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InGaAs resonant tunneling diode relaxation oscillator by direct optical injection
Solid-State Electronics 45 (2001) 1827±1830
Phase-locking of an InP/InGaP/InGaAs resonant tunneling diode relaxation oscillator by direct optical injection M. Kahn *, J. Lasri, M. Orenstein, D. Ritter, G. Eisenstein Department of Electrical Engineering, Technion ± Israel Institute of Technology, Haifa 32000, Israel Received 27 February 2001; accepted 3 April 2001
Abstract Optical-injection locking of an InP based resonant tunneling diode relaxation oscillator is demonstrated. The diode is an Al-free InP/InGaP/InGaAs structure. The characteristics of the fundamental oscillating line and its harmonic, including their phase noise are reported. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: GaInP/GaInAs resonant tunneling diode; On-wafer relaxation oscillator; Locking range; Aluminium free device
1. Introduction Optically controlled microwave oscillators have many applications in various communication systems. In particular, very high bit rate optical ®ber transmission systems require new solutions for clock synchronization of signal processing circuits. One such possible oscillator is based on a resonant tunneling diode (RTD) with a modulated optical signal input. The RTD oscillator can be optically injection locked so that the signal clock can be extracted and tracked. RTDs have been proven to be very fast switching devices, with self oscillations up to 700 GHz as obtained in an external cavity [1], and the feasibility of injection locking to an external modulated light source was demonstrated in an GaAs based RTD [2]. Recently, relaxation oscillators based on InP and GaAs RTDs have been intensively studied. This family of oscillators generates constant width pulses over a large range of repetition rates [3], and has proven to be easily integratable [4]. An optical clock recovery circuit based on injection locking of a self-oscillating RTD integrated with a photodiode was recently demonstrated [5]. To the
* Corresponding author. Address: OPTO+, Route de Nozay, 91460 Marcoussis, France. Tel.: +33-1-69634680; fax: +33-169631422. E-mail address: [email protected] (M. Kahn).
best of our knowledge, however, direct optical injection locking of InP based RTDs has to date not been demonstrated. We report here such direct optical injection locking in an InP based RTD relaxation-oscillator circuit. The RTD structures are fabricated in the InP/InGaAs/InGaP aluminum-free material system, following the work of Cohen and Ritter [6,7]. The aluminum-free system is advantageous because it allows integration with optoelectronic devices made of the InGaAsP material system. An on-wafer oscillator operating in the GHz range was demonstrated and locked via direct injection of a modulated 1550 nm wavelength optical signal. The locking characteristics of the fundamental oscillating line and its harmonics were studied in detail. 2. Device fabrication The epitaxial layers were grown on a h0 0 1i oriented semi-insulating InP substrate using a compact metalorganic molecular beam epitaxy apparatus [8]. Details of the growth procedure are described in Ref. [6]. The layer thick structure, shown in Fig. 1(a) consists of a 50 A In0:7 Ga0:3 P In0:64 Ga0:4 As well placed between two 35 A barriers. The tensile strain of the In0:7 Ga0:3 P layers is compensated by the compressive strain (1% lattice thick In0:6 Ga0:4 As mismatch) of the well, and by a 100 A thick undoped layer next to the barrier. A 1000 A
0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 1 7 0 - 8
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M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
and 0.12 V. A schematic diagram of the relaxation oscillator circuit is shown in Fig. 2(a). The device is shortcircuited through a large capacitor, and is biased in the negative resistance region, through an 18 GHz bias tee. This relaxation oscillator con®guration is slightly different than the one presented in Refs. [2,3,5] because the AC feedback loop is completely decoupled from the DC feed circuit. When the bias point of the device reaches the negative dierential resistance region, the RTD starts oscillating at a frequency determined primarily by the round trip time between the RTD and the AC short circuit. The exact bias point which determines the diode capacitance and hence its impedance matching to the transmission line, also aects the oscillation frequency. The generated voltage waveform across the RTD is a square wave corresponding to the successive switching of the operating point between peak and valley, as shown in Fig. 2(b). Because of the square wave nature of the oscillations higher order harmonics are generated. The full output spectrum for an oscillator placed in a microwave probe station with the shorted capacitor directly connected to the microwave probe is shown in Fig. 2(c). A fundamental oscillation frequency of 1.2 GHz and several harmonics are observed. The output power was 15 dBm.
