The effect of active layer design on the efficiency of InGaN light emitting diodes (LEDs) with the light emission in blue
(~420 nm) has been studied. Correlation between the internal quantum efficiency (IQE) and relative external quantum
efficiency (EQE) and salient features of structures on c-plane InGaN LEDs which contain multiple quantum wells
(MQWs) of different barrier height (either In0.01Ga0.99N or In0.06Ga0.94N barriers) and thickness (3 nm and 12 nm) as well
as different double heterostructure (DH) designs (3 nm, dual 3 nm, 6 nm, dual 6 nm, 9 nm and 11 nm) with inserted 3
nm In0.06Ga0.94N barrier. Pulsed electroluminescence (EL) and optical excitation power-dependent photoluminescence
(PL) measurements indicated that the thinner and lower In0.06Ga0.94N barriers bode well for high EQE and IQE.
Furthermore, increase of the effective active region thickness by multiple InGaN DH structures (dual, quad and hex)
separated by 3 nm In0.06Ga0.94N barriers is promising at high injection levels. Although increasing the single DH
thickness from 3 to 6 nm improves the peak relative EQE by nearly 3.6 times due to increased density of states and
increased emitting volume, the IQE suffers a nearly 30% loss. Further increase in the DH thickness to 9 and 11 nm
results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably
due to degradation of the InGaN layer. Increasing the number of 3 nm DH active regions with 3 nm In0.06Ga0.94N
barriers improves EQE, while still maintaining high IQE (above 95% at a carrier concentration of 1018 cm-3) and
showing negligible EQE degradation up to 550 A/cm2 due to increased emitting volume and high radiative
recombination coefficients and high IQE.
In this paper, the high-efficiency GaInP/GaAs/InGaAs triple-junction solar cells are investigated numerically by using
the APSYS simulation program. The solar cell structure used as a reference was based on a published article by Geisz et
al. (Appl. Phys. Lett. 91, 023502, 2007). By optimizing the layer thickness of the top and middle cells, the appropriate
solar cell structure which possesses high sunlight-to-energy conversion efficiency is recommended. At AM1.5G and one
sun, the conversion efficiency is improved by 2.3%. At AM0 and one sun, the conversion efficiency is improved by
4.2%. At AM1.5D and one sun, the conversion efficiency is improved by 1.3%. Furthermore, based on the optimized
structures, this device can achieve efficiencies of more than 40% at high concentrations. For the triple-junction solar cell
under AM1.5G solar spectrum, the conversion efficiency reaches 40.2% at 40 suns. For the device under AM0 solar
spectrum, the conversion efficiency reaches 36.2% at 30 suns. For the device under AM1.5D solar spectrum, the
conversion efficiency reaches 40.2% at 50 suns.
Effect of polarization on optical characteristics of blue InGaN LEDs with staggered QW are numerically investigated in this article by using APSYS simulation program. Specifically, band diagram, carrier distribution, and output power have been discussed. According to the simulation results, the structure of staggered QW is proposed to reduce the polarization-related effect; furthermore, the staggered QW structure together with thinner well width is beneficial for improvement of the output power of the blue InGaN SQW LEDs. In this work, the best optical performance is obtained when the quantum-well structure is designed as In0.20Ga0.80N (0.9 nm)-In0.26Ga0.74N (1.1 nm) owing mainly to the enhanced overlap of electron and hole wavefunctions inside the QW.
For III-nitride compound materials, the existence of spontaneous and piezoelectric polarizations results in strong electrostatic fields, which might strongly affect the optical properties of 405-nm InGaN laser diodes. In this work, for polarization-free purpose, the use of polarization-matched AlGaInN electron-blocking layer and barrier layer in the violet InGaN multiple-quantum-well laser diodes is proposed. The laser performance and optical characteristics of the violet laser diodes are numerically evaluated by using the LASTIP (abbreviation of LASer Technology Integrated Program) simulation program. The simulation results show that, when the original Al0.20Ga0.80N electron-blocking layer is replaced by the polarization-matched Al0.39Ga0.49In0.12N electron-blocking layer, the laser performance is slightly improved. However, on the other hand, when compared to the original InGaN laser diode, the violet InGaN laser diode with a polarization-matched Al0.33Ga0.45In0.22N barrier layer possesses an increase of the threshold current and a decrease of the slope efficiency. It is presumably due to the fact that the effective potential height of conduction band at the interface of barrier and electron-blocking layer is reduced, and the electron leakage current is correspondingly enhanced when the polarization-matched barrier layer is utilized.
