TFTs as photodetectors for optical interconnects

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Abstract—In this work we are looking at the prospect of using poly-silicon based Thin Film Transistors (TFTs) as photodetectors for optical interconnects that can detect light effectively at 1100nm wavelength from silicon based Light Emitting Diodes (LEDs). These TFTs were fabricated from laser crystallized silicon and were characterized under darkness and illumination. The photosensitivities of these devices were limited due to the presence of aluminium as their gate electrode but have shown us the possibility of a new approach to photodetection. Index _{Terms—Laser Crystallization, Optical Interconnect,} Photodetector, Silicon, Thin Film Transistor (TFT).

I. INTRODUCTION

N recent years there has been a lot of focus on replacing metallic interconnects with much faster optical interconnects for intra-chip communications. The optical interconnects have the advantage of providing higher bandwidth for data transfer while consuming less power [1].

One of the basic components of an optical interconnect is a Photodetector. The important properties of a photodetector are quantum efficiency, responsivity and speed. It is also important to have a lower leakage current. The optical absorption coefficient α of the material chosen greatly influences the responsivity of the photodetector [2].

There are different varieties of photodetectors namely p-n diode, p-i-n diode, Avalanche photodiode and Metal-Semiconductor-Metal (MSM) photodetector [3]. Many have addressed different approaches for these photodetectors [4]-[5]-[6]. In this paper we are looking at the novel approach of using TFTs for detecting light from silicon based LEDs at 1100nm wavelength [7]. One of the unique advantages of this approach is in using low temperature processing techniques in its fabrication compatible with CMOS post processing. We have also demonstrated in this paper that these TFTs were quite sensitive to light albeit having aluminium as its gate electrode.

II. EXPERIMENTAL

For this work we employed both p-Type TFTs and n-Type TFTs fabricated from laser crystallized silicon. The laser crystallization process involved a controlled lateral crystallization of the material. This was done by using pre-patterned a-Si lines prior to the deposition of a-Si layer using Low Pressure Chemical Vapour Deposition (LPCVD) at 5500 C which was laser crystallized using a green (515nm) laser as shown in Fig. 1 [8].

MESA+ Institute for Nanotechnology, Chair of Semiconductor Components, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands, E-mail: B.Rangarajan@ewi.utwente.nl

This resulted in the formation of grain boundaries which were preponderantly location-controlled thus making it possible to pre-define them.

The n+ and p+ regions were formed using ion implantation of dopants and activated using laser anneal. The bottom gate oxide was grown using thermal oxidation while the front gate oxide was deposited using Inductively Coupled Plasma Enhanced Chemical Vapour Deposition (IC-PECVD) at 1500 C [9]. All the TFTs had Aluminium as their front gate electrode which limited their direct illumination.

The TFTs were fabricated in such a way that their channels were - either parallel to the major grain boundaries ( TFT) or perpendicular to the major grain boundaries (⊥ TFT) as shown in Fig. 3. The width of the intrinsic region was 2µm while the length was 40 µm.

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TFTs as Photodetectors for Optical Interconnects

Balaji Rangarajan, Ihor Brunets, Jisk Holleman, Alexey Y. Kovalgin, Jurriaan Schmitz

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Fig. 2. (a) n-Type TFT (b) p-Type TFT

Fig. 3. (a) ⊥TFT (b)  TFT

Fig. 1. Crystallization process of a-Si layer Illustration of the crystallization step (a) Melting of the layer using Laser Beam (b) Crystallization process-Super lateral growth (c) Crystallized film with predefined grain boundaries.

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III. MEASUREMENTS

Output and Transfer Characteristics were measured under darkness and illumination covering different wavelengths using a Karl Suss Probe station.

An external light source was used for illumination. A wavelength carousel filter was fitted along with the light source in order to choose different wavelengths for the characterization of the devices.

IV. RESULTS AND DISCUSSION

Aluminium as a front gate electrode limited the direct illumination of the TFTs. The response of these TFTs could be attributed to the stray light entering the channel because of the light getting scattered along the edges of the metal as well as due to the back scattering of light via the oxide layer beneath it.

These TFTs were not fabricated with the intention of photodetection and so was the use of Aluminium as the gate electrode. They were instead used as test devices to look into the prospect of using them as photodetectors.

Under illumination the Drain current (Id) decreased for both

p-type and n-type ⊥ TFTs whereas it increased for p-type  TFTs whilst the change in the Drain current (Id) for n-type 

TFTs were relatively insignificant as shown Fig. 4 and Fig. 5.

These effects diminished with the increase in the Gate voltage (Vg) i.e. the influence of light on Id were significant

only in the sub-threshold region. With respect to wavelengths, infrared spectrum gave a predominant effect on p-type ⊥ TFTs while the white light had an upper hand on both n-type ⊥ TFTs and p-type  TFTs as shown Fig. 4 and Fig. 5.

