Photoluminescence and Band Energy Gap For Porous Silicon p-type with Different Current Density and Different Etching Time
1Mohammed Jabbar Hussein, *1W. Mahmood Mat Yunus, 2Halimah Mohamed Kamari
3Josephine Liew Ying Chyl,
 
Abstract
Photoluminescence (PL) for porous silicon lead to studied by many researcher .this phenomenon which has application in many devices. Porous silicon can be used in the optoelectronic devices and sensor. Therefore, the study to development porous silicon is very necessary,. In this study, porous silicon was prepared by chemical etching used silicon , the based electrolyte was ued HF acid with ethanol 1:1 . The porous silicon was characterized by Photoluminescence Spectrometer (PLS) and (SEM) Microscope. The range of the porosity (20.33 – 78.2) % and it is dependent on current density and etching time. The has Band gap energy which is from 1.81 ev to 2.o7 ev , the band gap energy is increased with increasing current density and increasing etching time
Introduction
Since the discovery of visible luminescence in the room temperature [1–5], porous silicon (PS) has become a subject of considerable interest, optoelectronic device [6,7]. many several methods [8–10] for fabricating PS from crystalline silicon wafers. The electrochemical etching [1,8]. Both method is the difference between them the chemical etching without using the external bias, therefore, considered this method the localized electrochemical process chemically [11]. Porous silicon was discovered in 1956 by Uhlir ( Uhlir 1956) while performing electro polishing experiments ,hydrofluoric acid (HF) was prepared in to silicon wafer by electrolyte containing . He found that under the appropriate conditions applied current and solution composition , the silicon did not dissolve uniformly but instead fine holes were produced, porous silicon formation was obtained using electrochemical dissolution of silicon wafer in aqueous or ethanoic HF solution The size dependency of the PL energy , which explains the efficient luminescence , causes the peaks to sift towards the higher energy or lower wavelength , as already reported [12,13] ,the red shift in PL peaks with decreasing average size of Si structure size in psi is considered to be strong evidence that the visible PL is caused by the quantum confinement effect [14] . The degree of the blue shift for psi usually depends preparation condition , Si resistivity , substrate ,type and dopant concentration , which can cause different modification of psi microstructure during anodisation [15] The values of the band gap energy of the porous silicon are the same range of the reported ( 1.5ev to 2.5 ev) [16.;17,18] In this work , porous silicon p-type was prepared by electrochemical etching technique and photoluminescence PL spectroscopy was used to determine the wavelength the emitted light .
Preparation porous silicon
All samples were prepared on (100) n- type silicon (Si) single crystal wafers of 537 thickness. Silicon substrates were cleaned by sonification for 5min in ethanol, and acetone. A Si substrate was placed at the bottom of a cylindrical Teflon cell and fixed by an aluminium plate as a backing material. A platinum (Pt) rod serves as a cathode perpendicular to the Si surface at a distance of (1cm). The samples were prepared with constant current, density, and etching time at a concentration of ethanol ( in the volume ratio of 1:1. Theaside is an essential ingredient for the anodical etching of . Ethanol was added into electrolyte to enhance the homogeneity and uniformity of the () surface because it acts as a promoting agent to increase the wettability of () surface and to remove the extraneous H2 bubbles that appear during the anodical etching process. In fact, ethanic solutions infiltrate the pores, while purely aqueous HF solution does not. This is very important for the lateral homogeneity and the uniformity in depth of the () layer. A digital current source () was used to supply constant current. Figure (1) shows the schematic diagram of all the elements used for the preparation of (). To generate the electron hole pairs, the surface of sample was illuminated with halogen lamp () during iodisation. For all samples, a voltage of was applied to the halogen lamp for illumination. The current densities used for samples are 10 mA/cm2, 20 mA/cm2, and 30 mA/cm2 with etching times of 20mins, 40mins, 60mins, and 80mins.

HF based electrolyte Pt electrode

Teflon cell
Si wafer Current source
AL plate

Figure 1: Schematic of electrochemical etching cell for iodisation of () samples

Figure (2):porous silicon (). a without light .b)with light .c) after remove the porous .

