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New a-Si:H two-terminal switching device for active display
]OURNA
ELSEVIER
L OF
Journal of Non-CrystallineSolids 198-200 (1996) 1134-1136
New a-Si:H two-terminal switching device for active display Domenico Caputo, Giampiero de Cesare * Department of Electronic Engineering, Unit,ersi~ of Rome 'La Sapienza ', via Eudossiana 18, 00184 Rome, Italy
Abstract
This work presents a new two-terminal switching amorphous silicon (a-Si:H) device for active flat panel display. The device is a multilayer stacked structure n - i - S p - i - n . The electrical characteristics of the diode are strongly determined by its geometry and by the doping level of the Sp layer. The current-voltage data show a symmetrical behaviour with an OFF current of 10- j j A and an ON/OFF current ratio of six orders of magnitude calculated at applied voltage of 3 and 1 V. An active matrix based on this device has been realized by using only two photolithographic masks.
1. Introduction
The use of a switching element in each pixel of active matrix displays (AMD) reduces the crosstalk problem [1]. Manufacturers are making a lot of effort to produce large area flat panel displays using thinfilm transistors (TFTs) based upon hydrogenated amorphous silicon (a-Si:H) [2]. However, many problems have still to be solved in TFT technology, as the complex fabrication process requires a large number of masks (from six to eight). An alternative approach to A M D fabrication is based on two-terminal switching devices with non-linear current-voltage characteristics, such as metal-insulator-metal (MIM) diodes [3], a diode ring [4] or back-to-back diodes [5]. The MIM diode is easy to realize using T a 2 0 5 or a-SiC thin layers but a serious problem is the large stray capacitance parallel to the device. In order to achieve large area homogeneity and reproducibility thick oxide layers are required. This fact leads to an increase of the switch operating voltage.
* Corresponding author. Tel: +39-6 4458 5431; fax: +39-6 474 2647; e-mail: [email protected].
The structures of the diode ring and back-to-back configurations consist of diodes connected respectively in parallel or in series with opposite polarities. The current-voltage ( / - V ) curves of the diode ring are made up of the forward characteristics which is based on the built-in potential of a p - i - n junction. The structure requires four masks to connect in parallel the two series of the diodes, and the quality factor increases (the device performance decreases) with the number of the stacked diodes. In the backto-back configuration the I - V behavior is based on the breakdown phenomenon of a junction. Due to the low reliability of the breakdown phenomenon the back-to-back approach has not been successful. In this paper we present a new device where the current through the n - i - S p - i - n structure arises from thermionic emission of electrons over the triangular barrier lowered by the applied voltage. The electrical behavior has been investigated pointing out the role of the central gp layer which governs either the OFF current level or the quality factor r / o f the diode. The best result are obtained when 8p is thinner than the Debye length in order to be completely depleted under all working conditions.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0022- 3 093(96)00063-4
D. Caputo, G. de Cesare / Journal of Non-Crystalline Solids 198-200 (1996) 1134-1136
1135
2. Device operation The basic structure of the device is reported in Fig. 1. It consists of two diodes n - i - ~ p and ~ p - i - n connected in series as in the 'back-to-back' configuration. A simple description of the operation of the device can be obtained from electrostatic consideration, since the structure can be modeled as a three plate capacitor as shown in Fig, 2a in equilibrium condition. The plates are the three doped layers, where the density of fixed charge, due to ionised dopant atoms and trapped carriers, is extremely high. Charge trapped in the intrinsic layers can be neglected since in this zone the defect density is orders of magnitude lower than in doped layers. Changes in the height of the triangular barrier @~ are expected [6] when a bias voltage is applied to the device (see Fig. 2b). In particular the voltage applied induces an electric field (E) which sums with the same or opposite sign to the pre-existing field in the reverse or forward biased diode respectively. The lowering of E lowers the barrier height from @b to @~, and drives the forward biased junction towards conduction. As a consequence current in the device increases exponentially. The high electric field in the reverse biased diode ensures a complete carrier collection. By inverting the sign of the applied voltage the bias of the two diodes inverts. The I - V characteristic shows a symmetrical behaviour because of the symmetry of the structure. For correct switch operation the device has to present a low current in the OFF condition and a high current in the ON condition. The key parameters in device design are the OFF current and the quality factor T/. In order to obtain a low B it is necessary to reduce recombination in the p layer,
Ec
Ev
n
n electron flow
_.=_
|
-~-
Fig. 2. Energy-band diagrams for the n - i - 6 p - i - n without (a) and with (b) applied voltage.
structure
while to obtain a low OFF current a high triangular barrier between the structure is needed. The first aim is achieved by using a material with low defect density which in doped amorphous films means using a low dopant concentration [7]. The lowest recombination is obtained when the p layer is completely depleted. At constant dopant concentration the thinner the layer the easier is the complete depletion. On the other hand the p layer should be thick a n d / o r doped enough to balance the charge in the n-layers, otherwise a decrease of built-in potential occurs. These considerations lead to a trade-off in doping and thickness of the p-doped layer.
