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28 Sep 1998

Volume 73, Issue 13, pp. 1757-1915

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Compensation doping of Ba0.7Sr0.3TiO3 thin films

M. Copel, J. D. Baniecki, P. R. Duncombe, D. Kotecki, R. Laibowitz, D. A. Neumayer, and T. M. Shaw

Appl. Phys. Lett. 73, 1832 (1998); http://dx.doi.org/10.1063/1.122297 (3 pages) | Cited 32 times

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We have investigated the effects of Mn impurities on Ba0.7Sr0.3TiO3 thin films using x-ray photoemission spectroscopy. Mn acts as an electron acceptor, compensating for the charge density found in nominally undoped films. This causes a greatly increased depletion width in acceptor-doped films. We also present evidence that acceptor-doped films have an increased barrier to thermionic emission of electrons from Pt contacts into the dielectric. This may explain the decrease in leakage current observed in some acceptor-doped titanates. © 1998 American Institute of Physics.
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79.60.Dp Adsorbed layers and thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
61.72.J- Point defects and defect clusters
79.40.+z Thermionic emission
71.55.Ht Other nonmetals

Incorporation and optical properties of magnesium in cubic GaN epilayers grown by molecular beam epitaxy

D. J. As, T. Simonsmeier, B. Schöttker, T. Frey, D. Schikora, W. Kriegseis, W. Burkhardt, and B. K. Meyer

Appl. Phys. Lett. 73, 1835 (1998); http://dx.doi.org/10.1063/1.122298 (3 pages) | Cited 22 times

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The incorporation and optical properties of Mg in cubic GaN (c-GaN) epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence (PL). By varying the Mg flux by more than four orders of magnitude, the incorporation of Mg saturates at high Mg flux and is limited to a value of about 5×1018 cm−3 due to the high volatility of Mg at growth temperature. In addition, we observe an accumulation of Mg at the GaN/GaAs interface due to a diffusion of Mg to the GaAs substrate. Low-temperature PL spectra reveal several well-separated lines. Besides a shallow acceptor level at EA ≅ 0.230 eV, additional Mg-related deep defect levels indicate an incorporation of Mg at off-gallium sites or as complexes. © 1998 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
71.55.Eq III-V semiconductors

Intense short-wavelength photoluminescence from thermal SiO2 films co-implanted with Si and C ions

J. Zhao, D. S. Mao, Z. X. Lin, B. Y. Jiang, Y. H. Yu, X. H. Liu, H. Z. Wang, and G. Q. Yang

Appl. Phys. Lett. 73, 1838 (1998); http://dx.doi.org/10.1063/1.122299 (3 pages) | Cited 39 times

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Intense short-wavelength photoluminescence (PL) observed at room temperature from thermal SiO2 films co-implanted with Si and C is reported. A flat Si profile was first implanted, followed by 1100 °C annealing for 60 min. C ions were subsequently used to be implanted into the same depth region. PL was observed from the as-implanted samples with and without annealing. The PL intensity increases with annealing temperature. Comparing the PL spectra and the PL dynamics of the C-implanted, annealed, Si-implanted (CIASI) SiO2 films with those from Si- and C-implanted SiO2 films suggests that the interaction of Si and C in SiO2 films plays an important role in the luminescence in CIASI SiO2 films. © 1998 American Institute of Physics.
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78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
61.72.up Other materials
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.80.Jh Ion radiation effects
61.72.Cc Kinetics of defect formation and annealing
61.82.Ms Insulators

Independent manipulation of density and size of stress-driven self-assembled quantum dots

I. Mukhametzhanov, R. Heitz, J. Zeng, P. Chen, and A. Madhukar

Appl. Phys. Lett. 73, 1841 (1998); http://dx.doi.org/10.1063/1.122300 (3 pages) | Cited 74 times

