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Si based metal–semiconductor–metal ultraviolet photodetectors manufactured using micromachining and nano-lithographic technologies
Thin Solid Films 520 (2012) 2158–2161
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Front and backside-illuminated GaN/Si based metal–semiconductor–metal ultraviolet photodetectors manufactured using micromachining and nano-lithographic technologies Alexandru Müller a,⁎, George Konstantinidis b, Maria Androulidaki b, Adrian Dinescu a, Alexandra Stefanescu a, Alina Cismaru a, Dan Neculoiu a, Emil Pavelescu a, Antonis Stavrinidis b a b
IMT-Bucharest, 32B Erou Iancu Nicolae, R-07719, Bucharest, Romania FORTH IESL Heraklion, Crete, Greece
a r t i c l e
i n f o
Article history: Received 15 December 2010 Received in revised form 14 September 2011 Accepted 15 September 2011 Available online 29 September 2011 Keywords: UV photodetector Responsivity GaN membrane e beam lithography
a b s t r a c t This paper presents the fabrication and characterization of GaN/Si based Ultraviolet (UV) Metal/Semiconductor/ Metal (MSM) photodetectors. The thin GaN membranes have been obtained by semiconductor micromachining techniques. The two MSM interdigitated structures are contrived of fingers and interdigit spacings 100 and 200 nm wide respectively, obtained by nanolithographic techniques on GaN. Responsivity measurements were performed using both front side as well as backside-illumination. For front side illumination and for a wavelength of 365 nm and 2.5 V bias the structure with 100 nm wide fingers/interdigit spacing, exhibited the high value of 1.45 A/W. Backside-illumination responsivity of the same structure was ~0.37 A/W at the same wavelength and bias. Backside-illuminated photodetctors are interesting in two dimensional UV CCD imaging array manufacturing. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many research efforts have been focused on semiconductor-based ultraviolet (UV) detectors in the last years. These devices have important commercial and scientific interest for engine control, astronomy, lithography aligners, combustion control systems, solar UV monitoring, space-to-space communications, detection of missiles etc. [1,2]. Silicon (Si) based UV photodetectors have some major drawbacks: degradation due to exposure to radiation with higher energy than the Si band-gap, reduced quantum efficiency in the deep-UV range (due to the passivation layers) and significant loss of effective area due to the needs to use filters in order to decrease the responsivity in the visible part of the solar spectrum. An alternative to the Si ones is wide band-gap III-nitrides based photodetectors. The UV detectors based on GaN, provide natural spectral selectivity, low dark currents, long-term stability and radiation hardness. GaN based photodetectors work without degradation in harsh conditions such as high power, high temperature, or space. GaN layers have been deposited on various substrates like SiC, Sapphire and also Silicon. The quality of the GaN layers grown by Metal Organic Chemical Vapour Deposition (MOCVD) on Silicon which is a critical factor for the performance of the detectors, improved drastically in the last years [3–6].
⁎ Corresponding author. Tel.: + 40 21 269 07 75; fax: + 40 21 269 07 72. E-mail address: [email protected] (A. Müller). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.09.045
Many photodetector devices are based on metal–semiconductor– metal (MSM) structures, due to their simplicity. The MSM configuration produces low dark current due to the rectifying nature of the contacts and the high resistivity semiconductor material. Additionally, its reduced parasitic capacitance and the low noise, render MSM very attractive for UV photodetection [7–9]. Recently, MSM photodetectors with very low dark current and responsivities in the range of 100 mA/W have been fabricated using GaN grown on silicon and standard lithographic processes [10]. UV photodetectors used in imaging systems have been described in [11]. Backside illumination experiments on MSM type UV photodetectors based on GaN grown on sapphire have been reported in [12]. The backside UV illumination through the transparent sapphire substrate was non-localized and avoided the shadowing of the GaN regions under the contact pads; photocarriers are generated also under the pads and not only under the MSM structure. This shadowing of the region under the pads is also present under top side illumination. To manufacture imaging systems, flip-chip mounting and backside illumination are used. Backside illumination experiments have been reported for photodiodes manufactured on AlxGa1 − xN grown on sapphire substrate used in focal plane array imagers [13 – 15]. The main disadvantage of the sapphire substrate is the lack of transparency to extreme UV light. Sapphire is also hard to etch, thus GaN membrane formation is also difficult. Therefore, an alternative approach to achieve backside illumination is to manufacture membrane supported UV photodetectors using micromachining technologies of GaN/Si. Reverchon et al. enounced the idea of backside
A. Müller et al. / Thin Solid Films 520 (2012) 2158–2161
illumination through a thin membrane in 2007 [16]. They have reported also the manufacturing of robust AlGaN membranes, using micromachining techniques (Bosch process). Membrane supported UV, photodetectors based on a MSM structure, with fingers and interdigit spacing 1 μm wide, obtained with optical lithography, have been manufactured by the authors in 2008 [17]; responsivity was measured using only front side illumination. Reverchon et al. [18] reported a membrane supported AlGaN focal plane array in 2009. Also, in 2009, a Schottky diode for UV photodetection, manufactured on a thin GaN membrane, was measured using backside illumination (Malinowski et al. [19]); very promising results have been obtained. The same authors have reported, in 2010, a MSM UV photodetector structure supported on a 200 nm thin AlGaN membrane [20]. The purpose of this paper is to present the fabrication and responsivity measurements using front side as well as backside illumination, for UV MSM photodetectors, manufactured on very thin GaN membranes. The MSM structures, with fingers and interdigit spacing 100 and respectively 200 nm wide, are manufactured using e-beam lithography; the nano-metric MSM structures significantly enhances the responsivity of the devices. The results and the analysis of front and backside-illumination responsivity measurements are presented.
