ALBERT

Notes: The isochronal and isothermal annealing characteristics of acceptor-doped GaAs:Be grown at low substrate temperatures (300 °C) by molecular-beam epitaxy (LTMBE) have been studied. The Be was introduced in a range of concentrations from 1016 to 1019 cm−3. Electrical measurements of as-grown material up to the highest Be concentration of 1019 cm−3 show that no free holes are contributed to the valence band even though Raman spectroscopy of the Be local vibrational mode indicates that the majority of the Be impurities occupy substitutional sites. It is proposed that Be acceptors are rendered inactive by the high concentration of AsGa-related native donor defects present in LTMBE material. The concentration of AsGa-related defects in the neutral charge state was estimated from infrared absorption measurements to be as high as 3×1019 cm−3. A distinct annealing stage at 500 °C, similar to that found in irradiation-damaged and plastically deformed GaAs, marks a rapid decrease in the concentration of AsGa-related defects. A second annealing stage near 800 °C corresponds to the activation of Be acceptors. The presence of gallium vacancies VGa was investigated by slow positron annihilation. Results indicate an excess concentration of VGa in LTMBE layers over bulk-grown crystals. Analysis of isothermal annealing kinetics for the removal of AsGa-related defects gives an activation energy of 1.7±0.3 eV. The defect removal mechanism is modeled with VGa-assisted diffusion of AsGa to As precipitates.

Notes: We have investigated the saturation phenomenon of the free carrier concentration in p-type GaAs and InP single crystals doped by zinc diffusion. The free hole saturation occurs at 1020 cm−3 for GaAs, but the maximum concentration for InP appears at mid 1018 cm−3. The difference in the saturation hole concentrations for these materials is investigated by studying the incorporation and the lattice location of the impurity zinc, an acceptor when located on a group III atom site. Zinc is diffused into the III-V wafers in a sealed quartz ampoule. Particle-induced x-ray emission with ion-channeling techniques are employed to determine the exact lattice location of the zinc atoms. We have found that over 90% of all zinc atoms occupy Ga sites in the diffused GaAs samples, while for the InP case, the zinc substitutionality is dependent on the cooling rate of the sample after high-temperature diffusion. For the slowly cooled sample, a large fraction (∼90%) of the zinc atoms form random precipitates of Zn3P2 and elemental Zn. However, when rapidly cooled only 60% of the zinc forms such precipitates while the rest occupies specific sites in the InP. We analyze our results in terms of the amphoteric native defect model. We show that the difference in the electrical activity of the Zn atoms in GaAs and InP is a consequence of the different location of the Fermi level stabilization energy in these two materials.

Notes: The effects of radiation damage and stoichiometry on the electrical activity of carbon implanted in GaAs are studied. Damage due to implantation of an ion heavier than C increases the number of C atoms which substitute for As (CAs). Creation of an amorphous layer by implantation and the subsequent solid phase epitaxy during annealing further enhances the concentration of CAs. However, the free carrier concentration does not increase linearly with increasing concentration of CAs due to compensating defects. Activation of implanted C is maximized by maintaining the stoichiometry of the substrate which reduces the number of compensating defects in the crystal. Under optimum conditions for carbon implanted at a dose of 5×1014 cm−2, the carbon acceptor activity can be increased from 2% to 65% of the total implanted carbon.

Notes: The lattice locations of Zn atoms in heavily Zn-doped InP single crystal have been investigated by ion channeling techniques. The InP samples were rapidly quenched in diffusion pump oil after high-temperature Zn diffusion. Ion channeling experiments performed along various major crystal axes suggest that a large fraction (20%–30%) of the Zn atoms are in the tetrahedral interstitial position in the InP lattice. It has been found that although the maximum hole concentration is not significantly affected by the cooling rate, there is a substantial increase in the incorporation of Zn on substitutional and tetrahedral interstitial lattice locations in the rapidly cooled samples as compared to the slowly cooled samples. The consequences of these results for understanding the mechanisms leading to the saturation of the free-hole concentration in compound semiconductors are discussed.

Notes: Diffusion profiles and the solubility of Cu in Ge were measured in the temperature interval 850–1200 K by means of the spreading-resistance technique. From these data it is concluded that the diffusion of Cu in Ge involves the interchange between a highly mobile interstitial configuration, Cui, and a practically immobile substitutional configuration, Cus, with the aid of vacancies, V, via the so-called dissociative mechanism, Cui+V(arrow-right-and-left)Cus. The excellent agreement of the values of the vacancy contribution to the tracer self-diffusion coefficient in Ge, as calculated from our diffusivity and solubility data on Cu in Ge, with directly measured values of the 71Ge tracer self-diffusion coefficient from the literature demonstrates that self-diffusion in Ge occurs via vacancies. A comparison with the mechanisms of Au and self-diffusion in Si is presented.

Notes: Diluted III–Nx–V1−x alloys were successfully synthesized by nitrogen implantation into GaAs, InP, and AlyGa1−yAs. In all three cases the fundamental band-gap energy for the ion beam synthesized III–Nx–V1−x alloys was found to decrease with increasing N implantation dose in a manner similar to that observed in epitaxially grown GaNxAs1−x and InNxP1−x alloys. In GaNxAs1−x the highest value of x (fraction of "active" substitutional N on As sublattice) achieved was 0.006. It was observed that NAs is thermally unstable at temperatures higher than 850 °C. The highest value of x achieved in InNxP1−x was higher, 0.012, and the NP was found to be stable to at least 850 °C. In addition, the N activation efficiency in implanted InNxP1−x was at least a factor of 2 higher than that in GaNxAs1−x under similar processing conditions. AlyGa1−yNxAs1−x had not been made previously by epitaxial techniques. N implantation was successful in producing AlyGa1−yNxAs1−x alloys. Notably, the band gap of these alloys remains direct, even above the value of y (y〉0.44) where the band gap of the host material is indirect. © 2001 American Institute of Physics.

