A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom, under EP/K014471/1 (Silicon Compatible GaN Power Electronics)

Foxon, CT

Guiney, I

Houston, PA

Humphreys, CJ

Jiang, S

Lee, KB

Novikov, SV

Qian, H

Vajargah, SH

Wallis, DJ

Zaidi, ZH

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel

Qian, H.

Lee, K.B.

Vajargah, S.H.

Novikov, S.V.

Guiney, I.

Zaidi, Z.H.

Jiang, S.

Wallis, D.J.

Foxton, C.T.

Humphreys, C.J.

Houston, P.A.

White Rose Research Online

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

H. Qian

K.B. Lee

S. Hosseini Vajargah

S.V. Novikov

I. Guiney

Z.H. Zaidi

S. Jiang

D.J. Wallis

C.T. Foxon

C.J. Humphreys

P.A. Houston

Crossref

Journal of Crystal Growth

Apollo (Cambridge)

Contents lists available at ScienceDirectJournal of Crystal Growthjournal homepage: www.elsevier.com/locate/jcrysgroNovel GaN-based vertical heterostructure ﬁeld eﬀect transistor structuresusing crystallographic KOH etching and overgrowthH. Qiana,⁎, K.B. Leea, S. Hosseini Vajargahb, S.V. Novikovc, I. Guineyb, Z.H. Zaidia, S. Jianga,D.J. Wallisb, C.T. Foxonc, C.J. Humphreysb, P.A. Houstonaa Department of Electronic and Electrical Engineering, University of Sheﬃeld, Sheﬃeld S1 3JD, UKb Department of Material Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UKc School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UKA R T I C L E I N F OCommunicated by Dr. T.F. KuechKeywords:A1. Wet etchingA1. Semi-polar (11-22) GaNA3. Molecular beam epitaxyA3. Metalorganic chemical vapour depositionB1. AlGaN/GaNB3. Vertical Heterostructure Field Eﬀect Transisto (VHFET)A B S T R A C TA novel V-groove vertical heterostructure ﬁeld eﬀect transistor structure is proposed using semi-polar (11-22)GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable adamage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecularbeam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essentialrequirement for the formation of a high mobility channel.1. IntroductionLateral GaN-based heterostructure ﬁeld eﬀect transistors (HFETs)has attracted much attention for power applications due to the wideband gap (3.4 eV) and the excellent transport properties of the two-dimensional electron gas (2DEG). Devices with voltage blockingcapability up to 650 V for power switching applications are commer-cially available [1] whilst devices with more than 1000 V breakdownvoltage (Vbr) have been experimentally established [2]. However,many issues have arisen at high voltage such as surface-related currentcollapse, buﬀer induced dynamic on-resistance and threshold voltagehysteresis. Hence, reliable devices with > 1000 V blocking capabilitywill likely require vertical structures. The development of GaN verticaltransistors so far are limited to the current aperture vertical electrontransistor [3–6], its variations [7–9] and metal-oxide-semiconductor(MOS) structures [10,11] with best reported Vbr around 1.6 kV. Thefabrication of these devices involves dry-etching in the channel regionwhich can cause issues such as leakage and threshold voltage instabil-ity. M. Kodama, et. al. [12] proposed a wet etching technique to removeplasma damage in the trench in an MOS structure where an inversionlayer is induced by a gate bias. However, MOS inversion structuresgenerally suﬀer from low electron mobility and hence low transcon-ductance. To address the above mentioned concerns, we propose anovel V-groove vertical HFET (VVHFET) structure which is free of dryetching damage and retains the advantage of the 2DEG. In this paperwe describe a structure which uses semi-polar (11-22) GaN to achievethe crystallographic wet etching of a V-groove and channel