GaN Multiple Quantum Research Articles

Excitation power dynamics of photoluminescence in InGaN∕GaN quantum wells with enhanced carrier localization

Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in InxGa1−xN∕GaN multiple quantum wells (x∼0.15) was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75meV in 2nm wells to about 250meV in 4nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak (0.5MV∕cm) built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest (4nm) wells. Increased incident photon energy resulted in an additional blueshift of the PL peak, which was explained by a nonthermalized distribution of localized carriers and/or carrier localization in the interface region. Our results are consistent with a concept of emission from partially relaxed large In-rich regions with internal band potential fluctuations, which are enhanced with increasing the growth time.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Reflection property of nano-acoustic wave at the air∕GaN interface

We present a study on the total internal reflection of nano-acoustic waves (NAWs) at the air∕GaN interface. The coherent NAW was generated and detected by piezoelectric InGaN∕GaN multiple quantum wells using a femtosecond transient transmission technique. With suitably designed sample structures, the fact that strain of the NAW experiences a sign change after total internal reflection at the air∕solid interface is examined directly. The surface roughness of the sample was found to distort the wave front of the NAW and to diminish the measured amplitude of the reflected NAW.

Generation and behavior of pure-edge threading misfit dislocations in InxGa1−xN∕GaN multiple quantum wells

The relaxation of the misfit strain by the formation of misfit dislocations in InxGa1−xN∕GaN multiple quantum wells grown by metal-organic chemical-vapor deposition was investigated by the cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. It is found that the misfit dislocations generated from strain relaxation are all pure-edge threading dislocations with burgers vectors of b=1∕3⟨112¯0⟩. The misfit dislocations arise from the strain relaxation due to the thickness of strained layer greater than the critical thickness. The relaxation of strained layer was mainly achieved by the formation of dislocations and localization of In, while the dislocations changed their slip planes from {0001}to{101¯0}. With the increasing temperature, the efficiency of photoluminescence decrease sharply. It indicates that the relaxation of the misfit strain has a strong effect on optical efficiency of film.

Well width dependence of disorder effects on the optical properties of AlGaN∕GaN quantum wells

We report on a temperature-dependent photoluminescence study of disorder effects on the optical properties of Al0.2Ga0.8N∕GaN multiple quantum wells as a function of the well width. It is found that the disorder-induced inhomogeneous broadening of the excitonic density of states increases with decreasing well width. A nonmonotonic temperature variation of the photoluminescence peak energy is observed, and interpreted as a crossover from a thermal to a nonthermal (trapped) distribution of recombining excitons amongst the band-tail states. The luminescence is quenched by two thermally activated mechanisms, and the dependence of the activation energies with well width is accounted for.

Facet degradation of (Al,In)GaN laser diodes

We study the facet degradation behavior of (Al,In)GaN multiple quantum well laser diodes. Water vapor causes a fast degradation due to facet oxidation of the uncoated facet. Degradation in an inert nitrogen atmosphere is slow and comparable to degradation of GaN laser diodes with coated facets. We also observe a reversible increase in the threshold current density due to a change in absorption caused by surface charges. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Electron Tunneling Barriers on the Performance of InGaN–GaN Ultraviolet Light-Emitting Diodes

InGaN–GaN multiple quantum well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) with and without n-AlGaN electron tunneling barriers (ETBs), grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD), are characterized by comparison with device simulation results. Compared with a conventional LED without the ETB, one of the proposed LEDs with the optimized ETB shows an 11% increase in normalized photodiode (PD) currents. We contribute this improvement to the reduced number of hot electron overflowing to the p-side from MQW by low-energy electron tunneling, which is consistent with the simulation results regarding the carrier distributions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells

We reported the effect of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells (MQWs) by photoluminescence (PL) and high-resolution x-ray diffraction techniques. The MQW samples on (0001)-plane sapphire substrates were prepared with a ramping method by metalorganic chemical deposition. It was found that the structural or interface quality of the MQW system improved as the deposition time of barrier layers increased from 10 to 14min, but lattice relaxation was still observed. The relaxation degree decreases from 33% to 6% as the deposition time increases. Temperature-dependent PL measurements from 12 to 300K indicated that the integrated PL intensities start to decay rapidly as temperature rises above 50K for the sample with the shorter deposition time, and above 100K for the sample with the longer deposition time. The luminescence thermal quenching of the two samples suggests the two nonradiative recombination centers based on a fit to Arrhenius plot of the normalized integrated PL intensity over the entire temperature range. The first centers at higher temperatures show less difference for the two samples. The second centers at lower temperatures can be attributed to the trapping of carriers at the rough interface for the sample with the shorter deposition time and to the thermal quenching of bound excitons for the sample with the longer deposition time, respectively.

