Resistivity dependence of minority carrier lifetime and cell performance in p-type dendritic web silicon ribbon

This study shows that the bulk lifetime in 95 μm thick p-type dendritic web silicon solar cells is a strong function of bulk resistivity. The higher the resistivity, the greater the bulk lifetime. This behavior is explained on the basis of dopant–defect interaction, which increases the lifetime limiting trap concentration with the addition of dopant atoms. Model calculations show that in the absence of doping dependence of bulk lifetime (τ), 2 Ω cm web should give the best cell efficiency for bulk lifetimes below 30 μs. However, strong doping dependence of bulk lifetime in p-web cells shifts the optimum resistivity from 2 to 15 Ω cm. Bulk lifetime in the as-grown web material was found to be less than 1 μs for all the resistivities. After the cell processing which involves phosphorus gettering, aluminum gettering, and SiN induced hydrogen passivation of defects, the bulk lifetime increased to 6.68, 11, 31 and 68.9 μs in 0.62, 1.37, 6.45 and 15 Ω cm p-type web material, respectively. Therefore, cell process induced recovery of lifetime in web is doping dependent, which favors high resistivity. Solar cells fabricated on 95 μm thick web silicon by a manufacturable process involving screen-printing and belt-line processing gave 14.5% efficient 4 cm² cells on 15 Ω cm resistivity. This represents a record efficiency for such a thin manufacturable screen-printed cell on a low-cost PV grade Si ribbon that requires no wafering or etching.     

An approach toward 25-percent efficient GaAs heteroface solar cells

To approach one-sun 25% efficiency in GaAs solar cells, it is necessary to improve the basic understanding of internal loss mechanisms by a combination of characterization techniques and computer models. A methodology is developed to measure and evaluate minority-carrier transport properties such as lifetime and recombination velocity throughout the device structure in a 21.2% GaAs cell. It is found that this cell has a recombination velocity of 1.25×10⁵ cm/s at the AlGaAs/GaAs interface and a base minority-carrier lifetime of 8 ns. Guidelines are provided to increase the efficiency of this cell to 24% with slightly increased surface passivation and base lifetime using effective recombination velocity and device modeling computer programs. Further device modeling is performed to show that efficiencies of 25% can be obtained using a modified heteroface structure with a moderate surface recombination and their relation to device design are fully understood     

Projections of GaAs solar-cell performance limits based ontwo-dimensional numerical simulation

The development of a comprehensive, two-dimensional numerical model for AlGaAs/GaAs solar cells is described. The model was used to identify loss mechanisms in present-day high-efficiency GaAs cells and to make realistic projections of attainable cell efficiencies. Numerical simulations show that achievable efficiencies of conventional heteroface cells made on high-quality GaAs films exceed 30% under 500 suns (AM1.5 direct spectrum). Both p-n and n-p cells are adversely affected by bandgap narrowing in p⁺ GaAs. For n-p cells, the use of heterojunction back-surface fields is advantageous and results in an increase of about 1.5% in efficiency. When the Shockley-Read-Hall lifetime parameters are set to infinity, the efficiency increases by only another 0.7%, which demonstrates that bulk material quality is not the major limiting factor in present-day cells. These efficiency projections, which are based on detailed device simulation and realistic material parameters, are only a few percentage points below the thermodynamic limit for GaAs cells     

Effect of trap location and trap-assisted Auger recombination onsilicon 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 (τ=10 ms at 200 ohm-cm, no intentional doping), high resistivity (⩾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_t-E_v⩽0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 μs at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be ~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 2×10^-14 cm³/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     

Effect of heat treatment on the nature of traps in epitaxial GaAs

By annealing v.p.e. GaAs at increasing temperatures, the commonly observed 0.83 eV electron trap in v.p.e. and bulk GaAs is removed and a 0.64 eV hole trap, also detected in l.p.e. GaAs, is introduced. The results indicate that these electron and hole traps are related to Ga and As vacancies, respectively.     

