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
Presented here is proof-of-principle that a thin single crystal semiconductor film—when twist-wafer bonded to a bulk single crystal substrate (of the same material)—will comply to the lattice constant of a different single crystal semiconductor thick film grown on its surface. In our experiment, a 100 Å film of GaAs was wafer bonded to a GaAs bulk substrate, with a large twist angle between their 〈110〉 directions. The resultant twist boundary ensures high flexibility in the thin film. Dislocation-free films of In0.35Ga0.65P(∼1% strain) were grown with thicknesses of 3000 Å, thirty times the Matthews–Blakeslee critical thickness, on twist-wafer-bonded films of GaAs.
The extent of relaxation and orientation of linearly graded InxAl1-xAs (x=0.05–0.25) buffers grown on GaAs were examined using a novel x-ray diffraction reciprocal-space mapping technique (kmap). Samples were grown at temperatures ranging from 370 to 550 °C. The fractional relaxation of the buffers grown between 470 and 550 °C was essentially identical (77%) and symmetric in orthogonal 〈110〉 directions. These buffers are believed to be in equilibrium indicating that the incomplete relaxation is not a kinetic effect. The extent of relaxation was less than that expected for equilibrium relaxation in the absence of dislocation–dislocation interactions indicating that such interactions must be considered to accurately predict the extent of relaxation. The saturation of the relaxation as a function of temperature indicates that at the grading rate used (8% In/μm or 0.69% strain/μm), we are not working in a growth regime where the relaxation is nucleation limited. In addition, all the buffers are slightly tilted with respect to the GaAs substrate about [11̄0] toward the [110] direction suggesting either a bias in the dislocation types in the boule-grown GaAs, or a bias in the way in which α and β dislocations interact with unintentional substrate miscuts.
An innovative compliant GaAs substrate was formed by wafer bonding a 30 Å GaAs layer to a bulk GaAs crystal with a large angular misalignment inserted about their common normals. InSb epitaxial layers, which is about 15% lattice mismatched to GaAs, have been grown on both compliant substrates and conventional GaAs substrates. Transmission electron microscopy studies showed that the InSb films grown on the compliant substrates have no measurable threading dislocations, whereas the InSb films on the conventional GaAs substrates exhibited dislocation densities as high as 1011 cm−2. The observations made here suggest that the defect-free heteroepitaxial growth of exceedingly large lattice-mismatched crystals can be achieved with compliant universal substrates.
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