Srinivasan Raghavan (Vasu)

Research:

• Growth of thin films, nano-structures and bulk crystals.
• Growth of group IIIA (Ga, In, Al) nitrides by chemical vapor deposition and wet chemical routes.
• Stress and defect structure evolution during growth of thin films
• Effects of stress on crystal properties and device performance

Research Areas:

Crystal Growth:
Developments in the science and technology of crystal growth have played a vital role in the rapid technological progress in the latter part of the 20th century.

Crystal growth finds applications in areas as diverse as

• bulk growth of Si ingots (100m) that drive the Pentium chips
• to epitaxial and non-epitaxial thin films (10-6to 10-9m)
• to “nano”wires and quantum dots (10-7 to 10-9m) which are hot research topics currently, but expected to be of technological importance in the future.

Professor Srinivasan Raghavan’s research interests encompass all the above areas of crystal growth. He is currently concentrating on growth of the group III-A(Al, Ga, In) nitrides by chemical vapor deposition with particular emphasis on the effects of stress on crystal growth, physical properties and device performance.

Group III-A nitrides: These nitrides by virtue of their band gap (~1-6 eV) are used in a number of opto-electronic applications such as blue, white and ultraviolet light emitting diodes, 400 nm laser diodes and could potentially be used in photovoltaics or solar cells with efficiencies >30%. They are also candidates for the next generation high power-high frequency electronics because of their large break down field strength, high electron saturation velocity and high thermal conductivity. (More) However, in spite of all the impressive strides made in technology, much of the basic science behind the growth of these materials still remains poorly understood. This is so because the poor stability of these nitrides along with the lack of lattice matched substrates makes growth relatively more complicated than for example Si or GaAs based devices. Si and GaAs devices are deposited on Si and GaAs substrates. However, bulk nitride substrates are yet unavailable because of difficulties in processing. Hence, growth is currently done heterogeneously on sapphire (Al2O3), SiC and Si substrates. The lack of wettability, the lattice mismatch between the nitrides and these substrates and the thermal expansion mismatch all contribute to stresses and defects which affect device properties and performance. Thus, growing nitrides is a challenge.

Stresses and Defects: Defects, such as grain boundaries, dislocations and cracks are always detrimental to device performance. In the case of nitride thin films, due to poor wetting of the substrates by the film, growth occurs by nucleation and coalescence of islands. Due to small levels of misalignment between the grains, the process of coalescence generates a grain boundary and dislocations to accommodate the misalignment. The process of coalescence also gives rise to a tensile growth stress. If in addition the epitaxial stresses and the thermal expansion mismatch stress are tensile, then cooling to room temperatures can cause cracking.

While defects are invariably detrimental to device performance, stress could also have benign effects. Strained (or stressed) silicon in which electron mobilities are twice as fast as the unstrained counter parts have been used in Intel Pentium 4 chips (http://www.penstarsys.com/editor/tech/cpu/amd/str_sil/index.html). In GaN/AlGaN high electron mobility transistors for high power-high frequency devices in mobile applications, piezo-electric (or stress induced) polarization effects yield 2DEGs (2-dimensional electron gases) with sheet carrier concentrations in excess of 1013/cm2 close to an undoped GaN/AlGaN interface, with 300 K mobilities greater than 1500 cm2/Vs. Piezo-electric fields have also been shown to effect the optical properties in GaN/AlGaN and GaN/InGaN quantum well structures.

Stress evolution and defect evolution during crystal growth are inter-dependent. For e.g. cracking occurs in response to a tensile stress. Thus, the two need to be studied in tandem. By understanding how these two are related during crystal growth, devices can be grown with low defect levels and the required levels of stress (or strain), a process called strain engineering, to yield the desirable properties.

Recent Publications:

1. Srinivasan Raghavan and Joan M. Redwing, “Growth Stresses and Cracking in MOCVD GaN films on (111) Si: Part I, AlN Buffer Layers,” Journal of Applied Physics, 98, 023514, 2005. Presented at the 19th Conference on Crystal Growth and Epitaxy, AACGE/West, Lake Tahoe.
2. Srinivasan Raghavan and Joan M. Redwing, “Growth Stresses and Cracking in MOCVD GaN films on (111) Si: Part II, Graded AlGaN Buffer Layers,” Journal of Applied Physics, 98, 023515, 2005. Presented at the 2004 MRS Fall Meeting, Boston.
3. Srinivasan Raghavan, Jeremy Acord and Joan M. Redwing,”Direct Evidence for Tensile Stresses due to Coalescence During Growth of GaN on Sapphire Using a 600 C AlN buffer layer,” Applied Physics Letters, 86, 261907, 2005.
4. Srinivasan Raghavan and Joan M. Redwing, “Effect of AlN interlayers on growth stresses in GaN films grown on (111) Si by MOCVD,” Applied Physics Letters, 87, 142101, 2005. Presented at the 16 th American Conference on Crystal Growth and Epitaxy, Big Sky Resort.
5. Srinivasan Raghavan, Xiaojun Weng, Elizabeth Dickey and Joan M. Redwing, “Growth Stress and TEM Studies of Structural Evolution During Metal Organic Chemical Vapor Deposition of GaN on (111) Si Using Graded AlGaN Buffer Layers,” Applied Physics Letters, 05/2005.

