Edward Cazalas

Hysteretic response of chemical vapor deposition graphene field effect transistors on SiC substrates

Graphene field effect transistors (GFETs) fabricated by chemical vapor deposition graphene deposited onto SiC substrates exhibit sensitivity to broadband visible light. The hysteretic nature of this GFET type was studied utilizing a new current-voltage measurement technique in conjunction with current-time measurements. This measurement method accounts for hysteretic changes in graphene response and enables transfer measurements that can be attributed to fixed gate voltages. Graphene hysteresis is shown to be consistent with electrochemical p-type doping, and current-time measurements clearly resolve a hole to electron to hole carrier transition in graphene with a single large change in gate voltage.

Position sensitivity of graphene field effect transistors to X-rays

Device architectures that incorporate graphene to realize detection of electromagnetic radiation typically utilize the direct absorbance of radiation by graphene. This limits their effective area to the size of the graphene and their applicability to lower-energy, less penetrating forms of radiation. In contrast, graphene-based transistor architectures that utilize the field effect as the detection mechanism can be sensitive to interactions of radiation not only with graphene but also with the surrounding substrate. Here, we report the study of the position sensitivity and response of a graphene-based field effect transistor (GFET) to penetrating, well-collimated radiation (micro-beam X-rays), producing ionization in the substrate primarily away from graphene. It is found that responsivity and response speed are strongly dependent on the X-ray beam distance from graphene and the gate voltage applied to the GFET. To develop an understanding of the spatially dependent response, a model is developed that inc...

Position-dependent and millimetre-range photodetection in phototransistors with micrometre-scale graphene on SiC

The extraordinary optical and electronic properties of graphene make it a promising component of high-performance photodetectors. However, in typical graphene-based photodetectors demonstrated to date, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size. For many optoelectronic device applications, it is desirable to obtain the photoresponse and positional sensitivity over a much larger area. Here, we report the spatial dependence of the photoresponse in backgated graphene field-effect transistors (GFET) on silicon carbide (SiC) substrates by scanning a focused laser beam across the GFET. The GFET shows a nonlocal photoresponse even when the SiC substrate is illuminated at distances greater than 500 µm from the graphene. The photoresponsivity and photocurrent can be varied by more than one order of magnitude depending on the illumination position. Our observations are explained with a numerical model based on charge transport of photoexcited carriers in the substrate.

Graphene field effect transistor-based detectors for detection of ionizing radiation

We present the results of our recent efforts to develop novel ionizing radiation sensors based on the nanomaterial graphene. Graphene used in the field effect transistor architecture could be employed to detect the radiation-induced charge carriers produced in undoped semiconductor absorber substrates, even without the need for charge collection. The detection principle is based on the high sensitivity of graphene to ionization-induced local electric field perturbations in the electrically biased substrate. We experimentally demonstrated promising performance of graphene field effect transistors for detection of visible light, X-rays, gamma-rays, and alpha particles. We propose improved detector architectures which could result in a significant improvement of speed necessary for pulsed mode operation.

Graphene-based neutron detectors

We are developing detector architectures and devices based on the novel carbon nanomaterial graphene, which has been shown to exhibit unusual electrical properties of potential use for next-generation radiation detectors. Of particular interest is the use of this technology to develop novel neutron detectors. To this end, we are studying architectures based on a neutron-absorbing converter material in conjunction with a graphene field-effect transistor (GFET). As an intermediate step towards the demonstration of GFET neutron detectors we utilize an alphasource to systematically study the effect of charge deposition on the device response. As an added benefit, this experiment helps us elucidate the important systematics of response to other types of radiation, including the dependence of the magnitude of graphene resistance modulation on the deposited energy and the dependence of device speed on the morphology of energy deposition.

Field effect photoconductivity in graphene on undoped semiconductor substrates

Due to its high charge carrier mobility, broadband light absorption, and ultrafast carrier dynamics, graphene is a promising material for the development of high-performance photodetectors. Graphene-based photodetectors have been demonstrated to date using monolayer graphene operating in conjunction with either metals or semiconductors. Most graphene devices are fabricated on doped Si substrates with SiO2 dielectric used for back gating. Here, we demonstrate photodetection in graphene field effect phototransistors fabricated on undoped semiconductor (SiC) substrates. The photodetection mechanism relies on the high sensitivity of the graphene conductivity to the local change in the electric field that can result from the photo-excited charge carriers produced in the back-gated semiconductor substrate. We also modeled the device and simulated its operation using the finite element method to validate the existence of the field-induced photoresponse mechanism and study its properties. Our graphene phototransistor possesses a room-temperature photoresponsivity as high as ~7.4 A/W, which is higher than the required photoresponsivity (1 A/W) in most practical applications. The light power-dependent photocurrent and photoresponsivity can be tuned by the source-drain bias voltage and back-gate voltage. Graphene phototransistors based on this simple and generic architecture can be fabricated by depositing graphene on a variety of undoped substrates, and are attractive for many applications in which photodetection or radiation detection is sought.

Modulation of graphene field effect by heavy charged particle irradiation

Device architectures based on the two-dimensional material graphene can be used for sensing of electromagnetic and particle radiation. The sensing mechanism may be direct, by absorbance of radiation by the graphene or the immediately adjacent material, and indirect, via the field effect principle, whereby the change in conductivity within a semiconducting absorber substrate induces electric field change at graphene. Here, we report on a graphene field effect transistor (GFET) sensitive to heavy charged particle radiation (α particles) at MeV energies by use of the indirect sensing mechanism. Both the continuous and discrete changes of graphene are observed, and the latter are attributed to single α particle interactions with the GFET. While this study provides the basis for understanding of the irradiation effects, it also opens prospects for the use of GFETs as heavy charged particle detectors.

Precision X-ray measurement of the position sensitivity of graphene field effect transistors

We have been exploring the graphene field-effect transistor (GFET) as a platform for detection of ionizing radiation, whereby the detection is achieved indirectly by use of the field effect in graphene, which is induced by the generation and transport of ionized charge carriers in the underlying undoped semiconductor substrate. An important characteristic of such a detector is scalability to large areas. Previous experimental studies suggest that the effective area significantly exceeds the size of graphene for field effect-based architectures, and this is also predicted from the operational principle of these devices. We describe the results of the experimental studies of GFETs on silicon carbide (SiC) substrates by use a microbeam X-ray facility, provided by the Advanced Photon Source at Argonne National Laboratory. The results confirm that the effective area of the GFET is significantly larger than that of graphene with response measured at distances as large as 1000 μm from 10-μm size graphene. A simple transport model has been developed and is used to explain the spatial dependence of the GFET response.

Detection of light, X-rays, and gamma rays using graphene field effect transistors fabricated on SiC, CdTe, and AlGaAs/GaAs substrates

Our work demonstrates the potential of gated graphene field effect transistors (GFETs) fabricated on a variety of undoped semiconductor substrates such as SiC, CdTe, and GaAs to sense ionizing radiation with promise of high sensitivity, low noise, low power, and room temperature operation. We exploit distinct material properties of different substrates to address different application regimes. Radiation detection with GFET is based on the high sensitivity of graphene resistivity on local electric field perturbations caused by ionized charges generated in the radiation absorbing semiconductor substrate. Light, X-rays, and gamma rays have been detected in our experiments.