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InAsSb-based XBnn bariodes grown by molecular beam epitaxy on GaAs
Journal of Crystal Growth 339 (2012) 31–35
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InAsSb-based XBnn bariodes grown by molecular beam epitaxy on GaAs Eliezer Weiss n, Olga Klin, Steve Grossmann, Noam Snapi, Inna Lukomsky, Daniel Aronov, Michael Yassen, Eyal Berkowicz, Alex Glozman, Philip Klipstein, Avraham Fraenkel, Itay Shtrichman SCD – Semi-Conductor Devices, P.O. Box 2250/99, Haifa 31021, Israel
a r t i c l e i n f o
abstract
Article history: Received 7 October 2011 Received in revised form 28 November 2011 Accepted 29 November 2011 Communicated by E. Calleja Available online 7 December 2011
XBnn mid-wave infrared (MWIR) detector arrays aimed at high operating temperature (HOT) applications, also known as barrier detectors or ‘‘bariodes’’, are based on device elements with an InAsSb/AlSbAs heterostructure. There is no depletion layer in the narrow bandgap active layer of such devices, suppressing the usual Generation-Recombination (G-R) and Trap Assisted Tunneling (TAT) mechanisms for dark current that exist in standard narrow bandgap diodes. This yields lower dark currents in bariodes than in diodes with the same bandgap wavelength. InAsSb-bariode detectors, grown on lattice matched GaSb substrates have been shown previously to exhibit low dark current densities of 10-7 A/cm2 at 150 K. In this communication we show crystallographic and electro-optical characteristics of bariode structures grown on GaAs. Although the 7.8% mismatch causes a high density of dislocations, the devices still demonstrate electr-optical performance comparable with equivalent structures grown on GaSb, both for test devices and for focal plane array detectors (FPAs) with a 640 512 pixel format and a 15 mm pitch. This is in contrast to the behavior reported for InAsSb pin photodiodes grown on lattice mismatched substrates. The large leakage currents seen in the latter and attributed to a TAT mechanism, do not occur in the InAsSb-based bariodes grown on GaAs. & 2011 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Antimonides B2. Semiconducting indium compounds B3. Infrared devices
1. Introduction A new family of III–V photonic infrared (IR) detectors, known as bariodes [1], has recently been proposed and studied [2,3]. These devices are based on a thin wide bandgap barrier layer (BrL) and a thick, narrow bandgap, photon absorbing active layer (AL). They are distinguished from standard narrow bandgap diodes by the fact that there is no depletion layer in the AL. Hence, the dark current from the AL due to generation-recombination in the depletion layer (G-R), or trap-assisted tunneling (TAT), which can be strong in diodes, is totally suppressed in bariodes. The dark current is then limited to the diffusion contribution, allowing a substantial increase of the operating temperature of the IR detector. Such detectors, operating at 150K with dark current densities of 10 7 A/cm2, were demonstrated with InAsSb-based bariodes, grown by molecular beam epitaxy (MBE), which were lattice matched to GaSb substrates [2,3]. The ability to operate a bariode IR detector at a higher temperature than a standard photodiode results in an improved range of solutions for various applications, especially where the size, weight, and power (SWaP) are critical.
n
Corresponding author: Tel.: þ 972 4 990 2521, fax: þ972 4 990 2686. E-mail address: [email protected] (E. Weiss).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.11.076
An important advantage of reduced SWaP is that the detector manufacturing costs can also be reduced, especially for large infrared imagers. To maximize this goal, large inexpensive substrates of high crystallographic quality are needed. The GaSb substrates, on which InAsSb-based bariodes are currently fabricated, come in sizes up to 300 , although 400 GaSb wafers are under development [4]. At these sizes, the wafers can accommodate only a few large (megapixel) focal plane arrays (FPAs). The use of an alternative substrate which is cheaper and larger is therefore a highly attractive proposition. GaAs, being widely used in the optoelectronic industry, is a low-cost, well-established candidate to replace GaSb. It is available in larger diameters with very good crystallographic quality and an epi-ready surface finish. However, the 7.8% lattice mismatch between InAs0.91Sb0.09 (or GaSb) and GaAs can cause a large number of dislocations in the epilayers. In photovoltaic detectors, which require very high material quality to avoid defect-related leakage currents, these dislocations can be harmful, especially for III–V materials. For example, InAs/GaSb type-II superlattice detectors have been demonstrated on GaAs for both the MWIR [5] and the LWIR [6,7] regions, using a thick GaSb metamorphic buffer layer, but their electrical performance was significantly impaired compared with the same detectors grown lattice-matched to GaSb. More sophisticated methods, like the interfacial misfit (IMF) array growth mode, were proposed to deposit high quality antimonide semiconductors on GaAs [8]. While good MWIR emitters such as lasers were prepared by this
32
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
method were demonstrated [9], their 99% misfit strain relaxation still left a substantial amount of threading dislocations (TDs) that propagated into the grown layer. At such high dislocation densities, IR photodiode detectors usually show increased dark currents due to the TDs crossing the depletion region of the pn junction. Since there is no depletion in the narrow bandgap AL of a bariode device, it may be less prone to interference with its performance by dislocations. To verify to what degree this dislocation-immunity exists, we have compared the electrooptical behavior of InAsSb-bariode structures grown on both GaAs and GaSb substrates. In conventional photodiodes fabricated in
InAsSb grown by MBE on different substrates, the reverse-bias leakage currents were reported to increase with the lattice mismatch [10,11], even when a specially designed buffer structure was used to lower the dislocation density at the pn junction (to 4 107 cm 2 for InAsSb on GaAs). The dominant dark current mechanism in pin photodiodes fabricated in InAsSb grown on GaAs was shown to be TAT related (at 50–150 K and low reverse-bias) [12], due to the dislocation-induced states in the depletion region. This appears not to be the case for InAsSb-based bariodes, as we show in this communication. When growing these bariodes on GaAs substrates we have used a thick GaSb buffer layer that had only a limited efficiency in reducing the mismatchinduced TDs passing through the devices. Nevertheless, the performance of the devices grown on GaAs is closely similar to that of bariodes grown lattice-matched to GaSb.
