The U.S. Supreme Court held that due process of law required appointment of counsel for young, inexperienced, illiterate, and indigent defendants in capital cases in:
Powell v. Alabama (1932)
This is the landmark case that established the right to counsel in capital cases when defendants are unable to adequately defend themselves due to various factors.
The other cases you listed address the right to counsel:
Johnson v. Zerbst (1938) dealt with the right to counsel for indigent defendants in federal felony cases.
Gideon v. Wainwright (1963) extended the right to counsel to all indigent defendants facing felony charges in state courts.
Mempa v. Rhay (1967) is not directly related to the right to counsel.
