- Email: [email protected]
InP separate absorption, grading and multiplication avalanche photodetectors
PERGAMON
Solid-State Electronics 43 (1999) 659±663
Fabrication and characterization of high-performance planar InGaAs/InP separate absorption, grading and multiplication avalanche photodetectors Wen-Jeng Ho a, Meng-Chyi Wu b, *, Yuan-Kuang Tu a a
Telecommunication Laboratories, Chunghwa Telecom Co., Ltd., Yang-Mei 32617, Taiwan Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
b
Received 3 September 1998; accepted 6 October 1998
Abstract In this article, we describe the fabrication and characterization of high-performance planar InGaAs/InP separate absorption, grading and multiplication avalanche photodetectors with double-diused guard rings. The fabricated devices exhibit a breakdown voltage VB of ÿ79± ÿ80 V, an extremely low dark current of 11.2 nA and a capacitance of 0.8 pF at 0.9VB, a maximum bandwidth of 2 GHz at the multiplication gain of 3, and a rise time of 126.4 ps and a pulsewidth of 206.8 ps for response speed. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction InGaAs/InP avalanche photodiodes (APDs) are of substantial interest for high-bit rate (r400 Mb s ÿ 1) optical ®ber communication at 1.3 and 1.55 mm that required high detector sensitivity. It is widely recognized that optical ®ber communication systems can be bene®tted from the use of APD due to the presence of photocurrent gain which increases the sensitivity of an optical receiver. Thus, APDs can enhance both the bandwidth and span length between repeaters over that can be achieved using other devices such as PIN photodiodes [1]. For this reason, considerable eorts during the past 10 years have been devoted to develop InP-based APDs for their potential application to long-distance optical ®ber communication systems in the 1.0±1.55 mm wavelength region. Mesa structures were ®rst proposed, but are often characterized by poor reliability, because of surface leakage current and instabilities. This is the reason why a planar structure is required for highly reliable avalanche photodiodes [2] since passivation techniques
* Corresponding author. Tel.: +886-35-715131/4038; fax: +886-35-715971; e-mail: [email protected]
can be entirely applied to minimize the surface leakage current. The planar InP-based separate absorption, grading and multiplication APD (SAGM-APD) structure is widely adopted for these applications due to its low dark current, high quantum eciency over a wide spectral region, and high gain-bandwidth product. In SAGM structure, the electric ®eld must be high enough to cause avalanche multiplication in the InP layer, yet low enough in absorbing InGaAs layer to prohibit the tunneling process, which would cause an unacceptably high leakage current. Moreover, a grading layer, usually composed of multiple thin InGaAsP layers, is inserted between InP and InGaAs to reduce the eect of the photo-excited hole pile-up at valence band oset [3]. To make a high-performance APD, it requires not only a good quality material and a careful structure design, but also a precise process control. However, the structure of planar APDs is complicated by the necessity to prevent premature low voltage breakdown at the junction periphery, which curves from the bulk to the surface of semiconductor. This situation is referred to as the `curvature eect' [4], and it commonly exists in diused or ion-implanted junction planar devices. There are two approaches to deal with this problem. One is to enhance the electric ®eld in the central active region of junction by selectively
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 8 ) 0 0 3 0 2 - 5
660
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
increasing the charge density under that region (separate absorption, grading charge, and multiplication APD; SAGCM-APD) [5]. The second approach is to introduce a guard ring (GR) at the junction periphery to reduce the electric ®eld and suppress edge breakdown (SAGM-GR APD) [6]. In this work, we present our study on the fabrication and characterization of planar InGaAs/InP SAGM-GR APDs with double-diffused junction.
