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High Responsivity Metal-Semiconductor-Metal (MSM) Photodiodes

Advisor: Paul R. Berger
Students: Wei Gao (graduated with Ph.D. in September 1995)
  Michael McCarthy (graduated with Master's thesis in November 1996)
  Sean L. Rommel (Ph.D. candidate on another project now)
  David Erby (graduated with Bachelor's in May 1998)
Collaborators: George Zydzik (AT&T Bell Labs)
  H. M. O'Bryan (AT&T Bell Labs)
  D. Sivco (AT&T Bell Labs)
  A. Y. Cho (AT&T Bell Labs)
  J. P. Lorenzo (Rome Lab, Hanscom AFB)
  K. Vaccaro (Rome Lab, Hanscom AFB)
  S. M. Spaziani (Rome Lab, Hanscom AFB)


Importance of the Problem:

Photodetectors are being used in more regions of everyday life from the bar code scanner at the grocery store, to the receiver for your remote control on the VCR, to the photoreceiver at the end of a fiber optic cable in a communication system. Since MSM diodes have such simple technology, any improvement in responsivity, the current limiting factor to their widespread use, will allow this class of photodetectors to supplant existing photodetectors in the marketplace.

Metal-semiconductor-metal (MSM) photodetectors offer an attractive benefit over alternative photodetectors such as conventional p-i-n photodiodes. An MSM photodetector consists of interdigitated Schottky metal contacts on top of an active (absorption) layer. An MSM photodetector is inherently planar and requires only a single photolithography step which is compatible with existing field effect transistor (FET) technology. MSM photodetectors are very high speed devices due to their low capacitance, and they typically have very low dark currents (current produced without incident light). However, the responsivity (total signal produced from a from a given optical input) is quite low compared to p-i-n photodiodes. The main causes for the low responsivity is the reflection from the surface metals and semiconductor surface, the finite carrier lifetime as the carriers traverse the gap between the electrodes before being collected, absorption of incident light outside the region in which photogenerated carriers can be collected by the electrodes, and surface recombination currents and deep traps within the semiconductor material which may lower the detected optical signal.

We are investigating ways to improve the responsivity of MSM photodiodes. By increasing the responsivity, simple easy to manufacture photodiodes sensitive to low-light levels would be feasible. This could be accomplished by investigations to suppress surface recombination through passivation, to minimize surface reflections and therefore collect a greater percentage of the incident light, to improve the carrier lifetime, and to better understand the internal gain mechanisms.

Brief Description of Work and Results:

A metal-semiconductor-metal (MSM) In0.53Ga0.47As photodiode using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes was investigated. CTO has greater transmission properties at the wavelengths appropriate for In0.53Ga0.47As photodiodes over conventional transparent conductors such as indium tin oxide (ITO). The CTO functions as a Schottky contact, an optical window and an anti-reflection (AR) coating. The transparent contact prevents shadowing of the active layer by the electrodes, thus allowing greater collection of incident light. The MSM photodiodes had 1 $\mu$m wide electrodes and 2 $\mu$m spacings and a 200 Å cap layer of In0.52Al0.48As for Schottky barrier enhancement. The 2000 Å CTO electrodes were reactively sputtered with an argon/oxygen plasma. MSM photodiodes with CTO electrodes and active area 75$\times$75 $\mu$m2 exhibited leakage currents of $\sim$ 4.8 $\mu$A at 10 V and soft breakdown voltages of $\geq$ 10 V. Similar MSM photodiodes with Ti/Au electrodes had leakage currents of $\sim$ 270 nA at 10 V and soft breakdown voltages of $\geq$ 13 V. Elevated dark current of the CTO photodiode is attributed to defect-related tunneling through the thin 200 Å In0.52Al0.48As layer which was damaged by the sputtering process. The barrier height ($\phi_{Bn}$) of CTO on i-In0.52Al0.48As was determined to be 0.47 eV, while the Ti/Au barrier height was 0.595 eV. The reduced barrier height for CTO is caused by tunneling through the sputter-damaged cap layer. Responsivity for 1.3 $\mu$m incident light was 0.49 A/W and 0.28 A/W, respectively, for the CTO and Ti/Au MSM photodiodes. No anti-reflection (AR) coating was utilized over the bare semiconductor surface. The CTO MSM photodiode shows a factor of almost two improvement in responsivity over conventional Ti/Au MSM photodiodes.

