表 面 物 理 学 江颖 量子材料中心 E. G. Wang 第六课: 表面热力学和动力学 E. G. Wang 本课内容提要: (一):表面热力学 表面热力学基本方程 表面张力和表面应力 晶体的表面能和平衡形状 表面的生长模式 (二): 表面动力学 表面生长动力学 表面二维岛的生长 表面二维和三维岛的退化 E. G. Wang 化学中的热力学和动力学 • 将化学反应应用于生产实践主要有两个方 面的问题: 1. 了解反应进行的方向和最大限度以及外界条 件对平衡的影响。 2. 知道反应进行的速率和反映的历程(即机 理)。 • 人们把前者归属于热力学的研究范围,把 后者归属于动力学的研究范围。 E. G. Wang 第一部分: 表面热力学 Josiah Willard Gibbs (1839-1903) E. G. Wang 前面主要讨论了“静止”表面的结构。这一节 将从宏观角度,运用平衡热力学基本理论研究晶 体表面的各种热现象。 研究宏观热现象的基本方法,是在实验的基 础上,用能量的观点研究温度对表面结构和表面 性质的影响。以平衡态的统计热力学理论为基础, 研究表面热现象及各种特征参数之间的关系,探 讨有关表面热力学、表面物理化学过程的基本规 律。 E. G. Wang Kinetic Processes and Surface in Equilibrium Surface is no static system. The characteristic bonding energy of an atom at a special site and the energy barriers between such sites determine the probability for adsorption, desorption and diffusion of atoms at a given temperature. Fig. 1. Schematic representation of fundamental atomic processes occuring during epitaxial growth. E. G. Wang Principle of detailed balance In the thermodynamic equilibrium all surface processes proceed in two opposite directions at equal rates according to the “principle of detailed balance”. Detailed balance means that the rate constants for the forward direction, rf, and the backward direction, rb, of a process satisfy the relation: rf /rb = exp(-ΔE/kBT), where ΔE is the energy difference between the initial and final states. Processes, such as adsorption and desorption, decay and formation of islands, etc. must obey the detailed balance. A surface after stopping growth for while at not too high temperature can be considered to be in thermodynamic equilibrium with the substrate and the surroundings, e.g., the rest gas. Crystal growth is clearly related to non-equilibrium kinetic processes. Yet the “principle of detailed balance” is still fulfilled. E. G. Wang Internal energy(内能): U = U(S,V,N) dU =(U/ S)V,N dS + ( U/V)S,N dV + ( U/N)S,V dN dU = T dS – P dV + μ dN The extensive property of the internal energy: U(λ S, λ V, λ N) = λ U(S,V,N) U=TS–PV+μN S dT – V dP + N d μ = 0 (Gibbs-Duhem 等式) 常用热力学函数: Enthalpy(焓): H = U + PV = T S + μ N Helmholtz free energy (自由能): F = U - TS = - P V + μ N Gibbs 函数(Gibbs 自由能) : G = F + PV = μ N Grand potential (巨势,自由能的差) Ω = F - G = - P V E. G. Wang 1 表面的热力学基本方程 U=TS–PV+μN For equilibrium system: T, P, μ are identical in different phases, S = S1 + S2 + Ss V = V1 + V2 + Vs N = N1 + N2 + Ns U=TS–PV+μN+γA γ = (U/ A)S,P,N -surface tension (表面能也称作表面张力) Enthalpy(焓): H = U + PV = T S + μ N + γ A Helmholtz free energy (自由能): F = U - TS = - P V + μ N+ γ A Gibbs 函数(Gibbs 自由能) : G = F + PV - γ A = μ N Grand potential(巨势,自由能的差): Ω = F - G = - P V+ γ A E. G. Wang 2 吉布斯定义的表面位置: V = V1 + V2 N = ρ 1V1 + ρ 2V2 In comparison to the total number of particles we have Ns = 0 & Vs=0 Enthalpy(焓): H = U + PV = T S + μ N + γ A Helmholtz free energy (自由能): F = U - TS = - P V + μ N+ γ A Gibbs 函数(Gibbs 自由能) : G = F + PV - γ A = μ N Grand potential(巨势,自由能的差): Ω = F - G = - P V+ γ A Gs = 0 Ω = Ω 1 + Ω 2 + Ω s= - P (V1+V2)+ γ A Ω s= γ A E. G. Wang 表面张力 = 表面应力? Surface tension Surface stress 液体:Yes! 