regrowth.2. Proposed design conceptThe schematic VVHFET device structure is shown in Fig. 1. Thestructure is formed on a (11-22) GaN substrate which is ~58.3° fromthe (0001) c-plane. The V-shaped groove is obtained by a self-limitingcrystallographic potassium hydroxide (KOH) wet etching. A c-planesloping side wall is exposed on which the 2DEG is subsequently formedduring regrowth. The other sloping side wall is made up of twoneighbouring m-planes from the m-plane family forming a “saw tooth”proﬁle. It should be noted that the m-plane cannot form a 2DEG as it isa non-polar plane. Detailed structural analyses will be discussed in thefollowing sections. Instead of an inversion layer in MOS structures, a2DEG on the c-plane side wall forms the conduction channel whichallows a lower on-resistance and higher transconductance. The deviceoperates in the same way as in an HFET where the conductance of the2DEG channel is controlled by the gate voltage and the thresholdvoltage can be controlled by optimizing the composition and thicknessof the AlGaN barrier. The oﬀ-state source-to-drain leakage is blockedby the p-type GaN current blocking layer (CBL). Notice that the 2DEGwould be depleted if the p-type GaN is placed too close to the channel.http://dx.doi.org/10.1016/j.jcrysgro.2016.12.025Received 20 June 2016; Received in revised form 10 October 2016; Accepted 7 December 2016⁎ Corresponding author.E-mail address: hqian2@sheﬃeld.ac.uk (H. Qian).Journal of Crystal Growth 459 (2017) 185–188Available online 08 December 20160022-0248/ © 2016 Published by Elsevier B.V.This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).MARKTherefore, an unintentionally doped (UID) GaN channel is neededduring the regrowth to separate the CBL layer and the 2DEG.3. Sample preparationIn order to establish the etching process, a simpliﬁed structure wasgrown on (10-10) m-plane sapphire by metal-organic chemical vapourdeposition (MOCVD). GaN grown on sapphire typically suﬀers frompoor crystal quality due to the lattice mismatch between (11-22) GaNand m-plane sapphire substrate [13–16]. In order to improve thecrystal quality, we employed an AlN interlayer technique as describedin [17]. First the sample was pre-cleaned in the reactor in a H2ambient. Next, a pulsed nucleation layer of 25 nm GaN was grown at800 °C, followed by a continuous growth of 700 nm GaN at 1080 °C. A15 nm AlN interlayer was subsequently grown at 1080 °C, followed byapproximately 2.3 µm of GaN. The purpose of the AlN interlayer was toact as a dislocation reduction layer, as well as a getter for unintentionaloxygen incorporation on semi-polar crystals. Prior to wet etching,100 nm SiNx was deposited by plasma enhanced chemical vapourdeposition as a hard mask. The sample was patterned by standardphotolithography with the opening orientated along the [1-100] and[-1-123] axes (Fig. 2) to obtain a smooth c-plane side wall. Reactive ionetching was performed to open the windows on the SiNx for wetetching. Subsequently, the sample was etched in 4 M KOH solution at90 °C for 23 min with an etch rate of ~98 nm/min. Lastly, the SiNxmask was removed by hydroﬂuoric acid.In order to verify the feasibility of regrowth, the sample wasdegreased by standard solvent cleaning and transferred (in air) to aplasma-assisted molecular beam epitaxy (PA-MBE) chamber for re-growth. 