- Email: [email protected]
Boron-doped nanocrystalline silicon germanium thin films for uncooled infrared bolometer applications
Infrared Physics & Technology 58 (2013) 32–35
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Boron-doped nanocrystalline silicon germanium thin films for uncooled infrared bolometer applications Rui Xu a, Wei Li b,⇑, Jian He a, Yan Sun a, Ya-Dong Jiang b a b
School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China
h i g h l i g h t s " Boron-doped nanocrystalline Si1xGex:H thin film was prepared by PECVD method. " Self-assembly circuit system was designed for the 1/f noise measurement. " The film with low resistivity, high TCR and lower 1/f noise was obtained.
a r t i c l e
i n f o
Article history: Received 2 October 2012 Available online 31 January 2013 Keywords: Silicon germanium thin film Nanocrystalline Uncooled infrared bolometer 1/f Noise
a b s t r a c t In this paper, boron-doped nanocrystalline Si0.78Ge0.22:H thin film is assessed for use as resistive sensing layer in uncooled infrared bolometer applications. The silicon germanium thin films were deposited by PECVD (plasma enhanced chemical vapor deposition) through decomposition of silane, germane and diborane diluted with argon at substrate temperature of 230 °C. Under optimum deposition parameters, the sensing films with modulate electrical resistivity (<104 X cm) and high temperature coefficient of resistance (TCR) (>3%/K) were obtained at room temperature. 1/f noise character in the form of the normalized Hooge parameter was measured in the frequency range of 1–64 Hz, resulting in a lower 1/f noise compared to other materials currently used for device application. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, infrared (IR) thermal imaging device has been widely studied due to its important application in military, medical and commercial area [1]. A popular detector used for IR imaging applications is the bolometer which is a thermal sensor whose resistance changes with temperature. It is important for a bolometer to have good absorption, a high temperature coefficient of resistance (TCR) and low 1/f noise to achieve a greater value of voltage responsivity (Rv) and detectivity (D) [2]. VOx [3] and doped a-Si:H [4,5] are the two most widely used sensing materials in industry for fabricating bolometers. VOx provides low resistivity and low 1/f-noise, however, its TCR is low. Moreover, VOx is not a standard material in IC technology and high temperature is needed to deposit it. Doped a-Si:H thin film, on the other hand, posses moderate TCR but high 1/f noise. Recent research reported a promising possibility of using amorphous SiGe alloys [6,7] and amorphous GexSi1xOy [8,9] compound for bolometer fabrication due to their high TCR, compatible with current MEMS (Micro-Electro-Mechan-
⇑ Corresponding author. E-mail address: [email protected] (W. Li). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.01.005
ical Systems) process and relatively low 1/f noise compared to amorphous Si. However they still exhibit high 1/f noise when compared to non-semiconductor materials [10]. It has been reported that the introduction of nanostructures in amorphous matrix can improve the stability and properties of amorphous thin films [11,12] as well as reduce the 1/f noise [13]. In this work, Borondoped nanocrystalline Si0.78Ge0.22:H thin film deposited by PECVD method diluted with argon was electrically characterized in order to assess this material for uncooled microbolometer applications.
2. Experimental details 2.1. Sample preparation Boron-doped nanocrystalline Si0.78Ge0.22:H thin films in the experiment were deposited in the conventional radio frequency (13.56 MHZ) PECVD system at substrate temperature of 230 °C, gas pressure of 60 Pa and power density of 0.1 W/cm2. The base pressure of the PECVD system was 2.0 104 Pa. High purity silane, germane and diborane mixed with argon were used as source gas for discharging. The boron doping concentration d (d = [B2H6]/ [SiH4] + [GeH4]) was 1% and the argon dilution ratio R (R = [Ar]/
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
33
[SiH4] + [GeH4] + [B2H6]) was 4. The films of 400 nm thickness were deposited on K9 glass substrate for resistivity and TCR measurements. The 1/f noise measurement was accomplished on the thin films (200 nm thick) deposited on substrates of PECVD Si3N4 on Si wafer in order to eliminate the noise from substrate. 2.2. Characterization and measurement methods The thickness and germanium content of the films were measured by spectrometric ellipsometer (SE850) and X-ray photoelectron spectroscopy (PH Quantera), respectively. X-ray diffraction (XRD) measurements were performed using a Philip X’pert Pro to confirm the presence of nano grains in the films. The electrical characterization was done with Keithley 4200 semiconductor characterization system in the I–V configuration with an evaporated coplanar metal aluminum electrode. The temperature dependence of dark resistivity q(T) was measured in an ESPEC ESL-02KA highlow temperature test chamber with a temperature stability of less than ±0.1 °C. The dark resistivity measurements were performed in the temperature range of 0–70 °C, at an interval of 5 °C in the dark environment. The temperature coefficient of resistance (TCR) of the film was deduced from q(T).
