Development of High Performance Ultraviolet and

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Development of high performance ultraviolet and near-infrared detector technologies

Ashok K. Sood, John W. Zeller, Parminder Ghuman, Sachidananda Babu, Russell D. Dupuis, et al.

Ashok K. Sood, John W. Zeller, Parminder Ghuman, Sachidananda Babu, Russell D. Dupuis, Harry Efstathiadis, "Development of high performance ultraviolet and near-infrared detector technologies," Proc. SPIE 10766, Infrared Sensors, Devices, and Applications VIII, 1076609 (18 September 2018); doi: 10.1117/12.2323642 Event: SPIE Optical Engineering + Applications, 2018, San Diego, California, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 9/20/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Development of High Performance Ultraviolet and Near-Infrared Detector Technologies Ashok K. Sood and John W. Zeller Magnolia Optical Technologies, Inc., 52-B Cummings Park, Suite 314, Woburn, MA, USA 01801 Parminder Ghuman and Sachidananda Babu NASA Earth Science Technology Office, Greenbelt, MD, USA 20771 Russell D. Dupuis School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA 30332 Harry Efstathiadis College of Nanoscale Science and Engineering, State University of New York Polytechnic Institute, 257 Fuller Road, Albany, NY, USA 12203 ABSTRACT Electro-optical detection in ultraviolet (UV) and near-infrared (NIR) bands has distinct advantages for various applications. UV/NIR wavelengths are desired for a variety of NASA, defense and commercial applications. While UV and NIR detection technologies are governed by similar physical principles, a major differentiating factor lies in the choice of detector materials. Using the GaN/AlGaN material system, we are developing avalanche photodiodes (APDs) as discrete devices with high gains and responsivities. These devices, based on high crystalline quality metal organic chemical vapor deposition (MOCVD) growth on lattice-matched GaN substrates, demonstrate uniform and reliable distribution of breakdown voltage and leakage currents with gains of above 106. For NIR detection we have employed epitaxial layer deposition of germanium on silicon for room temperature operation. This development is focused on demonstrating very low noise performance as a result of low dislocation densities and dark currents. Both these material/device technologies can be adapted to create arrays of detectors for a variety of applications. The primary objective in developing these sensing and imaging technologies is to advance the state-of-the-art to benefit diverse UV/NIR applications for NASA, defense, and commercial applications. Keywords: Photodetectors, avalanche photodiodes, germanium, GaN, AlGaN, MOCVD, thin films, large-area wafers

1. INTRODUCTION The various wavelength bands from the ultraviolet (UV) to far into the infrared (IR) of the electromagnetic spectrum have distinct properties affecting their utility and practicality for various sensing and imaging applications. For many defense applications, spectral information beyond that which can be provided by conventional visible sensors are desired, especially where covertness is required. Dealing with shorter wavelengths in the UV spectrum is often desired to maximize spatial resolution, since this allows for various pixel size and larger formats [1-3]. UV sensing applications include NASA applications, Defense applications, chemical and biological detection of surface residues and bio-aerosol agents, machine vision, and space research [4]. Near-infrared (NIR) photodetectors and detector arrays have traditionally been based on Group III-V compound semiconductor materials such as InGaAs [5]. However, these detector devices and arrays typically require cooling (e.g., with liquid nitrogen down to 77K), increasing their size, weight, power [6]. Due in part to recent improvements in techniques for depositing epitaxial layers of pure germanium, Ge presently offers a low-cost alternative to III-V materials such as InGaAs for developing sensors and detector arrays that operate over a significant portion of the NIR spectrum [7,8]. Ge epitaxial growth processes are compatible with both front- and backend CMOS fabrication technologies, enabling very small feature sizes and heterogeneous integration with CMOS circuitry. Unlike most III-V detectors, Ge-based devices can be designed for operation at room temperature (300 K) with significantly reduced size, weight and power in relation to cooled detectors and imaging arrays. Infrared Sensors, Devices, and Applications VIII, edited by Paul D. LeVan, Priyalal Wijewarnasuriya, Arvind I. D'Souza, Proc. of SPIE Vol. 10766, 1076609 · © 2018 SPIE CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2323642 Proc. of SPIE Vol. 10766 1076609-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 9/20/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

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Figure 1. Pictorial overview of: (a) CNSE, SUNY Poly campus in Albany, NY and (b) cleanroom facility. Magnolia Optical Technologies is collocated in CNSE complex and has access to the 300 mm wafer facilities.

