ShaoJun Ma

Strain-compensation measurement and simulation of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy using wafer-curvature

Precise strain compensation for lattice-mismatched quantum wells is crucial for obtaining high performance devices such as quantum well solar cells. High-accuracy in situ curvature monitoring is a more efficient tool to adjust growth conditions for perfect strain balancing, and we have achieved curvature measurement during growth of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy. We have also developed the curvature calculation model taking into account of thermal expansion and lattice relaxation effects based on Stoney’s equation. The measured periodical curvature behavior corresponds to the growth of compressive InGaAs well layers and tensile GaAsP barrier layers and fits perfectly with a theoretical curve assuming the structural parameters (thicknesses and atomic contents) obtained by x-ray diffraction analysis, confirming correctness of the developed calculation method. Considering the proper thermal expansion coefficients for InGaAs and GaAsP, we have obtained much accurate ...

Impact of Strain Accumulation on InGaAs/GaAsP Multiple-Quantum-Well Solar Cells: Direct Correlation between In situ Strain Measurement and Cell Performances

The effects of accumulating strain inside InGaAs/GaAsP multiple-quantum-well (MQW) solar cells were investigated and their correlation with in situ wafer curvature measurement was examined. The p–i–n GaAs solar cells, containing 20-period InGaAs/GaAsP MQWs in an i-GaAs layer, were fabricated by metalorganic vapor phase epitaxy. The strain inside MQWs was varied by changing In content in an InGaAs well, while maintaining other parameters. As evidenced by curvature transience, the excessive strain led to lattice relaxation, resulting in defects, dislocations, and poor crystal quality. Consequently, short circuit current density and open circuit voltage deteriorated, and solar cell performance degraded. The highest conversion efficiency was obtained in a strain-balanced MQW solar cell. InGaAs/GaAsP MQWs have a great potential for extending the absorption edge of GaAs cells and for enhancing the efficiency of III/V multijunction solar cells by current matching. Hence, the growth of InGaAs/GaAsP MQWs for photovoltaic application requires a strain monitoring system and careful control such that the accumulating strain is minimized.

Effects of Strain on the Performance of InGaAs/GaAsP Multiple-Quantum-Well Solar Cells Correlated with In situ Curvature Monitoring

The performance of InGaAs/GaAsP multiple-quantum-well (MQW) solar cells was evaluated and correlated with in situ curvature measurement. The average strain inside MQWs was varied by changing the GaAsP thickness. Strain-balanced MQW cells exhibited the best performance. Highly strained MQW cells suffered from defects and dislocations, resulting in poor efficiency. The average strain inside the structure had good agreement with cell properties. Strain-balanced InGaAs/GaAsP MQWs seem to have good potential for enhancing the efficiency of lattice-matched III–V/Ge multiple-junction solar cells. Hence, the crystal growth process should be precisely monitored and carefully controlled in a manner such that the extent of strain accumulation is minimized.

Open-circuit-voltage enhancement of the III-V super-lattice solar cells under optical concentration

The GaAs single junction solar cell containing the nano-structure of multiple quantum wells (MQWs) or super-lattice (SL) consist of the InGaAs/GaAsP strain-balanced wells and barriers grown by MOVPE was evaluated under the concentrated illumination. Defining the concentration ratio CJ as a ratio of saturated current density ratio of under the concentrated illumination versus 1SUN, the increasing dependency of open-circuit-voltage (VOC) on CJ showed steeper tendency for only SL inserted sample than the conventional multiple quantum wells (MQWs) case. This result indicated that the carrier transport enhancement by tunneling effect by thinning each barriers contribute the VOC increment at the high concentrated illumination.

