Low-frequency flicker noise in a MSM device made with single Si nanowire (diameter ≈ 50 nm)
© Samanta et al.; licensee Springer. 2013
Received: 14 February 2013
Accepted: 20 March 2013
Published: 10 April 2013
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© Samanta et al.; licensee Springer. 2013
Received: 14 February 2013
Accepted: 20 March 2013
Published: 10 April 2013
Low-frequency flicker noise has been measured in a metal-semiconductor-metal (MSM) device made from a single strand of a single crystalline Si nanowire (diameter approximately 50 nm). Measurement was done with an alternating current (ac) excitation for the noise measurement superimposed with direct current (dc) bias that can be controlled independently. The observed noise has a spectral power density ∝1/f α . Application of the superimposed dc bias (retaining the ac bias unchanged) with a value more than the Schottky barrier height at the junction leads to a large suppression of the noise amplitude along with a change of α from 2 to ≈ 1. The dc bias-dependent part of the noise has been interpreted as arising from the interface region. The residual dc bias-independent flicker noise is suggested to arise from the single strand of Si nanowire, which has the conventional 1/f spectral power density.
Exploring the fundamental properties of an individual silicon nanowire (Si NW) is important as it forms the backbone of the fabrication of single-nanowire nanoelectronic devices. There are reports on the development of Si NW-based nanoscale devices such as field-effect transistors (FETs) [1, 2] with wrap-around gates, surface-gated sensitive chemical and biomolecular sensors [3, 4], as well as nanoscale opto-electronic devices . In the context of such nanowire-based device, one important physical parameter is the low-frequency flicker noise, which has a direct impact on the device performance. In recent publications, it has been argued that flicker noise in qubits can lead to decoherence and can be the limiting factor in increasing the coherence time . While flicker noise in a sub-micron metal oxide semiconductor field-effect transistor (MOSFET) with varying channel width has been investigated for some time , there are no reports of measurements of the low-frequency flicker noise in Si NWs and nanowire-based devices particularly with diameters much less than 100 nm.
In this paper, we report the measurement of flicker noise in a metal-semiconductor-metal (MSM) device made from a single strand of a Si NW. In such a device, the flicker noise can come from the junction region where the metals make contacts with the semiconductor (MS junction) as well as from the single Si NW. The noise arising from the junction region can be large and can even mask the noise from the Si NW by a few orders. This is because the flicker noise is likely to arise from charge carrier density fluctuations due to trapping-detrapping in the junction region. By an innovative application of direct current (dc) bias (used for biasing the device) mixed with an alternating current (ac) bias (used for the noise measurements), we could suppress the noise from the junction region and observe the noise which likely arises from the single Si NW. The enabling physics that leads to suppression of the noise in the junction region on application of the dc bias is the collapse of the depletion region at the junction region by the applied dc bias.
The low-frequency flicker noise in most materials has a power spectral density (PSD) with 1/f frequency dependence and can serve as a diagnostic of the presence of structural defects arising from mobility fluctuations. In semiconductors, the 1/f noise can also arise from recombination-generation process . For the Si NW devices, proper estimation of the generic noise arising from nanowire itself is an essential device parameter for the better performance of low-noise electronics. The fluctuations in this cases arise from resistance fluctuations in a current biased system which shows up voltage fluctuations with PSD S V (f).
The Si NWs used in this experiment were fabricated by metal-assisted chemical etching  technique. The method leads to a dense array of single crystalline Si NWs with a diameter ranging from approximately 20 to 100 nm and lengths of more than 10 µm. A high-resolution transmission electron microscope (HRTEM) image shows the probable existence of an oxide layer with a thickness ≤ 2 nm at the surface. The Pt contacts (in the configuration of the MSM device) for the noise measurement were made by using e-beam-assisted local deposition of methylcyclopentadienyl platinum trimethyl precursor at a bias of 15 kV in a dual beam system FEI-HELIOS 600 (FEI Co., Hillsboro, OR, USA). The scanning electron microscopy (SEM) image of a single NW connected with four electrical contacts is shown in Figure 1c. The four electrical contacts allow us four-probe measurements of the resistance of the individual NW and hence its resistivity (ρ). The inner two electrodes were used for current-voltage (I − V) measurements in the MSM device configuration. Pt in a dual beam machine can also be deposited using Ga ions. However, to avoid damage as well as contamination from implanted Ga ions, we used e-beam-assisted deposition. We note that the Pt deposited from the decomposition of the high carbon-containing precursor is not pure Pt. Instead, it is a composite of carbon and Pt, which has been analysed before by our group for its physical characteristics and compositional details .
