High Dopant Densities Research Articles

Carrier–carrier Coulomb interactions reduce power factor in organic thermoelectrics

Organic semiconductors are excellent candidates for low temperature thermoelectric generators. However, such thermoelectric applications require materials be doped and highly conductive. Here, we show how doping affects the Seebeck coefficient in organic semiconductors using kinetic Monte Carlo simulations. Employing a hopping transport approach, we demonstrate that at high dopant loading, carrier–carrier interactions can reduce the Seebeck coefficient. This results in systems with intrinsic disorder, still following Heike's formula for thermopower at high dopant density. Reducing these carrier–carrier interactions results in an increased Seebeck coefficient and power factor. Specifically, a realistic reduction in carrier–carrier interactions can increase the power factor by more than a factor 15, increasing ZT above 1 for organic thermoelectrics.

Dependence of Photoluminescence Emission on Excitation Power and Temperature in Highly Doped 6H- SiC

The excitation power and temperature dependences of photoluminescence (PL) emission in 6H polytype of silicon carbide are investigated extensively. A sublinear power-function dependence of the donor-acceptor pair emission intensity on the excitation power is explained by a rate-equation model. A very low power exponent $(k=0.3)$ in a power-law dependence of PL intensity on the excitation power of $\mathrm{B}$-$\mathrm{N}$ codoped 6H-$\mathrm{Si}\mathrm{C}$ at room temperature indicates that three-particle Auger recombination is the main carrier recombination pathway. Auger recombination in an $\mathrm{Al}$-$\mathrm{N}$ codoped $\mathrm{Si}\mathrm{C}$ does not manifest as strongly as that of the $\mathrm{B}$-$\mathrm{N}$ codoped sample due to high density of ionized impurities which capture the nonequilibrium carriers faster. The ionized impurities form localized potential wells due to their random distribution, which is revealed by a blue shift of the PL band with increasing excitation intensity (approximately 16 meV per decade) and with decreasing temperature. Low-temperature Hall-effect measurement of the $\mathrm{Al}$-$\mathrm{N}$ codoped sample confirms the high density of ionized impurities and significantly reduced ionization energy of dopant states by approximately 53 meV due to the high dopant densities. These results could lead to realization of tunable $\mathrm{Si}\mathrm{C}$-based light sources in severe operating conditions.

The sub-band structure of atomically sharp dopant profiles in silicon

The downscaling of silicon-based structures and proto-devices has now reached the single-atom scale, representing an important milestone for the development of a silicon-based quantum computer. One especially notable platform for atomic-scale device fabrication is the so-called Si:P δ-layer, consisting of an ultra-dense and sharp layer of dopants within a semiconductor host. Whilst several alternatives exist, it is on the Si:P platform that many quantum proto-devices have been successfully demonstrated. Motivated by this, both calculations and experiments have been dedicated to understanding the electronic structure of the Si:P δ-layer platform. In this work, we use high-resolution angle-resolved photoemission spectroscopy to reveal the structure of the electronic states which exist because of the high dopant density of the Si:P δ-layer. In contrast to published theoretical work, we resolve three distinct bands, the most occupied of which shows a large anisotropy and significant deviation from simple parabolic behaviour. We investigate the possible origins of this fine structure, and conclude that it is primarily a consequence of the dielectric constant being large (ca. double that of bulk Si). Incorporating this factor into tight-binding calculations leads to a major revision of band structure; specifically, the existence of a third band, the separation of the bands, and the departure from purely parabolic behaviour. This new understanding of the band structure has important implications for quantum proto-devices which are built on the Si:P δ-layer platform.

The Influence of ITO Dopant Density on J-V Characteristics of Silicon Heterojunction Solar Cells: Experiments and Simulations

The TCO/a-Si:H(p) contact is a critical part of the silicon heterojunction solar cell. At this point, holes from the emitter have to recombine loss free with electrons from the TCO. Since tunneling is believed to be the dominant transport mechanism, a high dopant density in both adjacent layers is critical. In contrast to this, it has been reported that high TCO dopant density can reduce field effect passivation induced by the a-Si:H(p) layer. Thus, in this publication, we systematically investigate the influence of a thin (∼10 nm) ITO contact layer with dopant densities ranging from Nd = 1019 - 1021 cm-3 placed between an ITO bulk layer of 70 nm with Nd= 2·1020 cm-3 and the a-Si:H(p) emitter on the J-V characteristics, with the aim to find an optimum Nd. We accompanied our experiments by AFORS-HET simulations, considering trap-assisted tunneling and field dependent mobilities in the a-Si:H(p) layer. As expected, two regimes are visible: For low Nd the devices are limited by inefficient tunneling, resulting in S-shaped J-V characteristics. For high Nd a reduction of the field effect passivation becomes visible in the low injection range. We can qualitatively reproduce these findings using device simulations.

Numerical modeling of highly doped Si:P emitters based on Fermi–Dirac statistics and self-consistent material parameters

We have established a simulation model for phosphorus-doped silicon emitters using Fermi–Dirac statistics. Our model is based on a set of independently measured material parameters and on quantum mechanical calculations. In contrast to commonly applied models, which use Boltzmann statistics and apparent band-gap narrowing data, we use Fermi–Dirac statistics and theoretically derived band shifts, and therefore we account for the degeneracy effects on a physically sounder basis. This leads to unprecedented consistency and precision even at very high dopant densities. We also derive the hole surface recombination velocity parameter Spo by applying our model to a broad range of measurements of the emitter saturation current density. Despite small differences in oxide quality among various laboratories, Spo generally increases for all of them in a very similar manner at high surface doping densities Nsurf. Pyramidal texturing generally increases Spo by a factor of five. The frequently used forming gas anneal lowers Spo mainly in low-doped emitters, while an aluminum anneal (Al deposit followed by a heat cycle) lowers Spo at all Nsurf.