Carrier sweep-out time in InGaAs/GaAsP multiple quantum well solar cells by time-resolved photoluminescence: effects of well depth and barrier thickness

Sweep-out times for cross-well transport in p-i-n GaAs solar cells containing 20-period of InGaAs/GaAsP multiple quantum wells were evaluated by biased time-resolved photoluminescence. Effects of InGaAs well depth and GaAsP barrier thickness were analyzed and compared with simulation of carrier escape times from a single quantum well in a presence of an electric field. The tendencies and relative values of calculated escape rates suggested that the effective barrier height is the most important parameter to determine the carrier escape times in a quantum well. The dominant transport mechanism for the MQW in this work is most likely the thermionic emission from electron ground states followed by thermal assisted tunneling depending on the structure. In order to extend the absorption edge of the MQWs while keeping strain balance, thick GaAsP barriers with a small P content is preferable in the viewpoint of carrier extraction.

InGaAs/GaAsP asymmetric quantum wells for enhancing carrier escape through resonant tunneling

In the multiple quantum wells system, deep well supplies wide absorption range. But at the same time, the large band offset would increase the thermionic escape time constant of carriers exponentially. We utilized resonant tunneling effect to assist the carrier collection in InGaAs/GaAsP strain-balanced multiple quantum wells (MQWs). Through the optimization of confinement energy levels and barrier thicknesses, the collection time of photo-generated carriers is calculated to be reduced significantly from several ns to a few hundred ps compared with conventional multiple quantum wells. The asymmetric MQWs for tunneling-assisted carrier escape were fabricated successfully and the carrier collection time was investigated using time-resolved photoluminescence. The advantage of the tunneling design has been confirmed.

Optimization of Gas-Switching Sequence for InGaAs/GaAsP Superlattice Structures Using In situ Wafer Curvature Monitoring

By high-accuracy in situ curvature measurement during the growth of InGaAs/GaAsP superlattice structures by metal organic vapor phase epitaxy, we have successfully observed the effect of thin GaAs insertion layers between InGaAs wells and GaAsP barriers on strain control. By analyzing curvature transients, we found that an inadequate gas-switching sequence induces the carry over of indium from the InGaAs layer to the overlying GaAs insertion layer. The resulting carry-over layer has an estimated thickness of 0.6 nm and adversely affects the average strain of the structure. Through consideration of the kinetics of surface atoms, it has been revealed that an optimized gas-switching sequence with a 1 s hydrogen purge after the growth of InGaAs wells is effective for preventing the carry over.

Metalorganic vapor phase epitaxy growth of dual junction solar cell with InGaAs/GaAsP superlattice on Ge

The impact of growth temperature was investigated on the quality and interface abruptness of InGaAs/GaAsP multiple quantum wells (MQWs) grown on various misoriented substrates. The growth of MQWs on substrates with a larger misoriented angle required a lower temperature. Non-radiative carrier lifetimes in MQWs strongly depended on the quality and abruptness of MQWs. On the basis of this understanding, a dual junction cell consisting of InGaAs/GaAsP superlattice top cell and Ge bottom cell was successfully fabricated. The result encourages the application of InGaAs/GaAsP superlattice for better current balancing and higher efficiency by III-V/Ge multiple junction solar cells.

Evaluation of asymmetric tunneling-assisted structure for InGaAs/GaAsP MQWs solar cell

In the multiple quantum wells (MQWs) system, deep well supplies wide absorption range, but the large band offset would increase the carrier thermionic escape time exponentially. We utilized asymmetric tunneling-assisted structure in InGaAs/GaAsP strain-balanced MQWs to accelerate the carrier collection. The quantitative model of carrier collection time from this novel structure has been built and the collection time has been calculated to be 31% compared with conventional multiple quantum wells. The solar cell with designed tunneling-assisted structure has been fabricated and the external quantum efficiency of it has been compared with conventional MQWs structure. The temperature dependence has also been investigated.

A multi-step superlattice solar cell with enhanced subband absorption and open circuit voltage

In the report, we propose a multiple stepped superlattice (MS-SL) structure, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, for photovoltaic application. Comparison between the normal SL cell and MS-SL cell indicate that the step design in MS-SL cell enhanced sunband absorption to the host bulk material, while reduced overall recombination loss, exhibited a surprising advantage over SL cell in conversion efficiency. According to experimental results, discussions have been made on the competition process between carrier recombinations and carrier escape kinetics from the wells.