Select Career Publications:

1. Srinivasan Raghavan, Hsin Wang, Wallace G. Porter, Ralph B. Dinwiddie and Merrilea J. Mayo, “The Effect of Grain Size, Porosity and Yttria Content on the Thermal Conductivity of nanocrystalline Zirconia,” Scripta Materialia, 1998, 39[8],1119-1125. Presented at the 101st Annual Convention of the American Ceramic Society, Indianapolis.
2. Srinivasan Raghavan, Hsin Wang, Wallace G. Porter, Ralph B. Dinwiddie and Merrilea J. Mayo, “Thermal Properties of Trivalent and Pentavalent Co-doped Zirconia,” Acta Materialia, 2001, 49[1], 169-179.
3. Srinivasan Raghavan and Merrilea J. Mayo, “The Hot Corrosion Resistance of 20 mol % YTaO4 Stabilized Tetragonal Zirconia and 14 mol % Ta2O5 Stabilized Orthorhombic Zirconia for Thermal Barrier Coating Applications,” Surface and Coating Technology, 2002, 160, 187-196. Presented at the 103rd Annual Convention of the American Ceramic Society, Indianapolis
4. Srinivasan Raghavan, Hsin Wang, Wallace G. Porter, Ralph B. Dinwiddie, Robert Vassen and Merrilea J. Mayo, “20 mol % Y(Ta/Nb)O4 Doped Zirconia Thermal Barrier Coatings,”- Journal of The American Ceramic Society, 87 [3] 431-37 (2004). Presented at the 102nd Annual Convention of the American Ceramic Society, St. Louis.
5. Srinivasan Raghavan and Joan M. Redwing,” Intrinsic Stresses in AlN layers grown by MOCVD on (0001) sapphire and (111) Si substrates,” Journal of Applied Physics, 2004, 96, 2995-3003.
6. Srinivasan Raghavan and Joan M. Redwing, “In-situ stress measurements during the MOCVD growth of AlN buffer layers on (111) Si substrates,” Journal of Crystal Growth, 2004, 261, 294-300. Presented at the 15th American Conference on Crystal Growth and Epitaxy, Keystone.
7. Abhishek Jain, Srinivasan Raghavan and Joan M. Redwing, “Evolution of Surface Morphology and Film Stress during MOCVD growth of InN on Sapphire Substrates,” Journal of Crystal Growth, 2004, 269, 128-133.
8. Srinivasan Raghavan and Joan M. Redwing,” Intrinsic Stresses in AlN layers grown by MOCVD on (0001) sapphire and (111) Si substrates,” Journal of Applied Physics, 2004, 96, 2995-3003.
9. Jeremy D. Acord, Srinivasan Raghavan, David W. Snyder and Joan M. Redwing, “In-situ Stress Measurements During MOCVD Growth of High Al-content AlGaN on SiC,” , Journal of Crystal Growth, 2004, 272, 65-71.
10. A. Pogrebnyakov, J. M. Redwing, Srinivasan Raghavan, V. Vaithyanathan, D. G. Schlom, S. Y. Xu, Q. Li, D. A. Tenne, A. Soukiassian, X. X. Xi, M. D. Johannes, D. Kasinathan, W. E. Pickett, J. S. Wu, J. C. H. Spence, “Increasing Superconducting Transition Temperature in MGB2 by Strain Induced Bond-Stretching Mode Softening,” Physical Review Letters, 2004, 93[14], 147006-1-4. Presented at the 2003 MRS Fall Meeting, Boston
11. Srinivasan Raghavan, Xiaojun Weng, Elizabeth Dickey and Joan M. Redwing, “Correlation of Growth Stress and Structural Evolution During MOCVD of GaN on (111) Si,” Applied Physics Letters, 88, 41904, 2006.
12. Xiaojun Weng, Srinivasan Raghavan, J. Acord, A. Jain, E. Dickey and J. M. Redwing, “Evolution of Threading Dislocations in MOCVD-Grown GaN films on Si,” Journal of Crystal Growth, 2007, 300, 217-222
13. B. Sheldon, A. Bhandari, A. Bower, Srinivasan Raghavan and J. M. Redwing, “Steady State Tensile Stresses During Growth of Polycrystalline Films,” Acta. Materialia. , Accepted for Publication, May, 2007.

Conference Proceedings

1. J. M. Redwing, A. Pogrebnyakov, Srinivasan Raghavan, J. E. Jones, X. X. Xi, S. Y. Xu, Q. Li, Z. K. Liu, V. Vaithyanathan, D. G. Schlom, “Epitaxial Growth of MgB2 Thin Films by Hybrid Physical-Chemical Vapor Deposition,” MRS symposium proceedings, v. EXS, n. 3 Frontiers in Superconducting Materials-New Materials and Applications, 2004, p153-155.
2. X. Weng, Srinivasan Raghavan, S. Dickey and J. M. Redwing, “Stress and Microstructure Evolution in compositionally graded AlGaN buffer for GaN growth on Si,” MRS symp. Proceedings, V. 892, GaN, AlN, InN and related materials, 2006, p27-32