Types of Wafer Substrates

This section includes information on single element semiconductors, silicon on insulator, III-V Semiconductors, II-VI Semiconductors, Ternary Semiconductors, superconducting glass, and more.

1. Single Element Semiconductors

   1. Silicon, Si - the most common semiconductor, atomic number 14, energy gap Eg = 1.12 eV - indirect bandgap; crystal structure - diamond, lattice constant 0.543 nm, atomic concentration 5x1022 atoms/cm-3, index of refraction 3.42, density 2.33 g/cm3, dielectric constant 11.7, intrinsic carrier concentration 1.02 x 1010 cm-3, mobility of electrons and holes at 300 K: 1450 and 500 cm2/V-s, thermal conductivity 1.31 W/cm-oC, thermal expansion coefficient 2.6 x 10-6 1/oC, melting point 1414 oC; excellent mechanical properties (MEMS applications); single crystal Si can be processed into wafers up to 300 mm in diameter.
   2. Ge, C, Sb.

2. Silicon on Insulator (SOI)

   1. Only a thin layer on the surface of a silicon wafer is used for making electronic components; the rest serves essentially as a mechanical support. The role of SOI is to electronically insulate a fine layer of monocrystalline silicon from the rest of the silicon wafer. Integrated circuits can then be fabricated on the top layer of the SOI wafers using the same processes as would be used on plain silicon wafers. The embedded layer of insulation enables the SOI-based chips to function at significantly higher speeds while reducing electrical losses. The result is an increase in performance and a reduction in power consumption. There are two types of SOI wafers. Thin film SOI wafers have a device layer <1.5 ?m and thick film wafers have a device layer >1.5 ?m.
   2. Wafer bonding. - In this process the surface of two wafers are coated with an insulating layer (usually oxide). The insulating layers are then bonded together in a furnace creating one single wafer with a buried oxide layer (BOX) sandwiched between layers of semiconductor. The top of the wafer is then lapped and polished until a desired thickness of semiconductor above the BOX is achieved.
      
   3. SIMOX - Separation by Implantation of Oxide. In this process a bulk semiconductor wafer is bombarded with oxygen ions, crating a layer of buried oxide. The thickness of intrinsic semiconductor above the box is determined by the ion energy. An anneal reinforces Si-O bonds in the BOX.
      
   4. Smart Cut - The wafer bonding method is used to form the BOX, but instead of lapping off excess semiconductor (which is wasteful) a layer of hydrogen is implanted to a depth specifying the desired active layer of semiconductor. An anneal at ~500^oC splits the wafer along the stress plane created by the implanted hydrogen. The split wafer may then be reused to form other SIO wafers.
      

3. III-V Semiconductors

   1. Gallium Arsenide, GaAs - After silicon second the most common semiconductor, energy gap Eg = 1.43 eV, direct bandgap; crystal structure - zinc blend, lattice constant 5.65 Ang., index of refraction 3.3, density 5.32 g/cm³, dielectric constant 12.9, intrinsic carrier concentration 2.1 x 10⁶ cm^-3, mobility of electrons and holes at 300 K - 8500 and 400 cm²/V-s, thermal conductivity 0.46 W/cm-^oC, thermal expansion coefficient 6.86 x 10^{-6 o}C^-1; thermally unstable above 600 ^oC due to As evaporation; does not form sufficient quality native oxide; mechanically fragile; due to direct bandgap commonly used to fabricate light emitting devices; due to higher electron and hole mobilities, also foundation of the variety of high-speed electronic devices; bandgap can be readily engineered by forming ternary compounds based on GaAs, e.g. AlGaAs.
   2. Gallium Nitride, GaN - wide bandgap III-V semiconductor with direct bandgap 3.5 eV wide; among very few semiconductors capable of generating blue radiation, GaN is used for blue LEDs and lasers; intrinsically n-type semiconductor but can be doped p-type; GaN is formed as an epitaxial layer; Lattice mismatch remains a problem, creating a high defect density. Incorporation of Indium (In_xGa_1-xN) allows control of emission from green to violet (high and low In content respectively). GaN can also be used in UV detectors that do not respond to visible light. GaN has a Wurtzite(W) or Zinc Blend(ZB) crystal structure. Lattice constant [A] 3.189(W) 5.186(ZB); Density[g/cm³] 6.15(W) 6.15(ZB); Atomic concentration [cm^-3] 8.9 x 1022(W) 8.9 x 1022(ZB); Melting point [^oC] 2,500(W) 2,500(ZB); Thermal conduct.[W/cm ^oC] 1.3(W) 1.3(W); Thermal expansion coefficient[^oC^-1] ~1x10^-6; Dielectric constant (static) 8.9(W) 9.7(ZB); Refractive index 2.4(W) 2.3(ZB);
   3. GaP - Crystal structure zinc blend; Lattice constant [A] 5.45; Density [g/cm³] 4.14; Atomic concentration [cm^-3] 4.94 x 10²²; Melting point [^oC] 1457; Thermal conductivity [W/cm ^oC] 1.1; Thermal expansion coefficient[1/^oC] 4.65x10^-6; Dielectric constant 11.1; Refractive index 3.02; Energy gap [eV] 2.26; Type of energy gap: direct; Electron mobility [cm²/V sec] 250; Hole mobility [cm²/V sec] 150;