2. Device fabrication A schematic cross section of the planar SAGM-GR APD is shown in Fig. 1. The SAGM epitaxial layers are grown on a (100) S-doped InP substrate by metalorganic chemical vapor deposition (MOCVD). A typical layer structure consists of a 2 mm thick InP buer layer (n-type S-doped to 021016 cm ÿ 3), followed by a 3 mm thick n ÿ -InGaAs absorbing layer with a carrier concentration of 31014 cm ÿ 3. The three undoped InGaAsP grading layers (l = 1.5, 1.3 and 1.1 mm, 1000 AÊ each) were grown in sequence. Next, the n-InP multiplication layer of 1.2 mm is grown with a doping density of 2.1±2.51016 cm ÿ 3. The ®nal layer is a 3 mm thick undoped InP cap layer with 11015 cm ÿ 3 carrier concentration. The electrical ®eld drop is 20 kV/cm over the whole n ÿ -InGaAs absorbing layer. The low noise operation is obtained by using the n-type InP multiplication layer and InGaAs absorbing layer to provide pure hole injection with a larger impact ionization coecient of hole in InP [7, 8]. The thin InGaAsP grading layers between the InP and InGaAs heterointerface are used to reduce the hole pile-up. Device fabrication begins with the deposition of 1500 AÊ SiNx by plasma-enhanced chemical vapor deposition (PECVD), and then reactive ion etching (RIE)
Fig. 1. A schematic cross section of the SAGM-GR APD. The active diameter is 95 mm and the ring width is 20 mm with half inside the active region.
etches the guard ring window whose mid-diameter was 100 mm with a 20 mm spacing. Next, Zn-diusion was performed at 5008C for 16 min. Then a second SiNx was deposited and RIE opened the central active window with a 95 mm diameter which covers the inner annulus of guard ring. During the second Zn-diusion, an elevated temperature and a longer duration time were applied to keep the diusion front of the active junction to 0.25 mm away from the multiplication layer. At the same time, the guard ring under the SiNx is further driven-in under constant source condition, where the p ÿ -junction edge of the guard ring almost reaches the interface. This optimized diusion process cannot only improve the device characteristics, but also reduce the edge breakdown. Finally, a l/4 thick SiNx layer was deposited as anti-re¯ection (AR) coating, followed by p- and n-metal deposition using Cr/ AuZn/Au and AuGeNi/Au, respectively. Some devices were die/wire bonded on ceramic submount, and soldered onto SMA connector for further 3 dB bandwidth and pulse response measurements. 3. Results and discussion The reverse current±voltage (I±V) characteristic was measured using the HP4145B semiconductor parameter analyzer, both without light and with four dierent light intensities using a 1.55 mm laser coupled by means of a cleaved ®ber with a diameter of 10 mm which is in close proximity to the device. There is a ¯at region of d.c. photoresponse well before the onset of avalanche multiplication even though the InGaAs is undepleted in the active area of the device because the long-lived photo-generated carriers in the active area diuse [9] to the device periphery, where the InGaAs is depleted. The I±V results for a typical device are given in Fig. 2. A plot of the dark current, photocurrent, breakdown voltage, and multiplication gain as a function of reverse-bias voltage at 1.55 mm wavelength with 1.0 mW input light power for a typical APD is shown in Fig. 3. The breakdown voltage (VB) of the device is ÿ79± ÿ 81 V, in accordance to our calculation of 0.3 mm separation between the diusion front and the multiplication layer, and the punch-through voltage which corresponds to the extension from the depletion edge to the InGaAs absorption layer is ÿ46.5± ÿ 47.5 V. The device exhibits a very low dark current even at 90% breakdown voltage. The multiplied dark current is only ÿ11.2 nA, which implies a primary dark current of less than ÿ5 nA (<510 ÿ 5 A/cm2). The photocurrent starts to increase at the `punch-through' voltage of ÿ47.5 V, where the depletion edge extends to the heterointerface. The responsivity of AR-coated device is 1.0 A/W (Z = 80%) at the wavelength of 1.55 mm with an input power of 1.0 mW. The multiplication gain is
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
661
Fig. 4. A typical C±V trace for an SAGM-GR APD. Fig. 2. The reverse current±voltage (I±V) characteristic of a typical APD excited for both without light and with four dierent light intensities using a 1.55 mm laser.
calibrated assuming that it is unity at punch-through where the gain-quantum eciency product is determined to be 80%. Fig. 3 also shows the multiplication gain-voltage characteristic, which indicates a high gain over 40 at the multiplied dark current of ÿ100 nA. The measured C±V characteristic of the diode by HP 4280A is shown in Fig. 4. The chip capacitance is 0.8 pF at 0.9VB bias voltage. The reason why the C±V punch-through voltage (Vp0ÿ 50 V) lags the I±V punch-through voltage (Vp0ÿ 47.5 V) by 2.5 V is that the I±V punch-through may occur at the device edge, where there is a slight multiplication gain M of 1 discussed further in Fig. 5. That is, holes diuse to the periphery where they are swept out by the electric
Fig. 3. The dark current, photocurrent, and multiplication gain versus reverse bias voltage. The incident optical power was ®xed at 1.0 mW.
