Discuss the Graphene – Semiconductor Schottky spielention?
The voltage applied externally controls the electronic properties in modern electronics.
The external voltage causes an electric current to flow through a device, which alters the carrier concentration (Bartolomeo or Di 1-58).
The most common material is silicon. Other recent materials are organic conductors, carbon nano tubes and other non-traditional materials.
The screening of the electric field effect within metals takes place at extremely short distances, approximately 1nm. Therefore, it is necessary to have atomically thin metal film.
Peter Boehm was an organic chemist who synthesized graphene in 1962 from graphite dioxide solution.
Phillip Wallace first described the band structure of graphene 15 years prior to its synthesis.
Andre Geim, Konstantin Novoselov and other University of Manchester scientists synthesized graphene by using a simple, fast and inexpensive method. The tape was used to remove weakly bound layers of bulk graphite and then the layers were gently rubbed onto an oxidized silicon substrate.
This allows you to study and control the fabrication of 2D materials and their integration into devices.
Ferdinand Braun first examined the interaction between metals, semiconductors in 1874. He probed a lead sulfide crystalline at the point of the wire. The current flow was free in only one direction (Li et al 46-51, 2743-748).
Walter Schottky, who first observed rectifier behavior, realized that there was a Schottky Barrier across the junction.
Ralph Hartsough from the University of Kansas witnessed rectification when Carbon was joined to Silicon in the form of graphite or silicon to form a point connection.
Theodore Higier, from the National Bureau of Standards in 1962, found that graphite electrodes placed on p type Silicon had the behavior of ohmic contact.
Filippo Giannazzo from the National Research Council of Catania, Italy and his collaborators prepared the first Schottky Junction between SiC and graphene in 2009.
They measured the barrier height by scanning current spectroscopy.
Xinming Zhu and Hongwei Zhu made graphene-on-silicon Schottky joints using chemical vapor deposition in the following year.
It was demonstrated that photovoltaic performance and rectification characteristics were both possible.
The Nobel Prize was awarded to Andre Geim (and Konstantin Novoselov) in 2010 for their research on graphene.
Few Layers of Graphene, (FLG), and its Characteristics
Graphene can be described as a two-dimensional material where a single layer is made of carbon atoms and is tightly packed within the benzene ring structure.
It is also known as Few Layer Graphene (FLG). The study of FLG has revealed the properties and characteristics of carbon-based materials, such as graphites, nanotubes and others.
FLG films have an atomically thin layer and high quality. This allows for 2D electronic transfer at submicron ranges. (Novoselov 666–69; Supporting Online Materials).
FLG can be used to demonstrate a metal field effect transistor.
You can change the gate voltage to make the conducting channel as 2D electron or hole gases.
To prepare graphene films, plates are made from highly-oriented pyrolytic Graphite (HOPG), of 1mm thickness.
FLG films are produced up to 10um in size by
Multiple peelings of small mesas of HOPG.
Photograph of large multilayer graphene flake, thickness 3nm, layered on top of an Oxidized Si wafer
Atomic Force Microscopy’s (AFM) image shows single-layer graphene. The dark brown color corresponds to the SiO2 surface
Figure 1 illustrates the graphene single-layer and multilayer films.
These films are made into multi-terminal Hallbar device and placed on top of an oxide Si substrate. A gate voltage is applied.
FLG devices exhibit different electronic properties to thicker multilayer graphenes, and 3D graphite.
Figure 2 – Field effect of Few Layer Graphene – FLG
A Graphene’s resistivity vs. Gate voltage for different temperatures T=5, 700K and 300K, respectively
B – Graphene’s conductivity Vs gate voltage for T=70K
C – Hall coefficient vs Gate voltage
D – Temperature Vs Carrier Concentration (open circles-film in A; Squares – thicker FLG film; solidsquares – multilayer graphene
Figure 2 shows how Graphene resistivity, conductivity, and Hall coefficient depend on gate voltage.
The graph shows that Graphene’s resistivity has a sharp peak, which can be seen at several points. At high voltages it drops to 100.
Both sides of the resistivity peak show a linear increase in conductivity and sign reversal at the peak.
The figure also shows that there is a small overlap of the valence and conduction bands.
The gate voltage inducing a surface charge density is caused by which the Fermi energie position is shifted.
This is the permittivity for free space.
A semimetal of shallow overlap is converted into a conductor with full electrons or complete holes by creating a mixed state, in which both electrons as well as holes are present through the electric field doping (Figure 2).
Magnetic resistance and field effect measurements were used to determine the carrier mobilities for FLG. These mobilities varied from sample to sample, and ranged between 3,000 and 10,000. Carbon nanotubes show very high mobility while the multilayer graphenes display higher mobilities up to 15,000 at 300K or 60,000 respectively.
The band overlap between different FLG samples is 4-20 meV.
Also graphene is a linear energy dispersion material and has carriers with nil mass.
The ratio for different FLG devices is between 2.5 and 7, while it is 1.5 for multilayer graphene.
Graphene is a superior metal due to its unique characteristics.
Offers ballistic transportation
Scalability up to nm
Low resistance to on-off
Zero gap semi conductor, in which there is very little overlap between the conduction and the valence bands
Electric filed, also called the ambipolar electrical field effect, allows you to control the type (electron hole or hole) as well as the density of carriers.
The applied voltage can induce higher electron and hole concentrations and greater mobilities, up to 10,000
Band Structure and Properties for Graphene
Figure 3 illustrates graphene’s band structure.
