Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

doi:10.1016/j.solidstatesciences.2016.01.002

Solid State Sciences

Volume 54, April 2016, Pages 59-63

https://doi.org/10.1016/j.solidstatesciences.2016.01.002 Get rights and content

Highlights

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Photoreflectance was used to investigate the optical properties of free exciton in GaN.
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A strong correlation between electron concentration and exciton linewidths is observed.
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The exciton-acoustic phonon coupling is strongly related to the exciton kinetic energy and to the strain level.

Abstract

In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FX_A FX_B and FX_C, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FX_A, FX_B and FX_C. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

Graphical abstract

Introduction

Excitons in wide-gap semiconductors play an important role in optical phenomena since they have a large binding energy and could be stable at room temperature [1], [2], [3], [4], [5]. Many researches on excitons in GaN are part of a larger study concerning the development of a new generation of polariton devices based on wide bandgap semiconductor microcavities [4], [6], [7]. The transport and optoelectronic properties of these devices depend mainly on the vibrational properties of the material, which are influenced by several interactions involving excitons, free carriers, phonons, impurities, and defects [8]. Therefore, the knowledge of such interactions and their influence on the fundamental optical properties of GaN epilayers is crucial for further improvements of the performance of GaN-based optoelectronic devices. For this reason, massive researches have been done over the past decades to understand the scattering mechanism in GaN and particularly the exciton-phonon interaction [4], [8], [9], [10], [11], [12], [13], [14]. It was found that the temperature dependence of the exciton linewidth provides important information about the dynamics of the exciton-phonon interaction system [8], [9], [10], [11], [12], [13]. Surprisingly enough, most of these reports are limited to the FX_A and only little attention was paid to FX_B and FX_C. This can be explained by the lack of high quality materials and proper characterization tools to get such excitonic transitions with high spectral resolution. In fact, in most PL spectra reported so far on GaN, FX_A emission line is dominant, while FX_B and FX_C, under the best circumstances, appear only as a weak shoulder due to its fast decay via nonradiative process [13], and to the excitonic transfer towards the A exciton levels which are located at lower energy. This makes FX_B and FX_C hard to be spectrally resolved and investigated. Furthermore, in the case of reflectance measurement, spectra are dominated by fringes arising from the internal multiple reflections which can engender some errors under the fitting process [12].

Recent advances in growing epitaxial GaN with high quality [15], [16], [17], [18] as well as the progress achieved in optical characterization techniques, present the opportunity to study the excitons scattering processes in GaN. In this paper, PR spectroscopy is used to study the scattering properties of free excitons in GaN. The PR spectroscopy was selected because of its high sensitivity, non-destructive nature, and its precision in determining the energy position and the linewidth of optical transition compared to the conventional characterization methods [19]. The results show that the strength of the exciton-acoustic phonon scattering in GaN depends strongly on the exciton kinetic energy and on the strain level.

Section snippets

Experiments

The investigated GaN layers were grown on c-plane sapphire substrates in a vertical MOVPE reactor equipped with in-situ He–Ne laser reflectometry. Trimethylgallium (TMG) and NH₃ were used as the precursors of gallium and nitrogen, respectively. The carrier gas was a mixture of N₂ and H₂. After a nitridation step at 1080 °C in NH₃ + N₂ + H₂ ambient, SiN treatment was carried out by in-situ deposition of a thin SiN mask on the sapphire. The GaN buffer layer and GaN epilayer were grown at 600 °C

Results and discussions

Fig. 1 displays the low temperature (10 K) PL spectra of samples L1, L3, L4 and L6 normalized according to the NBE maximum peak emission. The PL spectra exhibit donor–acceptor pair (DAP) emissions which is consistent with the results reported in the literature [20], [21]. It is clearly observed from Table 1 that the DAP to NBE (DAP/NBE) luminescence intensity ratio decreases as the layer thickness increases and reaches the minimum value (∼1/5) for the thickest layer (sample L6). This behavior

Conclusion

The optical properties of GaN epilayers with different thickness have been investigated using PL and PR techniques. The results show that the impurity/defect concentration decreases as the layer thickness increases. This behavior is deduced from the decrease of the DAP/NBE intensity ratio observed in PL spectra and from the decrease of residual electron concentration obtained by Hall measurements. The transition lines from FX_B and FX_C are well resolved in the PR spectra. It was found that the

Acknowledgments

The authors acknowledge financial support from DGRST.