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Fig. 1. SEM photo of the GaN membrane UV detector structure with finger/interdigit spacing 100 nm wide; the inset presents a detail of the MSM structure obtained using a single metal/resist layer nanolithographic process.
micrograph of the MSM structure. The SEM is a “FEI Nova NanoSEM 630”, and it was used at 5 kV accelerating voltage with secondary electron detector.
2. Fabrication
3. Dark current and front and backside responsivity measurements
For GaN/Si UV photodetector fabrication, wafers purchased from NTT AT Japan, have been used. The thickness of the MOCVD grown GaN layer was 0.3 μm. Between the (111) oriented Si substrate and the GaN layer, a buffer layer containing various AlN and AlGaN layers, was grown. The commercial provider has not disclosed the exact composition and thickness of these layers. The total thickness of the buffer layer was 0.2 μm. The first step for the device fabrication was the patterning and deposition of the contacting pads. A Ni/Au layer (20 nm/500 nm) was deposited using lift-off techniques. For the interdigitated MSM structure, a direct writing Electron Beam Lithography (EBL) protocol was developed. Two different structures with fingers and interdigit spacing 100 nm and 200 nm wide respectively were processed, using Poly(methyl methacrylate) (PMMA) resist with a thickness of 200 nm. The MSM structure has 500 fingers and 499 interdigits spacing for the first structure and 250 fingers and 249 interdigits spacing for the second one. The length of the digits was 100 μm. A semitransparent Ni/Au (5 nm/10 nm) metallization was deposited, using lift-off technique. The wafers were then mounted on glass plates and the Si substrate was thinned down to 150 μm by chemo-mechanical lapping. For the formation of the membranes, an Al mask was patterned and deposited on the backside, to serve as an etch mask for the reactive ion etching process (RIE). The backside mask had 300 μm by 300 μm square openings aligned with the centers of the MSM structures on the front side. The silicon substrate was selectively etched in SF6 plasma. The selectivity between GaN and Si in SF6 plasma is of the order of 1/200, as it results from measurements performed by the authors. The nano-lithography on GaN for the interdigitated MSM structure is not a trivial process. The high resistivity of the un-doped GaN layer and the large atomic mass of Ga create charging effects and impede the evacuation of electrons injected by the e-beam. This enhances proximity effects and creates difficulties in the lift-off of Au [21]. Up to now sub-micron wide fingers and interdigit spacing have been obtained with sophisticated multi-layer nanolithographic processes [21,22]. Using the “e_Line”, e-beam lithography equipment from RAITH GmbH, the photodetector structures were patterned in a single electron resist layer — PMMA (950 k units). The fabrication was followed by metal evaporation and liftoff [23]. Fig. 1 presents a scanning electron microscope (SEM)
The dark current versus voltage, for the GaN membrane supported MSM photodetector structures, was measured using a Keithley 4200 semiconductor characterization system and the results are presented in Fig. 2 for both test structures. The dark current is approximately 80 pA for the 200 nm finger structure and approximately 300 pA for the 100 nm finger structure, at a bias of 2 V. These values are relatively high, compared with usual UV MSM photodetector structures and are related, in our opinion, to two different reasons: one related to the nano-metric interdigitated MSM structure, the other related to an increased gain of the membrane UV photodetector structure. The decrease of the width of the un-depleted region existing between the two space charge regions of the MSM structure, has an effect on the decrease of the resistance and the increase of the dark current. The nano-metric dimensions of the fingers and interdigit spacing have as effect a very narrow undepleted region or the overlapping of the two space charge regions from the two contacts of the MSM structure, at very low voltages (very close to zero bias) and as a consequence a lower resistance and a higher dark current. At “punch-trough”, the width of the un-depleted region vanishes. The effect of increasing of the dark current with decreasing of the digit/interdigit spacing width was recently (2010) analyzed in [24], using MgZnO MSM photodetectors, but was also observed by Palacios et al. [25] in 2002. They have manufactured UV photodectors on GaN/
Fig. 2. Dark current vs. applied voltage for the two different test structures.
