Avalanche photodiodes (APDs) have and will continue to be utilised in a diverse range of applications in the commercial, military and research sectors. In recent years, the primary driving force for research and development of APDs have been the rapid growth of the optical telecommunications industry. In telecommunications systems using silica fibres, in which the optimum transmission wavelengths for lowest loss are 1.3 μm and 1.55 μm, the photodiode detects the light emerging from the optical fibre and converts it into an electrical current. The basic photodiode is a reversed biased p-i-n structure in which the absorbed photons produce electron-hole pairs and generate current flow. The resulting electrical signal is weak however and requires amplification via analogue amplifiers which generate their own internal noise and would subsequently reduce the signal to noise ratio at the frequencies used in these systems (>2.5 Gbs-1).
It is well known that compared to regular photodiodes, APDs operate at fields near avalanche breakdown, where carrier multiplication process occurs, enabling amplification of weak signals. APDs are also key components in applications such as laser range finding, particle sizing and photon counting. APDs exceptional utility in these applications stems from their high sensitivities at low light levels. APDs also have the advantage of high speeds, low noise, fast rise times, and high quantum efficiencies.
The Separate Absorption Charge Multiplication (SACM) APD structure is typically used to enable efficient absorption of photons while allowing high multiplication values to be achieved [1]. The structure consists of a absorption layer and a multiplication layer that is separated by a highly doped charge sheet layer. The absorption layer uses a narrow band gap material for long wavelength absorption. However, narrow gap semiconductors are susceptible to tunnelling at high fields, hence electric field across this region is kept low to minimise tunnelling current, but is sufficient to rapidly sweep one photo-generated carrier type towards the multiplication region. The absorption layer is required to be sufficiently thick to ensure reasonable quantum efficiency. However, there is a trade-off between obtaining high quantum efficiency without degrading the APD frequency response due to the long carrier transit time. In applications where both high bandwidths and high absorption efficiencies are desired, thin absorption layers can be used as long as the coupling between incident light and the absorption region can be enhanced. An example would be edge-coupled devices. The multiplication region employs a wide band gap material, which experiences high electric fields so as to provide internal photocurrent gain by impact ionisation. The multiplication region should be thick enough to avoid excessive tunnelling leakage currents, and thin enough to achieve short carrier transit time for high speed and low noise performance. Both the absorption region and multiplication region are lightly doped (usually unintentionally doped) and is separated by a doped charged layer which acts as a field control layer to tailor the electric field profile across the APD. The charge sheet keeps the field across the absorption region well below the breakdown field even when the APD is biased near the limit of its operation (near breakdown of the multiplication region). The doped layer also ensures that the punch-through - the bias at which the depletion region reaches the absorption region- does not occur before the onset of avalanche multiplication.
The challenge of producing low noise, high power and high-speed APDs has motivated the detailed investigation of various semiconductor material systems such as Silicon (Si), Gallium Arsenide (GaAs), Aluminium Gallium Arsenide (AlGaAs) and Indium Phosphide (InP). Germanium (Ge) and Indium Gallium Arsenide/ Indium Phosphide (InGaAs/InP) APDs are commercially available for applications in the 0.8 - 1.7 μm infrared band, in particular for use at the low loss optical communications wavelengths. The InGaAs/InP APDs generally utilise InGaAs (with Indium content of 53% and Gallium content of 47%), which is lattice-matched to InP, as the absorption material and InP as the avalanche material.
InGaAs/InP APDs are limited in performance due to the high excess noise associated with the stochastic multiplication process within the InP multiplication layer. Furthermore, undesirable low field impact ionisation in the InGaAs absorption region has a very detrimental effect on the gain-bandwidth-product [2], [3] and excess noise [4], [5]. The presence of impact ionisation at relatively low fields InGaAs is due to the weak field dependence of the electron ionisation coefficients. Although the avalanche behaviour of InGaAs has been well documented, there has been no investigation on the excess noise performance, which could unambiguously confirm the reported ionisation coefficient ratios.
