We used the molecular beam epitaxial growth of CuCI on CaF2(I I I ) to determine if' scaling theory provides insight into the kinetic mechanisms of heteroepitaxy. %e measured quantitative surface topographs of several films representing the island nucleation, growth, and coalescence regimes of film growth with an atomic force microscope, and found that the static scaling exponent of all the surfaces was a 0.84 0.05. This a value is closer to theoretical predictions in which surface diAusion is the dominant smoothening mechanism than to those involving evaporation and recondensation.The classification scheme used to understand film growth modes over most of the last three decades was proposed by Bauer [I]. This paradigm recognized three different processes that have been named after some of their earliest investigators: Frank-van der Merwe (FM) for monolayer by monolayer growth [2], Volmer-Weber (VW) for initial film nucleation by 3D crystallite growth [3], and Stranski-Krastanov (SK) for formation of an initial uniform layer followed by 3D crystallite growth [4].All of these models are based on thermodynamic considerations, and have been discussed in detail previously [5,6]. The validity of the paradigm depends on the attainment of local equilibrium on the growing surface, which requires that the mass transport processes parallel to the macroscopic surface be fast compared to the flux of arriving species. In modern technological applications, the drive toward lower substrate temperatures and higher growth rates pushes practical growth of materials by vapor phase processes away from the 'idealized thermodynamic models toward a nonequilibrium or kinetic limit.Theoretical considerations showed that the thermodynamic models had to be modified to account for kinetic limitations [7]. The concept of scaling was introduced to the field by Family and Vicsek [8] to provide a framework for understanding the self-afine (or fractal-like [9]) topologies of the nonequilibrium surfaces. Most recently, a group of continuum models based on the competition between roughening of a surface caused by the stochastic arrival of depositing species and smoothening resulting from surface diffusion and other transport processes [10] have been proposed. Three of these models predict different surface topologies: Kardar-Parisi-Zhang (KPZ) [11], Wolf-Villain (WV) [12], and Villain [13] and Lai-Das Sarma (VLS) [14]. However, these models are implicitly valid only for homodeposition processes. The purpose of this paper is to see if some of the insights gained from these kinetic growth theories can be applied in understanding heteroepitaxy as well.According to scaling theory, the discreteness of the de-positing material is the main cause for the growing surface to become self-a%ne; the interface width g, i.e., the standard deviation of the surface height H, can be expressed in the form [8,15] & (t) =L 'f(t/L'), (I a) which reduces to z(t) =L2 for small L with t =const, and to &t'. (t ) = t ' as L =~, (lb) (Ic)where L is the length scale ...
Transmission and absorption spectra of CuCl epitaxial films are studied. Many structures are observed over the Z3 exciton region, corresponding to the quantization condition K" ^nnlL with odd n (K n : wave number). The absence of the states with even n is interpreted in terms of the parity selection rule in the confined exciton system. The value of the oscillator strength for the nth exciton is found to be proportional to Lin 1 for odd n and zero for even «, in accordance with theoretical expectation. As a polariton system, this belongs to a simple regime of long wavelength approximation.PACS numbers: 73.20.Dx, 68.55.Bd, 71.35,+z, 71.36.+C Quantum confinement effects of excitons in semiconductor systems with reduced dimensions have attracted considerable attention because of their potential application to nonlinear optical devices, and as a means of exploring the basic physics of the quantized excitonic states. The qualitative behavior of the exciton in confined systems can be characterized by the ratio of a film thickness L to the Bohr radius as of the exciton. In a quantum well with Lias ^ 1, the electron and hole levels are separately quantized in discrete subbands, leading to increases in both the binding energy and the oscillator strength of the exciton. In this confinement regime, the oscillator strength is independent of the electron and the hole quantum numbers (n e ,n n ), and the excitonic transitions obey the selection rule of An s=: n e -nh = 0.These properties of the exciton in quantum wells have been extensively studied in III-V compound systems both in theory and experiment [l]. For thin films outside the quantum well regime, the center-of-mass (cm.) motion of the exciton is quantized, while the relative motion is essentially identical to that in the bulk except for a possible distortion near the surfaces [2]. When long wavelength approximation (LWA) holds, L<£X (X: photon wavelength), this confined system can be regarded as an assembly of local oscillators characterized by the quantized exciton energy E n and its ^-dependent oscillator strength f n , where n is the quantum number of the exciton. Optical transitions are expected to obey the parity selection rule that only the transitions with odd quantum number n are allowed. The properties of cm. confinement have been studied in heterostructures of II-VI [3-6] and III-V [2,7] compounds. Alternating strengths of the resonant structures have been observed in luminescence excitation spectra [3-6] and luminescence spectra [7]. These spectra were analyzed in terms of the interference between the upper and lower polaritons (UP, LP) [8,9], but an analysis in terms of {/"} has not been done, probably because of the ambiguity in obtaining {f n } from luminescence measurements.Compared to the III-V and II-VI compounds, CuCl has a large binding energy (-190 meV) and a small Bohr radius (~7 A) of the exciton. The electron-hole Coulomb interaction will be strong enough to maintain the bulklike electron-hole relative motion, down to surprisingly small film thic...
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