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sapphire
Optical Materials xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical properties of plasma-assisted molecular beam epitaxy grown InN/sapphire Devki N. Talwar a,⇑, Ying Chieh Liao b, Li Chyong Chen c, Kuei Hsien Chen d, Zhe Chuan Feng b a
Department of Physics, Indiana University of Pennsylvania, 975 Oakland Avenue, 56 Weyandt Hall, Indiana, PA 15705-1087, USA Institute of Photonics and Optoelectronics, Department of Electrical Engineering, and Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 106-17, Taiwan, ROC c Center for Condensed Matter Sciences, National Taiwan University, Taipei 106-17, Taiwan, ROC d Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 13 November 2013 Accepted 14 April 2014 Available online xxxx Keywords: Photoluminescence Raman scattering IR spectroscopy PA-MBE InN/Sapphire
a b s t r a c t The optical properties of as-grown InN/sapphire films prepared by plasma assisted molecular beam epitaxy (PA-MBE) are characterized by photoluminescence (PL), Raman scattering (RS) and infrared (IR) reflectance techniques. The PL measurements have consistently exhibited lower values of InN band gaps providing clear indications of electron concentration dependent peak energy shifts and widths. The phonon modes identified by RS are found to be in good agreement with the grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. An effective medium theory used to analyze IR reflectance spectra of InN/sapphire films has provided reasonable estimates of free charge carrier concentrations. Published by Elsevier B.V.
1. Introduction Among the conventional group III-nitrides (InN, GaN, AlN), InN has been the least investigated compound semiconductor [1–5]. At the same time, it is perceived as one of the most intriguing materials with superior electronic, optical and transport properties. The lowest effective mass, largest electron saturation velocity, and highest mobility of InN have made it suitable for engineering high-efficiency solar cells, field-effect transistors and highperformance optoelectronic devices capable of operating at the optical-communication wavelength (i.e., k = 1.55 lm) [1,2]. Earlier absorption studies [6] on sputtered InN films with high electron concentration (>1019 cm3) and low electron mobility (<100 cm2/Vs) revealed larger band-gaps Eg 1.8–2 eV with no corresponding band-edge photoluminescence (PL). Recent measurements on InN epilayers grown by molecular beam epitaxy (MBE) provided plausible evidence of lower band-gaps [7–9] i.e., Eg < 1 eV. Subsequent studies on InN films prepared by MOCVD have also ascertained narrow band-gaps – rendering PL emission between 0.6 and 0.8 eV in sharp contrast to the higher Eg values cited before [6]. Although not intentionally doped, the electron charge density in as-grown MOCVD/MBE samples is found higher (g 1018–
⇑ Corresponding author. Tel.: +1 7243572371; fax: +1 7243573804. E-mail addresses: [email protected] (D.N. Talwar), [email protected] (Z.C. Feng).
1019 cm3) [7–10]. The origin of large discrepancy in Eg is still being debated – the explanations given for wider band-gap on sputter-grown materials include: (a) free-electron induced Burstein-Moss shift, (b) incorporation of oxygen to form larger bandgap In2O3, and (c) quantum-size effect in the InN nanocrystals [7–10]. While the narrow band-gap samples grown by MOCVD and/or MBE techniques are insinuated to have a better crystalline quality – the cause of a large free electron density and its impact on the electronic/vibrational properties of as-grown InN material samples is not well empathized. Due to band-gap controversy, the optical behavior of InN above absorption band edge is either sparsely known and/or contradictory [11–13]. The energy positions and relative amplitudes of specific absorption features in the dielectric functions are viewed as the most important parameters for extracting accurate band and/ or phonon structures [14–16]. While the optical properties of InN are strongly dependent on the growth processes – constructing a unique model for describing the meticulous dispersion relations of its dielectric function [17] is not a trivial matter. From the complementary experimental results of PL, Raman scattering (RS) and infrared (IR) reflectance, the investigations of optical properties for InN are expected to contribute to better understanding of its electronic and vibrational characteristics. At the same time these attributes may also be used as a criterion for assessing charge carrier concentration, crystalline quality and/or surface morphology of the InN material samples.
http://dx.doi.org/10.1016/j.optmat.2014.04.012 0925-3467/Published by Elsevier B.V.
Please cite this article in press as: D.N. Talwar et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.04.012
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D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
The purpose of this paper is to report results of a systematic experimental and theoretical study to comprehend electrical and optical properties of both In (rich) and N (rich) InN/sapphire films. The samples having different charge carrier concentrations are grown on low temperature AlN buffer layers using an SVTA (SVTV-2) plasma-assisted MBE (PA-MBE) system. Radio-frequency plasma source is the key component in the growth process to provide flux stability, material uniformity and purity. AlN buffer layer is used to reduce the lattice mismatch between InN epilayers and c-sapphire substrate. The procedures adopted in the PA-MBE growth are described in details elsewhere [10]. Hall measurements in the Van der Pauw configuration revealed all of the as-grown films exhibiting n-type conductivity. The free electron concentration g ranged from 1018–1019 cm3 with corresponding electron mobility l from 1044 to 200 cm2/V s. Only the hexagonal symmetry of InN (w) films is established by X-ray diffraction (XRD) with no traces of other polymorphs. The narrow profiles of h and h2h scans at the (0002) reflex indicated good crystalline quality. Surface morphology of all InN samples is examined by field-emission scanning electron microscopy (FE-SEM, JEOL-6700). Analysis of Rutherford back scattering data helped us evaluate composition and InN film thickness. Many samples are found to have film thickness d, varying between 150 and 700 nm. The materials are characterized by exploiting the conventional PL, RS and IR (cf. Section 2) spectroscopy. Our PL measurements (cf. Section 2.1) consistently provided lower values of fundamental band gap Eg between 0.65 eV and 0.80 eV. Major Raman active modes (cf. Section 2.2), in all InN samples with diverse charge carrier concentration, are recognized having no significant changes in their vibrational spectra – re-confirming good crystalline quality of the material samples. By using multi-layer formalism within an effective medium theory [18–19] we have analyzed the IR reflectance spectra (cf. Section 2.3) of InN/sapphire films to estimate the free charge carrier concentration. The results are compared and discussed with the existing data with concluding remarks presented in Section 3.
