In situ stress measurements during the MOCVD growth of AlN buffer layers on (1 1 1) Si substrates
Introduction
The epitaxial growth of GaN on Si (1 1 1) and Al2O3 (0 0 0 1) substrates always involves the growth of an intermediate buffer layer of AlN or GaN [1]. While on Al2O3 a thin (<20 nm), low temperature AlN or GaN (500–600°C) buffer layer that is subsequently annealed at the growth temperature of 1000–1100°C yields the best GaN films, on Si, a thicker (>30 nm) high temperature (>900°C) buffer layer works better [1], [2]. These results are largely empirical and a common framework to explain these differences is yet unavailable. In addition, while growth of GaN on sapphire is aided by the fact that on cooling from growth temperatures, the GaN epilayer is placed in compression due to the coefficient of thermal expansion (CTE) mismatch, on Si the GaN layer is placed in tension on cooling [1]. Hence, the growth of thick (>400 nm), crack-free GaN layers and heterostructures on Si substrates using a simple AlN buffer layer has been elusive until recently [3], [4]. It has been shown that the tensile thermal stress on Si can be mitigated, to some extent, by changes in the composition and structure of the buffer layer [2], [3], [4]. However, in spite of the fact that buffer layers strongly impact the quality of the epilayers, there are very few studies [5] in the literature on the growth of the AlN buffer layer itself, other than to study its effect on the GaN epilayer. Data on growth stresses in nitrides are also scarce [2], [3], [6], [7], [8], [9]. On Si, ex situ post growth studies performed using X-ray diffraction showed that the growth stresses are tensile with a value of +0.5 GPa in GaN grown on a high temperature (1100°C) AlN epilayer [9]. A similar value has been reported in GaN layers grown on sapphire from in situ studies [8]. Data on growth stresses in AlN on Si or sapphire by MOCVD are unavailable. Growth of AlN deposited on Si (1 1 1) by ultrahigh vacuum reactive sputter deposition at 810°C takes place under compression initially and then under a 0.5 GPa tensile stress [7]. Thus, in order to gain a better understanding of the growth of the buffer layer by MOCVD, stress evolution during growth of AlN on Si (1 1 1) substrates was monitored in situ using a multi-beam optical stress sensor (MOSS). Preliminary results on stress evolution during subsequent growth of the GaN epilayer are also presented.
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Experimental details
The substrates used in this study were 10 mm squares cleaved from 250 and 500 μm thick (1 1 1) Si wafers. Following cleaving, the substrates were cleaned in an ultrasonic bath using acetone, alcohol and DI water prior to growth. Growth was carried out in a vertical rotating (30 rpm) disk reactor with a water cooled quartz jacket that maintained a constant wall temperature of 60°C. A SiC coated graphite susceptor was used to heat the substrate by RF induction heating. The substrate temperature was
Film characteristics
The films grown as a part of this study were found to be of uniform thickness on visual inspection and based on cross-sectional SEM studies done on GaN films grown on Si under the same conditions, the total thickness variation is expected to be less than 2.5%. AFM studies of surface morphology in films of thickness greater than 200 nm thick showed that the AlN films grow by nucleation and coalescence of 3D islands. In agreement with earlier reports [5] the islands are equiaxed and faceted in
Discussion
Stresses in epitaxial III–V nitride thin films on Si are equi-biaxial and arise due to lattice mismatch, microstructural evolution and CTE mismatch. Epitaxial stresses arise because of the difference in lattice parameters between the substrate, the buffer and the epilayers. The epitaxial relationship in the GaN/AlN/Si system is (0 0 0 2)GaN||(0 0 0 2)AlN||(1 1 1)Si and GaN||AlN||Si [15]. Using 3.189, 3.112 and 3.84 Å as the inter-atomic spacings [1] corresponding to the above
Summary and conclusions
Growth stresses in AlN on (1 1 1) Si were found to be tensile right from the start of growth over a temperature range of 600–1100°C and growth rates of 0.1–1 nm/s. The tensile stress is believed to arise from a combination of epitaxial stresses and island coalescence stresses, though direct evidence of island coalescence was not obtained. There is a sharp transition in stress from >1 GPa above 900°C to <0.4 GPa below 800°C due to a transition in crystal structure from an epitaxially oriented
Acknowledgements
This work was supported by the National Science Foundation under grants number ECS-0093742 and DMR-0076312 and Air Products and Chemicals, Inc.
References (19)
- et al.
J. Crystal Growth
(2003) - et al.
J. Crystal Growth
(2003) J. Mech. Phys. Solids
(1996)- et al.
J. Crystal Growth
(1993) J. Phys. D: Appl. Phys.
(1998)- et al.
J. Appl. Phys.
(2001) - P. Rajagopal, T. Gehrke, J.C. Roberts, J.D. Brown, T.W. Weeks, E.L. Piner, K.J. Linthicum, in: C. Wetzel, E.T. Yu, J.S....
- et al.
MRS Internet J. Nitride Semicond. Res.
(1999) - et al.
Appl. Phys. Lett.
(1991)
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