One documented method to reduce the excess noise in InGaAs/InP APDs is by submicrometer scaling of the thickness of the InP multiplication region, as demonstrated in the excess noise measurement of thin InP p-i-n diodes [6], [7], [8].
Noise reduction in thin APDs have also been demonstrated for other materials, including GaAs [7], [8], [9], [10], [11], Si [12], GaInP [13], AlxGa1-xAs [7], [8], [14], [15], [16], Indium Aluminium Arsenide (InAlAs) [7], [8], [17], and SiC [18]. Low noise in thin APDs is achieved due the more ordered behaviour in the ionisation of carriers. Even better excess noise performance can be achieved if one carrier type has a higher probability of ionising in a thin structure.
InAlAs (with Indium mol fraction of 0.52 and Aluminium mol fraction of 0.48) is able to supersede InP as the multiplication layer in SACM-APDs [19], [20], [21], [22] due to its superior ionisation characteristics that include larger band gap [23] and very dissimilar electron and hole ionisation coefficients while remaining lattice matched to InP. Furthermore, InAlAs/InGaAs APDs have slightly different layer configuration to that of InGaAs/InP APDs. Electrons are the initiating carrier for multiplication in the InAlAs APDs, since electrons ionise more readily than holes in both InGaAs and InAlAs. These characteristics confer InAlAs with lower noise, higher gain bandwidth product and enhanced sensitivity compared to the InP/InGaAs APD. In addition to the lower excess noise factors and higher bandwidth, preliminary studies [24], [25] also suggest relatively small temperature dependence of breakdown voltages compared to InP. This weak temperature dependence of breakdown voltages is desirable so that accuracy of the temperature control of the APDs becomes less critical.
In recent years, InAlAs APDs have reported high sensitivities in high bit rate 10 Gb/s [21] optical communication applications. InGaAs/InAlAs APDs appear poised to replace InGaAs/InP APDs in very high sensitivity applications. High performance InGaAs/InAlAs APDs [19], [20], [21], [26], [27] have been reported while some Japanese research groups [28] prefer using more complicated InAlGaAs/InAlAs superlattices instead of bulk InAlAs for the avalanche region. InAlAs APDs have only recently been assessed in telecom APD receivers but have already set record sensitivities of -19.6 dBm at 40 Gb/s [29].
InGaAs-based APDs have a cut off absorption wavelength of 1.65 μm at room temperature, which covers the 1.55 μm telecommunications band. There is considerable interest in extending the light detection at wavelengths greater than 2 μm. The ability to detect weak optical signals beyond 2 μm at room temperature would be valuable, for instance, in the short wave infrared (SWIR) electromagnetic spectrum spanning the 1.4 μm to 3-μm region. Photodetectors sensitive to these wavelengths are useful in various applications such as spectral imaging, chemical sensing, environmental monitoring (remote sensing), optical eye-safe ranging, gas detection, medical diagnostics, high speed IR-imaging and free space communications.
The predominant technologies for SWIR detection use Mercury-Cadmium-Telluride (HgCdTe) material system. This material system suffers from poor uniformity, which leads to low yield and high costs. Lack of uniformity also severely undermines the focal plane array performance. The shortcomings for the HgCdTe system led to research efforts investigating alternative technologies for detection at these wavelengths. InxGa1-xAs alloy system with Indium content greater than 53% allows the wavelength range to be extended up to 2.5 μm [30], [31]. However, the increase in the Indium concentration is also accompanied by increase in strain and material defects in the InxGa1-xAs layer due to the lattice mismatch with the substrate and this require thick buffer layers and optimisation during growth. Narrow band gap materials such as Indium Antimonide (InSb) Indium Arsenide (InAs), Indium Gallium Arsenide Antimonide (InGaAsSb) detectors require cooling due to the high dark currents at ambient temperatures. InAsSb alloys can be grown compressively strained on InAs substrates, or lattice matched to GaSb substrates. The latter span the 1.7 μm to 4.2-μm spectral range. High performance operation is still limited to low temperatures. The requirement for cooling limits the operation lifetime, increases the weight and total cost as well as power budget to operate such detectors.
