3. Fabrication and Processing
In this chapter, all the major steps involved in the fabrication of a device are described. These
start with a description of the wafer growth and carry through to the techniques used in order
to realise the finished discreet device in a packaged form. This is followed by a summary, in
the form of a schematic block diagram, of the various processing steps used in the fabrication
of HBTs with conventional and ITO emitter ohmic contacts.
3.1 The Semiconductor Wafer
Some of the fundamental principles of semiconductor growth mechanisms and the associated practical methods used in the manufacture of wafers for device fabrication are discussed in this section.
Compound semiconductor materials can be realised by the formation of "solid solutions" of
two or more starting materials. These solutions occur when atoms of a different element are
able to substitute a given constituent of a material without altering its crystal structure. The
ability to do so by the new atom is referred to as its miscibility. In order that atoms can form
solid solutions over large ranges of miscibility, they must satisfy the Hume Rothery rules:
A two component alloy is known as a binary alloy; some common examples include GaAs,
AlAs, GaP, InP and InAs all of which have the Zincblende (diamond) crystal structure.
Similarly a ternary alloy is one with three components and a quaternary alloy is
one with four.
AlxGa1-xAs is a ternary compound where both
Ga and Al are from group III enabling Al to replace Ga on the alpha sites of the diamond
compound lattice. As the compound AlAs has the same structure as GaAs, this makes the
formation of solid solutions of Al in GaAs easy and preserves the same crystal structure over
the full range of Al substitution. The beta sites with As atoms are not altered in anyway. Thus
effectively, AlGaAs is an alloy of AlAs and GaAs.
Simultaneous replacement of atoms from alpha and beta sites of binary compounds allows
quaternary alloys to be formed. InxGa1-xAsyP1-y is one such example. This gives a more
flexible scheme for tailoring material properties. In this case, the binary compounds GaAs,
InP, InAs and GaP are part of the system and together they determine the limits to the range
of properties of the resultant compound.
Epitaxy refers to the ordered growth of one crystal upon another crystal [97]. Because of the
large range of possible semiconductor compounds and their alloys, it is rare in device
fabrication to grow bulk crystals of all these materials. Instead, it is more attractive to realise
the wider range of materials by epitaxial growth. This is partly due to the difficulties involved
in developing easy bulk crystal growth techniques for each new material and also because of
historical reasons. Two materials for which thorough research and bulk crystal growth and
polishing methods have already been developed are GaAs and InP.
There are three main modes of epitaxial growth: (a) monolayer, (b) nucleated and (c)
nucleation followed by monolayer. Monolayer growth occurs when the deposited atoms are
more strongly bound to the substrate than they are to each other. The atoms aggregate to
form monolayer islands of deposit which enlarge and eventually a complete monolayer
coverage has taken place. The process is repeated for subsequent layer growth. In case of
nucleated growth, the initial deposit atoms aggregate as small three-dimensional (3D) islands
which increase in size as further deposition continues until they touch and intergrow to form a
continuous film. This mode is favoured where the forces of attraction between the deposited
atoms is greater than that between them and the substrate. In the final mode, growth starts
with the formation of a single or few monolayers on the substrate followed by subsequent
nucleation of 3D islands on top of these monolayers.
Epitaxy, although a highly successful approach for growing a wide range of materials, none
the less suffers from an important constraint. Epitaxial growth requires that the atomic
spacing, the lattice constant, of the layer material not differ by more than a few percent and
that they have the same crystal structure. While most materials of interest have diamond
structure, satisfying the latter requirement, the lattice matching imposes a serious constraint on
the range of compositions that can be grown on a given bulk substrate. Although,
traditionally, these were provided by GaAs and InP, increasing use is being made of InAs and
GaSb as the substrate.
Figure 3.1: A plot of energy bandgap and lattice constant for major III-V compound
semiconductors.
