Physical Modelling of InGaAs-InAlAs APD and PIN Photodetectors for > 25Gb/s Data Rate Applications.O. S. Abdulwahid1, I. Kostakis2, S. G. Muttlak1, J. Sexton1, K.W. Ian2 and M. Missous1

1School of Electrical and Electronic Engineering, the University of Manchester, United Kingdom 2Integrated Compound Semiconductors, Manchester, United Kingdomm.missous@manchester.ac.uk

Abstract: Validated SILVACO physical models were exploited to optimize electrical and optical characteristics of 1.5µm wavelength InAlAs-InGaAs Avalanche photodiodes and InGaAs PIN diodes. Optimized SILVACO models were created by selectively thinning down the absorption layers to further reduce the carrier transit time. Further optimization was accomplished through scaling of the light window aperture and mesa area sizes with the aim of reducing the device capacitances. The optimized PIN diode provides a maximum opto-electric bandwidth of 35 GHz with a current responsivity of 0.4 A/W under -5V bias voltage and 10µW incident optical power. At 1µW incident optical power, the maximum opto-electric bandwidth and current responsivity of the optimized avalanche diode is 21 GHz and 1.4 A/W under -21.5V bias voltage. The optimized APD and PIN photo-detectors are capable of working at data rate of up to 25Gb/s and 40Gb/s respectively.

1. IntroductionSustained development efforts for high speed

photodetectors have been ongoing for many years in order to have a component that can be easily grown and fabricated and most importantly meet the increasing demand of high operating bandwidth and data rate as well as fulfilling low cost requirements for mass market adoption[1, 2]. Various structures [3-5] have been extensively studied and fabricated in order to realize a photodetector capable of maintaining a high-speed of operation, such as Avalanche diode (APD), PIN diode, and metal-semiconductor-metal (MSM) diode. However, the latter introduces a substantially large dark current which causes high shot noise [6, 7]. The perceived advantages of APD and PIN diodes make them highly preferred at the receiver front end to detect optical signals and convert them into electrical ones. To maintain high-data rate applications, the key factors of APD are high gain-bandwidth product and low excess noise factor[8]. III-V material systems and in particular In0.52Al0.48As-In0.53Ga0.47As are considered as one of the most promising technologies for the 1.2 to 1.6 µm wavelength range. More importantly, the use of InGaAs material to absorb incident light enables high 3-dB bandwidth while keeping the light window aperture at acceptable ranges for flexible alignment tolerances with fibres.

Such a photodetector which has wide operating bandwidth, low bias voltage of operation, and high responsivity is highly desired to achieve the maximum possible performance of receiver systems. Reduction of photodetector mesa area size is one way to minimize the RC time and thus improve the operating 3-dB bandwidth. Unfortunately, this lead

to inflexible alignment tolerances which in turn increase the cost of packaging and assembly [3]. PIN diodes are aimed for short distance application due to their lowest sensitives compared to APDs. In particular when an TIA amplifier is used, the maximum sensitivity of receivers is limited by the introduced noise of amplifier [4]. In terms of noise performance, PIN and APD photodiodes suffer from thermal and shot noises. However, the total noise of APD is significant due to the generated excess noise as a direct result of impact ionization process [7]. The design process of an APD is more complicated and needs more care to control its performance. Dark current and multiplication factor (M) are very sensitive to the thickness and doping profile of multiplication and charge sheet layers. The trade-off between achieving high signal-to-noise ratio (SNR) and low excess noise can be compensated by designing an APD with an appropriate (M) value. At equal absorber layer thickness, the applied electric field is higher in the case of an APD structure and this increase degrades transit time frequency due to the decrease of overshot drift velocity of electrons and holes [9]. InP and InAlAs materials are widely exploited as multiplication layers in APD photodetector based III-V semiconductor technology [9-13]. The InAlAs material is an electron multiplication material and offer lower k-ratio and better stability compared to InP material [10]. For the development and fabrication of such high data-rate APD or PIN diode it is more efficient to have a physical model that can accurately predict and analyse the effect of different parameters such as mesa area size, absorber layer thickness, and light window aperture size. This model can help to adequately mitigate any performance degradation and lowering the effective-cost of production. This

