research review Coalescence promises perfect
GaN boules Dislocation-free crystals can be made by forming GaN points seeds by the sodium-flux method and joining them together via coalescence
MAKERS of nitride LEDs, lasers and transistors all want to build their devices on large, dislocation-free substrates. Unfortunately, they don’t exist today, but they soon could thanks to the work of researchers at Osaka University, Japan.
Earlier this year, this team reported how they could make dislocation-free GaN crystals using a GaN point seed. Now they reveal how to join a pair of them together in a stainless steel tube that combines a melt of gallium, sodium and carbon with nitrogen gas held at a pressure of 3.6 MPa. After 200 hours of growth at 870 °C, crystals with dimensions of several millimetres forme that are free from dislocations associated with coalescence.
“We are now aiming to fabricate 8-inch GaN substrates by our newly developed coalescence process,” says corresponding author Mamoru Imade.
There are no reports of any GaN crystals of that size today. The incumbent method for producing boules of this wide bandgap material, HVPE, has enabled the fabrication of 4-inch substrates with a dislocation density of about 106
cm-2 .
Lower values are possible with techniques such as ammonothermal growth and a sodium-flux approach, but they yield smaller crystals.
Polish GaN crystal developer Ammono has reported that its ammothermal growth can yield 1-inch GaN crystals with a dislocation density of 5 x 103
cm-2 . However,
according to Imade, this technique suffers from a low growth rate and high levels of impurity in the crystals. In comparison, in the last few years he and his co-workers have used the sodium-flux technique to produce 2-inch crystals with a dislocation density of 104
cm-2 dislocation density of 108
on seeds with a cm-2
.
However, in their most recent work, the Osaka university researchers begin by producing a pair of GaN point seeds. To do this, they mount a sapphire plate with two 1.2 mm holes, separated by 0.5 mm,
a Horiba Imaging-CL DF-100. Any dark spots found in the images would result from non-radiative carrier recombination at dislocations.
For both types of crystal formed by coalescence, imaging revealed the absence of dark spots in the large areas apart from the coalescence boundary.
The sodium-flux method can yield GaN crystals grown on the point seed. 600 hours of growth results in the formation of this crystal,which is positioned against a backdrop of graph paper with a scale of 1 mm per division
on a 8 µm-thick c-plane GaN seed layer grown on sapphire. On this structure they deposit GaN by the sodium flux method.
Point seeds form through the apertures, and when a pair of them is arranged along the a-direction, they coalesce without generating dislocations at this interface. In comparison, arranging the point seeds along the m-direction produced inferior results, with voids appearing at the coalescence boundary.
To search for dislocations in the coalesced material, the team performed room- temperature cathodoluminescence imaging on cut and polished crystals with
However, near to this interface, dark spots were seen in material formed from seeds aligned along the m-direction. In contrast, no spots were visible in the crystal created from seeds in the a-direction.
Meanwhile, X-ray diffraction measurements of the full-width half maximum for GaN (0002) suggest that the crystalline quality of the material resulting from a-direction coalescence is as good as that distant from the coalescence boundary. This was not the case for m-direction coalescence.
Many researchers believe that substrates made from high quality crystals that are free from dislocations should lead to improved device performance, but this conjecture is yet to be fully tested.
“This is one of our current research topics,” says Imade. “We are now investigating device performance on these substrates.”
N. Dharmarasu et al. Appl. Phys. Express 5 091003 (2012)
The sodium-flux method that has been pioneered by researchers at Osaka University,Japan,has produced material with a diameter of up to 4 inches
October 2012
www.compoundsemiconductor.net 53
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