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n devices for photovoltaic applications
Materials Science and Engineering B80 (2000) 152– 155 www.elsevier.com/locate/mseb
Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications N. Konofaos a,*, E.K. Evangelou a, F. Scholz b, K. Zieger c, E. Aperathitis d a
Department of Physics, Uni6ersity of Ioannina, PO Box 1186, 45110 Ioannina, Greece b Stuttgart Uni6ersity, D-70550 Stuttgart, Germany c UMS GmbH, D-89081 Ulm, Germany d Microelectronics Research Group, Institute of Electronic Structure and Laser, FORTH-HELLAS, PO Box 1527, Heraklion 71110 Crete, Greece
Abstract GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current– voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime (~eff) and the diodes ideality factors. The values of the ~eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 mm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias. © 2001 Elsevier Science B.V. All rights reserved. Keywords: p/i/n Diodes; GaAs; Solar cells; Current conduction mechanisms
1. Introduction
2. Theory
The use of III – V p/i/n structures as photodetectors for converting irradiation into electricity either as solar cells or thermophotovoltaics have recently been the focus of intense investigation [1 – 4]. The dominant conduction mechanisms and the lifetime of the carriers in these structures (some of these structures being in multiple quantum well configuration) are very important for the output performance of the devices. These mechanisms are heavily dependent on the quality and geometry of the i-region and they differ depending whether the structure is under illumination or in the dark (in this work). The use of GaAs p/i/n diodes in photovoltaic applications has received a lot of attention during the last decade mainly due to the high efficiency of the cells [1,2,5,6]. Using electrical characterization techniques such as Admittance spectroscopy and current voltage measurements, allow the identification of the diode performance as a function of their structural configuration and in particular the i-region width.
The correct model describing the dc current –voltage dependence of a pn-AlGaAs/GaAs solar cell needs the analytical description of the current –voltage relation. The two-diode model, is very often used [2,8]. The equivalent circuit is shown in Fig. 1a. The equivalent devices correspond to the different dc current mechanisms with their respective voltage dependencies. The mechanisms are the minority carrier diffusion current Jd after Schockley via diode D1, the carrier recombination/generation current Jrg after Sah et al. Within the space charge region (scr) via diode D2 and the ohmic loss current Jsh via the shunt resistance Rp. The ohmic loss current dominates the reverse biased solar cell in the range below the breakdown voltage and the recombination/generation current dominates the forward biased GaAs solar cell up to voltages of about 0.8 V, whereas the diffusion current part is generally concealed in GaAs solar cells. The ac equivalent circuit of Fig. 1b is in accordance with the two-diode model. The devices correspond to the different current and charge storing mechanisms. These are the space charge region
* Corresponding author.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 6 4 1 - 3
N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155
capacitance Cscr, the diffusion admittance (Cd//Gd) of the non-equilibrium minority carriers in the diffusion regions and the recombination/generation admittance (Crg//Grg) of the free carriers within the space charge region, which neutralize each other [2,10]. The Space charge region capacitance is expressed by the relation [2,11]. Amrm0 C(V)= x(V)
(1)
where A is the diode area; x(V) is the space charge region and m0 and mr are the dielectric constants of vacuum and the material, respectively. Considering further inhomogeneous doping the capacitance dependence on the reverse bias can also be described as i C−i = C− (2) 0 (Vbi − V) where Vbi is the built-in voltage (i B 2 means falling doping gradient, i\ 2 increasing, i = 2 means uniform doping). Thus an accurate determination of the built-in potential is possible as well as of the exponent i via a linear regression. The effective lifetime of the free carriers within the space charge region is the important parameter. It can be expressed as [2] ~eff =
yA 2qnim0mr 4VdCscr0Grg0
(3)
where ni is temperature dependent intrinsic carrier concentration, Vd is found via the admittance measurements and Crg0 and Gscr0 are the values of the capacitance and the conductance of the recombination/ generation and the space charge region, respectively, extrapolated to zero bias. The expression in Eq. (3) can be derived from the basic relation of the time constant,
~eff =
153
nqniW 2 4Vdmrm0
bearing in mind Eq. (1) and the relation, Grg0 = A
yqniw0 4Vd~eff
where w0 = w(0) may be determined from the capacitance measurements after Eq. (1) with Csr0 =Cscr(0).