固体:No! E. G. Wang σ U=TS–PV+μN+γA γ = (U/ A)S,V,N dU = A Σij σij dεij ε Surface stress (应力张量) Linear elasticity theory Surface strain (应变张量) dU = T dS + S dT– P dV – V dP + μ dN + N dμ + γ dA + A d γ dU = T dS – P dV + μ dN + A Σij σij dεij S dT– V dP + N d μ + γ dA + A d γ - A Σij σij dεij = 0 dA /A = Σ d εij δij A d γ + S dT– V dP + N d μ + A Σ ( γij δij - σ ij ) d ε ij = 0 A d γ + (S1+S2 +Ss) dT– (V1 + V2 + Vs)dP + (N1 + N2 + Ns) d μ + A Σ ( γij δij - σ ij ) d ε ij = 0 Si dT – Vi dP + Ni dμ = 0; i=1,2 E. G. Wang (Gibbs-Duhem 等式) A d γ + Ss dT– Vs dP + Ns d μ + A Σ ( γij δij - σ ij ) d ε ij = 0 - Gibbs 吸附方程 A d γ + Ss dT– Vs dP + Ns d μ + A Σ ( γ ij δij - σ ij ) d ε ij = 0 - Gibbs 吸附方程 V = V1 + V2 N = ρ 1V1 + ρ 2V2 In comparison to the total number of particles we have Ns = 0 & Vs=0 E. G. Wang γ = γ(ε,T) Ss = -A(γ/T)ε σij = γδij+ ( γ / εij)T A special example is liquid surface where the surface atoms is free to rearrange themselves: σ = γ 若: γ / ε< 0 E. G. Wang Equilibrium Shape of Small Crystals Anisotropy of Surface Energy The surface free energy per unit area, γ, of a certain crystal surface varies with its crystallographic orientation characterized by the surface plane (hkl) or the surface normal n, i.e., γ=γ (hkl) or γ=γ(n). 晶体达到平衡时,其表面能为各晶面表面能的总和: ∑ γ (hkl) dAhkl 若以θ表示晶面的方向角,则表面能γ随方向角θ的不同而改变。根据表面能 的方向性推测晶体的平衡形状,最成功的方法是Wulff作图法。经过数学上 的严格证明,这一方法得到了公认。 3 E. G. Wang Fig. 3 shows the orientation dependence. Its nominal (1n) surface (n>>1) represents a vicinal (01) surface. It consists of a high number of (01) terraces separated by atomic steps of height a. With θ~1/n as the angle of orientation of [1n] against the [01] orientation, the step density is given as tan θ/a. If βs is the energy per step and γ(0) is the energy of a (01) face, the surface energy of a (1n) surface is γ(θ) = cosθγ(0)+βs(sinθ/a) The prefactor cosθ guarantees that the relative amount of the (01) terraces to the total surface area reduces with increasing angle θ. The interaction of steps has been neglected here. The increase of the angle from θ=0 to large values is accompanied by an increase of the step density. A proper expression for γ(θ) must hence include the interaction between steps. Fig. 3 A (1n) surface, which is slightly misoriented from the (01) surface. E. G. Wang E. G. Wang Wulff Construction Wulff Theorem: The equilibrium crystal shape (ECS) at constant temperature T with fixed volume V and chemical potential μ is determined by the minimal excess surface free energy with respect to the surface A Fs = A(V)γ(n)dA Subject to the constraint of fixed volume V = V(A)dV. The theorem states that the ECS is not necessarily that of the minimum surface area. It may be a complex polyhedron with the lowest total surface energy for a given volume. In the case of crystals the variation of γ with the normal n will produce, on each surface element dA, a force proportional to ∂γ/∂n, which will tend to alter its direction at the same time as γ tends to shrink its area. A minimal surface only occurs for a perfectly spherical Wulff plot, i.e., an isotropic excess surface free energy. The corresponding ECS is a sphere. E. G. Wang Wulff作图法 表面能是一标量,为了表示其与晶面方向的相关性,引入一矢 量 ,矢量的方向为晶面的法线方向,其长度正比于表面能数值。