Arlynn W. Smith

The effects of CdCl2 on the electronic properties of molecular‐beam epitaxially grown CdTe/CdS heterojunction solar cells

Significant improvements in CdTe/CdS solar cell efficiency are commonly observed as a result of a postdeposition CdCl2 dip followed by a 400 °C heat treatment during cell processing which increases CdTe grain size. In this paper, we investigate the electronic mechanisms responsible for CdCl2‐induced improvement in cell performance along with possible performance‐limiting defects resulting from this process in molecular‐beam epitaxy‐grown polycrystalline CdTe/CdS solar cells. Current density‐voltage‐temperature (J‐V‐T) analysis revealed that the CdCl2 treatment changes the dominant current transport mechanism from interface recombination/tunneling to depletion region recombination, suggesting a decrease in the density and dominance of interface states due to the CdCl2 treatment. It is shown that the change in transport mechanism is associated with (a) an increase in heterojunction barrier height from 0.56 to 0.85 eV, (b) a decrease in dark leakage current from 4.7×10−7 A/cm2 to 2.6×10−9 A/cm2 and, (c) an i...

Hydrodynamic Simulation of Semiconductor Devices

Abstract In this paper, we present an introduction to hydrodynamic-based simulation of semiconductor devices. A very general approach is given to illustrate a mathematical method of the solution. A general differential equation which is characteristic of the relevant semiconductor equations is first presented and discussed. Solution of this general differential equation forms the basis of the mathematical model employed. A discretization equation based on the general differential equation is next determined. From use of Newtons method, the system of equations arising from the discretization equation applied to the nodes in the simulation domain, can then be solved. This general technique is next applied to the specific problem of the hydrodynamic device simulation equations. The basic equations and their auxiliary relationships governing the hydrodynamic simulation are developed. The boundary conditions and material constituent relations are presented which complete the solution. Finally, we illustrate the method with several computational examples. The workings of silicon and GaAs based ballistic diodes are examined using the drift–diffusion and hydrodynamic simulations. Additionally, we examine a GaAs/AlGaAs heterostructure bipolar transistor. Both narrow and wide base devices are simulated to illustrate the importance and relevance of the hydrodynamic method in a realistic and important semiconductor device. It is shown that the hydrodynamic simulation recovers more of the important physics gosverning these devices than the standard drift–diffusion method.

Two-dimensional model of photon recycling in direct gap semiconductor devices

The effects of photon recycling are examined in a general, fully numerical, two-dimensional model accounting for the detailed geometry of the device and the spectral content of the recombined excess carriers. The primary component of this model is a three-dimensional ray tracing algorithm which encompasses effects such as wavelength dependent absorption and index of refraction, the angular dependence of transmissivity between layers in a heterostructure device, and multiple reflections within a device. This ray tracing preprocessing step is used to map all of the possible trajectories and absorption of various wavelengths of emitted light from each originating node within the device. These data are integrated into a macroscopic device simulator to determine the spatial and temporal location of the reabsorbed radiation within the geometry of the device. By incorporating the ray tracer results with the total quantity and spectral content of recombined carriers at each node within the simulation, the recycled generation rate can be obtained. To demonstrate the use of this model, the effects of photon recycling on the carrier lifetime in an InP/InGaAs double heterostructure photodiode are presented. Good agreement between the experimentally measured lifetime and that predicted using photon recycling is obtained.

Theoretical study of the effect of an AlGaAs double heterostructure on metal-semiconductor-metal photodetector performance

The impulse and square-wave input response of different GaAs metal-semiconductor-metal photodetector (MSM) designs are theoretically examined using a two dimensional drift-diffusion numerical calculation with a thermionic-field emission boundary condition model for the heterojunctions. The rise time and the fall time of the output signal current are calculated for a simple GaAs, epitaxially grown, MSM device as well as for various double-heterostructure barrier devices. The double heterostructure devices consist of an AlGaAs layer sandwiched between the top GaAs active absorption layer and the bottom GaAs substrate. The effect of the depth of the AlGaAs layer on the speed and responsivity of the MSM devices is examined. It is found that there is an optimal depth, at fixed applied bias, of the AlGaAs layer within the structure that provides maximum responsivity at minimal compromise in speed. >

Non-isothermal extension of the Scharfetter-Gummel technique for hot carrier transport in heterostructure simulations

Points out that the Scharfetter-Gummel technique is widely used in algorithms for the simulation of isothermal semiconductor devices. Recent interest in the modeling of ultrasmall devices requires a nonisothermal analysis, i.e., a hydrodynamic model. Several nonisothermal extensions to the Scharfetter-Gummel technique for carrier flux have been proposed for homostructure devices. An extension is presented which is suitable for the simulation of both the carrier flux and carrier energy flux equations in heterostructure devices, with the capability of using Fermi-Dirac statistics. Comparison with the extensions of other authors provides verification of the discretization formulation developed here. Limiting cases are discussed with suitable approximations. Calculated values of fluxes are presented for selected nonisothermal and degenerate cases to highlight the need for inclusion of the Fermi-Dirac statistics in the flux formulations. >

Non-Parabolic Hydrodynamic Formulations for the Simulation of Inhomogeneous Semiconductor Devices