Fig. 2. XRD spectra and orientation tendency of the Si0.78Ge0.22:H thin film.
2.3. 1/f Noise measurement system The 1/f noise measurement system was described in Fig. 1. A 9Volt power supply and a special resistor network, Rm, were applied to generate a bias current of 1 lA. A virtual-ground current amplifier (SR570) was used to measure the voltage fluctuations and amplify the signal .The amplifier was further amplified using a lownoise voltage amplifier (SR560) and sent to a spectrum analyzer (SR785), where the power spectral density (Sv) was calculated by Fourier transform in the frequency range of 1–64 Hz. In order to obtain better coverage over the interest frequency range, the measured voltage noise was recorded after 50 averages. During the measurement, the measuring circuit was placed in a metal box shield to reduce external noise disturbance. Both amplifier and spectrum analyzer have noise voltages on the order of nV, enabling high accuracy measurement.
Fig. 3. Aluminum electrode configuration for Si0.78Ge0.22:H thin film resistance measurement.
3. Results and discussion 3.1. Nanostructure in the film Fig. 2 shows the XRD spectra and orientation tendency of the Si0.78Ge0.22:H thin film. It is easily observed that there are three diffraction peaks centered at the angel of 28°, 48° and 58°, corresponding to the diffraction from (1 1 1), (2 2 0), and (3 1 1) planes of the silicon germanium film, respectively. This indicates that the testing thin film is partially crystallined. Based on the widths of the XRD lines, the average nano-grain size nearly 15–30 nm of different orientations in the testing thin films can be calculated
DUT
9V
SR570 SR560
Rm
SR785 (DSA)
Metal Box
Fig. 1. Schematic diagram of 1/f noise measurement system.
Fig. 4. I–V curves of Si0.78Ge0.22:H thin film with aluminum electrode measured at room temperature.
using Scherrer’s equation. The formation of the nanostructure can be correlated with the bombardment of argon plasma during the deposition process. Through the bombardment of Ar and Ar+, energy released at the growth zone will break up the weak and reform into strong Si–H and Ge–H bonds as well as polyhydride components of (GeH2)n and (SiH2)n modes at crystalline boundaries. Hence, more and more hydrogen reservoirs is appear, which is considered as fundamental for the growth of nanocrystalline grains. The more information about the nanostructure of the film can be seen in our previous work [14]. There is also a broad peak in the spectra between 20° and 30°, which is due to the glass substrate.
34
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
Fig. 5. Resistivity and TCR versus temperature of Si0.78Ge0.22:H thin film.
