The hetero-interfaces of an InGaAs/GaAsSb tunnel junction, embedded into two InP barrier layers, as used in a tandem solar cell, were studied. With regard to the sharpness of these interfaces, different surface preparations and growth procedures were tested. For that, several surface reconstructions of In0.53Ga0.47As grown by metal organic chemical vapor deposition (MOCVD) were investigated in-situ with reflectance difference spectroscopy (RDS) and analyzed in ultrahigh vacuum (UHV) with low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS). Depending on the annealing temperature, three different surface reconstructions of MOCVD prepared InGaAs were found: As-rich (4 x 3), (2 x 4) and III-rich (4 x 2)/c(8 x 2). Next, thin GaAs0.51Sb0.49 layers were grown on both As-rich and III-rich InGaAs surfaces. LEED patterns showed the expected c(4 x 4) reconstruction of the As-terminated GaAsSb surface for both preparations, also of the same quality. A significant difference for the Sb to As ratios of thin GaAsSb layers were measured by XPS, indicating that the unusual growth of GaAsSb on a III-rich InGaAs surface results in a sharper InGaAs/GaAsSb hetero-interface. Moreover, we searched for the best preparation procedure of the InP barrier layer on GaAsSb. RDS and a stable growth of the top cell suggested a favorable growth procedure by ramping the growth temperature from 500 to 600 degrees C during the growth of the InP layer. The challenge here was to prepare GaAsSb at the optimum temperature of 500 degrees C, to switch then to the optimum growth temperature of the subsequent layers of 600 degrees C and to produce a well-defined and sharp interface. (c) 2006 Elsevier B.V. All rights reserved.

GaAsSb/InP(1 0 0) hetero-interfaces were studied with regard to the performance of metal organic chemical vapor deposition (MOCVD)-grown p-type resonant tunneling diodes (RTDs). For that, thin InP layers were grown by MOCVD on the ternary compound GaAs0.5Sb0.5, which is lattice-matched to InP(1 0 0). Two different surface reconstructions of (1 0 0) GaAs0.5Sb0.5, similar to a reconstruction of either (1 0 0) GaAs or (1 0 0) GaSb, were used for preparing the InP/GaAs0.5Sb0.5 hetero-interfaces: the As-rich, c(4×4) and the Sb-rich, (1×3) reconstructions. The preparation of the RTDs was identical except for the Sb- versus As-rich reconstruction of GaAsSb. The RTDs with As-rich prepared GaAsSb/InP interfaces showed significantly more symmetric I–V characteristics than those with the Sb-rich interface preparation, demonstrating a clear advantage for the As-rich interface preparation. Surfaces were measured in-situ with reflectance difference spectroscopy (RDS) and analyzed in ultrahigh vacuum (UHV) with low-energy electron diffraction (LEED) with regard to the sharpness of the interface. The RD spectra of thin hetero-epitaxial InP layers grown on GaAsSb (1 0 0) were compared to the well-established RD spectrum of MOCVD-prepared homo-epitaxial, (2×1)-like reconstructed P-rich InP (1 0 0), that was used as reference for a well-defined surface. Growing InP on the c(4×4) reconstructed GaAsSb (1 0 0) surface resulted in a significantly sharper interface than InP growth on (1×3) reconstructed GaAsSb(1 0 0), a result that was also borne out by high-resolution X-ray diffraction spectra.

The thermal deoxidation procedure of GaSb(100) substrates has been investigated with in-situ reflectance difference spectroscopy (RDS). The “epi-ready” substrates were loaded in a metal-organic vapor phase epitaxy (MOVPE) reactor either “as-supplied” or after etching with HCl to remove the native oxide layer. Annealing between 475–575 ° C and in-situ monitoring reveals RDS features associated with the surface morphology and the development of oxide desorption. This process is supported by molecular hydrogen utilized as carrier gas. Photoemission spectroscopy was applied to benchmark the surface of selected samples with regard to the electronic structure and the chemical composition during the deoxidation of GaSb(100) substrates. Based on the in-situ and UHV data, a model of the oxide desorption process and recommendations for the GaSb substrate deoxidation procedure in MOVPE environment are proposed.

Aiming at the improvement of the conversion efficiency of a monolithic high-efficiency multi-junction solar cell based on the lattice constant of InP different types of low-band gap n/p solar cells were prepared on the lattice constant of InP via metal organic chemical vapor deposition (MOCVD) using only non-gaseous, so-called alternative precursors like tertiarybutylphosphine (TBP). Employing this less-toxic precursor compared to phosphine an InP single n/p solar cell was prepared as reference yielding the highest internal quantum efficiency reported in the literature. New materials were introduced on the lattice constant of InP, in particular GaAsSb (Egap=0.75eV) and InAlGaAs (Egap=1.03eV). The new absorber materials were compared to more established materials like InGaAs (Egap=0.75eV) and InGaAsP (Egap=1.03eV). It will be shown that the latter cell with 18% Al reached an internal quantum efficiency close to that of the InGaAsP cell.

MOVPE-grown GaSb(100) surfaces were investigated with in situ RAS, LEED, and UPS. The in situ signals turned out to be very useful in determining suitable growth parameters and in monitoring the critical deoxidation procedure. Undoped GaSb(100) films were grown with triethylantimony and triethylgallium as precursors. Characteristic differences in the in situ signals were correlated with surface sensitive measurements after having transferred the samples with a patented procedure from the MOVPE reactor into ultrahigh vacuum chambers. p-type doping was accomplished with the precursors ditertiarybutylsilane and carbon tetrabromide, that are sustainable alternatives compared to silane and carbon tetrachloride. LEED images of the surfaces displayed a c(2×6) surface reconstruction. Doping levels in the range from 1017 to 1020cm−3 were observed with SIMS and Hall measurements. Characteristic RAS peaks at the E1 and E1+Δ1 interband transitions were attributed to the linear electro-optic effect and were found to increase linearly with the dopant concentration.

