Design of Band Engineered HgCdTe nBn Detectors for ... - IEEE Xplore

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 3, MARCH 2015

Design of Band Engineered HgCdTe nBn Detectors for MWIR and LWIR Applications Nima Dehdashti Akhavan, Member, IEEE, Gregory Jolley, Gilberto A. Umana-Membreno, Jarek Antoszewski, and Lorenzo Faraone, Fellow, IEEE Abstract— In this paper, we present a theoretical study of mercury cadmium telluride (HgCdTe)-based unipolar n-type/barrier/n-type (nBn) infrared (IR) detector structures for midwave IR and longwave IR spectral bands. To achieve the ultimate performance of nBn detectors, a bandgap engineering method is proposed to remove the undesirable valence band discontinuity that is currently limiting the performance of conventional HgCdTe nBn detectors. Our proposed band engineering method relies on simultaneous grading of the barrier composition and doping density profiles, leading to efficient elimination of the valence band discontinuity. This allows the detector to operate at |Vbias | < 50 mV, rendering all tunneling-related dark current components insignificant and allowing the detector to achieve the maximum possible diffusion current limited performance. Index Terms— Band discontinuity, doping modulation, infrared (IR) detectors, mercury cadmium telluride (HgCdTe), n-type/barrier/n-type (nBn), unipolar barrier.

Fig. 1. Schematic of an nBn detector considered in this paper with graded layer on each side of the barrier.

I. I NTRODUCTION

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IGH-PERFORMANCE infrared (IR) detectors and imaging focal plane arrays have been dominated by mercury cadmium telluride (HgCdTe)-based photodiodes for decades. A convenient fabrication process combined with outstanding optoelectronic properties has been the driving force behind the outstanding success of HgCdTe-based photovoltaic technologies. However, the ever-increasing demand for even higher performance detectors that can also operate at higher device temperatures has been hampered by the intrinsic constraints of photodiodes based on a p-n junction technology. In particular, the performance of semiconductor-based optoelectronic detectors is limited by the dark current, since any fluctuations in dark current are directly translated into noise in the detector, which limits the achievable detectivity and performance. Hence, minimizing and, where possible, eliminating the individual dark current components is the principle technique whereby further performance improvements can be achieved. Consequently, a new class of IR detectors, termed n-type/barrier/n-type (nBn) detectors, has recently Manuscript received September 22, 2014; revised November 27, 2014; accepted January 3, 2015. Date of publication January 29, 2015; date of current version February 20, 2015. This work was supported by the Australian Research Council within the Super Science Fellowship Program through the Discovery Project Program under Grant DP120104835 and Grant FS110200022. The review of this paper was arranged by Editor J. Huang. The authors are with the Department of Electrical, Electronic and Computer Engineering, University of Western Australia, Crawley, WA 6009 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2389229

been introduced by Maimon and Wicks [1], which has been successfully implemented using InAs and HgCdTe materials [1]–[3]. An nBn detector comprises a midwave IR (MWIR) absorption n-type semiconductor layer, a wide-bandgap undoped and thin barrier layer, and a second n-type contact layer, as shown in Fig. 1. In principle, the nBn structure essentially eliminates any surface currents and Shockley–Read–Hall (SRH) currents due to the presence of the much wider bandgap barrier layer at the semiconductor/passivant interface, thus resulting in a lower total dark current. Correspondingly, the lower dark current translates into either higher temperature operation of the device for the same performance or higher performance at the same temperature. In addition, due to the wide energy gap of the barrier, the surface-related dark current may be significantly reduced or perhaps even eliminated in the nBn structure. Consequently, the very stringent surface passivation requirements for narrow bandgap semiconductor devices can be relaxed significantly [4]. There are other technological advantages related to the nBn structure in comparison with a p-n photodiode, such as better uniformity and higher yield, which have been well documented in the literature for III–V based nBn devices [5]–[7]. Despite all the advantages of nBn detectors outlined above, the implementation of this detector structure in the HgCdTe-based material system is not straightforward due to the existence of a valence banddiscontinuity (barrier) at the absorber–barrier interface, E V . This discontinuity leads to several issues that limit HgCdTe-based nBn detector performance [3], [8], [9]. 1) At very low temperatures, the low-energy minority carrier holes generated by optical absorption are not able