Luminescence in Highly Excited InGaN/GaN Multiple Quantum Wells Grown on GaN and Sapphire Substrates

N/GaN multiple quantum wells grown by MOCVD oversapphire and bulk GaN substrates. High excitation conditions enabled usto achieve a screening of the built-in fleld by free carriers. This allowed forthe evaluation of the in°uence of band potential °uctuations due to thevariation in In-content on e–ciency of spontaneous and stimulated emission.InGaN/GaN multiple quantum wells grown on bulk GaN substrate exhibit asigniflcantly lower stimulated emission threshold and thus enhanced lateralemission. Transient and dynamic properties of luminescence indicate asigniflcant reduction in compositional disorder in homoepitaxially grownstructures.PACS numbers: 78.45.+h, 78.47.+p, 78.67.De

Effect of gallium nitride template layer strain on the growth of InxGa1-xN∕GaN multiple quantum well light emitting diodes

GaN template layer strain effects on the growth of InGaN∕GaN light emitting diodes devices were investigated. Seven-period InGaN∕GaN multiple quantum well structures (MQW) were deposited on 5 and 15μm GaN template layers. It was found that the electroluminescence emission of the 15μm device was redshifted by approximately 132meV. Triple-axis x-ray diffraction and cross-sectional transmission electron microscopy show that the 15μm template layer device was virtually unstrained while the 5μm layer experienced tensile strain. Dynamic secondary ion mass spectrometry depth profiles show that the 15μm template layer device had an average indium concentration of 11% higher than that of the 5μm template layer device even though the MQW structures were deposited during the same growth run. It was also found that the 15μm layer device had a higher average growth rate than the 5μm template layer device. This difference in indium concentration and growth rate was due to changes in thermodynamic limitations caused by strain differences in the template layers.

Second harmonic generation in AlGaN, GaN and AlxGa1?xN/GaN multiple quantum well structures

Second harmonic generation coefficients of GaN and AlxGa1–xN (x=0.08) thin films deposited by MOCVD on a sapphire 0001 substrate were deduced through the standard Maker fringes method. Measurements were performed at λ=1064 nm using a Nd:YAG Q-Switched laser. The measured ratio between the d33 and d31 coefficients allowed one to retrieve information on the degree of crystallinity of the thin film samples that was found to be higher in thicker films. The presence of tiny oscillations in the SHG detected signal as a function of the incidence angle of the fundamental beam on the samples is discussed and related to the reflectance at the sample-air interfaces. Finally, nonlinear coefficients for three AlxGa1–xN/GaN multiple quantum well (MQW) samples (x=0.08 and x=0.15) were determined and compared to the values obtained for GaN crystalline thin films.

Calculation of Near-Infrared Intersubband Absorption Spectra in GaN/AlN Quantum Wells

Intersubband transitions in GaN multiple quantum wells (MQW) are expected to be applicable to 1-Tb/s all-optical switches. The near-infrared (1.33–2.15 µm) intersubband absorption spectra of GaN/AlN MQW samples have been theoretically investigated. The measured spectra have been well explained by a model in which zero-voltage drop across one quantum well period and 0.5- to 0.75-nm-thick interface graded layers were assumed. The effective electric field in the well was estimated to be greater than 5 MV/cm. For thick wells, absorption spectra are sensitive to the electric field, and hence to the strain, barrier thickness, and doping level. In thin wells, the effect of the electric field is negligible, but the quality of the interface affects the spectra. The calculation model has been applied to the design of coupled quantum wells (CQW), where faster optical response is expected. Higher-order intersubband absorption at wavelengths of less than 1 µm is also expected to be enhanced in CQWs.

Investigation of the piezoelectric polarization in (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy

We have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a redshift for small In contents while a blueshift was detected for higher In contents. This result is in qualitative agreement with self-consistent band profile calculations taking into account both band gap deformation potentials and piezoelectric polarization charges in these structures. However, the reduction of the polarization induced quantum-confined Stark effect is well in excess of that conventionally calculated for this material system. We attribute this observation to a substantial deviation of the piezoelectric polarization constants of strained layers from those calculated for unstrained material. This finding is shown to be in agreement with recent calculations of the piezoelectric polarization charges for biaxially strained (Al,Ga,In)N layers.

