\r\n\tHydrogen gas is the key energy source for hydrogen-based society. Ozone dissolved water is expected as the sterilization and cleaning agent that can comply with the new law enacted by the US Food and Drug Administration (FDA). The law “FDA Food Safety Modernization Act” requires sterilization and washing of foods to prevent food poisoning and has a strict provision that vegetables, meat, and fish must be washed with non-chlorine cleaning agents to make E. coli adhering to food down to “zero”. If ozone dissolved water could be successively applied in this field, electrochemistry would make a significant contribution to society.\r\n
\r\n\tOxygen-enriched water is said to promote the growth of farmed fish. Hydrogen dissolved water is said to be able to efficiently remove minute dust on the silicon wafer when used in combination with ultrasonic irradiation.
\r\n\tAt present researches on direct water electrolysis have shown significant progress. For example, boron-doped diamonds and complex metal oxides are widely used as an electrode, and the interposing polymer electrolyte membrane (PEM) between electrodes has become one of the major processes of water electrolysis.
\r\n\tThe purpose of this book is to show the latest water electrolysis technology and the future of society applying it.
Germanium based nano-electronic technology suffers from two major limitations. In order to produce high speed devices, n-type germanium is preferred over p-type germanium because electrons have a higher mobility than holes. However, the doping levels in n-type germanium are low . The second major limitation is that it is difficult to produce Ohmic contacts on n-type germanium [2, 3, 4, 5, 6, 7] because of Femi-level pinning. One way to achieve Ohmic contacts is by doping the semiconductor heavily enough so that tunneling is possible, this will be explained further in Section 1.1.2. However, heavy doping is a bulk process which is not always possible in n-type Ge. Other ways of producing Ohmic contacts are interface control processes like the local incorporation of dopant atoms at the metal-germanium interface or the insertion of an interlayer into the interface. The contact resistivity is very sensitive to the interlayer thickness and the temperature of annealing used during the fabrication process.
It has been demonstrated, in earlier studies , that NiGe/Ge and PdGe/Ge Schottky contacts have some of the lowest values of sheet resistivity in Ge-based technology. These contacts were also observed to remain stable over a wide temperature range during annealing [8, 9]. In this chapter we present the essential theoretical and experimental details required in order to make a comprehensive review of some of the interface control processes developed for the fabrication of NiGe/n-Ge and PdGe/n-Ge Schottky and Ohmic contacts; the review is presented in the next chapter.
Electrons in solids obey Fermi-Dirac statistics. At low temperatures, the distribution of electrons over a range of allowed energy levels at thermal equilibrium is given by,
where, kB is the Boltzmann constant. The function, f(E) is the Fermi-Dirac distribution function which gives the probability that an available energy state, E will be occupied by an electron at temperature, T on the Kelvin scale. The quantity, EF is the Fermi level. Figure 1 shows a schematic illustration of the dependence of the Fermi-Dirac distribution function on electron energy at various temperatures.
At T = 0 K, f(E) = 1 for E ≤ EF and f(E) = 0 for E > EF. This means that there is a 100% probability that all available energy states, up to the energy, EF, will be occupied at absolute zero, i.e., all energy levels up to EF are occupied at 0 K. As the temperature is increased to T1 and T2 some energy levels which were occupied will become vacant and some energy levels, above the Fermi energy, which were vacant at absolute zero will become occupied. The probability, f(E) at all temperature, T is equal to 0.5 when the energy E is equal to a quantity, μ called the chemical potential. At T ≈ 0 K, μ = EF and therefore f(EF) = 0.5.
In applying the Fermi-Dirac distribution to semiconductors, we must recall that f(E) is the probability of occupancy of an available state of energy, E. Thus, if there is no available electron state at the energy, E (e.g., if E is in the band gap of the semiconductor), there is no possibility of an electron having that energy. We can best visualize the relationship between f(E) and the band structure of a semiconductor by turning the f(E) versus E diagram on its side so that the E scale corresponds to the energies of the energy band diagram as shown in Figure 2.
Figure 2 represents intrinsic materials where the concentration of holes in the valence band is equal to the concentration of electrons in the conduction band and therefore the Fermi level EF lies near the middle of the band gap. In reality the effective densities of states, NC and NV in the conduction and valence bands respectively are slightly different because they depend on the effective inertial masses of the electrons and holes respectively, which are not the same. This causes the intrinsic Fermi level to be slightly displaced from the middle of the gap.
In n-type materials, there is a higher concentration of electrons in the conduction band than the hole concentration in the valence band. Thus, the Fermi level lies nearer the conduction band than the valence band, as shown in Figure 3.
In p-type materials there is a higher concentration of holes in the valence band compared with the electrons in the conduction band. The Fermi level therefore lies nearer the valence band than the conduction band, as seen in Figure 4.
In metals the valence and conduction bands overlap and there is not band gap. The Fermi level of a metal therefore lies in its conduction band, this fact will be referred to later on as we analyze Figure 13 in Section 1.1.3 and Figures 14 and 15 in Section 1.1.4.