Hence, it can be concluded that the Si0.78Ge0.22:H thin film embedded with nanocrystalline grains have been successfully obtained in our experiment using argon dilution. 3.2. Electrical properties of film Fig. 3 shows the structure of coplanar aluminum electrode on nanocrystalline Si0.78Ge0.22:H film. The resistance measurement was performed by current–voltage characteristics at an applied bias voltage of 0.1 V from the Keithley 4200-SCS system. From the I–V curves shown in Fig. 4, it can be easily obtained that the contact between aluminum and Si0.78Ge0.22:H thin film is Ohmic contact. The resistivity of the film can be obtained from q = Rld/ w, the resistance (R) of the film was obtained from I–V curves. The film thickness (d), width (l) of film and interelectrode distance (w) are 400 nm, 10 mm and 5 mm, respectively. The TCR value of film was directly calculated by using the expression TCR = (1/ R) (dR/dT), where R is the resistance of sample film and T is the temperature. The corresponding results are shown in Fig. 5. It can be seen that for sample prepared with some nanostructure in amorphous network and an identical boron doping level, the resistivity of nanocrystalline silicon germanium thin film is lower than 104 X cm and its TCR value is as high as 3.2%/K at room temperature. The low value of resistivity is attributed to the two reasons: (1) the doping boron atoms increase carrier concentration in the film, (2) The nano grains and crystal boundaries benefit the transfer of electron. 3.3. 1/f Noise properties of film Unlike other sources of noise, the origin of 1/f noise is still open to debate. A large number of theories have been developed. A general agreement was that 1/f noise is due to inhomogeneity and the defective structure in the films [15]. The 1/f noise can be expressed by Hooge’s formula [16]
Sv V
2
¼
aH N
1 f
Fig. 6. Noise voltage PSD of nanocrystalline Si0.78Ge0.22:H thin film at different frequency.
7 10 14/V2 is obtained by fitting the measured spectrum according to Eq. (1). Noise characteristics of this material is compared to other materials currently used for device application. As seen from Fig. 6, it is observed that nanocrystalline Si0.78Ge0.22:H thin film in our experiment exhibits lower 1/f noise than traditional nanocrystalline Si, a-SiGe and a-SiGeO thin films. This low 1/f noise is believed to be due to the introduction of nanostructures in amorphous matrix and this nanostructures cause the hydrogen variation as well as defects reduction [14]. Our results for SiGe films are consistent with the results obtained by S. Li for Si films [17,18]. On the other hand, The TCR value above 3%/K of this material is also obtained at room temperature. Therefore, by using nanocrystalline Si1xGex:H thin film instead of a-SiGe based thin film as sensing layer can significantly improve the device performance.
4. Conclusions
ð1Þ
where Sv represents the noise power spectral density as a function of frequency (f), V is the measurements of voltage, aH is Hooge coefficient and N is the number of fluctuators in the sample. In this paper, 1/f noise factor is defined as aH/N. Fig. 6 shows the noise voltage PSD at different frequency. The value of 1/f noise factor aH/N of
In this work, boron-doped nanocrystalline Si0.78Ge0.22:H thin film was deposited by PECVD diluted with argon at 230 °C. This low temperature process is fully compatible with MEMS technology. XRD spectroscopy was applied to verify the presence of nanostructure in the thin film. 1/f noise character of the film in the form of the normalized Hooge parameter was measured in the fre-
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
quency range of 1–64 Hz. It is found that nanocrystalline Si0.78Ge0.22:H thin film in our experiment exhibits lower 1/f noise by comparing the materials currently used for device application. This can be due to the introduction of nanostructure in the film. Besides, this sensing film with modulate electrical resistivity (<104 X cm) and high temperature coefficient of resistance (>3%/K) at room temperature was also obtained. All these excellent electrical properties make boron-doped nanocrystalline Si0.78Ge0.22:H thin film very suitable for the uncooled infrared bolometer applications. Acknowledgment The authors acknowledge the financial support of the China Scholarship Council. References [1] R.T.R. Kumar, B. Karunagaran, D. Mangalaraj, S.K. Narayandass, P. Manoravi, M. Joseph, V. Gopal, Study of a pulsed laser deposited vanadium oxide based microbolometer array, Smart Mater. Struct. 12 (2003) 188–192. [2] M.M. Rana, D.P. Butler, Radio frequency sputtered Si1xGex and Si1xGexOy thin films for uncooled infrared detectors, Thin Solid Films 514 (2006) 355–360. [3] C. Chen, X. Yi, J. Zhang, X. Zhao, Linear uncooled microbolometer array based on VOx, thin films, Infrared Phys. Technol. 42 (2001) 87–90. [4] J.L. Tissot, F. Rothan, C. Vedel, M. Vilain, J.J. Yon, LETI/LIR’s uncooled microbolometer development, Proc. SPIE 3436 (1998) 605–610. [5] T.R. Schimert, D.D. Ratcliff, J.F. Brady, S.J. Ropson, R.W. Gooch, B. Ritchey, P. McCardel, K. Rachels, M. Wand, M. Weistein, J. Wynn, Low-cost low-power uncooled a-Si-based micro infrared camera for unattended ground sensor applications, Proc. SPIE 3713 (1999) 101–111.