2. Experimental procedures The InAsSb-based bariode structures studied in this work were grown on either GaAs(100) or GaSb(100) substrates, in a Veeco Gen200 MBE machine equipped with group III SUMOs cells and group V valved crackers. The mismatched structures were grown on a 4 mm thick GaSb buffer layer (BfL), whereas the reference structures were grown directly onto GaSb (100) substrates. The principal layers in the bariode structures were a thick n-type InAsSb AL (1.5-3 mm), a thin n-type AlSbAs BrL (0.2–0.35 mm), and a thin (0.2–0.3 mm) n-type InAsSb contact layer (CL). Prior to processing, the grown wafers were characterized using high resolution X-ray diffractometry (HRXRD, Bruker D8 Discover) and Nomarski-contrast optical microscopy. Sister samples were also studied by high-resolution transmission electron microscopy (HRTEM, FEI Titan 80–300 keV) and atomic force microscopy (AFM, Veeco DI3100). The wafers were processed into single mesa devices, with side dimensions of between 50 and 300 mm, as well as into 640 512 pixel arrays, with a 15 mm pitch. In both cases the mesas were created by etching to a depth slightly greater than the thickness of the CL, and a common contact was made to the AL outside the active device area. The arrays were flip-chip bonded with indium bumps to SCD Pelican-D Read-Out Integrated Circuits (ROICs). The substrate was totally etched away, and no antireflection coating (ARC) was applied. The devices were characterized as a function of temperature, both electrically and optically.
Fig. 1. Cross-section (a) and tilted (b) HRTEM images of the interface between the GaAs substrate (bottom part) and the GaSb buffer layer (upper part) showing an ordered array of misfit dislocations.
Fig. 2. HRXRD curves of the epilayers grown on either GaAs (full) or GaSb (dashed) substrates after etching away the contact and barrier layers, selectively. While the InAsSb AL grown on GaSb has a narrow diffraction peak with an FWHM of less than 30 arcsec, the InAsSb AL and GaSb BfL grown on GaAs (full line) have a single, wide unresolved peak (FWHM 125 arcsec). Inset: a wide scan showing the diffraction peaks of the GaAs substrate (on the right) and the unresolved InAsSb AL and GaSb BfL (on the left).
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
3. Results and discussion The HRTEM images of the GaSb BfL – GaAs substrate interface, shown in Fig. 1, reveal a highly ordered array of misfit dislocations (MDs) with a pitch of 5.5 nm, indicating almost full relaxation of the lattice mismatch strain in the GaSb BfL. Dislocations on the (100) GaSb surface were made visible by etching with a H2O2:HCl-based solution [13] after chemically removing the InAsSb-based bariode structure. The etch pit density (EPD) in the GaSb BfL was of the order of 107 cm 2, due to the large density of MDs at the interface between it and the GaAs substrate. In contrast, the EPD in the GaSb substrates used for the lattice-matched structure was only few hundreds per cm2. The quality of the InAsSb ALs grown on either GaAs or GaSb was also
33
assessed by HRXRD, after etching away the contact and barrier layers, selectively, as shown in Fig. 2. The InAsSb AL grown on a GaSb substrate is almost completely lattice matched, and yielded a narrow diffraction peak with a (combined) full width at half maximum (FWHM) of less than 30 arcsec (dashed curve). The InAsSb AL and GaSb BfL grown on GaAs yielded a single, wide unresolved peak with a FWHM of 125 arcsec (full line). Note that the inset to Fig. 2 is a wide scan showing the diffraction peaks of the GaAs substrate on the right and the unresolved InAsSb AL and GaSb BfL on the left. The high threading dislocation (TD) density in the structure grown on GaAs and the low crystalline quality of the epilayers are reflected also in the surface morphology of the complete bariode structure. The Nomarski micrographs of structures grown lattice-matched to GaSb
Fig. 3. Surface morphology of ‘‘as-grown’’ bariode structures grown on either GaSb or GaAs substrates. (a, b) Nomarski micrographs of structures grown on GaSb (a) and GaAs (b). The bar length is 50 mm. (c) A 20 20 mm2 AFM image and (d) a line scan of the surface of a complete bariode structure grown on a GaAs substrate showing the cellular morphology of 10 nm high hillocks. The sides of the hillocks are 5 to 10 mm long.
