Inverted P-Down Design for High-Speed Photodetectors

hv photonics

Review Inverted p-down Design for High-Speed Photodetectors

Masahiro Nada 1,*, Fumito Nakajima 2, Toshihide Yoshimatsu 1, Yasuhiko Nakanishi 1, Atsushi Kanda 1, Takahiko Shindo 1, Shoko Tatsumi 1, Hideaki Matsuzaki 2 and Kimikazu Sano 1

1 NTT Device Innovation Center, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan; [email protected] (T.Y.); [email protected] (Y.N.); [email protected] (A.K.); [email protected] (T.S.); [email protected] (S.T.); [email protected] (K.S.) 2 NTT Device Technology Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan; [email protected] (F.N.); [email protected] (H.M.) * Correspondence: [email protected]

Abstract: We discuss the structural consideration of high-speed photodetectors used for optical communications, focusing on vertical illumination photodetectors suitable for device fabrication and optical coupling. We fabricate an avalanche photodiode that can handle 100-Gbit/s four-level pulse-amplitude modulation (50 Gbaud) signals, and pin photodiodes for 100-Gbaud operation; both are fabricated with our unique inverted p-side down (p-down) design.

Keywords: avalanche photodiode; photodiode; vertical illumination structure

1. Introduction Requirements for photodetectors in optical communications systems are always simple: a larger bandwidth to handle a higher bitrate that will lead to a larger transmission Citation: Nada, M.; Nakajima, F.; capacity; higher responsivities to extend the transmission distance and allow a larger Yoshimatsu, T.; Nakanishi, Y.; Kanda, splitting ratio of optical signals; a lower dark current to ensure reliability and improve A.; Shindo, T.; Tatsumi, S.; Matsuzaki, the signal-to-noise ratio; and a larger tolerance of assembling. These requirements are H.; Sano, K. Inverted p-down Design elementary but are sometimes a bottleneck limiting the capacity on optical networks. The for High-Speed Photodetectors. 400-Gbit/s direct detection system is a good example. The standardization on 400-Gbit/s Photonics 2021 8 , , 39. https:// Ethernet systems limits their transmission ability to 6 km [1], although past 100- and doi.org/10.3390/photonics8020039 200-Gbit/s Ethernet systems have been standardized to reach 10 km using pin photodiodes (pin-PDs). Additionally, discussions on 800-Gbit/s systems for both coherent systems and Received: 18 January 2021 direct-detection systems are on the horizon, but the outlook is still unclear. There is no doubt Accepted: 1 February 2021 Published: 4 February 2021 that better photodetectors are needed to accelerate such 400- and 800-Gbit/s applications. Up to now, we have developed high-speed and high-responsivity pin-PDs and

Publisher’s Note: MDPI stays neutral avalanche photodiodes (APDs), both with a vertical illumination structure. While the with regard to jurisdictional claims in waveguide structure is advantageous to mitigate the tradeoff between responsivity and published maps and institutional affil- bandwidth [2–5], the vertical illumination structure is still attractive in terms of device iations. fabrication, reliability, and assembly. Particularly, the inverted p-down design we have developed enables the vertical illumination structure to boost bandwidth while maintaining a certain responsivity by incorporating a hybrid absorber consisting of p-type and undoped absorbers while maintaining the benefits of the vertical illumination structure. The inverted p-down design also allows device fabrication using a simple etching process, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. eliminating ion-implantation or selective diffusion processes for the purpose of confining This article is an open access article the active region to the center of the device, which helps further reduce surface leakage. distributed under the terms and Here, we review and discuss the design of the inverted p-down structures for pin-PDs conditions of the Creative Commons and APDs. The inverted p-down APD is designed for high-sensitivity 100-Gbit/s four- Attribution (CC BY) license (https:// level pulse-amplitude modulation (PAM4) operation with a baud rate of 50 Gbaud. The creativecommons.org/licenses/by/ maximum bandwidth reaches 37 GHz and successfully demonstrates 100-Gbit/s PAM4 4.0/). operation with a receiver sensitivity of −15.5 dBm. The inverted p-down pin-PD with

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an active diameter of 10 µm exhibits over 70 GHz with 0.61 A/W at 2 V. Even at the bias voltage of 10 V, the dark current is less than 1 nA, which indicates the effectiveness of the inverted p-down design for stabilization on device surfaces.