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AKHAVAN et al.: DESIGN OF BAND ENGINEERED HgCdTe nBn DETECTORS

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TABLE I PARAMETERS U SED IN M ODELING OF MWIR AND LWIR HgCdTe nBn D ETECTORS

to overcome the valence band energy barrier, and hence, the device exhibits poor responsivity and detectivity. 2) Reduction of E V to a reasonably low value, by the adjustment of the Cd mole fraction in the barrier, results in a corresponding reduction of the barrier in the conduction band E C below a critical level, thus increasing majority carrier dark current and reducing responsivity (detectivity) at high temperatures. 3) Due to the existence of E V , depending on the wavelength of operation, a relatively high bias, typically greater than the bandgap energy, is required to collect all of the photogenerated minority carriers. This, in turn, leads to strong band-to-band tunneling (BTBT) and trapassisted tunneling (TAT) dark currents due to the high electric field within the barrier layer and any depletion layer. In principle, reduction or elimination of the valence band discontinuity E V can eliminate all of the above challenges, allowing HgCdTe nBn detectors to operate with |Vbias| < 50 mV. Consequently, in this paper, we propose a method that can effectively remove the valence band discontinuity in HgCdTe nBn detectors. Our method is based on band engineering of the barrier by simultaneous grading of the material composition and doping density profiles at the absorber–barrier interface. In Section III, we compare the performance of conventional (E V > 0) and band engineered (E V = 0) HgCdTe nBn detector structures for MWIR and longwave IR (LWIR) applications to estimate the ultimate performance of HgCdTe nBn detectors [3], [4], [9]. II. M ODELING AND D EVICE D ESIGN Numerical simulations of nBn structures have been performed using coupled equations for electrostatics (Poisson’s equation) and electron and hole current continuity equations (drift–diffusion model) using the commercial software Sentaurus. A 1-D model has been used, which includes the

electrical and optical properties of HgCdTe alloys that are based on previously published models [3], [10]. In addition, all important generation–recombination mechanisms, such as Auger, SRH, radiative, BTBT (which includes quantum tunneling models), and TAT, have been included in the steady-state drift–diffusion model at all locations within the device. In the case of BTBT, we use a convenient expression based on the Wentzel–Kramer–Brillouin approximation, which can easily be implemented for the purpose of drift–diffusion simulations and is given by [11], [12] √ √ 3/2 4 2m ∗ E g q2 m∗ F 2 exp − (1) RBTBT = √ 1/2 3q h¯ F 4 2π 2 h¯ 2 E g where the tunneling is expressed as a generation rate. F, m, and E g are the local electric field, effective mass, and bandgap, respectively. The TAT can be expressed as a slight modification of the SRH equation by adding a field effect factor n, p and is given by [10], [11], [13] RTAT =

pn − n 2i τ p0 Et τn0 −E t n + n + p + n exp exp i i 1+ p kB T 1+n kB T (2)

where n, p depends only on the electron and hole effective masses. τn0 and τ p0 are the electron and hole lifetimes, which are expressed as τn0 = (σn υth,n Ntrap ) −1 and τ p0 = (σ p υth, p Ntrap ) −1 1 1 υt h,n = 8k B T /πm ∗n 2 and υth, p = 8k B T /πm ∗p 2

(3) (4)

where σn, p are the capture cross sections and m n, p are the electron and hole effective masses. The light from an optical source has been introduced as a monochromatic wave with an incident power of 50 W/cm2 [8], [9]. Numerical modeling of the absorbed

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Fig. 2. Energy band profile in conventional HgCdTe nBn detector incorporating a graded barrier with E V > 0 for (a) MWIR and (b) LWIR. The valence band offset E V impedes the flow of minority carriers (holes) and results in performance degradation due to excessive BTBT, TAT, and noise. TB in Table I corresponds to the flat section of the barrier layer where composition is constant.

