Graphene-Semiconductor Schottky Junction: History, Characteristics And Applications

History of Graphene-Semiconductor Schottky Junction

Discuss about the Graphene - Semiconductor Schottky jucation?

In modern electronics, the electronic properties of the material are controlled by the voltage applied externally. The carrier concentration is varied by the externally applied voltage causing an electric current to flow through the device and the effect is called the electric field effect (Bartolomeo and Di 1-58). The traditional material includes the silicon and the recent non-traditional materials include organic conductors and carbon nano tubes. Since the electric field effect in metals is screened at extremely short distances of <1nm, atomically thin metal films are required. The report discusses the Graphene-semiconductor Schottky Junction with its history, characteristics, types, band structure and properties, significance and their applications.

Graphene was synthesized from graphite oxide solution in 1962 by the organic chemist Peter Boehm. Its band structure was described by Phillip Wallace 15 years before its synthesis. University of Manchester physicists Andre Geim, Konstantin Novoselov, and their colleagues  synthesized high quality graphene in a simple, quick and cheap method by peeling away weakly bound layers from bulk graphite with the tape and then gently rubbed those layers onto an oxidized silicon surface. Thus, it becomes possible to study the 2D material and also to control its fabrication and integration into devices.

The contact between metals and semiconductors was first studied in 1874 when Ferdinand Braun probed a lead sulfide crystal with the point of the metal wire and observed free flow of current in only one direction (Li et al 46-51, 2743-748). The rectifier behavior was first demonstrated by Walter Schottky who realized that a potential barrier called Schottky barrier exists across the junction. In 1914, Ralph Hartsough at the University of Kansas observed rectification when he adjoined Carbon in the form of graphite and Silicon to form a point contact. In 1962, George Harman and Theodore Higier from the National Bureau of Standards found that graphite electrodes on p-type Silicon had the behavior of ohmic contacts. In 2009, Filippo Giannazzo of the National Research Council in Catania, Italy, and his colleagues prepared the first Schottky junction between graphene and SiC. They measured its barrier height using scanning current spectroscopy.  In the subsequent year, Xinming Li and Hongwei Zhu prepared   graphene on-silicon Schottky junction using chemical vapor deposition. The rectification characteristics and photovoltaic performance were successfully demonstrated. In 2010, Andre Geim and Konstantin Novoselov were awarded the Nobel Prize for their research experiments conducted on the two-dimensional material graphene.

Characteristics of Few Layer Graphene (FLG)

Graphene is a two-dimensional material in which a single layer of carbon atoms is densely packed into the ring structure of benzene. It is referred to as Few Layer Graphene (FLG) and the properties of carbon based materials like graphites, nanotubes etc., are described by the study of FLG. FLG films are atomically thin with high quality to ensure that 2D electronic transfer is ballistic at submicron distances (Novoselov 666-69; Supporting Online Material).

A metallic field effect transistor is demonstrated using FLG. By changing the gate voltage, the conducting channel can be changed as both 2D electron and hole gases. Platelets of highly-oriented pyrolytic graphite (HOPG) of 1-mm thick are used to prepare the graphene films. FLG films upto 10µm in size are prepared by

repeated peeling of small mesas of HOPG.

Figure 1 – Graphene films

• Photograph of large multilayer graphene flake with thickness ≈3nm on top of an oxidized Si wafer
• Atomic Force Microscopy (AFM) image of single-layer graphene - dark brown corresponds to SiO2 surface

Figure 1 represents the multilayer and single-layer graphene films. These films are processed into multi-terminal Hall bar devices placed on top of an oxidized Si substrate and gate voltage  is applied. The electronic properties of FLG devices are different from that of the 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 for T=5, 70 and 300K for top to bottom curves 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; solid squares – multi layer graphene