2. Inverted p-down Design 2.1. Hybrid Absorber One key consideration to simultaneously achieve a larger responsivity and bandwidth is engineering layer structures to reduce the entire carrier transit time with as thick an absorber as possible. A hybrid absorber is effective for this [6]. Figure1a shows a schematic of a hybrid absorber. It consists of a p-type InGaAs absorber at the p-type contact side and an undoped InGaAs absorber at the opposite side. Under the operating bias voltage, the electric field is invoked only in the undoped absorber, while the p-type absorber remains electrically neutral. The electrons generated at the p-type absorber act diffusively with an electron diffusion coefficient of De. Note that an electron basically moves in both the direction of the p-type contact and its opposite side due to diffusive motion. The electron block layer helps electrons move toward the undoped absorber side. After the electrons are injected into the depleted (undoped) absorber, they drift together with the electrons generated in the undoped absorber. The drift velocity of electrons is governed by the saturation velocity of the electrons in InGaAs. The holes generated in the undoped absorber also are pushed toward the p-type InGaAs absorber by drift. They then behave as a majority carrier in the p-type InGaAs absorber. The electron drift velocity is one digit larger than that of the holes, and the dielectric relaxation time of the hole in the p-type absorber is negligibly smaller compared with other carrier transit times. Thus, in all carrier flows, we can take only electron diffusion and hole drift times into account for considering carrier transit time. Importantly, the responsivity of the hybrid absorber is determined Photonics 2021, 8, x FOR PEER REVIEW by its total thickness, including its p-type and undoped absorbers. To summarize, the 3 of 9 total carrier transit time in a hybrid absorber can be mitigated while maintaining a certain responsivity and, thus, leads to a larger bandwidth.

(a) (b) FigureFigure 1. (a) 1.schematic(a) schematic band band diagram diagram of ofa hybrid a hybrid absorber absorber and and (b)) calculatedcalculated carrier carrier transit transit time time against against various various thickness thickness ratiosratios of a p-type of a p-type absorber absorber to an to anentire entire hybrid hybrid absorber. absorber.

2.2. InvertedFigure p-down1b shows Design an example of carrier-transit-time behavior against the thickness ratio of the p-type absorber to the entire hybrid absorber. We calculate the carrier transit timeThe based vertical on theillumination charge-control structure model is with beneficial a hoe drift for velocityensuring of reliability 5.0 × 106 cm/s because and it can be anapplied electron to diffusionvarious structures coefﬁcient ofto 2.0confine× 102 thcme2 electric/s. The thicknessfield into ratio a device of 0 corresponds to avoid device failuresto a conventional owing to pinsurface absorber degradation. and that of 1Pr correspondsevious studies to a uni-traveling on avalanche carrier photodiodes (UTC) (APDs),absorber. which Regarding require this a calculation,tighter design we assume to make a thickness them reliable, of 600 nm have for the focused entire hybridon how to confineabsorber. the electric The carrier field transit into timethe central is at the region thickness of the ratio device of 0, mainly [7–9]. governedThe basic by idea the for a device structure to confine the electric field in the center of the device is to form a selectively doped region at the undoped layer [10–13]. Figure 2a shows a representative APD structure using selective diffusion techniques. The selectively doped p-type contact region in the top undoped layer defines the active region of the device. Thus, the sidewall of the device has no electric field in principle. A similar concept is achieved by the etching process by Levin et al. [14], as shown in Figure 2b. They used an etched mesa-form p-type contact layer rather than a p-type selective doping region, eliminating uncertainty in the active region caused by selective diffusion or the ion-implantation process. This concept has been extended to other mesa-type APDs [15,16].

(a) (b) Figure 2. Schematic cross-sections of avalanche photodiodes (APDs) with (a) a selective doping structure and (b) an etched mesa structure, both for ensuring reliability.

When we want to combine a hybrid absorber to boost the operating speed and sensitivity with the vertical illumination structure, however, we have to carefully consider the design. When a hybrid absorber is straightforwardly incorporated into the device configuration shown Figure 2b, a “terrace” region between the p-type contact layer and absorber will be formed, as shown in Figure 3. Considering this configuration, the active area is defined not by the p-type contact layer that has a smaller mesa shape, but by the

