- Email: [email protected]
Si Schottky diodes
Solid-State Electronics 48 (2004) 2219–2223 www.elsevier.com/locate/sse
Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes S. Venkatachalam a, R.T. Rajendra Kumar a, D. Mangalaraj Sa.K. Narayandass a, Kyunghae Kim b, Junsin Yi b a b
a,b,*
,
Department of Physics, Thin Film Laboratory, Bharathiar University, Coimbatore 641 046, India School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea Received 1 July 2003; received in revised form 31 March 2004; accepted 26 May 2004 Available online 6 August 2004
The review of this paper was arranged by Prof. Y. Arakawa
Abstract Zn0.52Se0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn0.52Se0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (d), strain (e), and the lattice parameter are calculated as 29.13 nm, 1.187 · 1015 lin/m2, 1.354 · 103 lin2 m4 and 5.676 · 1010 m respectively. From the I–V measurements on the Zn0.52Se0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 · 104 (305 K) respectively. The built-in potential, effective carrier concentration (NA) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 · 1015 cm3 and 1.359 eV respectively. 2004 Elsevier Ltd. All rights reserved. PACS: 71.20.Nr; 73.40.Lq; 73.61.Ga Keywords: Zn0.52Se0.48 thin films; Rutherford backscattering spectrometry; Zn0.52Se0.48/p-Si heterojunctions; Vacuum evaporation; Ideality factor; Barrier height
1. Introduction Wide-gap II–VI compounds are of special interest for solar energy conversion due to their high photosensitivity, direct optical transitions and high values of absorption coefficients (104 cm1) in the visible region of the spectrum. ZnxSe1x [x = 0.5] has been considered as a promising material for optoelectronic applications such
* Corresponding author. Tel.: +91 422 2425 458; fax: +91 422 2422 387. E-mail address: [email protected] (D. Mangalaraj).
as blue/green laser diodes [1]. ZnxSe1x is mainly used as buffer layer for the fabrication of high efficiency solar cells based on CuIn(S,Se)2 and CdTe absorbers [1–3]. The Zn-treatment allows for favorable modification of the absorber surface [4]. Also an intermixing at the interface reported by Heske et al. [5] has a positive effect on the solar cell performance. The evaluation of Schottky diode (SD) parameters provides a useful guidance for further characterizing these materials. I–V and C–V measurements are routinely used to extract various SD parameters. In this paper we report the composition and optical properties of Zn0.52Se0.48 thin films deposited at a substrate temperature of 250 C onto glass
0038-1101/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2004.05.082
2220
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
and Si substrates. RBS was used to determine the composition of the Zn0.52Se0.48 films and optical studies were used to evaluate the band gap of the deposited material. Structural and Raman studies were also carried out on Zn0.52Se0.48 thin films deposited onto Si substrates. The ideality factor, rectification factor were calculated from the dark I–V characteristics. The built-in potential, effective carrier concentration and barrier height were evaluated from the C–V measurements.
2. Experimental Thin film depositions were carried out by using a HINDHIVAC-12A4D coating unit and the deposition parameters are presented in Table 1. Prior to the deposition of the Zn0.52Se0.48 films, the Si wafers were cleaned by a standard process. Pure aluminium (99.999% Balzers, Switzerland) layer was deposited onto the back surface of silicon wafers by vacuum evaporation technique. Zn0.52Se0.48 alloy was thermally evaporated from a molybdenum boat onto well cleaned glass substrates and also onto the Si wafers under a pressure of 3 · 105 Torr, and Au was used as top electrode. The completed devices had an active area of about 2 · 106 m2. The RBS analysis was carried out to measure the stoichiometry of the film using a 2 MeV He + 1 beam with a detector angle of 168. Thicknesses of the deposited films was measured by multiple beam interference (Fizeau fringes) technique and also by RBS and it was found ˚. that the films have an average thickness of 1488 A 2.1. Structural analysis The structural analyses of the films were made by a X-ray diffractometer (SCINTAG; USA, Cu Ka radiation at k = 0.15418 nm) in the 2h ranges 20–80. The crystalline size (D) is calculated using the ScherrerÕs formula, D¼
0:94k b cos h
ð1Þ
where k––is the wavelength of the X-ray used, b––is the Full width half maximum, D––is the particle size value and h––is the angle between the incident and the scattered X-ray.