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
substrates (Fig. 3a) show smooth surfaces without any particular structure. In contrast, the surface of identical structures grown on GaAs (Fig. 3b) has a cellular morphology, with cell dimensions of 5 to 10 mm. AFM measurements show that the cellular morphology consists of hillocks 10 nm high (Fig. 3c,d). Our bariode structures grown on GaAs substrates clearly suffer from a high dislocation density and a reduced crystalline quality. Nevertheless, the electro-optical performance of these devices is similar to that of the bariodes grown lattice-matched to GaSb substrates. Arhenius plots of the dark current at operating bias for 200 200 mm2 test bariode devices grown on either GaAs (with a 3 mm thick AL) or GaSb (with a 1.5 mm thick AL) are shown in Fig. 4. Both sets of data fit a T3exp[( 340 meV)/kBT] expression above 150 K (where kB is the Boltzmann constant) [14], indicating that both devices have the same bandgap energy. Since the 340 meV activation energy is close to the optically measured bandgap, their dark currents at these temperatures are also diffusion limited. The current in the two structures differs by a factor of 2. This might be attributed to their different AL thicknesses, but may also be due, in part, to a difference in the density of gap states in the AL, which could affect the minority carrier lifetime, as is discussed in the next paragraph. Nevertheless, both devices, regardless of their different degree of crystalline perfection, have a low dark current density of 0.1 mA/cm2 at 150 K. The size dependence of the quantum efficiency (QE) for various devices from the two layers discussed above was measured, in order to estimate the minority carrier diffusion length. The results, for mesa dimensions of between 100 and 300 mm, are plotted in Fig. 5. As expected, the different AL thicknesses have a strong effect on the absolute QE values. Nevertheless, the size dependence of the QE’s in the two epilayers can be fitted to a function of the form QE¼QEN(L þ2LP)2/L2 where L is the mesa dimension and LP is the lateral diffusion length of the minority holes in the AL [15]. This lateral diffusion length is directly related to the minority carrier lifetime. It may be reduced relative to the bulk diffusion length by surface recombination, which if present in our case, appears to be relatively weak. Such fits are shown in Fig. 5 with QEN ¼68% and LP ¼8 mm for the non lattice matched sample and with QEN ¼ 25% and LP ¼16 mm for the lattice matched sample. The lower lateral diffusion length in the non lattice matched structure can be related to the presence of TDs in the AL of this sample. However, both values are comparable to the typical range of Lp values that we have observed from many of our lattice matched devices which is 10-25 mm [3,14,15]. It is notable,
Fig. 4. Arhenius plot of dark current (Id) for 200 200 mm2 test bariode devices grown on either GaAs () or GaSb (’) substrates. The AL thickness was 3 mm for the structure grown on GaAs and 1.5 mm for that grown on GaSb.
90 80 70 QE (%)
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AL=3 µm on GaAs
60
Lp = 8 µm AL=1.5 µm on GaSb
50
Lp = 16 µm
40 30 20 100
150
200
250
300
L [µm] Fig. 5. Quantum efficiency (QE) vs. mesa dimension for back illuminated test bariodes grown either on GaAs (circles) with an AL thickness of 3 mm or GaSb (squares) with an AL thickness of 1.5 mm. The devices were bonded to a silicon fan-out circuit. The n-substrates were not thinned and no ARC was applied.
Fig. 6. Electro-optical results of 640 512 pixel FPAs (15 mm pitch) fabricated using the same process, and at the same time, in two layers grown on either GaAs (FPAs CC and CE, full lines) or GaSb (FPA CD, dashed line) arrays. The AL thickness was 3 mm in both cases. The external QE (empty symbols) and Id (full symbols) are plotted against the applied bias (V) at 150 K.
therefore, that in spite of the high dislocation density in the epilayers grown on GaAs, there is, somewhat surprisingly, no significant difference in the minority carrier lifetime for lattice matched and lattice mismatched growths. The similar electrical properties of the test devices grown on the two substrate types is also reflected in the performance of full TV format FPAs. Fig. 6 shows plots of the external quantum efficiency (QE) and dark current (Id) vs. applied bias (V) for several 640 512 FPAs fabricated at the same time from identical bariode structures grown on either GaAs or GaSb. The characteristics in the two cases are similar, both with regard to the mean external QE without an ARC (60%) [16] and the small dark current density at an operating bias of 0.35 V (0.05 mA/cm2). In particular, the uniformities of the electro-optical characteristics of the arrays are also similar to one another: the QE and Id standard deviations are 3% and 10%, respectively, for the two bariode wafer growths, and their operabilities (percentage of working pixels) are 499.5% in each case. Furthermore, two FPAs taken from the same wafer with a GaAs substrate (samples CE and CC) show average QE and Id levels which are very close to one another. This absence of increased reverse-bias leakage currents in the InAsSb-based bariodes, even in the presence of a high TD density for the structure grown on GaAs, is in contrast to the strong deterioration
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
of performance reported for InAsSb-based pn and pin diodes as a function of lattice mismatch [10–12]. Besikci et al [12] showed that mismatch induced dislocations in InAsSb created states in the narrow energy bandgap of the pin diode depletion region which were efficient centers for TAT. In contrast, the depletion region of a bariode structure at its operating bias is confined to the wide-gap BrL. It is very likely that the TDs passing through this III–V material also form states in its energy gap. Yet even if this is the case, the wide bandgap of the BrL should render the TAT mechanism less efficient. Dislocation related states induced in the narrow bandgap InAsSb AL of the bariode structure reside only in flat-band regions, and thus cannot contribute to any TAT mechanism. This could make InAsSb-based bariodes more tolerant to the large lattice mismatch between the GaAs substrate and the BrL and AL of the structure.
