C. Awo-Affouda

Observation of crystallite formation in ferromagnetic Mn-implanted Si

Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.

Annealing temperature effects on the structure of ferromagnetic Mn-implanted Si

The dependence of the magnetization of Mn-implanted Si on the postimplant annealing temperature is studied. p-type Si wafers were implanted with 300keV Mn+ ions at 350°C to a fluence of 1×1016cm−2 and then annealed at 500–900°C for 5min. Ferromagnetic hysteresis loops were obtained at 10K using a superconducting quantum interference device magnetometer. The saturation magnetization increases with the postimplant annealing temperature, reaching an optimum field strength of 0.2emu∕g at 800°C. An out diffusion of Mn is observed at higher temperatures that coincides with a decrease in the saturation magnetization. The calculated point-defect profile that was generated by the implantation process peaks around the Mn-depleted region, suggesting that the residual implant damage may play a role in the ferromagnetic behavior of Mn-implanted Si.

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Electron-beam evaporated cobalt films on molecular beam epitaxy prepared GaAs(001)

We have deposited Co films on the GaAs(001) surface by using an e-beam evaporation method. The thicknesses of the Co films are measured by using x-ray reflectivity and Rutherford backscattering. The magnetic properties of the films have been measured using superconducting quantum interference device. The magnetization of the films was found to decrease with increasing film thickness. The slight degradation of magnetic properties is attributed to increasing roughness on the Co surface or the Co/GaAs interface during the Co deposition.

Anomalous Mn depth profiles for GaMnAs∕GaAs(001) thin films grown by molecular beam epitaxy

Mn concentration depth profiles in Mn-doped GaAs thin films grown at substrate temperatures of 580 and 250°C using various Mn cell temperatures have been investigated with dynamic secondary ion mass spectrometry and Auger electron spectroscopy. When the samples are grown at a low substrate temperature of 250°C, the Mn distributes uniformly. For the samples grown at a high substrate temperature of 580°C, the concentration depth profiles are easily fitted with a temperature-dependent Fermi function only if the Mn concentration is above the solubility limit. However, when the Mn concentration is below the solubility limit, unexpected peaks are observed in the concentration depth profiles.

Implantation damage study in ferromagnetic Mn-implanted Si

To investigate the influence of the residual implant damage and postimplant annealing upon the structure and magnetic properties of Mn-implanted Si, lattice disorder depth profiles were obtained from Rutherford backscattering spectroscopy (RBS)-channeling experiments on Mn-implanted ⟨100⟩ oriented p-type Si wafers. The defect concentration profiles were extracted from the RBS spectra using the two beam model. These profiles reveal a strong influence of the postimplant annealing temperatures upon the defects generated from implantation. Specifically, above 800°C, the backscattering yield from Si lattice defects decreases, which is coincident with a decrease in the magnetization. The evolution of the Mn concentration profiles and the magnetization suggest that the magnetization originates from Mn atoms located in the least damaged region.