Surface leakage investigation via gated

Surface leakage investigation via gated type-II InAs/GaSb long-wavelength infrared photodetectors G Chen, E K Huang, A M Hoang, S Bogdanov, S R Darvish et al Citation: Appl Phys Lett 101, 213501 (2012); doi: 10.1063/1.4767905 View online: http://dx.doi.org/10.1063/1.4767905 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i21 Published by the American Institute of Physics Related Articles Unipolar time-differential pulse response with a solid-state Charpak photoconductor Appl Phys Lett 101, 213503 (2012) High quality AlN grown on double layer AlN buffers on SiC substrate for deep ultraviolet photodetectors Appl Phys Lett 101, 192106 (2012) Polarity inversion and coupling of laser beam induced current in As-doped long-wavelength HgCdTe infrared detector pixel arrays: Experiment and simulation Appl Phys Lett 101, 181108 (2012) Quantum mechanical simulation of graphene photodetectors J Appl Phys 112, 084316 (2012) Transient photoresponse and incident power dependence of high-efficiency germanium quantum dot photodetectors J Appl Phys 112, 083103 (2012) Additional information on Appl Phys Lett Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors APPLIED PHYSICS LETTERS 101, 213501 (2012) Surface leakage investigation via gated type-II InAs/GaSb long-wavelength infrared photodetectors G Chen, E K Huang, A M Hoang, S Bogdanov, S R Darvish, and M Razeghia) Department of Electrical Engineering and Computer Science, Center for Quantum Devices, Northwestern University, Evanston, Illinois 60208, USA (Received 16 October 2012; accepted November 2012; published online 21 November 2012) By using gating technique, surface leakage generated by SiO2 passivation in long-wavelength infrared type-II superlattice photodetector is suppressed, and different surface leakage mechanisms are disclosed By reducing the SiO2 passivation layer thickness, the saturated gated bias is reduced to À4.5 V At 77 K, dark current densities of gated devices are reduced by more than orders of magnitude, with 3071 X cm2 differential-resistance-area product at À100 mV With quantum efficiency of 50%, the 11lm 50% cut-off gated photodiode has a specific detectivity of  1011 C 2012 Jones, and the detectivity stays above  1011 Jones from to À500 mV operation bias V American Institute of Physics [http://dx.doi.org/10.1063/1.4767905] Since Sai-Halasz et al.1 proposed the idea of type-II InAs/GaSb superlattice (T2SL) in 1970 s, this material system has demonstrated its ability to provide sensitive infrared detection from the short-wavelength infrared region (SWIR) to the very-long-wavelength infrared region (VLWIR)2–9 and is moving towards multi-spectral detection.10 However, the performance of T2SL photodetectors, especially in longwavelength infrared region (LWIR), is still limited by surface leakage due to the absence of an effective passivation technique that can protect mesas from sidewall leakage caused by chemically and mechanically aggressive focal plane array (FPA) fabrication steps Such effects become more severe as FPA pixel sizes scale down for higher resolution imagers Surface leakage is believed to originate from the abrupt termination of the periodic crystalline structure, contamination from processing, and fixed charges within the passivation layer, which can generate band bending on mesa-sidewalls This band bending causes electron accumulation or type inversion at sidewall surfaces, resulting in a conduction channel along sidewalls.11,12 Various attempts have been made to understand mesa sidewall surface physics and suppress surface leakage in the long-wavelength infrared photodiodes Measures to suppress this phenomenon developed in the past include the double heterostructure12 and hybrid graded doping profile13 in detector designs, use of inductively coupled plasma (ICP) dry etching,14 and various passivations.15–20 However, current physical understanding of surface leakage current does not provide sufficient control of its intensity In addition, SiO2 passivation, which is known to generate serious surface leakage in the long-wavelength infrared region, is still the dominant and most suitable passivation technique for T2SL FPA application