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A. Müller et al. / Thin Solid Films 520 (2012) 2158–2161
Saphire substrate, using a MSM structure with finger and interdigit spacing width of 4, 2, 1, and 0.5 μm. The increase of the dark current with the decrease of the finger/interdigit spacing was observed for all structures. The significant increase of the dark current and of the slope of d(logI)/dV, for the 0.5 μm wide finger/interdigit spacing structure, at voltages higher than 5 V (when the two space charge regions are overlapped) was also observed. It is also possible that the substrate removal process creates hole traps that could lead to gain (and dark current) increase, compared with UV photodetectors structures manufactured with GaN/Sapphire or GaN/Si. The membrane itself can have an influence in increasing the gain. In contrast with GaN/Sapphire and GaN/Si structures, in the case of the GaN membrane detectors, the photo-generated carriers are confined in the very thin membrane due to the very high barrier between the membrane and the air. This inhibits the carrier flow at moderate applied voltage and increases the dark current as well as the responsivity [26]. Responsivity measurements were performed using a special setup for both front and backside use. We employed a Xe lamp and a spectrometer with grating blaze at 300 nm. All spectra were taken using a lock-in amplifier and for the optical power we employed a calibrated power meter. A UV fiber was used for the backside illumination. The external responsivity was calculated according to the formula: RðλÞ ¼
Jph ðλÞ Popt ðλÞ
ð1Þ
In Eq. (1), Jph(λ) is the photocurrent density and Popt(λ) is the optical power density, defined as the power of the incident radiation on the photodetector divided by the spot area. The total area of the MSM structure was 10,000 μm 2 (digits + interdigits spacing for both structures) while the metal free (active) area was 5000 μm 2. However, the 5/10 nm Ni/Au contact has 50% transparency [27], thus the order of the “effective” active area of the detectors was ~7500 μm 2. This area was considered for the front side illumination responsivity estimation. For the backside responsivity, the entire area under the MSM structure is illuminated uniformly (10,000 μm 2). The measured front and backside illuminated responsivities for the 100 nm and 200 nm finger/interdigit spacing MSM membrane UV-photodetector structure at V = 2.5 V, are presented in Figs. 3 and 4 respectively. The maximum responsivity for our structures has been obtained at λ = 362 nm. For the 100 nm wide finger/interdigit spacing structure, front side illumination responsivity was 1.45 A/W and the backside illumination responsivity was 0.37 A/W. For the 200 nm wide finger/ interdigit spacing structure the maximum front side illumination responsivity was 0.127 A/W and the backside illumination responsivity
Fig. 3. Responsivity vs. wavelength for front side and backside-illuminated UV photodetector structures with fingers and interdigit spacing 100 nm wide, at a bias of 2.5 V.
Fig. 4. Responsivity vs. wavelength for front side and backside illuminated UV photodetector structures with fingers and interdigit spacing 200 nm wide, at a bias of 2.5 V.
was 0.022 A/W. This corresponds to a front to backside illumination responsivity ratio of 3.9 for the first structure and 5.9 for the second one. 4. Discussion of the results The values for the responsivity under front side illumination conditions are much higher than those reported for UV photodetectors manufactured on sapphire substrate and having fingers and interdigit spacing 0.5 μm wide [25] and demonstrate the advantage of using nanolithographic techniques for these devices. Also the results confirm the potential of GaN on silicon for photodetecting applications (as it was demonstrated in [10]). The results regarding backside illumination (Figs. 3 and 4) are very promising. The values of the responsivities for backside-illumination are a few times lower than those obtained for front side illumination of the same device. This result was expected because of the absorption in the GaN membrane layer. The membrane layer is composed of the 0.3 μm GaN layer on the top and also the 0.2 μm buffer layer on the bottom. It is known from the provider that the buffer is formed by different AlN and AlGaN layers. The number and thickness of the different layers, composing the buffer as well the exact composition of the AlGaN layers is unknown. The membrane is 0.5 μm thick, so it is expected to exhibit a higher absorption than the 0.3 μm and also a lower backside responsivity as compared to a pure 0.3 μm thick GaN membrane. So it is clear that the transmission of the 0.5 μm thick membrane is difficult to be predicted, the only solution to evaluate the results regarding the front side to backside illumination ratio for the measured structures, is the experimental measurement of its transmission coefficient. The measurement of transmission coefficient of the membrane was carried out using a 300 W Xe source connected with a Cornestone ¼ monochromator equipped with a 350 nm-blaze 1200 lines/mm diffraction grating. The output light from the monochromator was coupled to a solarization resistant fused-silica fiber. An UV-enhanced silicon photodetector in conjunction with a Newport power meter was used to measure the power of light at certain wavelengths after passing through the III-nitride based membrane and after passing through the opening remained upon the mechanical removal of the membrane. The results for the transmission coefficient are presented in Fig. 5. It is interesting to notice that the results obtained for the transmission are very close to those reported by Muth et al. [28] for a 0.4 μm GaN thin film (transmission probability). It seems that the contribution of the 0.2 μm thin buffer layer (formed from AlN and AlGaN layers) is equivalent for the absorption of UV light with 0.1 μm GaN.
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structures very promising for applications which need backside illumination (e.g. imaging systems). Acknowledgments The authors acknowledge Dr. Cristian Kusko from IMT-Bucharest for the assistance in transmission measurements. Dr. Alexandra Stefanescu and Dr. Alina Cismaru acknowledge the support of the Sectoral Operational Program Human Resource Development (SOP HRD) financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/89/1.5/S/63700. References Fig. 5. Transmission measurements on the 0.5 μm GaN (+ buffer) thin membrane. Inset contains a blow-up of the results in the wavelength region of interest.
