Aug 30, 1990 - 1 a48. El. Recent advances in the epitaxial growth of Hg,_,Cd,Te by. MOCVD on substrates including GaAs2 have increased the potential for ...
process, illustrating that the grating is not formed along the whole fibre length. This is probably because of spatial nonuniformities and temporal fluctuations in exposure caused by interactive coupling between the exposing eigen states in the slowly forming grating structure.
Conclusions: We have shown for the first time that the birefringence induced in germanosilicate optical fibres by exposure to 488nm light can be used to form polarisation convertors/filters in HiBi fibres. The devices operate by inducing a periodic rocking perturbation of the principal eigen axes of the fibre. They have applications as wavelength and polarisation selective filters. P. ST. 1. RUSSELL Physics Laboratory Uniuersity ofKent Canterbury C T 2 7 N R , United Kingdom
30th August 1990
D. P. HAND Optoelectronics Research Centre Uniuersity of Southampton Hampshire SO9 5 N H , United Kingdom
References 1
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HILL, K. 0..FUJII,Y., JOHNSON, D. C., and KAWASAKI, El. S.: ‘Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication’, Appl. Phys. Lett., 1978,32,pp. 6 4 7 4 9 STONE, J.: ‘Photorefractivityin GeO,doped silica fibers’, . I Appl. . Phys., 1987,62, pp. 43714374 HAND, D. P.. and RUSSELL, P. ST. J.: ‘Single-mode fibre grating written into Sagnac loop using photosensitive fibre: transmission filters’. IOOC‘89, Paper 21C3-4, Kobe, Japan, 1989 HANO,D. P., and RUSSELL, P. ST. J.: ‘Photo-induced refractive changes in germanosilicate fibres’, Opt. Lett., 1990, 15, pp. 102-104 MELTZ, G., MOREY, w. w., and GLENN, w. H.: ‘Formation of Bragg gratings in optical fibers by a transverse holographic method‘, Opt. Lett., 1989, 14, pp. 823-825 PARENT, M., BURES, J., LACROIX, s., and LAPIERRE,J.: ‘Proprietes de polarization des reflecteurs de Bragg induits par photosensibilitk dans les fibres optiques monomodes’, Appl. Opt., 1985, 24, pp. 354357 STOLEN, R. H., ASHKIN, A., PLEIBEL, W., and DZIEDZIC, 1. M.: ‘In-he fiber-polarization-rockingrotator and filter’, Opt. Lett., 1984, 9, pp. 30&303
Recent advances in the epitaxial growth of Hg,_,Cd,Te by MOCVD on substrates including GaAs2 have increased the potential for integration of the detector element as part of a monolithic circuit. Of particular interest is the integrated optical receiver comprising a metal-semiconductor field effect transistor (MESFET) and a metal-semiconductor-metal (MSM) photodetector which has recently been demonstrated in InGaAs.’ MSM structures, which comprise two Schottky contacts connected through the active layer, are one of the most promising detector alternatives for integrated optoelectronic circuits. This is because of process compatibility with planar MESFETs and characteristics of high speed response and low dark current. We report on the characteristics of the first MSM photodetectors fabricated on Hg, _,Cd,Te. The metals Au, Pd and Pt, previously identified as producing Schottky barriers on n-Hg, _,Cd,Te,“ were chosen as contact systems. Stoichiometries in the range x = 0.5 to 0.8 were investigated, corresponding to the near infrared spectral region. A double layer structure comprised of CdTe/n-Hg, _,Cd,Te on semi-insulating (2” missoriented 100) GaAs was used in the device fabrication. The 1.5pm thick layer of Hg,_,Cd,Te was formed by an interdiffused multiplayer process (IMP); with in situ annealing of the CdTe/HgTe multilayers at 360°C to produce a homogeneous alloy.’ A 30nm capping layer of CdTe was then epitaxially grown on the Hg,_,Cd,Te. As grown, the Hg, -,Cd,Te was n-type with carrier concentration ~ from in the range N , = 5 x lOI5 to 1 x l O ” ~ m -determined Hall measurements. In the processing of the devices, mesa regions (40pm x 80pm) were formed by chemical etching5 and a polyimide film was used to planarise the structure. Deposition of 30nm of Au, Pd or P t followed by 400nm of Au was by electron beam evaporation at a base pressure of Torr. The interdigitated pattern of the electrodes was subsequently formed by a lift-off technique, with electrode widths of 2.3pm separated by 3.3pm spacings as shown in Fig. 1. The devices were fabricated in an array structure which typically covered the surface of a quarter wafer of 2 diameter.
