ALD/ALE 2025 Session AF-WeA: Material Growth II

The group 13 nitrides, AlN, GaN, InN and their alloys, is an important class of semiconductors, forming the backbone for technologies such as light emitting diodes (LEDs) and high electron mobility transistors (HEMTs). Chemical vapor deposition (CVD) at high temperatures, 800-1200 °C, is the typical way of depositing these materials. But CVD of indium nitride (InN) is severely limited by the low thermal stability of the material,¹ and can thus only tolerate relatively low deposition temperature. As ALD is intrinsically low temperature, it is a logical path for InN films and has been demonstrated to deposit epitaxial InN films with excellent crystalline quality.^2,3 The two chemically and structurally closely related materials aluminium nitride (AlN) and gallium nitride (GaN) has been deposited by both plasma and thermal ALD, with ammonia (NH₃) as nitrogen precursor in thermal processes.

In this talk I will describe ALD of these semiconducting nitrides, with focus on the fundamental understanding of the ALD chemistry. ALD has been shown to be an enabler for the metastable InN and In_xGa_1-xN with x close to 0.5.⁴ By combining quantum-chemical density functional theory (DFT) calculations⁵ and in-situ techniques such as mass spectroscopy⁶, we can present a detailed surface chemical model for the ALD processes. I will also present results from our efforts understand the surface chemistry by in operando ambient pressure XPS. Finally, I will discuss limitations to thermal ALD of these nitrides and why plasma activation seems to be needed.

References

(1) Ivanov, S. V.; Shubina, T. V.; Komissarova, T. a.; Jmerik, V. N. J. Cryst. Growth 2014, 403, 83–89.

(2) Hsu, C.-W.; Deminskyi, P.; Martinovic, I.; Ivanov, I.; Palisaitis, J.; Pedersen, H. Appl. Phys Lett. 2020, 117, 093101.

(3) O’Brien, N. J.; Rouf, P.; Samii, R.; Rönnby, K.; Buttera, S. C.; Hsu, C.-W.; Ivanov, I. G.; Kessler, V.; Ojamäe, L.; Pedersen, H. Chem. Mater. 2020, 32, 4481–4489.

(4) Pedersen, H.; Hsu, C.; Nepal, N.; Woodward, J. M.; Eddy, C. R. Cryst. Growth Des. 2023, 23, 7010–7025.

(5) Rönnby, K.; Pedersen, H.; Ojamäe, L. J. Mater. Chem. C 2023, 11, 13935–13945.

(6) Mpofu, P.; Hafdi, H.; Niiranen, P.; Lauridsen, J.; Alm, O.; Larsson, T.; Pedersen, H. J. Mater. Chem. C 2024, 12, 12818–12824.

The miniaturization of electronic devices has intensified challenges related to electrical signal propagation delays, primarily due to increased wiring resistance and capacitance. As a response, there is a rising demand for low dielectric constant (low k) materials to enhance device performance. Silicon oxide-based dielectrics (SiCOH) are conventionally known as a low k material. However, the thermal and mechanical stability of these materials are significantly compromised due to their low film density. In this context, boron nitride (BN) films have emerged as promising candidates due to their low dielectric constant and high stability [1]. Hexagonal BN (hBN) is characterized by its layered structure and wide band gap (5.2 – 5.9 eV), and it is expected to exhibit low k values, particularly along the c-axis [2].

In this study, we explored the film formation process of hBN using a borazine-based precursor with N₂ and NH₃ plasma to achieve high-quality BN films with low k, high density, and high conformality. Deposition was conducted at 400 ℃ on a 300 mm Si substrate, resulting in an ideal hBN layered structure clearly revealed by cross-section TEM (Fig. 1a). It was possible to deposit even thicker layers while maintaining the hBN structure. Moreover, we have successfully deposited conformal hBN on a trench pattern for the first time (Fig. 1b), expanding the applicability and versatility of hBN. C-V measurements indicated a low k value of <3 for our hBN. Analysis through XPS and IR spectroscopy confirmed a stoichiometric BN composition containing a significant amount of B-N sp² bonds. By applying additional plasma treatments, we observed a reduction in k value along with increased density due to the promotion of sp² B-N bonding. The resulting hBN films exhibited a low k value at higher density than can be achieved with Si-based films (Fig. 2).