This is expectable because, the absorption coefficient of AlN [29] is a few times lower than of GaN [28] and the absorption coefficient in AlGaN has a value placed between the value for AlN and the value for GaN (depending of the Al fraction). At λ = 362 nm (where we have obtained the maximum responsivity for both structures) we obtained a value of the transmission coefficient close to 0.23 which indicates an expected front to backside illumination responsivity ratio of 4.3. This value is in good agreement with the results obtained for the structure with 100 nm finger interdigit spacings (10% difference) while for the structure having 200 nm finger interdigit spacings, the experimental results differ by 25% from the expected value. We believe that the errors can be reduced, by improving the experimental setup for responsivity measurements. It must be noticed that in the responsivity measurements, the rejection ratio is poor, especially in the 365–400 nm range; this can be related to the gain but further investigations are necessary to clarify more precisely the origin of this behavior of the membrane UV photodetectors. It is evident from the measurements, that the values obtained for the backside illumination responsivity are high enough to make the membrane supported UV photodetectors very promising for applications where backside illumination is needed. It must be pointed out that backside-illumination, using membrane supported UV photodectors, is localized. If the membrane area is small enough not to permit the exposure of the contact pads area, then only the semiconductor under the interdigitated MSM structure contributes to the photocurrent. It is practically the same area as for front side illumination.
5. Conclusions GaN membrane supported UV photodetectors have been manufactured using bulk micromachining and e-beam direct lithography on GaN/Si. MSM structures with fingers and interdigit spacings of 100 nm and 200 nm wide respectively have been implemented. The membranes are composed from a 0.3 μm thin GaN layer on the top and a 0.2 μm thin buffer (based on successive AlN and AlGaN layers) on the bottom. The UV detectors demonstrated high responsivities under top side illumination Backside-illumination responsivities were approximately 4–6 times lower than the front side illumination values, but high enough (tens to hundreds of mA/W) to make the
[1] E. Monroy, F. Omnes, F. Calle, Semicond. Sci. Technol. 18 (2003) R33. [2] M. Razeghi, Proc. IEEE 90 (2002) 1006. [3] A. Dadgar, P. Veit, F. Schulze, J. Blasing, A. Krtschil, Witte, H. Diez, A.R. Clos, Thin Solid Films 515 (2007) 4356. [4] W. Luo, X. Wang, H. Xiao, C. Wang, J. Ran, L. Guo, J. Li, H. Liu, Y. Chen, F. Yang, J. Li, Microelectron. J. 39 (2008) 1108. [5] E. Arslan, M.K. Ozturk, A. Teke, S. Ozcelik, E. Ozbay, J. Phys. D: Appl. Phys. 41 (2008) 155317. [6] Z.C. Feng, X. Zhang, S.J. Chua, T.R. Yang, J.C. Dang, G. Xu, Thin Solid Films 409 (2002) 15. [7] E. Monroy, F. Calle, F. Munoz, F. Omnes, Appl. Phys. Lett. 74 (1999) 3401. [8] J.C. Carrano, P.A. Grudowski, C.J. Eiting, R.D. Dupuis, J.C. Campbell, Appl. Phys. Lett. 70 (1997) 1992. [9] H. Huang, W. Yang, Y. Xie, X. Chen, Z. Wu, IEEE Electron Device Lett. 31 (2010) 588. [10] R.W. Chuang, S.P. Chuang, S.J. Chuang, J. Appl. Phys. 102 (2007) 073110. [11] J.L. Pau, C. Rivera, J. Pereiro, E. Muñoz, E. Calleja, U. Schühle, E. Frayssinet, B. Beaumont, J.P. Faurie, P. Gibart, Superlattices Microstruct. 36 (2004) 807. [12] H. Jiang, N. Nakata, G.Y. Zhao, H. Ishikawa, C.L. Shao, T. Egawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 40 (2001) L505. [13] R. McClintock, K. Mayes, A. Yasan, D. Shiell, P. Kung, M. Razeghi, Appl. Phys. Lett. 86 (2005) 11117. [14] J.P. Long, S. Varadaraajan, J. Matthewes, J.F. Schetzina, Opto-electron. Rev. 10 (2002) 251. [15] P. Lamarre, A. Hairston, S.P. Tobin, K.K. Wong, A.K. Sood, M.B. Reine, M. Pophristic, R. Birkham, I.T. Ferguson, R. Singh, C.R. Eddy Jr., U. Chowdhury, M.M. Wong, R.D. Dupuis, P. Kozodoy, E.J. Tarsa, Phys. Status Solidi A 188 (2001) 289. [16] J.L. Reverchon, J.A. Robot, J.P. Truffer, J.P. Caumes, I. Mourad, J. Brault, J.Y. Duboz, SPIE Eur. Remote Sens. 6744 (2007) 17. [17] A. Müller, G. Konstantinidis, M. Dragoman, D. Neculoiu, A. Kostopoulos, M. Androulidaki, M. Kayambaki, D. Vasilache, Appl. Optics 47 (2008) 1453. [18] J.L. Reverchon, S. Bansropun, J.A. Robo, J.P. Truffer, E. Costard, E. Frayssinet, J. Brault, F. Semond, J.Y. Duboz, M. Idir, SPIE Proceedings — Sens. Systems and Next-Generation Satellites XIII, 7474, 2009, p. 74741G. [19] P.E. Malinowski, J. John, J.Y. Duboz, G. Hellings, A. Lorenz, J.G. Rodriguez Madrid, C. Sturdevant, K. Cheng, M. Leys, J. Derluyn, J. Das, M. Germain, K. Minoglou, P. De Moor, E. Frayssinet, F. Semond, J.