IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 7, JULY 2011 919 GaInNAsSb/GaAs Photodiodes for Long-Wavelength Applications Siew Li Tan, Shiyong Zhang, Wai Mun Soong, Yu Ling Goh, Lionel J. J. Tan, Jo Shien Ng, John P. R. David, Igor P. Marko, Alfred R. Adams, Stephen J. Sweeney, and Jeremy Allam Abstract—GaInNAsSb p-i-n photodetectors on GaAs substrates capable of detecting wavelengths up to 1550 nm with a re- duced dark current are presented in this letter. Responsivities of 0.18 A/W at 1300 nm and 0.098 A/W at 1550 nm were achieved in devices with a ∼ 0.5-μm-thick GaInNAsSb p-i-n epitaxial layer with 10% In, 4.08% N, and 4.4% Sb. The absorption coefficient (α) spectra show that α is intrinsically higher than that of the indirect-gap Ge layer but ∼2.6 times lower than that reported for a In 0.53 Ga 0.47 As epitaxial layer at 1550 nm. The dark currents of the GaInNAsSb devices are found to be lower than not only those of the GaInNAs devices of a similar energy gap but also the state-of-the-art Ge/Si avalanche photodiodes. The lower dark currents in the GaInNAsSb devices compared with the GaInNAs devices can possibly be attributed to the reduction of defects in the Sb-containing epitaxial layer. Index Terms—Absorption coefficient, dark current, dilute ni- tride, InGaAsN, InGaAsNSb, photodetectors, quantum efficiency (QE), responsivity. I. I NTRODUCTION T HE dilute-nitride–arsenide material system has evolved from GaNAs to the more complex quaternary and pen- tanary alloys with the addition of In and/or Sb [2] since its introduction in the 1990s [1]. The interest in dilute-nitride- based optoelectronic devices has also grown from the earlier work in long-wavelength lasers on GaAs to a host of poten- tial applications including solar cells, optical modulators, and photodetectors [3]–[5]. Although the growth of high-quality dilute-nitride materials incorporating a sufficient amount of N remains challenging, the obvious advantages offered by the Manuscript received March 21, 2011; revised April 11, 2011; accepted April 11, 2011. Date of publication May 19, 2011; date of current version June 29, 2011. This work was supported in part by the Engineering and Physical Sciences Research Council under Grant EP/E065007, by the European Commission under Grant FP7-ICT-224142, and by the Technology Strategy Board under Grant “Extended Temperature Optoelectronics II.” The work of J. S. Ng was supported by The Royal Society under a University Research Fellowship. The review of this letter was arranged by Editor C. Jagadish. S. L. Tan, S. Zhang, Y. L. Goh, J. S. Ng, and J. P. R. David are with the Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. (e-mail: j.p.david@sheffield.ac.uk). W. M. Soong was with the Department of Electronic and Electrical Engi- neering, University of Sheffield, S1 3JD U.K. He is now with MIMOS Berhad, 57000 Kuala Lumpur, Malaysia (e-mail: soong.wm@mimos.my). L. J. J. Tan was with the Department of Electronic and Electrical Engineer- ing, University of Sheffield, S1 3JD U.K. He is now with Avago Technologies, Singapore 109673 (e-mail: lionel.tan@avagotech.com). I. P. Marko, A. R. Adams, S. J. Sweeney, and J. Allam are with the Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, U.K. (e-mail: j.allam@surrey.ac.uk). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2145351 growth of Ga(In)NAs(Sb) with a near lattice match to GaAs, such as cheaper substrates and mature processing technology, have driven continued efforts to improve the growth pro- cess [6]. GaAs-based avalanche photodiodes (APD) operating at telecommunication wavelengths can be made by using Ga(In)NAs(Sb) and utilizing the wider band-gap Al 0.8 Ga 0.2 As as the avalanche layer, which can be made very thin to achieve very low excess noise and high-speed operation [7]. At present, such APDs are commonly InP based, which use InGaAs as the absorber and either InP or InAlAs as the avalanche layer. While the dark currents in the InGaAs absorber can be low, high tunneling currents in very thin avalanche regions pose an upper limit on the gain–bandwidth product. We have previously reported photodiodes consisting of 0.5-μm-thick GaInNAs p + -i-n + epitaxial layers [4], which gave a responsivity of 0.11 A/W at 1280 nm and reverse-bias dark currents significantly lower than the results of comparable GaInNAs and GaInNAsSb diodes by other researchers. Since then, we have found that extending the absorption wavelength to 1550 nm with a higher concentration of N (> 4%) was impractical due to the rapid deterioration of device performance at high N content. For this reason, Sb was used in the GaInNAs growth as a surfactant to improve crystal quality [8] and achieve a higher degree of uniformity across devices. In particular, the use of Sb was found to reduce the background doping density in GaInNAs, enabling higher quantum efficiency (QE) in solar cells [5]. It is known that Sb is also incorporated as a constituent when used as a surfactant in GaInNAs, forming GaInNAsSb and further reducing the band gap allowing the production of GaInNAsSb-based lasers at 1.5 μm [6]. In this letter, we report on the electrical and optical characteristics of GaInNAsSb photodiodes with cutoff wavelengths between 1400 and 1600 nm, which would require at least 5% N in the absence of Sb. II. DEVICE STRUCTURE AND EXPERIMENTAL METHODS Two GaInNAsSb p + -i-n + diode wafers were grown on n + GaAs substrates using the VG V80H solid-source molecular- beam-epitaxy (MBE) system described in [4]. Samples A and B are similar in structural detail. They consist of 500-nm p + GaAs, 50-nm p + GaInNAsSb, 400-nm undoped GaInNAsSb, 50-nm n + GaInNAsSb, 300-nm n + GaAs, a 100-nm n + AlAs etch-stop layer, and a 200-nm n + GaAs buffer layer on an n + GaAs substrate. The GaInNAsSb epitaxial layers contain nominally 10% In and 3.8% N. The p + and n + GaAs layers 0741-3106/$26.00 © 2011 IEEE