IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 38. NO. 12. DECEMBER 1991 2713

The effective mobility versus effective electric field for nMOSFET’s with 67-A-thick gate oxide was studied. The effec- tive mobility value was calculated from the linear region of fD ver- SUS VG curves (VD = 100 mV) with W / L of 75 pm/60 pm. The linearly extrapolated threshold voltages were -0.17 and -0.14 V for oxynitride and control oxide devices, respectively. The maxi- mum effective mobility of oxynitride is 5 % lower than that of con- trol oxide. However, high-field mobility of oxynitirde shows 10% improvement. This characteristic is similar to that of the reoxi- dized-nitrided oxide 131. The transconductance degradation of con- trol oxide and oxynitride after substrate hot-electron injection (J = +O. 1 A/cm’) with 67-A-thick gate dielectrics was measured. Compared with control oxide, oxynitride gate dielectric shows sig- nificantly reduced voltage shift and peak-transconductance degra- dation which indicates less charge trapping and interface state gen- eration. The peak-transconductance and drain current degradation under channel hot-electron stress were investigated. Submicrome- ter nMOSFET’s = 0.5 pm) with 65-A-thick gate oxide were characterized. The drain current was measured in the triode region ( VD = 0.1 V, VG = 2 V) . Compared with control oxide, oxynitride samples show significantly less degradation under channel hot- electron stress. The device lifetime versus inverse drain used in hot-electron stress for nMOSFET’s with effective channel length of 0.5 pm and 65-A-thick gate oxide was characterized. The life- time of device was defined as 3% degradation of peak trans- conductance (AG,,,/G,,,). Compared with the control oxide, the oxynitride devices show approximately one order of magnitude longer lifetime under channel hot-electron stress. The device deg- radation as a function of gate bias after I O min stressing was mea- sured. Both devices show peak around VG = 2 V which is related to interface-state generation. Compared with reoxidized nitrided oxide, oxynitride devices show no significant degradation at high gate bias (VG = VD) . Gate-induced drain leakage (GIDL) current versus VDG for control and oxynitride was studied. Compared with control oxide, oxynitride gate dielectric show significantly reduced I G f D L after channel hot-carrier stress.

Hot-carrier effects and effective mobility of nMOSFET’s with oxynitiride gate dielectrics grown in N 2 0 were studied. The oxy- nitride devices exhibit less degradation under substrate hot-electron and channel hot-electron stress, in terms of transconductance (AG,,,/Gm) and drain current ( A f d / I d ) . Device lifetime of oxyni- tride samples is approximately one order of magnitude longer than that of control oxide counterpart. We believe that the improved device reliability of oxynitride devices is due to the nitrogen pileup at the interface which enhances the resistance to hot-carrier stress- ing [4]. These results suggest that the new oxynitride gate dielec- tric shows good promise as an alternative for future Ultra Large Scale Integration (ULSI)’ application.

This work was partially supported by SRCBEMATECH under Contract NO. 88-MC-505.

[ I ] H. Hwang et a l . . Appl. Phys. Lett.. vol. 57. p. 1010, 1990. 121 H. Hwang er a l . , in IEDM Tech Dig . . 1990. p. 42 1 . 131 T. Hori. IEEE Trans. Electron DPI*I’CES. vol. 37, p. 2058. 1990. 141 T. Hori. et a l . , IEEE Electron Devices Lett.. vol. 10. p. 64. 1989.

VA-I 10 Gb/s Monolithic Integrated MSM-Photodiode AlGaAs/GaAs-HEMT Optoelectronic Receiver-V. Hurm, J . Rosenzweig, M. Ludwig, W. Benz, R. Osorio, M. Berroth, A. Hiilsmann, G. Kaufel, K . Kohler. B. Raynor, and Jo. Schneider, Fraunhofer-Institut fur Angewandte Festkorperphysik, D-7800 Freiburg, Germany.

Several research groups have reported on the technology of monolithic integration of MESFET’s on GaAs with photodetectors for light of 0.85-pm wavelength. In this paper we present the first photoreceiver which is based on a metal-semiconductor-metal (MSM) phtotodiode and AlGaAs/GaAs HEMT’s.

The photoreceiver was fabricated using our established 0.5-pm recessed-gate process for double delta-doped quantum-well HEMT’s [ I ] , [ 2 ] . The following mean values for the enhancement and depletion HEMT parameters, respectively, have been ob- tained: threshold voltage: 0.1 and -0.5 V, transconductance: 500 and 390 ms/mm. source resistance: 0.7 and 0.6 O . mm, transit frequency: 35 and 30 GHz. This process now includes photo- diodes. A deep wet etch was used to deposit the photodiodes on an undoped GaAs buffer layer. The I-pm-wide photodiode fingers with 1.5-pm spacing were defined by electron-beam lithography and subsequent lift-off of Ti/Pt/Au Schottky metal. The dc reponsiv- ities of the photodiodes to light of 0.84-pm wavelength were 0.25 A/W for 4-V and 0.35 A/W for IO-V bias voltage, respectively. The dark current at 4 V was less than 2 nA for a photodiode with an active area of 25 x 25 pm‘.

