OPTO−ELECTRONICS REVIEW 17(1), 30–39 DOI: 10.2478/s11772−008−0050−z
Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl2,Ho S. BHUSHAN* and A. OUDHIA School of Studies in Physics, Pt Ravishankar Shukla University, Raipur (C.G.), 492010 India Results of the scanning electron microscopy (SEM), X−ray diffraction (XRD), optical absorption, photoconductivity (PC), and photoluminescence (PL) studies for the CdSSe:CdCl2,Ho films are presented in this paper. The SEM studies of different CdSSe films show a layered growth structure. A crystalline nature of the films is observed in the XRD studies. The regions with stacking fault were also observed in the X−ray diffractograms. The optical absorption spectra of these films show varia− tions corresponding to the band gaps and the grain−sizes obtained under various deposition conditions and also with anneal− ing. The effect of flux, impurities and annealing on the saturated photo to dark current ratio Ipc/Idc is observed in the PC rise and decay studies. The maximum value of Ipc/Idc ~107 is obtained for the impurity doped annealed films. The PL emission spectra of CdSSe films show two emission peaks associated with the annihilation of free excitons and the transitions between shallow donor and deep acceptor states. In CdSSe:CdCl2,Ho films, two PL emission peaks are observed at 495 nm and 545 nm corresponding to the transitions 5S2 ® 5I8 and 5F3 ® 5I8, respectively, in Ho. The effect of pH on PL and grain size is also included in the present studies.
Keywords: photoconduction, deposition from liquid phases, photoluminescence.
1. Introduction The PC and PL studies of CdS type materials are quite impor− tant because of their wide technological applications. These highly photosensitive and photoluminescent materials are used as sensitive photoconductors, IR detectors, solar cells, lamp phosphors, and display devices. Bube and co−workers [1,2] have extensively studied the photoconduction properties of CdS and CdSe single crystals, like sensitivity, IR quenching, temperature dependence, super linearity and slow growth of photocurrent. Similarly, the PL edge emission was intensively investigated in CdS crystals by several workers [3–5] and was associated with the excitonic transitions involving free excitons or defect−exciton complexes [6]. However, in recent years, the effect of alloying of CdS, CdSe, and other II–VI group compounds on the PC and PL properties has become a field of much interest. Gupta et al. have recently employed the PC studies to determine the barrier height at the inter−crystal− line boundary in CdSxSe1–x evaporated layers [7]. Similarly, studies on the application of high−speed CdS0.20Se0.80 photo− conductor as line image sensors [8] and the PEC cells made up of chemically deposited coupled films of CdS and CdSe exhib− iting higher sensitization, superior stability and smaller recom− bination rates [9] have also been reported. The PL spectral studies of CdSSe semiconductors also show interesting results. Shevel et al. have studied the localized electronic states created by the compositional disorder in CdSSe by employing the *e−mail:
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bhushan_312006@rediff mail.com
pico−second luminescence spectroscopy [10]. Further, the dis− order effects in CdSxSe1–x films were correlated with the local− ization of excitons observed in the PL spectra [11]. Encour− aged with these results, CdSSe was selected as the base mate− rial for the present study. The role of CdCl2 in re−crystallization and sensitization of CdS and CdSe is well known [12], so it was taken as a flux. Moreover, the ternary II–VI compounds like CdxZn1–xS and CdxPb1–xS show enhancement in PC and PL due to doping of lanthanides [13–16]. So,Ho was chosen as a dopant in the present study. Moreover, the well−separated en− ergy levels of Ho help in radiative transitions [17] and so the Ho doped inorganic materials are being used in luminescent devices like fluorescent lamps, cathode ray tubes and lasers [18]. This paper consists of the optical absorption, PC growth and decay, and PL emission spectral studies along with the SEM and XRD studies of some chemically deposited films of CdSSe. The changes observed with the variations in the composition, doping of flux and impurity and annealing are also discussed. Moreover, as Li et al. have reported the ef− fect of pH on the PL quantum yield of CdTe nano crystals such studies are also included in this paper [19].
