We have demonstrated a high-speed avalanche photodiode (APD) for a 10 Gbps optical communication
system. To achieve a high gain-bandwidth product and reliable operation, the reduction of
the multiplication layer thickness and an optimum design of the internal electric  eld distribution
are essential. One- and two-dimensional analysis were done for this purpose. The suppression of
edge breakdown can be achieved by precise control of the thickness of the multiplication layer and
charge densities of the  eld control layer. Furthermore we suggest a junction curvature shape having
negative curvature of the equi-potential line at the device edge. This new design successfully
suppressed edge breakdown. The fabricated APD shows high current gain without premature edge
breakdown, and a gain-bandwidth of above 80 GHz has been obtained

Kim, Sung June

Park, Chan Yong

Shin, Myoung Hoon

Yoon, Kyung Hun

Yun, Ilgu

Journal of the Korean Physical Society

SNU Open Repository and Archive

Journal of the Korean Physical Society, Vol. 45, December 2004, pp. S936∼S940Edge Breakdown Suppression of 10 Gbps Avalanche PhotodiodeK. H. Yoon,∗ M. H. Shin and C. Y. ParkXL-Photonics, Inc. Daejeon 305-343I. YunSchool of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749S. J. KimSchool of Electrical and Computer Sciences, Seoul National University, Seoul 151-742We have demonstrated a high-speed avalanche photodiode (APD) for a 10 Gbps optical commu-nication system. To achieve a high gain-bandwidth product and reliable operation, the reduction ofthe multiplication layer thickness and an optimum design of the internal electric field distributionare essential. One- and two-dimensional analysis were done for this purpose. The suppression ofedge breakdown can be achieved by precise control of the thickness of the multiplication layer andcharge densities of the field control layer. Furthermore we suggest a junction curvature shape hav-ing negative curvature of the equi-potential line at the device edge. This new design successfullysuppressed edge breakdown. The fabricated APD shows high current gain without premature edgebreakdown, and a gain-bandwidth of above 80 GHz has been obtained.PACS numbers: 85.60.Bt, 81.05.Ea, 78.66.FdKeywords: Avalanche photodiode, Optical communication, BreakdownI. INTRODUCTIONInP-based avalanche photodiodes (APD) have beenwidely used for fiber optic communications because oftheir advantages of high current gain and high sensitiv-ity. Much recent research has been focused on improvingoperational reliability [1,2] and a gain-bandwidth prod-uct [3], and reducing the excess noise factor [4,5].In the early stages of long wavelength optical fibercommunications, a Ge APD was widely used becauseof well-developed fabrication technologies. However thelight absorption properties of germanium at 1550 nmare poor, and the rate of ionization coefficients is nearlyequal to unity. The high electric field region is requiredto achieve avalanche multiplication, but the InGaAs ma-terial used for light absorption suffers from intolerablylarge tunneling leakage currents at high electric fields.For this reason, an InP/InGaAs APD with a separatedabsorption and multiplication (SAM) structure [6] hasbeen proposed, to obtain higher gain and reduce tun-neling current in the lower bandgap material of InGaAs.In this structure, an optical signal is converted into anelectrical signal in the InGaAs absorption layer and goesthrough avalanche multiplication in the InP multiplica-tion layer. A field control layer of moderately dopedInP is inserted to maintain a low electric field in the In-∗E-mail: khyoon@xl-photonics.com;GaAs absorption layer while supporting a high field inthe InP multiplication layer. Since the ionization coef-ficient of a hole is larger than that of an electron in anInP, the hole is usually chosen to be the type of injec-tion carrier in order to reduce the excess noise factor.When photo-generated holes are injected from the ab-sorption layer into the multiplication layer, a hole pile-upphenomenon has been found at the InGaAs-InP hetero-interface, which causes slow response and limits high-speed response of the APD device. This originates fromvalance band discontinuity and can be avoided by insert-ing mid-bandgap material nGaAsP, called the ”gradinglayer” [7,8].There are two critical issues for high speed APD. Oneis the reliability issue, and the other is how to obtaina high gain-bandwidth product. To solve the reliabil-ity issue, one must consider a planar structure. Due tofinite-size planar positive-negative (p-n) junctions, thereis a tendency toward enhancement of electric fields