Research On Silicon-based Nanaocrystalline Diamond Film Growth And Light-emitting Devices

In recent years,the deposition technique,growth mechanism and applications of nanocrystalline diamond film(NDF)have inspired enormous research intrests, and are becoming a hot area in the field of CVD diamond film research.As is known,diamond has a very wide bandgap of 5.47 eV and a large exciton binding energy of 80 meV,making it an excellent material for ultraviolet and visible light emitting devices.Unfortunately,as the dominant material for integrated circuits industry,silicon suffers from the difficulty of light generation due to its severe non-radiative recombination effect.Evidently,the integration of diamond with silicon so as to form electroluminescence(EL)devices is a promising solution and is expected to find wide potential applications in silicon-based optoelectronics.In this dissertation,approaches of depositing NDFs in PECVD and HFCVD systems have been developed;the thermal stability and luminescent properties of the NDFs were subsequently researched;Si-based diamond EL devices were fabricated and their carrier transport and EL properties were intensively investigated.The primary significant results were summarized as follows:(1)Utilizing conventional 13.56 MHz r.f-PECVD system and CO/H₂ gas mixture,large area(5×5 cm²)NDFs were deposited uniformly at low temperature(500℃)and low pressure(200-300 Pa)conditions.The optimum CO/H₂ ratio for NDF deposition was found to be 12:1,the NDF thus deposited was consisted of closely packed diamond grains with sizes of 20-30nm,featuring smooth surface roughness(rms＜15nm)and good adhesion to the substrate.Such NDFs is expected to find applications in mechanical and optical coating areas.(2)A simple and effective technique of NDFs deposition utilizing conventional CH₄/H₂ gas mixture in HFCVD system under low pressure conditions (～125 Pa)has been developed without Ar addition or substrate bias applied.The NDFs thus fabricated were characterized by low surface roughness and high optical transmittance.The effect of pressure on the growth mode,grain size,and growth rate of the NDFs were comprehensively investigated and the formation mechanism of NDFs under low pressures was elucidated.(3)Taking advantage of the rapid thermal process(RTP),the thermal stability of the sub-microcrystalline diamond film(sub-MDF),NDF and diamond-like carbon(DLC)film were comparatively investigated.Distinct features were observed for each kind of film:the sub-MDFs were stable up to 900℃whereas the surface layer became graphitized after RTP at 1100℃;the NDFs appeared to lose materials at grain boundaries after 800℃RTP whereas 1200℃RTP induced grain aggregation as well as the formation of SiO_x(1＜x＜2)nanowires on the film surface;the DLC films were stable up to 500℃and diamond nanocrystallites were formed after 700-900℃RTP treatment,high temperature RTP at 1100-1200℃leaded to the formation of SiO_x(1＜x＜2)nanowires and SiC interlayer.The chemical reactions occurred during the RTP treaments were investigated and the formation mechanism of the diamond nanocrystallites,SiO_x(1＜x＜2) nanowires and SiC interlayers were analyzed.(4)The photoluminesnence(PL)study of the NDFs with different grain sizes revealed opposite intensity variation of the 430 and 530nm PL peaks:with the reduction of the diamond grain size:the intensity of the 430nm peak increased whereas that of the 530nm peak decreased rapidly.With respect to this phenomenon,three possible origins of the 530 nm luminescence—boron doping,carrier recombination in amorphous carbon and surface hydrogenation—were investigated and excluded based on experiment results.Further research results based on low temperature-RT cathodoluminescence(CL)and monochromatic CL imaging indicated that the 530 nm luminescence center in NDFs consisted of four sharp peaks between 480 and 532nm whose intensity increased rapidly with the lowering of the temperature.(5)The room temperature electroluminescence(EL)from the diamond/Si heterojunctions was achieved for the first time utilizing heavily and lightly doped n-type and p-type(n⁺,n^-,p⁺)Si substrates coated with nanocrystalline diamond films with different grain sizes.It was found that (1)the electron tunneling effect occured for the diamond/n⁺-Si heterojunction under sufficient forward bias,inducing the defect related visible EL,whereas hardly any EL could be obtained under reverse biases; (2)for the diamond/n^--Si heterojunction,no EL could be obtained under forward bias,but under high reverse bias condition defect related EL and enhanced 630,740 and 757nm EL peaks could be obtained,owing to the hole injection from the n^--Si substrate into the diamond layer;(3)quite similar phenomenon was observed for the diamond/p⁺-Si heterojunction, except that the biases were reversed compared to that of the diamond/n^--Si heterojunction.The carrier transport and EL mechanisms of these heterojunctions were elucidated based on their current-voltage(â… -â…¤) characteristics and energy band diagrams.(6)The N₂ microplasma luminescence was excited in the diamond film-based Au/SiO_x/diamond/n⁺-Si and Au/diamond/n⁺-Si MIS structures when applied with high forward voltages.Upon increasing the forward voltage, the MIS devices experienced negative-differential resistance(NDR)effect in theirâ… -â…¤curves and strong N₂ microplasma luminescence(along with its half-frenquency replica)could be excited thereafter at higher voltages. Through the in-depth analysis of electron transportation in the MIS devices, it was pointed out that the N₂ microplasma was formed by the activation of air near the surface of the MIS device by the highly energetic electrons emitted from the MIS device under the high voltages.