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Modulation of threshold voltages in bidirectional a-Si:H switching devices
Journal of Non-Crystalline Solids 227–230 Ž1998. 1192–1195
Modulation of threshold voltages in bidirectional a-Si:H switching devices Domenico Caputo ) , Giampiero de Cesare Department of Electronic Engineering, Õia Eudossiana 18, 00184 Roma, Italy
Abstract The design and production of a two-terminal switching device with tunable threshold voltage are presented. The device consists of an nq–i–py–i–nq amorphous silicon stacked structure. The operation modes and the effect of the structure parameters on the threshold voltage are investigated with a numerical device model. Manufactured devices produced with varying the thickness of the different layers of the structure present threshold voltages ranging from 1 to 30 V. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon; Switching devices; Active matrix
1. Introduction The integration of a switching element in each pixel of active matrix displays ŽAMD. is a wellknown methodology to reduce crosstalk w1x. Amorphous-silicon-based devices are mainly used in this large area technology. Depending on the type of AMD and on the driving circuits, different threshold voltages are required. Two different approaches are used: one, based on three-terminal elements with an independent control gate such as thin-film transistors ŽTFTs., has supremacy in AMD fabrication due to the very good ratio between ON and OFF characteristics w2x. However, many problems are still to be solved in TFT technology, as the complex fabrication process requires many masks. The second ap-
proach uses a two-terminal swithcing device technology based on nonlinear elements. The applied bias voltage determines the ONrOFF condition. MIM devices are used in this case. As an example, recently the Philips group w3x reported an amorphous silicon nitride diode for the active matrix addressing of reflective display. An alternative two terminal device based on amorphous silicon has been presented in Ref. w4x. Here we report on an improvement by which we easily achieve devices with tunable threshold voltages. This versatility makes the switch suitable for several large area applications.
2. Structure and device operation
)
Corresponding author. Fax: q39-6 474 2647; e-mail: [email protected].
In Fig. 1 a sketch of the physical structure is reported. It consists of two junctions nq–i–py and py–i–nq connected in series as in the ‘back-to-back’
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 9 6 - 8
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 1. Structure of the device.
configuration. The device can operate in two different conditions, depending on the state of the central doped layer Žthe base of the device.. In Fig. 2a the band diagram under thermal equilibrium is shown. If the base is not completely depleted the device behaves as a back-to-back diode with an OFF current equal to the reverse saturation current of a p–i–n diode and the device is in its blocking state. Increasing the applied voltage the depth of the depletion region of the reverse biased junction increases ŽFig. 2b.. We define Vth as the threshold voltage at which the whole py layer becomes completely depleted. The device works now in a different operating condi-
Fig. 2. Energy band diagrams for the structure in thermal equilibrium Ža., at applied voltage less than the threshold voltage Žb., and after depletion of the central doped layer Žc..
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tion. As evident from the band diagram reported in Fig. 2c, at V ) Vth the applied voltage lowers the barrier height from F to F X and drives the forward biased junction towards conduction. The current through the structure arises from the thermionic emission of electrons over the triangular barrier and the current increases exponentially with voltage. By inverting the sign of the applied voltage the bias of the two diodes inverts and thus the device presents bidirectional characteristics. Since Vth is related to the depletion of the base, it mainly depends on the doping and thickness of the py layer and on the thickness of the intrinsic layers, as will be shown below in detail. An appropriate design of the device allows changing the threshold voltages to achieve both symmetrical and asymmetrical current vs. voltage Ž I–V . dependence.
3. Device modeling The device operation is now analysed by using a numerical model which solves the semiconductor equations based on a finite element method w5x. Following a well-known technique we assumed a one-dimensional structure divided in a grid of points. Each point has a set of parameters including free carrier mobility, energy gap, density of states distribution and capture cross sections. Dopant states, band tails, and deep states are taken into account in the density of states ŽDOS. distribution w6x. We made a parametric analysis to show which parameters affect Vth . In particular, in Figs. 3 and 4 we report I–V characteristics of symmetric devices as a function of the thickness and doping of the base, respectively. Numerical simulations confirm the qualitative description given in the previous section. In fact, since the transition from the blocking state to the ON condition occurs when the base is completely depleted, an increase of the thickness andror doping of this layer leads to an increase of the voltage at which the current increases exponentially Ž Vth .. In the inset we report Vth as a function of the base parameters. An almost linear dependence is found on both thickness and doping. We also note that I–V curves plotted with open circles refer to devices with base depleted at the junction formation and thus with
1194
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 3. Modeled current–voltage curves at different base thicknesses. In the inset Vth is plotted against the base thickness.