Allowed electron energy states can be produced in the forbidden band gap of a semiconductor by the introduction of impurities or defects in the crystal. A metal-semiconductor interface introduces incomplete covalent bonds and other lattice defects at the semiconductor surface, which may result in the creation of interface states in the band gap. To explain a way in which states may be created in the band gap, we could take the example of Ge doped with a donor impurity such as phosphorus (P) and an accepter like boron (B), as shown in Figure 5.
Phosphorus is in group V of the periodic table and is pentavalent. A P atom in the Ge lattice has the required number of valence electrons to complete the covalent bonding with four neighboring Ge atoms. The fifth valence electron of P does not fit into the bonding matrix of the Ge lattice and is therefore loosely bound to the P atom. Such electrons introduce energy levels very near the conduction band in the Ge band gap. These levels are occupied with electrons at 0 K and very little thermal energy is required to free them from the P atom, i.e., to excite them to the conduction band. At a temperature between 50 and 100 K, virtually all of the electrons in the impurity P levels are “donated” to the conduction band as shown in Figure 6.
Atoms like B from group III of the periodic table introduce accepter impurity levels in the Ge band gap near the valence band. B has only three valence electrons to contribute to the covalent bonding thereby leaving one incomplete bond. Such levels are not occupied by any electrons at 0 K. As the temperature is increased the thermal energy increases enough to excite electrons from the valence band into the impurity levels, leaving holes in the valence band, which become current carriers when an external field is applied, by the continuous “hopping” of electron across adjacent incomplete bonds.
The incomplete bonds at a metal-semiconductor interface introduce energy levels in the band gap in a way similar to those due to accepter impurities. Interface traps, which introduce energy levels in the Ge band gap, can be caused by a sudden termination of a Ge crystal lattice at a metal/germanium interface.
A reference energy, E0, called the vacuum energy, is the energy that an electron just “free” of a material would have in a vacuum. The work function of a semiconductor, Φs is defined as the energy required to move a unit electronic charge from the Fermi level to the vacuum level, i.e.,
where q is the electronic charge and EFS represents the Fermi energy of the semiconductor. Similarly the work function of a metal is,
where EFm represents the Fermi energy of the metal. The electron affinity, χs of a semiconductor is defined as the energy required to move a unit electronic charge from the conduction band edge to the vacuum level, i.e.,
Figure 7 is a schematic diagram of the band structures of a metal and a semiconductor before contact for the case where Φm > Φs, the semiconductor Fermi level, EFS is higher than that of the metal, EFm.
If Φm > Φs, the total energy of a metal/n-type semiconductor system could be reduced by moving electrons from the n-type semiconductor to the metal. When a metal is placed in contact with the semiconductor, therefore, electrons diffuse from the semiconductor to the metal in order to establish equilibrium. The electron diffusion causes the Fermi levels of the metal and the semiconductor to align at the same level throughout the interface region. Since the electron diffuse from the n-type semiconductor into the metal leaves behind uncompensated donor ions, a depletion region, W of an induced resultant positive charge is developed on the semiconductors side of the junction. A corresponding negative resultant charge is therefore induced on the metal side of the junction. The resultant positive charge from uncompensated donor ions in the depletion region matches the resultant negative charge induced on the metal, resulting in an electric field directed from the positive charge in the semiconductor to the negative charge in the metal. This causes the conduction energy band, Ec and the valence energy band, Ev of the semiconductor to bend in order to maintain continuity in the semiconductor band structure across the depletion region W, this is shown in Figure 8.
The electric field builds up to a magnitude where it eventually stops the electron diffusion across the junction, hence reaching a point of equilibrium. The corresponding induced equilibrium contact potential, Vo, across the junction, which prevents further electron diffusion from the semiconductor into the metal, is the difference in the work function potential energies, Φm − Φs, of the metal and the semiconductor, i.e., an energy of, q(Φm − Φs) is required for an electron to cross from the semiconductor to the metal. The barrier Vo can be raised or lowered by the application of a voltage across the junction.
When a forward-bias voltage V is applied to the barrier the contact potential is reduced from Vo to Vo − V, as shown in Figure 9. As a result, electrons in the semiconductor’s conduction band can diffuse across the depletion region to the metal. This gives us the forward current from the metal to the semiconductor, since the direction of electron flow is opposite to the associated current direction.
When a reverse-bias voltage Vr is applied to the metal/n-type semiconductor junction, the contact potential is increased from Vo to a large potential barrier for electron flow from the semiconductor to the metal of, Vo + Vr as shown in Figure 10. The electron flow from semiconductor to metal becomes negligible.
The reason that Ohmic contacts can be formed by heavy doping of the semiconductor, as mentioned in Section 1, is that even if this large, Vo + Vr barrier exists at the interface, the heavy doping reduces the width of the depletion region, W to an extent that is small enough to allow electrons to tunnel through this barrier.
In both the forward and reverse-bias cases, electrons in the metal need to tunnel through an energy barrier of height,
in order to get into the semiconductor. The quantity, ΦB, which will often also be labeled as ΦBn in this chapter, is referred to as the Schottky potential barrier height. This potential barrier height is unaffected by the bias voltage but any reverse current due to electron injection from the metal into the semiconductor depends on the size of the Schottky potential barrier height, ΦB. Electrons may flow from the metal to the semiconductor but the flow is retarded by the energy barrier, q(Φm − χs). The contact therefore acts as a diode with I–V characteristics of the form sketched in Figure 11.