Fig. 1. Room temperature I±V curve of a 160 lm2 GaInP/ thick GaInP barriers. GaInAs RTD with 35 A
In0:53 Ga0:47 As layer serves as the collector, and a 50 A thick undoped In0:53 Ga0:47 As layer adjacent to the emitter barrier serves as a spacer layer. The strained InGaP barrier layers in our devices replace the conventional AlInAs barrier layers. This approach is advantageous when aluminum-free structures are required. Although the theoretical critical thickness of In0:7 Ga0:3 P layers thicker than the theoretical limit is about 25 A, can be successfully grown, and strain compensation helps preventing the formation of defects in these layers as well [9]. The diodes were fabricated using a wet-etching process. The diode mesa is larger than the metal contact in order to enable light penetration into the device. The structure was coated by polyimide. Metallic contacts were evaporated through via holes in the polyimide.
3. Self oscillations The room temperature I±V curve of a 160 lm2 RTD is shown in Fig. 1(b). The peak voltage and current density are 6.2 kA/cm2 and 0.34 V while the peakto-valley current ratio and voltage dierence are 2.36
4. Optical injection locking Optical injection locking experiments were performed with the short circuit placed at about 3 cm from the probe, yielding a free-running oscillation frequency of 360 MHz. The self-oscillating RTD was then exposed to an optical signal at the wavelength of 1.5 lm modulated by a spectrally pure sinusoidal signal at a frequency close to that of the free running RTD oscillations. The modulated optical signal was delivered on top of the diode mesa next to the metal contact, by an optical ®ber. The illumination of the RTD with the optical signal causes the free running signal to shift its frequency. The underlying mechanism is believed to be the generation of electron±hole pairs in the collector depletion region, acting to decrease the bias across the structure which changes in turn the diode capacitance. For a modulation frequency close to the free-running signal, the RTD oscillation is synchronized to the injected signal. The spectra of the free-running and locked signals (measured with a resolution bandwidth of 3 kHz) are shown in Fig. 3(a) and (b), respectively. The phase noise of the free running oscillator, 78 dBc/Hz, measured at a 100 kHz oset, was reduced by 17 dB due to the optical injection locking. The locking range is de®ned as the frequency range within which the free-running oscillator with frequency x0 and power P0 , is synchronized to the external signal
M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
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Fig. 2. RTD relaxation oscillations. (a) Circuit diagram. (b) Measured waveform of oscillation. (c) Full spectra of oscillation. The length L of the AC feedback loop determines the fundamental frequency of oscillation.
Fig. 3. (a) Free-running fundamental oscillation peak at 360 MHz, with a feedback loop length 3 cm. (b) Optically locked oscillation peak. The phase noise is reduced by 17 dB.
at frequency xinj and power Pinj . The locking range is quanti®ed by the Adler equation [10]: r x0 Pinj Dxlock 2Q P0
where x0 =2Q is the cold cavity bandwidth. The results shown in Fig. 4 validate square root dependence of the locking range on the injection power, and from the experimental data we obtained a cold cavity bandwidth of 10 MHz.
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M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
as barriers, keeping the structure aluminum-free. Onwafer self-oscillations of the RTD were measured at 1.2 GHz, and locking between oscillations and a modulated optical signal was performed by direct optical injection on the device. The locking range was measured up to 400 kHz, with a linear dependence on the ratio between freerunning and optical injected signal amplitudes. Faster oscillations can be achieved by reducing the electrical AC feedback length, in a hybrid or monolithic circuit.
Acknowledgements
Fig. 4. Locking range vs. ratio between free-running and optical injected signal amplitudes.
We would like to thank S. Cohen for technical assistance in the growth and V. Sidorov for help in the processing.
References
Fig. 5. The spectra of an optically injection locked 10th harmonic signal.
Finally, we demonstrate that all the harmonics are phased locked simultaneously with the fundamental line. Fig. 5 shows injection locking of the 10th harmonic (with an optical injection at the fundamental frequency). The spectrum of the locked 10th harmonic exhibits a phase noise of 70 dBc/Hz at a 100 kHz oset.
