The simulation and optimization of the internal quantum efficiency of GaSb thermophotovoltaic cells with a box-shaped Zn diffusion profile (original) (raw)
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Performance Improvement of the GaSb Thermophotovoltaic Cells With n-Type Emitters
GaSb cells are commonly fabricated using Zn diffusion into n-GaSb, and in this paper they have been designed inversely using Te diffusion into unintentionally p-doped GaSb. Numerical simulation is used to analyze the cell performance. We found that a GaSb cell with n-type emitters showed a significantly higher output power density compared with that of the cell with p-type emitters under 1500 K-blackbody radiation. The performance improvement is owing to the good matching of the diffusion length of minority carriers with the depth of their moving regions. Several parameters that affect the cell performance were analyzed, such as doping depth, substrate thickness, and surface recombination velocity. The cell performance was evaluated under low temperature radiations and found to have huge potentials for the use in low-temperature thermophotovoltaic systems.
Journal of Electronic Materials, 1999
The GaInSb material system is attractive for application in thermophotovoltaic (TPV) cells since its band gap can be tuned to match the radiation of the emitter. At present, most of the TPV cells are fabricated using epitaxial layers and hence are expensive. To reduce the cost, Zn diffusion using elemental vapors in a semiclosed diffusion system is being pursued by several laboratories. In this paper, we present studies carried out on Zn diffusion into n-type (Te-doped) GaSb substrates in an open tube diffusion furnace. The dopant precursor was a 2000Å thick, zinc doped spin-on glass. The diffusion was carried out at temperatures ranging from 550 to 600°C, for times from 1 to 10 h. The diffused layers were characterized by Hall measurements using step-and-repeat etching by anodic oxidation, secondary ion mass spectrometry measurements, and TPV device fabrication. For diffusion carried out at 600°C, the junction depth was 0.3 µm, and the hole concentration near the surface was 5 × 10 19 /cm 3 . The external quantum efficiency, measured without any anti-reflection coating of the TPV cells fabricated using mesa-etching had a maximum value of 38%. Masked diffusion was also carried out by opening windows in a Si 3 N 4 coated, GaSb wafer. TPV cells fabricated on these structures had similar quantum efficiency, but lower dark current.
Solar Energy Materials and Solar Cells, 2019
N-type vapor diffusions in p-GaSb wafers is investigated using the group-VI elements S, Se and Te as diffusion sources. The group-VI elements are found difficult to diffuse in bare p-GaSb wafers because they react with Ga atoms to form compounds, thus hindering the diffusion. Deposition of a SiO layer on the p-GaSb surface is found to be crucial for realizing n-type diffusion. With this SiO coating layer, the concentration of group-VI atoms decreased to a low value limiting the reaction with Ga atoms. Se atoms are found to be good n-type diffusion sources. GaSb thermophotovoltaic cells with n-on-p structures are fabricated using Se diffusion. Compared with a traditional GaSb cell with a p-on-n structure, the cell with the n-on-p structure had a higher quantum efficiency at wavelengths beyond 1000 nm. The output power density was also 1.42 times higher compared to the traditional cell when measured with a 1300°C-SiN ceramic IR emitter.
Journal of Electronic Materials, 2003
A single-step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion pro le appropriate for high-ef ciency GaSb thermophotovoltaic (TPV) cells. The junction depth was controlled through monitoring of light current-voltage (I-V) curves (photovoltaic response) during the post-diffusion emitter-etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum ef ciencies (QEs) and the open-circuit voltages in the fabricated devices. An optimum junction depth for obtaining the highest QE and open-circuit voltage is presented based on diffusion lengths (or minority carrier lifetimes), carrier mobility, and the typical diffused impurity pro le in GaSb.
Thermophotovoltaic GaSb Cells Fabrication and Characterisation
For production of highly effective photoconverters semiconductor materials with strictly determined parameters are required. For thermophotovoltaic (TPV) GaSb cells homogeneous Te-doping level of (2-7).1017 cm-3 in the bulk semiconductor is required to produce high efficient PV cells by the Zn diffusion process. In this paper we present data on investigation of the performance of the cells obtained on different GaSb:Te wafers of (100) and (211) orientation. Based on classical I/V measurements and external quantum efficiency (EQE) curves, we analyze cell performances in order to improve all fabrication stages like wafer surface preparation, p-type GaSb emitter elaboration by the zinc diffusion process, antireflection coating deposition and contact realization. Today good performances are obtained on both 3.5 × 3.5 mm2 (211) and 10×10 mm2 (100) GaSb cells. We obtained EQE of 70-76 % in the 800-1600 nm range for the first one and 80-88 % in the same spectrum for the second one. Electri...
Diffusion Length of Holes in n-ZnSe Measured by Schottky Barrier Photovoltage Method
Japanese Journal of Applied Physics, 1994
The diffusion length (L ) of holes in undoped n-type ZnSe has been measured by using the Schottky barrier photovoltage method. The samples measured were single crystals and epitaxial layers which were prepared by metalorganic chemical vapor deposition. The values of L were 1 µm for single crystals and 0.15 µm for epilayers. The lifetimes estimated from those values were 4.5 ns and 0.10 ns, respectively.
