Crystallinity and microstructures of aluminum nitride films deposited on Si(111) substrates

doi:10.1016/S0038-1101(99)00307-X

Solid-State Electronics

Volume 44, Issue 4, 1 April 2000, Pages 747-755

https://doi.org/10.1016/S0038-1101(99)00307-X Get rights and content

Abstract

The crystallinity and microstructures of MOCVD AlN films deposited on Si(111) substrates with and without a buffer layer(s) were determined. The buffer layers were a thin 3C–SiC(111) layer produced via conversion of a Si(111) surface and a film stack consisting of graded-Al_xGa_1−xN/GaN/3C–SiC. A randomly oriented polycrystalline AlN film was obtained when this material was deposited directly on the Si(111). The use of a buffer layer led to the growth and coalescence of highly oriented AlN films produced by the coalescence of grains having average misalignments along the c-axis of 1.8° and that on the c-plane of 3.3°. The grains exhibited strongly faceted tips. The 2H–AlN(0001) films grown on a 3C–SiC(111) buffer layers showed adequate crystal perfection for use as a template for growth of single-crystal GaN and/or Al_xGa_1−xN films.

Introduction

Hexagonal 2H–AlN is a candidate material for high power, optical, and acoustic wave device applications. This is due to its high, direct band gap energy, high acoustic wave velocity and its solubility in GaN that allows the formation of AlGaN alloys and AlGaN/GaN heterojunctions. In addition, AlN is considered as a suitable intermediate buffer layer for GaN growth on Si(111) substrates. The epitaxial growth of AlN and GaN on Si(111) is of particular interest for potential integration of AlN and/or GaN devices with Si-based devices in one chip. However, low-defect, single-crystal epitaxial AlN is difficult to grow on Si(111), since the lattice mismatch is 18.8%.

The epitaxial relationship of a hexagonal AlN film and a cubic Si(111) substrate has been observed. Chubachi et al. [1] used metal-organic chemical vapor deposition and obtained single-crystal AlN(0001) on Si(111) with the AlN[11-20]//Si[110] relationship. The same crystallographic relationship was observed by Meng et al. [2] using reactive sputtering of AlN. Bourret et al. [3] used gas source MBE to grow AlN on Si(111) and observed two epitaxial relationships in one film, namely, AlN[2-1-10]//Si[02-2] and AlN[10-10]//Si[02-2]. Sanchez-Garcia et al. [4] deposited AlN on Si(111) by plasma-assisted molecular beam epitaxy and showed a single-crystal X-ray diffraction pattern of AlN. Although single-crystal-like AlN films have been obtained when deposited directly on Si, they either contained high densities of dislocations of approximately 3×10¹¹/cm² [2] or lacked long range order [3].

To improve the AlN crystallographic quality, attempts have been made to find a suitable buffer layer between the AlN and the Si substrate. Barski et al. [5] and Tamura and Hiroyama [6] used a thin (40 Å) crystalline layer of 3C–SiC(100) on Si(100) on which they deposited cubic AlN and GaN. Hong et al. [7] also used a thin 3C–SiC buffer layer for the deposition of hexagonal AlN on Si(111). The use of a 3C–SiC buffer layer between AlN(0002) and Si(111) seems to be a good approach for improving crystal perfection of the AlN, since the 3C–SiC(220) spacing (1.54 Å) is much closer to the AlN(11-20) spacing (1.56 Å) than the Si(220) spacing (1.92 Å) is to AlN(11-20).

This study investigated the crystallinity and the microstructures of MOCVD AlN films deposited on Si(111) with and without a buffer layer(s). The suitability of the deposited AlN for use as a substrate for GaN growth was also evaluated.

The AlN films were deposited using an RF heated, single-wafer, vertical, low-pressure chemical vapor deposition reactor, triethylaluminum (TEA) and ammonia as the source gases and H₂ as the carrier and diluent gas. The films were grown at 1100°C, and 45 torr. The gas flow rates were 0.134, 0.067, and 26×10⁻⁶ mol/min, respectively, for H₂, NH₃ and TEA. This yielded an AlN growth rate of approximately 11 Å/min.

The typical substrates used were Si(111) on which had been grown a thin (30–50 Å) 3C–SiC buffer layer via conversion of the surface by exposure to flowing ethylene at 970°C in an MBE reactor [8]. The two AlN films deposited on these substrates will be referred to as samples A1 and A2.

The second substrate used was bare Si(111). The film deposited on this substrate will be referred to as sample B. The third substrate was a film stack consisting of Si (on the bottom), SiC, AlN, GaN, and Al_xGa_1−xN alloys with x graded from 0 to 1. The layer thicknesses in the stack were 40 Å, 1000 Å, 5000 Å and 2000 Å, respectively, for SiC, AlN, GaN and the Al_xGa_1−xN alloys. The AlN film deposited on this film stack was referred to as sample C.

Section snippets

Results

The top surfaces of samples A1 and A2 were much rougher than those of samples B and C. The roughness values were measured by an atomic force microscope for the maximum thickness variation (Z_max) and root mean square thickness variation (rms) within an area of 1×1 μm and 5×5 μm. The results are tabulated in Table 1. The results show that sample B deposited directly on Si(111) had the smoothest surface. A comparison of surface morphologies for sample A1 and sample B is shown in Fig. 1(a) and (b).

