5非晶硅材料物理

大学本科必修课：《薄膜晶体管原理及应用》第5讲非晶硅材料物理任课教师：董承远上海交通大学电子工程系2015‐3‐30Solids(cont) Solids (cont.)Structure CrystalCrystal (cont.)Structure(cont)Structure(cont) Crystal (cont.)Imperfections in Crystals p yImpurities in CrystalsCrystalBondingEnergy Band in Single CrystalEnergy Band in Single Crystal (cont.)Influence of Defect on EBInfluence of Doping on EBp gDefinition of Amorphous StateProperties of Amorphous State p pof Amorphous State InstabilityDescription of Amorphous Structurep Localization in Amorphous StateLocalization in Amorphous State (cont.)pEnergy Band in Amorphous SemiconductorCrystal Structure of c‐SiExercise: The diameter of a silicon atom is d=0.235 nm, under the assumption that silicon atoms are hard spheres that just touch each th i th ili t l D t i th t l l tti t t other in the silicon crystal. Determine the crystal‐lattice constant (width of the unit cell, a).Covalent Bond in c‐SiSiSiSiSiEnergy Band in c‐SiDoping in c‐SiConduction in c‐SiExercise: P-type silicon has resistivity of 0.5 Ω·cm. Assuming μn=1450 cm2/V·s and μp=500 cm2/V·s, find the hole and electron concentrations. (Electronic Charge q=1.6×1019C)qτin c Photoconductivity ‐SiPhotoconductivity in c‐Si (cont.)Exercise: Please calculate the wave-length limit to induce34 photoconductivity effect in c-Si. (Plank’s Constant h=6.63×10-34 J·s, Speed of light in Vacuum c=3×1010cm/s)Why Amorphous Silicon in TFTs?•In an AMFPD incorporating TFTs, the substrate is usually a borosilicate glass and the area often greater than 1000 cm;devices cover an2•The thin film semiconductor in a TFT must be deposited on top of the substrate, usually from a gas phase precursor that can be easily distributed uniformly over a l arge area;•Of the materials, a‐Si:H currently dominates as the semiconductor material ofBenefits & Weakpoints of a‐Si TFTs •Good film deposition uniformity •Simple device structure•Low leakage current•High yield•Low mobility y•InstabilityStructure of a‐Si:H※The structure of four-fold-coordinated amorphous semiconductors consistspof randomly arranged tetrahedra of atoms.The bonding is generally covalent, thus this structure is referred to as a random covalent network.Bond distances are close to those of the corresponding crystalline materialsp g y(generally<±1%deviation).The major change is in the bond angle (distortions typically<±10°relative to the tetrahedral109°28’).c‐Si, a‐Si, and a‐Si:H Structure andBand DiagramC-Si a-Si a-Si:HC Si Si Si HLong Range Order v.s. Short Range OrderypDOS of n‐type a‐Si:HWeak BondsWeak BondsDeep StatesCovalent BondsQ: Why do the deep states locate at the position of E?of Trap States in a Description ‐Si•Acceptor‐like state: the localized states in the pp y g pupper half of the mobility gap closer to the conduction band edge, neutral when empty and charged when filled with an negativelyelectron.•Donor‐like state: the states in the lower part l k h h lof the gap closer to the valence band edge, positively charged when empty and neutral when filled with an electron.Modeling States in a‐Siof Localizedof Trap States in a(cont) Modeling ‐Si (cont.)‐Modeling of Carrier Concentrations in a Si-=vE dEE E 1⎰∞-free f g p ))()((q E E /)(-=ψvalues a KTypical of ‐Si:H at T=300 d ld l Trapped Electrons Trapped Holes Density of Ei(cm ‐3)Exponential Slope (mv)Density at Ei (cm ‐3)Exponential Slope (mV)Tail Statesn ti ～6.2×1013V nt ～30p ti ～2.7×1011V pt ～50States Deep n di ～1015V nd ～85p di ～1015V pd ～128n fi ～1.0×1010v =25.9 mVExercise : Plot the dependence of the numbers of trapped and free electrons on the Fermi voltage th and free electrons on the Fermi voltage.Hydrogenation in a‐Si•If the hydrogen content is increased above 10 at.%, then a polymeric material can be formed in which twoH atoms are bound to a single Si atom, which can thenonly form two bonds with other Si atoms to form achain‐like structure analogous to alkane chains incarbon systems.•A further increase in the hydrogen content can cause the out hydrogen to segregate from the siliconnetwork. The H atoms cluster together to form smallmicro‐voids in the silicon network that can be largec o o ds t e s co et o t at ca a geenough to contain hydrogen molecules. A‐Si:H with avery high hydrogen content is very porous, has a high y g y g y p,g defect density and poor electronic properties.Hydrogenation in a‐Si (cont.)Doping ‐Siin a•For crystalline material, doping takes place by “substitution ” behavior.•For amorphous material, doping relates to ionized state in the forbidden “ionized generation”gap. Doping is correlated to dangling bonds, tail states and also the hydrogen content.l l h hAs the •doping increases,number of dangling bonds increases and Fermi level shifts.Doping in a‐Si (cont.)Q: How about P-type doping process?in a(cont)Doping ‐Si (cont.)Doping efficiency is very low, typically between※Doping efficiency is very low typically between10-3and 10-1and is dependent upon theconcentration of the dopant gas in the gas phaseduring deposition.during deposition.Where Cg is the ratio of the dopant gas flow rate tothe silane gas flow rate.th il fl tConduction in a‐SiLow Temperature Conduction At low temperatures Variable Range Hopping Low Temperature ConductionAt low temperatures, Variable Range Hopping :In a-Si:H, an electron occupying ,py ga localized dangling bond defectstate just below the Fermi levelmay be in close proximity withan unoccupied localized defectstate just above the Fermi levelstate just above the Fermi level. Upon the application of anelectric field and almost noexpenditure of thermal energy,the electron may tunnel from thefirst state to the second.Average Temperature ConductionAverage Temperature ConductionAs the temperature rises, there is sufficient thermal energy for some electrons to be excited into the conduction band tail, leaving holes in the valance band tailthe valance band tail.Electrons in a band tail state are localized near a weak bond. Under the application of an electric field, the electron (and holes) may tunnel li i f l i fi ld h l(d h l)l from one localized state to another. This process is known as band tail hopping..The probability that an electron or hole occupied such a state is given b th F i f ti hi h i th t il f th di t ib tiby the Fermi function, which , in the tail of the distribution, approximates to a Boltzmann distribution:High Temperature ConductionQ: How to enter the extended state conduction except raising temperature?。