The resulting, I–V equation is similar in form to that of the p-n junction diode,
The reverse saturation current, I0 depends on the size of the energy barrier, q(Φm − χs) for electron injection from the metal into the semiconductor. This behavior of a metal/semiconductor contact is referred to as Schottky behavior as opposed to Ohmic behavior.
In many cases we wish to have an Ohmic metal/semiconductor contact, having linear I-V characteristics in both biasing directions. In n-type metal/semiconductor contacts, ideal (without Fermi level pinning) Ohmic behavior is observed if Φm < Φs. The separate energy bands for a metal and a semiconductor in this case are shown, before contact, in Figure 12.
After contact is made between the semiconductor and the metal, in the Φm < Φs (n-type) case, the Fermi levels become aligned at equilibrium by transferring electrons from the metal to the semiconductor and not by the transfer of electrons from the semiconductor to the metal. The energy band structure, at the interface, for this case is illustrated in Figure 13.
We see in Figure 13 that the conduction band of the semiconductor bends downwards towards the Fermi level of the metal at the interface, at equilibrium. What this means is that, since the Fermi level of a metal is in its conduction band as mentioned at the end of Section 1.1, the electrons in the metal are free to cross from their Fermi level straight into the conduction band of the semiconductor, i.e., electrons can flow unimpeded across the two conduction bands in both directions. Unlike in the rectifying contacts discussed earlier, no depletion region, W occurs in the semiconductor in this case.
The ideal energy band diagram for a metal semiconductor interface has two main limiting factors. Firstly, the ideal contact does not take the surface states into account between a metal and a semiconductor. Secondly, when a practical metal/semiconductor interface is made, a thin interfacial layer is present on the semiconductor surface. This thin layer could potentially be a native oxide or processing residue, which contains a large density of surface states (Dit), many with energies distributed within the band gap of the semiconductor. The physics of the junction is then no longer governed by the properties of the metal and semiconductor materials alone but is then largely governed by the properties of the semiconductor surface .
The height of the Schottky potential barrier (ΦB) is, in the ideal case, the difference between the metal work function (Φm) and the semiconductor’s electron affinity (χs). A thin native oxide or processing residue insulating layer at the metal/semiconductor interface causes an additional voltage drop (Vi) over the metal/semiconductor interface, which is determined by the charge (Qs) at the semiconductor surface and the capacitance at the interface layer (Ci). Therefore,
It is possible to define a neutral level, Φ0 in the interface energy band diagram. When the Fermi Level differs from the neutral level Φ0, a net charge (Qit) will be present at the semiconductor surface. Depending on the position of the surface states relative to Φ0, the semiconductor surface will either be positive or negatively charged. For a very large density of surface states (Dit), the potential barrier height is only dependent on the band gap of the semiconductor and the neutral level of the semiconductor. Fermi level pinning then takes place at the interface making the potential barrier height independent from the metal work function. If the density of surface states is modeled as being infinitely large then the potential barrier height would be pinned at (2/3)Eg, which is known as Bardeen’s limit . The formation of surface states is dependent on the bonding type of the semiconductor material . Covalent semiconductors such as germanium give rise to a large density of states at the surface due to the unsaturated bonds at the surface. For ionic semiconductors, the potential barrier height is more dependent on the metal work function . In the following section we will define a quantity, n called the ideality factor and the symbol, ΦBn will be used for the Schottky potential barrier height.
The emission of electrons across a Schottky potential barrier can be described by two mechanisms: thermionic emission (TE) and diffusion. In practice, the transport process is a combination of both. The thermionic emission theory is based on a heat-induced flow of charge carriers from a surface over a potential energy barrier and is derived from the following assumptions [12, 13]:
The energy barrier height (qΦBn) is greater than the thermal energy of the electrons determined by kBT, where kB is Boltzmann’s constant and T is the absolute temperature;
Thermal equilibrium is achieved at the plane that determines the emission;
Thermal equilibrium is not affected by the existence of a current flow. The two current fluxes, from the semiconductor to the metal and vice versa, can be superimposed;
The transfer of electrons across the interface of the metal and the semiconductor is the current limiting factor;
The electron mean-free-path should be bigger than the width of the region over, which a drop in potential energy, with a value of (kB T), occurs at the barrier.
where JT0 is the saturation current determined by
The constant A* is called the effective Richardson constant, q the electron charge, the electron effective mass and h Planck’s constant. The saturation current density (JT0) is therefore independent of the applied voltage. ΦBn is the zero bias effective Schottky potential barrier height which can be obtained using the intercepts of the straight lines obtained by the extrapolation of JT0 in the semi-log forward bias ln J–V characteristics according to [15, 16]:
The factor n is equal to 1 for an ideal diode which conforms to pure thermionic emission but usually has values between 1 and 2. It determines the departure from the ideal diode characteristics and therefore modifies the diode equation, it is called the ideality factor, as mentioned earlier. The values of n can be calculated from the slopes of the linear regions of the semi-log forward bias ln J–V characteristics. It can be determined assuming pure thermionic emission [15, 16] using:
The ideality factor is not a constant as it depends on the bias (V) and can only be specified for a particular point on a current-voltage characteristic curve.