®eld. It may take a slightly higher voltage to punchthrough to the InGaAs in the bulk of the device. In order to demonstrate the eectiveness of the structure in guarding against edge breakdown, a spatial pro®le of photocurrent response was measured by scanning a focused 1.55 mm wavelength with 1.0 mW light spot (approximately 10 mm in size) across the device [10]. Fig. 5 shows the photoresponse of the diode at various multiplication gains up to 9. However, as the multiplication gain is greater than 3, no excess edge gain due to junction curvature or surface gain due to high-®eld strength on the top surface is observed. Fig. 6 shows the relation between multiplication gain and incident optical power. The incident light is emitted from 1.3 and 1.55 mm wavelength semiconductor lasers and attenuated by a Tektronix OA5002 optical attenuator. The multiplication gain
Fig. 5. Line scanning of the APD photoresponse along the active area at dierent multiplication gains by using a 1.55 mm InGaAsP laser with a 1.0 mW power excitation.
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W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
Fig. 6. The relation between multiplication gain and incident optical power.
increases with decreasing the incident optical power. However, when the incident optical power is below ÿ50 dBm (10 nW), it becomes unstable due to the background noise interference. On the other hand, Fig. 6 also shows that the devices operated at a lower multiplication gain (for example, M = 1, just as the case of PIN photodiodes) do not saturate even at higher incident powers. In general, the APD with the multiplication gain is more sensitive than the PIN device. Fig. 7 shows the 3 dB bandwidth versus multiplication factor characteristic [11] measured with a 50 O load resistance by HP 8703A lightwave component analyzer. At room temperature, there is a minimum multiplication gain for which a substantial bandwidth is apparent. The maximum bandwidth is 2.0 GHz at
Fig. 7. The 3 dB bandwidth versus multiplication factor characteristic of SAGM-GR APDs. The maximum bandwidth and gain-bandwidth product at 2.0 and 10 GHz, respectively.
M = 3 for the 110 mm diameter device. The central portion of the dependence is in principle limited by the carrier transit time (ttr) and the RC time constant. In this work, the expected 3 dB electrical bandwidth is 2.5 GHz calculated using a device capacitance of 1.0 pF (which consists of 0.809 pF chip capacitance and 0.15 pF ceramic submount capacitance), the stray capacitance of 0.05 pF, and the photodiode series resistance of 10 O obtained. The 2 GHz bandwidth is consistent with an RC limit of 2.5 GHz combined by square law with a transit time limited bandwidth of 3.3 GHz. On the other hand, for 5 mm secondary transit and maybe 3 mm average primary transit distance, we can arrive from the quick estimation of f = 0.443/ttr, 0.443/[8 mm/(60 mm ns ÿ 1)] = 3.5 GHz, consistent with the above. Thus both RC and transit time limit the bandwidth. In the multiplication factor greater than 10, the bandwidth is limited by an avalanche build-up time. The gain-bandwidth (GBW) product deduced from the inverse linear relationship is about 10 GHz in this device. When the multiplication factor is less than 2, the edge of depletion region under photosensitive area does not reach the InGaAs absorption layer. The degradation of the 3 dB bandwidth under the multiplication factor of 2 is due to slow diusion of holes from the undepleted InGaAs layer. The bandwidth raises rapidly when the edge of the depletion region reaches the InGaAs absorption layer. The pulse response for the devices was measured by using a 1.55 mm InGaAsP injection laser driven by a Hamamatsu PLP-01 ps light pulser to generate the light pulse with a rise time of 30 ps and a pulse width of 60 ps at a repetition rate of 10 MHz and a 50 O load resistance. Fig. 8 shows the measured pulse response of an APD biased at ÿ79 V and M = 5 from a Tektronix 7854A sampling oscilloscope, indicating a rise time of 126.4 ps, a fall time of 403.8 ps, and a pulsewidth of 206.8 ps. The trailing edge of pulse response
Fig. 8. Pulse response for an SAGM-GR APD biased at ÿ79 V and M = 5 to a train of 10 MHz light pulse.