Vth s 0. This effect is due to the reduced thickness in Fig. 3 and to the low doping in Fig. 4, respectively. Fig. 5 reports the modeled I–V curve of the symmetric structure at different thicknesses of the intrinsic layers. The observed dependence of Vth is explained in terms of electrostatic effects. In fact, from Poisson’s equation an increase of the i-layer thickness causes a decrease of the depletion region in the base and thus an increase of Vth . This effect is evident in the inset where Vth as a function of the i-layer thickness is plotted. We note that the quality factors of the different curves do not change because they depend on the recombination occurring in the
Fig. 5. Modeled current–voltage curves at different intrinsic thicknesses. Crosses, solid squares, open circles, solid circle and triangles refer to 45, 60, 70, 85 and 100 nm, respectively. In the inset Vth is plotted against the intrinsic thickness. Lines are drawn as guides for the eyes.
base w6x whose parameters have not been varied in these simulations. In Figs. 3–5 we plot only the positive branch of the I–V curves because of the symmetry of the structure and thus of the simulation results. However, an interesting feature of our device is outlighted if we model I–V characteristics of an asymmetrycal structure with different thicknesses of the two intrinsic layers. In this case, the whole I–V curve is asymmetrical with two different threshold voltages. Each Vth depends on the corresponding intrinsic layer thickness. Thus the possibility to control independently each branch of the I–V curve is achieved. Finally, we would note that all the simulations present a saturation of the I–V characteristic due to space-charge limited current w7x.
4. Experimental results
Fig. 4. Modeled current–voltage curves at different base doping ŽNa.. Open circles, squares and solid circles refer to Na s 2=10 17 , Na s 5=10 17 Na s1=10 18 cmy3 , respectively. In the inset Vth is plotted against the base doping. Lines are drawn as guides for the eyes.
Devices were grown by plasma enhanced chemical vapor deposition ŽPECVD. on transparent conductive oxide ŽTCO., in a three-chamber system. A 5000-nm aluminum layer was vacuum evaporated onto the n-type layer in order to form the back contact. All devices were insulated using a mesa process consisting of wet etching photolithography of a-Si:H ŽHF:HNO 3 :CH 3 COOHs 1:2:2. followed
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 6. Measured I – V characteristics for three devices with different base thicknesses. The other deposition parameters are kept constant. Lines are drawn as guides for the eyes.
by a passivation step in H 2 O 2 . Without passivation a large number of devices were shunted or noisy. The analysis we made in the previous section concerned parameters easily controlled in the experiment. In Fig. 6 measured I–V curves of three symmetrical devices with different thickness of the base are presented. Details of deposition parameters are reported in Ref. w4x. Curves Ža., Žb., and Žc. refer to devices with expected base thickness equal to 6.5, 30 and 60 nm, respectively. As predicted by the model, the larger the thickness, the higher the threshold voltage. Comparing experimental results with Fig. 3, we see that for the 6.5-nm device current increases always exponentially showing Vth s 0 as in the curve with the lowest thickness in the simulation. We now note that in Fig. 6 devices with Vth greater than zero show asymmetrical I–V characteristics. This undesired result can be due to different causes, as the nonexact symmetry of the structure since the bottom and the front electrode differ or to problems induced by the etching process. Work is in progress to overcome this problem. A further confirmation of the predictions of the model comes from Fig. 7 which reports the measured I–V characteristics for three symmetrical structures with different thicknesses of intrinsic layer. Curves Ža., Žb. and Žc. refer to devices with i-layer thicknesses equal to 75, 140, and 210 nm, respectively.
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Fig. 7. Measured I – V characteristics for three devices with different intrinsic thickness. The other deposition parameters are kept constant. Lines are drawn as guides for the eyes.
As expected, threshold voltage increases cuncurrently with thickness. 5. Conclusions Modulation of threshold voltage of an amorphous silicon two-terminal switching device has been investigated, focusing on different sructure parameters. Simulations have outlighted the high sensitivity of the device to the thickness and doping of central layers. Model predictions are confirmed by manufactured devices showing threshold voltages ranging from 1 to 30 V. References w1x E. Kaneko, Liquid Crystal TV Display, KTK Scientific Publishers, 1987. w2x N. Ibaraki, Mat. Res. Symp. Proc. 336 Ž1994. 749, and references therein. w3x M.G. Pitt, N.A.J.M. van Aerle, F. Leehouts, J.M. Havekes, H.F. van Rooijen, H.W.M. Schell, F.C. van de Ven, SID Int. Symp. Digest XXVIII Ž1997. 473. w4x D. Caputo, G. de Cesare, J. Non-Cryst. Solids 199 Ž1996. 1134. w5x J.K. Arch, F.A. Rubinelli, J.Y. Hou, S.J. Fonash, J. Appl. Phys. 69 Ž1991. 7057. w6x D. Caputo, G. de Cesare, IEEE Trans. Electron Devices 43 Ž1996. 2109. w7x S.M. Sze, Physics of Semiconductor Devices, Wiley, 1981, p. 402.
