- Email: [email protected]
Band bending and interface state density in CdS thin-film MIS structures
994
Notes
expected to be introduced if the interface density is assumed uniform over the range. Institute of Radio Physics and Electronics, 92, Acharya Prafulla Chandra Road, Calcutta- 7oooo9, India
M. C. Asti A. N. DAW
Solid-St. Electron. 27, 1057 (1984). 4. S. M. Sze, Physics of Semiconductor Devices, pp.
3. A. N. Daw and P. Chattopadhyay,
437-438. Wiley, New York (1983). 5. D. L. Pulfrey, IEEE Trans. Electron Dev. ED-23, 587 (1976).
D. L. Pulfrey, Solid-St. Electron. 20, 455 (1977). H. C. Card and E. H. Rhoderick, J. Phys. D: Appl. Phys. 4, 1589 (1971). 8. K. K. Ng and.H. 6. Card, IEEE Trans. Electron Dev. ED-27. 716 (1980). 9. J. Shewchun; M.A. Green and F. D. King, Solid-St. Electron. 17, 563 (1974). 10. B. E. Deal, M. Sklar, A. S. Grove and E. H. Snow, J. Electrochem. Sot. 114, 266 (1967). 6. I.
REFERENCES
1. P. V. Halen, R. P. Mertens, R. J. V. Overstraeten, R. E. Thomas and J. V. Meerberaen. IEEE Trans. Electron Dev. ED-Xl, 507 (1978). 2. 0. M. Nielsen, IEE Proc. 127, Pt. I, 301 (1980).
BAND
BENDING AND INTERFACE STATE DENSITY Cd!3 THIN-FILM MIS STRUCTURES (Received 2 November 1985; in revised form 26 December
II-VI compound semiconductor have wide and direct band gaps so that they are promising for optoelectronic devices in the visible and near infrared region, such as solar cells and light emitting diodes. The problem is, however, that p-n iunctions are not readily obtained exceot for CdTe. If MIS (metal-insulator-semiconductor) structures with low interface state densities are obtained for the II-VI compounds, band bending similar to the p-n junction can be realized by applying a voltage to the metal plate of the MIS structure. With Si and GaAs, solar cells[l] and light emitting diodes[2] based on this idea have been already realized, respectively. The solar cells are called induced junction type[l], and grid electrodes between the insulator and the semiconductor collect the photocurrent. Thin-film transistors of the MIS type using semi-insulating CdS or CdSe have been reported[3,4]. However, the electron accumulation layers are utilized as the channels, that is, the semiconductor bands are bent downward at the insulator-semiconductor interface. In this note we report for the first time strong upward band bending in thin-flhn MIS structures, in which semiconducting CdS thin films am used as the semiconductor, and discuss the interface state density. An internal photovoltage due to the band bending was observed under illumination. The MIS structures were composed as Al-CdS-A&O,-AI glass. The AllO3 lihns were evaporated from sintered pellets by electron beam bombardment in a diffusion-pumped
-10
-5
“oL~AOoE (V)
5
10
Fig. 1. An example of high-frequency C-I’ characteristics. The frequency is 1 MHz and the voltage sweep rate is 0.1 V/set. The insert shows the fast-interface-state density determined by the Terman method for the upper half of the band gap.
