Checkbox checked by default on PHP Array

I have this array:

     $this->fields['menu']['fields']['_personalizado'] = array(

            'type'        => 'checkbox',

            'required'    => false,

            'placeholder' =>"Check this box if you dont't wnat to recieve customized job offers",
            'tooltip'     => __("Check this box if you dont't wnat to recieve customized job offers", 'listeo_core'),

            'name'        => '_personalizado',

            'label'       => "Check this box if you dont't wnat to recieve customized job offers",

            'placeholder' => '',

            'class'       => '',

            'options'   => array(

                'hide' => __("Check this box if you dont't wnat to recieve customized job offers", 'listeo_core' )

            ),
    );
}

I need to have this checkbox checked by default. I tried using 'checked' => '', 'checked' => 'checked', 'default' => true but nothing works.

I can't find the html code to add the "checked" attribute to it. It is simply created from this array.

Hope you can help me!