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
is due to the slow speed holes which travel by a diusion process in the neutral region. 4. Conclusions High-performance planar InGaAs/InP SAGM-GR APDs grown by MOCVD have been successfully fabricated and characterized. Double-diused guard rings are used to suppress the edge breakdown. These fabricated APDs exhibit a low dark current of only ÿ11 nA at 90% breakdown voltage, a high gain of 40 at the multiplied dark current of ÿ100 nA, and a gain-bandwidth product of 10 GHz. The SAGM-GR APDs with above-mentioned performance are well suited for highspeed and long-distance ®ber optical communications. Acknowledgements This work is supported by the Telecommunication Laboratories, Chunghwa Telecom Co., Ltd., and the National Science Council (NSC 88-2215-E-007-004).
663
References [1] Forrest SR. In: Willardson RK, Beers AC, editors. Semiconductors and semimetals, vol. 22, part D. New York: Academic, 1985. p. 329. [2] Taguchi K, Torikai T, Sugimoto Y, Makita K, Ishihara H. J Lightwave Technol 1988;6:1643. [3] Kuwatsuka H, Kito Y, Uchida T, Mikawa T. IEEE Photon Technol Lett 1991;3:1113. [4] Sze SM, Gibbons G. Solid-St Electr 1966;9:831. [5] Tarof LE, Knight DG, Fox KE, Miner CJ, Puetz N, Kim HB. Appl Phys Lett 1990;57:670. [6] Matsushima Y, Noda Y, Kushiro Y, Seki N, Akiba S. Electr Lett 1984;20:235. [7] Taguchi K, Torikal T, Sugimoto T, Makita K, Ishihara H. J Appl Phys 1986;59:476. [8] Mcintyre RJ. IEEE Trans Electr Dev 1966;13:164. [9] Gallant M, Zemel A. Appl Phys Lett 1988;52:1686. [10] Chi GC, Muehlner DJ, Ostermayer FW, Jr, Fruend JM, O'brien KJ, Pawelek R, Mccoy RJ, Smith RC, Mattera VD, Jr. IEEE Trans Electr Dev 1987;34:2265. [11] Tarof LE, Yu J, Bruce R, Knight DG, Baird T, Oosterbrink B. IEEE Photon Technol Lett 1993;5:672.
Solid-State Electronics 43 (1999) 659±663
Fabrication and characterization of high-performance planar InGaAs/InP separate absorption, grading and multiplication avalanche photodetectors Wen-Jeng Ho a, Meng-Chyi Wu b, *, Yuan-Kuang Tu a a
Telecommunication Laboratories, Chunghwa Telecom Co., Ltd., Yang-Mei 32617, Taiwan Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
b
Received 3 September 1998; accepted 6 October 1998
Abstract In this article, we describe the fabrication and characterization of high-performance planar InGaAs/InP separate absorption, grading and multiplication avalanche photodetectors with double-diused guard rings. The fabricated devices exhibit a breakdown voltage VB of ÿ79± ÿ80 V, an extremely low dark current of 11.2 nA and a capacitance of 0.8 pF at 0.9VB, a maximum bandwidth of 2 GHz at the multiplication gain of 3, and a rise time of 126.4 ps and a pulsewidth of 206.8 ps for response speed. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction InGaAs/InP avalanche photodiodes (APDs) are of substantial interest for high-bit rate (r400 Mb s ÿ 1) optical ®ber communication at 1.3 and 1.55 mm that required high detector sensitivity. It is widely recognized that optical ®ber communication systems can be bene®tted from the use of APD due to the presence of photocurrent gain which increases the sensitivity of an optical receiver. Thus, APDs can enhance both the bandwidth and span length between repeaters over that can be achieved using other devices such as PIN photodiodes [1]. For this reason, considerable eorts during the past 10 years have been devoted to develop InP-based APDs for their potential application to long-distance optical ®ber communication systems in the 1.0±1.55 mm wavelength region. Mesa structures were ®rst proposed, but are often characterized by poor reliability, because of surface leakage current and instabilities. This is the reason why a planar structure is required for highly reliable avalanche photodiodes [2] since passivation techniques
* Corresponding author. Tel.: +886-35-715131/4038; fax: +886-35-715971; e-mail: [email protected]
