Photoinduced spin alignment of the magnetic ions in (Ga,Mn)As

Time-resolved Kerr rotation measurement in the (Ga,Mn)As diluted magnetic semiconductor allows direct observation of the dynamical properties of the spin system of the magnetic ions and the spin-polarized holes. Experimental results show that the magnetic ions can be aligned by the polarized holes, and the time scales of spin alignment and relaxation take place in tens and hundreds of picoseconds, respectively. The Larmor frequency and effective g factor obtained in the Voigt geometry show an unusual temperature dependence in the vicinity of the Curie temperature due to the exchange coupling between the photoexcited holes and magnetic ions. Such a spin coherent precession can be amplified or destructed by two sequential excitation pulses with circularly copolarized or oppositely polarized helicity, respectively. (c) 2006 American Institute of Physics.

Effects of edge dislocations and intentional Si doping on the electron mobility of n-type GaN films

The effects of dislocations and Si doping on the electrical properties of n-type GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is found that both electron mobility and carrier concentration are strongly influenced by edge dislocations. A moderate Si doping during the GaN growth improves the electron mobility, but the best doping effect depends on the dislocation density of the sample. High quality about 4-mu m-thick MOCVD-grown GaN film with a room temperature electron mobility as high as 1005 cm(2)/V s is obtained by optimizing growth conditions. (c) 2006 American Institute of Physics.

Raman scattering study on vibrational modes in Ga1-xMnxN prepared by Mn-ion implantation

Raman scattering measurements have been carried out on ferromagnetic semiconductor Ga1-xMnxN prepared by Mn-ion implantation and post annealing. The Raman results obtained from the annealed and un-annealed Ga1-xMnxN demonstrate that crystalline quality has been improved in Ga1-xMnxN after annealing. Some new vibrational modes in addition to GaN-like modes are found in the Raman spectra measured from the Ga1-xMnxN where the GaN-like modes are found to be shifted in the higher frequency side than those measured from the bulk GaN. A new vibrational mode observed is assigned to MnN-like mode. Other new phonon modes observed are assigned to disorder-activated modes and Mn-related vibrational modes caused by Mn-ion implantation and post-annealing. (c) 2006 Elsevier Ltd. All rights reserved.

Effect of lightly Si doping on the minority carrier diffusion length in n-type GaN films

We investigate the effects of lightly Si doping on the minority carrier diffusion length in n-type GaN films by analyzing photovoltaic spectra and positron annihilation measurements. We find that the minority carrier diffusion length in undoped n-type GaN is much larger than in lightly Si-doped GaN. Positron annihilation analysis demonstrates that the concentration of Ga vacancies is much higher in lightly Si-doped GaN and suggests that the Ga vacancies instead of dislocations are responsible for the smaller minority carrier diffusion length in the investigated Si-doped GaN samples due to the effects of deep level defects. (c) 2006 American Institute of Physics.

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Aluminium doping induced enhancement of p-d coupling in ZnO

Valence-band type Auger lines in Al doped and undoped ZnO were comparatively studied with the corresponding core level x-ray photoelectron spectrography (XPS) spectra as references. Then the shift trend of energy levels in the valence band was that p and p-s-d states move upwards but e and p-d states downwards with increasing Al concentration. The decreased energy of the Zn 3d state is larger than the increased energy of the 0 2p state, indicating the lowering of total energy. This may indicate that Al doping could induce the enhancement of p-d coupling in ZnO, which originates from stronger Al-O hybridization. The shifts of these states and the mechanism were confirmed by valence band XPS spectra and 0 K-edge x-ray absorption spectrography (XAS) spectra. Finally, some previously reported phenomena are explained based on the Al doping induced enhancement of p-d coupling.

Growth and properties of magnetron cosputtering grown Mn(x)Ge(1-x)on Si(001)

We have grown MnxGe1-x films (x=0, 0.06, 0.1) on Si (001) substrates by magnetron cosputtering, and have explored the resulting structural, morphological, electrical and magnetic properties. X-ray diffraction results show there is no secondary phase except Ge in the Mn0.06Ge0.94 film while new phase appears in the Mn0.1Ge0.9 film. Nanocrystals are formed in the Mn0.06Ge0.94 film, determined by field-emission scanning electron microscopy. Hall measurement indicates that the Mn0.06Ge0.94 film is p-type semiconductor and hole carrier concentration is 6.07 X 10(19) cm(-3) while the MnxGe1-x films with x=0 has n-type carriers. The field dependence of magnetization was measured using alternating gradient magnetometer, and it has been indicated that the Mn0.06Ge0.94 film is ferromagnetic at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Silicon doping induced increment of quantum dot density

Low-indium-content self-assembled InGaAs/GaAs quantum dots (SAQD) were grown using solid-source molecular beam epitaxy (MBE) and investigated by atomic force microscopy and photoluminescence (PL) spectroscopy. Silicon, which was doped at different quantum dot (QD) growth stages, markedly increased the density of QD. We obtained high density In0.35Ga0.65As/GaAs(001) quantum dots of 10(11)/cm(2) at a growth temperature of 520degreesC. PL spectra and distribution statistics show the high quality and uniformity of our silicon-doped samples. The density increment can be explained using the lattice-hardening mechanism due to silicon doping.

