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Impact Ionization in Inas Electron Avalanche Photodiodes This is a repository copy of Impact ionization in InAs electron avalanche photodiodes. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/42655/ Article: Marshall, A.R.J., David, J.P.R. and Tan, C.H. (2010) Impact ionization in InAs electron avalanche photodiodes. IEEE Transactions on Electron Devices, 57 (10). pp. 2631-2638. ISSN 0018-9383 https://doi.org/10.1109/TED.2010.2058330 Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 10, OCTOBER 2010 2631 Impact Ionization in InAs Electron Avalanche Photodiodes Andrew R. J. Marshall, John P. R. David, Senior Member, IEEE, and Chee Hing Tan, Member, IEEE Abstract—A systematic study of impact ionization, avalanche possible. Ideally, either α or β should be equal to zero, such multiplication, and excess noise in InAs diodes has been carried that the ionization coefficient ratio k = α/β or k = β/α also out, confirming that avalanche multiplication is dominated by the becomes zero. In this ideal case, F asymptotically approaches 2 impact ionization of electrons. This results in highly desirable “electron avalanche photodiode” characteristics previously only as gain increases, under the local model of impact ionization demonstrated in HgCdTe diodes, which are discussed in detail. [2]. Furthermore, an APD’s bandwidth is also reduced with The suppression of excess noise by nonlocal effects, to levels below increasing values of k [3]. Unfortunately, in established wider the local model minimum of F = 2, is explained. An electron bandgap III–V materials, it has been found that both carrier ionization coefficient is calculated and shown to be capable of types undergo significant impact ionization resulting in k values modeling the electron impact ionization, which differs character- istically from that in wider bandgap III–V materials. between approximately 1 and 0.1 [1], [4]. It is for this rea- son that III–V APDs suffer from higher levels of noise than Index Terms —Avalanche photodiode (APD), electron avalanche silicon APDs. photodiode (e-APD), impact ionization, InAs, ionization coefficient. Amongst narrower bandgap materials, Hgx−1CdxTe and, in particular, Hg0.7Cd0.3Te have been the most widely investi- I. INTRODUCTION gated. Hg0.7Cd0.3Te APDs have been demonstrated with highly desirable and previously unachievable avalanche gain and HE PROCESS of impact ionization and its exploitation noise characteristics [5]. Measurements indicate that electron in avalanche photodiodes (APDs) has been thoroughly T impact ionization dominates the avalanche multiplication in investigated in most established semiconductor materials with Hg Cd0.3Te APDs, resulting in theoretically minimal excess bandgap energies greater than 1 eV. In contrast, there have been 0.7 noise and leading to the devices being referred to as electron few investigations in semiconductor materials with bandgaps APDs (e-APDs). However, Hg1 xCdxTe remains inaccessible below that of In0.53Ga0.47As and few reports of APDs detecting − to many and challenging to grow and process. Hence, it would at wavelengths beyond its 1.7-µm cutoff. There are, however, be desirable if similar performance could be achieved using emerging applications in the short-wave and midwave infrared, a more widely available III–V material. Recently published which could exploit APDs to improve overall system sensi- work has shown that InAs has the potential to meet this desire, tivity. These are typically low photon flux or high bandwidth exhibiting similar electron-dominated avalanche multiplication applications such as active imaging, hyperspectral imaging, and commensurate low excess noise [6], [7]. It is noted that atmospheric gas monitoring, and free space communications. Hence, there is increasing interest in the investigation of impact the use of InAs in APDs is not without its own issues. The ionization in narrower bandgap materials for potential use in maximum substrate diameter available at present is 3 in; InAs APDs. Furthermore, since the excess noise generated by any has an accepted predisposition for surface leakage currents and APD is dependent on the material properties of its gain medium, is one of the least well developed of the binary III–V alloys. there is a long standing cross-application interest in identifying However, with further development addressing the fabrication materials with favorable properties [1]. and surface passivation in particular, they may mature into a In all APDs, avalanche