35
[6] M. García, R. Ambrosio, A. Torres, A. Kosarev, IR bolometers based on amorphous silicon germanium alloys, J. Non Cryst. Solids 338–340 (2004) 744–748. [7] A. Kosarev, S. Rumyantsev, M. Moreno, A. Torres, S. Boubanga, W. Knap, SixGey:H-based micro-bolometers studied in the terahertz frequency range, Solid-State Electron. 54 (2010) 417–419. [8] E. Iborra, M. Clement, L.V. Herrero, J. Sangrador, IR uncooled bolometers based on amorphous GexSi1xOy on silicon micromachined structures, J. Microelectromech. Syst. 11 (2002) 322–329. [9] A. Ahmed, R.N. Tait, Noise behavior of amorphous GexSi1xOy for microbolometer applications, Infrared Phys. Technol. 46 (2005) 468–472. [10] M.M. Rana, D.P. Butler, Noise reduction of a-Si1xGexOy microbolometers by forming gas passivation, Thin Solid Films 516 (2008) 6499–6503. [11] E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, J. Pohl, Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution, J. Appl. Phys. 87 (2000) 3137–3142. [12] M.E. Gueunier, J.P. Kleider, R. Brüggemann, S. Lebib, P. Roca i Cabarrocas, R. Meaudre, B. Canut, Properties of polymorphous silicon–germanium alloys deposited under high hydrogen dilution and at high pressure, J. Appl. Phys. 92 (2002) 4959–4967. [13] S.B. Li, Z.M. Wu, Y.D. Jiang, W. Li, N.M. Liao, J.S. Yu, Structure and 1/f noise of boron doped polymorphous silicon films, Nanotechnology 19 (2008) 085706. [14] R. Xu, W. Li, J. He, Y. Sun, Y.D. Jiang, Investigation of nanocrystallization of aSi1xGex:H thin films diluted with argon in the PECVD system, J. Non-Cryst. Solids: NOC16378, 2013. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.026. [15] S.B. Li, Y.D. Jiang, Z.M. Wu, J. Wu, Z.H. Ying, Z.M. Wang, W. Li, G. Salamo, Origins of 1/f noise in nanostructure inclusion polymorphous silicon films, Nanoscale Res. Lett. 6 (2011) 281. [16] F.N. Hooge, T.G.M. Kleinpenning, L.K.J. Vandamme, Experimental studies on 1/f noise, Rep. Prog. Phys. 44 (1981) 479–532. [17] S.B. Li, Z.M. Wu, W. Li, N.M. Liao, J.S. Yu, Y.D. Jiang, Influence of microcrystallization on noise in boron-doped silicon film, Phys. Status Solidi A 204 (2007) 4292–4297. [18] S.B. Li, Z.M. Wu, Y.D. Jiang, W. Li, N.M. Liao, J.S. Yu, Noise in boron doped amorphous/microcrystallization silicon films, Appl. Surf. Sci. 254 (2008) 3274– 3276.
