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

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Vertical cavity surface emitting laser (VCSEL) diodes fabricated with inverted polarity, i.e. p-type bottom mirror and n-type top mirror, are reported with lower resistance and diode voltage and comparable output characteristics relative to similar conventional, non-inverted structures.Most laser diodes are fabricated on n-doped substrates with ndoped material below the active region and p-doped material above it. However, both circuit and device performance considerations for arrays of vertical cavity surface emitting lasers (VCSELs) motivate inverting this polarity so that the substrate and lower epitaxial layers are p-doped and the upper epitaxial layers are ndoped. From a circuit perspective, this latter configuration allows simple fabrication of common anode laser arrays that can be driven with open collector npn transistors which typically perform better than pnp transistors.While consideration of n-doped and p-doped mirror properties suggests that p-substrate VCSELs should perform better than their n-substrate counterparts [1], such experimental results have not been reported. Results from external cavity VCSELs based on psubstrates have been published [2] but are not readily compared with purely monolithic structures. A more recent report of similar VCSELs with both types of substrates indicated that the performance of the p-substrate devices lagged that of the n-substrate devices [3]. Here, we report state-of-the-art, oxide-confined, p-substrate VCSELs with characteristics equal to or better than comparable n-substrate devices.Material issues including the stability of dopants and the quality of substrates may have limited previous efforts to realise p-substrate VCSELs. Some p-type dopants such as zinc and beryllium are more diffusive or prone to surface segregation [4] than silicon, the most common n-type dopant. These properties can result in the movement of significant dopant concentrations from underlying material into the active region, reducing optical efficiency. This problem is enhanced in thick VCSEL structures with high aluminum content that require long growths at high temperatures. In general, p-type and semi-insulating substrates also have higher concentrations of defects than n-type substrates. Previous attempts at Sandia to realise p-substrate VCSELs using molecular beam epitaxy with beryllium doping have been unsuccessful. The structures presented here were produced with metalorganic chemical vapour deposition using CCI 4 as the source for carbon as a low diffusivity, more readily incorporated p-type dopant. A constant growth temperature of 750°C was used throughout the structures, except for the heavily doped contact layers. Decreasing the growth temperature in the active region could lead to enhanced carbon incorporation from any residual CCI 4 and was thus avoided [5]. The substrates were produced by vertical gradient freeze and had doping concentrations of 3 x 10 18 /= 3 of zinc or 2 x 10 18 /cm 3 of silicon. The etch pit density of the p-substrates was 5000/= 2 • Lasers made on two ...

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