W. Witthuhn

Band gap states of Ti, V, and Cr in 4H–silicon carbide

Band gap states of Ti, V, and Cr in n-type 4H–SiC were investigated by radiotracer deep level transient spectroscopy (DLTS). Doping with the radioactive isotopes 48V and 51Cr was done by recoil implantation followed by annealing (1600 K). Repeated DLTS measurements during the elemental transmutation of these isotopes to 48Ti and 51V, respectively, reveal the corresponding concentration changes of band gap states. Thus, six levels are identified in the band gap: Cr levels at 0.15, 0.18, and 0.74 eV, one V level at 0.97 eV, and two Ti levels at 0.13 and 0.17 eV below the conduction band edge.

Hydrogen passivation of silicon carbide by low-energy ion implantation

Ion implantation of deuterium is performed to investigate the mobility and passivating effect of hydrogen in epitaxial α-SiC (polytypes 4H and 6H). To avoid excessive damage and the resulting trapping of hydrogen, the implantation is performed with low energy (600 eV 2H2+). The 2H depth profile is analyzed by secondary ion mass spectrometry. Electrical properties are measured by capacitance–voltage profiling and admittance spectroscopy. In p-type SiC, hydrogen diffuses on a μm scale even at room temperature and effectively passivates acceptors. In n-type SiC, the incorporation of H is suppressed and no passivation is detected.

Epitaxial growth of CuGaS2 on Si(111)

We demonstrate the direct heteroepitaxial growth of the ternary semiconductor CuGaS2 on Si(111) substrates by means of molecular beam epitaxy. X-ray diffraction data prove the epitaxial growth of the CuGaS2 films in the highly ordered chalcopyrite structure. Using photoluminescence, we are able to detect strong excitonic emissions up to room temperature, while photocurrent spectra reveal the A, B, and C valence-to-conduction-band transitions as they are typical for the tetragonal chalcopyrite structure.

Optical properties of epitaxial CuGaS2 layers on Si(111)

Abstract CuGaS 2 films epitaxially grown on Si(111) substrates by molecular beam epitaxy from elemental sources were investigated by various optical methods. Photoluminescence studies at low temperatures reveal a strong dependence of the near-bandgap emission on the composition. Ga-rich films show predominantly donor–acceptor pair recombination, while additionally free-to-bound and excitonic transitions are found for stoichiometric and Cu-rich films. Low lineshape broadening of the free excitonic transitions FX A and FX B/C in the photoreflectance spectra up to room temperature demonstrates the high crystal quality. The assignment of these transitions is confirmed by polarisation dependent photocurrent measurements making use of the optical selection rules and the three different growth directions for the c -axis of CuGaS 2 relative to the Si-substrate. A fit to the photocurrent measurements yields the relative volume fraction of each c -axis orientation.

RECOIL IMPLANTATION OF RADIOACTIVE TRANSITION METALS AND THEIR INVESTIGATION IN SILICON BY DEEP-LEVEL TRANSIENT SPECTROSCOPY

Radioactive isotopes are produced by nuclear reactions in a thin target foil. The recoiling products are directly implanted into samples mounted off‐axis to the primary beam. Using proton or α beams and appropriate target foils, radioactive isotopes of Ti, V, Cr, Mn, and Co were implanted. The implantation parameters are presented and compared with other implantation techniques for radioactive isotopes. To demonstrate an application, a deep‐level transient spectroscopy measurement on 48V in silicon is presented. Ti and V correlated band‐gap levels were observed during the 48V decay.

Deep light ion lithography in PMMA — A parameter study

Abstract Three-dimensional microstructures have been produced in Polymethylmethacrylate (PMMA) by proton irradiations at 1.8 and 6.0 MeV and by irradiation with He+ ions at 1.0 and 1.8 MeV. The irradiated volumes were removed chemically by a properly adjusted etching process. The resulting structures are characterized by a root mean square roughness in the order of a few nanometers. The influence of the dose, the dose rate, the particle energy and the developer temperature is discussed. The parameter range for optimal irradiation and developing conditions is deduced.

Hetero-epitaxial growth of Cu(In,Ga)S2 on Si substrates

Abstract On the way to perfectly lattice-matched Cu(In,Ga)S 2 , we present the first CuGaS 2 films epitaxially grown on Si substrates and compare their structural properties to epitaxial CuInS 2 on the same type of substrate. Both of these chalcopyrites have been grown on sulfur-terminated Si(111) and showed a six-fold surface symmetry in low energy electron diffraction. Composition was controlled in and ex-situ using Auger electron spectroscopy and Rutherford backscattering, respectively. A detailed film analysis by means of X-ray diffraction methods yielded the lattice parameters of the epitaxial layers, wider rocking curves of the gallium compound as compared to the indium compound, and showed the suppression of twin formation in Ga-rich CuGaS 2 . The results indicate that a perfect lattice match between Cu(In (1− x ) Ga x )S 2 and Si will be attainable with x ≈0.5.

Deep levels of chromium in 4H-SiC

Abstract Deep levels of transition metals in 4H-SiC were investigated. A definite chemical identification is achieved by observing the elemental transmutation of radioactive isotopes. Epitaxial layers of n-type 4H-SiC were doped with the radioactive isotopes 48 V and 51 Cr by recoil implantation and subsequent furnace annealing at 1600 K. Repeated deep level transient spectroscopy (DLTS) measurements were performed during the elemental transmutation of these isotopes to 48 Ti and 51 V, respectively. In the case of 51 Cr, three levels at 0.74, 0.18 and 0.15 eV below E C disappear with a time dependence of the nuclear decay, i.e. these levels are due to chromium. In the case of the 48 V implantation, there is a comparatively strong tendency to form a compensated layer under identical implantation and annealing conditions. A level at E C — 0.97 eV is identified with vanadium in 4H SiC.

Structural properties of MBE grown Cu(In,Ga)S2 layers on Si

Abstract The epitaxial growth of the quaternary CuIn (1− x ) Ga x S 2 system with 0≤ x ≤1 on silicon substrates is investigated. Using molecular beam epitaxy, the layers were deposited on sulphur-terminated Si at a substrate temperature of typically 820 K. Reflection high-energy electron diffraction, Rutherford backscattering, X-ray diffraction, and transmission electron microscopy are employed to gain insight into the structural properties of the epitaxial layers with an emphasis on the interplay of lattice mismatch and cation sublattice ordering. All compounds grow epitaxially on Si(111). The quaternary films show a coexistence of chalcopyrite and metastable CuAu-type cation ordering. Lattice match to Si is found for gallium atomic fractions of x =0.41(2).

Band-gap level of the cadmium vacancy in CdTe

The reaction Ag Cd → Ag i + V Cd was used to create cadmium vacancies V Cd in the space charge region of a Schottky contact on p-type CdTe. At T = 380 K, the Ag i + ions were drifted out of the space charge region by the high electric field under reverse bias condition. After cooling down to 80 K with applied reverse bias, admittance spectroscopy measurements were performed. Only after this reverse bias annealing, an additional defect with a thermal activation energy of E V + 0.23(3) eV is observed. In other samples, the existence of V Cd after silver diffusion was monitored by perturbed angular correlation spectroscopy (PAC). Here, admittance spectroscopy reveals the same activation energy of the electrically dominating defects as given above. This result supports the assignment of this level to cadmium vacancy acceptors.