1

work concentrates on the optimization of two types of photodetectors (PIN diode, and APD) in order to enhance their capability of operating at data rates in excess of 25 Gb/s. Using full virtual wafer fabrication physical modelling (DC, AC, and optical characteristics), a 3D analytical models were built and simulated for both standard photodetectors using the ATLAS SILVACO tool. All simulations were performed for normal incidence devices. The modelled structures are validated by the fabricated devices in terms of electrical and optical characteristics. Three process factors namely: absorber thickness, light window aperture, and mesa area size were optimized to enable the photo-detectors to operate at data rate higher than 25 Gb/s. The optimized PIN photodetector has an opto-electric 3-dB bandwidth of 35GHz at -5V bias voltage when a 10µW optical power is incident, while the optimized APD has an opto-electric bandwidth of 21GHz and a multiplication gain of 3 at -21.6V and 1µW incident optical power. Both photo-detectors were grown using Molecular Beam Epitaxy (MBE) on 620µm thick semi-insulating InP substrates. The APD epi-layer structure was previously reported in our work [14]. The PIN structure comprises of undoped InGaAs material as an absorber sandwiched between two heavily doped ~0.1µm p-type In0.53Ga0.47As top and 0.5µm n-type In0.53Ga0.47As bottom contact layers as depicted in table 1. The absorber is relatively thick (~2µm) but this makes the PIN efficient at absorbing light in the 1.3 to 1.6 µm wavelength region.

2. Small signal RF equivalent circuit extraction of the APD and PIN diode.

Fig. 1 depicts the fabricated standard APD and PIN diode with a light window aperture of 30µm and 15µm respectively. As a part of the high-frequency characterization, on-wafer S11 reflection parameter measurements were performed using an Anritsu VNA from 40 MHz to 40GHz at different bias voltages. All measurements were performed in the dark at room temperature. Advanced Design System (ADS) tool was employed to extract the intrinsic and extrinsic component of the device such as series resistance (RS), junction resistance (RJ), junction capacitance (CJ), pad capacitance (CP) and pad inductance (LP). The simulated S11 of the ADS equivalent circuits were fitted with the experimental results to extract all parameters. Table 2 lists the

extracted parameters for the APD and PIN diodes when they are fully depleted.

The fully depleted junction capacitance of the standard APD is 162fF compared to 46fF for the standard PIN diode, and this is due to the larger light window aperture, mesa area size, and thinner depletion region of the APD. Larger device capacitance degrades the high-frequency performance, as the 3-dB optical bandwidth is mainly limited by the RC and transit time components as will be discussed in the next section. The highly doped top and bottom resulted in a relatively small series resistance of the APD and PIN diode which leads to improvements in the frequency response of the device when thinner intrinsic region widths are used. The extracted RS of the APD and PIN diode are 10Ω and 5Ω respectively. The APD series resistance is larger due to its epi-layer resistances introduced from several layers. More importantly, the separation between the top and bottom electrodes is larger in the case of the APD structure which has a great influence on the total series resistance as shown in Fig. 1. The effective mesa area sizes are 2205µm² and 961µm² for the standard APD and PIN diode respectively. Reducing the separation between the top and bottom electrodes helps minimize the series resistance of the APD. The optimized coplanar waveguide structures have CP and LP of ~10fF and 50pH respectively. The intrinsic cut off frequencies were evaluated for both standard structures using the usual expression 1/2π RSC J and found to be 100 GHz and 692 GHz for the APD and PIN diode respectively. However, in case of a 50Ω load the cut-off frequencies decrease to 16 GHz and 63 GHz for the APD and PIN diode respectively. Extracting the junction capacitance of the devices using the high-frequency small signal equivalent circuit model is necessary to validate the SILVACO physical model which exhibits almost the same value when the APD and PIN diode are fully depleted as will be discussed in section 4.

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TABLE 1: Epi-layer structures of InGaAs PIN diode

Material Doping (cm-3) Thickness (µm)

Top Contact p+-In0.53Ga0.47As ~1x1019 ~0.1

Absorber i-In0.53Ga0.47As Undoped ~2

Bottom Contact

n+-In0.53Ga0.47As ~1x1019 ~0.5

Substrate InP S.I. ~620

TABLE 2: APD and PIN diodes extracted parametersComponent APD PIN diode

CJ, fF 162 46

RJ, kΩ 15 50

RS, Ω 10 5

CP, fF 8 11

LP, pH 40 50

Top Electrode

Bottom Electrode

10µm13.5µm

15µm30µm

(b)(a)

Fig. 1 (a): Standard InAlAs-InGaAs APD with light window diameter of 30µm, (b) Standard InGaAs PIN diode with light window diameter of 15µm.(Images not to scale).