Fig. 4. Magnified valence band energy of the barrier layer in band engineered MWIR nBn detector. It can be observed that the proposed method of removing E V results in energy ripples smaller than 22 meV in the valence band.

Fig. 3. Required steps involved in the valence band discontinuity removal procedure in HgCdTe nBn detectors. First step is grading of the barrier Cd molar composition, as shown in the first row. Second row shows the grading of the acceptor and donor doping density profiles. Note that the doping profiles considered in this paper are not unique and different doping profiles can also be used. The calculated energy band diagram clearly shows that the valence band discontinuity has been eliminated for both types of detectors. TB in Table I corresponds to the flat section of the barrier layer where composition is constant.

light has been included using the optical absorption routine within the Sentaurus device simulator. Table I lists the material parameters used in the simulation of HgCdTe nBn detectors with λco = 5 μm and λco = 12 μm cutoff wavelengths for MWIR and LWIR detectors, respectively. Unless otherwise

stated, the values listed in Table I have been used throughout this paper. Using these parameters, the majority carrier diffusion length at T = 80 K is equal to 39 and 52 μm and in the case of minority carriers 28 and 47 μm for MWIR and LWIR detectors, respectively. In general, the valence band alignment in heterojunctions is not perfect and this imposes a serious constraint on the material system and alloy compositions that can be used in nBn structures. Utilizing materials that lead to a potential barrier in the valence band impedes the transport of minority carriers (holes) through the barrier layer, which necessitates the application of high bias voltages to collect the photogenerated minority carriers. Furthermore, this can lead to the formation of significant depletion regions, which degrades the performance of the detector with regard to dark current and operating temperature. Furthermore, BTBT and TAT may also take place. This is shown in Fig. 2 for an HgCdTe nBn detector. It shows the energy band diagram of a conventional HgCdTe nBn detector (E V > 0, Type I band alignment), with barrier composition of x B = 1 and zero bias applied to the detector contacts. The absorption layer is located to the right of each illustration, and the contact layer is located at the left side, while the barrier layer is sandwiched between the absorption layer and the contact layer. The bandgap of the barrier layer is larger than that of both the absorption layer and the contact layer. For the case of the band engineered nBn detector design considered in this paper (E V = 0), both the absorption and contact layers comprise Hg1−x Cdx Te with x = 0.3027 (5-μm cutoff) for MWIR and x = 0.2120 (12-μm cutoff) for LWIR at T = 80 K. The barrier layer is assumed to be an Hg1−x Cdx Te alloy with the molar concentration of Cd increasing at some distance away from the contact–barrier interface reaching its peak value of x = 1 at some distance from the contact–barrier interface. The barrier composition remains constant at x = 1, and then the Cd molar concentration starts to decrease until it reaches x = 0.3027 at some distance from the barrier–absorber interface, as shown in Fig. 3.


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Fig. 5. D ∗ versus inverse of temperature for (a) MWIR and (b) LWIR detectors, where TB and x B correspond to the barrier width and barrier composition in the flat region of the barrier layer. For MWIR nBn, a relatively large reverse bias is required in conventional devices to achieve the same performance as the band engineered detector, whereas for LWIR nBn, a reverse bias in the range of 200 mV is sufficient to reach the performance of a band engineered detector. For the case of detectors with E V = 0, x B = 1.0 to achieve the maximum theoretical performance in the band engineered devices.

Fig. 6. Jdark and Jphoto versus applied bias for (a) MWIR and (b) LWIR detectors. Note that the conventional MWIR nBn detector requires a relatively large bias to efficiently suppress the influence E V , since it has a barrier with x B = 0.60 compared with the LWIR detector with x B = 0.40.

