Kinetically stabilized high-temperature InN growth

Journal Pre-proofs Kinetically stabilized high-temperature InN growth G. Brendan Cross, Zaheer Ahmad, Daniel Seidlitz, Mark Vernon, Nikolaus Dietz, Daniel Deocampo, Daniel Gebregiorgis, Sidong Lei, Alexander Kozhanov PII: DOI: Reference:

S0022-0248(20)30097-X https://doi.org/10.1016/j.jcrysgro.2020.125574 CRYS 125574

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Journal of Crystal Growth

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23 May 2019 20 February 2020 22 February 2020

Please cite this article as: G. Brendan Cross, Z. Ahmad, D. Seidlitz, M. Vernon, N. Dietz, D. Deocampo, D. Gebregiorgis, S. Lei, A. Kozhanov, Kinetically stabilized high-temperature InN growth, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro.2020.125574

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Kinetically stabilized high-temperature InN growth G. Brendan Cross1, Zaheer Ahmad1, Daniel Seidlitz1,3, Mark Vernon1, Nikolaus Dietz 1, Daniel Deocampo2, Daniel Gebregiorgis2, Sidong Lei1, and Alexander Kozhanov1 1Department

of Physics and Astronomy, Center for Nano Optics, Georgia State University, 25 Park pl., Atlanta, GA 30303, USA

2Geosciences 3Institute

Department, Georgia State University, 25 Park pl., Atlanta, GA 30303, USA

of Solid State Physics, Technical University of Berlin, Hardenbergstrasse 36, Berlin, D-10623, Germany

Corresponding author:

Alexander Kozhanov, Postal address: Department of Physics and Astronomy Georgia State University Atlanta 25 Park Pl NE, rm.605 GA, 30303 USA e-mail: [email protected] phone:

+1(404)413-6084

fax:

+1(404)413-6025

Abstract We report on indium nitride growth on sapphire by migration-enhanced plasma-assisted metal organic chemical vapor deposition (MEPA-MOCVD).

The growth is studied in the

temperature range from 700 ˚C to 957 ˚C, well above the decomposition temperature of indium nitride in conventional MOCVD. Raman spectroscopy, atomic force microscopy, and X-ray diffraction indicate polycrystalline grainy InN film growth.

Keywords: B2. Semiconducting indium compounds, B2. Semiconducting III-V materials, A3. Migration enhanced epitaxy, A3. Metalorganic chemical vapor deposition, B1. Nitrides, B3. Solar Cells

1 Introduction Indium nitride (InN) and its ternary alloys are highly suitable for high-speed electronic and optoelectronic devices such as light emitting diodes and solar cells [1]. The InN bandgap of 0.69 eV at 300 K [2] allows tunability of the direct bandgap from the infrared into the ultraviolet [3] in ternary alloys with GaN and AlN by changing composition. However, growth of high crystal quality InN within the growth parameter window suitable for GaN and AlN is challenging [4,5]. One challenge is the significant difference between the decomposition temperature of InN between 500˚C and 700˚C [4,6–14] and the typical GaN and AlN growth temperatures above 950oC [8]. One of the methods used to synthesize InN at temperatures high enough to be compatible with aluminum and gallium nitride is high-pressure CVD (HPCVD) [15–19]. The decomposition of InN can be modified by using higher pressures due to the constant enthalpy associated with this process [20]. The drawback to using high pressure growth is the decreased growth rate and complexity of the growth systems that make it challenging for commercial use [19]. Another challenge associated with InN growth in traditional MOCVD systems is the degradation and unintentional doping of InN layers. This degradation is related to the hydrogen introduced from the decomposition of ammonia, the nitrogen precursor used in molecular beam epitaxy (MBE) and conventional MOCVD [8,13,14,21,22]. Plasma-assisted MBE (PA-MBE) [23–26] and plasma-enhanced atomic layer deposition (ALD) [27] have been shown to overcome some of these challenges. Using ionized nitrogen in PA-MBE offers improved InN crystal quality over conventional MBE growth methods [28]. MOCVD growth of InN could provide higher growth rates and would not require ultra-high vacuum.