3. Experimental Metal-organic vapor phase epitaxy was used to develop the GaAs devices. Four series of samples were prepared and studied each one with a different i-region width. The multilayer structure is shown in Fig. 1c. The performance of the devices as a function of the i-region width was examined by means of Admittance spectroscopy as well as classical current –voltage and capacitance –voltage characterization. Capacitance and loss measurements were carried out as a function of frequency with a computer-controlled HP4284A LCR meter at frequencies ranging from 20 Hz to 1 MHz and a test signal of 50 mVrms. The current –voltage (I–V) characteristics were measured using a Keithley 230 voltage source together with a 617 electrometer. The whole system was located inside a metal box to protect from electromagnetic interference.
4. Results and modeling Fig. 2 shows the dark I–V characteristics of the four devices. They are the typical diode curves of classical devices. Analysis of the curves can reveal the dominant
Fig. 1. (a) The 2-diode model of the GaAs solar cells for the dc-current voltage dependence (after [2]). (b) The ac equivalent circuit ([2]). (c) The device structure of the diodes presented in this work.
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N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155 Table 1 The results for the four samples examined
Fig. 2. The dark I – V characteristics of the four samples at room temperature. The plot shows the forward bias recombination/generation region and the reverse bias ohmic loss current region. + = c 1, = c2, = c 3, = c4.
Fig. 3. (a) The capacitance–voltage (C–V) of the four samples, for the frequency of 1 MHz. + = c1, = c 2, = c 3, = c4.
Sample
i-Region width (mm)
~eff (ps)
Built-in voltage (V)
i-Parameter
c1 c2 c3 c4
0.0 0.2 0.5 0.8
7.35 8.8 70 142
1.26 3.06 7.11 15.5
2 2 2 2
current mechanism by their shape. As derived from the plot shown in Fig. 2b, the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias below the breakdown voltage. There was no evidence for diffusion currents in the forward region for voltages up to 1 V. Fig. 3 depicts the capacitance –voltage curves of the four devices. Using the relations described above (Eqs. (1) and (2)), a plot of the 1/C i revealed a straight line for i=2 for all four samples, indicating that doping was uniform. Fig. 4 depicts such a plot for c3. Further analysis provided the minority carrier lifetime via the relation in Eq. (3). The built-in voltage was also calculated from the recombination/generation admittance and the values are shown in Table 1 together with the other measured parameters. Using those data, the doping profile inside the device base region was calculated for each sample. Fig. 5 depicts the doping concentration profile for sample c 1, showing that the average value of Nd is very close to the required values from the device construction. The calculated values prove that the two-diode model used was more than suitable to describe the diodes behavior. The samples behave as high quality p/i/n diodes, while the role of the i-region width can be understood from the above results. The larger the width, the larger the minority carriers lifetime. Compared with results published on similar structures in the past [2,10,11] these diodes exhibit shorter lifetime values. The admittance method first used by B. Reinicke et al. [2] is hereby confirmed as a method capable of studying GaAs solar cell structures.
5. Conclusions and future work
Fig. 4. Plot of l/Cb versus voltage of sample c 1, for i= 2. The calculated parameter is Vbi = 1.26 V, from the curve abscissa.
We studied the electrical characteristics of GaAs p/i/n diodes with different i-region widths, for use in photovoltaic applications. The dominant current mechanism was found to be that of recombination/generation in forward bias and the ohmic loss current in reverse. Calculation of the minority carrier lifetime (~eff) was possible as well as of the built-in potential. Those diodes have shown to behave like very good photovoltaic structures in the past [1] and the results above
N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155
155
Duong Sinh [9] will provide more details on the current and voltage losses of the diodes in conjunction with heterojunction band offsets. This procedure is currently on.
Acknowledgements The authors would like to express their gratitude to G. Seirinakis, presently with the State University of New York in USA and to G. Xilouris, presently with the National Technical University of Athens in Greece for their contribution to this work.
