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4 Apr 1994

Volume 64, Issue 14, pp. 1753-1882

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Resonant tunneling between parallel, two‐dimensional electron gases: A new approach to device fabrication using in situ ion beam lithography and molecular beam epitaxy growth

K. M. Brown, E. H. Linfield, D. A. Ritchie, G. A. C. Jones, M. P. Grimshaw, and M. Pepper

Appl. Phys. Lett. 64, 1827 (1994); http://dx.doi.org/10.1063/1.111768 (3 pages) | Cited 33 times

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Using the techniques of in situ focused ion beam lithography and molecular beam epitaxy regrowth high quality, patterned back gate, double quantum well devices have been fabricated. Independent ohmic contacts were made to the two two‐dimensional electron gases (2DEGs) using a ‘‘selective depletion’’ scheme, and using further gates the carrier densities in each well were controlled. Resonant tunneling between the two electron gases was observed as a function of carrier density in each 2DEG, and as a function of the bias applied between the two wells. Extremely large peak‐to‐valley ratios were observed, resulting from removal of unwanted parallel tunneling paths.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.40.Gk Tunneling
85.40.Hp Lithography, masks and pattern transfer

Determination of localized‐state distributions in amorphous semiconductors from transient photoconductivity

Hiroyoshi Naito, Jinli Ding, and Masahiro Okuda

Appl. Phys. Lett. 64, 1830 (1994); http://dx.doi.org/10.1063/1.111769 (3 pages) | Cited 12 times

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A method has been proposed for the determination of localized‐state distributions in amorphous semiconductors from transient photoconductivity using Laplace transforms. The method is valid in both pre‐ and post‐recombination regimes of transient photoconductivity. The applicability of the method is demonstrated in amorphous arsenic triselenide.  
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.40.+w Photoconduction and photovoltaic effects

Nitrogen‐doped ZnSe grown on 4°‐misoriented GaAs(100) and GaAs(211) by molecular beam epitaxy

Ziqiang Zhu, Takashi Ebisutani, Kazuhisa Takebayashi, Kiyotake Tanaka, and Takafumi Yao

Appl. Phys. Lett. 64, 1833 (1994); http://dx.doi.org/10.1063/1.111770 (3 pages) | Cited 3 times

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This letter presents the growth and characterization of N‐doped ZnSe grown on (100)GaAs misoriented 4° off toward (110), and (010), and (211)GaAs‐A and ‐B. The effects of the surface steps on the formation of deep donors are investigated by comparing photoluminescence properties and net‐acceptor concentration of ZnSe:N epilayers grown on misoriented (100) surfaces with those grown on the exact (100)GaAs. It is shown that the use of the misoriented (100) surfaces suppresses the generation of deep donors and enhances the net acceptor concentration. The effects of bond natures at Zn sites on the N‐incorporation process are studied through the comparison of the net‐acceptor concentration of N‐doped ZnSe epilayers grown on the (211)A surface with those on the (211)B surface. It is found that the N incorporation is limited by a single‐dangling bond at the Zn sites.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Empty state and filled state image of ZnGa acceptor in GaAs studied by scanning tunneling microscopy

Z. F. Zheng, M. B. Salmeron, and E. R. Weber

Appl. Phys. Lett. 64, 1836 (1994); http://dx.doi.org/10.1063/1.111771 (3 pages) | Cited 45 times

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See Also: Erratum

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ZnGa acceptor atoms in the first to sixth layer below the GaAs (110) cleavage plane have been identified. For the first time, we find that the empty state scanning tunneling microscopy image of a ZnGa acceptor is a characteristic equal latitude triangle‐shaped feature of ∼4 nm width with a (110) mirror plane. The filled state image, however, is a spherical feature of similar size. These unique features can be used as the signature for the identification of ZnGa in GaAs.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
71.55.Eq III-V semiconductors
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
61.72.S- Impurities in crystals

Bonding partner change reaction in oxidation of Ge on Si(001): Observation of two step formation of SiO2

K. Prabhakaran, T. Ogino, T. Scimeca, Y. Watanabe, and M. Oshima

Appl. Phys. Lett. 64, 1839 (1994); http://dx.doi.org/10.1063/1.111772 (3 pages) | Cited 11 times

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Oxidation of 5 Å Ge deposited at room temperature on Si(001) and the dependence of temperature on the oxidation behavior are investigated by employing synchrotron radiation photoelectron spectroscopy. The sample on exposure to air forms a mixture of Ge oxides and a small amount of Si oxides. Upon annealing, oxygen changes its bonding partner from Ge to Si forming SiO2 as the predominant final product. Two distinct steps have been observed in such a reaction. First step is the cleavage of all the Ge—O bonds and formation of Si—O bonds to form mainly Si suboxide. The second step is the rearrangement of Si—O bonds to form SiO2. The former one takes place in the temperature range 200–300 °C whereas the latter one in the range 300–600 °C.
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73.20.-r Electron states at surfaces and interfaces
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Electron mobility enhancement in Si using doubly δ‐doped layers

