Low-noise impact-ionization-engineered avalanche photodiodes grown on InP substrates

1722 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 12, DECEMBER 2002 Low-Noise Impact-Ionization-Engineered Avalanche Photodiodes Grown on InP Substrates S. Wang, J. B. Hurst, F. Ma, R. Sidhu, X. Sun, X. G. Zheng, A. L. Holmes, Jr., Member, IEEE, A. Huntington, L. A. Coldren, Fellow, IEEE, and J. C. Campbell, Fellow, IEEE Abstract—We report low noise multiplication region structures designed for avalanche photodiodes grown on InP substrates. By either implementing a single heterostructure or using a pseudograded structure in the multiplication region, better control of spatial distribution of impact-ionization for both injected and feedback carriers can be achieved; localization of the carrier impact ionization process has resulted in very low excess noise. Index Terms—Avalanche excess noise, avalanche multiplication, avalanche photodiode, impact ionization, photodetector, photodiode. A VALANCHE photodiodes (APDs) are frequently the photodetectors of choice in high-bit-rate, long-haul fiber-optic communication systems due to their internal gain. APDs can achieve 5–10 dB higher sensitivity than PINs, provided that the multiplication noise is low and the gain-bandwidth product of the APD is sufficiently high. In determining its gain, multiplication noise, and the gain-bandwidth product, the multiplication region of an APD plays a critical role. Submicrometer scaling [1]–[10] of the thickness of the multiplication region has been found to give lower multiplication noise and higher gain-bandwidth products in APDs. This is due to the nonlocal nature of impact ionization, which can be neglected if the thickness of the multiplication region is much greater than the “dead length,” which is the minimum distance carriers travel to gain sufficient energy to impact ionize. However, the minimum value to which the multiplication region can be scaled is ultimately limited by the onset of tunneling, which will result in excessive dark currents, especially in low band gap materials. Some wide bandgap materials [9], [11] were also found to have low excess noise even in large thickness ( 800 nm). We have demonstrated that Impact-Ionization-Engineering (I E) [12]–[15] of the multiplication region of an APD can achieve very low-noise by utilizing beneficially designed heterostructures. Monte Carlo simulation [16] has revealed spatial modulation of the carrier impact ionization process in the I E structures. In contrast to the “superlattice” [22], [23] and “staircase” [24] APD structures, the oper- Manuscript received July 15, 2002; revised August 26, 2002. This work was supported by the Defense Advanced Research Projects Agency through the Center for Heterogeneous Integrated Photonics and by the 3-D Imaging Program. S. Wang, J. B. Hurst, F. Ma, R. Sidhu, X. Sun, X. G. Zheng, A. L. Holmes, Jr., and J. C. Campbell are with the Department of Electrical and Computer Engineering, Microelectronics Research Center, The University of Texas at Austin, Austin, TX 78712 USA (e-mail: jcc@mail.utexas.edu). A. Huntington and L. A. Coldren are with the Department of Electrical and Computer Engineering, Optoelectronics Technology Center, University of California at Santa Barbara, Santa Barbara, CA 93106 USA. Digital Object Identifier 10.1109/LPT.2002.804651 Fig. 1. Schematic cross section of the APD structure. ating principle of the I E devices does not invoke heterojunction band discontinuities, as confirmed by Monte Carlo simulations in [16]. Owing to the relatively mature growth technology and the As, well-documented material characteristics of Al Ga As heterojunctions were used to study the GaAs–Al Ga transport mechanisms in the I E structures in [12]–[15]. These As-based low-noise structures have potential GaAs–Al Ga for operation at long-wavelengths (1.3 m, 1.55 m), as they can be incorporated with a GaAsSb multiple quantum well (MQW) absorption region [17] or by using wafer-bonding techniques [18]. In this letter, we report the direct growth of I E structures using InAlAs and InGaAlAs in the multiplication regions. These structures were chosen based on the following: 1) they worked well in lowering the excess noise factor in the GaAs–AlGaAs material system and 2) like AlGaAs, the InGaAlAs quaternary materials have a relatively wide range of bandgap energies. Compared to homojunction InAlAs and InP APDs, lower excess noise and comparable dark current have been achieved. Fig. 1 shows the device cross-sectional view for the InP-based I E APDs. Owing to the larger ionization coefficients of the electrons relative to holes, a p-“i”-n structure and top-illumination were adapted to ensure pure electron injection: the unintentionally doped multiplication region structure was 10 cm , 0.8 m) and sandwiched between p-type (3 10 cm , 0.5 m) In Al As layers, with n-type (5 a highly p-doped ( 5 10 cm , 30 nm) In Ga As contact layer on the top. After the formation of a p-metal contact (Ti–Pt–Au) using a standard lift-off process, the device mesa (160 m diameter) was etched in H PO : H O : H O (1 : 1 : 8) and immediately transferred to the PECVD chamber for SiO passivation ( 100 nm). After a bottom n-contact (AuGe–Ni–Au) was formed ( 100 nm), the sample was annealed at 430 C for 30 s. 1041-1135/02$17.00 © 2002 IEEE WANG et al.: LOW-NOISE IMPACT-IONIZATION-ENGINEERED AVALANCHE PHOTODIODES 1723 (a) Fig. 3. Measured photo-current and dark current curves for the single-well (dashed lines), pseudo-graded (dashed–dotted lines), and homojunction 200-nm In Al As (solid lines) APDs. The photo-current curves were taken using an Argon UV laser (351, 363 nm). (b) Fig. 2. Schematics and energy band diagrams of: (a) the single-well structure and (b) the pseudograded structure. Shown in Fig. 2 are the two I E multiplication region structures designed on InP-substrates. A single-well structure, as shown in Fig. 2(a), consisted of a 100-nm-thick In Al As