(a)
λ
(b)
λ
Fig. 1. (a) Room temperature PL spectra for two PA-MBE grown InN/sapphire Inrich (CD-1) and N-rich (CD-4) samples. (b) Temperature dependent PL spectra for sample CD-1.
2. Experimental results and discussions 2.1. Photoluminescence In Table 1, we have reported salient physical parameters for two sets of PA-MBE grown InN samples to appraise their electronic and vibrational properties. Fig. 1 shows PL spectra of CD-1 (In-rich) and CD-4 (N-rich) samples by using a Jobin-Yvon TRIAX-320 system equipped with a grating blazed at 2 lm. The measurements are performed by exciting a frequency-doubled Nd+-TAG laser (k = 532 nm) at room temperature (RT) with a spot diameter of 0.5–3 mm. A lead-sulfide (PbS) detector is employed for detecting luminescence. The PL spectra shown in Fig. 1a exhibited asymmetric line shapes with peaks centered near 0.65 eV for In-rich and near 0.80 eV for N-rich sample, respectively. Possible reasons, for blue-shift of PL peak and broader (33 meV) width in CD-4 are conjectured to Moss–Burstein effect related to the variation of charge carrier g, stoichiometry, and/or strain. Fig. 1b also reveals the decrease in PL peak intensity, broadening and slight red-shift with increase of temperature, which is attributed to lat-
tice expansion and electron–phonon interaction. The observed Eg (T) or PL energy shift is very well described by Pässler’s [20] theoretical methodology rather than Varshni’s [21] empirical approach. 2.2. Raman scattering In Fig. 2 we have displayed RT first-order RS spectra of phonons in the backscattering geometry ½zðx; yÞz] for three InN/sapphire samples (CC-21, CC-22 and CC-24) grown with 230, 300 and 350 W plasma power, respectively. The probing laser light of wavelength 532 nm is incident normal to the film surfaces. The pointgroup symmetry, for a polar wurtzite crystal with four atoms per unit cell, belongs to C 46v . Group-theoretical analyses have indicated A1, E1, 2E2 and 2B1 symmetry modes to be optically allowed. Except for the silent B1 mode, all other (A1, E1, and E2) phonons are Raman active. Due to macroscopic field associated with LO modes in InN, the polar A1 and E1 phonons split-up into longitudinal optical (LO)
Table 1 Summary of physical parameters of some PA-MBE grown InN/sapphire films. Sample
Plasma power (W)
Substrate temperature (°C)
Carrier concentration (cm3)
Mobility (cm2/Vs)
CC-21 CC-22 CC-24 CD-1 CD-4
350 300 230 80 90
460 460 460 460 460
9.488 7.014 5.679 1.534 2.180
251.2 225.1 386.7 938 887.7
E + 19 E + 19 E + 19 E + 19 E + 19
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D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
ω η ω η ω η
λ
ω
Fig. 2. Room temperature RS spectra for three PA-MBE grown InN/sapphire CC-21, CC-22 and CC-24 samples grown with different plasma power (cf. Table 1).
