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Amorphous and nanocrystalline p–i–n Si and Si,Ge photodetectors for structurally integrated O2 sensors
Available online at www.sciencedirect.com
Journal of Non-Crystalline Solids 354 (2008) 2606–2609 www.elsevier.com/locate/jnoncrysol
Amorphous and nanocrystalline p–i–n Si and Si,Ge photodetectors for structurally integrated O2 sensors Debju Ghosh a, Ruth Shinar b,*, Vikram Dalal a,b, Zhaoqun Zhou c, Joseph Shinar a,c a
Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA b Microelectronics Research Center, Iowa State University, Ames, IA 50011, USA c Ames Laboratory – USDOE and Department of Physics, Iowa State University, Ames, IA 50011, USA Available online 30 January 2008
Abstract Recent efforts to develop compact, field-deployable photoluminescence (PL)-based chemical and biological sensors have focused on structurally integrating an array of organic light emitting device (OLED) pixels, which serve as the excitation source, with a sensing film, and a thin-film photodetector (PD). To that end, VHF and ECR were used for fabricating and comparing amorphous and nanocrystalline p–i–n Si- and Si,Ge-based PDs for monitoring O2, which is preferably determined by monitoring the PL decay time, rather than the PL intensity, of the sensing film. This approach eliminates the need for frequent sensor calibration and, as pulsed OLED excitation is employed in this mode, the need for optical filters, which lead to bulkier sensors. Therefore, the development of the PDs also focused on increasing their speed, and understanding the factors affecting it, such as the device structure and boron diffusion during growth from the p+ to the i layer in p–i–n PDs. Incorporating a SiC buffer layer at the p+/i interface and a superstrate structure, where the p+ layer was grown last, increased the speed. The effects of Ge, p+ layer thickness, nanocrystallinity, defect states, and the illumination wavelength on the speed are also discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 85.60.Dw; 78.60.Fi; 07.07.Df; 81.05.Gc Keywords: Nanocrystalline; Silicon; Sensors; Plasma deposition
1. Introduction Photoluminescence (PL)-based oxygen sensors are often based on quenching of the PL intensity I and shortening of the PL decay time s of oxygen-sensitive dyes, such as Pt and Pd octaethylporphyrin (PtOEP and PdOEP, respectively), which are typically embedded in a thin sol–gel or polymeric film. The PL quenching is due to collisions of the dye molecules with O2 [1–4]. Monitoring O2 via s is preferable to I, as it eliminates the need for (i) frequent sensor calibration, since s is insensitive to changes in the intensity of the excitation source, minor film degradation, or background light [1–4], and (ii) optical filters, as s is mon*
Corresponding author. E-mail address: [email protected] (R. Shinar).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.065
itored during the off period of the pulsed PL excitation source. Like other PL-based sensors, the PL-based O2 sensors are attractive due to their high detection sensitivity and selectivity. Generally, however, field-deployability of PL-based sensors is limited due size, ease of fabrication, cost, and calibration/maintenance issues. The organic light emitting device (OLED)-based sensing platform presents an opportunity to alleviate these issues, by utilizing a compact OLED pixel array as the excitation source. This array can be structurally integrated with the sensing film in a uniquely simple way, resulting in an inexpensive and miniaturized module with a 2 mm thickness, determined largely by the thickness of the substrates used for the OLED and sensing film fabrication [3]. Moreover, OLEDs are flexible in design with tunable wavelength, and the ability
D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
2. Experimental The oxygen-sensitive dye was PdOEP (absorption and emission band peaks at 550 and 645 nm, respectively). PdOEP is attractive due to its long unquenched s0 1 ms, which ensures higher detection sensitivity than PtOEP, whose s0 90 ls. The dye was embedded in a polystyrene (PS) film. The OLED was a coumarin-doped tris(quinolinolate) Al (Alq3), whose EL spectrum is narrower than that of Alq3, resulting in a lower background light. The encapsulated OLED arrays were fabricated by thermally evaporating the organic layers on patterned indium tin oxide (ITO)coated glass, with 2 2 mm2 pixels resulting from mutually perpendicular stripes of the etched ITO anode and evaporated Al cathode. Continuous or pulsed OLEDs were operated in a forward bias of 15–25 V; pulse widths were 100–1000 ls. Details regarding OLED fabrication and operation are presented elsewhere [3,7]. The PDs were fabricated at 250 °C using PECVD; an ECR plasma for a-Si and a-(Si,Ge) deposition at 15 mTorr and a VHF plasma for a-Si and nc-Si deposition at 50 and 100 mTorr, respectively. The p–i–n structures were fabricated on an ITO-coated glass substrate that was protected from hydrogen plasma reduction with a 0.1-lm-thick ZnO layer RF sputter-grown at 30 W in Ar and oxygen. The p and i layers were tuned to match the dye PL and reduce the OLED background. The PDs were evaluated first by measuring their quantum efficiency (QE) and response using a pulsed LED with 50% duty cycle. Lock-in detection was used to reduce the noise. Chopped monochromatic light was also used to study the excitation wavelength kex-dependence of the PDs’ frequency response, and for comparison with the OLEDs.
