3.1.

Basic Device Characteristics

Figure 3(a) shows the drain current ($I_d$) response of the devices using a drain voltage ($V_d$) of 0 V. The devices were exposed to the $4.6\mu \mathrm{m}$ laser with a 2.0 s irradiation cycle (0.8 s on and 1.2 s off). The devices exhibited a definite photoresponse by modulation with an $I_d$ of $10.37\pm 0.15\mu \mathrm{A}$ as the photocurrent with a base $I_d$ of $0.86\mu \mathrm{A}$. Figure 3(b) shows the current-bias characteristics of the devices under various laser powers. The photocurrent showed a linear increase with variation of the laser light power and $V_d$ application. The maximum MWIR light responsivity of the devices was calculated to be $4.68\mathrm{A}/\mathrm{W}$ at a $V_d$ of 0.5 V. We also confirmed that the devices exhibited photoresponses under visible, near-IR, and MWIR around 3 to $5\mu \mathrm{m}$.

Fig. 3

Photoresponse characteristics of the devices under $4.6\mu \mathrm{m}$ light irradiation. (a) Drain current response of the devices under pulsed laser irradiation at $V_d$ of 0 V. (b) Drain current–voltage characteristics of the devices under various irradiation light powers.

3.2.

Graphene Channel Dependence of Photogating

The MWIR photoresponse of the devices was investigated, and Fig. 4 shows a comparison of the photocurrent characteristics of entirely and partially graphene-covered structures. The graphene/InSb contact region was $50\times 50\mu {\mathrm{m}}^2$ in both devices. The entirely graphene-covered device indicated a significant increase in the photocurrent, where the maximum photocurrent at a $V_d$ of 0.5 V reached $67.63\pm 4.03\mu \mathrm{A}$, whereas that of the partially graphene-covered device exhibited a maximum photocurrent of $9.44\pm 0.23\mu \mathrm{A}$.

Fig. 4

Comparison of photoresponse in devices with different graphene channel shapes. (a) Photocurrent as a function of $V_d$, with graphene channel connecting graphene/InSb contact entirely (red dotted) and partially (black solid) graphene coverage. Inset: schematic of respective devices. (b) Photocurrent–time characteristics of devices at $V_d$ of 0.5 V.

The photoresponse of the devices is mainly affected by the graphene/TEOS region because the photogating occurs in this region. Under positive $V_d$ application, photogenerated electrons and holes are separated in the p-InSb photosensitizer, and electrons in the vicinity of the graphene/InSb interface are injected into the graphene. On the other hand, photoelectrons excited in the InSb under the graphene/TEOS contact region accumulate at the TEOS/InSb interface owing to the depletion layer formed by $V_d$ application, which changes the graphene’s surface carrier density. As a result, the photocurrent of the devices can be amplified. This phenomenon is referred to as photogating. The remaining TEOS region can provide additional photogating by the photocarriers generated in the TEOS/InSb depletion layer, although it may be less effective for photoresponse enhancement because the carrier diffusion distance of the InSb is short, at around a few micrometers,⁴¹ and the effective photocarrier region is situated only in the vicinity of the graphene/TEOS region. The TEOS layer may also decrease the MWIR incident light power that penetrates to the InSb substrate. $\mathrm{TEOS}\text{-}{\mathrm{SiO}}_2$ has an absorption peak around 9 to $10\mu \mathrm{m}$; therefore, the effect of this decrease is negligible. The results show that photogating in the graphene/TEOS region plays a dominant role in the MWIR photoresponse.