The diffusion theory is based on the transport of charge carriers across a depletion region and is derived from the following assumptions :
The energy barrier height (qΦBn) is greater than the thermal energy of the electrons determined by kBT;
The effect of electron collisions taking place in the depletion region is included;
The current flow does not affect the carrier concentrations at the interface and in the semiconductor;
The impurity concentration of the semiconductor does not degenerate.
The diffusion current-voltage characteristics can be derived from the current density in the depletion region:
The diffusion current density in the x-direction depends on the electron charge, q the electron mobility, μn the electron concentration, n(x), the electric field at the barrier, E(x) and the diffusion coefficient for electrons, Dn. The current density can only be expressed in this form if the mobility and diffusion coefficient are independent of the electric field . The total current density, JD with an applied voltage across the barrier, V and temperature, T can be expressed, after applying Einstein’s relationship (Dn/μn = kT/q), in the form:
where the saturation current is,
The saturation current density, JD0 is determined by the effective density of states in the conduction band, NC, the built in potential, Vbi, the donor concentration, ND, the permittivity of free space, ε0 and the relative permittivity of the semiconductor material, εrs.
We can see that the expressions for the current density are similar for the thermionic emission and diffusion theory and are based on the saturation current density. However, the saturation current density for the thermionic emission theory, JT0 is more sensitive to the temperature while the saturation current density of the diffusion theory, JD0 is more sensitive to the applied voltage . It should be mentioned that a number of the results discussed in this chapter are for I–V as opposed to J–V characteristics, in such cases the equations presented in this section are applied with the incorporation of the perpendicular cross-sectional area of the current’s flow.
If a Schottky diode is connected to electrodes which give a maximum electric field, Em, there is what is referred to as an image-force which is the interaction due to the polarization of the conducting electrodes by the charged atoms of the sample. The image-force effect causes the energy barrier for electron transport across a metal-semiconductor interface to be lowered. The amount of barrier height reduction, ΔΦB is given by [15, 16],
In Schottky diodes, the depletion layer capacitance, C can be expressed as ,
where A is the cross-sectional area of the diode, V0 is obtained from the intercept of the C−2–V plot with the voltage axis and ND is the donor concentration of the n-type semiconductor substrate. The value of ND can be determined from the slope of the C−2–V plot using Eq. (17). The maximum electric field Em can be calculated using,
Due to the presence of surface states, an interfacial layer, microscopic clusters of metal-semiconductor phases and other effects, it is difficult to fabricate junctions with barriers near the ideal values predicted from the work functions and electron affinity. Therefore, measured barrier heights are used in device design and fabrication. In some semiconductors like Ge, the metal/semiconductor interface introduces states in the semiconductor band gap that pin the Fermi level at a fixed position, regardless of the metal used.
An example of a semiconductor Fermi level, EFS that is pinned well below the conduction band edge is in n-type GaAs. In this case there is a collection of interface states located at energy position that are 0.7–0.9 eV below the conduction band. These state are responsible for pinning the Fermi level as explained later in the next section. The Fermi level, EFS at the surface of the n-type GaAs is pinned at a position which is 0.8 eV below the conduction band edge, regardless of the choice of metal used, as shown in Figure 14. The Schottky barrier height is then determined from this pinning effect rather than by the work function of the metal. This means that electrons at the Fermi level of any metal in contact will always have to overcome the 0.8 eV barrier in order to cross over into the conduction band of the semiconductor.
A somewhat unique case of interest is in n-type InAs were EFS at the interface is not pinned below but above the conduction band edge of the semiconductor, as shown in Figure 15. The semiconductor conduction band edge bends downwards at the interface with the metal just as illustrated for Ohmic contacts by Figure 13 in Section 1.1.3. However, in this case, the bending of the semiconductor conduction energy band edge goes to a position, at the interface, which is below the Fermi levels of both the semiconductor EFS and the metal EFm. Regardless of the metal in contact, the semiconductor Fermi level, EFS remains in the conduction band (above EC) of both the semiconductor and the metal. Since the Fermi level is the highest energy level filled with electrons at 0 K, this means that the electrons can freely cross from the metal to the semiconductor and back, at any temperature. Excellent Ohmic contacts to n-type InAs can therefore be produced by the deposition of almost any metal as a contact because of this Fermi level pinning in the conduction band.
Fermi level pinning can be described by the theory of Metal Induced Gap States or MIGS . In a metal/semiconductor junction the free electron wave function can penetrate into the semiconductor band gap. This generates band gap states, which consist of donor and acceptor like states. As mentioned in Section 1.1.4, there is a charge neutrality level, Φ0 in the band gap where the gap-state charges are balanced. The Fermi level is pinned close to the charge neutrality level because of dipole formation. To prevent the Fermi level pinning, the free electron wave function penetration has to be reduced. This can be done by introducing a thin dielectric layer. Si3N4 [17, 18, 19] has low dielectric constant and moderately high band gap to prevent the free electron wave function from penetrating into the semiconductor band gap and hence releasing the Fermi level. Al2O3  has also been reported to reduce the Fermi level pinning effects.