H. H. Radamson, M. R. Sardela, O. Nur, M. Willander, B. E. Sernelius, W.‐X. Ni, and G. V. Hansson

Appl. Phys. Lett. 64, 1842 (1994); http://dx.doi.org/10.1063/1.111773 (3 pages) | Cited 17 times

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Large enhancements in the electron mobility are reported for structures containing a pair of closely spaced Sb δ‐doped layers in Si. The room‐temperature mobility is enhanced by a factor of 2 compared to corresponding uniformly doped layers of singly δ‐doped structures. Even higher mobilities were obtained by using a Schottky gate on top and applying a voltage to adjust the potential well. With an effective gate voltage of ∼−0.3 V the mobility was 1200 cm2 V−1 s−1 at room temperature, which is an enhancement by a factor of 10 relative to the layer with equivalent bulk doping concentration. The high mobility is attributed to wave functions with nodes at the δ‐doped layers.
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73.61.Cw Elemental semiconductors

Direct observation of exciton localization in a GaAs/AlGaAs quantum well

Yutaka Takahashi, Satoru S. Kano, Koji Muraki, Susumu Fukatsu, Yasuhiro Shiraki, and Ryoichi Ito

Appl. Phys. Lett. 64, 1845 (1994); http://dx.doi.org/10.1063/1.111774 (3 pages) | Cited 10 times

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We present the direct observation of exciton localization in a GaAs/AlGaAs quantum well. We have observed the two‐component exponential decay of photoluminescence from heavy‐hole excitons when the excitation density is very low. We have confirmed by measuring the lateral spatial motion of excitons that the fast component is attributable to the radiative recombination of free excitons while the slowly decaying component is due to the localized excitons.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Doping of ZnTe by molecular beam epitaxy

I. W. Tao, M. Jurkovic, and W. I. Wang

Appl. Phys. Lett. 64, 1848 (1994); http://dx.doi.org/10.1063/1.111775 (2 pages) | Cited 30 times

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We have grown Cl‐doped ZnTe under different growth conditions and N‐doped ZnTe on different orientations. n‐type doping was achieved for the first time by proper control of the Zn/Te beam flux. A p‐type doping level of 1×1020 cm−3, which is the highest reported, was obtained by substrate tilting. These phenomena can be analyzed by the surface bonding structure analogous to the impurity concentration in the III‐V compound semiconductors.
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61.72.uj III-V and II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Photoluminescence properties of Si1−xGexSi disordered superlattices

Akihiro Wakahara, Toshimichi Hasegawa, Kyosuke Kuramoto, Kam Koc Vong, and Akio Sasaki

Appl. Phys. Lett. 64, 1850 (1994); http://dx.doi.org/10.1063/1.111776 (3 pages) | Cited 9 times

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Si1−xGex/Si disordered superlattices (d‐SLs) are fabricated by solid source molecular beam epitaxy. Photoluminescence properties of Si1−xGex/Si d‐SL are studied at low temperature by comparing with a conventional ordered superlattice (o‐SL) where the macroscopic chemical composition is same as that of the d‐SL. Strong intensity of the PL peak compared with the o‐SL is observed in the d‐SL. Carrier confinement rather than localization effects by artificially introduced disordering enhances the luminescence process in SiGe strained layers.
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78.55.Hx Other solid inorganic materials
71.35.-y Excitons and related phenomena
78.66.Jg Amorphous semiconductors; glasses
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Low temperature growth of silicon‐boron layer by ultrahigh vacuum chemical vapor deposition

T. P. Chen, T. F. Lei, H. C. Lin, C. Y. Chang, W. Y. Hsieh, and L. J. Chen

Appl. Phys. Lett. 64, 1853 (1994); http://dx.doi.org/10.1063/1.111777 (3 pages) | Cited 5 times

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A polycrystalline silicon‐boron (Si‐B) layer with a thickness of 180 nm was grown on recrystallized amorphous silicon in an ultrahigh vacuum chemical vapor deposition (UHV/CVD) system using pure SiH4 and B2H6 (1% in H2). The growth temperature was as low as 550 °C. Auger electron spectroscopy and secondary ion mass spectroscopy showed that the boron concentration is extraordinarily high (2×1022 cm−3). From the analysis of transmission electron diffraction patterns, the phase of silicon hexaboride (SiB6) was found to be present in the as‐deposited Si‐B layer. After thermal annealing, most of the boron atoms in the Si‐B layer were found to be immobile. The presence of SiB6 in the Si‐B layer may lead to the reduction of boron diffusivity in the Si‐B layer during thermal annealing.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.Nq Composition and phase identification
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)