Therefore, it is crucial to achieve a deeper understanding of surface leakage physics and suppress surface leakage specifically in long-wavelength infrared SiO2 passivated photodiodes Recently, the gating technique has been demonstrated to have the ability to eliminate surface leakage11 in mid-wavelength a) Email: razeghi@eecs.northwestern.edu 0003-6951/2012/101(21)/213501/5/$30.00 infrared (MWIR) Pỵ-p-M-Nỵ T2SL photodetectors This technique can actively control the sidewall surface band bending through creating a metal-insulator-semiconductor (MIS) structure on the mesa sidewall Gated diodes (GD) demonstrated higher detectivity because of a strongly reduced leakage current at saturated gate bias However, this technique requires very high gate biases to be applied and has not been transferred to the long-wavelength infrared region because for small band gap active regions, the surface leakage in long-wavelength infrared Pỵ-p-M-Nỵ T2SL heterostructure photodetector is more severe and the leakage phenomenon might be more complicated The change of the surface potential is related to the band gap, doping level and the effective mass of the semiconductor.12 The Nỵ- and Pỵ-contact are large band gap heavily doped semiconductors,21 and the M-structure is a large band gap semiconductor with a much larger effective mass than the p- and Pỵ-regions.22 Therefore, for simplicity, the discussion is focused on the p-region while the influence of gate bias on the M-structure, Pỵ-, and Nỵ-contact is assumed to be less pronounced than the p-region Since we already know that applying negative gate bias can realize flat band condition and eliminate surface leakage,11 positive fixed charges must exist at the T2SL-SiO2 interface or within the SiO2 passivation layer When the gate bias is less than or equal to saturated negative gate bias (VG Ϲ Vsat) and not enough to generate a distinct field-induced depletion region at the M-structure surface, holes are accumulated at the surface of the p-region, and the space charge region (SCR) is still mainly at the metallurgical junction (Fig 1(a)) Since there is no significant change on the SCR region or type inversion occurring on the surface, the surface leakage current is eliminated Therefore, when VG Ϲ Vsat, the dark current density remains unchanged with respect to the gate bias and is identical to the bulk dark current When the gate bias is larger than Vsat but smaller than the threshold voltage of inversion (Vsat < VG < VT), fixed charges at the SiO2-T2SL interface or within the SiO2 passivation layer attract electrons, leaving behind an SCR of uncompensated ionized acceptor ions near the p-region surface 101, 213501-1 C 2012 American Institute of Physics V 213501-2 Chen et al Appl Phys Lett 101, 213501 (2012) FIG The schematic diagram of gated diode p-region surface condition at (a) accumulation, (b) depletion, and (c) inversion situation (Fig 1(b)) The surface generation-recombination (G-R) current associated with the surface depletion region is related to the surface field-induced depletion width, xsd When the mesa surface is depleted, the surface field-induced depletion width and hence surface G-R current increases with increasing gate bias,23 in which results in a rising reverse dark current As gate bias increases above VT, the field-induced depletion width reaches its maximum value (xsd max) and type inversion occurs on the surface (Fig 1(c)) Therefore, there is no further increase in the surface G-R current,23 and the dark current density remains unchanged with respect to gate bias However, since the surface field-induced depletion width is very small near the interface between the Pỵ- and p-region, under reverse diode operation bias, electrons in the p- and Pỵ-region can tunnel through the thin field-induced depletion region and get into the inversion channel Therefore, dark current density increases dramatically as diode reverse operation bias increases, which is usually observed in the long-wavelength infrared SiO2 passivated T2SL photodetectors In order to reduce the high saturated gate bias, two possible options are to apply: a high-k dielectric material or reduce the dielectric layer thickness The gate bias can be expressed by the