To improve the valence band alignment in HgCdTe nBn detectors (MWIR or LWIR), the donor and acceptor doping densities need to be graded in the vicinity of the barrier region. The distance over which the density of dopants is graded is in the order of 200 nm, depending on the barrier thickness. The grading of donor and acceptor densities is shown in Fig. 3. The proposed profile for grading of doping density is not unique, and other doping profiles may be used. It is worth noting that variations in the maximum value of acceptor density in the barrier do not have any significant influence on the final profile of the valence band. By simultaneously combining the grading profile of Cd molar fraction and acceptor/donor doping density, as explained above, it is possible to achieve a zero band offset and near-ideal valence band alignment in HgCdTe nBn detectors

under zero bias condition [14]. The resulting energy band diagrams in the presence of doping and composition grading are shown in Fig. 3. Small energy ripples in valence band energy can still occur, which is a result of introducing acceptor type impurity into the doping profile. The magnitude of these ripples is smaller than 22 meV, and hence it does not have any influence on the minority carrier (hole) transport in the valence band or on the final performance of the detectors. This has been shown in Fig. 4. Various modifications may be made to the described band engineering approach while maintaining the basic concept. For example, some detector device designs may be based on a ptype/barrier/p-type (pBp) structure, where instead of an n-type semiconductor for the absorption and contact layers, a p-type semiconductor is used. For such a pBp detector, the design goal for the barrier, absorber, and contact layer is to allow

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Fig. 7. D ∗ versus absorber layer doping density for (a) MWIR and (b) LWIR detectors. For MWIR, the valence band offset E V is larger than the E V in LWIR detector, and therefore, elimination of the valence band discontinuity has a profound effect in MWIR detector performance compared with LWIR.

alignment of the conduction band across the device under zero bias to facilitate minority carrier transport. Furthermore, different alloys in the barrier and absorber layers may be used [4], [15]. III. P ERFORMANCE C OMPARISON In this section, we compare the performance of conventional nBn (E V > 0) and band engineered nBn (E V = 0) detectors operating in the MWIR and LWIR spectral bands. In the case of the conventional nBn detectors, we have used the parameters reported in Table I. We considered an nBn detector with a 2-μm-thick contact layer, a 100-nm barrier layer, and a 10-μm-thick absorption layer. In our calculations, the doping density is assumed to be at 1015 cm−3 in the contact, barrier, and absorber layers. In the case of a band engineered nBn detector, all the parameters are the same as in Table I except for barrier composition and doping profiles, which are modified according to Fig. 3. The Cd composition in the contact and absorber layers is the same and equal to x = 0.3027 and x = 0.2120 at T = 80 K, which corresponds to the MWIR and LWIR range, respectively. The Cd composition of the barrier layer is set to x B = 1 for the band engineered nBn structure in which the valence band offset has been removed (E V = 0). For conventional nBn detector structures, we assumed x B = 0.60 for MWIR and x B = 0.40 for LWIR with valence band offset in place (E V > 0). Fig. 5 shows the detectivity versus inverse temperature for the two detector designs, where it can be observed that the D ∗ of the conventional nBn strongly depends on the applied bias due to the existence of the valence band discontinuity, whereas D ∗ of the band engineered nBn does not depend on bias. For the conventional nBn detector, a reverse bias equal to or greater than the bandgap is required to overcome the valence band discontinuity and allow the efficient flow of minority carriers (holes), whereas for the band engineered nBn detector, a reverse bias smaller than the bandgap energy is required to reach the maximum performance.

