Conductive-probe atomic force microscopy characterization of silicon nanowire
© Alvarez et al; licensee Springer. 2011
Received: 12 September 2010
Accepted: 31 January 2011
Published: 31 January 2011
The electrical conduction properties of lateral and vertical silicon nanowires (SiNWs) were investigated using a conductive-probe atomic force microscopy (AFM). Horizontal SiNWs, which were synthesized by the in-plane solid-liquid-solid technique, are randomly deployed into an undoped hydrogenated amorphous silicon layer. Local current mapping shows that the wires have internal microstructures. The local current-voltage measurements on these horizontal wires reveal a power law behavior indicating several transport regimes based on space-charge limited conduction which can be assisted by traps in the high-bias regime (> 1 V). Vertical phosphorus-doped SiNWs were grown by chemical vapor deposition using a gold catalyst-driving vapor-liquid-solid process on higly n-type silicon substrates. The effect of phosphorus doping on the local contact resistance between the AFM tip and the SiNW was put in evidence, and the SiNWs resistivity was estimated.
Silicon nanowires (SiNWs) are promising nanostructures which are expected to be integrated in building blocks for future microelectronics and optoelectronics devices [1–3]. Indeed, multiple studies have already shown the great potential of SiNWs as functional element to develop transistors , biosensors , memory applications , and as electrical interconnects . In addition, SiNWs offer an interesting geometry for light trapping and carrier collection which gives place to intensive investigations in the photovoltaic field [8, 9].
Several approaches and strategies exist to grow, deploy, and assemble SiNWs [10, 11]. In order to guide them, and more specifically to control the electrical properties of SiNWs, it is required to characterize their electronic transport properties.
Conductive-probe atomic force microscopy (CP-AFM)  reveals itself as a powerful current sensing technique for electrical characterizations in small-scale technologies, which could help us to explore the electrical properties and to reveal local conductivity fluctuations in SiNWs.
In this study, the authors focus on the CP-AFM characterization of horizontal SiNWs produced via in-plane solid-liquid-solid (IPSLS) method and phosphorus-doped vertical SiNWs obtained through vapor-liquid-solid (VLS) technique. Local resistance mapping and local current-voltage (I-V) measurements have been performed to evaluate the electrical properties of such semiconducting SiNWs.
n-Type phosphorous-doped SiNWs were grown by chemical vapor deposition through the gold-catalyzed VLS method as described in [15, 16], on n-type silicon substrates (3-5 mΩ cm). The SiNW growth temperature was in the range of 500-650°C, and the n-type doping was achieved by adding PH3 to SiH4, with PH3/SiH4 ratios which can vary from 0 to 2 × 10-2. Subsequent to the growth, the catalyst was removed, and in some cases, a rapid thermal annealing at 750°C for 5 min was done to activate dopant impurities. SiNWs were then embedded into spin-on-glass matrix in order to be planarized by chemical-mechanical polishing .
Sample description of vertical SiNWs analyzed by the CP-AFM technique
Growth temp. (°C)
Nominal impurity concentration
Undoped SiNWs/n-type Si (100)
Doped SiNWs/n-type Si (100)
5 min at 750°C
[P] ≈ 1 × 1018 cm-3
Doped SiNWs/n-type Si (100)
5 min at 750°C
[P] ≈ 1 × 1020 cm-3
Conductive-probe atomic force microscopy
Local electrical measurements were performed using a Digital Instruments Nanoscope IIIa Multimode AFM associated with the home-made conducting probe extension called "Resiscope" . This setup allows us to apply a stable DC bias voltage (from -10 to +10 V with 0.01 V resolution) to the device and to measure the resulting current flowing through the tip as the sample surface is scanned in contact mode. Local resistance values can be measured in the range of 102-1012 Ω, which allows investigations on a variety of materials [17, 18] and devices [19, 20]. Measurement accuracy based on calibrations is below 3% in the range of 102-1011 Ω, and it can reach 10% for higher resistance values.