Table 1 Deposition parameters for ZnSe film preparation Source
Tungsten and molybdenum
Substrate temperature Substrates Substrate–source distance Vacuum
250 C Glass and silicon 17 cm 3 · 105 Torr
The dislocation density (d) is the length of dislocation lines per unit volume of the crystal and has been calculated by using the formula d¼
1 D2
ð2Þ
The strain values (e) were evaluated by using the following relation k 1 e¼ b ð3Þ D cos h tan h where D––is the particle size, b––is the Full width half maximum. The lattice spacing (d) was calculated from the BraggÕs formula d¼
k 2 sin h
ð4Þ
where k––is the wavelength of the X-ray used and d––is the lattice spacing. The lattice parameter has been calculated by the following expression 1 ðh2 þ k 2 þ l2 Þ ¼ 2 a2 d
ð5Þ
where h, k, l represent the lattice planes. 2.2. Optical analysis Optical transmission of the Zn0.52Se0.48 films deposited onto micro glass glide was recorded using a UV–VIS–NIR Spectrophotometer as a function of wavelength in the range from 300 to 1100 nm. 2.3. Electrical analysis Electrical measurements were done in a rotary vacuum of 1.33 Pa at different temperatures (305–365 K) by using a cryostat equipped with a PT100 thermocouple. The capacitance values were measured using multifrequency LCR meter (4275HP).
3. Results and discussion A typical RBS spectrum of the film deposited onto glass substrates maintained at 250 C is shown in Fig. 1. The detailed analysis of the RBS spectrum shows that the films are nearly stoichiometry in nature with Zn:Se ratio of 0.52:0.48. The X-ray diffraction pattern of Zn0.52Se0.48 film deposited onto an Si substrate is shown in Fig. 2 and the structure of the Zn0.52Se0.48 film is identified as crystalline. The XRD patterns exhibit reflections corresponding to the cubic phase. The reflection in the (1 1 1) direction is observed at 2h = 27.19. The calculated values of lattice constant, particle size, strain,
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
6000
2000
1.8 micro-C of 2 MeV He(+1) 168 degrees RBS Thickness (x10e15 /cm2):1488 A0 Se/Zn-ratio (%): 52/48
Si
1800
Intensity (a.u)
Yield
4500 3000 1500 0
0
2221
1600 1400
LO (Zn0.52Se0.48)
1200
100
200 300 400 Channel Number
500
1000 200
600
300
400
500
600
700
800
Raman shift (cm-1)
Fig. 1. Rutherford backscattering spectrum of vacuum evaporated Zn0.52Se0.48 thin film.
Fig. 3. Raman spectra of Zn0.52Se0.48 thin film deposited onto an Si substrate at a substrate temperature of 250 C.
5000 Substrate: Si (100)
100
Transmittance (%)
Intensity (a.u)
4000
3000 Si
2000
1000 20
(111)
30
40 50 60 2θ (degrees)
70
80
Fig. 2. X-ray diffractogram of Zn0.52Se0.48 films deposited onto an Si substrate at a substrate temperature of 250 C.