Modulation of threshold voltages in bidirectional a-Si:H switching devices Domenico Caputo ) , Giampiero de Cesare Department of Electronic Engineering, Õia Eudossiana 18, 00184 Roma, Italy
Abstract The design and production of a two-terminal switching device with tunable threshold voltage are presented. The device consists of an nq–i–py–i–nq amorphous silicon stacked structure. The operation modes and the effect of the structure parameters on the threshold voltage are investigated with a numerical device model. Manufactured devices produced with varying the thickness of the different layers of the structure present threshold voltages ranging from 1 to 30 V. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous silicon; Switching devices; Active matrix
1. Introduction The integration of a switching element in each pixel of active matrix displays ŽAMD. is a wellknown methodology to reduce crosstalk w1x. Amorphous-silicon-based devices are mainly used in this large area technology. Depending on the type of AMD and on the driving circuits, different threshold voltages are required. Two different approaches are used: one, based on three-terminal elements with an independent control gate such as thin-film transistors ŽTFTs., has supremacy in AMD fabrication due to the very good ratio between ON and OFF characteristics w2x. However, many problems are still to be solved in TFT technology, as the complex fabrication process requires many masks. The second ap-
proach uses a two-terminal swithcing device technology based on nonlinear elements. The applied bias voltage determines the ONrOFF condition. MIM devices are used in this case. As an example, recently the Philips group w3x reported an amorphous silicon nitride diode for the active matrix addressing of reflective display. An alternative two terminal device based on amorphous silicon has been presented in Ref. w4x. Here we report on an improvement by which we easily achieve devices with tunable threshold voltages. This versatility makes the switch suitable for several large area applications.
2. Structure and device operation
)
Corresponding author. Fax: q39-6 474 2647; e-mail: [email protected].
In Fig. 1 a sketch of the physical structure is reported. It consists of two junctions nq–i–py and py–i–nq connected in series as in the ‘back-to-back’
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 9 6 - 8
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 1. Structure of the device.
configuration. The device can operate in two different conditions, depending on the state of the central doped layer Žthe base of the device.. In Fig. 2a the band diagram under thermal equilibrium is shown. If the base is not completely depleted the device behaves as a back-to-back diode with an OFF current equal to the reverse saturation current of a p–i–n diode and the device is in its blocking state. Increasing the applied voltage the depth of the depletion region of the reverse biased junction increases ŽFig. 2b.. We define Vth as the threshold voltage at which the whole py layer becomes completely depleted. The device works now in a different operating condi-
Fig. 2. Energy band diagrams for the structure in thermal equilibrium Ža., at applied voltage less than the threshold voltage Žb., and after depletion of the central doped layer Žc..
1193
tion. As evident from the band diagram reported in Fig. 2c, at V ) Vth the applied voltage lowers the barrier height from F to F X and drives the forward biased junction towards conduction. The current through the structure arises from the thermionic emission of electrons over the triangular barrier and the current increases exponentially with voltage. By inverting the sign of the applied voltage the bias of the two diodes inverts and thus the device presents bidirectional characteristics. Since Vth is related to the depletion of the base, it mainly depends on the doping and thickness of the py layer and on the thickness of the intrinsic layers, as will be shown below in detail. An appropriate design of the device allows changing the threshold voltages to achieve both symmetrical and asymmetrical current vs. voltage Ž I–V . dependence.
3. Device modeling The device operation is now analysed by using a numerical model which solves the semiconductor equations based on a finite element method w5x. Following a well-known technique we assumed a one-dimensional structure divided in a grid of points. Each point has a set of parameters including free carrier mobility, energy gap, density of states distribution and capture cross sections. Dopant states, band tails, and deep states are taken into account in the density of states ŽDOS. distribution w6x. We made a parametric analysis to show which parameters affect Vth . In particular, in Figs. 3 and 4 we report I–V characteristics of symmetric devices as a function of the thickness and doping of the base, respectively. Numerical simulations confirm the qualitative description given in the previous section. In fact, since the transition from the blocking state to the ON condition occurs when the base is completely depleted, an increase of the thickness andror doping of this layer leads to an increase of the voltage at which the current increases exponentially Ž Vth .. In the inset we report Vth as a function of the base parameters. An almost linear dependence is found on both thickness and doping. We also note that I–V curves plotted with open circles refer to devices with base depleted at the junction formation and thus with
1194
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 3. Modeled current–voltage curves at different base thicknesses. In the inset Vth is plotted against the base thickness.
