Doping Density Dependence Research Articles

M-plane GaN terahertz quantum cascade laser structure design and doping effect for resonant-phonon and phonon-scattering-injection schemes

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm−1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green’s function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature.

Improved conductivity and flatness of solution-processed transparent multilayer ZnO conductive films by stacking method

ZnO stacked films are prepared by a wet process under various conditions. Al and Ga are co-doped as dopants, i.e., producing AGZO films. The doping density dependence of the film resistivity shows that the best doping condition is 1.0 wt% of Ga-doping density and 0.1 wt% of Al-doping density, where the minimum sheet resistance of 1.2 × 105 Ω/□ and the best orientation of (002) surface are observed. The stacked AGZO films are prepared by two different methods to improve film properties: Method A is repeated provisional annealing, spin coating, and main annealing, where the maximum number of repetitions is three. On the other hand, method B is repeated provisional annealing and spin coating followed by annealing the stacked films only one time. As a result, the AGZO films produced by method B reveal a minimum surface roughness of about 3.2 nm. In addition, with an increase of the stacked layers, the sheet resistance is decreased.

Exciton-Trion Polaritons in Doped Two-Dimensional Semiconductors.

We present a many-body theory of exciton-trion polaritons (ETPs) in doped two-dimensional semiconductor materials. ETPs are robust coherent hybrid excitations involving excitons, trions, and photons. In ETPs, the 2-body exciton states are coupled to the material ground state via exciton-photon interaction, and the 4-body trion states are coupled to the exciton states via Coulomb interaction. The trion states are not directly optically coupled to the material ground state. The energy-momentum dispersion of ETPs exhibit three bands. We calculate the energy band dispersions and the compositions of ETPs at different doping densities using Green's functions. The energy splittings between the polariton bands, as well as the spectral weights of the polariton bands, depend on the strength of the Coulomb coupling between the excitons and the trions, which in turn depends sensitively on the doping density. The doping density dependence of the ETP bands and the charged nature of the trion states could enable novel electrical and optical control of ETPs.

Anomalous electron doping independent two-dimensional superconductivity

Transition metal (Co and Ni) co-doping effects are investigated on an underdoped Ca0.94La0.06Fe2As2 compound. It is discovered that electron doping from substituting Fe with transition metal (TM = Co, Ni) can trigger high- superconductivity around 35 K, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Remarkably, the critical temperature for the high- superconductivity remains constant against a wide range of TM doping levels. And the net electron doping density dependence of the superconducting based on the rigid band model can be nicely scaled into a single curve for Co and Ni substitutions, in stark contrast to the case of Ba(Fe1−xTMx)2As2. This carrier density independent superconductivity and the unusual scaling behavior are presumably resulted from the interface superconductivity based on the similarity with the interface superconductivity in a La2−xSrxCuO4-La2CuO4 bilayer. Evidence of the two-dimensional character of the superfluid by angle-resolved magneto-resistance measurements can further strengthen the interface nature of the high- superconductivity.

(Invited) Use of the Doping Dependence of Collection Efficiencies for Dye Sensitized Photocurrents to Demonstrate Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces

The different physical limits of an Onsager-based model for charge collection at a semiconductor electrode are revealed through an experimental examination of the doping density dependence of photocurrents at single crystal ruitile TiO2 electrodes sensitized with the N3 chromophore and a trimethine cyanine dye. As the doping density of the electrodes was varied from 1015 cm-3 to 1020 cm-3, three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at an electrode bias in the depletion region. At highly doped electrodes tunneling currents played a significant role, especially in the recombination processes. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.

Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces as Revealed by the Doping Density Dependence of the Collection Efficiency of Dye Sensitized Photocurrents.

The doping density dependence of photocurrents has been experimentally measured at single crystal rutile TiO2 electrodes sensitized with the N3 chromophore and a trimethine dye. As the doping density of the electrodes was varied from 10(15) to 10(20) cm(-3), three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at electrode bias in the depletion region. At highly doped electrodes, tunneling currents played a significant role especially in the recombination processes. These different forms of the current-voltage curves could be fit to an Onsager-based model for charge collection at a semiconductor electrode. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.

