InGaN Multi-quantum Well Research Articles

Fabrication of moth-eye structure on p-GaN layer of GaN-based LEDs for improvement of light extraction

Moth-eye structures were produced on a p-GaN top cladding layer by UV imprint and inductively coupled plasma (ICP) etch processes in order to improve the light extraction efficiency of GaN-based green light-emitting diodes (LEDs). The height and shape of moth-eye structures were adjusted by controlling the thickness of Cr mask layer and ICP etching time. The transmittance of LED device stacks with moth-eye structure was increased up to 1.5–2.5 times, compared to identical LED sample without moth-eye structure and the intensity of photoluminescence from the InGaN multi-quantum well layer of LED sample with moth-eye structure was 5–7 times higher than that of the LED sample without the moth-eye structure.

Carrier Dynamics of Deep-Level States in InGaN/GaN Multiquantum Wells

InGaN/GaN (3 nm/10 nm) multiquantum wells (MQWs) were grown on (0001) sapphire substrates by metal–organic chemical vapor deposition. To characterize the effect of internal electric fields on deep-level states in InGaN MQWs, deep-level transient spectroscopy (DLTS) and bias-dependent photoluminescence (PL) measurements were performed. The thermal activation energy of MQWs obtained from DLTS spectra decreased to 0.15 eV with an increase in reverse measure bias from -2 to -10 V, which indicates partially cancelled internal fields. The blue shift of PL peak energy was about 0.05 eV when the reverse measure bias was changed from 0 to -5 V. This is explained well by carrier dynamics in the conduction band of InGaN/GaN MQWs, which affects carrier recombination energies corresponding to the band edge to edge transitions with changing reverse measure bias.

Effects of Inhomogeneous Gain and Loss on Nitride-Based Vertical-Cavity Surface Emitting Lasers

Nitride-based vertical cavity surface emitting lasers (VCSEL) with hybrid mirror has been investigated. We further measured the as-grown samples (without top dielectric distributed Bragg reflector) by µ-photoluminescence, scanning near-field optical microscopy, and cathodoluminescence (CL). By different excitation power density, the experimental results indicated that the VCSEL devices have different lasing modes and spot sizes, represented inhomogeneous gain and loss distribution in the whole structure. The non-uniform emission intensity distribution including several bright spots of about 1 to 2 µm was observed for both VCSEL devices and as-grown samples, which was obtained from CL measurement, due to the effect of indium clusters and non-uniform micro-cavity resonant in InGaN multi-quantum wells (MQWs). The results show the significant influence on commercial applications such as light emitting diodes, lasers devices and so on.

The Effect of Temperature on the Efficiency of Nitride-based Multi-quantum well Light-emitting Diodes

AbstractWe have recently developed a self consistent light-emitting diode (LED) model that accounts for the current transport and internal heating in AlGaAs-GaAs LEDs. In this paper we extend the model to describe multi-quantum well (MQW) active regions and III-N materials, within the limits of the currently known values and temperature dependencies of the recombination parameters in these materials. The MQW description accounts for the effect of the reduced wave function overlap to the recombination. We present simulation results obtained for an InGaN MQW LED with 4 wells at selected temperatures and discuss the factors limiting the efficiency and luminescent output of LEDs.

New results on HVPE growth of AlN, GaN, InN and their alloys

AbstractA variety of new results on HVPE growth of group III nitride layers and heterostructures are described. Crack free from 10 to 30 μm thick AlN layers were grown on 100‐mm SiC substrates. Bowing of the AlN/SiC wafers was controlled below 10 microns. Low growth rate HVPE with deposition rate below 10 nm/min was demonstrated for GaN, AlN, and AlGaN. This process resulted in the first HVPE grown multi‐quantum wells (MQWs) and superlattices. P‐type as‐grown GaN<Mg> layers with acceptor concentrations up to 3×1019 cm–3 were demonstrated. InGaN layers from 0.002 to 2 μm thick were grown by HVPE for the whole composition range using c‐plane GaN/sapphire template substrates. Non‐polar InGaN layers were grown on a‐GaN/r‐sapphire templates and m‐plane GaN bulk substrates. Blue and green all‐HVPE LEDs were made utilizing InGaN MQWs grown on p‐GaN/sapphire templates. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Mg dopant on the degradation of InGaN multiple quantum wells in AlInGaN-based light emitting devices