4. II-VI Semiconductors

   1. examples: CdSe, CdTe, CdHgTe, ZnS.

5. Other Binary Semiconductors

   1. Silicon carbide, SiC - semiconductor featuring energy gap Eg = 2.9 -3.05 eV (wide bandgap semiconductor), indirect bandgap; SiC can be obtained in several polytypes- most common hexagonal in the form of either 4H or 6H polytypes; parameters vary depending on polytype; Intrinsically n-doped; p-type doping and n-type conductivity control can be obtained by doping with aluminum and nitrogen respectively. SiC features higher than Si and GaAs electron saturation velocity; excellent semiconductor but difficult and expensive to fabricate single-crystal wafers; excellent for high power, high temperature applications; SiC is closely lattice matched to GaN, has a thermal expansion coefficient close to GaN, and is available in both conductive and semi-insulating substrates. Thus it is often used as a substrate for GaN epitaxial layers. SiC wafers 75 mm in diameter are available commercially.
      

      Crystal Structure	3C-SiC (cubic)	6H-SiC (hexagonal)
      Lattice constant [A]	4.36	3.08
      Thermal conductivity [W/cm oC]	5
      Melting point	sublimes > 2,100 oC
      Dielectric constant	9.7
      Energy gap [eV]	2.3	3.03
      Electron Drift Mobility [cm2/V sec]	800	400
      Hole Drift Mobility [cm2/V sec]	40	100

6. Ternary Semiconductors

   1. Direct Bandgap Energy calculator for ternary semiconductor alloys.

8. Fiber Optic

   1. Silicon is the most economical IR material available. Both P-type and N-type substrates are acceptable for IR optics provided that they offer transmission greater than 50% in the 1.5 to 6 micron wavelength. Generally speaking, for N-type, the resistivity should be greater than 20 ohm-cm and, for P-type, greater than 40 ohm-cm. For use in the near IR, this is not so critical, but for applications for the far IR, resistivity can be critical. Float Zone Silicon most always meets criteria for optical use. Under certain circumstances, CZ silicon will also be adequate.
   2. - GaN - blue laser/LED
   3. - GaInN - green laser/LED
   4. - GaAs - red laser/LED

11. Fused Quartz

    1. Not to be confused with fused silica, fused quartz is naturally occurring crystalline silica (Silicon Dioxide) from stone, sand, rock or lumps in its melted form. Density (at 20° C): 2.2 g/cm³; Thermal Coefficient of Expansion: 5.5 x 10-7/°C; Strain Point: 1120°C; Refractive Index: 1.458; Dielectric Constant: 3.75.

12. Fused Silica

    1. Density (at 20° C): 2.2 g/cm³; Thermal Coefficient of Expansion: 5.5 x 10-7/°C; Strain Point: 990°C; Annealing Point: 1075°C; Softening Point: 1585°C; Refractive Index(l=589.3nm): 1.459; Dielectric Constant: 3.8.

13. Epitaxial Silicon

    1. Single crystal silicon grown on top of (usually) P+ or N+ type substrate.

14. Organic semiconductor

    1. New semiconductor material based completely on organic components (primarily carbon, hydrogen, and oxygen); these low-cost semiconductors can be formed as a thin film on any substrate. some organic semiconductors maintain their electrical properties even under strain, allowing for flexible substrates; much more resistive (~ 1014 ohm-cm) than inorganic semiconductors;