3. Physical Modelling and Optimization Details

The experimental characterization of the standard APD and PIN diode were accomplished to further investigate their electrical and optical performances. The electrical characteristics were measured in the dark and at room temperature. Capacitance-Voltage (C-V) measurement are crucial to validate the extracted CJ from the S11 reflection data and to extract the punch-through voltage of the APD as well as ensuring that the actual doping profile and thickness of the layers are close enough to the designed ones. The optical characterization details were discussed in [14]. This work firstly, uses numerical simulations as a tool to build a 3D quantitative and predictive physical model for the APD and PIN photo-detectors and to validate the measured electrical and optical data. Secondly, the successful and validated models were then used to investigate the effect of different parameters and optimize the performances of the photo-detectors. The APD structure requires more care to build and in the selection of the appropriate models since it is more complicated compared to the PIN diode. Shockley-Read-Hall (SRH) model and Fermi-Dirac statistics were used to model the generation-recombination and carrier drift-diffusion processes. To model the impact ionization process of the APD, an IMPACT SELBER model was used. The discussion and analysis of this model including the basic equations and the parameters required to activate it is presented in[14]. The optimization of our structures consists of two process factors which have significant effect on the high-frequency performances. The absorber thickness was thinned to be 0.5µm for the APD and PIN diode with the aim of reducing the transit time of the electrons and thus enhancing the 3-dB opto-electric bandwidth. However, a thinner depletion region results in a higher junction capacitance. So, further optimization was carried out by reducing the mesa area size, top gold electrode width, and the light window aperture. The latter was optimized to be 15µm, making the effective mesa area of both devices to be ~490µm². A smaller window aperture would increase the complexity of the packaging and assembly of the devices. The design process of the absorber layer determines the responsivity and the maximum operating 3-dB bandwidth of the photo-detector. The 3-dB bandwidth of the photo-detector is mainly constrained by the carrier transit time in the intrinsic regions. The carrier transit frequency FT is limited by the saturation drifts velocity (Vsat) of the electrons and thickness of the intrinsic regions (WI) and can be approximately calculated using the equation [4, 15]:

FT=0.45 x V sat

W I

(1)

FT can be maximized by thinning the intrinsic region thickness and/or by choosing a high drift velocity absorber material. However, another limitation, RC bandwidth (FRC), has to be considered also due to the delay time introduced by RC components. FRC can be theoretically estimated using the following expression:

FRC=1

2 π (R s+RL)C J

(2)

RL is 50Ω load resistance of the practical optical systems when a photo-detector is connected to a TIA amplifier. Minimizing the mesa area size reduces CJ

but this increases the contact resistance which is another limiting factor to improve RC bandwidth. Both terms FRC and FT determine the maximum 3-dB opto-electric bandwidth (F3dB) of the photo-detector as expressed by the following equation [16]:

F3dB =FRC

√1+(FRC /FT )2

(3)

Equation (3) shows that the total bandwidth is dominated by FRC when FRC < FT. Moreover, F3dB

reduces by a factor of 1/√2 when FT=FRC. The calculated 3-dB opto-electric bandwidth of the optimized APD is plotted in Fig. 2 as a function of the intrinsic region width. The intrinsic region is assumed to be fully depleted which is the case when the bias voltage is equal to -15V. Therefore, the plot in Fig. 2 is restricted to -15V< bias voltage < 90%VBR (breakdown voltage), with an internal gain of higher than 1. At bias voltages higher than 90%VBR, the saturated drift velocity of carriers start to decrease due to electron scattering from Г to L and X valleys at high electric fields causing it to degrade the 3-dB optical bandwidth as will be discussed in section 5 later on. For Fig. 2, the averaged drift velocity of electron and holes in the intrinsic region is taken from ref. [3, 16] with a value of ~5.5x106cm/s. The intrinsic region of the optimized APD includes both 0.5µm absorber layer and 0.2µm multiplication layer. Fig. 2 indicates that the 3-dB optical bandwidth is dominated by the carrier transit frequency for an intrinsic region thickness greater than 1µm. On the contrary, it is limited by the RC bandwidth for an intrinsic region thickness smaller than 0.5µm. The highest calculated F3dB of the optimized APD structure is 22.5 GHz when the intrinsic region width ranges between 0.6µm to 0.8µm. This means the optimum absorber thickness is in the range of 0.5µm. However, these calculated results do not take the effect of the

3

parasitic elements which can have a large impact at high operating frequencies.

