Large-Area Snse2/Gan Heterojunction Diodes Grown by Molecular Beam Epitaxy

Large-area SnSe2/GaN heterojunction diodes grown by molecular beam epitaxy

Cite as: Appl. Phys. Lett. 111, 202101 (2017); https://doi.org/10.1063/1.4994582 Submitted: 05 July 2017 . Accepted: 23 October 2017 . Published Online: 13 November 2017

Choong Hee Lee , Sriram Krishnamoorthy , Pran K. Paul, Dante J. O'Hara, Mark R. Brenner, Roland K. Kawakami, Aaron R. Arehart , and Siddharth Rajan

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Appl. Phys. Lett. 111, 202101 (2017); https://doi.org/10.1063/1.4994582 111, 202101

Large-area SnSe2/GaN heterojunction diodes grown by molecular beam epitaxy Choong Hee Lee,1 Sriram Krishnamoorthy,2 Pran K. Paul,1 Dante J. O’Hara,3 Mark R. Brenner,1 Roland K. Kawakami,3,4 Aaron R. Arehart,1 and Siddharth Rajan1 1Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA 2Department of Electrical and Computer Engineering, The University of Utah, Salt Lake City, Utah 84112, USA 3Program of Materials Science and Engineering, University of California, Riverside, California 92521, USA 4Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA (Received 5 July 2017; accepted 23 October 2017; published online 13 November 2017) We report on the synthesis and properties of wafer-scale two-dimensional/three-dimensional (2D/3D) n-SnSe2/n-GaN(0001) heterojunctions. The hexagonal crystal structure of crystalline SnSe2 grown by molecular beam epitaxy was confirmed via in-situ reflection high-energy electron diffraction and off-axis X-ray diffraction. Current-voltage (I-V) measurements of SnSe2/GaN diodes exhibited 9 orders of magnitude rectification, and the SnSe2/GaN heterojunction barrier height was estimated to be 1 eV using capacitance-voltage measurements and internal photoemission measurements. Vertical electronic transport analyzed using temperature-dependent I-V measurements indicates thermionic field emission transport across the junction. This work demonstrates the potential of epitaxial growth of large area high quality 2D crystals on 3D bulk semiconductors for device applications involving carrier injection across 2D/3D heterojunctions. Published by AIP Publishing. https://doi.org/10.1063/1.4994582

