Analysis of low-frequency noise in boron-doped polycrystalline silicon-germanium resistors

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Analysis of low-frequency noise in boron-doped polycrystalline silicon–germanium

resistors

Kun-Ming Chen, Guo-Wei Huang, D. Y. Chiu, Hsiang-Jen Huang, and Chun-Yen Chang

Citation: Applied Physics Letters 81, 2578 (2002); doi: 10.1063/1.1511815

View online: http://dx.doi.org/10.1063/1.1511815

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/81/14?ver=pdfcov Published by the AIP Publishing

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Analysis of low-frequency noise in boron-doped polycrystalline

silicon–germanium resistors

Kun-Ming Chen,a)Guo-Wei Huang,b) and D. Y. Chiu

National Nano Device Laboratories, Hsinchu 300, Taiwan, Republic of China Hsiang-Jen Huang and Chun-Yen Chang

Department of Electronics Engineering and Institute of Electronics, National Chiao-Tung University, Hsinchu, Taiwan, Republic of China

共Received 24 May 2002; accepted 12 August 2002兲

Low-frequency noise in boron-doped polycrystalline silicon–germanium共poly-Si1⫺xGex) resistors

at various temperatures is studied. The poly-Si1⫺xGexfilms with 0%⬃36% Ge content were grown

using ultrahigh vacuum chemical molecular epitaxy system. We find that the low-frequency noise in poly-Si1⫺xGex decreases with increasing Ge content, due to the lower potential barrier height of

grain boundaries in higher Ge content samples. Moreover, the low-frequency noise decreases with increasing temperature. These results are well explained by the carrier mobility fluctuation model. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1511815兴

In analog and radio frequency circuits, polycrystalline silicon 共poly-Si兲 films are frequently used for resistors, the gate material of metal-oxide-semiconductor field-effect tran-sistors, and the emitter contacts of bipolar junction transis-tors. Recently, polycrystalline silicon–germanium 共poly-Si1⫺xGex) has been shown to be an attractive alternative to

conventional poly-Si material for various integrated circuit applications.1–3By taking advantage of its lower processing temperature, thin film transistors can be fabricated with poly-Si1⫺xGex films with processing temperature not exceeding

550 °C.1 Furthermore, compatibility with existing silicon processing technology and the ability to adjust the threshold voltage by changing the Ge content have made heavily doped p-type poly-Si1⫺xGexa very promising gate-electrode

material for deep submicrometer complementary metal-oxide-semiconductor technologies.2,3 Because the low-frequency noise in transistors and resistors may contribute to the phase noise of the radio frequency circuits or systems, it is important to predict the amount of noise in them. Several researchers have studied the noise properties of poly-Si films.4 –7 Both carrier number fluctuations,5 and mobility fluctuations6,7 were supported for the possible mechanisms which can cause the frequency noise. However, the low-frequency noise in poly-Si1_⫺xGexresistor was less studied.8,9

In this letter, the low-frequency noise in boron-doped poly-Si1⫺xGex resistors at various temperatures is investigated.

The relationship of noise and Ge content in poly-Si1⫺xGex

resistors can be well predicted using the mobility fluctuation model.

In this work, poly-Si1⫺xGexfilms were grown by

ultra-high vacuum chemical molecular epitaxy system to a thick-ness of ⬃0.2␮m at 580 °C onto thermally grown silicon nitride. Pure disilane and germane were used as the source gases. The Ge content x in polycrystalline film was varied from 0 to 0.36. Boron atoms were implanted into the films by BF₂⫹ at an energy of 20 keV. After the ion implantation,

furnace annealing at 800 °C for 20 min and rapid thermal annealing at 1050 °C for 10 s were performed for dopant activation and uniform doping distribution. Kelvin resistor structures were fabricated and used to accurately measure resistivity. The dimension of all samples studied was 500

⫻10␮m2. Current–voltage characteristics of these poly-Si1⫺xGexresistors were measured using an HP4156A

semi-conductor parameter analyzer. The noise measurements were performed at various temperatures using a BTA9812B noise analyzer in conjunction with an HP35670A dynamic signal analyzer.

Figure 1 shows a typical result of the measured spectral density of the current noise in a poly-Si1⫺xGex resistor at

room temperature for various applied currents. The spectra reveal the presence of a large pure 1/f excess noise signal. As can be seen in Fig. 1, the noise decreases approximately inversely proportional to frequency. The exponent of the fre-quency slope of the noise varied between ⫺0.95 and ⫺1,

a兲_{Electronic mail: kmchen@ndl.gov.tw} b兲_{Electronic mail: gwhuang@ndl.gov.tw}

FIG. 1. Current noise spectrum vs frequency of a moderately doped (B

⫽6⫻1018_cm⫺3_{) poly-Si}

0.64Ge0.36resistor. Inset shows the current noise as

a function of applied current at f⫽10 Hz.

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 14 30 SEPTEMBER 2002

2578

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slightly increasing toward higher applied currents. Moreover, the variation of noise intensity varied as the square of cur-rent, as shown in the inset of Fig. 1. From these observations, the current noise can be normalized with frequency and the square of the current to permit a clear comparison of resistors with different Ge content. Figure 2 shows the normalized current noise in poly-Si1⫺xGexresistors as a function of the

Ge content. The low-frequency noise is almost independent of Ge content in heavily doped (B⫽1⫻1020cm⫺3) poly-Si1⫺xGex resistors. However, the noise decreases with

in-creasing Ge content in moderately doped samples (B⫽6

⫻1018_cm⫺3_{). As seen in Fig. 2, the poly-Si}

0.64Ge0.36 exhib-its a significantly lower noise level than the poly-Si, making poly-Si₁_⫺xGe_xfilms the preferred choice for analog resistors. In the generally accepted model of poly-Si, the material is viewed as composed of small crystallites joined together by grain boundaries.10,11Inside each crystallite, the atoms are arranged in a periodic manner forming small single crystals, while the grain boundaries are composed of disordered at-oms and contain large numbers of defects due to incomplete bonding. From the literature, the grain boundaries contain trapping states that are capable of trapping mobile carriers and contributing to the creation of space-charge potential barriers.12 The potential barriers will block the transport of free carriers between the grains, thereby reducing the appar-ent carrier mobility.12For low and moderately doped poly-Si, the sheet resistance Rs can be expressed as13

RS⫽const.

