrodes. In contrast, the field extends to deeper layers in case of groove-shaped electrode structure (Fig. 4b). The simulation reveals a strong and rather uniform electric field with depth within depth range of 10 µm. The field became weaker at depth z > 20 µm, and drops almost to zero for depth z> 40 µm, thus the better charge collection efficiency is expected for the groove-shaped electrode structure, as the charge from a larger volume is collected in this case.
2.4. Device fabrication The groove-shaped structure was fabricated according to geometry and dimensions used for the simulation described above. A planar structure, with same electrode width and electrode interval as for groove-shaped structure, has also been prepared on a neighbor area of the same diamond sample. The specific fabrication processes are shown in Fig. 4. The sample was first cleaned in a boiling 1:1:1 nitric, perchloric and sulphuric acid bath to remove surface contamination. The groove-shaped detector structure was firstly fabricated (Fig. 5a). The aluminum film was evaporated and photoresist layer was spun on the diamond surface in sequence, then the pattern of interdigitated electrodes was transferred by photolithography and wet etching of Al. Next, the diamond crystal was placed in an inductively coupled plasma (ICP) reactive-ion etching (RIE) system (Plasma Lab 80 Plus, Oxford Instruments) and etched for 30 min with 30 sccm flow rate of oxygen at pressure of 10 mTorr and 100 W bias power. For the first 2 min, 700 W ICP power was applied, then the etching continued for 28 min at 1,000 W ICP power to produce the groove-shaped structures. Finally, the titanium film (20 nm thick) capped with gold (100 nm) were deposited as electrodes by vacuum evaporation, and the Al mask was removed by sodium hydroxide wet etching. The planar UV detector structure was fabricated according to the process sequence shown in Fig. 5b. The resist (AZ6130, Shipley) was spun on the diamond surface, followed by the pattern of interdigitated electrodes transfer using a photolithography. Next, a Ti film capped with gold was deposited as electrode by vacuum evaporation, and the resist was removed by acetone wet etching. In order to form a good Ohmic contact, an annealing was carried out in vacuum at 400°C for 1 h to form the carbide TiC layer. A step profilometer (Dektak XT, Bruker) employed to measure etching morphology and depth, revealed the etched grooves in form of inverted trapezoid with depth of ≈9.2 µm and inclined sidewalls. The groove depth of about 10 µm has been chosen a compromise between the desire to obtain a really deep groove (several tens microns) and the restriction imposed by the resulting groove shape. Indeed, the side-walls of the groove was not strictly vertical, so the bottom width is narrower that the exit of the groove (see Fig. 6b,c). For the 9 µm deep groove the bottom narrows by approximately 2 times, and for deeper grooves it could completely shrink. The similar morphology of the groove electrodes was revealed with white light interferometry (Fig. 6a-c). The width of the top and bottom of the inverted trapezoid was about 10 µm and 4 µm, respectively, indication the side-wall slope of 17° with respect to vertical. The presence of Ti/Au layer both of side-walls and bottom of the grooves was confirmed with Energy Dispersive X-Ray Spectroscopy (EDX) as shown in Fig. 6d.
3. Device test 3.1. Dark current and I-V characteristics The I-V characteristics of the devices were measured by Keithley 4200-SCS Parameter Analyzer to evaluate the dark current and photocurrent, as a function of the applied voltage in the range of -30 V to 30 V. The maximum electric field E of ±1 V/µm was close to that corresponding to a velocity saturation of charge carriers in diamond [25]. The dark current revealed almost identical behavior, symmetric to polarity, for the two structures, as presented in Fig. 7. This indicates no significant change in surface conductivity has been introduced in the process of the detector fabrication. The photocurrent Iph was measured upon illumination by monochromatic light obtained with a monochromator coupled to a Xe lamp (LE-SP-MON-1000XTHP, LEOPTICS). The dependence of the photocurrent on bias voltage Iph(V) for the planar and groove-shaped electrodes, exited at wavelength λ = 220 nm, is shown in Fig. 8. The latter system demonstrates an enhanced sensitivity: it generates Iph ~ 28 nA at E = 1 V/µm, the 36% higher compared to Iph ~ 20.5 nA for the planar electrode system. No saturation in the photocurrent for both electrode configurations was observed within the voltage range used. Fig. 8 shows a slight non symmetric behavior, which may be ascribed from mechanism of trapping and releasing for carriers. For positive bias, photo generated carriers will be trapped by some kinds of capture centers, when the bias reversed, trapped carriers will be released.