2. Experimental procedures The films were deposited by dipping microscopic glass slides of dimension 24´75 mm in a chemical bath at 60°C for 1 h. Chemical bath was prepared using 1M solutions of cadmium acetate, thiourea, tri−ethanolamine, 0.01M solu−
Opto−Electron. Rev., 17, no. 1, 2009
tions of cadmium chloride and holmium oxide, and 0.42M solution of sodium selenosulfate in appropriate proportions in the presence of 30% aqueous (aq.) ammonia. Being insol− uble in water, solution of holmium oxide was prepared in di− lute sulfuric acid; solutions of all other chemicals were pre− pared in double distilled water. The solution of sodium selenosulfate was freshly prepared by mixing 12.5 gm of so− dium thiosulfate and 4 gm of selenium powder in a conical flask containing 100 ml double distilled water. This mixture was stirred and refluxed at 90°C for 5 h, after which it was filtered, resulting in a clear solution of around 0.42M so− dium selenosulfate. The pH value of the bath was ~9 for 30% concentration and ~11 for 90% concentration of aq. ammonia. In general, the films were prepared with 30% concentration of aq. ammonia but a few films were also pre− pared with 90% concentration of aq. ammonia, which were used to study the effect of increased alkalinity. After deposi− tion, the films were washed with distilled water to wash out the uneven overgrowth of grains at the surface and dried in open atmosphere at room temperature. The thickness of the films was measured by multiple beam interference method and was found to lie in the range from 0.1 µm to 0.9 µm. The photocurrents were measured by exposing the total area of the films. Coplanar electrodes of 1.5−mm width and 24−mm length were formed at a separation of 2 mm by applying col− loidal silver paint to the surface of the film for PC studies. The PL cells consisted of the films deposited on the sub− strates. An incandescent bulb of 100 W was used as the ex− citation source for PC growth and decay studies. The PL ex− citation source was a high−pressure Hg source, from which 365−nm radiation was selected by using the Carl−Zeiss inter− ference filter. The PL emissions were observed with the help of a prism monochromator, a RCA−6217 photomulti− plier tube and a sensitive polyflex galvanometer (10–9 A/mm). The optical absorption spectra were recorded with the help of a Shimadzu PharmaSpec−1700 spectrophoto− meter. XRD and SEM studies were performed at IUC−DAE Indore, using models Rigaku RU:H2R horizontal Rotaflex and JEOL−JSM 5600, respectively.
CdS0.70Se0.30 [Fig. 1(b)]. Figures 2 and 3 show the SEM mi− crographs of CdS0.95Se0.05:CdCl2,Ho and CdS0.70Se0.30: CdCl2,Ho films respectively for which the maximum PC and PL responses were observed, respectively. In these mi− crographs, the thicknesses of layers were found to be ~217 nm and ~139 nm, respectively. However, the thickness of layers decreased significantly to ~83 nm for CdS0.70Se0.30: CdCl2,Ho film prepared with 90% aq. ammonia shown in Fig. 4. It should be noted that the decrease in the thickness
Fig. 1. SEM micrographs of: (a) CdS0.95Se0.05 and (b) CdS0.70Se0.30 films, at 6000X.
3. Results and discussions 3.1. SEM studies The SEM micrographs of CdS0.95Se0.05 and CdS0.70Se0.30 films are shown in Figs. 1(a) and 1(b) respectively, each at a magnification of 6000. These compositions correspond to the maximum PC and PL response, respectively. A layered growth of leafy structures is observed in both the figures. The morphologies observed in these micrographs suggest the presence of strain or lattice mismatches, which result in imperfections like stacking faults or screw dislocations dur− ing the growth process [20]. Further, an increase in the num− ber of layers is observed with increasing percentage of Se. Using the scale shown in the figures, thickness of layers (marked with arrows) was calculated, which was found to be ~250 nm for CdS0.95Se0.05 [Fig. 1(a)] and ~125 nm for Opto−Electron. Rev., 17, no. 1, 2009
Fig. 2. SEM micrograph of CdS0.95Se0.05:CdCl2,Ho film, at 3500X.
S. Bhushan
31
Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl 2,Ho
Fig. 5. X−ray diffractogram of CdS0.95Se0.05 film.
Fig. 3. SEM micrographs of CdS0.70Se0.30:CdCl2,Ho film, at 2700X.
Fig. 6. X−ray diffractogram of CdS0.95Se 0.05:CdCl2,Ho film.
Fig. 4. SEM micrographs of CdS0.70Se0.30:CdCl2,Ho film (90% aq. ammonia) at 9000X.
of layers with increasing concentration of aq. ammonia cor− respond to the decrease in the particle size calculated by the XRD studies in similar films.