In this work, we explore migration-enhanced plasma-assisted (MEPA) MOCVD growth of InN at temperatures above its decomposition point. The concept of migration enhancement is that the increased kinetic energies of the plasma-activated nitrogen species stabilize the growth surface [19,29]. We analyze InN films grown using MEPA-MOCVD on sapphire substrates at growth temperatures ranging from 700˚C up to 957˚C, which is well above the reported decomposition temperature of InN. InN growth is achieved within the typical MOCVD growth temperature windows of AlN and GaN.

2 Experimental InN films were grown using MEPA-MOCVD on single side polished, c-plane (0001) Al2O3 wafers offcut at 0.2˚ towards m-plane. Trimethylindium (TMI) and nitrogen plasma were used as group-III and group-V precursors respectively. Sapphire wafers are placed horizontally with all flows coming from the top of the reactor, while the wafers are rotated slowly to ensure even flow distribution over the wafer. The sapphire wafers were cleaned in hydrogen plasma, from a RF hollow cathode plasma source at the top of the reactor, at 441˚C in the growth chamber, following that a low-temperature InN nucleation layer was deposited at 555˚C. The sample was then heated to the growth temperature which was varied from 700˚C to 957˚C. A thermocouple placed through the spindle of the wafer carrier was used to control its temperature as it was heated by the resistive heater from underneath. Temperature offsets between the thermocouple and sample surface were calibrated using a pyrometer and black body radiation spectral measurements. TMI was alternately pulsed through a showerhead input into the system at 10.2 μmol/min for one second and 0.4 μmol/min for 19 seconds, using N2 as a carrier gas. The N2 carrier flow was 250 and 10 sccm respectively. A nitrogen flow of 750 sccm passed through the plasma source with the plasma

power switching between 50 W for the first 6 seconds of the cycle and 400 W for the last 14 seconds. Growth was carried out at a reactor pressure of 4.1 Torr for all samples. More information about the reactor can be found in [19]. One significant difference to be noted between this work and [19] is the absence of any ammonia in the growth reactor. At the end of the growth cycle the plasma was stopped and the sample was cooled to room temperature in N2 flow, at which point it was removed from the MEPA MOCVD system. Room temperature Raman spectroscopy in backscattering geometry was used to investigate the structural composition of grown films. A 532 nm and 514 nm lasers were used as an excitation source. Phonon mode frequencies and full width at half-maximum (FWHM) were extracted using a multi-peak fit of the experimentally acquired Raman spectra for the samples in this study. X-ray diffraction scans were performed to analyze the crystal structure of the grown films using a Panalytical X’Pert PRO X-ray diffractometer with a Cu tube running at 45 kV and 40 mA. Atomic force

microscopy (AFM) was used to determine the surface morphology of grown films. Film thickness was determined by fitting interference fringes measured by Fourier-transform infrared (FTIR) spectroscopy using the multi-layer stack modelling method described in [30,31]. Hall transport measurements were used to evaluate the free carrier concentration and carrier mobility at room temperature and magnetic field of 0.5 T.

3 Results and Discussion A typical room-temperature FTIR reflectance spectrum measured on the InN films grown on Sapphire substrate is shown in Figure 1. Sample thicknesses were extracted by fitting the reflectance data within the 2200 cm-1 to 5300 cm-1 spectral range, thus ignoring absorption above

the band gap (below 5400 cm-1). Sample reflectance matched InN films modelled with thicknesses of 20-70 nm. Corresponding growth rates varied within 10-35 nm/hr, with no clear correlation to growth temperature. While the optical thicknesses of grown films were less than half of the wavelength, the modelled reflectance was calculated as a far-field response. The InN film and sapphire substrate phonon features were observed in the 450 cm-1 to 2200 cm-1 range. The absorption dip at 2150 cm-1 is typically associated with an impurities such as additional nitrogen or carbon and oxygen [32]. Further work would be necessary to establish what this feature is attributed to. No trends in phonon-related spectral features were observed for InN films grown at different temperatures.

