O. Briot

Raman determination of phonon deformation potentials in α-GaN

Abstract The effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by Metal Organic Vapor Phase Epitaxy on (0 0 0 1) direction on sapphire substrates is studied by Raman spectroscopy. Shifts in phonon frequencies are measured, which we correlate to the residual strain fields in the epilayers. Using stress calibration measurements taken from reflectance data, the biaxial pressure coefficients of mode frequencies are determined and used to calculate the corresponding deformation potentials.

Interplay of electrons and phonons in heavily doped GaN epilayers

Raman spectroscopy is used to analyze the effect of electrons on the lattice dynamics of unintentionally heavily doped GaN. The deposition temperature of GaN buffer layers on sapphire substrates is found to have an important influence on the presence of free carriers in GaN layers, evidenced by plasmon coupling to the A1(LO) phonon. Data from infrared measurements are used to calculate the Raman line shape of q=0 coupled A1(LO)-plasmon modes in a dielectric approach and give a good fit of the L−(q=0) component observed in Raman spectra. In particular, the fitting procedure applied to spatially resolved micro-Raman measurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons with wave vectors differing from the q=0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons, attributed to charge density fluctuations on account of its polarization properties.

Optical properties of GaN epilayers on sapphire

The optical properties of GaN epilayers grown by metal‐organic vapor‐phase epitaxy on (0001)‐oriented sapphire are investigated by means of photoluminescence, reflectance, and differential spectroscopy. We obtain quantitative information about the intrinsic or extrinsic nature of the 2 and 300 K photoluminescence features. From detailed investigations of the reflectance properties of these layers we can quantify the residual strain field in these layers and determine the GaN deformation potentials. Comparison of these values with quantities measured on other semiconductors with wurtzite symmetry is also addressed. Last we utilize photoreflectance spectroscopy to measure exciton binding energies.

Selectively excited photoluminescence from Eu- implanted GaN

The intensity of Eu-related luminescence from ion-implanted GaN with a 10nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300°C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to D05-F27 (∼622nm), D05-F37 (∼664nm), and D05-F17 (∼602nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the D05-F27 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356nm, and a broad subedge absorption band centred at ∼385nm. Marked differences in the shape of the D05-F27 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist ...

GALLIUM VACANCIES AND THE GROWTH STOICHIOMETRY OF GAN STUDIED BY POSITRON ANNIHILATION SPECTROSCOPY

We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 1016 to 1019 cm−3 when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 1020 to 1016 cm−3, demonstrating their role as compensating centers.

Indium nitride quantum dots grown by metalorganic vapor phase epitaxy

With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.With respect to growing indium nitride quantum dots with very low surface densities for quantum cryptography applications, we have studied the metalorganic vapor phase epitaxy of InN onto GaN buffer layers. From lattice mismatch results the formation of self-assembled dots. The effects of the growth temperature, V/III molar ratio, and deposition time are studied, and we demonstrate that quantum-sized dots of InN can be grown with a material crystalline quality similar to the quality of the GaN buffer layer, in densities of 107 to 108 cm−2. Such low densities of dots allow for the realization of experiments or devices in which a single dot is isolated, and may be used in the near future to produce single-photon sources.

Transmission electron microscopy characterization of GaN layers grown by MOCVD on sapphire

Abstract We characterize by transmission electron microscopy (TEM), GaN layers deposited by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire. Different GaN films with different surface morphologies have been observed and their crystallographic quality determined. Polarity and surface diffusion are the important factors that determine the surface morphology. The lack of an adapted buffer layer leads to a layer with a dominant N-polarity that contains many inversion domains (IDs) (of Ga-polarity) that grow faster than the surrounding material and form pyramids. All the flat unipolar GaN films we have observed have a Ga-polarity. Unipolarity (Ga-polarity) is realized with the recrystallization of the low temperature buffer layer or/and of the nitridation of the sapphire substrate. An intermediate cubic phase has been observed at the sapphire/buffer layer interface of optimized nitridated samples. In non optimized samples, IDs (of N-polarity) can remain near the buffer layer, but they tend to disappear during the growth of the Ga-polar GaN layer. A high growth temperature (about 1000 °C) was necessary to obtain flat GaN layers.

High-temperature annealing and optical activation of Eu-implanted GaN

Europium was implanted into GaN through a 10nm thick epitaxially grown AlN layer that protects the GaN surface during the implantation and also serves as a capping layer during the subsequent furnace annealing. Employing this AlN layer prevents the formation of an amorphous surface layer during the implantation. Furthermore, no dissociation of the crystal was observed by Rutherford backscattering and channeling measurements for annealing temperatures up to 1300°C. Remarkably, the intensity of the Eu related luminescence, as measured by cathodoluminescence at room temperature, increases by one order of magnitude within the studied annealing range between 1100 and 1300°C.

Properties of a photovoltaic detector based on an n-type GaN Schottky barrier

In this article, we report on the characterization of a photovoltaic detector based on an n-type GaN Schottky barrier. We first present the photovoltaic responsivity above the gap. Its spectrum is explained by the combined effects of absorption and diffusion. The hole diffusion length is estimated to be in the 0.1 μm range with a numerical model. The photoresponse below the gap is also investigated and it is shown that the current generated by the internal photoemission is the major contribution to the photocurrent at reverse biases at 80 K. At room temperature, an additional component to the photocurrent is clearly demonstrated and identified. This extra current stems from the existence of traps. Several spectroscopy techniques are used to characterize those traps. The supplementary current emitted from the traps in the depletion region accounts for the spectral and the temporal behavior of the Schottky photodetector at room temperature.

Coupling of GaN- and AlN-like longitudinal optic phonons in Ga1−xAlxN solid solutions

Long-wavelength optical phonons of Ga1−xAlxN solid solutions have been identified in a wide compositional range by Raman spectroscopy. The A1 and E1 polar phonon frequencies evolve continuously with x from one-member crystal to the other. The same behavior seems to hold true for the silent B1 mode, which manifests itself by an interference with an unidentified continuum. Coupling of the longitudinal-optic (LO) modes associated with the two types of bonds, via the macroscopic electric field, is treated by a generalization of the dielectric model [D. T. Hon and W. L. Faust, J. Appl. Phys. 1, 241 (1973)]. This approach accounts for the observed frequencies and supports the apparent one-mode behavior of the polar LO phonons.