The Schottky barrier height was measured for five different materials on undoped In0.52Al0.48As grown by molecular beam epitaxy (MBE). Of the materials tested, two were transparent conductors, indium-tin-oxide (ITO) and cadmium tin oxide (CTO) and for comparison, three were opaque metals, (Au, Ti and Pt). The barrier heights were measured using I-V measurements. Due to the high series resistance created by the undoped In0.52Al0.48As, the Norde method [J. Appl. Phys., 50, 5052 (1979)] was used to plot the I-V characteristics and extract the Schottky barrier height. The Schottky barrier heights were determined to be 0.639 eV, 0.637 eV, 0.688 eV, 0.640 eV, and 0.623 eV for ITO, CTO, Au, Ti, and Pt, respectively. Previously published results for Schottky barriers on In0.52Al0.48As are compared with our measurements.

Metal-semiconductor-metal (MSM) photodiodes with an In0.53Ga0.47As active region were investigated using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes to improve the low responsivity of conventional MSM photodiodes with opaque electrodes. CTO is suitable as a Schottky contact, an optical window and an anti-reflection (AR) coating. Responsivity of CTO-based MSM photodiodes without AR coating between the electrodes was twice (0.62 A/W) that of a similar MSM photodiodes with Ti/Au electrodes (0.30 A/W). A thin 800 Å In0.52Al0.48As layer is inserted below the electrodes to elevate the electrode Schottky barrier height. A digitally graded superlattice region (660 Å) was also employed to reduce carrier trapping at the In0.53Ga0.47As/In0.52Al0.48As heterointerface which acts to degrade photodiode bandwidth. Bandwidth was elevated nearly an order of magnitude over a previous MSM photodiode design with an abrupt heterointerface.

Metal-semiconductor-metal (MSM) photodiodes with electrodes fabricated from the transparent conductor cadmium tin oxide (CTO) have been shown to double photoresponsivity. Their bandwidths, however, are significantly lower than those of MSMs fabricated with standard Ti/Au contacts. Though MSMs are generally believed to be limited by the transit time of electrons, it is possible the larger resistivity of CTO has become a significant factor, making the MSMs RC time constant limited instead. Previous models of MSMs only account for one of the two back-to-back Schottky diodes. A new model which takes into account both the forward and reverse biased junctions has been developed from the small signal model of a Schottky diode. This new model was fit to data obtained from S-parameter measurements, and incorporates both the transit time response and RC time constant response.


For further information contact:

Paul R. Berger

Professor
Electrical and Computer Engineering
Physics

Director
Nanofabrication and Materials Processing Center (NanoMPC)
Nanoscale Patterning Laboratory
Nanoelectronics and Optoelectronics Laboratory (NOEL)
Polymer Device Laboratory (PDL)

Campus Address:
201 Caldwell Laboratory

Mailing Address:
Department of Electrical and Computer Engineering
The Ohio State University
205 Dreese Laboratory
2015 Neil Avenue
Columbus, OH 43210 USA 

Direct phone: (614) 247-6235 
EE Dept. FAX: (614) 292-7596
Email: pberger@ieee.org


Supported By: University of Delaware Research Foundation (UDRF)

Ph.D. Thesis:

1.
``Studies of High Performance InGaAs Metal-Semiconductor-Metal Photodiodes,'' Wei Gao, graduated September 5, 1995.
Master's Thesis:

1.
``Studies of Embedded Electrode GaAs Metal-Semiconductor-Metal Photodiodes,'' Michael McCarthy, graduated November 1996.

Invited Papers:

1.
``Metal-Semiconductor-Metal Photodiodes for Lightwave Communications and Detection,'' Paul R. Berger, IEEE Potentials April/May, pp. 25-29 (1996).

Patents:

1.
``Transparent/Opaque Electrode Metal-Semiconductor-Metal (MSM) Photodetector,'' Wei Gao and Paul R. Berger, (Docket No. UD95-8), submitted on October 10, 1995, U. S. Patent #5,777,390, Granted on July 7, 1998.

