sensors

Article The Fabrication and Characterization of InAlAs/InGaAs APDs Based on a Mesa-Structure with Polyimide Passivation

Jheng-Jie Liu 1, Wen-Jeng Ho 1,* , June-Yan Chen 1, Jian-Nan Lin 1, Chi-Jen Teng 2, Chia-Chun Yu 2, Yen-Chu Li 2 and Ming-Jui Chang 2

1 Department of Electro-Optical Engineering, National Taipei University of Technology, No. 1, Section 3, Zhongxial East Road, Taipei 10608, Taiwan 2 Tyntek Corp., No. 15, Kejung Rd., Chunan Science Park, Chunan, Miaoli County 350, Taiwan * Correspondence: [email protected]; Tel.: +886-2-2771-2171 (ext. 4639)

 Received: 14 June 2019; Accepted: 31 July 2019; Published: 2 August 2019 

Abstract: This paper presents a novel front-illuminated InAlAs/InGaAs separate absorption, grading, field-control and multiplication (SAGFM) avalanche (APDs) with a mesa-structure for high speed response. The electric fields in the InAlAs-multiplication layer and InGaAs-absorption layer enable high multiplication gain and high-speed response thanks to the thickness and concentration of the field-control and multiplication layers. A mesa active region of 45 micrometers was defined using a bromine-based isotropic wet etching solution. The side walls of the mesa were subjected to sulfur treatment before being coated with a thick polyimide layer to reduce current leakage, while lowering capacitance and increasing response speeds. The (VBR) of the proposed SAGFM APDs was approximately 32 V. Under reverse bias of 0.9 VBR at room temperature, the proposed device achieved dark current of 31.4 nA, capacitance of 0.19 pF and multiplication gain of 9.8. The 3-dB frequency response was 8.97 GHz and the gain-bandwidth product was 88 GHz. A rise time of 42.0 ps was derived from eye-diagrams at 0.9 VBR. There was notable absence of intersymbol-interference and the signals remained error-free at data-rates of up to 12.5 Gbps.

Keywords: avalanche photodiodes; InAlAs; InP; multiplication gain; polyamide passivation; eye-diagrams

1. Introduction Avalanche photodiodes (APDs) featuring a separate absorption and multiplication (SAM) layer-structure are widely used to produce low-noise optical detectors of high sensitivity [1,2]. InGaAs/InP APDs comprise an absorption layer of In0.53Ga0.47As (hereafter referred to as InGaAs) with a multiplication layer of InP. These devices have long been used as high-performance detectors in optical communication systems operating at wavelengths of 1310 or 1550 nm [3–5]. However, In0.52Al0.48As (hereafter referred to as InAlAs) is attracting considerable attention for use in the multiplication layer for three main reasons: (1) The electron/hole ionization coefficient of InAlAs is larger than that of InP [6–9]; (2) The electron mobility of InAlAs is greater than that of InP [10,11]; and (3) The ionization coefficient of InGaAs/InAlAs APDs is less sensitive than InGaAs/InP APDs to variations in temperature [6–9,12–15]. As a result, InGaAs/InAlAs APDs achieve far higher performance in terms of noise, gain-bandwidth, response time and resistance to temperature variation [14,16,17].

Sensors 2019, 19, 3399; doi:10.3390/s19153399 www.mdpi.com/journal/sensors Sensors 2019, 19, 3399 2 of 15

In this study, we fabricated front-side illuminated mesa-type InGaAs/InAlAs APDs based on a separate absorption, grading, field-control and multiplication (SAGFM) layer structure in conjunction with multi-step mesa etching and sulfur/polyimide passivation to achieve high-speed operations and high sensitivity [18–22]. Electric-field distribution in the multiplication and absorption layers is well controlled to enable high multiplication gain and low noise. The proposed device was evaluated in terms of dark current, breakdown voltage, multiplication gain, capacitance, incident optical power dynamic range, aging test, frequency response and transmission speeds at room temperature.

2. Experiments

2.1. Epitaxial Layer Design and the Calculation of Electric-Field Profiles The proposed avalanche (APD) was configured using separate absorption, grading, field-control and multiplication (SAGFM) layer-structure. Table1 details the parameters of the epitaxial layers, which were grown using molecular beam epitaxy (MBE) on semi-insulated InP wafers with (100) orientation and low etch pit density (EPD), measuring two inches in diameter and 350 µm in thickness. The quality of the epitaxial layers was verified via double-crystal X-ray diffraction (to check for lattice mismatch between hetero-junction layers less than 300 ppm), photoluminescence (to assess the optical properties of InGaAlAs, InAlAs, InGaAs and InP layers) and electrochemical capacitance-voltage (to measure the carrier concentrations of P+-InGaAs contact-layer, p-InP window-layer, i-InGaAs absorber, p-InAlAs field control-layer and n+-InAlAs contact-layer). Scanning electron microscopy (SEM) was used to measure the thickness of the epitaxial layers and characterize the interface between layers. Note that the concentration and thickness of the field control layer were factors of critical importance. All measurements pertaining to the epitaxial layers were confirmed by the foundry (Intelligent Epitaxy Technology Inc., Richardson, TX, USA). The electric field profiles of the multiplication and absorption layers were controlled by adjusting the doping and thickness of the p-InAlAs field-control layer, InAlAs multiplication layer and the InGaAs absorption layer. The electric field distribution was calculated using simulation software in accordance with the proposed layer parameters. The proposed SAGFM APD was fabricated using an un-doped InAlAs multiplication layer (0.3-µm-thick), an un-doped InGaAs absorption layer (1.2-µm-thick) and a p-InAlAs field-control layer (0.2-µm-thick) to enable high-speed operations.

Table 1. Details of epitaxial layers in SAGFM APD.

