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Phys. Status Solidi C 6, No. 6, 1480 – 1483 (2009) / DOI 10.1002/pssc.200881516
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current topics in solid state physics
Transport characteristics of indium nitride (InN) films grown by plasma assisted molecular beam epitaxy (PAMBE)
Andreas Knübel*,1, Rolf Aidam1, Volker Cimalla1, Lutz Kirste1, Martina Baeumler1, Crenguta-Columbina Leancu1, Vadim Lebedev1, Jan Wallauer2, Markus Walther2, and Joachim Wagner1 1 2
Fraunhofer Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany Albert-Ludwigs-University, Institute of Physics, Department of Molecular and Optical Physics, Stefan-Meier-Str. 19, 79104 Freiburg, Germany
Received 11 September 2008, revised 2 February 2009, accepted 10 February 2009 Published online 30 March 2009 PACS 68.55.-a, 73.61.Ey, 78.20.Ci, 78.55.Cr, 81.05.Ea, 81.15.Hi * Corresponding author: e-mail
[email protected], Phone: +49 761 5159-441, Fax: +49 761 5159-395
We report on the PAMBE-growth of high-quality InN layers with In polarity and on the influence of growth parameters such as Indium-to-nitrogen flux ratio as well as substrate temperature on the electronic transport characteristics of these layers. The flux ratio was changed between 0.94 to 1.18 near stoichiometric conditions and the growth temperature was varied between 400 and 485 °C as measured by calibrated pyrometer temperature. The InN films were grown on metalorganic vapour deposition (MOCVD) GaN templates 3” in diameter with InN layer thicknesses of 700 nm as determined by spectroscopic ellipsometry (SE). Under In-rich and near stoichiometric growth conditions a minimum surface root mean square roughness of 0.6 nm was achieved. High resolu-
tion X-ray diffraction show purely hexagonal phase growth and no evidence for the presence of cubic inclusions. InN layers grown under optimized growth conditions exhibit a narrow (FWHM 20 meV) near band edge low temperature photoluminescence (PL) spectrum with a PL peak energy of 0.67 eV, indicating an InN band gap energy of ≈ 0.7 eV, consistent with the dielectric function spectrum derived from SE measurements. Room temperature van-der-Pauw Hall measurements reveal high electron mobilities up to 1910 cm2/Vs with mean bulk electron concentrations as low as 5 x 1017 /cm3. Furthermore, efficient surface emission of THz radiation has been demonstrated.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction In recent years InN attracted much attention because its recently exposed narrowest bandgap energy (< 0.7 eV) among the group III-nitride semiconductors. As a consequence of the lower bandgap and the superior electron transport properties InN has emerged as a potentially important semiconductor for use in high frequency electronics [1, 2], optoelectronics [3] and photovoltaic applications [4]. In optoelectronics, InN covers when alloyed with aluminium and gallium nitride the whole spectral range between near-infrared (NIR) and the deep-ultraviolet (DUV) including the visible range. In addition, InN has opened a new field of interest as a THz emitting material [5]. In this field of application including THz imaging and time-domain spectroscopy, instrumentation is currently limited by the output power of
the semiconductor based emitter. InN is a promising semiconductor material for THz emission with an enhanced output power due to the specific band structure. Already at this early stage recent work shows that InN can exceed that of classical THz emitters such as GaAs and InAs [6]. InN based devices such as high cut-off frequency field effect transistors as well as THz emitters require optimized InN films with respect to high electron mobility, low free electron and defect densities along with flat surface morphology. 2 Experimental All InN films were deposited on MOCVD grown 3” GaN/sapphire (0001) templates in a Veeco Gen20 system equipped with conventional effusion cells. Activated nitrogen was supplied by a Veeco UNI© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Contributed Article Phys. Status Solidi C 6, No. 6 (2009)
Figure 1 Dependency of RHEED intensity (I) on In supply (II) over time at constant nitrogen flux. Insets in 1I show RHEED images recorded at different times.