There are principally three different methods of depositing metal on a substrate: plating, metal evaporation and sputtering. Metal plating is generally used to deposit thick layers. Only metal evaporation and sputtering were used in the work reported on in this chapter.
Evaporation techniques are based on heating up a source to a temperature where the material starts vaporizing. The vaporized material is then deposited on the sample and cools down forming a thin film. Thermal evaporation can either be achieved by heating the source with a resistive element or by using an electron beam. Resistive heating takes place by passing a current through a heating element, often made out of tungsten, which heats up a crucible containing the source material. Resistive evaporation has the disadvantage of potential contamination from the crucible if the melting temperature of the crucible is close to the melting temperature of the source material, resulting in a poor film quality. Electron beam evaporation uses an electron beam generated from a cathode to heat up the source material locally. The crucibles are water cooled to minimize contamination. The electron beam is generated by a thermionic emission filament and is accelerated towards the crucible using a high accelerating voltage. The beam is then focused into a spot on the surface of the source material and the interaction between the accelerated electrons and the source material will cause the material to start heating up and vaporize. The combination of local heating and water cooled sources prevents crucible metal contamination, resulting in a high purity film deposited on the substrate. The evaporation processes take place under high vacuum (10−3–10−4 mTorr) in order to create a mean free path of the evaporating flux, which is greater than the distance between the source and the sample.
While evaporation requires a source to be heated to produce a flux of gas, sputtering targets make use of a physical plasma process rather than heat. The plasma is formed using an inert gas (normally Argon) and is excited by either a direct current (DC) or radio frequency (RF) source. The target source is negatively biased and the plasma sputters neutral atoms of source material away from the target towards an anode, where the neutral atoms are deposited on the sample. Since a plasma is required, the working pressures of sputtering systems are relatively high (≈10−1 mTorr). The sputtering method used for most of the work reported on in this chapter is RF magnetron sputtering. Radio frequency magnetron sputtering is an enhanced sputter method which enables a higher deposition rate at low operating pressure together with the possibility of obtaining high quality films at low as well as high substrate temperatures. A schematic diagram of the experimental setup for this method is shown in Figure 16.
In the chamber filled with the Ar gas, a high voltage is applied at high frequency between the target and the sample. The surface atoms of the target material are removed and deposited onto the substrate by bombarding the target with the ionized Ar atoms. The magnet, located behind the target, enhances ionization and effectively directs the sputtered atoms towards the substrate.
To explain the process of cyclic stacked we take the example of the production of an NiGe layer on a Ge substrate. Multi layers of Ni and Ge are formed by RF magnetron sputtering on an n-type germanium substrate at room temperature and the average composition of the whole multi-layers is controlled so as to have a stoichiometric equivalence to the atomic ratio of Ni and Ge atoms of 1 to 1 as in the phase, NiGe . Figure 17 is a schematic diagram showing the sample configuration in a cyclically stacked Ni/Ge film.
The idea behind this stacking of layers is to suppress the reaction between Ni and the Ge substrate upon annealing. In this way it is possible to get a high-quality NiGe film with a smooth interface on the Ge substrate. It is hoped that this smooth interface would reduce the Fermi level pinning effects of the interface electron energy states.
Implanting atomic species like selenium (Se) into the surface of n-type germanium before metallization helps to reduce the Schottky barrier height by introducing local interfacial doping. In order to achieve a reasonable amount of implantation into the surface of the Ge substrate, the atoms to be implanted need to be energized to around 130 keV. The implantation is usually followed by heating at a high activation temperature to activate the diffusion of the dopant atoms further into the semiconductor surface, before metallization.
The results of sheet resistance measurements presented in this chapter were obtained using a four-terminal resistor structure also known as a Kelvin resistor  structure. The structure consists of four contact pads: two pads are connected to the doped bulk semiconductor material and two pads contact to the metal used to form the contact. Current is then passed through two terminals between the semiconductor and the metal and the corresponding voltage drop is measured using the other two terminals between the metal and the semiconductor. In this way a sheet resistivity, ρsh can be extracted.
The author would like to thank the Copperbelt University for the use of the institution’s facilities.
Myths provide cultural explanations for understanding the world. It contains creation stories and explains all the elements necessary to understand the universe . They are memory, a vehicle of cosmological message and identity . In Amazon, the myths rely on the animal metaphor as the social component of the cosmology . As far as Archaeology, the myth usually is represented in the ceramic support as Art.
Many archaeologists think that the art should be understood within a much broader semantic significance: (a) artistic manifestations, functioning as symbolism; (b) cosmological message vehicles serving to communicate social, political and religious values of a certain society; (c) or art to be on its own, as a social cohesion form or political control strategy, to demonstrate these same values to other peoples, as a form of ethnic identity .
Artistic manifestations filled the precolonial Amazon and invited the archaeologists to study these themes . However, it is still difficult to define the diverse variations of the art types of these societies, partly because there is no conceptual standardisation , or partly as the variability is not fully known, yet, due to the complexity and enormous size of the precolonial Amazon .