An emerging technology for 1 - 3 μm detection at room temperature is the type-II heterojunction superlattice structure. This structure was first proposed by Tsu and Esaki [32] and the first studies on the type-II InGaAs/GaAsSb material system grown by molecular beam epitaxy have been reported since 1978 [33], [34], [35]. The type-II heterojunction band structure has a lower effective band gap compared to the band gap of the constituent layers. The dark currents are predicted to be low due to the higher effective masses of the adjoining layer material systems compared to equivalent narrow band gap materials of the same detection range, thereby enabling higher temperature operation. The growth and processing and largely based on standard III-V technology, so cost and yield are not expected to be a major issue.
Photoluminescence up to a wavelength ~ 2.4 μm has been demonstrated in InGaAs/ GaAsSb type-II superlattice photodiodes grown on InP substrates in several research laboratories [36], [37]. p-i-n structures [38] and InP-based SACM-APDs [39] using type-II heterostructures have been reported operating up to 2.4 mm. However, there is nothing in literature that shows the avalanche properties of these heterostructures especially at low fields. It is important to understand the avalanche properties to enable optimisation of the APD structure design. Furthermore, combining the type-II absorber with InAlAs, which is lattice-matched to InP should give improved performance for long wavelength detection.
The motivation of this work is to measure and understand the impact ionisation behaviour of the InAlAs, InGaAs and type-II heterojunction superlattice structures, which leads to the optimal APD design for long wavelength applications. It is important to relate the measured characteristics directly to the material parameters and the band structure, thus enabling a better understanding to be achieved of the essential material properties affecting the avalanche process in these structures.
The work described in this thesis comprises a systematic investigation of the avalanche characteristics of InGaAs, InAlAs and type-II superlattice for applications in the telecommunications and next generation short-wave infrared avalanche photodiodes (APDs). This is done through the measurements of multiplication characteristics in p-i-n and n-i-p structures of varying thickness. The results of the respective material systems are interpreted using local and non-local models.
Chapter 1 serves to give an introduction to emerging APD technologies and the justification and motivation of the work presented in this thesis. An overview of the essential features for the APD designed for high-speed low noise applications is presented. The research objectives and organisation of thesis are outlined.
Chapter 2 presents the background theory of impact ionisation. The conventional analysis is briefly reviewed. The essential features of carrier transport and the material band structure affecting the ionisation process are described. The origin of the excess noise arising from the multiplication process is presented, followed by a discussion on the local and non-local theory on impact ionisation.
Chapter 3 describes the experimental technique of device fabrication process and optical and electrical characterisation of the fabricated devices. The experimental set up and technique for current-voltage, capacitance-voltage, multiplication and excess noise measurements are also described along with the procedure to eliminate errors.
Chapter 4 details the experimental results of the excess noise measurement of InGaAs.
Chapter 5 describes the results from the multiplication measurements of the InAlAs. The local effective impact ionisation coefficients are deduced from the local model. A simple correction to the non-local effects when modelling multiplication in thin devices is implemented and compared with the experimental results.
Chapter 6 describes the experimental results of excess noise measurements of InAlAs. The enabled ionisation coefficients, which include the effects of dead space, are deduced using a non-local model employing a set of recurrence equations. The modelled results are compared with the experimental results.
Chapter 7 details the preliminary multiplication measurements performed on the InGaAs/GaAsSb type-II heterojunction superlattice structure and the effective ionisation coefficients deduced from the measurements using a local model.
Chapter 8 summarises the achievement of the research objectives. Suggestions of future work are discussed.
[1] F. Capasso, A. Y. Cho, and P. W. Foy, "Low-dark-current low-voltage 1.3-1.6 µm avalanche photodiode with high-low electric field profile and separate absorption and multiplication regions by molecular beam epitaxy", Electron. Lett., vol. 20, no. 15, pp. 635-637, Jul. 1984.