The effect of ionization and displacement damage on minority carrier lifetime

Based on 1 MeV electrons and 40 MeV Si ion irradiations, the contribution of ionization and displacement damage to the decrease in the minority carrier lifetime of gate controlled lateral PNP (GLPNP) transistors is investigated by gate sweeping (GS) technique. Molecular hydrogen is employed to increase the ionization radiation sensitivity and help to understand the relationship between the minority carrier lifetime and ionization damage. Experimental results show that 1 MeV electrons mainly induce ionization damage to GLPNP transistors, 40 MeV Si ions primarily produce displacement defects in silicon bulk. For 40 MeV Si ions, with increasing the irradiation dose, the densities of interface trap and oxide charge are almost no change, the minority carrier lifetime obviously decreases. The decrease of the minority carrier lifetime is due to bulk traps induce by 40 MeV Si ions. For 1 MeV electrons, with increasing the irradiation dose, the densities of interface trap and oxide charge for the GLPNP with and without soaked in H₂ increase, and the minority carrier lifetime decreases. Compared with the GLPNP transistors without soaking in H₂, the density of the interface traps the irradiated GLPNP transistors by 1 MeV electrons and soaked in H₂ are larger and the minority carrier lifetime is lower. Therefore, both ionization and displacement damage can induce the decreases in the minority carrier lifetime including bulk minority carrier lifetime and surface minority carrier lifetime.     

Defect state assisted tunneling in intermediate temperature molecular beam epitaxy grown GaAs

Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by defect states can dominate, at some bias ranges, current transport in Schottky contacts to unannealed GaAs material grown at the intermediate temperature range of about 400°C. The deep defect states are modeled by quantum wells which trap electrons emitted from the cathode before re-emission to semiconductor. Comparison of theory with experimental data shows defect states of energies about 0.5 eVbelow conduction band to provide the best fit to data. This suggests that arsenic interstitials are likely to mediate this conduction. Comparison is also made between as-grown material and GaAs grown at the same temperature but annealed at 600°C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers.     

Characteristics of a GaAs spontaneous infrared source with 40 percent efficiency

The external quantum efficiency of a forward-biased GaAs p-n junction device selected for high efficiency measures 40.5, 32, and 7.3 percent at 20, 77, and 295°K, respectively. The optical exit path is through bulk material doped to a 10¹⁷donors/cm²level. The infrared emission is measured directly with a silicon solar cell. The effective transmissivity of the GaAs bulk device material is measured to be 42 and 8.3 percent at 77 and 295°K, respectively. The corresponding values for the internal quantum efficiency are 76 and 88 percent. The primary optical rise time measured for high level current pulsing conditions is 0.6 and 1.6 ns at 77 and 295°K, respectively.     

Characterization of deep traps in semi-insulating gallium arsenide

In this paper a number of deep traps, which play a crucial role in many macroscopic properties of semi-insulating GaAs have been characterized and analyzed. The main trap parameters (activation energy and capture cross section) were deduced by several experimental methods, based on thermally stimulated current and isothermal current transients. Results were compared with other studies of deep traps in SI GaAs performed with these and other methods as well as studies of deep levels in conductive GaAs. To enable these comparisons an analysis of the methods and usually employed calculation procedures was given as well as suggestions for the presentation of the trap parameters. It was also concluded that the observed traps are complex defects which correspond to deep traps observed by other authors in a variety of other SI GaAs materials. Some of them seem to include, as a part of the defect, a well known defect EL2 but there are also strong indications for involvement of other structural defects and/or common impurities.     

Effects of GaAs buffer layer and lattice-matching on deep levels in Zn(S)Se/GaAs heterostructures

The effects of GaAs buffer layer and lattice-matching on the nature of deep levels involved in Zn(S)Se/GaAs heterostructures are investigated by means of deeplevel transient spectroscopy (DLTS). The heterojunction diodes (HDs) where nZn(S)Se is grown on p⁺-GaAs by metalorganic vapor phase epitaxy are used as a test structure. The DLTS measurement reveals that when ZnSe is directly grown on a GaAs substrate, there exist five electron traps A-E at activation energies of 0.20, 0.23, 0.25, 0.37, and 0.53 eV, respectively. Either GaAs buffer layer and lattice-matching may reduce the incorporation of traps C, D, and E, implying that these traps are ascribed to surface treatment of GaAs substrate and to lattice relaxation. Concentration of trap B, which is the most dominant level, is proportional to the donor concentration. However, in the ZnSSe/GaAs sub. HD, another trap level, instead of trap B, locates at the almost same position as that of trap B, and it shows anomalous behavior that the DLTS peak amplitude changes drastically as changing the rate windows. This is explained by the defect generation through the interaction between sulfide and a GaAs substrate surface. For the trap A, the concentration is a function of donor concentration and lattice mismatch, and the origin is attributed to a complex of donor induced defects and dislocations.     

Diffused junction p+−n solar cells in bulk GaAs—I: Fabrication and cell performance

This paper describes the fabrication of solar cells made by a simple open tube p⁺-diffusion into bulk n-GaAs. In addition, cell performance is provided as an indicator of the quality of bulk GaAs for this application. Initial results using this technique (12.2% efficiency at AM1 for 0.5 cm² cells) are promising, and indicate directions for materials improvement. It is shown that the introduction of the diffusant (zinc) with point defects significantly affects the material properties and results in an increase in current capability.     