Figure 3 Graphene’s Band Structure
Pure crystal with linearly-dispersive valence, conduction bands and pure crystal that meet at discrete locations at which Fermi levels are present
The Fermi level may rise or fall depending on whether graphene is doped.
The incident light is sufficient to cause inter band transition. 2.3% is absorbed by crystals, which promotes electron migration to the conduction band. This leaves a hole.
If the light energie is too low, only intraband transitions within the valence bands occur
The graphene has a unique property that exhibits an electron mobility of more than 10,000.
It absorbs light across a wide spectrum, from UV to IR wavelengths.
The Fermi value of the metal is constant due to its high density.
However, the Fermi levels of graphene can be adjusted with the use of appropriate bias voltages or chemically doping the material for impurities.
The Fermi position can be affected by shining light onto graphene.
The Schottky barrier height depends on the Fermi level of graphene.
Two types of devices can also be produced by metal semiconductor contact.
The ohmic-joint devices are made with highly doped semiconductors. In these cases, the ratio of current and voltage is in accordance with ohm’s laws.
Schottky junction devices can be made from semiconductors that are lightly doped.
They can be used in both directions, with high currents and low resistances.
The Fermi level of the metal is different from that of the semiconductor.
This causes electrons to flow between the two until their Fermi levels are aligned.
This charge transfer reduces the region of free electrons in the semiconductor interface and leaves behind immobile positive charged.
This causes the conduction and valence band of the semiconductor to bend upwards at the interface.
The Fermi levels align at equilibrium.
The Schottky Barrier is created by the discontinuities in energy states.
Schottky barricades prevent electron flow between the metal and the semiconductor.
Schottky junctions can be made from a variety of semiconductor materials, including 3D organics, inorganics (two-layered semiconductors), 1D nanostructures, 0D quantites, and 2 layered semiconductors.
The Schottky junction formed with graphene makes up the building blocks of devices such as photo detectors, solar panels, LED’s and chemical sensors. Graphene is also mechanically strong, elastic, chemically stabilized, thermally conductive, and mechanically strong (Larsen et. al 38851-8858).
It’s best suited to sensing applications because every atom lives on graphene and has the largest contact area with its surroundings.
It works well with all standard thin film processing techniques.
Graphene, also known as gold, is atop of the n–doped semiconductor (gray), which produces an electric field. One-directional current flows from one material into the other.
– Graphene and Semiconductor work functions. (energy difference between the vacuum and Fermi levels). – Schottky barrier height. – Conduction band minimum.
– Valence band maximum
After contact is established, electrons will flow between graphene to semiconductor and bend the bands to align Fermi levels.
The Fermi level is also adjustable for graphene’s optical properties.
The device is responsive to input light fast and sensitive to faint lights (Lv et. al 1337-339).
On light fall, electron-hole pairs are created. They are collected in different areas of the circuit with an electrical field.
Figure 5 shows the graphene-silver photo diode.
In the semiconductor’s depletion layers, when the incident photon energy exceeds band gap electron-hole pair production occurs
The Schottky barrier height determines the number of electron-hole pairs that can be created in graphene.
The spectral range of optical signals detected in the Schottky junction can also be adjusted by adjusting the Schottky threshold height with the appropriate band gap
Figure 5 (a) shows that electron-holes form in the depletion layers of semiconductors when the incident photon energy becomes high.
The quantum efficiency and number of charge carriers produced per photon can be adjusted by adjusting the thickness.
Metals reflect most of the light, while graphene allows 98% to pass through the semiconductor.
A layer of oxide between graphene or semiconductor can reduce the barrier height.
Reduced dark current will improve Signal to Noise ratio.
Figure 5 (b), illustrates internal photo emission.
This is useful at longer wavelengths like IR where many semiconductors are sensitive at room temperatures.
The incident photons produce electron-hole pairs of graphene. These pair then enter the semiconductor and create current.
Graphene is a better choice than Si-based solar panels because of its advantages
Reduplication of material and
A simplified manufacturing process
Graphenesilicon solar cell with 2cm dimension
Exposure to sunlight causes the formation of charge carriers (electron holes pairs) in the semiconductor. These are then separated by the junction’s built-in potentio.
The graphene-silicon junction with the first graphene-silicon junction was only 1.7%. However, later doping graphene and stabilizing junctions achieved 15-20% conversion efficiency (Ayhan et. al 26866-26871 and Wu et. al 2486-2489).
Anti-reflection coatings such as titanium dioxid must be applied to shiny reflective graphene -Si interfaces to increase light penetration.
6.3 Other Applications
The ideal graphene/ni-type silicon (nSi), Schottky junction dodes have been fabricated. A new transport mechanism was demonstrated to describe the diode behavior (Sinha et. al 4660-664).
The ideal Metal Graphene semiconductor (MGS), ohmic contacts were formed with contact resistance lower than that of low doped Si. (Byun et.al 63-66).
Other major applications include sensors like bio-sensors (gas sensors), strain-gauge and pressure sensors as well as fuel and solar cell sensors.
These areas include textiles and fabrics for health care and electric power generation (Sharon, 145-165, Bououdina, et. 26-61)
Graphene is a 2-dimensional, crystallized allotrope of carbon.
It is one of the rare materials that has atom thinness.
Graphene is used in many areas, including nano electronics, biological engineering and energy storage.
It is also explained how it can be used.
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