can be entirely applied to minimize the surface leakage current. The planar InP-based separate absorption, grading and multiplication APD (SAGM-APD) structure is widely adopted for these applications due to its low dark current, high quantum eciency over a wide spectral region, and high gain-bandwidth product. In SAGM structure, the electric ®eld must be high enough to cause avalanche multiplication in the InP layer, yet low enough in absorbing InGaAs layer to prohibit the tunneling process, which would cause an unacceptably high leakage current. Moreover, a grading layer, usually composed of multiple thin InGaAsP layers, is inserted between InP and InGaAs to reduce the eect of the photo-excited hole pile-up at valence band oset [3]. To make a high-performance APD, it requires not only a good quality material and a careful structure design, but also a precise process control. However, the structure of planar APDs is complicated by the necessity to prevent premature low voltage breakdown at the junction periphery, which curves from the bulk to the surface of semiconductor. This situation is referred to as the `curvature eect' [4], and it commonly exists in diused or ion-implanted junction planar devices. There are two approaches to deal with this problem. One is to enhance the electric ®eld in the central active region of junction by selectively
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 8 ) 0 0 3 0 2 - 5
660
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
increasing the charge density under that region (separate absorption, grading charge, and multiplication APD; SAGCM-APD) [5]. The second approach is to introduce a guard ring (GR) at the junction periphery to reduce the electric ®eld and suppress edge breakdown (SAGM-GR APD) [6]. In this work, we present our study on the fabrication and characterization of planar InGaAs/InP SAGM-GR APDs with double-diffused junction.
2. Device fabrication A schematic cross section of the planar SAGM-GR APD is shown in Fig. 1. The SAGM epitaxial layers are grown on a (100) S-doped InP substrate by metalorganic chemical vapor deposition (MOCVD). A typical layer structure consists of a 2 mm thick InP buer layer (n-type S-doped to 021016 cm ÿ 3), followed by a 3 mm thick n ÿ -InGaAs absorbing layer with a carrier concentration of 31014 cm ÿ 3. The three undoped InGaAsP grading layers (l = 1.5, 1.3 and 1.1 mm, 1000 AÊ each) were grown in sequence. Next, the n-InP multiplication layer of 1.2 mm is grown with a doping density of 2.1±2.51016 cm ÿ 3. The ®nal layer is a 3 mm thick undoped InP cap layer with 11015 cm ÿ 3 carrier concentration. The electrical ®eld drop is 20 kV/cm over the whole n ÿ -InGaAs absorbing layer. The low noise operation is obtained by using the n-type InP multiplication layer and InGaAs absorbing layer to provide pure hole injection with a larger impact ionization coecient of hole in InP [7, 8]. The thin InGaAsP grading layers between the InP and InGaAs heterointerface are used to reduce the hole pile-up. Device fabrication begins with the deposition of 1500 AÊ SiNx by plasma-enhanced chemical vapor deposition (PECVD), and then reactive ion etching (RIE)
Fig. 1. A schematic cross section of the SAGM-GR APD. The active diameter is 95 mm and the ring width is 20 mm with half inside the active region.
etches the guard ring window whose mid-diameter was 100 mm with a 20 mm spacing. Next, Zn-diusion was performed at 5008C for 16 min. Then a second SiNx was deposited and RIE opened the central active window with a 95 mm diameter which covers the inner annulus of guard ring. During the second Zn-diusion, an elevated temperature and a longer duration time were applied to keep the diusion front of the active junction to 0.25 mm away from the multiplication layer. At the same time, the guard ring under the SiNx is further driven-in under constant source condition, where the p ÿ -junction edge of the guard ring almost reaches the interface. This optimized diusion process cannot only improve the device characteristics, but also reduce the edge breakdown. Finally, a l/4 thick SiNx layer was deposited as anti-re¯ection (AR) coating, followed by p- and n-metal deposition using Cr/ AuZn/Au and AuGeNi/Au, respectively. Some devices were die/wire bonded on ceramic submount, and soldered onto SMA connector for further 3 dB bandwidth and pulse response measurements. 