Interface trap and interface depletion in lattice-mismatched GaInAs/GaAs heterostructures

The effects of lattice mismatch on the deep traps and interface depletion have been studied for the Ga_0.92In_0.08As(p⁺)/GaAs(N) and Ga_0.92In_0.08As(n)/GaAs(SI) heterostructures grown by molecular beam epitaxy. We have used deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) and observed two hole traps, one at an energy ranging from 0.1 to 0.4 eV and the other at 0.64 eV, and two electron traps at 0.49 and 0.83 eV in the GalnAs/GaAsp ⁺-N junction sample. The hole trap appeared as a broad peak in the DLTS data and its energy distribution (0.1 ∼ 0.4 eV) was obtained by a simulation fitting of the peak. Concentration of this distributed hole trap increased as the in-plane mismatch increased, suggesting its relation to defects induced by lattice relaxation, whereas the other traps are from the bulk. The misfit dislocations are believed to be responsible for the interface trap. For the Ga_0.92In_0.08As(n)/GaAs(SI) samples, Hall effect measurements showed an increased interface depletion width of about 0.14 Μm for the 0.5 Μm thick layer and about 0.12 /gmm for the 0.25 Μm thick layer, while a corresponding GaAs/GaAs sample had only 0.088 Μm for the interface depletion width.     

The interdigitated back contact solar cell: A silicon solar cell for use in concentrated sunlight

The theoretical and experimental performance of an interdigitated back contact solar cell is described. This type of cell is shown to have significant advantages over a conventional solar cell design when used at high concentration levels, namely, reduced internal series resistance, nonsaturating open-circuit voltage, and an absence of shadowing by front surface contacting fingers. The results of a computer study are presented showing the effects of bulk lifetime, surface recombination velocity, device thickness, contact dimensions, and illumination intensity on the conversion efficiency and general device operation. Experimental results are presented for solar illumination intensities up to 28 W/cm².     

High quality GaAs qrowth by MBE on Si using GeSi buffers and prospects for space photovoltaics

III–V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightweight and inexpensive substrate. However, the primary obstacles blocking III–V/Si cells from achieving high performance to date have been fundamental material incompatibilities, namely the 4% lattice mismatch between GaAs and Si, and the large mismatch in thermal expansion coefficient. In this paper, we report on the molecular beam epitaxial (MBE) growth and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compositionally graded GeSi intermediate buffers grown by ultra‐high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si. GaAs cell structures were found to incorporate a threading dislocation density of 0.9–1.5×10 cm⁻², identical to the underlying relaxed Ge cap of the graded buffer, via a combination of transmission electron microscopy, electron beam induced current, and etch pit density measurements. AlGaAs/GaAs double heterostructures were grown on the GeSi/Si substrates for time‐resolved photoluminescence measurements, which revealed a bulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si. A series of growths were performed to assess the impact of a GaAs buffer layer that is typically grown on the Ge surface prior to growth of active device layers. We found that both the high lifetimes and low interface recombination velocities are maintained even after reducing the GaAs buffer to a thickness of only 0.1 μm. Secondary ion mass spectroscopy studies revealed that there is negligible cross diffusion of Ga, As and Ge at the III–V/Ge interface, identical to our earlier findings for GaAs grown on Ge wafers using MBE. This indicates that there is no need for a buffer to ‘bury’ regions of high autodoping, and that either pn or np configuration cells are easily accommodated by these substrates. Preliminary diodes and single junction AlGaAs heteroface cells were grown and fabricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricated on GaAs, Ge and Ge/GeSi/Si substrates show nearly identical I–V characteristics in both forward and reverse bias regions. External quantum efficiencies of AlGaAs/GaAs cell structures grown on Ge/GeSi/Si and Ge substrates demonstrated nearly identical photoresponse, which indicates that high lifetimes, diffusion lengths and efficient minority carrier collection is maintained after complete cell processing. Copyright © 2000 John Wiley & Sons, Ltd.     

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230°C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600°C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 ± 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly As_Ga antisite defects. According to X-ray diffraction and steady-state optical absorption data, the As_Ga concentration in the samples is 3 × 10¹⁹ cm^?3, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600°C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 ± 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.     