atthe edge of the active area. These enhanced edge fieldslead to larger gain at the device edges and premature”edge breakdown”. The key to avoiding edge breakdownis to reduce the electric field intensity at the device edge.There have been many efforts to solve this problem. Oneof them is low doping density at the device edge to reducethe electric field. This can be done by ion implantationor diffusion of different dopant materials at the deviceedge, but it is difficult to control the maximum electric-S936-Edge Breakdown Suppression of 10 Gbps Avalanche Photodiode – K. H. Yoon et al. -S937-field in the n-InP layer and the hetero-interface. The sec-ond method is the regrowth of the epitaxial layer afterselective etching of the field control layer at the deviceedge [9]. During the etching and regrowth process, thereis some charge accumulation in the regrown interface,identified by silicon acting as a donor. This interfacecharge makes it difficult to fabricate APD devices repro-ducibly and reliably.In the third approach, a junction of constant dopingdensity is shaped to create a wider multiplication regionwith consequently lower electric fields in the peripheralregion of the device. A floating guard ring is a similarconcept to realize the planar structure [10,11]. Floatingguard rings have the same doping as the central activeregion but are electrically isolated, which tends to lowerthe surface electric fields when the junction is biased.The widely used method for the planar type APD forhigher gigabit systems is the use of a floating guard ringstructure.Itzler [12] proposed InP/InGaAs APDs manufac-turable by using a multiple Zn diffusion process. In thisscheme, the floating guard ring and the shaped junc-tion profile created by double Zn diffusion are combined.An attractive aspect of this method is that the devicecan be fabricated by using a simple, mature diffusionprocess without regrowth or ion implantation, but theguard ring fabrication is very difficult for an APD hav-ing a high gain-bandwidth product. To obtain a highgain-bandwidth product, the multiplication layer has tobe very thin, so that formation of the floating guard ringis very difficult. We can form the floating guard ringand the shaped junction profile simultaneously by recessetching and a single diffusion process. This makes thedevice more reliable and manufacturable. In this paper,we calculated electric fields and avalanche gain factorsas a function of bias voltages and structural parame-ters. Two-dimensional analysis was done for the opti-mum floating guard ring structure design. Based on theresults, an APD for 10 Gbps was fabricated and mea-sured.II. DESIGN AND SIMULATION RESULTSA one-dimensional epitaxial layer structure and elec-tric field model of APD are shown in Figure 1. An n-InGaAs absorption layer (region 3 in Figure 1), an n-InPfield control layer (region 2 in Figure 1), an n-InP mul-tiplication layer (region 1 in Figure 1), and a p+ InPcontact layer are assumed to be deposited on an n+ InPsubstrate. Here, InGaAsP grading layers inserted be-tween the absorption layer and the field control layer areassumed to be parts of the absorption layer. These p+-i-n multiplication layer structures provide a uniform highelectric field, which can increase the impact ionizationcollision rate of a hole. The ratio α/β even approachesunity, this could reduce the carrier running path length,Fig. 1. Model of APD structure and electric field profiles.Fig. 2. Calculated EM , EH and avalanche gain factor as afunction of bias voltages.transit time and avalanche build-up time in turn.Let us assume that the x-coordinate is defined asshown in Figure 1, and a hole is injected at x = d1 + d2.Then the avalanche gain equation is given byM =11−∫W0β · exp{∫Wx(α− β)dx}dx, (1)where W = d1 + d2.It is our purpose to find a solution for the avalanchegain factor, M(V), as a function of bias voltage. Thesolution for the avalanche gain factor will provide thebreakdown voltage (VB) and other design parameters.Because the ionization coefficients are a function of theelectric field, we must find the E(x)-V relation. To findthe position-dependent electric field, the one-dimensionalPoisson equation was solved and numerical calculationwas carried out [13,14]. By using the parameters shownin Figure 1, the electric field has been successfully cal-culated as a function of bias voltage for a given layerstructure. In the calculation, we used ionization coeffi-cient data reported by Cook [15], and breakdown voltagedefined as the voltage where gain exceeds 100. Figure 2shows the calculated EM , EH and avalanche gain factoras a function of bias voltage.Based