Si/SiGe n-type inverted modulation doping using ion implantation

Inverted modulated-doped Si/Si1−xGex wafers have been prepared using ex-situ ion implantation of a virtual substrate for the doping followed by cleaning and the regrowth of the silicon quantum well in the growth system. This fabrication scheme attempts to circumvent the main problem in the growth of inverted modulation doped structures by chemical vapour deposition, which is the surface segregation and diffusion of n-type dopant causing high dopant densities in all subsequent layers after the introduction of n-type dopant to the growth chamber. Magneto-transport results will be shown for modulation-doped field effect transistor (MODFET) samples which have 1.7 K mobilities up to 73 000 cm2/V s for carrier concentrations of 4.25×1011 cm−2. Clear Shubnikov-de Haas oscillations and quantum Hall effect plateaux are visible demonstrating the quality of the samples produced using the technique.

Majority and minority electron and hole mobilities in heavily doped gallium aluminum arsenide

The majority electron and minority hole mobilities were calculated in Ga1−yAlyAs for donor densities between 1016 and 1019 cm−3. Similarly, the majority hole and minority electron mobilities were calculated for acceptor densities between 1016 and 1020 cm−3. The mole fraction of AlAs, y, varies between 0.0 and 0.3 in these calculations. All the important scattering mechanisms have been included. The ionized impurity and carrier–carrier scattering processes were treated with a quantum-mechanical, phase-shift analysis. These calculations are the first to use a phase-shift analysis for minority carriers scattering from majority carriers in ternary compounds such as Ga1−yAlyAs. The results are in good agreement with experiment for majority mobilities and predict that at high dopant densities minority mobilities should increase with increasing dopant density for a short range of densities. This effect occurs because of the reduction of plasmon scattering and the removal of carriers from carrier–carrier scattering due to the Pauli exclusion principle. These calculations do not treat the density-of-state modifications due to heavy doping, which should have only a small effect on the mobility at room temperature. The results are important for device modeling because of the need to have physically reasonable values for minority mobilities when simulating the electrical behavior of heterojunction bipolar transistors.

Calculated majority- and minority-carrier mobilities in heavily doped silicon and comparisons with experiment

As silicon devices approach 0.1 μm in size, it will be essential to have accurate values of the majority and minority mobilities of electrons and holes. These mobilities have been calculated in silicon for donor and acceptor densities between 1017 and 1020 cm−3. All the important scattering mechanisms have been included. The ionized impurity scattering has been treated with a quantum-mechanical phase-shift analysis. The results are in good agreement with experiment, but predict that the change of minority electron mobility with increasing dopant density should decrease slightly at high dopant densities for a small range of densities. This effect occurs mainly because of the reduction of plasmon scattering. Some recent experiments support these findings. In addition, the ionized impurity scattering rates calculated from the quantum-mechanical phase shifts and those rates calculated from the Born approximation are shown to differ by more than factors of 3. The Born approximation is not valid for low-energy carriers near band extrema. Carrier scattering rates are key input quantities for Monte Carlo calculations of carrier-velocity versus electric-field relations and of simulations of device behavior. Our calculations do not treat the density-of-states modifications due to heavy doping, which should have only a small effect on the mobility at room temperature.

Majority and minority electron and hole mobilities in heavily doped GaAs

The majority electron and minority hole mobilities have been calculated in GaAs for donor densities between 5×1016 and 1×1019 cm−3. Similarly, the majority hole and minority electron mobilities have been calculated for acceptor densities between 5×1016 and 1×1020 cm−3. All the important scattering mechanisms have been included. The ionized impurity and carrier–carrier scattering processes have been treated with a phase-shift analysis. These calculations are the first to use a phase-shift analysis for minority carriers scattering from majority carriers. The results are in good agreement with experiment, but predict that at high dopant densities minority mobilities should increase with increasing dopant density for a short range of densities. This effect occurs because of the reduction of plasmon scattering and the removal of carriers from carrier–carrier scattering because of the Pauli exclusion principle. Some recent experiments support this finding. These calculations do not treat the density-of-states modifications due to heavy doping, which should have only a small effect on the mobility at room temperature. The results are important for device modeling because of the need to have values for minority mobilities.

Electrical conduction in polycrystalline CdS films. II. Comparison of theory and experiment

For pt.I see ibid., vol.20, p.951-57 (1987). The electrical conduction in polycrystalline In doped CdS films has been studied over the temperature range 77-300 K using Hall effect and conductivity measurements. In the preceding paper a model for the electrical conduction in polycrystalline CdS has been developed. The comparison of the model to the experimental data gives a satisfactory interpretation of the transport parameters in the In doped CdS films. The conduction is limited by the energy barriers, even at high dopant densities, and tunnelling transport becomes predominant. The trap state density increases with increasing dopant density.

Influence of the dopant density profile on minority-carrier current in shallow, heavily doped emitters of silicon bipolar devices

Experimental determination of the dependence of recombination current in p + and n + regions on the dopant profile for shallow emitters of ion-implanted silicon solar cells is described. The results are analyzed by extending a previous analytical model for the transport of minority carriers in heavily doped regions. The extension accounts for an effective electric field, defined by heavy-doping effects at the surface, and suggests that the energy-gap narrowing for p + silicon is slightly smaller than that for n + silicon and/or that minority-carrier diffusivities are substantially lower than the majority-carrier ones at comparable dopant densities. The very high dopant densities achieved with the ion implantation/laser annealing technique provide an in situ surface passivation that supresses surface recombination and minimizes the emitter recombination current.