dislocation density and lattice spacing are presented in Table 2. Fig. 3 shows the Raman phonon spectrum of the Zn0.52Se0.48 films grown onto an Si substrate. The figure clearly shows the presence of longitudinal optical (LO) phonon mode (249.29 cm1). This longitudinal phonon mode confirms the crystalline nature of the deposited material. Similar results are available elsewhere [6–8]. Optical transmission spectrum of the films deposited at a substrate temperature of 250 C is shown in Fig. 4. The transparency of the as-deposited film is above 97% in the wavelength range of 730 nm. The T–k spectrum of the films with a sharp fall of transmittance at the band
27.19
[1 1 1]
5.676
3.277
60 40 20 0
400
600 800 Wavelength (nm)
1000
˚ Fig. 4. Transmittance spectra of Zn0.52Se0.48 thin film of 1488 A thickness deposited onto glass substrate.
edge once again confirms the crystalline nature of the films. Fig. 5 shows a plot between (ahm)2 and hm for a ˚ thick Zn0.52Se0.48 film. The band gap was ob1488 A tained by extrapolation of the plot of (ahm)2 versus hm and the estimated band gap value is 2.68 eV. The deposited films are found to have direct allowed transition. Zn0.52Se0.48/p-Si Schottky diodes were fabricated and Fig. 6 shows a cross sectional view of Zn0.52Se0.48/p-Si heterosturcture. Figs. 7 and 8 show the I–V plots measured at different temperatures in forward and reverse bias for a typical Zn0.52Se0.48/p-Si diode. In the forward bias the current increases exponentially with voltage. In
Table 2 Structural parameters of the Zn0.52Se0.48 films deposited onto Si substrates ˚) Thickness (A 2h [h k l] Lattice Lattice Particle ˚) ˚) constant (A spacing (A size (nm) 1488
80
29.03
Strain · 103 lin2 m4
Dislocation density · 1015 lin/m2
1.354
1.187
2222
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223 2.5x1015
5.0x10-6
15
2.0x10
305 K 345 K 365 K
1.5x1015
I(amp)
(αhν) 2 (eV/m )
2
4.0x10-6
1.0x1015
2.0x10-6 1.0x10-6
5.0x1014 0.0 1.2
3.0x10-6
0.0 1.6
2.0 2.4 hν (eV )
2.8
3.2
Fig. 5. The plots of (ahm)2 versus hm of Zn0.52Se0.48 thin film of ˚ deposited onto glass substrate. thickness 1488 A
0
2
4 6 V (volt )
8
10
Fig. 8. Reverse current versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
characteristic follows the standard diode equation [9] for forward bias below 0.4 V as given by qV I ¼ I 0 exp 1 ð6Þ nkT
Fig. 6. The cross sectional view of Zn0.52Se0.48/Si Schottky diodes.
6.0x10 -4 5.0x10 -4
3.0x10 -4 2.0x10 -4 305 K 345 K 365 K
1.0x10 -4 0.0
0
2
4
6 V (volt)
8
-8 -12
10
Fig. 7. Forward current versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
the reverse bias the current increases slowly with voltage and did not show any trend of saturation. In the forward bias, two distinct regions are observed. At low voltages, the current is linear with the applied voltage (ohmic behaviour) and at higher voltages I / V2, indicating the domination of space–charge limited current. This I–V
lnI(amp)
I (amp)
4.0x10 -4
where n is called the ideality factor, q is the electronic charge, k is the Boltzmann constant and T is the temperature. The ideality factor was determined at various temperatures using the plot between ln I and V (Fig. 9) and was found to be in the range from 1.749 to 1.769. When the ideal diffusion current is the dominating factor, then the value of the ideality factor (n) will be equal to 1, whereas this value will be equal to 2 when the recombination current is the dominating factor [10]. In the present case, the value of n varies between 1 and 2 and hence it can be viewed that both the ideal diffusion and recombination currents are comparable in this device. We have also evaluated the diode rectification factor from the ratio of forward to reverse bias current. The diode rectification factor had a maximum value of
-16 -20
305 K 345 K 365 K
-24 0
2
4
6 V (volt)
8
10
Fig. 9. Variation of ln I versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
ated as 5.907 · 1015 cm3 and 1.359 eV. The calculated values of Vbi, Vn, Nv and /Bn are given in Table 3.