New approach to the growth of low dislocation relaxed SiGe material

A. R. Powell, S. S. Iyer, and F. K. LeGoues

Appl. Phys. Lett. 64, 1856 (1994); http://dx.doi.org/10.1063/1.111778 (3 pages) | Cited 98 times

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In this growth process a new strain relief mechanism operates, whereby the SiGe epitaxial layer relaxes without the generation of threading dislocations within the SiGe layer. This is achieved by depositing SiGe on an ultrathin silicon on insulator (SOI) substrate with a superficial silicon thickness less than the SiGe layer thickness. Initially, the thin Si layer is put under tension due to an equalization of the strain between the Si and SiGe layers. Thereafter, the strain created in the thin Si layer relaxes by plastic deformation. Since the dislocations are formed and glide in the thin Si layer, no threading dislocation is ever introduced in to the upper SiGe material, which appeared dislocation free to the limit of the cross sectional transmission electron microscopy analysis. We thus have a method for producing very low dislocation, relaxes SiGe films with the additional benefit of an SOI substrate.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Low energy kinetic threshold in the growth of cubic boron nitride films

S. Kidner, C. A. Taylor, and Roy Clarke

Appl. Phys. Lett. 64, 1859 (1994); http://dx.doi.org/10.1063/1.111779 (3 pages) | Cited 44 times

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We report the growth of cubic boron nitride (cBN) films by magnetron sputtering on Si (100) substrates. The films are grown in the presence of negative substrate bias voltages and a nitrogen plasma produced by an electron cyclotron resonance source. We find evidence for a sharp low‐voltage threshold in the substrate bias (−105 V) beyond which the samples are predominantly cBN. The structural quality of the cBN films is optimized in a narrow range of voltages near this threshold. We discuss the important role of energetic ions in the formation of cBN in light of recent theoretical findings.
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81.15.Cd Deposition by sputtering
61.05.jh Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED)

Growth of high quality amorphous silicon films with significantly improved stability

Vikram L. Dalal, E. X. Ping, Sanjeev Kaushal, Mohan K. Bhan, and Mark Leonard

Appl. Phys. Lett. 64, 1862 (1994); http://dx.doi.org/10.1063/1.111780 (3 pages) | Cited 9 times

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We report on the results of a systematic series of experiments aimed at improving the stability of amorphous silicon (a‐Si:H) films. We find that very low levels (0.2–0.4 ppm) of compensation by boron, when combined with growth conditions that favor a robust microstructure in the material, results in films with significantly improved stability and very low defect density. The films were grown using a reactive plasma beam technique using a remote plasma beam from an electron‐cyclotron‐resonance source. The improvement in stability is seen under both short‐term (a few minutes) light soaking, and also under light soaking under high intensity (10×sun) illumination conditions. We conclude that the improvement in initial stability is a result of compensation of native donors (probably oxygen) in the material by boron. The surprising reduction in long term instability and corresponding defect density upon compensation implies that the microstructure and structural changes around the native impurities may play a role in the long‐term instability of the material.
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73.40.Ei Rectification
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Change in crystalline morphologies of polycrystalline silicon films prepared by radio‐frequency plasma‐enhanced chemical vapor deposition using SiF4+H2 gas mixture at 350 °C

Toshiki Kaneko, Masatoshi Wakagi, Ken‐ichi Onisawa, and Tetsuroh Minemura

Appl. Phys. Lett. 64, 1865 (1994); http://dx.doi.org/10.1063/1.111781 (3 pages) | Cited 52 times

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Polycrystalline silicon films have been deposited on glass substrates at 350 °C by radio‐frequency plasma‐enhanced chemical vapor deposition using a SiF4+H2 gas mixture. Crystalline fraction decreased abruptly with increasing gas flow ratio. Film structure drastically changed by increasing gas pressure from 0.4 to 2.0 Torr. At lower gas pressure, columnar crystals 30 nm in diameter grew from the glass substrates, while at higher gas pressure larger columnar crystals with a maximum diameter of approximately 100 nm grew on an amorphous Si layer approximately 170 nm thick.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology

Effect of the deposition sequence on the growth of YBa2Cu3O7−δ films by chemical vapor deposition

M. Matsubara, T. Morishita, and I. Hirabayashi

Appl. Phys. Lett. 64, 1868 (1994); http://dx.doi.org/10.1063/1.111782 (3 pages) | Cited 9 times

