Shamaita S. Shetu

Evidence of minority carrier injection efficiency >90% in an epitaxial graphene/SiC Schottky emitter bipolar junction phototransistor for ultraviolet detection

In this letter, we report the UV detection characteristics of an epitaxial graphene (EG)/SiC based Schottky emitter bipolar phototransistor (SEPT) with EG on top as the transparent Schottky emitter layer. Under 0.43 μW UV illumination, the device showed a maximum common emitter current gain of 113, when operated in the Schottky emitter mode. We argue that avalanche gain and photoconductive gain can be excluded, indicating minority carrier injection efficiency, γ, as high as 99% at the EG/p-SiC Schottky junction. This high γ is attributed to the large, highly asymmetric barrier, which EG forms with the p-SiC. The maximum responsivity of the UV phototransistor is estimated to be 7.1 A/W. The observed decrease in gain with increase in UV power is attributed to recombination in the base region, which reduces the minority carrier lifetime.

Si-adatom kinetics in defect mediated growth of multilayer epitaxial graphene films on 6H-SiC

We present a quantitative study on the growth of multilayer epitaxial graphene (EG) by solid-state decomposition of SiC on polar (c-plane Si and C-face) and non-polar (a and m planes) 6H-SiC faces, with distinctly different defect profiles. The growth rates are slower than expected from a mechanism that involves Si loss from an open and free surface, and much faster than expected for the nucleation of a defect-free EG layer, implying that defects in the EG play a critical role in determining the growth kinetics. We show that a Deal-Grove growth model, which assumes vertical diffusion of Si through these defects as the limiting factor for EG growth, is unsuitable for describing multilayer growth. Instead, we introduce a lateral “adatom” diffusion mechanism for Si out-diffusion, based on a modified Burton, Cabrera, and Frank model. In this model, defects in epitaxial graphene serve as sinks for Si desorption loss, taking the place of reactive sites, such as step edges for nucleation and growth of crystals p...

Study of Epitaxial Graphene on Non-Polar 6H-SiC Faces

We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. AFM surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasing stress with increasing temperature. These variations provide evidence of different EG growth mechanisms on non-polar and polar faces, likely due to differences in surface free energy. We also present differences between a-plane ( ) EG and m-plane ( ) EG in terms of morphology, thickness and Raman characteristics.

Study of SiC Epitaxial Growth Using Tetrafluorosilane and Dichlorosilane in Vertical Hotwall CVD Furnace

This paper presents one of the first comparative studies of distinctive results obtained using halogenated silicon precursors, dichlorosilane (SiH2Cl2, DCS) and tetrafluorosilane (SiF4, TFS) for SiC homo epitaxial growth. Both TFS and DCS possess very distinct properties that show specific influence on SiC growth. SiC epitaxial growth using TFS greatly suppresses parasitic deposition in the gas delivery system. Growth using TFS shows carbon mediated growth regime, and exhibits controlled doping concentration of the epilayer by an order of magnitude lower than that in the growth using DCS at the same C/Si ratio. Studies of epilayer surface morphology show that the epilayers from TFS growth have a specular surface in a wide C/Si range whereas in the growth using DCS, the epilayer surface roughness is strongly dependent on the C/Si ratio.

High gain bipolar photo-transistor operation in graphene/SiC Schottky interfaces: The role of minority carriers

We propose a new class of semiconductor transistor devices based on graphene/SiC and graphene/Si Schottky junctions that have the potential to be transformative. By using the graphene as collector/emitter in a bipolar transistor (BJT) and not as a channel material, there is relaxation of the tolerances in graphene thickness and quality, simplifying growth, device design and fabrication. This also enables the exploitation of engineered defects in thicker (2-5ML) graphene films for flexible electronics, currently not being considered, as well controlled uniform defects are preferred to localized random defect clusters. We will discuss an SiF4 based growth method that enables temperature programmed defect engineering. We will discuss the use of electron-beam induced current (EBIC) to characterize these materials. Based on recent results at our lab, a graphene/SiC Schottky junction behaves as a collector (GC) and an emitter (GE) in a BJT with common emitter gain, β>50, measured under phototransistor operation mode. The transparent graphene Schottky collector/emitter junction enables opto-electronic applications, minimizes series resistance in the device due to the thin graphene layer, and also minimizes charge storage time (diffusion capacitance), enabling high speed operation. Furthermore, the observation of β>50 with a GE-BJT demonstrates that significant minority carrier injection occurs in these Schottky junctions, contrary to what is commonly assumed. The injection of minority carriers has the ability to induce conductivity modulation in the underlying semiconductor, reducing overall device resistance. The role of minority carriers in Schottky Junctions will be discussed.

Gain of 56 in graphene silicon carbide schottky collector phototransistor (GSCBT) for radiation detection

Large low atomic number (Z) windows for novel solid state radiation detection schemes can be conceived by implementation of graphene semi-metal rectifying contacts on SiC epilayers and substrates. The low absorption of Graphene epilayers increases the incident flux/energy to the space charge region increasing detector charge collection. By implementation of a top graphene n-type schottky contact we report bipolar device operation in an epitaxial n-graphene/p-SiC/n-substrate vertical schottky collector bipolar transistor structure. The base current was supplied by e-beam irradiation of 15 keV on the p-SiC base layer. Transistor gains of up to 56 were obtained from the structure. Initial results suggest that gain improvements can be achieved by enhancement of the device interface, increased carrier lifetimes, and shortening the base region width.

Selective multimodal gas sensing in epitaxial graphene by fourier transform infrared spectroscopy

We report the sensing behavior of epitaxial graphene (EG) grown on C-face SiC substrates by infrared reflectance spectroscopy through molecular adsorption of NO2, NH3 and N2 gases. Fourier Transform Infrared Reflection (FTIR) measurements were performed on EG under gas exposure and it clearly exhibits an EG thickness dependence. By comparing the change in Ef under gas adsorption with the adsorbed impurity concentration as a function of EG thickness, the 3 gases were clearly distinguished, enabling a new paradigm for multi-modal gas sensing using optical interrogation of EG surfaces towards EG electronic or optical noses.