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hole saturation velocity and then, the transit time becomes shorter as the thickness ratio (a) (b) enlarges due to the shrinkage of the hole transit distance. Once the transit time reaches Figure 1. (a) schematic band diagram of a hybrid theabsorber minimum, and it(b increases) calculated again carrier due to transit the enlarged time against electron various drift time. thickness Explained above, ratios of a p-type absorber to an entire hybrid absorber.the carrier transit time dramatically changes in accordance with this ratio, even though the total thickness is fixed. The minimum carrier transit time of 2.2 ps appears at the ratio of 0.4. The total thickness of the entire hybrid absorber should be set in accordance with 2.2. Inverted p-downthe Design required responsivity; however, the hybrid absorber can reduce the carrier transit time The vertical illuminationcompared with structure a simple pinis beneficial or UTC absorber for ensuring configuration. reliability because it can be applied to various2.2. structures Inverted p-down to confine Design the electric field into a device to avoid device failures owing to surfaceThe vertical degradation. illumination Pr structureevious isstudies beneficial on for avalanche ensuring reliability photodiodes because it can (APDs), which requirebe applied a tighter to various design structures to make to confine them thereliable, electric have field intofocused a device on to how avoid to device confine the electricfailures field into owing the to surfacecentral degradation. region of Previousthe device studies [7–9]. on avalanche The basic photodiodes idea for (APDs), a device structure to whichconfine require the aelectric tighter designfield in to makethe center them reliable, of the havedevice focused is to on form how a to selec- confine the electric field into the central region of the device [7–9]. The basic idea for a device structure tively doped regionto at confine the undoped the electric layer field in [1 the0–13]. center Figure of the device 2a shows is to form a representative a selectively doped APD region at structure using selectivethe undoped diffusion layer techniques. [10–13]. Figure The2a shows selectively a representative doped p-type APD structure contact using region selective in the top undopeddiffusion layer defines techniques. the Theactive selectively region doped of the p-type device. contact Thus, region the in sidewall the top undoped of the layer device has no electricdefines field the in activeprinciple. region A of similar the device. concept Thus, is the achieved sidewall ofby the the device etching has nopro- electric cess by Levin et al.field [14], in as principle. shown A in similar Figu conceptre 2b. isThey achieved used by an the etched etching mesa-form process by Levin p-type et al. [14], as shown in Figure2b. They used an etched mesa-form p-type contact layer rather than contact layer rathera than p-type a selectivep-type selective doping region, doping eliminating region, uncertainty eliminating in the uncertainty active region in caused the by active region causedselective by selective diffusion diffusion or the ion-implantation or the ion-implantation process. This concept process. has been This extended concept to other has been extended tomesa-type other mesa-type APDs [15,16 ].APDs [15,16].

(a) (b)

FigureFigure 2. Schematic 2. Schematic cross-sections cross-sections of avalanche of avalanche photodiodes photodiodes (APDs) with (a )(APDs) a selective with doping (a) structurea selective and doping (b) an etched mesastructure structure, and both (b for) an ensuring etched reliability. mesa structure, both for ensuring reliability.

When we want to combine a hybrid absorber to boost the operating speed and When we wantsensitivity to combine with thea hybrid vertical abso illuminationrber to structure, boost the however, operating we have speed to carefully and sen- consider sitivity with the verticalthe design. illumination When a hybrid struct absorberure, however, is straightforwardly we have to incorporated carefully intoconsider the device the design. When a configurationhybrid abso shownrber is Figure straightforwardly2b, a “terrace” region incorporated between the into p-type the device contact layercon- and figuration shown Figureabsorber 2b, will a be“terrace” formed, asregion shown between in Figure3 the. Considering p-type contact this configuration, layer and the ab- active area is defined not by the p-type contact layer that has a smaller mesa shape, but by the sorber will be formed,p-type as absorber shown that in includesFigure a3. “terrace” Considering region; thus,this theconfiguration, confinement of the the electricactive field area is defined not isby not the achievable. p-type contact This is why laye ourr that inverted has a p-down smaller design, mesa which shape, achieves but by an the effective field confinement of a device as well as boosting operation speed and responsivity by introducing a hybrid absorber at the same time, is needed for the vertical illumination structure. Taken from the Section3, we present the practical detailed design of inverted p-

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p-type absorber that includes a “terrace” region; thus, the confinement of the electric field is not achievable. This is why our inverted p-down design, which achieves an effective p-type absorber that includes a “terrace” region; thus, the confinement of the electric field field confinement of a device as well as boosting operation speed and responsivity by Photonics 2021, 8, 39 is not achievable. This is why our inverted p-down design, which achieves4 an of 9 effective fieldintroducing confinement a hybrid of a device absorber as well at as the boosting same operation time, is speedneeded and for responsivity the vertical by illumination introducingstructure. Takena hybrid from absorber the next at the section, same ti weme, pris esentneeded the for practical the vertical detailed illumination design of inverted structure. Taken from the next section, we present the practical detailed design of inverted downp-down pin photodiodes pin photodiodes (pin-PDs) and (pin-PDs) an APD we and fabricated an AP andD discuss we fabricated their performance and discuss their perfor- p-down pin photodiodes (pin-PDs) and an APD we fabricated and discuss their perfor- formance optical for communications. optical communications. mance for optical communications.

