GaN Multiple Quantum Wells Research Articles

Photoluminescence study of semipolar {101¯1} InGaN∕GaN multiple quantum wells grown by selective area epitaxy

Semipolar InGaN∕GaN multiple quantum wells (MQWs) were fabricated on the {101¯1} facets of GaN pyramidal structures by selective area epitaxy. Optical properties of the MQWs were investigated by photoluminescence (PL) in comparison with (0001) MQWs. Compared with (0001) MQWs, the internal electric field in the {101¯1} MQWs was remarkably reduced, the PL peak redshifted monotonically with the increasing temperature, and the internal quantum efficiency was estimated to be improved by a factor of 3. These results suggest that the {101¯1} planes are promising for improving the performance of III-nitride light emitters owing to their surface stability and suppression of polarization effects.

Proposal and achievement of novel structure InN∕GaN multiple quantum wells consisting of 1 ML and fractional monolayer InN wells inserted in GaN matrix

The authors propose and demonstrate the fabrication of InN∕GaN multiple quantum well (MQW) consisting of 1 ML and fractional monolayer InN well insertion in GaN matrix under In-polarity growth regime. Since the critical thickness of InN epitaxy on GaN is about 1 ML and the growth temperature for 1 ML InN insertion can be remarkably higher, the proposed MQW structure can avoid/reduce generation of misfit dislocation, resulting in higher quality MQW-structure nature in principle than former InN-based MQWs. The proposed InN∕GaN MQWs are potentially applicable to room temperature operating excitonic devices working in short-wavelength visible colors.

Photocurrent gain mechanism in Schottky barrier photodiodes with negative average electric field

A photocurrent gain mechanism which takes advantage of the piezoelectric fields present in devices based on polar heterostructures is proposed. Piezoelectrically induced electric fields can be designed to generate a negative average electric field (NAF) region under certain bias conditions. For carrier transport limited by the barrier formed by the NAF region, photoinduced screening will result in photocurrent gain. This mechanism allows one to explain the experimental results obtained in Schottky barrier photodiodes with (In,Ga)N∕GaN multiple quantum wells embedded in their active region. Responsivities higher than 1A∕W and low dark currents below 10nA∕mm2 at forward voltage were obtained.

Dependence of carrier localization in InGaN∕GaN multiple-quantum wells on well thickness

Carrier localization in InGaN∕GaN multiple-quantum wells (MQWs) with three different well thicknesses was investigated optically using time-integrated and time-resolved microphotoluminescence spectroscopy. An anomalous temperature dependence of the photoluminescence peak energy was observed, as a consequence of local potential fluctuations. The carrier localization was more prominent in the case of MQWs with wide well thickness. The results indicate that the degree of potential fluctuation increases with increasing well thickness. Emission from quantum-dot-like states only became apparent in MQWs with wide well thickness, which supports the assertion that carrier localization in InGaN∕GaN MQWs is due to the formation of quantum dots.

Epitaxial growth and optical properties of semipolar (112¯2) GaN and InGaN∕GaN quantum wells on GaN bulk substrates

GaN and InGaN∕GaN multiple quantum well (MQW) were grown on semipolar (112¯2) GaN bulk substrates by metal organic vapor phase epitaxy. The GaN homoepitaxial layer has an atomically flat surface. Optical reflection measurements reveal polarization anisotropy for the A, B, and C excitons. Free A excitons dominate the photoluminescence (PL) spectrum at 10K and are accompanied by a weaker, sharp doublet emission due to neutral donor-bound excitons. The InGaN∕GaN MQW grown on a GaN homoepitaxial layer involves fast radiative recombination processes. The PL decay monitored at 428nm can be fitted with a double exponential curve, which has lifetimes of 46 and 142ps at 10K. These values are two orders of magnitude shorter than those in conventional c-oriented QWs and are attributed to the weakened internal electric field. The emissions from GaN and MQW polarize along the [11¯00] direction with polarization degrees of 0.46 and 0.69, respectively, due to the low crystal symmetry.

