Methods of forming a semiconductor device may include forming a tunnel oxide layer on a semiconductor substrate, forming a gate structure on the tunnel oxide layer, forming a leakage barrier oxide, and forming an insulating spacer. More particularly, the tunnel oxide layer may be between the gate structure and the substrate, and the gate structure may include a first gate electrode on the tunnel oxide layer, an inter-gate dielectric on the first gate electrode, and a second gate electrode on the inter-gate dielectric with the inter-gate dielectric between the first and second gate electrodes. The leakage barrier oxide may be formed on sidewalls of the second gate electrode. The insulating spacer may be formed on the leakage barrier oxide with the leakage barrier oxide between the insulating spacer and the sidewalls of the second gate electrode. In addition, the insulating spacer and the leakage barrier oxide may include different materials. Related structures are also discussed.