The design, growth by metal-organic chemical vapor deposition, and processing of an Ino,07G~~93As0~98N0~02 solar cell, with 1 .O eV bandgap, lattice matched to GaAs is described. The hole diffusion length in annealed, n-type InGaAsN is 0.6-0.8 p, and solar cell internal quantum efficiencies > 70% are obtained. Optical studies indicate that defects or impurities, from InGaAsN doping and nitrogen incorporation, limit solar cell performance.Multi-junction tandem solar cells are being developed as power sources for satellite systems operating in air mass zero (AMO) solar radiation. Models indicate that record efficiencies (= 38%) would be obtained for tandem cells where a 1.0 eV bandgap cell is added in series to proven InGaP-GaAs tandem structures.' The In,Ga,~,As,.,N, alloy system appears ideal for this application. Bandgaps of 5 1.0 eV are obtained for In,Ga,~,As,.,N, with minimal N concentrations ( y > 0.02), and the quaternary is latticematched to GaAs for compositions with x = 3y. '' Even at these low concenuations, N incorporation has proven problematic, and it remains a challenge to demonsuate thick (2-3 pm), high quality, In,Ga,.,As,.,N, (y > 0.02) epilayers needed for solar cell development. In this paper, we present a status report on In,Ga,~,As,.,N, properties, growth, and solar cell performance. , Under specialized conditions, we demonstate internal quantum efficiencies >70 9 ' 0 for I . O eV bandgap solar cells.The structures in this work were grown in a vertical flow, high speed rotaling disk, Emcore GS/3200 metalorganic chemical vapor deposition (MOCVD) reactor.In,Ga,-,As,,N, films were grown using trimethylindium (TMIn). lriniethylgallium (TMG), 100% arsine and dimethylhydrazine (DMf-Iy). Dirnetfiylhydrazine wits used x the nitrogen source since it has a lower disassociation temperature than ammonia and has a vapor pressurc or approximately 1 10 torr at 18°C. A significant increase in photoluminescence intensity was observed from these films following a post-growth anneaL4 Ex-situ, post -growth anneals were carried out in a rapid thermal anneal system under nitrogen using a sacrificial GaAs wafer in close proximity to the InGaAsN sample. The photoluminescence intensity was a maximum for samples annealed for either 700OC for 2 minutes or 65OOC for 30 minutes. Secondary ion mass spectrometry measurements showed the residual carbon concentration of similar films to be 6 -8~1 0 '~ cm". Carbon is incorporated during growth at sufficiently high levels to possibly cause the background p-type conductivity and the observed ex-situ annealing behavior.The optical properties of the InGaAsN films were extremely sensitive to N content, ex-situ annealing, and doping. Photoluminescence and optical absorption

QzqiTime-resolved photolttminescence spectroscopy has been used to investigate carrier decay dynamics~t ifl> in a InXGa, -XASI.-,,NV (x-O.03, y -0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photohrmineseence (PL) measurements, performed for various 4%L excitation intensities and sample temperatures, indicate that the broad PL emission at low -gm temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07-0.34 a ns are measured; these photo luminescence iifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, we observe an emission energy dependence of ,the decay lifetime at 10 K, whereby the Iifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations. @ 2000 American institute of l&ecentIy, the quatemary InGaAsN alloy system has attracted a great deal of attention due to its potential application in devices such as next generation multifunction solar cells and optoelectronic devices for optical I-7 The alloy is of fundamental and technocommunications. logical interest because it exhibits an extremely large band gap bowing coefficient (b --14eV) between the HI-N and HI-As bhanes.x The extremely large bowing coefficient permits the InXGal _XAsl _YNYquaternary alloy to maintain lattice match to GaAs, with a wide range of tunable band gap energies smaller than the GSAS band gap for x-3y. Studies of InGaAsN solar cell structures with 1 eV band gap have shown that the quatemary suffers from a short minority carrier diffusion length.z3 More recent work has found that significantly improved minority hole diffusion-lengths may be obtained by thermally annealing the InGaAsN after growth, although minority electron diffusion lengths remain short.' In this letter, we report the results of time-resolved PL spectroscopy studies of an InGaAsN epilayer. Tfds letter is one of the first investigations of the carrier dynamics witfdn InGaAsN.A 3-pm-thick, htGaAsN epilayer was grown at a growth temperature of 590 "C by metal organic chemicaI vapor deposition on a semi-insulating GSAS substrate and terminated with a 5 nm GaAs calp.Trimethylindium, trimethylgallium, arsine, and dimethy:lhydrazine were used as source gases. The nominal In and N molar fractions were 0.03 and 0.01, respectively. The Irr/N incorporation ratio of three has been shown to provide lattice match to GaAs!'9 As grown, the unintentionally doped lhtGaAsN film was p type. After growth, the sample was annealed at 600 "C for 30 min in a nitrogen ambient in order to improve the electrical and optic al properties of the material. 1 Photohtminescence (PL) measurements for various sample temperatures and excitation intensities were performed with the sample mounted on a cold -a)Et&~~i~rnaikjiimg@physksmed" finger and cooled by a closed-cycle helium refrigerator. The sample was optically pumped with 580 nm laser pulses of 10 ps width and ...

We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers.

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Electron and hole transport in compensated, InGaAsN (= 2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall nobilities reveal abroad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (>> mean free path) material inhomogeneities, not a random alloy-induced mobility edge.. ..