3. Results and discussion The reverse current±voltage (I±V) characteristic was measured using the HP4145B semiconductor parameter analyzer, both without light and with four dierent light intensities using a 1.55 mm laser coupled by means of a cleaved ®ber with a diameter of 10 mm which is in close proximity to the device. There is a ¯at region of d.c. photoresponse well before the onset of avalanche multiplication even though the InGaAs is undepleted in the active area of the device because the long-lived photo-generated carriers in the active area diuse [9] to the device periphery, where the InGaAs is depleted. The I±V results for a typical device are given in Fig. 2. A plot of the dark current, photocurrent, breakdown voltage, and multiplication gain as a function of reverse-bias voltage at 1.55 mm wavelength with 1.0 mW input light power for a typical APD is shown in Fig. 3. The breakdown voltage (VB) of the device is ÿ79± ÿ 81 V, in accordance to our calculation of 0.3 mm separation between the diusion front and the multiplication layer, and the punch-through voltage which corresponds to the extension from the depletion edge to the InGaAs absorption layer is ÿ46.5± ÿ 47.5 V. The device exhibits a very low dark current even at 90% breakdown voltage. The multiplied dark current is only ÿ11.2 nA, which implies a primary dark current of less than ÿ5 nA (<510 ÿ 5 A/cm2). The photocurrent starts to increase at the `punch-through' voltage of ÿ47.5 V, where the depletion edge extends to the heterointerface. The responsivity of AR-coated device is 1.0 A/W (Z = 80%) at the wavelength of 1.55 mm with an input power of 1.0 mW. The multiplication gain is
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
661
Fig. 4. A typical C±V trace for an SAGM-GR APD. Fig. 2. The reverse current±voltage (I±V) characteristic of a typical APD excited for both without light and with four dierent light intensities using a 1.55 mm laser.
calibrated assuming that it is unity at punch-through where the gain-quantum eciency product is determined to be 80%. Fig. 3 also shows the multiplication gain-voltage characteristic, which indicates a high gain over 40 at the multiplied dark current of ÿ100 nA. The measured C±V characteristic of the diode by HP 4280A is shown in Fig. 4. The chip capacitance is 0.8 pF at 0.9VB bias voltage. The reason why the C±V punch-through voltage (Vp0ÿ 50 V) lags the I±V punch-through voltage (Vp0ÿ 47.5 V) by 2.5 V is that the I±V punch-through may occur at the device edge, where there is a slight multiplication gain M of 1 discussed further in Fig. 5. That is, holes diuse to the periphery where they are swept out by the electric
Fig. 3. The dark current, photocurrent, and multiplication gain versus reverse bias voltage. The incident optical power was ®xed at 1.0 mW.
®eld. It may take a slightly higher voltage to punchthrough to the InGaAs in the bulk of the device. In order to demonstrate the eectiveness of the structure in guarding against edge breakdown, a spatial pro®le of photocurrent response was measured by scanning a focused 1.55 mm wavelength with 1.0 mW light spot (approximately 10 mm in size) across the device [10]. Fig. 5 shows the photoresponse of the diode at various multiplication gains up to 9. However, as the multiplication gain is greater than 3, no excess edge gain due to junction curvature or surface gain due to high-®eld strength on the top surface is observed. Fig. 6 shows the relation between multiplication gain and incident optical power. The incident light is emitted from 1.3 and 1.55 mm wavelength semiconductor lasers and attenuated by a Tektronix OA5002 optical attenuator. The multiplication gain
Fig. 5. Line scanning of the APD photoresponse along the active area at dierent multiplication gains by using a 1.55 mm InGaAsP laser with a 1.0 mW power excitation.
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W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
Fig. 6. The relation between multiplication gain and incident optical power.
increases with decreasing the incident optical power. However, when the incident optical power is below ÿ50 dBm (10 nW), it becomes unstable due to the background noise interference. On the other hand, Fig. 6 also shows that the devices operated at a lower multiplication gain (for example, M = 1, just as the case of PIN photodiodes) do not saturate even at higher incident powers. In general, the APD with the multiplication gain is more sensitive than the PIN device. Fig. 7 shows the 3 dB bandwidth versus multiplication factor characteristic [11] measured with a 50 O load resistance by HP 8703A lightwave component analyzer. At room temperature, there is a minimum multiplication gain for which a substantial bandwidth is apparent. The maximum bandwidth is 2.0 GHz at
Fig. 7. The 3 dB bandwidth versus multiplication factor characteristic of SAGM-GR APDs. The maximum bandwidth and gain-bandwidth product at 2.0 and 10 GHz, respectively.