Figure 3. Schematic cross-section of mesa-type avalanche photodiodes (APD) with hybrid absorber that uses n-down and p-up structure. FigureFigure 3. Schematic3. Schematic cross-section cross-section of mesa-type of avalanchemesa-type photodiodes avalanche (APD) photodiodes with hybrid absorber (APD) with hybrid ab- that3.sorber Inverted uses n-downthat p-down uses and n-down p-up Pin structure. Photodiodes and p-up structure. (Pin-PDs) 3. InvertedThe capacity p-down Pinof network Photodiodes systems (Pin-PDs) that require state-of-the-art pin photodiodes (pin- PDs)3. InvertedThe is now capacity approaching p-down of network Pin 800 systems PhotodiodesGbit/s, that which require uses (Pin-PDs) state-of-the-art a baud rate of pin 100 photodiodes Gbaud. Studies (pin- on such PDs)high-speed isThe now capacity approachingpin-PDs forof 800 network100-Gbaud Gbit/s, which systems operation uses athat baudwere requ rate conducted ofire 100 state-of-the-art Gbaud. but used Studies a onwaveguide pin photodiodes (pin- suchstructure high-speed [17,18]. pin-PDs Our forapproa 100-Gbaudch is developing operation were such conducted high-speed but used pin-PDs a waveguide with a vertical structurePDs) is [now17,18]. approaching Our approach is800 developing Gbit/s, suchwhich high-speed uses a baud pin-PDs rate with of a 100 vertical Gbaud. Studies on such illumination structure [19]. Since the operation voltage and operating electric field of pin- illumination structure [19]. Since the operation voltage and operating electric ﬁeld of PDshigh-speed are relatively pin-PDs lower forthan 100-Gbaudthose of avalanche operation photodiodes were (APDs),conducted the reliabilitybut used of a waveguide pin-PDsstructure are relatively[17,18]. lower Our than approa thosech of avalancheis developing photodiodes such (APDs), high-speed the reliability pin-PDs with a vertical ofpin-PDs pin-PDs can can be be ensured ensured regardless regardless of the devicedevice structure.structure. However, However, the the inverted inverted p- p-down downdesignillumination design contributes contributes structure to extremely to extremely [19]. lowSince low surface surfacethe operation leakage leakage of of current voltage andand and tolerance tolerance operating for for a a electric larger field of pin- largerdrivingPDs drivingare voltage. relatively voltage. Figure Figure lower 4 shows4 shows than a schematic a those schematic of cross-section cross-sectionavalanche of ofphotodiodes one one of of our inverted (APDs), p-down the reliability of p-downpin-PDs.pin-PDs pin-PDs. can be ensured regardless of the device structure. However, the inverted p-down design contributes to extremely low surface leakage of current and tolerance for a larger driving voltage. Figure 4 shows a schematic cross-section of one of our inverted p-down pin-PDs.

FigureFigure 4. 4.Schematic Schematic cross-section cross-section of one of ofone our of inverted our inverted p-down p-down pin photodiodes pin photodiodes (pin-PDs). (pin-PDs).

The layer structure includes a p-type contact layer, a p-type electron block layer, a The layer structure includes a p-type contact layer, a p-type electron block layer, a hybrid absorber consisting of p-type and undoped InGaAs absorbers, an InP collector, andhybrid an n-type absorber contact consisting layer, which of p-type are placed and onundoped top of the InGaAs InP substrate absorbers, starting an InP from collector, theand p-type an n-type contact contact layer. layer, To obtain which a verticalare placed and on backside top of the illumination InP substrate structure, starting the from the anti-reflectionp-type contact coat layer. is formed To obtain on the backsidea vertical of theand substrate. backside The illumination mirror on the structure, top reflects the anti- thereflection signal light coat from is formed the bottom on side;the backside thus, this structureof the substrate. has a two-path The mirror configuration. on the The top reflects Figure 4. Schematic cross-section of one of our inverted p-down pin photodiodes (pin-PDs). activethe signal region light of the from device the is bottom defined side; by the thus, area th ofis the structure mesa-form has n-type a two-path contact configuration. layer. The active region of the device is defined by the area of the mesa-form n-type contact The layer structure includes a p-type contact layer, a p-type electron block layer, a hybrid absorber consisting of p-type and undoped InGaAs absorbers, an InP collector, and an n-type contact layer, which are placed on top of the InP substrate starting from the p-type contact layer. To obtain a vertical and backside illumination structure, the anti- reflection coat is formed on the backside of the substrate. The mirror on the top reflects the signal light from the bottom side; thus, this structure has a two-path configuration. The active region of the device is defined by the area of the mesa-form n-type contact