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Boron-doped nanocrystalline silicon germanium thin films for uncooled infrared bolometer applications Rui Xu a, Wei Li b,⇑, Jian He a, Yan Sun a, Ya-Dong Jiang b a b
School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China
h i g h l i g h t s " Boron-doped nanocrystalline Si1xGex:H thin film was prepared by PECVD method. " Self-assembly circuit system was designed for the 1/f noise measurement. " The film with low resistivity, high TCR and lower 1/f noise was obtained.
a r t i c l e
i n f o
Article history: Received 2 October 2012 Available online 31 January 2013 Keywords: Silicon germanium thin film Nanocrystalline Uncooled infrared bolometer 1/f Noise
a b s t r a c t In this paper, boron-doped nanocrystalline Si0.78Ge0.22:H thin film is assessed for use as resistive sensing layer in uncooled infrared bolometer applications. The silicon germanium thin films were deposited by PECVD (plasma enhanced chemical vapor deposition) through decomposition of silane, germane and diborane diluted with argon at substrate temperature of 230 °C. Under optimum deposition parameters, the sensing films with modulate electrical resistivity (<104 X cm) and high temperature coefficient of resistance (TCR) (>3%/K) were obtained at room temperature. 1/f noise character in the form of the normalized Hooge parameter was measured in the frequency range of 1–64 Hz, resulting in a lower 1/f noise compared to other materials currently used for device application. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, infrared (IR) thermal imaging device has been widely studied due to its important application in military, medical and commercial area [1]. A popular detector used for IR imaging applications is the bolometer which is a thermal sensor whose resistance changes with temperature. It is important for a bolometer to have good absorption, a high temperature coefficient of resistance (TCR) and low 1/f noise to achieve a greater value of voltage responsivity (Rv) and detectivity (D) [2]. VOx [3] and doped a-Si:H [4,5] are the two most widely used sensing materials in industry for fabricating bolometers. VOx provides low resistivity and low 1/f-noise, however, its TCR is low. Moreover, VOx is not a standard material in IC technology and high temperature is needed to deposit it. Doped a-Si:H thin film, on the other hand, posses moderate TCR but high 1/f noise. Recent research reported a promising possibility of using amorphous SiGe alloys [6,7] and amorphous GexSi1xOy [8,9] compound for bolometer fabrication due to their high TCR, compatible with current MEMS (Micro-Electro-Mechan-
⇑ Corresponding author. E-mail address: [email protected] (W. Li). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.01.005
ical Systems) process and relatively low 1/f noise compared to amorphous Si. However they still exhibit high 1/f noise when compared to non-semiconductor materials [10]. It has been reported that the introduction of nanostructures in amorphous matrix can improve the stability and properties of amorphous thin films [11,12] as well as reduce the 1/f noise [13]. In this work, Borondoped nanocrystalline Si0.78Ge0.22:H thin film deposited by PECVD method diluted with argon was electrically characterized in order to assess this material for uncooled microbolometer applications.
2. Experimental details 2.1. Sample preparation Boron-doped nanocrystalline Si0.78Ge0.22:H thin films in the experiment were deposited in the conventional radio frequency (13.56 MHZ) PECVD system at substrate temperature of 230 °C, gas pressure of 60 Pa and power density of 0.1 W/cm2. The base pressure of the PECVD system was 2.0 104 Pa. High purity silane, germane and diborane mixed with argon were used as source gas for discharging. The boron doping concentration d (d = [B2H6]/ [SiH4] + [GeH4]) was 1% and the argon dilution ratio R (R = [Ar]/
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
33
[SiH4] + [GeH4] + [B2H6]) was 4. The films of 400 nm thickness were deposited on K9 glass substrate for resistivity and TCR measurements. The 1/f noise measurement was accomplished on the thin films (200 nm thick) deposited on substrates of PECVD Si3N4 on Si wafer in order to eliminate the noise from substrate. 2.2. Characterization and measurement methods The thickness and germanium content of the films were measured by spectrometric ellipsometer (SE850) and X-ray photoelectron spectroscopy (PH Quantera), respectively. X-ray diffraction (XRD) measurements were performed using a Philip X’pert Pro to confirm the presence of nano grains in the films. The electrical characterization was done with Keithley 4200 semiconductor characterization system in the I–V configuration with an evaporated coplanar metal aluminum electrode. The temperature dependence of dark resistivity q(T) was measured in an ESPEC ESL-02KA highlow temperature test chamber with a temperature stability of less than ±0.1 °C. The dark resistivity measurements were performed in the temperature range of 0–70 °C, at an interval of 5 °C in the dark environment. The temperature coefficient of resistance (TCR) of the film was deduced from q(T).
