Amorphization, recrystallization and end of range defects in germanium

The controlled doping of germanium by ion implantation is a process which requires basic research before optimization. For this reason, we have experimentally studied by transmission electron microscopy both the kinetics of amorphization and of recrystallization of Ge during ion implantation (Ge, P and B) and further annealing. As in Si, the crystalline to amorphous phase transition occurs through the linear accumulation of damage with the dose until a certain threshold is reached above which the material turns amorphous. We show that the Critical Damage Energy Density (CDED) model can be used in germanium to predict the existence, position and extension of amorphous layers resulting from the implantation of ions for almost all mass/energy/dose combinations reported here and in the literature. During annealing, these amorphous layers recrystallize by solid-phase epitaxy following an Arrhenius-type law which we have determined. We observe that this regrowth results in the formation of extended defects of interstitial type. During annealing these defects evolve in size and density following an Ostwald ripening mechanism which becomes non-conservative (defects “evaporate”) as the temperature is increased to 600 °C. These results have important implications for the modeling of diffusion of implanted dopant in Ge. Transient diffusion may also exist in Ge, driven by an interstitial component usually not evidenced under equilibrium conditions.     

Neutron-irradiation-induced defects in germanium: A Laplace deep level transient spectroscopy study

Deep level transient spectroscopy (DLTS), high-resolution Laplace DLTS (L-DLTS) and L-DLTS combined with uniaxial stress have been used in this work for characterization and identification of electrically active defects induced in Sb-doped germanium crystals by irradiation with fast neutrons. The samples were irradiated with relatively small doses of neutrons (≤5 × 10¹¹ cm⁻²) in order to produce uniformly distributed damage and to detect small defect clusters. It is found that for such low neutron doses in many respects the damage produced is similar to that resulting from electron irradiation. Vacancy-antimony (V-Sb) pairs uniformly distributed in the sample bulk are the dominant defects observed in the DLTS spectra. It is inferred from the L-DLTS measurements under application of uniaxial stress that the V-Sb pair has a trigonal symmetry in the doubly negatively charged state. It is argued that an electron trap with the activation energy for electron emission of 0.1 eV is related to an acceptor state of a small vacancy cluster located in highly damaged regions of the neutron-irradiated samples. L-DLTS measurements under application of uniaxial stress indicate that the symmetry of the defect is low, monoclinic-I, C_1h point group, or lower. Environment-induced broadening of the L-DLTS signal due to this centre prevents precise determination of the defect symmetry.     

Electrical properties of epitaxial germanium films on germanium

Thin films of germanium have been formed by vacuum deposition on single-crystal germanium (111) surfaces over a range of temperatures from 700° to 920°C. The films were carefully transferred to plane parallel sapphire substrates for measurements of their Hall mobility and Hall constant between 78° and 400°K. It was found that the Hall mobility values approached bulk values for films deposited onto Ge substrates near their melting point.     

Metallic impurities in germanium tetrafluoride and germanium dioxide prepared from it

The composition and contents of metallic impurities (in nonvolatile form) in isotopically unmodified and ⁷⁶Ge-depleted germanium tetrafluoride samples were determined by chemical atomic emission spectrometry. The germanium tetrafluoride samples were then converted to germanium dioxide. The impurity compositions of the resultant germanium dioxide samples were determined by laser mass spectrometry, atomic emission spectrometry, inductively coupled plasma mass spectrometry, and spark source mass spectrometry.     

Oxygen pressure and measurement temperature dependence of defects related bands in zinc oxide films

The ZnO films were fabricated by pulsed laser deposition at various oxygen pressure on single crystal silicon substrate. The structural and optical properties were investigated at various measurement temperature. The results showed that all the films have good c-axis preferred orientation. The different defects in films were fabricated which can be caused by various oxygen pressure. The films deposited at 1 Pa oxygen pressure have the most intense and narrow UV emission, and did not exhibit the deep band emission at the various measurement temperature. With the decrease of measurement temperature, the V_O-, O_i- and O_Zn-related band energy decreases, which is opposed to the V_Zn-related defects, meanwhile, the intensity of O_i-related emission peak has a sharp increase.     