Fabrication and characterization of 18.6% efficientmulticrystalline silicon solar cells

Solar cell efficiencies as high as 18.6%(1 cm² area) have been achieved by a process which involves impurity gettering and effective back surface recombination velocity reduction of 0.65 Ω-cm multicrystalline silicon (mc-Si) grown by the heat exchanger method (HEM). Contactless photoconductance decay (PCD) analysis revealed that the bulk lifetime (τ_b) in HEM samples after phosphorus gettering can exceed 100 μs. At these τ_b levels, the back surface recombination velocity (S_b) resulting from unoptimized back surface field (BSF) design becomes a major limitation to solar cell performance. By implementing an improved aluminum back surface field (Al-BSF), S_b values in this study were lowered from 8000-10000 cm/s range to 2000 cm/s for HEM mc-Si devices. This combination of high τ_b and moderately low S _b resulted in the 18.6% device efficiency. Detailed model calculations indicate that lowering S_b further can raise the efficiency of similar HEM mc-Si devices above 19.0%, thus closing the efficiency gap between good quality, untextured single crystal and mc-Si solar cells. For less efficient devices formed on the same material, the presence of electrically active extended defects have been found to be the main cause for the performance degradation. A combination of light beam induced current (LBIC) scans as well as forward-biased current measurements have been used to analyze the effects of these extended defects on cell performance     

Neutron irradiation of insulated gate field effect transistors

Insulated gate field effect transistors and polysilicon-gated capacitors were irradiated with fast (10 keV <E < 2 MeV) neutrons. As expected, damage to the bulk silicon was detected as a degradation in the minority carrier lifetime. Optically assisted electron injection was employed for the first time to examine neutral electron trap and fixed positive charge generation in the gate insulator of the devices. While fixed positive charge densities of ≤6 x 10¹⁰ cm⁻² were detected, little or no neutral electron trap generation was observed. The small density of coulombic defects observed in the insulator could be accounted for fully by the known flux of gamma rays associated with the neutron irradiation process. This indicates that fast neutrons passing through a thin gate oxide do not produce significant amounts of damage in the oxide. Somewhat surprisingly, it was found that 1.5 keV X-rays created similar lifetime degradation effects in the bulk silicon, as did fast neutrons, even though this photon energy is not believed to be capable of producing bulk damage in the form of atom displacement in either the semiconductor or the insulator. The minority carrier lifetime of the silicon could be restored to initial values following either neutron or x-ray exposure by annealing in H₂ for 30 min at 400° C.     

Properties of compensated GaAs light-emitting diodes following 60Co irradiation

The effect of gamma radiation on the light-emitting properties of compensated GaAs electroluminescent diodes has been investigated. For gamma doses up to 10⁸ rads(Si), a large exponential reduction in the external quantum efficiency is observed, accompanied by a small reduction in the total carrier lifetime. These results are consistent with a model wherein two different defects are introduced through irradiation. These defects in addition to increasing the number of nonradiative recombination centers can also degrade the radiation process directly. Isochronal annealing data on these irradiated diodes suggest that the defect responsible for degrading the radiative process anneals below 300°C, but that the defect responsible for decreasing the nonradiative lifetime is relatively unaffected. The exponential reduction of efficiency with irradiation, including an observed shift of spectral emission peak, as well as the annealing characteristic seen in these diodes suggests that the defect responsible for degrading the radiative process might be similar to the luminescent killer center associated with an arsenic vacancy.     

Hydrogen plasma passivation of bulk GaAs and Al0.3Ga0.7As/GaAs multiple-quantum-well structures on Si substrates

Hydrogen (H) plasma passivation effects on GaAs grown on Si substrates (GaAs on Si) are investigated in detail. H plasma exposure effectively passivates both the shallow and deep defects in GaAs on Si, which improves both the electrical and optical properties. It was found that the minority carrier lifetime is increased and the deep level concentration is decreased by the H plasma exposure. In addition, after H plasma exposure, room temperature photoluminescence (PL) for Al_0.3Ga_0.7As/GaAs multiple-quantum-well (MQW) on Si is enhanced with a decrease in the spectral width.     

Efficiency calculations for AlxGa1-xAs-GaAs heteroface solar cells

A computer-assisted analysis of the AlxGa1-xAs-GaAs heteroface solar cell is done to find the dependence of cell efficiency on substrate doping level. Assumptions for carrier lifetime needed for the evaluation of efficiency are based on measurements of experimertal AlxGa1-xAs-GaAs heteroface cells. The results show the doping range 10¹⁶to 10¹⁷cm^-3to be the best for heteroface solar cells, because experimental evidence suggests that the lifetimes required for high-efficiency cells are difficult to obtain at very low and very high doping levels. Calculations based on a T^3/2temperature dependence for lifetimes agree well with early experimental efficiency versus temperature measurements on GaAs cells, but do not explain the results for an AlxGa1-xAs-GaAs heteroface cell reported by Hovel (1975).