Reliable and understandable electrical measurements through CP-AFM setup require a well-characterized conductive tip. Depending on the experimental conditions, the AFM conductive tip should be the most suitable in terms of serial resistance that must be taken into account in the electrical analysis of SiNWs. B-doped diamond- and PtIr-coated Si cantilevers, with an intermediate spring constant of about 2 N/m, prove to be suitable for our experimental conditions, since measured resistance values are mostly greater than their intrinsic resistances that are estimated at 5-10 and 0.3-1 kΩ, respectively.
Results and discussion
In the same figure, the empty growth channel resulting from the unexpected cut of the wire with the AFM probe can also be noticed. Broken pieces of silicon remaining in the channel reveal a slight electrical conduction (1011 Ω) although they are electrically isolated through the undoped a-Si:H layer (1012 Ω). Possible explanations are that the whole surface of the remaining piece of silicon in contact with the a-Si:H layer fully contributes to decrease the electrical contact resistance or that the friction of the AFM tip induces charging effects which are electrically observable.
where R AFMtip is the intrinsic resistance of the AFM tip, R tip/SiNW refers to the contact resistance involving the AFM tip and the SiNW, R SiNW designates the intrinsic resistance of the SiNW, and R back the back contact resistance between the highly doped silicon wafer and the SiNW. The intrinsic resistance of the SiNW (R SiNW) is given by ρl/S where ρ, l, and S are the resistivity, the length of the wire, and the wire sectional area, respectively.
The presence of contact resistance often implies the presence of a barrier which gives rise to diode-like behavior or sigmoidal I-V characteristics. In some cases, a linear dependence on applied bias can be measured indicating that the barrier resistance involved in the contact resistance can be neglected. The contact resistance only consists then in a geometrical resistance which depends on the electrical radius . In order to estimate the geometrical resistance, the Wexler resistance model [24, 25] was used, which describes the transition between the diffusive and ballistic transport regimes in constricted contacts.
where K = λ/a is the ratio of the carrier mean free path, l, to the electrical radius, a, and Γ(K) is a monotonous function that takes the value 1 at K = 0 and decreases slowly reaching the limit of 0.694.
For the estimation of R tip/SiNW, the electrical radius was chosen equal to 10 nm, and the electron mean free path in the range 1-80 nm assuming bulk silicon values. From these calculations, the resistivity values were estimated to be in the range of 20-40 Ω cm for the undoped sample, 0.1-1.2 Ω cm for the intermediate doped sample, and 0.008-0.016 Ω cm for the highly doped sample. In terms of electrically active phosphorus, it corresponds to 1-2 × 1014, 0.5-7 × 1016, and 2-6 × 1018 cm-3, respectively. These values, extracted from bulk silicon values, indicate that the phosphorus incorporation is not fully activated despite the thermal anneal activation at 750°C. Recent results of CP-AFM show that phosphorus activation in SiNWs is enhanced at higher temperatures growth (T > 500°C) without the need of post-annealing treatment.
From the point of view of the CP-AFM measurements more accurate resistivity measurements could be achieved in the future making a pre-calibration of the technique using standard doped silicon wafers .
In this study, CP-AFM was used to electrically characterize horizontal and vertical SiNWs. CP-AFM technique reveals itself as a powerful tool for sensing current inhomogeneities that were observed in horizontal SiNWs pointing out an internal microstructure. In addition, local I-V measurements allowed us to put in evidence a SCLC transport regime that could be assisted by traps.
The effect of phosphorus doping on the local contact resistance was evidenced for vertical SiNWs, and resistivity values were estimated indicating that phosphorus incorporation was not fully activated.
conductive-probe atomic force microscopy
indium tin oxide
- I-V :
space-charge limited current
scanning electron microscopy
This study has been supported by the French Research National Agency (ANR) through Habitat intelligent et solaire photovoltaïque program (projet SiFlex n°ANR-08-HABISOL-010).
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