The Library of Congress subject heading for systematic theology is Theology, Doctrinal. Using this you will find a much broader array of materials than searching solely for the phrase "systematic theology."
The Library of Congress section for Doctrinal Theology is BT. Subdivisions of this field are listed below:
BT 10-1480 Doctrinal Theology
Subject headings may begin with
CATHOLIC CHURCH - DOCTRINES OR THEOLOGY, DOCTRINAL
Either of the subject headings may be refined by adding
HISTORY - EARLY CHURCH
You can also narrow your search by adding a keyword like a person or theological cocnetp, (e.g., RAHNER) or (e.g. HOLY SPIRIT)