However, many important paths have already been walked. Among the theoretical assumptions about art that found greater development in Amazon is the structuralist school and its variations, mainly through the works of Lévi-Strauss who regarded the art as an expression of communication and sociability . Gell  saw the artefacts as social and non-static agents, which highlighted the activities they were involved, such as rituals. In those magic gives a more pronounced property to the objects, what the author called incantation technologies. To understand the indigenous art the Amerindian Perspectivism , also under structuralist influence, has been used successfully, especially making the shamanistic relationships between men and animals more noticeable.
Having presented above the definition of art and studies in the Amazon, this text addresses the art in the Brazilian precolonial stilt villages of eastern Amazon societies, temporally between the 8th to the 10th A.D., thus, they no longer existed during the period of European conquest in the 16th century.
The stilt villages were ancient dwellings (palafittes) built on piles over a lake. Stumps or trunks of trees served as a support for the superior buildings of the villages . Traces of these wooden stilts are located within rivers and lakes and are shown only at the time of drought. This normally corresponds annually to October to December . Rest of the year, the palafittes are submerged. In some of these prehistoric sites, such as Coqueiro, there can be over three thousand stilts . This type of archaeological sites has been described in isolated cases in South America, namely in the reports of Amerigo Vespucci in 1499 on communities that lived in Stilt Houses on the Venezuelan coast. The stilt villages also appear in the reports of Amazon river mouth and from the Upper Amazon, near Peru . However, as known today, the prehistoric palafittes of Maranhão are the only ones preserved, in the entire American continent (Figure 1). The ancient stilt villages have been also common in the prehistory of Europe during the Neolithic age, for instance in northern Italy and Switzerland (Lake Constanza).
In the Amazonian eastern coast, the geographical area where the Stilt Houses are located is called Baixada Maranhense, in the State of Maranhão, Brazil. This region comprises approximately an area of 20,000 km2 within the legal Amazon and has more than 500,000 inhabitants. People are very poor, with the lowest HDI indices not only in Maranhão state but also in Brazil as a whole. Baixada Maranhense population lives from the subsistence of traditional agriculture, fishing, keeping small animals and growing vegetables. Santa Helena, Penalva, Pinheiro, and Viana are the main cities in this area.
It is assumed that the easy availability of food in the form of a rich variety of fish created a favourable situation for the sedentary housing of the human groups that occupied the region. It is also likely that these dwellings had defensive purposes. It also notable that the oxygen-free aquatic environment preserved the artefacts in such a way that even allowed the ink on the artefacts to remain visible. The straws of the cottage walls and roofs and utensils made from the hardwood, like oars and indigenous weapons (bordunas), also resisted the decomposition process over time.
The most striking Stilt House artefacts are the small objects, like plates and bowls, which most likely were used in serving foods or liquids in rituals. These utensils were made of good quality clay burnt at a very high temperature, giving rise to high-quality vessels. These small ceramics had very complex painted decoration with geometric motifs made with precision, indicating also that these vessels were used in rituals.
These pottery utensils formed an information channel of the social and ideological structure among the members of the society. As regards to their form and decoration, these vessels reflect mythical themes and/or were used in rituals . Archaeologists agree that the ceramics reflect the culture of a society and that the main social changes affect the production and types of vessels . So, the ceramics are vehicles of expression of ideological content. The most likely evidence of this is that they were painted, decorated, incised, modelled, with plastic decoration, aiming to reflect mythical or ideological themes.
Therefore, the art interpretations are cultural and in this sense adhere to rigid systems of social conviviality. In prehistory, its main material vehicles are small-scale art that is moveable (mobiliary art), such as decorated figurines and pottery and feather art; the graphic art, with parietal art and rock graphics and body painting, the latter very important, because, in general, it demonstrates the status of the individual, as in the case of chiefs and other leaders who had a high social position . Indigenous art is therefore the result of the identity relationship  and social conviviality between the indigenous groups .
Animal forms are recurring in the art of the Stilt Houses, being the most persistently various birds, but especially owls. The monkey and the jaguar are mostly illustrated Mammals, but also Amphibians (especially frogs) and reptiles (mainly snakes) are common. These animals form the appliques and figurines and are associated with indigenous myths some of which have been described by the missionary chroniclers of the 16th and 17th centuries and by anthropologists when working among indigenous Amazonian communities from the 19th century onward.
The ethnographic analogy shows that myths associating the snake to the creation, such as the canoe-snake, mother of the fish, for example, are recurring in Amazonian cosmologies . Whereas the king vulture is mentioned in Amazonian ethnography as a bird belonging to the realm of the dead.
Anaconda is a shamanic animal in the Amazon. Some peculiar ecological characteristics of these snakes could have attracted the attention of the indigenous people, as the specimens can reach up to 10 m in length and weigh more than 200 kg, is the heaviest animal in the Amazon. Also, the Anaconda has pale skin with black drawings served as an iconographic orientation, efficient camouflage, speed of attack in the water and lethargy on land, active at night, sensitive tongue for predate , powerful teeth and muscles and the females are larger and more aggressive than the males, predator of large mammals such as jaguars, deer and tapirs .
