654_ch4 - ECE 654 Prof. S. Mohammadi Solid State Devices II...

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ECE 654 Solid State Devices II Prof. S. Mohammadi - 75 - Chapter 4 Band Structure in Silicon and Compound Semiconductors In this chapter we will overview Si and Compound Semiconductor (CS) band structures and become familiar with the concept of semiconductor alloys. We will study bandgap engineering as well as strain engineering in lifting band degeneracy in various hetero-structures. Compound Semiconductors Nature has provided us with many semiconductor materials. Unfortunately, the quality of these semiconductors is often not good for making electronic devices and circuits. Over the past 50+ years, we have learned to synthesis semiconductors through various processes. For instance, to make Si wafers, we start by melting clean sand! and forming a molten slurry of Si in a crucible. A small crystalline seed of Si is then immersed in the crucible and withdrawn slowly as it is rotated. The result is a pure Si cylinder called ingot. Once Si ingot is cooled it is sawed into thin wafers of Si which then can be used for wafer processing. Currently 12” Si wafers are available for processing. There are other techniques to make Si and compound semiconductor ingots. Today, 8” GaAs wafers and 6” InP wafers are readily available. A number of compound semiconductor materials are formed by making an alloy using an epitaxial growth technique. Epitaxial films of such materials are formed on top of a compound semiconductor substrate. There are two widely-used techniques to do epitaxy, namely molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). While the two techniques are different by nature, the resulting films qualities are often comparable. Certain alloys are grown more easily using one technique compared to the other. For instance, an alloy of Fig. 4.1: Czochralski method to make Si ingot
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ECE 654 Solid State Devices II Prof. S. Mohammadi - 76 - InAlAs can be easily grown by MBE but is hard to grow with MOCVD, while InP can be easily grown by MOCVD but is harder to grow using MBE. Semiconductor substrates that are often used for epitaxy are Si GaAs, InP, SiC and GaP. Si substrate is used to grow epitaxial Si as well as epitaxial SiGe material. GaAs substrate is often used for growing AlGaAs alloy as well as InGaP alloy. InP substrate is used for growing InGaAs alloy and InAlAs alloy. SiC is used for SiC related materials as well as GaN related materials. Epitaxy is done using one of the following procedures: 1. Lattice Matched 2. Pseudomorphic 3. Metamorphic InSb AlAs Fig. 4.2. Relationship between bandgap, absorbed wavelength and Lattice Constant of several CS
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ECE 654 Solid State Devices II Prof. S. Mohammadi - 77 - In lattice-matched epitaxy, the epitaxial layer has the same lattice constant as the substrate. An example is AlGaAs alloy grown on GaAs substrate. An example is AlGaAs alloy on GaAs substrate. For Al mole fractions of smaller than 85% AlGaAs is a direct material and has almost the same lattice constant as GaAs. AlGaAs bandgap is higher than GaAs. Another example of
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654_ch4 - ECE 654 Prof. S. Mohammadi Solid State Devices II...

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