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A method to independently manipulate the density and size of stress-driven self-assembled quantum dots (QDs) is demonstrated in the InAs/GaAs material system. In bilayer stacks, different InAs deposition amounts in the initial (seed) and second layer are shown to enable independent control, respectively, of the density and the size distribution of the second layer QDs. The approach allows enhancing the average volume and improving the uniformity of InAs QDs, resulting in, respectively, low and room temperature photoluminescence at 1.028 eV (∼1.2 μm) and 0.955 eV (∼1.3 μm) with remarkably narrow linewidths of 25 and 29 meV for 1.74 ML (seed)/3.00 ML InAs stacking. © 1998 American Institute of Physics.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
85.35.-p Nanoelectronic devices
81.05.Ea III-V semiconductors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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
68.55.-a Thin film structure and morphology

Strain-dependent morphology of spontaneous lateral composition modulations in (AlAs)m(InAs)n short-period superlattices grown by molecular beam epitaxy

A. G. Norman, S. P. Ahrenkiel, H. Moutinho, M. M. Al-Jassim, A. Mascarenhas, J. Mirecki Millunchick, S. R. Lee, R. D. Twesten, D. M. Follstaedt, J. L. Reno, and E. D. Jones

Appl. Phys. Lett. 73, 1844 (1998); http://dx.doi.org/10.1063/1.122301 (3 pages) | Cited 32 times

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The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and (001)InP substrate. For samples grown under tension, transmission electron and atomic force microscopy reveal composition modulations along directions close to 〈310〉 coupled to a surface cusping. For samples grown under compression, we observe composition modulations roughly along the elastically soft 〈100〉 directions coupled to a surface rippling. For high strains (⩾0.7%), with individual InAs layer thicknesses ⩽1.6 monolayers, we observe weak or no composition modulations. © 1998 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.Ct Interface structure and roughness

Growth and optical investigation of strain-induced AlGaAs/GaAs quantum dots using self-organized GaSb islands as a stressor

T. Wang and A. Forchel

Appl. Phys. Lett. 73, 1847 (1998); http://dx.doi.org/10.1063/1.122302 (3 pages) | Cited 6 times

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The strain-induced quantum dots in a two-dimensional AlGaAs/GaAs quantum well plane are first reported to be fabricated by using self-organized GaSb islands as a stressor, which is confirmed by magnetophotoluminescence measurement. In the photoluminescence spectra, the emission peaks from this quantum dots and undisturbed quantum well are observed. In addition, the energy separation between these two peaks increases with the decrease of the thickness between the self-organized GaSb island and quantum well plane. When this thickness is 12.5 nm, the energy separation between these two peaks is about 21 meV. The calculation based on the strain distribution and parabolic potential model gives a good agreement with photoluminescence results. Furthermore, the optimum size of the self-organized GaSb islands as a stressor on the GaAs surface is about 36 nm. © 1998 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.20.Ls Magneto-optical effects

Temperature dependence of avalanche breakdown for epitaxial diodes in 4H silicon carbide

A. O. Konstantinov, N. Nordell, Q. Wahab, and U. Lindefelt

Appl. Phys. Lett. 73, 1850 (1998); http://dx.doi.org/10.1063/1.122303 (3 pages) | Cited 10 times

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The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with positive angle beveling and oxide passivation can withstand temperatures of up to 300–400 °C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas. © 1998 American Institute of Physics.
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85.30.Kk Junction diodes
77.22.Jp Dielectric breakdown and space-charge effects
73.61.Le Other inorganic semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
71.55.Ht Other nonmetals

Atomic structures of hydrogen-terminated Si(001) surfaces after wet cleaning by scanning tunneling microscopy

Katsuyoshi Endo, Kenta Arima, Toshihiko Kataoka, Yasushi Oshikane, Haruyuki Inoue, and Yuzo Mori

Appl. Phys. Lett. 73, 1853 (1998); http://dx.doi.org/10.1063/1.122304 (3 pages) | Cited 23 times