parallel capacitance formula r ¼ ee0 V=d; (1) where V is the saturated gate bias, e0 is the permittivity in vacuum, e is the SiO2 dielectric constant, assumed to be 3.9, r is the SiO2 surface charge density, and d is the dielectric thickness In this paper, we demonstrate high performance long-wavelength infrared gated diodes with low gate bias achieved by reducing the SiO2 passivation layer thickness In this work, a long-wavelength infrared T2SL Pỵ-p-M-Nỵ heterojunction24 was grown on a GaSb substrate with molecular beam epitaxy After a 1.5 lm n-doped InAsSb buffer layer, the device structure consisted of a 0.5 lm thick Nỵ-contact (nỵ $ 1018 cmÀ3), 0.5 lm thick lightly n-doped M-barrier,22 lm thick p-region (pÀ $  1016 cmÀ3), 0.5 lm thick Pỵ-contact (pỵ $ 1018 cm3) and capped with a Pỵ-type InAs capping layer The superlattice period in the p-region and Pỵ-contact region consisted of 13/7 monolayers (MLs) of InAs/GaSb and 7/11 MLs of InAs/GaSb respectively Both regions were doped with beryllium The Mbarrier and Nỵ-contact superlattice periods consisted of 18/ 3/5/3 MLs of InAs/GaSb/AlSb/GaSb in one period, and both were doped with silicon The material was processed into six dies of single element diodes with diameters ranging from 100 to 400 lm with the same processing procedures as those reported in Ref 25 One die (sample A) was left unpassivated for reference and the unpassivated diodes (UPD) were used for electrical and optical characterization The other dies (sample B, C, D, E, and F) were passivated with 10 nm, 20 nm, 60 nm, 300 nm, and 600 nm thick SiO2 dielectric layer, respectively, using plasma-enhanced chemical vapor deposition (PECVD) An additional metal gate was deposited on the mesa sidewall of half diodes in those samples, so that they contain both GD and ungated diode (UGD) In the final step, the top contacts were opened using electron cyclotron resonance etching All samples were wire-bonded onto 68-pin leadless ceramic chip carriers, loaded into a cryostat, and cooled down to 77 K for characterization Average I-V characteristics of five different sizes UPDs and UGDs are compared in Figure 2(a) All UGDs exhibit much leakier I-V characteristics than the UPDs because SiO2 passivation layers cause severe surface leakage for diodes in the long-wavelength infrared region UGDs with different SiO2 passivation layer thicknesses have the same I-V characteristics, which indicate that, above some threshold thickness, fixed charges in the SiO2 not have any effect on surface leakage Therefore, the fixed charges that cause band bending and surface leakage are mainly situated in the SiO2T2SL interface or the very thin SiO2 layer near the mesa sidewall Additional surface treatment to reduce the amount of fixed charges is required before or after SiO2 passivation As shown in Figure 2(b), the average dark current densities of GDs at VG ¼ Vsat are orders of magnitude lower than that of UPDs, and the corresponding saturated gate bias are shown in Table I At À100mV operation bias, at which the quantum efficiency (QE) is saturated, GDs exhibit one order of magnitude lower dark current than the UPDs, and more than one order of magnitude lower than the UGDs The 213501-3 Chen et al Appl Phys Lett 101, 213501 (2012) FIG (a) Electrical performance comparison between UPD and UGD with different SiO2 passivation layer thickness (b) Electrical performance comparison between UPD and GD at saturated gate bias (c) Correlation between gate bias and SiO2 passivation layer thickness TABLE I The differential-resistance-area product at À100 mV (RAÀ100 mV), saturated gate bias, and peak detectivity (D*) of UPD, UGD, and GD at saturation bias UPD Sample RAÀ100 mV (X cm2) Gate bias (V) D* (Jones) GD UGD A B C D E F B to F 389 N/A 2.5  1011 (0 mV) 2355 À4.5 5.9  1011 (70 mV) 3071 À7 7.0  1011 (70 mV) 2355 À15 6.4  1011 (70 mV) 2482 À60 5.8  1011 (70 mV) 2380 À120 6.3  1011 (70 mV) 6.8 N/A 1.7  1011 (0 mV) differential resistance area product at À100 mV (RAÀ100 mV) of UPDs, UGDs, and GDs are reported in Table I The best RAÀ100 mV value of the GD achieved is 3071 X cm2, which is $10 times higher than that of UPDs and $500 times higher than that of UGDs After eliminating the surface leakage, the