We investigated the effects of Mg dopant on the degradation of AlInGaN-based light emitting diodes (LEDs) and laser diodes (LDs) with InGaN multi-quantum wells (MQWs). Photoluminescence (PL) intensity of InGaN MQWs was significantly decreased with increasing the Mg intentional doping process in InGaN active region, indicating that Mg dopant could degrade the optical quality of InGaN MQWs. From secondary ion mass spectroscopy (SIMS) analysis of AlInGaN-based LDs grown on GaN/sapphire and GaN substrate with different dislocation densities, we found that Mg concentration of LD on GaN/sapphire was higher than that of LD on GaN substrate at the InGaN MQWs regions. Additionally, we observed that Mg atoms were significantly diffused from p-type layer to InGaN MQWs region in the LD structure after aging evaluation. From these results, we could conclude that Mg diffusion along threading dislocations is one of the major gradual degradation mechanisms of AlInGaN-based LD/LEDs during the device operation under high voltage condition.

Comparative investigation of InGaN quantum well laser diode structures grown on freestanding GaN and sapphire substrates

Comparative analysis of optical characteristics of In0.08Ga0.92N∕In0.03Ga0.97N multiquantum well (MQW) laser diode structures grown on freestanding GaN and on sapphire substrates is reported. Higher quantum efficiency, higher thermal activation energy, smaller Stokes-like shift, and shorter radiative lifetime are observed for InGaN MQWs on GaN substrate than those of the same MQWs on sapphire substrate. From time-resolved optical analysis, we find that not only an increase in nonradiative lifetime due to reduced dislocation density but also a decrease in radiative lifetime caused by suppressed piezoelectric field play an important role in enhancing optical properties of InGaN MQWs on GaN substrates.

Performance and optical characteristic of InGaN MQWs laser diodes

The performance of the InGaN multi-quantum wells (MQWs) laser diode structures has been numerically investigated by using ISE TCAD software. We investigated the effect of well numbers, barrier thickness and barrier doping on the output power, threshold current, and slope efficiency. All material parameters used in the model are evaluated based on the recent literature values. We observed the maximum output power and lower threshold current when the well number was two. Effective change in the output power and threshold current was observed with the variations in barriers thickness and doping level. Our results are in agreement with the experimental results observed by [S. Nakamura et al. Jpn. J. Appl. Phys. Part 2, 37, L1020 (1998) and S. Nakamura et al. Appl. Phys. Lett. 76, 22 (2000)].

High Power Continuous Wave Blue InAlGaN Laser Diodes Made by Plasma Assisted MBE

Room temperature, continuous wave operation of InGaN multi-quantum wells laser diodes made by rf plasma assisted molecular beam epitaxy at 411 nm wavelength is demonstrated. The threshold current density and voltage were 4.2 kA/cm and 5.3 V, respectively. High optical power output of 60 mW was achieved. The lifetime of these laser diodes exceeds 5 h with 2 mW of optical output power. The laser diodes are fabricated on low dislocation density bulk GaN substrates, at growth conditions which resembles liquid phase epitaxy. We demonstrate that relatively low growth temperatures (600–700◦C) pose no intrinsic limitations for fabrication of nitride optoelectronic components by plasma assisted molecular beam epitaxy.

Fast spin relaxation in InGaN/GaN multiple quantum wells

We report measurements of a fast spin relaxation in InGaN multi-quantum wells (MQW) using femtosecond circularly polarised pump–probe spectroscopy. These reveal quantum beats in the spin polarisation arising from a 1.6 meV fine structure splitting of the excitonic levels. We attribute this splitting to exchange interactions in the narrow quantum wells studied. The 5 K spin coherence time was measured to be 450 fs, a value which decreases by a factor of approximately two by 50 K. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN ∕ GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

No temperature dependence of spin relaxation in InGaN phase-separated quantum dots

We have investigated the spin relaxation in InGaN multiquantum wells (MQWs) at various temperatures (200–300K) using spin-dependent pump and probe measurements. The spin lifetime for In0.071Ga0.929N MQW clearly decreases with increasing temperature. This result indicates that longitudinal optical phonon scattering promotes the spin-flip under nonpure spin states in the conduction band. With increasing In molar fraction, the formation of In-rich quantum dots in InGaN MQW layers causes the suppression of the phonon scattering. The spin relaxation in In0.106Ga0.894N MQW is almost independent of temperature because the contribution of the impurity or defect scatterings is dominant to the spin-flip.

