Elsevier

Journal of Crystal Growth

Volume 378, 1 September 2013, Pages 372-375
Journal of Crystal Growth

Growth of high-quality CuCl thin films by a technique involving electron-beam irradiation

https://doi.org/10.1016/j.jcrysgro.2012.12.103Get rights and content

Abstract

We have developed the molecular-beam epitaxy (MBE) method for CuCl thin films. The crystalline quality has been improved by an electron beam irradiation before the MBE growth. The qualified films show an exceptionally high speed nonlinear response of excitons at room temperature, wherein the excitons decay radiatively before its coherence is destroyed by dephasing. The radiative decay time of the excitonic state in films with a thickness of hundred nanometers reaches the order of 100 fs, which is much faster than the dephasing process. The shapes of the measured degenerate four-wave mixing spectrum and the radiative decay profile closely reflect those of the calculated induced-polarization spectrum and the radiative decay profile obtained by real-time analysis, respectively.

Highlights

► We have developed molecular-beam epitaxy method for CuCl thin films. ► The crystalline quality has been improved by an electron beam irradiation before the MBE growth. ► The qualified films show an exceptionally high speed nonlinear response of excitons. ► The radiative decay time of the excitonic state in qualified films reaches the order of 100 fs.

Introduction

The exciton resonance is essential for the achievement of an efficient nonlinear optical response. However, processes accompanied by the creation of excitons have been considered to be unsuitable for the optical gate devices in which the information carrying capacity is the most important, since the radiative lifetime of excitons is too long; it is in the order of hundred picoseconds or longer [1], [2], [3], [4]. In nanocrystals, where the long wavelength approximation (LWA) is applied, the radiative decay rate of excitons increases with the system size due to the enhancement of wave coupling between light and excitons. In the LWA resume, light interact with nodeless-type (n=1) confined exciton and the enhancement is limited by the applicable range of the approximation, in which the spatial structure of light wave is neglected.

On the other hand, if the exciton coherences are extended to whole volume of the system in the size region over nanocrystals by improving the crystalline quality, wave functions of multinode-type (n>1) excitons can match with light wave with a wide range of the system despite the breakdown of LWA [5]. In such a regime, nonlinear optical response with an extremely high efficiency beyond the LWA regime is theoretically predicted [6]. In GaAs thin films, where the growth method of high-quality sample by molecular beam epitaxy (MBE) has been already established, observation of the degenerate four-wave mixing (DFWM) signal of n=2 exciton, which is forbidden in the LWA regime, is reported in Refs. [7], [8], where the signal reaches 25 times larger than that in bulk crystal and the fast response times below 10 ps is observed [7], [8]. If a new growth method for nanocrystals with high crystalline quality is established in a substance, where excitons are stable and strongly interacts with light more than GaAs, a high performance nonlinear optical device with ultrafast and high-efficient response may be realized.

I–VII semiconductor CuCl is a candidate for highly efficient optical devices in the near-ultraviolet region because of much stronger radiative coupling per unit cell volume [9] and huge exciton binding energy (about 200 meV). In the vicinity of room temperature, however, the nonlinear optical signal has never been observed because of the nonradiative process due to the interaction with the optical phonons is dominant. Thus, it has never been utilized except for the basic research at low temperatures. On the other hand, the exciton lifetime in CuCl is very short (below 1 ns [4]). Furthermore, the nonlocal theory predicted that a radiative width, which corresponds to an imaginary part of radiative correction due to long-range coherent coupling between light and multinode-type excitons, is several orders of magnitude larger than that in GaAs [10], [11], and efficient nonlinear signal and ultrafast response beyond the case of GaAs is expected. In the present work, we have established a growth method of high-quality CuCl thin film to realize ultrahigh-speed response overcoming dephasing by optical phonons.

Section snippets

Experimental procedure

Our growth method of CuCl thin films were based on molecular beam epitaxy (MBE) [12]. CaF2 (111) was used as a substrate. The lattice constant of CaF2 is 0.5463 nm and the lattice mismatch with CuCl, which is derived with the lattice constant of CuCl (0.5406 nm), is 1.0%. Therefore, the heteroepitaxial growth of CuCl thin films is expected to be achieved on CaF2 [13]. The substrates were derived from CaF2 ingot by a cleavage in air, and the thicknesses were adjusted to be 1 mm to avoid to appear

Results and discussion

Fig. 1 shows a comparison of surface morphology in a CuCl thin film with a thickness of 42 nm whether the electron beam is irradiated or not. The atomic force microscope (AFM) image of an area where the electron beam was not irradiated is shown in Fig. 1(a). Although MBE has succeeded in growing high quality thin films for III–V and II–VI semiconductors, the prominent surface roughness like aggregated island structures is observed over the whole of the surface, and epitaxial growth of CuCl on CaF

Conclusion

We have improved the growth method using MBE to be suitable for CuCl and successfully established the technique to obtain CuCl thin films with higher crystalline quality. As new established growth method, we have adopted an irradiation of electron beam, which can be derived from the typical system of reflection high-energy electron diffraction. The films grown using this technique show ultrafast radiative decays with the order of 100 fs, which are consistent with the theoretically predicted

Acknowledgment

The present work was supported by Grant for Basic Science Research Projects from The Sumitomo Foundation and by a Grant-in-Aid for Young Scientists (B) (24740210, 2012) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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