Fig. 5. The doping distribution inside the diode base region for sample c 1.
provide an explanation of their performance, regarding the minority carrier lifetime. The values of the lifetime are quite small when compared with the values reported in the literature [2,6,12]. In particular, for GaAs diodes, the calculated values are closer to those reported by Rancour et al. [6] and smaller to previously reported values by Reinicke et al. [2], while they are subsequently smaller to values recently reported for silicon solar cells [12]. These results will be used in conjunction with results taken after monitoring the I– V characteristics of these structures, processed as 1×1 cm solar cells, under solar illumination. Moreover, these admittance results on the p/i/n III –V devices will be used as a reference point for the design and development of more advanced and of high performance devices having p/i/n structures with multiple quantum wells in the i-region [7]. Further analysis using both the so called modulation capacitance voltage (MCV) method developed by Ngo
References [1] E. Aperathitis, C.G. Scott, D. Sands, V. Foukaraki, Z. Hatzopoulos, P. Panayotatos, Mater. Sci. Eng. B51 (1998) 85. [2] B. Reinicke, H. Eschrich, H.G. Wagemann, 11th E.C. Photovoltaic Solar Energy Conference Proceedings, Switzerland, 1992. [3] L.L. Kazmerski, J. Mater. Res. 13 (10) (1998) 2684. [4] C.K. Chia, J.P.R. David, G.J. Rees, P.N. Robson, S.A. Plimmer, R. Grey, Appl. Phys. Lett. 71 (26) (1997) 3877. [5] J.J. Liou, Solar Energy Mater. Solar Cells 29 (1993) 261. [6] D.P. Rancour, M.R. Melloch, R.F. Pierret, M.S. Lundstrom, M.E. Klausmeier-Brown, C.S. Kyono, J. Appl. Phys. 62 (4) (1987) 1539. [7] A.W. Bett, F. Dimoth, G. Stollwerck, O.V. Sulima, Appl. Phys. A 69 (1999) 119. [8] H.J. Hoval, Solar cells, in: Semiconductors and Semimetals, vol. 11, Academic Press, New York, 1975. [9] N.D. Sinh, F. Spiess, K. Kliefoth, W. Fuessel, Solar Energy Mater. Solar Cells 48 (1997) 43. [10] G. Friesen, H.A. Ossenbrink, Solar Energy Mater. Solar Cells 48 (1997) 77. [11] G. Ashkinazi, M. Meyler, M. Nathan, L. Zolotarevski, O. Zolotarevski, J. Appl. Phys. 75 (6) (1994) 3214. [12] T. Sasaki, J. Nishizawa, M. Esashi, J. Appl. Phys. 83 (8) (1998) 4069.
Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications N. Konofaos a,*, E.K. Evangelou a, F. Scholz b, K. Zieger c, E. Aperathitis d a
Department of Physics, Uni6ersity of Ioannina, PO Box 1186, 45110 Ioannina, Greece b Stuttgart Uni6ersity, D-70550 Stuttgart, Germany c UMS GmbH, D-89081 Ulm, Germany d Microelectronics Research Group, Institute of Electronic Structure and Laser, FORTH-HELLAS, PO Box 1527, Heraklion 71110 Crete, Greece
Abstract GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current– voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime (~eff) and the diodes ideality factors. The values of the ~eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 mm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias. © 2001 Elsevier Science B.V. All rights reserved. Keywords: p/i/n Diodes; GaAs; Solar cells; Current conduction mechanisms
1. Introduction
2. Theory
The use of III – V p/i/n structures as photodetectors for converting irradiation into electricity either as solar cells or thermophotovoltaics have recently been the focus of intense investigation [1 – 4]. The dominant conduction mechanisms and the lifetime of the carriers in these structures (some of these structures being in multiple quantum well configuration) are very important for the output performance of the devices. These mechanisms are heavily dependent on the quality and geometry of the i-region and they differ depending whether the structure is under illumination or in the dark (in this work). The use of GaAs p/i/n diodes in photovoltaic applications has received a lot of attention during the last decade mainly due to the high efficiency of the cells [1,2,5,6]. Using electrical characterization techniques such as Admittance spectroscopy and current voltage measurements, allow the identification of the diode performance as a function of their structural configuration and in particular the i-region width.