Figure(3) : SEM images of PSi a) – silicon wafer as scale 1m, b)-porous silicon (). as scale. c) porous silicon as scale 500 nm .d) porous silicon as scale .
Results and discussion
The optical properties of psi samples electrochemically etched at three different current density and by varying etching time. Figures (4- a,b,and c) shows the variation of PL spectra with etching time for the psi samples obtained at the current density 10 mA /cm2 , 20 mA/cm 2, and 30 mA/cm2. The intense luminescence spectra emitted from porous silicon structures formed on the samples. The pl peaks show a steady red color shift from 500 nm to 800 nm with increasing the etching time .

,
Figures (4-a,b,c) PL peaks for porous silicon samples prepared under differnet etching time with the current density a) 10 ma/cm2 ,b)20 mA/cm2 ,c)30 mA/cm2. respectively
The Figure (5) show the PL spectra of samples prepared by current density of 10, 20 and 30 , respectively ..The band gap energy () was inferred from (PL) wavelength (λ) using (. the relationship between band energy gap with the current density and etching time is increased the band energy gap when the current density and etching time increasing [19] . Figure (5-a,b) show the energy gap () variation of psi samples as a function of etching time and current density , respectively

Figures (5-a,b) Band energy gap as a function to a) etching time with different current density ,b) currents density with the different etching time . Respectively
Figure (6) show the band energy gap versus the porosity for three different current density of the samples and deferent etching time. The band gap energy value is not linearly increased with increasing porosity because the change in the structure size of the silicon. The results show the dependency of the band gab energy value to the current density especially in high porosity [16]

Figure (6) .band gap energy as a function to porosity with different current density.

)

T (min)

PL peak in.(a.u)

λ )

(ev)

Porosity (%)

10

20

1.5

678

1.82

20.33

40

4.3

648

1.90

40.6

60

11

636

1.94

48.2

80

13.6

627

1.97

51.2

20

20

3.2

662

1.86

29.04

40

4.6

640

1.93

50.3

60

11.2

628

1.96

57.6

80

17.1

612

2.01

65.8

30

20

4.4

640

1.93

38.2

40

13.7

627

1.97

59.6

60

16.9

614

2.01

68.7

80

19.8

595

2.07

78.2

Table (1) .the values of porosity, band gap energy, and PL peaks intensity for porous silicon with different current density and different etching time
Table (1) shows that the band gap energy increases from ( 1.82 ,1.90,194,and 1.97 ) ev to (1.86,1.91,196,and 2,01) in etching time (20,40,60,and 80 )min respectively when the current density increasing from 10 to 20 ,also the increases the band energy gap from ( 1.86,1.93,196,and 2,01) ev to ( 1.93,1.97,2.01 ,and 2.07 ) in the etching time20,40,60,and 80 )min respectively when the current density increasing from 20 to 30. The results show an increase the band energy gap when the etching time increasing. That mean the band energy gap depended of the current density and etching time . the results shown the band gap energy is increased with etching time , it is increase from (1.82,1.86,and 1.93 ) ev to (1.90 , 1.93 , and 1.97 ) ev in the current density (10,20,and 30) mA/cm2 respectively when the etching time increasing from 20 min to 40 min . as well to another etching time from 40 min to 60 min and from 60 min to 80min .
Conclusion.
In summary. The results show for the effects the etching time and current density to the band energy gap and the porosity. In this experimental the band energy gap is increased with increasing the current density the band gap energy increases from ( 1.81 ,1.88,194,and 1.99 ) ev to (1.86,1.91,196,and 2,01) in etching time (20,40,60,and 80 )min respectively when the current density increasing from 10 to 20 , . as well to another current density ,also the band energy gab is increased from (1.81,1.86,and 1.93 )ev to (1.88 , 1.91 , and 1.97 ) ev in the current density (10,20,and 30) mA/cm2 respectively when the etching time increasing from 20 min to 40 min . as well to another etching time .
Acknowledgment
The authors would like to thank Physics Department in the University Putra Malaysia for providing the research fealties.
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