The metallic contacts at the ends lead to the Schottky barrier (SB) formation in the junction region (see Figure 1b). The resulting MSM device can be modelled as two back-to-back Schottky diodes (SB1 and SB2) at the ends with a Si NW with resistance RNW connecting them. The current passing through such a device is mainly controlled by the barrier heights φ1 and φ2 at the two contacts SB1 and SB2, respectively. This device configuration also enabled us to do two-probe as well as four-probe measurements on the same Si NW, which then allows us to find the contact resistance RC, an important device parameter. The area of contact, AC, can be obtained from the SEM image of a given device from which a reliable estimate of specific contact resistivity ρC = ACRC can be obtained.
where V′ = V − I RNW, RNW. (In the equation above, φ1 is related to the terminal with V+ve.) I0 arises from thermoionic emission. The I − V data at low bias (< 0.5 V) as well as the fit to the data are shown in Figure 2a (solid line). Equation 1 fits the I − V data well, and we could obtain the barrier heights. For the data shown in Figure 2a, φ1≈ 0.1 eV and φ2≈ 0.04 eV. From the contact resistance RC measured as a function of bias, as depicted before, we obtained the bias-dependent specific contact resistance ρC in Figure 2b. With increase of bias, ρC is substantially reduced (by nearly a factor of 2). We limited the analysis to bias up to 1 V, because the variation of ρC saturates for bias that is much higher than the barrier heights φ1 and φ2.
The noise data reported here were taken with the contact with larger barrier height (φ1) forward biased. The dominant contribution to the contact noise as well as the contact resistance arises from this contact. On applying forward bias to this junction, the noise (as well as the contact resistance) is severely reduced. The other contact with much smaller barrier (φ2) has much less contribution to the contact noise. Thus, even if it is reversed biased (and the depletion width increases due to the reverse bias), its contribution still remains low.
Evaluation of the noise in a single Si NW needs to be put in perspective and compared with bulk systems. In noise spectroscopy, one often uses a quantitative parameter for noise comparison is the Hooge parameter . The spectral power of 1/f noise in many conductors often follows an empirical formula  where γH is the Hooge’s parameter, and N is the number of carriers in the sample volume (between voltage probe leads). γH is a useful guide when one compares different materials. Usually, a low γH is associated with a sample with less defect density that contributes to the 1/f noise arising from the defect-mediated mobility fluctuation . N can be calculated from carrier density n = 2×1017/cm3 and volume of the sample between two voltage leads (30×10−16 cm3). Our Si NW under test has N ≈ 600. We obtained γH ≃ 10 − 8 when we use the limiting value of the PSD for Vdc ≥ 0.2 V.
For bulk crystalline Si, the noise has been studied extensively both in low-doped and degenerately doped crystals  as well as in films [19, 20]. In bulk Si wafers with low doping concentration, the value of γH lies in the range of 10 − 7 to 10 − 2 with the exact value being a sensitive function of impurity and defect process conditions [15, 17]. For the Si NW, we observed that the value can even be lower. We note, however, that in this size range, it has not been established that such a scaling of spectral power with 1/N truly holds as there can also be significant surface contributions. Thus, the use of γH, as a parameter for comparison is done with caution. The intrinsic contribution in a NW can be large because N is small. In a NW, if the γH is indeed low as observed, this will mitigate the increase in the intrinsic noise on size reduction. For even smaller devices with smaller diameter, less dopant and closer contacts, N can even be below 10.
In this report, we propose a likely scenario of suppression of the junction noise by Vdc. The noise at the M-S contact can arise in the depletion region where the SB forms. The traps in the depletion region can lead to substantial noise due to trapping-detrapping of carriers. Such a noise has been observed also in the depletion region of MOSFETs . Flicker noise in sub-micron MOSFETs  have been investigated experimentally as well as theoretically, and it shows the existence of both 1/f2 and 1/f frequency components, where the 1/f2 component arises from charge exchange with traps in the oxide region. Application of the dc bias reduces the depletion width (ddw). In an ideal SB, ddw ∝ (ϕ − Vdc)1/2; for Vdc ≥ ϕ, ddw→0. In such case, the trapping centres are occupied and cannot contribute to the trapping-detrapping process generated noise. This leads to severe suppression of the noise in the junction region. Another strong evidence that the noise at the junction arises from the trap states in the depletion region is the value of the exponent α. It has been shown that existence of trap states in the depletion region can lead to a power spectrum of the type S v (f) ∝ 1/f α where α = 2 . We also found α ≈ 2 for a very low dc bias, when the observed noise is mainly due to the junction noise. α rapidly reduces to ≈ 1 for high Vdc. The suggested mechanism for noise reduction with applied Vdc is the controlling of ddw which can be a generic mechanism for an MSM device and thus has a general applicability for such junctions.
To summarize, we have measured the electrical noise in an MSM device consisting of a single stand of Si NW with a diameter of approximately 50 nm. The flicker noise as well as Nyquist noise was measured with ac excitation with a superimposed dc bias. On application of a dc bias, more than the Schottky barrier height reduces the contact resistance which leads to reduction of the Nyquist noise as well as the flicker noise (∝ 1/f α ) in the MSM device along with a change of α from 2 to ≃1. The part of the noise suppressed by dc bias has been interpreted as arising due to trapping-detrapping noise in the depletion region at the interface. The residual noise has been has been linked to the noise in the single Si NW, which has the conventional 1/f spectral power density with an estimated Hooge parameter γH ≃ 10 − 8.
High-resolution transmission electron microscopy.
The authors thank Nanomission, Department of Science and Technology, Govt. of India for financial support as sponsored projects UNANST-II and Theme Unit of Excellence in Nanodevice Technology.
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