100 nm UID GaN and 20 nm AlGaN were regrown atapproximately 750 °C with an N2 ﬂow of 1 sccm and RF power of200 W. The growth was under Ga-rich conditions with a Ga beamequivalent pressure of 2.1×10−7 Torr to ensure a smooth growth. Thesample was studied by scanning electron microscopy (SEM) andtransmission electron microscopy (TEM) analyses and presented inthe next section.4. Results and discussion4.1. KOH etching of (11-22) GaNWet etching of (11-22) GaN has been widely investigated in lightemitting diode (LED) applications and the chemical reaction mechan-ism and the resulting surface morphology are well understood in theliterature [18–22]. Due to the negligible contribution of spontaneouspolarization in semi-polar and non-polar GaN, hydroxide ions (OH-)Fig. 1. Schematic structure of the proposed VVHFET on a (11-22) GaN substrate.Fig. 2. (a) The schematic structure of a unit cell of wurtzite GaN oriented in [11–22] direction showing the crystallographic planes of the facets. The SEM images of the etched sample inthe (b) 22 µm opening region and (c) 2 µm opening region.H. Qian et al. Journal of Crystal Growth 459 (2017) 185–188186are able to access and react with surface Ga atoms following thereaction:O2GaN + 3H2 ⎯ →⎯⎯⎯ Ga2O3+2NH3KOHwhere the gallium oxide is subsequently dissolved in the KOH solution.However, the m-plane is more stable than the (11−22) plane due to theconﬁguration of the atoms where more negatively charged danglingbonds of the nitrogen atoms are presenting at the m-plane surface[21,22]. In the case of c-planes the strong negative surface polarizationrepels the negative OH- ions and the surfaces are almost immune toKOH.Fig. 2(a) illustrates the top view of a unit cell of wurtzite GaNorientated in [11–22] direction. Fig. 2(b) shows the SEM images of theetched region with an opening width of 22 µm which was used toidentify the crystal planes of the facets as revealed by the wet etching.Typical trigonal prism cells consisting of a c-plane and two m-planefacets from the m-plane family are observed in the central region. Thecrystallographic planes of the side walls can be identiﬁed from theprism-shaped features in Fig. 2(b) as well as that illustrated inFig. 2(a). In detail, a smooth c-plane (~58.3° slope) is obtained onthe left side wall. The top and bottom side walls (~62.5°) consist of c-and m-plane facets arranged in a step-like manner, whilst two m-planefacets are found on the right side wall (~31.7°).In comparison, the etching revealed a V-groove in the L-shapedregion with an opening width of ~2 µm. The side walls are in the sameconﬁguration as the 22 µm feature, but due to the reduced openingwidth, the (11-22) plane is fully masked by the slow etching c- and m-planes where an etch-stop is achieved. In this way, as the slopes of bothside walls are ﬁxed, the etch depth can be controlled by the openingwidth, which is more reliable compared to a time-based etching.However, when the sample is signiﬁcantly over-etched, the resultingfeature size can become larger than the as-patterned size due to afurther lateral etching (most likely in the m-plane) but at a signiﬁcantlyreduced etch rate compared to the (11−22) plane.4.2. MBE regrown structureMBE was chosen for the regrowth due to its relatively low growthtemperature to avoid mass transport at the high temperatures ( >1000 °C) as typically occurs in MOCVD growth [3,23]. The SEM imageafter the regrowth is shown in Fig. 3(a). In comparison to Fig. 2(b)(before regrowth), the formation of Ga droplets can be observed whichis expected from the Ga-rich growth conditions. Figs. 3(b) and (c)present high magniﬁcation SEM images of the c-plane side wall beforeand after regrowth, respectively. Small pits and ﬁssures can beobserved on the c-plane surface after regrowth which are thought toFig. 