Tuning of the electrical characteristics of organic bistable devices by varying the deposition rate of Alq(3) thin film

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Letter

Tuning of the electrical characteristics of organic bistable devices

by varying the deposition rate of Alq

3

thin ﬁlm

Po-Tsung Lee

a,b

, Tzu-Yueh Chang

a,b,*

, Szu-Yuan Chen

b

a

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC

b

Department of Photonics and Display Institute, National Chiao-Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC

a r t i c l e

i n f o

Article history:

Received 11 November 2007 Received in revised form 26 May 2008 Accepted 1 June 2008

Available online 14 June 2008

PACS: 73.40.Ns 73.50.Gr Keywords: Alq3

Organic bistable device

a b s t r a c t

Organic bistable devices with an Al/Alq3/n-type Si structure are investigated at different

deposition rates of Alq3thin ﬁlm. We can obtain current–voltage characteristics of these

devices similar to those of metal/organic semiconductor/metal structures that are widely used for organic bistable devices. The bistable effect of the Al/Alq3/n-type Si structure is

primarily caused by the interface defects at the Al/Alq3junction. Moreover, the electrical

properties of these devices can be modiﬁed and controlled by utilizing the appropriate deposition rates of the Alq3thin ﬁlm by thermal deposition. XPS, AFM, and GIXRD

mea-surements are performed to characterize the properties of Alq3thin ﬁlm and Alq3/Al

inter-face. This type of devices involves an extremely simple fabrication process and offers great potential in future advanced organic electronics.

1. Introduction

Due to advances in organic semiconductor materials, numerous organic conjugated materials have been exten-sively utilized in the production of electronic and opto-electronic devices[1–3]. Moreover, the demands for more accurate simulations in research and for consumer elec-tronic devices are increasing dramatically. Along with this trend, a tremendous demand for increased memory capac-ity is also evident. In order to satisfy this current demand, the capacity of Si-based memory has been augmented by scaling down its size[4]. However, scaling down will reach its limit in the near future; therefore, organic-based mem-ory [5–14] is one of the candidates of next generation

memory devices owing to the greater scope for better sca-lability offered by organic or polymeric mediums, low cost fabrication, and high mechanical ﬂexibility. Therefore, po-tential memory devices based on polymeric or organic materials have attracted rapidly growing interest and are being widely investigated.

In this work, organic bistable devices with Al/tri-(8-hydroxyquinoline) aluminum (Alq3) deposited on n-type

Si substrate are fabricated and investigated. This device shows distinct bistability with an ON/OFF current ratio over 106and a wide reading voltage range for differentiat-ing between ‘‘ON” and ‘‘OFF” states. The formation of the electrically bistable states is the result of electrons being trapped in the defects at the Schottky junction during elec-trical ﬁeld stressing. This study also provides a simple ap-proach, varying the deposition rate of the organic thin ﬁlm, using which the characteristics of electrical bistability of the device, e.g., threshold voltage, can be tuned or controlled. XPS, AFM, and GIXRD measurements are performed to help us understand the properties of Alq3 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.orgel.2008.06.003

* Corresponding author. Address: Department of Photonics and Insti-tute of Electro-Optical Engineering, National Chiao-Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC. Tel.: +886 3 5712121x59345; fax: +886 3 5735601.

E-mail address:lanceral@ms53.hinet.net(T.-Y. Chang).

Contents lists available atScienceDirect

Organic Electronics

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thin ﬁlm and Alq3/Al interface and explain the

experimen-tal results obtained. Besides, the simple structure of the reported device indicates that it can be easily embedded into the well-developed semiconductor fabrication processes.

2. Experiments

The bistable device consists of an organic layer inter-posed between two electrodes. The fabrication process of the device is described as follows. First, a 150-nm-thick Alq3organic layer is deposited on a cleaned 1Xcm

resistiv-ity n-type silicon wafer by thermal deposition method in a vacuum below 3 10 6_{Torr at room temperature. Then an}

80-nm-thick aluminum top-electrode is deposited on the organic layer through a shadow mask. The size of each Al electrode is 0.64 mm2_{. The deposition rates of the Alq}

3thin

ﬁlm are 0.05 nm/s, 0.15 nm/s, 0.2 nm/s, and 0.3 nm/s. The deposition rate is controlled by the setting temperature of the crucible and the corresponding setting temperature for each deposition rate is listed inTable 1. The current– voltage (I–V) characteristics are measured using a Hewlett Packard 4156A semiconductor parameter analyzer in an ambient environment. The capacitance–voltage (C–V) characteristics are recorded by an Agilent 4284A Precision LCR meter at a frequency of 1 MHz and amplitude of 25 mV. The surface morphology of the Alq3thin ﬁlm on

the Si wafer is obtained by using an atomic force micro-scope (AFM, DI-Veeco Instruments). The composition in the Alq3/Al and the atomic concentration of the Alq3are

analyzed using X-ray photoemission spectroscopy (XPS), while structural information is obtained via gracing inci-dence X-ray diffraction (GIXRD) analysis.