3 Layer Name Epitaxial Layer Thickness (µm) Concentration (cm− ) Contact P+-InGaAs 0.1 1.0–3.0 1019 × Window p-InP 0.35 5.0–6.0 1018 × Grading i-InGaAlAs 0.03 Undoped Absorber i-InGaAs 1.2 Undoped Grading i-InGaAlAs 0.03 Undoped Field control p-InAlAs 0.2 2.0 1017 × Multiplication i-InAlAs 0.3 Undoped Contact n+-InAlAs 1.0 5.0–6.0 1018 × Buffer n-InP 0.5 1.0–5.0 1017 × Semi-insulating – 350 – Sensors 2019, 19, 3399 3 of 15

2.2. Fabrication and Characterization of InAlAs/InGaAs APDs Epitaxial wafers were cut into samples measuring 1 1 cm2 for the fabrication of SAGFM APD × devices. After standard cleaning, a 5-µm-wide p+-InGaAs circular contact-ring (30-µm diameter) was Sensorscreated 2019 on, 19 the, x FOR front PEER surfaces REVIEW of the samples through the selective etching of InGaAs from InP3 using of 15 a solution of H3PO4:H2O2:H2O (1:1:38). Ti/Pt/Au (10 nm/20 nm/300 nm) films were then deposited processing.on the p+-InGaAs The samples contact-ring were th viaen E-beam subjected evaporation to annealing and in photolithographic a rapid thermal lift-o annealingff processing. (RTA) chamberThe samples at 420 were °C thenfor 1 subjectedmin to ensure to annealing the formation in a rapid of a thermalgood p- annealingohmic contact (RTA) [23]. chamber Bromine at- 420based◦C isotropicfor 1 min wet to ensure etching the solution formation (H3PO of4:HBr:K a good2Cr p-ohmic2O3; 1:1:1 contact) was [ used23]. Bromine-based to define a primary isotropic circular wet + mesaetching active solution region (H3 (40PO-4µm:HBr:K diamet2Cr2er)O3 ;as 1:1:1) far wasas the used n - toInAlAs define contact a primary layer circular as well mesa as activea secondary region circular(40-µm diameter)mesa active as region far as the(45- nµm+-InAlAs diameter) contact as far layer as the as wellsemi as-insulating a secondary InP circularsubstrate mesa to ensure active deviceregion isolation (45-µm diameter) [24]. The asetched far as sample the semi-insulating was then immersed InP substrate in hot H to2SO ensure4 solution device (60 isolation °C) for 60 [24 s]. followedThe etched by sample HF:H2O was (1:5) then treatment immersed for in 60 hot s in H 2orderSO4 solution to remove (60 native◦C) for oxides 60 s followed from the by smooth HF:H2O mesa (1:5) surface.treatment Sulfur for 60 passivation s in order towas remove performed native immediately oxides from after the smooth, by immersing mesa surface. the samples Sulfur passivationin (NH4)2Sx solutionwas performed for 40–60 immediately min in a const after,ant by temperature immersing thebath samples [25–27]. in Photosensitive (NH4)2Sx solution polyimide for 40–60 (CRC min 8000 in Seriesa constant Product temperature, Sumitomo bath Bakelite [25–27]. Co., Photosensitive Ltd.; Tokyo, polyimide Japan) was (CRC applied 8000 Series via spin Product,-on coating Sumitomo and patternedBakelite Co., for Ltd.;use as Tokyo, a capping Japan) layer was over applied the sulfur via spin-on passivated coating surface and patterned[28]. The polyimide for use as was a capping cured bylayer ramping over the up sulfur the temperature passivated surface(5 °C per [28 min)]. The until polyimide it reached was 350 cured °C, by where ramping it was up held the temperature for 30 min before(5 ◦C per being min) ramped until it reached down (5 350 °C◦ C, per where min) it to was room held temperature. for 30 min before The entire being curingramped process down (5 was◦C performedper min) to under room temperature.ambient N2. After The entirecuring, curing the thickness process wasof the performed polyimide under was ambientapproximately N2. After 4.0 µm.curing, Polyimide the thickness within of the the polyimide region of was the approximately n+-InAlAs contact 4.0 µm.-ring Polyimide was removed within the for region n-electrode of the metallization.n+-InAlAs contact-ring A N-ohmic was contact removed was for created n-electrode by evaporating metallization. AuGeNi/Au A N-ohmic (50 contact nm/500 was nm) created films by followedevaporating by AuGeNiannealing/Au in (50 an nm RTA/500 chamber nm) films at followed 350 °C for by annealing30 s under in ambient an RTA chamberN2 to ensure at 350 a ◦goodC for ohmic30 s under contact. ambient Note N that2 to ensure a smooth a good mesa ohmic is essential contact. for Note good that surface a smooth passivation mesa is essential effects. for Finally, good SiNxsurface was passivation deposited e fftoects. a thickness Finally, SiNxof 180 was nm deposited on the front to a surface thickness of ofthe 180 APD nm onas an the antireflective front surface coatingof the APD. Figure as an 1a antireflective presents a coating. schematic Figure illustration1a presents of the a schematic proposed illustration SAGFM APD. of the Figureproposed 1b presentsSAGFM APD.a top- Figureview schematic1b presents diagram a top-view of the schematic test pads diagram on APD ofchips the testfor the pads microwave on APD chips probe for used the tomicrowave characterize probe the usedfrequency to characterize and time response. the frequency Note andthat timethe metal response. bond Notepads that(Ti/Pt/Au; the metal 20 nm/30 bond nm/1000pads (Ti/ Pt nm)/Au; were 20 nm connected/30 nm/1000 to the nm) p- wereand connectedn-electrodes. to theWe p- sought and n-electrodes. to minimize Wethe soughtlength ofto connectionminimize the metallization length of connection in order metallization to reduce the in order inductance to reduce component the inductance and thereby component achieve and highthereby-frequency achieve response.high-frequency The performance response. The of performance the fabricated of the APD fabricated was characterized APD was characterized in terms of darkin terms current of dark-voltage current-voltage (I-V), capacitance (I-V), capacitance-voltage-voltage (C-V), photo (C-V), I-V, photomultiplication I-V, multiplication gain (M), time gain (M),and frequencytime and frequency response. response.

Figure 1. 1. SchematicSchematic diagram diagram showing showing InAlAs InAlAs/InGaAs/InGaAs separate separate absorption, absorption, grading, grading, field-control field-control and andmultiplication multiplication (SAGFM) (SAGFM) avalanche avalanche photodiode: photodiode: (a) Side-view, (a) Side-view, (b) top-view. (b) top-view.