at the lower frequencies, possibly due to defect states. As expected, a-(Si,Ge) PDs were slower than a-Si and nc-Si PDs. Additionally, VHF-grown PDs were better than ECR-grown PDs. These and other factors affecting the frequency response of the PDs are discussed next. 3.1. Amorphous Si /(Si,Ge) PDs The PDs were first tuned for the OLED excitation (550 nm) and PdOEP emission (640 nm). Fig. 1 shows the QE spectra of ECR a-Si- and a-(Si,Ge) PDs. As seen, the latter are preferred due to their match with the dye PL, and lower response at the EL band. However, a(Si,Ge) PDs were inferior due to their higher dark current and lower speed. 3.1.1. Effect of kex on the speed Fig. 2 shows the normalized response of the ECR PDs, using chopped monochromatic light, vs the chopper frequency, at different kex. As seen, the a-(Si,Ge) PDs were slower than the a-Si PDs, due to a higher defect density introduced by Ge. The effect of kex on the decrease of the normalized PD response with increasing frequency correlates with the QE dependence on kex. a-Si PDs were slowest at 650 nm and faster at 550 nm. a-(Si,Ge)-based PDs were faster at 650 nm, and slower at 550 and 600 nm. That is, the fastest response occurred when kex was near the peak QE, which was 550 nm for the a-Si PD and 650 nm for the a-(Si,Ge) PD. 3.1.2. Effect of boron diffusion during growth on the frequency response In p–i–n structures, boron diffuses from the p+ layer to the i layer during growth, resulting in a slower response. This diffusion depends on the doping level, i layer porosity, and deposition temperature and time. The plasma conditions were therefore tuned to minimize voids, and the deposition temperature was 250 °C, since below 200 °C the i layer deteriorates as the lower surface mobility of the
60
a-Si
40 30 20 10 0
3. Results and discussion As shown, the PD response decreased with increasing frequency of the pulsed light. It decreased more strongly
a-Si,Ge
50
Absolute QE (%)
to individually address the pixels enables their use for multianalyte detection in mixtures [5]. OLED-based oxygen sensors are also a basis for monitoring other analytes such as glucose, lactate, and ethanol, by monitoring oxygen consumption during the oxidation reactions of these analytes in the presence of the appropriate oxidase enzyme [6–8]. Initial results on the three-component OLED/sensor film/thin-film p–i–n PD integration [9] showed that a-(Si,Ge):H PDs tuned to the PL are usable for monitoring I; the detection sensitivity, however, was low and the speed too slow for monitoring s. This paper describes an approach to improve the sensitivity, by integrating instead a PECVD-grown nanocrystalline (nc)-Si PD. The advantages of the nc-Si PD over the amorphous PDs are discussed. Additionally, since monitoring s is preferable to I, the work focused on increasing the PDs’ speed, and understanding the factors affecting it.
2607
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550
600
650
700
750
800
Wavelength (nm) Fig. 1. The absolute quantum efficiency vs wavelength of ECR-grown a-Si- and a-(Si,Ge) PDs.