5. Conclusion Relaxation oscillators comprising InP based RTD were fabricated. Highly strained InGaP layers were used
[1] Brown ER, Soderstrom JR, Parker CD, Mahoney LJ, Molvar KM, McGill TC. Oscillation up to 712 GHz in InAs/AlSb resonant-tunneling diodes. Appl Phys Lett 1991;58:2291. [2] Higgins TP, Harvey JF, Struzebecher DJ, Paolella AC, Lux RA. Direct optical frequency modulation and injection locking of resonant tunnel diode oscillator. Electron Lett 1992;28:1574. [3] Brown ER, Parker CD, Verghese S, Geis MW, Harvey JF. Resonant-tunneling transmission-line relaxation oscillator. Appl Phys Lett 1997;70:2787. [4] Chen CL, Mathews RH, Mahoney LJ, Calwa SD, Sage JP, Molvar KM, Parker CD, Maki PA, Solner TCLG. Resonant-tunneling-diode relaxation oscillators. SolidState Electron 2000;44:1853. [5] Murata K, Sano K, Akeyoshi K, Shimizu N, Sano E, Yamamoto M, Ishibashi T. Optoelectronic clock recovery circuit using resonant tunneling diode and uni-travellingcarrier photodiode. Electron Lett 1998;34:1424. [6] Cohen GM, Ritter D. Room temperature operation of Gax In1 x P/Ga0:47 In0:53 as resonant tunneling diodes. J Cryst Growth 1998;188:359. [7] Cohen GM, Ritter D. Microwave performance of Gax In1 x P/ Ga0:47 In0:53 as resonnant tunnelling diodes. Electron Lett 1998;34:1267. [8] Hamm RH, Ritter D, Temkin H. Compact metalorganic molecular-beam epitaxy growth system. J Vac Sci Technol A 1994;12:2790. [9] Cohen GM, Zisman P, Bahir G, Ritter D. Growth of strained GaInP on InP by metalorganic molecular beam epitaxy for heterostructure ®eld eect transistor application. J Vac Sci Technol B 1998;16:2639. [10] Adler R. A study of locking phenomena in oscillators. Proc IRE 1946;34:351. Proc IEEE 1973;61:1380 [reprint].
Phase-locking of an InP/InGaP/InGaAs resonant tunneling diode relaxation oscillator by direct optical injection M. Kahn *, J. Lasri, M. Orenstein, D. Ritter, G. Eisenstein Department of Electrical Engineering, Technion ± Israel Institute of Technology, Haifa 32000, Israel Received 27 February 2001; accepted 3 April 2001
Abstract Optical-injection locking of an InP based resonant tunneling diode relaxation oscillator is demonstrated. The diode is an Al-free InP/InGaP/InGaAs structure. The characteristics of the fundamental oscillating line and its harmonic, including their phase noise are reported. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: GaInP/GaInAs resonant tunneling diode; On-wafer relaxation oscillator; Locking range; Aluminium free device
1. Introduction Optically controlled microwave oscillators have many applications in various communication systems. In particular, very high bit rate optical ®ber transmission systems require new solutions for clock synchronization of signal processing circuits. One such possible oscillator is based on a resonant tunneling diode (RTD) with a modulated optical signal input. The RTD oscillator can be optically injection locked so that the signal clock can be extracted and tracked. RTDs have been proven to be very fast switching devices, with self oscillations up to 700 GHz as obtained in an external cavity [1], and the feasibility of injection locking to an external modulated light source was demonstrated in an GaAs based RTD [2]. Recently, relaxation oscillators based on InP and GaAs RTDs have been intensively studied. This family of oscillators generates constant width pulses over a large range of repetition rates [3], and has proven to be easily integratable [4]. An optical clock recovery circuit based on injection locking of a self-oscillating RTD integrated with a photodiode was recently demonstrated [5]. To the
* Corresponding author. Address: OPTO+, Route de Nozay, 91460 Marcoussis, France. Tel.: +33-1-69634680; fax: +33-169631422. E-mail address: [email protected] (M. Kahn).