2.3. Electric Field Profile Calculation In this study, the InAlAs/InGaAs APD, light absorption and carrier multiplication processes were kept separate by employing an InGaAs absorption layer with a small band gap (Eg = 0.75 eV) and a InAlAs multiplication layer with a large band gap (Eg = 1.46 eV). Two InGaAlAs graded layers were used to shift the band gap from 0.75 eV (InGaAs) to 1.35 eV (InP) and from 0.75 eV (InGaAs) to 1.46 eV (InAlAs) to assist in the transport of carriers (generated in the absorption layer) into the multiplication and window layers. The function of the p-InAlAs field control layer was to

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2.3. Electric Field Profile Calculation In this study, the InAlAs/InGaAs APD, light absorption and carrier multiplication processes were kept separate by employing an InGaAs absorption layer with a small band gap (Eg = 0.75 eV) and a InAlAs multiplication layer with a large band gap (Eg = 1.46 eV). Two InGaAlAs graded layers were used to shift the band gap from 0.75 eV (InGaAs) to 1.35 eV (InP) and from 0.75 eV (InGaAs) Sensors 2019, 19, x FOR PEER REVIEW 4 of 15 to 1.46 eV (InAlAs) to assist in the transport of carriers (generated in the absorption layer) into the maintainmultiplication a high and electric window field layers.for the Themultiplication function of layer the p-InAlAs and a low field electric control field layer for wasthe toabsorption maintain layera high in electric order field to prevent for the multiplication high-field induced layer and current a low electrictunneling. field The for the InAlAs absorption multiplication layer in order layer to requiresprevent high-field an electric induced field intensity current tunneling. of >5 × 10 The5 V/cm InAlAs to multiplicationachieve carrier layer multiplication requires an electric via impact field 5 4 intensity of >5 10 V4/cm to achieve carrier multiplication via impact ionization and >2 10 V/cm to ionization and >2× × 10 V/cm to rapidly sweep out the carriers generated in the InGaAs× absorption layerrapidly (i.e., sweep prior outto recombination). the carriers generated This requires in the InGaAs the optimization absorption of layer electric (i.e., field prior distribution to recombination). in the absorptionThis requires and the multiplication optimization layers. of electric Typically, field distribution a SAGFM in APD the absorptionstructure would and multiplication require an electric layers. 5 5 Typically, a SAGFM APD structure5 5 would require an electric field distribution of 5 104 –8 105 V/cm field distribution of 5 × 10 –8 × 10 V/cm in the InAlAs multiplication layer and 2 × 10 –2 ×× 10 V/cm in the InAlAs multiplication layer and 2 104–2 105 V/cm in the InGaAs absorption layer. Figure2 in the InGaAs absorption layer. Figure 2× presents× the electric field profile, the calculation of which waspresents based the on electric the parameters field profile, of the the epitaxial calculation layer of whichin the SAG was basedFM APD on the(see parameters Table 1), as of a thefunction epitaxial of thelayer distance in the from SAGFM the p APD-n junction (see Table under1), asvarious a function reverse of bias the distancevoltages. fromOur results the p-n indicate junction that under the electricvarious fieldreverse intensity bias voltages. profile Our was results well indicate controlled that to the the electric range field required intensity for profile the was InAlAs well 5 5 controlled to the range required5 for5 the InAlAs multiplication layer (5 10 –8 104 V/cm)5 and the multiplication layer (5 × 10 –8 × 10 V/cm) and the InGaAs absorption layer× (2 × 10 –2 × 10 V/cm) InGaAs absorption layer (2 104–2 105 V/cm) when the APD was operated at a voltage of 0.9 V . when the APD was operated× at a voltage× of 0.9 VBR. BR 9 (a) V = 32.0 V (V ) 8 R BR PN junction (b) V = 28.8 V (0.9 V ) R BR ) 7 (a) (c) V = 17.9 V (V ) R P 6 (b) V/cm 5 (c)

10 5 ( Multiplication layer 4 Field control layer 3 Absorption layer 2 Electric field 1

0 0.0 0.3 0.6 0.9 1.2 1.5 1.8 Distance from pn Junction (μm) FigureFigure 2. 2. ElectricElectric field field profile, profile, calculated calculated as as a afunction function of of the the distance distance from from the the p p-n-n junction junction under under variousvarious reverse reverse bias bias voltages. voltages.

3.3. Results Results and and Discussion Discussion 3.1. DC Characteristics of SAGFM APD 3.1. DC Characteristics of SAGFM APD Figure3 presents dark forward I-V curves obtained from the proposed SAGFM APD before Figure 3 presents dark forward I-V curves obtained from the proposed SAGFM APD before and and after passivation/polyimide coating. The ideality factor (n) and reverse saturation current (J0) after passivation/polyimide coating. The ideality factor (n) and reverse saturation current (J0) of the of the InGaAs/InAlAs SAGFM APD (as extracted from dark I-V curves) were as follows: Before InGaAs/InAlAs SAGFM APD (as extracted from7 dark2 I-V curves) were as follows: Before passivation/coating (n = 2.79 and J0 = 3.21 10− A/cm ) and after passivation/coating (n = 1.86 and 0 −7 2 0 passivation/coating9 (n2 = 2.79 and J = 3.21 × 10× A/cm ) and after passivation/coating (n = 1.86 and J = J0 = 3.55 10− A/cm ). The reduction in n and J0 values after passivation are indications of suppressed 3.55 × 10−×9 A/cm2). The reduction in n and J0 values after passivation are indications of suppressed surface recombination and current leakage. The series resistance (RS) and shunt resistance (RSH) of surface recombination and current leakage. The series resistance (RS) and shunt resistance (RSH) of the SAGFM APD were as follows: Before passivation/coating (RS = 11.94 Ω and RSH = 29.7 MΩ) and the SAGFM APD were as follows: Before passivation/coating (RS = 11.94 Ω and RSH = 29.7 MΩ) and after passivation/coating (RS = 10.11 Ω and RSH = 34.3 MΩ). The increase in RSH demonstrates the after passivation/coating (RS = 10.11 Ω and RSH = 34.3 MΩ). The increase in RSH demonstrates the effectiveness of sulfur and polyimide passivation when applied to a mesa-structure SAGFM APD. All diode parameters of the SAGFM APD are listed in Table 2.

Sensors 2019, 19, 3399 5 of 15 effectiveness of sulfur and polyimide passivation when applied to a mesa-structure SAGFM APD. All diode parameters of the SAGFM APD are listed in Table2. Sensors 2019, 19, x FOR PEER REVIEW 5 of 15

10-1 10-2 10-3 10-4 10-5 10-6 10-7

10-8 10-9 -10

Dark current (A) current Dark 10 -11 10 With polyimide passivation 10-12 Without polyimide passivation 10-13 0.0 0.5 1.0 1.5 2.0 Voltage (V) FigureFigure 3. Dark3. Dark I-V I-V curvescurves of of SAGFM SAGFM avalanche avalanche photodiodes photodiodes (APD) (APD) before and before after and after passivation/polyimide coating. passivation/polyimide coating.

Table 2. Diode performance parameters of SAGFM APD. Table 2. Diode performance parameters of SAGFM APD.