1.8x1020
1/C 2 (F -2m4)
1.5x1020
1 MHz
1.2x1020
4. Conclusion
19
9.0x10
19
6.0x10
3.0x1019 0.0
-2
-1
0
1
2
3
4
5
V (volt)
Fig. 10. Dependence of 1/C2 value on applied voltage of Zn0.52Se0.48/Si Schottky diodes.
Table 3 Electrical parameters of the Zn0.5Se0.46/p-Si diodes Vbi (V)
d(1/C2)/dV · 1019 (m2/F)2/V
NA · 1015 (cm3)
Vn (V)
/Bn (eV)
1.02
2.6277
5.907
0.2001
1.359
kT q
˚ The Zn0.52Se0.48 thin films with a thickness of 1488 A each were deposited onto an Si substrate maintained at a substrate temperature of 250 C by using vacuum evaporation technique. The deposited film is polycrystalline having cubic structure and is oriented along (1 1 1) direction. In the Raman measurement, the presence of LO mode confirms the crystalline nature of the deposited material. From the I–V measurements we concluded that the ideality factor is the key role to identify the dominant transport mechanism in the prepared Zn0.52Se0.48/p-Si heterostructure. The drift and diffusion current are the dominant transport mechanism in the prepared diodes. It has been concluded from the C–V measurement that the examined heterostructures are abrupt heterojunctions.
References
1.04 · 104 at room temperature. The rectification factor decreases with the increase of temperature. The plot between the measured values of capacitance versus voltage for Zn0.52Se0.48/n-Si diodes is shown in Fig. 10. We obtained a straight line by plotting a curve between 1/C2 versus V, which implies a similar behaviour of an abrupt heterojunction [11]. The intercept of this plot at 1/C2 = 0 corresponds to the built-in potential Vbi, and is found to be 1.02 V. The value of barrier height [12] can be calculated from the measured value of Vbi as, /Bn ¼ V bi þ V n þ
2223
ð7Þ
where Vn = kT/q. Ln(Nv/NA), k is the Boltmann constant, T is the temperature, q is the charge of the electron, Nv is the density of states in the valance band and NA is the effective carrier concentration. From the slope of the 1/C2 versus voltage plot, the values of effective carrier concentration and barrier height are evalu-
[1] Konagai M, Ohtake Y, Okamoto T. Mater Res Soc Symp Proc 1996;426:1532. [2] Ohtake Y, Okamoto T, Yamada A, Konagai M, Saito K. Sol Energy Mater Sol Cells 1997;49:2699. [3] Turner AK, Woodcock JM, Ozsann ME, et al. Sol Energy Mater Sol Cells 1994;35:263. [4] Ramanathan KR, Wiesner H, Asher S, Wiles D, Bhattacharya N, Keane J, et al. In: Proceeding of the 2nd World Conference, Vienna, vol. 1, 1998. p. 477–81. [5] Heske C, et al. Appl. Phys. Lett. 1999;74:1451. [6] Sarigiannis D, Peck JD, Mountziaris TJ, Kioseoglou G, Petrou A. MRS Proc 2000;616:41. [7] Jan Ja-Chin, Kuo Shou-Yi, Yin Sun-Bin, Hsieh Wen-Feng. Chinese J Phys 2001;39:90. [8] Rajendra Kumar RT, Karunagaran B, Venkatachalam S, Mangalaraj D, Narayandass SK, Kesavamoorthy R. Mater Lett 2003;4431:1–6. [9] Rakhshani AE, Makdisi Y, Mathew X, Mathews NR. Phys stat Sol (a) 1998;168:177–87. [10] Sze SM. Semiconductor Devices, Physics and Technology. New York: John Wiley & Sons; 1985. [11] Goodman AM. J Appl Phys 1963;34:329. [12] Singh A, Cova P. J Appl Phys 1993;74:6714.
Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes S. Venkatachalam a, R.T. Rajendra Kumar a, D. Mangalaraj Sa.K. Narayandass a, Kyunghae Kim b, Junsin Yi b a b
a,b,*
,
Department of Physics, Thin Film Laboratory, Bharathiar University, Coimbatore 641 046, India School of Information and Communication Engineering, Sungkyunkwan University, Suwon, Korea Received 1 July 2003; received in revised form 31 March 2004; accepted 26 May 2004 Available online 6 August 2004
The review of this paper was arranged by Prof. Y. Arakawa
Abstract Zn0.52Se0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn0.52Se0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (d), strain (e), and the lattice parameter are calculated as 29.13 nm, 1.187 · 1015 lin/m2, 1.354 · 103 lin2 m4 and 5.676 · 1010 m respectively. From the I–V measurements on the Zn0.52Se0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 · 104 (305 K) respectively. The built-in potential, effective carrier concentration (NA) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 · 1015 cm3 and 1.359 eV respectively. 2004 Elsevier Ltd. All rights reserved. PACS: 71.20.Nr; 73.40.Lq; 73.61.Ga Keywords: Zn0.52Se0.48 thin films; Rutherford backscattering spectrometry; Zn0.52Se0.48/p-Si heterojunctions; Vacuum evaporation; Ideality factor; Barrier height
1. Introduction Wide-gap II–VI compounds are of special interest for solar energy conversion due to their high photosensitivity, direct optical transitions and high values of absorption coefficients (104 cm1) in the visible region of the spectrum. ZnxSe1x [x = 0.5] has been considered as a promising material for optoelectronic applications such
* Corresponding author. Tel.: +91 422 2425 458; fax: +91 422 2422 387. E-mail address: [email protected] (D. Mangalaraj).
as blue/green laser diodes [1]. ZnxSe1x is mainly used as buffer layer for the fabrication of high efficiency solar cells based on CuIn(S,Se)2 and CdTe absorbers [1–3]. The Zn-treatment allows for favorable modification of the absorber surface [4]. Also an intermixing at the interface reported by Heske et al. [5] has a positive effect on the solar cell performance. The evaluation of Schottky diode (SD) parameters provides a useful guidance for further characterizing these materials. I–V and C–V measurements are routinely used to extract various SD parameters. In this paper we report the composition and optical properties of Zn0.52Se0.48 thin films deposited at a substrate temperature of 250 C onto glass
0038-1101/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2004.05.082
2220
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
and Si substrates. RBS was used to determine the composition of the Zn0.52Se0.48 films and optical studies were used to evaluate the band gap of the deposited material. Structural and Raman studies were also carried out on Zn0.52Se0.48 thin films deposited onto Si substrates. The ideality factor, rectification factor were calculated from the dark I–V characteristics. The built-in potential, effective carrier concentration and barrier height were evaluated from the C–V measurements.
2. Experimental Thin film depositions were carried out by using a HINDHIVAC-12A4D coating unit and the deposition parameters are presented in Table 1. Prior to the deposition of the Zn0.52Se0.48 films, the Si wafers were cleaned by a standard process. Pure aluminium (99.999% Balzers, Switzerland) layer was deposited onto the back surface of silicon wafers by vacuum evaporation technique. Zn0.52Se0.48 alloy was thermally evaporated from a molybdenum boat onto well cleaned glass substrates and also onto the Si wafers under a pressure of 3 · 105 Torr, and Au was used as top electrode. The completed devices had an active area of about 2 · 106 m2. The RBS analysis was carried out to measure the stoichiometry of the film using a 2 MeV He + 1 beam with a detector angle of 168. Thicknesses of the deposited films was measured by multiple beam interference (Fizeau fringes) technique and also by RBS and it was found ˚. that the films have an average thickness of 1488 A 2.1. Structural analysis The structural analyses of the films were made by a X-ray diffractometer (SCINTAG; USA, Cu Ka radiation at k = 0.15418 nm) in the 2h ranges 20–80. The crystalline size (D) is calculated using the ScherrerÕs formula, D¼
0:94k b cos h
ð1Þ
where k––is the wavelength of the X-ray used, b––is the Full width half maximum, D––is the particle size value and h––is the angle between the incident and the scattered X-ray.
