0.12 Chapter 3. special classification of semiconductors.sec3.1  (Page 43/2)

Chapter 3. Special Classification of Semiconductors.

Section 3.1. Compound - Semiconductors.

Compound Semiconductors are the basis of a whole new branch of Science and Technology known as Photonics. Light Sources and Light Detectors belong to this discipline. III-V elements give rise to Compound Semiconductors which are suitable for Light Generation or Light detection. These III-V elements form alloys across the whole range of concentration at their growth temperature. This wide miscibility range allows alloys to be grown with band structures adjusted for specific applications. This leads to Band structure manipulation according to our specific needs. This is known as Band-gap Engineering. The common Alloys used in Photonics are as given below:

1. GaP(2.3eV, a = 5.42A°)____GaAs _x P _(1-x) ___________GaAs(1.42eV, 5.65A°): here x is the stoichometric coefficient and by adjusting ‘x’ , band-gap can be tailored from 1.42eV to 2eV.
2. InP(1.3eV,5.85A°)__________InGaP_________________GaP(2.3eV, 5.42A°).
3. GaAs(1.42eV, 5.65A°)____________GaAlAs__________________AlAs(2.2eV, 5.65A°).
4. GaAs(1.42eV, 5.65A°)________GaAsSb__________GaSb(0.65eV, 6.1A°).
5. GaAs(1.42eV, 5.65A°)_____GaInAs_________InAs(0.35eV, 6.05A°).
6. InP(1.3eV,5.85A°)_______InPAs__________InAs(0.35eV,6.05A°).
7. GaSb(0.7eV,6.1A°)______GaInSb_________InSb(0.15eV,6.5A°).

The three element alloys are TERNARY ALLOYS. Two from Group III and two from Group IV combine to form QUATERNARY ALLOYS.

Wide miscibility shown above has been translated into a topological diagram Figure 3.1 where Band-Gap versus Lattice Constant is plotted for the seven major Compound Semiconductors namely GaP, GaAs, GaSb, InAs, InSb , InP , AlAs, and AlSb and their derived Ternary Alloys.

As seen from Figure 3.1, none of the pure compounds listed have a direct band-gap more than 1.65μm for producing Visible Spectra Radiation. GaAs, InP, GaSb, InAs and InSb have direct band-gap but less than 1.65eV.

AlP, GaP, AlAs and AlSb all have Band-gap larger than 1.65eV but all are in-direct band-gap hence not suitable for optical generation. Hence for Optical LEDs we go for alloys of GaP and GaAs known as ternary alloy GaAs _(1-x) P _x . In Figure 3.2, the white light spectrum and the corresponding Band-gaps are shown.

Figure

Figure 3.1.Topoloigical diagram for Compound Semiconductors and their Ternary Compounds. The Solid lines indicate Direct Band Gap materials and dashed lines show In-direct Band Gap materials.

Figure 3.2. The plot of Band-Gap and the corresponding Wavelength versus Lattice Constant.

Figure 3.3. External Quantun Efficiency vs stoichiometric corefficient of Phosphorous in ternary alloy GaAs (1-x) P x .

For the manufacture of coloured LEDS we have to use GaAs _(1-x) P _x ternary alloy. This has a problem. Below x = 0.45 it is a direct band-gap material but at higher proportion of Phosphorous it becomes in-direct and its performance becomes very poor as shown in Figure 3.3. By doping with Nitrogen it can be restored to Direct Band-gap material and utilized for LED manufacturing. Table 3.1 tabulates the different ternary alloys used for manufacturing the spectrum coloured LEDs.