best of our knowledge, however, direct optical injection locking of InP based RTDs has to date not been demonstrated. We report here such direct optical injection locking in an InP based RTD relaxation-oscillator circuit. The RTD structures are fabricated in the InP/InGaAs/InGaP aluminum-free material system, following the work of Cohen and Ritter [6,7]. The aluminum-free system is advantageous because it allows integration with optoelectronic devices made of the InGaAsP material system. An on-wafer oscillator operating in the GHz range was demonstrated and locked via direct injection of a modulated 1550 nm wavelength optical signal. The locking characteristics of the fundamental oscillating line and its harmonics were studied in detail. 2. Device fabrication The epitaxial layers were grown on a h0 0 1i oriented semi-insulating InP substrate using a compact metalorganic molecular beam epitaxy apparatus [8]. Details of the growth procedure are described in Ref. [6]. The layer thick structure, shown in Fig. 1(a) consists of a 50 A In0:7 Ga0:3 P In0:64 Ga0:4 As well placed between two 35 A barriers. The tensile strain of the In0:7 Ga0:3 P layers is compensated by the compressive strain (1% lattice thick In0:6 Ga0:4 As mismatch) of the well, and by a 100 A thick undoped layer next to the barrier. A 1000 A
0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 1 7 0 - 8
1828
M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
and 0.12 V. A schematic diagram of the relaxation oscillator circuit is shown in Fig. 2(a). The device is shortcircuited through a large capacitor, and is biased in the negative resistance region, through an 18 GHz bias tee. This relaxation oscillator con®guration is slightly different than the one presented in Refs. [2,3,5] because the AC feedback loop is completely decoupled from the DC feed circuit. When the bias point of the device reaches the negative dierential resistance region, the RTD starts oscillating at a frequency determined primarily by the round trip time between the RTD and the AC short circuit. The exact bias point which determines the diode capacitance and hence its impedance matching to the transmission line, also aects the oscillation frequency. The generated voltage waveform across the RTD is a square wave corresponding to the successive switching of the operating point between peak and valley, as shown in Fig. 2(b). Because of the square wave nature of the oscillations higher order harmonics are generated. The full output spectrum for an oscillator placed in a microwave probe station with the shorted capacitor directly connected to the microwave probe is shown in Fig. 2(c). A fundamental oscillation frequency of 1.2 GHz and several harmonics are observed. The output power was 15 dBm.
Fig. 1. Room temperature I±V curve of a 160 lm2 GaInP/ thick GaInP barriers. GaInAs RTD with 35 A
In0:53 Ga0:47 As layer serves as the collector, and a 50 A thick undoped In0:53 Ga0:47 As layer adjacent to the emitter barrier serves as a spacer layer. The strained InGaP barrier layers in our devices replace the conventional AlInAs barrier layers. This approach is advantageous when aluminum-free structures are required. Although the theoretical critical thickness of In0:7 Ga0:3 P layers thicker than the theoretical limit is about 25 A, can be successfully grown, and strain compensation helps preventing the formation of defects in these layers as well [9]. The diodes were fabricated using a wet-etching process. The diode mesa is larger than the metal contact in order to enable light penetration into the device. The structure was coated by polyimide. Metallic contacts were evaporated through via holes in the polyimide.
3. Self oscillations The room temperature I±V curve of a 160 lm2 RTD is shown in Fig. 1(b). The peak voltage and current density are 6.2 kA/cm2 and 0.34 V while the peakto-valley current ratio and voltage dierence are 2.36
4. Optical injection locking Optical injection locking experiments were performed with the short circuit placed at about 3 cm from the probe, yielding a free-running oscillation frequency of 360 MHz. The self-oscillating RTD was then exposed to an optical signal at the wavelength of 1.5 lm modulated by a spectrally pure sinusoidal signal at a frequency close to that of the free running RTD oscillations. The modulated optical signal was delivered on top of the diode mesa next to the metal contact, by an optical ®ber. The illumination of the RTD with the optical signal causes the free running signal to shift its frequency. The underlying mechanism is believed to be the generation of electron±hole pairs in the collector depletion region, acting to decrease the bias across the structure which changes in turn the diode capacitance. For a modulation frequency close to the free-running signal, the RTD oscillation is synchronized to the injected signal. The spectra of the free-running and locked signals (measured with a resolution bandwidth of 3 kHz) are shown in Fig. 3(a) and (b), respectively. The phase noise of the free running oscillator, 78 dBc/Hz, measured at a 100 kHz oset, was reduced by 17 dB due to the optical injection locking. The locking range is de®ned as the frequency range within which the free-running oscillator with frequency x0 and power P0 , is synchronized to the external signal
M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
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Fig. 2. RTD relaxation oscillations. (a) Circuit diagram. (b) Measured waveform of oscillation. (c) Full spectra of oscillation. The length L of the AC feedback loop determines the fundamental frequency of oscillation.
