溅射制备非晶氮化镓薄膜的光学性能

Optical properties of amorphous GaN films deposited by sputtering

采用直流磁控溅射方法在不同的氩气-氮气(Ar-N2)气氛中制备了非晶氮化镓(a-GaN)薄膜.X射线衍射分析(XRD)和拉曼光谱(Raman)表明薄膜具有非晶结构.通过椭偏光谱(SE)得到薄膜的折射率和厚度都随着氩气分量的增多而增大.紫外—可见光谱(UV-Vis)的测量得到,当氩气分量R,即Ar/(Ar+N2),为0%时,薄膜的光学带隙为3.90eV,比晶体GaN(c-GaN)的较大,这主要是由非晶结构中原子无序性造成的;而当R增大时(10%—40%),薄膜的光学带隙降低为2.80—3.30eV,这可能是由于薄膜中存在未成键Ga原子引起的.对吸收带尾进行了拟合,得到在高能量和低能量范围的带尾态特征能量分别为0.257—0.338eV和1.44—1.89eV,表明a-GaN具有比c-GaN更宽的带尾态.在室温光致发光(PL)谱中,360nm处的发光峰来源于带间发射.

The amorphous GaN（a-GaN） films are deposited by direct current planar magnetron sputtering in different compositions of argon-nitrogen mixtures. X-ray diffraction patterns and the Raman spectra indicate that the films have amorphous structures. Spectroscopic ellipsometry shows that the refractive index and the film thickness increase with increasing argon content. The UV- Vis spectra indicate that the band gap of the film deposited without argon is 3.90 eV, which is much larger than that of crystalline GaN （c-GaN）, owing to the structural disorder. When the films are deposited at higher argon content, the band gap becomes much smaller （2.80-3.30 eV）, probably because of more excessive Ga in the films. The band tails extending to lower energies are modeled. Two parameters representing the energy broadening of the electronic transitions and the width or slope of the exponential tail are 0.257-0. 338 eV and 1.44- 1.89 eV, respectively, which are higher than that of c-GaN films, indicating that a-GaN films have wider absorption tails than the c-GaN films. Photoluminescence peak at 360 nm observed at room temperature comes from band-to-band emission.