In-situ electrical and structural characterization of individual GaAs nanowires

A method for probing the electrical and structural characteristics of individual as-grown III-V nanowires was studied. In-situ electrical characterization was performed in a focused ion beam / scanning electron microscopy system by using a fine nano-manipulator and ion beam assisted deposition. Transmission electron microscopy specimens of probed nanowires are prepared afterwards. This method would potentially allow the correlation of electrical and structural characteristics (e.g. crystal faults such as twinning) of the nanowire-substrate system. The challenge is in contacting the nanowires so that the electrical characteristics of the nanowire-substrate system can be extracted correctly.


Introduction
Several interesting optoelectronic applications are envisaged for III-V nanowires (NWs), amongst others for solar-cells and light emitting diodes [1]. To move towards devices, precise, homogenous positioning of high quality NWs on low-cost substrates is desired. This requires optimization of both the positioned growth, and the NW crystal structure. To use the growth substrate as an electrode for devices, it is also necessary to optimize the electrical properties of the combined NW-substrate system.
There exist several methods to synthesize NWs, but the most commonly used are based on the vapour-liquid-solid mechanism [2]. Here, a liquid catalyst droplet adsorbs precursor vapour, and facilitates the growth of a solid, crystalline NW. If grown on an appropriate substrate, the crystal growth will nucleate in such a manner that epitaxial growth is achieved. For GaAs NWs, as studied here, common catalysts include Au and Ga, the latter of which is termed self-catalyzed growth. For reproducibility and implementation into devices, positioned growth is required, by either controlling where the catalyst droplets are, or where the crystal can nucleate. Specifically, one way of doing this is by patterning a silicon oxide mask on the substrate [3]. The Ga droplets only form in the holes of the oxide mask, and thereby a position controlled NW growth is achieved.
In general, to measure the electrical properties of a system, it is important to be able to achieve good ohmic contacts. Particularly for GaAs, making ohmic contacts is challenging due to its high level of surface states, requiring proper pre-surface treatment and choice of best contact materials [4].
Here we studied a method to measure the electrical properties of as-grown GaAs NWs in a focused ion beam (FIB) system, and to subsequently characterize the same as-grown NW by (scanning) transmission electron microscopy ((S)TEM) for different NW-substrate systems. This should allow for the correlation of electrical and structural data of individual NWs, and to verify any effects of e.g. crystal defects, on device performance [5].

Materials and methods
The NWs used in this study were grown using the self-catalysed growth technique in a solid source Varian Gen II Modular molecular beam epitaxy system. The substrates used in this study were p-type Si(111) and Kish graphite. In total, four GaAs NW samples were studied. In sample A, intrinsic GaAs NWs were grown on a Si substrate for a duration of 25 min with an As flux of 5.6×10 -6 Torr, and at a substrate temperature of 640 °C. The nominal planar growth rate was 2 Å/s. Sample B was grown with the same growth conditions as sample A, but the NWs were Be-doped (p-doped). The temperature of the Be effusion cell was set to yield a nominal Be concentration of 3.5×10 18 cm -3 . Sample C was grown as sample B, but on a patterned Si substrate ( figure 1(a)). The substrate pattern was prepared by depositing a thin layer of SiO 2 (thickness < 60 nm) on the substrate by plasma-enhanced chemical vapour deposition method. Nano-hole array patterning (hole diameter < 200 nm) was done using standard e-beam lithography. Sample D had Be-doped GaAs NWs grown on Kish graphite, by a two-temperature technique [6].
After growth, the samples were mounted on aluminium scanning electron microscopy (SEM) stubs with conductive silver paint. The NWs were characterized in a FEI DualBeam FIB/SEM system (Helios NanoLab 600), equipped with an Omniprobe tungsten (W) nanomanipulator (AutoProbe 200), hereafter called W-probe, and a gas injection system with C-, Pt-and W-sources. The FIB has a Ga source and was operated at 30 kV. Electrical measurements were performed by connecting a Keithley 2636A source-measurement unit to the sample stage (ground) and W-probe. Before each electrical measurement, the W-probe was cleaned by ion sputtering to remove any contaminants and oxides that might interfere with the measurement.
TEM specimen preparation of as-grown NWs was performed in-situ in the same FIB/SEM. The as-grown NWs were extracted using the W-probe as a lift-out finger, before the specimen was thinned to electron transparency by ion sputtering. The final sputtering was done with a 5 keV ion beam. Before extraction, the NW was surrounded by carbon deposited by electron beam assisted deposition (EBAD) at 10 kV, to protect the area of interest against ion beam damage. The TEM characterization was performed on a JEOL 2010F operated at 200kV.

Results and discussion
To measure the electrical characteristics of the as-grown NW and substrate, current-voltage (I-V) measurements were performed, using the substrate as one electrode, and the W-probe as the other (figure 1). Different approaches to contact the Be-doped NW samples were studied. Initially, the W-probe was simply put in contact with the NW or the Ga droplet on the top, but such a contact was mechanically unstable due to drift and vibrations, leading to fluctuating and irreproducible results. The measurements also indicated a diode like behaviour of the system. A local deposition of W or Pt on the contact area between the probe and the NW by EBAD (a W or Pt weld) gave sufficient mechanical stability, but the signal was again diode-like. As EBAD deposited materials can have high carbon incorporation (from the precursor gas), ion beam assisted deposition (IBAD) was considered as a way to improve the contact. By attaching the W-probe to the NW tip using a local deposition of W by IBAD, (semi-) linear I-V characteristics could reliably be obtained (see figure 2(a)), which implies ohmic contacts. However, while the contact is ohmic, the system still has a significant resistance (~20-30 M ). A control measurement was performed in which intrinsic NWs were used (sample A). Intrinsic GaAs wires have a much higher resistivity (~10 G by two-probe measurements [6]), which should give a very low current. However, these control measurements consistently showed similar behaviour as the doped NWs (compare figure 2(a) and (b)). This indicates that the electrical characteristics do not originate from the NW itself. Most likely, W has been deposited along the length of the NW by deposition overspray, which is caused by the wide tails of the ion-beam profile. Consequently, the deposition method would need to be optimized with regards to spatial precision.