The correct model describing the dc current –voltage dependence of a pn-AlGaAs/GaAs solar cell needs the analytical description of the current –voltage relation. The two-diode model, is very often used [2,8]. The equivalent circuit is shown in Fig. 1a. The equivalent devices correspond to the different dc current mechanisms with their respective voltage dependencies. The mechanisms are the minority carrier diffusion current Jd after Schockley via diode D1, the carrier recombination/generation current Jrg after Sah et al. Within the space charge region (scr) via diode D2 and the ohmic loss current Jsh via the shunt resistance Rp. The ohmic loss current dominates the reverse biased solar cell in the range below the breakdown voltage and the recombination/generation current dominates the forward biased GaAs solar cell up to voltages of about 0.8 V, whereas the diffusion current part is generally concealed in GaAs solar cells. The ac equivalent circuit of Fig. 1b is in accordance with the two-diode model. The devices correspond to the different current and charge storing mechanisms. These are the space charge region
* Corresponding author.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 6 4 1 - 3
N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155
capacitance Cscr, the diffusion admittance (Cd//Gd) of the non-equilibrium minority carriers in the diffusion regions and the recombination/generation admittance (Crg//Grg) of the free carriers within the space charge region, which neutralize each other [2,10]. The Space charge region capacitance is expressed by the relation [2,11]. Amrm0 C(V)= x(V)
(1)
where A is the diode area; x(V) is the space charge region and m0 and mr are the dielectric constants of vacuum and the material, respectively. Considering further inhomogeneous doping the capacitance dependence on the reverse bias can also be described as i C−i = C− (2) 0 (Vbi − V) where Vbi is the built-in voltage (i B 2 means falling doping gradient, i\ 2 increasing, i = 2 means uniform doping). Thus an accurate determination of the built-in potential is possible as well as of the exponent i via a linear regression. The effective lifetime of the free carriers within the space charge region is the important parameter. It can be expressed as [2] ~eff =
yA 2qnim0mr 4VdCscr0Grg0
(3)
where ni is temperature dependent intrinsic carrier concentration, Vd is found via the admittance measurements and Crg0 and Gscr0 are the values of the capacitance and the conductance of the recombination/ generation and the space charge region, respectively, extrapolated to zero bias. The expression in Eq. (3) can be derived from the basic relation of the time constant,
~eff =
153
nqniW 2 4Vdmrm0
bearing in mind Eq. (1) and the relation, Grg0 = A
yqniw0 4Vd~eff
where w0 = w(0) may be determined from the capacitance measurements after Eq. (1) with Csr0 =Cscr(0).
3. Experimental Metal-organic vapor phase epitaxy was used to develop the GaAs devices. Four series of samples were prepared and studied each one with a different i-region width. The multilayer structure is shown in Fig. 1c. The performance of the devices as a function of the i-region width was examined by means of Admittance spectroscopy as well as classical current –voltage and capacitance –voltage characterization. Capacitance and loss measurements were carried out as a function of frequency with a computer-controlled HP4284A LCR meter at frequencies ranging from 20 Hz to 1 MHz and a test signal of 50 mVrms. The current –voltage (I–V) characteristics were measured using a Keithley 230 voltage source together with a 617 electrometer. The whole system was located inside a metal box to protect from electromagnetic interference.
4. Results and modeling Fig. 2 shows the dark I–V characteristics of the four devices. They are the typical diode curves of classical devices. Analysis of the curves can reveal the dominant
Fig. 1. (a) The 2-diode model of the GaAs solar cells for the dc-current voltage dependence (after [2]). (b) The ac equivalent circuit ([2]). (c) The device structure of the diodes presented in this work.
154
N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155 Table 1 The results for the four samples examined
Fig. 2. The dark I – V characteristics of the four samples at room temperature. The plot shows the forward bias recombination/generation region and the reverse bias ohmic loss current region. + = c 1, = c2, = c 3, = c4.
Fig. 3. (a) The capacitance–voltage (C–V) of the four samples, for the frequency of 1 MHz. + = c1, = c 2, = c 3, = c4.
Sample
i-Region width (mm)
~eff (ps)
Built-in voltage (V)
i-Parameter
c1 c2 c3 c4
0.0 0.2 0.5 0.8
7.35 8.8 70 142
1.26 3.06 7.11 15.5
2 2 2 2
current mechanism by their shape. As derived from the plot shown in Fig. 2b, the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias below the breakdown voltage. There was no evidence for diffusion currents in the forward region for voltages up to 1 V. Fig. 3 depicts the capacitance –voltage curves of the four devices. Using the relations described above (Eqs. (1) and (2)), a plot of the 1/C i revealed a straight line for i=2 for all four samples, indicating that doping was uniform. Fig. 4 depicts such a plot for c3. Further analysis provided the minority carrier lifetime via the relation in Eq. (3). The built-in voltage was also calculated from the recombination/generation admittance and the values are shown in Table 1 together with the other measured parameters. Using those data, the doping profile inside the device base region was calculated for each sample. Fig. 5 depicts the doping concentration profile for sample c 1, showing that the average value of Nd is very close to the required values from the device construction. The calculated values prove that the two-diode model used was more than suitable to describe the diodes behavior. The samples behave as high quality p/i/n diodes, while the role of the i-region width can be understood from the above results. The larger the width, the larger the minority carriers lifetime. Compared with results published on similar structures in the past [2,10,11] these diodes exhibit shorter lifetime values. The admittance method first used by B. Reinicke et al. [2] is hereby confirmed as a method capable of studying GaAs solar cell structures.
