 Nano Express
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Origins of 1/f noise in nanostructure inclusion polymorphous silicon films
Nanoscale Research Lettersvolume 6, Article number: 281 (2011)
Abstract
In this article, we report that the origins of 1/f noise in pmSi:H film resistors are inhomogeneity and defective structure. The results obtained are consistent with Hooge's formula, where the noise parameter, α _{H}, is independent of doping ratio. The 1/f noise power spectral density and noise parameter α _{H} are proportional to the squared value of temperature coefficient of resistance (TCR). The resistivity and TCR of pmSi:H film resistor were obtained through linear currentvoltage measurement. The 1/f noise, measured by a custombuilt noise spectroscopy system, shows that the power spectral density is a function of both doping ratio and temperature.
Introduction
Nanostructure semiconductor has been the focus of intense interest in recent years due to their extensive device application [1–6]. It is well known that hydrogenated polymorphous silicon is a nanostructure inclusion material [7–9]. Hydrogenated silicon films commonly exhibit high noise at low frequency (f). This noise has a spectral power density of the type S(f) ∝ 1/f ^{a}, where a is known as "1/f noise." However, lower noise materials are important for highperformance semiconductor devices. 1/f noise of amorphous and polycrystalline silicon has captured the attention of researchers in the field of electronics and physics for several decades [10]. Polymorphous silicon film is generally prepared by operating a strong hydrogendiluted silane plasma source at high pressure and power density [11]. Many efforts have been made concerning the growth process, microstructure, transport, and optoelectronic properties of pmSi:H films [12]. The results indicate that pmSi:H films show higher transport properties than aSi:H, a highly desirable trait for the production of devices, such as solar cells and thin film transistors. To date, pmSi:H investigations have focused on certain applications, but there is no study devoted to the 1/f noise of such materials except those by our group which have reported the dependence of 1/f noise on the change of material structure of silicon films [13–15]. In this article, we focus on the study of the origins of 1/f noise in pmSi:H and investigate the influence of boron doping ratio on 1/f noise in pmSi:H films.
Experimental
The pmSi:H films were obtained by using RF PECVD [11]. As shown in Figure 1a, Coplanar nickel electrodes (about 50 nm) were evaporated onto the pmSi:H films and lifted off to make linear IV contact. In Figure 1b, in order to reduce external noise disturbance, the measuring circuit was placed in a metal box. The noise and electrical measurements were performed at various temperatures using an ESL02KA thermostat. Hall measurements were performed using a BioRad HL5560 Hall system coupled with helium cryostat. The structure of pmSi:H films was characterized using a SE850 spectroscopic ellipsometer with Bruggeman effective medium model.
Results and discussions
The results in Table 1 show that pmSi:H films deposited at higher doping ratio were characterized by high hydrogen content and crystalline fraction, and negligible void fraction. As shown in Figure 2, because of its nanocrystalline nature, the crystalline Raman peak of pmSi:H exhibits a frequency downshift and peak broadening caused by a phonon confinement effect. A peak (I _{n}) is observed between 480 cm^{1} (I _{a}: amorphous silicon) and 520 cm^{1} (I _{c}: microcrystalline silicon). The crystalline volume fraction X _{C} of these films has been calculated from the relation X _{C} = (I _{n} + I _{c})/(I _{a} + I _{n} + I _{c}) [13]. In this study, the results have proven that the crystalline volume fractions (X _{C}) measured by SE and Raman spectroscopy are highly consistent.
Figure 3 shows a logarithmic plot of power spectral density, which is averaged over 30 measurements, versus frequency for different doping ratios in pmSi:H films at 300 K. The decrease of noise is inversely proportional to frequency. Moreover, the 1/f noise decreased with the increment of boron doping ratio in pmSi:H samples. Conventionally, the results of 1/f noise measurements are discussed using Equation 1 originally introduced by Hooge [16]:
where S _{v} is the noise power density at voltage V, α _{H} is the noise parameter, f is frequency, and N _{C} is the total number of charge carriers in a certain volume involved in noise generation. The total number of charge carriers, determined by Hall measurement, in conjunction with the dimension of the pmSi:H film resistor, determines the noise parameter α _{H} as a function of frequency. Our experimental results also demonstrate the 1/f noise power scales with the square of bias voltage, which is in agreement with the results of Fine et al. [17].