Vth s 0. This effect is due to the reduced thickness in Fig. 3 and to the low doping in Fig. 4, respectively. Fig. 5 reports the modeled I–V curve of the symmetric structure at different thicknesses of the intrinsic layers. The observed dependence of Vth is explained in terms of electrostatic effects. In fact, from Poisson’s equation an increase of the i-layer thickness causes a decrease of the depletion region in the base and thus an increase of Vth . This effect is evident in the inset where Vth as a function of the i-layer thickness is plotted. We note that the quality factors of the different curves do not change because they depend on the recombination occurring in the
Fig. 5. Modeled current–voltage curves at different intrinsic thicknesses. Crosses, solid squares, open circles, solid circle and triangles refer to 45, 60, 70, 85 and 100 nm, respectively. In the inset Vth is plotted against the intrinsic thickness. Lines are drawn as guides for the eyes.
base w6x whose parameters have not been varied in these simulations. In Figs. 3–5 we plot only the positive branch of the I–V curves because of the symmetry of the structure and thus of the simulation results. However, an interesting feature of our device is outlighted if we model I–V characteristics of an asymmetrycal structure with different thicknesses of the two intrinsic layers. In this case, the whole I–V curve is asymmetrical with two different threshold voltages. Each Vth depends on the corresponding intrinsic layer thickness. Thus the possibility to control independently each branch of the I–V curve is achieved. Finally, we would note that all the simulations present a saturation of the I–V characteristic due to space-charge limited current w7x.
4. Experimental results
Fig. 4. Modeled current–voltage curves at different base doping ŽNa.. Open circles, squares and solid circles refer to Na s 2=10 17 , Na s 5=10 17 Na s1=10 18 cmy3 , respectively. In the inset Vth is plotted against the base doping. Lines are drawn as guides for the eyes.
Devices were grown by plasma enhanced chemical vapor deposition ŽPECVD. on transparent conductive oxide ŽTCO., in a three-chamber system. A 5000-nm aluminum layer was vacuum evaporated onto the n-type layer in order to form the back contact. All devices were insulated using a mesa process consisting of wet etching photolithography of a-Si:H ŽHF:HNO 3 :CH 3 COOHs 1:2:2. followed
D. Caputo, G. de Cesarer Journal of Non-Crystalline Solids 227–230 (1998) 1192–1195
Fig. 6. Measured I – V characteristics for three devices with different base thicknesses. The other deposition parameters are kept constant. Lines are drawn as guides for the eyes.
by a passivation step in H 2 O 2 . Without passivation a large number of devices were shunted or noisy. The analysis we made in the previous section concerned parameters easily controlled in the experiment. In Fig. 6 measured I–V curves of three symmetrical devices with different thickness of the base are presented. Details of deposition parameters are reported in Ref. w4x. Curves Ža., Žb., and Žc. refer to devices with expected base thickness equal to 6.5, 30 and 60 nm, respectively. As predicted by the model, the larger the thickness, the higher the threshold voltage. Comparing experimental results with Fig. 3, we see that for the 6.5-nm device current increases always exponentially showing Vth s 0 as in the curve with the lowest thickness in the simulation. We now note that in Fig. 6 devices with Vth greater than zero show asymmetrical I–V characteristics. This undesired result can be due to different causes, as the nonexact symmetry of the structure since the bottom and the front electrode differ or to problems induced by the etching process. Work is in progress to overcome this problem. A further confirmation of the predictions of the model comes from Fig. 7 which reports the measured I–V characteristics for three symmetrical structures with different thicknesses of intrinsic layer. Curves Ža., Žb. and Žc. refer to devices with i-layer thicknesses equal to 75, 140, and 210 nm, respectively.
1195
Fig. 7. Measured I – V characteristics for three devices with different intrinsic thickness. The other deposition parameters are kept constant. Lines are drawn as guides for the eyes.
As expected, threshold voltage increases cuncurrently with thickness. 5. Conclusions Modulation of threshold voltage of an amorphous silicon two-terminal switching device has been investigated, focusing on different sructure parameters. Simulations have outlighted the high sensitivity of the device to the thickness and doping of central layers. Model predictions are confirmed by manufactured devices showing threshold voltages ranging from 1 to 30 V. References w1x E. Kaneko, Liquid Crystal TV Display, KTK Scientific Publishers, 1987. w2x N. Ibaraki, Mat. Res. Symp. Proc. 336 Ž1994. 749, and references therein. w3x M.G. Pitt, N.A.J.M. van Aerle, F. Leehouts, J.M. Havekes, H.F. van Rooijen, H.W.M. Schell, F.C. van de Ven, SID Int. Symp. Digest XXVIII Ž1997. 473. w4x D. Caputo, G. de Cesare, J. Non-Cryst. Solids 199 Ž1996. 1134. w5x J.K. Arch, F.A. Rubinelli, J.Y. Hou, S.J. Fonash, J. Appl. Phys. 69 Ž1991. 7057. w6x D. Caputo, G. de Cesare, IEEE Trans. Electron Devices 43 Ž1996. 2109. w7x S.M. Sze, Physics of Semiconductor Devices, Wiley, 1981, p. 402.