3.1.

Photogating Modulation in MWIR Photoresponse

The MWIR photoresponse of the graphene photodetector was initially investigated. Figure 3(a) shows the $I_{\text{photo}}$ response of the device using a $V_{\mathrm{sd}}$ of 1.0 V and a $V_{\mathrm{bg}}$ of 0 V at 50 K. The device was exposed to the $4.6\text{-}\mu \mathrm{m}$ laser in conjunction with a 2.0-s irradiation cycle (0.8 s on, 1.2 s off) and exhibited a definite photoresponse by generating an $I_{\text{photo}}$ of $2.07\pm 0.02\mu \mathrm{A}$ during irradiation. Figure 3(b) shows the back-gate current ($I_{\mathrm{bg}}$) during laser light irradiation. The photoresponse was associated with variations in $I_{\mathrm{bg}}$, which in turn were modulated by the electrical charge in the InSb photosensitizer.

Fig. 3

Time-dependent (a) $I_{\text{photo}}$ and (b) $I_{\mathrm{bg}}$ response of device under $4.6\text{-}\mu \mathrm{m}$ pulsed laser irradiation.

The effect of $I_{\mathrm{sd}}$ on $V_{\mathrm{bg}}$ was also investigated to further analyze the photogating effect. Figure 4(a) shows the experimentally determined gating responses under dark and illuminated conditions. The lowest $I_{\mathrm{sd}}$ was obtained at a $V_{\mathrm{bg}}$ of $\sim 10\mathrm{V}$, which corresponds to the Dirac point for graphene. Figures 4(b)–4(d) present magnified views of the gating responses around $V_{\mathrm{bg}}$ values of 0 and 23.5 V, and Fig. 4(b) demonstrates an 87.0-mV change in the response at a $V_{\mathrm{bg}}$ of 0 V. In contrast, Figs. 4(c) and 4(d) show only a slight $V_{\mathrm{bg}}$ variation of 0.5 mV.

Fig. 4

(a) Gating responses under dark (black solid line) and $4.6\text{-}\mu \mathrm{m}$ illuminated (red dotted line) conditions. (b)–(d) Magnified plots of drain current in vicinity of $V_{\mathrm{bg}}$ = (b) 0 and (c), (d) 23.5 V.

The photogating effect is primarily attributed to the behavior of photocarriers in the InSb photosensitizer. When a negative $V_{\mathrm{bg}}$ is applied to such a device, electron–hole pairs become separated in the p-doped InSb substrate, and the photoelectrons accumulate at the TEOS/InSb boundary. This surplus of electrons in the boundary region modulates the built-in electric field, which leads to the photogating effect. The results clearly indicate that the photogating effect enhances the photoresponse of the graphene by a factor of more than 100 compared with the inherent photoresponses of graphene obtained from photovoltaic,^10,25,41 bolometric,³⁵ and photothermoelectric effects.^42–44

The photogating effect is achieved by modulating $V_{\mathrm{bg}}$ via photocarriers generated in the InSb substrate. The sequence of events by which the photoresponse of the device appears in conjunction with the photogating effect can be summarized as follows. First, during light irradiation at wavelengths shorter than the cutoff wavelength of the InSb substrate, electron–hole pairs (photocarriers) are generated in the InSb. These photoelectrons/holes are subsequently separated via the application of $V_{\mathrm{bg}}$. Second, the photoelectrons accumulate in the depletion layer at the TEOS/InSb boundary as a consequence of $V_{\mathrm{bg}}$,²⁹ and $V_{\mathrm{bg}}$ is modulated by the internal electric field generated by the accumulated photoelectrons. Third, the field effect in the graphene is changed by the modulation of $V_{\mathrm{bg}}$. Specifically, variations in $V_{\mathrm{bg}}$ change the surface charge density of the graphene, and the corresponding change in the electrical signal from the graphene channel is detected as the photoresponse.

3.2.

Wavelength Dependence

The wavelength dependence of the device performance was also investigated, and Fig. 5 shows the photoresponse of the device during pulsed laser light irradiation at 0.6, 1.6, 8.0, and $9.6\mu \mathrm{m}$. The device evidently exhibited a photoresponse with $I_{\mathrm{bg}}$ modulation under irradiation at 0.6 and $1.6\mu \mathrm{m}$. However, the photoresponse of the device was found to decrease significantly at longer wavelengths such that, as shown in Figs. 5(c) and 5(d); no photoresponse was obtained during irradiation in the LWIR region at 8.0 and $9.6\mu \mathrm{m}$.

Fig. 5

Photoresponse of device at (a) 0.6, (b) 1.6, (c) 8.0, and (d) $9.6\mu \mathrm{m}$.

The photoresponse data obtained from a Si-based graphene FET were compared with those from an InSb-based graphene FET with both p-doped photosensitizers. A clear photoresponse was confirmed under irradiation at wavelengths shorter than the cutoff wavelength of the photosensitizer for both devices. Figure 6 presents the visible light photoresponse of the Si graphene FET and the MWIR photoresponse of the InSb graphene FET. The Si-based device produced a photoresponse under visible light irradiation [Figs. 6(a) and 6(b)], and the photoresponse characteristics together with the distinct back-gate voltage profile indicate the exact same behavior as that for the InSb-based graphene FET under MWIR irradiation [Figs. 6(c) and 6(d)].

Fig. 6

(a), (b) Visible photoresponse of Si graphene FET at 293 K and (c), (d) MWIR photoresponse of InSb graphene FET at 50 K, showing (a), (c) time-dependent $I_{\text{photo}}$ (black, solid) and gate current (red, dotted) and (b), (d) $I_{\text{photo}}$ values as functions of back-gate voltage. Excitation wavelengths: (a), (b) $0.6\mu \mathrm{m}$ and (c), (d) $4.6\mu \mathrm{m}$. Pulse periods: (a) 1.2 s on, 0.8 s off and (c) 0.8 s on, 1.2 s off.