M = 3 for the 110 mm diameter device. The central portion of the dependence is in principle limited by the carrier transit time (ttr) and the RC time constant. In this work, the expected 3 dB electrical bandwidth is 2.5 GHz calculated using a device capacitance of 1.0 pF (which consists of 0.809 pF chip capacitance and 0.15 pF ceramic submount capacitance), the stray capacitance of 0.05 pF, and the photodiode series resistance of 10 O obtained. The 2 GHz bandwidth is consistent with an RC limit of 2.5 GHz combined by square law with a transit time limited bandwidth of 3.3 GHz. On the other hand, for 5 mm secondary transit and maybe 3 mm average primary transit distance, we can arrive from the quick estimation of f = 0.443/ttr, 0.443/[8 mm/(60 mm ns ÿ 1)] = 3.5 GHz, consistent with the above. Thus both RC and transit time limit the bandwidth. In the multiplication factor greater than 10, the bandwidth is limited by an avalanche build-up time. The gain-bandwidth (GBW) product deduced from the inverse linear relationship is about 10 GHz in this device. When the multiplication factor is less than 2, the edge of depletion region under photosensitive area does not reach the InGaAs absorption layer. The degradation of the 3 dB bandwidth under the multiplication factor of 2 is due to slow diusion of holes from the undepleted InGaAs layer. The bandwidth raises rapidly when the edge of the depletion region reaches the InGaAs absorption layer. The pulse response for the devices was measured by using a 1.55 mm InGaAsP injection laser driven by a Hamamatsu PLP-01 ps light pulser to generate the light pulse with a rise time of 30 ps and a pulse width of 60 ps at a repetition rate of 10 MHz and a 50 O load resistance. Fig. 8 shows the measured pulse response of an APD biased at ÿ79 V and M = 5 from a Tektronix 7854A sampling oscilloscope, indicating a rise time of 126.4 ps, a fall time of 403.8 ps, and a pulsewidth of 206.8 ps. The trailing edge of pulse response
Fig. 8. Pulse response for an SAGM-GR APD biased at ÿ79 V and M = 5 to a train of 10 MHz light pulse.
W.-J. Ho et al. / Solid-State Electronics 43 (1999) 659±663
is due to the slow speed holes which travel by a diusion process in the neutral region. 4. Conclusions High-performance planar InGaAs/InP SAGM-GR APDs grown by MOCVD have been successfully fabricated and characterized. Double-diused guard rings are used to suppress the edge breakdown. These fabricated APDs exhibit a low dark current of only ÿ11 nA at 90% breakdown voltage, a high gain of 40 at the multiplied dark current of ÿ100 nA, and a gain-bandwidth product of 10 GHz. The SAGM-GR APDs with above-mentioned performance are well suited for highspeed and long-distance ®ber optical communications. Acknowledgements This work is supported by the Telecommunication Laboratories, Chunghwa Telecom Co., Ltd., and the National Science Council (NSC 88-2215-E-007-004).
663
References [1] Forrest SR. In: Willardson RK, Beers AC, editors. Semiconductors and semimetals, vol. 22, part D. New York: Academic, 1985. p. 329. [2] Taguchi K, Torikai T, Sugimoto Y, Makita K, Ishihara H. J Lightwave Technol 1988;6:1643. [3] Kuwatsuka H, Kito Y, Uchida T, Mikawa T. IEEE Photon Technol Lett 1991;3:1113. [4] Sze SM, Gibbons G. Solid-St Electr 1966;9:831. [5] Tarof LE, Knight DG, Fox KE, Miner CJ, Puetz N, Kim HB. Appl Phys Lett 1990;57:670. [6] Matsushima Y, Noda Y, Kushiro Y, Seki N, Akiba S. Electr Lett 1984;20:235. [7] Taguchi K, Torikal T, Sugimoto T, Makita K, Ishihara H. J Appl Phys 1986;59:476. [8] Mcintyre RJ. IEEE Trans Electr Dev 1966;13:164. [9] Gallant M, Zemel A. Appl Phys Lett 1988;52:1686. [10] Chi GC, Muehlner DJ, Ostermayer FW, Jr, Fruend JM, O'brien KJ, Pawelek R, Mccoy RJ, Smith RC, Mattera VD, Jr. IEEE Trans Electr Dev 1987;34:2265. [11] Tarof LE, Yu J, Bruce R, Knight DG, Baird T, Oosterbrink B. IEEE Photon Technol Lett 1993;5:672.