IN
1985)
vacuum system. The thickness was typically 13OOA. The CdS films were evaporated onto the A&O, films at 200°C to a thickness of about 1 pm from a Ta boat in a different vacuum system from the insulator tllm deposition. The base pressure and the deposition rate were mcstly 5 x 10m6Torr and 204OA/min, respectively. The donor concentrations Nd were determined from the slope of l/C2 - V characteristics at 1 MHz in the structure of Al-Cd,%AIrOr (50-100 &Au glass. This structure was rectifying, but the structure of Al-Cdl%Au glass, that is, the structure without the very thin Also, film was rarely rectifying. The values of Nd were rather large and varied in the range of 0.4-10 x 10i7/cm3, depending on the evaporation rate and the vacuum pressure. These N,, values were used in analysing C-V characteristics of the MIS structures. Al dots with an area of 0.025 cm2 were evaporated onto the CdS films as the ohmic electrodes. Figure 1 shows an example of MIS C-V characteristics measured at a frequency of 1 MHz and at a voltage sweep rate of O.l/V/sec. The calculated high-frequency C-Fcurve is also drawn in the figure. The experimental curve shows a large change in capacitance toward the calculated inversion capacitance, but it does not saturate. Inversion cannot be observed due to the low thermal generation rate of minority carriers in the wide gap semiconductor CdS, that is, around the inversion capacitance the system is under “deep depletion”. These large capacitance. changes suggest strong upward band bending under the negative bias. The magnitudes of capacitance changes in different samples were well consistent with their different Nd values, that is, it was larger in a sample with a lower donor concentration. (Compare Fig. 1 with Fig. 2.) The fast interface state density was determined from Fig. 1 by the Terman method[5] (highfrequency capacitance method), approximating that the film was a homogeneous material as a single crystal although the evaporated films on glass substrates were polycrystalline. The results for the upper half of the band gap are shown in the insert. The low density, less than 1 x lO”/cm*eV, was obtained in the midgap. The value of 1 x lO”/cn?eV is almost a lower limit of measurement by the Terman method. In Figure 2 is shown a capacitance increase under illumination. The insert shows possible photoexcitation processes which cause the capacitance increase. Electron-hole pairs which are photogenerated via direct band-to-band transitions and via deep traps in the depletion layer are separated by the band bending. The electrons am driven to the neutral region and the holes are stored at the semiconductor-insulator interface This redistribution of
Notes
Zl -10
I
I
-5
“O‘T&
( v 1
5
10
Fig. 2. Influence of illumination on the C-V characteristics. The frequency is 1 MHz and the voltage sweep rate is 0.1 V/xc. The insert shows possible photoexcitation processes which cause the capacitance increase. free carriers produces an internal photovoltage which cannot be measured directly because of the insulating layer. The photovoltage reduces the depletion layer width and leads to the capacitance increase. Also, a part of traps in the depletion layer are emptied by the light excitation and the resulting space charge contributes to the capacitance increase. (This phenomenon is the same as so-called photocapacitance[6] in Schottky junctions and p-n junctions.) Interface traps may contribute. However, since polycrystalline thin tihns generally contain many traps in the grain boundary region, the major contribution may possibly be due to bulk traps. The two origins of the capacitance increase were distinguished in our samples through the following experimental facts: The capacitance increase was composed of two parts, A and B. After the light was switched off part A did not change but part B attenuated gradually. The magnitude of part A did not vary with the light intensity but that of part B increased with the light intensity. Part A corresponds to the photocapacitance because the emptied traps in the depletion layer cannot capture electrons after the light is switched off, and the number of emptied traps is not determined by the light intensity but by the differences in the optical ionization cross sections for electrons and holes. Part B is due to the internal
FREQUENCY DEPENDENCE
995
photovoltage. The photovoltage attenuates after the light is switched off because the leakage current through the insulator carries away the stored holes at the semiconductor-insulator interface. This leakage should be reduced for better MIS operations. The internal photovoltage observed as a capacitance increase is a direct evidence of the upward band bending at the interface. If the photovoltage can be passed to the grid electrodes between the insulator and the semiconductor, thin-film solar cells of induced junction type will be realized. In summary, we have fabricated CdS thin-lilm MIS structures by vacuum evaporation. It was found that the bands at the surface could be bent strongly upward and that the fast interface state density was low, although the semiconductor was polycrystalline thin films. The observed internal photovoltage is an evidence of the upward band bending and also presents a possibility for the application of this structure to thin-film solar cells of induced junction type. Acknowledgements-We wish to thank T. Ohhata and M. Yoshimi for their assistance. A part of this work was supported financially by the Iwatani Naoji Foundation’s Research Grant. Department of Electrical Engineering Ju@rn SAWE KEUCHIFUJU Technical College Kyoto Institute of Technology YOSHITAKA WT.40 YUTAKA YOWCAWA Matsugasaki, Kyoto 606, Japan REFERENCES 1. T. Fuyuki, S. Moriuchi and H. Matsunami, Tech. Digest 1st Znt. Photovoltaic Science and Engng Conf. Kobe (Edited by Y. Hamakawa), p. 763. Japan Convention Services, Tokyo (1984). 2. C. N. Berglund, Appl. Phys. L&t. 9, 44 (1966). 3. For example, M. G. Miksic, E. S. Schlig and R. R. Haering, Solid-St. Electron. 7, 39 (1964). 4. For example, F. V. Shallcross, Proc. IEEE. 51, 851 (1963). 5. L. M. Terman, Solid-St. Electron. 5, 285 (1962). 6. For example, T. Takebe, H. Gno, J. Saraie and T. Tanaka, Phys. Stat. Sol. (a) 66, 725 (1981).