ParameterParameter SymbolSymbol WithWith Polyimide Polyimide WithoutWithout Polyimide Polyimide UnitUnit SeriesSeries resistance resistance RsRs 10.1110.11 11.9411.94 ΩΩ ShuntShunt resistance resistance RshRsh 34.334.3 29.729.7 MMΩΩ Ideality factor n 1.86 2.79 - Ideality factor n 1.86 9 2.79 7 - 2 Reverse Saturation Current Density J0 3.55 10− 3.21 10− A/cm Reverse Saturation Current Density J0 3.55× × 10−9 3.21× × 10−7 A/cm2

I I FigureFigure 4 presents the dark current ((IDD),), capacitance capacitance (C), (C), photocurrent photocurrent ( (Iphph) )and and multiplication multiplication gaingain (M) (M) as as a a function function of of reverse reverse bias bias voltage. voltage. Photocurrent Photocurrent measurements measurements were were obtained obtained under under illuminationillumination using using a a light light source source with with wavelength wavelength of of 1550 1550 nm nm and and optical optical power power of of 1 1µW.µW. The The punch punch throughthrough voltage voltage and and the the breakdown breakdown voltage ofof SAGFMSAGFM APDAPD were were respectively respectively 17.9 17.9 V V and and 32.0 32.0 V V (I D(IDof µ of10 10A), µA), as measured as measured at room at room temperature. temperature. At 0.9 AtVBR 0.9, the V darkBR, the current dark was current 31.4 nA, was the 31.4 capacitance nA, the capacitancewas 0.19 pF was and 0.19 the multiplicationpF and the multiplication gain was 9.8. gain At was a temperature 9.8. At a temperature of 300 K under of 300 1 µ WK under illumination, 1 µW illumination,the responsivity the responsivity and a multiplication and a multiplication gain were as gain follows: were 0.9 as Vfollows:BR (8.04 0.9 A/ WVBR and (8.04 9.8) A/W and and 0.95 9.8) VBR and(18.94 0.95 A V/WBR (18.94 and 23.1). A/W Theand maximum23.1). The maximum multiplication multiplication gain at V BRgainwas at 994.VBR was The 994. low The dark low current, dark current,low capacitance low capacitance and high multiplicationand high multiplication gain of the InGaAs gain of/InAlAs the InGaAs/InAlAs SAGFM APD demonstrate SAGFM APD the demonstrateapplicability the of applicability the proposed of device the proposed for high-speed device for optical high communication-speed optical communication systems. For the systems sake of. Fclarity,or the sake we calculated of clarity, thewe multiplicationcalculated the multiplication gain (M) of the gain APD (M) as follows:of the APD as follows:

I Iph IID ph − D MM= (1)(1) Ipho IDo I pho −I Do where I is the APD photocurrent that has been multiplied, I is the unmultiplied photocurrent, I is where Iphph is the APD photocurrent that has been multiplied, phoIpho is the unmultiplied photocurrent,D ID the APD dark current that has been multiplied and I is the unmultiplied dark current. is the APD dark current that has been multiplied andDo IDo is the unmultiplied dark current.

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10-3 1000 1.2 Dark current 10-4 Photocurrent @1 μW, T = 25oC Multiplication gain (M) 1.0 -5 10 Capacitance, f = 1MHz, T = 25oC

-6 100 0.8 10

-7 10 0.6 Sensors 2019, 19, 3399 10-8 6 of 15 10 0.4 Sensors 2019, 19, x FOR PEERCurrent (A) 10 REVIEW-9 6 of 15 -10 0.2 (pF) Capacitance 10-3 10 1000 gain (M) Multiplication 1.2 10-11 Dark current 10-4 Photocurrent @1 μW, T = 25oC 1 0.0 0 5 Multiplication10 15 gain (M)20 25 30 35 1.0 10-5 o Capacitance,Reverse f = bais1MHz, voltage T = 25 C (V) -6 100 0.8 10 Figure 4. Dark current, capacitance, photocurrent and multiplication factor of InGaAs/InAlAs -7 SAGFM APD as a function10 of reverse bias voltage. 0.6 10-8 10 0.4 Figure 5 presentsCurrent (A) -9the breakdown voltage of InP-based [29] and InAlAs-based APDs (current 10 work) as a function of operating temperature. Generally, the bias of an APD is controlled (pF) Capacitance with the 10-10 0.2 aim of preserving the gain of the device despite fluctuations in temperature. gain (M) Multiplication However, most APD 10-11 devices used in communication systems are meant to1 be operated0.0 at temperatures between −40 °C and 85 °C0 . Thus,5 the 10knowledge15 of20 how25 avalanche30 breakdown35 voltage changes with Reverse bais voltage (V) temperature could be useful. As shown in Figure 5, the breakdown voltage temperature coefficient (ΔVBRFigureFigure/ΔT) of 4.4. InPDarkDark-based current, current, and capacitance, capacitance,InAlAs-based photocurrent photocurrent APDs andwere andmultiplication 0.105 multiplication V/°C factor[29] and factorof InGaAs 0.028 of / InGaAs/InAlAsInAlAsV/°C, respectively. SAGFM TheseSAGFMAPD results as aAPD functiondemonstrate as a function of reverse that of biasreversethe voltage.InAlAs bias -voltage.based APD was less sensitive than the InP-based APD to variations in temperature. FigureFigure 5 5 6presents presents displays the the the breakdown breakdown photocurrent voltage voltage as of a InP-based of function InP-based [ of29 ] distance[29] and and InAlAs-based acrossInAlAs the-based APDs active APDs (current area (current of work) the work)aspropo a function sedas a mesafunction of- operatingstructure of operating SAGFM temperature. te mperature.APD Generally,(diameter Generally, of the 30 bias µm).the of bias anIncident APDof an is lightAPD controlled wasis controlled delivered with the with using aim the ofa aimpreservingdistributed of preserving thefeedback gain the of (gainDFB the device)of laser the via despitedevice a lens despite fluctuations fiber atfluctuations a wavelength in temperature. in temperature. of 1550 However, nm andHowever, most a power APD most of devices 1APD µW. devicesusedThe light in optical used spot inwas fiber optical moved communication fiber in steps communication of systems 2 µm. None are systems meant of the to arephotocurrent be operated meant to at becurves temperatures operated presented at between temperatures a significant40 C − ◦ betandspikeween 85 at◦ the−C.40 Thus, edge°C and of the 85the knowledge °C active. Thus, region; the of knowledge how however, avalanche theof how level breakdown avalanche of photocurrent voltage breakdown produced changes voltage with within changes temperature the active with temperaturecouldregion be was useful. uniform, could As be shown when useful. inthe As Figure multiplication shown5, the in breakdownFig uregain 5, was the voltage variedbreakdown temperaturebetween voltage 1 and coetemperature 10.ffi cientThis demonstrates (∆ VcoefficientBR/∆T) of (ΔVInP-basedthe BRefficacy/ΔT) of and of InP the InAlAs-based-based propose andd mesaInAlAs APDs structure-based were 0.105APDs in suppressing V /were◦C[29 0.105] and edge V/ 0.028°C-breakdown. [29] V/◦ andC, respectively. 0.028 It also V/ demonstrates°C, Theserespectively. results the Thesedemonstratehigh uniformity results demonstrate that of the the InAlAs-basedlayer that structure the InAlAs APD grown- wasbased by lessMBE APD sensitive in was terms less thanof sensitive thickness the InP-based than and the doping APDInP-based concentration to variations APD to variationsin temperature.the field in-control temperature. and multiplication layers. Figure 6 displays the photocurrent as a function of distance across the active area of the proposed mesa-structure SAGFM APD (diameter of 30 µm). Incident light was delivered using a 45 distributed feedback (DFB) laser via a lens fiber at a wavelength of 1550 nm and a power of 1 µW. The light spot was moved in steps of 2 µm. None of the photocurrent curves presented a significant spike at the edge of the active region; however, the level of photocurrent produced within the active region was uniform, when40 the multiplication gain was varied between 1 and 10. This demonstrates the efficacy of the proposed mesa structure in suppressing edge-breakdown. It also demonstrates the InP of temperature coefficient high uniformity of the layer structure grown by MBEis in 0.105 terms V/oC of [29] thickness and doping concentration in the field-control and multiplication35 layers.

