Yu Miyamoto

Epitaxial Growth Processes of Graphene on Silicon Substrates

Few-layers graphene is epitaxially grown on silicon substrates via SiC thin films inserted in between. We have conducted a detailed structural characterization of this graphene-on-silicon (GOS) material by Raman spectroscopy and transmission-electron microscopy, to obtain insights into the impacts of process parameters on defect formation. Results suggest that defects in graphene preferentially dwell at steps. Future flattening of the SiC surface, prior to graphene growth, is thus expected to contribute to the improvement of GOS quality.

Extraction of Drain Current and Effective Mobility in Epitaxial Graphene Channel Field-Effect Transistors on SiC Layer Grown on Silicon Substrates

We have fabricated and characterized field-effect transistors (FETs) with an epitaxial graphene channel on a SiC layer grown on a Si substrate. Epitaxial graphene can be formed on SiC substrates by thermal decomposition of its surface under an ultrahigh-vacuum (UHV) condition. To incorporate the thermal decomposition of SiC on Si substrates, we used an approach of growing a thin 3C-SiC(111) layer on Si substrates and subsequently annealing them in UHV to form graphene on the surface of the 3C-SiC layer. Backgate-field-effect transistors using the SiC layer as a gate insulator were characterized. Although a large amount of gate-leakage current is observed, the drain current modulation by backgate voltage is confirmed by extracting the channel current from the total drain current. The extracted channel current characteristics also suggest that the extracted effective mobility exceeds the universal mobility of bulk silicon under similar circumstances.

Ambipolar Behavior in Epitaxial Graphene-Based Field-Effect Transistors on Si Substrate

In this research, ambipolar behavior, which is one of graphenes unique characteristics, is studied for the epitaxial graphene formed on 3C-SiC grown on a Si substrate. The graphene channel is believed to be unintentionally p-type-doped at Dirac-point voltages of approximately +0.11 to +0.12 V. However, as drain voltage negatively increases, Dirac point voltage shifts. The drain current in the p-channel mode of operation saturates at a lower level than that in the n-channel mode of operation. These behaviors are caused by asymmetric carrier transport throughout channel-substrate heterojunctions (i.e., graphene, thin n-type SiC layer, and p-type Si substrate) and source/drain Schottky metal contacts. The interface between the p-type Si substrate and n-type SiC has a significant effect on transport in graphene channels. The results may be helpful for understanding transport in the device and for suppressing ambipolar operation, leading to a unipolar FET operation.

Epitaxial graphene top-gate FETs on silicon substrates

Graphene has a potential of extreme carrier mobility for both electrons and holes [1–4]. Therefore it is considered as a channel material of next-generation FETs. For this purpose, we study an epitaxial graphene on Si substrates [5,6] and applied it to backgate FETs [7]. In this paper, we report top-gate epitaxial graphene FETs (EGFETs) on Si substrates.

Optoelectronic application of multi-layer epitaxial graphene on a Si substrate

In this work, the epitaxial graphene channel formed on 3C-SiC grown on a Si substrate backgate transistor was designed, fabricated and characterized for electronic and optoelectronic applications. Even though a significant amount of the gate leakage current is observed, the experimental results show the device works as an n-type transistor as well as an infrared photovoltaic transistor with the backgate modulation. The observation of the ambipolar behavior verifies the unique property of the graphene layers. The epitaxial graphene is believed to be unintentionally p-type with the Fermi level offset around +0.11∼+0.12 V at the Dirac point. The drain saturated current of the graphene channel transistors is on the order of mA/mm. The photo-generated current can be achieved up to almost 20nA, corresponding to 0.06 mA/W in photo-responsivity at 0.5-V drain-source bias voltage and 0.5-V gate voltage. The backgate voltage tuning spectral characteristic is also demonstrated. The graphene based transistors have a potential application in infrared detection.

Epitaxial graphene field effect transistors on silicon substrates

We have fabricated and characterized the field effect transistors having epitaxial-graphene channel grown on Si substrates. Epitaxial graphene is usually formed on SiC substrates by ultrahigh-vacuum (UHV) annealing. We used an approach to grow 3C-SiC(111) on Si substrates and subsequently to anneal it in UHV to make few layers of graphene on the sample surface. Backgate transistors using the SiC layer as a gate insulator was characterized. Although significant gate leakage current is observed, the drain current modulation by the gate voltage is confirmed by extracting the channel current from the total drain current. The drain saturation current of the graphene-channel transistors is in the order of mA/mm due to the large contact resistance that should be minimized in future study.