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Photonics 2021 8 , , 39 layer. Since the “terrace” region between the InP collector and n-type contact5 oflayer 9 is formed in the undoped layer, strong confinement of the electric field can be achieved. The thickness of the hybrid absorber is 450 nm, and the thickness ratio of the p-type absorberSince the is “terrace”set to obtain region a betweenmaximum the bandwidth InP collector at and the n-type bias contactvoltage layer around is formed 2–3 V. in Com- paredthe undopedwith a conventional layer, strong confinementpin absorber, of thethe electric hybrid field absorber can be achieved.shortens the carrier transit time butThe increases thickness the of device the hybrid capacitance absorber isowing 450 nm, to andthe highly the thickness doped ratio p-type of the absorber. p-type The absorber is set to obtain a maximum bandwidth at the bias voltage around 2–3 V. Compared InP collector layer compensates for the increase in device capacitance. Since only the with a conventional pin absorber, the hybrid absorber shortens the carrier transit time photo-generatedbut increases the electrons device capacitancepass through owing the InP to the collector, highly dopedin other p-type words, absorber. the InP Thecollector is independentInP collector layerfrom compensatesthe behavior for of the holes increase that have in device low capacitance.drift velocity, Since the onlyInP collector the canphoto-generated extend the entire electrons thickness pass throughof the depl the InPeted collector, layers, inthus other reducing words, the device InP collector capacitance withis independenta marginal increase from the behaviorin carrier of transit holes that time. have low drift velocity, the InP collector can extendFigure the 5a entire shows thickness the frequency of the depleted characteristics layers, thus of reducing our fabricated device capacitanceinverted p-down with a pin- PDsmarginal with active increase diameters in carrier of transit 10 and time. 14 μm at 2 V. The 10-μm-diameter pin-PD exhibited a 70.1-GHzFigure bandwidth5a shows the while frequency the 14- characteristicsμm-diameter of pin-PD our fabricated exhibite invertedd 38.7 p-down GHz. Even pin- with PDs with active diameters of 10 and 14 µm at 2 V. The 10-µm-diameter pin-PD exhibited the vertical illumination structure, the 10-μm pin-PD achieved a sufficient bandwidth ap- a 70.1-GHz bandwidth while the 14-µm-diameter pin-PD exhibited 38.7 GHz. Even with plicablethe vertical to 100-Gbaud illumination systems, structure, including the 10- 800-µm pin-PDGbit/s networks. achieved a Figure sufficient 5b bandwidthshows the volt- ageapplicable dependence to 100-Gbaud of the bandwidth systems, including of these 800-Gbit/s pin-PDs. networks.The 14-μm Figure pin-PD5b shows shows the a peak bandwidthvoltage dependence of 40.4 GHz of at the 4.2 bandwidth V and almost of these completely pin-PDs. Thesaturates 14-µm until pin-PD the showsapplied a bias voltagepeak bandwidthof 10.2 V. The of 40.4 10-μ GHzm pin-PD at 4.2 V exhibited and almost interesting completely behavior. saturates The until bandwidth the applied of this pin-PDbias voltage initially of surges 10.2 V. and The 10-reachesµm pin-PD the peak exhibited bandwidth interesting of 70.9 behavior. GHz at The 2.4 bandwidthV. It then grad- uallyof thisdecreases pin-PD and initially finally surges shrinks and to reaches 50.3 GHz the peakat 10.2 bandwidth V. Note that of 70.9 the GHzdevice at 2.4capacitances V. It of eachthen graduallypin-PD should decreases be andalmost finally constant shrinks once to 50.3 they GHz are at fully 10.2 V.depleted Note that at the the device operating capacitances of each pin-PD should be almost constant once they are fully depleted at voltage. It is reasonable if we assume that the carrier transit time increases due to lowering the operating voltage. It is reasonable if we assume that the carrier transit time increases electrondue to velocity lowering in electron accordance velocity with in the accordance additional with bias, the additionaland the 10- bias,μm andpin-PD the 10-wellµm reflect thepin-PD delay wellin the reflect transit the time delay while in the the transit bandwidth time while of the the bandwidth 14-μm pin-PD of the is 14- governedµm pin-PD by the capacitance-resistanis governed by the capacitance-resistancece time constant. time constant.

(a) (b)

FigureFigure 5. (a) 5. Frequency(a) Frequency characteristics characteristics of our of our inverted inverted p-down p-down pin pin photodiodes photodiodes (pin-P (pin-PDs)Ds) withwith 10-10- and and 14-um 14-um active active diameters,diameters, and (b and) the ( bbias-voltage) the bias-voltage dependence dependence of 3-dB of 3-dB down down bandwidth bandwidth for for these pin-PDs.pin-PDs.

µ FigureFigure 6 6shows shows the the current-voltage current-voltage (I-V)(I-V) characteristicscharacteristics of of the the 10- 10-μmm pin-PD. pin-PD. The The in- input is a 1.3-µm wavelength and a −4-dBm power. Thanks to the inverted p-down put is a 1.3-μm wavelength and a −4-dBm power. Thanks to the inverted p-down design, design, the dark current is maintained at less than 1 nA for bias voltage up to 10 V. Judging thefrom dark the current observed is maintained photocurrent, at less the responsivitythan 1 nA for is 0.61bias A/W. voltage Thus, up theto 10 10- V.µm Judging pin-PD from theexhibited observed 0.61 photocurrent, A/W witha the bandwidth responsi ofvity over is 0.61 70 GHz A/W. simultaneously, Thus, the 10- evenμm pin-PD with the exhib- itedvertical 0.61 A/W illumination with a bandwidth structure. It of is interestingover 70 GHz that simultaneously, the photocurrent even remains with the the same vertical illuminationuntil 10 V, indicating structure. that It is avalanche interesting multiplication that the photocurrent still does not occur remains for the the entire same voltage until 10 V, indicating that avalanche multiplication still does not occur for the entire voltage region. This result also indicates that the gradual decrease in the bandwidth for additional bias

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voltage is independentregion. This of avalanche result also multiplicat indicates thation, the seemingly gradual due decrease to the in lowering the bandwidth of elec- for additional tron velocity.voltage bias voltage is independent is independent of avalanche of avalanche multiplicat multiplication,ion, seemingly seemingly due to due the tolowering the lowering of elec- of tronelectron velocity. velocity.

