The crystallinity of GaN layers grown at low temperature by organometallic vapor phase epitaxy on sapphire using dimethylhydrazine and triethylgallium has been studied with Raman spectroscopy, atomic force microscopy and Rutherford backscattering spectroscopy. The layers were grown in the temperature range from 520 degreesC to 660 degreesC. Amorphous, possibly non-stoichiometric Ga-rich layers were produced below 560 degreesC. Smooth layers of crystalline GaN with a disordered structure were produced between 560 degreesC and 600 degreesC. Rough but crystalline layers were produced at higher temperatures. The minimum temperature for production of crystalline layers occurs at about 580 degreesC.