Comparison of nickel silicide and aluminium ohmic contact metallizations for low-temperature quantum transport measurements
© Polley et al; licensee Springer. 2011
Received: 17 May 2011
Accepted: 3 October 2011
Published: 3 October 2011
We examine nickel silicide as a viable ohmic contact metallization for low-temperature, low-magnetic-field transport measurements of atomic-scale devices in silicon. In particular, we compare a nickel silicide metallization with aluminium, a common ohmic contact for silicon devices. Nickel silicide can be formed at the low temperatures (<400°C) required for maintaining atomic precision placement in donor-based devices, and it avoids the complications found with aluminium contacts which become superconducting at cryogenic measurement temperatures. Importantly, we show that the use of nickel silicide as an ohmic contact at low temperatures does not affect the thermal equilibration of carriers nor contribute to hysteresis in a magnetic field.
Aluminium has proven to be a versatile ohmic contact metallization, and for a time was the preferred choice for silicon integrated circuits . Aluminium has also been a common contact metallization for a variety of material systems such as gallium nitride , silicon carbide  and zinc oxide . Owing to this versatility, aluminium has seen continued use in silicon-based research, including recent quantum dot devices for the study of quantum transport in silicon towards the goal of solid-state quantum computation [5, 6].
However the characterization of such devices typically requires millikelvin temperatures, well below the normal-superconductor transition temperature of aluminium, T c = 1.175 K . Below this temperature, the aluminium contacts form a Bardeen-Cooper-Schrieffer (BCS) energy gap which manifests as an increased contact resistance near B = 0. The contact resistance increases exponentially as the temperature is reduced, with important ramifications for studies at very low temperatures and small magnetic fields. Such studies include the measurement of electron-nuclear interactions and dephasing times [8, 9], which are of critical importance for development in quantum computation [10–12]. Despite its versatility, aluminium is not an optimal metallization for low-temperature quantum transport measurements. As a result, it is important to consider alternative metallizations which do not undergo a superconducting transition at low temperatures.
In this article we examine nickel silicide (Ni x Si y ) as an alternative ohmic contact metallization to silicon for use at cryogenic temperatures. NiSi has already been integrated into current CMOS processes because of its low sheet resistivity and ability to form at narrow linewidths . It does not superconduct at any temperature and has recently been used in low-temperature transport measurements of a silicon nanowire quantum dot . In addition, the silicide has the attractive property that it can be formed at low-temperatures, with nickel rich phases (e.g. Ni2Si) forming at temperatures below 350°C . This property is crucial for the fabrication of atomic-precision donor-based devices where the aim is to measure transport through atomically positioned single dopants . This imposes a low thermal budget to prevent diffusion of the dopants. In this article we directly compare the electrical transport properties of aluminium and nickel silicide ohmic contacts to saturation dosed δ-layers of phosphorus in silicon. These δ-layers are fabricated using identical processes to atomic-scale devices patterned by scanning-tunnelling lithography . We find that nickel silicide ohmic contacts eliminate the zero-field resistance peak observed in aluminium contacts and do not introduce additional hysteresis in a magnetic field.
The devices were fabricated on a 1-10 Ωcm n-type Si(100) substrate, annealed to 1100°C in UHV by direct current heating to produce a 2 × 1 surface reconstruction. The surface was then δ-doped by saturation dosing with 1.1 Langmuir of PH3 gas at room temperature, followed by a 350°C anneal to incorporate the phosphorus into the silicon lattice . After encapsulating with 30 nm of epitaxial silicon, the sample was removed from UHV to be processed into Hall bar structures. This process is known to result in 2D carrier densities of ≈ 2 × 1014 cm-2 with dopant segregation confined to approximately 0.6 nm .
Electron-beam lithography and reactive ion etching were used to define the Hall bar mesas and ohmic contacts. A buffered hydrofluoric acid etch was used to remove the native oxide before the samples were loaded into a high vacuum (4 × 10-6 mbar) thermal evaporator. For the aluminium Hall bars, 80 nm of Al was evaporated followed by a 30-min anneal at ≈350°C in dry N2. The nickel silicide Hall bars received 60 nm of Ni with a 10-nm Ti capping layer to prevent oxidation . The sample was then annealed to 350°C in N2 for 30 min to yield the NiSi phase . The unreacted nickel and titanium were removed with a sulphuric acid- hydrogen peroxide etch before Ti/Au (10/60 nm) bond pads were patterned. The Ti/Au bilayer was required for successful ultrasonic gold-ball bonding, and while bulk titanium also has a superconducting transition at approximately 400 mK  it is known that in thin film superconductor-normal bilayers superconductivity is strongly suppressed [23, 24].
Initial magnetotransport characterization of these samples performed at 4.2 K revealed that both samples had carrier densities of (1.4 ± 0.1) × 1014 cm-2. Subsequent millikelvin temperature measurements were performed in a dilution refrigerator that allowed simultaneous measurement of both samples with perpendicular fields up to 8 T. Magnetotransport measurements were performed using standard low-frequency lock-in techniques with a 5 nA constant current.
Whilst pure nickel is ferromagnetic, previous theoretical study has concluded that transition metal silicides including NiSi are diamagnetic . However previous experimental results have indicated ambiguity in the magnetic properties of NiSi for fields below 200 mT at low temperatures . It is therefore important to determine whether the nickel silicide contacts used here have any influence on the measured magnetic field hysteresis.
We have compared the low-temperature magnetotransport properties of highly doped Si:P δ-layers with both nickel silicide and aluminium ohmic contacts. We have shown that a nickel silicide contact is comparable to aluminium, with the added advantage that nickel silicide does not transition to a superconducting state at low-temperatures (T < 200 mK). This eliminates the contact resistance peak around B = 0 observed with superconducting aluminium contacts, important for measurements of electron-nuclear interactions and de-phasing times. In addition, we have shown that nickel silicide contacts neither alter the thermal equilibration of carriers nor contribute to hysteresis in a varying magnetic field.
MYS acknowledges an Australian Government Federation Fellowship. WRC acknowledges funding from the Australian Research Council in the form of an Australian Post-Doctoral Fellowship.
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