Photoluminescence quenching in films of conjugated polymers by electrochemical doping

An important loss mechanism in organic electroluminescent devices is exciton quenching by polarons. Gradual electrochemical doping of various conjugated polymer films enabled the determination of the doping density dependence of photoluminescence quenching. Electrochemical doping was achieved by contacting the film with a solid electrochemical gate and an injecting contact. A sharp reduction in photoluminescence was observed for doping densities between 10${}^{18}$ and 10${}^{19}$ cm${}^{\ensuremath{-}3}$. The doping density dependence is quantitatively modeled by exciton diffusion in a homogeneous density of polarons followed by either F\"orster resonance energy transfer or charge transfer. Both mechanisms need to be considered to describe polaron-induced exciton quenching. Thus, to reduce exciton-polaron quenching in organic optoelectronic devices, both mechanisms must be prevented by reducing the exciton diffusion, the spectral overlap, the doping density, or a combination thereof.

Electron spin dynamics study of bulk p-GaAs: The screening effect

In this work, the electron spin dynamics of bulk p-GaAs doped with Be grown by molecular beam epitaxy is investigated by using the time-resolved magneto-optical Kerr rotation technique. The spin relaxation/dephasing times T1 and T2* are systematically investigated as a function of hole doping density and photo-excitation density as well as temperature. The complex hole doping density dependence of spin relaxation times T1 and T2* is observed experimentally, which agrees well with predictions of the kinetic spin Bloch equation theory published previously [J. Jiang and M. Wu, Phys. Rev. B 79, 125206 (2009)]. D'yakonov-Perel's mechanism [M. Dyakonov and V. Perel, Sov. Phys. Solid State 13, 3023 (1972)] is discussed to dominate the electron spin relaxation process in p-GaAs, with the effect of hole screening proven to play an important role.

4‐(dicyanomethylene)‐2‐methyl‐6‐(p‐dimethylaminostyryl)‐4H‐pyran (DCM)‐doping density dependence of luminescence spectra and white emission in polymer light‐emitting diodes

AbstractWe fabricated white polymer light‐emitting diodes (PLEDs), in which the active region is doped with a low‐weight molecule. The host polymer material is Poly(9,9‐didodecyl‐fluorenyl‐2,7‐yleneethylnylene) (PFO), while the guest luminescent low‐weight‐molecule is 4‐(Dicyanomethylene)‐2‐methyl‐6‐(p‐dimethylaminostyryl)‐4H‐pyran (DCM). The photoluminescence (PL) and electroluminescence (EL) spectra strongly depend on DCM‐doping‐density. However, the most suitable DCM‐doping‐densities for white emissions in PL and EL are slightly different. This discrepancy is caused by the difference in excitation efficiencies. The photo‐excitation system for PL measurement is better than the current injection for generating carriers efficiently. Thus, the realization of the white EL signal requires the larger DCM‐doping‐density than that of the white PL signal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cubic GaN/AlGaN Schottky-barrier devices on 3C-SiC substrates

In this work, we focus on the fabrication of cubic GaN based Schottky-barrier devices (SBDs) and measured current voltage ( I– V) characteristics and the critical field for electronic breakdown. Phase-pure cubic GaN and c-Al x Ga 1 − x N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 μm thick free-standing 3C-SiC (1 0 0) substrates, which were produced by HOYA Advanced Semiconductor Technologies Co., Ltd. The thickness of the c-GaN and c-Al 0.3Ga 0.7N epilayers were about 600 and 30 nm, respectively. Ni/In Schottky contacts 300 μm in diameter were produced on c-GaN and c-Al 0.3Ga 0.7N/GaN structures by thermal evaporation using contact lithography. A clear rectifying behavior was measured in our SBDs and the I– V behavior was analyzed in detail, indicating the formation of a thin surface barrier at the Ni–GaN interface. Annealing of the Ni Schottky contacts in air at 200 °C reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SBDs is investigated. The experimental values of breakdown voltage in c-GaN are in good agreement with theoretical values and show the same dependence on doping level as in hexagonal GaN. From our experimental data, we extrapolate a blocking voltage of 600 V in c-GaN films with a doping level N D = 5 × 10 15 cm −3.

