INDUSTRY CPV
Figure 3. A simulation and three-dimensional plot showing ternary composition verses bandgap for SiGeSn
lattice-matched III-V layers for forming higher-energy cells. According to analysis of the solar spectrum relative to the cell design, the germanium layer must be at least 5 µm-thick in order to absorb 85 percent of the solar radiation transmitted by the top III-V junctions in a multi-junction cell. Such a device will combine ultra-high efficiency with low manufacturing costs, because it can be initially formed on 150 mm silicon wafers, before being quickly transferred to a 200 mm platform.
We are playing our part in propelling multi-junction cells to higher efficiencies by developing a high quality group IV template of germanium that is grown directly onto a silicon wafer, and then adding a lattice-matched SiGeSn structure that will allow the formation of a 1eV junction. Subsequent addition of III-V materials allows the formation of upper junctions to create a device architecture with the potential to deliver very high efficiencies.
Figure 4. Efficiency evolution by the
application of both lattice and bandgap engineering can be enabled via incorporation of group IV SiGeSn alloys as shown by this modelling graph of efficiency with four different types of sub cell design (courtesy of Yong Hang Zhang, ASU)
This approach offers an ‘on-silicon’ technology for CPV that addresses mechanical issues – such as defect propagation, thermal mismatch, and cracking – and offers a roadmap to an ultra-high efficiency CPV sub-cell.
Building the platform
Our first objective has been to develop a virtual germanium substrate on silicon that offers the same characteristics as bulk germanium. To meet this criterion, our engineered substrate must: function as the bottom germanium junction; and provided a template for MOCVD that allows the growth of
To make this happen we have had to circumvent complications associated with direct growth of germanium on silicon – normally this leads to the growth of rough, highly defective layers, due to a significant difference in lattice constant between the two materials (it is 5.41 Å for silicon and 5.66 Å for germanium). We avoid this trap with a novel deposition approach, which was originally developed at Arizona State University and has been modified for pilot production at our Palo Alto plant. It is based on a low-temperature, ultra-high-vacuum (UHV) CVD process.
At our headquarters, we use this to deposit germanium on silicon in a custom-designed horizontal furnace that is capable of simultaneous growth on multiple 150 mm wafers. Germanium is synthesized at low growth temperatures on silicon (100) wafers using digermane (Ge2 and deuterated tin (SnD4
H6 ) ) precursors. For the
growth of thick, virtual germanium-on-silicon templates, a small amount of tin precursor is injected into the chamber during the growth. Its atomic concentration in the final layer is less than 0.5 percent, so it does not influence the structural, optical, or electrical properties of the germanium template.
However, at these levels tin is able to play a valuable role, modifying the germanium-silicon interface by promoting the formation of misfit dislocations. They are tied to this interface and do not propagate into the germanium layer. Attributes of our low temperature UHV-CVD technology include control of the tin content in the film and the opportunity to realise compositions well in excess of 0.5 percent. In the 1-2 percent range (as shown in figure 1) Ge1-x
becomes a true binary alloy – compared to germanium, it has a larger lattice parameter and greater optical absorption.
Snx
A larger lattice parameter is a tremendous asset, bringing a new degree of freedom to the design of the multi-junction sub cell devices. In the majority of today’s production-qualified III-V multi- junction devices, the fixed lattice parameter of the germanium wafer constrains III-V layer compositions and consequently the bandgaps of each of these layers. In contrast, the more flexible
52
www.compoundsemiconductor.net July 2013
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 |
Page 179