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Scanning tunneling microscopy observations are performed on a H-terminated Si(001) surface treated with HF solutions and ultrapure water with very low dissolved oxygen and total organic carbon contents. Over a large area, row structures are observed in [110] and [110] directions. Pyramidal-shaped etch pits are also observed, which are caused by anisotropic etching by OH ions. Detailed images clearly show 2×1 periodic structures. It is suggested that every other row of the ideally dihydride 1×1 surface is etched preferentially by OH ions. This explains the mechanism by which the smallest etch pits are formed. © 1998 American Institute of Physics.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
81.05.Cy Elemental semiconductors
81.65.Cf Surface cleaning, etching, patterning

Two-dimensional electron-gas density in AlXGa1−XN/GaN heterostructure field-effect transistors

Narihiko Maeda, Toshio Nishida, Naoki Kobayashi, and Masaaki Tomizawa

Appl. Phys. Lett. 73, 1856 (1998); http://dx.doi.org/10.1063/1.122305 (3 pages) | Cited 49 times

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We have calculated maximum two-dimensional electron-gas densities in AlXGa1−XN/GaN heterostructure field-effect transistors with wurtzite crystal structures in (0001) orientation, by self-consistently solving Schrödinger’s and Poisson’s equations, taking the piezoelectric effect into account. In order to obtain a guideline for increasing electron densities in the devices, we have examined dependences of the maximum electron densities on both Al compositions of AlXGa1−XN layers and lattice relaxations at the heterointerfaces. The maximum electron density was found to depend more strongly on the lattice relaxation than on the Al composition, which determines the conduction-band discontinuity. Controlling the lattice relaxation is shown to be crucial for obtaining high electron densities in the devices. © 1998 American Institute of Physics.
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85.30.Tv Field effect devices
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
85.50.-n Dielectric, ferroelectric, and piezoelectric devices

Band discontinuity fluctuations and local chemistry at the GaSe–Si interface

M. Zacchigna, L. Sirigu, J. Almeida, H. Berger, L. Gregoratti, M. Marsi, M. Kiskinova, and G. Margaritondo

Appl. Phys. Lett. 73, 1859 (1998); http://dx.doi.org/10.1063/1.122306 (3 pages) | Cited 4 times

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Photoemission spectromicroscopy data on Si–GaSe with 0.2 μm lateral resolution revealed laterally inhomogeneous interface Se–Si chemical reactions. The local Si 2p fits agrees with interface Si selenide parameters; the Se 3d peaks indicate a Si–selenide component; the Ga 3d peaks exhibit a metallic component whose intensity increases with Si coverage. This local interface reaction is related to lateral changes in the band lineup. © 1998 American Institute of Physics.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
79.60.Jv Interfaces; heterostructures; nanostructures
73.20.At Surface states, band structure, electron density of states

Influence of HCl on the chemical vapor deposition and etching of Ge islands on Si(001)

T. I. Kamins, G. A. D. Briggs, and R. Stanley Williams

Appl. Phys. Lett. 73, 1862 (1998); http://dx.doi.org/10.1063/1.122307 (3 pages) | Cited 14 times

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When HCl is added during the growth of Ge islands on Si(001) by chemical vapor deposition, the reduced Ge surface diffusion impedes island development. There is a shift in the relative populations of different island types even when other conditions such as temperature, coverage, and growth rate, are unchanged. The effect of HCl on the net rate of deposition is proportional to the square of the HCl partial pressure, suggesting a surface reaction with the Ge. When larger islands are etched with HCl at high enough temperature, they revert to a shape characteristic of smaller islands, confirming the reversibility of transformations from one island type to another. It has not proved possible to use etching to produce smaller and more uniform islands. © 1998 American Institute of Physics.
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81.05.Cy Elemental semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Cf Surface cleaning, etching, patterning
68.35.Fx Diffusion; interface formation
68.55.-a Thin film structure and morphology

Lateral index guiding in ZnCdSe quantum well lasers by selective implantation-induced disordering

M. Kuttler, M. Strassburg, U. W. Pohl, D. Bimberg, M. Behringer, and D. Hommel

Appl. Phys. Lett. 73, 1865 (1998); http://dx.doi.org/10.1063/1.122308 (3 pages) | Cited 2 times