•
3.2. XRD studies Figure 5 shows the X−ray diffractogram of CdS0.95Se0.05 film. The corresponding data are presented in Table 1. The maximum intensity peak observed at peak 4 in Fig. 5 was assigned to (200)c CdS. The diffraction peaks were assigned using JCPDS data CdS (cubic); JCPDS 10−454, CdS (hex− agonal); JCPDS 41−1049, CdSe (hexagonal) JCPDS 19− 191,Ho2O3; JCPDS 10−0194, CdCl2; JCPDS 9−401) and by comparing the evaluated values of the lattice constants with the reported once. Figures 6 and 7 show the X−ray diffracto− grams of CdS 0.95 Se 0.05 :CdCl 2 ,Ho and CdS 0.70 Se 0.30 : CdCl2,Ho films respectively. The corresponding data are compiled in Tables 2 and 3, respectively. The XRD pattern of CdS0.70Se0.30:CdCl2,Ho films prepared with 90% concen− tration of aq. ammonia is shown in Fig. 8 and the corre− sponding data are compiled in Table 4. It is evident from Figs. 5, 6, 7, and 8 that: all the diffractograms show an extraordinary broadening in the lines along with a small positive as well as nega− tive shift in the values of the diffraction angle as com− pared to the reported ones in the region 2q = 29°–31°. l
32
•
This may be attributed either to the existence of crystal− lites of characteristic shape, of some longer and some shorter dimensions, or involvement of certain kinds of lattice imperfections like stacking fault [21]. Since the X−ray diffractograms show a close stacking of cubic and hexagonal planes of CdS and CdSe/Ho2O3 in this region, such effects may be related to the presence of stacking fault. Grigorieva et al. have also reported presence of re− gions with stacking fault in CdSSe films [22], with increasing concentration of aq. ammonia, the peak intensities decrease considerably apart from this a broadening of lines and emergence of many new peaks either of the impurity like (222)c, (400)c and (411)c of Ho2O3, (101)h of CdCl2 or of CdS/CdSe like (100)h CdSe, (100)h CdS, and (002)h CdS is also ob− served. Moreover, the thickness of the films also de− creases from 0.58 to 0.35 µm with increasing concen− tration of aq. ammonia so, loss of material during re− −crystallization may be a possible reason for the de− crease in the peak intensities. However, decrease in the peak intensities in this case is also accompanied with a line broadening [FWHM = 0.72 in Fig. 6, 30% aq. am− monia to FWHM = 1.30° in Fig. 7, 90% (aq. ammonia), both calculated for (200)c CdS peak]. These effects can be associated to the decrease in the particle size as well [23], the maximum intensity peak is (200)c plane of CdS in all these films but the intensity and the FWHM of this peak vary with the incorporation of the impurities and the variations in the growth conditions. The average grain sizes for all the films were calculated for this peak by using Scherrer’s formula [24]. Before calculating
Opto−Electron. Rev., 17, no. 1, 2009
© 2009 SEP, Warsaw
Fig. 8. X−ray diffractogram of CdS0.70Se0.30: CdCl2,Ho film (90% aq. ammonia). Fig. 7. X−ray diffractogram of CdS0.70Se0.3: CdCl2,Ho film.
[26], a method used for grain size determination of ma− terials involving stacking faults. The (200)c CdS and (102)h CdSe peaks were used to calculate the com− pound fault probability in various films. The values calculated by both the methods, for different CdSSe films, are listed in Tables 1–4.
grain sizes, the values of lattice constants were cor− rected for instrumental and strain related broadening using Cohen’s least square extrapolation method [25]. Further, the grain sizes determined by Scherer’s method (S) were corrected by Warren’s method (W)
Table 1. XRD data of CdS0.95Se0.05 film. S.No. Peak
d value (Å)
Relative intensity (a.u.)
