R. Sporken

Molecular beam epitaxial growth of CdTe and HgCdTe on Si (100)

CdTe has been grown on Si(100) by molecular beam epitaxy. Two orientations can be obtained: (111)B CdTe when the CdTe is deposited directly on the Si(100) substrates, and (100)CdTe when an intermediate layer of ZnTe is grown first. The (111)B oriented layers are made of two domains which are rotated by 90°. A layer with only one domain can be grown on Si(100) misoriented by 8°, but the best misorientation for this purpose still needs to be found. These layers were characterized by reflection high‐energy electron diffraction, photoluminescence spectroscopy, scanning electron microscopy, and x‐ray diffraction. Hg1−xCdxTe has also been grown by molecular beam epitaxy on (111)B CdTe on Si(100).

Current status of direct growth of CdTe and HgCdTe on silicon by molecular‐beam epitaxy

CdTe and HgCdTe can be grown directly on Si(100) substrates by molecular‐beam epitaxy. The layers grow in the (111)B orientation. Single domain films are always obtained on Si(100) 8° off toward [011], whereas single and double domain films were obtained on nominal Si(100). A possible reason for the formation of these domains is discussed based on a microscopic model of the CdTe/Si interface. The structural quality of the layers is determined by double crystal x‐ray rocking curves. The narrowest rocking curves are obtained on single‐domain films grown on nominal Si(100) substrates; a full width at half‐maximum (FWHM) of only 230 arcsec was measured, compared to 460 arcsec on the best layer with two domains. For HgCdTe layers grown on CdTe/Si, rocking curves with 110 arcsec FWHM were measured; these layers are n‐type with electron mobilities above 5×104 cm2 V−1 s−1 at 23 K for a Cd mol % of 26%.

MOLECULAR BEAM EPITAXY AND CHARACTERIZATION OF CDTE(211) AND CDTE(133) FILMS ON GAAS(211)B SUBSTRATES

CdTe films were grown in both the (211) and (133) orientations on GaAs(211)B substrates by molecular beam epitaxy. The orientation of the epitaxy is dependent on the thermal cleaning process. Studies of these films included in situ reflected high‐energy electron diffraction, x‐ray double‐crystal diffractometry, transmission electron microscopy, and photoluminescence, which revealed high quality for both CdTe growth orientations, and especially for the CdTe(133). The lattice of the CdTe(211) growth tilts 3° with respect to its GaAs(211) substrate about the CdTe[011]//GaAs[011] coincidence axis. The CdTe(133) has no tilt with respect to its substrate, and its coincidence axes are CdTe[011]//GaAs[011] and CdTe[611]//GaAs[111].CdTe films were grown in both the (211) and (133) orientations on GaAs(211)B substrates by molecular beam epitaxy. The orientation of the epitaxy is dependent on the thermal cleaning process. Studies of these films included in situ reflected high‐energy electron diffraction, x‐ray double‐crystal diffractometry, transmission electron microscopy, and photoluminescence, which revealed high quality for both CdTe growth orientations, and especially for the CdTe(133). The lattice of the CdTe(211) growth tilts 3° with respect to its GaAs(211) substrate about the CdTe[011]//GaAs[011] coincidence axis. The CdTe(133) has no tilt with respect to its substrate, and its coincidence axes are CdTe[011]//GaAs[011] and CdTe[611]//GaAs[111].

The AlAl2O3 interface formation as studied by electron spectroscopies

Abstract The Al Al 2 O 3 (0001) interface formation has been investigated by low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), X-ray induced photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES). Aluminium reacts at the surface of alumina, giving rise to an aluminium-rich substoichiometric surface region, after a limited diffusion into the first atomic layers. At this point, a ( 31 × 31 ) R ± 9° reconstruction is observed by LEED, indentical to the diagram which is obtained after heating stoichiometric alumina above 1250°C. For coverages larger than 5 monolayers, metallic Al lines appear, the surface becomes rough, and the LEED patterns disappear due to charging effects. These observations are related to the formation of Al clusters above 5 monolayers. Further metal deposition leads to island growth and, for several tens of angstroms, the metallized surface becomes smooth again. The Al 2 O 3 (0001) Fuchs-Kliewer phonons are decreasing with increasing aluminum coverage. Theoretical considerations show that the presence of a uniform metallic layer on top of an ionic crystal such as Al 2 O 3 (0001), with a sharp unreacted interface, would immediately screen the surface modes of the substrate.

Growth and characterization of CdTe/Si heterostructures - effect of substrate orientation

Abstract Transmission electron microscopy and small-probe microanalysis have been used to compare the microstructure and compositional profiles of CdTe/Si heterostructures grown by molecular beam epitaxy on (001), (211) and (111) silicon substrates. Overall, our results have demonstrated that the final CdTe growth orientation is determined by careful preparation of the Si substrate surface, the nature of the interfacial layer, and the initial phase nucleation. Initial studies confirmed that growth on (001) was problematical, not only because of the large lattice mismatch between materials (approximately 19%), but also because the double-domain reconstruction of the Si substrate surface degraded epilayer quality. Growth of high quality, domain-free CdTe(111)B was achieved by offcutting the substrate with respect to the [110] surface direction, with an additional rotation about [110]. Alternatively, with intermediary buffer layers of Ge(001), perfect a /2〈110〉 Lomer edge dislocations accommodated the misfit at the CdTe/Ge interface, and the (001) orientation of the Si substrate was retained during CdTe growth. For (211)-oriented substrates a very thin (approximately 2 nm) buffer layer of ZnTe prior to CdTe deposition was sufficient to maintain the substrate orientation, although Zn diffusion was often observed during subsequent annealing. The growth of Cd 1−x Zn x Te(211)B (with x ∼2–4%) with intermediary CdTe buffer layers then provided substrates which were suitably lattice-matched for growth of HgCdTe. Finally, large-area, domain-free CdTe(111)B was achieved using As-passivated Si(111) substrates and thin (approximately 50 nm) ZnTe buffer layers.

