Introduction
Vanadium Oxide
(VO_{2}), which exhibits Metal Insulator transition (MIT) , is a
suitable material for making novel ultra low power switches. The MIT can be triggered
by thermal or electrical stimulus. In case of thermal triggering, at a critical
temperature of 340K, the VO_{2} samples undergo a metal insulator
transition where the resistance changes by 3 to 5 orders of magnitude depending
on if the VO_{2} sample is thin film or bulk. The thermal transition is
accompanied by phase transition from monoclinic to tetragonal. In the interest
of making low power high speed switches, the triggering needs to be electronic.
According to Mott`s theory of MIT a critical carrier density inside the VO_{2}
film will break the coulomb blockade and the sample would transition from
insulator to metal. Theoretically this transition would not cause a phase
transition of the material. In our research, funded by SRC and NRI, we are
exploring the possibility of electronically switching the VO_{2}
sample. We have prepared two/threeterminal devices of few hundred micrometers to nanometers.
Experiments show the samples undergo a semiconductor to metal transition at
certain critical voltage. For electronic switching, the contact, made of AuTi,
play a critical role. We have characterized the contacts to be ohmic based on
the work functions as well as experimental data. Electronic switching, which
inevitably results in some joule heating, might cause structural phase
transition as the critical temperature is reached. To account for this
phenomenon, we have developed an analytical thermal model of the VO_{2}
switch. This model can correlate the coupling effect between electronic and
thermal switching. Analysis and Results
Fig : Fabricated two terminal devices.
Characterizing the contactsThe electrical behavior of the contact depends on the relative workfunctions of VO2 and the contact material adjacent to the film.The contacts are made of AuTi alloy and Ti is adjacent to VO2. In order to make an
ohmic contact, the work function of metal contact must be less than that of VO_{2.}
Since the work function of Ti is 4.33 eV and that of VO_{2} is ~5.15 eV
(T < 340K) and 5.30 eV (T > 340 K), the contact is ohmic. Fig: Energy Band diagram of (a) isolated contact and VO2 (b) merged contact and VO2 Fig : (a) Contact resistivity (b) VO2 resistivity.
The ratio of contact resistivity
is around 75 across the transition temperature. The width dependence of contact
resistivity indicates crowding of current at the contact edges. The VO_{2} channel
resistivity changes 1250 times across the critical
temperature.
Thermal response during electronic switching
The VO2 devices are switched by applying short electrical pulse sweeps. As the voltage exceeds a critical value, the semiconductor to metal transition occurs. In order to decouple the electronic and thermal switching phenomenons, we need a model to predict the temperature profile of the VO2 film.
Fig: First order thermal circuit model.
The power flow and the temperature rise, with respect to the ambient, can be expressed in terms of the thermal resistor (Rth) and capacitor (Cth) using a first order thermal circuit model. The governing equations are,
The width of the switching pulses are varied and the results are tabulated below.
The first order thermal model, which assumes heat is absorbed by the VO 2 film, can be used to calculate the equivalent thermal capacitance and from this capacitance the equivalent volume is estimated. These results indicate temperature rise above the critical temperature. Including contacts in the thermal modelThe contacts , which are made of AuTi alloy absorb heat and slowly dissipate portion of it to the ambient. A detail thermal model includes the contacts.. Fig: Heat flow from VO2 film. The heat equation can be expressed as [1],
Simplifying using the thermal resistor and capacitors[1],
If the heat is
represented as a current source then above equation resembles Kirchhoff’s Current Law
from analogy. Thermal
capacitance describes the heat absorbing capability of a material, while
electrical capacitance describes the ability of accumulating electrical charges
of a material.The equation states that the heat flowing through the thermal
capacitance (the AC component)
plus the heat flowing through the thermal resistance (the DC component) equals
the total heat flowing through the material.
. Fig: Equivalent thermal circuit model of the two terminal device. The substrate plays a critical role in predicting the time dependent temperature profile of the VO2 film. The assumption of a perfectly heat absorbing substrate, which is based on the fact that the substrate is large compared to the device, results in the temperature rise of the VO2 film much less than the critical temperature. On the other hand if the substrate is assumed not be a good heat absorber, the temperature rise of the film would be higher than the critical temperature. The later result is similar to that of the first order approximation.
References
[1]
Wei
Huang; Ghosh, S.; Velusamy, S.; Sankaranarayanan, K.; Skadron, K.; Stan, M.R.;
, "HotSpot: a compact thermal modeling methodology for earlystage VLSI
design," Very Large Scale
Integration (VLSI) Systems, IEEE Transactions on , vol.14, no.5,
pp.501513, May 2006.
Collaborators Material Science and Engineering Department, UVa.
UVa NanoStar and ViNC (Virginia Nanoelectronics Consortium).
Members Mircea Stan (ECE) (PI)
Robert Weikle (ECE)
Jiwei Lu (MSE) Mehdi Sadi (ECE) S Kittiwatanakul (MSE)
B Percy (ECE)
Funding SRC and NRI
