Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 19

material engineering. At the microscale, chemical doping is used to fabricate active
electronic devices, such as diodes and transistors, at very large scale, and MEMS
devices with specific mechanical characteristics ( Dragoman and Dragoman 2001 ).
Chemical doping of intrinsic silicon relies on a controllable introduction of p- or
n-type impurities via ion implantation techniques or high-temperature diffusion
from liquid or solid sources. At nanoscale, although chemical doping is still used
in CNT transistors, for example, the diversity of functionalization techniques is
greatly enhanced. For instance, nanowires or quantum dots can be functionalized
also via electric fields, hydrogenation, oxygenation, adsorption of molecules or
biomolecules, or other methods discussed throughout the topic.
Deposition techniques for nanosized devices are based on chemical vapor
deposition (CVD), in which thin films, with thicknesses up to fractions of nm, are
deposited on a substrate using chemical reactions of specific gaseous components.
In general, these chemical reactions need high amounts of energy generated by (1)
plasma excitation, (2) optical excitation, or (3) heating the substrate at very high
temperatures. The first two processes work at lower temperatures than the last one.
CVD techniques comprise two basic methods: low-pressure CVD (LPCVD)
and plasma-enhanced CVD (PECVD). LPCVD requires electrical heated furnaces
where a very low pressure (at 0.1-0.7 torr) is maintained by a pumping system.
Thin films of a certain material are deposited on both faces of a wafer positioned
on a holder inside the furnace. For instance, thin films of SiO 2 (using as gaseous
components N 2 OandSiCl 2 H 2 at 900 ı C), polysilicon (from gaseous SiH 4 at 600 ı C),
or Si 3 N 4 (from gaseous components NH 3 and SiH 4 at 800 ı C),aswellasTi,Mo,
Cu, and Ta metallic thin films can be deposited using LPCVD.
PECVD is based on the plasma produced by a high-power radio frequency (RF)
source. The main advantage of PECVD is the lower temperature (100-300 ı C)
needed to heat the substrate. PECVD consists of a plasma reactor with a RF source;
a pumping system, which injects gases inside the reactor and produces a high
vacuum inside the reactor chamber; and two parallel plates. On one plate, the RF
signal is applied, and on the other, which contains also the wafer placed above an
electrical heater, the RF generator is grounded. PECVD is used to deposit Si x N y
(the nonstoichiometric form of silicon nitride), amorphous silicon, SiO 2 ,andCNTs
produced from a mixture of NH 3 and C 2 H 2 gases.
The CVD-based epitaxial techniques boosted the semiconductor industry
because they made possible the growth of monolayers of several AIII-BV
semiconductor heterostructures, such as InP/GaInAs or GaAs/AlAs, which are
widely used in the advanced nanoelectronic devices based on quantum wells, wires,
and dots. In the epitaxial growth based on CVD, a crystalline material is first
grown on the substrate, and an additional crystalline material can be subsequently
grown on top of it if their lattices match or are slightly different, case in which a
strain between the crystals is induced. This strain can be used, for instance, to tune
the bandgap of the resulting heterostructure to a desired value corresponding to a
specific emission wavelength in quantum well or quantum wire lasers.
Two CVD-based techniques are mainly used to grow heterostructures: metal
organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).