The spectral responsivity R = Iph/Pi of the detectors was also measured, where Pi is the incident optical power, defined as the power density of incident light multiplied by photosensitive area of 0.489 mm2 for each particular structure. The incident optical power was measured by HAMAMATSU S1226-44BQ photodiode exposed to illumination with the same optical area. The measured responsivities R for wavelengths of 220, 240, 280, 350 and 400nm are compared in Table. 1 for the two devices at maximum bias V = +30 V. A clear positive effect in the response with the groove-shaped structure takes place for the entire wavelength range explored. The maximum Rmax = 0.107 A/W was achieved at λ = 220 nm for groove-shaped structure, the 36% better than for the inplane electrodes. The responsivity monotonically reduces with wavelength as a result of optical absorption decrease. Even larger positive effect, up to 57%, was observed for the 3D structure at longer wavelengths of 240 and 280 nm, such benefit being comparable with charge collection increase for the diamond proton detector with similar 3D electrodes as reported by Forneris et al. [18].
Conclusions In summary, a diamond UV detector, with groove-shaped electrode structure, was designed to improve the photoresponse as compared to convention planar electrode geometry. The dimensions of the electrode grid, in particular the groove depth, were rationalized from consideration of UV photons penetration depth in diamond and simulation of electric field distribution for the two electrode geometries. The placing the electrodes deeper in diamond significantly improved the uniformity of the electric field, in which the photo-induced charge carriers drift to the collecting electrodes. As a result, the responsivity of the groove-shaped electrode structure in the spectral range of 220 – 280 nm increased up to 50% compared to that for the planar structure. These results are considered as the preliminary ones. A further improvement in sensitivity could be possible due to minimization of area of electrodes on the groove bottom to provide to avoid photon absorption by the metal electrodes, and due to a better quality diamond with a lower nitrogen impurity concentration.
Acknowledgments This work was financially supported by "the Fundamental Research Funds for the Central Universities"(HIT.NSRIF.2015040), "National Science Fund for Distinguished Young Scholars" (51625201), "National Natural Science Foundation of China" (51372053), "Innovative research group of National Natural Science Foundation of China" (11421091), "National Key Laboratory Funds" (914C490106150C49001) and "International Science & Technology Cooperation Program of China" (2015DFR50300). We are grateful to Wei Wang (HIT) for his work in simulation of electric field, and to Yufeng Zhang (HIT) for assistance in device fabrication.
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Author Biography Kang Liu was born in 1986. He received his M.Sc. in mechanics from Harbin Institute of Technology, China, in 2014, and he is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. His current research mainly involves the optoelectronic applications of wide bandgap semiconductor. Bing Dai, born in 1984, obtained his PhD degree in quantum engineering at Nagoya University, Japan, in 2013. He has currently focused on quantum devices such as high density MRAM and diamond devices consisting SiV color center, NV center. Victor Ralchenko is the head of Diamond Material Laboratory at General Physics Institute of Russian Academy of Sciences (GPI RAS) in Moscow, Russia and a professor at Harbin Institute of Technology, P.R. China. He received his MS in Quantum Electronics in 1976 at Moscow Institute of Physics and Technology, and PhD in Physics in 1989 at GPI RAS. His current research interests include CVD diamond growth, characterization and treatment, and applications of diamond materials in electronics and photonics. Yuanqin Xia was born in 1968. He received his M.Sc. in Physical Electronics from Harbin Institute of Technology, China, in 1994, and obtained his PhD in Physical Electronics from the same university in 2001. He is now working as a professor in Harbin Institute of Technology, focusing Optoelectronics technology. Baogang Quan was born in 1976. He obtained his M.Sc. in materials science and Engineering from Harbin Institute of