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Parabolic quantum wells have unique properties that make them attractive for certain applications. However, growing them with molecular beam epitaxy is much more difficult than growing standard rectangular wells, as parabolic wells require a smooth, precise variation of the composition during growth. Typically, such composition variations have been approximated using the digital alloy technique, but there are limits on the quality of wells that can be produced this way. In our approach, we instead create a smooth parabolic potential in Al x Ga 1 - x As by varying the Al cell flux as a function of time. To compensate for thermal lag in the effusion cell, we develop a simple linear dynamical model, which can be inverted to find the temperature input required for a desired composition profile. With this approach, the composition in a 3 THz Al x Ga 1 - x As parabolic quantum well can be controlled to a root-mean-squared error of 0.4% Al. This approach can be easily generalized to other structures, and, importantly, can be used at typical growth rates ( 1.5 - 2.5 A / s ), which allows for the growth of parabolic well arrays.

Temperature transient curves of molten tellurium have been measured by a laser flash method in the temperature range from 773 to 1173 K. The thermal diffusivity was calculated from the temperature transient caused by the laser pulse. Numerical fittings of the data were used to obtain both the thermal conductivity and specific heat as functions of temperature. The temperature-dependent specific heat results agree well with the previous published data. The thermal diffusivity and conductivity indicate a structure change in the liquid phase.

A five-fold increase in the size of dichalcogenide layer crystals grown by the iodine vapour transport technique results when the growth end of the ampoule is subjected to a periodic upward fluctuation of temperature during the growth period. The temperature oscillation method preferentially evaporates small nuclei and allows large crystals to grow at the expense of smaller ones. Due allowance must be made for thermal lag in the furnace by adjusting the durations of the heating and cooling cycles. Best results are then achieved if the amplitude of temperature fluctuation is of the same order as the height of the temperature gradient. Growth conditions have been investigated using the method for single crystals of SnS 2 , HfS 2 , GaSe and GaS to sizes up to 45 mm × 20 mm × 0.1 mm.

An increasing number of interesting new heterojunction structures rely on compositional gradients rather than abrupt heterojunctions. The use of rapid shuttering with a variable duty cycle is currently the most commonly used way of achieving the desired grading in molecular beam epitaxial (MBE) growth since thermal source ramping to achieve such grading suffers from problems of thermal lag and non-steady-state temperature distributions in the evaporation sources. In this work, we get around these obstacles by using reflective high energy electron diffraction (RHEED) oscillations to measure actual compositions during trial gradings and then use rhis direct data to fine tune the thermal ramps to achieve the desired compositional profile. The result is a graded layer with a precise reproducible profile grown without the complications of multiple interfaces and possible non-linear effective average properties inherent in the rapid shuttering Technique. These layers can then later be inserted during actual growth runs within more complex structures.