Doping
Doping
Intrinsic semiconductor is pure, has no impurity atoms and is called i-type semiconductor.
It is un-doped semiconductor and its conductivity is negligible so it cannot make a useful
device. Its number density of electrons is equal to the number density of holes. [ne = nh].
Its valence band is filled and the conduction band is empty. If its temperature is raised,
some of the valence electrons may get lifted to conduction band leaving behind holes in
the valence band. These electrons and holes can move randomly. With increasing
temperature, the resistivity of the material decreases and the conductivity increases.
If we intentionally modify an intrinsic semiconductor by adding external impurities, it gets
converted to extrinsic semiconductor. Such modification – i.e. the process of adding
impurities to intrinsic semiconductors to change their electrical properties – is called
doping. Mostly trivalent and pentavalent elements are used to dope these elements.
Generally, one impurity atom is added to about 108 atoms of a semiconductor.
The purpose of adding impurity in the semiconductor crystal is to increase the number of
free electrons or holes to make it conductive. If pentavalent impurity atoms having five
valence electrons is doped in an intrinsic semiconductor, it will create a larger number of
free electrons (negatively charged) than the number of holes (ne >nh) and it becomes N-
type semiconductor. However, if a trivalent impurity having three valence electrons is
added, the number of holes (positively charged) in the semiconductor will become more
than the number of electrons in it. (ne < nh) and it becomes P-type semiconductor. Typical
doping level is NA or ND = 1015–1018 per cc whereas for intrinsic ni = 1.3 x 1010 per cc.
Presence of doped impurity atoms in the extrinsic semiconductor leads to the formation of new energy levels within the bandgap. Energy levels created by N-type impurity (called electron donor or donor) are close to the conduction band. And a small amount of applied
Presence of doped impurity atoms in the extrinsic semiconductor leads to the formation of new energy levels within the bandgap. Energy levels created by N-type impurity (called electron donor or donor) are close to the conduction band. And a small amount of applied
energy can cause the transition from donor level to the conduction band. Similarly, the
energy levels created by P-type impurity (called electron acceptor or acceptor) are close
to the valence band. And it is rather easy to make it transit from donor level to the
conduction band.
Doping is done by: a) Molecular diffusion: transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion, resulting in a gradual mixing of materials like a drop of ink in a glass of water, and b) Ion implantation: Charged dopants (ions) are accelerated in an electric field and irradiated onto the wafer. The penetration depth of dopants is precisely decided by the acceleration voltage applied to the ions.
Epitaxial Growth:
If the deposited film and the substrate are of same material (e.g. Si on Si), it is homoepitaxy and if they are different, it is heteroepitaxy. Homoepitaxial films are purer than the substrate, could be made defect-free and can easily be doped. However, the Sublimation, vapour phase, liquid phase and the use of molecular beams are four routes of executing the epitaxial growth process. Epitaxy has led to the creation of new wide- bandgap optoelectronic devices such as LEDs and lasers, photon sources and detectors used in optical communications systems, High-speed and high frequency wireless communications devices. Epitaxy allowed fabricating multilayer heterojunction like quantum wells and superlattices. It also facilitates bandgap engineering of nanoscale devices.
Lattice matching in epitaxial growth:
If the epitaxial film and the substrate have different chemistry and thermal expansion, it often results in a lattice mismatch between the two. It affects the electronic properties and perfection of the interface. If the film and substrate lattice parameters differ substantially, two possibilities emerge. Either the two lattices undergo strain to accommodate their ular defects form at the interface (Relaxed epitaxy). Generally, the initial few epitaxial layers strain themselves to match the substrate crystallography and the subsequent layers are coherent epitaxial layers. Thus the strained-layer epitaxy generally prevails when the film and substrate are of dissimilar materials having similar crystal structure
Vapor phase epitaxy
VPE is also called Chemical Vapour Deposition (CVD). In this process, precursor gases such as SiCl4 and H2 are introduced in a reactor tube in which the substrate is kept in a quartz holder at a high temperature and suitably low pressure is also maintained. The deposition temperature may be as high as 1500 oC, the pressure could be a few mTorr and the rate of deposition could be as high as 2 micron per minute. The precursor gases are diluted in a carrier gas such as H2. Gases like Silane, Dichloro-silane and Trichloro- silane are also used as precursor (source) gas. These gases generally deliver an epitaxial layer of polysilicon at a relatively low temperature (~650 oC). The epitaxial constituents are transported in volatile form and they react at the substrate to form epi layer.