Fig. 2: calculated 3-dB optical bandwidth of the optimized InAlAs-InGaAs APD.

4. Dark Currents and C-V Characteristics The key fitting parameters used to fit the

simulated data to the experimental ones were reported in [14]. The dark currents of standard APD and PIN diode were measured up to -25V and -5V bias voltages respectively using a probe station under dark and at room temperature conditions. The modelling process starts with simulating I-V and C-V characteristics and maintaining a good agreement with experimental data, in order to build appropriate physical models and validate material parameters used which then can be used to simulate and predict the optical characteristics of higher frequency photo-detectors. The inclusion of different models is necessary to simulate the exact physical phenomena. IMPACT SLEB was used to model the impact ionization process of APD that causes the avalanche break down phenomena at high internal gain. Such phenomenon is quite difficult to model as it depends on several material parameters such as carrier’s impact ionization coefficient, applied electric field and doping profiles. The equations regarding carriers generation rate and calculation of electron and hole impact ionization coefficients used in the ATLAS SILVACO tool is discussed and analysed in details in our previous work [14]. The black and red lines in Fig. 3 which represent the simulated and measured dark currents of the standard APD and PIN diode are close to each other. The doping profile of the charge sheet layer plays an important role in determining the breakdown voltage (VBR) of the APD. Therefore, the doping was changed slightly to keep VBR at ~-23.7V for the standard and optimized APD. The doping profile of the charge sheet layer was set to ~6.5x1017

cm-3. Dark current is directly proportional to device mesa size; therefore, scaling of the device reduced the dark current of both photo-detectors. The optimized APD exhibits a dark current of 1.5nA at

(90%VBR) which is much lower than the InGaAs and Si-Ge APD reported in [5, 10, 17]. The electric field of our APD is greatly confined in InAlAs multiplication layer and only Shockley-Read-Hall process occurs in the InGaAs absorber layer and generates the dark current. Therefore the band-to-band tunnelling (BBT) model which calculates

-25 -20 -15 -10 -5 01.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

Voltage (V)

Cur

rent

(A)

𝑽𝑩𝑹=−23.7𝑽

Measured Standard APDSimulated Standard APDSimulated Opimized (smaller)APD

(a)

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 01.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

Voltage(V)

Cur

rent

(A)

Measured Standard P-I-N diodeSimulated Standard P-I-N diodeSimulated Optimized P-I-N diode

(b)Fig. 3: Measured and simulated dark currents of the standard and optimized InAlAs-InGaAs APD and InGaAs PIN diode.

The band-to-band tunnelling current was not taken into consideration in our model. Similarly, our PIN diode dark current of 0.5nA at -5V bias voltage is comparable to previously reported Ge and InGaAs PIN diodes in [15, 18]. These values make the devices appropriate candidates to achieve high SNR and high sensitivity optical communication receivers. The devices were also characterized with low-frequency AC signal to measure the total capacitances including the junction and parasitic capacitances at different bias conditions. The SILVACO models were used to simulate and fit the junction capacitances of the standard APD and PIN diode with the experimental data, as well as simulating the C-V data of the optimized (smaller and thinner) structures as depicted in Fig. 4. Fig. 4

4

FT

FRC

F3dB

shows excellent fit between the masured and simulated data for the standard APD and PIN diodes. The fully depletd capacitance occurs at bias voltages >-15V and >-3V for the APD and PIN diode respectively. The slope in Fig. 4 (a) indicates that the punch-through voltage (VPT) of the APD is ~-12.5V which is far enough from the VBR of ~-23.7V. It is clear that minimizing the light window aperture and mesa area size of the APD has resulted in a remarkable improvement in the fully depleted junction capacitance of the APD However, this is not the case for the PIN diode, where reducing the mesa area size was not sufficient enough to compensate the increase of the junction

-21 -18 -15 -12 -9 -6 -3 00

50

100

150

200

250

Voltage(V)

Cj(

fF)

Measured Standard APDSimulated Standard APDSimulated Optimized (smaller) APD

(a)

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 00

50

100

150

200

250

Voltage(V)

Cj(

fF)

Measured Standard P-I-N diodeSimulated Standard P-I-N diodeSimulated Optimized P-I-N diode

(b)Fig. 4: Measured and simulated C-V characteristics of the standard and optimized InAlAs-InGaAs APD and InGaAs PIN diode. Capacitance value due to thinning of the absorber thickness. Such an APD and PIN diode with junction capacitances of 72fF and 106fF respectively and relatively small series resistances should be suitable candidates for data rate applications higher than 25 Gb/s.