Mechanism of Current Leakage in Ni Schottky Diodes on Cubic GaN and AlxGa1-xN Epilayers

AbstractNi Schottky-diodes (SDs) 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN and AlxGa1-xN epilayers. Phase-pure cubic GaN and c-Al0.3Ga0.7N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 µm thick free-standing 3C-SiC (100) substrates. The quality of the cubic group III-nitride epilayers was checked by high resolution X-ray diffractometry, atomic force microscopy and photoluminescence at room temperature and at 2 K. Large deviations from the thermionic emission transport were observed in the current voltage (I-V) behavior of these SDs. Detailed analysis of the I-V characteristics at 300 K and at low temperature showed that a thin surface barrier is formed at the Ni semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier and reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SDs is in good agreement with theoretically calculated values and follows the expected trend. From these experimental data a blocking voltage of higher than 600V is extrapolated for c-GaN films with a doping level of ND = 5×1015 cm-3.

Room-temperature dephasing time of intersubband transitions in heavily-doped InGaAs/AlAs/AlAsSb coupled quantum wells

Room-temperature dephasing time (T2) estimates of 1.55 μm intersubband transitions (ISBT) are presented in InGaAs/AlAs/AlAsSb coupled double quantum wells. In this material, optimized for ISBT at communication wavelengths for all-optic switching application, we also studied the doping-density dependence of the four-wave-mixing decay time as a function of incident excitation intensity. A T2 value of about 292±40 fs is estimated at an intensity corresponding to the saturation intensity in samples that are doped to 2×1019 cm−3. This value is in good agreement with the value estimated from earlier calculations.

A reverse bias, tip-insulator-semiconductor tunnel diode model accounting for the delineation of a p/p+ junction using scanning tunneling microscopy

In this article, a tunneling-generation-avalanche model has been proposed to explain the reverse bias current–voltage behavior observed at a tip/air/p-type silicon junction. Based on this model, under conditions where the applied bias is more negative than the flat band voltage, the current will be dominated by generation processes, which has significant doping density dependence. Since mechanically cut tips, used in this work, can have complicated geometries, geometric effects, such as extended gates and concentration of the electrical field must be taken into account. By taking these factors into account, good agreement between theory and experiments can be achieved. Finally, in the presence of illumination, p/p+ junctions can be delineated successfully by taking advantage of the generation process. These results demonstrate that scanning tunneling microscopy can be used as a powerful tool for characterizing semiconductor devices.

Doping-density dependence of scanning tunneling spectroscopy on lightly doped silicon

The doping-density dependence of scanning tunneling spectroscopy on lightly doped hydrogen-terminated Si(100) (resistivities in the range of 0.2–12 Ω cm) was investigated in air with and without illumination. The observed doping-density dependence is consistent with a generation model in which the changes in the three-dimensional depletion region, induced by a scanning tunneling microscopy tip, contributes to changes in the concentration of thermally and/or photogenerated carriers in lightly doped samples. These results suggest that scanning tunneling spectroscopy can be used to image variations in dopant density in lightly doped samples.

Effect of doping density on capacitance of resonant tunneling diodes

We studied capacitance and parallel resistance in resonant tunneling diodes as a function of the doping density in the emitter and the collector regions. Capacitance was obtained by analyzing resonance in the admittance measured. Our data show that the capacitance varies with the doping density, and that the capacitance is smaller than the value expected from the growth parameters. Electron density modulation exists around the barriers, and capacitance has doping density dependence as a result of the modulation.

