E0 Critical Point Research Articles

Growth of ZnSe nanowires on quartz substrates and their photomodulated transmittance spectra

ZnSe nanowires were grown on fused quartz substrates by a vapor transport method in which the ZnSe powder is heated at 1100 °C in a tube of the furnace. The scanning electron microscopy images showed that the nanowires have the average diameter of ∼110 nm. The X-ray diffraction measurement indicates that the ZnSe nanowires were a single phase of zinc-blende ZnSe. The photomodulated transmittance (PT) measurements were performed for the ZnSe nanowires at temperatures between 15 and 300 K. The measured PT spectra revealed distinct structures of E0 critical point at ∼2.6−2.8 eV. This E0 critical point structure was successfully explained by adding an excitonic line shape to a one-electron line shape. The temperature dependence of E0 energy was analyzed using an analytical formula that takes into account the band-gap shrinkage effect by the electron-phonon interaction in semiconductors. The exciton binding energy of ZnSe nanowires was determined to be 20 meV.

Near-bandgap optical properties of pseudomorphic GeSn alloys grown by molecular beam epitaxy

We investigated the compositional dependence of the near-bandgap dielectric function and the E0 critical point in pseudomorphic Ge1-xSnx alloys grown on Ge (100) substrate by molecular beam epitaxy. The complex dielectric functions were obtained using spectroscopic ellipsometry from 0.5 to 4.5 eV at room temperature. Analogous to the E1 and E1+Δ1 transitions, a model consisting of the compositional dependence of relaxed alloys along with the strain contribution predicted by the deformation potential theory fully accounts for the observed compositional dependence in pseudomorphic alloys.

Birefringence and refractive indices of wurtzite GaN in the transparency range

Birefringence and anisotropic refractive indices of wurtzite GaN within the spectral range from 0.58 eV to 3.335 eV were determined combining optical retardation and spectroscopic ellipsometry measurements on a series of undoped m- and c-plane GaN bulk substrates grown by hydride vapor phase epitaxy. It is observable that the birefringence has a maximum close to the absorption edge and a weak broad minimum in near-IR range. A quantitative explanation of the whole data is given in terms of contributions to the optical response of GaN due to discrete excitons, Coulomb enhanced band-to-band optical transitions near the E0 critical point of the band structure, high-energy optical transitions, and infrared active optical phonon modes which are different for the ordinary and extraordinary waves both in magnitude and in spectral dependence.

Optical properties of pseudomorphic Ge1−xSnx (x = 0 to 0.11) alloys on Ge(001)

The characterization of the optical properties of pseudomorphic Ge1−xSnx/Ge/Si (x = 0 to 0.11) alloys from the IR to UV is presented. The Ge1−xSnx alloys were epitaxially grown on relaxed Ge grown on Si. Rutherford backscattering (RBS) and RBS ion channeling methods were used to confirm the Sn composition and substitutional nature of the Sn into the Ge lattice. The pseudomorphic nature of the Ge1−xSnx on Ge is confirmed using high resolution x-ray diffraction (HRXRD) and transmission electron microscopy. Although HRXRD reciprocal space maps indicated that the Ge1−xSnx was pseudomorphic to Ge, the shape of the Bragg peaks indicated that the sample surface was rough. The rough surface morphology is confirmed using atomic force microscopy. The complex dielectric function is reported in the IR, visible, and UV spectrum in the wavelength range of 0.2–5.06 eV. The E1, E1 + Δ1, E2, and E0 critical points are extracted using second and third derivative line shape fitting and are compared with the elastic theory calculations of strained Ge1−xSnx (x = 0 to 0.11) alloys and fully relaxed Ge1−xSnx (x = 0 to 0.11) alloys. The E0 critical point energies are observed to have slightly larger values than those calculated for completely relaxed Ge1−xSnx alloys due to the presence of compressive strain.