Enhanced optical properties of InGaN MQWs with InGaN underlying layers

The influence of an InGaN underlying layer under InGaN multiquantum wells (MQWs) on the optical properties was investigated. Two MQW structures with and without underlying layers were prepared by MOCVD on sapphire substrates. Optical properties were characterized by means of temperature-dependent photoluminescence (PL), PL excitation (PLE), space-resolved micro-PL (μ-PL) and time-resolved PL (TRPL) measurements. From the micro-PL mapping and temperature-dependent PL results, a more uniform emission distribution and an internal quantum efficiency of about 45% have been achieved with an InGaN layer below the MQWs. We observed an enhancement of both PL intensity and lifetime at room temperature for the InGaN MQWs grown on an InGaN underlying layer, compared to the InGaN MQWs without InGaN underlying layer.

High power blue–violet InGaN laser diodes grown on bulk GaN substrates by plasma-assisted molecular beam epitaxy

We report on the InGaN multi-quantum well laser diodes (LDs) made by RF plasma-assisted molecular beam epitaxy (PAMBE). The laser operation was demonstrated in a temperature range from 297 K up to 360 K with pulsed current injections using 50 ns pulses at 0.25% duty cycle. The threshold current density and voltage for these LDs were 9 kA cm−2 (680 mA) and 8.2 V respectively at 297 K. The slope efficiency is 0.35–0.47 W A−1. A high output power of 1.1 W was obtained during pulse operation for 3.6 A and 8.7 V. We compare parameters of laser diodes with two and five InGaN/InGaN quantum wells. The new, low temperature growth mechanism which enhances surface adatom kinetics, together with bulk GaN low dislocation density substrates allowed us to grow high quality laser diode structures. Our result indicates that there are no intrinsic limitations in the growth of nitride-based optoelectronic devices by PAMBE.

Fine-structure N-polarity InN∕InGaN multiple quantum wells grown on GaN underlayer by molecular-beam epitaxy

We have succeeded in the growth of very-fine-structure InN∕InGaN (3–16nm∕9nm) multi-quantum wells (MQWs) on GaN underlayer and characterized them by transmission electron microscopy (TEM), high-resolution x-ray diffraction (XRD), and photoluminescence (PL) at 13K. Clear satellite diffraction peaks and sharp heterointerfaces were observed by XRD and TEM, respectively. A single PL-emission peak at 1.75μm was observed in the ten-periods InN(16.1nm)∕In0.67Ga0.33N(9.2nm) MQWs. The use of the InGaN as a barrier layer instead of GaN resulted in remarkable reduction of lattice mismatch between the well and barrier, which was essential for the fabrication of MQWs with superior interface quality. This successful growth of fine-structure InN∕InGaN MQWs would be an important step for the application of InN in optical communication devices.

透明導電性基板としての酸化ガリウム単結晶の育成と評価

Large size single crystals of β-Ga2O3 with 1 inch in diameter have been grown by the floating zone technique. Wafers have been cut and fine polished in the (100), (010) and (001) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga2O3 as a substrate for optoelectric devices operating in the visible/near UV and with vertical current flow. Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated on β-Ga2O3 single crystal substrates. High-quality (0001) GaN epi-layers with a narrow bandedge luminescence are obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structures were also successfully grown. The first blue light-emitting diode (LED) on β-Ga2O3 with vertical current injection is demonstrated.

