H.W. Kunert

Studies of Piezoresistance and Piezomagnetoresistance in Si Whiskers at Cryogenic Temperatures

The temperature dependence of the electrical resistance of silicon whiskers was studied as their piezoresistance and piezomagnetoresistance at 4.2 K. A special technique was developed to study the whiskers in compressive and tensile strain. It was observed that the thermal strain due to the mounting of the whiskers on different substrates at helium temperatures may be high enough to ‘throw’ a crystal from the metallic side of the metal-insulator transition to insulating one and vice versa. A negative magnetoresistance in p-type silicon whiskers was found at 4.2 K, but in uniaxial compression a transition from negative magnetoresistance to ‘anomalous’ positive magnetoresistance was observed. The possibility to apply a ‘giant’ piezoresistance found in the vicinity of the metal-insulator transition in strain gauges and pressure sensors having a very high sensitivity to the measured value is discussed.

Defect luminescence bands in MOCVD-grown CdTe on GaAs

Photoluminescence (PL) spectra of epitaxial (100) and (111) CdTe on (100) GaAs and (211), (331), and dual epitaxy, (211) and (331) CdTe on (211) GaAs substrates, prepared by low-pressure metal-organic chemical vapour deposition, are presented. Steady-state PL measurements were carried out at temperatures ranging from 11.4 K to 200 K. The well-known 1.47 eV band was observed for all samples. PL bands at 1.36 and 1.32 eV, which were reported only once before for CdTe films were grown on (111) CdTe substrates, were observed for the dual epitaxy sample. These features are probably caused by the radiative recombination of electrons and holes trapped at a localised centre associated with a less-common structural defect.

Bound and free excitons in ZnO. Optical selection rules in the absence and presence of time reversal symmetry

The correlation between ionized donor bound exciton recombinations and neutral donor bound exciton recombinations in ZnO has been investigated. The experimental data obtained by means of magneto-photoluminescence (MPL) concerning charge state and localization energies of ionized and neutral donor bound excitons are in good agreement with theoretical predictions. The optical selection rules in absence and presence of time reversal symmetry (TRS) are investigated. It is shown that the inclusion of extra degeneracy due to TRS reveals a number of new states of the same symmetries and essentially does not change the existing optical selection rules.

Optical Properties of as-Grown and Proton Irradiated ZnO

Conventional photoluminescence spectroscopy (PL) shows several high energy optical transitions within the region 3.072 to 3.4345 eV from as-grown ZnO excited with a 257 nm UV line. Performing inelastic light scattering experiments (ILS), we probed some previously reported modes. Proton implantation caused drastic changes in the excitonic region and generates a broad band centered at about 650 nm (1.9eV) In addition, we observed from both sides of the ZnO sample implantation-induced high-energy vibrational modes centered at about 1350 and 1600 cm -1 (0.167 and 0.198 eV, respectively).

Allowed combinations and overtones of vibrational modes in wurtzite GaN

Abstract Using the standard method of placing displacement vectors upon each ion in the basis (Ga and N), and reducing this displacement representation we obtain eight allowed vibrational modes in crystals with the space group C 6v 4 . Decomposing the symmetrized Kronecker Product (KP) of the vector representation we obtain six first-order Raman active modes at the zone center of the Brillouin zone (BZ). We have determined the allowed combination and overtones of these first-order Raman modes (at k =0 ). Performing inelastic light scattering experiments (ILS) we have verified the derived selection rules for combinations and overtones.

Interaction of surface misfit and monatomic steps on unreconstructed {110} surfaces of anisotropic f.c.c. crystals I. Ledges parallel to rectangular surface unit cell axes

The difference between the bulk atomic spacings of a crystal and the spacings at and near the surface constitutes “surface misfit” and accordingly implies stressed surface layers. When an atomic step is formed on the surface, the stressed layers relax somewhat and induce stresses which act on the crystal “surface”. The surface stress distribution (needed to calculate the stress-strain fields generated within the crystal) resulting from the formation of a monatomic step on the {110} surface of an anisotropic f.c.c. crystal is derived. Both the tangential and normal components of the surface stresses are expressed in terms of the crystal parameters: the surface misfits (fx, fy, fz), the stiffness constants (c11, c12, c44), the anisotropy ratio A and the nearest-neighbour distance d. The monolayer atom-crystal interaction is modelled as a surface interaction potential V represented by a Fourier series truncated after six terms. The activation energies of surface self-diffusion are shown to be directly related to the Fourier coefficients Fi and to be anisotropic. This anisotropy is confirmed by empirical data for {110} surfaces of copper, nickel and rhodium. The results also imply the existence of upper and lower bounds for stiffness constants when activation energies are known, and vice versa.

Depth profiling of high-energy hydrogen-implanted 6H-SiC

The results of implanting silicon carbide with a 1-MeV proton beam at a dose of 1 x 10(17) cm(-2) are presented. Using high-resolution confocal Raman spectroscopy, we analyzed the depth profile of the implantation damage before and after thermal annealing. When it is applied to a high-refractive-index medium, such as SiC, this technique requires careful manipulation to ensure the correct interpretation of results. To this end we discuss a simple ray-tracking model that includes the effects of additional spherical aberration and of the Gaussian intensity profile of the excitation beam. In addition, infrared reflectance measurements show evidence of a well-defined step in the refractive-index profile at the expected implantation depth.

Stiffness constants for anisotropic hexagonal ultra thin overlayers

Abstract The elastic properties of overlayers are of interest in epitaxy, interplay of surface misfit and steps on crystal surfaces. The overlayer is represented by a thin slice from a bulk crystal regarded as an elastic continuum. In anisotropic crystals, the stiffness constants Cijkl are usually specified in principle axes (x0,y0,z0) and must be transformed to the local (x,y,z) axes, which are chosen to be convenient for the analysis of overlayers. We have computed a set of useful stiffness constant transformation relations for hexagonal ultra-thin overlayers, together with error probes i.e. equations for checking the correctness of transformed quantities when reduced to isotropic media. The transformed stiffness constants are expressed in terms of bulk ones and matrix elements that transform the principle axes to the local overlayer system. For the overlayer 11 2 2 the transformed stiffness constants are derived explicitly. The results can be applied directly to several phenomena that generate surface stresses such as: (i) surface misfit; (ii) dislocations; (iii) monatomic steps and others.

Stress Imposing During Microcrystals Characterization at Cryogenic Temperatures

A special method of stress imposing on semiconductor microcrystals/whiskers during their characterization at cryogenic temperatures has been developed and applied for the piezoresistance and piezo-magnetoresistance measurements at cryogenic temperatures down to 1.7 K. A set of substrates provided tensile and compressive stress of Si and Ge microcrystals. The method could be recommended for structurally perfect crystals at cryogenic temperatures; in this case a long-term stability of the properties is kept.

Raman and photoluminescence spectroscopy from N2+-ion implanted and α-irradiated and annealed GaN/sapphire

Abstract GaN on sapphire (SA) was both implanted by N 2 + ions and α-particle irradiated from the GaN and substrate sides. Irradiation-induced vibration modes were observed in GaN. Raman spectra from the substrate reveal more drastic changes. Photoluminescence spectroscopy shows a significant shift of the yellow band toward the higher energy. Annealing results in a good recovery of GaN, while the substrate retains substantial structural damage.