Origin of photoresponse in black phosphorus phototransistors

We study the origin of a photocurrent generated in doped multilayer black phosphorus (BP) phototransistors, and find that it is dominated by thermally driven thermoelectric and bolometric processes. The experimentally observed photocurrent polarities are consistent with photothermal processes. The photothermoelectric current can be generated up to a micrometer away from the contacts, indicating a long thermal decay length. With an applied source-drain bias, a photobolometric current is generated across the whole device, overwhelming the photothermoelectric contribution at a moderate bias. The photoresponsivity in the multilayer BP device is two orders of magnitude larger than that observed in graphene.

Figure 1

(a) Laser reflection image ($15\mu \mathrm{m}\times 15\mu \mathrm{m}$) of the BP device. Source and drain terminals are indicated. The light polarization used for the photocurrent and Raman spectroscopy are indicated. (b) Raman spectra of BP showing the prominent representative normal modes of the $\Gamma$ point optical phonons. Three other Raman-active modes are not observed because of selection rules for the scattering configuration [28]. (c) Electrical transfer characteristic of the device measured at different source-drain voltages $V_{\text{SD}}$, averaged over the positive and negative back gate voltage $V_{\text{BG}}$ sweeps. (d) Electrical conductivity $\sigma$ and the Seebeck coefficient $S$ extracted from the measured transfer characteristics (see text for details).

Figure 2

(a) Energy band diagram of the device at zero (top) and finite (bottom) source-drain bias. The polarities of the various photocurrents, i.e., thermoelectric, bolometric, photovoltaic, are indicated. (b) Simulated spatial profiles of the elevated electronic and phonon temperatures (i.e., $T_e$ and $T_{\text{ph}}$) due to local laser induced heating, as indicated. Ambient temperature $T_0=300\mathrm{K}$. (c) Simulated laser-scanned photothermoelectric currents at different incident power. (d) Simulated laser-scanned photobolometric currents at different applied source-drain bias.

Figure 3

Photocurrent cross sections extracted from photocurrent maps taken along the direction of maximum amplitude measured as a function of optical power density. The photocurrent is laser excited at a visible wavelength of 532 nm, at different (a) incident power and (b) applied source-drain bias. Similar measurements are done for an infrared wavelength of 1550 nm in (c) and (d). Shaded areas indicate contact regions of the device.