因 此, (hkl)即为表面能γ (hkl) 的矢径。 E. G. Wang Fig.9 A polar plot of the surface free energy for a 2D crystal (solid line) and the ECS based on the Wulff construction (dotted line). The higher the γ(n), the smaller the corresponding surface. In theoretical (table 2.1), γ(111)<γ(311)<γ(100)<γ(110), so (110) surface is negligible. Fig.10 Equilibrium shapes of a Si crystal based on the Wulff construction using (a) exp. values or (b) theo. values. (100), (311), (110), and (111), from black to white, surface orientations are considered. (Surface energies from Table 2.1). E. G. Wang Surface Energy and Morphology Facetting and Roughening The surface buckling happens on a mesoscopic length scale (large than atomic distances). For a small buckling surface, we have F’s = A’γ(θ)dA’ = Aγ(θ)dA/cos (θ) The assumption of a weak variation of γ withθyields an expansion of the integrand up to second order, F’s = γ(0)A + Adγ/d θ | θ=0 + ½ A θ 2[γ(0)+ d2γ/d θ 2 | θ=0 ]dA The first term gives the energy of the flat surface. The second one vanishes for symmetry reasons. The third term gives the energy due to surface buckling. For γ(0) + d2γ/d θ 2 | θ=0 >0, the flat surface is stable (or at least metastable), for its <0, the buckling surface is more stable. Fig.11 Small buckling of a surface. E. G. Wang 表面上薄膜生长的三种典型模式 层状生长 E. G. Wang 岛状生长 SK 生长 γs -the surface free energy of substrate-vacuum interface γo - the overlayer-vacuum interface γs/o - is the substrate-overlayer interface The equilibrium of the forces holds as γs = γs/o + γo cosφ with φ as the angle between the overlayer-vacuum face and the substratevacuum face. Set Δγ= γs/o + γo – γs For Frank-van der Merve (2D) mode, we have Δγ 0 with φ = 0; For Volmer-Weber (3D) mode, we have Δγ > 0 with φ > 0; For Stranski-Krastanov mode, we have Δγ 0 the first atomic layers (wetting layer) and Δγ > 0 for the islands. E. G. Wang 第二部分:表面动力学 E. G. Wang 一般来说,上述表面热力学理论只 能描述处于平衡状态下的表面形貌, 而在实际的薄膜生长过程中,体系往 往是处于非平衡状态,这时候则需要 用非平衡动力学的观点来描述。 E. G. Wang 3D Versus 2D Growth In thermodynamic equilibrium, there is no net growth. The crystal growth must be a non-equilibrium kinetic process. The resulting macroscopic state of the system depends on the reaction paths in the configuration space. Since the result is kinetically determined, the obtained state is not necessarily the most stable one. E. G. Wang The thermal stability of the Al thin films Initial surface 2.4ML (300nm x 300nm) 24 hours later( 500nm x 500nm) Thermally annealed (RT) Al film develops various heights enabling the comparison of the relative stability of islands with different heights E. G. Wang 动力学 • Kinetics is a concept only involved with the movement of objects. • Dynamics focuses on the forces and their effects. • In certain