TECHNOLOGY MICROSCOPY
This technique has a spatial resolution better than 0.3 nm, an analytical sensitivity of 1 atom per million and it can be used to study volumes greater than 106
nm3 .
Atom probe tomography is based on a combination of field ion microscopy and mass spectrometry (see Figure 2). Like the former technique, a needle shaped specimen with an end radius of less than 100 nm must be prepared before the measurement can begin. This specimen is cooled to cryogenic temperature under an ultra-high vacuum of around 10-11
mbar and subjected to an
Figure 2. Atom probe tomography can provide a three-dimensional map of a structure on the atomic scale
information, such as electron energy loss spectrometry (EELS), energy-dispersive X-ray emission spectrometry (EDS) and secondary ion mass spectrometry (SIMS). What is needed is a reliable technique that delivers precise, three- dimensional characterization at sub-nanometre levels. Such a technique can also be useful for determining dopant diffusion profiles around the drain/source, providing interface profiles between junctions, and uncovering the roughness of a multilayer contact.
Atom probe tomography is capable of doing all of this, and has the unique capability to combine detailed composition – with better resolution than EELS or SIMS – with the determination of structural features near the atomic scale. Three-dimensional maps can be constructed of detected atoms or molecules with equal efficiency (detection efficiency is greater than 50 percent) and without prior knowledge of the composition.
electric field, which creates an incredibly high DC field at the apex of the tip – it is tens of volts per nanometre. In addition to this field, voltage or laser pulses are applied to the specimen, triggering ionization of surface atoms that are subsequently field evaporated. The extracted ions are projected onto a position-sensitive detector that records their locations, which are linked to the former positions of the atoms on the tip. The isotopic identity of these atoms is revealed with a time-of-flight mass spectrometer that determines the mass-to-charge ratio of the ion. Armed with this information, it is possible to generate a three-dimensional reconstruction of the locations of tens of millions of neighbouring atoms.
Our team at the University of California, Santa Barbara, has used atom probe tomography to scrutinise nitride-based heterostructures. This effort kicked-off with a detailed study of a series of MBE-grown, GaN/AlN/GaN heterostructures, which were deposited on Ga-polar and N-polar GaN substrates using two different nitrogen sources: plasma nitrogen and ammonia.
The sample grown with a plasma nitrogen source has two AlN layers separated by a 20 nm-thick GaN layer. To study any effect of a gallium adlayer on the AlN/GaN interfaces, the first AlN layer was grown under metal-rich conditions, while the second was deposited under a slightly nitrogen- rich condition (prior to growth of the second AlN layer, the gallium adlayer was desorbed during a growth interruption, and no gallium was supplied during AlN growth). Note that when we switched to an ammonia source of nitrogen, we had intrinsically nitrogen-rich deposition conditions.
Figure 3. Three-dimensional atom distributions of plasma-assisted, MBE-grown (a) N-polar (b) Ga-polar Alx
Ga1-x N/GaN heterostructures, where the red and blue dots
represent aluminium and gallium atoms, respectively. A cylindrical volume with a 20 nm diameter was selected for the composition measurement from the centre data set of both samples, in order to make the interfaces perpendicular to the cylinder. (c) and (d) show chemical profiles of aluminium and gallium in the pure AlN layer indicated in red rectangle in (a) and (b)
54
www.compoundsemiconductor.net June 2013
Atom probe tomography allowed us to identify differences in the atomic structure of both these samples (see Figure 3). A compositional profile based on a cylindrical volume with a 20 nm diameter reveals aluminium compositions of the N-polar and Al-polar AlN layers of 0.98 and 1, respectively, which are in close agreement with the target compositions.
Our approach also exposes the asymmetric nature of the AlN/GaN interfaces in the metal- rich GaN/AlN/GaN heterostructure. For N-polar, chemical abruptness is observed at the top
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 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191