The effect of spontaneous and piezoelectric polarizations on optical characteristics of blue InGaN light-emitting diodes
(LEDs) is investigated in this work with the APSYS simulation program. Specifically, the band diagram, carrier
distribution, radiative recombination current, and light-current performance curves are discussed for various polarization
situations. According to the simulation results, the amount of polarization charges is dominated by piezoelectric
polarization. Therefore, the band diagram and overlap between electrons and holes in quantum wells are improved
effectively and the light-current performance is raised apparently as the piezoelectric polarization is removed and only
the spontaneous polarization is taken into account. The possible reason is that the influence of piezoelectric polarization
due to lattice constant mismatch on optical properties is severe than the spontaneous polarization due to asymmetry of
the wurtzite structure along the c-axis. Moreover, the simulation results suggest that the blue InGaN LED structures with
spontaneous polarization may provide higher output power but are more sensitive to temperature, in a range from 300 K
to 350 K, when compared to the LED structures with piezoelectric polarization.
Optical characteristics of deep ultraviolet light-emitting diodes with the consideration of spontaneous and piezoelectric
polarizations are studied in this article with the APSYS (Advanced Physical Model of Semiconductor Devices)
simulation program. The amounts of surface charges caused by different polarizations are calculated and compared.
Moreover, the band diagram, carrier distribution, radiative recombination current, and light-current performance curves
of the InAlGaN UV LED structures with different polarizations are also discussed and investigated. According to the
simulated results, we find that the influence of the spontaneous polarization is more apparent than the piezoelectric
polarization on band properties, carrier distribution, radiative recombination and output power in deep UV spectral
region. In other words, for nitride materials in deep UV region, the polarization resulted from lattice-mismatch is smaller
than that caused by asymmetry of the structure along the c-axis. This conclusion is quite different from the situation of
blue InGaN light-emitting diodes. For blue LEDs, the piezoelectric polarization is the dominant polarization mechanism
because the lattice mismatch between compound layers is a severe problem for these long-wavelength LED devices.
In reference to an AlInGaN UV LED fabricated in laboratory, the optical properties of the 370-nm UV LEDs are
investigated with a self-consistent APSYS simulation program. The optical performance of the UV LEDs with different
aluminum compositions in AlGaN electron blocking layer and different numbers of quantum wells are investigated in an
attempt to optimize the UV LED structure. The simulated results show that the electron leakage current can be
effectively reduced with the use of an AlGaN electron blocking layer with an aluminum composition of greater than 0.19,
and optimum performance may be obtained when the number of quantum wells is three. Since the built-in polarization is
one of the most important factors for the deterioration of III-nitride LED performance, the feasibility of using a latticematched
quaternary Al0.18In0.039Ga0.781N electron blocking layer in the UV LED to improve the LED performance is also
numerically studied. The simulated results suggest that with the use of a lattice-matched Al0.18In0.039Ga0.781N electron
blocking layer, the polarization charge density in each heterostructure interface is reduced, the electrostatic field in
quantum wells is reduced, and the maximum output power is sufficiently improved. The simulated results also indicate
that better LED performance may be obtained when the Al0.18In0.039Ga0.781N electron blocking layer has a higher pdoping
concentration due to reduced electron leakage and increased hole concentration in active region.
Referred to the laser structure and its experimental results obtained by Selmic et al. and Liu et al., optimized active
structure for the 1.55-μm quantum well lasers based on AlGaInAs material system is investigated. A structure with 1.2%
compressive-strained wells and a p-type AlInAs electron stopper layer of 20 nm thickness and 5×1023 m-3 doping
concentration is suggested. Using this structure the threshold current is reduced to 17.8 mA, and the electron overflow
percentage is decreased to 1.74% at 330 K. Furthermore, the characteristic temperatures of threshold currents are
enhanced to 55.6 K, 67.0 K, and 43.3 K in operating temperature ranges of 300 K~350 K, 300 K~330 K, and 330 K~350
K, respectively.
