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Register a new userSolid phase epitaxy of ferromagnetic MnAs dots on GaMnAs layers
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Formation of MnAs quantum dots in a regular ring-like distribution has been found on MBE-grown (GaMn)As surfaces after low-temperature annealing under As capping. The Mn was supplied by out-diffusing Mn interstitials from (GaMn)As. With 5 at% substitutional Mn the quantum dots appeared for (GaMn)As layers thicker than 500 A. For thinner layers the Mn-rich surfaces, presumably monolayer thick MnAs, are smooth and well-ordered (1x2), and are well suited for continued epitaxial growth.
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The kinetics of intrinsic and dopant-enhanced solid phase epitaxy (SPE) are stud- ied in buried amorphous Si (a-Si) layers in which SPE is not retarded by H. As, P, B and Al profiles were formed by multiple energy ion implantation over a con- centration range of 1 - 30 x 1019 /cm3. Anneals were performed in air over the temperature range 460-660 oC and the rate of interface motion was monitored us- ing time resolved reflectivity. The dopant-enhanced SPE rates were modeled with the generalized Fermi level shifting model using degenerate semiconductor statis- tics. The effect of band bending between the crystalline and amorphous sides of the interface is also considered.
The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the hole state as being extended or localized. The calculation leads to a phase diagram determining the metallic or non-metallic character of the sample. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1-0.25 known to occur among samples produced with the highest Curie temperatures. Both the non-metal-to-metal and the metal-to-non-metal transitions experimentally observed in the ferromagnetic regime are obtained, as the Mn concentration increases. An explanation is given for the occurrence of a maximal Curie temperature in ferromagnetic GaMnAs samples.
MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$cdot 10^{11}$ cm$^{-2}$ and $2.5cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.
Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe$_2$ mono- and multi-layers on SiO$_2$/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO$_2$/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe$_2$ monolayers over 1 cm$^2$. To grow MoSe$_2$ multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe$_2$ monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe$_2$ multilayers over 1 cm$^2$. Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10 % of Mn in MoSe$_2$ bilayers.
Chalcogen-hyperdoped silicon shows potential applications in silicon-based infrared photodetectors and intermediate band solar cells. Due to the low solid solubility limits of chalcogen elements in silicon, these materials were previously realized by femtosecond or nanosecond laser annealing of implanted silicon or bare silicon in certain background gases. The high energy density deposited on the silicon surface leads to a liquid phase and the fast recrystallization velocity allows trapping of chalcogen into the silicon matrix. However, this method encounters the problem of surface segregation. In this paper, we propose a solid phase processing by flash-lamp annealing in the millisecond range, which is in between the conventional rapid thermal annealing and pulsed laser annealing. Flash lamp annealed selenium-implanted silicon shows a substitutional fraction of around 70% with an implanted concentration up to 2.3%. The resistivity is lower and the carrier mobility is higher than those of nanosecond pulsed laser annealed samples. Our results show that flash-lamp annealing is superior to laser annealing in preventing surface segregation and in allowing scalability.
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