-F. Hochedez, B. Giordanengo, R. Mertens, IEEE Electron Device Lett. 30 (2009) 1308. [20] P.E. Malinowski, J.-Y. Duboz, J. John, C. Sturdevant, J. Das, J. Derluyn, M. Germain, P. de Moor, K. Minoglou, F. Semond, E. Frayssinet, J.-F. Hochedez, B. Giordanengo, C. van Hoof, R. Mertens, Proc. SPIE 7726 (2010) 772617. [21] T. Palacios, F. Calle, E. Monroy, F. Munoz, J. Vac. Sci. Technol., B 20 (2002) 2071. [22] P. Kirsch, M.B. Assouar, O. Elmazria, V. Mortet, P. Alnot, Appl. Phys. Lett. 88 (2006) 223504. [23] A. Müller, D. Neculoiu, G. Konstantinidis, G. Deligeorgis, A. Dinescu, A. Stavrinidis, A. Cismaru, M. Dragoman, A. Stefanescu, IEEE Electron Devices Lett. 31 (2010) 1398. [24] D. Jiang, X. Zhang, Q. Liu, Z. Bai, L. Lu, X. Wang, X. Mi, N. Wang, D. Shen, Mater. Sci. Eng. B 175 (2010) 41. [25] T. Palacios, E. Monroy, F. Calle, F. Omnes, Appl. Phys. Lett. 81 (2002) 1902. [26] A. Muller, G. Konstantinidis, M. Dragoman, D. Neculoiu, A. Dinescu, M. Androulidaki, M. Kayambaki, A. Stavrinidis, D. Vasilache, C. Buiculescu, I. Petrini, A. Kostopoulos, D. Dascalu, Microelectron. J. 40 (2009) 319. [27] P.C. Chang, C.H. Chen, S.J. Chang, Y.K. Su, C.L. Yu, P.C. Chen, C.H. Wang, Semicond. Sci. Technol. 19 (2004) 1354. [28] J.F. Muth, J.H. Lee, I.K. Shmagin, R.M. Kolbas, H.C. Casey Jr., B.P. Keller, U.K. Mishra, S.P. Den Baars, Appl. Phys. Lett. 71 (1997) 2572. [29] H. Demiryont, L.R. Thompson, G.J. Collins, Appl. Optics 25 (1986) 1311.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Front and backside-illuminated GaN/Si based metal–semiconductor–metal ultraviolet photodetectors manufactured using micromachining and nano-lithographic technologies Alexandru Müller a,⁎, George Konstantinidis b, Maria Androulidaki b, Adrian Dinescu a, Alexandra Stefanescu a, Alina Cismaru a, Dan Neculoiu a, Emil Pavelescu a, Antonis Stavrinidis b a b
IMT-Bucharest, 32B Erou Iancu Nicolae, R-07719, Bucharest, Romania FORTH IESL Heraklion, Crete, Greece
a r t i c l e
i n f o
Article history: Received 15 December 2010 Received in revised form 14 September 2011 Accepted 15 September 2011 Available online 29 September 2011 Keywords: UV photodetector Responsivity GaN membrane e beam lithography
a b s t r a c t This paper presents the fabrication and characterization of GaN/Si based Ultraviolet (UV) Metal/Semiconductor/ Metal (MSM) photodetectors. The thin GaN membranes have been obtained by semiconductor micromachining techniques. The two MSM interdigitated structures are contrived of fingers and interdigit spacings 100 and 200 nm wide respectively, obtained by nanolithographic techniques on GaN. Responsivity measurements were performed using both front side as well as backside-illumination. For front side illumination and for a wavelength of 365 nm and 2.5 V bias the structure with 100 nm wide fingers/interdigit spacing, exhibited the high value of 1.45 A/W. Backside-illumination responsivity of the same structure was ~0.37 A/W at the same wavelength and bias. Backside-illuminated photodetctors are interesting in two dimensional UV CCD imaging array manufacturing. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many research efforts have been focused on semiconductor-based ultraviolet (UV) detectors in the last years. These devices have important commercial and scientific interest for engine control, astronomy, lithography aligners, combustion control systems, solar UV monitoring, space-to-space communications, detection of missiles etc. [1,2]. Silicon (Si) based UV photodetectors have some major drawbacks: degradation due to exposure to radiation with higher energy than the Si band-gap, reduced quantum efficiency in the deep-UV range (due to the passivation layers) and significant loss of effective area due to the needs to use filters in order to decrease the responsivity in the visible part of the solar spectrum. An alternative to the Si ones is wide band-gap III-nitrides based photodetectors. The UV detectors based on GaN, provide natural spectral selectivity, low dark currents, long-term stability and radiation hardness. GaN based photodetectors work without degradation in harsh conditions such as high power, high temperature, or space. GaN layers have been deposited on various substrates like SiC, Sapphire and also Silicon. The quality of the GaN layers grown by Metal Organic Chemical Vapour Deposition (MOCVD) on Silicon which is a critical factor for the performance of the detectors, improved drastically in the last years [3–6].