4. Conclusion InAsSb-based bariodes can be fabricated successfully on alternative, non-lattice matched GaAs substrates. Furthermore, the indifference of the bariode device to the lattice mismatch with the substrate on which it is grown, and in particular to the dislocation density in the epilayers, opens the way to increasing the wavelength range of our current InAsSb-based bariodes from 4.1 mm (for InAsSb latticematched to GaSb) to 4.9 mm, thereby covering the whole midwavelength infrared (MWIR) atmospheric ‘‘window’’.
Acknowledgements The authors would like to acknowledge technical support from Mr. S. Greenberg and Mr. H. Geva for the smooth operation of the MBE growth equipment, to Ms. M. Menachem, Mr. Y. Caracenti, Mr. I. Bogoslavsky, Ms. N. Hazan, Mr. S. Weinstein Mr. D. Gur, and Ms. H. Paran, who have all contributed to the successful processing, packaging or characterization of the devices, and to Dr. Y. Kauffmann, Dr. T. Cohen-Hyam and Dr. R. Edrei for the TEM and AFM work performed at the Technion, Haifa. We are grateful to Dr. Z. Calahorra for coining the term ‘‘bariode’’.
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References [1] In 1919, William Henry Eccles coined the term diode from the Greek word dia, meaning ‘‘through’’, and ode (from odoB), meaning ‘‘path’’ (October 7th, 2011: /http://en.wikipedia.org/wiki/Diode#HistoryS). Bariode is a portmanteau of the term ‘‘barrier diode’’ that describes a semiconductor photodetector in which a clear path through the device is provided for minority carriers from the photon absorbing layer, while the path of its majority carriers is blocked by means of a barrier. An n-type bariode has an n-type AL and BrL, while in a p-type bariode, their polarities are reversed. [2] P. Klipstein, O. Klin, S. Grossman, N. Snapi, I. Lukomsky, M. Brumer, M. Yassen, D. Aronov, E. Berkowicz, A. Glozman, T. Fishman, O. Magen, I. Shtrichman, E. Weiss, Proceedings of SPIE 8012 (2011) 8012 2R and references therein. [3] P. Klipstein, O. Klin, S. Grossman, N. Snapi, I. Lukomsky, D. Aronov, M. Yassen, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, E. Weiss, Optical Engineering 50 (2011) 061002 and references therein. [4] M.J. Furlong, R. Martinez, S. Amirhaghi, D. Small, B. Smith, A. Mowbray, Proceedings of SPIE 8012 (2011) 801211. [5] B.-M. Nguyen, D. Hoffman, E.K.-w. Huang, S. Bogdanov, P.-Y. Delaunay, M. Razeghi, M.Z. Tidrow, Applied Physics Letters 94 (2009) 223506. [6] S. Abdollahi Pour, B.-M. Nguyen, S. Bogdanov, E.K. Huang, M. Razeghi, Applied Physics Letters 95 (2009) 173505. [7] M. Razeghi, E.K. Huang, B.-M. Nguyen, S. Abdollahi Pour, P.-Y. Delaunay, Proceedings of SPIE 7660 (2010) 7660 1F. [8] See for example M. Mehta, G. Balakrishnan, S. Huang, A. Khoshakhlagh, A. Jallipalli, P. Patel, M.N. Kutty, L.R. Dawson, D.L. Huffaker, Applied Physics Letters 89 (2006) 211110. [9] J. Tatebayashi, A. Jallipalli, M.N. Kutty, S.H. Huang, G. Balakrishnan, L.R. Dawson, D.L. Huffaker, Applied Physics Letters 91 (2007) 141102. [10] W. Dobbelaere, J. De Boeck, W. De Raedt, J. Vanhellemont, G. Zou, M. Van Hove, B. Brijs, R. Mertens, G. Borghs, MRS Symposium Proceedings 216 (1991) 181. [11] W. Dobbelaere, J. De Boeck, P. Heremans, R. Mertens, G. Borghs, W. Luyten, J. Van Landuyt, Applied Physics Letters 60 (1992) 3256. [12] C. Besikci, S. Ozer, C. Van Hoof, L. Zimmermann, J. John, P. Merken, Semiconductor Science and Technology 16 (2001) 992. [13] L. Reijnen, R. Brunton, I.R. Grant, Journal of Crystal Growth 275 (2005) e595. [14] P. Klipstein, O. Klin, S. Grossman, N. Snapi, B. Yaakobovitz, M. Brumer, I. Lukomsky, D. Aronov, M. Yassen, B. Yofis, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, E. Weiss, Proceedings of SPIE, 7660, 2010 7669 2Y. [15] P.C. Klipstein, O. Klin, S. Grossman, N. Snapi, B. Yaakobovitz, M. Brumer, I. Lukomsky, D. Aronov, M. Yassen, B. Yofis, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, Eliezer Weiss, Proceedings of SPIE, 7608, 2010 7608 1V. [16] The somewhat lower QE for the bariode grown on GaAs is most probably due to radiation absorption in the 4 mm undoped GaSb BfL in this sample.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InAsSb-based XBnn bariodes grown by molecular beam epitaxy on GaAs Eliezer Weiss n, Olga Klin, Steve Grossmann, Noam Snapi, Inna Lukomsky, Daniel Aronov, Michael Yassen, Eyal Berkowicz, Alex Glozman, Philip Klipstein, Avraham Fraenkel, Itay Shtrichman SCD – Semi-Conductor Devices, P.O. Box 2250/99, Haifa 31021, Israel
a r t i c l e i n f o
abstract
Article history: Received 7 October 2011 Received in revised form 28 November 2011 Accepted 29 November 2011 Communicated by E. Calleja Available online 7 December 2011