The indigenous peoples we mentioned in this text are the Tukano of the Tukano linguistic family who lives in the northwest of the Amazon; the Pano family Shipino who live between the Amazon and the Ucayali river in Peru; the Tupi peoples who have a wide dispersion in the lowlands of South America and countries like Bolivia, Uruguay and Argentina; the Waujá who live in the Xingu and belong to the Arawak family; the Panare and Timbira that are groups of the Jê family of Central Brazil and the already extinct groups of which we only have archaeological material such as the beautiful ceramic pots Tapajó and Konduri from the Lower Amazon in Brazil. Finally, there are the Warao who still live on stilts in Venezuela.
Regarding the cosmological aspects, the characteristics described above could contribute to the association of this powerful animal with the creation myths of the world and humanity. Among the Tukano, Hugh-Jones  the songs intoned by the shamans allude to the Amazon River as the terrestrial Anaconda and the Milky Way, the supernatural and creative Anaconda. Many Amazonian peoples compare the supernatural milk of the creative Anaconda with the milky-coloured sap of hallucinogenic plants of the genus Banisteriopsis, whose tree trunk is the metaphor of the body of the creative Great Anaconda .
In the polychrome ceramic material of the stilt villages, the principal iconographic element that stands out is the presence of curvy black or hook-shaped designs that fit together (Figure 2). The rim of these polychrome vessels is painted red. This iconography is recurrent in almost all the sites of this river. Although abstract, the iconography has a reading order in horizontal bands, filling the entire internal space of the vessel. The predominance of black colour could corroborate the suggestion that these images correspond to the black spots that the anaconda snake (Eunectes murinus) has on its back, as Roosevelt also interpreted in his study of anacondas and women-shamans in Marajo island.
This magical world is enhanced by music and dance. According to Barcelos , the snakes are part of a myth-musical repertoire among the Wauja, in which Kamalu Hai stands out, “the gigantic snake-canoe that carries on its back a long series of singing pots”. These ceramic cookwares are of different sizes and have a varied polyphony according to their function. In this sense, the snake-canoe could imply the origin of the ceramic activity among the Wauja, a pristine myth, therefore.
In the Amazon, the anaconda is associated with both the male universe, among the Tukano, as well as the female. Anaconda’s association with a shaman woman and creative deity are common in oral traditions in the Amazon, as among the Shipibo, according to Roosevelt . Indigenous peoples conceive of Anaconda as a dangerous ancestral spirit to the present day. She would be a master who governs the feminine part of the cosmos, which is the aquatic underworld. To Shipibo the anaconda is a woman shaman .
According to Roosevelt , many Amazonian peoples associate the Amazon River with Anaconda because these snakes dominate the aquatic landscape and because the meanders of the rivers imitate the movement of these reptiles. Thus, it is common for Anacondas drawings to be represented in community houses among the Tukano.
Roe  in his classic book The Cosmic Zygote reported among the Shipibo the mythological association of the creation of the world with the Anacondas, is also associated with the rituals of healing, divination, ceremonial dances and the creation of musical instruments. Among other groups, such as the Tupi-Gavião and Panare, Anaconda is a giant animal associated with the rainbow  or a celestial phenomenon among the Timbira in which the reptiles ends rest in the mouths of two anacondas . The rainbow would be a symbol of disease  and for Weiss  it would represent “something demonic, repulsive and detestable, as well as the anaconda”. Anaconda, therefore, inhabits a very large number of Amazonian myths, having as main characteristic the shamanic activities which include cosmological creation, under celestial aspects, and cultural properties associated with natural transformations of aquatic life and the water world.
The ceramic figurines indicate the presence of ritual . They are characterised by the representation of animals, especially the owl, the monkey, the turtle and the frog. Some of them are anthropomorphic or zoomorphic design, the zoomorphic being the most recurring. Many of them have a sculptural standard: the legs are open in the shape of a half-moon and some of them possess the feminine genitalia on display. A figurine in particular, in the form of an owl, is a rattler and features a small handle that possibly had the function of being hung.
The shamanic nature of these archaeological materials is also evidenced by the production of figurines, generally female, where there are small clay balls inside, which could be a rattle, known in the lowlands of South America as maracas, as communication between the living and the dead. According to Zerries , the maracá has always been the most important shamanic instrument in non-Andean South American cultures, since “the noise of the little stone or maracá seeds inside is interpreted as the voice of the spirits”. Thus, the maracá was considered an idol for the indigenous peoples of the lowlands of South America (Figure 3). Thus, in many cultures the owl is associated with death , and evil spirits .
These sound instruments are present in the ethnohistorical records of the colonial period such as Daniel , D’Abbeville  and D’Évreux  and also ethnography by anthropologists from the beginning of the 20th century. The maracá, therefore, is part of the shamanic paraphernalia since it can emit sound, thus, a form of communication between the different worlds in which the shaman acts. In this way, the rattle functions as a musical instrument whose sound together with the hallucinogens induce special sensations that alter their mental and psychological state. For the Warao, who still live in stilt houses in the Orinoco delta in Venezuela, the maracás have spiritual forces and their human forms refer to the ancestral shaman who visited heaven and was gifted with this instrument the Great Spirit of these water peoples .