OF MOS CAPACITANCE IN STRONG
INVERSION AND AT ELEVATED TEMPERATURES (Received 21 October 1985; in revised form 9 December 1985) It is well established[l] that the steady-state CV characteristics of the MOS capacitor in strong inversion are governed by minority-carrier response. The sources of minority carriers are due mainly to (a) generation and recombination of carriers at bulk traps in the depletion region, the “generation-recombination” mechanism, and (b) generation and recombination of carriers at the back contact followed by diffusion through the neutral region, the “diffusion” mechanism. In strong inversion, the contribution from interface states are insignificant because those states in the vicinity of the Fermi level are too far away from the midgap to be effective. At room temperature, minority carriers from bulk traps in the depletion region dominate the response; at some elevated temperature, carriers generated at the back contact and transported to the inversion region become dominant. The exact temperature at which transition from generation-recombination dominance to diffusion dominance depends on bulk trap density and doping concentration.
A simplified equivalent circuit for strong inversion, incorporating both mechanisms is reproduced[l] in Fig. 1, where C, and C, are the oxide and depletion capacitances, CO,
cd
0
Ggr T ii 1
Gd
1
1
1
Fig. 1. Equivalent circuit for an MOS capacitor in inversion. Applied frequency is assumed to be sutIiciently high so that the inversion capacitance can be neglected.
Notes
expected to be introduced if the interface density is assumed uniform over the range. Institute of Radio Physics and Electronics, 92, Acharya Prafulla Chandra Road, Calcutta- 7oooo9, India
M. C. Asti A. N. DAW
Solid-St. Electron. 27, 1057 (1984). 4. S. M. Sze, Physics of Semiconductor Devices, pp.
3. A. N. Daw and P. Chattopadhyay,
437-438. Wiley, New York (1983). 5. D. L. Pulfrey, IEEE Trans. Electron Dev. ED-23, 587 (1976).
D. L. Pulfrey, Solid-St. Electron. 20, 455 (1977). H. C. Card and E. H. Rhoderick, J. Phys. D: Appl. Phys. 4, 1589 (1971). 8. K. K. Ng and.H. 6. Card, IEEE Trans. Electron Dev. ED-27. 716 (1980). 9. J. Shewchun; M.A. Green and F. D. King, Solid-St. Electron. 17, 563 (1974). 10. B. E. Deal, M. Sklar, A. S. Grove and E. H. Snow, J. Electrochem. Sot. 114, 266 (1967). 6. I.
REFERENCES
1. P. V. Halen, R. P. Mertens, R. J. V. Overstraeten, R. E. Thomas and J. V. Meerberaen. IEEE Trans. Electron Dev. ED-Xl, 507 (1978). 2. 0. M. Nielsen, IEE Proc. 127, Pt. I, 301 (1980).
