Optical characterization of nanocrystalline boron nitride thin films grown by atomic layer deposition

    Optical Characterization of Nanocrystalline Boron Nitride Thin Films Grown by ATOMIC Layer Deposition Michael Snure, Qing Paduano, Merle Hamilton, Jodie Shoaf, J. Matthew Mann PII: DOI: Reference:

S0040-6090(14)00951-1 doi: 10.1016/j.tsf.2014.09.065 TSF 33754

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

17 April 2014 24 June 2014 28 September 2014

Please cite this article as: Michael Snure, Qing Paduano, Merle Hamilton, Jodie Shoaf, J. Matthew Mann, Optical Characterization of Nanocrystalline Boron Nitride Thin Films Grown by ATOMIC Layer Deposition, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.09.065

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ACCEPTED MANUSCRIPT Optical Characterization of Nanocrystalline Boron Nitride Thin Films Grown by ATOMIC Layer Deposition

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Michael Snure*a, Qing Paduanoa, Merle Hamiltona, Jodie Shoafb, J. Matthew Manna

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a) Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH, U.S.A. b) Wyle Laboratories, Inc., Wright-Patterson AFB, OH, U.S.A.

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Boron nitride thin films were grown on sapphire and Si substrates by atomic layer deposition

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from triethylborane (TEB) and NH3 precursors in the temperature range of 500 to 900 ºC. By varying the TEB exposure the film thickness can be controlled with < 1 nm precision. At 600

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ºC the process is self-limiting, but films are found to be amorphous. Films grown at higher temperatures were identified as sp2 BN, but the process is no longer self-limiting. From Raman

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and IR absorption spectroscopy films deposited at 900 ºC were identified as nanocrystalline sp2 BN with crystallite sizes in the range of 3 to 8 nm depending on NH3 dosage. Films deposited at

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lower temperatures had broad red shifted IR absorption peaks indicating the lack of long range ordering. The visible and UV optical properties of these films were characterized by UV Vis transmission measurements over the range of 800 to 190 nm. Nanocrystalline films are highly transparent over this range up to the band gap, which was measured to be in the range of 5.83 to 5.65 eV depending on the NH3 dosage.

Keywords: Boron Nitride, Atomic Layer Deposition, Thin Films, Fourier-Transform Infrared Spectroscopy, Raman Spectroscopy _____________________________________________________________________________

ACCEPTED MANUSCRIPT *Author to whom correspondence should be addressed; E-mail: [email protected], Tel: +1-973-528-8929

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Introduction Boron nitride is of interest for use in optoelectronic and electronic applications due to its

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exceptional properties. BN can form a number of different allotropes with either sp2 or sp3

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bonding. The sp2 bonded BN crystalizes in a hexagonal (h-BN) or rhombohedral (r-BN) phase,

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and sp3 BN crystallizes in a cubic (c-BN) or wurzite (w-BN) phase [1]. Similar to graphite, sp2 BN is a layered structure possessing an in-plane hexagonal lattice where the C-C bonds of

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graphite are replaced by partially ionic N-B bonds making it an atomically flat insulator. The hexagonal and rhombohedral structures differ only by the stacking sequence of these hexagonal

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layers. In h-BN layers are ordered in an ABA sequence and r-BN in an ABC sequence with an interplanar spacing of 0.333 and 0.3325 nm, respectively [2]. h-BN has been widely used as a

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structural material and lubricant due to its chemical and thermal stability. More recently h-BN has demonstrated its usefulness as a wide band semiconductor with a band gap of (5.9 eV) and transparency from ultraviolet (UV) to mid infrared [3,4]. Much of the interest in h-BN as an optical material was sparked by the demonstration of efficient UV emission from high quality bulk material grown by a high temperature high pressure method [5]. As a dielectric sp2 BN is quite interesting with a dielectric constant of 4, breakdown field of 10 MVcm-2, high thermal conductivity, and high chemical and thermal stability [6,7]. The layered 2 dimensional structure is ideal for both dielectric buffer layers and gate materials. Numerous chemical and physical vapor deposition techniques have been used to grow h-BN thin films [8-10]. Metal organic chemical vapor deposition (MOCVD) has been used to grow 100‘s of nm thick epitaxial h-BN [11] and r-BN [12] on AlN or sapphire; and metal catalyzed CVD has

