Carrier Dynamics in InGaN/GaN SQW Structure Probed by the Transient Grating Method with Subpicosecond Pulsed Laser

Abstract

Carrier dynamics in GaN and InGaN/GaN SQW structures were observed by using the transient grating (TG) method with sub-picosecond pulsed laser at room temperature. The diffusion coefficients (D) of photo-created carriers were estimated by the decay rate time of TG signals and the photoluminescence (PL) lifetime. It was found that D depends on the emission wavelength (In composition). The relationship between the emission efficiencies and carrier diffusion was considered in terms of the spatial inhomogeneity of In composition.Recently, InGaN/GaN-based light emitting diodes (LEDs) have been commercialized in ultraviolet (UV), blue, green, and amber spectral region [1,2]. In particular, the external quantum efficiency (h ext ) of about 20% is now achieved in blue (450 nm) LEDs. However, h ext values are still lower for LEDs out of this blue spectral range, but detailed reasons for the reduction of h ext have so far not been elucidated. In this study, we try to elucidate this reason by the viewpoint of carrier dynamics. The transient grating (TG) method which is one of third-order nonlinear spectroscopy, has been used for GaN to detect the nonlinear susceptibility [3], exciton dephasing time [4], time response of scattering [5], or quantum beat [6], etc. It is also a powerful tool to directly detect diffusion processes. By using this method, Haag et al. [7] have measured and reported the carrier diffusion in GaN. We observed the diffusion of heat energy generated by the nonradiative recombination of carriers in GaN and ZnSe by the TG method with nano-second pulsed laser [8,9]. In this work, we observed carrier diffusion in InGaN/GaN-based SQW structures in UV, blue, green, and amber spectral region by using the TG method with sub-picosecond pulsed laser.The samples used in this study were grown on a (0001) oriented sapphire (Al 2 O 3 ) substrate by a two-flow metalorganic chemical vapor deposition (MOCVD) technique [10]. The thickness of GaN bulk layer is 4 mm. GaN/InGaN SQW structure is composed of a GaN (1.5 mm), an n-GaN:Si (2.3 mm), an InGaN SQW (3 nm) and a GaN cap (5 nm) layer. LED structure of GaN/InGaN SQW is composed of GaN (30 nm), n-type 1 GaN : Si (5 mm), InGaN SQW (3 nm), p-type AlGaN : Mg (60 nm) and p-type GaN : Mg (150 nm).For the TG measurement, a mode-locked fiber laser and regenerative amplifier system (Clark) was used. The frequency doubled beam (388 nm) was used as pump beam. Pulse width, power, and repetition rate were 500 fs, 1 mW and 1 Hz, respectively. The pump beam was split into two coherence beams and crossed again in the sample with q = 30 and 120 . The modulation of carrier density (grating) is created in the sample by the interference pattern. The fundamental beam (775 nm) was used as a probe beam with the optical delay unit. The probe beam was partly diffracted, which was detected by a photomultiplier tube (Hamamatsu) and averaged with a boxcar integrator. For the time-resolved photoluminescence (TRPL) measurements, the frequency doubled beam of a mode-locked Ti : sapphire laser (Spec...

Carrier Dynamics in InGaN/GaN on the Basis of Different In Concentrations

“…The unique advantage of FWM is its capability to directly analyze transmission of carriers by changing the interval of optical interference patterns. Previous studies on InGaN materials using FWM technology were carried out based on heterostructure and single quantum well samples [20][21][22]. Okamoto et al [20] have proven that the considerable decrease in external quantum efficiency (η ext ) of light-emitting diode in InGaN refers not to the increase in non-radiative composite centers but the carrier delocalization caused by rapid diffusion.…”

“…Previous studies on InGaN materials using FWM technology were carried out based on heterostructure and single quantum well samples [20][21][22]. Okamoto et al [20] have proven that the considerable decrease in external quantum efficiency (η ext ) of light-emitting diode in InGaN refers not to the increase in non-radiative composite centers but the carrier delocalization caused by rapid diffusion. In previous studies, we proposed a free carrier grating decay time with different spatial periods to determine the bipolar diffusion coefficient, D = 2.1 cm 2 /s, with 470 ps effective carrier life for estimating the corresponding 40 cm 2 /Vs hole mobility and 10 18 cm −3 density of the carrier [21].…”

Carrier Dynamics in InGaN/GaN on the Basis of Different In Concentrations

“…The unique advantage of this technique is the possibility of direct analysis of carrier transport by varying a spacing of light interference pattern. Previous studies on InGaN materials by FWM technique were carried out in heterostructures and single-quantum well samples [20][21][22]. Okamoto et al [20] demonstrated that the main reason for the reduction of ηext for a large amount of In is not the increment of nonradiative recombination center but carrier delocalization due to fast diffusion.…”

“…Previous studies on InGaN materials by FWM technique were carried out in heterostructures and single-quantum well samples [20][21][22]. Okamoto et al [20] demonstrated that the main reason for the reduction of ηext for a large amount of In is not the increment of nonradiative recombination center but carrier delocalization due to fast diffusion. In our previous work, we presented that the decay times of free carrier gratings with various spatial periods allows the determination of bipolar diffusion coefficient D = 2.1 cm 2 /s, effective carrier lifetime of 470 ps, and estimated corresponding hole mobility of 40 cm 2 /Vs at carrier density of approximately 10 18 cm −3 [21].…”

Near-field scanning optical microscopic transient lens for carrier dynamics study in InGaN∕GaN

“…17,18 Also we reported the spatial resolved measurements of carrier and thermal dynamics by the transient lens ͑TL͒ technique. 19,20 The TG and TL techniques measure the third-order nonlinear response induced in the materials by the excitation laser.…”