XBnn mid-wave infrared (MWIR) detector arrays aimed at high operating temperature (HOT) applications, also known as barrier detectors or ‘‘bariodes’’, are based on device elements with an InAsSb/AlSbAs heterostructure. There is no depletion layer in the narrow bandgap active layer of such devices, suppressing the usual Generation-Recombination (G-R) and Trap Assisted Tunneling (TAT) mechanisms for dark current that exist in standard narrow bandgap diodes. This yields lower dark currents in bariodes than in diodes with the same bandgap wavelength. InAsSb-bariode detectors, grown on lattice matched GaSb substrates have been shown previously to exhibit low dark current densities of 10-7 A/cm2 at 150 K. In this communication we show crystallographic and electro-optical characteristics of bariode structures grown on GaAs. Although the 7.8% mismatch causes a high density of dislocations, the devices still demonstrate electr-optical performance comparable with equivalent structures grown on GaSb, both for test devices and for focal plane array detectors (FPAs) with a 640 512 pixel format and a 15 mm pitch. This is in contrast to the behavior reported for InAsSb pin photodiodes grown on lattice mismatched substrates. The large leakage currents seen in the latter and attributed to a TAT mechanism, do not occur in the InAsSb-based bariodes grown on GaAs. & 2011 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Antimonides B2. Semiconducting indium compounds B3. Infrared devices
1. Introduction A new family of III–V photonic infrared (IR) detectors, known as bariodes [1], has recently been proposed and studied [2,3]. These devices are based on a thin wide bandgap barrier layer (BrL) and a thick, narrow bandgap, photon absorbing active layer (AL). They are distinguished from standard narrow bandgap diodes by the fact that there is no depletion layer in the AL. Hence, the dark current from the AL due to generation-recombination in the depletion layer (G-R), or trap-assisted tunneling (TAT), which can be strong in diodes, is totally suppressed in bariodes. The dark current is then limited to the diffusion contribution, allowing a substantial increase of the operating temperature of the IR detector. Such detectors, operating at 150K with dark current densities of 10 7 A/cm2, were demonstrated with InAsSb-based bariodes, grown by molecular beam epitaxy (MBE), which were lattice matched to GaSb substrates [2,3]. The ability to operate a bariode IR detector at a higher temperature than a standard photodiode results in an improved range of solutions for various applications, especially where the size, weight, and power (SWaP) are critical.
n
Corresponding author: Tel.: þ 972 4 990 2521, fax: þ972 4 990 2686. E-mail address: [email protected] (E. Weiss).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.11.076
An important advantage of reduced SWaP is that the detector manufacturing costs can also be reduced, especially for large infrared imagers. To maximize this goal, large inexpensive substrates of high crystallographic quality are needed. The GaSb substrates, on which InAsSb-based bariodes are currently fabricated, come in sizes up to 300 , although 400 GaSb wafers are under development [4]. At these sizes, the wafers can accommodate only a few large (megapixel) focal plane arrays (FPAs). The use of an alternative substrate which is cheaper and larger is therefore a highly attractive proposition. GaAs, being widely used in the optoelectronic industry, is a low-cost, well-established candidate to replace GaSb. It is available in larger diameters with very good crystallographic quality and an epi-ready surface finish. However, the 7.8% lattice mismatch between InAs0.91Sb0.09 (or GaSb) and GaAs can cause a large number of dislocations in the epilayers. In photovoltaic detectors, which require very high material quality to avoid defect-related leakage currents, these dislocations can be harmful, especially for III–V materials. For example, InAs/GaSb type-II superlattice detectors have been demonstrated on GaAs for both the MWIR [5] and the LWIR [6,7] regions, using a thick GaSb metamorphic buffer layer, but their electrical performance was significantly impaired compared with the same detectors grown lattice-matched to GaSb. More sophisticated methods, like the interfacial misfit (IMF) array growth mode, were proposed to deposit high quality antimonide semiconductors on GaAs [8]. While good MWIR emitters such as lasers were prepared by this
32
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
method were demonstrated [9], their 99% misfit strain relaxation still left a substantial amount of threading dislocations (TDs) that propagated into the grown layer. At such high dislocation densities, IR photodiode detectors usually show increased dark currents due to the TDs crossing the depletion region of the pn junction. Since there is no depletion in the narrow bandgap AL of a bariode device, it may be less prone to interference with its performance by dislocations. To verify to what degree this dislocation-immunity exists, we have compared the electrooptical behavior of InAsSb-bariode structures grown on both GaAs and GaSb substrates. In conventional photodiodes fabricated in
InAsSb grown by MBE on different substrates, the reverse-bias leakage currents were reported to increase with the lattice mismatch [10,11], even when a specially designed buffer structure was used to lower the dislocation density at the pn junction (to 4 107 cm 2 for InAsSb on GaAs). The dominant dark current mechanism in pin photodiodes fabricated in InAsSb grown on GaAs was shown to be TAT related (at 50–150 K and low reverse-bias) [12], due to the dislocation-induced states in the depletion region. This appears not to be the case for InAsSb-based bariodes, as we show in this communication. When growing these bariodes on GaAs substrates we have used a thick GaSb buffer layer that had only a limited efficiency in reducing the mismatchinduced TDs passing through the devices. Nevertheless, the performance of the devices grown on GaAs is closely similar to that of bariodes grown lattice-matched to GaSb.