Fig. 2. XRD spectra and orientation tendency of the Si0.78Ge0.22:H thin film.
2.3. 1/f Noise measurement system The 1/f noise measurement system was described in Fig. 1. A 9Volt power supply and a special resistor network, Rm, were applied to generate a bias current of 1 lA. A virtual-ground current amplifier (SR570) was used to measure the voltage fluctuations and amplify the signal .The amplifier was further amplified using a lownoise voltage amplifier (SR560) and sent to a spectrum analyzer (SR785), where the power spectral density (Sv) was calculated by Fourier transform in the frequency range of 1–64 Hz. In order to obtain better coverage over the interest frequency range, the measured voltage noise was recorded after 50 averages. During the measurement, the measuring circuit was placed in a metal box shield to reduce external noise disturbance. Both amplifier and spectrum analyzer have noise voltages on the order of nV, enabling high accuracy measurement.
Fig. 3. Aluminum electrode configuration for Si0.78Ge0.22:H thin film resistance measurement.
3. Results and discussion 3.1. Nanostructure in the film Fig. 2 shows the XRD spectra and orientation tendency of the Si0.78Ge0.22:H thin film. It is easily observed that there are three diffraction peaks centered at the angel of 28°, 48° and 58°, corresponding to the diffraction from (1 1 1), (2 2 0), and (3 1 1) planes of the silicon germanium film, respectively. This indicates that the testing thin film is partially crystallined. Based on the widths of the XRD lines, the average nano-grain size nearly 15–30 nm of different orientations in the testing thin films can be calculated
DUT
9V
SR570 SR560
Rm
SR785 (DSA)
Metal Box
Fig. 1. Schematic diagram of 1/f noise measurement system.
Fig. 4. I–V curves of Si0.78Ge0.22:H thin film with aluminum electrode measured at room temperature.
using Scherrer’s equation. The formation of the nanostructure can be correlated with the bombardment of argon plasma during the deposition process. Through the bombardment of Ar and Ar+, energy released at the growth zone will break up the weak and reform into strong Si–H and Ge–H bonds as well as polyhydride components of (GeH2)n and (SiH2)n modes at crystalline boundaries. Hence, more and more hydrogen reservoirs is appear, which is considered as fundamental for the growth of nanocrystalline grains. The more information about the nanostructure of the film can be seen in our previous work [14]. There is also a broad peak in the spectra between 20° and 30°, which is due to the glass substrate.
34
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
Fig. 5. Resistivity and TCR versus temperature of Si0.78Ge0.22:H thin film.
Hence, it can be concluded that the Si0.78Ge0.22:H thin film embedded with nanocrystalline grains have been successfully obtained in our experiment using argon dilution. 3.2. Electrical properties of film Fig. 3 shows the structure of coplanar aluminum electrode on nanocrystalline Si0.78Ge0.22:H film. The resistance measurement was performed by current–voltage characteristics at an applied bias voltage of 0.1 V from the Keithley 4200-SCS system. From the I–V curves shown in Fig. 4, it can be easily obtained that the contact between aluminum and Si0.78Ge0.22:H thin film is Ohmic contact. The resistivity of the film can be obtained from q = Rld/ w, the resistance (R) of the film was obtained from I–V curves. The film thickness (d), width (l) of film and interelectrode distance (w) are 400 nm, 10 mm and 5 mm, respectively. The TCR value of film was directly calculated by using the expression TCR = (1/ R) (dR/dT), where R is the resistance of sample film and T is the temperature. The corresponding results are shown in Fig. 5. It can be seen that for sample prepared with some nanostructure in amorphous network and an identical boron doping level, the resistivity of nanocrystalline silicon germanium thin film is lower than 104 X cm and its TCR value is as high as 3.2%/K at room temperature. The low value of resistivity is attributed to the two reasons: (1) the doping boron atoms increase carrier concentration in the film, (2) The nano grains and crystal boundaries benefit the transfer of electron. 3.3. 1/f Noise properties of film Unlike other sources of noise, the origin of 1/f noise is still open to debate. A large number of theories have been developed. A general agreement was that 1/f noise is due to inhomogeneity and the defective structure in the films [15]. The 1/f noise can be expressed by Hooge’s formula [16]
Sv V
2
¼
aH N
1 f
Fig. 6. Noise voltage PSD of nanocrystalline Si0.78Ge0.22:H thin film at different frequency.