and transverse optical (TO) components. In RS one expects a total of six vibrational modes e.g., A1 (LO), A1 (TO), E1 (LO), E1 (TO), Ehigh and Elow 2 . 2 Our Raman study has identified (cf. Fig. 2) three strong phonon features corresponding to A1 (LO) 590 cm1; Ehigh 490 cm1; and 2 low 1 E2 mode 88 cm . In addition, the measurements in backscattering geometry have uncovered several extra phonon features (shown by vertical arrows in Fig. 2) near 160, 202, 410, 446, 475, and 562 cm1. The perusal of Fig. 2 reveals that the main A1 (LO) mode exhibits a relatively large FWHM 30 cm1and its frequency spans over a wide-range (i.e., 575–610 cm1). After careful analysis, the observed broad band is interpreted as a combination of A1 (LO) phonon centered at around 590–591 cm1 and E1 (LO) mode located at about 593–595 cm1. Our experiments (cf. Fig. 2) have also suggested that an E1 (TO) mode near 475 cm1 emerges in all InN samples as a low frequency shoulder of the strong Ehigh peak. In 2 some InN/sapphire films, at low frequency-edge of A1 (LO), the intensity of a phonon band near 562 cm1 exhibits a slight increase with increase of plasma power. Although we have no unambiguous interpretation for this 562 cm1 mode – relating it to the surface optical phonon cannot be completely ruled out [22]. In samples prepared with low plasma power (or lower g), we also observed a weak feature as a shoulder (near 446 cm1) on the lower energy side of a strong Ehigh mode. This trait, however, 2 becomes stronger in films prepared with high plasma power (or higher g). We assigned this characteristic as a low LO-plasmoncoupled (x) mode arising from the interaction of LO phonons through macroscopic field with collective excitations of free charge carriers. Again, on the low energy side (cf. Fig. 2), there appear two broad phonon bands near 160 and 202 cm1 – a region generally dominated by overtones of acoustical phonons. Another unidentified Raman active-mode near 410 cm1 (cf. Fig. 2) falls possibly within the forbidden gap of acoustical and optical phonon branches (225–445 cm1) of InN (w) [23]. Based on a recent lattice dynamical study of InN (w) by grazing inelastic X-ray scattering and ab initio calculations (see Fig. 3, Ref. [15]), the structure around 160 cm1 is attributed to an acoustic phonon near a critical-point A, while the band close to 202 cm1 is presumably ascribed as overtones of TA modes either near K or M critical point [15]. In the absence of realistic calculations of defect vibrational modes (e.g., an average–t–matrix Green’s function theory [19]) one cannot exclude the observed phonon feature near 410 cm1 as a gap-mode of N atoms surrounded by In vacancies in InN (w). Since as-grown InN/sapphire films exhibit n-type conductivity with electron concentration g >1018 cm3) one expects plasmon
η Fig. 3. Calculated LO phonon–plasmon coupled modes x± as a function of the free carrier concentration in InN.
qffiffiffiffiffiffiffiffiffi2ffi frequency xp ¼ 4mpgee1 to be higher at large g. Due to polar nate
ure of InN films one anticipates xP oscillations interacting strongly with LO-phonons to instigate (see Fig. 3) two coupled modes (x±). With increasing g, the upper mode x+ rises from A1 (LO) to higher
(a)
(b)
Fig. 4. Comparison of the experimental (open squares) room temperature infrared reflectivity spectra of PA-MBE grown InN/sapphire samples with the theoretical results (full lines) using the parameter values of Table 2: (a) for sample CD-10 and (b) for sample CD-14.
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D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
Table 2 Calculated optical constants of InN/sapphire films by fitting IR reflectivity spectra.
a b
Sample
xP (cm1)
xLO (cm1)
xTO (cm1)
c (cm1)
C (cm1)
Filma thickness, d (nm)
gb
CD-10 CD-14
1063.5 1406.4
590.5 589.1
474.0 474.2
193.2 156.4
5.71 6.47
239.658 280.882
5.0 (1018 cm3) 1.0 (1019 cm3)
Rutherford backscattering. Hall Measurement.
value while the lower mode x becomes closer to A1 (TO) frequency. For low g = 5.0 1017 cm3) the calculated x (269 cm1), xp (345 cm1) are smaller and x+ (629 cm1) differs from A1 (LO) by a few wave numbers (cf. Fig. 3). As g (3.0 1018 cm3) approaches close to the range of charge carriers in as-grown InN our simulations provided two LO-plasmon coupled modes near x (446 cm1) and x+ (930 cm1). While the low frequency coupled mode x is perceived in our RS studies (cf. Fig. 2) the higher energy mode x+ has not been detected – possibly due to a significantly large phonon broadening at higher g [16]. 2.3. IR reflectivity In addition to RS we have also examined phonon characteristics of PA-MBE grown samples by measuring RT reflectivity spectra using a Bruker IFS 120 v/S Fourier transform infrared (FTIR) spectrometer. In the energy range of 250–1500 cm1 the IR study is performed at near-normal incidence geometry (about 8° incidence angle) with a resolution of 2 cm1. In Figs. 4a and b, the results of IR spectra are displayed for InN/sapphire samples CD10 (d = 0.24 lm) and CD-14 (d = 0.28 lm). Numerical simulations of reflectivity are performed by adopting a standard methodology of multilayer optics within the transfer-matrix approach. We appraised the dielectric functions by incorporating both the lattice effects from optical phonons as well as free charge carriers. In InN films, the dielectric function which includes coupling between plasmon and phonons is evaluated by using the classical harmonic Lorentz oscillator approach [18]:
