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IEWELL, J.L., HARBISON,
I.P., SCHERER, A., LEE, Y.H., and FLOREZ, LT.:
‘Vertical-cavitysurface-emitting lasers: design, growth, fabrication,
characterisation’, IEEE J. Quantum Electron.. 1991, QEn,pp.
1332-1346
MATTHEWS,J.w., and BLMSLEE,
A.E.: ‘Defects in epitaxial
multilayers’, J. Crystal Growth, 1974, 27, pp. 118-125
WANG, c.A., GROVES. s.H., REINHOLD, J.H.,and CALAWA, D.R.:‘Critical
layer thickness of strained-layer InGaAdGaAs multiple quantum
wells determined by double crystal X-ray diffraction’, J. Electron.
Mater., 1993, 22, pp. 1365-1368
BENDER, G., LARKINS, E.c., SCHNEIDER, H., RAMON, I.D., and
KOIDL,P.: ‘Strain relaxation in high speed p-i-n photodetectors with
In&a,
,AdGaAs multiple quantum wells’, Appl. Phys. Lett., 1993,
63, pp. 292tL2922
GCQSSEN, K.W., CLTNNINGHAM, J.E., and IAN, W.Y.: ‘Electroabsorption
in ultranarrow-bamer GaAdAIGaAs multiple quantum well
modulators’, Appl. Phys. Lett., 1994, 64, pp. 1071-1073
FEWSTER, P.F.: ‘X-ray diffraction from low-dimensional structures’,
Semicond. Sri. Technol., 1993, 8, pp. 1915-1934
GREY, R., DAVID,I.P.R.,
CLAXTON,P.A.,
GONZALEZ SANZ, F., and
WOOOHEAD, I.: ‘Relaxation of strain within multilayer InGaAd
GaAs pseudomorphic structnres’, J. Appl. Phys., 1989.66, pp. 975977
MILLER, D.A.B.,
CHEMLA, D.S,
DAMEN. T.C.,
GOSSARD. A.C.,
WIEGMANN, w., WOOD,T.H.,
and BURRUS,C.A.:
‘Electric field
dependence of optical absorption near the band gap of quantum
well structures’,Phys. Rev. B., 1985, 32, pp. 1043-1060
Monolithic integration of optoelectronic
smart pixels
U. Kehrli, D. Leipold, K. Thelen, H.P. Schweizer,
P. Seitz and B.D. Patterson
Indexing t e r n : MMICs, Light entitring diodes. Photodiodes
An A l W G a A s layer structure, grown in a single step, and a
fabrication process has been developed for the monolithic
integration of cascadable optoelectronic smart pixels. Metal
semiconductor field-effect transistors (MESFETs), light-emitting
diodes (LEDs) and photodiodes PDs) are uad for the
integration. As an example a threshold circuit consisting of a
dual-photodiode input and a current balanced output containing
an LED is presented. The circuit shows a switching energy of 2pJ
and a minimum switching power of 3nW. The maximum light
output of the LED is 30mW with a contrast ratio =. 1000. The
overall power dissipation is I5mW.
LED
~
Experiment: The layer structure, which is grown in a single step by
metal organic chemical vapour deposition (MOCVD), is shown
schematically in Fig. 1. On an n+-doped GaAs substrate, first an
LED structure is grown, starting with an n-doped lower cladding
of 9oonm AL,,G%,,As. Next an 8nm undoped active &.mGa,.~As
quantum well (QW) is sandwiched between 6Onm thick undoped
&,Ga,.,As layers. As the upper cladding, a 480nm thick p-doped
A&,Ga,,,As layer is used. The LED structure ends with a 400nm
thick p + a n t a c t layer. On top of the LED the PD/MESFET layers
are grown. They consist of a Ipm thick GaAs p--buffer/absorber
layer, which is weakly doped by Zn diffused from the p+-contact
layer, a 200nm thick 10”m-3 ndoped GaAs channel, a lOnm
thick etch-stop layer of &,,Ga,,,As and a GaAs n + a n t a c t layer.
