A Novel Method to Fabricate Silicon Nanowire p–n Junctions by a Combination of Ion Implantation and in-situ Doping
© to the authors 2009
Received: 9 October 2009
Accepted: 14 October 2009
Published: 8 November 2009
We demonstrate a novel method to fabricate an axial p–n junction inside <111> oriented short vertical silicon nanowires grown by molecular beam epitaxy by combining ion implantation with in-situ doping. The lower halves of the nanowires were doped in-situ with boron (concentration ~1018cm−3), while the upper halves were doubly implanted with phosphorus to yield a uniform concentration of 2 × 1019 cm−3. Electrical measurements of individually contacted nanowires showed excellent diode characteristics and ideality factors close to 2. We think that this value of ideality factors arises out of a high rate of carrier recombination through surface states in the native oxide covering the nanowires.
In order to make use of silicon nanowires (Si NWs)  in nano-devices, selective doping to form p n junctions or p and n wells is a necessity. Till date, a host of devices with selectively doped Si NWs have been demonstrated [1, 2]. Out of them, axial p n and p n i junction in Si NWs have shown the potential to be used as solar cells. However, axial p n junctions in NWs grown by the vapor–liquid–solid (VLS) technique have mostly been fabricated by purely in-situ doping [3, 5, 6]. It has been observed that a pure in-situ doping to fabricate an axial junction may result in unwanted lateral doping  due to unavoidable dopant incorporations through the NW sidewalls by vaporsolid (VS) growth. On the other hand, ion implantation  which is the most widely used doping technique in very large scale integration (VLSI) fabrication can form well-confined doped regions when appropriately used with masking. Ion implantation has been used to fabricate the doped source and drain contacts  as well as the channel  in Si NW-based field effect transistors (FETs). But one of the principal reasons for not extensively using ion implantation to fabricate axial junctions in vertical NWs is possible irrecoverable implantation damages  that were observed in other low dimensional structures such as a FinFET . However, we have shown  that by choosing appropriate ion doses and energies, it is possible to uniformly dope vertical Si NWs of diameter in the range of 100 nm without leaving any residual structural defects in them. Separately, we have also demonstrated in-situ doping of molecular beam epitaxy (MBE)-grown Si NWs . S. Hoffmann et al.  have realized a p n junction in a Si NW purely by ion implantation. However, co-diffusion of acceptors and donors during annealing after such dual implantations of different ions (boron and phosphorus) often lead to the formation of acceptor–donor complexes [15, 16] that can anomalously increase the solubility of the donors in the acceptor-rich segments, thus affecting the p– and n– profiles.
In this paper, we demonstrate a novel approach to form an axial p–n junction in a Si NW by combining the above-mentioned ex-situ and in-situ doping techniques. First, we do a modulated in-situ doping with boron by homogeneously doping the lower half of the NW to make it p-type. The upper half of the NW is kept intrinsic (i-type) by simply switching off the boron source. This intrinsic upper half is subsequently converted to n-type by implanting it with phosphorus. We present the details of the fabrication process, the expected dopant profiles in the NW, and electrical characterization of individual NWp–n diodes and explain their typical current–voltage (I–V) curves.
Basics of the growth process including the mechanism of Si NW growth by MBE using Au seeds have already been reported earlier . The NWs were grown on 5″ p-type (boron doped, 5–10 Ω-cm) Si <111> wafers. A B-doped (B concentration ~1018 cm−3) Si buffer layer was grown first on a RCA-cleaned wafer at 525°C in order to provide a clean surface for NW growth and increase the density of the NWs. Afterward, a 1–2-nm thick Au film was deposited in-situ at the same temperature. The Au film subsequently broke into Au droplets to serve as the NW growth initiator . Immediately after this step, Si and B were co-evaporated for 45 min (B concentration ~1018 cm−3). At 45 min, the B source was switched off, while the Si source was kept on 45 min longer. Such a recipe should result in B-doped—intrinsic (p i) type NWs, since the boron diffusion in silicon is negligible at 525°C , i.e., the B atoms incorporated in the lower half will not diffuse into the upper half of the NW. Immamura et al.  verified this with Raman measurements on NWs grown by chemical vapor deposition (CVD) following a similar recipe as ours.
Results and Discussions
Details containing the dimensions, the measured ON/OFF current ratios, and ideality factors of the NWs whose I–V curves are shown in Fig. 3a
ON/OFF current ratio
Ideality factor (n)
where I S is the saturation current, V the applied voltage, k B the Boltzmann constant, T the temperature, and n the ideality factor of the diode.
The values of n for the three p–n junction NWs were 2.0, 1.8, and 1.7, respectively (see Table 1). Sah et al.  have found that the ideality factor of a p n diode can vary from 1 to 4 (or even higher in special cases) depending on what kind of current conduction mechanism is dominating. A value close to 2 for the ideality factor indicates that recombination across the p n junction through the surface states is dominant in the carrier transport mechanism. Our p n junction NWs are always covered with a 2–3 nm thick native silicon oxide with an estimated surface state density of 1.1 × 1010 cm−2. These surface states are in direct contact with the p n junction. Therefore we think that surface recombination is indeed playing a major role in the current conduction mechanism across the p n junction resulting in the extracted ideality factors close to 2.
In conclusion, we have demonstrated a novel method to form p–n junction NW diodes by combining two well-established doping techniques—in-situ doping and ion implantation, in succession. The measured NWs showed excellent diode characteristics with a high ON/OFF ratio. The ideality factors of the p–n junctions were close to 2 which points to significant carrier recombinations through the surface states.
The authors thank Mr. A. Frommfeld, Mr. K. U. Assmann, Ms. S. Hopfe, and Ms. C. Muenx for technical support. The authors acknowledge the financial support from the FP6 EU project ‘Nanowire based One Dimensional Electronics’ (NODE).
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