~5000A (AD
iiiii::ii::iii:i.::ii:iii:iiiiiiii:i:ii::i!ii+- 600.~, (n-type) ~- 550-~ (intrinsic) I00~, (p-type) • - 550~, (intrinsic)
i~- 600A(n-tyfe) GLASS
Fig. 1. Structure of the a-Si:H switching device.
3. Device fabrication and characterization Devices were grown by plasma enhanced chemical vapour deposition (PECVD) on transparent conductive oxide (TCO), in a three chamber system, with the following recipe: n l a y e r s - gas flow pure S i l l 4 : 4 0 sccm; silane diluted P H 3 : 1 0 sccm; pressure: 300 mTorr; Tdep: 205°C; power density: 0 . 0 4 W / c r u Z ; timedep: 180 s.
1136
D. Caputo, G. de Cesare /Journal of Non-Crystalline Solids 198-200 (1996) 1134-1136
i l a y e r s - gas flow pure S i l l 4 : 4 0 sccm; pressure: 680 mTorr; Tjep: 200°C; power density: 0.03 W / c m 2 ; tim%ep: 210 s. ~ p l a y e r - gas flow pure S i l l 4 : 4 0 sccm; helium diluted B 2 H 6 : 0 . 5 sccm; pressure: 500 mTorr; Td~p: 200°C; power density: 0.04 W / c m 2 ; timed~p: 20 s. Finally a 500 n m aluminum layer was vacuum evaporated onto the n-type layer in order to form the back contact. The estimated thicknesses of the layers are reported in Fig. 1. All devices were insulated using a mesa process consisting of wet etching photolithography of a-Si:H ( H F : H N O 3 : C H 3 C O O H = 1:2:2) followed by a passivation step in H 2 0 2. Without passivation a large number of devices appeared shunted or noisy. In Fig. 3 a current-voltage characteristic, performed by a source measurement unit (Keithley 236) is shown for a device with an active area of 36 X 10 -4 cm 2. The curve exhibits symmetrical behaviour with an O N / O F F current ratio calculated, at an applied voltage of 3 and 1 V respectively, of six orders of magnitude, with a non-linearity factor ~7 = 3.7. The value of the current at 0 V was in the range 10-12_10-13 A.
The n - i - S p - i - n device is easy to fabricate: example of an active matrix of 40 X 40 pixels realized in our laboratory is shown in Fig. 4. The pixel size is 630 X 630 t~m. The area of the switching device between the TCO film and the aluminum back contact is 150 X 60 Ixm. The matrix has been fabricated
Fig. 4. Particularof an active matrix of 40 X40 pixels. The pixel size is 630 X630 ixm. The area of the switching device between the TCO film and the aluminumback contact is 150X60 Ixm. with only two photolithographic masks: one for the patterning of the TCO and the second for the simultaneous lithography of the a-Si:H and metal films.
4. Conclusion We realized an active matrix suitable for flat panel display by using only two photolithographic masks. This has been possible due to a new two terminal switching amorphous silicon device. It consists of a n - i - 6 p - i - n structure showing a symmetrical I - V characteristic. A quality factor ~7 = 3.7 and an OFF current less then 10 11 A for a 6 x 6 m m device have been achieved. The easy fabrication process and the excellent O N / O F F ratio make this device very appealing for active matrix application.
-2
<
-4
References
-6
!-8 O ~2 -10 -12 -4
-3
-2
-1
0
1
2
3
4
Voltage (V) Fig. 3. Measured current-voltage characteristic for a 6 X6 mm device. The line is drawn as a guide for the eye.
[1] E. Kaneko, in: Liquid Crystal TV Display (KTK Scientific Publishers, Tokyo, 1987) p. 175. [2] N. Ibaraki, Mater. Res. Soc. Symp. Proc. 336 (1994) 749. [3] R. Vincenzoni, G. Leo and F. Galluzzi, Diamond and Rel. Mater. 3 (1994) 874. [4] K.H. Nicholas, et al., SID Proc. 32/4 (1991) 395. [5] J. Dresner, Appl. Phys. Lett. 48 (1986) 1006. [6] G. Masini, G. de Cesare and F. Palma, J. Appl. Phys. 77 (1995) 1133. [7] R.A. Street, CambridgeSolid State Science Series (1991) 135.