Figure 1. FTIR reflectance spectrum measured on InN sample grown on c-plane sapphire at T = 816˚C and for a c-plane sapphire substrate.

Structural properties of all grown films were evaluated using XRD measurements. Figure 2 shows a typical XRD diffractogram measured on the sample grown at 873˚C. For all analyzed samples (0002) InN, (0004) InN and the (0006) Al2O3 peaks were observed, located at 31.38˚, 64.90˚, and 41.85˚ respectively. Rotation in the φ direction did not show significant variation in the InN peak intensity, indicating a highly disordered crystalline film, affirming Raman results. The obtained XRD results are consistent with results previously reported by Gao et al. [33] for InN grown at 550˚C by MBE. However, Moret et al. [34] and Huang et al [35] identify the 33˚ peak as metallic indium, which might possibly be the case, given the growth temperatures being above the accepted temperature for InN decomposition [35].

Figure 2. XRD diffractogram of InN grown at 825 ˚C. Diffractogram demonstrates presence of InN. The peak at 33˚ has not been identified as [33] claims it belongs to InN and [34,35] identify it as belonging to In.

The Raman spectra measured for samples grown at different temperatures are shown in Figure 3a. The spectra, obtained with a 532 nm excitation source, reveal several phonon mode peaks which we attribute to the sapphire substrate. The peak located at 575 cm-1 is attributed to the Eg mode of the sapphire substrate [36]; the peaks located at 490 cm-1, and 587 cm-1 are related to the InN E2(high), and A1(LO) phonon modes respectively [30, 31]. Raman data for Raman shifts above and below the data presented were collected using a 514 nm excitation source, with no notable peaks at 2215 cm-1 or below 500 cm-1 leading us to believe that there is not a significant presence of metallic indium [39]. The presence of the InN E2(high) and A1(LO) phonon modes indicates that wurtzite InN is present. In the backscattering geometry, the InN B1(high), and E1(LO) Raman phonon mode responses should be disallowed, but can still be observed when the layer is disordered [32, 33]. For samples grown above 873˚C broad and narrow E2(high) Raman peaks were observed. Two E2(high) peaks were implemented in Raman spectra fitting model labeled as E2(high)sharp and E2(high)broad (Figure 3b) for these samples, while only one E2(high) peak was observed for samples grown below 873˚C. Figure 4 shows the full width at half the maximum height (FWHM) of the InN E2(high)sharp, E2(high)broad, A1(LO) peaks. The E2(high)sharp peak FWHM decreased as the growth temperature is increased. For the E2(high)broad and A1(LO) peak FWHMs there is no observable trend. The FWHM of the A1(LO) mode contains contributions from the phononplasmon interaction and point defects [34, 35].

Figure 3.(a) Raman spectra of InN films grown at different growth temperatures. Dashed lines indicate the phonon modes. (b) Raman spectra of the sample grown at 929˚C with (red) lines indicating individual phonon modes and (green) whole spectrum multi-peak fitting.

Figure 4. Temperature dependence of the E2(high) (empty squares for sharp peak, solid squares for broad peak) and A1(LO) phonon mode peak FWHM (triangles). Dashed line illustrates the trend.

Hall effect charge transport measurements were performed on the films at room temperature to extract free carrier concentrations of (3±2)×1020 cm-3 and electron mobilities ranging within 10 - 70 cm2/Vs. Both electron concentration and mobility did not show any trends as a function of growth temperature. AFM was used to analyze the surface morphology of the grown samples. Results of the AFM imaging are shown in Figure 5. All grown samples demonstrate a grainy surface with the median grain radius varying between 8.7 – 11.6 nm.

Figure 5. AFM micrographs of InN films. Growth temperatures are indicated in each image.