In Fig. 5, it can also be observed that conventional MWIR nBn devices require a large bias to reach a similar D ∗ value compared with a band engineered MWIR detector, whereas an LWIR nBn detector requires a very small reverse bias to exhibit a similar D ∗ compared with band engineered LWIR nBn detector. This is due to the fact that the LWIR device has a barrier composition of x B = 0.40, whereas the MWIR detector has a barrier composition of x B = 0.60, which results in smaller E V in the conventional LWIR detector compared with the MWIR detector. The requirement of higher Cd molar composition in the barrier layer of an MWIR detector results in a large is due to the fact that an MWIR detector has a larger E V in conventional devices, and thus, a higher bias is required to collect the photogenerated minority carriers [compare Fig. 6(a) and (b)]. In contrast, it is evident that Jdark and Jphoto in band engineered nBn detectors are almost independent of the applied bias, whereas in conventional detectors, a large bias value is required to reduce the impact of the valence band discontinuity and allow the flow of minority carriers. The onset of BTBT at high reverse bias shown in Fig. 6(b) is dependent on the E V and the grading of barrier doping, which results in a difference between band engineered detector and conventional detector designs. Fig. 7 shows the D ∗ versus absorber layer doping density at T = 80 K. It can be observed that the removal of E V has a profound influence on the performance of MWIR detectors, whereas it does not have any significant influence on the D ∗ in LWIR detectors. As explained previously, this degradation of performance in MWIR detectors is due to the high Cd molar composition in MWIR compared with LWIR detectors. A high Cd molar composition in the barrier layer significantly degrades the performance of MWIR detectors by imposing a large E V in the valence band, and consequently, a large E V impedes the flow of minority carriers in the valence band. Fig. 8 shows the dynamic resistance (Rd A) of the two detector designs along with zero-bias R0 A values predicted


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Fig. 8. Rd A versus inverse of temperature for (a) MWIR and (b) LWIR detectors. In the graphs, the bias is fixed at 200 mV, which corresponds to the flat region of dark current in Fig. 6, and the n-type doping density of the absorber layer is 1 × 1016 cm−3 , which is close to the optimum doping, as shown in Fig. 7.

by [16, Rule-07]. It can be observed that the conventional nBn detector has a lower Rd A in comparison with a band engineered detector, and it should be noted that a conventional nBn detector is not able to operate at low bias values, since the valence band offset E V does not allow the flow of minority carriers at zero bias. IV. C ONCLUSION Conventional IR nBn detectors based on HgCdTe naturally present a potential barrier in the valence band, which significantly degrades the performance of such detectors by introducing excessive SRH, BTBT, and TAT. We proposed a method based on simultaneous grading of barrier Cd molar fraction and acceptor/donor doping density that can eliminate the valence band discontinuity of HgCdTe-based nBn detectors. The simulation results performed on MWIR and LWIR detectors have clearly shown that the valence band discontinuity has a profound influence on the performance of MWIR detectors compared with LWIR detectors. Furthermore, it is shown that band engineered nBn structures with E V = 0 can provide optimum performance at very low bias, thus avoiding any high-field regions and eliminating all tunneling current-related components of the dark current.

[7] P. Klipstein et al., “XBn barrier detectors for high operating temperatures,” Proc. SPIE, vol. 7608, pp. 76081V-1–76081V-10, Jan. 2010. [8] N. D. Akhavan, G. Jolley, G. A. Umana-Membreno, J. Antoszewski, and L. Faraone, “Performance modeling of bandgap engineered HgCdTe-based nBn infrared detectors,” IEEE Trans. Electron Devices, vol. 61, no. 11, pp. 3691–3698, Nov. 2014. [9] A. M. Itsuno, J. D. Phillips, and S. Velicu, “Mid-wave infrared HgCdTe nBn photodetector,” Appl. Phys. Lett., vol. 100, no. 16, pp. 161102-1–161102-3, 2012. [10] P. Martyniuk and A. Rogalski, “Modelling of MWIR HgCdTe complementary barrier HOT detector,” Solid-State Electron., vol. 80, pp. 96–104, Feb. 2012. [11] X. Ji et al., “Deep-level traps induced dark currents in extended wavelength Inx Ga1−x As/InP photodetector,” J. Appl. Phys., vol. 114, no. 22, pp. 224502-1–224502-5, Dec. 2013. [12] K. Jó´zwikowski, M. Kopytko, A. Rogalski, and A. Jó´zwikowska, “Enhanced numerical analysis of current-voltage characteristics of long wavelength infrared n-on-p HgCdTe photodiodes,” J. Appl. Phys., vol. 108, no. 7, pp. 074519-1–074519-11, Oct. 2010. [13] W. D. Hu et al., “Analysis of temperature dependence of dark current mechanisms for long-wavelength HgCdTe photovoltaic infrared detectors,” J. Appl. Phys., vol. 105, no. 10, pp. 104502-1–104502-8, May 2009. [14] E. F. Schubert, L.-W. Tu, and G. J. Zydzik, “Elimination of heterojunction band discontinuities,” U.S. Patent 5 170 407 A, Dec. 8, 1992. [15] M. Kopytko, K. Jó´zwikowski, and A. Rogalski, “Fundamental limits of MWIR HgCdTe barrier detectors operating under non-equilibrium mode,” Solid-State Electron., vol. 100, pp. 20–26, Oct. 2014. [16] W. E. Tennant, “‘Rule 07’ revisited: Still a good heuristic predictor of p/n HgCdTe photodiode performance?” J. Electron. Mater., vol. 39, no. 7, pp. 1030–1035, Jul. 2010.

