We investigate the effect of hydrogen on morphological changes in GaN columns grown by electron cyclotron-resonance plasma-excited molecular beam epitaxy on sapphire (0 0 0 1) substrates. In GaN growth with a hydrogen flow of 6 seem, hexagonal columnar structure was obtained. Morphological changes of the GaN columns from hexagonal to triangle were observed by increasing the hydrogen flow rate. It is considered to be caused by preferential growth along three of the six [10 (1) over bar 10] directions, The surface of the triangle columns consisted of {10 (1) over bar3} and {10 (1) over bar1} faceted planes, Formation mechanism of the triangle columnar structure is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.