Breakdownvoltage (V) 45 InAlAs temperature coefficient 30 is 0.028 V/oC

40-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 o Temperature ( C) InP of temperature coefficient Figure 5. Breakdown voltage of InP-basedInP-based [ 29] and is InAlAs-basedInAlAs 0.105 V/-basedoC [29] APD (current work) as a function of temperature. 35

Breakdownvoltage (V) InAlAs temperature coefficient 30 is 0.028 V/oC

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 o Temperature ( C) Figure 5. Breakdown voltage of InP-based [29] and InAlAs-based APD (current work) as a function of temperature.

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Figure6 displays the photocurrent as a function of distance across the active area of the proposed mesa-structure SAGFM APD (diameter of 30 µm). Incident light was delivered using a distributed feedback (DFB) laser via a lens fiber at a wavelength of 1550 nm and a power of 1 µW. The light spot was moved in steps of 2 µm. None of the photocurrent curves presented a significant spike at the edge of the active region; however, the level of photocurrent produced within the active region was uniform, when the multiplication gain was varied between 1 and 10. This demonstrates the efficacy of the proposed mesa structure in suppressing edge-breakdown. It also demonstrates the high uniformity of the layer structure grown by MBE in terms of thickness and doping concentration in the field-control and multiplication layers. Sensors 2019, 19, x FOR PEER REVIEW 7 of 15

Active area 10-5

10-6

M=10 M=7 -7 10 M=5

Photocurrent (A) M=3 M=1 10-8 -20 -15 -10 -5 0 5 10 15 20 Distance (μm) FigureFigure 6 6.. PhotocurrentPhotocurrent as as a afunction function of of distance distance across across the the active active area area of of the the proposed proposed SAGFM SAGFM APD APD (i.e.,(i.e., diameter) diameter)..

FigureFigure 7 7 presents presents the the measured measured photocurrent photocurrent as a as function a function of incident of incident optical optical power power and reverse and reversebias voltage. bias voltage. Incident Incident light was light delivered was delivered using a DFB using laser a DFB via alaser lens via fiber a atlens a wavelengthfiber at a wavelength of 1550 nm. ofThe 1550 black nm. dashed The lines black in dashed Figure 7lines refer toin APD Figure devices 7 refer operating to APD with device constants operating responsivity with (R) constant across responsivitythe full range (R) of across incident the optical full range power. of Here,incident the optical responsivity power. of Here, 1.82 A the/W responsivity and 8.23 A/W of respectively 1.82 A/W andrefer 8.23 to InGaAs A/W respectively/InAlAs SAGFM refer APDs to InGaAs/InAlAs operated at 20 SAGFMV and 0.9 APDs VBR linearly operated across at 20 the V entire and range0.9 VBR of linearlyincident across optical the power. entire Ideally, range theof inci photocurrentdent optical values power. should Ideally, closely the matchphotocurrent the dashed values line, should which closelywould match indicate the that dashed the incident line, which optical would power indicate received that bythe the incident APD coveredoptical power a wide received dynamic by range the APD(DR). covered In this a study, wide thedynamic dynamic range range (DR). of In the this APD study, device the dynamic biased at range 20 V of (45.0 the dB)APD was device larger biased than atthat 20 ofV (45.0 the device dB) was biased larger at than 0.9 V thatBR (39.08 of the dB). device Under biased high at incident 0.9 VBR (39. optical08 dB). power Under and high high incident voltage opticalbiasing, power the multiplication and high voltage factor biasing, and photocurrentthe multiplication generated factor byand the photocurrent APD device generated were both by high. the APDThe internaldevice were series both resistance high. The and internal external series load resistance resistance associated and external with load high resistance output photocurrent associated withresulted high in output voltage photocurrent potential on theresulted two output in voltage terminals potential of APD on device.the two This output voltage terminals potential of wouldAPD device.induce This an increase voltage in potential the internal would shunt induce current an increase with the in result the thatinternal the outputshunt current photocurrent with the would result be thatsaturated the outp byut the photocurrent incident optical would power. be saturated In contrast, by underthe incident low incident optical optical power. power, In contrast, the measured under lowoutput incident photocurrent optical power, would the be measured limited by output dark current. photocurrent For the would sake be of clarity,limited weby calculateddark current. the Fordynamic the sake range of clarity, (DR) values we calculated as follows: the dynamic range (DR) values as follows:

PH DR (dB) 10 log . (2) PL where PH is the highest incident optical power (i.e., when output photocurrent is beginning to be saturated), PL is the lowest incident optical power (i.e., when the output photocurrent is equal to the dark current). Table 3 lists the calculated dynamic range of incident optical power.