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D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
a
550 nm 600 nm 650 nm
0.9
0.8
3.2. Nanocrystalline Si PDs 0.7
a-Si
3.2.1. The effect of the p+ layer thickness on the frequency response The p+ layer consisted of bandgap-graded SiC/a-Si/ncSi layers for improved bandgap matching, and hence better device performance. The nc-Si layer was lightly doped to facilitate its growth, and therefore post-growth anneal was used to diffuse boron from the highly doped a-Si to the nc-Si layer, reducing the series resistance. Fig. 4 shows the normalized PD response vs the pulsed LED frequency for PDs with 27 and 52 nm thick nc-Si layers in the p+ layer; the i layer was nc-Si. As seen, the device with the thicker layer is slower, possibly due to increased grain boundary defects.
0.6 1.0
Normalized Response
b 0.9
0.8
0.7
a-Si,Ge 0.6
0.5
0
50
100
150
200
250
300
Chopper Frequency (Hz) Fig. 2. The normalized response vs the chopper frequency at different kex of ECR-grown (a) a-Si and (b) a-(Si,Ge) PDs.
SiH3 radicals results in voids. Devices annealed above 250 °C showed slower responses. Additionally, the doping density was optimized in terms of the series resistance, the dark current, and the deposition time and rate. VHF-grown p–i–n devices were faster than ECR-grown devices, probably due to the faster deposition of the former, resulting in reduced boron diffusion. Fig. 3 shows the effect of boron diffusion in VHF PDs, where the p+ layer consists of a-Si/nc-Si/a-Si, and the i layer is a-Si.
1.0
n-i-p Normalized Response
Using a pulsed LED, the figure compares the normalized frequency response of p–i–n and n–i–p PDs, as well as a p–i–n structure with a SiC barrier layer at the p+/i interface. As seen, the speed increased by introducing the SiC barrier. A stronger improvement was observed when the p+ layer was grown last in an n–i–p superstrate structure.
0.9
SiC barrier layer
0.8
3.2.2. Comparison of a-Si and nc-Si PDs Fig. 5 compares VHF-grown PDs of comparable thickness; one with nc-Si in both the p+ and i layers, the other with a-Si only. As seen, the nc-Si PD showed a stronger reduction in the normalized response at low frequencies, possibly due to grain boundaries and a-Si/nc-Si interface defects, but it was faster at higher frequencies due to higher mobility in the i layer. The nc-Si PD was structurally integrated with an OLED/O2 sensor film module. Its response time was 250 ls, and the sensor was therefore operated in the I mode. We note that it was faster than the ECR-grown aSi PDs, whose response time was 2 ms.
1.0
Normalized Response
Normalized Response
1.0
0.8 SiC nc-Si a-Si
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27 nm nc-Si 0.7
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52 nm nc-Si 0
0
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Frequency (kHz)
Pulsed LED Frequency (kHz) Fig. 3. The normalized response of VHF-grown PDs vs the LED pulse frequency. The p+ and i layers were a-Si/nc-Si/a-Si and a-Si, respectively.
Fig. 4. The effect of the nc-layer thickness in the SiC/a-Si/nc-Si p+ layer (nc-Si i layer) on the VHF-grown PDs’ frequency response. Inset: the p+ layer structure.
D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
tion sensitivity S I0/I(100% O2) 47. Though higher sensitivities are obtained using the OLED/sensing film with commercial PDs, in particular when monitoring s, this result demonstrates the viability of the three-component integration.
1.0
p+:i layers:
Normalized Response
0.9
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4. Conclusions 0 .8
SiC/a-Si:a-Si 0.7
0.6 0
5
10
15
20
25
30
Pulsed LED Frequency(kHz) Fig. 5. The normalized response vs LED pulse frequency of (1) SiC/a-Si/ nc-Si (p+ layer):nc-Si (i layer) and (2) SiC/a-Si (p+ layer):a-Si (i layer) VHF-grown PDs.