Dielectric function and energy of the E 0 critical point of hexagonal GaN at 26 K studied by using spectroscopic ellipsometry

We report the dielectric function from 0.74 to 6.42 eV of hexagonal GaN at 26 K, along with the energies of its E0 exciton and critical point. The GaN sample is a 1.9 µm thick film deposited on a c-plane (0001) sapphire substrate by using molecular beam epitaxy. Data were obtained with a rotating-compensator spectroscopic ellipsometer. B-splines in a multilayer-structure calculation were used to extract the optical properties of the film. The 26 K E0 excitonic and critical-point structures were significantly blue-shifted and were sharpened relative to their room-temperature equivalents.

Identification of the additional transition around the E0 critical point in photoreflectance spectra of GaInAsSb epitaxial films

We present the analysis of the photoreflectance (PR) spectra line-shape of GaInAsSb films over GaSb. The studied samples were grown by liquid phase epitaxy technique. The growth parameters such as initial temperature and contact time were the same for all samples, but the precursor solution varied slightly in the As concentration. The cooling rate was different for each sample. The PR spectra were obtained at low temperature, modulated by a 632.8 nm He-Ne laser. These spectra were fitted using the third derivative lorentzian function but, for one of the samples exhibiting an additional structure around the E0 critical point oscillation, it was also necessary to use a first derivative lorentzian function, associated to an excitonic transition. This interpretation of the new PR feature was analyzed according to X-ray diffractograms. Comparing the FWHM of the main peak in the diffractograms it was found that the sample which exhibits the excitonic transition has the best crystalline quality, and was the sample with the lower cooling rate. The excitonic energy value obtained (9.7 meV) is in agreement with the value reported in the related literature.

Optical constants of cubic GaN/GaAs(001): Experiment and modeling

The optical constants ε(E)=ε1(E)+iε2(E) of unintentionally doped cubic GaN grown on GaAs(001) have been measured at 300 K using spectral ellipsometry in the range of 1.5–5.0 eV. The ε(E) spectra display a structure associated with the critical point at E0 (direct gap) and some contribution mainly coming from the E1 critical point. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden–Muñoz model dielectric function [M. Muñoz et al., J. Appl. Phys. 92, 5878 (2002)]. This model is based on the electronic energy-band structure near critical points plus excitonic and band-to-band Coulomb-enhancement effects at E0, E0+Δ0 and the E1, E1+Δ1, doublet. In addition to evaluating the energy of the E0 critical point, the binding energy (R1) of the two-dimensional exciton related to the E1 critical point was estimated using the effective mass/k⋅p theory. The line shape of the imaginary part of the cubic-GaN dielectric function shows excitonic effects at room temperature not withstanding that the exciton was not resolved.

Photoconductivity and Electroreflectance Study of Cubic GaN/GaAs(001) Heterostructures by Optical-Biasing Technique

Photoconductivity (PC) and electroreflectance (ER) spectrum measurements were carried out for the cubic GaN/GaAs and cubic AlGaN/GaN/GaAs heterostructures grown by LP-MOVPE. Both spectra contained significantly overlapped signals from the c-(Al)GaN epitaxial layer and the underlying GaAs layer, but the latter was successfully suppressed and distinguished from the former by the use of an additional steady-state illumination of Ar+ laser as an optical bias. The temperature dependence of the optically-biased PC spectrum showed that there remained structural disorder in the c-GaN layer indicated by the temperature-independent Urbach energy. From the temperature dependence of the ER spectrum, the E0 critical point of c-GaN observed likely corresponds not to excitonic transition but to free-carrier transition as it shifted by ∼30 meV to higher energies than the reported value, probably due to the dissociation of exciton by the relatively large modulation field applied to the c-(Al)GaN layer.

E 0 photoreflectance spectra of GaAs: Identification of the features related to impurity transitions

In the room-temperature photoreflectance spectra of moderately doped crystalline GaAs wafers near the fundamental E0 critical point, components corresponding to impurity transitions were identified. This was done by phase analysis of the spectra, and their variation with the laser-excitation density was examined. The dependences of the impurity component magnitude and the retardation phase on the excitation density were determined experimentally. It was established that the impurity component is strongly affected by the condition of the sample surface.