Soo Seok Kang

High photoresponsivity in an all-graphene p – n vertical junction photodetector

Intensive studies have recently been performed on graphene-based photodetectors, but most of them are based on field effect transistor structures containing mechanically exfoliated graphene, not suitable for practical large-scale device applications. Here we report high-efficient photodetector behaviours of chemical vapor deposition grown all-graphene p-n vertical-type tunnelling diodes. The observed photodetector characteristics well follow what are expected from its band structure and the tunnelling of current through the interlayer between the metallic p- and n-graphene layers. High detectivity (~10(12) cm Hz(1/2) W(-1)) and responsivity (0.4~1.0 A W(-1)) are achieved in the broad spectral range from ultraviolet to near-infrared and the photoresponse is almost consistent under 6-month operations. The high photodetector performance of the graphene p-n vertical diodes can be understood by the high photocurrent gain and the carrier multiplication arising from impact ionization in graphene.

High-performance graphene-quantum-dot photodetectors

Graphene quantum dots (GQDs) have received much attention due to their novel phenomena of charge transport and light absorption/emission. The optical transitions are known to be available up to ~6 eV in GQDs, especially useful for ultraviolet (UV) photodetectors (PDs). Thus, the demonstration of photodetection gain with GQDs would be the basis for a plenty of applications not only as a single-function device in detecting optical signals but also a key component in the optoelectronic integrated circuits. Here, we firstly report high-efficient photocurrent (PC) behaviors of PDs consisting of multiple-layer GQDs sandwiched between graphene sheets. High detectivity (>1011 cm Hz1/2/W) and responsivity (0.2 ~ 0.5 A/W) are achieved in the broad spectral range from UV to near infrared. The observed unique PD characteristics prove to be dominated by the tunneling of charge carriers through the energy states in GQDs, based on bias-dependent variations of the band profiles, resulting in novel dark current and PC behaviors.

Graphene p-n vertical tunneling diodes.

Formation and characterization of graphene p-n junctions are of particular interest because the p-n junctions are used in a wide variety of electronic/photonic systems as building blocks. Graphene p-n junctions have been previously formed by using several techniques, but most of the studies are based on lateral-type p-n junctions, showing no rectification behaviors. Here, we report a new type of graphene p-n junction. We first fabricate and characterize vertical-type graphene p-n junctions with two terminals. One of the most important characteristics of the vertical junctions is the asymmetric rectifying behavior showing an on/off ratio of ~10(3) under bias voltages below ±10 V without gating at higher n doping concentrations, which may be useful for practical device applications. In contrast, at lower n doping concentrations, the p-n junctions are ohmic, consistent with the Klein-tunneling effect. The observed rectification results possibly from the formation of strongly corrugated insulating or semiconducting interlayers between the metallic p- and n-graphene sheets at higher n doping concentrations, which is actually a structure like a metal-insulator-metal or metal-semiconductor-metal tunneling diode. The properties of the diodes are almost invariant even 6 months after fabrication.

Size-dependence of Raman scattering from graphene quantum dots: Interplay between shape and thickness

Raman-scattering behaviors have been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The peak frequencies of D and 2D bands are almost irrespective of d, and the intensity of the D band is larger than that of the G band over almost full range of d. These results suggest that GQDs are defective, possibly resulting from the dominant contributions from the edge states at the periphery of GQDs. The G band shows a maximum peak frequency at d = ∼17 nm, whilst the full-width half maximum of the G band and the peak-intensity ratio of the D to G bands are minimized at d = ∼17 nm. Since the average thickness of GQDs (t) is proportional to d, t can act as a factor affecting the d-dependent Raman-scattering behaviors, but they cannot be explained solely by the t variation. We propose that the abrupt changes in the Raman-scattering behaviors of GQDs at d = ∼17 nm originate from size-dependent edge-state variation of GQDs at d = ∼17 nm as d increases.

Graphene/Si‐Quantum‐Dot Heterojunction Diodes Showing High Photosensitivity Compatible with Quantum Confinement Effect

Graphene/Si quantum dot (QD) heterojunction diodes are reported for the first time. The photoresponse, very sensitive to variations in the size of the QDs as well as in the doping concentration of graphene and consistent with the quantum-confinement effect, is remarkably enhanced in the near-ultraviolet range compared to commercially available bulk-Si photodetectors. The photoresponse proves to be dominated by the carriertunneling mechanism.

