Taizo Masuda

Development of Hybrid Simulation for Supersonic Chemical Oxygen-Iodine Laser

DOI: 10.2514/1.20339 A numerical simulation method for a supersonic chemical oxygen–iodine laser is developed. The model is a combination of a three-dimensional computational fluid dynamics code without kinetics and a detailed onedimensional,multiple-leaky-stream-tubeskineticscode.Intheproposedmethod,thedetailed flowfieldcharacteristic is calculated by solving a full Navier–Stokes equation that does not involve chemical reactions, and the resultant temperature, velocity, and mixing characteristics are input to the kinetics code as its boundary conditions. A “nonuniform coefficient” is introduced to transform the fluid-dynamic mixing to the diffusive mixing term of the kinetics code. As a result, precise predictions of the gain distribution and laser output are given with a reasonable computational cost. The developed model is applied to the X-wing-type supersonic mixing chemical oxygen–iodine laser, which we have developed, and the calculated gain and output power are compared with the experimental results. The excellent agreements of calculated and experimental results show the validity of the developed method.

Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy

We have investigated ∼2.0 eV (AlxGa1−x)0.51In0.49P and ∼1.9 eV Ga0.51In0.49P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (AlxGa1−x)0.51In0.49P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (Voc) ranging from 1.29 to 1.30 V for Ga0.51In0.49P cells, and 1.35–1.37 V for (AlxGa1−x)0.51In0.49P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (Woc = Eg/q − Voc) of Ga0.51In0.49P cells to decrease from ∼575 mV to ∼565 mV, while that of (AlxGa1−x)0.51In0.49P cells remained nearly constant at 620 mV. The constant Woc as a function of substrate offcut for (AlxGa1−x)0.51In0.49P implies greater losses from non-radiative recombination compared with the Ga0.51In0.49P devices. In addition to larger Woc values, the (AlxGa1−x)0.51In0.49P cells exhibited significan...

Numerical simulation of an all gas-phase iodine laser based on NCl3 reaction system

A numerical simulation code for an all gas-phase iodine laser based on the NCl3 reaction system is developed. The model is a one-dimensional, multiple-leaky-stream-tubes kinetics code combined with all the known rate equations to date. To confirm the validity of this simulation code, the calculated results are compared with the experimental results obtained in other laboratories. The results of computer calculations utilizing this model are in good agreement with those experimental results. This agreement shows that the code is capable of precisely predicting the small signal gain and laser output for a given set of flow conditions. Using this simulation code, we defined the flow rates and the nozzle configuration that should allow laser oscillation based on NCl3 reaction system to be achieved. The calculations suggest that a laser output power of 410 mW can be obtained under optimum conditions with facilities available in our laboratory.

Effects of growth temperature and device structure on GaP solar cells grown by molecular beam epitaxy

Gallium phosphide (GaP) is an attractive candidate for wide-bandgap solar cell applications, possessing the largest bandgap of the III-arsenide/phosphides without aluminum. However, GaP cells to date have exhibited poor internal quantum efficiency (IQE), even for photons absorbed by direct transitions, motivating improvements in material quality and device structure. In this work, we investigated GaP solar cells grown by molecular beam epitaxy over a range of substrate temperatures, employing a much thinner emitter than in prior work. Higher growth temperatures yielded the best solar cell characteristics, indicative of increased diffusion lengths. Furthermore, the inclusion of an AlGaP window layer improved both open-circuit voltage and short wavelength IQE.

Characteristics of an all gas-phase iodine laser using molecular iodine as atomic iodine donor

The laser action of an all gas-phase iodine laser (AGIL), which uses molecular iodine as a source of iodine atoms, has been demonstrated. The laser is based on the energy transfer reaction between metastable NCl(a?1?) and ground state I(2P3/2) atoms, which are produced by the electric discharge of a mixture of I2 and He. At fixed flow rates of the chemical species, the laser output powers are measured at three different positions in a flow reactor. The output power is characterized by a function of the optical axis position and is in reasonable agreement with the numerical simulation. A repetitive pulse of laser output at 50?Hz with a duty factor of 40% is observed. The highest output power is 40?mW at 210?mm downstream from the mixing point of I/H/He and NCl3. This is 80% of the output power generated from the conventional system using HI as an iodine donor. The measured results of the time-resolved laser output power suggest that the output power of the I2-AGIL is more sensitive to the electric discharge plasma intensity as compared with that of the HI-AGIL. An AGIL operated using I2 could potentially have the same output power as that of an AGIL operated using HI if a continuous-wave electric discharge generator is used.