3. Results and discussion

Fig. 1shows typical I–V characteristics of the fabricated Al/Alq3/n-type Si structure. As can be seen, this device

exhibits two different conductance states at an identical applied voltage. The silicon electrode is kept at 0 V, and all bias conditions are applied on the aluminum electrode. At the ﬁrst bias (black curve inFig. 1), the voltage sweeps from 0 V to 10 V. Initially, the device exhibits low conduc-tance (OFF state). However, with an increased voltage, a transition from low conductance to high conductance (ON state) occurs at a threshold voltage of about 5 V, and then the device is maintained at a high conductance state. At the next bias (red curve inFig. 1), the device still holds at high conductance. Therefore, this device possesses the nature of bistablity. Furthermore, by applying a negative voltage form 0 V to 10 V, the device can be switched from

high conductance back to low conductance. The plot of ON/ OFF current ratio as a function of reading voltage is shown in the inset ofFig. 1. It is obvious that the device has a very wide reading voltage range with large ON/OFF current ra-tio which may reduce reading errors and increase the reli-ability of the device. For this reason, the tolerance of this device is large enough for external surrounding circuitry to adopt. The corresponding reading currents after ‘‘writ-ing” and ‘‘eras‘‘writ-ing” for the ﬁrst four cycles are shown in

Fig. 2.

At low bias of the ﬁrst bias, the current is very small cause electrons are obstructed by a barrier formed be-tween the Si substrate and Alq3. Thus, only a few

electrons can be injected into the organic active layer. Then most of them are further trapped by the defects in the bulk Alq3thin ﬁlm and at the interface of Schottky junction. As a

result, the device stays at high resistance. By applying a voltage above the threshold, the barrier can be overcome, and this enables numerous electrons to be injected into the active layer and the defects can be ﬁlled. Accordingly, electrons are transported easily into the active layer and drift unobstructedly towards the other end of the device. At the next bias, the device exhibits a resistance-like

Table 1

Alq3thin ﬁlm properties obtained from XPS measurements for different

deposition rates of Alq3thin ﬁlm under different setting temperatures

Deposition rate (nm/s) Setting temperature (°C) N (atom%) N/C

0.05 251 6.8 0.075

0.15 267 6.2 0.068

0.2 274 6 0.066

0.3 281 5.9 0.065

Fig. 1. Current–voltage characteristics of the fabricated device. The black and red curves represent writing and reading biases, respectively. The inset shows the voltage-dependent ON/OFF current ratio curve.

Fig. 2. The reading currents after ‘‘writing” and ‘‘erasing” of the reported device for the ﬁrst four cycles.

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characteristic when the reading voltage is larger than the energy barrier between Si and Alq3, that is, ohmic relation.

Thus, the ON state can be obtained for any reading voltage larger than the iso-type hetero-junction barrier between n-type silicon and Alq3, which is about 0.65 eV fromFig. 1.

The bistable characteristic of the Al/Alq3/n-type Si

structure mainly originates from the defects at the inter-face of Schottky junction.Fig. 3shows the C–V characteris-tic of the device. It can be seen from the curve that the device is kept at some capacitance value while the applied voltage is below the threshold. Then, the value changes into another lower capacitance value when the voltage ex-ceeds its threshold. The variation of capacitance could be ascribed to the defects in the bulk Alq3 thin ﬁlm and at

the interface of Schottky junction. At the initial stage of the applied voltage, few electrons are trapped by the de-fects in the low electrical field. Then, more and more elec-trons are trapped by the defects as the voltage increases. While the applied voltage is near the threshold, defects are filled sufficiently to make the device possess a metal-like property; consequently, the capacitance is converted

into a lower value. In addition, the I–V curve of the Al/Alq3/n-type Si structure with one small Al drop as the

top electrode does not exhibit bistability but rather diode behavior. This indicates that the interface property be-tween Al electrode and Alq3thin ﬁlm plays an important

role for bistability. A signiﬁcant chemical reaction occurs

Fig. 3. Capacitance–voltage characteristic of the device at a frequency of 1 MHz. The Si electrode is kept at 0 V, and the voltage on the Al electrode is swept form 5 V to 7 V.

Fig. 4. XPS curves of Al electrode and Al/Alq3interface of our reported

device.