R EFERENCES [1] S. Maimon and G. W. Wicks, “nBn detector, an infrared detector with reduced dark current and higher operating temperature,” Appl. Phys. Lett., vol. 89, no. 15, pp. 151109-1–151109-3, Oct. 2006. [2] J. W. Scott, C. E. Jones, E. J. Caine, and C. A. Cockrum, “Sub-pixel nBn detector,” U.S. Patent 2008 0 111 152 A1, May 15, 2008. [3] A. M. Itsuno, J. D. Phillips, and S. Velicu, “Design and modeling of HgCdTe nBn detectors,” J. Electron. Mater., vol. 40, no. 8, pp. 1624–1629, 2011. [4] P. Martyniuk, M. Kopytko, and A. Rogalski, “Barrier infrared detectors,” Opto-Electron. Rev., vol. 22, no. 2, pp. 127–146, Jun. 2014. [5] P. Klipstein, “Unipolar semiconductor photodetector with suppressed dark current and method for producing the same,” U.S. Patent 8 004 012 B2, Oct. 15, 2009. [6] P. Klipstein et al., “High operating temperature XBn-InAsSb bariode detectors,” Proc. SPIE, vol. 8268, pp. 82680U-1–82680U-8, Jan. 2012.

Nima Dehdashti Akhavan (M’08) was born in Tajrish, Iran, in 1982. He received the Ph.D. degree in microelectronics from the Tyndall National Institute, University College Cork, Cork, Ireland, in 2011. He has been a Post-Doctoral Researcher with the University of Western Australia, Crawley, WA, Australia, since 2012, where he is involved in the modeling and characterization of infrared detectors and quantum transport in nanostructures.

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Gregory Jolley received the Ph.D. degree in electrical and electronic engineering from Australian National University, Canberra, ACT, Australia, in 2010. He is currently a Research Fellow with the University of Western Australia, Crawley, WA, Australia. His current research interests include semiconductor material and device modeling, in particular, carrier transport.

Jarek Antoszewski received the Ph.D. degree from the Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, in 1982. He has been involved with the University of Western Australia Microelectromechanical Systems (MEMS) programs, which resulted in demonstration of the first worldwide monolithically integrated microspectrometer based on combined MEMS and HgCdTe technologies since 2004.

Gilberto A. Umana-Membreno received the Ph.D. degree in electrical and electronic engineering from the University of Western Australia, Crawley, WA, Australia, in 2007. He is currently a Senior Researcher with the Microelectronics Research Group, University of Western Australia.

Lorenzo Faraone (M’79-–SM’03–F’14) was born in Italy in 1951. He received the Ph.D. degree from the University of Western Australia (UWA), Crawley, WA, Australia, in 1979. He has been at UWA since 1987. His current research interests include compound semiconductor materials and devices, and microelectromechanical systems.