2. Experimental procedures The InAsSb-based bariode structures studied in this work were grown on either GaAs(100) or GaSb(100) substrates, in a Veeco Gen200 MBE machine equipped with group III SUMOs cells and group V valved crackers. The mismatched structures were grown on a 4 mm thick GaSb buffer layer (BfL), whereas the reference structures were grown directly onto GaSb (100) substrates. The principal layers in the bariode structures were a thick n-type InAsSb AL (1.5-3 mm), a thin n-type AlSbAs BrL (0.2–0.35 mm), and a thin (0.2–0.3 mm) n-type InAsSb contact layer (CL). Prior to processing, the grown wafers were characterized using high resolution X-ray diffractometry (HRXRD, Bruker D8 Discover) and Nomarski-contrast optical microscopy. Sister samples were also studied by high-resolution transmission electron microscopy (HRTEM, FEI Titan 80–300 keV) and atomic force microscopy (AFM, Veeco DI3100). The wafers were processed into single mesa devices, with side dimensions of between 50 and 300 mm, as well as into 640 512 pixel arrays, with a 15 mm pitch. In both cases the mesas were created by etching to a depth slightly greater than the thickness of the CL, and a common contact was made to the AL outside the active device area. The arrays were flip-chip bonded with indium bumps to SCD Pelican-D Read-Out Integrated Circuits (ROICs). The substrate was totally etched away, and no antireflection coating (ARC) was applied. The devices were characterized as a function of temperature, both electrically and optically.
Fig. 1. Cross-section (a) and tilted (b) HRTEM images of the interface between the GaAs substrate (bottom part) and the GaSb buffer layer (upper part) showing an ordered array of misfit dislocations.
Fig. 2. HRXRD curves of the epilayers grown on either GaAs (full) or GaSb (dashed) substrates after etching away the contact and barrier layers, selectively. While the InAsSb AL grown on GaSb has a narrow diffraction peak with an FWHM of less than 30 arcsec, the InAsSb AL and GaSb BfL grown on GaAs (full line) have a single, wide unresolved peak (FWHM 125 arcsec). Inset: a wide scan showing the diffraction peaks of the GaAs substrate (on the right) and the unresolved InAsSb AL and GaSb BfL (on the left).
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
3. Results and discussion The HRTEM images of the GaSb BfL – GaAs substrate interface, shown in Fig. 1, reveal a highly ordered array of misfit dislocations (MDs) with a pitch of 5.5 nm, indicating almost full relaxation of the lattice mismatch strain in the GaSb BfL. Dislocations on the (100) GaSb surface were made visible by etching with a H2O2:HCl-based solution [13] after chemically removing the InAsSb-based bariode structure. The etch pit density (EPD) in the GaSb BfL was of the order of 107 cm 2, due to the large density of MDs at the interface between it and the GaAs substrate. In contrast, the EPD in the GaSb substrates used for the lattice-matched structure was only few hundreds per cm2. The quality of the InAsSb ALs grown on either GaAs or GaSb was also
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assessed by HRXRD, after etching away the contact and barrier layers, selectively, as shown in Fig. 2. The InAsSb AL grown on a GaSb substrate is almost completely lattice matched, and yielded a narrow diffraction peak with a (combined) full width at half maximum (FWHM) of less than 30 arcsec (dashed curve). The InAsSb AL and GaSb BfL grown on GaAs yielded a single, wide unresolved peak with a FWHM of 125 arcsec (full line). Note that the inset to Fig. 2 is a wide scan showing the diffraction peaks of the GaAs substrate on the right and the unresolved InAsSb AL and GaSb BfL on the left. The high threading dislocation (TD) density in the structure grown on GaAs and the low crystalline quality of the epilayers are reflected also in the surface morphology of the complete bariode structure. The Nomarski micrographs of structures grown lattice-matched to GaSb
Fig. 3. Surface morphology of ‘‘as-grown’’ bariode structures grown on either GaSb or GaAs substrates. (a, b) Nomarski micrographs of structures grown on GaSb (a) and GaAs (b). The bar length is 50 mm. (c) A 20 20 mm2 AFM image and (d) a line scan of the surface of a complete bariode structure grown on a GaAs substrate showing the cellular morphology of 10 nm high hillocks. The sides of the hillocks are 5 to 10 mm long.
