An integrated circuit device includes a lower electrode, an upper electrode, and a dielectric layer structure between the lower electrode and the upper electrode, the dielectric layer structure including a first surface facing the lower electrode and a second surface facing the upper electrode. The dielectric layer structure includes a first dielectric layer including a first dielectric material and a plurality of grains extending from the first surface to the second surface and a second dielectric layer including a second dielectric material and surrounding a portion of a sidewall of each of the plurality of grains of the first dielectric layer in a level lower than the second surface. The second dielectric material includes a material having bandgap energy which is higher than bandgap energy of the first dielectric material.