The characteristics of amorphous silicon thin film

From Fig 2.4.2-1 the structure of amorphous silicon (a-Si) which is a structure with periodic atoms arrangement in a small area but a random atoms arrangement in large area, and there is a poor bonding energy between silicon atoms so it is easily to broken and forming dangling bond, these dangling bond cause a large defect density in amorphous silicon. Generally, the defect density of amorphous silicon is about 10²¹cm^-3, in order to reduce the dangling bond density, hydrogen will be induced during the manufacturing process to passivation (a-Si: H), hydrogen atoms can bond with dangling bond further to reduce the density of dangling bond.

Because of the atoms arrangement of amorphous silicon is a random arrangement, so the band gap definition of amorphous structure is not as the same as crystalline structure, because of in amorphous semiconductor there are some band tail at the edge of conduction band and valence band and these band tail which is caused

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by random atoms arrangement. In addition, defects can also form a defect state which located in the middle of the band gap, and these defects generate a continue state between conduction band and valence band, which is totally different with the band gap of crystalline silicon, any state between conduction band and valence band are not allow, so we cannot define the band gap of amorphous silicon clearly.

We will re-define the band gap and the energy state of amorphous silicon.

Firstly, instead of the strict definition of the highest energy of valence band and the lowest energy of conduction band of crystalline silicon, there is a similar definition which is called mobility edges as indicated by the symbol E_C and E_V. The energy state is divided into two parts: first one is extended states which is the energy state with the energy at E <E_V and E> E_C; the second one is localized states (localized states) which is the energy state with the energy at EC> E> EV. The carrier mobility in amorphous silicon is very low, in usually the electron mobility is around 1 ~ 10cm² / (V.s) and the hole mobility is around 0.01 ~ 0.1cm² / (V.s). The carrier mobility is near zero when the carrier is in localized states, usually the carrier mobility was cause by the mechanism of thermal effect or tunneling effect which we called hopping, therefore, the difference between the E_C and E_V at mobility edge was called the mobility gap, and this band gap does not have meaning of forbidden gap. The general mobility gap of amorphous silicon is around 1.7 ~ 1.8eV. Figure 2.4.2-2 is the fundamental model of amorphous silicon energy state, we can describe the band tail and defect state by using mathematics model. As near the central of mobility edge the mobility edge defect density of valence band and conduction band is exponential decay. The defect states are located in the middle of mobility gap and with a Gaussian distribution.

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Fig 2.4.2-1 The 1.atomic arrangement of crystalline, poly, amorphous silicon

Figure 2.4.2-2 The fundamental energy state model of amorphous silicon

Hydrogenated amorphous silicon thin-film is the alloy of silicon atoms and hydrogen atoms, the ratio of silicon and hydrogen and bonding structure will affect the film material, optical and electrical properties. Typically, there is a Staebler-Wronski effect in amorphous silicon application [16], this effect will lead to light-induced degradation [14.15]. Due to the great defect amount of amorphous silicon, after 1000 hours illumination the performance of amorphous silicon will shows a sharp decline, it is caused by the bonding structural changes due to illumination.

The most basic features of amorphous silicon is the special arrangement of atoms which shows an order arrangement in short-range and disorder arrangement in

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long-range which is a random network of covalent atomic structure, it is means for a single silicon atom the covalent bonds is form by four silicon atoms just as the same as single crystalline silicon and it is regularly arrangement for neighboring atoms, but there is an irregular arrangement for distant atoms.

Amorphous silicon is the direct band gap material, so the light absorption coefficient of amorphous silicon is very large and the energy gap is around 1.7eV.

However, the energy gap of amorphous silicon is tunable by different alloy ratio, and the tunable range is about 1.4eV ~ 2.0eV.

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Chapter3. Experimental Instruments and Methods

3.1 Scanning Electron Microsope (SEM)

The electron microscope uses electromagnetic lenses to focus the beam to produce an image. However, SEM and TEM differ in the way images are produced and magnified. SEM is used to view the surface or near surface of a sample, whereas TEM provides information of the internal structure of thin specimens. The scanning electron microscope as the name suggests scans across the specimen by the scan coils.

As the sample is scanned by the electron beam as shown in figure 3.1-1, it emits electrons and electromagnetic radiation. A detector counts the low energy secondary electrons (< 50 eV) or other radiation emitted. The image is produced by two dimensional intensity distributions by scanning a cathode ray tube (CRT) spot onto a screen and modulating the brightness by the amplified current from the detector.

Three dimensional samples change the way electrons are emitted and results in the appearance of a three dimensional image. Resolutions less than 1 nm may be achieved.

Figure 3.1-1 Schematic diagram of a scanning electron microscope.

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