Structural and optical properties of InGaN∕GaN multiple quantum wells grown on nano-air-bridged GaN template

Structural and optical properties of InGaN∕GaN multiple quantum wells (MQWs) grown on nano-air-bridged GaN template by metal organic chemical vapor deposition were investigated. The InGaN∕GaN MQWs on nano-air-bridged GaN demonstrate much better surface morphology, revealing low defect density ∼4×108cm−2 with step flow features measured by atomic force microscopy. The photoluminescence measurement shows one magnitude higher in intensity from less defective InGaN MQWs compared to that of the control InGaN MQWs. The improvement in photoluminescence of the InGaN MQWs is benefited from the reduction of threading dislocation density in the InGaN∕GaN active layers and GaN template, revealed from cross-sectional transmission electron microscopy. High resolution x-ray diffraction analysis results show higher indium mole fraction in the MQWs when grown on nano-air-bridged GaN template, due to the strain relaxation in the nano-air-bridged GaN template. This higher indium incorporation is consistent with the redshift of the photoluminescence peak.

Effects of the material polarity on the green emission properties of InGaN∕GaN multiple quantum wells

Green-light-emission InGaN∕GaN multiple quantum wells (MQWs) with different polarities were grown by metal organic chemical vapor deposition. A clear phase separation was observed both in the Ga- and N-polarity samples by high resolution transmission electron microscopy, corresponding to two InGaN-related emissions (In-rich dots and an InGaN matrix) seen in photoluminescence spectra. The dot-related emission in the Ga-polarity MQWs shows stronger carrier localization, as well as a weak influence of defects and temperature insensitivity, when compared to the N-polarity MQWs. In addition, efficient carrier transport, from the low-indium InGaN matrix to high-indium In-rich dots, was observed in the Ga-polarity structure, enhancing the function of quantum-dot structures with Ga polarity, and resulting in a high quantum yield of green light emission.

Effects of compressive strain on optical properties of InxGa1−xN∕GaN quantum wells

In 0.2 Ga 0.8 N ∕ Ga N multiple quantum well (MQW) blue light emitting diode (LED) structure was grown on a specially designed sapphire substrate (with increasing thickness from the edge to the center within a single wafer). X-ray diffraction revealed that the GaN lattice constant c decreases continuously from the edge to the center, indicating a continuous variation in the compressive strain. The spectral peak positions of the electroluminescence (EL) spectra exhibited a blueshift when probed at the edge as compared to the center, which is a direct consequence of the continuous variation in the compressive strain across the wafer. Based on the experimental results, a ratio of elastic stiffness constants (C33∕C13) for GaN was deduced to be ∼5.0±1.0, which was in agreement with the calculated value of ∼4.0. A linear relation of the EL emission peak position of LEDs with the biaxial strain was observed, and a linear coefficient of 19meV∕GPa characterizing the relationship between the band gap energy and biaxial stress of In0.2Ga0.8N∕GaN MQWs was also obtained.

Nonradiative traps in InGaN∕GaN multiple quantum wells revealed by two wavelength excitation

The authors report an investigation of two wavelength excited photoluminescence on InGaN∕GaN multiple quantum wells. It is found that with an addition below-gap excitation the photoluminescence intensity can be quenched by up to 50%. In addition, the decay time of localized carriers changes from 173.9to7.9ns. The experimental results can be well explained in terms of the electronic transitions involving deep defect states in the GaN barrier. Based on the variation of the photon energy of the below-gap excitation, the origin of the deep trap can be identified. The authors point out here that two wavelength excitation spectroscopy is a powerful tool to reveal nonradiative defects in optoelectronic devices.

Near ultraviolet emission from nonpolar cubic AlxGa1−xN∕GaN quantum wells

In this contribution the authors studied the optical properties of cubic AlxGa1−xN∕GaN single and multiple quantum wells. The well widths ranged from 2.5to7.5nm. Samples were grown by rf-plasma assisted molecular beam epitaxy on free standing 3C-SiC (001) substrates. During growth of Al0.15Ga0.85N∕GaN quantum wells clear reflection high energy electron diffraction oscillations were observed indicating a two dimensional growth mode. They observe strong room temperature, ultraviolet photoluminescence at about 3.3eV with a minimum linewidth of 90meV. The peak energy of the emission versus well width is reproduced by a square-well Poisson-Schrödinger model calculation.

