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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
Estimation Energy Band Gap of Au/ Nano-Crystal
Porous Silicon/Mono-Crystal Silicon Heterojunction
Hasan A. Hadi
Department of Physics, College of Education, Al-Mustansiriyah University, (Iraq)
Email: bojyotmtm@gmail.com, ha_yaba@yahoo.com
ABSTRACT
Porous silicon layers were fabricated on p-type crystalline silicon wafers using electrochemical etching ECE process.
An investigation of the dependence on applied current density to formed PS layer was made. Porosity of the porous
silicon layer and thickness were determined gravimetrically. Increasing the etching current density led t o increase
the surface porosity and thickness.The porosity varies between 44and 77% for current densities between 25 and 85
.The current density -voltage characteristics of Au/nano-crystal porous silicon/mono-crystal silicon
heterojunction was examined under 7.5 power density illuminations. From the experimental data set, the
maximum value of responsivity and band-gap energy of porous silicon are deduced to be 1.7A/W and 2.07 eV
respectively at etching current density.
Keyword: Porous Silicon; Electrochemical; Responsivity; Heterojunction; Quantum Confinement; Energy Gap
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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
INTRODUCTION 1
Electrochemical etching is one of the simplest and most reliable method used to synthesis porous silicon PS [1]. 2 The formation of PS (porous silicon) layers on crystalline Si (c-Si) wafers using electrochemical etching (ECE) 3 exhibits photoluminescent and electroluminescent properties similar to those of semiconductors with direct energy 4 gap [2].Since the porosity (ratio of voids to crystallites) of the PS layer depends on the effective distribution of the 5 voids fractions in the resulting silicon crystallites in the PS matrix, a variation of current density (for other etching 6 parameters such as concentration of the electrolyte, etching time and surface doping of the Si' substrate) can alter the 7 porosity of the sample [3]. Relation between porosity, thickness and etching current density leads to porous structure 8 which reduces the recombination center, thus giving information about lower defect densities in the interface of 9 heterojunction [1]. The absorption edge of PS samples exhibits a blue shift with increasing porosity. Also we have 10 observed that porous silicon has a quasidirect band gap [4].A lateral spatial variation of the porosity is observed for 11 PS layers formed on the Si surface. It has been shown; the selection of anodisation times and anodic current density 12 allows changing a peak position of the PL spectrum of porous silicon in the range from 580 to 780 nm. The 13 specified PL properties of porous silicon are connected with quantum size effects, i.e. with change of band diagram 14 and widening of effective band gap[5].The quantum confinement determines the enlargement of the band gap and 15 leads to the breakdown of the momentum conservation rule, allowing no-phonon optical transitions (like in the case 16 of a direct gap). But its main contribution consists in the appearance of new energy levels [6].The practical 17 applications are oriented towards the fabrication of new structures and devices. The compatibility of the 18 nanocrystalline silicon-based materials with the classic mono- and/or polycrystalline silicon (bulk o r thin films) 19 permits the use of these new materials for the integrated micro- and optoelectronics, photonic crystals, biomedical 20 applications or efficient sensors [7].The PS/c-Si interface to be a kind of heterojunction, and PS; layer is a wide band 21 gap semiconductor sensitive to visible light. Due to its extreme high resistivity, when a bias voltage is applied, the 22 major potential drop is produced across the PS layer. In this paper, we report the effect of applied current density on 23 thickness and porosity and the influence of these parameters on the photocurrent spectra of the obtained Au/porous 24 silicon/p-Si/Al heterojunction samples. Estimation energy band gap of Au/ nanocrystal porous silicon/mono-crystal 25 silicon heterojunction as a function of applied current density. 26
EXPERIMENTAL 27