Fig. 3. (a) Free-running fundamental oscillation peak at 360 MHz, with a feedback loop length 3 cm. (b) Optically locked oscillation peak. The phase noise is reduced by 17 dB.
at frequency xinj and power Pinj . The locking range is quanti®ed by the Adler equation [10]: r x0 Pinj Dxlock 2Q P0
where x0 =2Q is the cold cavity bandwidth. The results shown in Fig. 4 validate square root dependence of the locking range on the injection power, and from the experimental data we obtained a cold cavity bandwidth of 10 MHz.
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M. Kahn et al. / Solid-State Electronics 45 (2001) 1827±1830
as barriers, keeping the structure aluminum-free. Onwafer self-oscillations of the RTD were measured at 1.2 GHz, and locking between oscillations and a modulated optical signal was performed by direct optical injection on the device. The locking range was measured up to 400 kHz, with a linear dependence on the ratio between freerunning and optical injected signal amplitudes. Faster oscillations can be achieved by reducing the electrical AC feedback length, in a hybrid or monolithic circuit.
Acknowledgements
Fig. 4. Locking range vs. ratio between free-running and optical injected signal amplitudes.
We would like to thank S. Cohen for technical assistance in the growth and V. Sidorov for help in the processing.
References
Fig. 5. The spectra of an optically injection locked 10th harmonic signal.
Finally, we demonstrate that all the harmonics are phased locked simultaneously with the fundamental line. Fig. 5 shows injection locking of the 10th harmonic (with an optical injection at the fundamental frequency). The spectrum of the locked 10th harmonic exhibits a phase noise of 70 dBc/Hz at a 100 kHz oset.
5. Conclusion Relaxation oscillators comprising InP based RTD were fabricated. Highly strained InGaP layers were used
[1] Brown ER, Soderstrom JR, Parker CD, Mahoney LJ, Molvar KM, McGill TC. Oscillation up to 712 GHz in InAs/AlSb resonant-tunneling diodes. Appl Phys Lett 1991;58:2291. [2] Higgins TP, Harvey JF, Struzebecher DJ, Paolella AC, Lux RA. Direct optical frequency modulation and injection locking of resonant tunnel diode oscillator. Electron Lett 1992;28:1574. [3] Brown ER, Parker CD, Verghese S, Geis MW, Harvey JF. Resonant-tunneling transmission-line relaxation oscillator. Appl Phys Lett 1997;70:2787. [4] Chen CL, Mathews RH, Mahoney LJ, Calwa SD, Sage JP, Molvar KM, Parker CD, Maki PA, Solner TCLG. Resonant-tunneling-diode relaxation oscillators. SolidState Electron 2000;44:1853. [5] Murata K, Sano K, Akeyoshi K, Shimizu N, Sano E, Yamamoto M, Ishibashi T. Optoelectronic clock recovery circuit using resonant tunneling diode and uni-travellingcarrier photodiode. Electron Lett 1998;34:1424. [6] Cohen GM, Ritter D. Room temperature operation of Gax In1 x P/Ga0:47 In0:53 as resonant tunneling diodes. J Cryst Growth 1998;188:359. [7] Cohen GM, Ritter D. Microwave performance of Gax In1 x P/ Ga0:47 In0:53 as resonnant tunnelling diodes. Electron Lett 1998;34:1267. [8] Hamm RH, Ritter D, Temkin H. Compact metalorganic molecular-beam epitaxy growth system. J Vac Sci Technol A 1994;12:2790. [9] Cohen GM, Zisman P, Bahir G, Ritter D. Growth of strained GaInP on InP by metalorganic molecular beam epitaxy for heterostructure ®eld eect transistor application. J Vac Sci Technol B 1998;16:2639. [10] Adler R. A study of locking phenomena in oscillators. Proc IRE 1946;34:351. Proc IEEE 1973;61:1380 [reprint].