⁎ Corresponding author. Tel.: + 40 21 269 07 75; fax: + 40 21 269 07 72. E-mail address: [email protected] (A. Müller). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.09.045
Many photodetector devices are based on metal–semiconductor– metal (MSM) structures, due to their simplicity. The MSM configuration produces low dark current due to the rectifying nature of the contacts and the high resistivity semiconductor material. Additionally, its reduced parasitic capacitance and the low noise, render MSM very attractive for UV photodetection [7–9]. Recently, MSM photodetectors with very low dark current and responsivities in the range of 100 mA/W have been fabricated using GaN grown on silicon and standard lithographic processes [10]. UV photodetectors used in imaging systems have been described in [11]. Backside illumination experiments on MSM type UV photodetectors based on GaN grown on sapphire have been reported in [12]. The backside UV illumination through the transparent sapphire substrate was non-localized and avoided the shadowing of the GaN regions under the contact pads; photocarriers are generated also under the pads and not only under the MSM structure. This shadowing of the region under the pads is also present under top side illumination. To manufacture imaging systems, flip-chip mounting and backside illumination are used. Backside illumination experiments have been reported for photodiodes manufactured on AlxGa1 − xN grown on sapphire substrate used in focal plane array imagers [13 – 15]. The main disadvantage of the sapphire substrate is the lack of transparency to extreme UV light. Sapphire is also hard to etch, thus GaN membrane formation is also difficult. Therefore, an alternative approach to achieve backside illumination is to manufacture membrane supported UV photodetectors using micromachining technologies of GaN/Si. Reverchon et al. enounced the idea of backside
A. Müller et al. / Thin Solid Films 520 (2012) 2158–2161
illumination through a thin membrane in 2007 [16]. They have reported also the manufacturing of robust AlGaN membranes, using micromachining techniques (Bosch process). Membrane supported UV, photodetectors based on a MSM structure, with fingers and interdigit spacing 1 μm wide, obtained with optical lithography, have been manufactured by the authors in 2008 [17]; responsivity was measured using only front side illumination. Reverchon et al. [18] reported a membrane supported AlGaN focal plane array in 2009. Also, in 2009, a Schottky diode for UV photodetection, manufactured on a thin GaN membrane, was measured using backside illumination (Malinowski et al. [19]); very promising results have been obtained. The same authors have reported, in 2010, a MSM UV photodetector structure supported on a 200 nm thin AlGaN membrane [20]. The purpose of this paper is to present the fabrication and responsivity measurements using front side as well as backside illumination, for UV MSM photodetectors, manufactured on very thin GaN membranes. The MSM structures, with fingers and interdigit spacing 100 and respectively 200 nm wide, are manufactured using e-beam lithography; the nano-metric MSM structures significantly enhances the responsivity of the devices. The results and the analysis of front and backside-illumination responsivity measurements are presented.
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Fig. 1. SEM photo of the GaN membrane UV detector structure with finger/interdigit spacing 100 nm wide; the inset presents a detail of the MSM structure obtained using a single metal/resist layer nanolithographic process.
micrograph of the MSM structure. The SEM is a “FEI Nova NanoSEM 630”, and it was used at 5 kV accelerating voltage with secondary electron detector.