3. (a) The SEM images of etched (11-22) GaN after MBE regrowth under Ga-rich conditions. The high resolution SEM image of the c-plane sidewall (b) before regrowth, and (c)after regrowth. (d) The cross-sectional TEM image of the regrown c-plane side wall with EDS mapping showing success growth of Al0.1Ga0.9N.H. Qian et al. Journal of Crystal Growth 459 (2017) 185–188187be caused by the stress between AlGaN and GaN layers. Fig. 3(d) showsthe cross-sectional TEM image of the c-plane side wall with energydispersive X-ray spectroscopy (EDS) mapping showing the fraction ofthe Ga and Al atoms. No discernible interface is observed at theinterface between underlying GaN and regrown GaN which is anindication of the good quality achieved by wet etching. A uniformlygrown 20 nm AlGaN layer with a smooth interface with the underlyingGaN is observed which is essential for the formation of 2DEG. Thealuminium fraction of the AlGaN layer is 10% determined by EDS. Wehave calculated the statistical error in the measurement to be 2.7 at%.The thickness and Al fraction in the AlGaN layer are important incontrolling the mount of 2DEG and hence the Vth of the device. Itprovides the opportunity for enhancement-mode operation by optimiz-ing the regrowth conditions. Although the 10% AlGaN is somewhatlower than our expected value, it serves as a clear evidence of successfulregrowth of AlGaN layer. These results are very encouraging and provethe feasibility of the design.5. SummaryA novel design of VVHFET is proposed to achieve plasma damage-free vertical transistor structures. The device concept utilizes a semi-polar (11-22) GaN where the channel is formed on the c-plane side wallwith a slope of ~58° from the (11-22) plane. A self-limiting KOH wetetching technique was developed to enable the fabrication of the device.This technique may be used in other crystal orientations to obtain c-planes at diﬀerent slopes (e.g. 90° in m-/a- plane GaN), and presentother interesting vertical structure possibilities such as VMOSFETs.Finally, a channel regrowth was performed by MBE. TEM analysisrevealed a successful growth of Al0.1Ga0.9N barrier with a smoothinterface with the underlying GaN layer. The threshold voltage can becontrolled by optimizing the barrier properties. Studies on the elec-trical characteristics of the 2DEG and the realisation of a completedevice is underway.AcknowledgementThis work was funded by the Engineering and Physical SciencesResearch Council (EPSRC), United Kingdom, under EP/K014471/1(Silicon Compatible GaN Power Electronics).References[1] E.A.Jones, F.Wang, D.Costinett, Z.Zhang, B.Guo, B.Liu, R.Ren, Characterization ofan enhancement-mode 650-V GaN HFET, in: Proceedings of the 2015 IEEE EnergyConversion Congress and Exposition (ECCE), Montreal, QC, 2015, pp.400–407.[2] K.B. Lee, I. Guiney, S. Jiang, Z.H. Zaidi, H. Qian, D.J. Wallis, M.J. Uren, M. Kuball,C.J. Humphreys, Peter A. Houston, Enhancement-mode metal–insulator–semi-conductor GaN/AlInN/GaN heterostructure ﬁeld-eﬀect transistors on Si with athreshold voltage of +3.0 V and blocking voltage above 1000 V,, Appl. Phys.Express 8 (2015) 036502.[3] R. Yeluri, J. Lu, C.A. Hurni, D.A. Browne, S. Chowdhury, S. Keller, J.S. Speck,U.K. Mishra, Design, fabrication, and performance analysis of GaN vertical electrontransistors with a buried p/n junction,, Appl. Phys. Lett. 106 (2015) 183502.[4] S. Chowdhury, M.H. Wong, B.L. Swenson, U.K. Mishra, CAVET on bulk GaNsubstrates achieved with MBE-regrown AlGaN/GaN layers to suppress dispersion,,IEEE Electron Device Lett. 33 (1) (2012).[5] M.Sugimoto, H.Ueda, M.Kanechika, N.Soejima, T.Uesugi, T.Kachi, Vertical deviceoperation of AlGaN/GaN HEMTs on free-standing n-GaN substrates, in:Proceedings of the Power Conversion Conference – Nagoya, 2007. PCC '07,Nagoyapp. 2007, pp. 368–372.