Observation of Pumping Reaction in an Amine-Based All Gas-Phase Iodine Laser Medium

Gain measurement of all gas-phase iodine laser (AGIL) based on amine-based reactions is conducted. Three gaseous species, namely, NCl3, H, and HI are mixed in a glass tube, in which injection ports of each species are optimized using a numerical simulation code developed in our laboratory. The laser duct is attached downstream to the mixing tube, and a probe beam of 1315 nm passes at 23 cm downstream from the mixing point of H and HI. Hydrogen atoms are produced by the microwave discharge of the H2/He admixture. When NCl3 is not supplied, absorption by the 2P1/2–2P3/2 transition of iodine atoms is observed. When NCl3 is supplied, the absorption dip occasionally turns to the hump, which means that the energy transition from NCl(a1Δ) to iodine atoms results in population inversion. The observed small signal gain is 0.005%/cm. However, the reproducibility of the observed phenomenon is poor and presumably, some uncontrolled factor affects the gain evolution. To our knowledge, this is the first observation of a positive gain of the amine-based AGIL system.

Initiation strategies for simultaneous control of antiphase domains and stacking faults in GaAs solar cells on Ge

Incorporating a Ge junction into a lattice-matched GaInP/GaAs/GaInNAsSb triple-junction cell grown by molecular beam epitaxy (MBE) could enable concentrated efficiencies of ∼50%. Epitaxial integration allows lift-off and wafer bonding steps to be avoided, but growth of III–Vs on Ge by MBE can lead to antiphase domains (APD) and stacking fault pyramids (SFP), both of which diminish solar cell performance. Initiating growth by migration-enhanced epitaxy (MEE) is typically cited as necessary to obtain high-quality III–Vs on Ge. In this work, the authors show that typical MEE growth conditions force a compromise between APD height (hAPD) and SFP density (ρSFP). As APDs can readily self-terminate while SFPs cannot, a two-step initiation strategy was employed, where MEE is performed under conditions that minimize ρSFP followed by low-temperature MBE conditions that encourage APD termination. By doing so, the authors obtained ρSFP < 104 cm−2 with hAPD ≤ 57 nm. The authors also demonstrated that high-quality GaAs...

High-efficiency AlGaInP solar cells grown by molecular beam epitaxy

AlGaInP is an ideal material for ultra-high efficiency, lattice-matched multi-junction solar cells grown by molecular beam epitaxy (MBE) because it can be grown lattice-matched to GaAs with a wide 1.9–2.2 eV bandgap. Despite this potential, AlGaInP grown by molecular beam epitaxy (MBE) has yet to be fully explored, with the initial 2.0 eV devices suffering from poor performance due to low minority carrier diffusion lengths in both the emitter and base regions of the solar cell. In this work, we show that implementing an AlGaInP graded layer to introduce a drift field near the front surface of the device enabled greatly improved internal quantum efficiency (IQE) across all wavelengths. In addition, optimizing growth conditions and post-growth annealing improved the long-wavelength IQE and the open-circuit voltage of the cells, corresponding to a 3× increase in diffusion length in the base. Taken together, this work demonstrates greatly improved IQE, attaining peak values of 95%, combined with an uncoated AM1.5G efficiency of 10.9%, double that of previously reported MBE-grown devices.

Germanium solar cells grown by molecular beam epitaxy for lattice-matched, four-junction solar cells

We demonstrate Ge junctions grown by molecular beam epitaxy (MBE) for lattice-matched, four-junction solar cells. Integrating a Ge bottom cell beneath a 1.9 eV InGaP/1.4 eV Ga(In)As/1.0 eV dilute nitride stack could enable ultra-high efficiencies in a single growth step. In this work, we successfully created Ge junctions by growth of homoepitaxial n-Ge emitters using As2 as a dopant on p-Ge wafers within a III-V MBE system. The growth and post anneal conditions are shown to strongly influence open-circuit voltage (Voc) in epitaxial Ge solar cells. Voc = 0.174 V was obtained without a window layer, which is comparable to the Voc of diffused Ge cells fabricated by metal-organic-vapor-phase-epitaxy for triple-junction solar cells.

Direct-gap 2.1–2.2 eV AlInP solar cells on GaInAs/GaAs metamorphic buffers

AlInP offers the highest direct bandgap (Eg) among non-nitride III-V materials, making it attractive for top cell applications in 5-6 junction solar cells. We present novel 2.07-2.19 eV, direct-gap AlInP solar cells, grown on GaInAs/GaAs graded buffers by metal-organic chemical vapor deposition. Despite the high Al content of 36-39% in the active regions, SIMS results indicate oxygen concentrations less than 2.3×1016 cm-3. The AlInP devices we present here exhibit superior photovoltaic performance to GaP and are comparable to metamorphic GaInP solar cells, reaching a Eg-voltage offset of 0.58 V. Design enhancements based on device and material characterization led to improvements of up to 65% in short circuit current density from our first-generation AlInP devices. The promising results in this work provide an alternative path towards realizing high-Eg top junctions with applications in upright metamorphic multijunction solar cells.