2.
``Asymmetric Contacted Metal-Semiconductor-Metal (MSM) Photodetector,'' Paul R. Berger and Wei Gao, (Docket No. UD94-13), submitted on October 10, 1995, U. S. Patent #5,780,916, Granted on July 14, 1998.

Recent Publication Activity:

1.
"Equivalent Circuit Modeling of Metal-Semiconductor-Metal Photodiodes with Transparent Conductor Electrodes," Sean L. Rommel, David N. Erby, Wei Gao, Paul R. Berger, G. Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, Proceedings of SPIE's Optoelectronics '97: Photodetectors - Materials and Devices II, Vol. 2999, pp. 86-91 (1997).

2.
``In0.53Ga0.47As MSM Photodiodes with Transparent CTO Schottky Contacts and Digital Superlattice Grading,'' Wei Gao, Paul R. Berger, George Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, IEEE Transactions on Electron Devices, 44, pp. 2174-2179 (1997). PDF (242 kB)

3.
``Transparent and Opaque Schottky Contacts on In0.52Al0.48As,'' Wei Gao, Paul R. Berger, Robert G. Hunsperger, G. Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, Applied Physics Letters, 66, pp. 3471-3473 (1995). PDF (67 kB)

4.
``Long wavelength metal-semiconductor-metal photodiodes with transparent cadmium tin oxide Schottky contacts,'' Wei Gao, Al-Sameen Khan, Paul R. Berger, Robert Hunsperger, George Zydzik, H. M. O'Bryan, D. Sivco, A. Y. Cho, Proceedings of the LEOS 1994 Summer Topical Meeting, Lake Tahoe, NV, pp. 63-64 (1994).

5.
``In0.53Ga0.47As Metal-Semiconductor-Metal Photodiodes with Transparent Cadmium Tin Oxide Schottky Contacts,'' Wei Gao, Al-Sameen Khan, Paul R. Berger, Robert Hunsperger, George Zydzik, H. M. O'Bryan, D. Sivco, and A. Y. Cho, Applied Physics Letters, 65, pp. 1930-1932 (1994). PDF (75 kB)

Recent Conference Activity:

1.
``Equivalent Circuit Modeling of MSM Photodiodes with Transparent Conductor Electrodes,'' Sean L. Rommel, David Erby, Wei Gao, Paul R. Berger, G. Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, submitted to SPIE's Optoelectronics '97: Photodetectors: Materials and Devices II in San Jose, CA, February 8-14 (1997).
2.
``Inverted, Substrate-Removed MSM and Schottky Diode Optical Detectors,'' S. M. Spaziani, K. Vaccaro, E. A. Martin, P. R. Berger, and J. P. Lorenzo, 8th International Conference on Indium Phosphide and Related Materials in Schwäbish Gmünd, Germany, April 21-25 (1996).

3.
``InGaAs Metal-Semiconductor-Metal Photodiodes with Transparent Schottky Contacts and Digital Superlattice Grading,'' Wei Gao, Paul R. Berger, Robert G. Hunsperger, George Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, 15th North American Conference on Molecular Beam Epitaxy in College Park, MD, September 17-20 (1995).

4.
``Transparent Schottky Contacts on In0.52Al0.48As,'' Wei Gao, Paul R. Berger, Robert G. Hunsperger, George Zydzik, W. W. Rhodes, H. M. O'Bryan, D. Sivco, and A. Y. Cho, 37th Electronic Materials Conference in Charlottesville, VA, June 19-21 (1995).

5.
``High Responsivity MSM Photodetectors,'' Paul R. Berger, Workshop on Compound Semiconductor Materials and Devices in New Orleans, LA, February 20-22 (1995).

6.
``Long Wavelength Metal-Semiconductor-Metal Photodiodes with Transparent Cadmium Tin Oxide Schottky Contacts,'' Wei Gao, Al-Sameen Khan, Paul R. Berger, Robert Hunsperger, George Zydzik, H. M. O'Bryan, D. Sivco, and A. Y. Cho, IEEE/LEOS Summer Topical Meeting on Optoelectronic Materials Growth and Processing in Lake Tahoe, NV (1994).



 
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Paul R. Berger
1998-10-06