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.
The composition and stain distributions in the InGaN epitaxial films are jointly measured by employing various x-ray diffraction (XRD) techniques, including out-of-plane XRD at special planes, in-plane grazing incidence XRD, and reciprocal space mapping (RSM). It is confirmed that the measurement of (204) reflection allows a rapid access to estimate the composition without considering the influence of biaxial strain. The two-dimensional RSM checks composition and degree of strain relaxation jointly, revealing an inhomogeneous strain distribution profile along the growth direction. As the film thickness increases from 100 nm to 450 nm, the strain status of InGaN films gradually transfers from almost fully strained to fully relaxed state and then more In atoms incorporate into the film, while the near-interface region of InGaN films remains pseudomorphic to GaN.
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.
This paper investigates the major structural parameters,such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition,using an in-plane grazing incidence x-ray diffraction technique.The results are analysed and compared with a complementary out-of-plane xray diffraction technique.The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method.The method for directly determining the in-plane lattice parameters of the GaN layers is also presented.Combined with the biaxial strain model,it derives the lattice parameters corresponding to fully relaxed GaN films.The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established,reaching to a maximum level of-0.89 GPa.
A method to calculate the reflectivity of the coated cavity facet was proposed, and the distribution of the optical power near the two coated cavity facets was calculated for GaN-based laser diodes. A new design method for reducing the optical power at the two cavity facets without changing the output power of laser diodes was discussed, which is helpful to optimize the cavity facet coating and raise the threshold current at which catastrophic optical damage occurs.
We have fabricated InGaN-based superluminescent diodes(SLDs)with one-sided oblique facet.The characteristics of the SLDs and laser diodes with the same cavity length(800 lm)were compared.The typical peak wavelength and the full width at half maximum of the spectrum in superluminescence regime are 445.3 and 7.7 nm for the SLDs with 800 lm cavity length.The characteristics of the SLDs with different cavity length were also demonstrated in a comparative way.It is found that the gain of the InGaN multi-quantum wells in blue spectral range is a linear function of the current density below gain saturation region.The lasing threshold current turns out to be higher for the shorter SLD(S-SLD)(400 lm),but the output light intensity of the longer SLD(800 lm)is higher than that of the S-SLD under the same current density.The gain saturation phenomenon was observed in S-SLD when it was biased at a current density larger than 27.5 kA/cm2.The increase of junction temperature was identified as the main reason for gain saturation through spectra analysis.
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.
An InGaN/GaN p-i-n solar cell inserted with a 5-nm low-temperature(LT)GaN interlayer between the p-GaN cap layer and the InGaN i-layer is grown on a c-plane sapphire substrate by metal organic chemical vapor deposition.The effects of the LT GaN interlayer on the performance of the InGaN/GaN solar cells are investigated.It is found that the LT-GaN interlayer prevents the extension of threading dislocations from the InGaN layer to the p-GaN layer and improves the crystal quality of both the p-GaN cap layer and the InGaN i-layer,ultimately leading to an increasing external quantum efficiency and photocurrent density of the InGaN/GaN solar cells.
This paper reports that a long delay between the beginning of pumping current pulse and the onset of optical pulse is observed in InGaN laser diodes.The delay time decreases as the pumping current increases,and the speed of the delay time reduction becomes slower as the current amplitude increases further.Such delay phenomena are remarkably less serious in laser diodes grown on GaN substrate than those on sapphire.It attributes the delay to the traps which cause a large optical loss by saturable absorption and retard the laser action.The traps can be bleached by capturing injected carriers.The effect of GaAs laser irradiation on InGaN laser action demonstrates that the traps responsible for the delay are deep centres which can be filled by the photo-assisted processes.
Using the finite-element method,the thermal resistances of GaN laser diode devices in a TO 56 package for both epi-up configuration and epi-down configuration are calculated.The effects of various parameters on the thermal characteristics are analysed,and the thicknesses of the AlN submount for both epi-up configuration and epi-down configuration are optimized.The obtained result provides a reference for the parameter selection of the package materials.