Strong potential profile fluctuations and effective localization process in InGaN∕GaN multiple quantum wells grown on {10-1m} faceted surface GaN template

Cathodoluminescence (CL) and time-resolved photoluminescence (TRPL) spectroscopy were used to investigate the relation between the surface morphology and emission efficiency in 10× InGaN(3nm)∕GaN(4nm) quantum wells (QWs) deposited by plasma-assisted molecular beam epitaxy (MBE). For this study, two QWs with peak emission around 405nm but grown on different surface morphologies have been investigated. A strong increase in the emission efficiency was observed in the QWs grown on {10-1m} faceted surface GaN template (m>2) as compared to those grown on an atomically smooth template. CL mapping and temperature-dependent PL studies revealed that the QWs grown on the faceted surface GaN epilayer exhibit much stronger in-plane indium content fluctuations and larger width PL peak in the temperature range of 8–300K. We found that the use of {10-1m} faceted surface GaN template resulted in strong potential profile fluctuations (PPFs) inducing different localization centers at different energy levels. We found that the deeper the corresponding fluctuation of the energy level, the weaker the decrease of the PL intensity with increasing temperature, the higher the PL decay time (τPL) in the whole temperature range and the slower the collapse of τPL. Our results demonstrate that the use of {10-1m} faceted surface morphology GaN template is an amplifying process of the PPFs which favors a regime dominated by the recombination of localized carriers.

Indium surfactant effect on AlN∕GaN heterostructures grown by metal-organic vapor-phase epitaxy: Applications to intersubband transitions

We report on a dramatic improvement of the optical and structural properties of AlN∕GaN multiple quantum wells (MQWs) grown by metal-organic vapor-phase epitaxy using indium as a surfactant. This improvement is observed using photoluminescence as well as x-ray diffraction. Atomic force microscopy shows different surface morphologies between samples grown with and without In. This is ascribed to a modified relaxation mechanism induced by different surface kinetics. These improved MQWs exhibit intersubband absorption at short wavelength (2μm). The absorption linewidth is as low as 65meV and the absorption coefficient is increased by 85%.

Metalorganic vapor phase epitaxy grown InGaN∕GaN light-emitting diodes on Si(001) substrate

We present GaN-based light emitting diode structures on a Si(001) substrate. The 2.3μm thick, crack-free layers were grown by metalorganic vapor phase epitaxy using a high-temperature AlN seed layer and 4° off-oriented substrates. This allows us to grow a flat, fully coalesced, and single crystalline GaN layer on Si(001). For preventing crack formation, four AlN interlayers were inserted in the buffer structure. The optically active layers consist of five-fold InGaN∕GaN multiple quantum wells showing a bright electroluminescence at 490nm at room temperature. The crystallographic structure was analyzed by x-ray diffraction measurements and the optical properties were determined by photo- and electroluminescence.

Effect of annealing on optical properties of InGaN/GaN multiple quantum wells

Site-selective photoluminescence, photoluminescence excitation, and time-resolved luminescence in as-grown and thermally treated In0:15Ga0:85N / GaN multiple quantum wells (MQWs) was investigated as a function of well width in the temperature range of 10n300 K. Thermal annealing at 800 ‐ C for 30 min monitored by microstructure imaging was shown to result in an alteration of MQWs optical properties intricately depending on the well thickness. The observed blue shift of the luminescence band and pronounced changes in the absorbance indicate a remarkable interdiffusion of indium at the quantum wellnbarrier interface for MQWs with thin (2 nm) wells. Meanwhile for thicker (3 nm) wells, a pronounced red shift of the luminescence band and an increase in the luminescence decay time was observed and attributed to electronnhole wave function separation facilitated by the smoothened band potential prole. In the thickest wells (4 nm), annealing resulted in even more pronounced improvement of microstructure, which led to a noticeable reduction of the localization energy of the electronic excitations (an annealing-invoked luminescence peak blue shift that overweighs the red one caused by intrinsic eld) and to suppression of nonradiative recombination (an increase in luminescence efcienc y). The results are accounted for in terms of annealing-invoked InnGa interdiffusion, which behaves as either diffusion of indium to barriers or iuphilli diffusion within the wells depending on the well width.

Midinfrared intersubband absorption in lattice-matched AlInN∕GaN multiple quantum wells

We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInN∕GaN multiple-quantum-wells. A clear absorption peak is observed around 3μm involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInN∕GaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82In0.18N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInN∕GaN system is very promising to achieve crack-free and low dislocation density structures dedicated to intersubband devices in the 2–4μm wavelength range.