ELSEVIER
L OF
Journal of Non-CrystallineSolids 198-200 (1996) 1134-1136
New a-Si:H two-terminal switching device for active display Domenico Caputo, Giampiero de Cesare * Department of Electronic Engineering, Unit,ersi~ of Rome 'La Sapienza ', via Eudossiana 18, 00184 Rome, Italy
Abstract
This work presents a new two-terminal switching amorphous silicon (a-Si:H) device for active flat panel display. The device is a multilayer stacked structure n - i - S p - i - n . The electrical characteristics of the diode are strongly determined by its geometry and by the doping level of the Sp layer. The current-voltage data show a symmetrical behaviour with an OFF current of 10- j j A and an ON/OFF current ratio of six orders of magnitude calculated at applied voltage of 3 and 1 V. An active matrix based on this device has been realized by using only two photolithographic masks.
1. Introduction
The use of a switching element in each pixel of active matrix displays (AMD) reduces the crosstalk problem [1]. Manufacturers are making a lot of effort to produce large area flat panel displays using thinfilm transistors (TFTs) based upon hydrogenated amorphous silicon (a-Si:H) [2]. However, many problems have still to be solved in TFT technology, as the complex fabrication process requires a large number of masks (from six to eight). An alternative approach to A M D fabrication is based on two-terminal switching devices with non-linear current-voltage characteristics, such as metal-insulator-metal (MIM) diodes [3], a diode ring [4] or back-to-back diodes [5]. The MIM diode is easy to realize using T a 2 0 5 or a-SiC thin layers but a serious problem is the large stray capacitance parallel to the device. In order to achieve large area homogeneity and reproducibility thick oxide layers are required. This fact leads to an increase of the switch operating voltage.
* Corresponding author. Tel: +39-6 4458 5431; fax: +39-6 474 2647; e-mail: [email protected].
The structures of the diode ring and back-to-back configurations consist of diodes connected respectively in parallel or in series with opposite polarities. The current-voltage ( / - V ) curves of the diode ring are made up of the forward characteristics which is based on the built-in potential of a p - i - n junction. The structure requires four masks to connect in parallel the two series of the diodes, and the quality factor increases (the device performance decreases) with the number of the stacked diodes. In the backto-back configuration the I - V behavior is based on the breakdown phenomenon of a junction. Due to the low reliability of the breakdown phenomenon the back-to-back approach has not been successful. In this paper we present a new device where the current through the n - i - S p - i - n structure arises from thermionic emission of electrons over the triangular barrier lowered by the applied voltage. The electrical behavior has been investigated pointing out the role of the central gp layer which governs either the OFF current level or the quality factor r / o f the diode. The best result are obtained when 8p is thinner than the Debye length in order to be completely depleted under all working conditions.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0022- 3 093(96)00063-4
D. Caputo, G. de Cesare / Journal of Non-Crystalline Solids 198-200 (1996) 1134-1136
1135
2. Device operation The basic structure of the device is reported in Fig. 1. It consists of two diodes n - i - ~ p and ~ p - i - n connected in series as in the 'back-to-back' configuration. A simple description of the operation of the device can be obtained from electrostatic consideration, since the structure can be modeled as a three plate capacitor as shown in Fig, 2a in equilibrium condition. The plates are the three doped layers, where the density of fixed charge, due to ionised dopant atoms and trapped carriers, is extremely high. Charge trapped in the intrinsic layers can be neglected since in this zone the defect density is orders of magnitude lower than in doped layers. Changes in the height of the triangular barrier @~ are expected [6] when a bias voltage is applied to the device (see Fig. 2b). In particular the voltage applied induces an electric field (E) which sums with the same or opposite sign to the pre-existing field in the reverse or forward biased diode respectively. The lowering of E lowers the barrier height from @b to @~, and drives the forward biased junction towards conduction. As a consequence current in the device increases exponentially. The high electric field in the reverse biased diode ensures a complete carrier collection. By inverting the sign of the applied voltage the bias of the two diodes inverts. The I - V characteristic shows a symmetrical behaviour because of the symmetry of the structure. For correct switch operation the device has to present a low current in the OFF condition and a high current in the ON condition. The key parameters in device design are the OFF current and the quality factor T/. In order to obtain a low B it is necessary to reduce recombination in the p layer,
Ec
Ev
n
n electron flow
_.=_
|
-~-
Fig. 2. Energy-band diagrams for the n - i - 6 p - i - n without (a) and with (b) applied voltage.
structure
while to obtain a low OFF current a high triangular barrier between the structure is needed. The first aim is achieved by using a material with low defect density which in doped amorphous films means using a low dopant concentration [7]. The lowest recombination is obtained when the p layer is completely depleted. At constant dopant concentration the thinner the layer the easier is the complete depletion. On the other hand the p layer should be thick a n d / o r doped enough to balance the charge in the n-layers, otherwise a decrease of built-in potential occurs. These considerations lead to a trade-off in doping and thickness of the p-doped layer.