" ~eðxÞ ¼ e1 1 þ
#
x2p x2LO x2TO ; 2 2 ðxTO x iCxÞ xðx þ icÞ
ð1Þ
qffiffiffiffiffiffiffiffiffi2ffi where, the term xp ¼ 4mpgee1 in Eq. (1) represents the plasma free quency; g stands for the free-carrier density; me is the effective mass; c (C) signifies plasmon (phonon) damping coefficient; xLO and xTO symbolize the A1 (LO) and A1 (TO) phonon frequency. The dielectric function of sapphire substrate is evaluated by incorporating contributions of its optical phonons only. For simulating the IR spectra of InN/sapphire films the best-fit parameters (see Table 2) required in Eq. (1) are evaluated by exploiting a regression method – minimizing the mean-square deviations between theoretical and experimental reflectivity values. From Fig. 4a and b the typical RT IR reflectance spectra (open squares) of InN/sapphire samples (CD-10 and CD-14) are found to be in very good agreement with the simulated curves (solid lines). The free carrier concentrations estimated g (4.73 1018 cm3 and 8.47 1018 cm3) by fitting IR reflectivity of the two samples are found a little smaller (cf. Table 2) than the values determined by Hall measurements. 3. Summary and conclusions In summary, the as-grown PA-MBE n-type InN/sapphire films are characterized by PL, RS and IR reflectance spectroscopy. The PL measurements have resolutely divulged lower values of InN band gaps Eg (0.65–0.80 eV) with strong indications of free
carrier concentration dependent peak energy shifts and widths. A classical parameterized model applied to simulate IR spectra has helped us evaluate free-charge carrier concentration in n-type InN/saphire films. The phonon features identified by RS are found to be in good agreement with grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. Acknowledgements The author (D.N.T) acknowledges useful discussions on the subject matter with M.D. Tiwari of Indian Institute of Information Technology, Allahabad, India and the support that he received through an Innovation Grant from the School of Graduate Studies at Indiana University of Pennsylvania. The work at the National Taiwan University was supported by Grants NSC 98-2221-E-002015-MY3 and by NTU Excellent Research Projects 10R80908 and 102R890954. References [1] Hadis Morkoç, Nitride Semiconductor Devices – Fundamentals and Applications, Wiley, VCH, 2013. [2] Zhe Chuan Feng, III-Nitride Devices and Nanoengineering, Imperial College Press, 2008. [3] Tomohiro Yamaguchi, Tsutomu Araki, Yasushi Nanishi, Proc. SPIE 7939, Gallium Nitride Mater. Dev. VI 34 (2011) 793904, doi:10.1117/12.874840. [4] Tanya Paskova, Drew A. Hanser, Keith R. Evans, Proc. IEEE 98 (2010) 1324, http://dx.doi.org/10.1109/JPROC.2009.2030699. [5] A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, J. Appl. Phys. 94 (2003) 2779 (and references cited therein). [6] T.L. Tansley, C.P. Foley, J.Appl. Phys 59 (1986) 3241; C. Wetzel, T. Takeuchi, S. Yamaguchi, H. Katoh, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 1994; Q. Guo, A. Yoshida, Jpn. J. Appl. Phys. Part 1 33 (1994) 2453; W.Z. Shen, L.F. Jiang, H.F. Yang, F.Y. Meng, H. Ogawa, Q. Guo, Appl. Phys. Lett. 80 (2002) 2063. [7] V.Yu. Davydov, A.A. Klochikhin, R.P. Seisyan, V.V. Emtsev, S.V. Ivanov, F. Bechstedt, J. Furthmuller, H. Harima, A.V. Mudryi, J. Aderhold, O. Semchinova, J. Graul, Phys. Status Solidi B 299 (2002) R1. [8] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967. [9] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549. [10] 10 Yee Ling Chung. Xingu Peng, Ying Chieh Liao, Shude Yao, Li Chyong Chen, Kuei Hsien Chen, Zhe Chuan Feng, Thin Solid Films 519 (2011) 6778. [11] Z.G. Qian, W.Z. Shen, H. Ogawa, Q.X. Guo, J. Phys. Condens. Matter. 16 (2004) R381. [12] V. Darakchieva, P.P. Paskov, E. Valcheva, T. Paskova, M. Schubert, C. Bundesmann, H. Lu, W.J. Schaff, B. Monemar, Superlattices Microstruct. 36 (2004) 573. [13] T. Inushima, K. Fukui, H. Lu, W.J. Schaff, Appl. Phys. Lett. 92 (2008) 171905. [14] Bin Liu, Rong Zhang, Zi Li Xie, Xiang Quian Xiu, Liang Li, Jie Ying Kong, Hui Qiang Yu, Pin Han, Shu Lin Gu, Yi Shi, You Dou Zheng, Chen Guang Tang, Yong Hai Chen, Zhan Guo Wang, Sci China Ser G–Phys. Mech. Astron. 51 (2008) 237. [15] J. Serrano, A. Bosak, M. Krisch, F.J. Manjón, A.H. Romero, N. Garro, X. Wang, A. Yoshikawa, M. Kuball, Phys. Rev. Lett. 106 (2011) 205501. [16] S. Schöche, T. Hofmann, V. Darakchieva, N. Ben Sedrine, X. Wang, A. Yoshikawa, M. Schubert, J. Appl. Phys. 113 (2013) 013502. and references cited therein. [17] Devki N. Talwar (unpublished). [18] Devki N. Talwar, Z.C. Feng, T.-R. Yang, Phys. Rev. B85 (2012) 195203. [19] Devki N. Talwar, Z.C. Feng, J. Fu Lee, P. Becla, Phys. Rev. B87 (2013) 165208. [20] R. Pässler, J. Appl. Phys. 83 (1998) 3356. [21] Y.P. Varshni, Phys. Rev. B2 (1970) 3952. [22] S. Sahoo, M.S. Hu, C.W. Hsu, C.T. Wu, K.H. Chen, L.C. Chen, A.K. Arora, S. Dhara, Appl. Phys. Lett. 93 (2008) 233116. [23] J.B. Wang, Z.F. Li, P.P. Chen, Wei Lu, T. Yao, Acta Mater. 55 (2008) 183.
