The effect of Co ion implantation on Ge1−xMnx films

Ge_1−xMn_x (x = 0, 0.013, 0.0226, 0.0339, 0.0565, 0.0678, 0.0904, 0.113) films prepared by magnetron sputtering at 773 K had a Ge cubic structure except for x = 0.1130. Co ion implantation into these films can effectively prevent the formation of a second phase. Both single-doped and co-doped samples were ferromagnetic at room temperature. The d-d exchange interaction between the interstitial Mn (MnT) and the substituted Mn (Mn_Ge) resulted in ferromagnetism in the sputtered films. Since Co ion implantation destroyed the Mn_T-Mn_Ge-Mn_T complex, the saturated magnetization decreased. Hall measurements revealed that the Co ion implanted films were n-type semiconductors, and the anomalous Hall Effect (AHE) suggested the ferromagnetism was carrier-mediated in the implanted films.     

Structural, magnetic and electronic transport properties of MnxGe1−x/Ge(0 0 1) films grown by MBE at 350 °C

We report on the structural, magnetic and electronic transport properties of thin Mn_xGe_1−x films grown at 350 °C. Isolated Mn₅Ge₃ nanoclusters, about 100 nm in size, were formed at the top surface of the film, dominating the magnetic properties of the whole film. Electronic transport properties show Mn doping effect indicating the presence of substitutional Mn ions dispersed in the Ge host, contributing to the formation of a Mn_xGe_1−x diluted phase. Electrical behaviour indicates a saturation effect with the raise of the nominal Mn concentration in the film, above x ≅ 0.03.     

Magnetization of epitaxial MnGe alloys on Ge(1 1 1) substrates

The structural and magnetic properties of an Mn rich solid phase epitaxy Mn_xGe_1−x alloy grown on a clean 2 × 8-Ge(1 1 1) surface, with a Curie temperature of about 300 K are investigated. Magneto-optical  Kerr effect infers the existence of in-plane easy magnetization direction. We describe the epitaxial registry condition, the room temperature—zero field magnetic structure observed by magnetic force microscopy and the magneto-optical properties. The observations are consistent with the formation of epitaxial Mn₅Ge₃ alloy, with a modulated magnetic structure characterized by asymmetric 180° Bloch walls consisting of a vortex-like configuration of the local magnetization.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Nanoscratch study of Zn1−xMnxO heteroepitaxial layers

We investigated the nanotribological properties of Zn_1−xMn_xO epilayers (0 ≤ x ≤ 0.16) grown by molecular beam epitaxy (MBE) on sapphire substrates. The surface roughness and friction coefficient (μ) were analyzed by means of atomic force microscopy (AFM) and hysitron triboscope nanoindenter techniques.The nanoscratch system gave the μ value of the films ranging from 0.17 to 0.07 and the penetration depth value ranging 294-200 nm when the Mn content was increased from x = 0 to 0.16. The results strongly indicate that the scratch wear depth under constant load shows that higher Mn content leads to Zn_1−xMn_xO epilayers with higher shear resistance, which enhances the Mn-O bond. These findings reveal that the role of Mn content on the growth of Zn_1−xMn_xO epilayers can be identified by their nanotribological behavior.     

MnxGe1−x thin layers studied by TEM, X-ray absorption spectroscopy and SQUID magnetometry

Mn_0.06Ge_0.94 samples have been grown by molecular-beam epitaxy on Ge(0 0 1)2 × 1. High-resolution transmission electron microscopy shows the coexistence of an ordered diluted Mn_0.06Ge_0.94 film and of nanoscopic crystallites, which were identified as Mn₅Ge₃ by electron diffraction. The magnetic properties of the Mn_0.06Ge_0.94 samples show a superposition of a paramagnetic behavior, due to the interaction of Mn atoms diluted in the Ge host, and a ferromagnetic behavior attributed to the Mn₅Ge₃ crystallites dispersed into the films. The Mn L_2,3 X-ray absorption spectra of the Mn_0.06Ge_0.94 films exhibit a lineshape typical of metallic Mn, with considerably reduced multiplet structure.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

Manipulating Ge quantum dots on ultrathin SixGe1−x oxide films using scanning tunneling microscope tips

Germanium quantum dots (QDs) were extracted from ultrathin Si_xGe_1−x oxide films using scanning tunneling microscope (STM) tips. The extraction was most efficiently performed at a positive sample bias voltage of +5.0 V. The tunneling current dependence of the extraction efficiency was explained by the electric field evaporation transfer mechanism for positive Ge ions from QDs to STM tips. Ge QDs (∼7 nm) were formed and isolated spatially by extracting the surrounding Ge QDs with an ultrahigh density of >10¹² cm⁻². Scanning tunneling spectroscopy of the spatially-isolated QDs revealed that QDs with an ultrahigh density are electrically-isolated from the adjacent dots.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.     