The monolithic integrated optoelectronic receiver consists of an MSM photodiode, a transimpedance amplifier, and a 5 0 4 output buffer. The transimpedance stage is composed of two enhancement transistors (gate widths 40 pm), two I-kO NiCr thin-film load re- sistors, and a 50042 NiCr feedback resistor. The output stage is a source-follower with a constant current load (gate widths 80 pm). All high-frequency measurements on the receiver were performed on-wafer using CASCADE probes. The photodiode was irradiated by 0.84-pm light from a high-speed ORTEL laser diode via a sin- gle-mode fiber. The current driving the laser diode was modulated to obtain up to 0.8-mW peak-to-peak modulated optical signals. The -3-dB bandwidth for sinusoidal modulated incident light lies at 8.2 GHz. The circuit response to pulse-modulated non-return- to-zero (NRZ) optical signals was tested at data rates up to IO GB/s using an ANRITSU pulse pattern generator. The eye diagram of the output voltage demonstrates that the optoelectronic receiver op- erates successfully for a IO-Gb/s NRZ pseudorandom data stream of length 2’ - 1 b.

[ I ] K. Kohler. P. Ganser. K. H. Bachem. M. Maier. J. Homung. and A. Hiilsmann, in Proc. Inr. Svmp. on GaAs and Related Compounds. 1990. Inst. Phys. Conf. Ser., 112. p. 521. 1990.

121 A. Hiilsmann, G. Kaufel. K. Kohler, B . Raynor, K. H. Glorer, E. Olander. B. Weismann, J . Schneider. T. Jakobus. in Proc. In t . Sytnp. on GaAs and Related Compounds, 1990. Inst. Phys. Conf. Ser.. 112, p. 429. 1990.

VA-2 Ultra-High Speed p-i-nlHBT Monolithic OEIC Photo- receiver-K. D. Pedrotti, R. L. Pierson, Jr., R. B. Nubling, C. W. Farley, E. A. Sovero, and M. F. Chang, Rockwell Intema- tional Corp., 1049 Camino Dos Rios, Thousand Oaks, CA 91360.

A novel combination of a p-i-n photodetector with GaAs/AIGaAs HBT has resulted in what we believe is the highest speed OEIC reported to date [ I ] , [2]. This circuit has the additional virtue of requiring no additional processing steps beyond those nor- nally used in producing our HBT’s. The circuit reported here achieved a bandwidth of 7 GHz with a transimpedance of 1650 fl and an averaged input current noise of 4.3 pA/Hz’/’. The mea- sured detector quantum efficiency of 0.35 at 0.82 pm implies a sensitivity of -17.5 dBm at a IO-’ BER into the 7-GHz band- width. This should allow operation to 9 Gb / s at this sensitivity.

The photodetector is fabricated directly from the same layers used for the HBT. The base-collector junction is used for the photode-

2714 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 38. NO 12. DECEMBER 1991

tector. This junction consists of the heavily doped GaAs p f base layer, a lightly doped GaAs n-collector layer and a more heavily doped n-type GaAs subcollector layer. Contacts are made to the base and subcollector layers during the corresponding steps of the HBT fabrication. A ring electrode structure was used with a di-

151 1. D. Crow, in Tech. Dig. on Optical Fiber Cornmunication Conf.. 1989 (Optical Soc. of America. Washington, DC), p. 83, and Tech.

1ooc3 1989 (Kobe* Japan). 4 9 p. 86.

ameter of 20 pm. No additional changes in our standard HBT pro- cess were made in either epitaxy or processing to fabricate these

sponse into 50 0 in exceSS of GHz. A leakage current of40 nA was observed at the typical bias of - 3 V. With minor modifications

VA-3 Ultra-Compact Monolithic Integration of Polarization

nings, R. J . Hawkins,* and C. Caneau, Bellcore, 331 N t ~ m a n Springs Rd, Red Bank, NJ 07701-7040.

devices, These detectors with a IOO-fF capacitance exhibited a re- Diversity Waveguide Photodiodes-R’ J‘ Deri, E. c‘ M ’ Pen-

to the device structure such as using a wider bandgap subcollector, a slightly wider collector region, and an underlying dielectric mir- ror combined with appropritate front surface coating to provide a resonant cavity, the quantum efficiency of this detector can be greatly improved while maintaining full compatibility with the HBT.

The transistors used are our baseline design [3] with a 1.3 x 3 pm emitter, emitter doping of 5 x IO” cm-3, base doping of 4 x IOl9 cm-3. and collector doping of 1 X 10l6 cm-3. The structure

Monolithic optoelectronic integration using optical waveguides is of current interest to enhance OEIC functionality with on-chip optical signal processing. A major drawback to monolithic inte- grated optics has been the large size, typically several millimeters or more, required for routing structures such as directional couplers with their associated input/output branching guides [ I ] . The size of conventional couplers, patterned laterally in the substrate plane, has been limited by lithographic and branching constraints. Here we demonstrate a novel approach using

results in a transistor with a 0 of 80 and an F, of 60 GHz. These are typically biased with a current of - 3 mA.