The zincblende InxGa1-xN, AlxGa1-xN, and AlxIn1-xN alloys are studied by numerical analysis based on first-principles calculations. The results show that the lattice constant of the three alloys obeys the Vegard's law. For InxGa1-xN the direct band gap bowing parameter obtained with the equilibrium lattice constant is 1.890 ± 0.097 eV. For AlxGa1-xN the direct and indirect bowing parameters of 0.574 ± 0.034 eV and 0.055 ± 0.038 eV are obtained, and there is a direct-indirect crossover near x = 0.56. For AlxIn1-xN the direct and indirect bowing parameters of 3.5694 ± 0.028 eV and 0.1953 ± 0.054 eV are obtained, and there is a direct-indirect crossover near x = 0.807
The 1.3-μm semiconductor material systems are numerically studied with a LASTIP simulation program. The optimum active layer materials of AlGaInAs/InP and InGaNAs/GaAs system are suggested. For the AlGaInAs/InP system, we optimize the structure by varying the number of quantum wells, the linear GRINSCH, and the compensated tensile strain in barriers. The optimized active structure possesses four quantum wells, linear GRINSCH, and a compensated tensile strain in the barrier of 0.325% at an emission wavelength of 1.3 μm. The characteristic temperature can be improved to 99.4K, 51.0K, and 68.6K as it is operating among 288K~318K, 318K~348K, and 288K~348K respectively. Furthermore, the optimized structure can also enhance the stimulated recombination rate and reduce the Auger recombination rate because of the compensated tensile strain in barriers. The simulation results show that the active layer with a certain amount of compensated tensile strain in barriers is beneficial for improving the laser performance. On the other hand, the performance of the InGaNAs/GaAs lasers with quantum wells of different compressive strains is investigated. The wavelength of InGaNAs/GaAs system is about 1.3 μm if the Ga composition in quantum wells is 0.54. The results of numerical simulation suggest that the stimulated recombination rate is larger and the Auger recombination rate is smaller when the Ga composition in quantum well is 0.50.
The ultraviolet AlGaInN light-emitting diode under study is grown on a c-face sapphire substrate by low-pressure horizontal-flow metalorganic chemical vapor deposition (MOCVD). With increasing input current from 10 to 100 mA, the main peak of the emission wavelength shifts from 368 to 372 nm. The room-temperature output power is 0.8 mW at 20 mA. Under continuous-wave operation, an output power of 4 mW is achieved at a driving current of 125 mA. The simulation program, advanced physical model of semiconductor devices (APSYS), is used to fit in our experimental results in order to obtain an optimized structure. The device performance affected by piezoelectric and thermal effects is studied via drift-diffusion model for carrier transport, optical gain and loss. The optical performance of the ultraviolet light-emitting diodes with different numbers of quantum wells at various temperatures is numerically investigated. Preliminary simulated results indicate that when the number of quantum wells is 5 to 7, better output performance is obtained. To raise the internal efficiency and radiative recombination rate, a current blocking layer SiO2 is used to guide and confine current flows through active region.
The optical properties of the violet-blue InGaN quantum-well lasers with an emission wavelength of 400-480 nm are studied with a LASTIP simulation program. Assuming that the InxGa1-xN/InyGa1-yN heterostructure has a band-offset ratio of 7/3, our simulation results indicate that the use of an AlGaN blocking layer can help reduce the electronic current overflow, and the non-uniform carrier distribution in the quantum wells plays an important role in the laser performance. If the piezoelectric effect is taken into account, the lowest threshold current of the violet-blue InGaN quantum-well lasers is obtained when the number of InGaN well layers is two if the emission wavelength is shorter than 412 nm, and one if the emission wavelength is longer than 412 nm. At a laser wavelength of 478 nm, the slope efficiency of the InGaN single quantum-well laser is decreased by ~2.4% and that of the double quantum-well laser is decreased by ~13.9% when the thermal effect is taken into account.
The wurtzite AlxGa1-xN, InxGa1-xN, and AlxIn1-xN alloys are studied by numerical simulation based on first-principles calculations. For AlxGa1-xN the Vegard's law deviation parameter is 0.018 ± 0.001 Å for the a lattice constant and -0.036 ± 0.005 Å for the c lattice constant. For InxGa1-xN that is 0.047 ± 0.011 Å for the a lattice constant and -0.117 ± 0.026 Å for the c lattice constant. For AlxIn1-xN that is 0.063 ± 0.014 Å for the a lattice constant and -0.160 ± 0.015 Å for the c lattice constant The results indicate that the band gap bowing parameters obtained with the equilibrium lattice constant and with the lattice constants derived from the Vegard's law are 0.341 ± 0.035 eV and 0.351 ± 0.043 eV respectively for AlxGa1-xN, 1.782 ± 0.076 eV and 1.916 ± 0.068 eV respectively for InxGa1-xN, and 3.668 ± 0.147 eV and 3.457 ± 0.152 eV respectively for AlxIn1-xN.
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