As seen from the output characteristics of these TFTs, it can be clearly stated that the orientation of the major grain boundaries along the channel plays a crucial role in determining the type of response under illumination. The increase in Id for  TFTs can be attributed to the minimal

influence of the grain boundaries as they are oriented in parallel to the path of the movement of the carriers though n-Type  TFTs have registered a very minimal change in Id

under illumination. In general  TFTs have higher Drain current (Id) when compared to ⊥ TFTs as seen in Fig. 4 and

Fig. 5. The decrease in Id for the ⊥ TFTs cannot be

well-accounted. Many previous studies have claimed for an increase in the conductivity of poly-silicon under illumination [10]-[11]. In the present case the responses of  TFTs and ⊥ TFTs are complementary to each other. This needs further analysis in order to well document the effect of illumination in ⊥ TFTs with an applied gate voltage.

0.E+00 2.E-08 4.E-08 6.E-08 0 2 4 Vd (V) 6 Id ( A )

Dark 350-600nm >1000nm White Light

4.5E-08 4.8E-08 2.5 Vd (V) 4.5 Id ( A ) -2.4E-09 -1.6E-09 -8.0E-10 0.0E+00 -5 -2.5 Vd (V) 0 Id ( A )

-5.0E-10 -2.5E-10 0.0E+00 -5 -2.5 Vd (V) 0 Id ( A ) -2.4E-09 -1.6E-09 -8.0E-10 0.0E+00 -5 -2.5 Vd (V) 0 Id ( A )

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Fig. 4. Output Characteristics under darkness and illumination (a) n-Type ⊥TFT (b) n-Type TFT

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Fig. 5. Output Characteristics under darkness and illumination (a) p-Type ⊥TFT (b) p-Type TFT

V

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V

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0.E+00 3.E-10 6.E-10 9.E-10 0 2 4 Vd (V) 6 Id ( A )

7.E-10 9.E-10 2.5 Vd (V) 4.5 Id ( A )

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V. CONCLUSION

The above measurements have shown the possibility of using TFTs as Photodetectors but their performances were limited due to the presence of the aluminium as their gate electrode. These TFTs will be further redesigned in order to better understand their characteristics. True compatibility with CMOS post processing can be achieved with silicon by depositing a-Si using Plasma Enhanced Chemical Vapour Deposition (PECVD) instead of LPCVD.

ACKNOWLEDGMENT

This research is funded by the Dutch Ministry of Economic Affairs under the project MEMPHIS Smartmix.

REFERENCES

[1] Michael R. Feldman, Sadik C. Esener, Clark C. Guest, and Sing H. Lee, “Comparison between optical and electrical interconnects based on power and speed considerations”, Applied Optics / Vol. 27, No. 9 / 1 May 1988

[2] K. Schneider, H. Zimmermann, “Highly Sensitive Optical Receivers”, Springer 2006, pp. 13-34.

[3] S. M. Sze, “Physics of Semiconductor Devices”, 2nd Edition, A Wiley-Interscience Publication.

[4] Prabhakar Bandaru, Subal Sahni, Eli Yablonovitch, Hyung-Jun Kim, and Ya-Hong Xie, “The Fabrication of p-Ge/n-Si Photodetectors, Compatible With Back-End Si CMOS Processing, by Low Temperature (< 400 °C) Molecular Beam Epitaxy and Electron-Beam Evaporation”, Mat. Res. Soc. Symp. Proc. Vol. 796 © 2004 Materials Research Society.

[5] O. I. Dosunmu, M. K. Emsley, and M. S. Ünlü, “High Speed Resonant Cavity Enhanced Ge Photodetectors on Si Reflecting Substrates for 1550 nm Operation”, 0-7803-8491-1/04/$20.00 © 2004 IEEE. [6] Chi On Chui, Ali K. Okyay, and Krishna C. Saraswat, “Effective Dark

[7] G. Piccolo, T. Hoang, J. Holleman, A Y. Kovalgin, J. Schmitz, “Silicon LEDs with Antifuse Injection” Group IV Photonics, 2008 5th IEEE International Conference Page(s):49 – 51

[8] I. Brunets, J. Holleman, A.Y. Kovalgin, A. Boogaard, J. Schmitz, “Low temperature Fabricated TFTs on Polysilicon Stripes”, Electron Devices, IEEE Transactions on Volume 56, Issue 8, Aug. 2009 Page(s):1637 - 1644

[9] A. Boogaard, A.Y. Kovalgin, I. Brunets, A.A.I. Aarnink, J. Holleman, R.A.M. Wolters, J. Schmitz, “Characterization of SiO2 films deposited at low temperature by means of remote ICPECVD”, Surface & Coatings Technology 201 (2007) 8976–8980.

[10] Dinesh Prasad Joshi and Devesh Prasad Bhatt, “Theory of Grain Boundary Recombination and Carrier Transport in Polycrystalline Silicon Under Optical Illumination”, IEEE Transaction on Electron Devices, NO. I. January 1990.

[11] D. Z. Mitic and D. M. Petkovic, “Carrier Recombination at Grain Boundary and Fermi Level in Polysilicon Films under Optical Illumination”, PROC 22nd International Conference on Microelectronics (MIEL 2000), VOL 1, NIS, Serbia, 14-1 7 May, 2000.