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P DR (dB) = 10 log H . (2) PL where PH is the highest incident optical power (i.e., when output photocurrent is beginning to be saturated), PL is the lowest incident optical power (i.e., when the output photocurrent is equal to the dark current). Table3 lists the calculated dynamic range of incident optical power. Sensors 2019, 19, x FOR PEER REVIEW 8 of 15

Incident Power (W)

-2 1 n 10 n 100 n 1 μ 10 μ 100 μ 1 m 10 I @ 0.9 V , 1550 nm ph BR -3 10 I @ 0.9 V D BR I @ 20 V, 1550 nm 10-4 ph I @ 20 V D R=8.23 A/W 10-5

10-6

ID= 31.4 nA 10-7 R=1.82 A/W

-8 10 ID= 10.4 nA 10-9 39.1 dB

Photocurrent/Dark current (A) 45.0 dB 10-10 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 Incident Power (dBm)

Figure 7. Measured photocurrent as a function of incident optical power and reverse bias voltage. Figure 7. Measured photocurrent as a function of incident optical power and reverse bias voltage. Table 3. Dynamic range of incident optical power. Table 3. Dynamic range of incident optical power. DR (dB) PH (W) PL (W) @0.9 V 39.08DR (dB) 1.78PH (W)10 5 PL 2.2(W) 10 9 BR × − × − @[email protected] V VBR 45.0239.08 1.78 × 1010−45 2.2 5.6× 10−910 9 × − × − @20 V 45.02 1.78 × 10−4 5.6 × 10−9 Figure8 presents the reverse dark current of four SAGFM APDs as a function of aging time. Figure 8 presents the reverse dark current of four SAGFM APDs as a function of aging time. Current leakage and reliability are important issues when dealing with mesa-structure APD devices. Current leakage and reliability are important issues when dealing with mesa-structure APD devices. Before conducting aging tests, we first screened test samples to obtain reference data for comparisons. Before conducting aging tests, we first screened test samples to obtain reference data for The aging conditions included stress reverse current of 100 µA and stress temperature of 85 ◦C in comparisons. The aging conditions included stress reverse current of 100 µA and stress temperature an air atmosphere. As shown in Table4, testing began after approximately 100–200 h. The samples of 85 °C in an air atmosphere. As shown in Table 4, testing began after approximately 100–200 h. were measured at room temperature to monitor the dark current at 25 V and then returned to the The samples were measured at room temperature to monitor the dark− current at −25 V and then aging system to repeat the procedure. We did not detect a significant variation in dark current after returned to the aging system to repeat the procedure. We did not detect a significant variation in aging for 1344 h (i.e., 15–20 nA with S.D. <1.6 nA, which is nearly identical to the values obtained in dark current after aging for 1344 h (i.e., 15–20 nA with S.D. <1.6 nA, which is nearly identical to the the screening stage). These results clearly demonstrate the efficacy of the proposed sulfur/polyimide values obtained in the screening stage). These results clearly demonstrate the efficacy of the passivation scheme for the treatment of SAGFM APD with a mesa-structure. proposed sulfur/polyimide passivation scheme for the treatment of SAGFM APD with a mesa-structure.

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10-7 Aging condition: 85oC, -25 V

10-8

Chip 1 Chip 2 Chip 3

Dark Dark current RTat (A) Chip 4 10-9 100 1000

Aging time (hours) FigureFigure 8. 8.Variations Variations in in dark dark current current measured measured at at RT RT after after aging. aging. Aging Aging conditions conditions included included reverse reverse darkdark current current of of 100 100µ µAA at at 85 85◦ C°C in in air air atmosphere. atmosphere. Table 4. Variations in the dark current of SAGFM APDs after stressed aging. Table 4. Variations in the dark current of SAGFM APDs after stressed aging.

Aging Time (hours) Chip 1 ID (nA) Chip 2 ID (nA) Chip 3 ID (nA) Chip 4 ID (nA) Aging Time (hours) Chip 1 ID (nA) Chip 2 ID (nA) Chip 3 ID (nA) Chip 4 ID (nA) 00 15.515.5 17.917.9 16.816.8 17.6 17.6 7272 16.616.6 18.918.9 17.517.5 18.8 18.8 168168 15.015.0 20.220.2 18.018.0 19.5 19.5 336 15.2 19.5 17.1 23.5 336 15.2 19.5 17.1 23.5 504 16.3 18.8 18.4 21.2 504 16.3 18.8 18.4 21.2 672 16.8 19.7 17.7 20.1 840672 15.416.8 17.819.7 17.417.7 20.1 18.8 1008840 16.715.4 17.517.8 18.517.4 18.8 17.6 11761008 17.616.7 18.517.5 20.818.5 17.6 18.4 13441176 15.917.6 18.818.5 19.020.8 18.4 17.9 15121344 16.415.9 19.418.8 18.419.0 17.9 18.4 16801512 17.816.4 19.919.4 16.818.4 18.4 18.8 18481680 17.417.8 18.319.9 17.416.8 18.8 19.3 20161848 16.517.4 19.718.3 18.017.4 19.3 20.5 Standard Deviation2016 (S.D.; nA) 0.816.5 0.819.7 1.018.0 20.5 1.6 Standard Deviation (S.D.; nA) 0.8 0.8 1.0 1.6 3.2. AC Characteristics of SAGFM APD 3.2. AC Characteristics of SAGFM APD Figure9a presents the measured frequency response ( f 3-dB) of the SAGFM APD under incident opticalFigure power 9a of presents 1.0 µW the and measured reverse bias frequency voltage response of 0.9 VBR (f,3- asdB) measuredof the SAGFM under APD load under resistance incident of 50opticalΩ using power a HP of 8703A1.0 µW lightwave and reverse component bias voltage analyzer. of 0.9 V WeBR, measuredas measuredf 3-dB underof 8.97 load GHz resistance at the chipof 50 level,Ω using which a HP corresponds 8703A lightwave to an operating component bit rateanalyzer. of >10 We Gbps. measured Figure9 fb3-dB presents of 8.97 GHzf 3-dB asat the a function chip level, of multiplicationwhich corresponds gain from to an 2 tooperating 50. The highest bit rate f of3-dB >10bandwidth Gbps. Figure (approximately 9b presents 9 f GHz)3-dB as was a function obtained of withinmultiplication a multiplication gain from gain 2 to range 50. The of 5–10. highest The f3f-dB3-dB bandwidthbandwidth (approximately increased with 9 multiplicationGHz) was obtained gain betweenwithin a 2 multiplication and 4, due to a gain reduction range inof capacitance 5–10. The f associated3-dB bandwidth with increased an increase with in the multiplication depletion width gain ofbetween the APD 2 andunder 4, reverse due to abias reduction voltage. in The capacitancef 3-dB bandwidth associated also with decreased an increase when in multiplication the depletion gainwidth exceeded of the 10, APD due to under an increase reverse in bias avalanche voltage. build-up The f time3-dB bandwidth with an increase also in decreased multiplication when gain.multiplication The avalanche gain build-upexceeded time 10, due constant to an increased increase linearlyin avalanche with thebuild multiplication-up time with gain an andincrease is also in proportionalmultiplication to gain. the ionization The avalanche coeffi cientbuild ratio-up time (k) [ 30constant]. In general, increase thed linearly gain-bandwidth with the multiplication (GB) product ofgain APD and is inversely is also proportional proportional to to the the mean ionization drift time coefficient of carriers within ratio ( thek) avalanche[30]. In genera regionl, and the isgain proportional-bandwidth to (GB) the ionizationproduct of coe APDfficient is inversely ratio [31 ].proportional Therefore, theto the k-ratio mean should drift time be as of small carriers as within the avalanche region and is proportional to the ionization coefficient ratio [31]. Therefore,