The ceramic figurines were important products of the indigenous art in the Stilt Houses. Sometimes, they represented mainly hand-shaped geometrical figures. Often those represented also animals such as amphibians, fish, mammals and birds, which were similar to those among the Tapajó and Konduri peoples . These animals were represented in a naturalistic style, thus preserving the identifiable traces of the species. A good example is the squirrel monkey (Saimiri collinsi) figurine, in which we can identify its furry ears, and in another piece, the torn mouth, typical of these primates  (Figure 4).
In turn, the frogs are associated with fertility, which is most likely due to the aquatic environment in which these societies lived. Themes associated with frogs are also common in the Mesoamerica  and the Caribbean  (Figure 5).
Regard to iconography, it happens through geometric lines or traces that delimit patterns within the stylistic composition of the vase: they are Greeks, zigzags or spirals taking up the interior of the pieces. We can see, that there are two opposite iconographic fields divided by one or two lines across the piece. Mostly the motifs differ in these two fields. For instance, when the square motifs were used in one artistic field, then the circular elements were selected for the opposite one. Red and black were the predominant colours, which were painted on cream engobe or white.
According to Prous , some of the motifs resemble the Tupi iconography and he associated them with the custom and practice of eating human flesh (anthropophagic) ritual, such as the representation of the intestine and the brain. However, the pottery shapes at the Stilt Houses, as well as their technologies, such as anti-plastic and quality of clay burning, are very distinct from those of the Tupi. Therefore, Stilt Houses’ ceramics show better production control and technological quality.
Perspectivism deriving from structuralism serves the most fruitful theoretical and methodological discussions that apply to the study of archaeological pottery of the stilt villages. The study of the iconographic motifs, as well as their repetition and pattern, in addition to the ethnographic bibliographical revision of the Amazon, show that the geometric motifs of the ceramics are, in general, depictions of the skin or feathers of some animals, especially those of the top of the food chain, such as snakes and owls.
This chapter has shown that the pre-colonial Amazon peoples were well adapted to the environment and produced a rich art of strong social cohesion. Two types of the artefact of mobiliary art and one of the graphic art stand out in the conception of the indigenous art of stilt villages. The mobiliary art corresponds to the figurines, generally representing animals (zoomorphic design) and sometimes human beings mixed with animals and the appliques showing different animal shapes, especially mammals, amphibians and birds. Therefore, the art reveals the Perspectivism associated with the cosmologies involving mythical concepts. From the graphic art stands out the black and red paintings on white and cream engobe of the ceramic vessels. They contain geometric shapes which take up two distinct geometric fields, forming motifs resembling the skin of predatory animals.
Guss  has pointed out when postulating the relationship between myth and artefacts, that objects act as “subtexts” that provide an understanding of the functioning of society bringing them closer to their origins. In this sense, the artefacts imitate primaeval objects, as they are copies of this primordial world. This concept refers to what Gell  would call the object’s enchantment.
Very likely the representation of animals in appliques and painting of Stilt Houses ceramics have a close connection with the creation myths among the pre-colonial peoples in Amazon. Snakes are mainly dwelling in the river, and therefore associated with the origin of human life. The Amazonian rivers, on the other hand, have many meanders remembering the shape of the undulating movement of the snakes. Serpents are also associated with fertility and in an aquatic environment where stilts were found, these myths could be very important as social cohesion.
In the river environment of Amazon frogs are other important animals . These amphibians were associated with the fertility and they produce hallucinogenic substances that allowed the shamans to make their spiritual journeys . Many Indian ceramics have frog-shaped appliques and the small size of these vessels indicates their use for the consumption of liquid drinks. Some scholars have argued that the abstract drawings in ceramics paintings and appliques could originate from the view of the visual illusions (phosphenes) caused by hallucination.
On the other hand, the representation of mammals at the top of the food chain such as jaguars and monkeys may be associated with Amerindian perspectivism. Animals with aggressive characteristics in the ceramic appliques likely represented for the Indians a metaphor of power. It is also possible that these ceramics belonged to the chiefs. Often the animal itself was not represented, but the paintings of the vessels alluded only the skin of these animals.
The bright colours of the Anacondas, as well as their ecological characteristics such as constriction and their large size, played an important role in their choice to symbolise the canoe snake. Nothing better than a strong and large animal to be used as a transport vehicle to populate the villages. The fact of knowing how to swim, like a canoe, corroborates that Anaconda has adequate properties for its appropriation as a narrative. In this sense, the Anaconda had a higher status in the animal hierarchy, appearing only in painting and not in effigies.
Finally, it has to be considered that the contemporary Stilt Houses, most of them comprise the temporality of 800 to 1000 AD. So the long-range of their artistic ideologies in an extensive area, indicating firstly a cultural homogeneity of these societies, and, secondly, makes us think, even if hypothetically, that the existences of these chiefdoms of large regional scale between the 8th and 10th AD coincide with the pinnacle of the precolonial Amazonian societies.