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Electronic processes in adatom dynamics at epitaxial semiconductor surfaces studied using MBE-STM combined system

Kiyoshi Kanisawa 

NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi 243-0198, Japan

     It is essential to understand the epitaxial growth mechanism in order to precisely control thin film and nanostructure fabrication. In situ observation is a powerful tool for achieving atomic-level accuracy. For the molecular beam epitaxy (MBE) of III-V compound semiconductors, reflection high-energy electron diffraction (RHEED) [1] and the scanning electron microscopy (SEM) [2] are widely used in situ monitoring methods. At one extreme, real-time microscopy of transient process in 100 fs, i.e., atomic-scale imaging during single phonon absorption or emission, is long-awaited, though it has not been realized yet. Alternatively, in situ observation by scanning tunneling microscopy (STM) during MBE has been achieved [3]. During STM imaging at the MBE growth temperature T, however, the object of interest changes frequently and spectroscopic characterization is still difficult. The energy resolution is inherently limited by the thermal fluctuation (kBT = 68.9 meV at 800 K; kB: Boltzmann constant). At present, time-dependent topographic imaging of the geometric transition at the scanning time in units of seconds is possible. More rapid scanning or a complementary nanoprobing technique is demanded to approach the growth dynamics more directly.

     Epitaxial growth consists of microscopic processes at the surface. Among these processes, adatom dynamics is quite typical. According to ab initio-based theoretical calculations, substrate temperature shows limited contribution to adatom dynamics from the entropy and the statistics of kinetic energies [4, 5]. This means that higher temperature stimulates thermodynamic events more frequently, but the temperature does not change the elemental microscopic processes of events. This is consistent with previous STM experiments in an ultra-high vacuum (UHV) at the room temperature. It has been well confirmed that such non-real-time topographic STM characterizations still catch rich traces of thermodynamic events expected during MBE growth [6 - 8]. It is possible to further take advantage of this feature of UHV-STM by performing characterization at lower cryogenic temperature.

     In this talk, adatom dynamics at the surface of InAs, one of the most important narrow-gap semiconductors, is discussed on the basis of low-temperature STM (LT-STM) observation of as-grown (111)A surface by MBE. LT-STM characterization is a promising complement to in situ STM observation during MBE. At the cryogenic temperature, the thermal fluctuation effect is markedly weakened and energy resolution becomes very high (kBT = 0.43 meV at 5 K). Such high resolution allows us to interpret thermodynamic phenomena from the viewpoint of quantum mechanics. Since atomic structures have very long lifetimes at the cryogenic temperature, stable spectroscopy is possible repeatedly. For the interpretation of the desorption event, knowledge obtained by an atom manipulation at the compound semiconductor surfaces [9, 10] was applied. The desorption (detachment by phonon excitation) is regarded as an adatom 'pick-up' event (detachment by electron excitation). This scheme replaces the spontaneous and incontrollable phonon process with an intentional and controllable electron process. Using STM, we can easily control both the excitation energy and the frequency by the bias voltage and the magnitude of the tunneling current. Phonon frequency of 10 THz, for example, is regarded to be the inelastic excitation period of 100 fs. Electron injection by tunneling at the period of 100 fs corresponds to the tunneling current 1013 electrons/s = 1.6 × 10-6 A. If there is an inelastic event, it is immediately detected in situ by the tunneling current measurement at picometer spatial resolution. The desorption dynamics of cation adatoms was modeled in the context of the transition state theory. The model is based on the rate equation for the adatom charge state determined by the ratio of the electron lifetime in the adatom resonance state to the electron excitation period of the phonon-assisted inelastic tunneling. The predicted desorption temperature range excellently agreed with the reported one [11]. Thus, the equivalence of excitation sources is verified [12]. This equivalence allows us to make a giant leap in our understanding of epitaxial growth mechanism. It is suggested that adatom density at the MBE growth temperature is almost equal to that measured at 5 K and that the electron accumulation layer is maintained as it is induced by the MBE growth. Adatom desorption dynamics is found to be correlated to the surface electron accumulation. These results demonstrate that the LT-STM experiment is an excellent method for exploring individual quantum mechanical dynamic processes expected during the epitaxy.



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Presentation: Invited oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 6, by Kiyoshi Kanisawa
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-15 04:29
Revised:   2013-04-15 05:55