Melting and Crystallization of Amorphous Germanium Films on Insulating Substrate By Atmospheric Pressure Micro-Thermal-Plasma-Jet
Authors: Higashi Seiichiro , Harada Hiromu , Nakatani Taichi 
Published: February 2020

Fabrication of complementary metal oxide semiconductor (CMOS) on interlayer dielectric films is important to realize 3-dimentional (3D) integration which are promising for higher integrity beyond the scaling limits. Because of the high carrier mobility compared to that of silicon (Si), crystalline-germanium (c-Ge) is very attractive as a new channel material for 3D devices. In this work, rapid melting and crystalline growth of amorphous germanium (a-Ge) thin films on insulating substrate induced by atmospheric pressure micro-thermal-plasma-jet (μ-TPJ) irradiation have been investigated.

80 to 150-nm-thick a-Ge films were formed on quartz substrate by plasma enhanced chemical vapor deposition (PECVD) using GeH4 and H2 at 200 °C.

Phosphorus (P) ion implantation at the doses up to 5.0×1015 cm-2 (100 keV) was performed to some samples and then the films were dry etched to form strips with the width of 2 mm. After 550 nm SiO2 capping layer deposition by remote-PECVD, μ-TPJ was irradiated under atmospheric pressure with supplying power (P) of 0.8-1.1 kW, Ar gas flow rate (f) of 1.0 L/min, and scanning speed (v) of 600-1800 mm/s to perform crystallization and impurity activation simultaneously. The distance between the plasma source and substrate (d) was set at 2.0 mm. After formation of contact holes, Al electrodes were formed by vacuum evaporation and TiN electrodes were formed by sputtering to compare contact characteristics. The shape and the grain boundaries (GBs) of these Ge patterns were investigated by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) method.

Crystallized Ge films showed intrinsic p-type conduction when no doping to the samples was performed. They showed good Ohmic contacts with Al, and hole concentration of ~1×1018cm-3 and mobility of ~100 cm2V-1s-1 were observed in samples with small grain size. By performing lateral crystallization and obtaining large grain, the hole concentration significantly decreased to 5.2×1016 cm-3, and a high hole mobility of 1070 cm2V-1s-1 were obtained. These results suggest that defect induced hole generation was significantly suppressed by enlargement of grain size in Ge strips. Thin film transistors fabricated under the same crystallization condition showed a high on/off ration of 1.4×104 and a mobility of 196 cm2V-1s-1. Small field effect mobility compared with bulk mobility is attributed to the poor interfacial quality between SiO2/Ge.

In the case of heavily-phosphorus-doped and μ-TPJ crystallized Ge films, we obtained good Ohmic contacts not only by TiN electrodes, but by Al electrode also. We obtained a high electron concentration of 2.0×1020 cm-3 and mobility of 150 cm2V-1s-1, respectively. The efficiency of activated electron conc. / phosphorus conc. reached 54 %, and this high efficiency activation achieved good Ohmic contacts irrespective of electrode metal. EBSD images of the crystallized Ge, the maximum grain size was 28 µm in length and most of the GBs showed Σ3 coincidence site lattice (CSL).

By decreasing the phosphorus doping concentration, the mobility increased markedly as shown in the graph. Although there exist large scatter in mobility values, a highest electron mobility of 2720 cm2V-1s-1 has been obtained so far. μ-TPJ is a promising method for formation of high quality Ge crystalline films on insulating substrate.

Figure 1