Table 1 Deposition parameters for ZnSe film preparation Source
Tungsten and molybdenum
Substrate temperature Substrates Substrate–source distance Vacuum
250 C Glass and silicon 17 cm 3 · 105 Torr
The dislocation density (d) is the length of dislocation lines per unit volume of the crystal and has been calculated by using the formula d¼
1 D2
ð2Þ
The strain values (e) were evaluated by using the following relation k 1 e¼ b ð3Þ D cos h tan h where D––is the particle size, b––is the Full width half maximum. The lattice spacing (d) was calculated from the BraggÕs formula d¼
k 2 sin h
ð4Þ
where k––is the wavelength of the X-ray used and d––is the lattice spacing. The lattice parameter has been calculated by the following expression 1 ðh2 þ k 2 þ l2 Þ ¼ 2 a2 d
ð5Þ
where h, k, l represent the lattice planes. 2.2. Optical analysis Optical transmission of the Zn0.52Se0.48 films deposited onto micro glass glide was recorded using a UV–VIS–NIR Spectrophotometer as a function of wavelength in the range from 300 to 1100 nm. 2.3. Electrical analysis Electrical measurements were done in a rotary vacuum of 1.33 Pa at different temperatures (305–365 K) by using a cryostat equipped with a PT100 thermocouple. The capacitance values were measured using multifrequency LCR meter (4275HP).
3. Results and discussion A typical RBS spectrum of the film deposited onto glass substrates maintained at 250 C is shown in Fig. 1. The detailed analysis of the RBS spectrum shows that the films are nearly stoichiometry in nature with Zn:Se ratio of 0.52:0.48. The X-ray diffraction pattern of Zn0.52Se0.48 film deposited onto an Si substrate is shown in Fig. 2 and the structure of the Zn0.52Se0.48 film is identified as crystalline. The XRD patterns exhibit reflections corresponding to the cubic phase. The reflection in the (1 1 1) direction is observed at 2h = 27.19. The calculated values of lattice constant, particle size, strain,
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
6000
2000
1.8 micro-C of 2 MeV He(+1) 168 degrees RBS Thickness (x10e15 /cm2):1488 A0 Se/Zn-ratio (%): 52/48
Si
1800
Intensity (a.u)
Yield
4500 3000 1500 0
0
2221
1600 1400
LO (Zn0.52Se0.48)
1200
100
200 300 400 Channel Number
500
1000 200
600
300
400
500
600
700
800
Raman shift (cm-1)
Fig. 1. Rutherford backscattering spectrum of vacuum evaporated Zn0.52Se0.48 thin film.
Fig. 3. Raman spectra of Zn0.52Se0.48 thin film deposited onto an Si substrate at a substrate temperature of 250 C.
5000 Substrate: Si (100)
100
Transmittance (%)
Intensity (a.u)
4000
3000 Si
2000
1000 20
(111)
30
40 50 60 2θ (degrees)
70
80
Fig. 2. X-ray diffractogram of Zn0.52Se0.48 films deposited onto an Si substrate at a substrate temperature of 250 C.
dislocation density and lattice spacing are presented in Table 2. Fig. 3 shows the Raman phonon spectrum of the Zn0.52Se0.48 films grown onto an Si substrate. The figure clearly shows the presence of longitudinal optical (LO) phonon mode (249.29 cm1). This longitudinal phonon mode confirms the crystalline nature of the deposited material. Similar results are available elsewhere [6–8]. Optical transmission spectrum of the films deposited at a substrate temperature of 250 C is shown in Fig. 4. The transparency of the as-deposited film is above 97% in the wavelength range of 730 nm. The T–k spectrum of the films with a sharp fall of transmittance at the band
27.19
[1 1 1]
5.676
3.277
60 40 20 0
400
600 800 Wavelength (nm)
1000
˚ Fig. 4. Transmittance spectra of Zn0.52Se0.48 thin film of 1488 A thickness deposited onto glass substrate.