Figure 6. Current-voltage (I-V) characteristics of our inverted p-down pin photodiodes (pin-PD) with a 10-μm activeFigureFigure area 6. 6. Current-voltage diameter.Current-voltage (I-V) (I-V) characteristics characteristics of ofour our inverted inverted p-down p-down pinpin photodiodes photodiodes (pin-PD) (pin-PD) withwith a a 10- 10-μµmm active active area area diameter. diameter. 4. Inverted p-down Avalanche Photodiodes (APDs) 4. Inverted p-down Avalanche Photodiodes (APDs) While the4. pinInverted photodiodes p-down (pin-PDs) Avalanche were Photodiodes designed to (APDs) achieve 100-Gbaud operation, While the pin photodiodes (pin-PDs) were designed to achieve 100-Gbaud operation, cutting-edge avalancheWhile thephotodiodes pin photodiodes (APDs) (pin-PDs) are being were designed designed for to 100G-achieve four-level 100-Gbaud operation, cutting-edge avalanche photodiodes (APDs) are being designed for 100G- four-level pulse- pulse-amplitudecutting-edge modulation avalanche (PAM4) photodiodes(50 Gbaud) operation(APDs) are with being a higher designed sensitivity for 100G- to four-level amplitude modulation (PAM4) (50 Gbaud) operation with a higher sensitivity to reach reach over a 10-km transmission [20,21]. Figure 7a shows a schematic cross-section of our pulse-amplitudeover a 10-km transmission modulation [(PAM4)20,21]. Figure(50 Gbaud)7a shows operation a schematic with a cross-sectionhigher sensitivity of our to fabricated inverted p-down APD. Compared with the inverted p-down pin-PDs, the epi- reachfabricated over a inverted 10-km transmission p-down APD. [20,21]. Compared Figure 7a with shows the a inverted schematic p-down cross-section pin-PDs, of our the taxial structure becomes more complex. The layer structure includes a p-type contact layer, fabricatedepitaxial structureinverted p-down becomes APD. more Compared complex. The with layer the inverted structure p-down includes pin-PDs, a p-type the contact epi- p-type electron block layer, a hybrid absorber consisting of p-type and undoped InGaAs taxiallayer, structure p-type electron becomes block more layer, complex. a hybrid The laye absorberr structure consisting includes of a p-type p-type andcontact undoped layer, absorbers, a 90-nm-thickp-typeInGaAs electron absorbers, InAlAs block aavalanche 90-nm-thicklayer, a hybrid layer InAlAs sandwiched absorb avalancheer consisting between layer of sandwichedn-type p-type and and p-type between undoped n-type InGaAs and field control layers,absorbers,p-type an ﬁeld InP a 90-nm-thick control edge-field layers, buffer InAlAs an InPlayer, avalanche edge-ﬁeld and an la n-type bufferyer sandwiched layer,contact and layer. between an n-type n-type contact and layer. p-type field control layers, an InP edge-field buffer layer, and an n-type contact layer.

(a) (b) (a) (b) Figure 7. (a)Figure Schematic 7. (a cross-section) Schematic cross-section of our inverted of our p-down inverted avalanche p-down photodiodes avalanche (APD)photodiodes and (b (APD)) its electric-field and profile. (b) its electric-fieldFigure profile. 7. (a) Schematic cross-section of our inverted p-down avalanche photodiodes (APD) and (b) its Theelectric-field thickness profile. of the hybrid absorber is 500 nm. Seen from the pin-PDs discussed in The thicknessthe Section of the3 hybrid, the inverted absorber p-down is 500 designnm. Seen efficiently from the decreases pin-PDs the discussed surface leakagein current. the previous section,However,The the thickness weinverted also haveof p-down the to hybrid consider des ignabsorber theefficiently electric is 500 decreases field nm. of Seen the the from internal surface the region pin-PDsleakage in thediscussed case of in an the previous section, the inverted p-down design efficiently decreases the surface leakage