The single cell back-side contact and has been already shown in Fig. 1 was used to papered porous layer. In 28 this cell, the Si wafer is placed on a copper disk and sealed through an O-ring, so that only the front side of the 29 sample is e xposed to the electrolyte. The copper disk has to be cleaned with a lapping machine in order to remove 30 the oxide film that forms itself after many etching processes. The silicon was used as an anode while the cathode 31 was of gold mesh to papered porous layer by electrochemical etching (ECE) technique. In this study, we used p-type 32 silicon wafer was doped with boron to resistivity of 11-16 Ω.cm with (111) orientation. After cleaning processing 33 and to get ohmic back- side contact of substrate, aluminum th ick film was deposited by thermal evaporation. The 34 evaporation is performed in a vacuum pressure of torr, using an evaporation plant model “E306 A 35
manufactured by Edwards high vacuu m”. After the evaporation process, the thicknes s of evaporated film on a glass 36 substrate is measured using gravimetric method and was 88-90nm. Porous silicon etching was performed in HF 37 (40%): methanol (99%) = 1:1 solution with different etching current densities (25, 45, 65 and 85) and 38 process duration of 1 min in the dark and room temperature (Table 1). The virgin wafer is first weighed before 39 anodisation (), then just after anodisation () and finally after dissolution of the whole porous layer in a molar 40 NaOH aqueous solution (). Uniform and rapid stripping in the NaOH solution is obtained when the PS layer is 41 covered with a small amount of ethanol which improves the infiltration of the aqueous NaOH in the pores. The 42 porosity is given simply by the equation [8]: 43 44
(1) 45
46 From these measured masses, it is also possible to determine the thickness of the layer according to the equation [8]: 47 48
(2) 49
50 51
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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
Table 1.The calculated values of porosity and thickness 52
Etching current density
Thickness nm
Porosity %
25
56
44
45
73
60
65
85
72
111
77
53
54
Figure 1: Schematic of the single cell back-side designed porous silicon fabrication system 55
where is the silicon density and the etched surface, or measuring, with a profile meter, the depth of the hole left 56
after diss olution of the PS layer[8].In gravimetr ic method, “Mettler AE-160 digital with accuracy of gm” is 57 used to weight the samples. The topography of the PS surface is investigated with optical microscopy. Computerize 58 optical microscope provided with camera model Olympus BX 51n is used.J-V measurements of photodetectors at 59 reverse bias under illumination are carried out using 100W halogen lamp .The optical power of the lamp was varied 60 through adjust the input electrical voltage with aid of a variac. The light power measurement is performed using 61 calibrated Si power meter model 401C. 62 Spectral Responsivity R measurements were done using a monochromatic model Jobin Yvon Division Instruments 63 in the spectral range of (400-900) nm. Power calibration is done using calibrated Si power meter. R is calculated by 64 measuring the photocurrent as function of wavelength and then divides it on power as in the following relationship 65 [9]: 66
(3) 67
where: is the photogenerated current due to the absorption of the incident light power at a given 68
wavelength.The value of energy gap of porous silicon is determined by the photoresponse spectrum curve between 69 photocurrent and energy of quanta of the incident light. 70
RESULT AND DISCUSSION 71
One of the most important characteristics of a porous silicon layer is its porosity, defined as the fraction of air 72 inside the porous layer. The porosity and the thickness of the PS layer can be controlled by controlling the 73 anodisation parameters such as current density and etching time.The porosity varies between 44and 77% for current 74 densities between 25 and 85 . As shown in table1 the porosity increases when the current density increases 75 and this results agree with [10].These porosity values of porous silicon layers obtained by the Eq. (1) to show the 76
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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
77