Please cite this article in press as: D.N. Talwar et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.04.012
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical properties of plasma-assisted molecular beam epitaxy grown InN/sapphire Devki N. Talwar a,⇑, Ying Chieh Liao b, Li Chyong Chen c, Kuei Hsien Chen d, Zhe Chuan Feng b a
Department of Physics, Indiana University of Pennsylvania, 975 Oakland Avenue, 56 Weyandt Hall, Indiana, PA 15705-1087, USA Institute of Photonics and Optoelectronics, Department of Electrical Engineering, and Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 106-17, Taiwan, ROC c Center for Condensed Matter Sciences, National Taiwan University, Taipei 106-17, Taiwan, ROC d Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 115, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 13 November 2013 Accepted 14 April 2014 Available online xxxx Keywords: Photoluminescence Raman scattering IR spectroscopy PA-MBE InN/Sapphire
a b s t r a c t The optical properties of as-grown InN/sapphire films prepared by plasma assisted molecular beam epitaxy (PA-MBE) are characterized by photoluminescence (PL), Raman scattering (RS) and infrared (IR) reflectance techniques. The PL measurements have consistently exhibited lower values of InN band gaps providing clear indications of electron concentration dependent peak energy shifts and widths. The phonon modes identified by RS are found to be in good agreement with the grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. An effective medium theory used to analyze IR reflectance spectra of InN/sapphire films has provided reasonable estimates of free charge carrier concentrations. Published by Elsevier B.V.
1. Introduction Among the conventional group III-nitrides (InN, GaN, AlN), InN has been the least investigated compound semiconductor [1–5]. At the same time, it is perceived as one of the most intriguing materials with superior electronic, optical and transport properties. The lowest effective mass, largest electron saturation velocity, and highest mobility of InN have made it suitable for engineering high-efficiency solar cells, field-effect transistors and highperformance optoelectronic devices capable of operating at the optical-communication wavelength (i.e., k = 1.55 lm) [1,2]. Earlier absorption studies [6] on sputtered InN films with high electron concentration (>1019 cm3) and low electron mobility (<100 cm2/Vs) revealed larger band-gaps Eg 1.8–2 eV with no corresponding band-edge photoluminescence (PL). Recent measurements on InN epilayers grown by molecular beam epitaxy (MBE) provided plausible evidence of lower band-gaps [7–9] i.e., Eg < 1 eV. Subsequent studies on InN films prepared by MOCVD have also ascertained narrow band-gaps – rendering PL emission between 0.6 and 0.8 eV in sharp contrast to the higher Eg values cited before [6]. Although not intentionally doped, the electron charge density in as-grown MOCVD/MBE samples is found higher (g 1018–
⇑ Corresponding author. Tel.: +1 7243572371; fax: +1 7243573804. E-mail addresses: [email protected] (D.N. Talwar), [email protected] (Z.C. Feng).
1019 cm3) [7–10]. The origin of large discrepancy in Eg is still being debated – the explanations given for wider band-gap on sputter-grown materials include: (a) free-electron induced Burstein-Moss shift, (b) incorporation of oxygen to form larger bandgap In2O3, and (c) quantum-size effect in the InN nanocrystals [7–10]. While the narrow band-gap samples grown by MOCVD and/or MBE techniques are insinuated to have a better crystalline quality – the cause of a large free electron density and its impact on the electronic/vibrational properties of as-grown InN material samples is not well empathized. Due to band-gap controversy, the optical behavior of InN above absorption band edge is either sparsely known and/or contradictory [11–13]. The energy positions and relative amplitudes of specific absorption features in the dielectric functions are viewed as the most important parameters for extracting accurate band and/ or phonon structures [14–16]. While the optical properties of InN are strongly dependent on the growth processes – constructing a unique model for describing the meticulous dispersion relations of its dielectric function [17] is not a trivial matter. From the complementary experimental results of PL, Raman scattering (RS) and infrared (IR) reflectance, the investigations of optical properties for InN are expected to contribute to better understanding of its electronic and vibrational characteristics. At the same time these attributes may also be used as a criterion for assessing charge carrier concentration, crystalline quality and/or surface morphology of the InN material samples.
http://dx.doi.org/10.1016/j.optmat.2014.04.012 0925-3467/Published by Elsevier B.V.
Please cite this article in press as: D.N. Talwar et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.04.012
2
D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
The purpose of this paper is to report results of a systematic experimental and theoretical study to comprehend electrical and optical properties of both In (rich) and N (rich) InN/sapphire films. The samples having different charge carrier concentrations are grown on low temperature AlN buffer layers using an SVTA (SVTV-2) plasma-assisted MBE (PA-MBE) system. Radio-frequency plasma source is the key component in the growth process to provide flux stability, material uniformity and purity. AlN buffer layer is used to reduce the lattice mismatch between InN epilayers and c-sapphire substrate. The procedures adopted in the PA-MBE growth are described in details elsewhere [10]. Hall measurements in the Van der Pauw configuration revealed all of the as-grown films exhibiting n-type conductivity. The free electron concentration g ranged from 1018–1019 cm3 with corresponding electron mobility l from 1044 to 200 cm2/V s. Only the hexagonal symmetry of InN (w) films is established by X-ray diffraction (XRD) with no traces of other polymorphs. The narrow profiles of h and h2h scans at the (0002) reflex indicated good crystalline quality. Surface morphology of all InN samples is examined by field-emission scanning electron microscopy (FE-SEM, JEOL-6700). Analysis of Rutherford back scattering data helped us evaluate composition and InN film thickness. Many samples are found to have film thickness d, varying between 150 and 700 nm. The materials are characterized by exploiting the conventional PL, RS and IR (cf. Section 2) spectroscopy. Our PL measurements (cf. Section 2.1) consistently provided lower values of fundamental band gap Eg between 0.65 eV and 0.80 eV. Major Raman active modes (cf. Section 2.2), in all InN samples with diverse charge carrier concentration, are recognized having no significant changes in their vibrational spectra – re-confirming good crystalline quality of the material samples. By using multi-layer formalism within an effective medium theory [18–19] we have analyzed the IR reflectance spectra (cf. Section 2.3) of InN/sapphire films to estimate the free charge carrier concentration. The results are compared and discussed with the existing data with concluding remarks presented in Section 3.