50
40
PECVD-grown PDs were fabricated for structural integration with an OLED/PL-based O2 sensor film module, as the basis for compact sensors for various analytes. The oxygen is monitored via its effect on the PL intensity I or decay time s; the latter eliminates the need for frequent sensor calibration or optical filters. Therefore, factors affecting the PDs’ speed were evaluated. These include device configuration (p–i–n or n–i–p), p+ layer structure and thickness, introduction of a SiC layer at the p+/i interface, incorporation of Ge, and crystallinity of the p+ and i layers. Boron diffusion into the i layer, incorporation of Ge, and grain boundary defects in nc-Si PDs affect the PDs’ speed. An O2 detection sensitivity of 47 was obtained in the I mode with a SiC/a-Si/nc-Si p+ layer and nc-Si i layer PD, demonstrating the viability of the integrated system. The PD response time, however, was 250 ls; additional studies are therefore necessary to improve it for operation in the s mode. Acknowledgments
I 0/ I
30
We thank Max Noack for helpful discussions. Ames Laboratory is operated by ISU for USDOE under Contract DE-AC 02-07CH11358. This work was partially supported by NSF and by the Director for Energy Research, Office of Basic Energy Sciences, USDOE.
20
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0 0
20
40
60
80
100
% Oxygen Fig. 6. I0/I vs gas-phase O2 concentration using the three-component integrated [coumarin-doped Alq3 OLED]/[PdOEP-doped polystyrene sensor film]/[VHF-grown SiC/a-Si/nc-Si (p+ layer):nc-Si (i layer) PD].
3.3. OLED/sensor film/thin-film PD structure Fig. 6 shows I0/I (I0 is the intensity in 100% Ar) vs gasphase O2 using coumarin-doped Alq3 OLEDs, PdOEPdoped PS films, and VHF-grown nc-Si PDs. As seen, this calibration curve is linear up to 40% O2, and the detec-
References [1] O.S. Wolfbeis (Ed.), Fiber Optic Chemical Sensors and Biosensors, CRC Press, BocaRaton, FL, 1991. [2] Y. Amao, Michrochim. Acta 143 (2003) 1. [3] R. Shinar, Z. Zhou, B. Choudhury, J. Shinar, Anal. Chim. Acta 568 (2006) 190. [4] P. Roche, R. Al-Jowder, R. Narayanaswamy, J. Young, P. Scully, Anal. Bioanal. Chem. 386 (2006) 1245. [5] R. Shinar, C. Qian, Y. Cai, Z. Zhou, B. Choudhury, J. Shinar, Proc. of SPIE 6007 (2005) 600710. [6] Z. Rosenzweig, R. Kopelman, Sensors Actuators B 35&36 (1996) 475. [7] B. Choudhury, R. Shinar, J. Shinar, J. Appl. Phys. 96 (2004) 2949. [8] R. Shinar, D. Ghosh, B. Choudhury, M. Noack, V.L. Dalal, J. Shinar, J. Non-Cryst. Solids 352 (2006) 1995.
Journal of Non-Crystalline Solids 354 (2008) 2606–2609 www.elsevier.com/locate/jnoncrysol
Amorphous and nanocrystalline p–i–n Si and Si,Ge photodetectors for structurally integrated O2 sensors Debju Ghosh a, Ruth Shinar b,*, Vikram Dalal a,b, Zhaoqun Zhou c, Joseph Shinar a,c a
Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA b Microelectronics Research Center, Iowa State University, Ames, IA 50011, USA c Ames Laboratory – USDOE and Department of Physics, Iowa State University, Ames, IA 50011, USA Available online 30 January 2008
Abstract Recent efforts to develop compact, field-deployable photoluminescence (PL)-based chemical and biological sensors have focused on structurally integrating an array of organic light emitting device (OLED) pixels, which serve as the excitation source, with a sensing film, and a thin-film photodetector (PD). To that end, VHF and ECR were used for fabricating and comparing amorphous and nanocrystalline p–i–n Si- and Si,Ge-based PDs for monitoring O2, which is preferably determined by monitoring the PL decay time, rather than the PL intensity, of the sensing film. This approach eliminates the need for frequent sensor calibration and, as pulsed OLED excitation is employed in this mode, the need for optical filters, which lead to bulkier sensors. Therefore, the development of the PDs also focused on increasing their speed, and understanding the factors affecting it, such as the device structure and boron diffusion during growth from the p+ to the i layer in p–i–n PDs. Incorporating a SiC buffer layer at the p+/i interface and a superstrate structure, where the p+ layer was grown last, increased the speed. The effects of Ge, p+ layer thickness, nanocrystallinity, defect states, and the illumination wavelength on the speed are also discussed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 85.60.Dw; 78.60.Fi; 07.07.Df; 81.05.Gc Keywords: Nanocrystalline; Silicon; Sensors; Plasma deposition