Figure 4 shows the relative voltage noise power S _{v}/V ^{2} at 100 Hz. We obtained that S _{v}/V ^{2} is constant at voltage less than 1 V, which indicates that 1/f noise in pmSi:H film resistor does not originate from the resistance fluctuations at 100 Hz under our experimental conditions. PmSi:H film is generally accepted as inclusion material in nanocrystalline and nanosized clusters [18]. The above results indicate that pmSi:H films are far from being homogeneous, and thus, one could predict that their electronic properties are affected by heterogeneity. For the clarification of our results, the structure and 1/f noise variations in amorphous, microcrystalline, and pmSi:H films were compared [13]. The results demonstrate the dependence of 1/f noise in silicon film on the structure variation. Paul and Dijkhuis [19] proved the influence of metastable defect creation on the noise intensity in hydrogenated amorphous silicon. Hence, we also believe that the defects and heterogeneity cause 1/f noise in pmSi:H.
The temperature dependence of 1/f noise in pmSi:H film resistor was also measured at 100 Hz for the various boron doping pmSi:H film resistors at temperatures ranging from 300 to 420 K. In Figure 5a, the 1/f noise in pmSi:H film resistor decreases with the increasing temperature. From the theoretical model proposed by Richard, there is a correlation between S _{v} and the temperature coefficient of resistance (TCR) given by Equation 2 [20]:
where is the average voltage biased on the sample, 〈(ΔT)^{2}〉 is meansquare temperature fluctuation, and β is the value of TCR [13]. In the case of our measurement condition, the value of and 〈(ΔT)^{2}〉 is the same for each film resistor. Therefore, the power spectral density of 1/f noise in pmSi:H film resistors is proportional to squared β (S _{v}(f) ∝ β^{2}). The TCR is a function of resistivity in pmSi:H film resistors, which means that resistance fluctuation is another origin of 1/f noise in the pmSi:H resistors when the measurement temperature changed significantly. Figure 5b shows that the temperature dependence of the total charge carrier number in the measured volume also decreases with increasing boron doping ratio. The more highly doped the sample (such as sample A) the fewer the dangling bonds and defects. Therefore, the variation in the total charge carrier number for the higherdoped pmSi:H sample is lower. From Equation 1, we obtain
For each measured sample here, the values of N _{C}, f, and V ^{2} are constant. The value of noise parameter α _{H} at 100 Hz is plotted against temperature for different doping ratios as shown in the inset of Figure 5b. The noise parameter α _{H} for the pmSi:H film resistors in this study is also a function of the squared TCR (α_{H} ∝ β^{2}). It demonstrated that the resistance fluctuation of the film samples also resulted in the variation of noise parameter when the measurement temperature changed dramatically.
Conclusions
The results of this study demonstrated that the origins of 1/f noise in nanostructure inclusion pmSi:H are the inhomogeneity and the defective structure in the films. The power spectral density of 1/f noise is inversely proportional to boron doping ratio, which is consistent with Hooge's formula. The value of S _{v}/V ^{2} is constant when the voltage is less than 1 V, demonstrating that resistance fluctuation is not the origin of 1/f noise in pmSi film resistors in the case of constant temperature. At 100 Hz, the temperature dependence of 1/f noise indicates that the power spectral density and the noise parameter α _{H} are proportional to the squared TCR. It has also been proven that the resistance fluctuation of the film samples also results in the variation of noise parameter when the measurement temperature changed dramatically.
Abbreviations
 TCR:

temperature coefficient of resistance.
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Acknowledgements
This work was partially supported by National Science Foundation of China via grant No. 60901034 and 60425101.
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Authors' contributions
SL designed the experiments, carried out the sample preparation and 1/f noise measurement. JW and ZY worked on organize data. All authors participated in discussion on writing. All authors read and approved the final manuscript.
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Keywords
 Power Spectral Density
 Silicon Film
 Noise Parameter
 Film Resistor
 Boron Doping