ELECTRONICS LElTERS 24th November 1994
PD
rn
Fig. 1 Schematic layer sequence of cascodoble optoelectronic smart
pixel
The fabrication process requires nine photolithographic steps
and is based on mesa isolation. The backside of the n+-substrate is
used as the ohmic contact, formed by G e - A N i A u , to the n-side
of all LEDs. For the ohmic contacts to the MESFETs and the
PDs, an Ni/Ge-Au/Ni/AuPt [5] metallisation is evaporated on the
top n+-contact layer. Both contacts are annealed in the same step
at 440°C in a forming gas ambient. The individual MESFETs and
PDs are separated by magnetic field enhanced reactive ion etching
(MIE). After the deposition of a l00nm thick SIN, dielectric film
by plasma enhanced chemical vapour deposition (PECVD), contact areas to the n-ohmic contacts of the MESFETs and PDs as
well as to the p-ohmic contact areas of the LEDs and the PDs are
opened by reactive ion etching (RIE). The gate areas of the MESFETs are then opened by RIE. The dielectric film is used as a
mask for a selective, recess wet etch of these areas using the
Ab,Gq,As layer as an etch-stop [a]. The wet etch produces an
undercut of -2OOnm under the dielectric film. In the next step, a
Ti/Pt/Au metallisation is applied by a liftoff technique. This metallisation is used as a Schottky metal for the gates of the MESFETs,
as an ohmic tunnelling contact to the p+-layer and as a fust wiring
level. The LEDs, PDs and MESFETs are then isolated by a second MIE mesa etch step. A 3OOnm dielectric film isolates the main
wiring level, which is made by the deposition of TdAI over the
whole wafer. These metals are patterned by wet etching.
$04
0
so 2
a00
Introduction: Monolithic integrated optoelectronic smart pixels are
of current interest in the field of parallel optical interconnects,
optical computing and optoelectronic neural networks [l, 21. To
integrate different functional devices such as detectors, transistors
and light sources, the compatibility of the materials and the whole
structure must be considered. Approaches to this integration have
included selective area growth, regrowth or vertical integration [3,
41. In this Letter we present a layer structure which uses vertical
integration and a fabrication process which allows the integration
of cascadable optoelectronic smart pixels with different electrooptical functions.
MESFET
0
~
2
4
6
drain voltage, V
450 600
750 goo
wavelength,nm
Fig. 2 PD quantum efficiency and LED emission spectrum against wavelength
a Drain current-voltagecharacteristics for MESFET vertical scale is
normalised with respect to gate width (mA/mm)
Results: The characteristics were measured for devices processed
on the same wafer. The MESFET shows a maximum transconductance of 80mS/mm and a threshold voltage of -1.7V. The
drain current-voltage characteristics are shown in Fig. 2 left. A
transit frequency of f r = 1.2GHz and a maximum oscillation frequency fmaX = 2GHz were measured at a gate source voltage of
OV. The peak emission wavelength of the LED was set to be
790nm, where the quantum efficiency of the PD is 0.85. Therefore
the emitted light can eficiently be detected on the same wafer
(Fig. 2, right). The PD capacitance was measured to be 100nF/cmz
at a reverse voltage of A V . The LED has an efficiency of 0.008W/
A and a cutoff frequency of 150MHz at a current of IO&.
The fabricated cascadable optoelectronic smart pixels are
threshold circuits. The example is schematically drawn in Fig. 3,
and consists of a dual-photodiode differential input (PDI, PD2)
c o ~ e c t e dto a balanced LED driver (MI,M2, LED). The LED is
turned off when the input power of the switching beam (P_I,cJ
Vol.30 No.24
2069
exceeds that of the reference beam (P,</).
The switching threshold
can thus be controlled by the reference beam. The LED driver is
current-balanced: if the LED is off, the current flows through
MESFET MI, and the LED is turned on by pinching off MI.
f-?
Fig. 3 Schematic diagram of threshold circuit including parasitic LEDs
connected to MESFETs and PDs
WOODWARD, T.K.,
LENTINE, A.L.,
CHIROVSKY, L.M.F.,
and
ASARO, L.AD.: ‘GaAdAIGaAs FET-SEED receiverltransmitters’.
OSA Proc. Photonics in Switching, 1994, Vol. 16
T., JIMBO, T., and UMENO, M.: ‘Monolithic integration of
AlGaAsJGaAs MQW laser diode and GaAs MESFET grown on Si
using selective regrowth’, IEEE Photonics Technol. Lett., 1992, 4,
pp. 612414
CHENG, J , mou, P., SUN, s.z., HERSEE, s., MYERS, D.R.,ZOLPER, J., and
VAWTER, G.A.: ‘Surface-emitting laser-based smart pixels for twodimensional optical logic and reconfigurable optical
interconnections’, IEEE J. Quantum Electron., 1993, QE29, pp.
