Vanya Darakchieva

Infrared dielectric functions and phonon modes of high-quality ZnO films

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Two-dimensional electron gas density in Al1−xInxN/AlN/GaN heterostructures (0.03≤x≤0.23)

Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegards law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ~17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1-x Inx N/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03=x=0.23 for 6 nm thick barriers and 0.07=x=0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1) × 1013 cm-2 and (2.2±0.1) × 1013 cm-2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1) × 1013 cm-2 is obtained for a nearly LM AlInN barrier with ~14.5% indium on GaN as thin as 6 nm.

High-quality bulk a-plane GaN sliced from boules in comparison to heteroepitaxially grown thick films on r-plane sapphire

Thick GaN bars with [1120] orientation have been sliced from GaN boules grown on freestanding films by hydride vapor phase epitaxy (HVPE) in the [0001] direction. High-resolution x-ray diffraction and transmission electron microscopy have been used to study the structural quality and defect distribution in the material in comparison to heteroepitaxially grown thick HVPE-GaN films grown in the [1120] direction on (1102)-plane sapphire. It is demonstrated that while the heteroepitaxial material possesses a high density of stacking faults and partial dislocations, leading to anisotropic structural characteristics, the (1120)-plane bulk GaN, sliced from boules, exhibits low dislocation density and narrow rocking curves with isotropic in-plane character.

On the lattice parameters of GaN

The lattice parameters of low-defect density, undoped bulk GaN fabricated by hydride vapor phase epitaxy (HVPE) on (0001) sapphire and subsequent substrate removal, are precisely determined using high-resolution x-ray diffraction. The obtained values, c=5.18523A and a=3.18926A, are compared with the lattice parameters of freestanding HVPE GaN from different sources and found to be representative for state-of-the-art undoped HVPE bulk GaN material. A comparison with bulk GaN fabricated by the high-pressure technique and homoepitaxial GaN is made, and significant differences in the lattice parameters are found. The observed differences are discussed and a possible explanation is suggested.

Effects of strain and composition on the lattice parameters and applicability of Vegard's rule in Al-rich Al1-xInxN films grown on sapphire

The lattice parameters and strain evolution in Al1−xInxN films with 0.07⩽x⩽0.22 grown on GaN-buffered sapphire substrates by metal organic vapor phase epitaxy have been studied by reciprocal space mapping. Decoupling of compositional effects on the strain determination was accomplished by measuring the In contents in the films both by Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Differences between XRD and RBS In contents are discussed in terms of compositions and biaxial strain in the films. It is suggested that strain plays an important role for the observed deviation from Vegard’s rule in the case of pseudomorphic films. On the other hand, a good agreement between the In contents determined by XRD and RBS is found for Al1−xInxN films with low degree of strain or partially relaxed, suggesting applicability of Vegard’s rule in the narrow compositional range around the lattice matching to GaN.

Lattice parameters of GaN layers grown on a-plane sapphire: Effect of in-plane strain anisotropy

We have studied GaN films grown on a-plane sapphire by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. The in-plane lattice parameter was determined from sets of equivalent interplanar distances measured for six different directions in order to examine the effect of strain anisotropy. It is found that, in both types of films, the obtained six values of the in-plane lattice parameter can be grouped around two values. The strain anisotropy is estimated to have different value in the films grown by the two techniques and possible explanations are suggested.

Free electron behavior in InN: On the role of dislocations and surface electron accumulation

The free electron behavior in InN is studied on the basis of decoupled bulk and surface accumulation electron densities in InN films measured by contactless optical Hall effect. It is shown that the variation in the bulk electron density with film thickness does not follow the models of free electrons generated by dislocation-associated nitrogen vacancies. This finding, further supported by transmission electron microscopy results, indicates the existence of a different thickness-dependent doping mechanism. Furthermore, we observe a noticeable dependence of the surface electron density on the bulk density, which can be exploited for tuning the surface charge in future InN based devices.

Anisotropy, phonon modes, and free charge carrier parameters in monoclinic β -gallium oxide single crystals

There is growing interest in low-symmetry metal oxides because of their potential use in high-power electronics capable to sustain very high voltages. Very little is known about their fundamental physical properties, such as transverse and longitudinal optical phonon modes, dielectric constants, and how free charge carriers couple with lattice vibrations. This lack of information is partially due to the complexity by which these properties intertwine due to the low symmetry crystal systems. Here, the authors describe a general pathway to the analysis of long-wavelength experiments for monoclinic and triclinic crystal systems, and they report for the first time a complete set of phonon modes for the monoclinic phase of gallium oxide. These parameters may arrive just in time to support computational optimization of charge and heat transport for device designs. The concept for analysis of long wavelength properties in monoclinic and triclinic crystal systems can help access a widely uncharted field in condensed matter physics.

Deformation potentials of the E1 (TO) and E2 modes of InN

The deformation potentials of the E1(TO) and E2 modes of InN are determined by combining infrared spectroscopic ellipsometry, Raman scattering, and x-ray diffraction measurements, and using a reported value of the mode Gruneisen parameter. The deformation potentials are obtained for two sets of stiffness constants. Strain-free values of the InN E1(TO) mode of 477.9 cm−1 and of the E2 mode of 491.1 cm−1 have been determined.

Lattice parameters, deviations from Vegard’s rule, and E2 phonons in InAlN

The lattice parameters of InxAl1-xN in the whole compositional range are studied using first-principle calculations. Deviations from Vegards rule are obtained via the bowing parameters, delta(a)=0. ...