BAND
BENDING AND INTERFACE STATE DENSITY Cd!3 THIN-FILM MIS STRUCTURES (Received 2 November 1985; in revised form 26 December
II-VI compound semiconductor have wide and direct band gaps so that they are promising for optoelectronic devices in the visible and near infrared region, such as solar cells and light emitting diodes. The problem is, however, that p-n iunctions are not readily obtained exceot for CdTe. If MIS (metal-insulator-semiconductor) structures with low interface state densities are obtained for the II-VI compounds, band bending similar to the p-n junction can be realized by applying a voltage to the metal plate of the MIS structure. With Si and GaAs, solar cells[l] and light emitting diodes[2] based on this idea have been already realized, respectively. The solar cells are called induced junction type[l], and grid electrodes between the insulator and the semiconductor collect the photocurrent. Thin-film transistors of the MIS type using semi-insulating CdS or CdSe have been reported[3,4]. However, the electron accumulation layers are utilized as the channels, that is, the semiconductor bands are bent downward at the insulator-semiconductor interface. In this note we report for the first time strong upward band bending in thin-flhn MIS structures, in which semiconducting CdS thin films am used as the semiconductor, and discuss the interface state density. An internal photovoltage due to the band bending was observed under illumination. The MIS structures were composed as Al-CdS-A&O,-AI glass. The AllO3 lihns were evaporated from sintered pellets by electron beam bombardment in a diffusion-pumped
-10
-5
“oL~AOoE (V)
5
10
Fig. 1. An example of high-frequency C-I’ characteristics. The frequency is 1 MHz and the voltage sweep rate is 0.1 V/set. The insert shows the fast-interface-state density determined by the Terman method for the upper half of the band gap.
IN
1985)
vacuum system. The thickness was typically 13OOA. The CdS films were evaporated onto the A&O, films at 200°C to a thickness of about 1 pm from a Ta boat in a different vacuum system from the insulator tllm deposition. The base pressure and the deposition rate were mcstly 5 x 10m6Torr and 204OA/min, respectively. The donor concentrations Nd were determined from the slope of l/C2 - V characteristics at 1 MHz in the structure of Al-Cd,%AIrOr (50-100 &Au glass. This structure was rectifying, but the structure of Al-Cdl%Au glass, that is, the structure without the very thin Also, film was rarely rectifying. The values of Nd were rather large and varied in the range of 0.4-10 x 10i7/cm3, depending on the evaporation rate and the vacuum pressure. These N,, values were used in analysing C-V characteristics of the MIS structures. Al dots with an area of 0.025 cm2 were evaporated onto the CdS films as the ohmic electrodes. Figure 1 shows an example of MIS C-V characteristics measured at a frequency of 1 MHz and at a voltage sweep rate of O.l/V/sec. The calculated high-frequency C-Fcurve is also drawn in the figure. The experimental curve shows a large change in capacitance toward the calculated inversion capacitance, but it does not saturate. Inversion cannot be observed due to the low thermal generation rate of minority carriers in the wide gap semiconductor CdS, that is, around the inversion capacitance the system is under “deep depletion”. These large capacitance. changes suggest strong upward band bending under the negative bias. The magnitudes of capacitance changes in different samples were well consistent with their different Nd values, that is, it was larger in a sample with a lower donor concentration. (Compare Fig. 1 with Fig. 2.) The fast interface state density was determined from Fig. 1 by the Terman method[5] (highfrequency capacitance method), approximating that the film was a homogeneous material as a single crystal although the evaporated films on glass substrates were polycrystalline. The results for the upper half of the band gap are shown in the insert. The low density, less than 1 x lO”/cm*eV, was obtained in the midgap. The value of 1 x lO”/cn?eV is almost a lower limit of measurement by the Terman method. In Figure 2 is shown a capacitance increase under illumination. The insert shows possible photoexcitation processes which cause the capacitance increase. Electron-hole pairs which are photogenerated via direct band-to-band transitions and via deep traps in the depletion layer are separated by the band bending. The electrons am driven to the neutral region and the holes are stored at the semiconductor-insulator interface This redistribution of
Notes
Zl -10
I
I
-5
“O‘T&
( v 1
5
10
