Dear colleagues,
I believe that the impurities/composition inhomogeneities and structural defects which may occur in the deposited thin-film materials (both metals and insulators/oxides) also play a huge role in both the performance and long-term reliability of the device along with the interface morphology between the metal and insulator since they are associated with energy levels in the forbidden gap.
How one can control the influence of the microstructure and defects of thin-film materials (metals Au, Al, Cr and Ti, and oxides Al2O3, Cr2O3 and TiO2 were deposited by e-beam evaporation system) used in the rectenna? What special technique modes can be applied for depositing a thin film of material by e-beam deposition & thermal evaporation onto a GaAs substrate or onto previously deposited layers in order to control the aforementioned parameters?
I apologize for taking your valuable time.
Best regards and thanks in advance,
Konstantin.
Hi,
I had same issue and I tired this for 3 years, but both the electron beam and thermal evaporator both cannot be controlled for ultra-thin films and its interfaces required for devices such as MIM Tunnel Diodes for Rectenna and MTJs or Spin-Valves.
Out of both the processes, I found that E-Beam Evaporation is better with very low deposition rates, if you have in-situ crystal that monitors the rate of deposition, the interfaces and the average roughness will be better(but not as required) at very low deposition rates, as low as 10A0 or 1nm or less.
I also tried to use substrate heating to improve the crystallinity, uniformity and the quality of the interfaces, but it did not help me much.
Ideal way of making the MIM tunnel diodes would be ALD, where the metal and oxide/insulator layers can be deposited layer by layer.
Dear colleague Konstantin Gochua,
Have a nice time and good day.
To control the micro structure and defects of thin-film materials; there are many parameters you can check according to your available instruments.
1- Deposition rate, You can use a low deposition rate.
2- Deposition temperature; use different substrate temperature.
3- Annealing, use different annealing temperature.
To get a good results by using 2 and 3 try to make DTA or DSC to your sample to get the glass transition temperature, Tg , crystallization temperature, Tc and the melting point, Tm. The values of the mentioned parameters; Tg, Tc and Tm give you the temperature range that the you can use through the substrate temperature and different annealing temperature to get effective results.
I wish this is useful and help you.
Yours
A. El-Denglawey
Hi,
E-Beam and Thermal Evaporation are used for thicker films. For the devices like MIM Tunnel diodes and MTJs where you need ultra thin layers in the order of 1nm-2nm thick, the average RMS roughness that you can measure by AFM will come around 15nm-35nm for the films deposited by any PVD methods, even after annealing. I agree that annealing will make the film crystalline and dense, but it wont reduce the surface roughness as the grain size of the metals you deposit with PVD is larger.
On top of such rough surfaces if you deposit 1nm-2nm of the tunnel oxide which needs to be conformal, otherwise you'll end up shorting the two metal electrodes and your IV will be resistive.
Hence, to get a perfect MIM tunnel diodes, you must use ALD, where the deposition is done layer by layer and the surface roughness which depends on the grain size will be in the range of the atomic dimensions.
For more detail analysis on this, about the surface roughness and the IV characteristics, please refer to my papers on MIM Tunnel Diodes for Rectenna applications.
Dear colleagues,
Thank you for your prompt reply.
It is known that, the MIIM architecture not only allows insulators dominated by emission based conduction to be used in temperature insensitive diodes, but actually takes advantage of defect conduction to improve asymmetry through “defect enhanced direct tunneling (DEDT)’’. Hence, it is found that diode performance is enhanced by a mechanism termed DEDT [Alimardani, N. & Conley, J. F. ].
However, on the other hand, the defect sites are regions where leakage currents can pass between the electrodes and can therefore have detrimental effects on the I-V characteristics of the device.
What kind of trade-off can be implied in this case in order to obtain an optimal value of the diode performance?
Best regards and thanks in advance,
Konstantin.
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