The Preparation And Device Applications Of Nanocrystalline Ge Films By Thermal Annealing And Laser Annealing Techniques

Nowadays,the high performance microelectronic(or nano-electronic)and optoelectronic devices with high speed,multiple-function,low power dissipation and miniaturization,are highly desired in order to meet the requirements of today’s information age.Consequently,it is necessary to explore new functional semiconductor materials and devices.Although Si still dominates the microelectronics industry,GeSi and Ge-based materials,including nanocrystalline Ge(nc-Ge)films,Ge nanowires and Ge quantum dots have attracted more and more attention recently.Compared with Si,the electron and hole mobility of Ge are much higher,which can be used to fabricate high mobility transistors.On the other hand,the band gap of Ge is smaller than that of Si,which is located in the near-infrared range.As a result,it can be used to fabricate infrared light emitting devices and infrared photodetectors that are needed in monolithic optoelectronic integration.In addition,its smaller carrier effective mass as well as higher dielectric constant causes a large Bohr radius,which make it easily to observe the quantum confinement effect and therefore to tail the band structures in nano-scaled materials.Recently,light emitting devices,floating gate memory,high mobility thin film transistor,photodetectors and solar cell base on nanocrystalline Ge(nc-Ge)have been reported.In order to further improve the performance of nc-Ge-based devices,one of the interesting topics is to fabricate size controllable nc-Ge on cheap substrates such as glass plates by a suitable approach and to study the electronic and optical properties of prepared materials.Based on our previous work,we firstly studied the microstructure,optical and electrical properties of hydrogenated amorphous Ge(a-Ge:H)films prepared by plasma enhanced chemical vapor deposition(PECVD)technique.It is found that the microstructures and luminescent characteristics are changed with the decrease of film thickness.Then nc-Ge single layer and nc-Ge/SiNx multilayers were obtained by thermally annealing and laser annealing a-Ge:H single layer and a-Ge:H/SiN,multilayers.Especially,combining the laser crystallization technique and multilayered structures,we can prepare size controllable nc-Ge films on inexpensive glass substrate.The structural,optical and electrical properties,particularly the carrier transport process were studied.And we succeeded to obtain nc-Ge materials with the hole mobility as high as 211.7cm2V-s-1.Finally,we tried to prepare nc-Ge-based devices,including floating gate memory and photodetector and their performance was studied.The main results of this thesis are as following:(1)Hydrogenated amorphous Ge films with various thicknesses(160nm-5nm)were prepared by PECVD technique and the structures as well as optical and electrical properties of the samples were systematically studied.It is found that with the film thickness decrease to less than 10nm,the optical band gap of sample slightly increases while the conductivity reduces.Further study shows that the mechanism of carriers is dominated by extended states.We also study its photoluminescence(PL)characteristics.It is found that the PL spectra of thick sample(>10nm)are composed of two peaks,corresponding to the recombination between tail states and tail states to defect states.As the film thickness decreases to less than 10nm,however,only one PL peak is shown,whose intensity is enhanced and activation energy for non-radiative process is increased.The corresponding luminescence is originated from the recombination between tails states.These results indicate that the defect states in ultra thin a-Ge:H films maybe effectively passivated with the reduction of film thickness,and the luminescence related to defect states is quenched.Meanwhile,the size effect along the vertical direction cause intensive radiative recombination.(2)We prepared high quality nc-Ge films by thermal annealing a-Ge:H films at different temperature under N2 atmosphere.The effect of annealing on microstructure and optoelectronic performance of samples was studied.It is found that a-Ge:H films start to crystallize at 400℃.After the annealing temperature increases to between 450℃ and 600℃,nc-Ge films with the grain size of 10-30nm was formed.The electrical tests indicate that the room temperature conductivity of crystallized sample is 6 orders higher than that of as-grown films and reaches as high as 14.2 S cm-1,while the Hall mobility is enhanced by 3 orders and reaches up to 211.7 cm2V-1s-1.It is the highest value of nc-Ge films prepared by thermal annealing technique to our acknowledgement,which indicates the high quality of our prepared samples.Moreover,we obtained size controllable nc-Ge multilayered structure by thermal annealing the deposited a-Ge:H/SiNx multilayers.The corresponding room temperature conductivity and Hall mobility are 5.5×10-4 S cm-1 and 5.0 cm2V-1s-1,respectively.It is also found that the carrier transport in multilayers is dominated by nc-Ge sublayer.In addition,an intense visible light emission was also observed at room temperature.(3)We developed the KrF pulses excimer laser crystallization technique to crystallize a-Ge:H single layer and a-Ge:H/SiNx multilayers.From the Raman scattering spectra and transmission electron microscope(TEM)images,the Ge nanocrystals can be formed under a relatively low laser fluence compared with Si.And we found that during the evolution from amorphous to nanocrystalline phase,the optical band gap of both nc-Ge single layer and multilayer shrinks with the increase of laser fluence.As to multilayers,the experiments showed that the band gap also decreases for sample with thicker Ge sublayer.These results demonstrate that we can effectively control the grain size of nc-Ge and prepared corresponding materials by designing the film thickness,sample structure and laser fluence.We also systematically studied the carrier transport process of nc-Ge single layer and multilayers and compared with that of thermal annealed samples.It is found that the electrical conduction is the same as that of thermally annealed one,which is determined by nc-Ge phase.According to the experiments,the Hall mobility of singled and multilayered nc-Ge sample is 39.4 cm2 V-1 s-1 and 2.4 cm2 V-1 s-1,respectively.Our results indicate that laser crystallization can be used to avoid long-time thermal treatment at high temperature,and therefore it is properly to fabricate high quality nc-Ge films with large area on inexpensive substrate.(4)We tried to fabricate floating gate memory and photodetector by using our prepared nc-Ge films and studied the device performance.The changes of the surface potential of SiNx(23nm)/nc-Ge(4nm)/SiNx(5nm)/p-Si structures were evaluated,by Kelvin probe microscope(KFM),which indicates that the charges can be injected and stored in nc-Ge-based structures.Then we test the capacitance-voltage(C-V)characteristics under high frequency of Al/SiNx(23nm)/nc-Ge(4nm)/SiNx(5nm)/p-Si structure and observed its charge storage behavior.It indicates that both electron and hole can be stored in the floating gate structure.Meanwhile,we fabricated metal-insulator-semiconductor(MIS)structured photodetector based on 3 periods of SiOx/nc-Ge and found that it has good device performance in the wavelength range of 300-1200nm.For SiOx(12nm)/nc-Ge(5nm)sample,the responsibility is about 600mA/cm2 and the corresponding external quantum efficiency is above 90%under 1V bias.