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We have investigated the possibility of layer‐by‐layer growth in the metalorganic chemical vapor deposition of YBa2Cu3O7−δ. In this study, the source gases were supplied alternately by various sequences in the form of a subunit cell block. Films deposited on MgO and SrTiO3 (STO) substrates were investigated by x‐ray diffraction, scanning electron microscopy, and atomic force microscopy. We found the surface morphology and composition of the deposited films were highly dependent on the selection of the blocks and the deposition sequence. The best film on the MgO or STO substrates were obtained by sequence starting with the Ba layer, then followed by the /Cu,Y,Cu/ block, and ending up with the Cu‐O chain layer. This result can be explained by the good wettability of Ba onto the substrates and the suppression of forming impurity phases such as barium cuprates.
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74.72.-h Cuprate superconductors
74.78.-w Superconducting films and low-dimensional structures
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.-a Thin film structure and morphology

CeO2: An alternative insulator material for superconducting field effect devices

A. Walkenhorst, M. Schmitt, H. Adrian, and K. Petersen

Appl. Phys. Lett. 64, 1871 (1994); http://dx.doi.org/10.1063/1.111783 (3 pages) | Cited 25 times

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The properties of high‐Tc superconducting field‐effect devices using CeO2 as an insulating layer have been studied. The dielectric constant and hence the achieved charge transfer for constant gate voltage and equivalent geometry is smaller as compared to the established SrTiO3 dielectric. This is mostly compensated by better insulating properties. The observed field effects for a fixed amount of charge transfer are very similar to those obtained with the SrTiO3 dielectric, indicating that the observed field effects are truly due to changes in the charge carrier density and not due to field‐induced stresses in the multilayer structure.
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85.30.Tv Field effect devices
74.72.-h Cuprate superconductors

Novel YBa2Cu3O7−x and YBa2Cu3O7−x/Y4Ba3O9 multilayer films by bias‐masked ‘‘on‐axis’’ magnetron sputtering

Jun‐Hao Xu, Guo‐Guang Zheng, A. M. Grishin, B. M. Moon, K. V. Rao, and John Moreland

Appl. Phys. Lett. 64, 1874 (1994); http://dx.doi.org/10.1063/1.111784 (3 pages) | Cited 3 times

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In situ YBa2Cu3O7−x (YBCO) films have been fabricated on SrTiO3 (001) and LaAlO3 (001) substrates by on‐axis biased‐radio‐frequency magnetron sputtering in Ar‐10% O2 at total pressures as low as 3 Pa (3×10−2 mbar) and a deposition rate 210 nm/h. Negative oxygen ion‐resputtering has been considerably reduced by introducing a biased copper mask between the substrate and target. The surface morphology and physical properties of the films are greatly improved on applying a positive dc substrate bias with respect to the grounded deposition chamber. We have obtained superconducting YBCO films with transport critical current as high as 106 A/cm2 at 77 K and low normal‐state resistivity by this approach. Scanning tunneling microscopy analyses of the films with the best superconducting properties reveal a spiral growth mechanism. However, films deposited by negative dc bias under identical sputtering conditions are insulating. From x‐ray θ‐2θ and rocking curve measurements, we identify the insulating films to be c‐axis oriented Y4Ba3O9 (YBO) films. Furthermore, YBCO films could be grown on the YBO layers without any degradation of TC and c‐axis orientation. This novel bias sputtering feature gives us a unique opportunity to produce superconductor/insulator, YBCO/YBO, multilayers from a single YBCO target.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents
74.72.-h Cuprate superconductors

Growth and microstructure of interfacially oriented large‐crystalline‐grain C60 sheets

A. Fartash

Appl. Phys. Lett. 64, 1877 (1994); http://dx.doi.org/10.1063/1.111785 (3 pages) | Cited 15 times

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C60 films reaching sheet thicknesses of ∼10 μm have been grown in vacuum by using a custom‐built effusion cell generating a beam of C60 molecules at flux rates several times higher than rates achieved by conventional methods. High quality films are grown epitaxially on Muscovite 2M1 mica substrates with double domain structures. X‐ray diffraction measurements show that in the [111] direction, the structural coherency of the films reaches ∼5000 Å. Since with increasing thickness the mosaic structures of the films do not disorder significantly, the film grains remain well oriented away from the interface.
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61.46.-w Structure of nanoscale materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.-a Thin film structure and morphology

Photon‐stimulated desorption of H− from a BaO surface

D. H. Baker, R. L. Champion, L. D. Doverspike, and Yicheng Wang

Appl. Phys. Lett. 64, 1880 (1994); http://dx.doi.org/10.1063/1.111786 (3 pages)

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Photon‐stimulated desorption of H− ions has been observed from a BaO surface for photon energies in the range of 3–5 eV, with a yield that increases sharply for energies above the threshold at 3 eV. A possible explanation is that a chemisorbed H atom captures a photoemitted electron and desorbs as H− via the Menzel–Gomer–Redhead mechanism.
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79.20.Ds Laser-beam impact phenomena
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
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