edge once again confirms the crystalline nature of the films. Fig. 5 shows a plot between (ahm)2 and hm for a ˚ thick Zn0.52Se0.48 film. The band gap was ob1488 A tained by extrapolation of the plot of (ahm)2 versus hm and the estimated band gap value is 2.68 eV. The deposited films are found to have direct allowed transition. Zn0.52Se0.48/p-Si Schottky diodes were fabricated and Fig. 6 shows a cross sectional view of Zn0.52Se0.48/p-Si heterosturcture. Figs. 7 and 8 show the I–V plots measured at different temperatures in forward and reverse bias for a typical Zn0.52Se0.48/p-Si diode. In the forward bias the current increases exponentially with voltage. In
Table 2 Structural parameters of the Zn0.52Se0.48 films deposited onto Si substrates ˚) Thickness (A 2h [h k l] Lattice Lattice Particle ˚) ˚) constant (A spacing (A size (nm) 1488
80
29.03
Strain · 103 lin2 m4
Dislocation density · 1015 lin/m2
1.354
1.187
2222
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223 2.5x1015
5.0x10-6
15
2.0x10
305 K 345 K 365 K
1.5x1015
I(amp)
(αhν) 2 (eV/m )
2
4.0x10-6
1.0x1015
2.0x10-6 1.0x10-6
5.0x1014 0.0 1.2
3.0x10-6
0.0 1.6
2.0 2.4 hν (eV )
2.8
3.2
Fig. 5. The plots of (ahm)2 versus hm of Zn0.52Se0.48 thin film of ˚ deposited onto glass substrate. thickness 1488 A
0
2
4 6 V (volt )
8
10
Fig. 8. Reverse current versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
characteristic follows the standard diode equation [9] for forward bias below 0.4 V as given by qV I ¼ I 0 exp 1 ð6Þ nkT
Fig. 6. The cross sectional view of Zn0.52Se0.48/Si Schottky diodes.
6.0x10 -4 5.0x10 -4
3.0x10 -4 2.0x10 -4 305 K 345 K 365 K
1.0x10 -4 0.0
0
2
4
6 V (volt)
8
-8 -12
10
Fig. 7. Forward current versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
the reverse bias the current increases slowly with voltage and did not show any trend of saturation. In the forward bias, two distinct regions are observed. At low voltages, the current is linear with the applied voltage (ohmic behaviour) and at higher voltages I / V2, indicating the domination of space–charge limited current. This I–V
lnI(amp)
I (amp)
4.0x10 -4
where n is called the ideality factor, q is the electronic charge, k is the Boltzmann constant and T is the temperature. The ideality factor was determined at various temperatures using the plot between ln I and V (Fig. 9) and was found to be in the range from 1.749 to 1.769. When the ideal diffusion current is the dominating factor, then the value of the ideality factor (n) will be equal to 1, whereas this value will be equal to 2 when the recombination current is the dominating factor [10]. In the present case, the value of n varies between 1 and 2 and hence it can be viewed that both the ideal diffusion and recombination currents are comparable in this device. We have also evaluated the diode rectification factor from the ratio of forward to reverse bias current. The diode rectification factor had a maximum value of
-16 -20
305 K 345 K 365 K
-24 0
2
4
6 V (volt)
8
10
Fig. 9. Variation of ln I versus voltage characteristics of Zn0.52Se0.48/Si Schottky diodes.
S. Venkatachalam et al. / Solid-State Electronics 48 (2004) 2219–2223
ated as 5.907 · 1015 cm3 and 1.359 eV. The calculated values of Vbi, Vn, Nv and /Bn are given in Table 3.
