InGaN/GaN p-i-n solar cells,each with an undoped In0.12Ga0.88N absorption layer,are grown on c-plane sapphire substrates by metal-organic chemical vapor deposition.The effects of the thickness and dislocation density of the absorption layer on the collection efficiency of InGaN-based solar cells are analyzed,and the experimental results demonstrate that the thickness of the InGaN layer and the dislocation density significantly affect the performance.An optimized InGaNbased solar cell with a peak external quantum efficiency of 57% at a wavelength of 371 nm is reported.The full width at half maximum of the rocking curve of the(0002) InGaN layer is 180 arcsec.
We have investigated the distribution of the electric field in p-i-n type and separate absorption and multiplication(SAM) type GaN avalanche photodiodes under different reverse bias values.We have also analyzed the influences of the parameters of each layer,including width and concentration,on the distribution of the electric field,especially on the breakdown voltage.It is found that a relatively high concentration of p-GaN(higher than 1×10 18 cm 3) and low carrier concentration of i-GaN(lower than 5×10 16 cm 3) are helpful to restrict the electric field and reduce the breakdown voltage.In a SAM(p-i-n-i-n) structure,a suitable choice should be made for the concentration and thickness of the intermediate n-GaN layer in order to decrease breakdown voltage and prevent the device from degenerating into a p-i-n structure.Finally,the optimized material parameters of each layer are proposed.
AlN epilayers are grown directly on sapphire(0001)substrates each of which has a low temperature AlN nucleation layer.The effects of pretreatments of sapphire substrates,including exposures to NH3/H2and to H2only ambients at different temperatures,before the growth of AlN epilayers is investigated.In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2only ambient,and are characterized by six 60°-apart peaks with splits in each peak in(10ˉ12)phi scan and two sets of hexagonal diffraction patterns taken along the[0001]zone axis in electron diffraction.These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2ambient.AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2ambient are Al-polar.Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers.We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.
The influences of polarization and p-region doping concentration on the photocurrent response of Al0.4Ga0.6N/Al0.4Ga0.6N/Al0.65Ga0.35N p–i–n avalanche photodetector are studied in a wide range of reverse bias voltages.The simulation results indicate that the photocurrent under high inverse bias voltage decreases with the increase of polarization effect,but increases rapidly with the increase of effective doping concentration in p-type region.These phenomena are analyzed based on the calculations of the intensity and distribution of the electric field.A high p-region doping concentration in the p–i–n avalanche photodetector is shown to be important for the efficient compensation for the detrimental polarization-induced electrostatic field.
In situ optical reflectivity measurements are employed to monitor the GaN epilayer growth process above a low-temperature GaN buffer layer on a c-plane sapphire substrate by metalorganic chemical vapour deposition.It is found that the lateral growth of the GaN islands and their coalescence are promoted in the initial growth stage if optimized nitridation time and temperature are selected when the substrate is pre-exposed to ammonia.As confirmed by atomic force microscopy observations,the quality of the GaN epilayers is closely dependent on the surface morphology of the nitridated buffer layer,especially grain size and nucleation density.
The effects of Mg-induced net acceptor doping concentration and carrier lifetime on the performance of a p-i-n InGaN solar cell are investigated. It is found that the electric field induced by spontaneous and piezoelectric polariza- tion in the i-region could be totally shielded when the Mg-induced net acceptor doping concentration is sufficiently high. The polarization-induced potential barriers are reduced and the short circuit current density is remarkably increased from 0.21 mA/cm2 to 0.95 mA/cm2 by elevating the Mg doping concentration. The carrier lifetime determined by defect density of i-InGaN also plays an important role in determining the photovoltaic properties of solar cell. The short circuit current density severely degrades, and the performance of InGaN solar cell becomes more sensitive to the polarization when carrier lifetime is lower than the transit time. This study demonstrates that the crystal quality of InGaN absorption layer is one of the most important challenges in realizing high efficiency InGaN solar cells.
Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well(MQW) solar cells are investigated. It is found that due to the reduction of piezoelectric polarization and the enhancement of tunneling transport of photo-generated carriers in MQWs, the external quantum efficiency(EQE) of the solar cells increases in a low energy spectral range(λ > 370 nm) when the barrier thickness value decreases from 15 nm to 7.5 nm. But the EQE decreases abruptly when the barrier thickness value decreases down to 3.75 nm. The reasons for these experimental results are analyzed. We are aware that the reduction of depletion width in MQW region, caused by the high resistivity of the p-type GaN layer may be the main reason for the abnormally low EQE value at long wavelengths(λ > 370 nm).