In comparison to these UV detection technologies, avalanche photodiodes (APDs) based on AlGaN semiconductor alloys offer the advantages of high optical gains, high sensitivity, low dark currents, and chemical and thermal stability for many demanding applications [4]. Since the operating wavelength range of AlxGa1-xN APDs can be adjusted by controlling the Al concentration in the material, they can detect photons over a large portion of the UV spectrum. In addition, development of arrays of APD pixels as small as 4 μm is achievable with this material technology [1]. Magnolia Optical Technologies has employed separate approaches for developing high-performance UV and NIR detector technologies. Ge-on-Si NIR photodetector devices have been developed on 300 mm (12”) Si wafers at fabrication facilities at the College of Nanoscale Science and Engineering (CNSE) at State University of New York Polytechnic Institute (SUNY Poly), located in Albany, NY (Figure 1). These fully-equipped 300 mm facilities with large-area Si/Ge growth/processing tools enable development of cutting-edge Ge-on-Si technology with small feature sizes for NIR sensor applications. The epitaxial growth of Ge for fabrication of room temperature operation Ge-on-Si photodetectors is undertaken at these facilities using widely installed manufacturing infrastructure.

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Figure 2. (a) High-temperature MOCVD system with close-coupled showerhead at Georgia Tech; and (b) III-nitride MOCVD growth chamber of system, open for loading wafers [10].

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To advance the technology y for UV deteection, Magnollia has collaboorated with thee Georgia Instiitute of Technology to develop GaN N/AlGaN PIN UV-APDs U andd UV-APD arraays on lattice-m matched substrrates for high-sensitivity UV V sensing and imagingg. Figure 2 sh hows the upgrraded AIXTRO ON MOCVD growth reactoor at Georgia Tech, which provides p uniformity control for grow wth of high quality q GaN annd AlGaN matterial with dopping for UV-A APD applicatioons [10]. mance, small unit u cell and pixel size GaN//AlGaN UV-A APD detectors and arrays havve been and are a being High perform developed too enable high reesolution imaging over UV bands of interesst.

2. UV GAN N/ALGAN AVALANCH A E PHOTODIODES 2.1 UV-APD D fabrication GaN/AlGaN PIN UV-APD D structures andd devices have been fabricateed on GaN subsstrates with a focus f on addresssing the technologicaal issues associiated with crysstalline defectss and crack formation. Metalorganic chem mical vapor deeposition (MOCVD) iss utilized for ep pitaxial growthh of the GaN/A AlGaN PIN UV V-APD detectoors and arrays. Growth of AllGaN for high gain UV V-APD arrays has traditionaally been restriccted to the usee of lattice-missmatched substrates such as SiC and sapphire duee to the lack off availability off native III-N substrates s [11]]. However, thhe lattice mism match and diffeerence in thermal expaansion coefficieents between such foreign suubstrates can lead to cracking and/or bowingg of material sttructures and other sttrain-induced defects, d resultiing in high leeakage current and prematurre microplasm ma breakdown prior to reaching avaalanche breakd down [12]. To T improve thhe growth proocess, low dislocation densiity n-type GaN N “freestanding” subbstrates are em mployed to succcessfully improove the crystalline and structtural quality off the epitaxial layers l by minimizing the defect den nsity. Al0.05Ga G 0.95N UV-AP PDs have alsoo been grownn on sapphire substrates to provide comparative analysis of thee influence of leakage l currentt on the perform mance and longgevity of the devices d [10,13]. Fabrication of o PIN GaN/AllGaN UV-APD Ds is initiated with w mesa form mation employiing inductivelyy coupled plasm ma (ICP) etching. Mettal stacks for contacts c are theen deposited byy electron beam m (e-beam) evaaporation and annealed a for thhe ohmic contact layerrs. The fabricated devices are a then passivvated by SiO2 using u plasma-eenhanced chem mical vapor deeposition (PECVD). The T passivation n layer is designed to reducee the leakage current c throughh the mesa siddewalls, prevennting the devices from m undergoing premature breeakdown. Finnally, additionnal metal stackks forming meetal interconneects and bonding padss are deposited d by e-beam evvaporation. Figgure 3(a) show ws a schematic cross-sectionaal view of the epitaxial layers and AlGaN A UV-APD D device struccture, and Figuure 3(b) featurees a top-view scanning s electrron microscopyy (SEM) image of the APD physical layout showinng the metal coontact pads andd circular mesa [12].