[2] G. Kahraman, N. E. A. Saleh, W. L. Sargeant, and M. C. Teich, "Time and frequency response of avalanche photodiodes with arbitrary structure", IEEE Trans. Electron Dev., vol. 39, no. 3, pp. 553-560, Mar. 1992.
[3] L. E. Tarof, J. Yu, R. Bruce, D. G. Knight, T. Baird, and B. Oosterbrink, "High-frequency performance of separate-absorption, grading, charge, and multiplication InP/InGaAs avalanche photodiodes", IEEE Photon. Tech. Lett., vol. 5, no. 6, pp. 672-674, Jun. 1993.
[4] J. S. Ng, C. H. Tan, J. P. R. David, and G. J. Rees, "Effect of impact ionization in the InGaAs absorber on excess noise of avalanche photodiodes", IEEE J. Quantum Electron., vol. 41, no. 8, pp. 1092-1096, Aug. 2005.
[5] N. Duan, S. Wang, X. G. Zheng, X. Li, N. Li, J. C. Campbell, C. Wang, and L. A. Coldren, "Detrimental effect of impact ionization in the absorption region on the frequency response and excess noise performance of InGaAs-InAlAs SACM avalanche photodiodes", IEEE J. Quantum Elect., vol. 41, no. 4, pp. 568-572, Apr. 2005.
[6] K. F. Li, S. A. Plimmer, J. P. R. David, R. C. Tozer, G. J. Rees, P. N. Robson, C. C. Button, and J. C. Clark, "Low avalanche noise characteristics in thin InP p+-i-n+ diodes with electron initiated multiplication", IEEE Photon. Tech. Lett., vol. 11, no. 3, pp. 364-366, Mar. 1999.
[7] P. Yuan, C. C. Hansing, K. A. Anselm, C. Lenox, H. Nie, A. L. Holmes, B. G. Streetman, and J. C. Campbell, "Impact ionization characteristics of III-V semiconductors for a wide range of multiplication region thicknesses", IEEE J. Quantum Elect., vol. 36, no. 2, pp. 1982-04, Feb. 2000.
[8] M. A. Saleh, M. M. Hayat, P. P. Sotirelis, A. L. Holmes, J. C. Campbell, B. E. A. Saleh, and M. C. Teich , "Impact ionization and noise characteristics of thin III-V avalanche photodiodes", IEEE Trans. Electron Dev., vol. 48, no. 12, pp. 2722-2730, Dec. 2001.
[9] S. A. Plimmer, J. P. R. David, D. C. Herbert, T. W. Lee, G. J. Rees, P. A. Houston, R. Grey, P. N. Robson, A. W. Higgs, and D. R. Wight, "Investigation of impact ionization in thin GaAs diodes", IEEE Trans. Electron Dev., vol. 43, no. 7, pp. 1066-1072, Jul. 1996.
[10] C. Hu, K. A. Anselm, B. G. Streetman, and J. C. Campbell, "Noise characteristics of thin multiplication region GaAs avalanche photodiodes", Appl. Phys. Lett., vol. 69, no. 24, pp. 3734-3736, Dec. 1996.
[11] K. F. Li, D. S. Ong, J. P. R. David, G. J. Rees, R. C. Tozer, P. N. Robson, and R. Grey, "Avalanche multiplication noise characteristics in thin GaAs p+-i-n+ diodes", IEEE Trans. Electron Dev., vol. 45, no. 10, pp. 2102-2107, Oct. 1998.
[12] C. H. Tan, J. C. Clark, J. P. R. David, G. K. Rees, S. A. Plimmer, R. C. Tozer, D. C. Herbert, D. J. Robbins, W. Y. Leong, and J. Newey, "Avalanche noise measurements in thin Si p+-i-n+ diodes", Appl. Phys. Lett., vol. 76, no. 26, pp. 3926-3928, Jun. 2000.
[13] C. H. Tan, R. Ghin, J. P. R. David, G. J. Rees, and M. Hopkinson, "The effect of dead space on gain and excess noise in In0.48Ga0.52P p+-i-n+ diodes", Semicond. Sci. Technol., vol. 18, no. 8, pp. 803-806, Aug. 2003.