Abstract Hydrodynamic models are becoming prevalent design tools for small scale devices and other devices in which high energy effects can dominate transport. Most current hydrodynamic models use a parabolic band approximation to obtain fairly simple conservation equations. Interest in accounting for band structure effects in hydrodynamic device simulation has begun to grow since parabolic models cannot fully describe the transport in state of the art devices due to the distribution populating non-parabolic states within the band. This paper presents two different non-parabolic formulations of the hydrodynamic model suitable for the simulation of inhomogeneous semiconductor devices. The first formulation uses the Kane dispersion relationship ( k ) 2 2m = W(1 + αW) . The second formulation makes use of a power law { ( k ) 2 2m = xW y } for the dispersion relation. Hydrodynamic models which use the first formulation rely on the binomial expansion to obtain moment equations with closed form coefficients. This limits the energy range over which the model is valid. The power law formulation readily produces closed form coefficients similar to those obtained using the parabolic band approximation. However, the fitting parameters (x, y) are only valid over a limited energy range. The physical significance of the band non-parabolicity is discussed as well as the advantages/disadvantages and approximations of the two non-parabolic models. A companion paper describes device simulations based on the three dispersion relationships; parabolic, Kane dispersion and power law dispersion.

Comparison of Non-Parabolic Hydrodynamic Simulations for Semiconductor Devices

Abstract Parabolic drift-diffusion simulators are common engineering level design tools for semiconductor devices. Hydrodynamic simulators, based on the parabolic band approximation, are becoming more prevalent as device dimensions shrink and energy transport effects begin to dominate device characteristics. However, band structure effects present in state-of-the-art devices necessitate relaxing the parabolic band approximation. This paper presents simulations of ballistic diodes, a benchmark device, of Si and GaAs using two different non-parabolic hydrodynamic formulations. The first formulation uses the Kane dispersion relationship in the derivation of the conservation equations. The second model uses a power law dispersion relation { (h k ) 2 2m = x W y }. Current-voltage relations show that for the ballistic diodes considered, the non-parabolic formulations predict less current than the parabolic case. Explanations of this will be provided by examination of velocity and energy profiles. At low bias, the simulations based on the Kane formulation predict greater current flow than the power law formulation. As the bias is increased this trend changes and the power law predicts greater current than the Kane formulation. It will be shown that the non-parabolicity and energy range of the hydrodynamic model based on the Kane dispersion relation are limited due to the binomial approximation which was utilized in the derivation.

Effect of trap location and trap-assisted Auger recombination on silicon solar cell performance

Model calculations were performed to investigate and quantify the effect of trap location and trap-assisted Auger recombination on silicon solar cell performance. Trap location has a significant influence on the lifetime behavior as a function of doping and injected carrier concentration in silicon. It Is shown in this paper that for a high quality silicon (/spl tau/=10 ms at 200 ohm-cm, no intentional doping), high resistivity (/spl ges/200 ohm-cm) is optimum for high efficiency one sun solar cells if the lifetime limiting trap is located near midgap. However, if the trap is shallow (E/sub t/-E/sub v//spl les/0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 /spl mu/s at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be /spl sim/0.2 ohm-cm, regardless of the trap location. It is shown that the presence of a shallow trap can significantly degrade the performance of a concentrator cell fabricated on high-resistivity high-lifetime silicon material because of an undesirable injection level dependence in the carrier lifetime. The effect of trap assisted Auger recombination on the cell performance has also been modelled in this paper. It is found that the trap-assisted Auger recombination does not influence the one sun cell performance appreciably, but can degrade the concentrator cell performance if the trap-assisted Auger recombination coefficient value exceeds 2/spl times/10/sup -14/ cm/sup 3//s. Therefore, it is necessary to know the starting lifetime as well as trap location in order to specify base resistivity in order to predict or achieve the best cell performance for a given one sun or concentrator cell design. >

Re‐evaluation of the derivatives of the half order Fermi integrals

The Fermi integrals of half orders are important in the simulation of semiconductor transport processes. Several of these integrals (−1/2, 1/2, 3/2, 5/2) have been recently retabulated since the 1938 study by McDougall and Stoner [Phil. Trans. Roy. Soc. A 237, 67 (1938)], but the derivatives were not re‐evaluated. The original integral values were calculated without the aid of high speed computers by using approximate series evaluation and tabulations of exponentials and zeta functions. In addition, a discrepancy was found in the literature since the original study in 1938. The second derivative of F1/2 has been mistakenly represented as being proportional to a Fermi integral of another order. This article tabulates the half order Fermi integrals from −1/2 to 5/2 over the reduced energy range −5 to 20 in 0.25 increments. The first two derivatives of F−1/2 are also calculated by numerical integration and tabulated to aid in interpolation. It is shown that the second derivative of F1/2 is not proportional t...

Numerical Examination of Photon Recycling as an Explanation of Observed Carrier Lifetime in Direct Bandgap Materials

Photon recycling is examined as an explanation for the observed large carrier lifetimes in an InP/InGaAs photodiode. This effect extends the effective carrier lifetime within a device by re-absorbing a fraction of the photons generated through radiative band-toband recombination events. In order to predict the behavior of this carrier generation, photon recycling has been added to our two-dimensional macroscopic device simulator, STEBS-2D. A ray-tracing preprocessing step is used to map all of the possible trajectories and absorption of various wavelengths of emitted light from each originating node within the device. The macroscopic simulator uses these data to determine the spatial location of the re-absorbed radiation within the geometry of the device. By incorporating the ray tracer results with the total quantity and spectral content of recombined carriers at each node within the simulation, the recycled generation rate can be obtained. A practical application of this model is presented where the effects of photon recycling are used as a possible explanation of the discrepancy between the theoretically predicted and experimentally observed radiative recombination rate in a double heterostructure photodetector.