M ETA L-S E M I C 0 N DUCTO R-M ETAL ( MS M ) PHOTODETECTORS FABRICATED O N M O C V D GROWN Hg,-,Cd,Te Indexing terms: Semiconductor deuices and materials, Photodetectors
The fabrication of the first metal-semiconductor-metal photodetectors on Hg, .,Cd,Te is reported using MOCVD grown layers on GaAs substrates. An epitaxial CdTe overlayer has been incorporated in the device structure for the enhancement of Schottky harrier characteristics. The interdigitated devices (2.3 pm electrode width, 3.3 pm spacing) exhibited a breakdown voltage of -6OV and responsivities of more than I.OA/W at a wavelength of 1-3pm and bias voltage of 4OV. Over the range of bias voltage examined, the dark leakage current of the detectors was dependent on the choice of contact metal, with minimum values of lOnA at < I V for Pt/CdTe/Hg, _,Cd,Te. The semiconductor Hg, -,Cd,Te has attracted considerable interest as a material for infrared detectors in optical fibre communications.’ Hg, -.Cd,Te possesses a low intrinsic carrier concentration, high absorption coefficient and flexibility of bandgap tuning allowing 1.3, 1.5 or 2.5pm detection. 1 a48
Fig. 1 Scanning electron micrograph showing M S M detector fabricated on CdTeIHg, _,Cd,Te/GaAs
A typical current/voltage curve for the MSM detectors fabricated on CdTe/Hg,_,Cd,Te is shown in Fig. 2, with a breakdown voltage exhibited in the range from 60 to 80V. These values of breakdown voltage were significantly greater than the 20V measured for Hg,-,Cd,Te without the CdTe overlayer, an effect which has been attributed to a reduction in the magnitude of surface leakage current. The breakdown voltages for the CdTe/MOCVD detectors were also considerably higher than for equivalent devices on GaAs and InAlAs/ InGaAs.‘ Plots of dark current as a function of applied bias voltage for devices having Au, Pd or P t barrier layers are shown in Fig. 3. At a bias voltage of 5 V, the respective values of leakage current were 0.08pA for Pt, 0.2pA for Au and 5pA for Pd; corresponding to a similar ordering as the reverse current of Schottky barrier diodes fabricated with these metals4 The magnitude of dark leakage current for MSM detectors with Pt metallisation in Fig. 2 was comparable with devices fabricated on InA1As/InGaAs.6 Current transport in
El .ECTRONICS LETTERS 25th October 1990 Vol.26
No. 22
The permission of the Executive General Manager, Telecom Australia Research Laboratories, to publish this letter is acknowledged.