5. Optical and Noise Characteristics The photo-currents of the standard devices were

measured using a 1.55 µm laser light. The incident optical powers on the APD and PIN diode were -30dBm and -20dBm respectively. The measured dc responsivity without AR coating layer is 9A/W and 0.7A/W at (90%VBR) and -5V bias for the APD and PIN diode respectively. APD and PIN diode models were optically characterized by taking into account the generation process of the electron-hole pair in the absorber layer. In SILVACO, different parameters have to be considered to calculate the photo-current such as material quantum efficiency, material absorption coefficient, and the effect of absorption losses and transmission and reflection factors [19]. The same models and fitting parameters of the dark current and C-V characteristics were used. The physical models are in excellent agreement with the fabricated devices as is seen in the black and red lines of Fig. 5 (a) and (b). The optimized APD (Fig. 5 (a)) has virtually a flat photo-current between VPT

and VBR which results in a practically constant internal gain. At VBR the photo-current raises significantly due to the occurrence of a large number of impact ionization events resulting in high internal gain. The extracted VBR and VPT from the photo-current data agree well with the values from dark current and C-V data. The optimized APD photo-current is ~1.4µA at (90%VBR) corresponding to a dc current responsivity of 1.4 A/W. The optimized APD could not maintain a high responsivity as a result of thinner absorber thickness. On the other hand, the optimized PIN diode has a photo-current of 4.6µA giving a current responsivity of 0.46 A/W at -5V bias voltage. At the same absorber thickness and incident optical power, the APD provides higher responsivity compared to the PIN diode due to the multiplication process of the Photo-current. The slope in Fig. 5 (a) shows that at voltages >VPT, the carrier start to enter into the multiplication region and generate the impact ionization process which leads to an increase in the photo-current. Under a uniform electric field, the excess noise factor (F) as a function of the applied bias voltages was estimated using the following equation[20]:

F =K x M+(1-K) x (2-1M

)(4)

5

VPT=-12.5V

-25 -20 -15 -10 -5 01.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

Voltage(V)

Cur

rent

(A)

Measured Standard APDSimulated Standard APDSimulated Optimized APD

(a)

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 00.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

7.00E-06

8.00E-06

Voltage(V)

Cur

rent

(A)

(b)Fig. 5: Measured and simulated I-V photo-currents of the standard and optimized InAlAs-InGaAs APD and InGaAs PIN diode.

Where K is the ratio of hole (αp) to electron (αn) impact ionization coefficients. αp and αn were theoretically calculated using the equations and impact statement parameters of InAlAs material that can be found in our previous work [14]. The internal gain (M) was calculated for the measured and simulated photo-currents as shown in Fig. 6. The low internal gain of ~3 at (90%VBR) of the optimized APD is mainly caused by the decrease of the electric field in the multiplication layer, where it was found that the electric field is close to 613 kV/cm compared to a value of ~700 kV/cm for the standard APD design. Low internal gain introduced less excess noise of ~2.2 for the optimized APD (at 90%VBR bias voltage, and M=~3) which is comparable to the value reported in [8, 21, 22]. Less excess noise is highly important to achieve high (SNR) value. Furthermore, it was observed that reducing the absorber thickness of the APD by ~60% led to an increase in electric field in the absorber layer by 194%. A high electric field affects the drift saturation velocity of the carries

and degrades the carrier transit frequency. So, care has to be taken to choose the optimum absorber thickness. The photo-detector noise is another figure of merit which determines the maximum achievable SNR and data-rate. The total noise of PIN diode is given by [7]:

iPIN=√2 e ( I ph+ I D+ I B ) x B+

√ 4 KTBReq

(5)

The term (√2 e ( I ph+ I D + I B ) x B) refers to the shot noise caused by the dark and photo-current. Where e is the charge of an electron in coulomb unit, Iph is the photo-current, ID is the dark current, IB is the bulk current, and B is the bandwidth of noise measurement. The second term (√4 KTB / Req) is the thermal Johnson noise, where K the Boltzmann constant, T is the temperature of the photodetector, Req is equal to (RS+RL). In case of APD, the equivalent noise is expressed by [7]:

iAPD=√2e ( I ph+ ID+ I B ) x BF M 2+

√ 4 KTBReq

(6)