Fig. 5. Electrical properties of the device with different deposition rates of the Alq3thin ﬁlm: (a) deposition-rate dependent threshold voltage, (b)

deposition-rate dependent ON/OFF current ratio, and (c) threshold– voltage dependent retention time. Data points shown in (a) and (b) are average values measured from our fabricated devices.

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at the interface when aluminum is thermally deposited on the Alq3thin ﬁlm[15]. The resulting product, supportively

consisting of Al–O interactions, serves as interface traps and makes carriers be poorly injected through the Schottky junction interface. For this reason, trapping charges at the interface between Alq3and Al primarily control the

switch-ing mechanism. Fig. 4 shows the XPS curves of the Al electrode and the Alq3/Al interface of the reported device,

which clearly conﬁrms the existence of Al–O compound at the Alq3/Al interface.

The electrical behavior of the device can be modiﬁed by varying the deposition rate of the organic active layer.

Fig. 5a and b shows the deposition rate effect on the threshold voltage and ON/OFF current ratio of the device: both decrease with an increase in the deposition rate of Alq3 thin ﬁlm. In addition, as can be seen inFig. 5c, the

retention time is dependent on the threshold voltage of the device. Since the threshold voltage can be tuned by adjusting the deposition rate of the organic thin ﬁlm, the retention time can be extended by reducing the deposition rate of the organic thin ﬁlm.

Previous reports on the morphology of the organic thin ﬁlm indicate that roughness decreases with the deposition rate[16,17]. That is to say, the effective surface area be-tween Alq3and Al can be adjusted by regulating the

depo-sition rate of Alq3. For that reason, a higher deposition rate

introduces a relatively small amount of defects at the

Schottky junction interface.Fig. 6shows the AFM images of the Alq3 thin ﬁlms deposited at 0.05 nm/s, 0.15 nm/s,

0.2 nm/s, and 0.3 nm/s, respectively. The corresponding surface roughness means are 0.38 nm, 0.35 nm, 0.31 nm, and 0.17 nm. These reveal that the deposition rate of the Alq3thin ﬁlm is a major factor in the adjustment of

effec-tive contact surface area between Alq3 and Al. Effective

contact surface area will affect the amount of the interface defects of the device. As a result, the relative amount of the defects at the Schottky junction interface can be modified by controlling the deposition rate of the organic thin film. Furthermore,Fig. 7shows gracing incidence X-ray diffrac-tion curves of the Alq3 thin film deposited at different

rates. It is obvious that all Alq3thin ﬁlms are with

amor-phous diffraction patterns. In other words, crystallization does not occur in all organic thin films. That is, threshold voltage, ON/OFF current ratio, and retention time are not related to crystallization quality of thin film. They are clo-sely related to the film roughness, as shown by the AFM images inFig. 6. Besides, it has been demonstrated that the atomic N/C ratio of the Alq3 thin film changes with

the deposition rate of the Alq3thin ﬁlm[16,17]. At a higher

deposition rate, that is, higher temperature condition, the Alq3molecule structure disintegrates to release

N-contain-ing species due to the decomposition energy of Alq3being

smaller than its sublimation energy. It is also shown that the Alq3 thin ﬁlm deposited at a lower deposition rate

Fig. 6. AFM images of the Alq3thin ﬁlm deposited on n-type Si wafer at four different deposition rates. Surface roughness means are 0.38 nm, 0.35 nm,

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contains a greater atomic concentration of nitrogen and a higher atomic N/C ratio. The corresponding concentrations of N and atomic N/C ratios from XPS measurements for dif-ferent deposition rates of Alq3thin ﬁlm are given inTable

1, which clearly indicates the same trend discussed above. Moreover, the electrons being injected into the Alq3thin

ﬁlm undergo a repulsive force generated by the negatively charged nitrogen atoms which is the result of the electro-negativity of a nitrogen atom being larger than that of a carbon or oxygen atom for a neutral Alq3molecule[18].

Hence, the electrons in the Alq3 thin ﬁlm with smaller

N/C ratio experience less repulsive force [17]. In other words, an increase in the deposition rate of the Alq3thin

ﬁlm can extend the hopping distance and raise hopping frequency of electrons in the Alq3thin ﬁlm.

From above discussions, two ﬁndings can be made to explain the results obtained inFig. 5a and b. First, it is obvi-ous that threshold voltage decreases with increasing depo-sition rate because of a smaller amount of defects at the Schottky junction interface at a higher deposition rate. Sec-ond, the same relationship for the ON/OFF current ratio is because a smaller amount of nitrogen atoms are available to prevent the electrons from hopping in the Alq3thin ﬁlm,

hence increase the low conductance state current and de-crease the ON/OFF current ratio at a higher deposition rate. Additionally, the distribution of defects at the interface corresponds to the trapping energy [19]. These defects can be classiﬁed roughly into two groups: low trapping en-ergy defects (Elow) and high trapping energy defects (Ehigh).