Figure 2 shows the dependencies of Graphene’s resistivity , conductivity  and Hall coefficient  on the gate voltage . The graph shows that  exhibits sharp peak to several  and at high  it decays to 100 . On both sides of the resistivity peak, the conductivity increases linearly with  and at the peak  exhibits sign reversal. Also, it is indicated in the figure, there exists a small overlap  between the valence band and conduction band. A surface charge density of    is induced by the gate voltage and the position of Fermi energy  is shifted.  represents the permittivity of free space and  represents the permittivity of ,  is the electron charge and  refers to the thickness of the  layer. For    

The shallow-overlap semimetal is transformed into conductor with either whole electrons or whole holes through a mixed state in which both holes and electrons are present by the electric field doping (Figure 2). Through magnetoresistance and field effect measurements, the carrier mobilities  in FLG are measured and they found to vary from sample to sample between 3,000 and 10,000  The typical mean free path is  Carbon nanotubes exhibit very high mobility and the multilayer graphenes exhibit higher mobilities upto 15,000  at 300K and 60,000 . The band overlap  in FLG for different samples varies from 4 to 20 meV. Also, graphene has linear energy dispersion and carriers with nil mass. Different FLG devices exhibit the ratio of  between 2.5 and 7 while for multilayer graphene, it is 1.5. The important characteristics that make graphene the best metal are

• Linear I-V characteristics
• Huge sustainable currents
• Offers ballistic transport
• Scalability to nm size
• Modest on-off resistance ratio
• Zero gap semi conductor in which there is a tiny overlap between the valence and the conduction bands
• The type (electron or hole) and density of carriers can be controlled by electric filed which is called ambipolar electric field effect.
• Higher electrons and hole concentrations of and higher mobilities upto 10,000  are induced by the applied voltage

Graphene's band structure and properties

The graphene’s band structure is shown in Figure 3

  Figure 3 – Graphene’s band structure

• Pure crystal with linearly dispersive valence and conduction bands that meet at discrete points at which Fermi level is present
• If graphene is doped, the Fermi level can rise or fall. The incident light  is high enough to induce inter band transition and 2.3% of the light is absorbed by the crystal promoting an electron to the conduction band leaving a hole
• If the light energy is too low, only intra band transitions in the valence band occur

The important property of the graphene material is that it exhibits electron mobility exceeding 10,000  when deposited on  or  Another important property of graphene is that it has no band gap. Hence it can absorb light across a broad spectrum from the UV to IR wavelengths. Because of the high density of the states of metal, the Fermi level of the metal is constant. But the Fermi level of the graphene may be tuned by applying appropriate bias voltage or chemically doping the material with impurities. Shining light on graphene also affects the position of Fermi level. The Schottky barrier height of the junction mainly depends mainly on the graphene’s Fermi level.

Through metal semiconductor contact, two kinds of devices are produced. They are

• Ohmic junction and
• Schottky junction

The ohmic junction devices are made from highly doped semiconductors in which the ratio between current and voltage follows ohm’s law.  The Schottky junction devices are which are made from lightly doped semiconductors. They exhibit high current and low resistance in one direction and negligible current and high resistance in the other direction.

The Fermi level  differs for the metal and the semiconductor. This disparity causes       electrons to flow from one to the other until their Fermi levels align. This charge transfer depletes a region of free charges inside the semiconductor interface leaving immobile positive charges. This causes the semiconductor’s valence and conduction bands to bend upward at the interface. At equilibrium, the Fermi levels have aligned. The discontinuities in the allowed energy states produce the Schottky barrier. The flow of electrons from the metal to the semiconductor is blocked by the Schottky barrier.

The semiconductor materials used for creating Schottky junctions with graphene include 3D organics and inorganics, 2 layered semiconductors, 1D nanostructures and 0D quantum dots. This Schottky junction with graphene forms the building block of devices like photo detectors, solar cells, LED’s, chemical sensor etc., Also, graphene material is mechanically strong, elastic, chemically stable and thermally conductive (Larsen et al 38851-8858). It is best suited for sensing applications since every atom resides on graphene’s surface and it offers the largest possible contact area with its environment. It is also compatible with standard thin film processing techniques. Figure 4 shows the Graphene-semiconductor schottky junction.