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
substrates (Fig. 3a) show smooth surfaces without any particular structure. In contrast, the surface of identical structures grown on GaAs (Fig. 3b) has a cellular morphology, with cell dimensions of 5 to 10 mm. AFM measurements show that the cellular morphology consists of hillocks 10 nm high (Fig. 3c,d). Our bariode structures grown on GaAs substrates clearly suffer from a high dislocation density and a reduced crystalline quality. Nevertheless, the electro-optical performance of these devices is similar to that of the bariodes grown lattice-matched to GaSb substrates. Arhenius plots of the dark current at operating bias for 200 200 mm2 test bariode devices grown on either GaAs (with a 3 mm thick AL) or GaSb (with a 1.5 mm thick AL) are shown in Fig. 4. Both sets of data fit a T3exp[( 340 meV)/kBT] expression above 150 K (where kB is the Boltzmann constant) [14], indicating that both devices have the same bandgap energy. Since the 340 meV activation energy is close to the optically measured bandgap, their dark currents at these temperatures are also diffusion limited. The current in the two structures differs by a factor of 2. This might be attributed to their different AL thicknesses, but may also be due, in part, to a difference in the density of gap states in the AL, which could affect the minority carrier lifetime, as is discussed in the next paragraph. Nevertheless, both devices, regardless of their different degree of crystalline perfection, have a low dark current density of 0.1 mA/cm2 at 150 K. The size dependence of the quantum efficiency (QE) for various devices from the two layers discussed above was measured, in order to estimate the minority carrier diffusion length. The results, for mesa dimensions of between 100 and 300 mm, are plotted in Fig. 5. As expected, the different AL thicknesses have a strong effect on the absolute QE values. Nevertheless, the size dependence of the QE’s in the two epilayers can be fitted to a function of the form QE¼QEN(L þ2LP)2/L2 where L is the mesa dimension and LP is the lateral diffusion length of the minority holes in the AL [15]. This lateral diffusion length is directly related to the minority carrier lifetime. It may be reduced relative to the bulk diffusion length by surface recombination, which if present in our case, appears to be relatively weak. Such fits are shown in Fig. 5 with QEN ¼68% and LP ¼8 mm for the non lattice matched sample and with QEN ¼ 25% and LP ¼16 mm for the lattice matched sample. The lower lateral diffusion length in the non lattice matched structure can be related to the presence of TDs in the AL of this sample. However, both values are comparable to the typical range of Lp values that we have observed from many of our lattice matched devices which is 10-25 mm [3,14,15]. It is notable,
Fig. 4. Arhenius plot of dark current (Id) for 200 200 mm2 test bariode devices grown on either GaAs () or GaSb (’) substrates. The AL thickness was 3 mm for the structure grown on GaAs and 1.5 mm for that grown on GaSb.
90 80 70 QE (%)
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AL=3 µm on GaAs
60
Lp = 8 µm AL=1.5 µm on GaSb
50
Lp = 16 µm
40 30 20 100
150
200
250
300
L [µm] Fig. 5. Quantum efficiency (QE) vs. mesa dimension for back illuminated test bariodes grown either on GaAs (circles) with an AL thickness of 3 mm or GaSb (squares) with an AL thickness of 1.5 mm. The devices were bonded to a silicon fan-out circuit. The n-substrates were not thinned and no ARC was applied.
Fig. 6. Electro-optical results of 640 512 pixel FPAs (15 mm pitch) fabricated using the same process, and at the same time, in two layers grown on either GaAs (FPAs CC and CE, full lines) or GaSb (FPA CD, dashed line) arrays. The AL thickness was 3 mm in both cases. The external QE (empty symbols) and Id (full symbols) are plotted against the applied bias (V) at 150 K.
therefore, that in spite of the high dislocation density in the epilayers grown on GaAs, there is, somewhat surprisingly, no significant difference in the minority carrier lifetime for lattice matched and lattice mismatched growths. The similar electrical properties of the test devices grown on the two substrate types is also reflected in the performance of full TV format FPAs. Fig. 6 shows plots of the external quantum efficiency (QE) and dark current (Id) vs. applied bias (V) for several 640 512 FPAs fabricated at the same time from identical bariode structures grown on either GaAs or GaSb. The characteristics in the two cases are similar, both with regard to the mean external QE without an ARC (60%) [16] and the small dark current density at an operating bias of 0.35 V (0.05 mA/cm2). In particular, the uniformities of the electro-optical characteristics of the arrays are also similar to one another: the QE and Id standard deviations are 3% and 10%, respectively, for the two bariode wafer growths, and their operabilities (percentage of working pixels) are 499.5% in each case. Furthermore, two FPAs taken from the same wafer with a GaAs substrate (samples CE and CC) show average QE and Id levels which are very close to one another. This absence of increased reverse-bias leakage currents in the InAsSb-based bariodes, even in the presence of a high TD density for the structure grown on GaAs, is in contrast to the strong deterioration
E. Weiss et al. / Journal of Crystal Growth 339 (2012) 31–35
of performance reported for InAsSb-based pn and pin diodes as a function of lattice mismatch [10–12]. Besikci et al [12] showed that mismatch induced dislocations in InAsSb created states in the narrow energy bandgap of the pin diode depletion region which were efficient centers for TAT. In contrast, the depletion region of a bariode structure at its operating bias is confined to the wide-gap BrL. It is very likely that the TDs passing through this III–V material also form states in its energy gap. Yet even if this is the case, the wide bandgap of the BrL should render the TAT mechanism less efficient. Dislocation related states induced in the narrow bandgap InAsSb AL of the bariode structure reside only in flat-band regions, and thus cannot contribute to any TAT mechanism. This could make InAsSb-based bariodes more tolerant to the large lattice mismatch between the GaAs substrate and the BrL and AL of the structure.