Pressure dependence of Mn2+ luminescence in differently sized ZnS : Mn nanoparticles

The pressure behavior of Mn2+ emission in the 10-, 4.5-, 3.5-, 3-, and 1-nm-sized ZnS:Mn2+ nanoparticles is investigated. The emission shifts to lower energies with increasing pressure, and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. The pressure coefficient increases with the decrease in particle size with the 1-nm-sized particles as an exception. Pressure coefficient calculations based on the crystal field theory are in agreement with the experimental results. The pressure dependence of the emission intensity is also size dependent. For nanoparticles 1 and 3 nm in size, the luminescence intensity of Mn2+ decreases dramatically with increasing pressure, while, for bulk and particles with average sizes of 3.5, 4.5, and 10 nm, the luminescence intensity of Mn2+ is virtually unchanged at different pressures. The bandwidth increases faster with increasing pressure for smaller particles. This is perhaps due to the fact that there are more Mn2+ ions at the near-surface sites and because the phonon frequency is greater for smaller particles. These new phenomena provide some insight into the luminescence behavior of Mn2+ in ZnS:Mn2+ nanoparticles.

The micro-magnetic structures of Mn+ ion-implanted GaSb

The micro-magnetic structures of Mn+ ion-implanted GaSb are studied using a magnetic force microscope (MFM). MFM images reveal that there are many magnetic domains with different magnetization directions in our samples. The magnetic domain structures and the magnetization direction of typical MFM patterns are analyzed by numeric simulation.

(Ga,Mn,N) compounds growth with mass-analyzed low energy dual ion beam deposition

The (Ga,Mn,N) samples were grown by the implantation of low-energy Mn ions into GaN/Al2O3 substrate at different elevated substrate temperatures with mass-analyzed low-energy dual ion beam deposition system. Auger electron spectroscopy depth profile of samples grown at different substrate temperatures indicates that the Mn ions reach deeper in samples with higher substrate temperatures. Clear X-ray diffraction peak from (Ga,Mn)N is observed in samples grown at the higher substrate temperature. It indicates that under optimized substrate temperature and annealing conditions the solid solution (Ga,Mn)N phase in samples was formed with the same lattice structure as GaN and different lattice constant. (C) 2003 Elsevier Science B.V. All rights reserved.

Temperature behaviour of the orange and blue emissions in ZnS : Mn nanoparticles

The temperature dependences of the orange and blue emissions in 10, 4.5, and 3 nm ZnS:Mn nanoparticles were investigated. The orange emission is from the T-4(1)-(6)A(1) transition of Mn2+ ions and the blue emission is related to the donor-acceptor recombination in the ZnS host. With increasing temperature, the blue emission has a red-shift. On the other hand, the peak energy of the orange emission is only weakly dependent on temperature. The luminescence intensity of the orange emission decreases rapidly from 110 to 300 K for the 10 nm sample but increases obviously for the 3 nm sample, whereas the emission intensity is nearly, independent of temperature for the 4.5 nm sample. A thermally activated carrier-transfer model has been proposed to explain the observed abnormal temperature behaviour of the orange emission in ZnS:Mn nanoparticles.

In situ doping of 3C-SiC grown on (0001) sapphire substrates by LPCVD

The heteroepitaxial growth of n-type and p-type 3C-SiC on (0001) sapphire substrates has been performed with a supply of SiH4+C2H4+H-2 system by introducing ammonia (NH3) and diborane (B2H6) precursors, respectively, into gas mixtures. Intentionally incorporated nitrogen impurity levels were affected by changing the Si/C ratio within the growth reactor. As an acceptor, boron can be added uniformly into the growing 3C-SiC epilayers. Nitrogen-doped 3C-SiC epilayers were n-type conduction, and boron-doped epilayers were p-type and probably heavily compensated.

Effect on the optical properties and surface morphology of cubic GaN grown by metalorganic chemical vapor deposition using isoelectronic indium surfactant doping

The surfactant effect of isoelectronic indium doping during metalorganic chemical vapor deposition growth of cubic GaN on GaAs (1 0 0) substrates was studied. Its influence on the optical properties and surface morphology was investigated by using room-temperature photoluminescence (PL) and atomic force microscopy. It is shown that the sample with small amount of In-doping has a narrower PL linewidth, and a smoother surface than undoped cubic GaN layers. A slight red shift of the near-band-edge emission peak was observed. These results revealed that, for small TMIn flow rates, indium played the role of the surfactant doping and effectively improved the cubic GaN film quality; for large TMIn flow rates, the alloying formation of Ga1-xInxN might have occurred. (C) 2002 Elsevier Science B.V. All rights reserved.

(Ga,Mn,As) compounds grown on semi-insulating GaAs with mass-analyzed low energy dual ion beam depositionw

The (Ga,Mn,As) compounds were obtained by the implantation of Mn ions into semi-insulating GaAs substrate with mass-analyzed low energy dual ion beam deposition technique. Auger electron spectroscopy depth profile of a typical sample grown at the substrate temperature of 250degreesC showed that the Mn ions were successfully implanted into GaAs substrate with the implantation depth of 160 nm. X-ray diffraction was employed for the structural analyses of all samples. The experimental results were greatly affected by the substrate temperature. Ga5.2Mn was obtained in the sample grown at the substrate temperature of 250degreesC. Ga5.2Mn, Ga5Mn8 and Mn3Ga were obtained in the sample grown at the substrate temperature of 400degreesC. However, there is no new phase in the sample grown at the substrate temperature of 200degreesC. The sample grown at 400degreesC was annealed at 840degreesC. In this annealed sample Mn3Ga disappeared, Ga5Mn8 tended to disappear,Ga5.2Mn crystallized better and a new phase of Mn2As was generated. (C) 2002 Elsevier Science B,V. All rights reserved.