Optical and structural properties of GaN materials and structures grown on Si by metalorganic chemical vapor deposition

GaN thin films, undoped, Si- and Mg-doped, and InGaN–GaN multiple quantum well (MQW) structures have been grown on Si (001) substrates with specially designed composite intermediate layers consisting of an ultra-thin amorphous Si layer and a GaN/AlGaN multilayered buffer by low pressure metalorganic chemical vapor deposition. Their optical and structural properties were studied by a variety of characterization techniques, including X-ray diffraction (XRD), Raman scattering, Fourier transform infrared reflectance, photoluminescence, scanning and transmission electron microscopy. Wurtzite GaN structure with the single crystalline grain size up to ∼2 μm is confirmed, and material properties are improved by adjusting the growth conditions and buffer layer structural design.

Optical Characterization of InGaN/GaN MQW Structures without In Phase Separation

Photoluminescence and cathodoluminescence spectroscopies are used to investigate the properties of the band edge emission of InGaN/(In)GaN multiple quantum well (MQW) structures which do not show evidence of phase separation in high resolution electron microscopy. The data still show a clear low energy peak in the spectra, about 0.1 eV below the main exciton peak. Possible interpretations of this second peak are discussed.

Threading Dislocations and Optical Properties of GaN and GaInN

We categorized threading dislocations in GaN and GaInN multiple quantum wells and epitaxially lateral overgrown GaN into three types of line defects (edge, screw and mixed dislocations), and investigated the optical properties. It was confirmed by cathodoluminescence measurements that not only screw and mixed dislocations but also edge dislocations act as non-radiative centers in GaN. Epitaxial lateral overgrowth (ELO) technique can reduce the densities of all line-defects in a several μm wide wing region. Growth steps in the wing region were disturbed by the defects which were left in a seed region, and a complicated structure was formed at the surface of GaN and GaInN layers grown on ELO-GaN at low temperature. We believe that this surface structure formed by high supersaturation is a cause of In compositional spatial fluctuation and phase separation of GaInN alloy.

GaN-Based High Power Blue-Violet Laser Diodes

The epitaxial lateral overgrowth (ELO) technique is an important technology for improving the characteristics of GaN-based laser diodes (LDs). The photoluminescence intensities from GaN and GaInN multiple quantum well active layers in the ELO-GaN wing region were found to be higher than those in the seed region. This indicates that the density of dislocations in the wing region could be reduced significantly. This is evidenced by dislocation densities of less than 106 cm-2 as determined from transmission emission microscopy and etching-pit-density measurements. The cleaved facets of LDs on ELO-GaN and sapphire were observed by atomic forced microscopy. Although the roughness of GaN cleaved facets on sapphire were high (Ra>10 nm), the roughness in the ELO-GaN wing region was found to be as smooth as that of GaAs cleaved facet (Ra<1 nm). The characteristics of LDs on ELO-GaN were found to be superior to those on sapphire as a result of smoother facets and lower dislocation densities.

Impact of In content on the structural and optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy

AbstractWe have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a red-shift for small In contents while a blue-shift was detected for higher In contents. This result is in qualitative agreement with band profile calculations taking into account both the band gap deformation potentials and the piezoelectric polarization in these structures. However, the magnitude of the shift is well in excess of the calculated one. We attribute this finding to a substantial deviation of the piezoelectric constants from those calculated for unstrained material. Finally, we estimate the piezoelectric polarization of InGaN/GaN for linear and non-linear terms in strain.

Optical Properties and Lasing in (In, Al)GaN Structures

We achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. Lasing modes of a single microcavity were examined from 20 to 300 K and gain mechanisms were compared to those of a thick GaN epilayer. We have also systematically studied InGaN/(In)GaN multiple quantum wells as a function of well and barrier thickness. We demonstrate that the stimulated emission threshold and photoluminescence (PL) decay time are strongly dependent on the well and barrier thickness. The experimental results indicate that the enhanced optical quality of samples with larger barrier thicknesses can he readily applied to the fabrication of InGaN/(In)GaN laser diodes.

MOCVD growth, stimulated emission and time-resolved PL studies of InGaN/(In)GaN MQWs: well and barrier thickness dependence

Optically pumped stimulated emission (SE) and time-resolved photoluminescence (TRPL) of InGaN/(In)GaN multiple quantum wells (MQWs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD) were systemically studied as a function of well and barrier thickness. The SE threshold pumping density and photoluminescence (PL) decay time were found to be strongly dependent on the well and barrier thickness. As the barrier thickness increases, the PL efficiencies and room temperature PL decay time significantly increase, which can be attributed to the improved structural quality, as seen by the reciprocal lattice mapping results. The lowest SE threshold density of 58 kW/cm 2 was obtained for the 3.0 nm well and 15.0 nm barrier sample. The experimental results indicate that the enhanced optical quality of samples with larger barrier thicknesses can be readily applied to the fabrication of InGaN/(In)GaN LDs.