[14] S. A. Plimmer, J. P. R. David, G. J. Rees, R. Grey, D. C. Herbert, D. R. Wright, and A. W. Higgs, "Impact ionization in thin AlxGa1-xAs (x = 0.15-0.30) p-i-n diodes", J. Appl. Phys., vol. 82, no. 3, pp. 1231-1235, Aug. 1997.
[15] C. H. Tan, J. P. R. David, S. A. Plimmer, G. J. Rees, R. C. Tozer, and R. Grey, "Low multiplication noise in thin Al0.6Ga0.4As avalanche photodiodes", IEEE Trans. Electron Dev., vol. 48, no. 7, pp. 1310-1317, Jul. 2001.
[16] B. K. Ng, J. P. R David, R. C. Tozer, M. Hopkinson, G. Hill, and G. J. Rees, "Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes", IEEE Trans. Electron Dev., vol. 48 , no. 10, pp. 2198-2204, Oct. 2001.
[17] C. Lenox, P. Yuan, H. Nie, O. Baklenov, J. C. Campbell, Jr. A. L. Holmes, and B. G. Streetman, "Thin multiplication region InAlAs homojunction avalanche photodiodes", Appl. Phys. Lett., vol. 73, no. 6, pp. 783-784, Aug. 1998.
[18] B. K. Ng, J. P. R David, R. C. Tozer, G. J. Rees, Y. Feng, J. H. Zhao, and M. Weiner, "Nonlocal effects in thin 4H-SiC UV avalanche photodiodes", IEEE Trans. Electron Dev., vol. 50, no. 8, pp. 1724-1732, Aug. 2003.
[19] G. Kinsey, C. C. Hansing, Jr. A. L. Holmes, B. G. Streetman, J. C. Campbell, and A. G. Dentai, "Waveguide In0.53Ga0.47As-In0.52Al0.48As avalanche photodiode", IEEE Photon. Tech. Lett., vol. 12, no. 4, pp. 416-418, Apr. 2000.
[20] T. Nakata, I. Watanabe, K. Makita, and T. Torikai, "InAlAs avalanche photodiodes with very thin multiplication layer of 0.1mm for high-speed and low-voltage-operation optical receiver", IEE Electron. Lett., vol. 36, no. 21, pp. 1807-1809, Oct. 2000.
[21] T. Nakata, T. Takeuchi, I. Watanabe, K. Makita, and T. Torikai, "10Gbit/s high sensitivity, low-voltage-operation avalanche photodiodes with thin InAlAs multiplication layer and waveguide structure", IEE Electron. Lett., vol. 36, no. 24, pp. 2033-2034, Nov. 2000.
[22] G. Kinsey, J. C. Campbell, and A. G. Dentai, "Waveguide avalanche photodiode operating at 1.55 µm with a gain-bandwidth product of 320GHz", IEEE Photon. Tech. Lett., vol. 13, no. 8, pp. 842-844, Aug. 2001.
[23] J. R. Waldrop, E. A. Kraut, C. W. Farley, and R. W. Grant, "Measurement of InP/In0.53Ga0.47As and In0.53Ga0.47As/In0.52Al0.48As heterojunction band offsets by x-ray photoemission spectroscopy", J. Appl. Phys., vol. 69, no. 1, pp. 372-378, Jan. 1991.
[24] D. J. Massey, "Impact ionization in submicron Silicon and In0.52Al0.47As", 2006.
[25] J. S. Ng, L. J. J. Tan, D. S. G. Ong, C. H. Tan, and J. P. R. David, "Temperature dependence of breakdown voltage of InP and InAlAs avalanche photodiodes", Proceedings for the 20th Annual Meeting of the IEEE Lasers & Electro-Optics Society (LEOS), Oct. 2007.