Bulb HF plasma source. The wafers were backside metallised with 500 nm Mo to provide efficient heat transfer. The GaN templates were outgased at 450 °C for 15 minutes in a separate preparation chamber. All samples grown consist of a 50 nm thick PAMBE GaN layer which was grown in the Ga-rich regime, were excess Ga was desorbed from the surface. Then a 700 nm thick InN film was grown. In order to adjust the In-to-nitrogen ratio we used a constant gas flow of 3.1 sccm at 350 W output power for all samples and altered the In cell temperature and thus the In flux; a beam equivalent pressure (BEP) of 25.7 x 10-8 Torr is defined in the following as an In flux of 100 %. The growth was monitored in-situ using reflection high electron energy diffraction (RHEED). Temperature measurement was performed with an IRCON Modline 3V pyrometer that measures the blackbody radiation intensity between 0.9
Figure 2 Sensivity of Hall mobility on In-to-N ratio and substrate temperature. Electron mobility results were found at stoichiometric growth and substrate temperatures around 475 °C. The maximum electron mobility of µ = 1910 cm2/Vs at a mean bulk electron concentration of 5 x 1017 /cm3 is indicated by a white triangle. www.pss-c.com
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Figure 3 RT electron mobility (red squares) and carrier concentration (blue triangles) versus InN film thickness [10-12]. The solid black square and triangle represent the best result in this work, i.e. an electron mobility of 1910 cm2/Vs with a bulk electron concentration of 5 x 1017 /cm3 at 700 nm film thickness.
and 0.97 µm. Initial calibration of the infrared thermometer was carried out by means of the well-known melting point of aluminium at 660 °C correcting for the emissivity factor ε. The growth temperature was varied between 400 and 485 °C. The pyrometer was used within its specified temperature range with a lower limit of 460 °C. Below that we extrapolated the pyrometer temperature with a calibration procedure by means of thermocouple temperature. Near stoichiometric growth conditions were approached by adjusting the In flux and monitoring the specular RHEED spot intensity. The RHEED intensity drops in the case of In excess on the wafer surface and is associated with a streaky 2-dimensional (2-D) growth mode whereas an increase of RHEED intensity is observed if an excess of N2 is present on the surface. However, a continuous deficiency of metal leads to 3-D growth. Figure 1 shows the direct relation between the relative In flux and the RHEED intensity. Note that the signal shown in figure 1I is the envelope of the actual RHEED signal modulated in amplitude because of wobbling of the rotating substrate holder. The area indicated in red is defined as the region where an excess of In and N2 dominates, respectively, and the In-to-N ratio is out of equilibrium. If the RHEED signal is approaching a constant intensity in the area marked in green, the amount of metallic In on the wafer surface is constant and the In-to-N ratio is nearly unity. When the In shutter is opened the signal immediately drops down and then recovers in the first three minutes which is attributed to the nucleation and coalescence of InN islands. The inset a) of Fig. 1I shows a diffuse and streaky RHEED pattern with low contrast that gets brighter after repeated decreasing in 1% steps of relative In flux shown in Fig. 1II and inset b) of Fig. 1I. Inset c) then shows the rapid transition from 2-D to 3-D growth resulting in a bright and spotty RHEED pattern. After another cycle of In flux adjustment the RHEED signal approaches © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Knübel et al.: Transport characteristics of indium nitride films grown by (PAMBE)
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a constant between 70 and 80 of relative RHEED intensity scale which lies within the green area. This agrees with the inset d) which reveals a streaky and sharp RHEED pattern with high contrast. The corresponding relative In flux in Fig. 1II approximates a constant value of ~ 85 % indicated by a red line which relates to a BEP of 21.8 x 10-8 Torr. The influence of impinging In/N ratio and growth temperature on transport characteristics and surface morphology has been investigated by many groups for both Inpolar and N-polar InN [7-9]. As-grown samples were probed by optical microscopy, atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), spectroscopic ellipsometry (SE), low temperature photoluminescence (PL) spectroscopy and room temperature Hall measurements in Van-der-Pauw geometry. 3 Results and discussion For a detailed investigation of the InN film properties special attention was paid on the dependence of electron transport characteristics on In/N flux ratio and growth temperature. The transport characteristics of all the InN films grown in this work were assessed by room temperature (RT) Hall measurements in a Van-der-Pauw set-up. Figure 2 shows a contour plot of
Figure 5 (2 x 2) µm2 AFM images of an InN film grown in the slightly In-rich regime. In evidence are step flow growth and spiral hillocks. Left: height image, right: amplitude image. The rms roughness is 0.6 nm.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 6 Low temperature PL spectra of a 701 nm thick InN film excited at λ = 1064 nm with different pump intensities. Sample temperature was set to 20 K.