Fig. 2. Influence of illumination on the C-V characteristics. The frequency is 1 MHz and the voltage sweep rate is 0.1 V/xc. The insert shows possible photoexcitation processes which cause the capacitance increase. free carriers produces an internal photovoltage which cannot be measured directly because of the insulating layer. The photovoltage reduces the depletion layer width and leads to the capacitance increase. Also, a part of traps in the depletion layer are emptied by the light excitation and the resulting space charge contributes to the capacitance increase. (This phenomenon is the same as so-called photocapacitance[6] in Schottky junctions and p-n junctions.) Interface traps may contribute. However, since polycrystalline thin tihns generally contain many traps in the grain boundary region, the major contribution may possibly be due to bulk traps. The two origins of the capacitance increase were distinguished in our samples through the following experimental facts: The capacitance increase was composed of two parts, A and B. After the light was switched off part A did not change but part B attenuated gradually. The magnitude of part A did not vary with the light intensity but that of part B increased with the light intensity. Part A corresponds to the photocapacitance because the emptied traps in the depletion layer cannot capture electrons after the light is switched off, and the number of emptied traps is not determined by the light intensity but by the differences in the optical ionization cross sections for electrons and holes. Part B is due to the internal
FREQUENCY DEPENDENCE
995
photovoltage. The photovoltage attenuates after the light is switched off because the leakage current through the insulator carries away the stored holes at the semiconductor-insulator interface. This leakage should be reduced for better MIS operations. The internal photovoltage observed as a capacitance increase is a direct evidence of the upward band bending at the interface. If the photovoltage can be passed to the grid electrodes between the insulator and the semiconductor, thin-film solar cells of induced junction type will be realized. In summary, we have fabricated CdS thin-lilm MIS structures by vacuum evaporation. It was found that the bands at the surface could be bent strongly upward and that the fast interface state density was low, although the semiconductor was polycrystalline thin films. The observed internal photovoltage is an evidence of the upward band bending and also presents a possibility for the application of this structure to thin-film solar cells of induced junction type. Acknowledgements-We wish to thank T. Ohhata and M. Yoshimi for their assistance. A part of this work was supported financially by the Iwatani Naoji Foundation’s Research Grant. Department of Electrical Engineering Ju@rn SAWE KEUCHIFUJU Technical College Kyoto Institute of Technology YOSHITAKA WT.40 YUTAKA YOWCAWA Matsugasaki, Kyoto 606, Japan REFERENCES 1. T. Fuyuki, S. Moriuchi and H. Matsunami, Tech. Digest 1st Znt. Photovoltaic Science and Engng Conf. Kobe (Edited by Y. Hamakawa), p. 763. Japan Convention Services, Tokyo (1984). 2. C. N. Berglund, Appl. Phys. L&t. 9, 44 (1966). 3. For example, M. G. Miksic, E. S. Schlig and R. R. Haering, Solid-St. Electron. 7, 39 (1964). 4. For example, F. V. Shallcross, Proc. IEEE. 51, 851 (1963). 5. L. M. Terman, Solid-St. Electron. 5, 285 (1962). 6. For example, T. Takebe, H. Gno, J. Saraie and T. Tanaka, Phys. Stat. Sol. (a) 66, 725 (1981).
OF MOS CAPACITANCE IN STRONG
INVERSION AND AT ELEVATED TEMPERATURES (Received 21 October 1985; in revised form 9 December 1985) It is well established[l] that the steady-state CV characteristics of the MOS capacitor in strong inversion are governed by minority-carrier response. The sources of minority carriers are due mainly to (a) generation and recombination of carriers at bulk traps in the depletion region, the “generation-recombination” mechanism, and (b) generation and recombination of carriers at the back contact followed by diffusion through the neutral region, the “diffusion” mechanism. In strong inversion, the contribution from interface states are insignificant because those states in the vicinity of the Fermi level are too far away from the midgap to be effective. At room temperature, minority carriers from bulk traps in the depletion region dominate the response; at some elevated temperature, carriers generated at the back contact and transported to the inversion region become dominant. The exact temperature at which transition from generation-recombination dominance to diffusion dominance depends on bulk trap density and doping concentration.
A simplified equivalent circuit for strong inversion, incorporating both mechanisms is reproduced[l] in Fig. 1, where C, and C, are the oxide and depletion capacitances, CO,
cd
0
Ggr T ii 1
Gd
1
1
1
Fig. 1. Equivalent circuit for an MOS capacitor in inversion. Applied frequency is assumed to be sutIiciently high so that the inversion capacitance can be neglected.