Figure 2: (a ,b, c , and d) optical micrographs of PS surface on p-Si formed at different current densities and right 78 inset of figures (a,b,c and d) photograph of PS surface image by camera digital HD Samsung 5x at the same 79 condition 80 81 relation between the porosity and the sensitivity of the formed junction between the crystalline silicon and the PS 82 layer. sometimes the increased of porosity . 83 The topography of the PS surface was investigated and the porous morphology formation has been confirmed using 84 optical microscopy. Figure 2 (a,b,c and d) display the micrographs of porous silicon synthesized with d ifferent 85 etching current densities (25, 45,64, and 85) respectively for 1 min etching time. It is observed that current 86 density plays a significant role in controlling porous morphology.This confirms anodic dissolution of silicon surface 87 leading to porous structure formation and variation of silicon surface colors indicate these surfaces are having 88 different band gap and hence give different photoluminescence. The etched layer have different colors between 89 golden and yellow, also some time close to red resulting may be sub oxide of, this color indicates increasing of band 90 gap of the silicon after etching (visible photoluminescence) as shown in Fig.2 (a,b,c and d). 91 It is observed that current density plays a significant role in controlling porous morphology. Increasing the etching 92 current density leads to increasing the density and size of the pores the etched region gave different color which 93 confirms changing of silicon band gap and also indicates different pore size. Figure 3 show the illuminate d reversed 94 (J-V) characteristics of the Au/PS/p-Si heterojunction samples with different etching current densities under dark 95 and white tungsten lamp 7.5 illuminating power density. This figure explains the J-V characteristics 96 sandwiches Au/PS/p-Si/Al structure under illumination with different etching current densities (25, 45,65,and 97 85)Operating the detector under external reveres bias voltage causes the depletion region to be extended. 98 So, a large photon number of incident beam will constitute the electron-hole current. .It can be seen that the reverse 99 current value at a given voltage for the Au/PS/p-Si/Al heterojunction under illumination is higher than that in the 100 dark. The effect of etching current density it clear ,when the photocurrent decreases with increasing preparation 101 current density because the increasing value of resistivity is due to increasing the thickness and porosity of PS layer. 102
The photo current can be found in another form as a function of the quantum efficiency [ 11],
, 103
where is the reflectivity and is the quantum efficiency. This equation agrees with the present results shown in 104 Fig.3, where photocurrents increases when illumination all Au/PS/p-Si devices. 105 It is obvious from Fig.3 the photocurrent increas es sharply with increasing light intensity at above voltage -6 volt 106 which is very close to the breakdown. The spectral responsivity of structures is investigated in the wavelength range 107 of 400 – 1000 nm with 1V bias and by Eq. (3) was calculated. Photocurrent represents an important parameter which 108 effect on spectral responsivity, the operating spectral range of the photodetector was in the range of (400-800) nm. 109 Fig. 4 (A1, A2, A3 and A4) show the variation of responsivity as a function of wavelength for A u/PS/p-Si/Al 110 devices prepared by different current-density (25, 45, 65 and85 mA/cm2) with constant etching time 1min. All the 111 samples show approximately similar spectral responses regions and the differences appeared to be in the amplitude, 112 with different etching current density. Also these values of responsivity of PS/p -Si were higher than conventional 113 diode because of the porous surface is perfect in trapping photons and the surfaces are well passivity with very low 114
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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
concentration of surface reco mbination as well as the reflectivity of porous Si for visible and near ultra violet 115 regions is very low. Also Raid considers the total sensitivity of the photodetector is the sum two terms; = 116
+ , where is the sensitivity of the Al/PS Schottky contact and is the sensitivity of the PS/c-117