[6] S. Chowdhury, B.L. Swenson, M.H. Wong, U.K. Mishra, Current status and scope ofgallium nitride-based vertical transistors for high-power electronics application,,Semicond. Sci. Technol. 28 (2013) 074014.[7] I.C. Kizilyalli, O. Aktas, Characterization of vertical GaN p–n diodes and junctionﬁeld-eﬀect transistors on bulk GaN down to cryogenic temperatures,, Semicond.Sci. Technol. 30 (2015) 124001.[8] H. Nie, Q. Diduck, B. Alvarez, A.P. Edwards, B.M. Kayes, M. Zhang, G. Ye,T. Prunty, D. Bour, I.C. Kizilyalli, 1.5 kV and 2.2 m Ohm cm2 vertical GaNtransistors on bulk-GaN substrates,, IEEE Electron Device Lett. 35 (9) (2014)939–941.[9] M. Okada, Y. Saitoh, M. Yokoyama, K. Nakata, S. Yaegassi, K. Katayama, M. Ueno,M. Kiyama, T. Katsuyama, T. Nakamura, Novel vertical heterojunction ﬁeld-eﬀecttransistors with re-grown AlGaN/GaN two-dimensional electron gas channels onGaN substrates,, Appl. Phys. Express 3 (2010) 054201.[10] T. Oka, T. Ina, Y. Ueno, J. Nishii, 1.8 mΩ& cm2 vertical GaN-based trench metal–oxide–semiconductor ﬁeld-eﬀect transistors on a free-standing GaN substrate for1.2 kV-class operation,, Appl. Phys. Express 8 (2015) 054101.[11] T. Oka, Y. Ueno, T. Ina, K. Hasegawa, Vertical GaN-based trench metal oxidesemiconductor ﬁeld-eﬀect transistors on a free-standing GaN substrate withblocking voltage of 1.6 kV,, Appl. Phys. Express 7 (2014) 021002.[12] M. Kodama, M. Sugimoto, E. Hayashi, N. Soejima, O. Ishiguro, M. Kanechika,K. Itoh, H. Ueda, T. Uesugi, T. Kachi, GaN-based trench gate metal oxidesemiconductor ﬁeld-eﬀect transistor fabricated with novel wet etching,, Appl. Phys.Express 1 (2008) 021104.[13] F. Scholz, Semipolar GaN grown on foreign substrates: a review,, Semicond. Sci.Technol. 27 (2012) 024002.[14] G. Zhao, L. Wang, S. Yang, H. Li, H. Wei, D. Han, Z. Wang, Anisotropic structuraland optical properties of semi-polar (11-22) GaN grown on m-plane sapphire usingdouble AlN buﬀer layers,, Sci. Rep. 6 (2016) 20787.[15] S. Ploch, T. Wernicke, D.V. Dinh, M. Pristovsek, M. Kneissl, Surface diﬀusion andlayer morphology of (11-22) GaN grown by metal-organic vapor phase epitaxy,, J.Appl. Phys. 111 (2012) 033526.[16] T.J. Baker, B.A. Haskell, F. Wu, J.S. Speck, S. Nakamura, Characterization of planarsemipolar gallium nitride ﬁlms on sapphire substrates,, Jpn. J. Appl. Phys. 45 (6)(2006).[17] M. Pristovsek, M. Frentrup, Y. Han, C.J. Humphreys, Optimizing GaN (11-22)hetero-epitaxial templates grown on (10-10) sapphire,, Phys. Status Solidi B 253(1) (2016) 61–66.[18] D.A. Stocker, E.F. Schubert, J.M. Redwing, Crystallographic wet chemical etchingof GaN,, Appl. Phys. Lett. 73 (1998) 2654.[19] Y. Jung, K.H. Baik, M.A. Mastro, J.K. Hite, C.R. Eddy Jr., J. Kim, Chemical etchingbehaviors of semipolar (11-22) and nonpolar (11-20) gallium nitride ﬁlms,, Phys.Chem. Chem. Phys. 16 (2014) 15780.[20] S. Jung, K.R. Song, S.N. Lee, H. Kim, Wet chemical etching of semipolar GaNplanes to obtain brighter and cost-competitive light emitters,, Adv. Mater. 25(2013) 4470–4476.[21] K.H. Baik, H.Y. Song, S.M. Hwang, Y. Jung, J. Ahn, J. Kimb, Etched surfacemorphology of heteroepitaxial nonpolar (11-20) and semipolar (11-22) GaN ﬁlmsby photo enhanced chemical wet etching,, J. Electrochem. Soc. 158 (4) (2011)D196–D199.[22] D. Zhuang, J.H. Edgar, Wet etching of GaN, AlN, and SiC: a review,, Mater. Sci.Eng. R 48 (2005) 1–46.[23] S. Heikman, S. Keller, S.P. DenBaars, U.K. Mishra, Mass transport regrowth of GaNfor ohmic contacts to AlGaN/GaN,, Appl. Phys. Lett. 78 (2001) 2876.H. Qian et al. Journal of Crystal Growth 459 (2017) 185–188188

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Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

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