Higher deposition rate of Alq3thin ﬁlm introduces

smooth-er Alq3surface roughness, and may produce less Ehigh. A

sample with a smaller threshold voltage, resulting from higher deposition rate and smaller surface roughness of Alq3, exhibits shorter retention time probably due to less

Ehigh. The trapped electrons are more easily released from

the Elowdefects at the Schottky junction interface.

There-fore, the electrons can not be kept longer in the Elowdefects

and the device has shorter retention time, as shown in

Fig. 5c. Consequently, the deposition rate of Alq3thin ﬁlm

has a signiﬁcant effect on the electrical properties of the organic bistable devices, e.g., threshold voltage, ON/OFF current ratio, and retention time. The electrical character-istics of the device can be optimized and tuned according to our needs for different situations based on the trends obtained in these experiments. Of course some tradeoffs must be made.

4. Conclusions

In summary, the current–voltage characteristics of the organic bistable device, Al/Alq3/n-type Si, are investigated.

This bistability results from the interface defects at the Alq3/Al junction. Promising results for thermal deposition

with controllable ﬁlm quality by varying the deposition rate of Alq3thin ﬁlm are also provided. The properties of

Alq3thin ﬁlm and Alq3/Al interface are obtained by XPS,

AFM, and GIXRD measurements. Owing to the simple structure of the device, the organic electronic memory de-vice can be embedded into the conventional silicon-based fabrication processes. Furthermore, this device has great potential for high-density data storage, low-cost memory applications in future nanoelectronics.

References

[1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995)

1789.

[3] C.D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari, J.M. Shaw, Science 283 (1999) 822.

[4] B. Eitan, P. Pavan, I. Bloom, E. Aloni, A. Frommer, D. Finzi, IEEE Electron. Dev. Lett. 21 (2000) 543.

[5] R.S. Potember, T.O. Poehler, D.O. Cowan, Appl. Phys. Lett. 34 (1979) 405.

[6] L.P. Ma, J. Liu, Y. Yang, Appl. Phys. Lett. 80 (2002) 2997.

[7] S. Möller, C. Perlov, W. Jackson, C. Taussig, S.R. Forrest, Nature 426 (2003) 166.

[8] A. Bandyopadhyay, A.J. Pal, Adv. Mater. 15 (2003) 1949.

[9] R.J. Tseng, J. Huang, J. Ouyang, R.B. Kaner, Y. Yang, Nano. Lett. 5 (2005) 1077.

[10] C.W. Chu, J. Ouyang, J.-H. Tseng, Y. Yang, Adv. Mater. 17 (2005) 1440. [11] Q. Ling, Y. Song, S.J. Ding, C. Zhu, D.S.H. Chan, D.-L. Kwong, E.-T. Kang,

K.-G. Neoh, Adv. Mater. 17 (2005) 455.

[12] C.-H. Tu, Y.-S. Lai, D.-L. Kwong, IEEE Electron. Dev. Lett. 27 (2006) 354.

[13] S. Paul, A. Kanwal, M. Chhowalla, Nanotechnology 17 (2006) 145. [14] L. Li, Q.-D. Ling, S.-L. Lim, Y.-P. Tan, C. Zhu, D.S.H. Chan, E.-T. Kang,

K.-G. Neoh, Org. Electron. 8 (2007) 401.

[15] M.G. Mason, C.W. Tang, L.-S. Hung, P. Raychaudhuri, J. Madathil, D.J. Giesen, L. Yan, Q.T. Le, Y. Gao, S.-T. Lee, L.S. Liao, L.F. Cheng, W.R. Salaneck, D.A. dos Santos, J.L. Bre˙das, J. Appl. Phys. 89 (2001) 2756. [16] L.F. Cheng, L.S. Liao, W.Y. Lai, X.H. Sun, N.B. Wong, C.S. Lee, S.T. Lee,

Chem. Phys. Lett. 319 (2000) 418.

[17] C.B. Lee, A. Uddin, X. Hu, T.G. Anderson, Mater. Sci. Eng. B 112 (2004) 14.

[18] R. Trensch, F.J. Himpsel, S. Kakar, L.J. Terminello, C. Heske, T. van Buuren, V.V. Diuh, H.W. Lee, K. Pakbaz, G. Fox, I. Jime˙nez, J. Appl. Phys. 86 (1999) 88.

[19] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. Chapter 7, p. 380.

Fig. 7. Gracing incidence X-ray diffraction (GIXRD) curves of the Alq3thin