Study On Polycrystalline Germanium Films Prepared By Pulsed Laser Deposition

At present,the research and application of semiconductor film have become hot spots.As a narrow bandgap material,germanium is of great research value in photodetectors and modulators,thin film transistors as well as thin film solar cells due to its high carrier mobility and high IR band absorption coefficient.The polycrystalline germanium film faces the problems of high preparation temperature and difficult film formation alone.Pulsed laser deposition as an advanced thin film preparation technology can produce high quality thin films at low temperature.Therefore,it is of great significance to study the preparation of polycrystalline germanium films by pulsed laser deposition technology.Germanium films were deposited on silicon substrates via KrF excimer laser.High-purity germanium was used as target and Ar gas was introduced as background atmosphere during the deposition.The effects of different Ar atmosphere pressure,substrate temperature,pulse energy,laser pulse frequency,and deposition time on the structure transformation,the deposition rate and surface morphology of Ge films were investigated.The influence of annealing temperature and annealing time on the morphology and crystal structure of Ge films was investigated by annealing treatment of the amorphous films.Experimental parameters of pulsed laser deposition of germanium thin film were studied.Under the condition that the atmosphere pressure is 10Pa,the substrate temperature is 300℃,the pulse energy is 300mJ and the laser pulse frequency is 5Hz,the surface roughness of the prepared film with uniform particle size is low,and the structure of germanium film is compact.The thickness of the film was analyzed by elliptical polarizing spectrometer.When the atmospheric pressure is 10Pa,the substrate temperature is 300℃,the pulse energy is 300mJ,the laser pulse frequency is 7Hz,the deposition rate of the film is higher,reaching 0.77?/s.But due to the deterioration of the film surface morphology and structure of high frequency,5Hz is the optimized deposition parameters,and the deposition rate is 0.60?/s.Raman and XRD spectra are used to analyze the structure of the film.The temperature of the silicon substrate plays an important role in the films transition from amorphous to crystalline.The degree of order improves while the temperature rises,and the film completely crystallizes at 300℃,in this case,the peak of Raman spectrum is at297cm^-1.The film has a（111）preferred orientation,and the full width at half maximum（FWHM）becomes narrow.The film is a polycrystalline germanium film of high quality.The crystallization temperature of germanium film is lower than the general preparation method.An effective method to improve the crystal structure of film is annealing treatment.In this paper,the amorphous germanium films deposited at 100℃were treated by in situ annealing.The surface quality and crystallinity of the films obtained after annealing at 300℃are inferior to those prepared at substrate temperature of300℃by PLD.While the annealing temperature is 450℃,and maintains for 30minutes,the film has a compact as well as homogeneous surface.Raman peak is located at 298.2cm^-1,and the FWHM is 9.5cm^-1.The intensities of the diffraction peaks corresponding to（111）,（220）and（311）orientations of germanium films all increase,therefore,Ge film has a well polycrystalline structure,and the structure is in favor of the germanium film as a narrow band gap semiconductor material in thin film solar batteries.