(a)
λ
(b)
λ
Fig. 1. (a) Room temperature PL spectra for two PA-MBE grown InN/sapphire Inrich (CD-1) and N-rich (CD-4) samples. (b) Temperature dependent PL spectra for sample CD-1.
2. Experimental results and discussions 2.1. Photoluminescence In Table 1, we have reported salient physical parameters for two sets of PA-MBE grown InN samples to appraise their electronic and vibrational properties. Fig. 1 shows PL spectra of CD-1 (In-rich) and CD-4 (N-rich) samples by using a Jobin-Yvon TRIAX-320 system equipped with a grating blazed at 2 lm. The measurements are performed by exciting a frequency-doubled Nd+-TAG laser (k = 532 nm) at room temperature (RT) with a spot diameter of 0.5–3 mm. A lead-sulfide (PbS) detector is employed for detecting luminescence. The PL spectra shown in Fig. 1a exhibited asymmetric line shapes with peaks centered near 0.65 eV for In-rich and near 0.80 eV for N-rich sample, respectively. Possible reasons, for blue-shift of PL peak and broader (33 meV) width in CD-4 are conjectured to Moss–Burstein effect related to the variation of charge carrier g, stoichiometry, and/or strain. Fig. 1b also reveals the decrease in PL peak intensity, broadening and slight red-shift with increase of temperature, which is attributed to lat-
tice expansion and electron–phonon interaction. The observed Eg (T) or PL energy shift is very well described by Pässler’s [20] theoretical methodology rather than Varshni’s [21] empirical approach. 2.2. Raman scattering In Fig. 2 we have displayed RT first-order RS spectra of phonons in the backscattering geometry ½zðx; yÞz] for three InN/sapphire samples (CC-21, CC-22 and CC-24) grown with 230, 300 and 350 W plasma power, respectively. The probing laser light of wavelength 532 nm is incident normal to the film surfaces. The pointgroup symmetry, for a polar wurtzite crystal with four atoms per unit cell, belongs to C 46v . Group-theoretical analyses have indicated A1, E1, 2E2 and 2B1 symmetry modes to be optically allowed. Except for the silent B1 mode, all other (A1, E1, and E2) phonons are Raman active. Due to macroscopic field associated with LO modes in InN, the polar A1 and E1 phonons split-up into longitudinal optical (LO)
Table 1 Summary of physical parameters of some PA-MBE grown InN/sapphire films. Sample
Plasma power (W)
Substrate temperature (°C)
Carrier concentration (cm3)
Mobility (cm2/Vs)
CC-21 CC-22 CC-24 CD-1 CD-4
350 300 230 80 90
460 460 460 460 460
9.488 7.014 5.679 1.534 2.180
251.2 225.1 386.7 938 887.7
E + 19 E + 19 E + 19 E + 19 E + 19
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D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
ω η ω η ω η
λ
ω
Fig. 2. Room temperature RS spectra for three PA-MBE grown InN/sapphire CC-21, CC-22 and CC-24 samples grown with different plasma power (cf. Table 1).
