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Carbon doping and V-Pits in AlGaN/GaN HEMT structures on silicon

Verkerke, Maurits (2015) Carbon doping and V-Pits in AlGaN/GaN HEMT structures on silicon.

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Abstract:A high electron mobility transistor (HEMT) is a transistor with a channel with high electron mobility. An important performance characteristic of a gallium nitride (GaN) HEMT is its high breakdown voltage. To improve this voltage, the GaN (grown via metal-organic chemical vapor deposition) is doped with carbon atoms, yet this increases the density of V-pits, which lower the device quality. In order to restore low V-pit density, pentane is used as carbon source, allowing higher growth pressure. The effect of the growth parameters pressure and dopant type on the V-pit density and morphology are studied in this work, through examination of three species of samples: 1) low pressure samples with high carbon concentration through precursor doping, 2) high pressure samples with low (non-intentional) carbon concentration and 3) high pressure samples with high carbon concentration through pentane doping. Automated optical inspection and scanning electron microscopy are used to characterize the surface morphology of the samples. Transmission electron microscopy (TEM) is used to study the origin of the V-pit. Increasing the pressure by 165 mbar and the surface temperature by 70°C) lowers the V-pit density by a factor 3±0.7. Adding pentane further decreases the density a factor 10±5. TEM measurements attribute the change in density to an increase in lateral growth of a factor 2.7±0.5 over 70°C and 165 mbar. Doping a sample with carbon increases the average pit diameter and pit depth by a factor 3.5±1.5. Using pentane as dopant changes the facet orientation of the V-pits from {1¯101}-planes to {11¯22}-planes. Based on these results a simple model for formation and closure of V-pits is proposed. This model states that V-pits originate when the vertical growth is disturbed. Growth parameters temperature, pressure and carbon concentration affect the lateral growth rate, which eventually can close the pit if it is high enough to overcome the disturbance.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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