The circuit topology uses a single-transistor transimpedance am- plifier with a 1750-0 feedback resistor directly connected between the collector and the base of the input transistor. This is in contrast

( i ) vertically defined couplers with all critical dimensions con- trolled by precision epitaxy,

ii) optical-to-electronic conversion for minimal guide branching problems,

to the usual topology used for OEIC receivers in which a follower which achieves ultra-compact (< 100 pm), high-performance de- stage is included in the feedback loop. This is usually done to allow vices. This approach is applied to polarization diversity photode- for level shifts required for biasing of the collector-base junction tection, which produces two photocurrent outputs proportional to of the input transistor or biasing of an input cascode stage. The guide input intensities in each of two orthogonal polarization states. GaAs/AIGaAs HBT’s used here have close to their optimum per- This function is necessary for polarization-independent coherent formance, unlike silicon transistors, at 0-V collector to base bias receivers employing polarization diversity architectures [2]. making this topology desirable. This configuration also minimizes Our device consists of an input rib waveguide, a vertically de- the number of devices in the feedback loop and thereby reduces the fined coupler, a vertically coupled mesa p-i-n photodiode (PD). a delay around the loop, increasing the phase margin of the amplifier second coupler, and a second PD, all serially connected. A gold to 50-0 loads directly. cover on the first coupler ensures that only TE-polarized light is

As with previously demonstrated OEIC receiver approaches coupled into the first PD: the second coupler/PD pair collects the combining MSM detectors with MESFET’s on GaAs [4], [5] this remaining TM light. Component lengths are 79, 54 pm for couplers approach yields itself easily to ready integration with more com- and 21, 31 pm for PD’s, for - 200-pm total detector length. Fab- plex circuits with no sacrifice in yield. For example, this circuit rication is simple, involving a single OMCVD growth on a planar was fabricated on the same wafer as high-speed multiplexers, de- InP: Fe substrate followed by a conventional metallization and wet multiplexers, AGC amplifiers, and high-speed (15-GHz clock) etching with features >5 pm. The device consists of p-In- 500-gate arrays. GaAsP/i-InGaAs/n-InGaAsP PD layers above semi-insulating

In summary we have demonstrated an extremely wide-bandwidth coupler layers (two InGaAsP : Fe guides, InP : Fe cladding). The monolithic OEIC receiver with high gain and low noise. This dem- n-InGaAsP provides both electrical contact and optical “imped- onstrates the ability of HBT technology to provide low-noise and ance matching” [3] to minimize PD length (for high polarization high-bandwidth performance in a preamplifier combined with an selectivity) while maintaining high quantum efficiency. PD dimen- easily and naturally integrable optical detector. A novel circuit to- sions (35-pm width necessary to intercept all diffracted light, I-pm pology is also demonstrated to yield good high-speed performance depletion width) are suitable for high-speed detection. Semi-insu- with GaAs/AlGaAs HBT’s. With minor modifications and im- lating guides permit connection of two devices for low-noise, bal- provements both speed and quantum efficiency should be easily anced reception with on-chip photocurrent subtraction [4]. increased. This approach can be easily extended into the Fabricated devices exhibit low PD leakage at -8 V (5 I O nA InGaAs/InAIAs or InGaAs/InP system to provide operation at 1.3 before. 5 6 0 nA after passivation) and low WG propagation loss and 1.55 pm. (0.5 dB . cm) at the 1.52-pm test wavelength. These are the first

reported semi-insulating 111-V WG’s; their low loss despite antic- [I] H . Yano, K. Aga, M . Murata. H. Kaniei, G. Sasaki, and H. Hayashi,

in Tech Dig. on Oprical Fiber Cornmunicarion C o n t . 1991 (Opt. Soc. of America. Washington, DC). paper TuBl.

[2] S. Chandrasekhar, A. G. Dentai, C. H. Joyner. B. C. Johnson, A . H. Gnauck, G. J . Qua. Electron. Le f t . . vol. 26. paper no. 22. p. 1880. 1990.

131 P. M . Asbeck. M. F. Chang. J . A. Higgins. N . H. Sheng. G . J. Sul- livan. K. C. Wang, IEEE Trans. Electron. Devices. vol. 36. no. I O , p. 2032, 1989.

141 B. Brar. S. Beccue, M. K. Kilcoyne. K . Pedrotti. G. Robinson, Proc. V I E . vol. 994. Optoelectron. Muter. D e i k e s . Packaging Intercon- nects 11. 1988. p. 65.

ipated Fe deep-level absorpiton, is significant for an entire class of future high-speed, waveguide OEIC’s. On-chip quantum efficien- cies of 90, 80% are obtained for TE. TM polarization, respec- tively. Typical polarization selectivities were 1 I dB(TE) and 14 dB(TM) optical extinction, somewhat lower than those predicted (15 dB TE, 30 dB TM) due to stray light (imperfect input coupling) and deviations from the ideal structure. Nonetheless, the achieved extinction ratios are more than adequate for coherent receivers in network applications: only 1 dB of total (TE plus TM) IF signal variation due to input polarization fluctuations is expected.