SensorsSensors 20192019,, 1919,, x 3399 FOR PEER REVIEW 1010 of of 15 15 Sensors 2019, 19, x FOR PEER REVIEW 10 of 15 the k-ratio should be as small as possible to improve the GB-product. Overall, the gain-bandwidth possible to improve the GB-product. Overall, the gain-bandwidth product of the proposed SAGFM productthe k-ratio of theshould proposed be as smallSAGFM as APDpossible reached to improv 88 GHz.e the GB-product. Overall, the gain-bandwidth productAPD reached of the 88proposed GHz. SAGFM APD reached 88 GHz. -35 (b) Wavelength: 1550 nm -35 (a) Wavelength: 1550 nm Incident Power: 1 μW (a) Power:Wavelength: 1 μW 1550 nm (b) Wavelength: 1550 nm Reverse bias: 0.9 V Incident Power: 1 μW Power: 1 μW BR -40 Reverse bias: 0.9 V BR -40 10 f = 8.97 GHz 3dB 10 f = 8.97 GHz GBP = 88 GHz 3dB

-45 GBP = 88 GHz

-45 Power (dBm)

Power (dBm) -50

-50 3-dB bandwidth (GHz) 3-dB bandwidth (GHz)

-55 1 -550.13 5 10 15 1 1 10 100 0.13 5 10 15 1 10 100 Frequency (GHz) Multiplication gain Frequency (GHz) Multiplication gain Figure 9. (a) Measured frequency response of SAGFM APD under incident optical power of 1.0 µW Figure 9. (a) Measured frequency response of SAGFM APD under incident optical power of 1.0 µW andFigure reverse 9. (a) bias Measured voltage frequency of 0.9 VBR response; (b) Measured of SAGFM f3-dB APD bandwidth under incident as a function optical of power multiplication of 1.0 µW and reverse bias voltage of 0.9 VBR; (b) Measured f3-dB bandwidth as a function of multiplication factorand reverse from 2 bias to 50 voltage. of 0.9 VBR;(b) Measured f 3-dB bandwidth as a function of multiplication factor factorfrom 2from to 50. 2 to 50. Figure 10 presents eye diagrams of the InGaAs/InAlAs SAGFM APD, under a fixed multiplicationFigureFigure 10 10 gainpresents presents of 10 eye using eye diagrams nonreturn diagrams of the -to of InGaAs- zero the (NRZ)/ InGaAs/InAlAsInAlAs pseudorandom SAGFM APD, SAGFM codes under APD, with a fixed a under length multiplication aof fixed231-1, 31 31 undermultiplicationgain of operating 10 using gain nonreturn-to-zerospeeds of 10 correspondingusing nonreturn (NRZ) to pseudorandom-to bit-zero rates (NRZ) of (a) codespseudorandom 10, (b) with 11, a(c) length 12 codes and of 2with(d)-1, 12.5 a under length Gb/s. operating of A 2 rise-1, timeunderspeeds of operating correspondingapproximately speeds to38.9 corresponding bit– rates42.0 ps of was (a) 10, derivedto (b)bit 11,rates from (c) 12of the and(a) eye10, (d) -(b)diagrams. 12.5 11, Gb (c)/s. 12 AObserve riseand time(d) that 12.5 of we approximately Gb/s. detecte A rised a notabletime38.9–42.0 of approximatelyabsence ps was of intersymbol derived 38.9– from42.0-interference ps the was eye-diagrams. derived and thefrom signals Observethe eye remained-diagrams. that we error detectedObserve-free at that adata notable -werates detecte of absence upd to a 12.5notableof intersymbol-interference Gbps. absence Figure of intersymbol 11 presents and - theinterference the eye signals diagrams and remained the obtained signals error-free remained from the at error InGaAs/InAlAsdata-rates-free at of data up SAGFM-rates to 12.5 of Gbps.up APD to operated12.5Figure Gbps. 11 under presents Figure multiplication the11 presents eye diagrams gains the eye obtainedof diagrams(a) 3, (b) from 5, obtained the(c) InGaAs10, (d) from 20,/InAlAs the(e) 30 InGaAs/InAlAs SAGFM with the APD bit rate operated SAGFM fixed underat APD 10 Gb/s.operatedmultiplication A 10under Gb/s gains multiplication built of- (a)in 3,eye (b) gains mask 5, (c) of was10, (a) (d) 3, used (b) 20, 5, (e) to (c) 30 evaluate 10, with (d) the 20, thebit (e) APDrate 30 with fixed operated the at 10bit withGb rate/s. fixed a A suitable 10 at Gb 10/s multiplicationGb/s.built-in A eye 10 mask Gb/s gain; was built i.e., used- inwhere to eye evaluate the mask transmitted the was APD used operated signals to evaluate withwould a suitable thebe not APD present multiplication operated within with gain; the aeye i.e., suitable wheremask region.multiplicationthe transmitted We determined gain; signals i.e., th where wouldat multiplication the be nottransmitted present gains withinsignalsof 5 and thewould 10 eyewere be mask thenot optimal region.present choices. within We determined theWe eyesought mask that to characterizeregion.multiplication We determined the gains dynamic of th 5at and multiplication range 10 were of the the optimalgains AC of optical choices.5 and 10 power Wewere sought the by optimal obtaining to characterize choices. eye We diagrams the sought dynamic ofto InGaAs/InAlAscharacterizerange of the AC the SAGFM optical dynamic power APD range operated by obtaining of at the 10 eye Gb/s AC diagrams opticalwith various of power InGaAs incident by/InAlAs obtaining optical SAGFM powers eye APD diagrams(−17.22 operated dBm, of at InGaAs/InAlAs10 Gb/s with various SAGFM incident APD operated optical powers at 10 Gb/s ( 17.22 with dBm, various12.22 incident dBm, optical7.22 dBm,powers0.22 (−17.22 dBm dBm, and −12.22 dBm, −7.22 dBm, −0.22 dBm and 0.78 − dBm) at room− temperature,− as shown− in Figure 12. Similarly,−0.7812.22 dBm) dBm, atthe room− 7.22 transmitted temperature, dBm, − 0.22 signals dBm as shown should and in 0.78 not Figure dBm) be 12 present . at Similarly, room within temperature, the the transmitted eye maskas shown signals region. in should Figure This not was 12. be achieveSimilarly,presentd within when the transmitted thethe eyedynamic mask signals range region. of should This incident was not achievedoptical be present power when within ranged the dynamic the from eye − range12. mask22 dBm of region. incident to 0.78 This optical dBm. was achievepower rangedd when from the dynamic12.22 dBm range to 0.78of incident dBm. Thus, optical the power dynamic ranged range from of AC −12. incident22 dBm opticalto 0.78 powerdBm. Thus, the dynamic range− of AC incident optical power was approximately 13 dB, which is less than Thus, the dynamic range of AC incident optical power was approximately 13 dB, which is less than thatwas of approximately DC incident 13optical dB, which power is (45 less dB). than Table that of5 lists DC incidenttime response optical and power calculation (45 dB). of Table f3-dB5 datalists usingthattime of responsevalues DC incident obtained and calculation optical from eyepower of diagramf 3-dB (45data dB). measurements. using Table values 5 lists obtained time response from eye and diagram calculation measurements. of f3-dB data using values obtained from eye diagram measurements.