Heterogeneous integration of two-dimensional (2D) and been exploited to form type-III heterojunctions with black 27 18 three-dimensional (3D) materials could enable device architec- phosphorus and WSe2. tures that are not possible for conventional semiconductor het- In this paper, we report on the growth and electronic erojunctions. The absence of out-of-plane chemical bonds in properties of SnSe2/GaN heterojunctions. The combination 2D layered materials enables flexibility for epitaxy of 3D mate- of such a high electron affinity low bandgap material such as 1,2 rials, and can therefore enable combinations of materials for SnSe2 with a wide bandgap material such as GaN presents a devices such as heterojunction bipolar transistors (HBTs), verti- unique heterojunction combination that is not possible with cal tunneling devices,3 and hot electron transistors.4 the III-Nitride system alone. While the bandgap of InGaN The synthesis of 2D/3D heterojunctions has been investi- can be tuned to be as low as 1 eV, lattice mismatch between gated extensively using mechanically exfoliated 2D crystals InN and GaN (11%) makes it very challenging to grow high transferredontobulkcrystals5–8 and wafer-scale chemical composition InGaN on GaN. 9,10 11–13 vapor transport or chemical vapor deposition growth of The epitaxial growth of SnSe2 on GaN was performed in 2D materials on epitaxial templates and molecular beam epi- a Veeco GEN930 MBE system with a standard thermal effu- taxy (MBE). The method used in this work, MBE, offers some sion cell for Ga and Sn. A valved cracker source (with the distinct advantages due to the ability to realize sharp interfaces, cracker zone at 950 C) was used to evaporate Se. The sam- excellent control of background impurities, and powerful in ple surfaces were monitored in-situ by reflection high-energy situ characterization techniques.14,15 Previous work on MBE electron diffraction (RHEED) operated at 15 keV. The struc- growth of metal dichalcogenides (MoSe2,HfSe2,WSe2,and tural quality of the SnSe2 films was evaluated through X-ray SnSe2) on 3D substrates has shown epitaxial registry between diffractometry (XRD) (Bruker, D8 Discover) and Raman the 2D material and 3D bulk substrates.15–18 spectroscopy (Renishaw) equipped with a 514 nm laser. The To date, band lineups for various heterojunctions thickness of the SnSe2 film was measured by X-ray reflec- between 2D and 3D materials have been proposed. For tometry (XRR) (Bruker, D8 Discover). Atomic force micros- instance, type-I band alignment was demonstrated in copy (AFM) (Bruker Icon 3) was used to examine the 19 20 21,22 28 n-MoS2/p-Si, p-MoS2/n-SiC, and p-MoS2/n-GaN. surface morphology of the film. VESTA software was used Unlike transition metal dichalcogenides, Sn has two oxida- to generate graphical illustrations of the SnSe2 crystal tion states (Sn2þ and Sn4þ) which give two stoichiometric structure. phases, SnSe and SnSe2. SnSe is an orthorhombic layered Semi-insulating and n-type (0001) oriented GaN/sap- 23 24 structure with p-type conductivity, while SnSe2 is intrin- phire substrates were used for the study. Pre-growth surface sically an n-type semiconductor25 and is known to have two preparation included solvent cleaning followed by a 1 h 6h crystal structures. One is the 2 H phase with D (P63/mmc) 400 C anneal under ultra-high vacuum conditions symmetry and the other is the 1 T phase with D3d (P3m1) (1 109 Torr). Samples were then loaded into the growth 10 symmetry. The bulk 1 T phase of SnSe2 has been reported to chamber (base pressure 7 10 Torr) and exposed to the have a direct energy bandgap of 1 eV (Refs. 25 and 26) with Ga polish procedure to remove gallium sub-oxides on the an electron affinity of 5 eV.18 This high electron affinity has GaN surface prior to the growth. The procedure used is as

0003-6951/2017/111(20)/202101/5/$30.00 111, 202101-1 Published by AIP Publishing. 202101-2 Lee et al. Appl. Phys. Lett. 111, 202101 (2017)

