A detailed analysis based on first-principles calculations with self-energy corrections is combined with photoemission spectroscopy to determine the origin of features observed in reflectance anisotropy spectroscopy ͑RAS͒ at semiconductor surfaces. Using the InP(001)(2ϫ4) surface as a model case we obtain quantitative agreement between slab calculations and low-temperature RAS measurements. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the absorption processes causing the anisotropy signal take place in the uppermost few atomic layers of the substrate. RAPID COMMUNICATIONS R16 336PRB 61 W. G. SCHMIDT et al. RAPID COMMUNICATIONS R16 338PRB 61 W. G. SCHMIDT et al.

The oxidation of anion-and cation-rich indium phosphide ͑001͒ has been investigated by exposure to unexcited molecular oxygen. Indium phosphide oxidation is an activated process and strongly structure sensitive. The In-rich ␦(2ϫ4) surface reacts with oxygen at 300 K and above. Core levelx-ray photoemission spectra have revealed that the O 2 dissociatively chemisorbs onto the ␦(2ϫ4), inserting into the In-In dimer and In-P back bonds. By contrast, the P-rich (2ϫ1) reconstruction does not absorb oxygen up to 5ϫ10 5 Langmuir at 300 K, as judged by the unperturbed reflectance difference spectrum and low energy electron diffraction pattern. Above 455 K, oxygen reacts with the (2ϫ1) inserting preferentially into the In-P back bonds and to a lesser extent into the phosphorus dimer bonds.

The laboratory performance of CIGS (Cu(In,Ga)Se 2) based solar cells (20.8% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use density functional theory with the B3PW91 hybrid functional that we validate to provide very accurate descriptions of the band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the conduction band offset (CBO) of perfect CuInSe 2 /CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the valence band offset, and we find that Cd impurities do not significantly affect the CBO. Thus we show that Cu vacancies at the interface play the key role in enabling the tunability of CBO. We predict that Na further improves the CBO through electrostatically elevating the valence levels to decrease the CBO, explaining the observed essential role of Na for high performance. Moreover we find that K leads to a dramatic decrease in the CBO to 0.05 eV, much better than Na. We suggest that the efficiency of CIGS devices might be improved substantially by tuning the ratio of Na to K, with the improved phase stability of Na balancing phase instability from K. All these defects reduce interfacial stability slightly, but not significantly.

An apparatus is described here in detail for the transfer of a sample from a metalorganic chemical vapor deposition ͑MOCVD͒ reactor to an ultrahigh-vacuum ͑UHV͒ chamber without introducing any contamination. The surface of the sample does not change during transfer as is borne out by the identical reflectance difference ͑RD͒ spectrum measured first in the MOCVD reactor, i.e., in situ, and afterwards again in the UHV chamber. Making use of the earlier apparatus a semiconductor can be grown in the MOCVD reactor and can afterwards be investigated with any desired tool of surface science, in particular also those that require UHV. All the data collected in UHV can be identified with the RD spectrum measured already in the MOCVD reactor. Several examples are presented here for data collection in UHV on III-V semiconductors grown in the MOCVD reactor. They illustrate the ease and reliability of the here described apparatus for contamination-free sample transfer. Signals are presented in particular for the genuine MOCVD-grown P-rich seemingly (2 ϫ1)/(2ϫ2)InP(100) reconstructed surface that until now can only be investigated in UHV if one makes use of the sample transfer system described in this article.

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