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(a) Figure 3.. (a) Device strructure cross-secction of GaN PIN P UV-APD on o bulk GaN suubstrate. (b) Topp-view SEM im mage showing contact pads and d circular mesa area a of photodiode [12].

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2.2 UV-APD D device chara acterization Atomic-forcee microscopy (AFM) ( is empployed to charaacterize the suurface propertiees of the UV-A APD arrays. Figure F 4 plots the root mean square (RMS) surfacce roughness with w various AF FM scan sizes of AlGaN struuctures grown on freestanding GaN N substrates, an nd on sapphiree substrates forr comparison [14]. The insetts of Figure 4 show s the 5×5 μm μ 2 scan surface imagges. Both types of UV-APD epitaxial strucctures had well-developed atoomic step-flow w morphologiess, and no significant deefects were obsserved on the surfaces s from the t AFM analyysis.

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Figure 5. Current density y and gain plotted vs. reverse biaas for an AlGaN UV-APD havinng mesa diameteer of 30 μm withh and U illumination.. Inset shows daark current and photocurrent p plottted over 99-1011 V reverse bias range [12]. without UV

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The dark current, photocurrent, and avalanche gain were measured for an AlGaN UV-APD with 30 μm diameter circular mesa as a function of reverse bias, which are plotted in Figure 5. The avalanche gain was calculated as the difference between the reverse-biased photocurrent and the dark current, divided by the difference between the low-bias photocurrent and the dark current: =

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Under dark conditions the UV-APD device demonstrated low leakage current of under 1 pA (corresponding to a dark current density of less than 10-7 A/cm2) up to a reverse voltage of around 60 V [12]. Above 60 V, the dark current rose sharply with increasing reverse bias, indicating active impact ionization in the multiplication region. The photocurrent remained constant up to around 60 V, and then for higher values of reverse bias rose noticeably over the dark current background (see Figure 5, inset). At the onset of avalanche breakdown (102 V) the avalanche gain for this device reached a maximum value of above 2×106, indicating a strong avalanche multiplication process. Furthermore, no microplasma breakdown or edge breakdown due to sidewall damage were found to occur, which is attributed to the low damage etching process and high-quality dielectric passivation utilized for the UV-APD devices. The reverse bias dependent spectral response of an Al0.05Ga0.95N PIN UV-APD devices was likewise characterized at room temperature. As shown in Figure 6, a 70 μm diameter AlGaN UV-APD demonstrated peak responsivity of 43.4 mA/W at 354 nm under zero bias, corresponding to an external quantum efficiency (EQE) of ~16% [15]. However, at a reverse bias of 80 V closer to the breakdown voltage, the peak responsivity increased to 221.8 mA/W at 362 nm, corresponding to an EQE of ~94%. The AlGaN device also exhibited an absorption cutoff wavelength of 370 nm at zero-bias, about 10 nm shorter than that of similar GaN PIN UV-APDs.

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3. NIR GE-ON-SI PIN PHOTODETECTORS 3.1 NIR detector fabrication Working with Ge-on-Si growth presents exciting opportunities for the development of high-performance vis-NIR capable detector devices. Incorporating Ge into the CMOS growth process allows the detector operating wavelength to be extended into the NIR while taking advantage of Si-based fabrication technology and tools [16]. Our next goal is to further demonstrate and improve the capability of growing Ge-on-Si detector devices and arrays on large-area Si wafers to benefit defense and commercial applications.

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Figuree 7. Cross-sectio onal STEM imagge of vis-NIR dettector device layyer structure, shoowing portion off Cu contact [17]].