Figure 4 – Graphene-semiconductor Schottky Junciton

• Graphene (gold) atop on the n-doped semiconductor (gray) that produces an electric field and one-directional current flow from one material to the othe
• Energy band diagram of graphene-semiconductor schottky junction;

 – Graphene work function; - Semiconductor work function (energy difference between the vacuum level and Fermi level)  - Schottky barrier height;  - conduction band minimum;
- valence band maximum

Graphene-Semiconductor Schottky junction and its characteristics

When the contact is first established, electrons will flow from semiconductor to graphene and bend the bands in order to align Fermi levels

6.1       Photo detector

The optical properties of graphene are also tunable as the Fermi level. Due to high carrier mobility and ultra-broadband response to light, the graphene-semiconductor junction is an ideal photo detector. The device responds to input light quickly and it is also sensitive to faint light (Lv et al 1337-339). Electron-hole pairs are generated on light fall and they are collected in different parts of the circuit with an electric field. Figure 5 shows the graphene semiconductor photo diode.

Figure 5 – Graphene semiconductor photo detector

• When the incident photon energy exceeds the band gap  electron-hole pairs are generated in the semiconductor’s depletion layer
• When the Schottky barrier height , the electron-hole pairs are generated in the graphene. By adjusting the schottky barrier height with appropriate band gap, the spectral range of optical signals detected by the schottky junction can be tuned

When the incident photon energy is high, electron-holes are produced in the depletion layer of semiconductor as shown in Figure 5 (a). By adjusting the thickness of the layer, the number of charge carriers generated per photon or quantum efficiency can be adjusted. Metals reflect much of the light while graphene allows 98% of the light to pass through into the semiconductor. The barrier height can be improved by introducing an interfacial layer, generally an oxide layer, between graphene and semiconductor. The Signal to Noise ratio is improved by the reduction of dark current. The second mode is internal photo emission which is illustrated in Figure 5 (b). This is useful at long wavelengths like IR to which many semiconductors are insensitive at room temperature. The incident photons generate electron-hole pairs in graphene which then pass into the semiconductor and contribute to current.

6.2 Solar Cells

The advantages of using graphene in solar cells over Si based solar cells are

• Improved Performance
• Reduction of the amount of materials and
• Simplified manufacturing process
• Graphene-silicon solar cell of 2cm dimension

• On exposed to sunlight, charge carriers (electron hole pairs) are generated in the semiconductor and separated by built-in potential created by the junction

Figure 6 shows the Graphene-semiconductor solar cell. The first graphene-silicon schottky junction provided Power Conversion Efficiency of only 1.7% and later with improved method of doping graphene and stable junction, 15-20% conversion efficiency is attained (Ayhan et al 26866-26871; Wu et al 2486-2489). To maximize the amount of light penetrating into the semiconductor, anti-reflection coating such as titanium di oxide has to be added to the shiny reflective graphene-Si interface.

6.3       Other Applications

Ideal graphene/n-type silicon (n-Si) Schottky junction diodes are fabricated and a new transport mechanism is demonstrated to describe the ideal diode behavior (Sinha et al 4660-664). The ideal Metal Graphene Semiconductor (MGS) ohmic contact was formed with contact resistance less than  with low doped Si (Byun et al 63-66). Metal-semiconductor-metal (MSM) photodetectors based on graphene/p-type Si Schottky junctions are fabricated and characterized (An et al 1-5). The other major applications include sensors such as bio-sensors, gas sensors, strain-gauge, pressure sensors, chemi-sensors, fuel and solar cells. The application areas include health care, textiles and fabrics, bio-devices and electric power generation (Sharon et al 145-165; Bououdina et al 26-61)

Conclusion

Graphene is the 2-dimensional, crystalline allotrope of carbon. It is one of the incredible materials with atom thinness. Graphene finds its enormous applications in nano electronics, biological engineering, optical electronics, photo voltaic cells, energy storage etc., In this report, the history of Graphene, graphene semiconductor schottky junction, its characteristics, band structure, types including Few Layer Graphene (FLG) and properties are discussed. The applications are also elucidated.

References

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