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Local implantation-induced disordering of ZnCdSe quantum wells is applied to generate lateral index guided II–VI lasers. Lateral selectivity of implanted nitrogen ions is achieved by metal stripes also used for contacts. Secondary ion mass spectroscopy and luminescence prove that quantum well intermixing can be generated without subsequent thermal treatment. Lateral index guiding in II–VI lasers after implantation is demonstrated by recording the near-field pattern, yielding a lateral index step of the order of 10−3. First full index-guided injection lasers with an implantation-induced lateral confinement are processed. Threshold current density is reduced by 40% and emission characteristics of these lasers are strongly improved. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
61.72.uj III-V and II-VI semiconductors
42.60.By Design of specific laser systems

Fabrication of sub-μm bipolar transistor structures by scanning probe microscopy

Shachar Richter, David Cahen, Sidney R. Cohen, Konstantin Gartsman, Vera Lyakhovitskaya, and Yishay Manassen

Appl. Phys. Lett. 73, 1868 (1998); http://dx.doi.org/10.1063/1.122309 (3 pages) | Cited 5 times

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We show how sub-μm sized transistor structures (down to 50 nm cross section) can be fabricated by thermally assisted electromigration of mobile dopants inside the semiconductor CuInSe2. Small device structures are fabricated by application of an electric field to the sample via the contact, defined by a conducting atomic force microscope tip. The structures are characterized by nm scale scanning spreading resistance and scanning capacitance measurements to reveal the inhomogeneous doping profiles created by the electric field. © 1998 American Institute of Physics.
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85.30.Pq Bipolar transistors
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
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
85.35.-p Nanoelectronic devices

Microscopic characterization of hot-electron spreading and trapping in SiO2 films using ballistic electron emission microscopy

B. Kaczer, H.-J. Im, J. P. Pelz, and R. M. Wallace

Appl. Phys. Lett. 73, 1871 (1998); http://dx.doi.org/10.1063/1.122310 (3 pages) | Cited 4 times

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Ballistic electron emission microscopy was used to study three-dimensional spreading and trapping of charge in buried, 10 nm thick SiO2 films following hot-electron injection at one location. The trapped charge was found to be distributed approximately uniformly across the thickness of the oxide, and spread laterally ∼50 nm, much more than the oxide film thickness. This large spreading can be explained by a combination of strong hot electron scattering and strong suppression in trapping efficiency at higher injected electron dose. © 1998 American Institute of Physics.
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73.61.Ng Insulators
73.50.Fq High-field and nonlinear effects

Electron emission from amorphous carbon nitride films

A. Modinos and J. P. Xanthakis

Appl. Phys. Lett. 73, 1874 (1998); http://dx.doi.org/10.1063/1.122311 (3 pages) | Cited 19 times

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We propose a model of field electron emission from amorphous carbon nitride films. According to this model, emission occurs at protrusions of the film surface. Most of the emitted electrons come from states with energy within a rather narrow region about the Fermi level, which shifts with nitrogen concentration but remains within the mobility gap of the film. The model reproduces reasonably well the observed variation of the emitted current with the externally applied electric field and with the nitrogen concentration. © 1998 American Institute of Physics.
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73.61.Ng Insulators
79.70.+q Field emission, ionization, evaporation, and desorption
71.20.Ps Other inorganic compounds

Thermal stability of 2H-implanted n- and p-type GaN

S. J. Pearton, R. G. Wilson, J. M. Zavada, J. Han, and R. J. Shul

Appl. Phys. Lett. 73, 1877 (1998); http://dx.doi.org/10.1063/1.122324 (3 pages) | Cited 12 times

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Implantation of 2H+ into n- and p-type GaN creates high resistivity material in which the resistance displays activation energies of 0.8 and 0.9 eV, respectively. Annealing at 500 °C restores the initial, preimplanted resistance of the n-GaN, due to removal of the deep trap states created by the ion stopping. By contrast, in p-type GaN annealing at 500 °C produces motion of the implanted deuterium and formation of Mg–H complexes that keep the resistance high. About 20% of the deuterium remains in n-GaN even after annealing at 1200 °C, where it decorates the residual implant damage. In p-type GaN all of the deuterium is evolved from the crystal by 1000 °C. © 1998 American Institute of Physics.
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72.80.Ey III-V and II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
85.40.Ry Impurity doping, diffusion and ion implantation technology
72.20.Fr Low-field transport and mobility; piezoresistance

