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High speed (≥6 GHz) InGaAs/InP avalanche photodiodes grown by gas source molecular beam epitaxy with a thin quaternary grading layer for high bit
rate (≥5 Gbit/s) systems
G. Ripoche, J.-L. Peyre, M. Lambert, S. Mottet
To cite this version:
G. Ripoche, J.-L. Peyre, M. Lambert, S. Mottet. High speed (≥6 GHz) InGaAs/InP avalanche photodiodes grown by gas source molecular beam epitaxy with a thin quaternary grading layer for high bit rate (≥5 Gbit/s) systems. Journal de Physique III, EDP Sciences, 1993, 3 (9), pp.1761-1767.
�10.1051/jp3:1993102�. �jpa-00249037�
Classification
Physi<.s Ab,ifi.act.I 42.805
High speed (m 6 GHz) InGaAs/InP avalanche photodiodes
grown by gas source molecular beam epitaxy with a thin
quaternary grading layer for high bit rate (m 5 Gbit/s) systems
G. Ripoche ('), J.-L. Peyre ('), M. Lambert (2) and S. Mottet (3) ('1 Alcatel Alsthon Recherche, route de Nozay, 91460 Marcoussis. France (2) Alcatel Optronics, route de Nozay, 91460 Marcoussis, France
f) CNET, 2 route de Tr6gastel, 22300 Lannion, France
lRe<.erred 10 Nr)i,ember 1992, ieiised 8 Mai<.h 1993, a<.<.epted 9 Maich 1993)
Rksumk.- Des photodiodes h avalanche (PDA) lnGaAs/lnP rapides pour communications
optiques h haut ddbit (m 5 Gbit/s) ont 6td rdalis6es sur des hdt6rostructures dlabordes par dpitaxie
par jets moldculaires h sources gaz (EJM-SG). Conformdment aux indications de la moddlisation,
une fine couche quatemaire de transition a permis d'dviter le « stockage » des trous h l'interface InGaAs/lnP. Des PDA ayant une bande passante et un produit gain x bande passante respective-
ment supdrieurs h 6 GHz et h 50 GHz, un faible courant d'obscuritd et pr6sentant une excellente stabilitd, ont dtd obtenues avec
un rendement de fabrication dlevd.
Abstract.- High speed InGaAs/InP avalanche photodiodes (APD) grown by gas source
molecular beam epitaxy (GSMBE) for high bit rate (m 5 Gbit/s) optical communications systems have been successfully processed. As predicted by modelling, a thin quatematy grading layer practically suppresses « hole pile up » at the InGaAs/lnP hetero-interface. Devices with bandwidth
over 6 GHz, gain-bandwidth product in excess of 50 GHz, low dark current and excellent stability
have been achieved with a high fabrication yield.
1. Introduction.
High speed InGaAs/InP avalanche photodiodes (APD) are required for long haul terrestrial or submarine optical communications systems operating at high bit rates (5 to 10 Gbit/s) in the
wavelength range 1.3-1.55~m. Separate absorption, grading and multiplication region (SAGM) structure allows high responsivity, low noise and fast response provided layer parameters (doping, thickness) verify tight constraints. A large increase of the bandwidth of usual APD'S, typically 2.5 GHz, needs first of all to eliminate the « hole pile up » due to the
valence band discontinuity at the InGaAs/InP hetero-interface which is the main bandwidth limitation, and also to reduce the carrier transit time through the absorption region and to decrease the avalanche build-up time by multiplication layer doping enhancement.
Different structures of high speed APD'S have been recently investigated to overcome these
1762 jOURNAL DE PHYSIQUE III N° 9
limitations, either with thin quaternary layers [1-4] or with a compositionally graded quaternary layer [5, 6].
A SAGM-APD model has been developed and bandwidths were calculated for a grading
region constituted of 1, 2 or 3 thin quaternary layers of composition chosen in order to divide the large InGaAs/InP valence band step in much smaller ones, indicating that a thin (700 h)
quaternary layer with an appropriate composition could be a simple efficient solution.
Gas source molecular beam epitaxy (GSMBE) is a suitable growth technique for such
structures because it provides an accurate control of epitaxial layer thickness, doping and
composition with good uniformity on large area wafers. Several GSMBE-APD wafers with layer parameters as defined by modelling have been successfully processed in a planar
structure, favorable to achieve reliable devices.