and transverse optical (TO) components. In RS one expects a total of six vibrational modes e.g., A1 (LO), A1 (TO), E1 (LO), E1 (TO), Ehigh and Elow 2 . 2 Our Raman study has identified (cf. Fig. 2) three strong phonon features corresponding to A1 (LO) 590 cm1; Ehigh 490 cm1; and 2 low 1 E2 mode 88 cm . In addition, the measurements in backscattering geometry have uncovered several extra phonon features (shown by vertical arrows in Fig. 2) near 160, 202, 410, 446, 475, and 562 cm1. The perusal of Fig. 2 reveals that the main A1 (LO) mode exhibits a relatively large FWHM 30 cm1and its frequency spans over a wide-range (i.e., 575–610 cm1). After careful analysis, the observed broad band is interpreted as a combination of A1 (LO) phonon centered at around 590–591 cm1 and E1 (LO) mode located at about 593–595 cm1. Our experiments (cf. Fig. 2) have also suggested that an E1 (TO) mode near 475 cm1 emerges in all InN samples as a low frequency shoulder of the strong Ehigh peak. In 2 some InN/sapphire films, at low frequency-edge of A1 (LO), the intensity of a phonon band near 562 cm1 exhibits a slight increase with increase of plasma power. Although we have no unambiguous interpretation for this 562 cm1 mode – relating it to the surface optical phonon cannot be completely ruled out [22]. In samples prepared with low plasma power (or lower g), we also observed a weak feature as a shoulder (near 446 cm1) on the lower energy side of a strong Ehigh mode. This trait, however, 2 becomes stronger in films prepared with high plasma power (or higher g). We assigned this characteristic as a low LO-plasmoncoupled (x) mode arising from the interaction of LO phonons through macroscopic field with collective excitations of free charge carriers. Again, on the low energy side (cf. Fig. 2), there appear two broad phonon bands near 160 and 202 cm1 – a region generally dominated by overtones of acoustical phonons. Another unidentified Raman active-mode near 410 cm1 (cf. Fig. 2) falls possibly within the forbidden gap of acoustical and optical phonon branches (225–445 cm1) of InN (w) [23]. Based on a recent lattice dynamical study of InN (w) by grazing inelastic X-ray scattering and ab initio calculations (see Fig. 3, Ref. [15]), the structure around 160 cm1 is attributed to an acoustic phonon near a critical-point A, while the band close to 202 cm1 is presumably ascribed as overtones of TA modes either near K or M critical point [15]. In the absence of realistic calculations of defect vibrational modes (e.g., an average–t–matrix Green’s function theory [19]) one cannot exclude the observed phonon feature near 410 cm1 as a gap-mode of N atoms surrounded by In vacancies in InN (w). Since as-grown InN/sapphire films exhibit n-type conductivity with electron concentration g >1018 cm3) one expects plasmon
η Fig. 3. Calculated LO phonon–plasmon coupled modes x± as a function of the free carrier concentration in InN.
qffiffiffiffiffiffiffiffiffi2ffi frequency xp ¼ 4mpgee1 to be higher at large g. Due to polar nate
ure of InN films one anticipates xP oscillations interacting strongly with LO-phonons to instigate (see Fig. 3) two coupled modes (x±). With increasing g, the upper mode x+ rises from A1 (LO) to higher
(a)
(b)
Fig. 4. Comparison of the experimental (open squares) room temperature infrared reflectivity spectra of PA-MBE grown InN/sapphire samples with the theoretical results (full lines) using the parameter values of Table 2: (a) for sample CD-10 and (b) for sample CD-14.
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D.N. Talwar et al. / Optical Materials xxx (2014) xxx–xxx
Table 2 Calculated optical constants of InN/sapphire films by fitting IR reflectivity spectra.
a b
Sample
xP (cm1)
xLO (cm1)
xTO (cm1)
c (cm1)
C (cm1)
Filma thickness, d (nm)
gb
CD-10 CD-14
1063.5 1406.4
590.5 589.1
474.0 474.2
193.2 156.4
5.71 6.47
239.658 280.882
5.0 (1018 cm3) 1.0 (1019 cm3)
Rutherford backscattering. Hall Measurement.
value while the lower mode x becomes closer to A1 (TO) frequency. For low g = 5.0 1017 cm3) the calculated x (269 cm1), xp (345 cm1) are smaller and x+ (629 cm1) differs from A1 (LO) by a few wave numbers (cf. Fig. 3). As g (3.0 1018 cm3) approaches close to the range of charge carriers in as-grown InN our simulations provided two LO-plasmon coupled modes near x (446 cm1) and x+ (930 cm1). While the low frequency coupled mode x is perceived in our RS studies (cf. Fig. 2) the higher energy mode x+ has not been detected – possibly due to a significantly large phonon broadening at higher g [16]. 2.3. IR reflectivity In addition to RS we have also examined phonon characteristics of PA-MBE grown samples by measuring RT reflectivity spectra using a Bruker IFS 120 v/S Fourier transform infrared (FTIR) spectrometer. In the energy range of 250–1500 cm1 the IR study is performed at near-normal incidence geometry (about 8° incidence angle) with a resolution of 2 cm1. In Figs. 4a and b, the results of IR spectra are displayed for InN/sapphire samples CD10 (d = 0.24 lm) and CD-14 (d = 0.28 lm). Numerical simulations of reflectivity are performed by adopting a standard methodology of multilayer optics within the transfer-matrix approach. We appraised the dielectric functions by incorporating both the lattice effects from optical phonons as well as free charge carriers. In InN films, the dielectric function which includes coupling between plasmon and phonons is evaluated by using the classical harmonic Lorentz oscillator approach [18]:
