Chemistry enables life. Chemistry also enriches life. For thousands of years, human beings have been — Physical Chemistry Chemistry Question
From sand to semiconductors
Chemistry enables life. Chemistry also enriches life. For thousands of years, human beings have been putting sand to good use. Glass was made from sand. Lenses were made from glass and were used to make telescopes, microscopes, eyeglasses, and glassware for chemical experiments.
More recently, sand became a starting material for semiconductors. One of the most abundant elements in the Earth's crust is silicon, which is found in compounds containing Si–O bonds. Silica (SiO2) is present in abundance at the earth’s surface.
Figure 17–1. β–cristobalite, one structure of silica. [VISUAL] How many Si and O atoms exist in the unit cell of β–cristobalite?
Model Answer
Si : (1/ 8 × 8) + (1/ 2 × 6) + (1 × 4) = 8
O : 1 × 16 = 16
Suggest the hybridized orbital of Si for this structure and guess the bond angle of O–Si–O.
Model Answer
sp3 ; 109.5°
SiO2 is very unreactive, yet it reacts with HF. The reaction with HF may be used to etch glass or in semiconductor manufacturing:
SiO2(s) + 6 HF(aq) → A(aq) + 2 H+(aq) + 2 H2O(l)
Draw the molecular structure of A.
Model Answer
octahedral:
[VISUAL]
Silicon can be obtained by heating silica and coke (a form of carbon) at 3000 °C in an electric arc furnace.
Write a balanced equation for the reaction of SiO2 with carbon. In this case, assume that only one kind of gas is formed whose Lewis structure should show formal charges.
Model Answer
Since Lewis structure of the gas shows the formal charges, it should not be O=C=O, but :C≡O: where atom C has the formal charge of –1 and atom O has the formal charge of +1. Therefore, the balanced equation for the reaction is
SiO2(s) + 2 C(s) → Si(s) + 2 CO(g)
Sketch the molecular orbitals of the gas formed from the reaction above.
Model Answer
[VISUAL]
To obtain ultrapure silicon, crude silicon is treated either with Cl2 gas to give B or with HCl gas to give C.
Write a balanced equation for the reaction of Si with Cl2.
Model Answer
Si(s) + 2 Cl2(g) → SiCl4(l) : from the Merck Index
Schenk in Handbook of Preparative Inorganic Chemistry Vol. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) pp 682–683.
Predict the molecular structure of B.
Model Answer
Tetrahedral
[VISUAL]
Is the product C from the following reaction (1) polar or not? Draw the three–dimensional structure of C and sketch the direction of its dipole moment, if any:
Si(s) + 3 HCl(g) → C(g) + H2(g) (1)
Model Answer
C: SiHCl3 , polar
[VISUAL]
The reverse reaction of (1) is spontaneous at 1000 °C, depositing ultrapure silicon. The final purification of the silicon takes place by a melting process called zone refining. This process depends on the fact that the impurities are more soluble in the liquid phase than in the solid phase (Figure 17.2). The zone refining procedure can be repeated until the desired level of purity (less than 0.1 ppb impurity) is obtained.
Figure 17.2. Zone refining of silicon
[VISUAL]
How many atoms per gram in the silicon wafer have been replaced by impurity atoms when the impurity level is 0.1 ppb?
Model Answer
(1 g / 32.066 g mol–1) × 0.1×10−9 × 6.02×1023 = 1.9×1012
Like all semiconductors, high–purity silicon fails to conduct electrical current until a minimum electrical voltage is applied, but at higher voltages it conducts moderately well. Semiconducting properties of silicon can be improved significantly by doping. Doping is the addition of a minor amount of a different element.
When a small number of boron atoms replace silicon atoms in solid silicon, what is the charge carrier? What is the name for this type of doped–semiconductor?
Model Answer
In a Si wafer doped with B atoms, holes exist that neighboring electrons can move into, thus causing electrical conductivity. Therefore, holes are the charge carriers. This kind of doped–semiconductor is the p–type semiconductor.
Draw a band diagram that can explain conductivity improvement upon replacement of some silicon atoms with boron atoms. Show in your drawing the band gap change after doping.
Model Answer
[VISUAL]