4. Conclusion InAsSb-based bariodes can be fabricated successfully on alternative, non-lattice matched GaAs substrates. Furthermore, the indifference of the bariode device to the lattice mismatch with the substrate on which it is grown, and in particular to the dislocation density in the epilayers, opens the way to increasing the wavelength range of our current InAsSb-based bariodes from 4.1 mm (for InAsSb latticematched to GaSb) to 4.9 mm, thereby covering the whole midwavelength infrared (MWIR) atmospheric ‘‘window’’.
Acknowledgements The authors would like to acknowledge technical support from Mr. S. Greenberg and Mr. H. Geva for the smooth operation of the MBE growth equipment, to Ms. M. Menachem, Mr. Y. Caracenti, Mr. I. Bogoslavsky, Ms. N. Hazan, Mr. S. Weinstein Mr. D. Gur, and Ms. H. Paran, who have all contributed to the successful processing, packaging or characterization of the devices, and to Dr. Y. Kauffmann, Dr. T. Cohen-Hyam and Dr. R. Edrei for the TEM and AFM work performed at the Technion, Haifa. We are grateful to Dr. Z. Calahorra for coining the term ‘‘bariode’’.
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References [1] In 1919, William Henry Eccles coined the term diode from the Greek word dia, meaning ‘‘through’’, and ode (from odoB), meaning ‘‘path’’ (October 7th, 2011: /http://en.wikipedia.org/wiki/Diode#HistoryS). Bariode is a portmanteau of the term ‘‘barrier diode’’ that describes a semiconductor photodetector in which a clear path through the device is provided for minority carriers from the photon absorbing layer, while the path of its majority carriers is blocked by means of a barrier. An n-type bariode has an n-type AL and BrL, while in a p-type bariode, their polarities are reversed. [2] P. Klipstein, O. Klin, S. Grossman, N. Snapi, I. Lukomsky, M. Brumer, M. Yassen, D. Aronov, E. Berkowicz, A. Glozman, T. Fishman, O. Magen, I. Shtrichman, E. Weiss, Proceedings of SPIE 8012 (2011) 8012 2R and references therein. [3] P. Klipstein, O. Klin, S. Grossman, N. Snapi, I. Lukomsky, D. Aronov, M. Yassen, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, E. Weiss, Optical Engineering 50 (2011) 061002 and references therein. [4] M.J. Furlong, R. Martinez, S. Amirhaghi, D. Small, B. Smith, A. Mowbray, Proceedings of SPIE 8012 (2011) 801211. [5] B.-M. Nguyen, D. Hoffman, E.K.-w. Huang, S. Bogdanov, P.-Y. Delaunay, M. Razeghi, M.Z. Tidrow, Applied Physics Letters 94 (2009) 223506. [6] S. Abdollahi Pour, B.-M. Nguyen, S. Bogdanov, E.K. Huang, M. Razeghi, Applied Physics Letters 95 (2009) 173505. [7] M. Razeghi, E.K. Huang, B.-M. Nguyen, S. Abdollahi Pour, P.-Y. Delaunay, Proceedings of SPIE 7660 (2010) 7660 1F. [8] See for example M. Mehta, G. Balakrishnan, S. Huang, A. Khoshakhlagh, A. Jallipalli, P. Patel, M.N. Kutty, L.R. Dawson, D.L. Huffaker, Applied Physics Letters 89 (2006) 211110. [9] J. Tatebayashi, A. Jallipalli, M.N. Kutty, S.H. Huang, G. Balakrishnan, L.R. Dawson, D.L. Huffaker, Applied Physics Letters 91 (2007) 141102. [10] W. Dobbelaere, J. De Boeck, W. De Raedt, J. Vanhellemont, G. Zou, M. Van Hove, B. Brijs, R. Mertens, G. Borghs, MRS Symposium Proceedings 216 (1991) 181. [11] W. Dobbelaere, J. De Boeck, P. Heremans, R. Mertens, G. Borghs, W. Luyten, J. Van Landuyt, Applied Physics Letters 60 (1992) 3256. [12] C. Besikci, S. Ozer, C. Van Hoof, L. Zimmermann, J. John, P. Merken, Semiconductor Science and Technology 16 (2001) 992. [13] L. Reijnen, R. Brunton, I.R. Grant, Journal of Crystal Growth 275 (2005) e595. [14] P. Klipstein, O. Klin, S. Grossman, N. Snapi, B. Yaakobovitz, M. Brumer, I. Lukomsky, D. Aronov, M. Yassen, B. Yofis, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, E. Weiss, Proceedings of SPIE, 7660, 2010 7669 2Y. [15] P.C. Klipstein, O. Klin, S. Grossman, N. Snapi, B. Yaakobovitz, M. Brumer, I. Lukomsky, D. Aronov, M. Yassen, B. Yofis, A. Glozman, T. Fishman, E. Berkowicz, O. Magen, I. Shtrichman, Eliezer Weiss, Proceedings of SPIE, 7608, 2010 7608 1V. [16] The somewhat lower QE for the bariode grown on GaAs is most probably due to radiation absorption in the 4 mm undoped GaSb BfL in this sample.