the Hg,-,Cd,Te detectors was well modelled by a process of electron drift at low voltages (< 1 V) and by thermionic emission at an applied bias of z 1 V. This model, as outlined by
P. W. LEECH N. PETKOVIC P. J. GWYNN G. N. PAIN J. THOMPSON
29th August 1990
Research Laboratories, Telecom Australia 770 Blackburn Road, PO Box 249 Clayton, Victoria 3168, Australia
References J., MACKETT, r., JENKIN, G.T., NGUYEN DUY, T., and FORI, r.: ’The growth of CdHgTe by metal organic chemical vapour deposition for optical communication devices’, J. Crystal Growth, 1988,86,p. I17 PAIN, G. N., BHARATULA, N., ELMS, T. J., GWYNN, P., KIBEL, M., KWIETNIAK,M. s., LEECH, r., PETKOVIC, N., SANDFORU, c., THOMPSON, J., WARMINSKI, T., GAO, D.. GLANVILL, S. R., ROSSOUW, C. J., SEVENSON, A. W., WILKINS, S. W., and WIELUNSKI, L.: ‘Large-area HgTe-CdTe superlattices and Hg, -.Cd,Te multilayers on GaAs and sapphire grown by low temperature metal organic chemical vapour deposition’, J. Vac. Sci. Techno/., 1990, AS, (Z),p. 1067 YANG, L., SUUBO, A. S., TSANG, W. I., GARBINSKI, P. A., and CARMARDA, R . M.:‘Monolithically integrated InGaAs/InP MSM-FET photoreceiver prepared by chemical beam epitaxy’, IEEE Photonics Technol. Lett., 1990, 2, (l), p. 59 LEECH, P. w., and KIBEL, M. H.:‘Properties of Schottky diodes on n-type Hg,+,Cd,Te’. Proc. 1990 Workshop on the Physics and Chemistry of Mercury Cadmium Telluride and Related 11-VI compounds, San Francisco LEECH, P. W., KIBEL, M. H., and GWYNN, P. J.: ‘The chemical etching of 11-VI/GaAs heterostructures in aqueous I : KI : HBr solutions’, J . Electrochem. Soc., 1990, 137, (2), p. 705 SODLE, J. B. D., XHUMACHUI, H., LEBLANC, H. P., BHAT, R., and KOZA, M. A.: ‘High-speed performance of OMCVD grown InAIAskInGaAs MSM photodetectors at 1.5pm and 1.3pm wavelengths’, IEEE Photonics Techno/. Lett., 1989, 1, @), p. 250 SZE, s. M., COLEMAN, D.J., and LOYA, A.:‘Current transportin metalsemiconductor-metal (MSM) structures’, Solid Stare Electronics, 1971,14, p. 1209
1 THOMPSON,
2
vo1tage.V
p5
Fig. 2 Current-voltage plot for CdTeIHg, -.Cd,Te MSM detector
3
Pt metallisation
4
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6 Pt
7 m
, , , / , n , ,
,
, , , , ,
/ ,
Fig. 3 Dark leakage current against bias voltage for MSM detectors fabricated using different metallisation systems
Sze, corresponds to a hole barrier height, dg, much greater than the electron barrier height, &, resulting in domination by the electron current.’ The photoresponsivity of the devices, as measured by a laser diode at a wavelength of 1.3pm, is shown in Fig. 4. The responsivity was a function of decreasing level of optical power and increasing bias voltage. Responsivities of >l.OA/W have been measured, with the largest values in the nanoWatt range of input power. The values of responsivity > 1 A/W have been confirmed by measurements of spectral response of the devices. In addition, from capacitance-voltage measurements, the capacitance of the MSM devices was measured as 25 fF.
IC
=
10
-.-
=
20
-A-
IC0
optical input pomer b %
Fig. 4 Currenf-voltage characteristics optical input power and applied bias
-m- v = 5
-*v -0- V = 15 -0-
V
loco I013r.
of M S M detectors against
v = 30
-0- v = 40 -A- V = 5 0 V = 60
ELECTRONICS LETTERS 25th October 1990 Vol. 2 6
COMPUTER DESIGN OF BROADBAND LUMPED COMPONENT MICROWAVE PHASE SHIFTER AMPLIFIERS Indexing terms. Microwave circuits and systems, Computer aided circuit design
Broadband microwave phase shifts can he obtained using two amplifiers with different phase delays. A computer program which modifies circuits, by adding and removing components and nodes as necessary, has designed two amplifiers having a gain phase difference of 90” hut otherwise similar performance. Introduction: Microwave phase shifts need to be controlled for use in phased arrays and a 90” phase difference is essential in balanced modulators. A common method of obtaining phase shift is by switching between different lengths of transmission lines which have the disadvantage of being frequency dependent. An alternative is to use lumped component amplifier circuits having different phase delays. To investigate this technique, two amplifier circuits having a 90” gain phase difference with respect to each other have been designed. A computer program for circuit reduction and growth’ has been used to design these controlled phase shift amplifiers. Previously it was used for equivalent circuit modelling of microwave passive’ and active’ components and the design of microwave transistor a1nplifiers.4.~ The program alters a circuit so that its performance fits a set of target parameters as closely as possible. The error function calculated is a weighted sum of squares of the individual s-parameters at a number of
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