Figure 10. Cont.

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FigureFigure 10. 10. EyeEye diagrams diagrams ofof SAGFM-APDSAGFMSAGFM--APDAPD operated operatedoperated under underunder a aa multiplication multiplicationmultiplication gain gaingain of ofof 10 1010 and andand at atat bit bitbit rates ratesrates of of(ofa ) ((a 10,a)) 10,10, (b ) ((b 11,b)) 11,11, (c) (( 12cc)) 12 and12 andand (d ) ((d 12.5d)) 12.512.5 Gb Gb/sGb/s/s. ..

Figure 11. Eye diagrams of SAGFM-APD operated under multiplication gains of (a) 3, (b) 5, (c) 10, Figure 11. Eye diagrams of SAGFM SAGFM-APD-APD operated under multiplication multiplication gains gains of of ( (aa)) 3, 3, ( (bb)) 5, 5, ( c) 10, (((dd))) 20, 20,20, ( ((ee))) 30 3030 at at a a bit bit rate rate of of 10 10 Gb/s GbGb/s/s...

Sensors 2019, 19, 3399 12 of 15 Sensors 2019, 19, x FOR PEER REVIEW 12 of 15

Figure 12. EyeEye diagrams diagrams of of SAGFM SAGFM-APD-APD operated operated unde underr incident incident powers powers of ( ofa) (−a17.22) 17.22 dBm, dBm, (b) − −(b12.22) 12.22 dBm, dBm, (c) − (c7.22) 7.22 dBm, dBm, (d) (−d0.22) 0.22 dBm, dBm, (e) (0.78e) 0.78 dBm dBm at ata bit a bit rate rate of of 10 10 Gb/s Gb/ swith with a a multiplication multiplication − − − gain of 10. Note: Note: ( (aa––dd)) The The vertical vertical scale scale is is 15 15 mV; mV; ( (ee–,ff)) thethe verticalvertical scalescale isis 4040 mV.mV.

TableTable5. 5.Time Time response response and calculationand calculation of f 3-dB ofdata f3-dB using data values using obtained values from obtained eye diagram from measurements. eye diagram measurements. Speeds @M = 10 Rise Time (ps) Calculate f 3-dB (GHz) SpeedsBit rate: @M 10 Gb = 10/s Rise 42.0 Time (ps) Calculate 8.33 f3-dB (GHz) BitBit rate: rate: 10 11 GbGb/s/s 41.542.0 8.438.33 BitBit rate: rate: 11 12 GbGb/s/s 40.641.5 8.628.43 Bit rate: 12.512 Gb/s Gb/s 38.940.6 8.998.62 MultiplicationBit rate: 12.5 Gain Gb/s @10 Gb/s –38.9 –8.99 MultiplicationM =G3ain @10 Gb/s 53.2-- 6.58-- MM = 35 43.853.2 7.996.58 M = 10 43.3 8.08 M = 5 43.8 7.99 M = 20 46.0 7.61 MM = 1030 49.143.3 7.138.08 M = 20 46.0 7.61 M = 30 49.1 7.13 Incident power (dBm) @10 Gb/s, M = 10 -- --

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Table 5. Cont.

Speeds @M = 10 Rise Time (ps) Calculate f 3-dB (GHz) Incident power (dBm) @10 Gb/s, M = 10 –– 17.22 50.0 7.00 − 12.22 48.7 7.19 − 7.22 43.3 8.08 − 2.22 35.3 9.91 − 0.22 42.4 8.25 − 0.78 43.3 8.08

4. Conclusions This paper presents the fabrication and characterization of InAlAs/InGaAs SAGFM APDs with a mesa-structure to enable high-speed operations. The side walls of the mesa underwent sulfur treatment before being coated with a thick polyimide passivation layer, to reduce current leakage, reduce capacitance, increase response speeds and enhance reliability. The passivation effects were shown to reduce surface recombination and current leakage, as indicated by a reduction in ideality factor and reverse saturation current. The breakdown temperature coefficient of InGaAs/InAlAs APD was approximately 0.028 V/◦C, which indicates that the proposed device is highly insensitive to temperature variations. Under 0.9 breakdown voltage biasing, we obtained impressive results in terms of dark current (31.4 nA), capacitance (0.19 pF) and multiplication gain (10) at room temperature. The dynamic range of DC incident optical power was approximately 45 dB, which is higher than that of AC incident optical power (13 dB). We observed only a negligible change in dark current (i.e., 15–20 nA) after aging for 1344 h. We also obtained f 3-dB of 8.91 GHz and gain-bandwidth product up to 88 GHz. In eye-diagrams, there was also a notable absence of intersymbol-interference and the signals remained error-free at data-rates of up to 12.5 Gbps.

Author Contributions: All of the authors conceived the experiments; W.-J.H. designed, analyzed and wrote the first draft of the paper; J.-J.L., J.-Y.C., J.-N.L., Y.-C.L. and M.-J.C. performed the experiments; C.-J.T. and C.-C.Y. conducted APD chip measurements; all authors contributed to the discussion. Funding: The authors would like to thank the Tyntek Corporation for their financial support and the authors would like to thank the Ministry of Science and Technology of the Republic of China for their financial support under Grant MOST 106-2221-E-027-101-MY3. Conflicts of Interest: The authors declare no conflict of interest.

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