Ilhom Saidjafarzoda, Mohammad Adnan, Shukla Deepa, Grasso John, Willis Brian G, Okyay Ali Kemal, Biyikli Necmi
Department of Electrical & Computer Engineering, University of Connecticut, 371 Fairfield Way, Storrs, Connecticut 06269, United States.
Department of Materials Science & Engineering, University of Connecticut, 97 North Eagleville Road, Storrs, Connecticut 06269, United States.
ACS Appl Mater Interfaces. 2021 Feb 24;13(7):8538-8551. doi: 10.1021/acsami.0c21128. Epub 2021 Feb 10.
We report on the low-temperature growth of crystalline GaO films on Si, sapphire, and glass substrates using plasma-enhanced atomic layer deposition (PEALD) featuring a hollow-cathode plasma source. Films were deposited by using triethylgallium (TEG) and Ar/O plasma as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed within 150-240 °C substrate temperature and 30-300 W radio-frequency (rf) plasma power ranges. Additionally, each unit AB-type ALD cycle was followed by an Ar plasma annealing treatment, which consisted of an extra (50-300 W) Ar plasma exposure for 20 s ending just before the next TEG pulse. The growth per cycle (GPC) of the films without Ar plasma annealing step ranged between 0.69 and 1.31 Å/cycle, and as-grown refractive indices were between 1.67 and 1.75 within the scanned plasma power range. X-ray diffraction (XRD) measurements showed that GaO films grown without Ar plasma annealing exhibited amorphous character irrespective of substrate temperature and rf power values. With the incorporation of the Ar plasma annealing process, the GPC of GaO films ranged between 0.76 and 1.03 Å/cycle along with higher refractive index values of 1.75-1.79. The increased refractive index (1.79) and slightly reduced GPC (1.03 Å/cycle) at 250 W plasma annealing indicated possible densification and crystallization of the films. Indeed, X-ray measurements confirmed that plasma annealed films grow in a monoclinic β-GaO crystal phase. The film crystallinity and density further enhance (from 5.11 to 5.60 g/cm) by increasing the rf power value used during Ar plasma annealing process. X-ray photoelectron spectroscopy (XPS) measurement of the β-GaO sample grown under optimal plasma annealing power (250 W) revealed near-ideal film stoichiometry (O/Ga of ∼1.44) with relatively low carbon content (∼5 at. %), whereas 50 W rf power treated film was highly non-stoichiometric (O/Ga of ∼2.31) with considerably elevated carbon content. Our results demonstrate the effectiveness of Ar plasma annealing process to transform amorphous wide bandgap oxide semiconductors into crystalline films without needing high-temperature post-deposition annealing treatment.
我们报道了使用具有空心阴极等离子体源的等离子体增强原子层沉积（PEALD）技术在硅、蓝宝石和玻璃衬底上低温生长结晶GaO薄膜的情况。分别使用三乙基镓（TEG）和Ar/O等离子体作为金属前驱体和氧共反应物来沉积薄膜。在150 - 240°C的衬底温度和30 - 300W的射频（rf）等离子体功率范围内进行了生长实验。此外，每个AB型ALD循环之后进行Ar等离子体退火处理，该处理包括在下次TEG脉冲之前额外进行20s的（50 - 300W）Ar等离子体暴露。没有Ar等离子体退火步骤的薄膜的每循环生长速率（GPC）在0.69至1.31 Å/循环之间，并且在所扫描的等离子体功率范围内，生长后的折射率在1.67至1.75之间。X射线衍射（XRD）测量表明，无论衬底温度和rf功率值如何，没有进行Ar等离子体退火生长的GaO薄膜都表现出非晶特性。随着Ar等离子体退火工艺的引入，GaO薄膜的GPC在0.76至1.03 Å/循环之间，同时折射率更高，为1.75 - 1.79。在250W等离子体退火时折射率增加（1.79）且GPC略有降低（1.03 Å/循环），这表明薄膜可能发生了致密化和结晶。实际上，X射线测量证实等离子体退火后的薄膜以单斜β - GaO晶相生长。通过增加Ar等离子体退火过程中使用的rf功率值，薄膜的结晶度和密度进一步提高（从5.11 g/cm³提高到5.60 g/cm³）。在最佳等离子体退火功率（250W）下生长的β - GaO样品的X射线光电子能谱（XPS）测量显示接近理想的薄膜化学计量比（O/Ga约为1.44），碳含量相对较低（约5 at.%），而50W rf功率处理的薄膜化学计量比严重偏离（O/Ga约为2. — 31），碳含量大幅升高。我们的结果证明了Ar等离子体退火工艺在无需高温沉积后退火处理的情况下将非晶宽带隙氧化物半导体转变为结晶薄膜的有效性。