The chemical reaction of precursor and carrier gases results in a reversible reaction, such as SiCl4 + 2H2 Si + 4HCl through which a simultaneously competing deposition and etching reaction takes place. The proportion of the chemicals in the precursor determines whether the growth would dominate or the etching would dominate the process. The reactor wall id often hot and the substrate holder is tilted to allow fresh and equally unused precursor gases to all the substrates. The constituent reactions of the above overall reaction are as follows and 14 species in the above reaction were detected at 1200 oC by Infrared spectroscopy, mass spectroscopy and Raman spectroscopy.
SiCl4 + H2 SiHCl3 + HCl
SiHCl3 + H2 SiH2Cl2 + HCl
SiH2Cl2 SiCl2 + H2
SiCl2 + H2 Si + 2HCl
Doping is done by: a) Molecular diffusion: transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion, resulting in a gradual mixing of materials like a drop of ink in a glass of water, and b) Ion implantation: Charged dopants (ions) are accelerated in an electric field and irradiated onto the wafer. The penetration depth of dopants is precisely decided by the acceleration voltage applied to the ions.
Epitaxial Growth:
The process of depositing a crystalline layer over a crystalline substrate is called Epitaxy
and the deposited overlayer is called an epitaxial film or epitaxial layer. Such films can be
grown from gaseous or liquid precursors (starting chemicals) and the substrate itself acts
as the seed, whose crystallographic orientation continues in the layer being deposited. If
the layer being deposited has a random orientation, it is non-epitaxial. The epitaxial layer
can be doped during its deposition, by adding impurities such as arsine, phosphine or
diborane to the precursor.
If the deposited film and the substrate are of same material (e.g. Si on Si), it is homoepitaxy and if they are different, it is heteroepitaxy. Homoepitaxial films are purer than the substrate, could be made defect-free and can easily be doped. However, the Sublimation, vapour phase, liquid phase and the use of molecular beams are four routes of executing the epitaxial growth process. Epitaxy has led to the creation of new wide- bandgap optoelectronic devices such as LEDs and lasers, photon sources and detectors used in optical communications systems, High-speed and high frequency wireless communications devices. Epitaxy allowed fabricating multilayer heterojunction like quantum wells and superlattices. It also facilitates bandgap engineering of nanoscale devices.
Lattice matching in epitaxial growth:
If the epitaxial film and the substrate have different chemistry and thermal expansion, it often results in a lattice mismatch between the two. It affects the electronic properties and perfection of the interface. If the film and substrate lattice parameters differ substantially, two possibilities emerge. Either the two lattices undergo strain to accommodate their ular defects form at the interface (Relaxed epitaxy). Generally, the initial few epitaxial layers strain themselves to match the substrate crystallography and the subsequent layers are coherent epitaxial layers. Thus the strained-layer epitaxy generally prevails when the film and substrate are of dissimilar materials having similar crystal structure
Vapor phase epitaxy
VPE is also called Chemical Vapour Deposition (CVD). In this process, precursor gases such as SiCl4 and H2 are introduced in a reactor tube in which the substrate is kept in a quartz holder at a high temperature and suitably low pressure is also maintained. The deposition temperature may be as high as 1500 oC, the pressure could be a few mTorr and the rate of deposition could be as high as 2 micron per minute. The precursor gases are diluted in a carrier gas such as H2. Gases like Silane, Dichloro-silane and Trichloro- silane are also used as precursor (source) gas. These gases generally deliver an epitaxial layer of polysilicon at a relatively low temperature (~650 oC). The epitaxial constituents are transported in volatile form and they react at the substrate to form epi layer.
The chemical reaction of precursor and carrier gases results in a reversible reaction, such as SiCl4 + 2H2 Si + 4HCl through which a simultaneously competing deposition and etching reaction takes place. The proportion of the chemicals in the precursor determines whether the growth would dominate or the etching would dominate the process. The reactor wall id often hot and the substrate holder is tilted to allow fresh and equally unused precursor gases to all the substrates. The constituent reactions of the above overall reaction are as follows and 14 species in the above reaction were detected at 1200 oC by Infrared spectroscopy, mass spectroscopy and Raman spectroscopy.
SiCl4 + H2 SiHCl3 + HCl
SiHCl3 + H2 SiH2Cl2 + HCl
SiH2Cl2 SiCl2 + H2
SiCl2 + H2 Si + 2HCl
The main advantages of the vapor phase epitaxy (VPE) are the ability to grow very good
quality layers, with high growth rate (higher than μm min –1). Its principle is relatively
simple and allows great flexibility in the level or type of doping. VPE can handle several
large wafers, which is particularly desirable for photovoltaic applications.
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