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Photonics 2021, 8, 39 7 of 9 current. However, we also have to consider the electric field of the internal region in the case of an APD. A bias voltage as large as 30 V needs to be applied to this APD, and the electricAPD. field A bias in voltageeach layer as largereaches as 30several V needs hundred to be appliedkV/cm, unlike to this in APD, pin-PDs. and the During electric such field a situation,in each layer the reachesedge field several invoked hundred in the kV/cm, edge region unlike of in each pin-PDs. mesa During can be such problematic. a situation, Particularly,the edge field if the invoked edge field in the reaches edge regionthe avalan of eachche layer, mesa canavalanche be problematic. multiplication Particularly, will be if enhancedthe edge at field the reacheslocal region, the avalanche leading layer,to an avalancheabnormal multiplicationbreakdown. The will edge-field be enhanced buffer at the layerlocal helps region, mitigate leading this. to anThe abnormal edge-field breakdown. buffer layer The separates edge-field the buffer avalanche layer layer helps and mitigate n- typethis. contact The edge-field layer spatially, buffer weakening layer separates the ed thege avalanchefield by the layer n-type and contact n-type layer contact at the layer avalanchespatially, layer. weakening The edge-field the edge buffer field by layer the also n-type is exposed contact layerto the at edge the field; avalanche however, layer. the The n-typeedge-field field-control buffer layerlayer alsolowers is exposed the electric to the field edge in the field; edge-field however, buffer the n-type layer, field-control as shown in layerFigure lowers 7b. Figure the electric 7b is the field electric in the edge-fieldfield profile buffer of the layer, APD as under shown the in Figuregain condition7b. Figure of7 b aroundis the 5. electric field profile of the APD under the gain condition of around 5. FigureFigure 8a8 showsa shows the the current-voltage current-voltage (I-V) (I-V) characteristics characteristics of our of ourfabricated fabricated inverted inverted p- downp-down APD APD with with a 14- aμ 14-m µactivem active diameter. diameter. We Wecan can find find no noabnormal abnormal breakdown. breakdown. After After the the on-seton-set voltage voltage of of14.5 14.5 V, V,the the photocurrent photocurrent gradually gradually increases increases according according to tothe the avalanche avalanche gain,gain, then then reaches reaches break break down down at at 30 V.V. FigureFigure8 b8b shows shows the the frequency frequency response response of this of this APD. APD.The The peak peak bandwidth bandwidth of 37 of GHz 37 GHz appears appears at 0.79 at A/W0.79 A/W at a at 1.3- aµ 1.3-m-wavelengthμm-wavelength input. input. While Whilethe responsivitythe responsivity increases increases according according to the to increasethe increase in the in the avalanche avalanche gain, gain, the the bandwidth band- widthgradually gradually decreases. decreases. However, However, a bandwidth a bandwidt of overh of 28over GHz, 28 GHz, which which is sufficient is sufficient to operate to operateat 100-Gbit/s at 100-Gbit/s PAM4, PAM4, is maintained is maintained at a responsivity at a responsivity of 1.95 of A/W. 1.95 A/W.

(a) (b)

Figure 8. (a) Current-voltageFigure (I-V) 8. characteristics(a) Current-voltage and (b (I-V)) frequency characteristics characteristics and (b of) frequency fabricated characteristics inverted p-down of fabricated avalanche photodiodes (APD). inverted p-down avalanche photodiodes (APD).

WeWe mounted mounted this this APD APD onto onto an an optical optical rece receiveriver together together with with a a commercially commercially availableavaila- bletrans-impedance trans-impedance amplifier amplifier and and conducted conducted a a bit-error-ratio bit-error-ratio (BER) (BER) measurement measurement to to confirm con- firmthe the capability capability of the of APDthe APD to handle to handle a 100-Gbit/s a 100-Gbit/s PAM4 PAM4 optical optical signal. signal. We used We our used in-house our in-housesemiconductor semiconductor optical amplifieroptical amplifier assisted extended assisted reachextended electro-absorption reach electro-absorption modulator in- modulatortegrated withintegrated distributed with feedbackdistributed laser feedb (AXEL)ack laser with (AXEL) a wavelength with a of wavelength 1294.82 nm of and 1294.82an average nm and launch an average power launch of +8.4 power dBm asof a+8.4 transmitter dBm as a [ 22transmitter]. A commercially [22]. A commer- available ciallydigital available signal processordigital signal was processor connected was to the connected transmitter to the and transmitter receiver to and measure receiver receiver to − measuresensitivity receiver at a BER sensitivity of 2.4 × at10 a 4BER, which of 2.4 corresponds × 10−4, which to the corresponds error-free condition to the error-free assuming conditionKP-4 forward assuming error KP-4 corrector forward (FEC). error Figure corrector9 shows (FEC). the Figure BER characteristics. 9 shows the BER A minimum characteristics.receiver A sensitivity minimum of receiver−15.5 dBmsensitivity in the outerof −15.5 optical dBm modulation in the outer amplitude optical modulation (OMA outer) amplitudewas achieved (OMA at outer) the APD was gain achieved of 5 for at athe 100-Gbit/s APD gain PAM4 of 5 for optical a 100-Gbit/s signal. ThePAM4 BER optical charac- signal.teristics The of BER the characteristics optical receiver of with the aoptical pin-PD receiver also is with shown a pin-PD in this figure.also is Comparedshown in this with the receiver sensitivity of the pin-PD receiver of −10.3 dBm, the APD receiver exhibited a 5.2-dB improvement.

