**4. Formation mechanism of the nanostructures**

Electrochemical dissolution of Si, in the second step of etching, is highly influenced by hole current density at the interface of Si/electrolyte. Hole drift current density in semiconductors can be found by,

$$J\_P = e \times p \times \mu\_p \times E \left(\frac{A}{cm^2}\right) \tag{4}$$

Where e is electric charge (1.6 × 10<sup>−</sup>19 coulomb), p is the concentration of holes/ cm3 , *μp* is the hole mobility (cm<sup>2</sup> /V s) and E is the applied electric field (V/cm).

Eq. 4 confirms the dependence of the etching rate with the electric field strength. According to **Figure 3b** the pyramid perimeters are experiencing a higher etching rate at the beginning of the etching process. This can be attributed to maximum electric field intensity at those regions.

Due to uneven textured Si surfaces, the electric field is not uniform at the interface. As a result, the hole drift current density is not uniform over the surface of the samples and different areas are experiencing different dissolution rates during the anodic etching.

The electric field intensity generated at textured Si (anode) is modeled and simulated utilizing COMSOL, electrostatics module. To study the evolution mechanism we have considered the 3D schematic geometry and initial electric field distribution shown in **Figure 4** when voltage is applied to the backside of the textured Si. Simulation results shows that the electric field is not uniform and the lowest belongs to tips and lateral edges of the pyramids, and highest is for pyramid perimeters.

To present the development of the structures, COMSOL deformed geometry (dg) physics interface is applied to all domains. A prescribed mesh velocity is assigned to the boundary between Si and electrolyte (pyramidal texture) and a prescribed mesh displacement is assigned to boundaries all around the block. Moving mesh velocity is defined proportional to electric field.

**Figure 5** clearly shows that the etching and developing porous silicon start at the edges of the pyramids bases where the electric field has the maximum value.

As the etching extends, pyramid faces are etched away while lateral edges remain as connecting walls between the pyramids. **Figure 6** clearly illustrates the residual walls between the structures.

*Nanostructures*

ohmic regions.

where *Eeff* is the enhanced electric field and βtol is average field gain coefficient. By comparing Eqs. (1) and (2) and considering a constant *σGas*, βtol of gas ionization sensors can be estimated by dividing the slopes of I-V characteristics of the

> *SlopeGIS SlopePPL*

where *SlopeGIS* is the slope of I-V discharge graph of the nanowire based GIS and *SlopePPL* is the slope of the I-V discharge graph of the parallel plate in their

Silicon nanostructures are fabricated using chemical/electrochemical technique [43]. Samples of p-type <100> silicon wafers (380 ± 10 μm thickness with resistiv-

cleaned using RCA technique. In the first step of etching, samples were textured by pyramidal structures through anisotropic etching in tetramethylammonium hydroxide (TMAH) based solution. The solution for anisotropic etching, made of equal amount of TMAH and isopropyl alcohol (IPA) (5 wt% each), was used to etch the samples for 20 minutes. The temperature of the solution was kept constant at 90°C using an oil bath system. A condenser covered the etchant container in order

*Experimental results of the evolution of the nanostructures. (a) Anisotropic etching of <100> p-type silicon at 90°C for 20 minutes in 5 wt% TMAH and 5 wt% IPA resulted in formation of pyramids that covers the surface completely. (b) Top view of the electrochemically etched textured silicon after 10 minutes of etching. (c) Top view of the electrochemically etched silicon after 70 minutes. (d) Top view of the electrochemically etched silicon after 70 minutes. (d-inset) Tilted view of the final nanostructures illustrates the existence of residual walls* 

ity about 5–10 Ω cm from Silicon Material Inc.) were cut in 1 × 1 cm2

(3)

pieces and

device with a parallel-plates in the ohmic region.

**3. Fabrication of silicon nanostructures**

β*tol* = \_\_\_\_\_\_\_

**104**

**Figure 3.**

*around the structures with the thickness of 100 nm.*

#### **Figure 4.**

*Distribution of electric field in the textured Si surface is simulated by COMSOL electrostatics module. (a) Top view and (b) tilted view of the 3D structures shows that the electric field strength is not uniform and the lowest belongs to vertex and sides (shown by dark blue) and highest is for pyramid perimeters (shown in yellow/red).*

#### **Figure 5.**

*(a) Top view and (b) tilted view of COMSOL deformed geometry. The figures show that as the etching starts, pyramid perimeters are experiencing higher etching rates due to higher electric filed strength.*

**107**

**Figure 7.**

**Figure 6.**

*Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon…*

*DOI: http://dx.doi.org/10.5772/intechopen.84264*

**5. Device fabrication and sensor setup**

*while pyramid faces are etched away. (a) Top view, (b) tilted view.*

the schematic illustration of the gas ionization cell.

Fabricated Si nanostructures were incorporated as the anode in gas ionization cell. The cathode is a piece of p-type Si cleaned with RCA method and coated with a 5 μm aluminum (Al) layer using thermal evaporation technique. The electrodes are separated, using an insulating thin film (double sided adhesive tape), by a narrow gap but wide enough to allow the flow of the gases through the cell. **Figure 7** shows

*COMSOL deformed geometry shows that the extended etching results in residual walls around the structures,* 

Torr prior to

The device was placed in the gas chamber and vacuumed to 10<sup>−</sup><sup>5</sup>

introduce each gas to the chamber. The electrodes are connected to two source measure units (SMUs) of a HP4155 semiconductor parameter analyzer and I-V characteristics of the device was conducted by sweeping the voltage of the anode

*Schematic of Si nanowires-based GITS. Fabricated sample is applied as the anode and it attached to SMU #1 of the parameter analyzer. An Al coated Si is applied as the cathode. Electrodes are separated by two pieces of double sided adhesive tape. The other sides are left open to facilitate the gas flow through the sensor.*

*Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon… DOI: http://dx.doi.org/10.5772/intechopen.84264*

**Figure 6.**

*Nanostructures*

**106**

**Figure 5.**

**Figure 4.**

*(a) Top view and (b) tilted view of COMSOL deformed geometry. The figures show that as the etching starts,* 

*Distribution of electric field in the textured Si surface is simulated by COMSOL electrostatics module. (a) Top view and (b) tilted view of the 3D structures shows that the electric field strength is not uniform and the lowest belongs to vertex and sides (shown by dark blue) and highest is for pyramid perimeters (shown in yellow/red).*

*pyramid perimeters are experiencing higher etching rates due to higher electric filed strength.*

*COMSOL deformed geometry shows that the extended etching results in residual walls around the structures, while pyramid faces are etched away. (a) Top view, (b) tilted view.*
