Optically-addressed Visible/MWIR two-color photodetectors using monolithically-integrated CdTe and InSb diodes

We report an optically-addressed, two-color and two-terminal photodetector consisting of monolithically integrated and lattice-matched CdTe on top of InSb diodes grown on an InSb substrate. The CdTe PIN diode with an 820 nm cut-off wavelength and the InSb PIN diode with a 5.5 μm cut-off wavelength are electrically connected through a highly conductive (< 0.1 Ω-cm²) n-CdTe/p-InSb heterovalent interface. To achieve an InSb diode with low dark current of < 10^-6 A/cm² at 77 K it is critical to keep the n-type background doping concentration of the InSb absorber lower than 10¹⁵ cm^-3, otherwise the band bends too much and the band-to-band tunneling current is too high. However, the epi-grown undoped InSb usually has an unintentional n-type doping concentration higher than 10¹⁶ cm^-3. Here, we propose to use the undoped InSb substrate as the absorber of the InSb diode because it has an n-type background concentration of ~ 10¹⁴ cm^-3. To achieve such an InSb diode, a p-type InSb layer is directly grown on the undoped InSb substrate after careful surface oxide removal treatment. Indium is soldered on the backside of the InSb substrate to create an n-type Ohmic contact. To achieve the CdTe diode, it is necessary to deposit p-type amorphous silicon on top of a 2.5-μm-thick undoped CdTe absorber to circumvent the p-type doping difficulty for CdTe. The optical-addressing for this two-color photodetector can be realized by using an optical bias (a 633 nm CW laser) to illuminate the device or not, so that the spectral responsivity of the device can be switched between the MWIR band and the visible band respectively. Device performance, including the dark current, the spectral responsivities under different optical bias conditions, responsivities vs. optical-bias power relations, and the optical crosstalk between the two diodes will be presented at the conference.