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Controlled nano-roughening of the GaN surface by post-growth thermal annealing
2024, Applied Surface Science
GaN-based semiconductor structures have been widely used in photonic, electronic, and optoelectronic devices. However, due to the lack of lattice-matched substrates for GaN, several schemes related to material growth and device structure, including surface roughening, have been proposed to improve the efficiency of GaN-based devices. In our work, we developed an experimental approach to achieving a controllable roughening of the GaN epilayer surface by thermal post-annealing. The degree of surface roughness, i.e., nanometric modification, is controlled by the number of annealing cycles. The samples were annealed at 1200 °C under N₂ ambient. Under this condition, the first stage of GaN decomposition occurs. This annealing process led to the formation of Ga-rich GaN nanoparticles at the film surface with no significant change in the dislocation density. Furthermore, this treatment greatly decreases the compressive stress degree, resulting in a considerable improvement in the optical properties of the GaN samples.
Correlation of structural and optical properties of AlGaN films grown on SiN-treated sapphire by MOVPE
2021, Materials Science and Engineering: B
We investigated the structural and optical properties of AlGaN films grown on SiN-treated sapphire substrates, without and with GaN-template, by atmospheric pressure metal organic vapor phase epitaxy. The samples were characterized using high-resolution X-ray diffraction (HR-XRD), time-resolved photoluminescence (TR-PL), and photoreflectance (PR) spectroscopies. Furthermore, the carrier mobility was determined from Hall-effect measurements. When the AlGaN (GaN-template) layer thickness increases up to 0.6 µm (1.3 µm), an increase in the PL decay times is observed and correlated with the transition from 3D to 2D growth mode resulting in a decrease in the dislocations density as obtained from the HR-XRD measurements. Beyond the aforementioned layer thicknesses, we observed a deterioration in the PL transient corresponds to an increase in the density of V_Al-related complexes during the relaxation process, which act as non-radiative recombination centers. Our observations strongly suggest that this type of defects influences the carrier transport and carrier recombination process in the AlGaN layers. Furthermore, our results reveal a phenomenological linear relationship between the internal electric field, obtained from the PR measurements, and the dislocations density. This finding predicts an increase in the GaN internal electric field by about 147 KV/cm when the Al content is increased to 7% in the AlGaN layers. We attribute this increase to a rise in the polarization-induced electric field due to Al incorporation in the AlGaN layer. Based on the obtained correlation between the internal electric field and the dislocation density, we propose an experimental approach that can be utilized to determine the internal electric field, at zero dislocation density, which is very important for designing high-efficient electronic and optoelectronic devices.
Time-resolved photoluminescence and photoreflectance spectroscopy of GaN layers grown on SiN-treated sapphire substrate: Optical properties evolution at different growth stages
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The P5 emission energy is varied between 3.46 and 3.48 eV depending on the GaN layer thickness. Such an emission (peak P5) has been reported in the literature and is associated with the radiative recombination of excitons bound to neutral donors (D°X) [17,20,21]. The variation in the energy position of P5 from sample to sample is attributed to the variation of residual strain with the layer thickness [8].
In this paper, we present a systematic study of the optical properties evolution of GaN films during the complete growth process on SiN-treated sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy. The growth process was monitored using in-situ laser reflectometry and was interrupted at different stages to obtain the studied samples. The obtained samples were ex-situ characterized by means of photoluminescence (PL), photoreflectance (PR) and time-resolved PL (TRPL) spectroscopies. The PL emission from the samples of the initial growth stages originates from nano-crystallite and defect states due to the 3D growth mode. However, with increasing layer thickness, the 2D growth mode is established, and the PL spectrum is dominated by free-exciton emission. The electric field extracted by applying the Franz-Keldysh oscillation (FKO) theory on the PR spectra shows a trend to decrease as the GaN layer thickness is increased. For fully coalesced layers, the FKO totally disappears, and the PR spectrum is dominated by free-exciton transitions. TRPL measurements demonstrate the contribution of two processes to the PL decay, i.e., fast and slow components. While the slow decay time reveals the same sensitivity to different types of dislocations (twist and tilt mosaics), the fast decay time is more affected by the twist mosaic than by the tilt one.
Effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN grown by MOVPE
2017, Optik
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This causes unavoidable errors under the estimation of the linewidth. Furthermore, in the case of reflectance measurement, fringes arising from the internal multiple reflections, which dominate the spectra, influence the accuracy of obtained linewidths and errors would be generated under the fitting process [12,17]. A poor quality of the sample may also plague the analysis of experimental linewidth and consequently distort the determination of the excitonic parameters, i.e. GaN of high quality provides, in fact, more precise values.
In this work, the effects of thermal ionized-impurities and mosaicity on the excitonic properties of GaN are systematically investigated. For this reason, a number of GaN epilayers with different impurity/defect concentrations have been grown on c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Variable-temperature photoreflectance (PR) spectroscopy is used, in the temperature range from 10 K to 300 K, to study the thermal broadening of excitonic transitions. A comprehensive comparison between the experimental data and the available theoretical models is presented. The results show that the theoretical models, in which the exciton scattering with thermal-ionized impurity is ignored, fail to account for the experimental data. However, when the exciton-ionized impurity interaction is considered, the simulation results become in good agreement with the experimental data. Based on the last approach, the parameters that describe the thermal broadening of the exciton linewidth are accurately evaluated and discussed. It is found that the exciton-LO phonon coupling strength, which suffers from scattering in the literature, is almost constant for the different samples (Γ_LO ≈ 255 ± 15meV), and it is independent to impurity concentration. Furthermore, it is observed that the free exciton transitions have not the same sensitivity to the tilt and twist mosaic interaction, and the C-exciton undergoes a stronger dislocation scattering than FX_B and FX_A.
Exciton-phonon interaction in planar nitride nanostructures: The case of acoustic phonons
2023, Physical Review B

View full text

Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experiments

Results and discussions

Conclusion

Acknowledgments

Solid State Sic

Solid State Sic

Solid State Sic

Superlattices Microstruct.

Superlattices Microstruct.

Superlattices Microstruct.

Curr. Appl. Phys.

Mater. Res. Bull.

J. Cryst. Growth

Superlattices Microstruct.

Mater. Sci. Semicond. Process.

Solid State Commun.

Phys. Rev. B

Appl. Phys. Lett.

Phys. Rev. B

Phys. Rev. B