point of view, we can say kinetics only deals with the motions of objects, and dynamics with the reason why and how they moves. E. G. Wang 表面动力学 • We can consider the growth on the surface as a kind of chemical reaction. • Kinetics: the influence of external macroscopic variables on the overall reaction rate (temperature, pressure, relative concentration) • Dynamics: the detailed atomic motions that characterize an elementary act of reaction. • Sometimes these two terms are not distinguishable. E. G. Wang Generic Understanding of Growth Growth, by definition, is nonequilibrium in nature, and in many cases is far from equilibrium. A specific growth mode is selected by the interplay between thermodynamics and growth kinetics. Thermodynamics Kinetics (various atomic rate processes) Atom Dynamics A: formation of a surface vacancyadatom pair,or their recombination. B: association or dissociation of adatoms with an atomic cluster & cluster diffusion. C: diffusion of a surface vacancy ,especially toward the lattice step. D: falling off a lattice step of an adatom. E: diffusion of an adatom & its long range interactions with other adatoms. G: diffusion,dissociation & activation of a ledge atom. H: dissociation & activation of a kink atom. • 实验表明,参与表面各种原子过程的原子 扩散能力可以用表面扩散系数来描述。表 面扩散系数与扩散原子的跳跃几率有关,可 以表示为: • 这里D0 是尝试频率 (1012-1013), Vs 是能量势 垒, kB 是玻耳兹曼常数, T 是温度。 E. G. Wang 表面原子的扩散 -长程跳步 (jump over long distance) 原子每次跳步的距离为晶格常数的整数倍, 即原子每次跳 步的长度大于一个最近邻位。 -交换扩散机制 跳步扩散 (hopping)或替换扩散(exchange)。替换扩散机制产 生的物理原因是由于系统应保持沿扩散路径的断键数目最少。 初态 跳步扩散 替换扩散 过渡态 末态 Philosophy If we can establish EVERY correspondence between Atomic Rate Process Morphological Evolution then in principle we should be able to select a preferred growth mode via precise control of the various rate processes. Two growth models - Step flow growth Burton et al, Phil. Trans. R. Soc. London Ser. A 243 299(1951) 台阶指的是两层台面(terrace)之间的边界,沿着晶体某一指数 面切割表面通常都会在表面产生台阶,这种含有台阶的面叫做邻 面(vicinal surface)。由于表面上原子的吸附能力强烈地依赖于吸附 位的最近邻原子数,而位于台阶处的原子比位于表面上的原子具 有更多的配位数,因此沉积原子与台阶的键合更强。这一理论假 定在邻面上,如果沉积原子运动速率较高,在新的沉积原子到来 之前基底上已有的扩散单原子就能够到达台阶处并与之结合。这 样一来,通过沉积原子和台阶的键合使台阶不断前进,从而使生 长连续进行,这种生长叫做模式叫做step flow模式 。较高温度和 较低的沉积速率时更容易造成形成这种生长模式。 (沉积原子运动快) Two growth models - 形核和成岛生长模式 一般薄膜的生长温度较低,这样沉积原子在基底上的运动较 慢,在新的沉积原子到来之前基底上的原子不能运动到台阶处。 这些沉积原子就会在表面(台面)上游走,在原子行走过程中, 如果能碰到同类原子,它们便结合在一起,形成原子团。如果这 个原子团满足一定的能量关系,就会增加它们在基底表面上的居 留时间,就有与其它原子集结的更大几率,这就是成核。成核以 后形成的原子团并不是稳定的,还存在原子团的离解过程。只有 在一定条件下满足一定的能量关系,原子团才不再离解,随外来 原子加入或热处理,原子团不断长大。这个一定数量原子构成的 原子团即为前述的临界核。达到临界核以后,若继续入射原子到 达基底表面,那么原子团不断长大成粒子簇,即岛。这样生长就 会通过岛的长大,结合方式进行。 临界尺寸 在经典成核理论中,一个晶核的形成主要取决于Gibbs自由能 的变化量。随着晶核尺寸的增加,会出现一个临界晶核尺寸nc, 使得Gibbs自由能的变化量取极大值。小于这个尺寸的晶核随着粒 子数的增加Gibbs自由能的变化量不断增加,晶核长大的几率比退 化的几率小,是不稳定晶核;相反,大于这个尺寸的晶核,随着 粒子数的增加Gibbs自由能的变化量不断下降,晶核长大的几率比 退化的几率大,是稳定晶核,如图 所示。在数学上,临界晶核尺寸可 以这样确定: G(n) 0 n nnc 临界尺寸 在二维亚单层生长中,假定已知临界尺寸为i, 由速率方程可 以导出标度关系 这里N是总的岛密度,D是原子在基底表面上的扩散速度,F是沉 积速率,Ei是键能,i是临界尺寸。运用方程, 通过实验测量就可 以推导出微观参数。比如,测量岛密度随沉积流量的变化规律, 可以得到临界尺寸大小i , 再测量岛密度随温度的变化规律,那 么由已知的i , 就可以得到扩散势垒Ed和前因子D0的大小。