Notes: We have developed a low-temperature particle detector that uses a novel quasiparticle trapping mechanism to funnel athermal phonon energy from an 80 mg Ge absorber into a 1.6 mg doped Ge thermistor via a superconducting Al film. We report on pulse height spectra obtained at 320 mK by scanning a 241Am alpha source along the device, and show that up to 20% of the energy deposited in the Ge absorber by a 5.5 MeV alpha particle interaction can be collected into a thermistor via quasiparticle trapping. We show that this device is sensitive to the position of an alpha particle interaction in the Ge absorber for interaction distances of up to 5 mm from a quasiparticle trap. © 1995 American Institute of Physics.

Notes: A copper-related shallow acceptor complex has been discovered in germanium single crystals grown in vacuum and doped with arsenic and copper. Photothermal ionization spectroscopy of samples quenched from 673 K reveals two sets of hydrogenic lines with ground state binding energies of 9.15 and 10.05 meV. The line intensity ratios between corresponding transitions (1s-np) of the two hydrogenic series follow a Boltzmann dependence. This shows that the two series belong to the same impurity complex with a split ground state. Taking into account the crystal growth conditions together with the changes in the donor concentration deduced from variable temperature Hall effect measurements, we conclude that arsenic and substitutional copper form the new acceptor complex A(Cus,As) since copper is the only fast-diffusing species at this low annealing temperature. This complex is expected to have C3v symmetry in agreement with preliminary piezospectroscopy measurements. © 1996 American Institute of Physics.

Notes: Scientific interest, technological promise, and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor crystals. This review of mostly recent activities begins with an introduction to some past classical experiments which have been performed on isotopically controlled semiconductors.A review of the natural isotopic composition of the relevant elements follows. Some materials aspects resulting in part from the high costs of enriched isotopes are discussed next. Raman spectroscopy studies with a number of isotopically pure and deliberately mixed Ge bulk crystals show that the Brillouin-zone-center optical phonons are not localized. Their lifetime is almost independent of isotopic disorder, leading to homogeneous Raman line broadening. Studies with short period isotope superlattices consisting of alternating layers of n atomic planes of 70Ge and 74Ge reveal a host of zone-center phonons due to Brillouin-zone folding. At n(approximately-greater-than)40 one observes two phonon lines at frequencies corresponding to the bulk values of the two isotopes. In natural diamond, isotope scattering of the low-energy phonons, which are responsible for the thermal conductivity, is very strongly affected by small isotope disorder. Isotopically pure 12C diamond crystals exhibit thermal conductivities as high as 410 W cm−1 K−1 at 104 K, leading to projected values of over 2000 W cm−1 K−1 near 80 K.The changes in phonon properties with isotopic composition also weakly affect the electronic band structures and the lattice constants. The latter isotope dependence is most relevant for future standards of length based on crystal lattice constants. Capture of thermal neutrons by isotope nuclei followed by nuclear decay produces new elements, resulting in a very large number of possibilities for isotope selective doping of semiconductors. This neutron transmutation of isotope nuclei, already used for homogeneous doping of floating zone Si with P, holds perhaps the biggest promises for isotopically controlled semiconductors and is discussed in some detail. Local vibrational modes of low-mass impurities are sensitive to the mass of the impurity as well as the masses of the host atoms neighboring the impurity. High-resolution infrared-absorption studies of O in Ge crystals of different isotopic composition demonstrate the extreme simplification in such spectra which is achieved by isotope control. Interdiffusion of GaAs and Ge isotope superlattices with 0.1–1 μm thick layers have been studied with secondary-ion-mass spectroscopy. This kind of internal diffusion avoids the problems with surface effects and can produce accurate data without the need for radioactive tracers. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor materials. © 1995 American Institute of Physics.

Notes: The effect of In and Ga coimplanted with C into GaAs on the concentration of CAs has been investigated by local vibrational mode spectroscopy, Rutherford backscattering, particle-induced x-ray emission, and Hall effect. The dose of the C was fixed at 5×1014 cm−2, 27 keV, while the doses of either In, 185 keV, or Ga, 160 keV, ranged from 5×1013 to 5×1015 cm−2. Based on the Hall-effect and local vibrational mode data, 99% of the C, when implanted alone and annealed, is not located as isolated, substitutional atoms on either sublattice, but in an inactive complex. The coimplanted group-III species acts to increase both the concentration of CAs ([CAs]) and the sheet hole concentration. For coimplant doses of 5×1013 and 5×1014 cm−2, these values are in good agreement. Increasing the dose of the group-III coimplanted ion to 5×1015 cm−2 results in a hole concentration that is 45% less than the [CAs] and the coimplanted ions begin to occupy nonsubstitutional sites. The reduction in the concentration of holes due to CAs at the highest dose of coimplanted ion appears to be caused by a compensating defect which limits the maximum sheet hole concentration obtainable by the coimplantation technique in GaAs.