Photonics 2021, 8, x FOR PEER REVIEW 8 of 9

figure. Compared with the receiver sensitivity of the pin-PD receiver of −10.3 dBm, the Photonics 2021, 8, 39 APD receiver exhibited a 5.2-dB improvement. 8 of 9

Figure 9. Bit-error-ratio BER characteristics of a fabricated avalanche photodiode (APD) receiver assumingFigure 9. Bit-error-ratio KP4 FEC for error-free BER characteristics operation. of a fabricated avalanche photodiode (APD) receiver assuming KP4 FEC for error-free operation. 5. Conclusions 5. ConclusionsWe reviewed the design and benefits of the inverted p-down structure for photodetectors andWe discussedreviewed theirthe design performance. and benefits The invertedof the inverted p-down p-down design allowedstructure the for incorpora- photode- tiontectors of aand hybrid discussed absorber their to performance. a mesa-type configuration The inverted thatp-down confined design the allowed electric the field incor- into theporation center of of a the hybrid device. absorber The incorporation to a mesa-type of a configuration hybrid absorber that resulted confined in the an overelectric 70-GHz field bandwidthinto the center and of a the 0.61-A/W device. responsivityThe incorporatio withn aof 10- a µhybridm pin absorber photodiode resulted (pin-PD), in an evenover with70-GHz the bandwidth vertical illumination and a 0.61-A/W structure, responsivity while the with field a confinement10-μm pin photodiode that the mesa-type (pin-PD), configurationeven with the providedvertical illumination contributed structur to an extremelye, while the low field dark confinement current of lessthat than the mesa- 1 nA, eventype configuration at 10 V. The inverted provided p-down contributed avalanche to an photodiodes extremely low (APD) dark also current included of less a hybrid than 1 absorber.nA, even at A 10 responsivity V. The inverted of 1.95 p-down A/W was aval obtainedanche photodiodes at over 28 GHz, (APD) which also included was sufficient a hy- bandwidthbrid absorber. to handleA responsivity a 100-Gbit/s of 1.95 four-level A/W was pulse-amplitude obtained at over modulation 28 GHz, which (PAM4) was signal. suffi- Thecient optical bandwidth receiver to fabricatedhandle a 100-Gbit/s with this APDfour-level exhibited pulse-amplitude a −15.5-dBm modulation sensitivity. (PAM4) signal. The optical receiver fabricated with this APD exhibited a −15.5-dBm sensitivity. Author Contributions: Conceptualization, M.N.; methodology, M.N., F.N., A.K., T.Y., Y.N., S.T. and T.S.; investigation, M.N., F.N., A.K., T.Y., Y.N., S.T. and T.S.; writing—original draft preparation, M.N.; Author Contributions: Conceptualization, M.N.; methodology, M.N., F.N., A.K., T.Y., Y.N., S.T., writing—reviewand T.S.; investigation, and editing, M.N., allF.N., authors; A.K., T.Y., supervision, Y.N., S.T., H.M. and and T.S.; K.S.; writin projectg—original administration, draft prepara- H.M. andtion, K.S. M.N.; All writing—review authors have read and and edit agreeding, all to authors; the published supervision, version H. ofM. the and manuscript. K.S.; project administra- Funding:tion, H.M.This and researchK.S. All authors received ha nove external read and funding. agreed to the published version of the manuscript. InstitutionalFunding: This Review research Board received Statement: no externalNot applicable.funding. InformedInstitutional Consent Review Statement: Board Statement:Not applicable. Not applicable. DataInformed Availability Consent Statement: Statement:Not Not applicable. applicable. ConflictsData Availability of Interest: Statement:The authors Not declareapplicable. no conflict of interest. Conflicts of Interest: The authors declare no conflicts of interest. References 1.ReferencesIEEE P802.3cu 100 Gb/s and 400 Gb/s over SMF at 100 Gb/s per Wavelength Task Force. Available online: http://www.ieee802 .org/3/cu/index.html (accessed on 4 January 2021). 1. IEEE P802.3cu 100 Gb/s and 400 Gb/s over SMF at 100 Gb/s per Wavelength Task Force. Available online: 2. Runge, P.; Zhou, G.; Beckerwerth, T.; Ganzer, F.; Keyvaninia, S.; Seifert, S.; Ebert, W.; Mutschall, S.; Serger, A.; Schell, M. http://www.ieee802.org/3/cu/index.html (accessed on 4 January 2021). Waveguide Integrated Balanced Photodetectors for Coherent Receivers. IEEE J. Sel. Top. Quantum Electron. 2017, 24, 6100307. [CrossRef] 3. Beling, A.; Campbell, J. InP-based high-speed photodetectors. J. Lightwave Technol. 2009, 27, 343–355. [CrossRef] 4. Beling, A.; Campbell, J. High-speed photodiodes. J. Sel. Top. Quantum Electron. 2014, 20, 3804507. [CrossRef] Photonics 2021, 8, 39 9 of 9

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