Schottky barrier engineering in III–V nitrides via the piezoelectric effect

E. T. Yu, X. Z. Dang, L. S. Yu, D. Qiao, P. M. Asbeck, S. S. Lau, G. J. Sullivan, K. S. Boutros, and J. M. Redwing

Appl. Phys. Lett. 73, 1880 (1998); http://dx.doi.org/10.1063/1.122312 (3 pages) | Cited 65 times

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A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes. © 1998 American Institute of Physics.  
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73.61.Ey III-V semiconductors
85.30.Hi Surface barrier, boundary, and point contact devices
73.20.At Surface states, band structure, electron density of states
73.30.+y Surface double layers, Schottky barriers, and work functions
85.30.Kk Junction diodes
77.65.Bn Piezoelectric and electrostrictive constants
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Effect of surface layer on optical properties of GaN and InxGa1−xN upon thermal annealing

Eunsoon Oh, Bongjin Kim, Hyeongsoo Park, and Yongjo Park

Appl. Phys. Lett. 73, 1883 (1998); http://dx.doi.org/10.1063/1.122313 (3 pages) | Cited 14 times

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We investigated the effect of rapid thermal annealing (RTA) on the optical properties of GaN and InxGa1−xN. It was found that some of the changes in the photoluminescence spectra of GaN upon annealing were associated with the properties of the surface layer. For example, a new low-temperature photoluminescence line associated with donor–acceptor pair (DAP) recombination appeared at around 3.40 eV upon annealing, but disappeared after etching off the surface layer. This indicates that the acceptors responsible for the DAP emission were created only near the surface. After the RTA process, the near-band-edge emission was blueshifted, and the relative intensity of yellow luminescence with respect to the near-band-edge emission was increased, which were also attributed to the influence of the surface layer. The thermal annealing effect of an InxGa1−xN multiple quantum well structure is also discussed.© 1998 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing
81.65.Cf Surface cleaning, etching, patterning

Parametric amplification of electromagnetic waves in low-dimensional electron systems

S. A. Mikhailov

Appl. Phys. Lett. 73, 1886 (1998); http://dx.doi.org/10.1063/1.122315 (3 pages) | Cited 1 time

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We propose a parametric amplifier (oscillator) of electromagnetic radiation, based on the time-dependent modulation of the confining potential in low-dimensional electron systems like arrays of quantum dots and wires (or classical disks and strips), and calculate their operating characteristics. The operating frequency of the devices can be tuned by external (for instance, magnetic) fields. © 1998 American Institute of Physics.
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73.23.-b Electronic transport in mesoscopic systems
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
41.20.Jb Electromagnetic wave propagation; radiowave propagation

Blue SrS:Cu thin-film electroluminescent devices grown by hot-wall deposition using successive source supply

Koutoku Ohmi, Kazuharu Yamabe, Haruki Fukada, Toshihiko Fujiwara, Shosaku Tanaka, and Hiroshi Kobayashi

Appl. Phys. Lett. 73, 1889 (1998); http://dx.doi.org/10.1063/1.122316 (3 pages) | Cited 9 times

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SrS:Cu, Cl thin-film electroluminescent (TFEL) devices have been prepared by hot-wall deposition using a successive supply of source materials. The TFEL devices show blue EL emission with the chromaticity coordinate of (0.19, 0.28). The luminance and the efficiency at 1 kHz drive are 100 cd/m2 and 0.1 lm/W, respectively. EL emission is observed at both the leading and the trailing edge of the applied pulse voltage. The presence of trailing edge emission implies that Cu+ centers are ionized during EL excitation, and then electrons are subsequently captured by ionized Cu2+ centers giving rise to the EL emission. © 1998 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Pump-probe spectroscopy of band tail states in metalorganic chemical vapor deposition-grown InGaN