A DFT study of the electronic and magnetic properties of Fe2MnSi1−xGex alloys

We performed density functional theory (DFT) calculations to study the structural, electronic and magnetic properties of Fe₂MnSi_1−xGe_x alloys (x=0, 0.25, 0.50, 0.75, and 1.00). The lattice constant is found to increase linearly as a function of Ge concentration with a decrease in the formation energy. The total magnetic moment is found to be 3 μ_B for all alloys with the most contribution from Mn local magnetic moments. Iron atoms, however, exhibit much smaller spin moments about 10% of the bulk value. It seems that due to the proximity of Fe, magnetic moments have been induced on the sp atoms, which couple antiferromagnetically with Fe and Mn spin moments. Although, the band gap remains almost constant (0.5 eV), the spin–flip gap decreases as a function of x.     

Surface morphology of Mn implanted Ge(1 0 0): A systematic investigation as a function of the implantation substrate temperature

Ge (1 0 0) wafers were implanted with 100 keV Mn⁺ ions with a dose of 2 × 10¹⁶ ions/cm² at different temperatures, ranging from 300 to 573 K. The surface morphology of implanted samples, analyzed with scanning electron microscopy and atomic force microscopy measurements, reveals for the 300-463 K implant temperature range the formation of a surface swelled and porous film, containing sponge-like structures. On the contrary, samples implanted in the 513-573 K temperature range present an atomically flat surface, with a roughness less than 1 nm, indicating that crystalline order has been preserved. X-ray photoemission spectroscopy depth profiling measurements indicate the presence of adsorbed oxygen in the porous layer of lower-temperature implanted samples, as well the presence of a large Mn concentration below the expected end of range for impinging ions. Mn and O concentrations at anomalously great depths are maximum in the 413 K implanted sample, indicating that the phenomenon of ion beam induced porosity is best favored at a well defined temperature.     

Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash

In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 °C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a complex nature of this superstructure with double-hexagonal symmetry and (2√3×2√3)-R30° periodicity. Among the possible candidate phases, including metastable FeSi₂ with a CaF₂ structure and FeSi_1+x with a CsCl structure, the best match of the interatomic distances to the measured 14.4 Å × 14.4 Å unit cell dimensions pointed to the hexagonal Fe₂Si (Fe₂Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I-V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to β-FeSi₂.     

Growth and properties of magnetron cosputtering grown MnxGe1−x on Si (001)

We have grown Mn_xGe_1−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 Mn_0.06Ge_0.94 film while new phase appears in the Mn_0.1Ge_0.9 film. Nanocrystals are formed in the Mn_0.06Ge_0.94 film, determined by field-emission scanning electron microscopy. Hall measurement indicates that the Mn_0.06Ge_0.94 film is p-type semiconductor and hole carrier concentration is 6.07×10¹⁹ cm⁻³ while the Mn_xGe_1−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 Mn_0.06Ge_0.94 film is ferromagnetic at room temperature.     

Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds

We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge_1 ? xMn_x are formed with Ge_1 ? xMn_x in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge_1 ? xMn_x phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge₃Mn₅) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge_1 ? xMn_x critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge_1 ? xMn_x solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge_1 ? xMn_x and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge_1 ? xMn_x phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.     

Phase transformations in the Mn-Ge system and in Ge x Mn1 ? x diluted semiconductors

Results of an X-ray diffraction study as well as magnetic and electrical measurements of the solid-state reactions in Ge/Mn polycrystalline films of an 80/20 atomic composition have been presented. It has been shown that the ferromagnetic Mn₅Ge₃ phase is formed first on the Ge/Mn interface after annealing at ??120°C. The further increase in the annealing temperature to 300°C leads to the beginning of the synthesis of the Mn₁₁Ge₈ phase, which becomes dominating at 400°C. The existence of new structural transitions in the Mn-Ge system in the region of ??120 and ??300°C has been predicted on the basis of the presented results and results obtained earlier when studying solid-state reactions in different film structures. The supposition about the general chemical mechanisms of the synthesis of the Mn₅Ge₃ and Mn₁₁Ge₈ phases during the solid-state reactions in the Ge/Mn films of the 80/20 atomic composition and the phase separation in Ge _x Mn_{1 ? x} (x > 0.95) diluted semiconductors has been substantiated.     