GaAsSb was grown lattice matched on InP(100) by metalorganic vapor-phase epitaxy (MOVPE). The surfaces of the samples were observed in the MOVPE reactor with reflectance anisotropy (RA) spectroscopy during and after growth. RA spectra taken during growth were similar to RA spectra of surfaces stabilized with TESb. However, the RA spectrum changed significantly and led to an As-rich surface with a higher degree of atomic order while supplying only TBAs. As- and Sb-rich GaAsSb surfaces were transferred into ultrahigh vacuum without any contamination and subsequently characterized with low-energy electron diffraction. There was great similarity of the different group-V-rich surface reconstructions of GaAsSb to the reconstructions known from their related binary compounds: As-rich GaAs0.51Sb0.49 showed a clear c(4×4) reconstruction well known from GaAs(100), whereas Sb-rich GaAs0.51Sb0.49 showed a (1×3) reconstruction, which was observed on GaSb(100) surfaces.

Focus of the paper is the MOVPE process development for Al‐containing antimonides on GaSb substrates, which has been proven to be specifically challenging. First, the deoxidation of GaSb substrates was investigated. It was found that Ga2O3 is reduced to volatile Ga2O at moderate temperatures by molecular hydrogen used as carrier gas in the MOVPE environment. For the deposition of the Al‐containing antimonides DMEAA (dimethylethylamine alane) was used as Al‐precursor. Unfortunately this precursor suffers from severe pre‐reactions with other metalorganics (MO) and a low vapor pressure. To meet the challenging demands of the industrial growth of Al‐containing antimonides an AlX2600‐G3 Planetary Reactor® with new 9×2‐inch substrate configuration was developed. This is the first multiwafer reactor that was specifically designed for the growth of antimonides. Major advantages of this reactor are the reduction of pre‐reactions, higher MO efficiency as well as excellent layer homogeneity and reproducibility.

Epitaxial GaSb(100) semiconductor films were prepared in an AIX200 reactor using triethylantimony and triethylgallium as precursors, MOVPE growth of (100) surfaces was investigated and monitored in situ with reflectance anisotropy/difference spectroscopy (RDS). RDS signals helped in improving the growth parameters. i.e. growth temperature, growth rate, and Sb/Ga ratio. Using unfavourable growth parameters (high flow rates or adverse Sb/Ga ratio) promoted the growth of 3D islands with micrometer diameters as monitored by in situ RDS and imaged by scanning electron microscopy. There was a concomitant increase in the RDS intensities by more than one order of magnitude. This effect was used to obtain improved growth parameters for preparing high quality films with specular surfaces and state-of-the-art carrier concentration (N-p < 3 x 10(16) cm(-3)). For the first time, MOVPE-prepared GaSb(1 00) surfaces were measured by RDS in the spectral range from 0.8 to 5.0 eV. (C) 2002 Elsevier Science B.V. All rights reserved.

Two-photon photoemission was employed to study the electronic structure of the ordered (2×4)-reconstructed In-rich surface of InP(100). Dangling bond surface states, two unoccupied and one occupied, were identified near the Γ-point of the surface Brillouin zone in good agreement with the theoretical predictions of Schmidt et al. (Phys. Rev. B 61 (2000) R16335). In addition, one photon photoemission from occupied electronic states in the band gap was observed that is tentatively ascribed to small In-droplets or clusters.

MOCVD-preparation of ordered [100] surfaces of InP and GaP was monitored in-situ with reflectance difference/anisotropy spectroscopy (RDS/RAS). RDS was measured at 20 K after contamination-free transfer of the sample to UHV. Specific RD spectra with the highest peaks and fine structure were correlated with different ordered surface reconstructions. The ordered In-rich InP[100] surface was investigated with femtosecond 2PPE. Surface states and surface resonances near the Γ-point showed up as peaks in the 2PPE spectrum in agreement with theoretical predictions.

Reflectance difference spectroscopy was measured in the metal organic chemical vapor deposition reactor and also in UHV at 20 K. It revealed a characteristic negative peak at the low energy side that was indicative of the specific surface reconstruction. This peak disappeared completely if the sample was kept within a narrow intermediate temperature range. At 20 K the negative peak appeared at 2.4 eV for the Ga-terminated (2×4)-reconstructed surface and at 2.6 eV for the P-terminated (2×1)/(2×2)-reconstructed surface. RDS for the two different surface reconstructions displayed strong structures also in the range of the bulk transitions. A characteristic zig-zag pattern was observed in the STM image of the P-terminated surface.

MOVPE-preparation of highly ordered InP(100) and GaP(100) surfaces was monitored with in-situ reflectance difference spectroscopy (RDS). Specific ordered P-terminated and ordered cation-terminated surface reconstructions were identified with specific structured RD spectra with the highest peaks. After contamination-free transfer of the samples to UHV, RDS measurements were performed also at 20 K. The experimental RD spectrum for the In-terminated, (2×4) reconstructed InP(100) surface shows a remarkable similarity to a recently published theoretical spectrum, whereas there is only moderate similarity between the experimental RD spectrum for the (2×4) reconstructed Ga-terminated GaP(100) surface and a recently proposed theoretical spectrum.