~5000A (AD
iiiii::ii::iii:i.::ii:iii:iiiiiiii:i:ii::i!ii+- 600.~, (n-type) ~- 550-~ (intrinsic) I00~, (p-type) • - 550~, (intrinsic)
i~- 600A(n-tyfe) GLASS
Fig. 1. Structure of the a-Si:H switching device.
3. Device fabrication and characterization Devices were grown by plasma enhanced chemical vapour deposition (PECVD) on transparent conductive oxide (TCO), in a three chamber system, with the following recipe: n l a y e r s - gas flow pure S i l l 4 : 4 0 sccm; silane diluted P H 3 : 1 0 sccm; pressure: 300 mTorr; Tdep: 205°C; power density: 0 . 0 4 W / c r u Z ; timedep: 180 s.
1136
D. Caputo, G. de Cesare /Journal of Non-Crystalline Solids 198-200 (1996) 1134-1136
i l a y e r s - gas flow pure S i l l 4 : 4 0 sccm; pressure: 680 mTorr; Tjep: 200°C; power density: 0.03 W / c m 2 ; tim%ep: 210 s. ~ p l a y e r - gas flow pure S i l l 4 : 4 0 sccm; helium diluted B 2 H 6 : 0 . 5 sccm; pressure: 500 mTorr; Td~p: 200°C; power density: 0.04 W / c m 2 ; timed~p: 20 s. Finally a 500 n m aluminum layer was vacuum evaporated onto the n-type layer in order to form the back contact. The estimated thicknesses of the layers are reported in Fig. 1. All devices were insulated using a mesa process consisting of wet etching photolithography of a-Si:H ( H F : H N O 3 : C H 3 C O O H = 1:2:2) followed by a passivation step in H 2 0 2. Without passivation a large number of devices appeared shunted or noisy. In Fig. 3 a current-voltage characteristic, performed by a source measurement unit (Keithley 236) is shown for a device with an active area of 36 X 10 -4 cm 2. The curve exhibits symmetrical behaviour with an O N / O F F current ratio calculated, at an applied voltage of 3 and 1 V respectively, of six orders of magnitude, with a non-linearity factor ~7 = 3.7. The value of the current at 0 V was in the range 10-12_10-13 A.
The n - i - S p - i - n device is easy to fabricate: example of an active matrix of 40 X 40 pixels realized in our laboratory is shown in Fig. 4. The pixel size is 630 X 630 t~m. The area of the switching device between the TCO film and the aluminum back contact is 150 X 60 Ixm. The matrix has been fabricated
Fig. 4. Particularof an active matrix of 40 X40 pixels. The pixel size is 630 X630 ixm. The area of the switching device between the TCO film and the aluminumback contact is 150X60 Ixm. with only two photolithographic masks: one for the patterning of the TCO and the second for the simultaneous lithography of the a-Si:H and metal films.
4. Conclusion We realized an active matrix suitable for flat panel display by using only two photolithographic masks. This has been possible due to a new two terminal switching amorphous silicon device. It consists of a n - i - 6 p - i - n structure showing a symmetrical I - V characteristic. A quality factor ~7 = 3.7 and an OFF current less then 10 11 A for a 6 x 6 m m device have been achieved. The easy fabrication process and the excellent O N / O F F ratio make this device very appealing for active matrix application.
-2
<
-4
References
-6
!-8 O ~2 -10 -12 -4
-3
-2
-1
0
1
2
3
4
Voltage (V) Fig. 3. Measured current-voltage characteristic for a 6 X6 mm device. The line is drawn as a guide for the eye.
[1] E. Kaneko, in: Liquid Crystal TV Display (KTK Scientific Publishers, Tokyo, 1987) p. 175. [2] N. Ibaraki, Mater. Res. Soc. Symp. Proc. 336 (1994) 749. [3] R. Vincenzoni, G. Leo and F. Galluzzi, Diamond and Rel. Mater. 3 (1994) 874. [4] K.H. Nicholas, et al., SID Proc. 32/4 (1991) 395. [5] J. Dresner, Appl. Phys. Lett. 48 (1986) 1006. [6] G. Masini, G. de Cesare and F. Palma, J. Appl. Phys. 77 (1995) 1133. [7] R.A. Street, CambridgeSolid State Science Series (1991) 135.