bulk dark current characteristic should be achieved, and all GDs saturate at the same level, which is confirmed in Figure 2(b) The best saturated gate bias achieved is À4.5 V with 10 nm SiO2 passivation layer The linear correlation between the saturated gate bias and the thickness of SiO2 (Figure 2(c)) follows the Eq (1), and the calculated SiO2 fixed charged density is estimated to be about  1012 cmÀ2 The reverse dark current densities at À0.5 V and À1 V diode operation biases are shown in Figure as a function of the applied gate bias According to our previous discussion, the surface depletion width is believed to be at its maximum under no applied gate bias and in the case where a positive gate bias is applied In this scenario, the surface G-R current will not change with gate bias; the mesa surface is inverted, and a leakage channel is formed causing the high tunneling leakage current observed As gate bias becomes increasingly negative towards À7 V, the reverse dark current density decreases and the mesa surface begins to be less depleted Since the diode operation bias is fixed for each individual curve, the decrease in dark current density seen does not come from bulk current but a decreasing surface G-R current, which is the dominant dark current mechanism in this region When comparing the dark current between the two curves at the same gate bias, the surface G-R current at À1 V is found to be larger than that at À0.5 V, which is caused by an increasing surface depletion width (xsd) at higher diode reverse operation bias according to Ref 23, which is why the surface G-R current at À1 V is larger than that at À0.5 V When gate bias is beyond Vsat (À9.5 V Ϲ VG Ϲ À7 V), the reverse dark current density is independent of gate bias FIG The correlation between the reverse dark current density and gate bias of sample C at different diode operation bias 213501-4 Chen et al FIG (a) The peak responsivity (at 9.9 lm) and (b) QE at peak responsivity at different diode operation bias at 77 K but is found to be dependent on the operation bias, which means that the surface leakage current is eliminated and the mesa surface is under accumulation Since the reverse dark current density at À1 V operation bias is one order of magnitude higher than that at À0.5 volt, this means that the dominant dark current mechanism in this region is the bulk tunneling current rather than bulk G-R or diffusion current The peak responsivity (k ¼ 9.9 lm) and QE at peak responsivity of UPDs are shown in Figures 4(a) and 4(b) At À100 mV operation bias, the diode exhibits a saturated peak responsivity of 4.0 A/W and saturated quantum efficiency of 50% at peak responsivity At 77 K, the 50% cut-off wavelength and 100% cut-off wavelength of this detector are 11 lm and 13 lm, respectively The small bias dependence seen in the optical behavior is from the doping level of M-barrier, which causes a small conduction band misalignment between the M-structured superlattice and the p-region.24,26 Since after passivation, optical measurements are not possible, the detectivity values for UGDs and GDs are calculated based on their electrical measurements and the responsivity from UPDs Figure shows detectivities of UPDs, Appl Phys Lett 101, 213501 (2012) UGDs, and GDs at different operational bias The detectivities of UPD and UGD decrease very quickly with operation bias because of the noise caused by surface leakage Between and À500 mV of operation bias, the detectivity of UPD drops 10 times, and that of UGD drops by 32 times In contrast, in the whole operation range, detectivities of GDs in all samples are higher than those of UPDs and UGDs, and detectivities of GDs all stay in the level above  1011 Jones This is very important for the practical use of this technology because the detector can operate under a much lower bias, reducing noise and power consumption, and has a much wider operation range for FPA applications Sample C has the highest detectivities and reaches its maximum detectivity  1011 Jones at À70 mV, which is 2.8 times higher than the maximum detectivity of UPD and times higher than the maximum value of UGD Detectivities of other samples are reported in Table I In summary, we demonstrated