FIG. 1. (a)–(d) RHEED patterns of SnSe2 and GaN along the [1120] and [1010] azimuthal directions. (e) XRD spectra of SnSe2 on the GaN/sapphire substrate exhibiting the (001) family of diffraction peaks. (f) XRD u scan of GaN (103) and SnSe2 (101) planes confirms the basal plane alignment. follows. The GaN surface was exposed to a Ga flux of lm2 region. As shown in Fig. 2(b), two characteristic Raman 8 1 10 Torr at 400 C until the RHEED intensity active modes for SnSe2 at 112 (in-plane mode, Eg) and 1 dropped. The substrates were then heated to 700 C for 186.27 (out-of-plane mode, A1g)cm are present in the 30 min to recover the GaN RHEED pattern, followed by a spectrum, which corresponds to 1 T phase SnSe2 as reported ramp down to the growth temperature of 210 C. The sub- for the MBE grown18 and exfoliated bulk film.7,29 The corre- strate temperature was measured using a thermocouple sponding 1 T SnSe2 crystal structure is shown in the inset of attached to the continuous azimuthal rotation (CAR) sub- (b). The asterisk indicates the Raman modes at 419 and 1 3 strate heater. 570 cm for sapphire (A1g) and GaN (E2), respectively. For growth of SnSe2, the Se:Sn beam equivalent pressure For electrical characterization of the SnSe2/GaN het- (BEP) flux ratio (measured using a nude ion gauge with a erojunction, Ti/Au/Ni contacts were evaporated using tungsten filament) was maintained at 250. The surface was e-beam evaporation to form ohmic contacts to the SnSe2. covered with Se by opening the Se shutter for two minutes. The contact to the n-GaN layer was formed by an indium Growth was then initiated by opening the Sn shutter. This dot. Inductively coupled plasma reactive ion etching (ICP- procedure is qualitatively similar to that described previously RIE) with BCl3/Ar chemistry was used for the device for the growth of GaSe on GaN.14 Growth was carried out for mesa isolation ð14 14 lm2Þ.Hallmeasurementson 1 hour and terminated by closing all shutters and immediately SnSe2 films on semi-insulating GaN substrates were found cooling down the sample to room temperature. to exhibit n-type conductivity with a carrier concentration Figures 1(a)–1(d) show the RHEED patterns of the GaN of 1:3 1019 cm3 and an electron mobility of 4.7 cm2 1 1 substrate before growth and the SnSe2 film after growth was V s . completed, along the [1120] and [1010] directions. The To investigate vertical transport, n-type SnSe2 films streaky RHEED patterns observed in both azimuthal orienta- were grown on the MBE-grown n-GaN (100 nm–1018 cm3 tions indicate 2-dimensional growth with azimuthally Si-doped) layer on n-GaN/Sapphire substrates. The struc- aligned to the GaN substrate ([1120] SnSe2//[1120] GaN and tural and surface characteristics of these films were similar [1010] SnSe2//[1010] GaN). The RHEED spacing for the to the films (described earlier in this work) on insulating GaN and SnSe2 patterns was found to have a ratio of 0.85, substrates. Vertical current-voltage characteristics of the which matches the experimentally expected ratio (0.848) n-SnSe2/n-GaN isotype heterojunction diode were mea- ˚ from bulk in-plane lattice constants of SnSe2 (a ¼ 3.76 A) sured by applying a bias to SnSe2 with respect to GaN [Fig. and GaN (a ¼ 3.189 A).˚ We conclude that the hexagonal 3(a)]. The I-V characteristics at room temperature showed basal plane lattice for SnSe2 and GaN materials is aligned 9 orders of magnitude rectification at 6 1 V and exhibited along the same crystallographic direction despite their large an ideality factor of 1.1 in the linear region of the semi-log lattice mismatch of 18%. forward bias curve. An optical microscopy image of the Figure 1(e) shows the on-axis (001)-oriented high reso- lution XRD spectrum of the SnSe2/GaN structure, and the SnSe2 peaks are found to be at the theoretically expected positions. No other phases were observed in the scan. The thickness of the crystalline SnSe2 film was determined by X-ray reflection measurements to be 21 nm. Off-axis azimuthal scan [Fig. 1(f)] was done using a thick (>200 nm) SnSe2 film. A full range of 360 scans (u) for the SnSe2 (101) plane and GaN (102) plane were done, and six peaks were found at the identical azimuth angles for both SnSe2 and GaN, confirming that the two hexagonal unit cells are epitaxially aligned. This is in agreement with our conclusion from RHEED measurements. FIG. 2. (a) 2 lm 2 lm atomic force microscopy image of SnSe after Figure 2(a) shows the surface morphology of SnSe . 2 2 growth with the RMS roughness of 0.99 nm. (b) Raman spectra of SnSe2 on The RMS roughness was calculated to be 0.99 nm for a 4 the GaN/sapphire substrate with characteristic Eg and A1g peaks. 202101-3 Lee et al. Appl. Phys. Lett. 111, 202101 (2017)

FIG. 3. (a) The I-V characteristic of the SnSe2/GaN junction shows 9 orders of magnitude rectiﬁcation and an ideality factor of 1.1. The inset of (a) shows the optical microscopy image of the devices with different dimensions. (b) C-V characteristic of 2 the SnSe2/GaN diode. The 1/C is linear with respect to voltage and 0.99 V of built-in potential was extracted.