Technology, China, in 2003, and received his PhD in condensed matter physics from Institute of physics, Chinese Academy of Sciences, China, in 2006. He is working as a engineer in Institute of physics, Chinese Academy of Sciences, He is mainly engaged in micro/nanofabrication technologies. Jiwen Zhao, born in 1994, obtained bachelor's degree in composite materials and engineering in Harbin Institute of Technology, and continued to complete his doctorate degree in material science and engineering in HIT. The major theme in his research is the diamond growth and applications of diamond in thermotics. Guoyang Shu was born in 1992. He received his M.Sc. in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. His current research focusing on crystal growth kinetics for diamond. Mingqi Sun was born in 1990. she is a candidate of PhD program in Harbin Institute of Technology, China, majoring in materials science and Engineering. Her current research topic is about developing thermal interface materials based on diamond. Ge Gao was born in 1992. She received his M.Sc. in mechanics from Harbin Institute of Technology, China, in 2016, and She is a candidate of PhD program in Harbin Institute of Technology, majoring in materials science and Engineering. Her subject concerns Micro mechanical properties of ceramics. Lei Yang was born in 1983. He received his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is working as a engineer in Harbin Institute of Technology, researching measurement based on XPS.
Pei Lei was born in 1986. He received his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2016, and he is a now working as a researcher at Harbin Institute of Technology, focusing optical and electrical properties of inorganic thin film. Jiaqi Zhu, born in 1974, has obtained his PhD in materials science and Engineering from Harbin Institute of Technology, China, in 2004. Now, he is a professor working in Harbin Institute of Technology. His current interests concern optical and electrical properties of inorganic thin film. Jiecai Han was born in 1966, he is a professor working in Harbin Institute of Technology, and was elected academician of the Chinese Academy of Sciences in 2015. He Mainly engaged in the mechanics of composite materials. At the same time, he was vice president of Harbin Institute of Technology.
Figure Captions Fig. 1. Collecting carriers with planar (left) and buried (right) electrode structures. Fig. 2. Schematic view of UV detector geometries (a) with groove-shaped electrode (right) and the in-plane electrode structure (left) fabricated on the same diamond sample. The groove’s bottom and sidewalls are metalized. Sketch of the electrical connections of (b) the planar detector and (c) the groove-shaped structure detector. Fig. 3. Raman spectrum (a) of diamond sample. The band at 1410 cm-1 is due to photoluminescence signal from nitrogen-vacancy NV0 center. Optical transmission spectrum (b). Inset: the penetration depth spectrum for wavelengths 220 ~ 300 nm. Fig. 4. Simulation of electric field distributions for planar structure (a) and groove-shaped structure (b) of finger-type electrodes. The black horizontal line marks the depth of 10 µm. The color bars show the electric filed E strength, which changes from zero to 1.5 V/µm. Note that the field is weak at this depth for the planar electrodes configuration. Fig. 5. Schematics of device fabrication processes for groove-shaped(a) and planar(b) structures. Fig. 6. White light interferometer image (a); a pseudo-color (b) and gray (c) presentations of a part of the pattern with a groove at a higher magnification. Elemental composition of the metal electrode on the bottom and side-wall of the groove as measured by EDX spectroscopy (d). Fig. 7. The dark current vs bias voltage for planar (circles) and groove-shaped (squares) electrode structures. Fig. 8. The measured photocurrent vs bias voltage for planar and groove-shaped structures exited by 220 nm light.
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Table. 1. Responsivities Rgr and Rpl for groove-shaped structure and planar structure, respectively, and their ratio at bias V = +30 V. Wavelength, λ (nm) Rgr (A/W)
Rpl (A/W) Rgr/Rpl
220
240
280
350
400
0.107±0.00
0.032±0.00
0.022±0.00
0.013±0.00
0.005±0.00
5
5
3
2
2
0.078±0.00
0.021±0.00
0.014±0.00
0.009±0.00
0.004±0.00
5
5
3
2
2
1.37
1.52
1.57
1.44
1.25