摘要:

A natural monolayer {111} superlattice (the CuPt ordered structure) is formed spontaneously during organometallic vapor phase epitaxial (OMVPE) growth ofGa0.52In0.48P.The extent of this ordering process is found to be a strong function of the input partial pressure of the phosphorus precursor during growth due to the effect of this parameter on the surface reconstruction and step structure. Thus, heterostructures can be produced by simply changing the flow rate of the P precursor during growth. It is found, by examination of transmission electron microscope (TEM) and atomic force microscope (AFM) images, and the photoluminescence (PL) and PL excitation (PLE) spectra, that order/disorder (O/D) (really less ordered on more ordered) heterostructures formed by decreasing the partial pressure of the P precursor during the OMVPE growth cycle at a temperature of 620 °C are graded over several thousands of Å whenPH3is the precursor. The ordered structure from the lower layer persists into the upper layer. Similarly, D/O structures produced by increasing thePH3flow rate yield PL spectra also indicative of a graded composition at the heterostructure. The grading is not reduced by a 1 h interruption in the growth cycle at the interface. Similar heterostructures produced at 670 °C using tertiarybutylphosphine (TBP) as the P precursor show a totally different behavior. Abrupt D/O and O/D heterostructures can be produced by abruptly changing the TBP flow rate during the growth cycle. PL and PLE studies show distinct peaks closely corresponding to those observed for the corresponding single layers. TEM dark field images also indicate that the interfaces in both for D/O and O/D heterostructures are abrupt. The cause of the difference in behavior for TBP andPH3is not clear. It may be related to the difference in temperature. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365387

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The structures of a series of(TiSe2)n(NbSe2)msuperlattices grown through controlled crystallization of designed multilayer reactants have been studied. X-ray diffraction of the data of the superlattices after crystallization show considerable preferred orientation, with the basal plane of the dichalcogenide structure parallel to the substrate to within 0.1°. Lattice refinement using the observed (00l) diffraction maxima yields lattice parameters along thecaxis that are consistent with those expected based on the target superlattices and lattice parameters of the binary constituents. These (00l) diffraction data, however, contain no information about the crystalline structure in theabplane of the superlattice associated with the preferredc-axis orientation. Off-specular x-ray diffraction (XRD), scanning electron microscopy, and scanning transmission x-ray microscopy (STXM) were used to explore the structure and homogeneity of the superlattices in theabplane. XRD results rule out preferred long-range orientational order of theabplane. Between grains, both the backscattered electron images and STXM images show grain domain structure in theabplane with a characteristic grain domain size of approximately 50 &mgr;m. X-ray absorption microscopy in the STXM mode obtained at the TiL2,3edge shows that the titanium in the superlattices is present as both octahedral Ti consistent with theTiSe2structure and metallic Ti. A comparison of the data obtained from these techniques highlights chemical information, which can be deduced on a submicrometer range from the space resolved spectra obtained using STXM. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365388

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The interfacial interaction between phosphorus-doped amorphous silicon hydrogen alloy((n+)a-Si:H)and thermal evaporated Al-1&percent;Si layer after furnace annealing in the temperature range from 150 to 250 °C has been investigated in detail. The scanning electron microscope photographs show that many dendrites were formed on the original(n+)a-Si:Hsurface at annealing temperature higher than 170 °C. Raman spectroscopy and transmission electron diffraction show that the original(n+)a-Si:Hfilm has been converted to polycrystalline Si with the crystalline Si dendrites on top. The drastic increase (∼4orders of magnitude) of electrical conductivity of the 200 °C annealed(n+)a-Si:Hfilms with the Al-1&percent;Si removed is caused by the formation of polycrystalline silicon percolation channel in the background area between dendrites. Auger spectroscopy also provides evidence that no aluminum is incorporated into the converted film during silicon recrystallization and thus no SiAl alloy is formed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365389

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In view of (1×1):2H dihydride/(2×1):H monohydride reconstruction, structural stability of (100) surfaces of both cBN and diamond was comparatively investigated by semiempirical molecular orbital methods using isoelectronic clusters ofB52N42H80−2n(10−),N52B42H80−2n(10+),andC94H80−2n,to model (100)B and (100)N of cBN, and diamond surface, respectively, wheren=0,1, 2, or 3. Thendenotes the number of monohydride dimers formed. These clusters were nanometer-sized pyramidal crystallites bound by four of {111} faces and one (100). The (100)N of cBN was found unique because of the great stability as (1×1):2H dihydride phase, which retains the bulk structure truncated at the surface without reconstruction and is expected to be chemically inert. This passivation seems to be related to the difficulty in chemical vapor deposition of high quality cBN. The (100)B of cBN was predicted to stabilize as (2×1):H monohydride phase as much as hydrogenated (100) of diamond does. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365376