Charge transfer and electronic doping in nitrogen-doped graphene.

Understanding the modification of the graphene’s electronic structure upon doping is crucial for enlarging its potential applications. We present a study of nitrogen-doped graphene samples on SiC(000) combining angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS). The comparison between tunneling and angle-resolved photoelectron spectra reveals the spatial inhomogeneity of the Dirac energy shift and that a phonon correction has to be applied to the tunneling measurements. XPS data demonstrate the dependence of the N 1s binding energy of graphitic nitrogen on the nitrogen concentration. The measure of the Dirac energy for different nitrogen concentrations reveals that the ratio usually computed between the excess charge brought by the dopants and the dopants’ concentration depends on the latter. This is supported by a tight-binding model considering different values for the potentials on the nitrogen site and on its first neighbors.

Heteroepitaxy of CdTe on GaAs and silicon substrates

Abstract CdTe can be grown directly by molecular beam epitaxy on substrates such as GaAs or silicon, which exhibit very large lattice mismatches of 14.6% and 19% respectively. The occurrence of dual epitaxy, which has been previously reported for growth on (100)GaAs, has also been found recently for growth on (211)GaAs. The (133)CdTe-(211)GaAs hetero-interface presents a smooth continuation of the tetrahedral bond network from GaAs to CdTe, which is not the case for the (211)CdTe-(211)GaAs interface. Single-domain, twin-free CdTe(111)B films are currently obtained on Si(100) surface where single atomic steps are dominant. The crystalline quality of CdTe/Si films has been dramatically improved as confirmed by X-ray diffraction, photoluminescence and electron microscopy investigations. The narrowest rocking curves obtained for as-grown epilayers are 70 arcsec for (133)CdTe/(211)GaAs, 50 arcsec for a flash-annealed (211)CdTe/(211)GaAs and 140 arcsec for (111)B CdTe/(100)Si. These results confirm that the CdTe/GaAs and CdTE/Si composite substrates should be viewed as prime candidates to replace bulk CdTe substrates.

New development on the control of homoepitaxial and heteroepitaxial growth of CdTe and HgCdTe by MBE

Abstract It is reported that rotation twins as well as reflection twins are easily formed in CdTe and HgCdTe grown by MBE in the (111) orientation. Twinning can be avoided by carefully controlling the substrate preparation and by applying very stringent growth conditions, mostly for the stability of Hg pressure and the real surface temperature of the substrate, which is extremely difficult to control when the substrate rotates. A comparison between HgCdTe twinned layers and twin-free layers has shown that electrically active acceptors and high hole mobility are associated with the presence of reflection twins and/or mercury-rich alloy zones due to Hg overpressure during the growth. Twin-free HgCdTe layers can exhibit etch pit density count two orders of magnitude lower than twinned layers. Twin-free CdTe layers have been grown on GaAs and Si substrates. Excellent thickness uniformities have been reported: 0.24% for the standard deviation of a 2-inch diameter CdTe layer grown on GaAs(100) and 2.3% for a 5-inch diameter CdTe grown on Si(100).

Vibrational study of the SiO2/Si interface by high resolution electron energy loss spectroscopy

By using high resolution electron energy loss spectroscopy (HREELS) the surface phonon spectra of SiO2(0001) and of thick amorphous SiO2 layers have been measured. The spectra recorded on the crystalline surface are characterized by three first order peaks at 498, 798, and 1176 cm−1. The HREELS data could be interpreted by the dielectric theory applied to anisotropic material, and the optical constants were compared to infrared spectroscopy values. HREELS spectra as a function of oxide thickness could be obtained from oxidized Si(100) wafers by sputter erosion. Peak positions shifted to lower frequencies and intensities decreased regularly with decreasing oxide thickness. At the interface a transition layer of about 25 A was observed and carbon contamination was identified. The dielectric theory applied to a thin homogeneous supported film cannot account for the observed frequency shift.

The effect of As passivation on the molecular beam epitaxial growth of high-quality single-domain CdTe(111)B on Si(111) substrates

The effect of As passivation of Si(111) substrates on the subsequent molecular beam epitaxial growth of CdTe(111) is investigated through a detailed comparison of the microstructures of two types of films. The film grown on a substrate treated with a Te flux is found to exhibit a rough film-substrate interface and has very poor crystalline quality with a (111)A orientation. In contrast, a CdTe film grown under identical conditions except for the Si substrate treated with an As flux is observed to have an atomically abrupt film-substrate interface and a single-domain structure in the technologically more relevant (111)B orientation. A growth mechanism for the formation of these high-quality single-domain CdTe(111)B films is proposed.