Discussion

The microstructure in the AlN films observed in this study may have resulted from the specific deposition conditions employed or as a result of the substrate surfaces having only a thin film of converted SiC rather than a thicker (0.2–0.5 micron) deposited layer of SiC or for both reasons. Our previous study [8] showed that the thickness of converted 3C–SiC varied greatly within a film and its top surfaces were very rough. The rough surfaces likely provided a large number of low energy sites

Conclusion

The growth of highly textured 2H–AlN(0001) films on Si(111) substrates was significantly improved by the use of a 3C–SiC buffer layer. The buffer layers were formed by the conversion of Si(111) surfaces without an additional deposition of a 3C–SiC layer by CVD growth. The top surfaces of the converted SiC layers were very rough and resulted in the nucleation, growth and coalescence of the polycrystalline AlN films. The AlN grains were well aligned crystallographically. The crystallographic

Acknowledgements

The authors of NCSU acknowledge the support of the Semiconductor Research Corporation and Motorola Inc. R. Davis was supported in part by Kobe Steel, Ltd. University Professorship.

References (8)

Y. Chubachi et al.
Thins Solid Films
(1984)
W.J. Meng et al.
Appl. Phys. Lett.
(1991)
A. Bourret et al.
J. Appl. Phys.
(1998)
M.A. Sanchez-Garcia et al.
MRS Internet Journal, Nitride Semicond. Res.
(1998)

There are more references available in the full text version of this article.

Cited by (17)

Role of 3C-SiC intermediate layers for III-nitride crystal growth on Si
2011, Journal of Crystal Growth
Citation Excerpt :
III-nitride epilayers have been utilized in high-frequency and high-power electronic devices [1,2] as well as optical emitting devices [3,4]. Recently, III-nitride epilayers have been grown on Si substrates, since large diameter Si substrates are readily available and III-nitride device fabrication using Si-device production lines are expected to have low cost [5–15,17–19]. The epitaxy of III-nitrides on Si substrates is considerably more difficult than that on other substrates such as 6H-SiC or sapphire, mainly due to two reasons: First, Si substrates can be quickly damaged by Ga and nitrogen sources such as trimethylgalium (TMGa) and ammonia needed for III-nitride deposition [5,6].
The role of 3C-SiC intermediate layers in III-nitride crystal growth has been studied by observing III-nitride epilayers grown on Si substrates. We found that better quality epilayers were obtained by using such intermediate layers than by direct growth on Si substrates. In the case of III-nitride epilayers grown directly on Si, the layers grown at the initial stage are not flat. High-resolution transmission electron microscopy observations showed that a non-crystalline layer exists at the interface between the AlN layer and the Si substrate. Thus, the initial growth of III-nitride becomes disordered. On the other hand, the interface between 3C-SiC and the AlN layer is atomically flat, and there is no non-crystalline layer present. We concluded that III-nitride epilayers on Si substrates with 3C-SiC intermediate layers are promising for the fabrication of vertical devices.
Structural analysis of metalorganic chemical vapor deposited AIN nucleation layers on Si (1 1 1)
2004, Journal of Crystal Growth
AlN nucleation layers are being investigated for growth of GaN on Si. The microstructures of high-temperature AlN nucleation layers on Si (1 1 1) substrates grown by metalorganic chemical vapor deposition with trimethylaluminum (TMA) pretreatments have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM results show that with TMA pretreatment, AlN nucleates under a pseudo-two dimensional growth mode due to the enhanced lateral growth rate and improved wetting property of AlN on silicon. In addition, with TMA pretreatment, absence of amorphous SiN_x layer at the interface demonstrates good crystalline quality. Transmission electron diffraction patterns reveal epitaxial crystallographic orientation: $AlN[0 0 0 1]||Si[1 1 1]$ and $AlN[1 1 2 ̄ 0]||Si[1 1 0]$ . High-resolution TEM measurements also indicate a 5:4 lattice matching relationship for AlN and Si along the Si [1 1 0] direction.
Substrates for gallium nitride epitaxy
2002, Materials Science and Engineering: R: Reports
In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior.
The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented.
The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO₂, and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.
Metal organic vapor phase epitaxy growth of (Al)GaN heterostructures on SiC/Si(111) templates synthesized by topochemical method of atoms substitution
2017, Physica Status Solidi (A) Applications and Materials Science
Impact of growth conditions on stress and quality of aluminum nitride (AlN) thin buffer layers
2016, Physica Status Solidi (B) Basic Research
New Technology Approaches
2016, III-V Compound Semiconductors: Integration with Silicon-Based Microelectronics

View all citing articles on Scopus

View full text

Crystallinity and microstructures of aluminum nitride films deposited on Si(111) substrates

Abstract

Introduction

Section snippets

Results

Discussion

Conclusion

Acknowledgements

Thins Solid Films

Appl. Phys. Lett.

J. Appl. Phys.

MRS Internet Journal, Nitride Semicond. Res.