Improvement the efficiency of SnO2/n-Si detector by engraving method using a CNC machine

Tin oxide was deposited by using vacuum thermal method on silicon wafer engraved by Computer Numerical Controlled (CNC) Machine. The inscription was engraved by diamond-made brine. Deep 0.05 mm in the form of concentric squares. Electrical results in the dark were shown high value of forward current and the high value of the detection factor from 6.42 before engraving to 10.41 after engraving. (I-V) characters in illumination with powers (50, 100, 150, 200, 250) mW/cm2 show Improved properties of the detector, Especially at power (150, 200, 250) mW/cm2. Response improved in rise time from 2.4 μs to 0.72 μs and time of inactivity improved 515.2 μs to 44.2 μs. Sensitivity angle increased at zone from 40o to 65o.


Introduction
Tin oxide, in its volumetric state, is one of the insulators, but it turns into a semiconductor if it is deposited form thin film [1]. The transition from the insulator to the semiconductor occurs due to deviation at valence in oxidation preparation [2]. SnO 2 thin film can be prepared in a variety of ways physical and chemical. The interest in heterostructures with silicon as one of the components is due to the practical significance of multilayered structures of materials in modern electronic engineering.
Along with semiconductor heterostructures, it is also interesting to investigate liquid crystal (LC)-semiconductor interfaces [3].
Properties of SnO 2 /n-Si heterojunctions of the symmetric type, 52 it acts as a solar cell or detector [4]. Thin films of SnO 2 which are growing on silicon surfaces have possess certain characteristics which make them interesting for photovoltaic applications [5]. The efficiency of the detector depends on the surface area exposed to radiation [6] . The area can be increased by chemical etching using hydrofluoric acid to obtain nano structure in the silicon surface [7], or by developing nanoparticles of tin oxide on flat surfaces of silicon [8]. The research aims is making engraving in a silicon chip in a mechanical manner using CNC and coating the tin oxide to manufacture a detector.

1-CNC machine
Computer Numerical Control (CNC) a machine that can be controlled by computer. It is an innovative addition to cutting and drilling operations, especially for operations that need high accuracy [9]. The CNC machine used in the research of the type used for drilling circuit boards and working on three axes this machine is works by program written in high level language known G-cod. In this research a program was written to make concentric squares and a depth of 0.05mm as follows: The program was repeated four times, and the length was reduced to xaxis and y-axis by 1mm at every time.
In the engraving process a diamondmade brine was used. The machine is set to rotate the cutter speed up to 4200 rpm, and quickly engraving up to 1 cm per minute.

2-Silicon wafer
In this research's n-type Silicon wafer was used with (111) direction. Silicon was cleaning by washing using distilled water Then wash with methanol for 10 minutes because it is a strong solvent and then wash with water then wash with ethanol for 10 minutes and then with distilled water [10]. The silicon was cut into 1.5 cm 2 pieces. Then put it in CNC machine to be engraving in four concentric squares. Some samples were left without engraving to use them as reference.

3-Thin film of SnO 2
Tin oxide thin films were prepared by using vacuum thermal method. A quantity of pure tin powder 99.9% was put in boat of tungsten, at height of 10 cm from silicon pieces. The pressure inside the chamber was reduced to 1.4 x 10 -4 mbar, then tin was evaporated tin to growing on silicon wafer. It was then put into a 650-degree oven for 1 hour, the tin then oxidizes and becomes tin film produces our SnO 2 / n-Si device. Fig.1 shows the x-ray analysis of SnO 2 films deposited on the glass and oxidate in the same way, confirming the tin oxide to the tin oxide. Fig.2 shows the image of the detector used in this research. To calculate thickness, the weighted method represented by Eq. (1) [11]. (1) when t: thickness, m 2 -m 1 : difference weight between m 2 and m 1 , ρ: density of material, A: The area is added to the increase due to engraving. Fig.3 show the (I-V) characters of SnO 2 /n-Si in the dark using (digital multimeter (Victor VC97). The forward current shows an increase in the current value of the detector, and this increase is due to the increase of the detector area. The value of the voltage barrier V b has not changed significantly. In reverse, the reverse current of the detector is also increased in engraving detector especially when increasing the voltages. Rectification factor is equal to the ratio between the forward current to the reverse current at a same voltage [12], reaches to 6.42 for smooth and equal to 10.41 for engraved detector by an increase of 61.18%.

2-(I-V) Characterization in illumination
The effect of the incident light on the detector was studied in power (50, 100, 150, 200, 250) mW/cm 2 . Fig.4(a,  b, c, d, e) shows the increase reverse current when illumination for the engraving detector compared with smooth detector, and came higher than the results of the researcher (a) [12]. The Fig.4(f) shows the variation of photo current and power intensity with constant bias at 9 volt, smooth detector reached to Saturation current after 150mW while engraving detector continue to rise. The increase in the area of the detector led to an increase in the photo current, in addition to the fact that the engraving has caused a rough surface reduced the value of the reflection. The results of the engraving detector in general were inferior to the detector deposited by the SnO 2 thin film of the nanoparticle [13] or the SnO 2 thin film deposited on the porous silicon [14]. The reason for the engraving detector being that the increase in the area of linear areas either in the porous detectors or nanoparticles, the increase in areas for all detectors.

3-Response time
Pulsed laser was used with a wavelength of 900 nm with wave width 2 µs with a lead 120 µs was used to find response time. The detector is connected to an external voltage of 9V across serial resistant 1 KΩ, and from them signal received by digital oscilloscope. Fig.5 shows the pulse shape received. Rise time calculates from 10% to 90% of wave rise time. Time of inactivity calculates from 90% to 10 % of wave relaxation time. The results showed that the time of the rise of the smooth detector in 900 nm was 2.4 µs while the detector was faster than times 0.72 µs. Time of inactivity of the smooth detector was 515.2 µs while the detector was also faster than times 44.2 µs. The response speed in the engraving detector was slightly higher than the detector deposited by the SnO 2 tin film of the nanoparticle [13] or the SnO 2 tin film deposited on the porous silicon [14]. The results indicate crystal defects works to minimize the mobility of carriers. This means that the engraving process has caused crystalline defects due to heat generation during drilling [12].  Fig.6 show the relationship between the angle of incident light and the photo current. Helium Neon Laser was used at wavelength 633 nm / 1mW. The angle was changed from 5 degree to 95 degree. The results indicate high of photo current value from 5 o to 30 o For both detectors and the detector with engraving showed an increase in current value at the angle 40 o to 65 o The maximum angle which promise is in 55 o due to the vertical of the light incident on the edges of the engraving [12]. This increase did not occur in nanotubes or porous detectors [13,14] because their surfaces did not contain deep reductions.

Conclusions
Silicon wafer can be engraving from CNC machine by diamond-made brine, then deposition thin films on them to fabicate a detector. The engraving detector has high sensitivity to incidence light. It has high response time compared to another detector of the same type. The detector response was able to increase with increasing the sensitivity angle. Therefore, the detector prepared in this way done quickly and have a specifications that can be exploited for industrial production. This method is characterized by non-use chemical solutions that are deactivate when used, In addition to being a safety.