2. Fabrication
3. Dark current and front and backside responsivity measurements
For GaN/Si UV photodetector fabrication, wafers purchased from NTT AT Japan, have been used. The thickness of the MOCVD grown GaN layer was 0.3 μm. Between the (111) oriented Si substrate and the GaN layer, a buffer layer containing various AlN and AlGaN layers, was grown. The commercial provider has not disclosed the exact composition and thickness of these layers. The total thickness of the buffer layer was 0.2 μm. The first step for the device fabrication was the patterning and deposition of the contacting pads. A Ni/Au layer (20 nm/500 nm) was deposited using lift-off techniques. For the interdigitated MSM structure, a direct writing Electron Beam Lithography (EBL) protocol was developed. Two different structures with fingers and interdigit spacing 100 nm and 200 nm wide respectively were processed, using Poly(methyl methacrylate) (PMMA) resist with a thickness of 200 nm. The MSM structure has 500 fingers and 499 interdigits spacing for the first structure and 250 fingers and 249 interdigits spacing for the second one. The length of the digits was 100 μm. A semitransparent Ni/Au (5 nm/10 nm) metallization was deposited, using lift-off technique. The wafers were then mounted on glass plates and the Si substrate was thinned down to 150 μm by chemo-mechanical lapping. For the formation of the membranes, an Al mask was patterned and deposited on the backside, to serve as an etch mask for the reactive ion etching process (RIE). The backside mask had 300 μm by 300 μm square openings aligned with the centers of the MSM structures on the front side. The silicon substrate was selectively etched in SF6 plasma. The selectivity between GaN and Si in SF6 plasma is of the order of 1/200, as it results from measurements performed by the authors. The nano-lithography on GaN for the interdigitated MSM structure is not a trivial process. The high resistivity of the un-doped GaN layer and the large atomic mass of Ga create charging effects and impede the evacuation of electrons injected by the e-beam. This enhances proximity effects and creates difficulties in the lift-off of Au [21]. Up to now sub-micron wide fingers and interdigit spacing have been obtained with sophisticated multi-layer nanolithographic processes [21,22]. Using the “e_Line”, e-beam lithography equipment from RAITH GmbH, the photodetector structures were patterned in a single electron resist layer — PMMA (950 k units). The fabrication was followed by metal evaporation and liftoff [23]. Fig. 1 presents a scanning electron microscope (SEM)
The dark current versus voltage, for the GaN membrane supported MSM photodetector structures, was measured using a Keithley 4200 semiconductor characterization system and the results are presented in Fig. 2 for both test structures. The dark current is approximately 80 pA for the 200 nm finger structure and approximately 300 pA for the 100 nm finger structure, at a bias of 2 V. These values are relatively high, compared with usual UV MSM photodetector structures and are related, in our opinion, to two different reasons: one related to the nano-metric interdigitated MSM structure, the other related to an increased gain of the membrane UV photodetector structure. The decrease of the width of the un-depleted region existing between the two space charge regions of the MSM structure, has an effect on the decrease of the resistance and the increase of the dark current. The nano-metric dimensions of the fingers and interdigit spacing have as effect a very narrow undepleted region or the overlapping of the two space charge regions from the two contacts of the MSM structure, at very low voltages (very close to zero bias) and as a consequence a lower resistance and a higher dark current. At “punch-trough”, the width of the un-depleted region vanishes. The effect of increasing of the dark current with decreasing of the digit/interdigit spacing width was recently (2010) analyzed in [24], using MgZnO MSM photodetectors, but was also observed by Palacios et al. [25] in 2002. They have manufactured UV photodectors on GaN/
Fig. 2. Dark current vs. applied voltage for the two different test structures.
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Saphire substrate, using a MSM structure with finger and interdigit spacing width of 4, 2, 1, and 0.5 μm. The increase of the dark current with the decrease of the finger/interdigit spacing was observed for all structures. The significant increase of the dark current and of the slope of d(logI)/dV, for the 0.5 μm wide finger/interdigit spacing structure, at voltages higher than 5 V (when the two space charge regions are overlapped) was also observed. It is also possible that the substrate removal process creates hole traps that could lead to gain (and dark current) increase, compared with UV photodetectors structures manufactured with GaN/Sapphire or GaN/Si. The membrane itself can have an influence in increasing the gain. In contrast with GaN/Sapphire and GaN/Si structures, in the case of the GaN membrane detectors, the photo-generated carriers are confined in the very thin membrane due to the very high barrier between the membrane and the air. This inhibits the carrier flow at moderate applied voltage and increases the dark current as well as the responsivity [26]. Responsivity measurements were performed using a special setup for both front and backside use. We employed a Xe lamp and a spectrometer with grating blaze at 300 nm. All spectra were taken using a lock-in amplifier and for the optical power we employed a calibrated power meter. A UV fiber was used for the backside illumination. The external responsivity was calculated according to the formula: RðλÞ ¼
Jph ðλÞ Popt ðλÞ
ð1Þ
In Eq. (1), Jph(λ) is the photocurrent density and Popt(λ) is the optical power density, defined as the power of the incident radiation on the photodetector divided by the spot area. The total area of the MSM structure was 10,000 μm 2 (digits + interdigits spacing for both structures) while the metal free (active) area was 5000 μm 2. However, the 5/10 nm Ni/Au contact has 50% transparency [27], thus the order of the “effective” active area of the detectors was ~7500 μm 2. This area was considered for the front side illumination responsivity estimation. For the backside responsivity, the entire area under the MSM structure is illuminated uniformly (10,000 μm 2). The measured front and backside illuminated responsivities for the 100 nm and 200 nm finger/interdigit spacing MSM membrane UV-photodetector structure at V = 2.5 V, are presented in Figs. 3 and 4 respectively. The maximum responsivity for our structures has been obtained at λ = 362 nm. For the 100 nm wide finger/interdigit spacing structure, front side illumination responsivity was 1.45 A/W and the backside illumination responsivity was 0.37 A/W. For the 200 nm wide finger/ interdigit spacing structure the maximum front side illumination responsivity was 0.127 A/W and the backside illumination responsivity
Fig. 3. Responsivity vs. wavelength for front side and backside-illuminated UV photodetector structures with fingers and interdigit spacing 100 nm wide, at a bias of 2.5 V.