In this paper, we report on the modelling, the fabrication and performances of the first planar
APD'S grown by GSMBE. Bandwidths over 6 GHz at a gain comprised between 2 and 8 have
been achieved for non optimized components with a high fabrication yield.
2. Diode modelling.
The dynamic operation of SAGM lnGaAs/InP APD'S is simulated by adding to the drift- diffu~ion current densities of the basic static model [7, 8] a term of displacement current
density
I~ =
~~~ ~~
at
This displacement current is of major importance in the simulation of the transient behaviour of heterojunction devices, since it describes the capacitive effects due to the free carrier
accumulation and storage at heterointerfaces [9].
The re;ponse time of the diodes, of which the 3 dB-bandwidth is determined, is deduced from the transient response to photon flux rise and fall heavyside functions for a bias condition chosen ~o as to obtain a gain of 10 under standard illumination. Furthermore, the maximum electric field in the graded region has to be limited to reasonable values to avoid the inter-band tunnel effect which appear~ for fields higher than 1.5 x 10~ V. cm~ ' This mechani~m would lead to a prohibitive dark current contribution of the absorbing ternary layer and therel'ore impose~ some of the design limitations.
A purpose of the study is to analyse the influence oi the difierent layers on the frequency response oi the device. The computed results are summarized in the table I. They show the influence oi the valence band di~continuities oi the transition region, the absorbing layer
thickne,s and the multiplying InP layer doping nn the 3 dB-bandwidth oi the devices. ln
particular, we can observe that ior a given absorbing layer thickne~~, the 3 dB-bandwidth increa;e~ when the total valence band di~continuity 10.38 eV) i~ shared through the graded region in step; of small amplitude (typically lower than 0.15 mev ). To achieve high bandwidth
operation diiferent number oi layers in the graded region may be u;ed. Re;ults show that even
one layer in the transition region (APD 6), which corresponds to the easiest ~tructure to fabricate, can give very intere~ting performances, very clo;e by those of more ~ophisticated
,tructure~ (APD 4 or APD 5), provided that the coillpo~ition of the cluaternary layer is properly
adju;ted (Qi
~
l.18 ~m), the valence band di;continuity being then shared in ~ equal part;
(U.19 eV).
Following thi~ statement, APD 6 with a single quaternary II.18 ~m) layer in the graded region have been fabricated with a doping level in the fi-InP multiplying layer of 4.5 x10'~~cm~~ and
an ab~orption thicLne~, oi 1.5 ~m, which I, sufiicient to get high sensitivity at 1.55 ~111 wavelength.
Table I. Simulated 3 dB-band~,idths of the APD'S. The stt.u(.lures APD to APD6 dij$ei by
the number. of'quatet.nary laj~eis in the graded ie,qion and their c.ompositions. The thickness of
each quateinai=v layer. is assumed to be 7501. Fat- APD6, dijfiet.ent absoibin,q layer.
thi(.I.nesses hat,e been studied. The 3 dB-bandwidth at M
=
10 has been c.omputed for m>o
typical multiplyin,q layer dopin,q let,els. Some published iesiilts hai~e been reported for (.omparison (a) 5.5 GHz for a,qain of 5 ~~ith a multiplying layer doped at 4.5 x 10'~ cm~ ~
Ii- (b) About 7 to 8 GHz for APD'S grown by CBE [2]. (c) 7.5 GHz for small area planar APD'S ~~ith multiplying layer doped at 4 x 10'~ cm~ .~ [3].
3 dB-bandwidth
Graded region Absorbing Doping level of the n InP
layer multiplying layer
Quaternary Valence band thickness
compositions discontinuities 2.3 x 10'~ x 10'6
APDI Qi (1.50 0.07 eV 2.0 ~m 4.0 GHz 5.5 GHz (a)
Q~ (1.30 0.07 eV
0.24 eV
APD2 Q~ (1.25 ~m) 0.17 eV 2.0 ~m 5.0 GHz
Q~ (1.18 ~m) 0.02 eV
0.19 eV
APD3 Qi (1.30 ~m) 0.13 eV 2.0 ~m 5.3 GHz 7.2 GHz
Q2 (1.08 ~m) 0.13 eV
0.12 eV
APD4 Qj (1.50 ~m) 0.07 eV 2.0 ~m 5.3 GHz 7.2 GHz (b)
Q2 (1.30 ~m) 0.06 eV Q~ (1.08 ~m) 0.13 eV 0.12 eV
APDS Qj (1.38 ~m) 0.I eV 2.0 ~m 5.3 GHz 7.2 GHz
Q~ (1,18 ~m) 0.08 eV
Q~ (1.03 ~m) 0.09 eV 0,10 eV
APD6 Qi (1.18 ~m) 0.19 eV 2.0 ~m 4.9 GHz 6.8 GHz
0.19 eV
1.7 ~m 7.2 GHz
1.5 ~m 7.5 GHz (c)