ACCEPTED MANUSCRIPT been used to controllably grow few monolayer thick h-BN on metal foil [13]. For many applications a deposition technique with near monolayer thickness control on insulating or

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semiconducting substrates is required. Atomic layer deposition (ALD) is a widely used technique

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to conformally deposit dielectrics like Al2O3 and HfO2 with monolayer control [14]. ALD has also been used to deposit a number of metal nitrides including h-BN. Previous work on ALD

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deposition of h-BN used a metal halide precursor, boron tribromide (BBr3), and ammonia as

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process gases [15, 16]. An inert transport gas (Ar or N2) was required to prevent gas phase decomposition of BBr3. These films were reportedly unstable. Our initial attempts to reproduce

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ALD deposition of BN from BBr3 and NH3 produced similarly unstable films. Ammonium bromide crystals formed on the surface of these films after a few days of exposure to air. In this

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paper we present work on UV through IR optical characterization of ALD grown BN thin films on Si and sapphire substrates. To eliminate air stability issues a metal organic boron source and

Experimental

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NH3 were used as precursors.

Boron nitride films were grown by ALD on (001) Si and (0001) sapphire substrates in a cold walled vertical reactor. Triethylborane (TEB) was used for the boron precursor, NH3 for the nitrogen precursor, and H2 as the transport and purge gas. A TEB flow rate of 10 μmol/min, NH3 flow rate between 1.7 and 14 mmol/min, and H2 flow rate of 7 mmol/min at a pressure of 1.3 kPa were used. The TEB was delivered from a bubbler held at a constant temperature of 10 ºC using H2 as the carrier gas. Substrate temperatures in the range of 500 to 900ºC were used in this study. ALD cycles of the sequence TEB exposure, H2 purge, NH3 exposure, and H2 purge were used. The TEB exposure time was varied over the range 2-32 s and NH3 over the range 2-6 s. The number of deposition cycles was varied from 2 to 100.

ACCEPTED MANUSCRIPT Film thickness was determined from x-ray reflectance (XRR) measurements using an Empyrean X’pert Pro in grazing incidence. Thickness was calculated assuming a two layer model and a BN

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density between 2 and 2.5 g/cm3. Surface topography was analyzed by atomic force microscopy

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(AFM) using a Bruker Dimension Icon in tapping mode using an OTESPA tip. Infrared absorption was measured using a PerkinElmer 400 FTIR spectrometer over the range of 700 to

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3000 cm-1. Films deposited on Si substrates were used for FTIR due to the limited IR

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transmission window of sapphire. Raman measurements were performed using a Renishaw inVia system with a 1 mW 488nm excitation source, 20 µm slits, and 3000 line/mm grating. UV and

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visible optical properties of films on sapphire were measured by transmission spectroscopy preformed using a PerkinElmer Lambda 900 UV/visible scanning spectrophotometer in

Results

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transmission mode over the wavelength range of 800 nm-190 nm.

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Atomic layer deposition is capable of achieving atomic layer thickness control by using sequential self-limiting reactions. The temperature window where ALD is a self-limiting process is typically quite narrow. Below this temperature window there may be insufficient thermal energy for films to grow and above this temperature window the reactants could decompose allowing them to pileup. The deposition rate was studied as a function of TEB, NH3, and H2 purge time at substrate temperatures between 500 and 900 ºC. Fig. 1 shows the growth rate per cycle as a function of TEB exposure time for substrate temperatures of 600 (a), 650, 800, and 900 ºC (b). At substrate temperatures of 550 ºC or below, there was no measurable growth. At 600 ºC the process is self-limiting where the growth rate saturates at ~0.7 Å/per cycle around 24 s of TEB. By increasing the temperature to 650 ºC or above, the process is moved above the selflimiting growth window. At these temperatures the growth rate increases continually with TEB

ACCEPTED MANUSCRIPT exposure time and substrate temperature. Varying NH3 and H2 exposure times from 2 to 6 s had no observable effect on the deposition rate. The NH3 exposure time was found to impact

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material crystallinity as shown below.

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Surface morphology of these films is strongly influenced by the deposition temperature. Fig. 2 shows AFM micrographs of films with similar thickness deposited on sapphire at 600 (a) and

Increasing the temperature outside of the self-limiting growth regime increases the

roughness.

