In mid-infrared region, at low bias, only Saracatinib the signal around 5 μm is clearly visible, indicating excitation of holes into the valence band continuum states where
the holes can easily reach the contact. As the applied voltage is increased, the PC at longer wavelength appears and grows rapidly, and at |U b |>2 V, both mid- and long-wave signals become comparable. We suppose that the long-wave photoresponse is caused by the excitation of holes to a shallow level confined in QD near the valence band edge with subsequent field-assisted tunneling through a barrier. Figure 4 Relative photoresponse and responsivity. (a) Relative photoresponse of the device in long- and mid-wave regions. (b) Responsivity at λ=5 and 8 μm as a function of applied bias. Solid curves learn more are the best fit of experimental data to expression (1). The sample temperature is 90 K. To check this interpretation, the voltage dependence of the mid-wave photoresponse (λ = 5 μm) and long-wave PC (λ = 8 μm) was analyzed separately. The inherent feature of tunneling mechanism of carrier escape is the exponential dependence of PC intensity I on the applied
voltage. Finkman and co-workers  proposed a simple equation which follows from the WKB approximation: where I 0 is the intensity prefactor, m ∗ is the hole effective mass, V B is the tunneling barrier height, d is the contact separation, U 0 is the built-in voltage, and q is the elementary charge. The results of the fitting analysis for both bias polarities are presented in Figure 4b by solid lines. It is clear that the 5- μm PC is not characterized well by Equation 1. On the contrary, the theoretical curves show good agreement with the 8- μm experimental data. From the best fit, we derive the barrier height V B =12 meV for negative bias and 19 meV for positive bias. The built-in voltage was found to be U 0=0.68 and 0.94 V for U b <0 and U b >0, respectively. These values are typical for p-type Ge/Si QDIPs . Figure 5 shows the spectral
response measured with an applied voltage of 2 V in the temperature range of 90 to 120 K. The long-wave signal rapidly decreases at high temperatures because the probability of occupation Etofibrate of the dot excited states increases with temperature thus blocking the interlevel transitions. Figure 5 Responsivity spectra measured at temperatures from 90 to 120 K. The applied voltage is 2 V. Conclusions In summary, we report a normal incidence broadband mid-IR Ge/SiGe quantum dot photodetector on SiGe virtual substrate with a background limited performance at 100 K. The detector exhibits photoresponse in both the 3- to 5- μm and 8- to 12- μm spectral regions. The operating wavelength range of the device can be varied via the bias voltage. The long-wave responsivity measured at 90 K (approximately 1 mA/W) is higher or selleck chemicals llc comparable to previously reported values for Ge/Si QDIPs [13, 14] and SiGe/Si QWIPs  at much lower temperatures (10 to 20 K).