layer at the p-side followed by a 100-nm-thick In Ga Al As quaternary layer. The lower bandgap energy of In Ga Al As (estimated to be 1.25 eV [19]) as compared to In Al As results in a lower car. There are relatively rier ionization threshold energy ( few ionization events in the In Al As layer, owing to the combined effects of “dead space” [21] and the higher threshold energy in In Al As. Once the electrons reach the In Ga Al As layer, they tend to ionize quickly because it has a lower threshold energy and the electrons have gained energy in the In Al As portion of the multiplication region [15]. The secondary hole ionization rate also peaks in the In Ga Al As well. However, after these secondary holes lose energy through impact ionization in the well region, their ionization probability is low in the In Al As region due to its higher hole-ionization threshold energy. This significantly reduced the number of secondary hole ionization events. Therefore, in this single-well structure, both electron and hole ionization are expected to be localized in the In Ga Al As well, while the ionization of both carrier types are suppressed in the In Al As layer. The increased localization of the multiplication process relative to homojunction APDs is the origin of the low excess noise characteristics. The structure shown in Fig. 2(b) was implemented on an InP-substrate using lattice-matched InAlGaAs quaternary materials as the multiplication region, based on a similar structure As–GaAs that was reported in [14]. Utilizing in Al Ga the decreasing band gap energies (thus decreasing ionization Al As with decreasing Al threshold energies) in In Ga content [19], the structure consisted of an 80-nm In Al As layer followed by layers of 60-nm In Ga Al As, 40-nm In Ga Al As, and 70-nm In Ga Al As. The “graded” layer thickness were selected to decrease with decreasing bandgap energy because the dead space decreases . As discussed in detail in [14], the ionization events with layers, for electrons are expected to localize in the lowwhile the ionization of the secondary holes can be suppressed because the holes enter materials with increasing ionization threshold energies as they travel toward the p-region. This As structure, as demonstrated previously in GaAs–Al Ga [14], is expected to give low noise. Fig. 3 shows the – curves for the single-well structure and the graded-structure for ultraviolet (351, 363 nm) laser illumination (from the top of the mesa) and in the dark. For comparison, a nominal 200-nm In Al As homojunction structure was also fabricated and tested. All three structures exhibited flat low-bias photocurrent responses and high photocurrent gain. The dark currents of the two heterostructures in the low gain regime are comparable to that of the homojunction In Al As, whereas at voltages near breakdown, two heterostructures exhibited higher dark current as a result of increased tunneling in the narrower bandgap materials. Compared to the graded-structure, the slightly higher dark current of the single-well structure at lower bias voltages was consistent with its narrower intrinsic region thickness. of the three structures were The excess noise factors measured using an HP8970B noise figure meter with a standard noise source. The measurement bandwidth was 4 MHz. Since the noise has been proved experimentally to be independent of center-frequency, the criteria for choosing a center-frequency (40–300 MHz) was the minimization and stability of the background noise level. An Argon ultraviolet laser was used to ensure pure electron injection conditions in the gain and noise measurements. Plotted in Fig. 4 are the excess noise factor curves for the three structures; theoretical curves based on local-field theory [20] are also plotted for reference purpose. Both the single-well structure and the graded-structure have 1724 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 12, DECEMBER 2002 Fig. 4. Measured excess noise factor F (M ) curves for the single-well (triangles), pseudograded (squares), and homojunction 200-nm In Al As (circles) APDs. The excess noise was measured under the same laser illumincation as in Fig. 3. an excess noise level that corresponds to an effective [20] of 0.1, which is well below that of the homojunction 200-nm to 0.3). It is also much lower than In Al As ( to 0.5). To that of commercial InP–InGaAs APDs ( the best of our knowledge, this is the lowest noise achieved, to date, in APDs that are directly grown on InP substrates. As demonstrated in other I E APDs in [12]–[15], the excess noise factor curves of the two heterostructures are below the cal[20] for lower gain values ( culated curve for a specific ) and above them for higher gains ( ), as shown in Fig. 4. Consequently, these heterostructures are particularly promising in the low-gain regime although relative to homojunction APDs lower noise can be achieved even in the high-gain region. The mechanisms that lead to the different noise-gain regimes are still under investigation. We suggest that these low-noise I E APDs may prove useful in a “quasi-pin” mode, i.e., in a low to moderate gain level. These APDs working in the “quasi-pin” mode have the potential to yield improved sensitivities over PIN photodiodes with very little noise penalty, while avoiding the critical bias requirements and higher dark current associated with the conventional APD mode of operation. In conclusion, we report impact-ionization-engineered APDs that were grown on InP substrates. By using either a single-well structure or a pseudograded structure based on InAlAs–InGaAlAs materials, lower excess noise and comparable low dark current were achieved compared to homojunction InAlAs APDs. Once incorporated into separate-absorption-charge-multiplication (SACM) structures with an In Ga As absorber, these structures may provide significant performance improvement at 1.3 or 1.55 m. REFERENCES [1] J. C. 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