SnSe2/GaN diodes with metal contacts is shown in the inset ðE00Þ can be used whether the transport is governed by of Fig. 3(a). Multiple devices were measured under the thermionic emission (TE), thermionic field-emission (TFE), same conditions and showed identical I-V characteristics, or field-emission (FE), where E00 is defined from material indicating uniform electrical characteristics of large-area parameters as32,33 SnSe2/GaN diodes (see supplementary material). 2 Capacitance-voltage (C-V) measurements showed typical qh ND;GaN E00 ¼ ; C-V characteristics of a reverse-biased Schottky diode [Fig. 2 eGaNe0mGaNm0 3(b)]. Since the SnSe2 unintentional doping is significantly higher than the GaN doping density, the depletion region lies where mGaN is the electron effective mass for GaN, eGaN the almost entirely in the GaN. Therefore, the relationship dielectric constant of GaN, e0 the vacuum permittivity, m0 between capacitance and voltage can be approximated as the electron rest mass, ND;GaN the donor concentration of 2 1=C 2=ðqNDeGaNe0ÞðV Vbi kT=qÞ,whereq is the GaN, and q the electronic charge. The calculated value of electron charge, k is the Boltzmann constant, T is the tempera- E00 is 19.1 meV which is comparable to kT. Hence, the ture, ND is the doping density in GaN, eGaN ¼ 9:7e0 is the thermionic field emission (TFE) model was considered for dielectric constant of GaN, and V is the reverse-bias voltage.30 the current transport mechanism. The current in TFE in the 34,35 The capacitance data yield a linear 1/C2 dependence on the forward bias region is expressed as voltage [inset of Fig. 3(b)], and the extracted built-in voltage pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 AA T qpE00ðÞ/B V þ Vn of the SnSe2/GaN heterojunction was 0.98 6 0:02 V. A dop- I ¼ 18 TFE ing concentration of 2:1 10 for GaN was extracted from E00 kTcosh kT the measurement (see supplementary material). The conduc- qV qðÞ/ þ V qV tion band offset was determined using the expression, exp n B n exp 1 ; qDE ¼ qV þ E E E E ,whereE and kT E0 gkT c bi ðÞc F GaN ðÞc F SnSe2 c EF are the conduction band edge and Fermi level, and the where Vn is ðÞEC;GaN EF;GaN =q, /B is the junction barrier Fermi level positions in GaN and SnSe2 were estimated using the Joyce-Dixon approximation.31 The extracted heterojunc- height, A is the diode area, and A is the Richardson constant (26.4 A/cm2 K2 for GaN), tion barrier height was estimated to be 1 eV. SnSe2/GaN diodes were characterized using current- E E ¼ E coth 00 ; voltage-temperature (I-V-T) measurements in the tempera- 0 00 kT ture range of 100–400 K in steps of 50 K [Fig. 4(a)]. To determine the conduction mechanism, characteristic energy and g is the ideality factor,

FIG. 4. (a) Temperature dependent I-V characteristics of the SnSe2/GaN diode. (c) Measured ideality factor as a function of temperature. g exhibits a 15.8 meV characteristic energy. (d) Forward-bias I-V-T data plotted with fits of the theoretical thermionic field-emission (TFE) I-V relationship. A characteristic energy of 15.8 meV and a barrier height of 0.84 V best fit the data over the temperature ranging from 100 K to 400 K. 202101-4 Lee et al. Appl. Phys. Lett. 111, 202101 (2017)

FIG. 5. (a) Internal photoemission (IPE) measurement results of the SnSe2/GaN diode at 300 K. The linear ﬁt corresponds to a barrier height of 1.03 eV. (b) Band diagram of SnSe2/ GaN with the 1 eV barrier. The Fermi level (orange dots) is shown at zero energy.

