Solid‐Phase Crystallization of GeSn Thin Films on GeO₂‐Coated Glass

Abstract

Polycrystalline Ge thin films are promising candidates for next‐generation thin‐film transistors. However, the difficulty in Fermi‐level control due to the high density of defect‐induced acceptors has been one of the main problems for device applications. Herein, GeO2 preparation and Sn addition are combined in the advanced low‐temperature (375 °C) solid‐phase crystallization of Ge layers that have been recently developed. The GeO2 underlayer works effectively at a low Sn composition (< 4%), enlarging the grain size by a magnification of ≈5. An appropriate amount of unsubstituted Sn reduces acceptor defects and achieves a minimum hole concentration of 3 × 1016 cm−3 by combining post‐annealing (500 °C). This hole concentration is the lowest level for undoped polycrystalline Ge‐based thin films, which allows control over their Fermi level widely by impurity doping. Although the lower hole concentration provides a higher energy barrier height of the grain boundary, the hole mobility is maintained at 190 cm2 V−1 s−1 owing to the large grains (15 μm diameter). This achievement paves the way for the implementation of poly‐Ge‐based thin films in various semiconductor devices, including advanced thin‐film transistors.

Acceptor defects in polycrystalline Ge layers evaluated using linear regression analysis

“…The carrier mobility reached the highest values (690 and 370 cm 2 V –1 s –1 for holes and electrons, respectively) for poly-Ge films, even on a flexible plastic substrate 36 , 39 . In addition, Sn doping in Ge passivated the acceptor defects and reduced its p to the order of 10 16 cm –3 39 , 40 . However, despite the long history of poly-Ge thin films, the behavior of acceptor defects and their levels has not yet been systematically investigated.…”

Strain-dependent grain boundary properties of n-type germanium layers

“…In recent years, it has been discovered that in the solid-phase crystallization (SPC) of Ge thin films, increasing the density of amorphous Ge precursors or adding impurities significantly affects the crystallinity of the resulting poly-Ge thin films 32 – 34 . We achieved the lowest hole concentration (2 × 10 16 cm −3 ) for a poly-Ge thin film 35 and also realized control of n-type conduction by doping with impurities (Sb, As, and P) 36 . The carrier mobility reached the highest value for a poly-Ge thin film (holes: 690 cm 2 V −1 s −1 , electrons: 450 cm 2 V −1 s −1 ) 37 , 38 .…”

n-Type Polycrystalline Germanium Layers Formed by Impurity-Doped Solid-Phase Growth

“…In recent years, the hole mobility of p-type polycrystalline Ge layers has been remarkably improved by controlling the amorphous precursor states in SPC. − We demonstrated a p-channel thin-film transistor (TFT), which was the highest performing low-temperature TFT using a polycrystalline Ge layer. , The high crystallinity also contributed to the reduction of acceptor defects , and increased the dopant activation rate, which achieved n-type conduction control using Sb and As as dopants. , In this study, we examined the SPC of P-doped amorphous Ge layers for the first time and compared the effects of dopant species on the growth behavior and electrical properties of Ge layers. Appropriate amounts of P-doping improved the grain size and electron mobility μ n of the Ge layer, demonstrating the best characteristics of the low-temperature n-type Ge layer.…”

“…39−42 We demonstrated a p-channel thin-film transistor (TFT), which was the highest performing low-temperature TFT using a polycrystalline Ge layer. 43,44 The high crystallinity also contributed to the reduction of acceptor defects 42,45 and increased the dopant activation rate, which achieved n-type conduction control using Sb and As as dopants. 46,47 In this study, we examined the…”