Tikrit Journal of Pure Science

p orous Silicon (PS) Layers were prepared by photochemical etching using Sun Light as a Source of energy for the first time instead of industrial Sources of light Such as Lasers, Halogen and tungsten lamps. That was used to preparation (PS) by photochemical etching. Silicon Wafers n-type (100), resistivity (10Ω.cm), (40%) HF Acid, with different illumination intensities(power density) (1758, 3956, 6182 and 8902)mw /cm 2 were obtained using different Lenses with different diameters (40, 60, 75 and 90) mm, focal length (13,18,41,45) cm respectively and constant etching time (60min). The morphology of the surface was studied by using an (atomic force microscope) (AFM) and (scanning electron microscope) (SEM) results shows formation of groups of pores and crystals in (nm)size with different diminution are distributed in etched area in wavy forms. The thickness of porous layers ranged from (50.06 3.68, 3.79 and 3.39) nm and the diameter of the particles are(84.06, 59.42, 51.12 and 34.8)nm respectively. The current voltage characteristics for AL thin /n-si/PS/AL thin device fabricated by sun light photochemical etching technique showed a rectifying behavior which improved with increasing light intensity.


Introduction
Crystalline silicon is the main material for microelectronic technology but its application is limited du to its relatively small and indirect band gap of approximately (1.12 ev)]1[.In 1990 Canham found strong visible room temperature photoluminescence emitted form porous silicon (PS) ]2[.an explosion of studies has been initiated to develop silicon based electroluminescent devices from this material commonly referred to as porous silicon ]3[.porous silicon (PS) is a complex network of pores separated by thin columns and contains nano-meter sized Si crystallites ]4[.(PS) is quickly becoming an increasingly important and versatile electronic material in today's fabrication technology .Reducing dimensionality of bulk silicon to nano-scale silicon (PS) leads to appreciable changes in optical, electrical and electronic properties.2)with suitable slope making the plat facing sun light with constant etching time (60min).Bubbles were observed during the etching process, indicating the propagation of the etching process and hence the formation of porous silicon.
The morphology of silicon nano crystallites studied by the atomic force microscope (AFM)and scanning electron microscope(SEM).Also we studied (I-V) characteristic to measure the electrical properties and other related properties of the silicon nano crystallites Layer after etching.An ohmic contact for the prepared samples are produced from both sides (bulk & porous) by evaporating Aluminum(AL).Silver past used to connect the copper wires on the surface of the samples .

2-3. Porosity of (PS)
The porosity (P) is defined as the fraction of void space within the porous layer and can be determined easily by weight measurements .The porosity of the layer can be determined by weighting the silicon substrate before and after etching process (m 1 and m 2 ) respectively and again after removing the porous silicon layer by using a molar NaOH or KOH (m 3 ).The porosity is given by the following equation: The value of porosity of PS sample shown in table (1).The porosity increases with increasing the light intensity as show in table (1), and this lead to decrease the charge carriers (electrons) in the porous Layer.The porosity is strongly depends on the preparation conditions, these conditions include HF concentration, etching time, illumination wavelength and power density of the illumination light]9[ .Electrical Properties: I-V measurement were recorded for the samples AL thin /n-Si/PS/AL thin structures fabricated by sun light photo chemical etching (SLPCE) on n-type (100) Si with different light intensities (power densities) (1.7584,3.9564, 6.181875 and 8.9019) w/cm 2 shown in Fig( 9) the rectification improves with increasing of light intensity.This behavior is expected when we probe the change in size of nano structures and porosity .

Fig ( 1 )
Fig (1) The band gap energy (E g ) as a function of the crystal diameter ]10[.

FigFigFig ( 4 )
Fig (2) a schematic the Diagram of the sun light photochemical etching (SLPCE) Result and Discussion a-The surface morphology ]AFM[ An atomic force microscope (AFM) was used to study the surface morphology of the porous silicon produced by sun light photochemical etching .(AFM) Images for surface morphology showed The effect of light intensity on surface porosity roughness, particle size that can be calculate and gives the distribution rat of the crystalline particle size on the surface as show in fig (3).To study the effect of the light intensity on morphology of the (PS) layer, four samples of (100) n-type silicon, 10(Ω.cm)resistivity immersed in (40%) HF acid for 60 min constant etching time and different light intensities (1758, 3956, 6182 and 8902) mw/cm 2 focus on mirror like side of silicon samples .AFM studies shows that, The thickness of porous layers ranged from (50.06 3.68, 3.79 and 3.39) nm ,and the diameter of the particles are(84.06,59.42, 51.12 and 34.8) nm respectively we observe that the nano crystals size reduced with increasing light intensity .

Fig ( 7 )
Fig (7)Distribution of statistical size nano particles made up on surface of the sample (d) in fig .(3)b-The surface morphology by scanning electron microscope (SEM):The study of the surface samples is removed by the photochemical etching method showed that the areas appear to dissolve in the middle of the samples and the superficial dimensions]1-D[ increase with the intensity of the light on them can be explained in this