Interface-controlled layer exchange in metal-induced crystallization of germanium thin films

Abstract

Low-temperature synthesis of polycrystalline germanium (poly-Ge) thin films is of great interest in thin-film photovoltaic and electronics applications. We demonstrate metal (Al)-induced crystallization to form poly-Ge thin films on both glass and polymer substrates at temperatures as low as 200 °C. An interfacial diffusion control layer, intentionally interposed between the Al and the underlying amorphous Ge (a-Ge) layer, is found to achieve layer exchange while suppressing uncontrolled Ge crystallization within the bilayer samples. Germanium thin films with micron-size grains and (111)-preferred orientation are prepared by controlled Ge nucleation and Ge lateral overgrowth of Al during a-Ge crystallization.

Nucleation and growth kinetics during metal-induced layer exchange crystallization of Ge thin films at low temperatures

“…[1][2][3][4][5] The methods to prepare polycrystalline thin films can be separated into two major categories: (1) Thin film deposition at substrate temperatures of 300$600 C to form the polycrystalline phase directly; [6][7][8] (2) amorphous thin film deposition with subsequent thermal processes such as MIC, solid phase crystallization, and laser annealing. [1][2][3][4][5][6]9 In MIC, Al, Au, and Ag have been shown to catalyze the crystallization of amorphous germanium (a-Ge) at temperatures in the range 150 $ 250 C, which is significantly lower than that for a-Ge solid phase crystallization without the presence of these metals. 1,[3][4][5]10 Such a low-temperature crystal growth process enables the use of inexpensive substrates, e.g.…”

“…[1][2][3][4][5][6]9 In MIC, Al, Au, and Ag have been shown to catalyze the crystallization of amorphous germanium (a-Ge) at temperatures in the range 150 $ 250 C, which is significantly lower than that for a-Ge solid phase crystallization without the presence of these metals. 1,[3][4][5]10 Such a low-temperature crystal growth process enables the use of inexpensive substrates, e.g. glass or flexible polymer sheets that are of interest for large-area electronics and solar photovoltaic technologies.…”

“…22 X-ray photoelectron spectroscopy with in situ heating gives a qualitative interpretation of crystallization kinetics, inferred from the time evolution of the area fraction of the transformed phase on the sample surface. 1 Optical microscopy has been employed to visualize nucleation and growth behavior and to correlate the nucleation density with the crystallized fraction of thin films. 23 However, largely missing from these studies has been the quantitative analysis of both nucleation and growth behavior at various stages of thermal annealing.…”

The Larger Grain and (111)-Orientation Planes of Poly-Ge Thin Film Grown on SiO<sub>2</sub> Substrate by Al-Induced Crystallization

“…Kurosawa et al recently studied Al-induced crystallization of the a-Ge thin film on SiO 2 substrate, and they have acquired the preferentially (111)-orientation planes (~68%) poly-Ge thin film by decreasing the thickness of Al and Ge thin film to 50 nm [6]. Hu et al have achieved the (111)-orientation planespoly-Ge (~70%) thin film on SiO 2 substrate by using the GeO x diffusion control interlayer structure during Al-induced crystallization [7]. From Al-induced crystallization of the a-Ge thin film in our first experiments, we have gained (111)-orientation planes (~90%) poly-Ge thin film grown on SiO 2 substrate by lowing the annealing temperature to 325˚C and forming AlO x diffusion control interlayer at the same time.…”

Direct synthesis of highly textured Ge on flexible polyimide films by metal-induced crystallization

“…Because the solid-phase crystallization of amorphous Ge (a-Ge) requires temperatures higher than 400 C, 11,12 metal-induced crystallization has gained much attention as a way to lower the crystallization temperature of a-Ge. [13][14][15][16][17][18][19][20][21] Some researchers achieved polycrystalline Ge (poly-Ge) on plastic substrates; however, they had problems on the crystal quality of the resulting Ge layer or the expensive catalytic metals. [19][20][21] Recently, we found that using Al as a catalyst and controlling annealing temperature (180-375 C) allowed for large-grained, (111)-oriented poly-Ge on glass via exchanges between the Ge and Al layers.…”