INDUSTRY EPIWAFERS
measurement at 405 nm, 633 nm and 950 nm). Since the frequency of the Fabry-Pérot oscillations decreases with increasing wavelength, the 950 nm measurement is most appropriate for thick layers deposited with a high growth rate. Meanwhile, the signal generated by reflectance from the 633 nm source is more suitable for scrutinising thinner layers. 405 nm emission is absorbed in III-V materials, making this reflectance measurement highly surface sensitive. It offers insights into surface morphology and interface quality, and can determine tunnel junction thickness.
Keeping it flat If a fab is to have a high yield, it must produce wafers with minimal bow, both during and after growth. Historically, curvature measurements have focused on nitride films grown on foreign substrates, due to the high degree of strain in these materials – meanwhile, measurements on lattice-matched III-Vs have been neglected. But even with the latter material system, wafer bow and warp can occur during cooling, due to differences in the thermal mismatch of the substrate and the deposited layers. And when it comes to triple-junction structures, strain engineering becomes essential, due to the growth of pseudomorphic and metamorphic structures.
At Fraunhofer Institute for Solar Energy Systems, scientists employ the EpiCurve TT in the production of multi-junction solar cells, where it is used for managing the strain and optimising the process. The wafer bows significantly when intentionally lattice-mismatched III-V layers are deposited on germanium, and its curvature is strongly aspheric after intentional buffer relaxation for metamorphic growth (see Figure 3 a).With LayTec’s advanced resolution curvature technology, it is possible to distinguish between spherical curvature and asphericity (Figures 3b and 3c). The resulting signal helps a process engineer to optimise the growth of the buffer and its relaxation at early, decisive stages of the epitaxial process.
Researchers are striving to increase the efficiency of multi-junction cells, because this should make CPV technology more competitive. New designs are being pursued, which are often more complex than their predecessors, due to the addition of a fourth junction or the use of an inverted architecture. We can help these ambitious efforts – we can provide the tools for in-situ monitoring, and thanks to our close collaboration with leading researchers in academia, we can offer advice to industry customers from our resulting extensive know-how in analysing and understanding in-situ data associated with epitaxial processes. Our hope is that we will play a key role in driving up the yield and quality of multi-junction solar cells, and ultimately the rapid growth of this industry.
© 2013 Angel Business Communications. Permission required.
At Ioffe Physical Technical Institute, researchers produce multi-junction solar cells using an Aixtron MOCVD tool equipped with a LayTec’s EpiRAS TT system. These cells have been deployed in a concentrating photovoltaic system
Further reading J.F. Geisz
et.al. J. of Cryst. Growth 310 2339-2344 (2008)
W. Guter
et.al. Appl. Phys. Lett. 94 223504 (2009) N.A. Kalyuzhnyy
et.al. Proceedings of the 24th European Photovoltaic Solar Energy Conference (2009)
August / September 2013
www.compoundsemiconductor.net 47
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178