(h k l)
Lattice constant (Å)
observed
reported
observed
reported
observed
reported
1
3.35
3.36
30.27
100
(111)c CdS
a = 4.02
a = 4.13
2
3.05
3.1608
56.65
100
(101)h CdS
a = 3.99
a = 4.30
3
3.01
3.28
78.83
65.70
(101)h CdSe
c = 6.923
c = 7.03
4
2.84
2.90
100
40
(200)c CdS
a = 5.70
a = 5.82
5
2.69
2.55
37
40
(102)h CdSe
c = 7.33
c = 7.03
6
2.39
2.45
22.96
25
(102)h CdS
a = 4.06
a = 4.13
7
2.15
2.15
20.98
82
(110)h CdSe
a = 4.30
a = 4.30
8
2.04
2.05
17.68
80
(220)c CdS
a = 5.78
a = 5.82
9
1.76
1.753
46.64
60
(311)c CdS
a = 5.85
a = 5.82
10
1.67
1.68
23.08
10
(222)c CdS
a = 5.79
a = 5.82
Grain size for (200)c CdS peak
13.04 nm (S)
8.62 nm (W)
Table 2. XRD data of CdS0.95Se0.05:CdCl2,Ho film. S.No. Peak
d value (Å)
Relative intensity (a.u.)
(h k l)
Lattice constant (Å)
observed
reported
observed
reported
1
3.50
3.51
26.79
65.85
2
3.35
3.36
30.87
100
(111)c CdS
a = 5.80
a = 5.82
3
3.30
3.35
32.07
59
(002)h CdS or (101)h CdCl2
c = 6.72
c = 6.75
4
3.04
3.06
47.84
100
(222)c Ho2O3
a = 10.53
a = 10.6
5
3.00
3.16
50.23
100
(101)h CdS
a = 3.87
a = 4.13
6
2.85
2.90
61.33
40
(200)c CdS
a = 5.71
a = 5.82
7
2.59
2.55
20.50
40.59
(102)h CdSe
a = 4.28
a = 4.3
8
2.06
2.05
20.20
80
(220)c CdS
a = 5.83
a = 5.82
9
1.76
1.75
31.89
60
(311)c CdS
a = 5.83
c = 5.82
10
1.67
1.68
18.40
10
(222)c CdS
a = 5.81
a = 5.82
Opto−Electron. Rev., 17, no. 1, 2009
(002)h CdSe
S. Bhushan
observed
reported
c = 6.98
c = 7.02
Grain size for (200)c CdS peak
9.35 nm (S)
8.29 nm (W)
33
Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl 2,Ho Table 3. XRD data of CdS0.70Se0.30:CdCl2,Ho film. S.No. Peak 1
d value (Å)
Relative intensity (a.u.)
(h k l)
observed
reported
observed
reported
3.86
3.72
21.76
23.90
(100)h CdSe
Lattice constant (Å) observed
reported
Grain size for (200)c CdS peak
a = 4.45
a = 4.3
11.97 nm (S)
2
3.04
3.28
43.64
65
(101)h CdSe
a = 3.9
a = 4.3
3
2.98
3.16
71.46
100
(101)h CdS
c = 6.54
c = 6.75
4
2.81
2.90
91.84
40
(200)c CdS
a = 5.63
a = 5.82
5
2.65
2.55
27.69
30
a = 3.84
a = 3.84
6
2.34
2.45
18.58
25
(102)h CdSe (400)c H2O3 (102)h CdS
a = 3.86
a = 4.13
7
2.11
2.15
17.38
82.49
(110)h CdSe
a = 4.29
a = 4.30
8
1.76
1.75
43.16
60
(311)c CdS
a = 5.85
a = 5.82
9
1.65
1.68
17.86
30
(222)c CdS
a = 5.82
a = 5.82
5.90 nm (W)
Table 4. XRD data of CdS0.70Se0.30:CdCl2,Ho film with 90% ammonia. S.No. Peak
d value (Å) observed
reported
Relative intensity (a.u.) observed
(h k l)
reported
Lattice constant (Å) observed
reported
1
3.88
3.72
20.56
100
(100)h CdSe
a = 4.45
a = 4.3
2
3.48
3.58
24.46
100
(100)h CdS
a = 4.05
a = 4.13
3
3.27
3.35
37.35
59
c = 6.67
c = 6.75
4
3.21
3.28
40.76
65.85
(002)h CdS or (101)h CdCl2 (101)h CdSe
c = 6.56
c = 7.02
5
3.17
3.16
33.15
100
(101)h CdS
a = 4.15
a = 4.13
6
3.03
3.06
37.94
100
(222)c Ho2O3
a = 10.52
a = 10.6
7
2.99
2.45
40.04
25
(102)h CdS
a= 3.88
a = 4.13
8
2.81
2.90
48.86
40
(200)c CdS
a = 5.62
a = 5.82
9
2.75
2.55
27.15
35
(102)h CdSe
c = 7.13
a = 7.03
10
2.69
2.65
24.76
90
(400)c Ho2O3
a = 10.61
a = 10.6
11
2.57
2.49
19.60
47.54
(411)c Ho2O3
a = 10.56
a = 10.6
12
2.16
2.15
18.28
82
(110)h CdSe
a = 4.32
a = 4.30
13
2.05
2.05
18.88
80
(220)c CdS
a = 5.82
a = 5.82
14
1.76
1.75
26.79
45
(311)c CdS
a = 5.82
a = 5.82
15
1.66
1.68
16.30
10
(222)c CdS
a = 5.75
a = 5.82