on this model, we calculated breakdown volt-ages as a function of the multiplication layer width (d1).Figure 3 shows the calculated breakdown voltages as afunction of the multiplication layer width (MLW) for sev--S938- Journal of the Korean Physical Society, Vol. 45, December 2004Fig. 3. Calculated breakdown voltages as a function ofmultiplication layer width and carrier densities of field controllayer.Fig. 4. Calculated breakdown voltages as a function ofmultiplication layer width and field control layer width.eral carrier densities of the field control layer. Figure 4shows the calculated breakdown voltages as a functionof the multiplication layer width for three different casesof field control layer thickness. Breakdown voltage de-creases as MLW increases from 0.1 µm to the minimumVB , defined as w0, while breakdown voltage increases asMLW increases in the region where MLW > w0. Eachcurve has the minimum VB marked with an arrow in thefigure, and the minimum point w0 is independent of thecarrier density or the thickness of the field control layer.In a design of APD with a shaped junction profile, theMLW at the active region is less than that at the deviceedge or periphery. Thus, if the MLW at the active regionis less than w0, then w0 is always located between activeregion and device edge, where maximum gain is obtained.When the MLW at the device periphery is within w0, theMLW at the device edge is greater than that at the devicecenter in the active region, which means edge breakdown.Thus the MLW at the active region should be greaterthan w0, to suppress the edge breakdown. Since a smallerMLW yields a higher gain-bandwidth product, we mustfind a structure having small w0. As shown in Figure 3Fig. 5. Calculated breakdown voltages as a function ofmultiplication layer width for three cases of absorption layerthicknessFig. 6. EM ad EH curves at M = 100 as a function ofMLW for different charge densities of field control layer.and Figure 4, the minimum VB depends neither on thecarrier density nor on the thickness of the field controllayer.Figure 5 shows the breakdown voltage changes withthe MLW for several cases of the InGaAs absorption layerthickness. The w0 depends sharply on the InGaAs layerthickness. Figure 6 shows the EM and EH for M = 100as a function of the MLW for several cases of chargedensities of the field control layer. For stable operationand high performance, EH should be sufficiently highto make photo-generated holes drift away, but should below enough to suppress the tunneling and avalanche mul-tiplication in the lower bandgap material InGaAs. Thisregion is the shadowed band in Figure 6. For 10 Gbpsoperation, MLW should be less than 0.3 µm. From Fig-ure 5, the InGaAs absorption layer thickness should beless than 1.0 µm, and charge density of the field controllayer should be as high as 3.5 × 1012 /cm2.The effect of floating guard ring structure on thedevice performance was analyzed by solving the two-dimensional semiconductor equations. Figure 7 (a)shows the conventional floating guard ring, and FigureEdge Breakdown Suppression of 10 Gbps Avalanche Photodiode – K. H. Yoon et al. -S939-Fig. 7. (a) Conventional shallow floating guard ring and(b) proposed deep floating guard ring. (c) Electric field profilealong the device.Fig. 8. Typical current-voltage characteristics of fabri-cated APD.7 (b) shows the proposed deep floating guard ring. Thedeep floating guard ring has negative curvature of theequi-potential line at the device edge. Figure 7 (c) showsthe simulated electric field profile over the device alongthe cross cut shown in Figure 7 (a) and (b). In the caseof the deep floating guard ring, the electric field at thedevice edge is efficiently suppressed.III. DEVICE FABRICATION ANDMEASUREMENTMetal-organic chemical vapor deposition (MOCVD)grown epitaxial wafers were used for the fabrication ofthe APD. An undoped InGaAs layer lattice-matched onan InP substrate was used for the absorption layer. Ab-sorption layer thickness was reduced to 0.8 µm for 10Gbps operation. The charge density of the field controllayer was 3.5 × 1012 /cm2 to increase the electric field inthe InP multiplication layer for higher gain while main-taining a low electric field in the InGaAs absorption layerto avoid tunneling phenomena. InGaAsP grading layersare inserted to remove hole pile-up at the InGaAs-InPhetero-interface.The p-junction is formed by the thin film depositionFig. 