1.2 ~m 8,I GHz
1764 jOURNAL DE PHYSIQUE III N° 9
3. Diode fabrication.
High speed APD heterostructures were grown on 2" diameter wafers in a Varian GSMBE system, equipped with solid gallium, silicon and indium cells, phosphorus and arsenic being generated from 100 fb PH~ and AsHi by thermal cracking [10]. The growth temperature was
about 500 °C and the pressure inside the growth chamber was in the range of 10-5 mbar. Low growth rates (= I ~m/h allow a good control of the layer thickness and a high quality of the
interfaces. InP doping levels from x 10'~ cm~ to I
x 10'? cm~ and InGaAs doping levels
between x 10'~ cm~~ to 5
x 10'~ cm~~, needed to achieve performant devices, could be
controlled. The excellent uniformity in doping and thickness on the whole wafers was verified
by photoluminescence mapping. Different compositions of lattice-matched InGaASP material
are commonly available allowing several grading regions to be grown.
The structure shown in figure consists of 5 epitaxial layers, successively an InP buffer
layer (n = 5 x 10'~ cm~ ~, 0.5 ~m),
an InGaAs absorption layer (n = 2 x 10'~ cm~~, 1.5 ~m),
a thin InGaASP grading layer (Q = I. 18 ~m, n
= 2 x 10'~ cm~ ~, 700 1),
an InP multiplication layer (n
=
4.5 x10'~cm~~, 0.6 ~m) and
an undoped InP cap layer (n
=
5 x10'~cm~~,
1.8 ~m). After growth, the wafers presented a good morphology and a very low defect density.
Cd active area P+
Be guard-ring (P"
TPIAU Si~N~
lnP-n"
lnP-n lnGaAsP-n lnGaAs-n
lnP-n
imp-n+
AuGeNi Fig. I. Cross section of a planar front-illuminated InGaAs/InP APD.
The processing technology has already produced reliable InGaAs/InP APD'S which have satisfied the qualification tests for submarine optical communications systems. A cross section of the front illuminated planar structure is shown in figure I. The guard ring consists of a
selective Be+ ion implanted region subsequently annealed to produce a graded p-n junction.
Then Cd impurities are selectively diffused to form the p+ n active junction. Plasma deposited
silicon nitride is used as passivation layer and as antireflective layer. TilPt/Au and AuGeNilAu metallizations are finally deposited for p+ and n+ contacts. Photosensitive area diameter is 30 ~m with the bondpad over diffused region or 50 ~m with a throughout bondpad.
4. Device characteristics.
A plot of dark current and multiplication gain i,eisus reverse voltage in a planar APD is shown
in figure 2. Total dark current is low, typically 60 nA with best values about 10 nA at a gain of
io2 io4
~c
c lo io3
_o
fl
fi -
+
~
~
$
_
t i~
4~
%
1o 1
0
everse oltage (Vi
Fig. 2. Dark current and multiplication gain i,eisus reverse voltage in a planar APD.
10. Multiplied dark current is in the range of 0.8 nA to 4 nA. This demonstrates a good surface
passivation by silicon nitride film and a good quality of GSMBE material. Maximum
multiplication gain of about 35 under ~W incident light power were obtained for almost all diodes. Sensitivity at a gain of I is 0.9 A/lV and 0.95 A/lV at 1.3 ~m and 1.55 ~m wavelength respectively.
The low punch-through voltages of the guard ring and active junctions observed for APD'S of several samples were favorable in order to achieve wide bandwidth at low multiplication gain (w 2). In the other hand, the onset of multiplication gain in the active area at values close to 3 displaced the inferior limit of the 3 dB bandwidth i,s, gain towards gains of about 4.
