Fast Lateral Epitaxial Overgrowth of Gallium Nitride by Metalorganic Chemical Vapor Deposition Using a Two-Step Process
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rate varies approximately linearly with the V/Ill ratio, up to the point where { 10 1 11 facets are formed, which results in a jagged morphology and a drop of the lateral growth rate. For device applications it is desirable to maximize the lateral growth rate while avoiding the formation of jagged sidewalls. In this paper we report on a two-step process that results in a reduction of the growth time necessary to achieve full coalescence of adjacent LEO stripes. The overgrowth is initiated in conditions that lead to smooth sidewalls, after which the growth parameters are changed to increase the lateral growth rate. We show that the stripe morphology is essentially determined by the growth conditions of the first overgrowth step, which greatly extends the range of conditions to achieve smooth LEO stripes. EXPERIMENT The LEO GaN was grown on 2 1dm-thick GaN prepared by MOCVD on 2-inch diameter sapphire wafers using a conventional two-step process. 21 Samples were coated with 200-nmthick Si0 2 using plasma-enhanced chemical vapor deposition, and 5 pm-wide openings oriented in the direction' 6 were patterned using standard UV photolithography and wet chemical etching. The mask width was varied to give fill factors 19of 0.01 to 0.5, corresponding to mask widths of 495 to 5 pm. The LEO growth was performed at 76 Torr and 1060'C using hydrogen as the carrier gas. No dopants were intentionally introduced during growth. In this paper we discuss the properties of six samples whose growth parameters are listed in Table I. Uncoated samples were characterized by scanning electron microscopy (SEM) using a JEOL 6300F field emission microscope operating at 15 kV. Specimens for transmission electron microscopy (TEM) were prepared by wedge polishing followed by standard Ar+ ion milling. Images were recorded on a JEOL 2000FX microscope operated at 200 kV. The roomtemperature photoluminescence was excited using a HeCd laser (325 nm, -20 mW/cm 2) and detected using a 1/8 m grating monochromator and a photomultiplier tube. RESULTS Figure l(a,b) shows SEM micrographs of sample A, overgrown in a single 15-minute step using 3.6 slm of NH 3 , which results in jagged sidewalls (I 10111 facets). Figure l(c,d) shows micrographs of sample B, grown under the same conditions except that the overgrowth was initiated under half the NH 3 flow for the first 3 minutes. Sample B shows smooth vertical sidewalls ({ I 20}) and a slightly larger lateral growth rate. Although the samples shown in Fig. I were grown consecutively using the same GaN/A120 3 substrate and patterning run, the elimination of the 110 11 facets following the overgrowth initiation at low V/III ratio is reproducible and appears to be insensitive to process parameters such as threading dislocation density of GaN/A120 3 substrate, fill factor, and growth parameters of the second step. Table I Precursor flow and step durations Step 1 TMGa NH 3 Duration
Step 2 TMGa
NH 3
(s)
(pmol/min)
(slm)
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Sample
Duration
A A2
(s) 900 3600
(pmol/min) 100 100
(slm) 3.6 3.6
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