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nm.

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900 ºC (b). The films deposited at 600 ºC are smooth with an average surface roughness of 0.2

The surfaces of these films are covered with nm sized grains that become

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increasingly rough with deposition temperature. The RMS roughness is 0.2, 0.3, 0.7, and 1.1 nm for films deposited at 600, 650, 800, and 900 ºC, respectively.

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The bonding characteristics of films were characterized using Raman and IR spectroscopy,

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which are quite sensitive to B-N bonds, allowing us to distinguish between sp2 bonded and sp3

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bonded BN. However, due to the structural and bond similarities between the hexagonal and rhombohedral phases, distinguishing between these phases is extremely difficult as demonstrated [2,12]. The IR transmission spectra of films deposited at 600, 650, 800, and 900 ºC on Si (001) are shown in Fig. 3 a). Four absorption peaks are observed in these films around 2500, 1370, 1120, and 800 cm-1. Peaks at 1370 and 800 cm-1 are characteristic of sp2 BN and are associated with the in-plane stretching and out-of-plane bending, respectively [17, 18]. The peak at 2500 cm-1 can be assigned to B-H [19] and the peak at 1120 cm-1 to B-C [20,21] The B-H bond is only observed in the low temperature samples. In the 600 ºC film a B-C bond is clearly observed. As the growth temperature is increased there is a distinct shift and narrowing of the in-plane B-N absorption peak, which indicates an increase in sample ordering due to improved crystallinity.

ACCEPTED MANUSCRIPT The very broad and red shifted peaks in the low temperature films closely resemble amorphous BN [22, 23].

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The effect of NH3 dosage on film structure was also examined by FTIR. Fig. 3 b) shows the IR transmission spectra of films deposited on Si (001) at 900 ºC by 50 cycles of 4/2/4/2 with

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ammonia flow rates of 1.7 and 14 mmol/min. Only absorption peaks assigned to sp2 BN are

The shift of the out-of-plane B-N absorption peak to lower

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with reduced NH3 exposure.

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observed in these films. A red shift in both the in-plane and out-of-plane B-N peaks is observed

frequency, as compared to the bulk value (817 cm-1), is caused by a combination of increased

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inter-planer spacing and reduced crystallite size. According to Rozenberg et al. [23] the out-ofplane B-N absorption shown in Fig. 3b) are characteristic of turbostratic BN with a crystalline

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At 950 ºC less than 15% of the NH3 is decomposed, and this concentration

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size < 20 nm.

precipitously drops at lower temperatures [24].

As such growth temperature has an even

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stronger impact on the structure of these films than NH3 exposure time. In order to gain a better understanding of the TEB decomposition step in our process, Si substrates were exposed to only TEB at 600 and 900 ºC. TEB is known to begin decomposition via β-elimination above 300 ºC, which at higher temperature will completely decompose into borane and ethylene [25]. TEB exposure of 50 cycles of 24/2/0/2 at 600 ºC and 20 cycles of 2/2/0/2 at 900 ºC were used for this experiment. Absorption peaks corresponding to the B-H and B-C vibrations are observed in the 600 ºC film, while only the B-C is found at 900 ºC, Fig. 4. From Fig. 3 and 4 we can infer a sequential growth reaction starting with decomposition of TEB into BC followed by reaction with ammonia to form BN and CH4. The presence of B-C in low temperature films is due to insufficient thermal energy to activate ammonia for the second half of the reaction. The observed B-H in both the BC and BN films grown at low temperatures is also

ACCEPTED MANUSCRIPT quite interesting. Previous reports on growth of BxC from TEB also show the presence of B-H in films grown in this temperature range [25, 26], where desorption of H was found to be the rate

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limiting factor in growth of these films [26]. H may play a similar rate limiting role in our

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process at low temperature. This will be the focus of future work.

