A side barrier is provided between columnar dots each constituted by directly stacking respective quantum dots in seven or more layers. Out of respective side barrier layers composing the side barrier, each of the lower side barrier layers (four layers of the undermost layer to the fourth layer from the bottom) is formed as a first side barrier layer into which a tensile strain is introduced, and each of the upper side barrier layers (three layers of the fifth layer to the uppermost layer from the bottom) is formed as a second side barrier layer which has no strain.