Fig. 4. Responsivity vs. wavelength for front side and backside illuminated UV photodetector structures with fingers and interdigit spacing 200 nm wide, at a bias of 2.5 V.
was 0.022 A/W. This corresponds to a front to backside illumination responsivity ratio of 3.9 for the first structure and 5.9 for the second one. 4. Discussion of the results The values for the responsivity under front side illumination conditions are much higher than those reported for UV photodetectors manufactured on sapphire substrate and having fingers and interdigit spacing 0.5 μm wide [25] and demonstrate the advantage of using nanolithographic techniques for these devices. Also the results confirm the potential of GaN on silicon for photodetecting applications (as it was demonstrated in [10]). The results regarding backside illumination (Figs. 3 and 4) are very promising. The values of the responsivities for backside-illumination are a few times lower than those obtained for front side illumination of the same device. This result was expected because of the absorption in the GaN membrane layer. The membrane layer is composed of the 0.3 μm GaN layer on the top and also the 0.2 μm buffer layer on the bottom. It is known from the provider that the buffer is formed by different AlN and AlGaN layers. The number and thickness of the different layers, composing the buffer as well the exact composition of the AlGaN layers is unknown. The membrane is 0.5 μm thick, so it is expected to exhibit a higher absorption than the 0.3 μm and also a lower backside responsivity as compared to a pure 0.3 μm thick GaN membrane. So it is clear that the transmission of the 0.5 μm thick membrane is difficult to be predicted, the only solution to evaluate the results regarding the front side to backside illumination ratio for the measured structures, is the experimental measurement of its transmission coefficient. The measurement of transmission coefficient of the membrane was carried out using a 300 W Xe source connected with a Cornestone ¼ monochromator equipped with a 350 nm-blaze 1200 lines/mm diffraction grating. The output light from the monochromator was coupled to a solarization resistant fused-silica fiber. An UV-enhanced silicon photodetector in conjunction with a Newport power meter was used to measure the power of light at certain wavelengths after passing through the III-nitride based membrane and after passing through the opening remained upon the mechanical removal of the membrane. The results for the transmission coefficient are presented in Fig. 5. It is interesting to notice that the results obtained for the transmission are very close to those reported by Muth et al. [28] for a 0.4 μm GaN thin film (transmission probability). It seems that the contribution of the 0.2 μm thin buffer layer (formed from AlN and AlGaN layers) is equivalent for the absorption of UV light with 0.1 μm GaN.
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structures very promising for applications which need backside illumination (e.g. imaging systems). Acknowledgments The authors acknowledge Dr. Cristian Kusko from IMT-Bucharest for the assistance in transmission measurements. Dr. Alexandra Stefanescu and Dr. Alina Cismaru acknowledge the support of the Sectoral Operational Program Human Resource Development (SOP HRD) financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/89/1.5/S/63700. References Fig. 5. Transmission measurements on the 0.5 μm GaN (+ buffer) thin membrane. Inset contains a blow-up of the results in the wavelength region of interest.
This is expectable because, the absorption coefficient of AlN [29] is a few times lower than of GaN [28] and the absorption coefficient in AlGaN has a value placed between the value for AlN and the value for GaN (depending of the Al fraction). At λ = 362 nm (where we have obtained the maximum responsivity for both structures) we obtained a value of the transmission coefficient close to 0.23 which indicates an expected front to backside illumination responsivity ratio of 4.3. This value is in good agreement with the results obtained for the structure with 100 nm finger interdigit spacings (10% difference) while for the structure having 200 nm finger interdigit spacings, the experimental results differ by 25% from the expected value. We believe that the errors can be reduced, by improving the experimental setup for responsivity measurements. It must be noticed that in the responsivity measurements, the rejection ratio is poor, especially in the 365–400 nm range; this can be related to the gain but further investigations are necessary to clarify more precisely the origin of this behavior of the membrane UV photodetectors. It is evident from the measurements, that the values obtained for the backside illumination responsivity are high enough to make the membrane supported UV photodetectors very promising for applications where backside illumination is needed. It must be pointed out that backside-illumination, using membrane supported UV photodectors, is localized. If the membrane area is small enough not to permit the exposure of the contact pads area, then only the semiconductor under the interdigitated MSM structure contributes to the photocurrent. It is practically the same area as for front side illumination.
5. Conclusions GaN membrane supported UV photodetectors have been manufactured using bulk micromachining and e-beam direct lithography on GaN/Si. MSM structures with fingers and interdigit spacings of 100 nm and 200 nm wide respectively have been implemented. The membranes are composed from a 0.3 μm thin GaN layer on the top and a 0.2 μm thin buffer (based on successive AlN and AlGaN layers) on the bottom. The UV detectors demonstrated high responsivities under top side illumination Backside-illumination responsivities were approximately 4–6 times lower than the front side illumination values, but high enough (tens to hundreds of mA/W) to make the
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