MnxGe1－x稀磁半导体薄膜的结构研究

利用X 射线衍射(XRD)和X射线吸收精细结构(XAFS)方法研究了磁控共溅射方法制备的Mn_xGe_1-x薄膜样品的结构随掺杂磁性原子Mn含量的变化规律.XRD结果表明，在Mn的含量较低(7.0%)的Mn_0.07Ge_0.93样品中，只能观察到对应于多晶Ge的XRD衍射峰，而对Mn含量较高(25.0%, 36.0%)的Mn_0.25Ge_0.75和Mn关键词： 磁控溅射 XRD XAFS xGe_1-x稀磁半导体薄膜')" href="#">Mn_xGe_1-x稀磁半导体薄膜     

Electrical characteristics of hole resonant tunneling diodes with high Ge fraction (x > 0.4) Si/strained Si1−xGex/Si(1 0 0) heterostructure

Effectiveness of a Ge fraction modulated spacer in hole resonant tunneling diodes (RTDs) with Si/strained Si_1−xGe_x heterostructures epitaxially grown on Si(1 0 0) was investigated to improve the electrical characteristics at higher temperatures. Electrical characteristics measured for 30 RTDs, with the modulated spacer at higher Ge fraction (x = 0.48) on a single wafer, show that the deviation of the peak current and voltage at the resonant peak falls in ranges of ±25% and ±10%, respectively. For the RTDs, negative differential conductance (NDC) characteristics are obtained even at higher temperatures around 230 K than that for the RTDs with x = 0.42. The result indicates that the introduction of higher Ge fraction is effective for NDC in RTD at higher temperature.     

Modification of Cd1−xMnxTe crystal surface layers by nanopulsed laser irradiation

The surface modification of Cd_1−xMn_xTe (x = 0-0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity measurements at wavelengths 0.53 and 1.06 μm. Laser irradiation energy density, E varied in the range of 0.1-0.6 J/cm². Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12-0.14 J/cm² for x ≤ 0.2, in the case of x = 0.3 this value is about 0.35 J/cm². The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21-0.25 J/cm², the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn.     

Influence of composition on electrical and optical properties of new chalcogenide thin films from Ge-Se-Tl system

Bulk Ge₂₀Se_80−xTl_x (x ranging from 0 to 15 at%) chalcogenide glasses were prepared by conventional melt quenching technique. Thin films of these compositions were prepared by thermal evaporation, on glass and Si wafer substrates at a base pressure of 10⁻⁶ Torr. X-ray diffraction studies were performed to investigate the structure of the thin films. The absence of any sharp peaks in the X-ray diffractogram confirms that the films are amorphous in nature. The optical constants (absorption coefficient, optical band gap, extinction coefficient and refractive index) of Ge₂₀Se_80−xTl_x thin films are determined by absorption and reflectance measurements in a wavelength range of 400-900 nm. In order to determine the optical gap, the absorption spectra of films with different Tl contents were analyzed. The absorption data revealed the existence of allowed indirect transitions. The optical band gap showed a sharp decrease from 2.06 to 1.79 eV as the Tl content increased from 0% to 15%. It has been found that the values of absorption coefficient and refractive index increase while the extinction coefficient decreases with increase in Tl content in the Ge-Se system. These results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. DC electrical conductivity of Ge₂₀Se_80−xTl_x thin films was carried out in a temperature range 293-393 K. The electrical activation energy of these films was determined by investigating the temperature dependence of dc conductivity. A decrease in the electrical activation energy from 0.91 to 0.55 eV was observed as the Tl content was increased up to 15 at% in Ge₂₀Se_80−xTl_x system. On the basis of pre-exponential factor, it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges.     

Chemical reactions of water molecules on Ru(0 0 0 1) induced by selective excitation of vibrational modes

Tunneling electrons in a scanning tunneling microscope were used to excite specific vibrational quantum states of adsorbed water and hydroxyl molecules on a Ru(0 0 0 1) surface. The excited molecules relaxed by transfer of energy to lower energy modes, resulting in diffusion, dissociation, desorption, and surface-tip transfer processes. Diffusion of H₂O molecules could be induced by excitation of the O-H stretch vibration mode at 445 meV. Isolated molecules required excitation of one single quantum while molecules bonded to a C atom required at least two quanta. Dissociation of single H₂O molecules into H and OH required electron energies of 1 eV or higher while dissociation of OH required at least 2 eV electrons. In contrast, water molecules forming part of a cluster could be dissociated with electron energies of 0.5 eV.