the low field electron mobilities as a function of the calibrated pyrometer temperature and relative In flux. Electron mobility is colour coded. Highest electron mobilities were measured on samples grown at around 475 °C and nearly stoichiometric growth conditions. In the following we concentrate on the characteristics of an InN layer grown under optimised conditions, which are a In/N ratio of 1.06 and a temperature TP ~ 475 °C. A mean bulk electron concentration of 5 x 1017 /cm3 of our record sample is a rough estimate assuming a constant carrier concentration over the entire film disregarding two electron accumulation layers, at the surface and at the InN/GaN interface. For comparison Fig. 3 shows a compilation of published RT Hall measurements and electron concentrations [10-12] with those presented in this work. A black circle and triangle indicates an improved transport characteristic at film thicknesses of 700 nm. In order to examine the structural quality of this InN/GaN/Al2O3 (0001) sample HRXRD ω-scans of the 00.2 and 10.2 InN and GaN reflections were performed (Fig. 4). While the full width at half maximum (FWHM) of the InN 00.2 reflection (385 arcsec) is only slightly higher than the FWHM of the GaN 00.2 reflection (288 arcsec), the 10.2 reflection of InN (FWHM: 1364 arcsec) is considerable broader than the one of GaN buffer layer (FWHM: 733 arcsec). This higher broadening of the 10.2 reflection of InN compared to that of the underlying GaN can be attributed to higher density of edge-type dislocations in the InN layer. Although thicker InN films showed smaller FWHM values [13], 700 nm thick films are well suited for use as THz surface emitters. Additional a HRXRD phase analysis was performed for this InN/GaN/Al2O3 (0001) sample using a Θ-2Θ scan and an azimuthal scan (phiscan) for the cubic InN 200 reflection (2Θ = 36.04°, polar angle 54.74°). Both measurements showed no evidence of neither the cubic InN polytype nor metallic In. Figure 5 shows the (2 x 2) µm2 height and amplitude AFM image of the sample with a root mean square (rms)
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roughness of 0.6 nm and step flow growth with spiral hillocks. The low-temperature PL spectrum recorded at T = 20 K shows a narrow PL peak centred ~0.67 eV with a FWHM of 20 meV (Fig. 6). This result suggests a possible band gap of InN at around 0.7 eV. Spectroscopic ellipsometry (SE) was used to asses the dielectric function spectrum and the thickness of the InN layer. The refractive index n and the extinction coefficient k are deduced from a multilayer parametric model fit to the pseudo-dielectric functions [14] revealing a bandgap of 0.68 eV and a InN film thickness of 701 nm. From the latter a growth rate of 0.5 µm/h can be extracted for In-rich grown InN films being nearly constant for growth temperatures between 420 and 475 °C. Within Figure 7 Spectral amplitude of the measured THz emission for this temperature range we get a loss in growth rate of only two different InN samples and for GaAs as a reference. 2 % approaching 475 °C, which is quite consistent with the work of Gallinat et al. [15] References A band gap energy of around 0.7 eV, as indicated by the present PL and SE data, is in agreement with band gap [1] Y. C. Kong, Y. D. Zheng, and Y. Shi, Solid State Electron. data reported in the recent literature for state-of-the-art 49, 199-203 (2005). high-quality InN layers [16]. [2] Y.-S. Lin, S.