741-756
SHIH, Y.-c., MURAKAMI, M., WILKIE, E.L., and CALLEGARI, A.c.:
‘Effects of interfacial microstructureon the uniformity and thermal
stability of AuNiGe ohmic contact to n-type GaAs’, J. Appl. Phys.,
1987,62, pp. 582-590
DESALVO,G.C.,
TSENG,W.F.,and COMAS,I.:‘Etch rates and
selectivities of citric acidhydrogen peroxide on GaAs, AI,,,Ga.,As,
IQ,Ga.,As, In&.3q.4As, and InF”, J. Electrochem. Soc.. 1992,
139, (3), pp. 831-835
EGAWA,
The dimensions of the MESFET gates in the diagram are given by
length/width
The output power is 30pW in the ON state, and a contrast ratio
greater than IO00 has been estimated (Fig. 4). The minimnm
switching power is measured to be 3nW at vanishing reference
power. At a differential input power of 5OnW between Pref and
PlWlrrh,
the circuit has a switching delay of 4 3 p , which corresponds to a switching energy of 2 pJ. The photodetector area is
twice 50 x S o p 2 and the whole circuit, consisting of two PDs,
two MESFETs and an LED, occupies 200 x 2 0 0 p 2 . The power
dissipation remains constant at 15mW.
Noise and small-signal performance of three
different monolihic InP-based 10Gbit/s
photoreceiver OElCs
D. Kaiser, F. Besca, H. GroBkopf, I. Gyuro,
J.-H. Reemtsma and W. Kuebart
Indexing terms: Integrated optoelectronics, Optical receivers,
Semiconductor device noise
Three circuit concepts (high impedance, common gate, and
transimpedance) for a 10GbiUs monolithic receiver OEIC
consisting of an InGaAsiInP pin photodiode and InAlAsflnGaAsJ
InP HEMTs are compared in terms of noise and small-signal
performance using on-wafer measurements. A total equivalent
input noise current of 13.5pNdHz within the bandwidth of the
transimpedance circuit is the lowest value ever reported for a
monolithic InP-based IOGhiUs reiver OEIC.
Ught input power, n W
171(111
Fig. 4 Switching behaviour of threshold circuit
P m f = IlSnW, V,, = 4V, Vss = 2-V, V, = 1.5V
Conclusion: We have developed a layer structure grown in a single
step and a fabrication process for the monolithic integration of
cascadable optoelectronic smart pixels. A threshold circuit with
low switching power and switching energy has been demonstrated.
The performance of such circuits is suitable for application in the
fields of parallel optical interconnects and optoelectronic neural
networks.
Acknowledgments: The authors respectfully acknowledge the discussions with J.E. Epler, K.H. Gulden and M. Moser and for the
support of W. Bachtold, W. Kiindig and J. Mlynek.
For long-haul lightwave communication systems there is an
increasing interest in IOGbit/s components. However, it has not
been clear to date which concept will be successful in providing a
high-performance but lowcost optical receiver. Monolithically
integrated receiver front ends offer lower parasitics than hybrid
solutions, which is essential for obtaining the highest bit rates.
However, obviously the noise of the integrated devices is still too
high. Today the best monolithic result reported is a 7.4GHz bandwidth circuit with a sensitivity of -17.3dBm at a bit error ratio of
Ik9for a lOGbit/s NRZ signal [I]. The sensitivity was traced hack
to a total medium input noise current of 25 pA/dHz. We present
on-wafer measurements on three different circuit concepts for an
InP-based monolithic photoreceiver using an InGaAdInP pin photodiode and InAIAs/InGaAflnP HEMTs. The circuits with
increasing complexity, from a simple high impedance OEIC over a
common gate circuit with source follower up to a transimpedancecascode circuit, are shown in Fig. 1 and are compared in terms of
bandwidth, responsivity, electrical output matching, and noise current.
0 IEE 1994
23 September 1994
Electronics Letters Online No: 19941380
U. Kehrli, D. Leipold, K. Thelen, H.P. Schweizer, P. Seitz and B. D.
Patterson (Paul Scherrer Institute, Baaherstr 569, Ch-8048 Zurich,
Switzerlad
References
1 BROWN,J.J.,
GARDNER, IT., and FORREST,S.R.:
‘An integrated,
optically powered, optoelectronic ‘smart’ logic pixel for
interconnection and computing applications’, IEEE J. Quantum
Electron., 1993, QE29, pp. 715-726
2070
HI
GO
TI
lbiiiil
Fig.1 Circuit diagrams of three different realised receiver-OEIC concepts
ELECTRONICS LETTERS 24th November 7994
Vol. 30
No. 24
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