冑

T exp

冉

q␾B

kT

冊

, 共1兲

where T is the temperature,␾_〉is the potential barrier height, and k is Boltzmann’s constant. To determine the barrier heights of the grain boundaries, the sheet resistance has been determined as a function of the measurement temperature for poly-Si and poly-Si0.64Ge0.36 samples. In Fig. 3, the loga-rithm of the normalized sheet resistance is plotted as a func-tion of reciprocal temperature. For heavily doped samples, the sheet resistance contains barrier and bulk grain compo-nents, so the bulk resistance must be subtracted from Eq.共1兲. The obtained values for␾_〉are listed in Table I. It is shown that the barrier height is lower for the Si1⫺xGe1-x samples compared to the Si samples at equal doping levels. In the

case of poly-Si, both p- and n-type doped material will show a similar trapping behavior. In the case of poly-Ge, the traps at the grain boundaries are p type, and the energy levels of the traps shift toward the valence band. Hence, Si₁_⫺xGe_xhas a lower potential barrier for the boron-doped samples.14It is believed that the observed 1/f noise in poly-Si is attributed to carrier mobility fluctuations occurring in the space charge regions near the grain boundary. From the model proposed by Luo, the normalized current noise can be expressed as7

SI⫻ f I2 ⫽ 1 Neff

冉

vr vd

冊

2 _q2_d_␣ 3␧kTAexp

冉

q␾B kT

冊

, 共2兲

where SI is the measured current noise spectral density, I is

the bias current, f is the frequency, Neffis the effective num-ber of large-barrier grains in the conduction path, vr is the

recombination velocity,vd is the diffusion velocity,␧ is the

dielectric constant, A is the cross section of the resistor, d is the width of a one-sided space charge region, and ␣ is the noise parameter for the grains. Substituting d ⫽(2␧␾〉/qn)1/2in Eq.共2兲, we have

SI⫻ f

I2 ⬀

冑␾

Bexp

冉

q␾B

kT

冊

, 共3兲

Hence, the noise will depend on the barrier height according to Eq. 共3兲. For moderately doped samples, the difference in the barrier height can lead to a factor of 6 difference in noise between Si and Si0.64Ge0.36. For both materials, the potential barriers are lower with increasing dopant concentration and the relative difference becomes smaller, so that the effect of the potential barriers becomes less important. For heavily doped samples, the potential barrier height only contributes approximately a factor 1.5 to the difference in noise.

FIG. 2. Normalized current noise as a function of Ge content at different

boron doping levels. FIG. 3. Normalized sheet resistance of poly-Si and poly-Si_{boron concentrations at different temperatures.} 0.64Ge0.36for two

TABLE I. Grain boundary energy barriers of boron-doped poly-Si and poly-Si0.64Ge0.36samples for two boron concentrations.

Sample q␾_〉共meV兲 B 6⫻1018_cm⫺3 _{B 1}_⫻1020_cm⫺3 Poly-Si 61 14 Poly-SiGe 27 9 2579

Appl. Phys. Lett., Vol. 81, No. 14, 30 September 2002 Chenet al.

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The temperature dependence of the low-frequency noise in poly-Si1⫺xGex resistors was also measured. The

normal-ized spectral noise density for the poly-Si1⫺xGex resistors

over the temperature range from room temperature to 200 °C is shown in Fig. 4. It is seen that the low-frequency noise decreases with increasing temperature. For resistors with higher grain boundary potential barriers 共e.g., moderately doped poly-Si兲, the low-frequency noise depends on the tem-perature. On the other hand, the noise signal in resistors with a lower potential barrier共e.g., heavily doped poly-Si兲 is char-acterized by rather weak temperature dependence. The ex-perimentally observed behavior is in agreement with Eq.共2兲. It shows that the model of Luo is useful to predict the low-frequency noise in poly-Si and poly-Si₁_⫺xGe_xresistors.

In conclusion, the noise properties of poly-Si₁_⫺xGe_xare comparable with those of poly-Si at heavy boron doping lev-els. On the other hand, the noise in moderately doped

resis-tors decreases with increasing Ge content. On the basis of their superior noise characteristics, poly-Si1⫺xGex resistors

are preferable to poly-Si resistors for analog circuit applica-tions. For poly-Si1⫺xGexwith higher Ge content, the

poten-tial barrier of the grain boundary is lower than in poly-Si films, thereby reducing the noise from the grain boundary. Furthermore, we find that the noise in moderately doped re-sistors decreases with increasing temperature. These noise characteristics can be well predicted by using the carrier mo-bility fluctuation model.

The authors would like to thank Dr. H.C. Lin for experi-mental assistance. This work was supported by the R.O.C.’s National Science Council through Contract No. NSC91-2721-2317-200.

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FIG. 4. Normalized current noise as a function of temperature for poly-Si and poly-Si0.64Ge0.36resistors at different boron doping level.

2580 Appl. Phys. Lett., Vol. 81, No. 14, 30 September 2002 Chenet al.