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Mott Transition Switch

Introduction

Vanadium Oxide (VO2), 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 VO2 samples undergo a metal insulator transition where the resistance changes by 3 to 5 orders of magnitude depending on if the VO2 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 VO2 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 VO2 sample. We have prepared two/three-terminal 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 Au-Ti, 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 VO2 switch. This model can correlate the coupling effect between electronic and thermal switching.

Analysis and Results



Fig : Fabricated two terminal devices.
Characterizing the contacts
The electrical behavior of the contact depends on the relative work-functions of VO2 and the contact material adjacent to the film.The contacts are made of Au-Ti 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 VO2. Since the work function of Ti is 4.33 eV and that of VO2 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 VO2 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 VO2 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 model

The contacts , which are made of Au-Ti 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 early-stage VLSI design," Very Large Scale Integration (VLSI) Systems, IEEE Transactions on , vol.14, no.5, pp.501-513, May 2006.



Collaborators

Material Science and Engineering Department, UVa.

UVa NanoStar  and ViNC (Virginia Nano-electronics 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




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