" ~eðxÞ ¼ e1 1 þ
#
x2p x2LO x2TO ; 2 2 ðxTO x iCxÞ xðx þ icÞ
ð1Þ
qffiffiffiffiffiffiffiffiffi2ffi where, the term xp ¼ 4mpgee1 in Eq. (1) represents the plasma free quency; g stands for the free-carrier density; me is the effective mass; c (C) signifies plasmon (phonon) damping coefficient; xLO and xTO symbolize the A1 (LO) and A1 (TO) phonon frequency. The dielectric function of sapphire substrate is evaluated by incorporating contributions of its optical phonons only. For simulating the IR spectra of InN/sapphire films the best-fit parameters (see Table 2) required in Eq. (1) are evaluated by exploiting a regression method – minimizing the mean-square deviations between theoretical and experimental reflectivity values. From Fig. 4a and b the typical RT IR reflectance spectra (open squares) of InN/sapphire samples (CD-10 and CD-14) are found to be in very good agreement with the simulated curves (solid lines). The free carrier concentrations estimated g (4.73 1018 cm3 and 8.47 1018 cm3) by fitting IR reflectivity of the two samples are found a little smaller (cf. Table 2) than the values determined by Hall measurements. 3. Summary and conclusions In summary, the as-grown PA-MBE n-type InN/sapphire films are characterized by PL, RS and IR reflectance spectroscopy. The PL measurements have resolutely divulged lower values of InN band gaps Eg (0.65–0.80 eV) with strong indications of free
carrier concentration dependent peak energy shifts and widths. A classical parameterized model applied to simulate IR spectra has helped us evaluate free-charge carrier concentration in n-type InN/saphire films. The phonon features identified by RS are found to be in good agreement with grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. Acknowledgements The author (D.N.T) acknowledges useful discussions on the subject matter with M.D. Tiwari of Indian Institute of Information Technology, Allahabad, India and the support that he received through an Innovation Grant from the School of Graduate Studies at Indiana University of Pennsylvania. The work at the National Taiwan University was supported by Grants NSC 98-2221-E-002015-MY3 and by NTU Excellent Research Projects 10R80908 and 102R890954. References [1] Hadis Morkoç, Nitride Semiconductor Devices – Fundamentals and Applications, Wiley, VCH, 2013. [2] Zhe Chuan Feng, III-Nitride Devices and Nanoengineering, Imperial College Press, 2008. [3] Tomohiro Yamaguchi, Tsutomu Araki, Yasushi Nanishi, Proc. SPIE 7939, Gallium Nitride Mater. Dev. VI 34 (2011) 793904, doi:10.1117/12.874840. [4] Tanya Paskova, Drew A. Hanser, Keith R. Evans, Proc. IEEE 98 (2010) 1324, http://dx.doi.org/10.1109/JPROC.2009.2030699. [5] A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, J. Appl. Phys. 94 (2003) 2779 (and references cited therein). [6] T.L. Tansley, C.P. Foley, J.Appl. Phys 59 (1986) 3241; C. Wetzel, T. Takeuchi, S. Yamaguchi, H. Katoh, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 1994; Q. Guo, A. Yoshida, Jpn. J. Appl. Phys. Part 1 33 (1994) 2453; W.Z. Shen, L.F. Jiang, H.F. Yang, F.Y. Meng, H. Ogawa, Q. Guo, Appl. Phys. Lett. 80 (2002) 2063. [7] V.Yu. Davydov, A.A. Klochikhin, R.P. Seisyan, V.V. Emtsev, S.V. Ivanov, F. Bechstedt, J. Furthmuller, H. Harima, A.V. Mudryi, J. Aderhold, O. Semchinova, J. Graul, Phys. Status Solidi B 299 (2002) R1. [8] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 80 (2002) 3967. [9] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549. [10] 10 Yee Ling Chung. Xingu Peng, Ying Chieh Liao, Shude Yao, Li Chyong Chen, Kuei Hsien Chen, Zhe Chuan Feng, Thin Solid Films 519 (2011) 6778. [11] Z.G. Qian, W.Z. Shen, H. Ogawa, Q.X. Guo, J. Phys. Condens. Matter. 16 (2004) R381. [12] V. Darakchieva, P.P. Paskov, E. Valcheva, T. Paskova, M. Schubert, C. Bundesmann, H. Lu, W.J. Schaff, B. Monemar, Superlattices Microstruct. 36 (2004) 573. [13] T. Inushima, K. Fukui, H. Lu, W.J. Schaff, Appl. Phys. Lett. 92 (2008) 171905. [14] Bin Liu, Rong Zhang, Zi Li Xie, Xiang Quian Xiu, Liang Li, Jie Ying Kong, Hui Qiang Yu, Pin Han, Shu Lin Gu, Yi Shi, You Dou Zheng, Chen Guang Tang, Yong Hai Chen, Zhan Guo Wang, Sci China Ser G–Phys. Mech. Astron. 51 (2008) 237. [15] J. Serrano, A. Bosak, M. Krisch, F.J. Manjón, A.H. Romero, N. Garro, X. Wang, A. Yoshikawa, M. Kuball, Phys. Rev. Lett. 106 (2011) 205501. [16] S. Schöche, T. Hofmann, V. Darakchieva, N. Ben Sedrine, X. Wang, A. Yoshikawa, M. Schubert, J. Appl. Phys. 113 (2013) 013502. and references cited therein. [17] Devki N. Talwar (unpublished). [18] Devki N. Talwar, Z.C. Feng, T.-R. Yang, Phys. Rev. B85 (2012) 195203. [19] Devki N. Talwar, Z.C. Feng, J. Fu Lee, P. Becla, Phys. Rev. B87 (2013) 165208. [20] R. Pässler, J. Appl. Phys. 83 (1998) 3356. [21] Y.P. Varshni, Phys. Rev. B2 (1970) 3952. [22] S. Sahoo, M.S. Hu, C.W. Hsu, C.T. Wu, K.H. Chen, L.C. Chen, A.K. Arora, S. Dhara, Appl. Phys. Lett. 93 (2008) 233116. [23] J.B. Wang, Z.F. Li, P.P. Chen, Wei Lu, T. Yao, Acta Mater. 55 (2008) 183.
Please cite this article in press as: D.N. Talwar et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.04.012