9. two-dimensional gain profile across the devices.Fig. 10. Experimentally measured bandwidth of fabricatedAPD.of ZnP composition followed by rapid thermal annealing.This process is the same as the method for manufactur-ing the reliable planar PIN photodiodes. The recessedjunction profile was obtained by etching an InP top layerbefore Zn diffusion. This process needs no epitaxial re-growth or double-diffusion of Zn. The junction curvaturecan be obtained by well controlled reactive-ion etching(RIE) and single diffusion of Zn. The MLW of the cen-ter active region can be controlled to be less than 0.3µm by adjusting the diffusion time, while the MLW ofthe device edge could be made greater than that of thecenter active region by RIE of the center active region.The difference of the MLW of the two regions can becontrolled by RIE time. The floating guard ring regionwas also etched out to form a deep floating guard ring.To reduce the junction capacitance, back surface illumi-nation geometry was introduced. Front p-metalizationwas made by using Ti/Pt/Au alloy. The Back surfaceof the device was thinned and polished. SiNx dielectric-S940- Journal of the Korean Physical Society, Vol. 45, December 2004layer was deposited on the back surface of the device toreduce the reflection of incident light from the air to theInP interface.Typical current-voltage (I-V) curves of fabricatedAPD under dark condition and microscope lamp illumi-nation are shown in Figure 8. The punchthrough volt-age is 14 V and breakdown voltage is 27 V. From theI-V measurement we can expect that the MLW of thecentral active region is about 0.25 µm. Figure 9 showsthe gain profiles measured across the fabricated APD.At 0.6 VB , this device shows sufficient photocurrent andan avalanche gain factor higher than 2. Even M = 10 (V= 0.9 VB) and M = 15 (V = 0.96 VB) avalanche gainfactors at the device edges are lower than those of thecentral active region. This is attributed to the strongguard ring structure of our new design and the exactanalysis and tightly controlled fabrication process. Theexperimentally measured bandwidth of the devices showsthat the APD has a gain-bandwidth product of above 80GHz (Figure 10). Finally, 10 Gbps hybrid integrated re-ceiver modules have been made by using this APD andpre-amplifier. The modules have shown a significant sen-sitivity improvement of at least 5 to 6 dB over similarreceivers based on PIN photodiodes.IV. CONCLUSIONSIn conclusion, we have demonstrated a high perfor-mance APD for 10 Gbps operation. One-dimensionalelectric field profile analysis was done for the optimumepitaxial layer design. From this analysis, optimum mul-tiplication layer thickness and absorption layer thicknesswere found for stable operation. With this optimizedepitaxial layer structure and the newly proposed deepfloating guard ring structure, we have fabricated APDwith high current gain without edge breakdown. Thefabrication process needs only one growth and single dif-fusion of Zn, which makes the devices more reliable andreproducible. The fabricated APD shows a high gain-bandwidth product of above 80 GHz.REFERENCES[1] J. Yao, K. K. Loi, P. Baret, S. Kwan and M. A. Itzler,LEOS 2001, 699 (2001).[2] K. S. Hyun, Y. H. Kwon and I. Yun, J. Korean Phys.Soc. 44, L779 (2004).[3] T. Nakata, T. Takeuchi, K. Makita and T. Torikai, LEOS2001, 770 (2001).[4] H. S. Kim, J. H. Choi, H. M. Bang, J. Burm, Y. Jee andS. W. Yun, J. Korean Phys. Soc. 39, 28 (2001).[5] Y. Kang, P. Mages, A. Pauchard, A. R. Clawson, S. S.Lau, Y. H. Lo and P. K. L. Yu, LEOS 2001, 772 (2001).[6] K. Nishida, K. Taguchi and Y. Matsumoto, Appl. Phys.Lett. 35, 251 (1979).[7] S. R. Forrest, O. K. Kim and R. G. Smith, Appl. Phys.Lett. 41, 95 (1982).[8] J. C. Campbell, W. S. Holden, G. J. Qua and A. G.Dentai, J. Quatum. Electron. 21, 1743 (1985).[9] L. E. Tarof, D. G. Knight, K. E. Fox, C. J. Miner, N.Puetz and H. B. Kim, Appl. Phys. Lett. 57, 670 (1990).[10] D. E. Ackley, J. Hladky, J. J. Jange, S. Mason, G. Er-ickson, G. H. Olsen, V. S. Ban, Y. Liu and S. R. Forrest,IEEE Photon. Tech. Lett. 2, 571 (1990).[11] S. R. Cho, S. K. Yang, J. S. Ma, J. S. Yu, S. D. Lee, A.G. Choo and T. I. Kim, J. Korean Phys. Soc. 38, 182(2001).[12] M. A. Itzler, K. K. Loi, S. McCoy, and N. Codd, OFC2000, 126 (2000).[13] C. Y. Park, K. S. Hyun, S.G. Kang, and H. M. Kim,Appl. Phys. Lett. 67, 37789 (1995).[14] K. S. Hyun and C. Y. Park, J. Appl. Phys. 81, 974(1997).[15] L. W. Cook, G. E. Bulman and G. E. Stillman, Appl.Phys. Lett. 40, 589 (1982).

Edge Breakdown Suppression of 10 Gbps Avalanche Photodiode

https://s-space.snu.ac.kr/bitstream/10371/8919/1/%5b2004-4%5d%20EDGE%20BREAKDOWN%20SUPPRESSION%20OF%2010%20GBPS%20AVALANCHE%20PHOTODIODE.pdf

Edge Breakdown Suppression of 10 Gbps Avalanche Photodiode

Abstract

Similar works

Full text

Available Versions

SNU Open Repository and Archive