E E E rectification with a built-in voltage of 1 V. The I-V-T and g ¼ 0 ¼ 00 coth 00 : kT kT kT IPE measurements were used to measure the heterojunction barrier height, which is found to be approximately 1 eV. The The values of the ideality factor (gÞ of the diode at different transport mechanism was identified to be thermionic field temperatures were extracted from the slope of the linear por- emission. A type-I heterojunction band line-up is inferred tion of lnI versus V plot (see supplementary material) and are from electrical and optical measurements. Despite the large plotted in Fig. 4(b). g closely follows the TFE curves for an lattice mismatch, it is shown that the heterojunction between E00 of 15.8 meV, which is close to the theoretical value cal- the 2D material and the 3D bulk is well-explained using con- culated above. The measured E00 was used to fit the I-V-T ventional 3D/3D heterojunction transport theory. This dem- data using the TFE model yielding the best fit with the bar- onstration of the 2D/3D heterojunction demonstrates the rier height of 0.84 eV, shown in Fig. 4(d). In addition, the potential for 2D/3D heterojunctions for high performance barrier height lowering for TFE is estimated to be 0.08 eV device applications. using the expression,36 See supplementary material for I-V data for multiple 3 2=3 D/ ¼ E2=3V1=3; GaN/SnSe2 diodes and extracted doping density profile of B 2 00 bi GaN layer. where Vbi is the built-in potential. Therefore, taking into This research was supported by the Office of Naval account the barrier lowering, the effective barrier height is Research (Dr. Paul A. Maki) Devices, Architectures for 0.92 eV which is close to the value obtained from the C-V Terahertz Electronics (DATE) project, National Science measurement. Foundation Major Research Initiative (NSF DMR-423 The internal photoemission (IPE) measurement was also 1429143), The Ohio State University Materials Research used to determine the heterojunction barrier height. Figure Seed Grant Program, and Northrop Grumman Aerospace 5(a) shows measured photocurrent with an electrometer as a Systems (NGNEXT). function of photon energy. Zero-bias was applied to the junction with an incident photo energy varying from 0.5 to 1.6 eV. The photo-yield was extracted from the quadratic 1A. Koma, K. Sunouchi, and T. Miyajima, Microelectron. Eng. 2(1), 37 dependence on incident photon energy, and the extracted 129–136 (1984). 2A. Koma, J. Cryst. Growth 201–202, 236–241 (1999). barrier height was found to be 1.03 eV with 620 meV of 3 error (0.987 of R-squared). For comparison, our estimate of S. Krishnamoorthy, E. W. Lee, C. H. Lee, Y. Zhang, W. D. McCulloch, J. M. Johnson, J. Hwang, Y. Wu, and S. Rajan, Appl. Phys. Lett. 109(18), DEc from C-V measurements and I-V measurements was 183505 (2016). 0.98 6 0:02 eV and 0.92 eV, respectively. 4S. Vaziri, G. Lupina, C. Henkel, A. D. Smith, M. Ostling,€ J. Dabrowski, G. Lippert, W. Mehr, and M. C. Lemme, Nano Lett. 13(4), 1435–1439 Figure 5(b) shows the band diagram of SnSe2/GaN with (2013). a heterojunction barrier height of 1 eV based on electrical 5B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, and optical measurements. The heterojunction barrier height Nature Nanotechnol. 6(3), 147–150 (2011). is close to the value expected from the electron affinity dif- 6B. Radisavljevic, M. B. Whitwick, and A. Kis, Appl. Phys. Lett. 101(4), 18 043103 (2012). ference between SnSe2 and GaN estimated. The characteri- 7T. Pei, L. Bao, G. Wang, R. Ma, H. Yang, J. Li, C. Gu, S. Pantelides, S. zation of this 2D/3D heterojunction shows the potential for Du, and H.-j. Gao, Appl. Phys. Lett. 108(5), 053506 (2016). heterogeneous integration of narrow gap and wide gap mate- 8D. Ovchinnikov, A. Allain, Y.-S. Huang, D. Dumcenco, and A. Kis, ACS rials. The fact that the electronic transport in this 2D/3D het- Nano 8(8), 8174–8181 (2014). 9 erojunction is not dominated by interface recombination R. Vaidya, M. Dave, S. S. Patel, S. G. Patel, and A. Jani, Pramana–J. Phys. 63(3), 611–616 (2004). suggests that large-area MBE-grown 2D/3D heterojunctions 10M. Dave, R. Vaidya, S. Patel, and A. Jani, Bull. Mater. Sci. 27(2), could be utilized for vertical device applications. 213–216 (2004). In summary, we have demonstrated large area hetero- 11L. Ma, D. N. Nath, E. W. Lee II, C. H. Lee, M. Yu, A. Arehart, S. Rajan, junctions of layered-SnSe on GaN using MBE. We charac- and Y. Wu, Appl. Phys. Lett. 105(7), 072105 (2014). 2 12M. R. Laskar, L. Ma, S. Kannappan, P. S. Park, S. Krishnamoorthy, D. N. terized the isotype heterojunction between n-SnSe2 and Nath, W. Lu, Y. Wu, and S. Rajan, Appl. Phys. Lett. 102(25), 252108 n-GaN with I-V and C-V measurements showing 9 orders of (2013). 202101-5 Lee et al. Appl. Phys. Lett. 111, 202101 (2017)

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