3.3. Optical absorption spectral studies Figure 9 shows the optical absorption spectra of different CdSSe films. These films prepared with different ratios of S and Se (not shown here) show a monotonic variation in the band gap over the whole composition range (CdS, 2.42 eV, CdS0.40Se0.60, 2.13 eV), indicating the formation of a com− mon lattice of CdSSe through solid solutions. Curves 1 and 2 represent the optical absorption spectra of CdS0.95Se0.05 and CdS0.70Se0.30 films, respectively. A steep increase in the absorption corresponding to the onset of the band−to−band transition is observed in curve 1 as compared to curve 2). The absorption edges are steep for the un−doped films but are flat and extended in case of the impurity−doped films showing emergence of more energy levels in the band gap region due to impurities. It is also observed that the op− tical absorption decreases in impurity−doped films
34
Grain size for (200)c CdS peak
5.10 nm (S) 4.27 nm (W)
CdS0.95Se0.05:CdCl2,Ho and CdS0.70Se0.30 CdCl2,Ho (curves 3 and 4, respectively) as compared to the corresponding un−doped films (curves 1 and 2). Further, a blue shift in the absorption edges [calculated from the Tauc’s plot, shown in Fig. 9(b)] and the energies corresponding to the onset of di− rect band−to−band transition (obtained by extending the re− gion of the absorption curve showing a steep rise, to the x− axis) in CdS0.95Se0.05:CdCl2,Ho and CdS0.70Se0.30:CdCl2,Ho films prepared with 90% concentration of aq. ammonia (shown in curves 5 and 6, respectively) as compared to simi− lar films prepared with 30% aq. ammonia (curves 3 and 4), suggests a decrease in the particle size. It is known that in the nano crystalline materials the energy spectrum is quan− tized with energy spacing relative to the particle size. Apart from this, the highest occupied valence band and the lowest occupied conduction band are shifted to the more positive and more negative values respectively, resulting in a blue
Opto−Electron. Rev., 17, no. 1, 2009
© 2009 SEP, Warsaw
Fig. 9. Optical absorption spectra of different CdSSe films: 1. CdS0.95Se0.05, 2. CdS0.70Se0.30, 3. CdS0.95Se0.05:CdCl2,Ho, 4. CdS0.70Se0.30: CdCl2,Ho, 5. CdS0.95Se0.05:CdCl2,Ho (90% aq. ammonia). 6. CdS0.70Se0.30:CdCl2,Ho (90% aq. ammonia) 7. Annealed CdS0.95Se0.05, 8. Annealed CdS0.95Se0.05:CdCl2,Ho (a). Tauc’c plot between (ahn)2 vs. hn for CdS0.70Se0.30:CdCl2,Ho films: 1. Prepared with 30% aq. ammonia, 2. Prepared with 90% aq. ammonia (b).
shift in the absorption edge/onset of direct band−to−band ab− sorption. The decrease in the integrated area under the ab− sorption curves for the films prepared with higher con− centration of aq. ammonia can be related to the decrease in the thickness of the film. The optical absorption spectra of the annealed CdS0.95Se0.05 and CdS0.95Se0.05:CdCl2,Ho films observed in curves 7 and 8, respectively, show a considerable enhance− ment in the optical densities of the films after annealing. The onset of the direct band−to−band transition shows a red shift related to an increase in the particle size due to anneal− ing, as compared to the corresponding un−annealed films (curves 1 and 3). In addition, a hump is also observed be− tween 425–525 nm in the absorption curves of the impu− rity−doped films (curves 3, 4, 5, 6), which corresponds to the absorption related to the transition 5I7 ® 5F3 in Ho [27].
