Nanoscale Res Lett. 2011; 6(1): 281.

# Origins of 1/*f *noise in nanostructure inclusion polymorphous silicon films

^{1}State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China

^{2}Arkansas Institute for Nanoscale Materials Science and Engineering, University of Arkansas, Fayetteville, AR 72701, USA

^{3}Department of Electronics and Information, Hang Zhou Dianzi University, Hangzhou, 310018, China

^{}Corresponding author.

Received 2010 Dec 18; Accepted 2011 Apr 4.

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## Abstract

In this article, we report that the origins of 1/*f *noise in *pm*-Si: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 *pm*-Si:H film resistor were obtained through linear current-voltage measurement. The 1/*f *noise, measured by a custom-built 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 high-performance 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 hydrogen-diluted 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 *pm*-Si:H films [12]. The results indicate that *pm*-Si:H films show higher transport properties than *a*-Si:H, a highly desirable trait for the production of devices, such as solar cells and thin film transistors. To date, *pm*-Si: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 *pm*-Si:H and investigate the influence of boron doping ratio on 1/*f *noise in *pm*-Si:H films.

## Experimental

The *pm*-Si:H films were obtained by using RF PECVD [11]. As shown in Figure , Coplanar nickel electrodes (about 50 nm) were evaporated onto the *pm*-Si:H films and lifted off to make linear *I*-*V *contact. In Figure , 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 ESL-02KA thermostat. Hall measurements were performed using a BioRad HL5560 Hall system coupled with helium cryostat. The structure of *pm*-Si:H films was characterized using a SE850 spectroscopic ellipsometer with Bruggeman effective medium model.

**Schematic of measurement system**. **(a) **Schematic of coplanar electrode configuration for thin *pm*-Si:H film resistance measurement; **(b) **schematic diagram of low-frequency noise measurement system.

## Results and discussions

The results in Table show that *pm*-Si:H films deposited at higher doping ratio were characterized by high hydrogen content and crystalline fraction, and negligible void fraction. As shown in Figure , because of its nanocrystalline nature, the crystalline Raman peak of *pm*-Si: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.

Structure and electrical properties for different doping ratios in *pm*-Si:H film

**Raman spectroscopy of polymorphous silicon samples**. Raman spectroscopy for *pm*-Si:H samples (A, B, C, D), the crystal volume fractions *X*_{C }(%) obtained by Raman is consistent with the results from SE measurements.

Figure shows a logarithmic plot of power spectral density, which is averaged over 30 measurements, versus frequency for different doping ratios in *pm*-Si: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 *pm*-Si:H samples. Conventionally, the results of 1/*f *noise measurements are discussed using Equation 1 originally introduced by Hooge [16]:

**Log-log plot of power spectra density for various doping ratios in ***pm*-films at 50 mV bias.

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 *pm*-Si: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 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 *pm*-Si:H film resistor does not originate from the resistance fluctuations at 100 Hz under our experimental conditions. *Pm*-Si:H film is generally accepted as inclusion material in nanocrystalline and nanosized clusters [18]. The above results indicate that *pm*-Si: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 *pm*-Si: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 *pm*-Si:H.

**Relative noise power at 100 Hz vs voltage**. Relative noise power demonstrates the dependence of 1/*f *noise in silicon film on structure variation.

The temperature dependence of 1/*f *noise in *pm*-Si:H film resistor was also measured at 100 Hz for the various boron doping *pm*-Si:H film resistors at temperatures ranging from 300 to 420 K. In Figure , the 1/*f *noise in *pm*-Si: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]:

**Temperature dependence of 1/***f *noise in *pm*-Si:H film. **(a) **Temperature dependence of 1/*f *noise in *pm*-Si:H film. Inset: temperature dependence of TCR value for samples with various doping ratios; **(b) **temperature dependence of total carriers number (*N*_{C}) on **...**

where is the average voltage biased on the sample, 〈(Δ*T*)^{2}〉 is mean-square temperature fluctuation, and *β *is the value of TCR [13]. In the case of our measurement condition, the value of and 〈(Δ*T*)^{2}〉 is the same for each film resistor. Therefore, the power spectral density of 1/*f *noise in *pm*-Si:H film resistors is proportional to squared *β *(*S*_{v}(*f*) ∝ β^{2}). The TCR is a function of resistivity in *pm*-Si:H film resistors, which means that resistance fluctuation is another origin of 1/*f *noise in the *pm*-Si:H resistors when the measurement temperature changed significantly. Figure 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 higher-doped *pm*-Si: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 . The noise parameter *α*_{H }for the *pm*-Si: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 *pm*-Si: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 *pm*-Si 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.

## Competing interests

The authors declare that they have no competing interests.

## 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.

## Acknowledgements

This work was partially supported by National Science Foundation of China via grant No. 60901034 and 60425101.

