Near infrared intersubband transitions in delta-doped InAs/AlSb multi-quantum wells

Intersubband transitions (ISBTs) in Si doped narrow InAs/AlSb multiple quantum wells (MQWs) were investigated for well widths, $d$, ranging from 5 nm down to 1.8 nm with 10, 20 or 60 periods. As the well width decreased, the ISBT signal of the MQWs decreased. However, it persisted down to $d$ = 2.1 nm with a sheet doping density in each quantum well of 9 x 10$^{12}$ cm$^{-2}$ and 60 periods. The ISBT signal observed for $d$ = 2.1 nm was peaked at an energy of 670 meV at 77K. A large linewidth increase was also observed for the narrowest wells ($d \quad \le $ 3 nm). In order to study the origin of the linewidth broadening, we measured the electron mobility of the samples and found that the mobility showed $d^{6}$ dependence for $d \quad \le $ 3 nm. We show that the linewidth broadening is well explained by the reduction in the mean free time as the well width decreased.