To clarify the influence of the graphene/TEOS region on the responsivity enhancement, the MWIR photoresponse characteristics obtained with different graphene/TEOS cross-sectional areas were compared. Figure 5(a) shows a schematic diagram of a device structure. In the region with graphene, $S_{\mathrm{CH}}$, $L$, and $W$ are defined as the graphene/TEOS region, length, and width, respectively. As shown in Fig. 5(b), the photoresponse increased with an increase in $S_{\mathrm{CH}}$. Next, the effects of the graphene channel length $L$ with a fixed $W$ of $100\mu \mathrm{m}$ and $W$ with a fixed $L$ of $200\mu \mathrm{m}$ were investigated. Figures 5(c) and 5(d) show that the photocurrent increased as $L$ and $W$ increased.

Fig. 5

(a) Schematic of graphene channel in the devices, indicating each symbolic character $S_{\mathrm{CH}}$, $W$, and $L$. (b)–(d) Photocurrent characteristics in graphene photodetectors for various graphene-channel cross-sectional areas (b) $S_{\mathrm{CH}}$, (c) lengths $L$, and (d) widths $W$.

The photoresponse in graphene FETs with photogating is dependent on the graphene-channel aspect ratio because the devices operate under the same principles as metal–oxide–semiconductor FETs.^42,44 By contrast, the devices with graphene/InSb heterojunction structures exhibit greater photocurrent when the graphene-channel $W$, $L$, and $S_{\mathrm{CH}}$ increase. An entirely covered graphene channel enables the injected photocarriers in the graphene/InSb heterojunction region to spread to the drain electrode in all directions. This increases the effective region of the photogating, which multiplies the photocurrent. In addition, the photocurrent is nonlinearly increased with $W$. Although an increase in $L$ increases only the graphene/TEOS region for the photogating effect, an increase in $W$ increases both the graphene/TEOS region and graphene/InSb heterojunction region. The heterojunction region size does not affect the graphene/InSb Schottky barrier height but changes the flow rate of the photocarriers between graphene and InSb. Both the increase in photocarriers injected into graphene and the improvement of the responsivity due to photogating cause a nonlinear increase.

3.3.

Dark-Current Reduction

Next, the dark-current dependence on the device structures was investigated. Figures 6(a) and 6(b) show the $V_d$-dependent dark-current characteristics with different graphene/InSb heterojunction cross-sectional areas and dopant types of the InSb substrate. The devices were fabricated to be entirely covered with a graphene channel, with $S_{\mathrm{CH}}$ and the graphene/n-InSb contact region ($S_{\mathrm{CI}}$) set as $50\times 50\mu {\mathrm{m}}^2$, $100\times 100\mu {\mathrm{m}}^2$, and $200\times 200\mu {\mathrm{m}}^2$, respectively, as shown in Fig. 6(c). The maximum dark current decreased with decreasing $S_{\mathrm{CI}}$, and the bias region of low dark current within $5\mu \mathrm{A}$ expanded from 62 to 73 and 85 mV in graphene/p-InSb devices and from 54 to 708 and 711 mV in graphene/n-InSb devices according to decreasing $S_{\mathrm{CI}}$. These results indicate that $S_{\mathrm{CI}}$ significantly affects the photoresponse and operating characteristics of both dopant types.

Fig. 6

(a), (b) Dark-current characteristics in (a) graphene/p-InSb and (b) graphene/n-InSb devices with various carrier-injection areas $S_{\mathrm{CI}}$. (c) Schematic illustration of the devices. Energy band diagram of (d) graphene/p-InSb and (e) n-InSb heterojunction under application of reverse bias.

The dark-current behavior was investigated using a band model of a graphene/InSb heterojunction, as shown in Figs. 6(d) and 6(e). A reverse bias decreases the Schottky barrier height (${\mathrm{\Phi }}_B$), which corresponds to the work function difference between the Fermi level of graphene and the valence band of n-InSb, and causes a leak current for both graphene/p-InSb and graphene/n-InSb structures. The work function of InSb is around 4.77 eV⁶³ and is very close to that of the graphene at around 4.5 to 5.0 eV.^64–66 Since the work function of graphene is strongly affected by the surface condition^45,67 in which the adhesion of moisture in the atmosphere or residue causes hole doping,⁶⁸ the Fermi level $E_F$ of graphene is lowered from the neutral point. This lower shift of the Fermi level in graphene increases ${\mathrm{\Phi }}_B$ and suppresses the dark-current leak at the graphene/n-InSb heterojunction.

