Optical properties and carrier dynamics in differently strained GaN epilayers grown on Si by MOVPE (original) (raw)
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Optical investigations and strain effect in AlGaN/GaN epitaxial layers
Journal of Physics: Conference Series
Al x Ga 1-x N epilayers with x ranging from 0.20 to 0.50 have been grown on c-plane sapphire substrate by metal-organic chemical vapor deposition. The thickness, composition, strain and stress values of the AlGaN were determined by high resolution X-ray diffraction. The optical properties of the epilayers were studied using photoluminescence (PL) and reflectivity measurements. The effect of the stress on the bandgap can be explained by room temperature PL. The temperature dependent PL result shows the well-known "S-shape" behavior.
Effect of AlGaN/GaN Strained Layer Superlattice Period on InGaN MQW Laser Diodes
Physica Status Solidi (a), 1999
AlGaN/GaN strained layer superlattices have been employed in the cladding layers of InGaN multi-quantum well laser diodes grown by metalorganic chemical vapor deposition (MOCVD). Superlattices have been investigated for strain relief of the cladding layer, as well as an enhanced hole concentration, which is more than ten times the value obtained for bulk AlGaN films. Laser diodes with strained layer superlattices as cladding layers were shown to have superior structural and electrical properties compared to laser diodes with bulk AlGaN cladding layers. As the period of the strained layer superlattices is decreased, the threshold voltage, as well as the threshold current density, is decreased. The resistance to vertical conduction through p-type superlattices with increasing superlattice period is not offset by the increase in hole concentration for increasing superlattice spacing resulting in higher voltages.
Journal of Applied Physics, 2008
We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well ͑MQW͒ structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3-1.9 m wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures ͑Ͼ720°C͒ due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 10 20 cm −3 . By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.
Applied Physics Letters, 2000
A number of distributed Bragg reflectors ͑DBRs͒ based on AlN/GaN quarterwave layers have been grown on ͑0001͒ sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00820-2͔
Semiconductor Science and Technology, 2006
High-quality 400 µm thick GaN has been grown by hydride vapour phase epitaxy (HVPE) on (0 0 0 1) sapphire with a 2 µm thick AlN buffer layer. The material's crystalline quality and homogeneity was verified by x-ray diffraction (XRD), low-temperature photoluminesence (LT-PL) and LT cathodoluminescence. Plan-view transmission electron microscopy images reveal a low dislocation density of ∼1.25 × 10 7 cm −2. The residual stress of the material was studied by two complementary techniques. LT-PL spectra show the main neutral donor bound exciton line at 3.4720 eV. This line position suggests virtually strain-free material with a high crystalline quality as indicated by the small full width at half maximum value of 0.78 meV. The presence of well resolved A-and B-free excitons in the LT-PL spectra and the absence of a yellow luminescence band prove the high quality of the HVPE-GaN in terms of purity and crystallinity. These findings are consistent with the XRD results, implying the high crystalline quality of the material grown. Hence, the material studied is well suited as a lattice parameter and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications. Strain-free homoepitaxy on native substrates is needed to decrease considerably the defect density and in that way an improvement of the device's performance and lifetime can be achieved.
Optical characterization of AlN/GaN heterostructures
2003
AlN/GaN/sapphire heterostructures with AlN gate film thickness of 3-35 nm are characterized using photoreflectivity ͑PR͒ and photoluminescence ͑PL͒ spectroscopy. Under a critical AlN film thickness, the luminescence from the GaN channel layer near the interface proves to be excitonic. No luminescence related to the recombination of the two-dimensional electron gas ͑2DEG͒ is observed, in spite of high 2DEG parameters indicated by Hall-effect measurements. The increase of the AlN gate film thickness beyond a critical value leads to a sharp decrease in exciton resonance in PR and PL spectra as well as to the emergence of a PL band in the 3.40-3.45 eV spectral range. These findings are explained taking into account the formation of defects in the GaN channel layer as a result of strain-induced AlN film cracking. A model of electronic transitions responsible for the emission band involved is proposed.
Journal of Applied Physics, 2000
GaN epilayers and GaN/AlGaN quantum wells ͑QWs͒ were grown by molecular beam epitaxy on GaN͑0001͒ single crystal substrates. Transmission electron microscopy ͑TEM͒ was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 10 5 cm Ϫ2 is deduced from TEM imaging. Low temperature ͑1.8 K͒ photoluminescence ͑PL͒ of homoepitaxial GaN reveals PL linewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxial GaN/Al 2 O 3 layer. A 2 nm thick GaN/Al 0.1 Ga 0.9 N QW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxial QW is shown to be comparable to that measured on heteroepitaxial QWs grown either on sapphire or silicon substrates.
physica status solidi (c), 2005
Subpicosecond time-resolved photoluminescence (TRPL) has been used to compare the room temperature carrier dynamics in Al 0.1 Ga 0.9 N/Al 0.3 Ga 0.7 N multiple quantum well (MQW) structures simultaneously deposited on a high quality free standing HVPE GaN substrate (dislocation density ~ 1x10 7 cm -2 ) and 1 µm MOCVD GaN template on sapphire. The PL lifetime of ~ 500 ps in the MQW on GaN substrate is about 5 times longer than that for the MQW on GaN template, with a concomitant increase in CW PL intensity. This behavior is attributed primarily to an increase in nonradiative lifetime associated with a 100 times reduction in dislocation density in the GaN substrate. The observation that the PL lifetime in the MQW falls short of the ~ 900 ps dominant decay time in the GaN substrate may be indicative of generation of additional defects and dislocations due to substrate surface preparation, strain relaxation, and nonoptimal growth temperature associated with the difference in heating of the thin GaN template on sapphire and the thick GaN substrate. An extended PL rise time of greater than 20 ps for the MQW emission when above barrier pumping is employed implies that both wells and barriers are of high quality.