1. Introduction Photoluminescence (PL)-based oxygen sensors are often based on quenching of the PL intensity I and shortening of the PL decay time s of oxygen-sensitive dyes, such as Pt and Pd octaethylporphyrin (PtOEP and PdOEP, respectively), which are typically embedded in a thin sol–gel or polymeric film. The PL quenching is due to collisions of the dye molecules with O2 [1–4]. Monitoring O2 via s is preferable to I, as it eliminates the need for (i) frequent sensor calibration, since s is insensitive to changes in the intensity of the excitation source, minor film degradation, or background light [1–4], and (ii) optical filters, as s is mon*
Corresponding author. E-mail address: [email protected] (R. Shinar).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.065
itored during the off period of the pulsed PL excitation source. Like other PL-based sensors, the PL-based O2 sensors are attractive due to their high detection sensitivity and selectivity. Generally, however, field-deployability of PL-based sensors is limited due size, ease of fabrication, cost, and calibration/maintenance issues. The organic light emitting device (OLED)-based sensing platform presents an opportunity to alleviate these issues, by utilizing a compact OLED pixel array as the excitation source. This array can be structurally integrated with the sensing film in a uniquely simple way, resulting in an inexpensive and miniaturized module with a 2 mm thickness, determined largely by the thickness of the substrates used for the OLED and sensing film fabrication [3]. Moreover, OLEDs are flexible in design with tunable wavelength, and the ability
D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
2. Experimental The oxygen-sensitive dye was PdOEP (absorption and emission band peaks at 550 and 645 nm, respectively). PdOEP is attractive due to its long unquenched s0 1 ms, which ensures higher detection sensitivity than PtOEP, whose s0 90 ls. The dye was embedded in a polystyrene (PS) film. The OLED was a coumarin-doped tris(quinolinolate) Al (Alq3), whose EL spectrum is narrower than that of Alq3, resulting in a lower background light. The encapsulated OLED arrays were fabricated by thermally evaporating the organic layers on patterned indium tin oxide (ITO)coated glass, with 2 2 mm2 pixels resulting from mutually perpendicular stripes of the etched ITO anode and evaporated Al cathode. Continuous or pulsed OLEDs were operated in a forward bias of 15–25 V; pulse widths were 100–1000 ls. Details regarding OLED fabrication and operation are presented elsewhere [3,7]. The PDs were fabricated at 250 °C using PECVD; an ECR plasma for a-Si and a-(Si,Ge) deposition at 15 mTorr and a VHF plasma for a-Si and nc-Si deposition at 50 and 100 mTorr, respectively. The p–i–n structures were fabricated on an ITO-coated glass substrate that was protected from hydrogen plasma reduction with a 0.1-lm-thick ZnO layer RF sputter-grown at 30 W in Ar and oxygen. The p and i layers were tuned to match the dye PL and reduce the OLED background. The PDs were evaluated first by measuring their quantum efficiency (QE) and response using a pulsed LED with 50% duty cycle. Lock-in detection was used to reduce the noise. Chopped monochromatic light was also used to study the excitation wavelength kex-dependence of the PDs’ frequency response, and for comparison with the OLEDs.
