SCI 3421-2 Past Paper Examination 93-94 Solutions

Part A

1.

Fig.1 Vacancy diffusion

Fig.2 Interstitial diffusion

2. The preferred crystallographic directions within a preferred crystallographic plane, along which a dislocation may move, constitute a slip system. A ductile metal has many slip systems, e,g. Ag has 12, namely {111}<110>.

3. It is the phenomenon whereby a ductile metal becomes harder & increases in yield strength, after receiving much plastic deformation at room temperature, i.e. "cold worked". This processing treatment increases the dislocation density. Due to the repulsive nature of dislocation-dislocation interaction, the resistance to dislocation movement is enhanced, and the critical stress necessary to deform the material is raised.

4. As see in Fig.3 below, The two outer probes carry current, and the two inner ones monitor the voltage drop. In the usual case, the four probes are collinear, equally spaced, and places for from the edge of the sample being tested; then

This technique should be adopted when ss to be measured is expected to be high, and the errors due to contact and spreading resistance between current-carrying probe and the sample surface have to be eliminated.

5. For minority carriers, the classical measurement of Haynes and Shockley shown in Fig. 5.1 determines m directly. The procedure is as follows. An electric field is established along a length of the semiconductor. A point-contact emitter is pulsed to inject minority carriers into the bar at point A. When the extra packet of carriers reach the collector at point B, they can be extracted and will show as a pulse on the oscilloscope. The mobility is given by

Where V is the applied sweep voltage, t the time between pulse injection and its appearance at the collector, d the emitter-collector separation, and L the length of the bar. As the arrival time increases, the pulse amplitude decreases, and the pulse width increases. The decrease in amplitude is primarily because of carrier recombination and can be used to deduce lifetime. The increase in width occurs because the carriers do not remain closely bunched but diffuse out from the high-concentration central region. Note that if the emitter-collector spacing is too great, the carriers will all have recombined before reaching the collector. For Ge, whose lifetime may be in the millisecond range, spacing of up to 1 cm can be conveniently used. Material with lower lifetimes and mobility require commensurately lesser spacing. There are a number of experimental variations that may afford advantages in special circumstances. For example, instead of using the emitter contact of Fig. 5.1, partially injecting end contacts or light flashes can be used for carrier injection and the sweep field can be pulsed.

6. Thermogravimetry is the experimental method of measuring the sample wieght as a function of time, when the sample is either held at an elevated temperature or heated at a linear rate. It may be applied to the steady of any physical or chemical change involving mass variation, e.g. sublimation, condensation, oxidation, decomposition.

7. Differential scanning caorimetry: T1=T2, measure diffence in input energy required to maintain equal temperatures H1-H2

Part B
 
8.  (i)  Vacancy. (Interstitial defects: uncommon) 

Since N1=N*exp(-Q/kT1) and N2=N*exp(-Q/kT2), 

Where N* is total number of atomic sites and Q the energy of formation for a vacancy defect, 

\ N1/N2=103=exp(-Q/kT1+Q/kT2) 

or Q=ln103xkx(1/T2-1/T1)-1 

=0.119 or 0.2 eV 
¡@  (ii)  Interstitial: carbon in iron (steel); 

Substititional: zinc for copper (brass). 

Impurity atoms tend to form interstitional defects among the host atoms if the former are much smaller in atomic size than the latter; on the other hand, Substititional defects are favored if the two are roughly the same in size, they have approximately equal electronegativities and they, in the form of pure elements, adopt the same crystal structure. 
¡@  (iii)  Edge dislocation: Beurgers vector ^ to dislocation; 

Screw dislocation: Burgers vector // to dislocation; 

Mixed dislocation: Burgers vector at the angle to dislocation. 
¡@  (iv)  ¡@ 

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However, in the presence of twinning: 

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Where the two sides are mirror images of each other, any measurement across the sample crystal crosses many twin boundaries and the a/b anisotropy is averaged out. 

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¡@  (v)  A: by electrochemical etching of silicon in HF, to attain porosity (percentage of void volume) up to ~ 80%, so that the pores are large enough to meet me another, leaving behind a forest of pillars or strands of beads; 

B: ordinary silicon has an indirect band gap which furthermore is rather narrow, and can therefore emit IR radiation with low efficiency, but when made ¡§porous¡¨, silicon has a direct gap which is widened (due to quantum size effects), and it emits visible light efficiently (~1% effeciency) when excited by UV (photoluminescence) or , technologically more imporantly, by current injection (electroluminescence). 

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9. (I) At r = ro, equilibrium separation, net force between ions

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(II) Applied stress = 48.8kN/(p 0.952cm2)<yield strength, \ elastic deformation

Therefore, Change in length, D l, satisfies

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Change in radius, D r, is related to Poisson¡¦s ratio n thus

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(III)

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And is quite large, \ material is ductile.

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The color area is the contact area. The contact area A is calculated as follow:

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10. Thermal evaporation \ high vacuum required.

Hence, need 4 more components:

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¡@  Components  Example 
1.  Roughing pump  Rotary vane pump 

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2.  High vacuum pump  Differential pump 
3.  Pressure gauge for low vacuum  Pirani gauge 

(measure thermal conductivity of residue gas in vacuum chamber) 
4.  Pressure gauge for high vacuum  Penning gauge 

(measure cold cathode discharge current) 

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11. (I) UPS is based on the photoelectric effect. The sample is irradiated with a monochromatic light, in the UV region, and we measure the kinetic energies of the photoelectrons (i.e. electrons ejected from the surface into vacuum, constituting the photoemission current). The kinetic energy of the photoelectrons Ek is related to the binding energy Eb of its original energy level in the solid:

Where hn is the UV photon energy and Fsa, the difference in work function between sample and electron energy analyzer. We may thus deduce Eb. Thus, the occupied bands of the solid can be studies.

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(II) Hydrogen discharge lamp: lower hn , need monocheomator but does not required differential pumping

Inert-gas discharge lamps (Ne, Ar, He): higher hn , no monochromator but no window, therefore need differential pumping.

Synchrotron radiation: wide spectral range, high fluency, large degree of polarization but need monochromator and very expensive

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(III) Non-dispersive, e.g. retarding field analyzer or time of flight analyzer

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Dispersive, e.g. electrostatic cylindrical mirror analyzer

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Or magnetic double-focusing analyzer or electrostatic hemispherical analyzer

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THE END