The appearance of large grain structures (approximately 35 – 80 nm in radius) on top of a small-grain film is correlated with the appearance of the “sharp” E2(high) peaks in the Raman spectrum (Fig.2, Fig.3). This change in structure could be considered similar to changes observed by Ploch et al. [44], but the growth temperatures are about 350˚C higher than those reported. The FWHM of the E2(high) peak is related to the number of structural defects of the film and can be used to characterize the crystallinity of an InN epilayer [42]. It can be concluded that the large coalesced grains observed on the surface of samples grown at temperatures higher than 873˚C are wurtzite InN reflected in the “sharp” Raman E2(High) peak. The “broad” E2(high) peak is attributed to the small-grain InN observed in all studied samples. The disallowed A1(TO), B1(high), and E1(LO) Raman phonon mode peaks may originate from the disordered small-grain

InN, but these features and the relative intensities of the phonon peaks could also be related to crystallite size [45]. The surface morphology of the InN films revealed by the AFM indicate that due to the thin nature of the films, hopping conductivity [46] and surface effects [47,48] are likely to dominate the electron transport. This needs to be confirmed via temperature dependent charge transport measurements. Growth systems typically used for III-Nitrides growth use H2, NH3, and N2 as nitrogen precursors. There is an abundance of literature focusing on InN thermal stability [5, 6, 13, 14, 20, 49–52] in these systems. In this work a system utilizing ionized nitrogen was used. One aspect of this process that is different is that nitrogen ions are directly introduced into the system by using the nitrogen plasma source. Since ionized nitrogen has a different free energy this could be what is driving the reaction at higher temperatures and relatively low pressure even though Ambacher et al [6] indicate that nitrogen partial pressures should be on the order of 10 bar to grow at these temperatures. A combination of plasma activated nitrogen and the lack of hydrogen in the growth reaction [13, 14] could be contributing factors to this phenomenon. The probable presence of metallic indium in the growth could lead people to believe that the sample crystallizes during cool down from an indium-rich mixture of indium and InN, however the dependence of growth rate on the presence of ionized nitrogen does seem to indicate to the contrary [53]. Further work is necessary to establish the presence of impurities such as carbon, from the breakdown of the TMI, or excess nitrogen, carbon and oxygen as may be indicated by the high carrier concentration and absorption peak in the FTIR spectra.

4 Conclusions In summary, InN films were grown at temperatures varying from 700˚C to 957˚C using plasma-activated nitrogen as the group V precursor in the MEPA-MOCVD system. InN growth was confirmed by Raman spectroscopy and XRD. Broad and sharp E2(high) Raman peaks were used to refine the model spectra for samples grown above 873˚C. We attributed them to disordered small grain InN and coalesced crystalline large grain InN as revealed by the AFM. Hall transport measurements revealed that changes in free carrier concentration and mobilities are independent of growth temperature and are likely defined by surface and possibly hopping electron transport. We demonstrated indium nitride growth above its decomposition temperature, which gives more options to explore the growth of ternary alloys and heterostructures with GaN and AlN. The presence of impurities (possibly nitrogen per FTIR results), or the lack of hydrogen in the growth reaction could be factors allowing for growth above previously accepted decomposition temperatures, however the theoretical work describing the growth of InN using ionized nitrogen should be further explored. Further experimental work will need to be done to eliminate the effect of impurities and explore the growth parameter space to produce atomically flat films useful for semiconductor devices.

Acknowledgments: This work was supported by the DOE grant NA-22-WMS-#66204 via PNNL subcontract and NSF grant #EAR-1029020. Authors are thankful to Pete Walker for helpful discussions and help with growth facilities maintenance.

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Highlights:



InN was grown on sapphire substrates by migration enhanced plasma-assisted MOCVD



Growth temperature varied within 700-957oC, above InN decomposition temperature



Raman and XRD indicate polycrystalline InN



AFM reveals granular surface with grain size increasing with temperature