Can't figure out what the machine names of widget types are

So I'm working on developing module and one of the things the module does is create a field in certain content types. This field is a boolean (type: list_boolean) but what I'm trying to do is to get it so the default widget is the single radio button rather than the list radio button. Here is my code as of now:

<?PHP

function mcc_notify_enable() {

// Check if our field is not already created.
  if (!field_info_field('notify_emailb')) {
    $field = array(
        'field_name' => 'notify_emailb', 
        'type' => 'list_boolean', 
        'settings' => 
        array(
        'label' => 'Notify opt-ins',
        'description' => 'blah blah',
                
        )
        ,
    );
    field_create_field($field);
    

// Create the instance on the bundle.
    $instance = array(
        'field_name' => 'notify_emailb', 
        'entity_type' => 'node', 
        'label' => 'Notify opt-ins', 
        'description' => 'blah blah',
        // If you don't set the "required" property then the field wont be required by default.
        'required' => TRUE,
        'bundle' => 'article',
        'widget' => array(
            'type' => 'list_boolean',
        ), 
    );
    field_create_instance($instance);
  }

  
}

?>

I know that there is a 'default widget' property in the field_create_field function. However, I don't know the machine name of the widget I want. Where would I go to find that information? I've searched the docs and even searched the database but came up with nothing. What is the machine name for field type boolean, widget type "Single on/off checkbox"? You'd think this information would be easy to find.