at the lower frequencies, possibly due to defect states. As expected, a-(Si,Ge) PDs were slower than a-Si and nc-Si PDs. Additionally, VHF-grown PDs were better than ECR-grown PDs. These and other factors affecting the frequency response of the PDs are discussed next. 3.1. Amorphous Si /(Si,Ge) PDs The PDs were first tuned for the OLED excitation (550 nm) and PdOEP emission (640 nm). Fig. 1 shows the QE spectra of ECR a-Si- and a-(Si,Ge) PDs. As seen, the latter are preferred due to their match with the dye PL, and lower response at the EL band. However, a(Si,Ge) PDs were inferior due to their higher dark current and lower speed. 3.1.1. Effect of kex on the speed Fig. 2 shows the normalized response of the ECR PDs, using chopped monochromatic light, vs the chopper frequency, at different kex. As seen, the a-(Si,Ge) PDs were slower than the a-Si PDs, due to a higher defect density introduced by Ge. The effect of kex on the decrease of the normalized PD response with increasing frequency correlates with the QE dependence on kex. a-Si PDs were slowest at 650 nm and faster at 550 nm. a-(Si,Ge)-based PDs were faster at 650 nm, and slower at 550 and 600 nm. That is, the fastest response occurred when kex was near the peak QE, which was 550 nm for the a-Si PD and 650 nm for the a-(Si,Ge) PD. 3.1.2. Effect of boron diffusion during growth on the frequency response In p–i–n structures, boron diffuses from the p+ layer to the i layer during growth, resulting in a slower response. This diffusion depends on the doping level, i layer porosity, and deposition temperature and time. The plasma conditions were therefore tuned to minimize voids, and the deposition temperature was 250 °C, since below 200 °C the i layer deteriorates as the lower surface mobility of the
60
a-Si
40 30 20 10 0
3. Results and discussion As shown, the PD response decreased with increasing frequency of the pulsed light. It decreased more strongly
a-Si,Ge
50
Absolute QE (%)
to individually address the pixels enables their use for multianalyte detection in mixtures [5]. OLED-based oxygen sensors are also a basis for monitoring other analytes such as glucose, lactate, and ethanol, by monitoring oxygen consumption during the oxidation reactions of these analytes in the presence of the appropriate oxidase enzyme [6–8]. Initial results on the three-component OLED/sensor film/thin-film p–i–n PD integration [9] showed that a-(Si,Ge):H PDs tuned to the PL are usable for monitoring I; the detection sensitivity, however, was low and the speed too slow for monitoring s. This paper describes an approach to improve the sensitivity, by integrating instead a PECVD-grown nanocrystalline (nc)-Si PD. The advantages of the nc-Si PD over the amorphous PDs are discussed. Additionally, since monitoring s is preferable to I, the work focused on increasing the PDs’ speed, and understanding the factors affecting it.
2607
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 1. The absolute quantum efficiency vs wavelength of ECR-grown a-Si- and a-(Si,Ge) PDs.
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D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
a
550 nm 600 nm 650 nm
0.9
0.8
3.2. Nanocrystalline Si PDs 0.7
a-Si
3.2.1. The effect of the p+ layer thickness on the frequency response The p+ layer consisted of bandgap-graded SiC/a-Si/ncSi layers for improved bandgap matching, and hence better device performance. The nc-Si layer was lightly doped to facilitate its growth, and therefore post-growth anneal was used to diffuse boron from the highly doped a-Si to the nc-Si layer, reducing the series resistance. Fig. 4 shows the normalized PD response vs the pulsed LED frequency for PDs with 27 and 52 nm thick nc-Si layers in the p+ layer; the i layer was nc-Si. As seen, the device with the thicker layer is slower, possibly due to increased grain boundary defects.
0.6 1.0
Normalized Response
b 0.9
0.8
0.7
a-Si,Ge 0.6
0.5
0
50
100
150
200
250
300
Chopper Frequency (Hz) Fig. 2. The normalized response vs the chopper frequency at different kex of ECR-grown (a) a-Si and (b) a-(Si,Ge) PDs.
SiH3 radicals results in voids. Devices annealed above 250 °C showed slower responses. Additionally, the doping density was optimized in terms of the series resistance, the dark current, and the deposition time and rate. VHF-grown p–i–n devices were faster than ECR-grown devices, probably due to the faster deposition of the former, resulting in reduced boron diffusion. Fig. 3 shows the effect of boron diffusion in VHF PDs, where the p+ layer consists of a-Si/nc-Si/a-Si, and the i layer is a-Si.
1.0
n-i-p Normalized Response
Using a pulsed LED, the figure compares the normalized frequency response of p–i–n and n–i–p PDs, as well as a p–i–n structure with a SiC barrier layer at the p+/i interface. As seen, the speed increased by introducing the SiC barrier. A stronger improvement was observed when the p+ layer was grown last in an n–i–p superstrate structure.