Transport properties of low-resistance ohmic contacts to InP

The transport properties of conventional Au-based low-resistance ohmic contacts to n- and p-type InP have been investigated. For n-type InP, a good agreement between the minimum specific contact resistance and the bulk doping density is observed in accordance with an inverse bulk doping density dependence for the specific contact resistance. Drift and diffusion across a thermodynamically stable metalphosphide-InP junction is found to be the rate-limiting step of the low-resistance contacts to n-type InP. Tunneling is not observed, but rather a significantly effective Schottky barrier lowering is initially responsible for the low-resistance contacts. Significant Schottky barrier lowering is also observed for ohmic contacts to p-type InP, but no total transition from predominately thermionic emission to drift and diffusion is observed for low-resistance contacts to p-type InP, indicating that the contacts are not fully developed, and that further reduction of the specific contact resistance can be expected.

Doping-density dependence of photoluminescence in highly Si-doped GaAs/AlxGa1-xAs quantum wells from below to above the metallic limit.

The development of the photoluminescence spectra with doping density has been studied for a series of single GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells center doped with Si. The trends observed are found to be very different from those observed for equivalent doping levels in bulk GaAs. In particular, excitons continue to dominate the radiative recombination at doping levels right up to the metallic limit. The electron density at which the exciton is quenched is found to be considerably higher (about ${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) in the center-doped case than has previously been demonstrated for modulation-doped quantum wells (about 4\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$). This result is understood in terms of the difference in phase-space filling for the center-doped quantum well when compared with the modulation-doped case. The impurity concentration at which the material becomes degenerate is also found to be significantly higher than in bulk GaAs. The recombination dynamics at the metallic limit have been studied using a time-resolved photoluminescence technique and demonstrate that localization, resulting from either interface roughness or the high doping level, is reduced as the density is increased above the degenerate limit. The apparent relaxation of momentum conservation observed in the photoluminescence spectra for highly doped samples is interpreted as being due to recombination from strongly localized holes.

Persistent photocurrent decay mechanisms by capture of photoelectrons in GaAs-AlxGa1-xAs heterostructures.

We introduce a method for separating multiple-rate decay mechanisms of persistent photoconductivity (PPC) in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures. The derivative of the decay of measured conductivity or carrier number density with respect to the logarithm of time (1 msec--1000 sec) shows a slowly varied base-line structure with some pronounced peaks. The temperature dependence of the varied base line agrees with the temperature dependence of the tunneling barrier. The temperature dependence of the base-line structure and the positions of the pronounced peaks was analyzed to yield capture energies and corresponding lifetime prefactors. The collected evidence demonstrates that the combination of measured capture energies and lifetime prefactors can distinguish the microscopic contributions to PPC decay in the doped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As layer of samples from other capture mechanisms. (1) The short-lifetime prefactor (${10}^{\mathrm{\ensuremath{-}}8}$--${10}^{\mathrm{\ensuremath{-}}10}$ sec) and associated capture energies are in good agreement with the results from other work on the DX-center capture mechanism. (2) This short-lifetime prefactor in a narrow range of temperature compares with prefactors longer by as much as 10 orders of magnitude in adjacent ranges of temperature, which can be identified as tunneling related PPC decay of the two-dimensional electron gas into shallow or deep donors in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As layer. (3) The mechanism with short-lifetime prefactors cannot lie on the GaAs side of the heterojunction because of the observed strong gate-bias dependence and doping-density dependence of the decay magnitude.

Carrier recombination and lifetime in highly doped silicon

The predominance of phonon-assisted band-band Auger recombination in highly doped silicon is demonstrated by showing that no recombination mechanism involving common (unavoidable) defects in silicon can yield carrier lifetimes that are consistent with the measured lifetimes, which exhibit an inverse-quadratic doping-density dependence, and/or with their temperature dependence. Both trap-assisted-Auger and Shockley-Read-Hall recombination mechanisms are considered, and dependences of the defect density on the doping density, which are implied by theory and experiment, are accounted for.