-H. Koa, and S. S. H. Hsu, Appl. Phys. Lett. 90, InN is a very effective surface emitter of THz radiation 142111 (2007). [3] J. Wu, W. Walukiewicz, and W. J. Schaff, Appl. Phys. Lett. upon excitation by fs-laser [5, 6]. The efficiency critically 80, 3967 (2002). depends on the transport properties [6, 17]. Figure 7 shows [4] J. Wu, W. Walukiewicz, and S. Kurtz, J. Appl. Phys. 94, the spectral amplitude of the THz signal, measured in re6477 (2003). flection geometry for two representative InN layers in [5] R. Ascazubi, I. Wilke, K. Denniston, H. Lu, and W. J. comparison to a GaAs emitter. The results of these investiSchaff, Appl. Phys. Lett. 84, 4810 (2004). gations revealed that the low background electron concen[6] V. Cimalla, B. Pradarutti, G. Matthäus, C. Brückner, S. Rietration and the high mobility achieved in this study indeed hemann, G. Notni, S. Nolte, A. Tünnermann, V. Lebedev, improves the THz emission characteristics as a conseand O. Ambacher, Phys. Status Solidi B 244, 1829–1833 quence of the photo Dember effect as described in Ref. [6]. (2007). Moreover, the comparison in Fig. 7 also confirms the pre[7] K. Xu and A. Yoshikawa, Appl. Phys. Lett. 83, 251 (2003). viously proposed assumption [6] that the atomically [8] Y. Nanishi, Y. Saito, and T. Yamaguchi, Jpn. J. Appl. Phys. smooth InN surfaces increases the THz emission power. 42, 2549 (2003). Consequently, thin layers with a thickness below 1 µm ex[9] T. Ive, O. Brandt, M. Ramsteiner, M. Giehler, H. Kostial, hibiting smooth surfaces and transport properties demonand K. H. Ploog, Appl. Phys. Lett. 84, 1671 (2004). strated in this work are well suited for use as THz surface [10] H. Lu, W.J. Schaff, L. F. Eastman, J. Wu, and R. J. Molnar, emitters. MRS Symp. Proc. 743, L4.10 (2003). 4 Conclusions In this work we have demonstrated that PAMBE is a suitable technique to grow high quality InN films without any indication of cubic phase. Layers grown in the slightly In-rich regime revealed excellent surface morphology with a rms roughness of 0.6 nm and step flow growth. We were able to show a strong dependence of the RT electron mobility on In-to-nitrogen ratio and substrate temperature. The InN films presented in this work were optimized on high electron mobility and low free carrier density. The good surface and transport properties of these InN layers enable the application in efficient THz surface emitters even for thin layers with a thickness below 1 µm. Acknowledgements We would like to thank Stefan Müller for providing the MOCVD-grown GaN templates.
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[11] H. Lu, W. J. Schaff, L. F. Eastman, and C. E. Stutz, Appl. Phys. Lett. 82, 1736 (2003). [12] W. J. Schaff, H. Lu, L. F. Eastman, W. Walukiewicz, K. M. Yu, S. Keller, S. Kurtz, B. Keyes, and L. Gevilas, in: Proc. Electrochem. Soc. PV 2004-06: State-of-the-Art Program on Compound Semiconductors XLI- and Nitride and Wide Bandgap Semiconductors for Sensors, Photonics, and Electronics V, edited by V. H. Ng and A. G. Baca (The Electrochemical Society, Inc., 2004), p. 358. [13] X. Wang, S.-B. Che, Y. Ishitani, and A. Yoshikawa, Jpn. J. Appl. Phys. 45, L730 (2006) [14] R. Goldhahn, P. Schley, and W. J. Schaff, J. Cryst. Growth 288, 273–277 (2006). [15] C. S. Gallinat, G. Koblmüller, J. S. Brown, and J. S. Speck, J. Appl. Phys. 102, 064907 (2007) [16] C. S. Gallinat, G. Koblmüller, and J. S. Speck, Appl. Phys. Lett. 89, 032109 (2006). [17] V.M. Polyakov and F. Schwierz, Semicond. Sci. Technol. 22, 1016 (2007). © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