the gating technique in long-wavelength infrared photodetectors, and effective suppression of the surface leakage generated by SiO2 passivation layer By minimizing the SiO2 passivation layer thickness, we confirmed that the origin of surface leakage is from fixed charges at the SiO2/T2SL interface or within the very thin SiO2 passivation layer near the surface, and that a thicker SiO2 passivation layer will not further affect surface leakage It means additional surface treatment is required before or after passivation to remove the fixed charges Also, the saturated gate bias of gated diodes has been reduced to À4.5 V, with RAÀ100 mV of 3071 X cm2, and detectivity of  1011 Jones at 77 K, which implies that the gating technique has great potential for FPA application Moreover, gated diode offers a much wider operation range than those of UGDs and UPDs and preserves detectivity above  1011 Jones, which is very important in FPA applications Most importantly, the gating technique reveals the evolution of surface leakage with gate bias, which facilitates further research work on surface leakage suppression The authors acknowledge the support, interest, and encouragement of Dr Fenner Milton, Dr Meimei Tidrow, and Dr Joseph Pellegrino from the U.S Army Night Vision Laboratory and Dr William Clark from U.S Army Research Office G A Sai-Halasz, R Tsu, and L Esaki, Appl Phys Lett 30, 651 (1977) M Razeghi, “Focal plane arrays in type-II superlattice,” U.S Patent No 6864552 (8 March 2005) A M Hoang, G Chen, A Haddadi, S Abdollahi Pour, and M Razeghi, Appl Phys Lett 100, 211101 (2012) B.-M Nguyen, G Chen, A M Hoang, S A Pour, S Bogdanov, and M Razeghi, Appl Phys Lett 99, 033501 (2011) S A Pour, E K.-w Huang, G Chen, A Haddadi, B.-M Nguyen, and M Razeghi, Appl Phys Lett 98, 143501 (2011) B M Nguyen, G Chen, M A Hoang, and M Razeghi, IEEE J Quantum Electron 47, 686 (2011) B M Nguyen, S Bogdanov, S Abdollahi Pour, and M Razeghi, Appl Phys Lett 95, 183502 (2009) M Walther, J Schmitz, R Rehm, S Kopta, F Fuchs, J Fleibner, W Cabanskib, and J Zieglerb, J Cryst Growth 278, 156 (2005) V Swaminathan, J W Little, and R L Tober, Proc SPIE 6127, 61270R (2006) 10 E K Huang, A Haddadi, G Chen, B M Nguyen, M A Hoang, R McClintock, M Stegall, and M Razeghi, OSA Opt Lett 36, 2560 (2011) FIG Detectivity of UPDs, UGDs, and GDs at saturation gate bias calculated at different operation bias The calculation of detectivity bases on the inset equation, with Ri is the responsivity, J is the dark current density, q is the charge of electron, kb is the Boltzmann constant, T is the temperature, and RA is the differential resistance-area product 213501-5 11 Chen et al G Chen, B.-M Nguyen, A M Hoang, E K Huang, S R Darvish, and M Razeghi, Appl Phys Lett 99, 183503 (2011) 12 P.-Y Delaunay, A Hood, B.-M Nguyen, D Hoffman, and M Razeghi, Appl Phys Lett 91, 091112 (2007) 13 S Bogdanov, B M Nguyen, A M Hoang, and M Razeghi, Appl Phys Lett 98, 183501 (2011) 14 E K.-w Huang, D Hoffman, B.-M Nguyen, P.-Y Delaunay, and M Razeghi, Appl Phys Lett 94, 053506 (2009) 15 A Gin, Y Wei, J Bae, A Hood, J Nah, and M Razeghi, Thin Solid Films 447–448, 489 (2004) 16 A Hood, P Y Delaunay, D Hoffman, B.-M Nguyen, Y Wei, and M Razeghi, Appl Plys Lett 90, 233513 (2007) 17 E Plis, J B Rodriguez, S J Lee, and S Krishna, Electron Lett 42, 1248 (2006) 18 A Gin, Y Wei, A Hood, A Bajowala, V Yazdanpanah, M Razeghi, and M Z Tidrow, Appl Phys Lett 84, 2037 (2004) Appl Phys Lett 101, 213501 (2012) 19 R Rehm, M Walther, F Fuchs, J Schmitz, and J Fleissner, Appl Phys Lett 86, 173501 (2005) F Szmulowicz and G J Brown, Infrared Phys Technol 53, 305–307 (2010) 21 B M Nguyen, D Hoffman, E K Huang, P Y Delaunay, and M Razeghi, Appl Phys Lett 93, 123502 (2008) 22 B.-M Nguyen, M Razeghi, V Nathan, and G J Brown, Proc SPIE 6479, 64790S (2007) 23 A S Grove and D J Fitzgerald, Solid-State Electron 9, 783 (1966) 24 B M Nguyen, D Hoffman, P Y Delaunay, E K Huang, M Razeghi, and J Pellegrino, Appl Phys Lett 93, 163502 (2008) 25 A Hood, D Hoffman, B.-M Nguyen, P.-Y Delaunay, E Michel, and M Razeghi, Appl Phys Lett 89, 093506 (2006) 26 D Hoffman, B M Nguyen, E K Huang, P Y Delaunay, M Razeghi, M Z Tidrow, and J Pellegrino, Appl Phys Lett 93, 031107 (2008) 20