0.9
SiC barrier layer
0.8
3.2.2. Comparison of a-Si and nc-Si PDs Fig. 5 compares VHF-grown PDs of comparable thickness; one with nc-Si in both the p+ and i layers, the other with a-Si only. As seen, the nc-Si PD showed a stronger reduction in the normalized response at low frequencies, possibly due to grain boundaries and a-Si/nc-Si interface defects, but it was faster at higher frequencies due to higher mobility in the i layer. The nc-Si PD was structurally integrated with an OLED/O2 sensor film module. Its response time was 250 ls, and the sensor was therefore operated in the I mode. We note that it was faster than the ECR-grown aSi PDs, whose response time was 2 ms.
1.0
Normalized Response
Normalized Response
1.0
0.8 SiC nc-Si a-Si
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27 nm nc-Si 0.7
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p-i-n
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52 nm nc-Si 0
0
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Frequency (kHz)
Pulsed LED Frequency (kHz) Fig. 3. The normalized response of VHF-grown PDs vs the LED pulse frequency. The p+ and i layers were a-Si/nc-Si/a-Si and a-Si, respectively.
Fig. 4. The effect of the nc-layer thickness in the SiC/a-Si/nc-Si p+ layer (nc-Si i layer) on the VHF-grown PDs’ frequency response. Inset: the p+ layer structure.
D. Ghosh et al. / Journal of Non-Crystalline Solids 354 (2008) 2606–2609
tion sensitivity S I0/I(100% O2) 47. Though higher sensitivities are obtained using the OLED/sensing film with commercial PDs, in particular when monitoring s, this result demonstrates the viability of the three-component integration.
1.0
p+:i layers:
Normalized Response
0.9
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4. Conclusions 0 .8
SiC/a-Si:a-Si 0.7
0.6 0
5
10
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20
25
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Pulsed LED Frequency(kHz) Fig. 5. The normalized response vs LED pulse frequency of (1) SiC/a-Si/ nc-Si (p+ layer):nc-Si (i layer) and (2) SiC/a-Si (p+ layer):a-Si (i layer) VHF-grown PDs.
50
40
PECVD-grown PDs were fabricated for structural integration with an OLED/PL-based O2 sensor film module, as the basis for compact sensors for various analytes. The oxygen is monitored via its effect on the PL intensity I or decay time s; the latter eliminates the need for frequent sensor calibration or optical filters. Therefore, factors affecting the PDs’ speed were evaluated. These include device configuration (p–i–n or n–i–p), p+ layer structure and thickness, introduction of a SiC layer at the p+/i interface, incorporation of Ge, and crystallinity of the p+ and i layers. Boron diffusion into the i layer, incorporation of Ge, and grain boundary defects in nc-Si PDs affect the PDs’ speed. An O2 detection sensitivity of 47 was obtained in the I mode with a SiC/a-Si/nc-Si p+ layer and nc-Si i layer PD, demonstrating the viability of the integrated system. The PD response time, however, was 250 ls; additional studies are therefore necessary to improve it for operation in the s mode. Acknowledgments
I 0/ I
30
We thank Max Noack for helpful discussions. Ames Laboratory is operated by ISU for USDOE under Contract DE-AC 02-07CH11358. This work was partially supported by NSF and by the Director for Energy Research, Office of Basic Energy Sciences, USDOE.
20
10
0 0
20
40
60
80
100
% Oxygen Fig. 6. I0/I vs gas-phase O2 concentration using the three-component integrated [coumarin-doped Alq3 OLED]/[PdOEP-doped polystyrene sensor film]/[VHF-grown SiC/a-Si/nc-Si (p+ layer):nc-Si (i layer) PD].
3.3. OLED/sensor film/thin-film PD structure Fig. 6 shows I0/I (I0 is the intensity in 100% Ar) vs gasphase O2 using coumarin-doped Alq3 OLEDs, PdOEPdoped PS films, and VHF-grown nc-Si PDs. As seen, this calibration curve is linear up to 40% O2, and the detec-
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