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. 2024 May 21;18(20):12760-12770.
doi: 10.1021/acsnano.3c11856. Epub 2024 May 10.

Engineering Graphene Phototransistors for High Dynamic Range Applications

Shadi Nashashibi  1 , Stefan M Koepfli  1 , Raphael Schwanninger  1 , Michael Baumann  1 , Michael Doderer  1 , Dominik Bisang  1 , Yuriy Fedoryshyn  1 , Juerg Leuthold  1
Affiliations

Engineering Graphene Phototransistors for High Dynamic Range Applications

Shadi Nashashibi et al. ACS Nano. .

Abstract

Phototransistors are light-sensitive devices featuring a high dynamic range, low-light detection, and mechanisms to adapt to different ambient light conditions. These features are of interest for bioinspired applications such as artificial and restored vision. In this work, we report on a graphene-based phototransistor exploiting the photogating effect that features picowatt- to microwatt-level photodetection, a dynamic range covering six orders of magnitude from 7 to 107 lux, and a responsivity of up to 4.7 × 103 A/W. The proposed device offers the highest dynamic range and lowest optical power detected compared to the state of the art in interfacial photogating and further operates air stably. These results have been achieved by a combination of multiple developments. For example, by optimizing the geometry of our devices with respect to the graphene channel aspect ratio and by introducing a semitransparent top-gate electrode, we report a factor 20-30 improvement in responsivity over unoptimized reference devices. Furthermore, we use a built-in dynamic range compression based on a partial logarithmic optical power dependence in combination with control of responsivity. These features enable adaptation to changing lighting conditions and support high dynamic range operation, similar to what is known in human visual perception. The enhanced performance of our devices therefore holds potential for bioinspired applications, such as retinal implants.

Keywords: adaptability; air stable; bioinspired; enhancement; graphene; high dynamic range; phototransistor.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Basic working principles of the proposed graphene-based photodetector. (a) Visualization of the enhanced photogating device. The device consists of an absorbing silicon substrate (Si), an insulating alumina gate oxide (Al2O3), a graphene patch (Gr), interdigitated source/drain gold finger contacts (Au), an alumina passivation layer (Al2O3), and a semitransparent gold top-gate (Au). Photogenerated electrons (blue) in the substrate lead to image charges in the graphene patch (red), which affect the conductivity of the graphene patch. (b) Biasing scheme for the enhanced photogating detector. The three voltages VSD, VTG, and VBG are the source-drain voltage, top-gate voltage, and bottom-gate voltage, respectively. (c) Exemplary band diagram of the device structure (vertical cut) showcasing the absorption of photons, the movement of electrons (blue) and holes (red) in the Si absorber layer, and the photogating effect Δn on the graphene patch.
Figure 2
Figure 2
Bottom-gate control of standard interfacial photogating devices under 532 nm illumination. (a) False-colored scanning electron microscopy (SEM) image of a standard device without interdigitated finger contacts or a semitransparent top-gate. (b) Biasing schematic of the standard device with two bias voltages, namely, the source-drain voltage VSD and the bottom-gate voltage VBG. (c) Photocurrent magnitude measured under a source-drain voltage sweep. This behavior nicely follows the expected trend with VSD given by eq 1. (d) Channel resistance measured under a bidirectional bottom-gate voltage sweep with and without optical input. Under illumination, the gating curve moves toward the right due to the photogating effect. (e) Bottom-gate voltage dependence of the photocurrent for the device measured in (d) with a source-drain voltage VSD of 0.1 V. (f) Band diagrams shown for three distinct bottom-gate voltage conditions. Case 1: The highest response is obtained for the condition with depletion mode biasing of the Si substrate. Case 2: The dip in photocurrent occurs due to the presence of the Dirac point. Case 3: Under positive bottom-gate voltages, the silicon substrate is driven into accumulation leading to a diminishing photoresponse.
Figure 3
Figure 3
Performance of interfacial photogating devices with a semitransparent top-gate under 532 nm illumination and a source-drain voltage VSD of 0.1 V. (a) False-colored SEM image showing an interfacial graphene photogating device with a semitransparent top-gate. (b) Biasing schematic of the device with three control voltages, namely, the bottom-gate voltage VBG, the source-drain voltage VSD, and the top-gate voltage VTG. (c) Band diagram for a device with a semitransparent top-gate. The bottom- and top-gate voltages control the Fermi level in the absorbing Si substrate EF,Si and the top-gate EF,Au, respectively, and thus tune the graphene Fermi level EF,Gr. (d) Graphene channel resistance measured in dependence of the bottom-gate and top-gate voltage under dark conditions. (e, f) Photoresponse in dependence of the bottom- and top-gate voltages for 601 and 7.7 nW, respectively, of optical power. The maximum photocurrent is marked with a magenta-colored star. (g–i) The photocurrent (left axis) for the three horizontal labeled lines highlighted in magenta in (f) are shown in more detail together with the gating resistance curve (right axis). The location of the Dirac point (black dotted line) has a detrimental effect on the highest achievable photoresponse.
Figure 4
Figure 4
Performance of interfacial photogating devices with rectangularly shaped channels under 532 nm illumination and a source-drain voltage VSD of 0.1 V. (a) False-colored SEM image of a standard interfacial photogating device with a rectangularly shaped graphene channel with width w and length l. The aspect ratio a of the channel is defined as l/w. (b) Maximal photocurrent in dependence of the channel aspect ratio. The black curve shows a numerical fit obtained with the equation Iph = c/a, where c is a proportionality constant.
Figure 5
Figure 5
Performance of interfacial photogating devices with interdigitated finger contacts under 532 nm illumination and a source-drain voltage VSD of 0.1 V. (a) SEM image of an interfacial photogating device with an interdigitated source-drain finger structure to contact the 10 × 10 μm graphene patch. The fingers have a finger width wf and finger distance df as defined in the image. (b) Biasing scheme for a device with interdigitated fingers with source-drain voltage VSD and bottom-gate voltage VBG. (c) Maximal photocurrent for different interdigitated finger configurations. By decreasing the finger distance, the channel aspect ratio is artificially reduced and the photoresponse increases while keeping the photodetector area constant. The dotted reference line denotes the photoresponse for a standard device with a 10 × 10 μm graphene channel.
Figure 6
Figure 6
Performance metrics of the enhanced interfacial photogating device under 532 nm illumination and a source-drain voltage VSD of 0.1 V. Photocurrent (a) and responsivity (b) for an optical power sweep across six orders of magnitude for a standard and an enhanced device. The standard device has a 10 × 10 μm channel without any enhancements, while the enhanced device incorporates a finger structure (df = 1.2 μm and wf = 0.8 μm) and a semitransparent top-gate biased at the optimal operating point (VTG = 1.5 V). In both cases, the bottom-gate voltage has been set to −0.9 V providing optimal conditions for photodetection. (c) Power sweep measurements at different bottom-gate voltages shifting the response curve. This displays the ability to tune the photocurrent to the desired range and adapt the responsivity depending on the ambient light conditions. (d) Frequency response of the enhanced device for a mechanically chopped optical input at 532 nm wavelength at the ideal operating point. The two measurement ranges correspond to two different chopper blades used to measure the two modulation ranges from 50 to 1000 Hz and 1–10 kHz.
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