Self-Diffusion as A Limiting Factor of a-SiGe Crystallization

Abstract

Highly doped (∼1018 to 1021cm−3) polycrystalline Si1-xGex films, crystallized from amorphous (a) state at relative low temperatures, are prospective materials in a variety of applications, such as liquid-crystal displays, solar cells and integrated thermoelectric sensors on large-area glass substrates. Since the nature of the grains in the crystallized film defines properties such as carrier mobility, the nucleation and growth process of the a-SiGe films is of fundamental interest. We have studied the crystallization of undoped and highly doped (B or Ga) amorphous SiGe films. The films were deposited by RFCVD or molecular beam on oxidized (001)Si and for TEM study on cleaved NaCl. The incubation time and grain growth rate were studied by means of in situ TEM using a heating stage. The crystallization process in undoped SiGe followed Avrami relationship. An average grain size between 0.1 and 2μm was observed. However, the highly p-doped (with B or Ga) SiGe films crystallized to a stable nanocrystalline structure (grain size <10nm). The process of the a-SiGe crystallization is explained on the basis of self-diffusion. During the first stage, the nucleation of crystals is accompanied with nonequilibrium vacancy generation at the amorphous/crystalline interface. During the second stage, the growth of crystals takes place by vacancy outdiffusion which is hindered by B and Ga interaction with vacancies.

Crystallization of amorphous Si0.5Ge0.5films studied by means ofin-situX-ray diffraction andin-situtransmission electron microscopy

“…The change in n reflects a typical crystallization process in thin films, which starts as a 3D process when the crystals are small and transforms to a 2D process when the grain size D , reaches the film thickness d . When N is high and vg low, as was observed earlier in Sil-,Ge,v : H : B films (Edelman et al 1995) and Sil-,Ge, : Ga (Edelman et al 1996(Edelman et al , 1997, the crystallization resulted in a nanocrystalline structure, typical of a 3D mode, at temperatures between 500 and 850°C. However, in this case, the Sio.5Geo.5 films (both undoped and B doped) crystallized in a mode in which N is relatively low but wg is relatively high.…”

Low Temperature Reoxidation Mechanism in Nanocrystalline TiO[sub 2−δ] Thin Films

“…Rutherford backscattering spectroscopy ͑RBS͒ examination of the films showed that the O/Ti ratio is about 2/1, and no contaminations were detected by Auger electron spectroscopy ͑AES͒. 12 The sensing behavior during exposure to ambient oxygen between 100 and 325°C was reported in a previous paper. 15 The response is significant between 200 and 325°C, but a steady state is not attained within 1 h and the recovery is slower than the response, leading to a drift in the electrical conductance during exposure to successive pulses of oxygen.…”

“…The TiO 2 films were deposited on oxidized Si substrates with embedded Au interdigital electrodes, and were subsequently annealed at 400°C in vacuum ͑0.008 Pa͒ for 170 h. Microstructural characterization of the TiO 2 films was reported elsewhere. [12][13][14] The electrical conductance of TiO 2 specimens was monitored in an environmental chamber during exposure to air ͑practically oxy-gen͒ when the pressure inside the chamber increased from a base level of 0.008 to 3.1 Pa by letting air leak into the chamber for 1 h, and immediately afterwards during re-evacuation back to the base pressure for another 1 h. It took ϳ1 min for the system to pressurize and ϳ1 s to re-evacuate. These tests were done at constant temperatures between 100 and 325°C.…”

“…The microstructure and crystallization during annealing of the films have been reported in previous papers. [12][13][14] The films are largely amorphous in the as-deposited state and crystallize to nanocrystalline rutile ͑ϳ20 nm average grain size͒ after annealing at 400°C or above. Rutherford backscattering spectroscopy ͑RBS͒ examination of the films showed that the O/Ti ratio is about 2/1, and no contaminations were detected by Auger electron spectroscopy ͑AES͒.…”