At wavelengths longer than the bandgap wavelength, the photoresponses of both devices were decreased significantly. The MWIR and LWIR photoresponses of the Si-based graphene FET were especially weak, in the vicinity of only several mA/W. We also ascertained that the InSb-based graphene FET did not exhibit a photoresponse under LWIR light irradiation. In addition, generation of photocarriers in the photosensitizers under longer wavelength light was not evident, based on modulation of the gate current. Therefore, it can be concluded that the MWIR photoresponse of the InSb-based graphene FET was enhanced by the photogating effect.

3.3.

Temperature Dependence

The effect of temperature on the MWIR photoresponse was assessed, and Fig. 7 shows the photoresponses obtained from the device at 150 and 200 K. At 150 K, the photogating effect was weakened by the generation of hot carriers, and so the inherent photoresponse of the graphene itself played the dominant role. At 200 K, $I_{\text{photo}}$ was evidently not synchronized with the laser pulses. In addition, $I_{\mathrm{bg}}$ exhibited greater fluctuations in response to thermally generated carriers during pulsed irradiation compared with fluctuations at lower temperatures. Consequently, the photogating effect was weakened and an MWIR photoresponse could not be confirmed. At high temperatures, hot carriers would be expected to be generated in the InSb substrate as a consequence of thermal excitation, leading to changes in $V_{\mathrm{bg}}$ due to the varying concentrations of photocarriers.

Fig. 7

Time dependence of $I_{\text{photo}}$ (black solid lines) and $I_{\mathrm{bg}}$ (red dotted lines) for device at (a) 150 and (b) 200 K.

3.4.

Device-Size Dependence

The effect of the graphene channel dimensions on the MWIR photoresponse was also evaluated. Figure 8 plots the photoresponses of two devices having different channel aspect ratios but the same total channel area of $100\mu {\mathrm{m}}^2$. The photoresponse is seen to increase from $1.15\pm 0.02$ to $4.90\pm 0.39\mu \mathrm{A}$ upon increasing the ratio of the width ($W$) of the channel to its length ($L$) of gap between electrodes. The devices operate under the same principles as metal–oxide–semiconductor FETs, so changing the $W/L$ ratio causes a resistance change in the graphene channel.

Fig. 8

(a) Time dependence of $I_{\text{photo}}$ for devices with different graphene channel widths ($W$) and lengths ($L$). Black solid line: $W=5\mu \mathrm{m}$, $L=20\mu \mathrm{m}$ right image in (b). Red dotted line: $W=20\mu \mathrm{m}$, $L=5\mu \mathrm{m}$ left image in (b). Optical microscopy images of graphene channels.

The effects of the graphene channel shape on the MWIR photoresponse were evaluated in more detail, and Fig. 9 plots $I_{\text{photo}}$ for $W/L$ ratios from 0.25 to 8.00. It is noteworthy that these $I_{\text{photo}}$ values have been normalized with respect to the graphene field effect mobility (${\mu }_{\mathrm{FE}}$), because ${\mu }_{\mathrm{FE}}$ varied slightly even though the Dirac point for all devices was tuned to be about the same at 0 to 10 V. Assigning a value of 1 to $I_{\text{photo}}$ for an aspect ratio of 8, $I_{\text{photo}}$ values of 0.04, 0.08, 0.17, 0.26, 0.49, and 0.66 were obtained for aspect ratios of 0.25, 0.5, 1, 2, 4, and 6, respectively. It is evident that the resulting plot describes an almost linear correlation between $I_{\text{photo}}$ and the aspect ratio of the channel. This line can be fit using the exponential expression shown in the figure. The results indicate that the MWIR photoresponse was affected by the shape of the graphene channel but not by the irradiated area or the total incident light power.

Fig. 9

Normalized $I_{\text{photo}}$ values of devices with different aspect ratios.

Figure 10(a) shows the MWIR responses of devices with channels having a fixed width of $5\mu \mathrm{m}$ and lengths of 5, 10, 15, or $20\mu \mathrm{m}$. The responsivity was calculated from the photocurrent, the incident light intensity of $46.2\mu \mathrm{W}/{\mathrm{mm}}^2$, and the irradiated surface area, which is determined by the graphene channel size and the photocarrier diffusion distance in the InSb photosensitizers. The responsivity increases significantly with decreasing channel area, with values of 61.7, 68.1, 159.1, and $321.0\mathrm{A}/\mathrm{W}$ corresponding to areas of 100, 75, 50, and $25\mu {\mathrm{m}}^2$, respectively.

Fig. 10

(a) MWIR responsivity of devices with different graphene channel areas and (b) $I_{\text{photo}}$ values over time for devices with $100\text{-}\mu {\mathrm{m}}^2$ (black solid line) and $25\text{-}\mu {\mathrm{m}}^2$ (red dotted line) graphene channel areas from which the data in (a) were obtained.

In general, the responsivity of a conventional photodetector is proportional to the irradiated area, because incident light energy is directly converted to a signal current/bias. In contrast, the photoresponse of a graphene photodetector using the photogating effect is not based on the direct collection of photoelectrons/holes from the photosensitizers but on modulation of the field effect around the graphene channel. A sufficiently strong field-effect modulation can be obtained via the localized accumulation of photoelectrons/holes in a depletion layer. Therefore, expansion of the irradiated area is not required in order to enhance the responsivity of such devices. A decrease in the graphene channel area leads to a change in the aspect ratio, which causes an increase in the photocurrent. The results of this work demonstrate that increasing the aspect ratio of the graphene channel decreases the device area and also improves the responsivity.