Dislocation Mechanisms in the GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy
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EXPERIMENTAL The substrates used for our LEO experiments were 2.7-µm-thick GaN seed layers grown by MOVPE on a sapphire substrate. A 0.2-µm-thick SiO2 mask layer was grown on top by tetraethoxysilane (TEOS)-based chemical vapor deposition at ~600°C, and growth windows were patterned by standard optical lithography and etching processes. The pattern consisted of 3- or 4-µm-wide stripe openings parallel to the [1100] direction separated by 8-µm-wide mask areas. In one sample (sample no. 3), the stripes (5.5 µm in width) were arranged in a radial pattern, allowing comparisons to be made between the LEO process along different lateral growth directions. The samples were grown in a vertical HVPE reactor [5]. The samples were introduced into the growth region of the system under flowing NH3 and N2. The growth was initiated by the introduction of HCl to the Ga source region. The sample was kept at a constant temperature and gas phase ambient during growth. The five growth conditions investigated are listed in Table 1. Many factors influence the growth of HVPE GaN. We have altered the growth temperature in this study since it is the primary variable affecting the growth behavior. Table 1 Growth conditions, growth rate, and defect structure Sample 1 2 3a 3b 4 5
Tg (°C) 1050 1050 1075 1075 1100 1100
NH3 flow rate (sccm) 600 1000 1000 1000 600 1000
HCl flow rate (sccm) 18 18 22.5 22.5 18 18
Vertical growth rate (µm/h) 8 13 24 24 9.6 5.8
Lateral growth rate (µm/h) 3.6 6 14 2.4 20 20
c-axis tilt 7° 8° 8° 10° 3° 5°
RESULTS AND DISCUSSION A. Growth morphology Triangular-shaped ridge growth was observed at 1050°C over 3-µm-wide stripe openings parallel to the [1100] direction in sample no. 1 [Fig. 1(a)]. The growth facets are curved before coalescing into a continuous film. In sample no. 2, higher NH3 flux (1000 sccm) results in higher growth rate. The lateral/vertical growth rate ratio (= 0.45) and the ridge growth morphology remain the same as in sample no. 1, but coalescing has occurred, and the film has become continuous. In sample no. 3a, higher (∼2×) lateral and vertical growth rates with a lateral/vertical rate ratio of about 0.6 were observed on stripe windows parallel to the [1100] direction. The increase in growth rate is mostly due to the higher HCl flow rate. In sample no. 3b, the stripe windows are parallel to the [1120] direction, and the lateral growth rate is much smaller (2.4 µm/hr), resulting in a lateral/vertical rate ratio of ∼ 0.1. From the scanning electron microscope images, we learned that on [1120]-oriented windows, the LEO is on (1101) and (1101) facets, while on [1100]-oriented windows, the growth is on (1122) and (1122) facets. Our observations imply a much slower LEO rate on (1101) and (1101) facets. Similarly strong dependencies of growth rate on facet
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(a) (b) Fig. 1 Cross-sectional SEM images of GaN grown on windows parallel to the [1100] direction: (a) sample no. 1 grown for one hour. (b) sample no. 5 grown for one hour. plane have also been reported for the lateral growth of G
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