Monte Carlo模拟和实验上都已经证实了标度关系的存在。 休息15分钟 E. G. Wang 亚单层生长时表面上典型的二维岛 当沉积流量F=0.167ML/s, 覆盖率Θ=0.12ML时, 在 Ag/Pt(111)系统中得到的二维 岛STM像。 (a)=110K, (b)=280K 。 Diffusion-Limited Aggregation ( DLA ) Witten and Sander, Phys. Rev. Lett.47, 1400 (1981) Citation > 3700 Nucleation without surfactant Diffusion-Limited-Aggregation (DLA) Hit and Stick 岛的平均分支宽度 b (average brach thinkness) 为1个原子宽度 ( b≈1)。 Witten and Sander, Phys. Rev. Lett.47, 1400 (1981) Nucleation without surfactant Extended Diffusion Limited Aggregation Adatoms can relax along island edges. 这时候沉积原子会沿着岛 边缘扩散,稳定位置是近邻原 子数大于等于2的位置。 在三 角格子上,就会得b为4个原子 宽度的分形岛; 在正方格子上 不存在这个生长区域。 Zhang, Chen, and Lagally, Phys. Rev. Lett.73, 1829 (1994) Corner Crossing (Irriversible case) No Corner Crossing Corner Crossing Zhong, Zhang, Zhang, Lagally, PRB 63, 113403 (2001) “Rules of Thumbs” Based on atomistic mechanisms Increase of the degree of local relaxation will lead to a fractal to compact transition Either through Growth temperature Or Deposition flux 二维原子岛退化与动力学标度理论 E. G. Wang Evolution of 2-Dimensional Nanostructures Power-law Scaling: A (t0 t )2 A: island size : scaling exponent t: time Gibbs-Thomson Formula: eq r exp rk T B eq Two limiting cases: Diffusion-limited: = 1/3 interface transfer-limited: = 1/2 J. G. Mclean, et. al, Phys. Rev. B 55, 1811 (1997). E. G. Wang Transition from 1D to 2D Island Decay on Aniosotropic Surfaces X , Y (t0 t ) a: = 0.33 (X direction) = 0 (Y direction) b: = 0.33 (X direction) = 0.33 (Y direction) Morgenstern等用变温STM细致地研究了二维岛Ag在各向异性表面Ag(110)的退化。他们发现 当温度在175-220K时,观测到准一维的退化行为(a);而在大约220K左右,体系由准一维退化 转变到二维退化(b)。从图(a) 和图(b) 可以明显地看出在不同温度区域内,岛的退化行为明显 不同。图(c) 显示体系的退化表现出很好的标度行为。根据已有的标度理论的结果,他们用 =0.33的标度率来拟合实验结果。图4.1中的实线为拟合曲线。似乎拟合曲线与实验结果符合 较好,但是事实上在低温区域中,沿x方向盘的退化标度指数不等于0.33,而应该是1/2。 E. G. Wang Theoretical Model Island Decay on Anisotropic Surfaces Dy Dx y Ky Kx 2Ly The outer boundary x 2Ry Kox 2Lx Koy 2Rx 在各向异性表面上二维岛的示意图。下标x和y分别表示不同的方向。 x 和 表示岛的线张力,Kx,Ky是原子在岛边的动能系数,Dx和Dy分别表示沿x和y 方向的扩散常数,Kox和Koy表示原子在外边界的动能系数。 E. G. Wang y Scaling Properties of Island Decay on Anisotropic Surfaces Low Temperature Regime: Diffusion Limited Attachment-Detachment Limited Lx (t0 t ) 1/ 2 Ly const. High Temperature Regime: Diffusion Limited Attachment-Detachment Limited E. G. Wang Lx (t0 t )1/ 3 Lx (t0 t ) 1/ 2 Theory contrast Experiments Theoretical Scaling exponent Anisotropic : 1/2 Isotropic : 1/3 E. G. Wang Quasi-1D and 2D Decay of Islands on Anisotropic Surfaces Quasi-1D (Low T regime) disappear y 2D (High T regime) x E. G. Wang Scaling Low of 2D Island Decay X , Y (t0 t ) Symmetry of Surface Decay Mode Diffusion Attachmentlimited Detachment limited Isotropic 2D Decay 1/3 1/2 Anisotropic Quasi-1D Decay (low T) 1/2 1/2 1/3 1/2 Anisotropic E. G. Wang 2D Decay (high T) ES势垒对原子扩散的控制及反常ES势垒效应 E. G. Wang 2D ES barrier Ehrlich & Hudda, J. Chem. Phys. 44, 1039 (1966). Schwoebel & Shipsey, J. Appl. Phys. 37, 3682 (1966). E. G. Wang Ehrlich-Schwoebel (ES) Barrier ES barrier: An additional potential energy barrier against the descent of