The bulk current is extremely small and thus can be neglected. The total noises of the standard and optimized photodetectors were theoretically calculated at 1Hz as shown in table 3. The noise characteristics are estimated at bias voltages of (90%VBR) and -5V for the APD and PIN diode respectively. In the case of a PIN diode, it is clear that the noise of standard and optimized PIN diodes is dominated by the thermal noise caused by the equivalent resistances of (RS+RL). The existence of internal gain (M) resulted in a high shot noise of ~37pA/√ Hz for the standard APD. However, this was reduced to ~2.8pA/√ Hz for the optimized APD since (M) was decreased to 3. The frequency photo response (S21) of the standard APD and PIN diode was on wafer characterized using 1.55µm modulated laser and a Lightwave Component Analyser (HP 8703A). S21 represents the opto-electric response and is given by the ratio of photo-current to the optical power in dB unit. Process simulations were initiated to virtually simulate the S21 response of the optimized models. Fig. 7 depicts the normalised measured and simulated frequency photo response of the standard and optimized APD and PIN diode at (90%VBR) and -5V bias voltages respectively.

6

VPT

VBR

Measured Standard APDSimulated Standard APDSimulated Optimized APD

Fig. 6: Measured and simulated internal gain of the standard and optimized InAlAs-InGaAs APD.

The S21 response of PIN diode was shifted by -20 dB in order to separate it from the APD response. The measured 3-dB opto-electric bandwidth of the standard APD and PIN is 6.7 GHz and 20 GHz which agrees well with simulated ones. The maximum opto-electric bandwidth of the optimized PIN diode is 35 GHz which is comparable to the reported value in [15], though the light window aperture is 3 times smaller than our design. The simulated opto-electric bandwidth of optimized APD is 21 GHz and agrees well with the theoretically calculated one of 22.5 GHz when the intrinsic region width is 0.7 µm (0.5 µm absorber layer, and 0.2 µm multiplication layer). The optimized APD provides a gain-bandwidth of ~63 GHz at (90%VBR). The simulation process has shown that the bandwidth of the optimized APD decreased to ~14 GHz when the applied bias voltage reached -22V. This is mainly due to electron scattering from Г to L and X valleys at high electric fields. As a result, electrons have higher effective mass and, thus lower drift velocity. Experimental work of these proposed high-frequency photodetectors will be carried out to further

investigate their characteristics and validate the physical models used.

6. ConclusionOptimized 1.5µm wavelength InAlAs/InGaAs

avalanche and InGaAs PIN photo-diodes were physically modelled using ATLAS SILVACO tool to investigate their electrical and optical performances. The maximum opto-electric bandwidth of the optimized APD and PIN diode are 21 GHz and 35 GHz at (90%VBR) and -5V bias voltages respectively. The obtained electrical and optical characteristics as well as the calculated noise Performances are good enough to exploit these photo-detectors in the integration of 25 Gb/s and 40 Gb/s optical receivers. DBR layers can be buried at the bottom of the epi-layers structure of the diodes to enhance the current responsivity.

Fig. 7: Normalized S21 response of the InAlAs-InGaAs APD and InGaAs PIN diode, (red and black are the measured and simulated standard APD, blue is the simulated optimized APD, green and brown refer to the measured and simulated standard PIN diode, and purple is the simulated optimized PIN diode).

Acknowledgments

We are grateful for the support of the UK’s Engineering and Physical Sciences Research Council under grant (EPSRC-EP/P006973/1) “Future Compound Semiconductor Manufacturing Hub”.

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Marris-Morini et al., "Zero-bias 40Gbit/s germanium waveguide photodetector on silicon," Optics express, vol. 20, no. 2, pp. 1096-1101, 2012.

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TABLE 3: NOISE characteristics of the standard and optimized APD and PIN diode

Parameter

Standard APD

Optimized APD

Standard PIN

Optimized PIN

ID(nA) 11 0.5 2 1.5Iph(µA) 9 1.4 6.7 4Req(Ω) 60 60 55 55

F ~6 ~2.2 Free FreeShot noise (pA/

√Hz)

~37 ~2.8 ~1.46 ~1.1

Thermal noise (pA/

√Hz)

~16.5 ~16.6 ~17.3 ~17.3

Net

Measured Standard APDSimulated Standard APDSimulated Optimized APD

Excess Noise Factor (F)

B.W=35 GHz

B.W=21 GHz

@ -5V

@(90%VBR)

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