Admittance spectroscopy of InGaNAs layers in solar cells

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Study of GaP nucleation layer grown on Si by PE-ALD

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Characterization of GaP/Si heterojunctions by space charge capacitance measurements

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Interface Properties of GaP/Si Heterojunction Fabricated by PE-ALD

International audienceThe properties of n‐GaP/p‐Si interface as well as their influence on solar cell performance are studied for GaP layers grown by low‐temperature (380 °C) plasma‐enhanced atomic layer deposition (PE‐ALD). The influence of different plasma treatments and RF power values are explored. The increase of RF power leads to a growth transition from amorphous (a‐GaP) to microcrystalline GaP (μc‐GaP) with either amorphous‐GaP/Si or epitaxial‐GaP/Si interface, respectively. However, when continuous hydrogen plasma is used the amorphous‐GaP/Si interface exhibits better photovoltaic performance compared to the epitaxial one. Values of open circuit voltage, Voc = 0.45–0.55 V and internal quantum efficiencies, IQE > 0.9 are obtained for amorphous‐GaP/Si interfaces compared to Voc = 0.25–0.35 V and IQE < 0.45 for epitaxial‐GaP/Si interfaces. According to admittance spectroscopy and TEM studies the near‐surface (30–50 nm) area of the Si substrate is damaged during growth with high RF power of hydrogen plasma. A hole trap at the level of EV + (0.33 ± 0.02) eV is detected by admittance spectroscopy in this damaged Si area. The damage of Si is not observed by TEM when the deposition of the structures with epitaxial‐GaP/Si interface is realized by a modified process without hydrogen plasma indicating that the damage of the near‐surface area of Si is related to hydrogen plasma interaction

Influence of PE-ALD of GaP on the Silicon Wafers Quality

International audienceAn attractive method of low-temperature plasma-enhanced atomic layer 7 deposition (PE-ALD) of GaP on silicon wafer was recently proposed. In the 8 present paper, the influence of the growth process on the quality of silicon 9 wafers is explored by space charge capacitance techniques, C-V profiling and 10 deep level transient spectroscopy (DLTS). No DLTS peak is observed for 11 PE-ALD GaP deposited onto n-type wafer, meaning that the defect concentra-12 tion is very low (less than 1 Â 10 12 cm À 3) and that the growth process does 13 not affect the properties of the n-Si wafer. For boron-doped p-type silicon, 14 C-V profiling shows that there is no deactivation of boron doping after the 15 PE-ALD process, as could have been expected from the presence of hydrogen 16 in the plasma. Measurements on the reference Schottky diodes formed on 17 the p-type Si wafer reveal the presence of the well-known Fe interstitial 18 defects at the position E V þ 0.38 eV with a concentration of 3 Â 10 13 cm À 3. 19 PE-ALD of GaP leads to a modification of the response of this defect and to 20 the appearance of another response in the low temperature range, possibly 21 related to changes in the Fe interstitial defect environment or configuration. 22 However, deep-levels were not detected in p-Si after PE-ALD, meaning that 23 the quality of p-Si does not degrade. 24 1. Introduction 25 High-efficiency and low-cost of solar cells are the driving forces 26 for the success of photovoltaics in terrestrial applications. The 27 highest efficiencies are obtained for multi-junction solar cells 28 based on III-V compounds (GaAs, GaInP, GaNAs, etc.). In 2015 29 multi-junction solar cell based on III-V semiconductors have 1 achieved record efficiency of 46% (concen-2 trator). [1] For space applications, where the 3 key factors are the efficiency and resistance 4 to radiation triple-junction solar cells based 5 on the GaInP (1.85 eV)/GaAs (1.42 eV)/Ge 6 (0.7 eV) system are industrially used. 7 However, cost of wafers (Ge, GaAs, InP, 8 etc.) and expensive growth methods 9 (molecular-beam (MBE) and vapor-phase 10 epitaxy (VPE)) are major drawbacks to the 11 extension of low-cost solar cells based on 12 III-V compounds to terrestrial usage. On 13 the other hand, silicon industry is much 14 more developed than that of III-V semi-15 conductors and silicon is one of the most 16 abundant element in the Earth, so the cost 17 of growth and processing for silicon solar 18 cells is much lower. Thus, majority of 19 terrestrial solar cells (90%) are fabricated 20 from silicon. However, record efficiency 21 solar cell based on silicon is slightly above 22 26% [2] and it almost reaches the theoretical 23 limit for single-junction silicon solar cell. 24 PV requires new approaches for high-25 efficiency and low-cost solar cells, which 26 can combine the advantages of III-V multi-27 junction and silicon solar cells. Therefore, fabrication of multi-28 junction solar cells with active layers of III-V compounds on 29 silicon wafers is a very promising approach for the photovoltaic 30 industry. Growth of III-V semiconductors on silicon wafers is a 31 real challenge for scientists, because such a technology can open 32 the way to the fabrication of optoelectronic integrated circuits. 33 Gallium phosphide is one of the most perspective candidates for 34 fabrication of multi-junction solar cells on silicon wafers. Firstly, 35 lattice mismatch between Si and GaP is only 0.4% so GaP can be 36 grown on silicon for the fabrication of the bottom subcell based 37 on GaP/Si heterojunction by different epitaxial methods (MBE 38 and VPE). [3,4] Further, novel materials (In)GaP(NAs) with small 39 nitrogen content (called dilute nitrides) can be lattice-matched to 40 GaP. These materials can be grown as active layers for top 41 subcells with a wide range of bandgap values, 1.5-2.1 eV. [5] 42 Previous efforts on the growth of dilute nitrides GaPNAs on Si 43 wafers have not allowed one to reach high efficiencies due to the 44 low quality of layers in solar cells. [6] Temperature of 800-900 C 45 required during growth process is a possible reason for the low 46 performance of solar cells for MBE and VPE epitaxial methods

Defect properties of InGaAsN layers grown as sub-monolayer digital alloys by molecular beam epitaxy

International audienceThe defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Eg = 1.03 eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5 nm) between wide GaAsN (7–12 nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900 nm and 1200 nm thick (less 1 × 1015 cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900 nm. An increase in thickness to 1200 nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5 eV (NT = 8.4 × 1014 cm−3) and a shallow defect level at 0.20 eV. The last one leads to the appearance of additional doping, but its concentration is low (NT = 5 × 1014 cm−3) so it does not affect the photoelectric properties. However, further increase in thickness to 1600 nm, leads to significant growth of its concentration to (3–5) × 1015 cm−3, while the concentration of deep levels becomes 1.3 × 1015 cm−3. Therefore, additional free charge carriers appearing due to ionization of the shallow level change the band diagram from p-i-n to p-n junction at room temperature. It leads to a drop of the external quantum efficiency due to the effect of pulling electric field decrease in the p-n junction and an increased number of non-radiative recombination centers that negatively impact lifetimes in InGaAsN