gain is accompanied by an increase in useful technology. noise power, characterized by the gain-dependent excess noise Predicting or explaining the impact ionization characteristics factor F . In order to minimize F and maximize the extent to of a material from its band structure is difficult, as, usually, the which an APD’s gain can be exploited before its noise begins energies required for impact ionization to take place lie above to dominate the system noise, the electron ionization coefficient the first conduction band. In InAs and Hg0.7Cd0.3Te, however, α and the hole ionization coefficient β should be as disparate as a relatively simple case can be made for preferential electron impact ionization [5], [6], [8]. In these materials, electrons Manuscript received March 31, 2010; revised June 4, 2010; accepted can be heated by an electric field to energies well in excess June 23, 2010. Date of publication August 9, 2010; date of current version of the bandgap energy, while still confined to the Γ valley September 22, 2010. This work was supported by the Electro Magnetic Remote of the first conduction band. Due to the low polar optical Sensing Defence Technology Centre Patent GB0723858.7. The review of this paper was arranged by Editor L. Lunardi. phonon-dominated scattering experienced by the electrons in The authors are with the Department of Electronic and Electrical En- the Γ valley, they should be able to rapidly attain energies gineering, The University of Sheffield, S1 3JD Sheffield, U.K. (e-mail: well in excess of the bandgap and, hence, have the potential to andy.marshall@sheffield.ac.uk; j.p.david@sheffield.ac.uk; c.h.tan@sheffield. ac.uk). impact ionization, as long as suitable states are available for the Digital Object Identifier 10.1109/TED.2010.2058330 carriers. In contrast, the relatively flat heavy hole band limits the 0018-9383/$26.00 © 2010 IEEE 2632 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 10, OCTOBER 2010 TABLE I TABLE II COMPARISON OF THE SATELLITE VALLEY SEPARATION STRUCTURES OF THE DIODES CHARACTERIZED AS DETERMINED BY ENERGIES IN SELECTED MATERIALS SIMS AND C–V MEASUREMENTS (BRACKETED VALUES IN ITALICS) rate at which holes heat up. Hence, it can be hypothesized that electron ionization will occur more readily than hole ioniza- II. DEVICE GROWTH AND FABRICATION tion in InAs and Hg0.7Cd0.3Te, particularly under low electric fields. Table I shows the published X and L valley separation Homojunction InAs p-i-n and n-i-p diodes were grown by energies for a number of materials with respect to their Γ valley molecular beam epitaxy on InAs substrates. The growth condi- minimum. The full band diagrams are given in [10] for InAs, tions given in [17] were developed through trial growths, with GaAs, and InP and in [12] for Hg0.7Cd0.3Te. the primary aim being to reduce the defect density and reverse In InAs at 77 K, the spin-orbit split-off energy is approxi- leakage current. Minimizing the unintentional background dop- mately equal to the bandgap energy. This has led to an oppos- ing concentration in the intrinsic layer was also a growth target, ing hypothesis of preferential hole impact ionization, through so that the diodes could become fully depleted at low reverse so-called resonant impact ionization, initiated by holes with biases, and the electric field profile could be approximated to near-zero momentum. This was the explanation offered by that of an ideal p-i-n type diode. All p-i-n diodes included Mikhailova et al. [13] for their reported finding that β was an AlAs0.16Sb0.84 minority electron diffusion blocking layer much greater than α in InAs. However, using a Monte Carlo under the p-type InAs contact layer [17]. Circular mesa diodes model, Brennan and Mansour [14] were unable to replicate the of 25, 50, 100, and 200-µm radii were then fabricated by wet α published by Mikhailova et al., instead reporting it to be much etching. higher and exhibiting notably weaker electric field dependence. The grown doping concentrations and layer thickness, as The α modeled by Brennan and Mansour was broadly sup- shown in Table II, were primarily confirmed by secondary ion ported by Marshall et al. [6] who found experimentally that mass spectroscopy (SIMS). Capacitance–voltage (C–V ) mea- α was much greater than β in InAs at room temperature. It surements at 77 K were also used to confirm the approximate is noted, however, that the conditions to support resonant hole intrinsic width and the background doping concentration.
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