annealing in air/oxygen will not help to obtain a semiconducting film

1) annealing in vacuum will have some effect.

2) But annealing in a reducing atmosphere will greatly help (N2, N2+H2, cracked ammonia.  Obviously annealing in H2 atmosphere is not advisable in laboratories.

One can get away from the annealing route very conveniently:

But Before answering your problem, and give an appropriate direction, please let me know the following. This will help me in directing you correctly.

With your rf magnetron sputtering technique:

1) what kind of a target are you using - (ZnO oxide target) ?

   or a metallic (Zinc) target, and then doing reactive sputtering in (Ar+O2). 

K. Sreenivas

Thank you for your support, I am sputtering from a stoichiometric ZnO target, using a system with no substrate heating, and only a pure Ar atmosphere.

To futrther clarify, my samples use a two stage annealing process. The ZnO  is deposited on silicon, and then annealed in either vacuum or atmosphere. Then I depsoit my insulator by RF sputtering, which requires annealing in atmosphere at a minimum of 550OC.

I have tried annealing the ZnO in vacuum, and later in atmosphere after BTO has been deposited, however exposed ZnO shows the high ressitance/non-contact behavious I mentioned earlier.

David

Dear David,

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In your previous reply what is BTO, and how is it coming into the picture.

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By the way

1) If I understand you correctly you are trying to make a simple MOS device structure.

and for the three letters in the abbreviation "MOS"

2) For "S" you want the ZnO as a semiconductor

3) For "O" you are trying to deposit another insulator.

4) and for "M" you are trying to put Nickel - Titanium electrode contacts.

Am I correct in the above assumption. Pl. correct me.

K. Sreenivas

BTO is Bismuth Titanate Oxide, It is the insulative ferroelectric material that I am working with. For the MOS device I have used Titanium contacts on the metal and on the semiconductor.

Dear Daivd,

Now I understand your device structure.

1) You are making either an MFS device useful for memory applications.

2) and by combining BTO and ZnO you are possibly aiming towards the fabrication of an optically modulated spatial light modulator.

Both these ideas have been there in literature since a long time, and somehow due to the technical challenges, researchers have not been able to combine two appropriate and convenient materials to see the desired effects.

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a) Interfacing two oxide films (one of which is a semiconductor-ZnO), and the other a typical insulator ( cum ferroelectric -BTO)

Both these layers demand conflicting and opposite processing requirements, when one is grown over the other, and to handle this is quite challenging.

You must develop a smart method.

Because to be a good semiconductor, ZnO needs an O2 reduced environment, or other reducing environment (e.g., vacuum, H2, N2 , N2+H2, cracked ammonia).

whereas BTO needs an oxygen rich environment to form into a good insulating film.

b) finally you should have appropriate ohmic contacts at top and bottom.

    A good ohmic contact on top of BTO, and equally 

    a good ohmic contact underneath the ZnO film.

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c) The way to tackle this problem can be as follows;

i) You should deposit simply ZnO films first, and understand their semiconducting properties well, by doing Hall measurements on a simple non reacting substrate ( say glass/silicon).

  As you have an oxide (ZnO) target, try to control your oxygen partial pressure carefully, and obtain a transparent conduclting film. There is a lot of literature on this aspect, and ho to achieve a semiconducting film.

But remember you may be able to achieve the desired carrier concentration (n), and n-type behavior, but your mobility will be quite low.

So what we are saying is it will be a real challenge to get properties close to that of a doped silicon substrate.

ii) Once you know how to make a good zno simiconducting film, try to grow this semiconducting film on a suitable metal layer. This will be your bottom contact.

    - BY the wya why don't you develop a semiconducting ZnO layer on highly conduting ITO coated substrate. This will give you a substrate on which you have a transparent electrode, through which light can be shined.

iii)  Now when you grow BTO layer on top of your ZnO. Please remember it should be a very quick process. Because you will need an oxygen rich atompsphere to get a good BTO, and if you take a lot of time to deposit BTO, then you will allow oxygen diffusion into the underlying ZnO and disturb its semiconducting properties.

    One possible solution is not to use rf magnetron sputtering for depositing BTO.

    See if anyone has a PLD (pulsed layer deposition unit). Make a BTO target, and flash a thin layer of BTO quickly.

  Now if the BTO needs to be ferroelectric, it must be crystalline (or polycrystalline), then you need some insitu temperaure during BTO deposition by PLD.

Hopefully a thin layer of BTO ( which is a good insulator) can suffice. and PLD will be a good option.

In this way you can develop the desired structure.

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Don't go to high temperatures while growing BTO over ZnO, this will disturb your underlying ZnO.

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Alternately think of tackling the high temperature grown BTO (ferroelectric and insulator) first, and then deposit your ZnO (semiconducting) layer which does not need high temperatures.

These are some of the thoughts I had which you may find useful.

K. Sreenivas

 Dear Professor Sreenivas,

Thank you very much for your input. I have done some additional reviewing of litriture which concludes what you say about the non-compatibility of these two methods. I am goign to start looking for another route to take my work down, and start looking for a semiconductor that is compatible with the fabrication ethods of BTO. Thank you again very much for your support, It is highly appreciated.

Best wishes

David