H. Teisseyre

Lattice parameters of gallium nitride

Lattice parameters of gallium nitride were measured using high‐resolution x‐ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.

Mechanism of yellow luminescence in GaN

We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.

Thermal expansion of gallium nitride

Lattice constants of gallium nitride (wurzite structure) have been measured at temperatures 294–753 K. The measurements were performed by using x‐ray diffractometry. Two kinds of samples were used: (1) bulk monocrystal grown at pressure of 15 kbar, (2) epitaxial layer grown on a sapphire substrate. The latter had a smaller lattice constant in a direction parallel to the interface plane by about 0.03%. This difference was induced by a higher thermal expansion of the sapphire with respect to the GaN layer. However, this thermal strain was created mainly at temperatures below 500–600 K. Above these temperatures the lattice mismatch in parallel direction diminished to zero at a temperature of about 800 K.

Temperature dependence of the energy gap in GaN bulk single crystals and epitaxial layer

We performed optical‐absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free‐electron concentrations (Burstein–Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free‐electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free‐electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein–Moss effect.

Effect of growth polarity on vacancy defect and impurity incorporation in dislocation-free GaN

We have used positron annihilation, secondary ion mass spectrometry, and photoluminescence to study the point defects in GaN grown by hydride vapor phase epitaxy (HVPE) on GaN bulk crystals. The results show that N polar growth incorporates many more donor and acceptor type impurities and also Ga vacancies. Vacancy clusters with a positron lifetime τD=470±50ps were found near the N polar surfaces of both the HVPE GaN layers and bulk crystals.

Piezoelectric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k×p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor.

The microstructure of gallium nitride monocrystals grown at high pressure

Abstract This work shows the results of X-ray diffractometric measurements performed on gallium nitride (wurtzite structure, (00.1) oriented plates) bulk crystals grown using the high-pressure (12–15 kbar)-high-temperature (about 1800 K) method. The examinations included: rocking-curve analysis, reciprocal lattice mapping, topography and measurements of lattice parameters. Monocrystals of size up to about 1 mm exhibit a very high crystallographic perfection (rocking curves of 20–30 arc sec). Bigger crystals possess a mosaic structure (0.1–1 mm crystallites separated by 1–3 arc min angle boundaries) visualised by X-ray topography. A model of the creation of those low-angle boundaries is proposed. It is based on the following observations: (i) the mosaic crystals are dome-shaped; (ii) the concave side is a “nitrogen-terminating” one, which grows faster; (iii) this side possesses smaller lattice parameters with respect to the “gallium-terminating” side. We have related the decrease of the lattice parameters to the gallium precipitation (observed in electron microscopy) beneath the “nitrogen-terminating” side. The difference between the lattice parameters on the two sides of the crystal causes its bending, which is then relaxed by emission of the low-angle boundaries.

Lattice constants, thermal expansion and compressibility of gallium nitride

High-resolution X-ray diffraction measurements can be performed at variable temperatures and pressures. The usefulness of such experiments is shown when taking gallium nitride, which is a wide-band semiconductor, as an example. The GaN samples were grown at high pressures (bulk crystals) and as epitaxial layers on silicon carbide and sapphire. The X-ray examinations were done at temperatures of 293-750 K and at pressures of up to 8 kbar. The results served for an evaluation of the basic physical properties of gallium nitride; namely lattice constants, thermal expansion and compressibility. The comparison of monocrystals with epitaxial layers grown on highly mismatched substrates provided important information about the influence of the substrate on the crystallographic perfection of the layers.

High Quality Homoepitaxial GaN Grown by Molecular Beam Epitaxy with NH3 on Surface Cracking.

Epitaxial GaN films have been grown on GaN single-crystal substrates, using on surface cracked ammonia as nitrogen precursor for molecular beam epitaxy. With this approach excellent optical and structural properties are achieved. Low-temperature photoluminescence shows well-resolved excitonic lines with record low linewidths as narrow as 0.5 meV. The transitions are attributed to excitons bound to neutral donors ((D°, X)1 at 3.4709 eV and (D°, X)2 at 3.4718 eV) and to a neutral acceptor ((A°, X) at 3.4663 eV). In addition, free exciton lines are observed at 3.4785 eV, 3.4832 eV, and 3.499 eV for excitons A, B, and C, respectively.

Complete in-plane polarization anisotropy of the A exciton in unstrained A-plane GaN films

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure.