Optimization of the Number of Stacks in the Submonolayer Quantum Dot Heterostructure for Infrared Photodetectors

We studied the optical, electrical, and spectral properties of InAs submonolayer quantum dot infrared photodetectors with different number of stacks. Three samples with 4, 6, and 8 dot stacks were grown by molecular beam epitaxy under identical conditions. Increasing the number of stacks results in a gradual shift in the photoluminescence ground-state transition energy of the samples from 1.195 to 1.111 eV. Cross-sectional transmission electron microscopy images confirm increase in dot size with increasing number of stacks from 4 to 8. Samples with 4 and 6 stacks measured moderately uniform dot size distribution and with further increasing the number of stacks 4 to 8 variations in dot sizes along with improper dot size formation were observed. The activation energy of the samples was measured by both optical and electrical methods increase with increasing number of dots. All photodetectors exhibit a photocurrent peak in the range of 7.3-7.8 mu m at 77 K at an applied bias of -1 V. Highest peak responsivity value of 0.04523 A/W at 77 K was observed from the 6 stacked sample, which was highest among the three samples. It also exhibited highest detectivity of 5E9 Jones with lowest noise current density among the others. The sample with 6 dot stacks is the best as it exhibited lowest dark current density of 6.1 (10(-7) A/cm(2) and highest operating temperature of 110 K).