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F Figure 8. Sequenttial flowchart suummarizing and illustrating Ge-oon-Si photodetecctor fabrication process p [18].

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For fabrication of the NIR Ge-on-Si PIN photodetectors, we have chosen a two-step growth method that reduces dark/leakage current through deposition of high crystal quality, low dislocation density epitaxial layers forming the PIN detector devices. This two-step growth process involves initial low temperature epitaxial growth of boron-doped p-Ge to form a thin strain-relaxed seed/buffer layer, and successive high temperature growth to deposit a thicker intrinsic Ge absorbing film, as seen in the scanning transmission electron microscopy (STEM) image in Figure 7 [17]. This growth process is intended to provide reduced surface roughness and thus lower dark current performance. The Ge depositions were performed using a 300 mm reduced-pressure chemical vapor deposition (RPCVD) system with germane as the precursor and hydrogen as the carrier gas. Following the high temperature growth steps, annealing at 600°C was performed [16]. Doped n+ regions were formed by ion implantation of phosphorus. A layer of oxide (SiO2) was then deposited over the detector surfaces to isolate states at the layer interface from the signal carrying layers and prevent communication between the interface states and the intrinsic Ge layer, as well as to further reduce traps contributing to leakage current [5,19]. Windows were then opened in the oxide for the top metal contacts [20]. A flowchart and pictorial summarization of the fabrication process is provided in Figure 8 [18]. 3.2 NIR detector electrical characterization Electrical testing of the opto-electronic characteristics of the fabricated photodetector devices was performed using a probe station, Keithley 2400 Source Meter, and fiber-coupled broadband tungsten-halogen source emitting primarily over the 1000-1700 nm NIR spectrum. Figure 9 shows the plotted current-voltage (I-V) dark current and photocurrent plotted measurement data for a device with an n-region doping level of ~1019 cm-3 [21]. At -1 V, the measured dark current was 0.93 μA. The photocurrent at -1 V for this device was 36.6 μA, corresponding to a photocurrent to dark current ratio of about 40. The zero-bias photocurrent was 35.6 μA, showing only a 20% drop in photocurrent from -4 V to 0 V bias, which is attributed to a strong built-in electric field in the intrinsic region of the detector device. This detector device also exhibited a forward-to-reverse (dark) current ratio of 103 at ±1 V. 1.E-02 Dark Current 1.E-03 Photocurrent

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4. SUMMARY AND CONCLUSIONS The development of GaN/AlGaN UV-APDs and APD arrays on GaN substrates with low dislocation densities for UV detection addresses the technological issues associated with crystalline defects and crack formation in UV-APDs that have traditionally been detrimental to device performance and reliability. MOCVD epitaxial growth of AlGaN UVAPDs on native GaN substrates was utilized to suppresses microplasma breakdown, providing low dark current densities with comparatively high photocurrents and responsivities over portions of the UV spectrum. These improvements in GaN/AlGaN APD performance and reliability are key towards the development and successful implementation of robust, highly sensitive, high gain UV-APD detector arrays that can benefit future advanced defense systems and other NASA and Defense applications. The high gain GaN AlGaN detectors and arrays developed by Magnolia with Georgia Tech are advancing the state-of-the-art and provide enhanced capabilities for UV detection. For detection of NIR wavelengths, the room temperature operation Ge-on-Si photodetectors developed by Magnolia and fabricated on 300 mm (12”) Si wafers derive benefits from CMOS processing tools and technology. Photodetectors capable of room temperature operation were epitaxially grown using CMOS-compatible fabrication techniques. The two-step low/high temperature fabrication process utilized yielded high-quality material growth for improved NIR detection performance. High crystalline quality of the devices was confirmed through various structural characterization methods, including scanning transmission electron microscopy (STEM).