7 10 14/V2 is obtained by fitting the measured spectrum according to Eq. (1). Noise characteristics of this material is compared to other materials currently used for device application. As seen from Fig. 6, it is observed that nanocrystalline Si0.78Ge0.22:H thin film in our experiment exhibits lower 1/f noise than traditional nanocrystalline Si, a-SiGe and a-SiGeO thin films. This low 1/f noise is believed to be due to the introduction of nanostructures in amorphous matrix and this nanostructures cause the hydrogen variation as well as defects reduction [14]. Our results for SiGe films are consistent with the results obtained by S. Li for Si films [17,18]. On the other hand, The TCR value above 3%/K of this material is also obtained at room temperature. Therefore, by using nanocrystalline Si1xGex:H thin film instead of a-SiGe based thin film as sensing layer can significantly improve the device performance.
4. Conclusions
ð1Þ
where Sv represents the noise power spectral density as a function of frequency (f), V is the measurements of voltage, aH is Hooge coefficient and N is the number of fluctuators in the sample. In this paper, 1/f noise factor is defined as aH/N. Fig. 6 shows the noise voltage PSD at different frequency. The value of 1/f noise factor aH/N of
In this work, boron-doped nanocrystalline Si0.78Ge0.22:H thin film was deposited by PECVD diluted with argon at 230 °C. This low temperature process is fully compatible with MEMS technology. XRD spectroscopy was applied to verify the presence of nanostructure in the thin film. 1/f noise character of the film in the form of the normalized Hooge parameter was measured in the fre-
R. Xu et al. / Infrared Physics & Technology 58 (2013) 32–35
quency range of 1–64 Hz. It is found that nanocrystalline Si0.78Ge0.22:H thin film in our experiment exhibits lower 1/f noise by comparing the materials currently used for device application. This can be due to the introduction of nanostructure in the film. Besides, this sensing film with modulate electrical resistivity (<104 X cm) and high temperature coefficient of resistance (>3%/K) at room temperature was also obtained. All these excellent electrical properties make boron-doped nanocrystalline Si0.78Ge0.22:H thin film very suitable for the uncooled infrared bolometer applications. Acknowledgment The authors acknowledge the financial support of the China Scholarship Council. References [1] R.T.R. Kumar, B. Karunagaran, D. Mangalaraj, S.K. Narayandass, P. Manoravi, M. Joseph, V. Gopal, Study of a pulsed laser deposited vanadium oxide based microbolometer array, Smart Mater. Struct. 12 (2003) 188–192. [2] M.M. Rana, D.P. Butler, Radio frequency sputtered Si1xGex and Si1xGexOy thin films for uncooled infrared detectors, Thin Solid Films 514 (2006) 355–360. [3] C. Chen, X. Yi, J. Zhang, X. Zhao, Linear uncooled microbolometer array based on VOx, thin films, Infrared Phys. Technol. 42 (2001) 87–90. [4] J.L. Tissot, F. Rothan, C. Vedel, M. Vilain, J.J. Yon, LETI/LIR’s uncooled microbolometer development, Proc. SPIE 3436 (1998) 605–610. [5] T.R. Schimert, D.D. Ratcliff, J.F. Brady, S.J. Ropson, R.W. Gooch, B. Ritchey, P. McCardel, K. Rachels, M. Wand, M. Weistein, J. Wynn, Low-cost low-power uncooled a-Si-based micro infrared camera for unattended ground sensor applications, Proc. SPIE 3713 (1999) 101–111.
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