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The valence band spectra of a vacuum cleaved CuInSe2 (011)surface were measured with synchrotron radiation at photon energies between 16 and 95 eV. The strong dependence of the photoionization cross section of atomic levels between 28 and 60 eV is used to divide the valence band emissions into contributions from Se 4p and Cu 3d states in order to map the respective partial density of states. The derived partial density of Cu 3d states to the total valence band density of states is around 50&percent; in the upper part of the valence band and about 75&percent; at its maximum corresponding to non-bonding Cu d states. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365390

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We prove the effective Hall factor inp-GaAs to be larger than values discussed in the literature up to now. The scattering rates for the relevant scattering mechanisms inp-GaAs have been recalculated after critical testing the existing models. These calculations allow to deduce theoretical drift and theoretical Hall mobilities as functions of temperature which can be compared with measured data. Theoretical Hall factors in the heavy and light hole bands and an effective Hall factor result. The calculated room temperature values of the drift mobility and of the effective Hall factor are118 cm2/V sand 3.6, respectively. The fitted acoustic deformation potentialE1=7.9 eVand the fitted optical coupling constantDK=1.24×1011 eV/mare close to values published before. It is shown that the measured strong dependence of the Hall mobility on the Hall concentration is not mainly caused by scattering by ionized impurities but by the dependence of the effective Hall factor on the hole concentration. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365391

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A photoinduced “transient thermoelectric effect” (TTE) has been measured for ap-GaAs crystal using a tunable pulsed laser, over the laser energy range 0.93–1.80 eV, laser intensity0.2–130 mJ/cm2,time range 1 ns–1 ms, and temperature range 4.2–50 K, with special attention to native defects of EL2 centers, whose ground state(EL20)and excited state(EL2ex)are located, respectively, at 0.76 and 1.80 eV above the top of the valence band (their energy difference&sgr;ex=1.04 eV). After laser irradiation at one end of the sample, a TTE voltage is induced within a rising time&tgr;r(1.0–1.5 &mgr;s) due to hole diffusion, followed by exponential decay with multiple decay times&tgr;1–&tgr;5that depend on the laser energy, its intensity, and the temperature. The decay time&tgr;1is assigned to relate to photoexcited electron diffusion in the conduction band and others&tgr;2–&tgr;5with electron recombinations with photogenerated holes in the valence band via EL2 centers inp-GaAs, for which a rough evaluation of the capture cross section is made. Based on the experimental data, we have discussed the photoinduced carrier generation/recombination processes in three laser energy ranges with the two boundaries&sgr;exand the band-gap energyEg(=1.50 eV); regions I(E<&sgr;ex),II(&sgr;ex⩽E<Eg),and III(E⩾Eg).For these three energy regions, we have carried out computer simulations for the photoinduced TTE voltage profiles by solving one-dimensional transport equations for photogenerated electrons and holes, in qualitative agreement with the observations. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365192

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this article, the first calculations of hole initiated interband impact ionization in bulk zincblende and wurtzite phase GaN are presented. The calculations are made using an ensemble Monte Carlo simulation including the full details of all of the relevant valence bands, derived from an empirical pseudopotential approach, for each crystal type. The model also includes numerically generated hole initiated impact ionization transition rates, calculated based on the pseudopotential band structure. The calculations predict that both the average hole energies and ionization coefficients are substantially higher in the zincblende phase than in the wurtzite phase. This difference is attributed to the higher valence band effective masses and equivalently higher effective density of states found in the wurtzite polytype. Furthermore, the hole ionization coefficient is found to be comparable to the previously calculated electron ionization coefficient in zincblende GaN at an applied electric field strength of 3 MV/cm. In the wurtzite phase, the electron and hole impact ionization coefficients are predicted to be similar at high electric fields, but at lower fields, the hole ionization rate appears to be greater. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365392

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In our present study, the origin of the increase in resistivity of polycrystallineMn0.47Zn0.47Fe2.06O4with increasing oxygen partial pressure was investigated by measuring thermoelectric power and electrical resistivity, and through analysis of grain size. The ferrous ion(Fe2+)concentration of the samples was estimated using the thermoelectric power data and it indicated that the increase of oxygen partial pressure accompanied only a 0.5 wt. &percent; decrease in the concentration ofFe2+.The decrease inFe2+concentration failed to explain the order of magnitude increase in resistivity. Preferential oxidation of the grain boundaries did not contribute to the increase in resistivity since all the samples were cooled under the same conditions, i.e., constant oxidation potential. Impedance spectroscopy revealed that the increase in resistivity arose from the increase in resistivity of the grain boundary; this is discussed in terms of the microscopic shape factor, the brick-layer model, and the Maxwell–Wagner model. It is suggested that the increase in resistivity, with increasing oxygen partial pressure, originates from the increase in the microscopic shape factor of the grain boundary. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365393

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This paper reports on theptontransition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range300 K⩽T⩽315 K,dc conductivity in the temperature range100⩽T⩽300 K,and optical band gap(Egopt)have been carried out forSe75In25−xPbx(x=0,5,10,15)samples. Thep-ntransition occurs with very low addition of Pb impurity (5 at. &percent;). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads ton-type conduction in these glasses. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365397

出版商:AIP    

年代:1997

数据来源: AIP