and electrons becoming located at centres with large recom− bination centres. A saturated photocurrent occurs due to bal− ancing of the generation and recombination effects. Simi− larly decay curves consist of fast decrease followed by a very low decrease. Fast decrease is related to direct recom− bination effects, where as the slow variation is related to a slow release of the trapped electrons from deep traps formed in CdS/CdSe system. The long rise and decay of photo− current in the ternary alloys of CdSSe is attributed to the sta− tistical fluctuations of local composition leading to the cre−
3.4. PC studies Figure 10 shows the PC growth and decay curves of various CdSSe films. All these curves show a rapid rise of photo− current in the early part, which is followed by a slower rise finally leading to a saturation photocurrent. The PC in CdS is sensitized with incorporation of CdSe in CdSSe films. According to Bube [2] the initial rapid rise of photocurrent in such semiconductors occurs as the hole demarcation level moves down near to the sensitizing levels in CdSSe due to generation of carriers under irradiation. The slow rise corre− sponds to the time required for readjustment of empty and filled bands in the forbidden gap, hole becoming located at sensitization centres with small recombination cross−section Opto−Electron. Rev., 17, no. 1, 2009
Fig. 10. PC rise and decay curves for various CdS0.95Se0.05 films: 1. CdS0.95Se0.05, 2. CdS0.95Se0.05:CdCl2, 3. CdS0.95Se0.05:CdCl2,Ho. 4. Annealed CdS0.95Se0.05, 5. Annealed CdS0.95Se0.05:CdCl2, 6. Annealed CdS0.95Se0.05:CdCl2,Ho.
S. Bhushan
35
Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl 2,Ho ation of potential barriers and wells, which cause the local− ization of excited carriers [10]. The high photosensitivity in II–VI compounds is associated with the presence of com− pensated acceptors. In the CdS/CdSe films deposited by CBD method an excess of Cd (equivalent to a sulfur/sele− nium vacancy) is naturally incorporated as interstitial at− oms, which act as donors. These donors compensate the cat− ion vacancies (acceptors) either already present in the mate− rial or created by the nonstoichiometry. The halide ions in CdCl2 incorporated as flux also act as donors, which further sensitize the films through the same mechanism. The triva− lent rare−earth ion donates an extra electron introduced for each RE3+ incorporated, which becomes free at room tem− perature and above. This explains the high photosensitivity in the presence of Ho. The effect of annealing was also ob− served in doped and un−doped CdS0.95Se0.05 films annealed at 350°C for 3 minutes. Annealing produces a red shift in the energies corresponding to the onset of the direct absorp− tion in the optical absorption spectra, showing formation of bigger grains due to re− crystallization. In fact both anneal− ing and incorporation of CdCl2 are known to promote re− −crystallization of CdS/CdSe grains, which reduces the number of grain boundaries, i.e., the number of high resis− tance paths, resulting in higher photo−response. The high photosensitization of CdSSe systems on annealing is also associated with the formation of excess Cd [28]. The values of Ipc, Idc and the ratio Ipc/Idc, lifetime, mobility and trap depths for different films are compiled in Table 5. Thus, a maximum value of I pc /I dc = 4.5´10 5 appears for CdS0.95Se0.05 films, which improves to 9.5´105 with addi− tion of CdCl2 and to 2.1´106 due to addition of Ho. The an− nealed films show further improvement in this ratio. The method of evaluating the lifetime, mobility, and trap− depths is described in our earlier publication [13]. The values of thus obtained parameters for different CdSSe films are compiled in Table 5. It is noticed that the high photo− sensitization in the presence of impurities and due to
annealing corresponds to the higher values of lifetime and mobility.
3.5. PL spectral studies Figure 11 shows the PL emission spectra of various CdSSe films obtained at 300 K for the compositions CdS (curve 1) to CdS0.60Se0.40 (curves 2–6). The PL emission spectrum of CdS consists of a single peak where as the CdSSe films pre− pared with different ratios of S and Se show two broad PL emission peaks. The peak positions are compiled in Table 6. The maximum PL intensity was observed in CdS0.70Se0.30 film so these films were doped with impurities for further studies.
