Low temperature plasma enhanced deposition of GaP films on Si substrate

Abstract : Amorphous and microcrystalline GaP films were deposited on Si substrates by time modulated plasma enhanced deposition from trimethylgallium and phosphine using constant hydrogen plasma at a temperature of 250–380 °C. Amorphous GaP films obtained at constant low radio-frequency (RF) power (20 W) mode exhibit the broad feature at 350–360 cm−1 and a shoulder at 370–390 cm−1 in Raman spectra. Amorphous films have smooth surface with root-mean-square (RMS) roughness decreasing from 0.9 to 0.2 nm with increasing deposition temperature from 250 to 380 °C. Small amounts of 3–5 nm GaP nanocrystal inclusions in the amorphous matrix are formed at 380 °C. An increase of RF power to 100 W during Ga and P deposition steps leads to the formation of a GaP crystalline phase as confirmed by transmission electron microscopy. Two peaks in the Raman spectra at 365 and 402 cm−1, which correspond to GaP TO-LO duplet, were observed in this case. However, the microcrystalline GaP layers have rough surface with RMS roughness of 6 nm. A critical role of the hydrogen plasma for GaP growth mechanism is demonstrated. Indeed an effect of the phosphorous accumulation in the deposition chamber during phosphine plasma assisted decomposition step is shown. The hydrogen plasma treatment after P deposition step allows one to avoid an influence of excess phosphorous to the growth process providing a way to obtain ALD like growth of GaP with good thickness uniformity.