[26] S. Tanaka, S. Fujisaki, Y. Matsuoka, T. Tsuchiya, S. Tsuji, K. Ito, T. Toyonaka, H. Matsuda, and A. Miura, "Highly sensitive and high reliable APD for 10 Gb/s optical communication systems", 28th Eur. Conf. Optical Comm. 2002, Paper 10.5.3, 2002.
[27] E. Yagyu, E. Ishimura, M. Nakaji, T. Aoyagi, and Y. Takuda, "Simple planar structure for high-performance AlInAs avalanche photodiodes", IEEE Photon. Tech. Lett., vol. 18, no. 1, pp. 76-78, Jan. 2006.
[28] I. Watanabe, M. Tsuji, K. Makita, and K. Taguchi, "A new-planar-structure InAlGaAs-InAlAs superlattice avalanche photodiode with a Ti-implanted guard-ring", IEEE Photon. Tech. Lett., vol. 8, no. 6, pp. 827-829, Jun. 1996.
[29] K. Makita, T. Nakata, S. K, and T. Takeuchi, "40Gbps waveguide photodiodes", NEC J. Adv. Tech., vol. 2, no. 3, pp. 234-240, 2005.
[30] M. D'Hondt, I. Moerman, and P. Demeester, "Dark current optimisation for MOVPE grown 2.5mm wavelength InGaAs photodetectors", IEE Elect. Lett., vol. 34, no. 9, pp. 910-912, Apr. 1998.
[31] J. John, L. Zimmermann, S. Nemeth, T. Colin, P. Merken, S. Borghs, and C. Van Hoof, "Extended InGaAs on GaAs detectors for SWIR linear sensors", Proc. of SPIE, vol. 4369, pp. 692-697, 2001.
[32] G. A. Sai-Halasz, R. Tsu, and L. Esaki, "A new semiconductor superlattice", Appl. Phys. Lett., vol. 30, no. 12, pp. 651-653, Jun. 1977.
[33] G. A. Sai-Halasz, L. L. Chang, J. -M. Welter, C. -A. Chang, and L. Esaki, "Optical absorption of In1-xGaxAs-GaSb1-yAsy superlattices", Solid State Comm., vol. 27, no. 10, pp. 935-937, Sep. 1978.
[34] Y. Sugiyama, T. Fujii, Y. Nakata, S. Muto, and E. Miyauchi, "Conduction-band edge discontinuity of InGaAs/GaAsbSb heterostructures lattice-matched to InP grown by molecular beam epitaxy", J. Crys. Growth, vol. 95, pp. 363-366, 1989.
[35] J. F. Klem and S. R. Kurtz, "Growth and properties of GaASb/InGaAs superlattices on InP ", J. Crys. Growth, vol. 111, no. 1-4, pp. 628-632, 1991.
[36] A. Yamamoto, Y. Kamamura, H. Naito, and N. Ionue, "Optical properties of GaAs0.5Sb0.5 and In0.53Ga0.47As/GaAs0.5Sb0.5 type II single hetero-structures lattice-matched to InP substrates grown by molecular beam epitaxy", J. of Crys. Growth, vol. 201-202, pp. 872-876, 1999.
[37] H. Takasaki, Y. Kamamura, T. Katayama, A. Yamamoto, H. Naito, and N. Inoue, "Photoluminescence properties in In0.53Ga0.47As/GaAs0.5As0.5 type-II quantum well structures lattice-matched to InP", Appl. Surface Science, vol. 159-160, pp. 528-531, 2000.
[38] R. Sidhu, N. Duan, J. C. Campbell, and Jr. A. L. Holmes, "A long-wavelength photodiode on InP using lattice-matched GaInAs-GaAsSb type-II quantum wells", IEEE Photonics Tech. Lett., vol. 17, no. 12, pp. 2715-2717, Dec. 2005.
[39] R. Sidhu, L. Zhang, N. Tan, N. Duan, J. C. Campbell, Jr. A. L. Holmes, C. -F. Hsu, and M. A. Itzler, "2.4 mm cutoff wavelength avalanche photodiode on InP substrate", IEEE Electron. Lett., vol. 42, no. 3, Feb. 2006.