Effect of thermal damage on optical and structural properties of In 0.08Ga 0.92N/In 0.02Ga 0.98N multi-quantum wells grown by MOCVD

Effects of thermal damage on the optical and structural properties of InGaN/InGaN multi-quantum wells (MQWs) were investigated by using photoluminescence (PL), high-resolution X-ray diffraction (HRXRD), and atomic force microscopy (AFM). The five-period In 0.08Ga 0.92N/In 0.02Ga 0.98N MQWs were grown on sapphire using metalorganic chemical vapor deposition (MOCVD). To investigate the influence of thermal damage on InGaN MQWs, the MQW sample was heated to the high-temperature for p-type GaN layer growth and cooled down and its structural and optical properties were compared with the MQW sample cooled down without high-temperature thermal treatment. The surface morphology of InGaN MQWs changes from a spiral to a step structure after the high-temperature treatment, which is induced by the desorption and the surface migration of In and Ga adatoms during the high-temperature treatment. In HRXRD (0 0 0 4) ω / 2 θ scan, the intensity fringes between high-order satellite peaks disappear, which suggests that the interface quality of InGaN MQWs be also deteriorated by high-temperature treatment. Temperature dependence of PL measurement shows that InGaN MQWs without high-temperature thermal treatment had higher internal quantum efficiency and lower localization effect.

Optical study of spin injection dynamics in InGaN∕GaN quantum wells with GaMnN injection layers

The spin injection dynamics of GaMnN∕InGaN multiquantum well (MQW) light emitting diodes (LEDs) grown by molecular beam epitaxy were examined using picosecond-transient and circularly polarized photoluminescence (PL) measurements. Even with the presence of a room temperature ferromagnetic GaMnN spin injector, the LEDs are shown to exhibit very low efficiency of spin injection. Based on resonant optical orientation spectroscopy, the spin loss in the structures is shown to be largely due to fast spin relaxation within the InGaN MQW, which itself destroys any spin polarization generated by optical spin orientation or electrical spin injection. Typical photoluminescence decay times were 20–40ns in both commercial GaN MQW LEDs with emission wavelengths between 420–470nm and in the GaMnN∕InGaN multi-quantum well MQW LEDs. In the wurtzite InGaN∕GaN system, biaxial strain at the interfaces give rise to large piezoelectric fields directed along the growth axis. This built-in piezofield breaks the reflection symmetry of confining potential leading to the presence of a large Rashba term in the conduction band Hamiltonian which is responsible for the short spin relaxation times.

Polarization field effects on the recombination dynamics in low-In-content InGaN multi-quantum wells

The interplay of polarization fields and free carrier screening in In x Ga 1− x N/GaN ( 0.03 < x < 0.07 ) multiple quantum wells is studied by combining photoluminescence (time-integrated and time-resolved) and cathodoluminescence studies, in an excitation density range from 10 8 to 10 12 cm −2 of generated e–h pairs. For such low In content, the quantum-confined Stark effect is verified to rule the recombination dynamics, while effects of carrier localization in potential fluctuations have a minor role. Efficient field screening is demonstrated in CL steady-state high-injection conditions and in PL time-resolved experiments at the maximum excitation density. Under recovered nearly flat band conditions, quantum confinement effects are revealed and a high and possibly composition-dependent bowing parameter is extrapolated. Information on radiative and non-radiative rates for carrier recombination in the wells is obtained, both from steady-state and from time-resolved experiments, modelling the carrier dynamics in the framework of a theoretical rate equation model, which calculates electronic states and recombination rates in the nanostructure by coupling complete self-consistent solutions of Schrödinger and Poisson equations.

Optical and electrical characterization of (Ga,Mn)N/InGaN multiquantum well light-emitting diodes

(Ga,Mn)/N/InGaN multiquantum well (MQW) diodes were grown by molecular beam epitaxy (MBE). The current-voltage characteristics of the diodes show the presence of a parasitic junction between the (Ga,Mn)N and the n-GaN in the top contact layer due to the low conductivity of the former layer. Both the (Ga,Mn)N/InGaN diodes and control samples without Mn doping show no or very low (up to 10% at the lowest temperatures) optical (spin) polarization at zero field or 5 T, respectively. The observed polarization is shown to correspond to the intrinsic optical polarization of the InGaN MQW, due to population distribution between spin sublevels at low temperature, as separately studied by resonant optical excitation with a photon energy lower than the bandgap of both the GaN and (Ga,Mn)N. This indicates efficient losses in the studied structures of any spin polarization generated by optical spin orientation or electrical spin injection. The observed vanishing spin injection efficiency of the spin light-emitting diode (LED) is tentatively attributed to spin losses during the energy relaxation process to the ground state of the excitons giving rise to the light emission.