Size-dependent radiative decay processes in graphene quantum dots

Radiative decay processes have been studied in graphene quantum dots (GQDs) by varying their size. The photoluminescence (PL) decay traces are well fitted to a biexponential function with lifetimes of τ1 and τ2, indicating their fast and slow components, respectively. The τ1 is almost constant, irrespective of the average GQD size (da) for two excitation wavelengths of 305 and 356 nm. In contrast, the τ2 decreases as da increases for da ≤ ∼17 nm, but da > ∼17 nm, it increases with increasing da for both the excitation wavelengths, similar to the size-dependent behaviors of the time-integrated PL peak energy. We propose that the τ1 and τ2 originate from size-independent fast band-to-band transition and size-dependent slow transition resulting from the edge-state variation at the periphery of GQDs, respectively.

Energy transfer from an individual silica nanoparticle to graphene quantum dots and resulting enhancement of photodetector responsivity

Förster resonance energy transfer (FRET), referred to as the transfer of the photon energy absorbed in donor to acceptor, has received much attention as an important physical phenomenon for its potential applications in optoelectronic devices as well as for the understanding of some biological systems. If one-atom-thick graphene is used for donor or acceptor, it can minimize the separation between donor and acceptor, thereby maximizing the FRET efficiency (EFRET). Here, we report first fabrication of a FRET system composed of silica nanoparticles (SNPs) and graphene quantum dots (GQDs) as donors and acceptors, respectively. The FRET from SNPs to GQDs with an EFRET of ∼78% is demonstrated from excitation-dependent photoluminescence spectra and decay curves. The photodetector (PD) responsivity (R) of the FRET system at 532 nm is enhanced by 100∼101/102∼103 times under forward/reverse biases, respectively, compared to the PD containing solely GQDs. This remarkable enhancement is understood by network-like current paths formed by the GQDs on the SNPs and easy transfer of the carriers generated from the SNPs into the GQDs due to their close attachment. The R is 2∼3 times further enhanced at 325 nm by the FRET effect.

Light-induced negative differential resistance in graphene/Si-quantum-dot tunneling diodes

One of the interesing tunneling phenomena is negative differential resistance (NDR), the basic principle of resonant-tunneling diodes. NDR has been utilized in various semiconductor devices such as frequency multipliers, oscillators, relfection amplifiers, logic switches, and memories. The NDR in graphene has been also reported theoretically as well as experimentally, but should be further studied to fully understand its mechanism, useful for practical device applications. Especially, there has been no observation about light-induced NDR (LNDR) in graphene-related structures despite very few reports on the LNDR in GaAs-based heterostructures. Here, we report first observation of LNDR in graphene/Si quantum dots-embedded SiO2 (SQDs:SiO2) multilayers (MLs) tunneling diodes. The LNDR strongly depends on temperature (T) as well as on SQD size, and the T dependence is consistent with photocurrent (PC)-decay behaviors. With increasing light power, the PC-voltage curves are more structured with peak-to-valley ratios over 2 at room temperature. The physical mechanism of the LNDR, governed by resonant tunneling of charge carriers through the minibands formed across the graphene/SQDs:SiO2 MLs and by their nonresonant phonon-assisted tunneling, is discussed based on theoretical considerations.

Strong enhancement of emission efficiency in GaN light-emitting diodes by plasmon-coupled light amplification of graphene

Recently, we have demonstrated that excitation of plasmon-polaritons in a mechanically-derived graphene sheet on the top of a ZnO semiconductor considerably enhances its light emission efficiency. If this scheme is also applied to device structures, it is then expected that the energy efficiency of light-emitting diodes (LEDs) increases substantially and the commercial potential will be enormous. Here, we report that the plasmon-induced light coupling amplifies emitted light by ∼1.6 times in doped large-area chemical-vapor-deposition-grown graphene, which is useful for practical applications. This coupling behavior also appears in GaN-based LEDs. With AuCl3-doped graphene on Ga-doped ZnO films that is used as transparent conducting electrodes for the LEDs, the average electroluminescence intensity is 1.2-1.7 times enhanced depending on the injection current. The chemical doping of graphene may produce the inhomogeneity in charge densities (i.e., electron/hole puddles) or roughness, which can play a role as grating couplers, resulting in such strong plasmon-enhanced light amplification. Based on theoretical calculations, the plasmon-coupled behavior is rigorously explained and a method of controlling its resonance condition is proposed.