This methodology also facilitated control over the crystalline orientation of hBN. We found certain processing conditions led to the deposition of both less and highly ordered hBN. C-V measurements across varying oriented films demonstrated a decrease in k values correlating with order, while I-V measurements confirmed that order in crystallinity contributed to the reduction of leakage current. We established a relationship between the crystallinity of hBN, the k value, and leakage current. This research created high quality conformal hBN, demonstrating significant potential for practical applications as low dielectric constant films for high-performance semiconductor devices.

References

[1] C. M. Lin, et al., Adv. Mater. Technol., 2022, 2200022

[2] Y. Hattori, et al., ACS Appl. Mater. Interfaces, 2016, 8, 27877-27884

Indium nitride (InN) possesses high electron mobility, approximately 3000–4000 cm²/V·s) and a low bandgap (~0.7 eV), making it an ideal material for high-speed transistors and RF devices that require low-power, high-performance operation. Additionally, InN exhibits excellent infrared absorption properties, enabling its applications in infrared detectors, high-efficiency solar cells, and optical sensors. Recently, as device sizes have decreased and structures become more complex, research on atomic layer deposition (ALD) for fabricating InN thin films has gained significant momentum. However, previous InN ALD research has primarily focused on plasma-enhanced processes, which pose challenges such as (1) difficulty in achieving uniform film deposition on three-dimensional structures with high aspect ratios and (2) potential substrate damage due to high-energy plasma exposure. To address these limitations, we explored the feasibility of thermal ALD for InN deposition. Thermal ALD for InN is challenging due to the high deposition temperatures required, low reactivity of reactants, and the tendency of indium compounds to oxidize easily, leading to poor film quality and low deposition efficiency. In this research, we employed hydrazine and hot-wire-activated NH₃ as reactants for thermal InN ALD. We conducted a comparative study using trimethylindium (TMI) and InPALA as In precursors. The physical and chemical properties of the deposited InN thin films were characterized using X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD).

Electron-enhanced atomic layer deposition (EE-ALD) (Figure 1a) and electron-enhanced chemical vapor deposition (EE-CVD) (Figure 1b) can be employed for the low temperature deposition of thin films using metal precursors with various reactive background gases (RBGs).EE-CVD expands on the previous report of TiN EE-ALD using alternating Ti(N(CH₃)₂)₄ (tetrakisdimethylamino titanium (TDMAT)) and electron beam exposures with NH₃ RBG.During EE-CVD, the electron beam and the RBG are present continuously.Together with the RBG and electron beam incident on the surface, the metal precursor is pulsed into the vacuum chamber to control the film growth.

In this work, the metal precursors were TDMAT, Si₂H₆, and W(CO)₆.The RBGs were O₂, NH₃, CH₄, and H₂.The studies demonstrated TiO₂ EE-ALD and SiN, SiO₂, SiC_x, Si, W₂N, WO₃, and WC_x EE-CVD. To illustrate EE-ALD, TiO₂ EE-ALD was performed at T < 80 °C using alternating TDMAT and electron beam exposures together with O₂ RBG.The growth rate for the TiO₂ EE-ALD was ~0.7 Å/cycle.The TiO₂ EE-ALD films were nearly stoichiometric, displayed crystallinity, and were smooth as measured by atomic force microscopy (AFM).

To illustrate EE-CVD, SiC_x EE-CVD was performed at T < 100 °C using repeating Si₂H₆ pulses with continuous electron beam and CH₄ RBG exposures.XPS revealed a 1:1 Si:C stoichiometry for a CH₄ RBG pressure of 0.45 mTorr and C-rich films for higher CH₄ RBG pressures.The SiC EE-ALD growth rate was ~0.4 Å per Si₂H₆ pulse.The stoichiometric SiC EE-CVD films were smooth as measured by AFM.

W₂N EE-CVD was also performed at T < 120 °C using repeating W(CO)₆ pulses with continuous electron beam and NH₃ RBG exposures (Figure 2a).The W₂N EE-CVD growth rate was ~0.17 Å per W(CO)₆ pulse.The W₂N films had a resistivity of ~450 μΩ-cm.The W₂N EE-CVD films also displayed crystallinity and high purity (Figure 2b).This work shows that EE-ALD and EE-CVD with various RBGs can deposit a broad range of materials at low temperatures including oxides, nitrides and carbides.