1 指導教授：劉致為 博士 學生：梁啟源 台灣大學光電工程學研究所 Enhancement of Metal-Oxide- Semiconductor Tunneling Photodetectors and Light Emitting Diode 金氧半穿隧光偵測器及 發光二極體的增進

2 Outline Introduction ……………………………………..3 NMOS photodetector…………………………..4 SOI NMOS photodetector……………………..9 Metal-HfO2-Silicon LED …………… Surface plasmon enhanced transmission..34 Summary………………………………………..40

3 The electro-optical products may be one of the killer applications in the future Si market. The worldwide revenue of the optical semiconductor is ~6% (~10 B) of the total semiconductor revenue. The ITRS has predicted that the incorporation of electro-optical components into CMOS compatible process is needed in 2004 to achieve System-on-a-Chip (SOC). Si-based optoelectronics - low cost, high reliability, VLSI compatible Introduction

4 Al gate The dark and photo- currents are relatively constant in the log scale at large gate bias. I light intensity  => I ph  NMOS detector I-V characteristic

5 NMOS detector operation principle Positive gate bias Deep depletion Soft-pinning of V ox

6 V ox pinning Simulation results V g falls on Si substrate. Soft-pinning of V ox

7 Minority carrier distribution

8 Excess carriers generated before 46um could contribute to total current.

9 SOI MOS Photodetector Responsivity and Bandwidth Previously reported tunneling MOS photodetectors (PD) couldn’t promote it’s speed to GHz => far from application The absorption length of 850 nm lightwave in Si (~16 um) is much larger than the Si depletion width.

10 Photo-generated carriers in the bulk neutral region. => collected by gate electrode through the slow diffusion process. The large diffusion current limits the device bandwidth to hundreds MHz.

11 Simulation Details Proposed MOS/SOI structure carried out by ISE.

12 Simulation Details Thick buried oxide => stops the diffusion current from substrate thin absorption layer => makes sure that the device is fully-depleted during operation. The absorption region and ground electrode are separated by oxide and connected by buffer layer. The grid structure of Al gate electrode => allows the light directly exposures on the absorption region.

13 Results and Discussion The diffusion current is eliminated in SOI-MOS PD to increase speed => total photocurrent is reduced Thicker absorption region increase the responsivity but reduces bandwidth.

14 Results and Discussion Under inversion bias (V g > 0), most voltage drops falls on Si substrate and the depletion width increases with V g. => Tunneling MOS diode is deeply depleted under inversion bias.

15 Results and Discussion SOI-MOS devices with different absorption layer thickness. Thicker absorption layers => higher responsivity => bandwidth decreases from 30 GHz to 0.9 GHz. The absorption region outside the depletion region produces the diffusion current and reduces the bandwidth.

16 Results and Discussion Band diagrams of the SOI-MOS PDs with different buffer layer doping along the route of hole current. A energy barrier for hole is observed for device with buffer layer doping.

17 Results and Discussion The photocurrent of device with cm -3 buffer layer doping rises and falls quickest. SOI-MOS PD generates more photocurrent than the bulk devices, unlike the result in DC condition.

18 Depletion region Vn: electron saturation velocity Vp: hole saturation velocity Analytical Model

19 Fourier transform : Considering depletion capacitance C d and series resistance R s : Analytical Model

20 Because of symmetry, we can just analyze right half side of the device, and transform it into equalized circuit. There are three currents with different capacitance and resistance. The total frequency response was formed by linearly adding currents together. Analytical Model

21 Buffer layer of doping is depleted and holes from center are blocked by depletion region. Only holes generated in the outside part could transport to ground contact. => lower responsivity Analytical Model

22 The parameter values used in the analytical model are given as followed. Analytical Model

23 The analytical model precisely fits the simulation results. Analytical Model

24 Bandwidth of SOI PD For t si >0.5um ， diffusion process ↑ ； bandwidth ↓ For t si >0.5um ， capacitance ↑ ； bandwidth ↓

25 Bandwidth of SOI PD As tsi ↑ ， diffusion length↑ ， bandwidth↓

26 for TE wave for TM wave Bragg reflector Transform matrix of layer n reflectance transmittance

27 Bragg reflector Reflectivity has period relation with silicon thickness

28 Reflectivity has minimum values at 850nm and 1300nm. Bragg reflector

29 When incident angle is smaller than 40 o, reflectivity of SOI device is smaller than bulk one. Bragg reflector

30 SiO2 2nm Si 0.56um SiO2 0.25um Bragg reflector The TEM and thickness of SOI MOS PD T ox (2nm) << 850nm, gate oxide was neglected.

31 The measured photocurrent of ~30 o incident light is 80% higher than normal incident one. Bragg reflector

32 Metal-Insulator-silicon tunneling LED Light emitted from electron hole recombination at accumulation

33 The light efficiency of metal-HfO2-silicon LED is 4 times larger than MOS LED. k↑ capacitance( ) ↑ charge (Q=CV) ↑ recombination ↑ efficiency ↑ Metal-Insulator-silicon tunneling LED

34 Surface plasmon Enhanced transmission Surface plasmon at metal-dielectric interface resonance with TM light.

35 Surface plasmon Enhanced transmission Dispersion relation Drude model

36 SP right at light line  imaginary k z Surface plasmon Enhanced transmission Radiative resonance

37 Surface plasmon Enhanced transmission single subwavelength hole transmission increase larger than area T. W. Ebbesen, Nature (London) 391, 667 (1998)

38 Surface plasmon Enhanced transmission The periodicity of the array determines the position of the peaks, independent of metal hole diameter and film thickness.

39 Surface plasmon Enhanced transmission a=316nm, r=50nm, and h=100nm. 2nm

40 Summary Photodetectors using NMOS tunneling structures are demonstrated. The novel SOI-MOS PDs can reach high bandwidth (22 GHz) and are fully compatible with ULSI technology. The device structure could be optimized with Bragg reflector by tuning the doping and thickness of epi-layers.

41 The light emission from Al/HfO2/p-Si tunneling diodes has an enhanced intensity because of the high dielectric constant of HfO2 to increase the hole concentration at Si/HfO2 interface Surface plasmon could enhance the transmission through hole array on metal film. The size of hole array is predicted. Summary