Unless your settlement says otherwise, there is nothing in the law that says you cannot go back to work for them, so you may certainly fill out an application.
However, you need to be aware that there is also nothing in the law that says they have to hire you back. You can fill out the application; they can say no.
Please note that I am not saying they WILL refuse to hire you back; just that they are not required to do so.
NOTE: In some states, an employer is required to give preference in hiring to a former employee who left because of a workers comp issue. I do not know if California is one of them. This assumes that the employee is qualified for the position s/he is applying for.