The two dimension photoresponses at a gain of 10 at 1.3 ~m show a good gain uniformity
across the active area. Moreover, characteristics such as breakdown voltage, dark current,
maximum gain measured on devices distributed across the wafer had very close values, significant a high uniformity of the GSMBE heterostructures. As a result, fabrication yield
over 75 fb were obtained for different wafers.
1766 JOURNAL DE PHYSIQUE III N° 9
An excess noise factor of 6 was measured at a gain of 10 instead of'3 for classical APD'S,
due to a higher electric field in the multiplication region of the high speed APD'S.
The frequency response as a function of the multiplication gain of APD'S mounted on
stripline alumina submounts was investigated using a 0.1-20 GHz HP optical components analyser, equipped with a high frequency 1.3 ~m laser and a calibrated 20 GHz detector. The 3 dB-bandwidth i,eisus gain of an APD is shown in figure 3. This curve is typical of a device with low punch through voltages. At low gain (<2), the frequency limitation is due to
insufficient depletion of the absorption layer and not to hole trapping. At intermediate gain, the carrier transit time limits the bandwidth at values of about 6 GHz in spite of slightly too low
multiplication layer doping (n
=
4.5 x 10'~ cm~ ~). For gain m 10, the avalanche build up time limits the frequency response. So, gain-bandwidth products of 55 GHz were obtained at high gain.
GHz
6#
) 10 cI
m
e
i
lo loo
Multiplication gain Fig. 3. -dB-bandwidth i,ei.itt.I multiplication gain in a planar APD.
The ability of these APD'S to be used in high bit rate receivers was demonstrated by the eye
diagram shown in figure 4 obtained at a gain of 10 in the case of 5 Gbit/s RZ pulse pattern.
Accelerated aging tests under severe conditions (175 °C/100 ~A) are in progress on these APD'S in order to verify their reliability. After 2 000 h, no significant changes in dark current
characteristics have been observed.
5. Conclusion.
High speed planar InGaAs/InP APD'S grown by GSMBE have been successfully processed for the first time. The thin quaternary grading layer with I,18 ~m composition revealed efficient in
suppressing hole trapping at the InGaAs/InP interface as predicted by modelling. So, bandwidths over 6 GHz at gains comprised between 2 and 8 were obtained for non-optimized APD'S showing an excellent frequency behaviour even at gain as low as 2. Moreover, the
uniformity of GSMBE wafers allows a high fabrication yield. Aging tests indicate a good long
term stability compatible with submarine optical systems requirements.
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Id', ~~'° ~'* -.Oz -,_;I-'.' *-W," Jm '°~ ~j~OQ ~
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Fig. 4. Eye diagram of the 5 Gbit/s RZ pattern at a multiplication gain of IO-
Acknowledgments.
The authors would like to thank P.Aubert, G.Grandpierre, J.-P. Hdbert, J.Lohdac,
P. Pagnod, J.-G. Provost and B. Simon for their technical assistance.
This work was supported by France Telecom.
References
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[2] Campbell J. C. et al., High speed InP/InGaASP/InGaAs APD'S grown by chemical beam epitaxy, QttcInfiim Ele<.tioii. 24 (1988) 496.
[3] Torikai T. et al., Small area planar InGaAs APD'~ with 7.5 GHz wide bandwidth, Proc. OFC'BB (1988).
[4] Kuchibhotla R., Campbell J. C.. Tsa C., Tsang W. T. and Choa F. S., InP/InGaASP/InGaAs SAGM avalanche photodiodes with delta-doped multiplication layer, Ele<.fi.r)ii. Lent. 27 (1991) 1361.
[5) Kuwatsuka H., Kito Y., Uchida T. and Mikawa T., High speed InP/InGaAs avalanche photodiodes
with a compositionally graded quatemary layer, Proc. ECOC'91 (1991) 249.
[6] Kuebart W., Eisele H., Scherb J., Kimmerle J.. Wiedemann P, and Korber W., Continuous graded IO Gb/s planar InGaAs/lnP SAGM-APD grown by LP-MOVPE, Proc. ECOC°91 (1991) 253.
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[8] Baccarani G.. Rudan M., Guerrieri R, and Ciampolini P., Physical models for numerical device simulation, Process and Device Modelling, Vol, I (North-Holland, 1986) l07.
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lO] Zhang G., Hakkarainen T., Rakennus K., Tappura K.. Asonen H. and Pessa M., Low dark current
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