#### Open access

This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

## References

- Li SS, Xia JB. Electronic structure of a hydrogenic acceptor impurity in semiconductor nano-structures. Nanoscale Res Lett. 2007;2:554. doi: 10.1007/s11671-007-9098-9. [Cross Ref]
- Arendse CJ, Malgas GF, Muller TFG, Knoesen D, Oliphant CJ, Motaung DE, Halindintwali S, Mwakikunga BW. Thermally induced nano-structural and optical changes of nc-Si:H deposited by hot-wire CVD. Nanoscale Res Lett. 2009;4:307. doi: 10.1007/s11671-008-9243-0. [PMC free article] [PubMed] [Cross Ref]
- Li SS, Xia JB. Linear Rashba model of a hydrogenic donor impurity in GaAs/GaAlAs quantum wells. Nanoscale Res Lett. 2009;4:178. doi: 10.1007/s11671-008-9222-5. [PMC free article] [PubMed] [Cross Ref]
- Li SB, Han L, Chen Z. The interfacial quality of HfO
_{2 }on silicon with different thicknesses of the chemical oxide interfacial layer. J Electrochem Soc. 2010;157:G221. doi: 10.1149/1.3483789. [Cross Ref] - Li SS, Xia JB. Electronic structures of GaAs/AlxGa1-xAs quantum double rings. Nanoscale Res Lett. 2006;1:167. doi: 10.1007/s11671-006-9010-z. [Cross Ref]
- Zhou M, Zhou JY, Li RS, Xie EQ. Preparation of aligned ultra-long and diameter controlled silicon oxide nanotubes by plasma enhanced chemical vapor deposition using electrospun PVP nanofiber template. Nanoscale Res Lett. 2010;5:279. doi: 10.1007/s11671-009-9476-6. [PMC free article] [PubMed] [Cross Ref]
- Voyles PM, Gerbi JM, Treacy MMJ, Gibson JM, Abelson JR. Absence of an abrupt phase change from polycrystalline to amorphous in silicon with deposition temperature. Phys Rev Lett. 2001;86:5514. doi: 10.1103/PhysRevLett.86.5514. [PubMed] [Cross Ref]
- Chaâbane N, Kharchenko AV, Vach H, Roca i Cabarrocas P. Optimization of plasma parameters for the production of silicon nano-crystals. New J Phys. 2003;5:37.1.
- Fontcuberta i Morral A, Roca i Cabarrocas P, Clerc C. Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements. Phys Rev B. 2004;69:125307. doi: 10.1103/PhysRevB.69.125307. [Cross Ref]
- Yoon HP, Yuwen YA, Kendrick CE, Barber GD, Podraza NJ, Redwing JM, Mallouk TE, Wronski CR, Mayer TS. Enhanced conversion efficiencies for pillar array solar cells fabricated from crystalline silicon with short minority carrier diffusion lengths. Appl Phys Lett. 2010;96:213503. doi: 10.1063/1.3432449. [Cross Ref]
- Li SB, Wu ZM, Jiang YD, Yu JS, Li W, Liao NM. Growth mechanism of microcrystalline and polymorphous silicon film with pure silane source gas. J Phys D. 2008;41:105207. doi: 10.1088/0022-3727/41/10/105207. [Cross Ref]
- Butté R, Vignoli S, Meaudre M, Meaudre R, Marty O, Saviot L, Roca i Cabarrocas P. Plasma enhanced chemical vapor deposition of amorphous, polymorphous and microcrystalline silicon films. J Non-Cryst Solids. 2000;266-269:263.
- Li SB, Wu ZM, Jiang YD, Li W, Liao NM, Yu JS. Structure and 1/f noise of boron doped polymorphous silicon films. Nanotechnology. 2008;19:085706. doi: 10.1088/0957-4484/19/8/085706. [PubMed] [Cross Ref]
- Li SB, Wu ZM, Li W, Liao NM, Yu JS, Jiang YD. Influence of microcrystallization on noise in boron-doped silicon film. Phys Status Solidi A. 2007;204:4292. doi: 10.1002/pssa.200723235. [Cross Ref]
- Li SB, Wu ZM, Jiang YD, Li W, Liao NM, Yu JS. Noise in boron doped amorphous/microcrystallization silicon films. Appl Surf Sci. 2008;254:3274. doi: 10.1016/j.apsusc.2007.11.004. [Cross Ref]
- Hooge FN. 1/f noise is no surface effect. Phys Lett A. 1979;29:139. doi: 10.1016/0375-9601(69)90076-0. [Cross Ref]
- Fine BV, Bakker JPR, Dijkhuis JI. Long-range potential fluctuations and 1/
*f *noise in hydrogenated amorphous silicon. Phys Rev B. 2003;68:125207. doi: 10.1103/PhysRevB.68.125207. [Cross Ref] - Nguyen-Tran T, Suendo V, Roca i Cabarrocas P, Nittala LN, Bogle SN, Abelson JR. Fluctuation microscopy evidence for enhanced nanoscale structural order in polymorphous silicon thin films. J Appl Phys. 2006;100:094319. doi: 10.1063/1.2360381. [Cross Ref]
- Paul AWE, Dijkhuis JI. Resistance fluctuations in hydrogenated amorphous silicon: Thermal equilibrium. Phys Rev B. 1998;58:3904. doi: 10.1103/PhysRevB.58.3904. [Cross Ref]
- Voss RF, Clark J. Flicker (1/
*f*) noise: equilibrium temperature and resistance fluctuations. Phys Rev B. 1976;13:556. doi: 10.1103/PhysRevB.13.556. [Cross Ref]