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InGaAs/InP MOS Device for Single Photon Detection, Amplification, and Wavelength Conversion Yimin Kang, Kai Zhao and Yu-Hwa, Lo Department of Electrical and Computer Engineering, Jacobs School of Engineering Email:
[email protected] University of California, San Diego, La Jolla, CA 92093-0407
are recombined with the injected electrons at the InP p/n junction (now in forward bias) and produce photons. These photons, millions of times more than the incoming photon(s), have the InP bandgap energy and can be easily detected by Si detectors. The entire detection process is illustrated in Figure 2.
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Introduction Photon emission is an effective mechanism to produce and transmit information for interconnecting and imaging applications such as real-time monitoring of integrated circuits at work [1]. However, most materials, including Si, have extremely low quantum efficiency (e.g. 10-3 to 10-5) for light emission. In this paper, a unique device is proposed to detect single IR photons from low efficiency sources and to convert a single-photon signal into a much stronger optical signal at a wavelength easily detectable by a Si CCD or CMOS sensor. The three functions: single photon detection, signal amplification, and wavelength conversion, are integrated in one device. This could pave the way for optical interconnect without dedicated high efficiency light sources such as lasers or LEDs.
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Experiment Results and Discussion The SAM APD structure consists of a 0.65micron thick InP multiplication region and a 1.5-micron thick InGaAs absorption region. The active MOS capacitor window was a 50-micron diameter circular dot where a sputtered gate SiO2 is 600Å thick. The periphery area adjacent to the MOS was covered with thick SiO2 of 2000 Å. Semi-transparent Ti/Au was deposited on top of the gate oxide and Ge/Au was deposited on the bottom of the substrate as electrodes. The devices were wirebonded onto a ceramic chip carrier and mounted in a cryogenic dewar with a quartz window, through which the device was imaged to a Si CCD. At 77K, the MOS-APD device was biased periodically with a triangular voltage pulse having a width of 10ms and a repetition rate of 30Hz. The voltage ramped from +20V down to –64V in 7.5ms and ramped back to +20V in 2.5 ms, as shown in Figure 3. The device enters the avalanche regime near the peak of the negative bias. An avalanche event was triggered when a photo-excited or thermally excited carrier was present in the high field region. To verify the single-carrier sensitivity, we chose thermally excited carrier for the measurement because at 77K, the chance of having more than one thermal generation event in the InGaAs region over a single pulse period is essentially zero. On the other hand, it is hard to calibrate a quasi-single-photon source to assure < 1 photon per pulse period. A typical MOS-APD response to a single photo or thermal carrier is shown in Figure 3. The current response shows a superposition of a displacement current due to the CdV/dt term, where C is the capacitance, and the selfquenched photo response. The sharp current peak at 63V was due to the avalanche event inside InP. The area under the peak gives rise to the total multiplied charges created by avalanche before it is quenched. The avalanche gain obtained from the integration of the peak current is in the range of 108 – 1010 for different devices we tested.
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Device Description The device has an MOS-APD structure, containing a MOS structure in series with an avalanche region found in conventional APDs. For a cutoff wavelength of 1550 nm, an InGaAs layer is introduced as the absorption layer. Avalanche in an MOS device has been studied since mid-1970s, and the effect of Negative Avalanche Feedback (NAF) has been considered as the underlining mechanism for single carrier detection and automatic quenching of the avalanche process to prevent current run-away. A simple description of the NAF effect is given in the following. When avalanche happens, the impact-ionized charges are blocked by the insulating oxide and accumulated at the oxide/semiconductor interface. The accumulated charges form an inversion layer to shield the electric field from penetration into the semiconductor. When the field-shielding reaches a threshold value, the impact ionization efficiency in semiconductor drops considerably and the avalanche event is quenched [2], as shown in Figure 1. Recently NAF has been used to the design of a new class of Si APDs[3-5] In this work, MOS structure was added onto conventional InP/InGaAs separate-absorptionmultiplication (SAM) APDs. When the avalanche is quenched, holes created by impact ionization are accumulated at the oxide/semiconductor interface. When the applied voltage is reversed, these accumulated holes
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Infrared emission was observed when the accumulated carriers (holes in this case) are recombined with the injected electrons when the voltage became positive. Because the photoemission happens in InP, the emission wavelength is equal to the InP bandgap energy and can be easily detected by a Si detector. Therefore, the tremendous avalanche gain of 108 – 1010 leads to the generation of a strong emission at a shorter wavelength. This demonstrates that a single MOS-APD device can combine all three functions together: single-photon detection, amplification, and wavelength conversion. A single-carrier-triggered MOS-APD photoemission captured by a Si CCD is shown in Figure 4. 4.
Summary
We demonstrated, for the first time, an InGaAs/InP based MOS-APD capable of single photon
detection up to 1550 nm wavelength. The device characteristics make it a promising candidate for long wavelength single photon transducer which could have tremendous impact on optical interconnect and imaging. Reference: [1]. M.Remmach, R. Desplats, P. Perdu, J.P. Roux, M. Vallet, S. Dudit, P. Sardin, D. Lewis, Microelectronics Reliability, 44 (2004), 1715-1720. [2] A.B. Kravchenko, et. al., J. Quantum Electron, 8 (1978), 1086, 1399. [3] L.F. Lou and G. L. Tettemer, J. Appl. Phys., 66 (1989),2678-2688. [4] N.Y. Sadygov, et. al., Optical Memory and Neural Networks, Proc. SPIE, 1621 (1991), 158-168. [5] D.Bisello, et. al. , Nucl. Instr. And Meth., A 367
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Fig. 1. Schematic illustration of InGaAs/InP MOS-APD for single photon detection, amplification, and wavelength converted emission.
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Fig. 3. Electrical response under a triangular bias from +20V to -64 V. The self-quenched response peak was triggered (in this specific case) by a single dark carrier.
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Fig. 2. Test setup for single photon optical transducer. The CCD detects the image of single near IR photons emitted from low efficiency sources.
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Fig. 4. Single-photon image on an off-the-shelf CCD using the setup in Fig. 2.
