Microcrystalline Silicon-Germanium Thin Films By VHF-PECVD

Hydrogenated microcrystalline silicon germanium (μc-SiGe:H) films as a narrowband gap material, which is one of the most promising absorption materials forapplication in multi-junction solar cells due to its high absorbing coefficient andbandgap tailorable property. However, with the increasing of Ge content, the defects inthe films increase, which impede the practical application ofμc-SiGe:H in solar cells.In this work, a series of silicon-germanium thin films were prepared withSi₂H₆+GeH₄ and SiH₄+GeH₄ by using VHF-PECVD technique. The performance offilms with the increase of Ge content was research. We found the films whichdeposited by Si₂H₆ and GeH₄ show a slower Ge incorporation rate than the seriesgrown by SiH₄+GeH₄, and also the structure show almost no order deteriorating withthe increasing of Ge content. The films grown by Si₂H₆ and GeH₄ mixed gases have ahigher photosensitivity. Compared with SiH₄ and GeH₄, the microcrystalline silicongermaniumthin grown by Si₂H₆ and GeH₄ has certain advantages.SiH₄ and GeH₄ mixed source gases were used to systematically investigate theinfluence of depositing parameters such as substrate temperature, the total gas flow,glow discharge power and deposition pressure, on the microstructure andphotoelectronic properties ofμc-SiGe:H. It was found that the increase of temperature,films grow towards ordering; With the increase of total gas flow, the growth rateincreased, the crystal volume fraction decreased; With the increase of power, thestructural ordering of films increased but the defect density also increased when thepower exceeded a certain value due to bombardment of positive ions on the growing surface, result in deterioration of the photoelectronic properties of films; Whenreaction pressure is too large, the films show a structure deterioration, and the defectdensity increased when reaction pressure is too small.Finally, the hydrogenated microcrystalline silicon germanium film withcrystallization of 42.7%, Ge content of 20% and the photosensitivity over 103 wasobtained with substrate temperature of 190℃, 10% GeH₄ concentration, glowdischarge power of 12W and deposition pressure of 100Pa.