Si heterojunction [12].Two peaks are observed in Fig.4 (A1), a maximum responsivity of 0.9 A/W was observed at 118 600 n m and 0.7 A/W at 750nm. The photoresponse of device shaft toward the short-wavelength, at short 119 wavelength incident photon energy which is large than the energy gap implies a considerable increase in the 120 responsivity and this increase relates to the high absorption coefficient. The value of energy g ap is determined by the 121 photoresponse spectrum curve between photocurrent and energy of quanta of the incident light. In the case of nano - 122 or micro-porous silicones, quantum confinement causes spatial fluctuations of the effective band gap as can be seen 123 in figures 4(B1),5(B2),6 (B3),and 7(B4) respectively. In Fig.4(B1) the energy gap of PS/p -Si was 1.66 eV and it 124 represent the total of energy of the heterojunction, where the energy gap of c-Si is 1.12 eV. The enhancing of band 125 gap in PS is connected with quantum-size effect. The main quantum confinement effect is represented by the 126 appearance of new energy levels in the silicon band gap. Therefore, the contribution of the formulation of the porous 127 silicon layer at 25 mA/cm2 etching current density was 0.54eV. 128 129
130
Figure 3: Photocurrent of PS/p-Si heterojunction as a function of reverse bias at different etching current density 131 and illumination 7.5 132
133
134
Figure 4: (A1) the responsivity as function of wavelength and (B1) the spectrum of photocurrent of Au/PS/p-Si/Al 135
structure vs. incident photon for. 136
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OALib PrePrints | http://dx.doi.org/10.4236/oalib.preprints.1200044 | CC-BY 4.0 Open Access
Received: 2014/06/24, published: 2014/06/30
Fig. 5 (A2 and B2) show the responsivity as a function of (400-1000) nm wavelength and the spectrum of 137 photocurrent of Au/PS/p-Si/Al structure vs. incident photon at.current density. In Fig. 5(A2) are shown 138 three peaks of responsivity were (0.28, 0.4 and 0.44) A/W at (450,600 and 700) nm respectively. Also and result of 139 quantum confinement, all these peaks in visible reg ion result of porous formation and corresponding in Fig.5 (B2) 140 energy gap of 1.778 eV. Therefore, the contribution of the formulation of the porous silicon layer at 25 mA/cm2
141 etching current density was 0.658eV. 142 Fig.6 (A3 and B3) show the responsivity as a function of (400-1000) nm wavelength and the spectrum of 143 photocurrent of Au/PS/p-Si/Al structure vs. incident photon at65 mA/cm^2.current density. Fig. 6(A3) shown one 144 peak of responsivity was 1.7A/W at 500 n m and this may due to the excessive etching process which leads to 145 increase of porosity of the porous silicon layer and hence improve the sensitivity of the formed junction between the 146 crystalline silicon and the PS layer. Fig.6 (B3) energy gap of PS/p-Si was 2.07 eV. Therefore, the contribution of the 147 formulation of the porous silicon layer at 65mA/cm2 etching current density was 0.95eV. 148 Fig.7 (A4 and B4) show the responsivity as a function of (400-1000) nm wavelength and the spectrum of 149 photocurrent of Au/PS/p-Si/Al structure vs. incident photon at85 mA/cm^2.current density. In Fig. 7(A4) are shown 150 two peaks of responsivity were (0.6 and 0.33) A/W at (450 and 750) nm respectively. Also and result of quantum 151 confinement, a ll these peaks in visible region near UVand IR regions result of porous formation and corresponding 152 in Fig.7 (B4) energy gap of 1.778 eV. Therefore, the contribution of the formulation of the porous silicon layer at 153 85mA/cm2 etching current density was 0.658eV 154 155
156
Figure 5: (A2) the responsivity as function of wavelength and (B2) the spectrum of photocurrent of Au/PS/p -Si/Al 157 structure vs. incident photon for . 158
159
Figure 6: (A3) the responsivity as function of wavelength and (B3) the spectrum of photocurrent of Au/PS/p-Si/Al 160 structure vs. incident photon for . 161
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162 Figure 7: (A4) the responsivity as function of wavelength and (B4) the spectrum of photocurrent of Au/PS/p-Si/Al 163 structure vs. incident photon for . 164
CONCLUSION 165
Porous silicon morphology formation synthesized by electrochemical etching has been confirmed using optical 166 microscopy investigation. The porosity and thickness of porous silicon are found strongly dependence on etching 167 current density. The main quantum confinement effect is represented by the appearance of new energy levels in the 168 silicon band gap .In this work, The PS band gap in the vicinity of PS/c-Si heterojunction for p-type can be 169 determined by new method dependence of the short-circuit photocurrent of structure on photon energy hν 170
REFERENCE 171
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