1.8x1020
1/C 2 (F -2m4)
1.5x1020
1 MHz
1.2x1020
4. Conclusion
19
9.0x10
19
6.0x10
3.0x1019 0.0
-2
-1
0
1
2
3
4
5
V (volt)
Fig. 10. Dependence of 1/C2 value on applied voltage of Zn0.52Se0.48/Si Schottky diodes.
Table 3 Electrical parameters of the Zn0.5Se0.46/p-Si diodes Vbi (V)
d(1/C2)/dV · 1019 (m2/F)2/V
NA · 1015 (cm3)
Vn (V)
/Bn (eV)
1.02
2.6277
5.907
0.2001
1.359
kT q
˚ The Zn0.52Se0.48 thin films with a thickness of 1488 A each were deposited onto an Si substrate maintained at a substrate temperature of 250 C by using vacuum evaporation technique. The deposited film is polycrystalline having cubic structure and is oriented along (1 1 1) direction. In the Raman measurement, the presence of LO mode confirms the crystalline nature of the deposited material. From the I–V measurements we concluded that the ideality factor is the key role to identify the dominant transport mechanism in the prepared Zn0.52Se0.48/p-Si heterostructure. The drift and diffusion current are the dominant transport mechanism in the prepared diodes. It has been concluded from the C–V measurement that the examined heterostructures are abrupt heterojunctions.
References
1.04 · 104 at room temperature. The rectification factor decreases with the increase of temperature. The plot between the measured values of capacitance versus voltage for Zn0.52Se0.48/n-Si diodes is shown in Fig. 10. We obtained a straight line by plotting a curve between 1/C2 versus V, which implies a similar behaviour of an abrupt heterojunction [11]. The intercept of this plot at 1/C2 = 0 corresponds to the built-in potential Vbi, and is found to be 1.02 V. The value of barrier height [12] can be calculated from the measured value of Vbi as, /Bn ¼ V bi þ V n þ
2223
ð7Þ
where Vn = kT/q. Ln(Nv/NA), k is the Boltmann constant, T is the temperature, q is the charge of the electron, Nv is the density of states in the valance band and NA is the effective carrier concentration. From the slope of the 1/C2 versus voltage plot, the values of effective carrier concentration and barrier height are evalu-
[1] Konagai M, Ohtake Y, Okamoto T. Mater Res Soc Symp Proc 1996;426:1532. [2] Ohtake Y, Okamoto T, Yamada A, Konagai M, Saito K. Sol Energy Mater Sol Cells 1997;49:2699. [3] Turner AK, Woodcock JM, Ozsann ME, et al. Sol Energy Mater Sol Cells 1994;35:263. [4] Ramanathan KR, Wiesner H, Asher S, Wiles D, Bhattacharya N, Keane J, et al. In: Proceeding of the 2nd World Conference, Vienna, vol. 1, 1998. p. 477–81. [5] Heske C, et al. Appl. Phys. Lett. 1999;74:1451. [6] Sarigiannis D, Peck JD, Mountziaris TJ, Kioseoglou G, Petrou A. MRS Proc 2000;616:41. [7] Jan Ja-Chin, Kuo Shou-Yi, Yin Sun-Bin, Hsieh Wen-Feng. Chinese J Phys 2001;39:90. [8] Rajendra Kumar RT, Karunagaran B, Venkatachalam S, Mangalaraj D, Narayandass SK, Kesavamoorthy R. Mater Lett 2003;4431:1–6. [9] Rakhshani AE, Makdisi Y, Mathew X, Mathews NR. Phys stat Sol (a) 1998;168:177–87. [10] Sze SM. Semiconductor Devices, Physics and Technology. New York: John Wiley & Sons; 1985. [11] Goodman AM. J Appl Phys 1963;34:329. [12] Singh A, Cova P. J Appl Phys 1993;74:6714.