3.30 Sspd_chapter 1_part 11_solid state of matter_continued_crystal  (Page 2/4)

In 1980’s with the development of low attenuation optical fiber , short haul-heavy traffic and long haul communication turned towards Optical Fiber Communication. In communication there is a heavy premium on Band Width and Optical fiber could offer immense amount of BW at very low cost. This gave a tremendous boost to R&D in optoelectronic devices and optoelectronic chips. The material of choice could only be Compound Semiconductors and their alloys. The compound semiconductors are direct band-gap materials offering high radiative quantum efficiency.

Indium Antimonide (InSb) was the first III-V compound semiconductor to be discovered in 1950. Because of narrow band-gap, E _g = 0.17eV, InSb is the suitable material for fabricating optoelectronic devices in far infrared spectrum because 0.17eV band-gap corresponds to critical wavelength λ _c = (1.24/ E _g )= 7.3μm. This critical wavelength will decide the optical emission and absorption wavelength.

The invention of semiconductor laser and the discovery of Gunn-Effect brought GaAs ( E _g = 1.43eV) and InP ( E _g = 1.35eV) to a position of eminence. The need for visible light Light Emitting Diodes by necessity led to the importance of GaP( E _g =2.1 eV) and its alloys. 2.1eV band-gap corresponds to a critical wavelength of 0.59μm which falls in visible part of the spectrum. By suitable alloying , any critical wave length in the range 0.4μm to 0.7 μm can be achieved and 0.4μm to 0.7 μm is the visible part of the spectrum. The alloying process, to achieve a suitable critical wavelength, is known as BAND-GAP ENGINEERING. We will deal with this topic in PHOTONICS Chapter.

The choice of different Binary Compounds provides us a wide range of Band-gaps ranging from InSb of band-gap 0.17eV to SiC of band-gap 2.99eV. SiC enabled the fabrication of Blue Laser diodes which are used to sense the pixels of DVD (Digital Versatile Disc). The advent of DVD has greatly enhanced the storage capacity. CD stores 700MB whereas DVD stores 4GB and more.

Here it may also be pointed out that Extreme Ultra-Violet photolithography is enabling the fabrication of nano-electronics chips or realization of lateral feature size less than 100nm as required in Ultra-Large-Scale-Integrated (ULSI) Chips. This again is dependent upon research in very wide band-gap material typically a band-gap of 12.4eV corresponding to a critical wavelength of 0.1μm.

The ease of alloying Binary Compounds into Ternary Compounds such as AlGaAs and Quaternary Compound such as InGaAlAs makes the use of compound semiconductor in photonics very attractive and convenient.. The alloyed Binary compounds gives a continuous and monotonic variation in Band-gap energy according the stoichometric coefficient of the alloyed material. This continuous variation gives the Photonic Scientists a more varied option in terms of band-structure, electronic properties and optical properties. The alloying process can be achieved by Molecular Beam Epitaxy(MBE). This alloying process also enables the fabrication of Hetrostructure Junctions which is much more power efficient as compared to Homojunctions. Homojunctions are junctions formed by P type and N Type of same material whereas Hetrojunctions are formed by P Type and N Type of different materials. Table(1.26) tabulates the electronic properties of Compound Semiconductors.