M. H. Bode

Band-gap engineering in Cu(In,Ga) Se2 thin films grown from (In,Ga)2Se3 precursors

Abstract A three-stage process starting with the deposition of (In,Ga) 2 Se 3 precursor films has been successful in the fabrication of graded band-gap Cu(In,Ga)Se 2 thin films. In this work we examine (1) the reaction of Cu + Se with (In,Ga) 2 Se 3 , which leads to a spontaneous grading in the Ga content as a function of depth through the film, and (2) modification of the Ga content in the surface region of the film through a final deposition of In + Ga + Se. We show how band-gap grading can be enhanced by the formation of non-uniform precursors, how counterdiffusion limits the degree of grading possible in the surface region, and how the Cu x Se secondary phase acts to homogenize the film composition.

Structure, chemistry, and growth mechanisms of photovoltaic quality thin‐film Cu(In,Ga)Se2 grown from a mixed‐phase precursor

The formation chemistry and growth dynamics of thin‐film CuInSe2 grown by physical vapor deposition have been considered along the reaction path leading from the CuxSe:CuInSe2 two‐phase region to single‐phase CuInSe2. The (Cu2Se)β(CuInSe2)1−β (0<β≤1) mixed‐phase precursor is created in a manner consistent with a liquid‐phase assisted growth process. At substrate temperatures above 500 °C and in the presence of excess Se, the film structure is columnar through the film thickness with column diameters in the range of 2.0–5.0 μm. Films deposited on glass are described as highly oriented with nearly exclusive (112) crystalline orientation. CuInSe2:CuxSe phase separation is identified and occurs primarily normal to the substrate plane at free surfaces. Single‐phase CuInSe2 is created by the conversion of the CuxSe into CuInSe2 upon exposure to In and Se activity. Noninterrupted columnar growth continues at substrate temperatures above 500 °C. The addition of In in excess of that required for conversion produce...

Ordered vacancy compound CuIn3Se5 on GaAs (100): Epitaxial growth and characterization

Epitaxial growth of the ordered vacancy compound CuIn3Se5 has been achieved on GaAs (100) by molecular beam epitaxy from Cu2Se and In2Se3 sources. Electron probe microanalysis and x‐ray diffraction have confirmed the composition for the 1‐3‐5 phase and that the films are single‐crystal CuIn3Se5 (100). Transmission electron microscopy characterization of the material also showed it to be single crystalline. Structural defects in the layer consisted mainly of stacking faults. Photoluminescence measurements performed at 7.5 K indicate that the band gap is 1.28 eV. Raman spectra reveal a strong polarized peak at 152 cm−1, which is believed to arise from the totally symmetric vibration of the Se atoms in the lattice.

Observation of CuPt‐ordered CuInSe2

Long‐range order of the CuPt type has been observed in the I‐III‐VI2 material CuInSe2. The ordering was observed by transmission electron diffraction and by high resolution transmission electron microscopy. Comparison with simulated images confirms the CuPt‐type ordering and shows the high degree of ordering. Extrinsic stacking faults were found near domain boundaries of the CuPt‐type ordered material. During irradiation with the electron beam, the transformation from CuPt ordered to the sphalerite or chalcopyrite phase of CuInSe2 was observed.

Low‐band‐gap Ga0.5In0.5P grown on (511)B GaAs substrates

The band gap and microstructure of Ga0.5In0.5P have been shown to vary with deposition conditions. However, growth on (511)B GaAs substrates has been reported to give Ga0.5In0.5P with band gaps close to that of disordered material. It is shown here, that with appropriate selection of the growth parameters, Ga0.5In0.5P can be grown with low band gap and significant ordering on even the (511)B substrates, implying that surface steps play an important role in the ordering process. For the lattice‐matched composition, a band gap of 1.83 eV was obtained using low growth temperature (575 °C), low growth rate (0.55 μm/h), and high phosphine pressure (5 Torr).

Microcharacterization of polycrystalline semiconductor thin films for photovoltaic applications

We have investigated the properties of individual grains in polycrystalline thin films of CuInSe2 and CdTe by transmission electron microscopy (TEM), transmission electron diffraction (TED), scanning electron microscopy (SEM), and energy dispersive x‐ray spectroscopy (EDS) in a Scanning Transmission Microscope (STEM). TED experiments showed that most grains possess the chalcopyrite structure, which is expected for CuInSe2. In some cases, however, a complex arrangement of different phases was found within a single grain, allowing a glimpse at the kinetics of the grain boundaries, EDS spectra were recorded in a SEM. The spectra show unambiguously the presence of grains with disparate composition. To assess the compositional changes within single grains, EDS spectra were taken at various locations on the grains in a STEM. Using the total photon count as a measure of the local grain thickness and the ratio of SeKα/SeLα intensities to correct for absorption losses, we can analyze the relative concentrations of...

Determination of the Order Parameter by Quantitative Tem Techniques

To quantitatively measure the order parameter in ordered III-V materials we have developed two techniques based on electron microscopy. The first technique calculates the diffraction pattern of ordered material quantitatively and fits the calculated data to experimentally obtained data. A test sample, specifically grown for this technique, shows that the method can determine the average order parameter with high accuracy. We found an order parameter of 0.34, which coincides with the value found by piezo-reflectance measurements. To measure the order parameter on a smaller length scale, we use a technique based on image processing of high-resolution TEM images. By comparing ordered and disordered parts of the image, we calculate the relative order parameter on an atomic length scale. Analyzing the data provides information about the development of the ordering in the crystal.

Microcharacterization of CuInSe 2 Grown by Coevaporation and Selenization

Thin films of CuInSe 2 , grown by coevaporation or by selenization of a Cu-In precursor, were analyzed in a scanning transmission electron microscope. While the coevaporated film shows clear evidence of second phases (Cu 2 Se) around the individual grains, no second phases could be found in the selenized material. Structural characterization also showed the presence of two ordered phases in the coevaporated films, the ordered-vacancy compound CuIn 2 Se 3.5 , and a CuPt-ordered phase of CuInSe 2 .

TEM investigations on ordered phases in CuInSe2

Two different ordered phases of CuInSe2 have been investigated by transmission electron microscopy (TEM) methods: the so‐called ‘‘ordered vacancy compound,’’ found in Cu‐poor material, and a new structure, CuPt‐ordered material, which has been found only in Cu‐rich material so far. Both phases may have a strong influence on the performance of solar cells—the ordered vacancy compound by forming the actual heterointerface in a solar cell—the CuPt‐ordered material through effects on the band structure of the material. Here, we present first TEM experiments to identify the ‘‘ordered vacancy compound,’’ and we show the existence of the CuPt‐ordered phase.

Growth and characterization of spontaneously-ordered AlInAs and GaInAs on InP

Spontaneous CuPt-type ordering of Group III atoms on the (111) subplanes of the GaInAs2 and AlInAs2 epitaxially deposited by atmospheric pressure organometallic vapor phase epitaxy is observed by transmission electron microscopy. We find positive correlation between the observation of CuPt-like (111) superlattice diffraction spots in transmission electron diffraction (TED) patterns and reduced band gap energies, with a reduction of more than 75 meV for GaInAs2 and 25 meV for AlInAs2. For these materials, ordering depends strongly on growth temperature, but only moderately on substrate misorientation. Room temperature time-resolved photoconductivity of ordered GaInAs2 exhibit 50 microsecond(s) ec decay and behavior indicative of carrier localization.