atoms at step edges. The barrier primarily controls interlayer mass transport. Low ES barrier results in smooth films, whereas high ES barrier results in rough films. E. G. Wang Islands Decay Time (s) Ag/Ag(111) S. J. Chey, et al. Surf. Sci. 419, L100 (1998) E. G. Wang Time (s) Time (s) Cu/Cu(111) Cu/Cu(100) M. Giesen, et al. PRL 80,552(1998) Maozhi Li, et al. PRL 86,2345(2001) Two Categories of Interlayer Mass Transport Mechanisms Li et al., Phys. Rev. Lett. 86, 2345(2001) ASD SSD Any-Site Descent Mechanism Selective-Site Descent Mechanism Kinked site Straight step E. G. Wang ASD Mechanism: initially equal terraces Start: 13,000 atoms in 7 layers equal terrace widths #1 #7 Time = 0 s T2 T4 T6 Time = 120 s vd = 0.65 eV vt = 0.35 eV vs = 0.43 eV vsk = 0.40 eV @T=400 K E. G. Wang ASD Mechanism: initially unequal terraces T1 T3 T5 Time = 0 s T3 T4 T5 T6 Time = 600 s T3 T4 T5 T6 T=400 K E. G. Wang Time = 900 s SSD Mechanism: initially equal terraces Start:: 6,000 atoms in 5 layers equal terrace widths #1 #5 Time = 0 s Time = 2.0x104 s vd = 0.80 eV vt = 0.43 eV vs = 0.62 eV vsk = 0.52 eV @T=400 K E. G. Wang fcc(111) and fcc(100) Li et al., Phys. Rev. Lett. 86, 2345(2001) island decay on fcc(111) surfaces Small energy barriers ASD mechanism Vs=0.43 eV,Vsk=0.40 eV constant slope equal terrace widths fcc(111) Slope becomes steeper fcc(100) Vs=0.62 eV,Vsk=0.52 eV island decay on fcc(100) surfaces E. G. Wang large energy barriers SSD mechanism fcc(111) and fcc(100) Li et al., Phys. Rev. Lett. 86, 2345(2001) island decay on fcc(111) surfaces Small energy barriers ASD mechanism Vs=0.43 eV,Vsk=0.40 eV constant slope equal terrace widths fcc(111) Slope becomes steeper fcc(100) Vs=0.62 eV,Vsk=0.52 eV island decay on fcc(100) surfaces E. G. Wang large energy barriers SSD mechanism fcc(111) and fcc(100) Li et al., Phys. Rev. Lett. 86, 2345(2001) island decay on fcc(111) surfaces Small energy barriers ASD mechanism Vs=0.43 eV,Vsk=0.40 eV constant slope equal terrace widths fcc(111) Slope becomes steeper fcc(100) Vs=0.62 eV,Vsk=0.52 eV island decay on fcc(100) surfaces E. G. Wang large energy barriers SSD mechanism Observed Rapid Decay at T=300 K “Avalanche Mechanism” Cu/Cu(111) Cu/Cu(100) Site independent Site dependent “Robust Model” (J. F. Wendelken @ ORNL) E. G. Wang Concluding Remarks All adatoms are very busy moving around on surface in all possible possibledirections. directions. As we learn more and more about the growth kinetics, we can eventually devise ways to manipulate atomic motion and dictate on how they should behave collectively to please us. Searching for counter-intuitivity ! E. G. Wang 参考书: 1. F. Bechstedt, Principles of Surface Physics, Springer-Verlag, 2003. 2. M. C. Desjonqùeres and D. Spanjaard, Concepts in Surface Physics, Springer-Verlag, 1996. 3. Zangwill, Physics at Surfaces, Cambridge, 1998 4. 黄昆,固体物理,高等教育出版社 5. 王恩哥,薄膜生长中的表面动力学(I,II),物理学进展, Vol.23, 2003 6. 吴自勤、王兵,薄膜生长,科学出版社,2005 E. G. Wang 下一节课 分组讨论:1,2,3,4 每组20分钟,10分钟提问 E. G. Wang 谢谢! E. G. Wang