In Ga N ∕ Ga N multi-quantum-well structures on (111)-oriented bonded silicon-on-insulator substrates

We report growth of InGaN∕GaN multiple quantum wells (MQWs) on (111)-oriented bonded silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition (MOCVD). Prior to MOCVD growth of MQWs, about a 1.2μm thick GaN layer was deposited on SOI substrate with a high-temperature transitional buffer layer. The growth conditions were tuned to realize blue-green emission peaks centered around 420–495nm from such MQWs on SOI. X-ray diffraction, atomic force microscopy, scanning electronic microscopy, and photoluminescence techniques were used to characterize these MQWs. Such an approach to realize multicolor light-emitting layers on SOI substrates is suitable for the integration of InGaN∕GaN-based optoelectronic structures on SOI-based micro-optoelectromechanical systems and sensors.

High efficiency and brightness of blue light emission from dislocation-free InGaN∕GaN quantum well nanorod arrays

We report on the optical properties of blue-light-emitting, dislocation-free InGaN∕GaN multiple quantum well (MQW) nanorod arrays (NRAs) with high brightness and high efficiency. The InGaN MQW NRAs were grown by metal-organic hydride vapor phase epitaxy and the optical properties were investigated in detail by photoluminescence (PL), PL excitation (PLE) and time resolved PL. We observed a large Stokes-like shift between InGaN PL emission and PLE absorption edge due to the influence of built-in internal electrical field, reflecting the coherent growth of MQW along the NRA growth direction. From the temperature-dependent PL, we extracted a PL intensity ratio at 300to15K of ∼55.4% and large thermal activation energy of ∼171meV from the InGaN∕GaN MQW NRAs. Time-resolved PL results showed almost the same decay time of the InGaN emission at 20 and 300K. From the results, the optical properties are dominated by the radiative recombination process and are insensitive to temperature due to large thermal activation energy, indicating that carriers in InGaN wells are well confined by the GaN barriers without an influence of other non-radiative processes. Therefore, we conclude that internal quantum efficiency and extraction efficiency of MQW NRAs are significantly enhanced by a drastic suppression of non-radiative centers inside NRAs and a large surface area to active volume area ratio of NRAs, respectively.

Influence of dislocations on photoluminescence of InGaN∕GaN multiple quantum wells

The influence of dislocations on photoluminescence (PL) of InGaN∕GaN multiple quantum wells (MQWs) is investigated by triple-axis x-ray diffraction (TAXRD), transmission electron microscopy (TEM), and PL spectra. The ω scan of every satellite peak by TAXRD is adopted to evaluate the mean screw and edge dislocation densities in MQWs. The results show that dislocations can lead to a reduction of the PL-integrated intensity of InGaN∕GaN MQWs under certain conditions, with edge dislocations playing a decisive role. Additionally, the dislocations can broaden the PL peak, but the effect becomes evident only under the condition when the interface roughness is relatively low.

Ultraviolet electroabsorption modulator based on AlGaN∕GaN multiple quantum wells

An ultraviolet electroabsorption modulator based on AlGaN∕GaN quantum wells is demonstrated. Enhanced excitonic absorption in the quantum wells at around 3.48eV was achieved using a Schottky contact to partially cancel the polarization-induced electric fields in the quantum well layers. A change in the absorption coefficient greater than 4×104cm−1 was obtained for a modest reverse bias of 10V. The observed blueshift in the exciton energy was smaller than that predicted by theoretical calculations. This is accounted for by variations in the background carrier concentration in the wells with reverse bias.

Room temperature near-ultraviolet emission from In-rich InGaN∕GaN multiple quantum wells

We grew In-rich InGaN∕GaN multiple quantum wells (MQWs) using growth interruption (GI) by metalorganic chemical vapor deposition. The quality of overgrown InGaN∕GaN QW layers in MQWs was largely affected by the crystalline quality and interfacial abruptness of the underlying QW layer. Introduction of 10s GI was very effective in improving the crystalline quality and interfacial abruptness of InGaN QW layers, and we grew a ten periods of 1-nm-thick In-rich InGaN∕GaN MQW with 10s GI and obtained a strong near-ultraviolet (UV) emission (∼390nm) at room temperature. We believe that use of less than 1-nm-thick In-rich InGaN MQW can be a candidate for near-UV source, which might replace the conventional low-indium content (<10%), thicker InGaN QW layer.