Fig. 11. PL emission spectra of CdSSe films prepared with different ratios of S and Se: 1. CdS, 2. CdS0.95Se0.05, 3. CdS0.90Se0.10, 4. CdS0.80Se0.20, 5. CdS0.70Se0.30, 6. CdS0.60Se0.40.
Table 5. Values of Idc, Ipc, Ipc/Idc, lifetime t, mobility µ, and trap depth E for different un−annealed and annealed CdSSe films. No.
Sample
Idc (µA)
Ipc (µA)
Gain (Ipc/Idc)
Life time t (s)
Mobility µ c m2/V (s)
Trap depth (eV)
E1 = 0.8703 E2 = 0.8809 E1 = 0.8694 E2 = 0.8572 E1 = 0.8423 E2 = 0.8279
Un−annealed 1
CdS0.95Se0.05
0.2
89.8
4.5×105
38.62
31
2
CdS0.95Se0.05;CdCl2
0.2
190.56
9.5×105
57.22
44.2
3
CdS0.95Se0.05; CdCl2,Ho
0.1
215.25
2.1×106
60.06
88
Annealed 4
CdS0.95Se0.05
0.05
280.50
5.6×106
80
186
5
CdS0.95Se0.05; CdCl2
0.05
483.30
9.6×106
86.60
295
6
CdS0.95Se0.05; CdCl2,Ho
0.05
615.50
1.21×107
89.13
362
36
Opto−Electron. Rev., 17, no. 1, 2009
E1 = 0.8373 E2 = 0.8243 E1 = 0.8116 E2 = 0.8041 E1 = 0.8052 E2 = 0.7608 © 2009 SEP, Warsaw
In CdS, the PL emission was observed at ~515 nm corre− sponding to its band gap 2.42 eV calculated by the absorp− tion studies. At low temperatures the PL edge emission in CdS was attributed to the transitions associated with the de− fect−exciton complexes by Thomas and Hopfield [6]. How− ever, at room temperature only free excitons have sufficient binding energy (~0.029 meV) to sustain the thermal dissoci− ation (kT ~0.025 meV). Jeong and Yu [29] have recently re− ported PL in CdS single crystals grown by sublimation method. They observed a broad and intense excitonic emis− sion peak corresponding to 2.44 eV at 298 K, which was re− lated to Ex (A) free exciton and their LO− phonon replica. Similarly, in the present case also the emission of CdS can be attributed to the excitonic transitions involving free excitons. Table 6. Wavelengths and intensities of PL emission peaks of vari− ous CdSSe films. No.
Sample
Wavelength (nm) 515
Intensity (a.u.) 62
1
CdS
2
CdS0.95Se0.05
503, 525
50, 42
3
CdS0.90Se0.10
503, 528
70, 65
4
CdS0.80Se0.20
503, 540
78, 75
5
CdS0.70Se0.30
503, 560
110, 100
6
Cd0.60Se0.40
503, 579
85, 80
7
CdS0.70Se0.30:CdCl2
503, 560
125, 120
8
CdS0.70Se0.30:CdCl2,Ho
495, 545
130, 125
9
CdS0.70Se0.30:CdCl2,Ho (90% ammonia)
490, 540
160, 135
The CdS0.95Se0.05 film (curve 2) shows two broad emis− sion peaks at ~503 nm and ~526 nm. Owing to the similar excitonic nature of the edge emissions in both CdS and CdSe the emission observed at ~503 nm can be associated with the annihilation of free excitons. The broadening of this excitonic peak can be attributed to the localized exci− tonic states with a broad distribution of energies [10] in al− loys like CdSSe along with other effects like exciton−lattice coupling and scattering. The second peak observed at ~526 nm shifts towards higher wavelength with increasing mol% of Se (curves 3–6). This emission can be associated with the donor− acceptor transitions. In CdS type materials, the in− corporation of cations (excess Cd in present case) introduce shallow donor levels (like S vacancies with ionization ener− gies ~0.03 eV) where as that of anions (S or Se in present case) introduce deep acceptor levels (Cd vacancies with ion− ization energy typically ~1.1 eV for sulfides and ~0.6 eV for selenides) for the charge compensation of the system. In the present method of preparation the excess Cd was produced naturally, which was confirmed by EDX studies reported elsewhere [30], where as the source of sulfur and selenium was Na2SSeO3. Sulfur was also produced by both, thiourea and sodium thiosulphate. The thiosulphate anion in Na2S2O3 characteristically reacts with dilute acid to produce Opto−Electron. Rev., 17, no. 1, 2009