Raman spectrum from a film deposited on sapphire at 900 ºC by 50 cycles of 4/2/4/2 with an

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ammonia flow rate of 14 mmol/min is shown in Fig. 5. Only one peak at 1369.7 cm-1 is

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observed, which can be assigned to the characteristic E2g mode of sp2 BN [18, 27]. The full width at half maximum (FWHM) of this peak is 40 cm-1. Compared to bulk BN, which has an E2g peak

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at 1366 cm-1 and FWHM of ~9cm-1, this peak is quite broad and blue shifted. As with IR absorption spectroscopy these deviations from the bulk values are ascribed to reduced crystallite

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size. Rechter et al. [28] and Campbell and Fauchet [29] developed models to explaining the broadening and shifting of Raman modes in nanocrystalline materials through relaxation of the These models were

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phonon selection caused by phonon confinement to a finite volume.

extended to BN by Nemanich et al. [27] to estimate the crystallite size. Accordingly films deposited at 900 ºC should have a crystallite size of ~5nm, which is consistent with IR absorption.

UV and visible optical properties of BN films deposited on sapphire substrates were characterized by transmission measurements in the range of 190 to 800 nm. The spectra for films deposited by 50 cycles of 4/2/4/2 with NH3 flow rates of 14 mmol/min at 800 and 900 ºC are plotted in Fig. 6 a). The film deposited at 800 ºC has a long absorption tail extending beyond the range of the spectrometer while, transmission of the 900 ºC film remains flat around 98% transmittance up to the band edge. Using the absorption spectrum the band gap (Eg) can be estimated for direct band gap materials using the relation α = C(hv-Eg)2/hv, where C is a

ACCEPTED MANUSCRIPT constant and hv is the photon energy [30]. Fig. 6 b) shows the Tauc plot for the films deposited at 900 ºC at NH3 flow rates ranging from 1.7 to 14 mmol/min. By extrapolating the linear portion of

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this curve, Eg was estimated to be 5.63, 5.75 and 5.85 eV for NH3 flow rates of 1.7, 7, and 14

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mmol/min, respectively. These values are consistent with previous reports for BN [31, 32]. The decrease in Eg with NH3 flow rate may be attributed to a decrease in film order or higher

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concentrations of C impurities due to low NH3 concentration.

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Conclusion

BN thin films were deposited by ALD from TEB and NH3 precursors on sapphire and Si (001)

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substrates with thickness control of < 1nm per cycle. The process was self-limiting only in a narrow temperature regime around 600 ºC, and above this temperature the growth rate increased

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continually. From Raman and IR absorption measurements films grown at low temperature were

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identified as amorphous, whereas films grown at 900 ºC were found to nanocrystalline with sp2

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bonded B-N. These high temperature films are quite transparent though the entire visible spectrum with a measured band gap ranging from 5.63 to 5.85 eV depending on NH3 concentrations. Reducing the temperature or the NH3 dosage was shown to have a great impact on crystallinity by limiting the amount of available reactive NH3 to form BN and sweep away C. Acknowledgements:

The authors would like to acknowledge support from the Air Force Office of Scientific Research under contract number 13RY03COR (Program Manager Kenneth Goretta).

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ACCEPTED MANUSCRIPT Figure Captions: Fig.1 Growth rate as a function of triethylborane (TEB) exposure time at substrate temperatures

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of 600 °C (a), 650, 800 and 900 °C (b) for 100 cycles on sapphire.

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Fig. 2 AFM images (1 µm x 1 µm)of films grown at 600 ºC using 100 cycles of 24/2/4/2 (a) and at 900 ºC using 100 cycles of 2/2/4/2 (b).

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Fig. 3 Infrared transmission spectra of films grown at four different temperatures from 600 to

DSP substrate was used as the background.

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900 ºC (a) and at three different NH3 flow rates from 1.4 to 5.6 mmol/min. A blank Si (001)

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Fig. 4 Infrared transmission spectra of films deposited from 50 cycles of TEB at 600 ºC and 20 cycles at 900 ºC.

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Fig. 5 Raman spectra of films grown at 900 ºC using 100 cycles of 2/2/4/2 on sapphire.

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Fig. 6: Transmission spectra of films deposited on sapphire at 800 and 900 ºC (a) and (αhv)2 vs.

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photon energy for films deposited at different NH3 flow rates (b).

ACCEPTED MANUSCRIPT Highlights: Atomic layer deposition of BN with < 1 nm per cycle deposition rates

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A narrow self-limiting temperature window was found

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Nanocrystalline h-BN films with a wide transparence window with Eg up to 5.85 eV

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