Short interval open tube diffusion of Zn in GaAs at low temperatures
Semiconductor Science and Technology, 1999
Short interval open tube Zn diffusion in GaAs at low temperatures is studied for application to heterostructure lasers. The electrochemical capacitance voltage (ECV) profiling technique is used to obtain the carrier concentration versus depth profiles for Zn diffused samples. Diffusion rate is found to be somewhat different from the values obtained for longer durations employing similar techniques. Results are applied to improve the ohmic contact quality for Al-free semiconductor lasers grown in our laboratory. No deterioration is observed in the light versus current (L-I ) characteristics of these devices fabricated after Zn diffusion.
Evidence of enhanced Zn-diffusion observed during the growth of Inverted Metamorphic Solar Cells
2019 IEEE 46th Photovoltaic Specialists Conference (PVSC)
Zinc-diffusion can induce multiple failures in the electrical performance of a multijunction solar cell. In this work, we show an important Zn-diffusion from the AlGaInP back-surface-field layer to the emitter of the GaInP top cell of an inverted multijunction solar cell. Through the analysis of different doping profiles, we provide strong evidence that the diffusion mechanism is (1) triggered by the growth of the tunnel junction cathode and (2) involves point defects. We analyze the implications of Zn-diffusion on the bandgap, the rear-passivation and the minority carrier quality of the GaInP solar subcell by relating the electrical performance of different samples to its corresponding doping profile.
Direct Auger recombination and density-dependent hole diffusion in InN
Scientific Reports, 2018
Indium nitride has a good potential for infrared optoelectronics, yet it suffers from fast nonradiative recombination, the true origin of which has not been established with certainty. The diffusion length of free carriers at high densities is not well investigated either. Here, we study carrier recombination and diffusion using the light-induced transient grating technique in InN epilayers grown by pulsed MOCVD on c-plane sapphire. We show that direct Auger recombination governs the lifetime of carriers at densities above ~10 18 cm −3. The measured Auger recombination coefficient is (8 ± 1) × 10 −29 cm −3. At carrier densities above ~5 × 10 19 cm −3 , we observe the saturation of Auger recombination rate due to phase space filling. The diffusion coefficient of holes scales linearly with carrier density, increasing from 1 cm 2 /s in low-doped layers at low excitations and up to ~40 cm 2 /s at highest carrier densities. The resulting carrier diffusion length remains within 100-300 nm range, which is comparable to the light absorption depth. This feature is required for efficient carrier extraction in bipolar devices, thus suggesting MOCVD-grown InN as the material fit for photovoltaic and photonic applications. Indium nitride with a direct band gap of 0.7 eV 1 is an attractive material for infrared optoelectronics. However, InN layers of high quality are difficult to obtain. In addition to structural problems, InN suffers from high residual electron density (n 0) caused by abundant point defects. n 0 can be diminished by growing thick InN layers using molecular beam epitaxy (MBE) 2 , but this is an expensive and hardly scalable approach. Other growth techniques were also employed, including metalorganic chemical vapor deposition (MOCVD) 3 , chemical vapor deposition 4 , sputtering 5 , or even sol-gel spin coating 6. The typical n 0 values, however, remain in the range from 10 18 cm −3 to mid-10 19 cm −3. It is likely that InN-based devices will have to operate at high electron densities, thus, it is essential to understand the impact of high carrier density on carrier dynamics. Carrier lifetime dependence on their density τ(n) is a powerful tool to reveal the dominating recombination mechanisms. Mainly linear or sublinear dependences were observed in InN layers by using the time-resolved photoluminescence, differential reflectance, or light-induced transient gratings (LITG) techniques. Based on these results, it was argued that Shockley-Read-Hall (SRH) 7-9 , Auger recombination in degenerate plasma 10 , or trap-assisted Auger recombination 11 were the dominant recombination mechanisms in InN. Carrier transport, especially that of minority holes, is less investigated. It was theoretically predicted that the room temperature hole mobility μ h can reach 220 cm 2 /Vs in low-doped InN, but should drop rapidly with n 0 above 10 17 cm −3 12. Experimentally, several techniques were used to measure μ h at fixed hole density. μ h = 17-36 cm 2 /Vs was estimated from sheet conductivity against sample thickness in Mg-doped layers at (1.4-3.0) × 10 18 cm −3 13. Variable magnetic field Hall measurements provided the mobility of heavy and light holes of 50 cm 2 /Vs and 600 cm 2 /Vs, respectively, in a sample with Mg doping at 3 × 10 20 cm −3 14. Hall measurements in InN layers Mg doped in a wide range from 10 18 to 10 20 cm −3 revealed p-type conductivity with similar μ h of 20-30 cm 2 /Vs 15. Application of LITG technique allowed for measuring the mobility of minority holes, which was ~40 cm 2 /Vs in high-quality MBE layers with n 0 in the mid-10 17 cm −3 16,17. This work is focused on the study of carrier dynamics in a wide range of carrier densities. Epilayers with different residual carrier densities were fabricated, while the increasing photoexcited carrier densities were generated using femtosecond laser pulses to ensure high time resolution. LITG technique is exploited to simultaneously extract the carrier lifetimes and their diffusion coefficients at different stages of the decay of nonequilibrium