colloidal sulphur [31] according to the following reaction S2O32– (aqueous) + 2H+(aqueous) = S(s) + SO2(g) + H2O(l). In this reaction, sulfur is produced in the solid phase. The H+ ions required for this reaction may be available from the acidic compounds used in the present method of prepa− ration of films. Moreover, higher volumes of Na2SSeO3 were added for increasing the mol% of CdSe, thus the for− mation of more deep acceptors associated with S and Se could be expected. The variations of peak positions with the different concentration of acceptor states appear as a natural consequence of inter−impurity transitions between donor and acceptor levels. Figure 12 shows the PL emission spectra of different CdS0.70Se0.30 films. The emission intensity was increased in CdS0.70Se0.30:CdCl2 film (curve 2) but the peak positions re− mained similar as those observed in CdS0.70Se0.30 (curve 1). However, in CdS0.70Se0.30:CdCl2,Ho film (curve 3) new peaks of higher intensities were observed at 495 nm and 545 nm. The emission peak at 545 nm (green luminescence) cor− responds to the transition 5S2 ® 5I8 in Ho [32]. Similarly, the emission at 495 nm corresponds to the transition 5F3 ® 5I in Ho [27]. Curve 4 shows the PL emission spectrum of 8 CdS0.70Se0.30:CdCl2,Ho film prepared with 90% aq. ammo− nia. Although the films deposited with increasing concen− tration of aq. ammonia were thinner, these films show the maximum PL intensity, which can be related to the nano crystalline effects. The decrease in the intensity of XRD peaks as well as the particle size in this film was also related to the nano crystalline effects (Sect. 3.2). Patel and co−work− ers [33] have reported that in powder phosphors of CdS:Te, the PL increases upon exposure to ammonia, which they at−
Fig. 12. PL emission spectra of various CdS0.70S0e.30 films: 1. CdS0.70Se0.30, 2. CdS0.70Se0.30:CdCl2 3. CdS0.70Se0.30:CdCl2,Ho, 4. CdS0.70S0.30:CdCl2,Ho (90% aq. ammonia).
S. Bhushan
37
Photoconductivity and photoluminescence in chemically deposited films of CdSSe:CdCl 2,Ho tributed to the inter relationship between magnitude of the electric field in the near surface region and the PL intensity. Thus, it can be inferred that in the present system also, the increasing concentration of ammonia results in some sur− face effects; predominant in the nano crystalline material that subsequently produces changes in the PL intensities.
4. Conclusions The chemically deposited films of various CdSSe films showed layered growth morphology in the SEM studies. The nature of the films was observed to be highly crystalline in the XRD studies. The line broadening of particular peaks in the X−ray diffractograms of different films was associated with the stacking faults and hence, the average grain sizes calculated by Scherrer’s formula were modified using War− ren’s method. The peak intensities were reduced and broad− ened markedly along with emergence of some new peaks with the increasing concentration of ammonia in the chemi− cal bath. These observations can be explained by the nano− crystalline effects. The on−set of the direct transitions in the absorption spectra showed a blue shift in the films prepared with a higher concentration of ammonia and a red shift in the annealed films. The absorption spectra of the Ho doped films showed presence of absorption corresponding to the energy levels of Ho. The enhancement in the ratio Ipc/Idc was related to the increase in the lifetime and mobility of the charge carriers with the doping of flux and impurity in the un− annealed film. However, along with all these factors the creation of excess Cd and the re−crystallization resulting in bigger particle size were found to be responsible for the high value of the ratio Ipc/Idc ~107 in the impurity doped annealed films. PL emission spectra of CdSSe films consisted of two peaks associated with the radiative decay of the free excitons and donor−acceptor transitions where as in pres− ence of Ho, the two peaks observed at 495 nm and 545 nm were corresponding to the transitions 5S2 ® 5I8 and 5F3 ® 5I respectively, in Ho. 8
Acknowledgements Authors are grateful to IUC−DAE, Indore, for providing fa− cilities for the XRD and SEM studies. They are also thank− ful to UGC, New Delhi, for granting Teacher−Fellowship to the co−author A. Oudhia.
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