New hydrogen donors in germanium

The electrical properties of n-type germanium single crystals irradiated with protons were studied by measuring capacitance-voltage characteristics. The thermal treatment of irradiated samples at 200–300°C leads to the formation of highly mobile shallow donor centers. The coefficient of diffusion of these donors is equal to that of atomic hydrogen with allowance for capture on traps. It is concluded that atomic hydrogen plays the role of a shallow donor in germanium.     

Inducing imperfections in germanium nanowires

Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for optical,electrical,and thermophysical applications.Additionally,the incorporation of metal impurities in semiconductor nanowires could substantially alter their electronic and optical properties.In this highlight article,we review our recent progress and understanding in the deliberate induction of imperfections,in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities.The role of catalysts and catalyst-nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored.Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor nanowires via catalyst and/or interfacial manipulation."Epitaxial defect transfer"process and catalyst-nanowire interfacial engineering are employed to induce twin defects parallel and perpendicular to the nanowire growth axis.By inducing and manipulating twin boundaries in the metal catalysts,twin formation and density are controlled in Ge nanowires.The formation of Ge polytypes is also observed in nanowires for the growth of highly dense lateral twin boundaries.Additionally,metal impurity in the form of Sn is injected and engineered via third-party metal catalysts resulting in above-equilibrium incorporation of Sn adatoms in Ge nanowires.Sn impurities are precipitated into Ge bi-layers during Ge nanowire growth,where the impurity Sn atoms become trapped with the deposition of successive layers,thus giving an extraordinary Sn content (＞6 at.％) in Ge nanowires.A larger amount of Sn impingement (＞9 at.％) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity trapping.     

Ultimate-strength germanium nanowires

Semiconducting nanowires (NWs) are important "building blocks" for potential electrical and electromechanical devices. Here, we report on the mechanical properties of supercritical fluid-grown Ge NWs with radii between 20 and 80 nm. An analysis of the bending and tensile stresses during deformation and failure reveals that while the NWs have a Young's modulus comparable to the bulk value, they have an ultimate strength of 15 GPa, which is the maximum theoretical strength of these materials. This exceptional strength is the highest reported for any conventional semiconductor material and demonstrates that these NWs are without defect or flaws that compromise the mechanical properties.     

Creep of germanium

Isothermal creep, as well as the response to incremental stress and temperature changes, were studied in germanium single crystals oriented for double slip, in the range 470 to 700° C. The stress-sensitivity of the compressive creep rate In / In is numerically close to 3 at low strains, but increases appreciably with deformation. This effect, and a similar strain dependence of the activation energy as determined by thermal cycling, are explained in terms of the curvature of the creep curves on the basis of Boltzmann's superposition principle. The Peierls barrier seems to be an important obstacle to dislocation movement at relatively low temperatures, when S-shaped creep curves are observed. Other barriers, with greater heights, seem to become increasingly effective above about 550° C. Although dislocation loops, and the formation and break-up of dipoles were observed by TEM, recovery mechanisms involving self-diffusion did not appear to make a substantial contribution to the creep within the range of temperatures used.     

Divacancies at room temperature in germanium

We have studied the annealing of vacancy defects in neutron and proton irradiated germanium. After neutron irradiation the Sb-doped samples were annealed at 473, 673 and 773 K for 30 min. The positron lifetime was measured as a function of temperature (30 - 295 K). A lifetime component of 330 ps with no temperature dependence is observed in as irradiated samples, identified as the positron lifetime in a neutral divacancy. The average positron lifetime in the samples annealed at 473 K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upward in the band gap. Proton irradiation of germanium at 37 K with subsequent room temperature annealing also resulted in a similar lifetime component 315 ps, in good agreement with the neutron irradiation experiment.     

Early stage donor-vacancy clusters in germanium

There is considerable experimental evidence that vacancies in Ge dominate several solid state reactions that range from self-diffusivity to metal and dopant transport. It is therefore vital that we fully understand how vacancies interact with other point defects in Ge. Here we have a look at the properties of small donor-vacancy (Sb _n V _m with m,n ≤ 2) complexes in Ge by ab-initio density functional modeling. Particular attention has been payed to binding energies and to the electronic activity of the complexes. We found that all aggregates may contribute to the n→ p type conversion that is typically observed under prolonged MeV irradiation conditions. In general, Sb _n V _m defects are double acceptors. It is also suggested that spontaneous formation of Sb₃V complexes may limit the activation level of donors introduced by ion implantation.