5. Conclusions and future work
Fig. 4. Plot of l/Cb versus voltage of sample c 1, for i= 2. The calculated parameter is Vbi = 1.26 V, from the curve abscissa.
We studied the electrical characteristics of GaAs p/i/n diodes with different i-region widths, for use in photovoltaic applications. The dominant current mechanism was found to be that of recombination/generation in forward bias and the ohmic loss current in reverse. Calculation of the minority carrier lifetime (~eff) was possible as well as of the built-in potential. Those diodes have shown to behave like very good photovoltaic structures in the past [1] and the results above
N. Konofaos et al. / Materials Science and Engineering B80 (2001) 152–155
155
Duong Sinh [9] will provide more details on the current and voltage losses of the diodes in conjunction with heterojunction band offsets. This procedure is currently on.
Acknowledgements The authors would like to express their gratitude to G. Seirinakis, presently with the State University of New York in USA and to G. Xilouris, presently with the National Technical University of Athens in Greece for their contribution to this work.
Fig. 5. The doping distribution inside the diode base region for sample c 1.
provide an explanation of their performance, regarding the minority carrier lifetime. The values of the lifetime are quite small when compared with the values reported in the literature [2,6,12]. In particular, for GaAs diodes, the calculated values are closer to those reported by Rancour et al. [6] and smaller to previously reported values by Reinicke et al. [2], while they are subsequently smaller to values recently reported for silicon solar cells [12]. These results will be used in conjunction with results taken after monitoring the I– V characteristics of these structures, processed as 1×1 cm solar cells, under solar illumination. Moreover, these admittance results on the p/i/n III –V devices will be used as a reference point for the design and development of more advanced and of high performance devices having p/i/n structures with multiple quantum wells in the i-region [7]. Further analysis using both the so called modulation capacitance voltage (MCV) method developed by Ngo
References [1] E. Aperathitis, C.G. Scott, D. Sands, V. Foukaraki, Z. Hatzopoulos, P. Panayotatos, Mater. Sci. Eng. B51 (1998) 85. [2] B. Reinicke, H. Eschrich, H.G. Wagemann, 11th E.C. Photovoltaic Solar Energy Conference Proceedings, Switzerland, 1992. [3] L.L. Kazmerski, J. Mater. Res. 13 (10) (1998) 2684. [4] C.K. Chia, J.P.R. David, G.J. Rees, P.N. Robson, S.A. Plimmer, R. Grey, Appl. Phys. Lett. 71 (26) (1997) 3877. [5] J.J. Liou, Solar Energy Mater. Solar Cells 29 (1993) 261. [6] D.P. Rancour, M.R. Melloch, R.F. Pierret, M.S. Lundstrom, M.E. Klausmeier-Brown, C.S. Kyono, J. Appl. Phys. 62 (4) (1987) 1539. [7] A.W. Bett, F. Dimoth, G. Stollwerck, O.V. Sulima, Appl. Phys. A 69 (1999) 119. [8] H.J. Hoval, Solar cells, in: Semiconductors and Semimetals, vol. 11, Academic Press, New York, 1975. [9] N.D. Sinh, F. Spiess, K. Kliefoth, W. Fuessel, Solar Energy Mater. Solar Cells 48 (1997) 43. [10] G. Friesen, H.A. Ossenbrink, Solar Energy Mater. Solar Cells 48 (1997) 77. [11] G. Ashkinazi, M. Meyler, M. Nathan, L. Zolotarevski, O. Zolotarevski, J. Appl. Phys. 75 (6) (1994) 3214. [12] T. Sasaki, J. Nishizawa, M. Esashi, J. Appl. Phys. 83 (8) (1998) 4069.