3.4.

Performance Evaluation of Improved Structured Device

The MWIR photoresponse performance of the devices that combines the aforementioned features was evaluated. The devices were fabricated as fully covered with a graphene channel of $20\times 20\mu {\mathrm{m}}^2$ as the MWIR light irradiation area, and the graphene/n-InSb contact region $S_{\mathrm{CI}}$ was decreased to $7\times 7\mu {\mathrm{m}}^2$, as shown in Fig. 7(a). Figure 7(b) shows the $V_d$-dependent dark/photocurrent characteristics of the devices. The devices exhibited diode characteristics, and the dark current was suppressed within $5\mu \mathrm{A}$ in a bias region of $-200$ to 200 mV and within 1 nA in a bias region of $-2.5$ to 27.7 mV. The devices exhibited negative photocurrent in a negative $V_d$ region and positive photocurrent in a positive $V_d$ region under $6.4\mathrm{mW}/{\mathrm{cm}}^2$ MWIR light irradiation. Figure 7(c) shows the MWIR performance of the devices with the dark current of $-0.96\mathrm{nA}\pm 0.58\mathrm{pA}$ and photocurrent of $-8.67\pm 0.08\mathrm{nA}$ at a $V_d$ of $-1\mathrm{mV}$, which corresponds to a noise equivalent power (NEP) of $0.43\mathrm{pW}/{\mathrm{Hz}}^{1/2}$. The performance of the graphene photodetectors is more dependent on the device fabrication process and the graphene synthesis method than on the device structure. The NEP value was significantly improved from $94.5\mathrm{nW}/{\mathrm{Hz}}^{1/2}$ in our previous work,¹⁷ where the graphene and other device elements were prepared in the same manner.

Fig. 7

Photoresponse characteristics in graphene/n-InSb photodetectors. (a) Optical microscopy image of the device. (b) Dark current (black, solid) and photocurrent (red, dotted) voltage characteristics of the devices. (c) Photocurrent–time characteristics of the devices under reverse bias application of $-1\mathrm{mV}$. (d), (e) Schematic of energy band diagram of the devices under (d) reverse bias of $-0.2\mathrm{V}$ and (e) forward bias of 0.1 V as indicated in (b).

The devices exhibited a high responsivity that exceeded 100% of the external quantum efficiency under a large bias application. The responsivity reached $6.14\mathrm{A}/\mathrm{W}$ at $V_d$ at $-0.2\mathrm{V}$ and $12.6\mathrm{A}/\mathrm{W}$ at $V_d$ at 0.1 V. Figures 7(d) and 7(e) show band models of the graphene/n-InSb heterojunction under reverse and forward bias. Under reverse-bias negative $V_d$ application, the Fermi level is upshifted in the graphene. ${\mathrm{\Phi }}_B$ is further decreased with the upshift of the Fermi level by photogenerated electrons in the graphene from $E_F$ to $E_{F\text{photo}}$. The photogenerated electrons can obtain enough energy to generate an avalanche multiplication process at the depletion layer formed in the graphene/n-InSb heterojunction. Since the carrier transportation has not alienated the negative $V_d$ region, the dark current is also increased with the photocurrent. By contrast, a positive $V_d$ application and photogenerated holes decrease ${\mathrm{\Phi }}_B$, and the photogenerated electrons in the InSb and the holes in graphene are transported between each other. A further increase of positive $V_d$ promotes the recombination of the photogenerated electrons and injected holes in n-InSb, and the photocurrent decreases. The results obtained here indicate that the high performance of the devices stands on the formation of Schottky barriers at the interface of the graphene/n-InSb heterojunction and photocarrier transportation.