As mentioned this is not possible.
however, I would be wary of trying to overcome a standard functionality "deficiency " like this by implementing an island. If you take this path - all your custom applications will behave differently from the SAP standard ones - which is not really an ideal situation.
Instead, add your concern to the sticky at the top of this forum - the functionality to "clear" a field (or select all on field on entry which is what is happening with the tab selection) might be added in future releases.
I think however, that you'll find that the "select all on tab" behaviour is an browser "functionality" rather than anything specifically that SAP has implemented - if you try using a different browser, you may not find the same thing happening (although with the browsers I've checked it does).
The same thing happens with any HTML forms - just try yourself. e.g.[external site with details on HTML forms and input fields|http://www.w3schools.com/html/html_forms.asp]
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