This problem is best appreciated by a graph of the energy gap versus lattice constant for major
III-V compounds, as shown in Figure 3.1. This is also known as a phase
diagram. For a possible range of ternary alloy systems, a solid line is generated between the
starting binary materials. In the case of a quaternary compound, the boundary is laid out by four
intersecting lines.
The ability to tailor the bandgap of III-V alloys to a desired wavelength makes them
particularly attractive for optoelectronic applications. The dark currents are significantly
reduced in compound detectors from their Ge and Si predecessors. In addition, heterojunction
structures can be easily used to enhance their high speed operations. One such system that is
of great interest is the InP/InGaAs heterostructure which is sensitive to 1.55mm wavelength. In this case the composition of
In0.53Ga0.47As ternary compound, with the
desired bandgap of 0.75 eV, is dictated by the lattice matching constraint to the InP substrate.
Lattice mismatch, on the other hand, manifests itself in the form of dislocation-induced junction
leakage and low quantum efficiency in optoelectronic devices.
The three basic and most commonly used compound semiconductor growth techniques are
discussed below. These are Liquid Phase Epitaxy (LPE), Metal Organic Chemical Vapour
Epitaxy (MOCVD) and Molecular Beam Epitaxy (MBE). MOCVD is also sometimes
referred to as MOVPE (Metal Organic Vapour Phase Epitaxy).
LPE refers to the growth of semiconductor crystals from a liquid solution at temperatures well
below their melting point [98]. This is made possible by the fact that a mixture of a
semiconductor and a second element has a lower melting point than the pure semiconductor
alone. Thus, for example, the melting point of a mixture of GaAs and Ga is considerably
lowered from 1238 ºC, the melting point of pure GaAs. The actual melting point of the
mixture is determined by the proportion of the constituent Ga and GaAs.
For example, in the growth of GaAs, LPE is commenced by placing a GaAs seed crystal in
solution of liquid Ga and GaAs which is molten at a temperature below the melting point of
the seed. As the solution is cooled, a single crystal GaAs begins to grow on the seed leaving a
Ga rich liquid mixture with an even lower melting point; further cooling causes more GaAs to
crystalise on the seed.
By this technique single crystals can be grown at low enough temperatures to avoid the
problems associated with impurity introduction at temperatures near the melting point of the
crystal. It is particularly useful for growing III-V compounds based on Ga and In as these
metals form solutions at conveniently low temperatures. LPE remains a successful production
technique for structures that do not require thin, uniform and high quality epitaxial layers
needed for microwave devices.
This involves the forced convection of the metal organic vapour species over a heated
substrate [99]. Those molecules striking the heated crystal release the desired species,
resulting in crystal growth. The chemical process involved is quite simple in that an alkyl
compound for the group III element and a hydride for group V element decompose in the
500 ºC to 800 ºC temperature range to form the III-V compound semiconductor.
Equations (eqn. 3.1) and (eqn. 3.2) represent, to
a first approximation, the gas phase reactions
that occur during MOCVD growth of GaAs, while the corresponding reactions for the growth
of AlAs is given by equations (eqn. 3.3) and (eqn. 3.4).
Hence, in order to grow AlxGa1-xAs, a combination
of the above reactions is used and the mole fraction 'x' is determined by their
relative ratios. Similarly, in the growth of InGaAsP/InP heterostructures, the band gap is
controlled by the ratio of AsH3/PH3
while the ration between group III elements (i.e. TMGa/TMIn or TEGa/TEIn) determines the
lattice matching to the InP substrate. Doping is achieved by introducing the respective n-type
(H2Se or H2S) and p-type (demethyl zinc DMZn
/ Zn(CH3)2) gases to the reactor.
3.1.1 Compound Semiconductor Materials
3.1.2 Epitaxy
3.1.2.1 Epitaxial Lattice Matching
3.1.3 Growth Techniques: LPE, MOCVD, MBE
3.1.3.1 Liquid Phase Epitaxy - LPE
3.1.3.2 Metal Organic Chemical Vapour Deposition - MOCVD