ACKNOWLEDGEMENTS This research is and has been funded by the National Aeronautics and Space Administration (NASA), Contract No. 80NSSC18C0093, and the Defense Advanced Research Projects Agency (DARPA). The views, opinions, and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

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[9] Choi, S., Kim, H. J., Zhang, Y., Bai, X., Yoo, D., Limb, J., Ryou, J.-H., Shen, S.-C., Yoder, P. D., and Dupuis, R. D., “Geiger-mode operation of GaN avalanche photodiodes grown on GaN substrates,” IEEE Photon. Tech. Lett. 21(20), 1526-1528 (2009). [10] Sood, A. K., Zeller, J. W., Puri, Y. R., Dupuis, R. D., Detchprohm, T., Ji, M.-H., Shen, S.-C., Babu, S., Dhar, N. K., and Wijewarnasuriya, P., “Development of high gain GaN/AlGaN avalanche photodiode arrays for UV detection and imaging applications,” Int. J. Engr. Res. Tech. 10(2), 129-150 (2017). [11] Zhou, Q., McIntosh, D. C., Lu, Z., Campbell, J. C., Sampath, A. V., Shen, H., and Wraback, M., “GaN/SiC avalanche photodiodes,” Appl. Phys. Lett. 99, 131110 (2011). [12] Kim, J., Ji, M.-H., Detchprohm, T., Ryou, J.-H, Dupuis, R. D., Sood, A. K., and Dhar, N. K., “AlxGa1-xN ultraviolet avalanche photodiodes with avalanche gain greater than 105,” IEEE Photon. Tech. Lett. 27(6), 642-645 (2015). [13] Zhang, Y., Shen, S.-C., Kim, H. J., Choi, S., Ryou, J.-H., Dupuis, R. D., and Narayan, B., “Low-noise GaN ultraviolet p-i-n photodiodes on GaN substrates,” Appl. Phys. Lett. 94, 10309 (2009). [14] Kim, J., Ji, M.-H., Detchprohm, T., Dupuis, R. D., Ryou, J.-H., Sood, A. K., Dhar, N. K., and Lewis, J., “Comparison of AlGaN p-i-n ultraviolet avalanche photodiodes grown on free-standing GaN and sapphire substrates,” Appl. Phys. Expr. 8(12), 122202 (2015). [15] Sood, A. K., Zeller, J. W., Welser, R. E., Puri, Y. R., Ji, M.-H, Kim, J., Detchprohm, T., Dupuis, R. D., Dhar, N. K., and Wijewarnasuriya, P., “Development of high gain avalanche photodiodes for UV imaging applications,” Proc. SPIE 9609, 96090X (2015). [16] Sood, A. K., Richwine, R. A., Puri, Y. R., Olubuyide, O. O., DiLello, N., Hoyt, J. L., Akinwande, T. I., Balcerak, R. S., Horn, S., Bramhall, T. G., and Radack, D. J., “Design considerations for SiGe-based near-infrared imaging sensor,” Proc. SPIE 6940, 69400M (2008). [17] Zeller, J. W., Rouse, C., Efstathiadis, H., Haldar, P., Dhar, N. K., Lewis, J. S., Wijewarnasuriya, P., Puri, Y. R., and Sood, A. K., “Design and development of wafer-level near-infrared micro-camera,” Proc. SPIE 9609, 96090O (2015). [18] Zeller, J. W., Rouse, C., Efstathiadis, H., Haldar, P., Lewis, J. S., Dhar, N. K., Wijewarnasuriya, P., Puri, Y. R., and Sood, A. K., “Development of silicon-germanium visible-near infrared arrays,” Proc. SPIE 9854, 985408 (2016). [19] Michel, J., Liu, J., and Kimerling, J. C., “High-performance Ge-on-Si photodetectors,” Nature Photon. 4(8), pp. 527-534 (2010). [20] Sood, A. K., Richwine, R. A., Puri, Y. R., DiLello, N., Hoyt, J. L., Dhar, N., Balcerak, R. S., and Bramhall, T. G., “Development of SiGe arrays for visible-near IR imaging applications,” Proc. SPIE 7780, 77800F (2010). [21] Zeller, J. W., Rouse, C., Efstathiadis, H., Dhar, N. K., Wijewarnasuriya, P., and Sood, A. K., “Germanium photodetectors fabricated on 300 mm silicon wafers for near-infrared focal plane arrays,” Proc. SPIE 10404, 104040H (2017).

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