382
P. Kumar
the holes play the main role: in fact, porous silicon formation is a self-regulated
mechanism, with hole depletion as the controlling agent. The dissolution reaction
initiates at defects on the surface of silicon, the pores are formed and their walls
are eroded until they are emptied of the holes. This formation process passivates
them from further attack, and the reaction proceeds at the pore tip only. The overall
etching process is self-adjusting and the physical properties of porous silicon, i.e.,
shape, diameter of pores, the porosity, and the thickness of the formed porous layer is
given by the electrochemical parameters only. The physical properties of PS can be
varied with extreme control by changing of the electrochemical etching parameters,
e.g., current density, substrate doping level and type or HF concentration (Kumar
and Huber 2007; Kumar 2011). In addition, when the feature size of the pores of
PS is less than a few nanometers, various quantum-size effects occur, which make
PS even more fascinating. Many theories on the PS formation mechanisms have
been reported since its discovery. In the Fig. 3 it is shown that by changing the HF
concentration in etching solution the pore diameter and porosity of PS can be varied
that results in the different color of PS. In Fig. 3, SEM images, the color observation
and photoluminescence spectroscopy measurements for three different samples were
shown. From the figure it is clear that with increase in ethanol to HF concentration
by keeping other parameters constant the pore diameter and porosity is increased.
As Si is indirect bandgap material and does not show any PL therefore is limited in
many of the technology. However, PS shows the PL properties as shown in Fig. 3
and open up several optical applications. With increase in pore diameter in the PS
sample the distance between two pores is decreased results in the decrease of Si
crystalline size. When the size of Si crystallites among the pore reduces less than the
bohr’s exciton radius, i.e., 4.9 nm for Si, several quantum effect starts to occur and
that resulted in the photoluminescence appearance in PS. The physical properties of
PS are determined by two large groups of factors, the first group of factors includes
doping type and level and potential that affect carrier density on the surface of a
pore bottom and second group of factors includes HF concentration and current
density that affect only the dissemination of the reactions. In this chapter, the effect
of HF concentration on physical properties by keeping other parameters constant
as shown in Fig. 3 was discussed. Si surface is hydrophobic in nature, therefore
ethanol is usually added to aqueous etching solution to increase the wettability of
the PS surface. Ethanol is very important for the lateral homogeneity of the PS
layer in depth because ethanoic solution infiltrates the pores, while pure aqueous HF
solution does not. Further during the etching the hydrogen evolution in bubbles form
takes place and stick on the Si surface in pure aqueous solutions whereas they are
easily removed if ethanol is present. With increase of HF concentration in etching
electrolyte, the pore diameter and porosity of PS decreases that can be understood
as follows: both the pore diameter and wall thickness for highly doped p-type Si, are
largely determined by the thickness of the space-charge layer formed at the interface
of electrolyte and Si. Thus, in general, the thickness of the space-charge layer has the
same order of magnitude as pore diameter. The wall thickness is generally less than
twice the space-charge layer thickness. The wall region is depleted of carriers and
is thus not conductive due to the overlapping of two space-charge regions entering
P. Kumar
the holes play the main role: in fact, porous silicon formation is a self-regulated
mechanism, with hole depletion as the controlling agent. The dissolution reaction
initiates at defects on the surface of silicon, the pores are formed and their walls
are eroded until they are emptied of the holes. This formation process passivates
them from further attack, and the reaction proceeds at the pore tip only. The overall
etching process is self-adjusting and the physical properties of porous silicon, i.e.,
shape, diameter of pores, the porosity, and the thickness of the formed porous layer is
given by the electrochemical parameters only. The physical properties of PS can be
varied with extreme control by changing of the electrochemical etching parameters,
e.g., current density, substrate doping level and type or HF concentration (Kumar
and Huber 2007; Kumar 2011). In addition, when the feature size of the pores of
PS is less than a few nanometers, various quantum-size effects occur, which make
PS even more fascinating. Many theories on the PS formation mechanisms have
been reported since its discovery. In the Fig. 3 it is shown that by changing the HF
concentration in etching solution the pore diameter and porosity of PS can be varied
that results in the different color of PS. In Fig. 3, SEM images, the color observation
and photoluminescence spectroscopy measurements for three different samples were
shown. From the figure it is clear that with increase in ethanol to HF concentration
by keeping other parameters constant the pore diameter and porosity is increased.
As Si is indirect bandgap material and does not show any PL therefore is limited in
many of the technology. However, PS shows the PL properties as shown in Fig. 3
and open up several optical applications. With increase in pore diameter in the PS
sample the distance between two pores is decreased results in the decrease of Si
crystalline size. When the size of Si crystallites among the pore reduces less than the
bohr’s exciton radius, i.e., 4.9 nm for Si, several quantum effect starts to occur and
that resulted in the photoluminescence appearance in PS. The physical properties of
PS are determined by two large groups of factors, the first group of factors includes
doping type and level and potential that affect carrier density on the surface of a
pore bottom and second group of factors includes HF concentration and current
density that affect only the dissemination of the reactions. In this chapter, the effect
of HF concentration on physical properties by keeping other parameters constant
as shown in Fig. 3 was discussed. Si surface is hydrophobic in nature, therefore
ethanol is usually added to aqueous etching solution to increase the wettability of
the PS surface. Ethanol is very important for the lateral homogeneity of the PS
layer in depth because ethanoic solution infiltrates the pores, while pure aqueous HF
solution does not. Further during the etching the hydrogen evolution in bubbles form
takes place and stick on the Si surface in pure aqueous solutions whereas they are
easily removed if ethanol is present. With increase of HF concentration in etching
electrolyte, the pore diameter and porosity of PS decreases that can be understood
as follows: both the pore diameter and wall thickness for highly doped p-type Si, are
largely determined by the thickness of the space-charge layer formed at the interface
of electrolyte and Si. Thus, in general, the thickness of the space-charge layer has the
same order of magnitude as pore diameter. The wall thickness is generally less than
twice the space-charge layer thickness. The wall region is depleted of carriers and
is thus not conductive due to the overlapping of two space-charge regions entering