T. J. Schmidt, Yong-Hoon Cho, G. H. Gainer, J. J. Song, S. Keller, U. K. Mishra, and S. P. DenBaars

Appl. Phys. Lett. 73, 1892 (1998); http://dx.doi.org/10.1063/1.122317 (3 pages) | Cited 26 times

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Nanosecond nondegenerate optical pump-probe experiments have been performed on InGaN thin films and InGaN/GaN multiple quantum wells. Bleaching of absorption of the localized band tail states was observed with increasing excitation density (Iexc) of the pump pulse. The dynamics of the bleaching was found to depend on the localization depth of the band tail states and on Iexc. With high Iexc, large blueshifts in the spontaneous emission luminescence peaks were also observed, the magnitude of which was again found to depend on the localization depth of the band tail states. Stimulated emission is observed from the samples with increasing Iexc and correlates with significant changes in the behavior of the absorption bleaching. The observed bleaching dynamics of the band tail states are well explained by considering the effective lifetime of the band tail states as measured by time-resolved photoluminescence experiments. © 1998 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.47.-p Spectroscopy of solid state dynamics
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.45.+h Stimulated emission
73.61.Ey III-V semiconductors

Thermoelectric figure of merit of metal–semiconductor barrier structure based on energy relaxation length

B. Moyzhes and V. Nemchinsky

Appl. Phys. Lett. 73, 1895 (1998); http://dx.doi.org/10.1063/1.122318 (3 pages) | Cited 18 times

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The thermoelectric figure of merit is calculated for a compound material comprising thin semiconductor and wider metallic layers. The layers are perpendicular to the direction of current. The semiconductor barriers exclude electrons with energies ϵ<μ from the current. This exclusion increases thermopower. One may obtain a material with a very high ZT if the distance between the barriers is on the order of the energy relaxation length. This material should have the resistivity characteristic of a metal and the thermopower characteristic of a semiconductor. An additional significant rise in ZT can be achieved by increasing the contact area at the metal–semiconductor interface. © 1998 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
72.20.Pa Thermoelectric and thermomagnetic effects
73.50.Lw Thermoelectric effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Strain-induced quantum confinement of electron gases: The observation of quantum dot levels

W. G. Stallard, A. S. Plaut, S. Thoms, M. C. Holland, S. P. Beaumont, C. R. Stanley, and M. Hopkinson

Appl. Phys. Lett. 73, 1898 (1998); http://dx.doi.org/10.1063/1.122319 (3 pages) | Cited 3 times

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Zero-dimensional electron gases (0DEGs) have been fabricated by the strain patterning of a GaAs/AlAs heterojunction using amorphous carbon stressors. We have observed radiative recombination of electrons with both acceptor-bound and strain-confined holes. For our smallest 0DEGs, quantum dot electron levels are clearly resolved. In finite magnetic field we have mapped the evolution of these electron states, from which we estimate the number of electrons per dot to be 15. © 1998 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.61.Ey III-V semiconductors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
78.20.Ls Magneto-optical effects
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Barrier lowering in dye-sensitized porous- TiO2 solar cells

Th. Dittrich, P. Beer, F. Koch, J. Weidmann, and I. Lauermann

Appl. Phys. Lett. 73, 1901 (1998); http://dx.doi.org/10.1063/1.122320 (3 pages) | Cited 4 times

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The current–voltage characteristics of dye-sensitized porous-TiO2(por-TiO2) solar cells are investigated in the dark and under illumination at light intensities up to 1500 W/m2 and temperatures between −5 and 80 °C. In the dark, the barrier height and the ideality factor of the por-TiO2/electrolyte contact are 0.67 eV and 1.05, respectively. The very low effective Richardson constant indicates the importance of diffusion for transport. A current-dependent effective barrier height has been established under illumination of dye-sensitized por-TiO2 solar cells. The barrier lowering effect should be caused by the low neutralization rate of the positively charged dye radicals in the electrolyte. © 1998 American Institute of Physics.
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84.60.Jt Photoelectric conversion
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